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EP4636109A1 - Steel sheet and method for manufacturing steel sheet - Google Patents

Steel sheet and method for manufacturing steel sheet

Info

Publication number
EP4636109A1
EP4636109A1 EP23903529.8A EP23903529A EP4636109A1 EP 4636109 A1 EP4636109 A1 EP 4636109A1 EP 23903529 A EP23903529 A EP 23903529A EP 4636109 A1 EP4636109 A1 EP 4636109A1
Authority
EP
European Patent Office
Prior art keywords
steel sheet
less
hot
content
rolling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP23903529.8A
Other languages
German (de)
French (fr)
Inventor
Kyohei ISHIKAWA
Takafumi Yokoyama
Kengo Takeda
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4636109A1 publication Critical patent/EP4636109A1/en
Pending legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a steel sheet and a manufacturing method of the steel sheet.
  • steel sheets for a vehicle in order to reduce a weight of a vehicle body and improve fuel efficiency in consideration of the global environment, there is a significantly increasing demand for high strength steel sheets having a small sheet thickness and excellent formability.
  • the steel sheets for a vehicle particularly for high strength steel sheets used for vehicle body frame components, a high strength is required, and furthermore, high formability for wide applications is required.
  • vehicle components are formed by pressing or the like, the vehicle components are required to have excellent formability (for example, uniform elongation and bendability) even with a high strength.
  • Patent Document 1 discloses a high strength steel sheet having a thickness middle part and a surface layer soft part formed on one side or both sides of the thickness middle part, in which, in a cross section of the high strength steel sheet, a microstructure of the thickness middle part includes, by area ratio, tempered martensite: 85% or more, one or two or more of ferrite, bainite, pearlite, and residual austenite: less than 15% in total, and quenched martensite: less than 5%, a microstructure of the surface layer soft part includes, by area ratio, ferrite: 65% or more, pearlite: 5% or more and less than 20%, one or two or more of tempered martensite, bainite, and residual austenite: less than 10% in total, and quenched martensite: less than 5%, a thickness of the surface layer soft part formed on one side or both sides is more than 10 ⁇ m and is 15% or less of a sheet thickness, an average interval between the pearlite and the pearlit
  • Patent Document 2 discloses a technique for controlling an in-plane anisotropy of bendability of a steel sheet to smaller in addition to achieving a high hole expansion ratio as a workability. This makes it possible to perform press forming into particularly severe shapes, and allows greater freedom in cutting from steel sheets.
  • a steel sheet having smaller bending anisotropy than the steel sheets described in Patent Documents 1 and 2 is required from a viewpoint of absorbing energy during collision.
  • shapes of members of vehicle have become complicated, and it is required to improve a degree of freedom in assembly of the components.
  • the steel sheet described in Patent Document 2 is preferable in that the degree of freedom in assembly of the components is improved by reducing bending anisotropy of the steel sheet, the steel sheet cannot cover a high strength steel sheet required for the impact absorbing member, for example, a strength region of 1180 MPa or more.
  • the present invention has been contrived in view of the above-described circumstances, and an object thereof is to provide a steel sheet having excellent bendability and small bending anisotropy, and a manufacturing method of the steel sheet.
  • the present inventors have studied in detail a chemical composition, a microstructure, and manufacturing conditions that affect bending anisotropy of the steel sheet. As a result, the present inventors found that bending anisotropy can be reduced by strictly controlling anisotropy of a structure of a surface layer with a predetermined chemical composition and microstructure.
  • the present invention has been contrived in view of the above findings.
  • the gist of the present invention is as described below.
  • the chemical composition and microstructure of a steel sheet according to an embodiment of the present invention (hereinafter, may be simply referred to as the steel sheet according to the present embodiment), and a rolling condition and an annealing condition, and so on of a manufacturing method capable of manufacturing the steel sheet will be described in detail below.
  • the steel sheet according to the present embodiment is applied not only to a steel sheet with no plating layer on a surface thereof but also to a base metal of a steel sheet with a plating layer formed on a surface thereof.
  • the "base metal” is a part mainly including Fe in a thickness middle region of a plated steel sheet from which a plating layer has been removed.
  • Such a plated steel sheet includes, for example, a hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface thereof and a hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface thereof.
  • Main conditions described below are common to the base metal of the hot-dip galvanized steel sheet, the hot-dip galvannealed steel sheet, or the like.
  • Carbon (C) is an essential element for high-strengthening of the steel sheet.
  • the C content is set to 0.09% or more.
  • the C content is preferably 0.11% or more.
  • the C content is set to 0.25% or less.
  • the C content is preferably 0.20% or less from the viewpoint of suppressing a deterioration in press formability and weldability.
  • Silicon (Si) is a solid solution strengthening element and is an effective element for high-strengthening of the steel sheet.
  • the Si content is set to 0.01% or more.
  • the Si content is preferably 0.30% or more.
  • the Si content is set to 2.00% or less.
  • the Si content is preferably 1.50% or less.
  • Mn is an element that has an action of improving the hardenability of steel and is effective for obtaining a desired microstructure.
  • the Mn content is set to 1.5% or more.
  • the Mn content is preferably 2.0% or more.
  • the Mn content is set to 3.5% or less.
  • the Mn content is preferably 3.0% or less.
  • the P content is an element that segregates at grain boundaries, thereby embrittling steel and deteriorating bendability. Therefore, the P content is preferably as small as possible and may be 0%. Considering time and cost for removing P, the P content is set to 0.100% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less.
  • the S content is an element that forms a sulfide-based inclusion and deteriorates bendability. Therefore, the S content is preferably as small as possible and may be 0%. Considering time and cost for removing S, the S content is set to 0.0500% or less. The S content is preferably 0.0100% or less, more preferably 0.0030% or less, and still more preferably 0.0010% or less.
  • N is an element that forms coarse nitrides in the steel sheet, and deteriorates bendability and hole expansibility of the steel sheet.
  • the N content is set to 0.0100% or less.
  • the N content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.
  • the N content may be set to 0.0001% or more.
  • the N content may be set to 0.0005% or more.
  • O is an element that forms coarse oxides in steel and deteriorates bendability and hole expansibility. In a case where the O content is more than 0.0060%, the above deteriorations become significant. Therefore, the O content is set to 0.0060% or less.
  • the O content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less.
  • the O content is preferably as small as possible. However, it is not economically preferable that the O content be less than 0.0001% due to an excessive increase in cost. Therefore, the O content may be set to 0.0001% or more. The O content may be set to 0.0010% or more.
  • the steel sheet according to the present embodiment may contain the above elements and a remainder of Fe and impurities.
  • the impurities are elements that are mixed in from a raw material such as ore or a scrap or due to a variety of factors in manufacturing steps during industrial manufacturing of steel and are allowed to an extent that the properties of the steel sheet according to the present embodiment are not adversely affected.
  • the impurities also include elements that are not intentionally added to the steel sheet.
  • the steel sheet according to the present embodiment may further contain the following one or two or more elements (optional elements) of Al, Cr, Mo, W, Co, B, Nb, Ti, V, Ta, Sn, Sb, As, Ni, Cu, Ca, Zr, Mg, and REM to improve the various properties. Since these elements do not need to be contained, the lower limits thereof in content are 0%. In addition, even in a case where the following elements are contained as impurities, the effects of the steel sheet according to the present embodiment are not impaired.
  • Al is an element that has a deoxidizing action on the steel and. Therefore, Al may be contained in steel.
  • the Al content is preferably set to 0.001% or more.
  • the Al content is preferably 0.005% or more.
  • the Al content is set to 1.000% or less.
  • the Al content is preferably 0.900% or less.
  • Cr is an element that increases hardenability and is effective in high-strengthening of the steel sheet. Therefore, Cr may be contained in the steel.
  • the Cr content is preferably 0.001% or more.
  • the Cr content is set to 2.000% or less.
  • Mo is an effective element for reinforcing the steel sheet. Therefore, Mo may be contained in steel.
  • the Mo content is preferably 0.010% or more.
  • B is an element that suppress a generation of ferrite and pearlite in a cooling process from austenite and promote the generation of a low temperature transformation structure such as bainite or martensite.
  • B is an element useful for high-strengthening of the steel sheet. Therefore, B may be contained in steel.
  • the B content is preferably 0.0001% or more.
  • the B content is set to 0.0100% or less.
  • Nb is an effective element for controlling the morphology of carbide and the inclusion of Nb refines the structure, it is also an effective element for improving the toughness of the steel sheet. Therefore, Nb may be contained in steel. In order to obtain the effects of Nb, the Nb content is preferably set to 0.01% or more.
  • the Nb content is set to 0.50% or less.
  • Ti is an element that is important in controlling the morphology of carbide, and is an element that promotes an increase in strength of ferrite by being contained in a large amount. Therefore, Ti may be contained in steel. In order to obtain the effect, the Ti content is preferably set to 0.0001% or more.
  • the Ti content is set to 0.5000% or less.
  • V is an effective element for controlling the morphology of carbide and the inclusion of V refines the structure, it is also an effective element for improving the toughness of the steel sheet. Therefore, V may be contained in steel. In order to obtain the effects of V, the V content is preferably 0.001% or more.
  • the V content is set to 0.500% or less.
  • Ta is an effective element for controlling the morphology of carbide and improving strength of the steel sheet. Therefore, Ta may be contained in steel. In order to obtain the above-described effect, the Ta content is preferably 0.001% or more.
  • the Ta content is set to 0.100% or less.
  • the Ta content is more preferably 0.020% or less.
  • the Ta content is still more preferably 0.011% or less.
  • Sn is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet.
  • Sn may cause a decrease in hole expansibility and bendability of the steel sheet due to the embrittlement of ferrite. Therefore, the Sn content is preferably as small as possible.
  • the Sn content is set to 0.050%.
  • the Sn content is more preferably 0.040% or less.
  • the Sn content may be 0%. However, reducing the Sn content to less than 0.001% results in excessive increase in refining cost. Therefore, the Sn content may be set to 0.001% or more.
  • Sb is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet.
  • Sb is likely to segregate at grain boundaries, and may cause the embrittlement of the grain boundaries, a decrease in ductility, and a decrease in hole expansibility and bendability. Therefore, the Sb content is preferably as small as possible.
  • the Sb content is set to 0.050% or less.
  • the Sb content is more preferably 0.040% or less.
  • the Sb content may be 0%. However, reducing the Sb content to less than 0.001% results in excessive increase in refining cost. Therefore, the Sb content may be set to 0.001% or more.
  • the As content is preferably as small as possible.
  • the As content is set to 0.050% or less.
  • the As content is more preferably 0.040% or less.
  • the As content may be 0%.
  • reducing the As content to less than 0.001% results in excessive increase in refining cost. Therefore, the As content may be set to 0.001% or more.
  • Ni is an effective element for improving strength of the steel sheet. Therefore, Ni may be contained in steel. In order to obtain the above-described effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.010% or more. On the other hand, in a case where the Ni content is too large, ductility of the steel sheet may decrease, and thus hole expansibility and bendability may decrease. Therefore, the Ni content is set to 1.000% or less. The Ni content is preferably 0.800% or less.
  • Cu is an element that contributes to an improvement in strength of the steel sheet. Therefore, Cu may be contained in steel.
  • the Cu content is preferably 0.001% or more.
  • the Cu content is set to 1.000% or less.
  • the Cu content is preferably 0.500% or less.
  • Ca is an element that can control the morphology of sulfide with a small amount. Therefore, Ca may be contained in steel.
  • the Ca content is preferably 0.001% or more.
  • the Ca content is set to 0.050% or less.
  • the Ca content is preferably 0.030% or less.
  • Zr is an element that can control the morphology of sulfide with a small amount. Therefore, Zr may be contained in steel.
  • the Zr content is preferably 0.001% or more.
  • the Zr content is set to 0.050% or less.
  • the Zr content is preferably 0.040% or less.
  • Mg is an element that controls the morphologies of sulfide and oxide and contributes to an improvement in bendability of the steel sheet. Therefore, Mg may be contained in steel.
  • the Mg content is preferably 0.0001% or more.
  • the Mg content is set to 0.050% or less.
  • the Mg content is preferably 0.040% or less.
  • REM stands for rare earth metal (rare earth element). REM is an element that effectively acts to control the morphology of sulfide even in a case where the amount thereof is small. Therefore, REM may be contained in steel. In order to obtain the above-described effect, the REM content is preferably 0.001% or more. However, in a case where the REM content is too large, the workability, fracture resistance, hole expansibility, and bendability may decrease due to the generation of a coarse REM oxide. Therefore, the REM content is preferably set to 0.100% or less, and more preferably 0.059% or less.
  • REM is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu).
  • the "REM” mentioned in the present embodiment includes one or more selected from these rare earth elements, and the “REM content” is the total amount of the rare earth elements.
  • the chemical composition of the steel sheet according to the present embodiment can be obtained by the following method.
  • the chemical composition of the steel sheet described above may be measured using a general chemical composition.
  • the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-nondispersive infrared absorption method.
  • the chemical composition may be analyzed after removing the plating layer by mechanical grinding.
  • a C concentration is measured in the sheet thickness direction of the steel sheet using glow discharge optical emission spectrometry (GDS), and a position where the C concentration in a range of 200 ⁇ m deep in the sheet thickness direction from the point at which the detection of C is started is at a minimum value is determined as the boundary between the plating layer and the steel sheet.
  • GDS glow discharge optical emission spectrometry
  • the term "surface (of the steel sheet)” refers to the above-described boundary being regarded as the “surface (of the steel sheet)" in a steel sheet having a plating layer on a surface thereof.
  • microstructural fractions are indicated by area ratios. Accordingly, unless otherwise specified, "%" represents "area%".
  • the surface serving as a reference of the 1/4 depth position means the surface of the base steel sheet excluding the plating layer in a case of the plated steel sheet.
  • the microstructure at a 1/4 depth position includes, 0% to 40% of ferrite and bainite in total, 50% to 100% of martensite, and 0% to 10% of pearlite and residual austenite in total.
  • Ferrite is a soft phase generated by performing dual phase annealing or performing slow cooling after holding in the annealing step.
  • a hard phase such as martensite
  • ductility of the steel sheet is improved.
  • bainite is a phase generated by holding at 350°C to 450°C for a certain period of time during cooling after holding at the annealing temperature. Bainite is softer than martensite, and thus has an effect for improvement of ductility. However, in order to achieve a predetermined strength, it is necessary to limit the area ratio of bainite as in the case of ferrite.
  • the total of the area ratios of ferrite and bainite is set to 40% or less.
  • the total of the area ratios of ferrite and bainite is preferably 30% or less. Since ferrite and bainite may not be contained, the lower limit thereof is 0%. In addition, the area ratio of each of ferrite and bainite is not limited.
  • the area ratio of martensite is set to 50% or more.
  • the area ratio of martensite is preferably 70% or more, and more preferably 85% or more. Therefore, a high tensile strength (for example, a tensile strength of 1180 MPa or more) is easily ensured. From the viewpoint of strength, the area ratio of martensite may be 100%.
  • the "martensite” refers to both of fresh martensite and tempered martensite. That is, in the present embodiment, the "area ratio of martensite” refers to the “total of area ratios of fresh martensite and tempered martensite”.
  • the "fresh martensite” is martensite containing no iron-based carbide.
  • the "tempered martensite” is martensite containing an iron-based carbides.
  • the area ratio of tempered martensite is desirably high, but the area ratio of each of fresh martensite and tempered martensite is not limited.
  • the degree of tempering may be adjusted within a general range according to the required strength, toughness, hole expansibility, and ductility level.
  • Pearlite is a hard structure in which soft ferrite and hard cementite are arranged in layers, and is a structure that contributes to an improvement in tensile strength of the steel sheet.
  • residual austenite is a structure that contributes to an improvement in elongation by transformation induced plasticity (TRIP). Since pearlite and residual austenite may not be contained, the lower limit of the area ratio of each of pearlite and residual austenite is 0%.
  • Pearlite is a structure having cementite in the structure and consumes carbon (C) in steel that contributes to an improvement in strength. Therefore, in a case where the area ratio of pearlite is excessive, strength of the steel sheet decreases. In addition, in a case where the area ratio of residual austenite becomes excessive, the grain size of residual austenite increases. Such residual austenite having a large grain size becomes coarse and hard martensite after deformation. In this case, the origins of cracks are likely to occur, and bendability deteriorates. Therefore, the total of the area ratios of pearlite and residual austenite is set to 0% to 10%. For example, the area ratio of pearlite is set to 5.0% or less. The area ratio of pearlite is preferably 3.0% or less, and more preferably 1.0% or less. The area ratio of residual austenite is set to, for example, 10.0% or less. The area ratio of residual austenite is preferably 5.0% or less.
  • a region to be residual austenite is determined in a predetermined observation region, and then the identification of ferrite, bainite, martensite, or pearlite is performed in the same observation region.
  • the area and the area ratio of residual austenite are measured by the following method. That is, an observation surface is finished by colloidal silica polishing or electrolytic polishing, at a position of 1/4 of the sheet thickness away from the surface of the steel sheet, the diffraction electrons are measured by EBSD attached to a scanning electron microscope in a square region of 100 ⁇ m ⁇ 100 ⁇ m at intervals of 0.2 ⁇ m (lattice-like arrangement) in the thickness direction and the rolling direction respectively, and the obtained pseudo-Kikuchi pattern is analyzed to identify a crystal orientation and crystal system.
  • the preparation conditions of sample are within the range of the conditions recommended in the Japan Society of Materials Science standard "Standard for Crystal Orientation Difference Measurement for Material Evaluation by Electron Backscatter Diffraction (EBSD) Method".
  • EBSD Electron Backscatter Diffraction
  • the areas and the area ratios of ferrite, bainite, martensite, and pearlite are measured by the following method.
  • the same square region as that for the observation of the residual austenite described above is etched with a Nital solution and photographed (a magnification: 5000 times) using a field emission scanning electron microscope (FE-SEM).
  • the ratio of each structure obtained by the point counting method at intervals of 2 ⁇ m (lattice-like arrangement) from the obtained structure photograph is defined as an area ratio of each structure.
  • Ferrite has a granular or needle-like morphology and contains no iron-based carbide inside.
  • Bainite has a lath-like morphology (lath structure), and is a region in which an iron-based carbide having a major axis of 20 nm or longer is present in the lath structure and the carbide is stretched in the same direction.
  • Martensite has a lath-like morphology (lath structure), and is a region containing no iron-based carbide having a major axis of 20 nm or longer in the lath structure.
  • Tempered martensite has a lath-like morphology (lath structure), and is a region in which an iron-based carbide having a major axis of 20 nm or longer is present in the lath structure and the carbide is stretched in different directions.
  • regions where ferrite and cementite are lamellar are defined as pearlite.
  • the above-described visual identification is not performed on the region determined as residual austenite in the preceding identification of residual austenite.
  • the area ratio is calculated as a ratio to a total area of the observation range including the residual austenite region.
  • microstructures The determination of the microstructures is generally performed as a normal operation by those skilled in the art and can be easily determined by those skilled in the art.
  • a region from the surface of the steel sheet to 20 ⁇ m along the sheet thickness direction of the steel sheet according to the present embodiment is defined as a surface layer soft part.
  • a region starting from the surface of the steel sheet and ending at a depth of 20 ⁇ m from the surface is defined as a surface layer soft part.
  • ferrite is 50% or more, and the total of one or two or more of martensite, bainite, pearlite, and residual austenite is 0% to 50%.
  • the area ratio of ferrite in the surface layer soft part is set to 50% or more.
  • the area ratio of ferrite in the surface layer soft part is more preferably 60% or more.
  • the upper limit of the area ratio of ferrite may be 100%.
  • the total of the area ratios of one or two or more of martensite, bainite, pearlite, and residual austenite in the surface layer soft part is more than 50%, bendability of the steel sheet according to the present embodiment may deteriorate. Therefore, the total of the area ratios of one or two or more of martensite, bainite, pearlite, and residual austenite in the surface layer soft part is set to 50% or less. Since martensite, bainite, pearlite, and residual austenite of the surface layer soft part may not be contained, each lower limit thereof is 0%.
  • the method of calculating the area ratio of each microstructure at the 1/4 depth position can be used for the identification of each microstructure and the calculation of the area and area ratio except for the measurement position.
  • the measurement area is a square region from the surface of the steel sheet to a depth of 20 ⁇ m (a square region of 20 ⁇ m in the sheet thickness direction and 20 ⁇ m in the rolling direction, with the surface of the steel sheet as one side).
  • Hs/Hc which is a ratio of Vickers hardness Hs of the surface layer soft part of the steel sheet according to the present embodiment to Vickers hardness Hc of the 1/4 depth position, is 0.65 or less. In a case where the Hs/Hc is 0.65 or less, bendability of the steel sheet can be improved.
  • the Vickers hardness of the surface layer soft part of the steel sheet and the Vickers hardness of the 1/4 depth position can be measured by the following procedure.
  • a sheet thickness cross section parallel to a plane formed by the rolling direction and the sheet thickness direction of the steel sheet is mirror-finished by mechanical polishing.
  • the Vickers hardness (HV) is measured at 12 points on a straight line parallel to the rolling direction at respective depth points of a distance (depth) of 5 ⁇ m, a distance (depth) of 10 ⁇ m, a distance (depth) of 15 ⁇ m, and a distance (depth) of 20 ⁇ m with an indentation load of 20 gf.
  • the measurement points of 12 points at each sheet thickness position are measured with an interval of 3 or larger indentation sizes.
  • the average value of the Vickers hardness of 10 points excluding the lowest and the highest values among the 12 points measured at each depth position is defined as the Vickers hardness at that depth position.
  • the average value of the Vickers hardness at each depth position is defined as the Vickers hardness (Hs) of the surface layer soft part of the steel sheet.
  • the Vickers hardness (HV) is measured at 12 points on a straight line parallel to the rolling direction at a distance (depth) of 1/4 of the sheet thickness from the surface of the steel sheet toward the inside of the sheet thickness with an indentation load of 20 gf.
  • the average value of the Vickers hardness of 10 points excluding the lowest and the highest values among the measured 12 points is defined as the Vickers hardness of the 1/4 depth position.
  • the interval between the measurement points is preferably set to a distance of 4 times or longer the indentation.
  • distance of 4 times or longer the indentation refers to a distance obtained by multiplying a diagonal length of an indentation produced by a diamond indenter by a value of 4 times or more when measuring the Vickers hardness.
  • the ratio of the L cross section to the C cross section of Taylor factor M value is 0.95 or more. That is, M L /M c , which is a ratio of Taylor factor M value M L of a L cross section to Taylor factor M value M c of a C cross section, is 0.95 or more. M L /M c is more preferably 0.97 or more. M L /M c may be 1.20 or less. M L /M c is more preferably 1.10 or less.
  • the M value is a ratio of yield stress to critical resolved shear stress in uniaxial tension of polycrystalline substance, and is called Taylor factor.
  • the M value of the surface layer soft part defined as described above can be said to be the most significant feature of the steel sheet according to the present embodiment.
  • the crystal orientation of the surface layer soft part of the steel sheet according to the present embodiment is controlled so that the ratio of the L cross section to the C cross section of the Taylor factor M value is 0.95 or more, thereby solving the problem of in-plane anisotropy of bending deformation of the steel sheet.
  • the reason for this is not clear, but is considered as follows. It is known that crack initiation during bending deformation is related to a formation of shear bands, and the crystal texture strongly influences the shear band formation.
  • the M value is the sum of shear slip, and a large M value means that unevenness due to slip is likely to be formed on the outermost surface. It is considered that such unevenness of the outermost surface leads to a generation of cracks of bending deformation. Therefore, by bringing the ratio of the M value of the L cross section to the M value of the C cross section close to 1, it is possible to effectively relax the in-plane anisotropy of the bending property of the steel sheet.
  • the Taylor factor M value is obtained by the following procedure. Using an electron backscatter diffraction (EBSD) device attached to a scanning electron microscope, in a region (L cross section) having a depth of 20 ⁇ m in the sheet thickness direction from the surface, that is the direction perpendicular to the steel sheet surface and a length of 50 ⁇ m in the rolling direction, and a region (C cross section) having a depth of 20 ⁇ m in the sheet thickness direction and a length of 50 ⁇ m in the direction perpendicular to the rolling direction, a measurement is performed at intervals (pitches) of 0.2 ⁇ m in the sheet thickness direction and the rolling direction or the direction perpendicular to the rolling direction.
  • EBSD electron backscatter diffraction
  • Grain Average Misorientation is calculated from the measured data. Then, a region where the grain average misorientation is less than 0.5° is regarded as ferrite, and data of only that part is extracted. For the extracted crystal orientation data of only ferrite, calculation is performed by a calculation mode of "Taylor factor" using the OIM software. Only ⁇ 110 ⁇ 111> is considered as the slip system to be considered at this time.
  • strain tensor a tensor represented by the following formula (1) is used.
  • the average value of the M values of all ferrite grains in the measurement field is defined as the M value of ferrite in the surface layer soft part of the cross section.
  • the M value of the L cross section and the C cross section is obtained, and the ratio is calculated. [Formula 1] 1 0 0 0 0 0 0 0 0 ⁇ 1
  • the morphology of island-shaped hard phase in the surface layer soft part is also an important factor for decreasing the anisotropy of bendability.
  • the aspect ratio of the island-shaped hard phase in the surface layer soft part is 3.0 or less, and the major axis of the island-shaped hard phase is 5.0 ⁇ m or shorter.
  • the aspect ratio of the island-shaped hard phase may be 1.0 or more. in a case where the aspect ratio of the island-shaped hard phase in the surface layer soft part is 3.0 or less and the major axis of the island-shaped hard phase is 5.0 ⁇ m or shorter, the anisotropy of bendability can be reduced.
  • the major axis of the island-shaped hard phase may be 1.0 ⁇ m or longer.
  • the island-shaped hard phase is a phase composed of one or more of martensite and residual austenite.
  • the island-shaped hard phase is a region surrounded by other phases (ferrite, bainite, or pearlite).
  • the aspect ratio of the island-shaped hard phase can be calculated by determining each region of ferrite, bainite, martensite, pearlite, and residual austenite in a region having a depth of 20 ⁇ m in the sheet thickness direction from the surface and a length of 50 ⁇ m in the rolling direction by the above-mentioned means, and measuring major axes and minor axes of the island-shaped hard phase (a phase composed of one or more of martensite and residual austenite) present in the observation field.
  • the major axis and the minor axis are determined as follows.
  • the rolling direction is defined as the x-axis and the island-shaped hard phase is sandwiched between 2 parallel lines perpendicular to the x-axis from both sides.
  • the distance between the 2 parallel lines is defined as a size of the island-shaped hard phase in the rolling direction.
  • the direction inclined by 1° from the rolling direction is defined as the x-axis, and the size of the island-shaped hard phase in the 1° direction is determined.
  • the size of the island-shaped hard phase is determined assuming the inclination of the x-axis from the rolling direction of 1° increments to 179°.
  • 180 data are obtained for the size of the island-shaped hard phase, which are obtained by 1° increments from 0° direction to 179° direction.
  • the largest value among the 180 data is defined as the major axis of the island-shaped hard phase.
  • the smallest value is defined as the minor axis of the island-shaped hard phase.
  • the value obtained by dividing the obtained major axis by the minor axis is defined as the aspect ratio of the island-shaped hard phase.
  • the major axis and the aspect ratio are determined for all island-shaped hard phases present in the observation field.
  • the average value of the major axes of all island-shaped hard phases in the observation field is defined as the major axis of the island-shaped hard phase of the surface layer soft part of the steel sheet.
  • the average value of the aspect ratios of all island-shaped hard phases in the observation field is defined as the value of the aspect ratio of the island-shaped hard phase of the surface layer soft part.
  • the rolling direction is determined by the following method.
  • a test piece is collected so that the sheet thickness cross section of the steel sheet can be observed.
  • the sheet thickness cross section of the collected test piece is finished by mirror polishing and observed with an optical microscope.
  • the observation range is 500 ⁇ m wide and an entire sheet thickness, and a region where a brightness is dark is determined as an inclusion.
  • a sheet thickness cross section observed first by the above method as a reference planes parallel to a plane rotated by 5° in a range of 0° to 180° with the sheet thickness direction as an axis are cross-sectional observed.
  • the average values of obtained lengths of long axes of inclusions in each cross section are calculated for each cross section.
  • the direction parallel to the long axis direction of the inclusion in the cross section where the average value of the lengths of the long axes of the inclusions is the maximum is determined as the rolling direction.
  • a Z plane of the sheet (a plane perpendicular to both a longitudinal direction and a width direction of the sheet) is polished to a depth of 1/4 ⁇ t of the sheet thickness, and finished by mirror polishing, and then a Mn concentration map of a region of 500 ⁇ m ⁇ 500 ⁇ m is obtained by EPMA.
  • the longitudinal direction of a stripe pattern is determined as the rolling direction.
  • the rolling direction of the steel sheet may be determined without the above determination method. Even in a case where the steel sheet is processed into a component, the rolling direction can be determined by the method described above for a weakly processed part (for example, a flat part which is not relatively processed) of the component.
  • softening the surface layer of the steel sheet according to the present embodiment has a further favorable effect in addition to the above effect.
  • softening of the surface layer of the steel sheet effectively works for "suppression of in-plane anisotropy of bending property" which is the greatest effect of the present invention.
  • the reason for this is not clear, but the following is considered.
  • the steel sheet according to the present embodiment utilizes a phase utilizing transformation as a microstructure, and grains and precipitates (Fe carbides) are likely to have anisotropic shapes.
  • additional heat treatment or decarburization is applied. It is considered that the anisotropy of the form of grains and precipitates is also relieved, and the morphology is improved especially in the surface layer of the steel sheet, which is effective in suppressing the in-plane anisotropy of bending property.
  • a tensile strength (TS) as a strength that contributes for a reduction in weight of a vehicle body of a vehicle is preferably set to 1180 MPa or more.
  • strength of the steel sheet is preferably 1400 MPa or more, and more preferably 1470 MPa or more.
  • ⁇ L / ⁇ C which is a ratio of the critical bending angle ⁇ L in the direction along the rolling direction to the critical bending angle ⁇ C in the direction perpendicular to the rolling direction and the sheet thickness direction, is preferably set to more than 0.80.
  • the ⁇ L / ⁇ C is preferably 0.85 or more, and more preferably 0.90 or more.
  • the critical bending angle ⁇ C is preferably set to 90° or more in a case where the tensile strength is 1180 to 1470 MPa, and more preferably 95° or more, and still more preferably 100° or more.
  • the critical bending angle ⁇ C is preferably set to 80° or more in a case where the tensile strength is more than 1470 MPa, and more preferably 85° or more, and still more preferably 90° or more.
  • the tensile strength (TS) can be obtained by performing a tensile test according to JIS Z 2241: 2022 with a JIS No. 5 tensile test piece collected from the steel sheet in a direction perpendicular to the rolling direction and the sheet thickness direction.
  • the critical bending angle can be determined by a bending test in accordance with VDA (Verband der Automobilindustrie) 238-100.
  • the critical bending angle in a case where the direction parallel to the rolling direction is defined as the bending edge line is regarded as the critical bending angle ⁇ L in the direction along the rolling direction.
  • the critical bending angle in a case where the bending edge line is defined as the direction parallel to the direction perpendicular to the rolling direction and the sheet thickness direction is regarded as the critical bending angle ⁇ C in the direction perpendicular to the rolling direction and the sheet thickness direction.
  • a deformation during collision in a strength member associated with the steel sheet according to the present embodiment is mainly bending deformation.
  • the bending deformation in this case is very complicated, and it should be considered that the bending deformation of the member is not uniformly generated due to the anisotropy of the shape of the member, the anisotropy of the deformation stress during collision, and so on.
  • This non-uniformity of deformation can be considered as a model in which a minute bending deformation is bent in various directions in each local region of the steel sheet surface.
  • the bending deformation intensively progresses in a part (bending direction) where a resistance to the bending deformation is small, and a fracture in the part occurs and the undeformed region expands, so that an impact absorption amount of the entire member is decreased.
  • the in-plane anisotropy of the critical bending angle by appropriately controlling the in-plane anisotropy of the critical bending angle, the local deformation of the member is avoided, and strain due to deformation during collision is propagated to the entire member. This makes it possible to improve the shock absorbability of the member.
  • the sheet thickness of the steel sheet according to the present embodiment is not limited, but is preferably 0.5 mm or more in consideration of an application to an impact absorbing member.
  • the sheet thickness is more preferably 1.0 mm or more.
  • the sheet thickness of the steel sheet may be, for example, 3.0 mm or less.
  • the sheet thickness is more preferably 2.0 mm or less.
  • the thickness of the steel sheet is obtained by measuring the thickness of at least 3 points of the steel sheet and calculating an average value thereof.
  • the steel sheet according to the present embodiment may have a hot-dip galvanized layer on the surface thereof. Corrosion resistance is improved by providing a plating layer on the surface. In a case where there is a concern about holes due to corrosion in a steel sheet for a vehicle, the steel sheet cannot be thinned to a certain sheet thickness or less in some cases even in a case where the high-strengthening is achieved.
  • One purpose of the high-strengthening of the steel sheet is to reduce the weight by making the steel sheet thinner. Accordingly, even in a case where a steel sheet is developed, the part where the steel sheet is to be applied is limited in a case where the steel sheet has low corrosion resistance.
  • the steel sheet is subjected to plating such as hot-dip galvanizing with high corrosion resistance.
  • the steel sheet components are controlled as described above, and hot-dip galvanizing is thus possible.
  • the hot-dip galvanized layer may also be a hot-dip galvannealed layer.
  • Whether or not the steel sheet has a hot-dip galvanized layer or a hot-dip galvannealed layer is determined by the following method. Using an energy dispersive X-ray spectrometer (EDS), a line analysis is performed in a direction perpendicular to the interface between the plating layer and a matrix phase in a sample cross section containing the plating layer and the matrix phase. The concentration profile of each element is obtained from the plating layer side toward the matrix phase direction.
  • EDS energy dispersive X-ray spectrometer
  • the hot-dip galvannealed layer is determined to be present, and otherwise, the hot-dip galvanizing layer is determined to be present. Note that the quantification of the concentration is performed by the ZAF method.
  • the steel sheet according to the present embodiment may be a steel sheet processed into a component shape. That is, the steel sheet according to the present embodiment may be a component including the steel sheet. Even when the steel sheet is processed into the component, the same effects as those of the steel sheet according to the present embodiment can be obtained since the above-described features are inherited.
  • a part a flat part, for example, with a radius of curvature R of 1000 mm or more
  • an amount of processing is relatively small.
  • the steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (V). Note that unless otherwise specified, the temperature described below refers to surface temperature of a steel sheet or a slab.
  • a slab having the same chemical composition as the steel sheet according to the present embodiment described above is heated and hot-rolled to obtain a hot-rolled steel sheet.
  • Slab heating conditions in the hot rolling are not limited, but the slab is preferably heated to 1100°C or higher. In a case where the heating temperature is lower than 1100°C, the material is likely to be insufficiently homogenized.
  • an inlet temperature of the first finishing stand (the stand of the first pass of finish rolling) is set so that the average temperature from the surface to 10% of the thickness is 850°C or lower.
  • the average temperature from the surface to 10% of the thickness can be estimated. Further, finish rolling cannot be performed since a temperature gradient of the steel sheet is small and a rolling load increases in a case where a temperature in a region of more than 10% of the sheet thickness becomes 850°C or lower, water cooling may be performed so that only the surface layer is cooled before the start of finish rolling in order to provide an appropriate temperature gradient in the sheet thickness direction.
  • the manufacturing method of the slab is not limited.
  • the slab is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be manufactured by an ingot-making method or a thin slab casting method.
  • the heating of the slab may be omitted.
  • the steel sheet (hot-rolled steel sheet) after the hot rolling step is coiled at a coiling temperature of 550°C to 450°C.
  • cooling conditions to the coiling temperature are not particularly limited.
  • the coiling temperature is preferably set to 450 °C or higher.
  • the steel sheet (hot-rolled steel sheet) after coiling is held in the temperature higher than the coiling temperature and 650°C to 500°C for 1 hour or longer.
  • the holding time may be set to 6 hours or shorter.
  • the method of holding such a temperature range is not limited, but for example, a coil may be inserted into a soaking furnace held at the corresponding temperature. Generally, holding in the temperature range for 1 hour or longer is not performed since pickling property deteriorates.
  • fine ferrite By holding at 650°C to 500°C for 1 hour or longer, fine ferrite can be formed on the surface layer of the steel sheet.
  • a ferrite recrystallization in a strain accumulation part of the surface layer can be sufficiently progressed during a temperature rising in the annealing step, and the unrecrystallized ferrite which causes the anisotropy of bendability can be reduced.
  • M L /M c which is the ratio of Taylor factor M value M L of the L cross section to Taylor factor M value M c of the C cross section, can be 0.95 or more.
  • the steel sheet (hot-rolled steel sheet) after the holding step is descaled as necessary by pickling or the like using a known method, and then cold rolling is performed under the conditions that the sheet thickness reduction is 20% to 80% and the average dynamic friction coefficient of the last two passes is 0.025 or more to obtain a cold-rolled steel sheet.
  • the average dynamic friction coefficient of the last two passes is preferably set to 0.035 or more, more preferably set to 0.055 or more.
  • the thickness reduction can be expressed by (1-t1/t0) ⁇ 100 (%), where t0 is a sheet thickness before cold rolling and t1 is a sheet thickness after cold rolling.
  • the average dynamic friction coefficient of the last two passes is, with the measured rolling load, obtained by using the dynamic mean deformation resistance equation of Gokyu and Kihara, the roll flattening equation of Hitchcock, and the cold rolling theoretical equation of Bland and Ford.
  • the cold-rolled steel sheet after the cold rolling step may be subjected to a treatment such as degreasing according to a known method, if necessary.
  • Hs/Hc can be set to 0.65 or less.
  • the microstructure of the surface layer soft part can be made to have 50% or more of ferrite and 0 to 50% of the total of one or two or more of as-quenched martensite, pearlite, and residual austenite.
  • the holding time in the annealing temperature is preferably 60 seconds or longer. In a case where the holding time in the annealing temperature is shorter than 60 seconds, decarburization does not sufficiently progress, and the ferrite fraction of the surface layer soft part may not be 50% or more.
  • the steel sheet in the manufacturing of a cold-rolled steel sheet (hot-dip galvanized steel sheet) having a hot-dip galvanized layer on a surface thereof, in a process that cooling is performed after annealing (annealing and cooling process), in a state in which the temperature of the surface of the steel sheet is higher than 425°C and lower than 600°C, the steel sheet may be subjected to hot-dip galvanizing by being immersed in a plating bath at the same temperature.
  • a cold-rolled steel sheet (hot-dip galvannealed steel sheet) having a hot-dip galvannealed plating on the surface thereof
  • an alloying heat treatment of heating the steel sheet at higher than 450°C and lower than 600°C may be further performed subsequent to the hot-dip galvanizing step to form a hot-dip galvannealed plating.
  • the cold-rolled steel sheet after the annealing step is cooled to a temperature range of 50°C to 250°C at a cooling rate of 30 °C/s or faster, thereby untransformed austenite is transformed into martensite.
  • the cold-rolled steel sheet is tempered at 200°C to 350°C for 1 second or longer, thereby a structure mainly having martensite (tempered martensite) can be stably obtained at the 1/4 depth position.
  • tempering step is not an essential step.
  • the cold-rolled steel sheet after the hot-dip galvanizing step or the cold-rolled steel sheet after the hot-dip galvanizing step and the alloying step is cooled to a temperature range of 50°C to 250°C, and then tempered for 1 second or longer at a temperature range of 200°C to 350°C.
  • the tempering temperature is set to 350°C or lower.
  • the tempering temperature is preferably 325°C or lower, and more preferably 300°C or lower.
  • the tempering temperature is set to 200°C or higher.
  • the tempering temperature is preferably 220°C or higher, and more preferably 250°C or higher.
  • the tempering time may be 1 second or longer, but is preferably 5 seconds or longer, and more preferably 10 seconds or longer in order to perform a stable tempering treatment.
  • the tempering time is preferably 750 seconds or shorter, and more preferably 500 seconds or shorter.
  • the cold-rolled steel sheet after the tempering step may be cooled to a temperature at which skin pass rolling can be performed, and then subjected to skin pass rolling.
  • the cooling after annealing is water spray cooling, dip cooling, air-water cooling, or the like in which water is used, it is preferable to perform pickling, and then plating with a small amount of one or two or more of Ni, Fe, Co, Sn, and Cu before skin pass rolling in order to remove an oxide film formed by contact with water at a high temperature and improve chemical convertibility of the steel sheet.
  • the small amount refers to a plating amount of about 3 to 30 mg/m 2 on the steel sheet surface.
  • the shape of the steel sheet can be adjusted by the skin pass rolling.
  • the elongation ratio of the skin pass rolling is preferably 0.05% or more.
  • the elongation ratio of the skin pass rolling is more preferably 0.10% or more.
  • the elongation ratio is preferably set to 1.00% or less.
  • the elongation ratio is more preferably 0.75% or less, and still more preferably 0.50% or less.
  • Annealing Step Remarks Dew Point (°C) Annealing Temperature (°C) Holding Time (s) Presence or Absence of Plating Presence or Absence of Alloying 21 6 799 89 Absence Absence Comparative Example 22 -2 832 105 Presence Presence 23 Fractured in Cold Rolling Step 24 -1 760 136 Absence Absence 25 5 766 136 Absence Absence 26 -9 820 147 Presence Presence 27 6 790 136 Presence Absence 28 -16 755 123 Presence Presence 29 -20 820 155 Presence Presence 30 -9 800 136 Presence Absence 31 Fractured in Cold Rolling Step 32 0 880 105 Absence Absence 33 -12 840 123 Presence Absence 34 2 890 136 Presence Presence 35 -2 888 123 Presence Presence 36 -45 820 100 Presence Presence 37 14 730 155 Presence Absence 38 -13 830 20 Presence Presence 39 -10 850 80 Presence Presence Invention Example 40 Fractured
  • the microstructure of the 1/4 depth position and the microstructure of the surface layer soft part were evaluated by the method described above.
  • the direction of rolling of the steel sheet was known from the manufacturing process, and therefore the determination of the rolling direction was not performed.
  • the tensile strength (TS), the critical bending angle (C-axis bending angle) ⁇ C , and the anisotropy of the critical bending angle were evaluated in the following manner.
  • the tensile strength (TS) was obtained by performing a tensile test according to JIS Z 2241: 2022 with a JIS No. 5 tensile test piece collected from the cold-rolled steel sheet in a direction perpendicular to a rolling direction. In a case where the tensile strength was 1180 MPa or more, the steel sheet was determined as having a high strength.
  • the critical bending angle was determined by the bending test described above in accordance with VDA (Verband der Automobilindustrie) 238-100. In a case where the tensile strength was 1180 to 1470 MPa, it was determined as being acceptable when the critical bending angle ⁇ C was 90° or more. In a case where the tensile strength was more than 1470 MPa, it was determined as being acceptable when the critical bending angle ⁇ C was 80° or more. When the above criteria were not satisfied, it was determined as not having excellent bendability and being unacceptable.
  • the reason why the criteria of the critical bending angle ⁇ C is changed depending on the tensile strength as described above is to take the following background into consideration.
  • the absolute value of the critical bending angle increases with decreasing tensile strength.
  • the absolute value of the critical bending angle is at a level comparable to that of the steel material of the invention example.
  • the critical bending angles of Nos. 21 and 24 are only high because they are steel materials having relatively low tensile strength. In other words, in a case where the steel sheet has the same chemical composition and tensile strength as Nos. 21 and 24 and satisfies Hs/Hc, which is the feature of the present application, the critical bending angle is likely to be further increased.

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Abstract

A steel sheet according to one aspect of the present invention has a predetermined chemical composition, Hs/Hc, which is a ratio of Vickers hardness Hs of a surface layer soft part to Vickers hardness Hc of a 1/4 depth position, is 0.65 or less, in ferrite of the surface layer soft part, ML/Mc, which is a ratio of Taylor factor M value ML of a L cross section to Taylor factor M value Mc of a C cross section, is 0.95 or more, and an aspect ratio is 3.0 or less and a major axis is 5.0 µm or shorter of island-shaped hard phase of the surface layer soft part.

Description

    TECHNICAL FIELD
  • The present invention relates to a steel sheet and a manufacturing method of the steel sheet.
  • Priority is claimed on Japanese Patent Application No. 2022-200340, filed on December 15, 2022 , the content of which is incorporated herein by reference.
    • BACKGROUND ART
  • With regard to steel sheets for a vehicle, in order to reduce a weight of a vehicle body and improve fuel efficiency in consideration of the global environment, there is a significantly increasing demand for high strength steel sheets having a small sheet thickness and excellent formability. Among the steel sheets for a vehicle, particularly for high strength steel sheets used for vehicle body frame components, a high strength is required, and furthermore, high formability for wide applications is required.
  • In addition, since vehicle components are formed by pressing or the like, the vehicle components are required to have excellent formability (for example, uniform elongation and bendability) even with a high strength.
  • As such a high strength steel sheet, Patent Document 1 discloses a high strength steel sheet having a thickness middle part and a surface layer soft part formed on one side or both sides of the thickness middle part, in which, in a cross section of the high strength steel sheet, a microstructure of the thickness middle part includes, by area ratio, tempered martensite: 85% or more, one or two or more of ferrite, bainite, pearlite, and residual austenite: less than 15% in total, and quenched martensite: less than 5%, a microstructure of the surface layer soft part includes, by area ratio, ferrite: 65% or more, pearlite: 5% or more and less than 20%, one or two or more of tempered martensite, bainite, and residual austenite: less than 10% in total, and quenched martensite: less than 5%, a thickness of the surface layer soft part formed on one side or both sides is more than 10 µm and is 15% or less of a sheet thickness, an average interval between the pearlite and the pearlite in the surface layer soft part is 3 µm or more, and Vickers hardness (Hc) of the thickness middle part and Vickers hardness (Hs) of the surface layer soft part satisfy 0.50 ≤ Hs/Hc ≤ 0.75.
  • In addition, Patent Document 2 discloses a technique for controlling an in-plane anisotropy of bendability of a steel sheet to smaller in addition to achieving a high hole expansion ratio as a workability. This makes it possible to perform press forming into particularly severe shapes, and allows greater freedom in cutting from steel sheets.
    • Citation List Patent Document
    • SUMMARY OF INVENTION Technical Problem
  • In an impact absorbing member of a vehicle, a steel sheet having smaller bending anisotropy than the steel sheets described in Patent Documents 1 and 2 is required from a viewpoint of absorbing energy during collision. In recent years, shapes of members of vehicle have become complicated, and it is required to improve a degree of freedom in assembly of the components. Although the steel sheet described in Patent Document 2 is preferable in that the degree of freedom in assembly of the components is improved by reducing bending anisotropy of the steel sheet, the steel sheet cannot cover a high strength steel sheet required for the impact absorbing member, for example, a strength region of 1180 MPa or more.
  • The present invention has been contrived in view of the above-described circumstances, and an object thereof is to provide a steel sheet having excellent bendability and small bending anisotropy, and a manufacturing method of the steel sheet. Solution to Problem
  • The present inventors have studied in detail a chemical composition, a microstructure, and manufacturing conditions that affect bending anisotropy of the steel sheet. As a result, the present inventors found that bending anisotropy can be reduced by strictly controlling anisotropy of a structure of a surface layer with a predetermined chemical composition and microstructure.
  • The present invention has been contrived in view of the above findings. The gist of the present invention is as described below.
    1. (1) A steel sheet according to a first aspect in the present invention containing, as a chemical composition, by mass%:
      • C: 0.09% to 0.25%;
      • Si: 0.01% to 2.00%;
      • Mn: 1.5% to 3.5%;
      • P: 0.100% or less;
      • S: 0.0500% or less;
      • N: 0.0100% or less;
      • O: 0.0060% or less;
      • Al: 0% to 1.000%;
      • Cr: 0% to 2.000%;
      • Mo: 0% to 1.000%;
      • W: 0% to 1.000%;
      • Co: 0% to 0.500%;
      • B: 0% to 0.0100%;
      • Nb: 0% to 0.50%;
      • Ti: 0% to 0.5000%;
      • V: 0% to 0.500%;
      • Ta: 0% to 0.100%;
      • Sn: 0% to 0.050%;
      • Sb: 0% to 0.050%;
      • As: 0% to 0.050%;
      • Ni: 0% to 1.000%;
      • Cu: 0% to 1.000%;
      • Ca: 0% to 0.050%;
      • Zr: 0% to 0.050%;
      • Mg: 0% to 0.050%;
      • REM: 0% to 0.100%, and
      • a remainder containing Fe and impurities,
      • in a microstructure at a 1/4 depth position that is a position away from a surface by 1/4 of a sheet thickness along a sheet thickness direction, by area ratio,
      • a total of ferrite and bainite is 0% to 40%,
      • martensite is 50% to 100%, and
      • a total of pearlite and residual austenite is 0% to 10%, and
      • in a microstructure of a surface layer soft part that is a region from the surface to 20 µm along the sheet thickness direction, by area ratio,
      • ferrite is 50% or more, and
      • a total of one or two or more of martensite, bainite, pearlite, and residual austenite is 0% to 50%,
      • Hs/Hc, which is a ratio of Vickers hardness Hs of the surface layer soft part to Vickers hardness Hc of the 1/4 depth position, is 0.65 or less,
      • in the ferrite of the surface layer soft part, ML/Mc, which is a ratio of Taylor factor M value ML of a L cross section to Taylor factor M value Mc of a C cross section, is 0.95 or more, and
      • an aspect ratio is 3.0 or less and a major axis is 5.0 µm or shorter of island-shaped hard phase of the surface layer soft part.
    2. (2) A second aspect of the present invention, in the steel sheet of the first aspect,
      in which θL/θC, which is a ratio of a critical bending angle θL in a direction along a rolling direction to a critical bending angle θC in a direction perpendicular to the rolling direction and the sheet thickness direction, may be more than 0.80.
    3. (3) A third aspect of the present invention, in the steel sheet of the first aspect or second aspect,
      in which a tensile strength may be 1180 MPa or more.
    4. (4) A fourth aspect of the present invention, in any one of the steel sheet of the first aspect to third aspect,
      • in which the critical bending angle θC may be 90° or more in a case where the tensile strength is 1180 to 1470 MPa, and
      • the critical bending angle θC may be 80° or more in a case where the tensile strength is more than 1470 MPa.
    5. (5) A fifth aspect of the present invention, in any one of the steel sheet of the first aspect to fourth aspect,
      in which a sheet thickness may be 1.0 mm or more.
    6. (6) A sixth aspect of the present invention, in any one of the steel sheet of the first aspect to fifth aspect.
      • in which the chemical composition may contains, by mass%, one or two or more of
      • Al: 0.001% to 1.000%,
      • Cr: 0.001% to 2.000%,
      • Mo: 0.010% to 1.000%,
      • W: 0.001% to 1.000%,
      • Co: 0.010% to 0.500%,
      • B: 0.0001% to 0.0100%,
      • Nb: 0.01% to 0.50%,
      • Ti: 0.0001% to 0.5000%,
      • V: 0.001% to 0.500%,
      • Ta: 0.001% to 0.100%,
      • Sn: 0.001% to 0.050%,
      • Sb: 0.001% to 0.050%,
      • As: 0.001% to 0.050%,
      • Ni: 0.010% to 1.000%,
      • Cu: 0.001% to 1.000%,
      • Ca: 0.001% to 0.050%,
      • Zr: 0.001% to 0.050%,
      • Mg: 0.0001% to 0.050%, and
      • REM: 0.001% to 0.100%.
    7. (7) A seventh aspect of the present invention may have a hot-dip galvanized layer on the surface in any one of the steel sheet of the first aspect to sixth aspect.
    8. (8) An eighth aspect of the present invention may have a hot-dip galvannealed layer on the surface in any one of the steel sheet of the first aspect to sixth aspect.
    9. (9) A ninth aspect of the manufacturing method of the present invention may include:
      • a hot rolling step of heating a slab containing, as a chemical composition, by mass%,
      • C: 0.09% to 0.25%;
      • Si: 0.01% to 2.00%;
      • Mn: 1.5% to 3.5%;
      • P: 0.100% or less;
      • S: 0.0500% or less;
      • N: 0.0100% or less;
      • O: 0.0060% or less;
      • Al: 0% to 1.000%;
      • Cr: 0% to 2.000%;
      • Mo: 0% to 1.000%;
      • W: 0% to 1.000%;
      • Co: 0% to 0.500%;
      • B: 0% to 0.0100%;
      • Nb: 0% to 0.50%;
      • Ti: 0% to 0.5000%;
      • V: 0% to 0.500%;
      • Ta: 0% to 0.100%;
      • Sn: 0% to 0.050%;
      • Sb: 0% to 0.050%;
      • As: 0% to 0.050%;
      • Ni: 0% to 1.000%;
      • Cu: 0% to 1.000%;
      • Ca: 0% to 0.050%;
      • Zr: 0% to 0.050%;
      • Mg: 0% to 0.050%;
      • REM: 0% to 0.100%, and
      • a remainder containing Fe and impurities, and performing hot rolling that starts finish rolling under a condition that an average temperature from a surface to 10% of a thickness is 850°C or lower at a first finishing stand to obtain a hot-rolled steel sheet;
      • a coiling step of coiling the hot-rolled steel sheet at a coiling temperature of 550°C to 450°C after the hot rolling step;
      • a holding step of holding the hot-rolled steel sheet at a temperature higher than the coiling temperature and 650°C to 500°C for 1 hour or longer,
      • a cold rolling step of performing cold rolling on the hot-rolled steel sheet under conditions that a sheet thickness reduction is 20% to 80% and an average dynamic friction coefficient of last two passes is 0.025 or more after the holding step to obtain a cold-rolled steel sheet; and
      • an annealing step of annealing the cold-rolled steel sheet by holding in a temperature range of 740°C to 900°C at an atmosphere having a dew point of -30°C to 20°C for 60 seconds or longer after the cold rolling step.
    Advantageous Effects of Invention
  • According to the aspect of the present invention, it is possible to provide a steel sheet having excellent bendability and small bending anisotropy, and a manufacturing method of the steel sheet.
    • DESCRIPTION OF EMBODIMENTS
  • The chemical composition and microstructure of a steel sheet according to an embodiment of the present invention (hereinafter, may be simply referred to as the steel sheet according to the present embodiment), and a rolling condition and an annealing condition, and so on of a manufacturing method capable of manufacturing the steel sheet will be described in detail below.
  • The steel sheet according to the present embodiment is applied not only to a steel sheet with no plating layer on a surface thereof but also to a base metal of a steel sheet with a plating layer formed on a surface thereof. Here, the "base metal" is a part mainly including Fe in a thickness middle region of a plated steel sheet from which a plating layer has been removed. Such a plated steel sheet includes, for example, a hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface thereof and a hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface thereof. Main conditions described below are common to the base metal of the hot-dip galvanized steel sheet, the hot-dip galvannealed steel sheet, or the like.
    • <Chemical Composition>
  • First, the chemical composition of the steel sheet according to the present embodiment will be described. The symbol "%" indicating an amount of each element in the chemical composition means mass% unless otherwise specified.
    • [C: 0.09% to 0.25%]
  • Carbon (C) is an essential element for high-strengthening of the steel sheet. In a case where the C content is less than 0.09%, a sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.09% or more. The C content is preferably 0.11% or more.
  • On the other hand, in a case where the C content is more than 0.25%, weldability decreases and bendability deteriorates. Therefore, the C content is set to 0.25% or less. The C content is preferably 0.20% or less from the viewpoint of suppressing a deterioration in press formability and weldability.
    • [Si: 0.01% to 2.00%]
  • Silicon (Si) is a solid solution strengthening element and is an effective element for high-strengthening of the steel sheet. In order to obtain the above effect, the Si content is set to 0.01% or more. The Si content is preferably 0.30% or more.
  • On the other hand, in a case where Si is excessively contained, the steel sheet is embrittled, and manufacturability and workability may decrease. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.50% or less.
    • [Mn: 1.5% to 3.5%]
  • Mn is an element that has an action of improving the hardenability of steel and is effective for obtaining a desired microstructure. In a case where the Mn content is less than 1.5%, obtaining a desired microstructure becomes difficult. Therefore, the Mn content is set to 1.5% or more. The Mn content is preferably 2.0% or more.
  • On the other hand, in a case where the Mn content is more than 3.5%, a formation of ferrite in the surface layer of the hot-rolled steel sheet or the steel sheet after annealing is suppressed, and bendability and bending anisotropy cannot be improved. Therefore, the post-press working collapse properties deteriorate. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.0% or less.
    • [P: 0.100% or Less]
  • P is an element that segregates at grain boundaries, thereby embrittling steel and deteriorating bendability. Therefore, the P content is preferably as small as possible and may be 0%. Considering time and cost for removing P, the P content is set to 0.100% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less.
    • [S: 0.0500% or Less]
  • S is an element that forms a sulfide-based inclusion and deteriorates bendability. Therefore, the S content is preferably as small as possible and may be 0%. Considering time and cost for removing S, the S content is set to 0.0500% or less. The S content is preferably 0.0100% or less, more preferably 0.0030% or less, and still more preferably 0.0010% or less.
    • [N: 0.0100%]
  • N is an element that forms coarse nitrides in the steel sheet, and deteriorates bendability and hole expansibility of the steel sheet. In a case where the N content is more than 0.0100%, since the above deteriorations become significant, the N content is set to 0.0100% or less. The N content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.
  • On the other hand, in a case where the N content is set to less than 0.0001%, the manufacturing cost increases significantly. Therefore, the N content may be set to 0.0001% or more. The N content may be set to 0.0005% or more.
    • [O: 0.0060% or Less]
  • O is an element that forms coarse oxides in steel and deteriorates bendability and hole expansibility. In a case where the O content is more than 0.0060%, the above deteriorations become significant. Therefore, the O content is set to 0.0060% or less. The O content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less.
  • The O content is preferably as small as possible. However, it is not economically preferable that the O content be less than 0.0001% due to an excessive increase in cost. Therefore, the O content may be set to 0.0001% or more. The O content may be set to 0.0010% or more.
  • The steel sheet according to the present embodiment may contain the above elements and a remainder of Fe and impurities. Here, the impurities are elements that are mixed in from a raw material such as ore or a scrap or due to a variety of factors in manufacturing steps during industrial manufacturing of steel and are allowed to an extent that the properties of the steel sheet according to the present embodiment are not adversely affected. The impurities also include elements that are not intentionally added to the steel sheet.
  • The steel sheet according to the present embodiment may further contain the following one or two or more elements (optional elements) of Al, Cr, Mo, W, Co, B, Nb, Ti, V, Ta, Sn, Sb, As, Ni, Cu, Ca, Zr, Mg, and REM to improve the various properties. Since these elements do not need to be contained, the lower limits thereof in content are 0%. In addition, even in a case where the following elements are contained as impurities, the effects of the steel sheet according to the present embodiment are not impaired.
    • [Al: 0% to 1.000%]
  • Al is an element that has a deoxidizing action on the steel and. Therefore, Al may be contained in steel. In order to obtain the above effect, the Al content is preferably set to 0.001% or more. The Al content is preferably 0.005% or more.
  • On the other hand, in a case where Al is excessively contained, the effect is saturated, and not only does the cost increase, but the transformation temperature of the steel rises, and the load in hot rolling increases. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.900% or less.
    • [Cr: 0% to 2.000%]
  • Cr is an element that increases hardenability and is effective in high-strengthening of the steel sheet. Therefore, Cr may be contained in the steel. For the high-strengthening of the steel sheet by increasing the hardenability by Cr, the Cr content is preferably 0.001% or more. On the other hand, in a case where the Cr content is more than 2.000%, Cr may segregate at a center part of the steel sheet and form a coarse Cr carbide, and the cold formability may be decreased. Therefore, the Cr content is set to 2.000% or less.
    • [Mo: 0% to 1.000%]
  • Mo is an effective element for reinforcing the steel sheet. Therefore, Mo may be contained in steel. For the high-strengthening of the steel sheet by Mo, the Mo content is preferably 0.010% or more.
  • On the other hand, in a case where the Mo content is more than 1.000%, the cost increases and a coarse Mo carbide is formed, which may decrease the cold workability of the steel sheet. Therefore, the Mo content is set to 1.000% or less.
    • [W: 0% to 1.000%]
  • W is an element that forms a carbide and is an effective element for high-strengthening of the steel sheet. Therefore, W may be contained in steel. In order to obtain the above-described effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more. The W content is still more preferably 0.010% or more.
  • On the other hand, even in a case where W is excessively contained, the effect is saturated, and the cost increases. Therefore, in a case where W is contained, the W content is set to 1.000% or less. The W content is more preferably 0.600% or less. The W content is more preferably 0.100% or less.
    • [Co: 0% to 0.500%]
  • Co is an effective element for improving strength of the steel sheet. Therefore, Co may be contained in steel. In order to obtain the above-described effect, the Co content is preferably 0.010% or more, and more preferably 0.050% or more.
  • On the other hand, in a case where the Co content is too large, there is a concern that ductility of the steel sheet may reduce, leading to a decrease in hole expansibility and bendability. Therefore, the Co content is set to 0.500% or less. The Co content is more preferably 0.400% or less.
    • [B: 0% to 0.0100%]
  • B is an element that suppress a generation of ferrite and pearlite in a cooling process from austenite and promote the generation of a low temperature transformation structure such as bainite or martensite. In addition, B is an element useful for high-strengthening of the steel sheet. Therefore, B may be contained in steel. In order to obtain the above-described effect by B, the B content is preferably 0.0001% or more.
  • On the other hand, in a case where the B content is more than 0.0100%, a coarse B oxide or boride, which become origins where voids start to form during press forming, is generated in steel, and the workability of the steel sheet may deteriorate. Therefore, the B content is set to 0.0100% or less.
    • [Nb: 0% to 0.50%]
  • Since Nb is an effective element for controlling the morphology of carbide and the inclusion of Nb refines the structure, it is also an effective element for improving the toughness of the steel sheet. Therefore, Nb may be contained in steel. In order to obtain the effects of Nb, the Nb content is preferably set to 0.01% or more.
  • On the other hand, in a case where the Nb content is more than 0.50%, a large number of coarse Nb carbides are precipitated, which become origins where voids start during press forming, and the workability of the steel sheet may deteriorate. Therefore, the Nb content is set to 0.50% or less.
    • [Ti: 0% to 0.5000%]
  • Ti is an element that is important in controlling the morphology of carbide, and is an element that promotes an increase in strength of ferrite by being contained in a large amount. Therefore, Ti may be contained in steel. In order to obtain the effect, the Ti content is preferably set to 0.0001% or more.
  • On the other hand, in a case where the Ti content is more than 0.5000%, a coarse Ti oxide or Ti carbonitride may be present in steel, thereby decreasing the workability of the steel sheet. Therefore, the Ti content is set to 0.5000% or less.
    • [V: 0% to 0.500%]
  • Since V is an effective element for controlling the morphology of carbide and the inclusion of V refines the structure, it is also an effective element for improving the toughness of the steel sheet. Therefore, V may be contained in steel. In order to obtain the effects of V, the V content is preferably 0.001% or more.
  • On the other hand, in a case where the V content exceeds 0.500%, a large number of fine V carbides may be precipitated, and strength of the steel sheet is increased and ductility significantly deteriorates, resulting in a reduction of workability. Therefore, the V content is set to 0.500% or less.
    • [Ta: 0% to 0.100%]
  • Ta is an effective element for controlling the morphology of carbide and improving strength of the steel sheet. Therefore, Ta may be contained in steel. In order to obtain the above-described effect, the Ta content is preferably 0.001% or more.
  • On the other hand, in a case where the Ta content is too large, a large number of fine Ta carbides may be precipitated, which may cause a decrease in ductility of the steel sheet, resulting in a decrease in hole expansibility and bendability of the steel sheet. Therefore, the Ta content is set to 0.100% or less. The Ta content is more preferably 0.020% or less. The Ta content is still more preferably 0.011% or less.
    • [Sn: 0% to 0.050%]
  • Sn is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet. In addition, Sn may cause a decrease in hole expansibility and bendability of the steel sheet due to the embrittlement of ferrite. Therefore, the Sn content is preferably as small as possible. In order to suppress the decrease in hole expansibility and bendability of the steel sheet, the Sn content is set to 0.050%. The Sn content is more preferably 0.040% or less. The Sn content may be 0%. However, reducing the Sn content to less than 0.001% results in excessive increase in refining cost. Therefore, the Sn content may be set to 0.001% or more.
    • [Sb: 0% to 0.050%]
  • Similar to Sn, Sb is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet. Sb is likely to segregate at grain boundaries, and may cause the embrittlement of the grain boundaries, a decrease in ductility, and a decrease in hole expansibility and bendability. Therefore, the Sb content is preferably as small as possible. In order to suppress the property decreases by the above actions, the Sb content is set to 0.050% or less. The Sb content is more preferably 0.040% or less. The Sb content may be 0%. However, reducing the Sb content to less than 0.001% results in excessive increase in refining cost. Therefore, the Sb content may be set to 0.001% or more.
    • [As: 0% to 0.050%]
  • Similar to Sn and Sb, As is an element that can be contained in the steel sheet in a case where a scrap is used as a raw material of the steel sheet. As is likely to segregate at grain boundaries, and may cause a decrease in hole expansibility and bendability. Therefore, the As content is preferably as small as possible. In order to suppress the decrease in hole expansibility and bendability, the As content is set to 0.050% or less. The As content is more preferably 0.040% or less. The As content may be 0%. However, reducing the As content to less than 0.001% results in excessive increase in refining cost. Therefore, the As content may be set to 0.001% or more.
    • [Ni: 0% to 1.000%]
  • Ni is an effective element for improving strength of the steel sheet. Therefore, Ni may be contained in steel. In order to obtain the above-described effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.010% or more. On the other hand, in a case where the Ni content is too large, ductility of the steel sheet may decrease, and thus hole expansibility and bendability may decrease. Therefore, the Ni content is set to 1.000% or less. The Ni content is preferably 0.800% or less.
    • [Cu: 0% to 1.000%]
  • Cu is an element that contributes to an improvement in strength of the steel sheet. Therefore, Cu may be contained in steel. In order to obtain the above-described effect, the Cu content is preferably 0.001% or more. However, in a case where the Cu content is too large, red shortness may occur and the productivity in hot rolling may decrease. Furthermore, in a case where the Cu content is too large, hole expansibility and bendability may decrease due to the formation of a coarse inclusion. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.500% or less.
    • [Ca: 0% to 0.050%]
  • Ca is an element that can control the morphology of sulfide with a small amount. Therefore, Ca may be contained in steel. In order to obtain the above-described effect, the Ca content is preferably 0.001% or more. However, in a case where the Ca content is too large, a coarse Ca oxide may be generated, and the Ca oxide becomes origins where cracks start during cold forming, and as a result, hole expansibility and bendability may deteriorate. Therefore, the Ca content is set to 0.050% or less. The Ca content is preferably 0.030% or less.
    • [Zr: 0% to 0.050%]
  • Zr is an element that can control the morphology of sulfide with a small amount. Therefore, Zr may be contained in steel. In order to obtain the above-described effect, the Zr content is preferably 0.001% or more. However, in a case where the Zr content is too large, a coarse Zr oxide may be generated, and hole expansibility and bendability may decrease. Therefore, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less.
    • [Mg: 0% to 0.050%]
  • Mg is an element that controls the morphologies of sulfide and oxide and contributes to an improvement in bendability of the steel sheet. Therefore, Mg may be contained in steel. In order to obtain the above-described effect, the Mg content is preferably 0.0001% or more. However, in a case where the Mg content is too large, hole expansibility and bendability may decrease due to the formation of a coarse inclusion. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less.
    • [REM: 0% to 0.100%]
  • REM stands for rare earth metal (rare earth element). REM is an element that effectively acts to control the morphology of sulfide even in a case where the amount thereof is small. Therefore, REM may be contained in steel. In order to obtain the above-described effect, the REM content is preferably 0.001% or more. However, in a case where the REM content is too large, the workability, fracture resistance, hole expansibility, and bendability may decrease due to the generation of a coarse REM oxide. Therefore, the REM content is preferably set to 0.100% or less, and more preferably 0.059% or less.
  • Here, REM is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). The "REM" mentioned in the present embodiment includes one or more selected from these rare earth elements, and the "REM content" is the total amount of the rare earth elements.
  • The chemical composition of the steel sheet according to the present embodiment can be obtained by the following method.
  • The chemical composition of the steel sheet described above may be measured using a general chemical composition. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). In addition, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-nondispersive infrared absorption method. In a case where the steel sheet is provided with a plating layer on a surface thereof, the chemical composition may be analyzed after removing the plating layer by mechanical grinding. Regarding the boundary between the plating layer and the steel sheet, a C concentration is measured in the sheet thickness direction of the steel sheet using glow discharge optical emission spectrometry (GDS), and a position where the C concentration in a range of 200 µm deep in the sheet thickness direction from the point at which the detection of C is started is at a minimum value is determined as the boundary between the plating layer and the steel sheet. In the following description, the term "surface (of the steel sheet)" refers to the above-described boundary being regarded as the "surface (of the steel sheet)" in a steel sheet having a plating layer on a surface thereof.
    • <Microstructure of 1/4 Depth Position>
  • Next, the microstructure at a 1/4 depth position (a position away from the surface by 1/4 of the sheet thickness along the sheet thickness direction) in the steel sheet according to the present embodiment will be described.
  • In the description of the microstructure of the steel sheet according to the present embodiment, microstructural fractions are indicated by area ratios. Accordingly, unless otherwise specified, "%" represents "area%". In the present embodiment, the surface serving as a reference of the 1/4 depth position means the surface of the base steel sheet excluding the plating layer in a case of the plated steel sheet.
  • In the steel sheet according to the present embodiment (including a steel sheet, a hot-dip galvanized steel sheet, and a hot-dip galvannealed steel sheet), the microstructure at a 1/4 depth position (a position away from the surface by 1/4 of the sheet thickness along the sheet thickness direction, i.e., a direction perpendicular to the surface of the steel sheet) includes, 0% to 40% of ferrite and bainite in total, 50% to 100% of martensite, and 0% to 10% of pearlite and residual austenite in total.
    • (Total of Area Ratios of Ferrite and Bainite Is 0% to 40%)
  • Ferrite is a soft phase generated by performing dual phase annealing or performing slow cooling after holding in the annealing step. In a case where ferrite is mixed with a hard phase such as martensite, ductility of the steel sheet is improved. However, in order to achieve a predetermined strength, it is necessary to limit the area ratio of ferrite.
  • In addition, bainite is a phase generated by holding at 350°C to 450°C for a certain period of time during cooling after holding at the annealing temperature. Bainite is softer than martensite, and thus has an effect for improvement of ductility. However, in order to achieve a predetermined strength, it is necessary to limit the area ratio of bainite as in the case of ferrite.
  • Therefore, the total of the area ratios of ferrite and bainite is set to 40% or less. The total of the area ratios of ferrite and bainite is preferably 30% or less. Since ferrite and bainite may not be contained, the lower limit thereof is 0%. In addition, the area ratio of each of ferrite and bainite is not limited.
    • (Area Ratio of Martensite Is 50% to 100%)
  • Since martensite is a hard structure, it contributes for an improvement in tensile strength. In order to obtain a desired strength, the area ratio of martensite is set to 50% or more. The area ratio of martensite is preferably 70% or more, and more preferably 85% or more. Therefore, a high tensile strength (for example, a tensile strength of 1180 MPa or more) is easily ensured. From the viewpoint of strength, the area ratio of martensite may be 100%.
  • In the present embodiment, the "martensite" refers to both of fresh martensite and tempered martensite. That is, in the present embodiment, the "area ratio of martensite" refers to the "total of area ratios of fresh martensite and tempered martensite". The "fresh martensite" is martensite containing no iron-based carbide. In addition, the "tempered martensite" is martensite containing an iron-based carbides.
  • From the viewpoint of toughness, hole expansibility, and ductility, the area ratio of tempered martensite is desirably high, but the area ratio of each of fresh martensite and tempered martensite is not limited. The degree of tempering may be adjusted within a general range according to the required strength, toughness, hole expansibility, and ductility level.
    • (Total of Area Ratios of Pearlite and Residual Austenite Is 0% to 10%)
  • Pearlite is a hard structure in which soft ferrite and hard cementite are arranged in layers, and is a structure that contributes to an improvement in tensile strength of the steel sheet. In addition, residual austenite is a structure that contributes to an improvement in elongation by transformation induced plasticity (TRIP). Since pearlite and residual austenite may not be contained, the lower limit of the area ratio of each of pearlite and residual austenite is 0%.
  • Pearlite is a structure having cementite in the structure and consumes carbon (C) in steel that contributes to an improvement in strength. Therefore, in a case where the area ratio of pearlite is excessive, strength of the steel sheet decreases. In addition, in a case where the area ratio of residual austenite becomes excessive, the grain size of residual austenite increases. Such residual austenite having a large grain size becomes coarse and hard martensite after deformation. In this case, the origins of cracks are likely to occur, and bendability deteriorates. Therefore, the total of the area ratios of pearlite and residual austenite is set to 0% to 10%. For example, the area ratio of pearlite is set to 5.0% or less. The area ratio of pearlite is preferably 3.0% or less, and more preferably 1.0% or less. The area ratio of residual austenite is set to, for example, 10.0% or less. The area ratio of residual austenite is preferably 5.0% or less.
  • Next, methods for identification of each microstructure and for area ratio calculation of the 1/4 depth position will be described.
  • For the identification of each microstructure and the area ratio calculation, first, a region to be residual austenite is determined in a predetermined observation region, and then the identification of ferrite, bainite, martensite, or pearlite is performed in the same observation region.
  • The area and the area ratio of residual austenite are measured by the following method. That is, an observation surface is finished by colloidal silica polishing or electrolytic polishing, at a position of 1/4 of the sheet thickness away from the surface of the steel sheet, the diffraction electrons are measured by EBSD attached to a scanning electron microscope in a square region of 100 µm × 100 µm at intervals of 0.2 µm (lattice-like arrangement) in the thickness direction and the rolling direction respectively, and the obtained pseudo-Kikuchi pattern is analyzed to identify a crystal orientation and crystal system. The preparation conditions of sample are within the range of the conditions recommended in the Japan Society of Materials Science standard "Standard for Crystal Orientation Difference Measurement for Material Evaluation by Electron Backscatter Diffraction (EBSD) Method". The region detected as an FCC phase from the measurement data is regarded as residual austenite, and its area and area ratio are measured.
  • Note that the method of determining the rolling direction will be described later.
  • The areas and the area ratios of ferrite, bainite, martensite, and pearlite are measured by the following method. The same square region as that for the observation of the residual austenite described above is etched with a Nital solution and photographed (a magnification: 5000 times) using a field emission scanning electron microscope (FE-SEM). The ratio of each structure obtained by the point counting method at intervals of 2 µm (lattice-like arrangement) from the obtained structure photograph is defined as an area ratio of each structure.
  • Each structure is determined as follows. Ferrite has a granular or needle-like morphology and contains no iron-based carbide inside. Bainite has a lath-like morphology (lath structure), and is a region in which an iron-based carbide having a major axis of 20 nm or longer is present in the lath structure and the carbide is stretched in the same direction. Martensite has a lath-like morphology (lath structure), and is a region containing no iron-based carbide having a major axis of 20 nm or longer in the lath structure. Tempered martensite has a lath-like morphology (lath structure), and is a region in which an iron-based carbide having a major axis of 20 nm or longer is present in the lath structure and the carbide is stretched in different directions. In addition, regions where ferrite and cementite are lamellar are defined as pearlite.
  • Note that in the identification of ferrite, bainite, martensite, and pearlite, the above-described visual identification is not performed on the region determined as residual austenite in the preceding identification of residual austenite. However, the area ratio is calculated as a ratio to a total area of the observation range including the residual austenite region.
  • The determination of the microstructures is generally performed as a normal operation by those skilled in the art and can be easily determined by those skilled in the art.
  • In a case where the total area ratio of the structures obtained by the above evaluation method is different from 100%, a value obtained by multiplying the area ratio of each structure by 100/(total area ratio of structures) is defined as the area ratio of each structure.
    • <Microstructure of Surface Layer Soft part>
  • A region from the surface of the steel sheet to 20 µm along the sheet thickness direction of the steel sheet according to the present embodiment is defined as a surface layer soft part. In other words, a region starting from the surface of the steel sheet and ending at a depth of 20 µm from the surface is defined as a surface layer soft part.
  • In the microstructure of the surface layer soft part of the steel sheet according to the present embodiment, by area ratio, ferrite is 50% or more, and the total of one or two or more of martensite, bainite, pearlite, and residual austenite is 0% to 50%.
    • [Area Ratio of Ferrite Is 50% or More]
  • Ferrite improves ductility of the steel sheet when mixed with a hard phase such as martensite. In order to improve bendability of the steel sheet, the area ratio of ferrite in the surface layer soft part is set to 50% or more. The area ratio of ferrite in the surface layer soft part is more preferably 60% or more. The upper limit of the area ratio of ferrite may be 100%.
    • [Total of Area Ratio of One or Two or More of Martensite, Bainite, Pearlite, and Residual Austenite Is 0% to 50%]
  • In a case where the total of the area ratios of one or two or more of martensite, bainite, pearlite, and residual austenite in the surface layer soft part is more than 50%, bendability of the steel sheet according to the present embodiment may deteriorate. Therefore, the total of the area ratios of one or two or more of martensite, bainite, pearlite, and residual austenite in the surface layer soft part is set to 50% or less. Since martensite, bainite, pearlite, and residual austenite of the surface layer soft part may not be contained, each lower limit thereof is 0%.
  • Next, methods for identification of each microstructure and for area ratio calculation in the surface layer soft part will be described.
  • The method of calculating the area ratio of each microstructure at the 1/4 depth position can be used for the identification of each microstructure and the calculation of the area and area ratio except for the measurement position. The measurement area is a square region from the surface of the steel sheet to a depth of 20 µm (a square region of 20 µm in the sheet thickness direction and 20 µm in the rolling direction, with the surface of the steel sheet as one side).
    • <Ratio of Vickers Hardness of Surface Layer Soft part to Vickers Hardness of 1/4 Depth Position>
  • Hs/Hc, which is a ratio of Vickers hardness Hs of the surface layer soft part of the steel sheet according to the present embodiment to Vickers hardness Hc of the 1/4 depth position, is 0.65 or less. In a case where the Hs/Hc is 0.65 or less, bendability of the steel sheet can be improved.
  • The Vickers hardness of the surface layer soft part of the steel sheet and the Vickers hardness of the 1/4 depth position can be measured by the following procedure. First, a sheet thickness cross section parallel to a plane formed by the rolling direction and the sheet thickness direction of the steel sheet is mirror-finished by mechanical polishing. Within the polished surface, in accordance with JIS Z 2244-1:2020, from the surface of the steel sheet toward the inside of the sheet thickness, the Vickers hardness (HV) is measured at 12 points on a straight line parallel to the rolling direction at respective depth points of a distance (depth) of 5 µm, a distance (depth) of 10 µm, a distance (depth) of 15 µm, and a distance (depth) of 20 µm with an indentation load of 20 gf. The measurement points of 12 points at each sheet thickness position are measured with an interval of 3 or larger indentation sizes. The average value of the Vickers hardness of 10 points excluding the lowest and the highest values among the 12 points measured at each depth position is defined as the Vickers hardness at that depth position. The average value of the Vickers hardness at each depth position is defined as the Vickers hardness (Hs) of the surface layer soft part of the steel sheet.
  • Similarly, the Vickers hardness (HV) is measured at 12 points on a straight line parallel to the rolling direction at a distance (depth) of 1/4 of the sheet thickness from the surface of the steel sheet toward the inside of the sheet thickness with an indentation load of 20 gf. The average value of the Vickers hardness of 10 points excluding the lowest and the highest values among the measured 12 points is defined as the Vickers hardness of the 1/4 depth position.
  • Note that the interval between the measurement points is preferably set to a distance of 4 times or longer the indentation. The term "distance of 4 times or longer the indentation" as used herein refers to a distance obtained by multiplying a diagonal length of an indentation produced by a diamond indenter by a value of 4 times or more when measuring the Vickers hardness.
    • <Ratio of Taylor Factor M Value of Ferrite In Surface Layer Soft part between L Cross Section and C Cross Section>
  • In ferrite of the surface layer soft part, the ratio of the L cross section to the C cross section of Taylor factor M value is 0.95 or more. That is, ML/Mc, which is a ratio of Taylor factor M value ML of a L cross section to Taylor factor M value Mc of a C cross section, is 0.95 or more. ML/Mc is more preferably 0.97 or more. ML/Mc may be 1.20 or less. ML/Mc is more preferably 1.10 or less.
  • The M value is a ratio of yield stress to critical resolved shear stress in uniaxial tension of polycrystalline substance, and is called Taylor factor. The M value of the surface layer soft part defined as described above can be said to be the most significant feature of the steel sheet according to the present embodiment. The crystal orientation of the surface layer soft part of the steel sheet according to the present embodiment is controlled so that the ratio of the L cross section to the C cross section of the Taylor factor M value is 0.95 or more, thereby solving the problem of in-plane anisotropy of bending deformation of the steel sheet. The reason for this is not clear, but is considered as follows. It is known that crack initiation during bending deformation is related to a formation of shear bands, and the crystal texture strongly influences the shear band formation. The M value is the sum of shear slip, and a large M value means that unevenness due to slip is likely to be formed on the outermost surface. It is considered that such unevenness of the outermost surface leads to a generation of cracks of bending deformation. Therefore, by bringing the ratio of the M value of the L cross section to the M value of the C cross section close to 1, it is possible to effectively relax the in-plane anisotropy of the bending property of the steel sheet.
  • The Taylor factor M value is obtained by the following procedure. Using an electron backscatter diffraction (EBSD) device attached to a scanning electron microscope, in a region (L cross section) having a depth of 20 µm in the sheet thickness direction from the surface, that is the direction perpendicular to the steel sheet surface and a length of 50 µm in the rolling direction, and a region (C cross section) having a depth of 20 µm in the sheet thickness direction and a length of 50 µm in the direction perpendicular to the rolling direction, a measurement is performed at intervals (pitches) of 0.2 µm in the sheet thickness direction and the rolling direction or the direction perpendicular to the rolling direction. The value of Grain Average Misorientation (GAM) is calculated from the measured data. Then, a region where the grain average misorientation is less than 0.5° is regarded as ferrite, and data of only that part is extracted. For the extracted crystal orientation data of only ferrite, calculation is performed by a calculation mode of "Taylor factor" using the OIM software. Only {110}<111> is considered as the slip system to be considered at this time. As the strain tensor, a tensor represented by the following formula (1) is used. The average value of the M values of all ferrite grains in the measurement field is defined as the M value of ferrite in the surface layer soft part of the cross section. The M value of the L cross section and the C cross section is obtained, and the ratio is calculated.
    [Formula 1] 1 0 0 0 0 0 0 0 − 1
    • <Aspect Ratio and Major Axis of Island-shaped Hard Phase in Surface Layer Soft part>
  • The morphology of island-shaped hard phase in the surface layer soft part is also an important factor for decreasing the anisotropy of bendability. The aspect ratio of the island-shaped hard phase in the surface layer soft part is 3.0 or less, and the major axis of the island-shaped hard phase is 5.0 µm or shorter. The aspect ratio of the island-shaped hard phase may be 1.0 or more. in a case where the aspect ratio of the island-shaped hard phase in the surface layer soft part is 3.0 or less and the major axis of the island-shaped hard phase is 5.0 µm or shorter, the anisotropy of bendability can be reduced. The major axis of the island-shaped hard phase may be 1.0 µm or longer. The island-shaped hard phase is a phase composed of one or more of martensite and residual austenite. The island-shaped hard phase is a region surrounded by other phases (ferrite, bainite, or pearlite).
  • The aspect ratio of the island-shaped hard phase can be calculated by determining each region of ferrite, bainite, martensite, pearlite, and residual austenite in a region having a depth of 20 µm in the sheet thickness direction from the surface and a length of 50 µm in the rolling direction by the above-mentioned means, and measuring major axes and minor axes of the island-shaped hard phase (a phase composed of one or more of martensite and residual austenite) present in the observation field. The major axis and the minor axis are determined as follows.
  • First, for one hard island-shaped phase, it is considered that the rolling direction is defined as the x-axis and the island-shaped hard phase is sandwiched between 2 parallel lines perpendicular to the x-axis from both sides. The distance between the 2 parallel lines is defined as a size of the island-shaped hard phase in the rolling direction. Further, the direction inclined by 1° from the rolling direction is defined as the x-axis, and the size of the island-shaped hard phase in the 1° direction is determined. In the same manner, the size of the island-shaped hard phase is determined assuming the inclination of the x-axis from the rolling direction of 1° increments to 179°. In this way, 180 data are obtained for the size of the island-shaped hard phase, which are obtained by 1° increments from 0° direction to 179° direction. The largest value among the 180 data is defined as the major axis of the island-shaped hard phase. The smallest value is defined as the minor axis of the island-shaped hard phase. The value obtained by dividing the obtained major axis by the minor axis is defined as the aspect ratio of the island-shaped hard phase.
  • In the present embodiment, the major axis and the aspect ratio are determined for all island-shaped hard phases present in the observation field. The average value of the major axes of all island-shaped hard phases in the observation field is defined as the major axis of the island-shaped hard phase of the surface layer soft part of the steel sheet. In addition, the average value of the aspect ratios of all island-shaped hard phases in the observation field is defined as the value of the aspect ratio of the island-shaped hard phase of the surface layer soft part.
  • In the present embodiment, the rolling direction is determined by the following method.
  • First, a test piece is collected so that the sheet thickness cross section of the steel sheet can be observed. The sheet thickness cross section of the collected test piece is finished by mirror polishing and observed with an optical microscope. The observation range is 500 µm wide and an entire sheet thickness, and a region where a brightness is dark is determined as an inclusion. Next, a sheet thickness cross section observed first by the above method as a reference, planes parallel to a plane rotated by 5° in a range of 0° to 180° with the sheet thickness direction as an axis are cross-sectional observed. The average values of obtained lengths of long axes of inclusions in each cross section are calculated for each cross section. The direction parallel to the long axis direction of the inclusion in the cross section where the average value of the lengths of the long axes of the inclusions is the maximum is determined as the rolling direction.
  • In a case where no inclusion having a large aspect ratio is found, a Z plane of the sheet (a plane perpendicular to both a longitudinal direction and a width direction of the sheet) is polished to a depth of 1/4×t of the sheet thickness, and finished by mirror polishing, and then a Mn concentration map of a region of 500 µm × 500 µm is obtained by EPMA. In a case where a solidification segregation of Mn is measured in a stripe shape, the longitudinal direction of a stripe pattern is determined as the rolling direction.
  • Note that in a case where the rolling direction of the steel sheet is known in advance, the rolling direction of the steel sheet may be determined without the above determination method. Even in a case where the steel sheet is processed into a component, the rolling direction can be determined by the method described above for a weakly processed part (for example, a flat part which is not relatively processed) of the component.
  • Generally, a technique for improving bendability of a steel sheet by softening the surface layer of the steel sheet is known. However, softening the surface layer of the steel sheet according to the present embodiment has a further favorable effect in addition to the above effect. Specifically, softening of the surface layer of the steel sheet effectively works for "suppression of in-plane anisotropy of bending property" which is the greatest effect of the present invention. The reason for this is not clear, but the following is considered. The steel sheet according to the present embodiment utilizes a phase utilizing transformation as a microstructure, and grains and precipitates (Fe carbides) are likely to have anisotropic shapes. In order to realize softening in the steel sheet having such structure, for example, additional heat treatment or decarburization is applied. It is considered that the anisotropy of the form of grains and precipitates is also relieved, and the morphology is improved especially in the surface layer of the steel sheet, which is effective in suppressing the in-plane anisotropy of bending property.
    • <Mechanical Properties> [Tensile Strength Is 1180 MPa or More] [θL/θC, Which Is Ratio of Critical Bending Angle θL In A Case Where Direction Along Rolling Direction Is Defined As Bending Edge Line to Critical Bending Angle θC In A Case Where Direction perpendicular to Rolling Direction and Sheet Thickness Direction Is Defined as Bending Edge Line, Is More Than 0.80] [Critical Bending Angle θC Is 90° Or More In A Case Where Tensile Strength Is 1180 to 1470 MPa, and Critical Bending Angle θC Is 80° Or More In A Case Where Tensile Strength Is More Than 1470 MPa]
  • In the steel sheet according to the present embodiment, a tensile strength (TS) as a strength that contributes for a reduction in weight of a vehicle body of a vehicle is preferably set to 1180 MPa or more. From the viewpoint of impact absorption properties, strength of the steel sheet is preferably 1400 MPa or more, and more preferably 1470 MPa or more.
  • From the view point of bendability, θL/θC, which is a ratio of the critical bending angle θL in the direction along the rolling direction to the critical bending angle θC in the direction perpendicular to the rolling direction and the sheet thickness direction, is preferably set to more than 0.80. The θL/θC is preferably 0.85 or more, and more preferably 0.90 or more.
  • The critical bending angle θC is preferably set to 90° or more in a case where the tensile strength is 1180 to 1470 MPa, and more preferably 95° or more, and still more preferably 100° or more. The critical bending angle θC is preferably set to 80° or more in a case where the tensile strength is more than 1470 MPa, and more preferably 85° or more, and still more preferably 90° or more.
  • The tensile strength (TS) can be obtained by performing a tensile test according to JIS Z 2241: 2022 with a JIS No. 5 tensile test piece collected from the steel sheet in a direction perpendicular to the rolling direction and the sheet thickness direction.
  • The critical bending angle can be determined by a bending test in accordance with VDA (Verband der Automobilindustrie) 238-100. The critical bending angle in a case where the direction parallel to the rolling direction is defined as the bending edge line is regarded as the critical bending angle θL in the direction along the rolling direction. The critical bending angle in a case where the bending edge line is defined as the direction parallel to the direction perpendicular to the rolling direction and the sheet thickness direction is regarded as the critical bending angle θC in the direction perpendicular to the rolling direction and the sheet thickness direction.
  • In the steel sheet according to the present embodiment, particularly, impact absorbing property can be enhanced by appropriately controlling bending anisotropy. The reason for this is not clear, but is considered as follows. It is assumed that a deformation during collision in a strength member associated with the steel sheet according to the present embodiment is mainly bending deformation. The bending deformation in this case is very complicated, and it should be considered that the bending deformation of the member is not uniformly generated due to the anisotropy of the shape of the member, the anisotropy of the deformation stress during collision, and so on. This non-uniformity of deformation can be considered as a model in which a minute bending deformation is bent in various directions in each local region of the steel sheet surface. Then, the bending deformation intensively progresses in a part (bending direction) where a resistance to the bending deformation is small, and a fracture in the part occurs and the undeformed region expands, so that an impact absorption amount of the entire member is decreased. In the steel sheet according to the present embodiment, by appropriately controlling the in-plane anisotropy of the critical bending angle, the local deformation of the member is avoided, and strain due to deformation during collision is propagated to the entire member. This makes it possible to improve the shock absorbability of the member.
    • <Sheet Thickness>
  • The sheet thickness of the steel sheet according to the present embodiment is not limited, but is preferably 0.5 mm or more in consideration of an application to an impact absorbing member. The sheet thickness is more preferably 1.0 mm or more. The sheet thickness of the steel sheet may be, for example, 3.0 mm or less. The sheet thickness is more preferably 2.0 mm or less.
  • The thickness of the steel sheet is obtained by measuring the thickness of at least 3 points of the steel sheet and calculating an average value thereof.
  • The steel sheet according to the present embodiment may have a hot-dip galvanized layer on the surface thereof. Corrosion resistance is improved by providing a plating layer on the surface. In a case where there is a concern about holes due to corrosion in a steel sheet for a vehicle, the steel sheet cannot be thinned to a certain sheet thickness or less in some cases even in a case where the high-strengthening is achieved. One purpose of the high-strengthening of the steel sheet is to reduce the weight by making the steel sheet thinner. Accordingly, even in a case where a steel sheet is developed, the part where the steel sheet is to be applied is limited in a case where the steel sheet has low corrosion resistance. As a method for solving the problems, it is considered that the steel sheet is subjected to plating such as hot-dip galvanizing with high corrosion resistance. In the steel sheet according to the present embodiment, the steel sheet components are controlled as described above, and hot-dip galvanizing is thus possible. The hot-dip galvanized layer may also be a hot-dip galvannealed layer.
  • Whether or not the steel sheet has a hot-dip galvanized layer or a hot-dip galvannealed layer is determined by the following method. Using an energy dispersive X-ray spectrometer (EDS), a line analysis is performed in a direction perpendicular to the interface between the plating layer and a matrix phase in a sample cross section containing the plating layer and the matrix phase. The concentration profile of each element is obtained from the plating layer side toward the matrix phase direction. In a case where a region, which has a high Fe concentration of 5% or more with respect to an average composition of 2 µm depth in the thickness direction from the surface of the plating layer, of 1 µm or more is present at the interface, the hot-dip galvannealed layer is determined to be present, and otherwise, the hot-dip galvanizing layer is determined to be present. Note that the quantification of the concentration is performed by the ZAF method.
  • The steel sheet according to the present embodiment may be a steel sheet processed into a component shape. That is, the steel sheet according to the present embodiment may be a component including the steel sheet. Even when the steel sheet is processed into the component, the same effects as those of the steel sheet according to the present embodiment can be obtained since the above-described features are inherited.
  • In a case where the measurement is performed on the component, it is preferable to measure a part (a flat part, for example, with a radius of curvature R of 1000 mm or more) where an amount of processing is relatively small.
    • <Manufacturing Method>
  • The steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (V). Note that unless otherwise specified, the temperature described below refers to surface temperature of a steel sheet or a slab.
    1. (I) A hot rolling step of heating a slab having the same chemical composition as the steel sheet described above and performing hot rolling that starts finish rolling under a condition that an average temperature from a surface to 10% of a thickness is 850°C or lower at a first finishing stand to obtain a hot-rolled steel sheet.
    2. (II) A coiling step of coiling the hot-rolled steel sheet at a coiling temperature of 550°C to 450°C after the hot rolling step.
    3. (III) A holding step of holding the hot-rolled steel sheet at a temperature higher than the coiling temperature and 650°C to 500°C for 1 hour or longer.
    4. (IV) A cold rolling step of performing cold rolling on the hot-rolled steel sheet under conditions that a sheet thickness reduction is 20% to 80% and an average dynamic friction coefficient of last two passes is 0.025 or more after the holding step to obtain a cold-rolled steel sheet.
    5. (V) An annealing step of annealing the cold-rolled steel sheet by holding in a temperature range of 740°C to 900°C at an atmosphere having a dew point of -30°C to 20°C for 60 seconds or longer after the cold rolling step
  • Hereinafter, each step will be described.
    • [Hot Rolling Step]
  • In the hot rolling step, a slab having the same chemical composition as the steel sheet according to the present embodiment described above is heated and hot-rolled to obtain a hot-rolled steel sheet. Slab heating conditions in the hot rolling are not limited, but the slab is preferably heated to 1100°C or higher. In a case where the heating temperature is lower than 1100°C, the material is likely to be insufficiently homogenized.
  • In the finish rolling, an inlet temperature of the first finishing stand (the stand of the first pass of finish rolling) is set so that the average temperature from the surface to 10% of the thickness is 850°C or lower. By setting the average temperature from the surface to 10% of the thickness to 850°C or lower at inlet side of the first finishing stand, fine and soft ferrite having an average grain size of 5 µm or smaller can be generated on the surface layer of the steel sheet from cooling after rolling to coiling process. By forming such a structure, it is possible to preferentially accumulate a shearing strain of cold rolling in a vicinity of the surface layer in the subsequent cold rolling step.
  • By measuring the temperature of the steel sheet before the finish rolling and performing a simulation based on the obtained temperature, the average temperature from the surface to 10% of the thickness can be estimated. Further, finish rolling cannot be performed since a temperature gradient of the steel sheet is small and a rolling load increases in a case where a temperature in a region of more than 10% of the sheet thickness becomes 850°C or lower, water cooling may be performed so that only the surface layer is cooled before the start of finish rolling in order to provide an appropriate temperature gradient in the sheet thickness direction.
  • The manufacturing method of the slab is not limited. The slab is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be manufactured by an ingot-making method or a thin slab casting method.
  • In a case where a steel piece obtained by continuous casting can be subjected to the hot rolling step while it maintains a sufficiently high temperature, the heating of the slab may be omitted.
    • [Coiling Step]
  • In the coiling step, the steel sheet (hot-rolled steel sheet) after the hot rolling step is coiled at a coiling temperature of 550°C to 450°C. After hot rolling ends, cooling conditions to the coiling temperature are not particularly limited.
  • By coiling the hot-rolled steel sheet at the coiling temperature of 550°C to 450°C, fine ferrite can be formed in the surface layer of the steel sheet. In a case where the coiling temperature is too low, a fracture may be occurred during cold rolling, and therefore the coiling temperature is preferably set to 450 °C or higher.
    • [Holding Step]
  • In the holding step, the steel sheet (hot-rolled steel sheet) after coiling is held in the temperature higher than the coiling temperature and 650°C to 500°C for 1 hour or longer. The holding time may be set to 6 hours or shorter. The method of holding such a temperature range is not limited, but for example, a coil may be inserted into a soaking furnace held at the corresponding temperature. Generally, holding in the temperature range for 1 hour or longer is not performed since pickling property deteriorates.
  • By holding at 650°C to 500°C for 1 hour or longer, fine ferrite can be formed on the surface layer of the steel sheet. By forming such a structure, it is possible to preferentially accumulate a shearing strain of cold rolling in a vicinity of the surface layer in the subsequent cold rolling step. In addition, a ferrite recrystallization in a strain accumulation part of the surface layer can be sufficiently progressed during a temperature rising in the annealing step, and the unrecrystallized ferrite which causes the anisotropy of bendability can be reduced. Further, since the shear strain of the cold rolling is preferentially accumulated in the vicinity of the surface layer, it is possible to make the generated recrystallized ferrite extremely fine and make the aspect ratio of the hard phase generated in the cooling process after heating in the annealing process low. Furthermore, the texture of the surface layer soft part becomes suitable. That is, in ferrite of the surface layer soft part, ML/Mc, which is the ratio of Taylor factor M value ML of the L cross section to Taylor factor M value Mc of the C cross section, can be 0.95 or more.
    • [Cold Rolling Step]
  • In the cold rolling step, the steel sheet (hot-rolled steel sheet) after the holding step is descaled as necessary by pickling or the like using a known method, and then cold rolling is performed under the conditions that the sheet thickness reduction is 20% to 80% and the average dynamic friction coefficient of the last two passes is 0.025 or more to obtain a cold-rolled steel sheet.
  • By performing cold rolling under the conditions that the sheet thickness reduction is 20% to 80% and the average dynamic friction coefficient of the last two passes is 0.025 or more, strain can be accumulated intensively in the surface layer of the steel sheet. By sufficiently progressing the recrystallization of fine ferrite structure of the surface layer during temperature increasing in the annealing step, it is possible to reduce unrecrystallized ferrite that causes the increase in bending anisotropy. This makes it possible to bring ML/Mc close to 1. In addition, it is possible to make the aspect ratio of the hard phase generated during cooling after heating in the annealing process low. Furthermore, the texture of the surface layer soft part becomes suitable. That is, ML/Mc can be set to 0.95 or more.
  • The average dynamic friction coefficient of the last two passes is preferably set to 0.035 or more, more preferably set to 0.055 or more.
  • The thickness reduction can be expressed by (1-t1/t0) × 100 (%), where t0 is a sheet thickness before cold rolling and t1 is a sheet thickness after cold rolling. The average dynamic friction coefficient of the last two passes is, with the measured rolling load, obtained by using the dynamic mean deformation resistance equation of Gokyu and Kihara, the roll flattening equation of Hitchcock, and the cold rolling theoretical equation of Bland and Ford.
  • The cold-rolled steel sheet after the cold rolling step may be subjected to a treatment such as degreasing according to a known method, if necessary.
    • [Annealing Step]
  • In the annealing step, the steel sheet (cold-rolled steel sheet) after the cold-rolling step is annealed by holding in the temperature range of 740°C to 900°C at the atmosphere having the dew point of -30°C to 20°C for 60 seconds or longer. The holding time in the temperature range of 740°C to 900°C may be set to 400 seconds or shorter. Following heating and holding in the annealing step, the cold-rolled steel sheet may be subjected to cooling, holding, quenching, and tempering in order to form a desired structure in the surface layer soft part and the 1/4 depth position. Further, hot dip galvanizing or alloying may be performed.
  • By setting the atmosphere during temperature rising and heating in the annealing step as described above, decarburization of the surface layer of the steel sheet during annealing can be progressed, and the formation of ferrite in the surface layer of the steel sheet can be promoted at a maximum heating temperature and during subsequent cooling. As a result, Hs/Hc can be set to 0.65 or less. In this way, the microstructure of the surface layer soft part can be made to have 50% or more of ferrite and 0 to 50% of the total of one or two or more of as-quenched martensite, pearlite, and residual austenite.
  • The holding time in the annealing temperature is preferably 60 seconds or longer. In a case where the holding time in the annealing temperature is shorter than 60 seconds, decarburization does not sufficiently progress, and the ferrite fraction of the surface layer soft part may not be 50% or more.
    • [Hot-Dip Galvanizing] [Alloying]
  • In the manufacturing of a cold-rolled steel sheet (hot-dip galvanized steel sheet) having a hot-dip galvanized layer on a surface thereof, in a process that cooling is performed after annealing (annealing and cooling process), in a state in which the temperature of the surface of the steel sheet is higher than 425°C and lower than 600°C, the steel sheet may be subjected to hot-dip galvanizing by being immersed in a plating bath at the same temperature. In addition, in the manufacturing of a cold-rolled steel sheet (hot-dip galvannealed steel sheet) having a hot-dip galvannealed plating on the surface thereof, for example, an alloying heat treatment of heating the steel sheet at higher than 450°C and lower than 600°C may be further performed subsequent to the hot-dip galvanizing step to form a hot-dip galvannealed plating.
    • [Tempering Step]
  • The cold-rolled steel sheet after the annealing step is cooled to a temperature range of 50°C to 250°C at a cooling rate of 30 °C/s or faster, thereby untransformed austenite is transformed into martensite.
  • In the tempering step, the cold-rolled steel sheet is tempered at 200°C to 350°C for 1 second or longer, thereby a structure mainly having martensite (tempered martensite) can be stably obtained at the 1/4 depth position.
  • Note that the tempering step is not an essential step.
  • In a case where the hot-dip galvanizing step and/or the alloying step is performed, the cold-rolled steel sheet after the hot-dip galvanizing step or the cold-rolled steel sheet after the hot-dip galvanizing step and the alloying step is cooled to a temperature range of 50°C to 250°C, and then tempered for 1 second or longer at a temperature range of 200°C to 350°C. In a case where the tempering temperature is higher than 350°C, the steel sheet strength decreases. Accordingly, the tempering temperature is set to 350°C or lower. The tempering temperature is preferably 325°C or lower, and more preferably 300°C or lower. On the other hand, in a case where the tempering temperature is lower than 200°C, the tempering becomes insufficient, and bendability and hydrogen embrittlement resistance deteriorate. Accordingly, the tempering temperature is set to 200°C or higher. The tempering temperature is preferably 220°C or higher, and more preferably 250°C or higher.
  • The tempering time may be 1 second or longer, but is preferably 5 seconds or longer, and more preferably 10 seconds or longer in order to perform a stable tempering treatment. On the other hand, since long tempering decreases the steel sheet strength, the tempering time is preferably 750 seconds or shorter, and more preferably 500 seconds or shorter.
    • [Skin Pass Step]
  • The cold-rolled steel sheet after the tempering step may be cooled to a temperature at which skin pass rolling can be performed, and then subjected to skin pass rolling. In a case where the cooling after annealing is water spray cooling, dip cooling, air-water cooling, or the like in which water is used, it is preferable to perform pickling, and then plating with a small amount of one or two or more of Ni, Fe, Co, Sn, and Cu before skin pass rolling in order to remove an oxide film formed by contact with water at a high temperature and improve chemical convertibility of the steel sheet. Here, the small amount refers to a plating amount of about 3 to 30 mg/m2 on the steel sheet surface.
  • The shape of the steel sheet can be adjusted by the skin pass rolling. The elongation ratio of the skin pass rolling is preferably 0.05% or more. The elongation ratio of the skin pass rolling is more preferably 0.10% or more. On the other hand, in a case where the elongation ratio of the skin pass rolling is high, the area ratio of residual austenite decreases, and ductility deteriorates. Therefore, the elongation ratio is preferably set to 1.00% or less. The elongation ratio is more preferably 0.75% or less, and still more preferably 0.50% or less.
    • Examples
  • The present invention will be described in greater detail with reference to examples. However, conditions of the examples are merely an example adopted to confirm the operability and the effects of the present invention, and the present invention is not limited to these condition examples. The present invention may adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
  • Slabs having chemical compositions shown in Tables 1A and 1B were cast. The slabs after casting were subjected to hot rolling under the conditions shown in Tables 2A and 2B. After rolling, coiling and holding were performed under the conditions shown in Tables 2A and 2B. After holding, cold rolling was performed under the conditions shown in Tables 2A and 2B to obtain cold-rolled steel with sheet thickness of 1 mm to 2 mm, and annealing was performed under the conditions shown in Tables 3A and 3B. Note that the cold-rolled steel sheets after the annealing step were cooled to the temperature range of 50°C to 250°C at the cooling rate of 30 °C/s or faster.
  • In the examples in which "presence" is written in "presence or absence of plating", hot-dip galvanizing was performed by immersing the steel sheet in a plating bath having an equivalent temperature in a state where the surface temperature of the steel sheet was higher than 425°C and lower than 600°C. Further, in the examples in which "presence" is written in "presence or absence of alloying", an alloying heat treatment of heating to higher than 450°C and lower than 600°C was performed. [Table 1A]
    Component Chemical Composition (mass%), Remainder: Fe and Impurities
    C Si Mn P S N O Al Cr Mo W Co
    a 0.12 0.48 2.6 0.010 0.0030 0.0043 0.0035 0.030 0.086
    b 0.14 0.17 2.9 0.014 0.0051 0.0032 0.0051 0.060 0.224
    c 0.23 0.03 3.1 0.016 0.0080 0.0047 0.0035 0.060
    d 0.21 0.50 2.4 0.017 0.0072 0.0035 0.0038 0.040 0.065
    e 0.24 1.00 2.3 0.007 0.0047 0.0015 0.0011 0.800
    f 0.24 0.45 1.6 0.012 0.0027 0.0025 0.0034 0.020 0.100
    g 0.11 0.19 1.8 0.016 0.0083 0.0017 0.0025 0.040 0.600
    h 0.12 1.60 2.6 0.015 0.0072 0.0030 0.0011 0.040
    i 0.19 1.80 2.7 0.017 0.0067 0.0034 0.0023 0.400
    j 0.09 1.90 2.5 0.008 0.0026 0.0046 0.0037
    k 0.23 0.12 2.8 0.013 0.0077 0.0054 0.0011 0.040 1.560
    l 0.22 0.31 1.7 0.013 0.0007 0.0042 0.0011 0.040 0.800
    m 0.06 0.30 2.6 0.014 0.0021 0.0037 0.0012
    n 0.28 0.80 1.8 0.004 0.0080 0.0018 0.0014
    o 0.16 2.50 2.4 0.008 0.0072 0.0017 0.0034
    p 0.14 0.60 1.1 0.012 0.0047 0.0030 0.0025
    q 0.12 0.60 3.6 0.011 0.0096 0.0034 0.0011
    r 0.23 0.40 2.3 0.160 0.0076 0.0014 0.0025
    s 0.17 1.50 2.5 0.008 0.0600 0.0032 0.0016
    u 0.18 0.60 2.8 0.015 0.0140 0.0600 0.0017 0.050
    v 0.25 0.60 2.5 0.020 0.0180 0.0014 0.0400 0.040
    w 0.25 0.50 3.2 0.010 0.0030 0.0035 0.0012
    The underline indicates that the underlined item falls outside the scope of the present invention.
    [Table 1B]
    Component Chemical Composition (mass%), Remainder: Fe and Impurities Remarks
    B Nb Ti V Ta Sn Sb As Ni Cu Ca Zr Mg REM
    a 0.0012 0.02 0.0200 Invention Steel
    b 0.0010 0.02 0.0345 0.011 0.001 0.011
    c 0.0015 0.0200
    d 0.0012 0.0200 0.120
    e 0.0018 0.0180
    f 0.0012 0.05 0.0120 0.260 0.250
    g
    h 0.0020 0.02 0.0120
    i 0.0024 0.03 0.0160 0.004 0.030
    j 0.0025 0.45 0.005
    k 0.0060 0.011
    l 0.3400 0.059
    m Comparative Steel
    n
    o
    p
    q
    r
    s
    u
    v
    w Invention Steel
    [Table 2A]
    Component Test No. Hot Rolling Step Holding Step Cold Rolling Step
    Average Temperature From Surface to 10% of Thickness at First Finishing Stand of Finish Rolling (°C) Coiling Temperature (°C) Holding Temperature After Coiling (°C) Holding Time After Coiling (hr) Sheet Thickness Reduction (%) Average Dynamic Friction Coefficient of Last Two Passes
    a 1 788 494 609 5 67 0.049
    a 2 848 535 583 3 65 0.042
    a 3 826 508 600 5 56 0.043
    b 4 815 456 577 2 62 0.039
    b 5 828 548 564 4 68 0.041
    b 6 845 481 577 3 52 0.054
    c 7 827 540 510 5 58 0.046
    c 8 819 534 650 5 65 0.054
    c 9 834 496 585 4 68 0.059
    d 10 824 497 593 4 58 0.034
    e 11 825 498 567 1 48 0.041
    e 12 834 491 611 6 52 0.044
    e 13 814 529 579 3 51 0.043
    f 14 814 506 619 4 78 0.051
    g 15 825 531 551 2 28 0.034
    h 16 845 529 580 4 55 0.028
    i 17 813 526 540 4 61 0.058
    j 18 811 482 567 4 51 0.057
    k 19 817 500 547 5 49 0.047
    l 20 822 515 566 4 64 0.059
    [Table 2B]
    Component Test No. Hot Rolling Step Holding Step Cold Rolling Step
    Average Temperature From Surface to 10% of Thickness at First Finishing Stand of Finish Rolling (°C) Coiling Temperature (°C) Holding Temperature After Coiling (°C) Holding Time After Coiling (hr) Sheet Thickness Reduction (%) Average Dynamic Friction Coefficient of Last Two Passes
    m 21 844 520 549 5 65 0.051
    n 22 819 533 604 2 55 0.047
    o 23 833 523 596 5 Fractured in Cold Rolling Step
    p 24 827 538 574 4 59 0.051
    q 25 834 488 597 3 65 0.057
    r 26 814 519 598 3 55 0.055
    s 27 821 508 567 4 62 0.050
    u 28 813 494 616 3 59 0.057
    v 29 837 487 618 2 54 0.049
    a 30 920 494 592 5 49 0.050
    a 31 830 350 541 3 Fractured in Cold Rolling Step
    c 32 841 532 460 3 48 0.050
    c 33 820 520 567 0.2 56 0.040
    e 34 818 483 585 2 18 0.060
    e 35 845 517 574 4 50 0.015
    f 36 822 520 619 4 50 0.033
    f 37 822 489 541 2 51 0.051
    f 38 823 535 572 2 67 0.041
    w 39 805 510 520 1 55 0.038
    c 40 810 400 520 1 Fractured in Cold Rolling Step
    The underline indicates that the underlined item falls outside the scope of the present invention.
    [Table 3A]
    Test No. Annealing Step Remarks
    Dew Point (°C) Annealing Temperature (°C) Holding Time (s) Presence or Absence of Plating Presence or Absence of Alloying
    1 -8 790 147 Presence Presence Invention Example
    2 2 784 136 Presence Absence
    3 -16 811 123 Presence Absence
    4 0 790 155 Presence Presence
    5 -2 775 155 Presence Absence
    6 -10 789 123 Absence Absence
    7 14 840 123 Presence Absence
    8 -20 832 190 Presence Presence
    9 -2 883 60 Presence Absence
    10 -1 810 123 Presence Presence
    11 -7 880 75 Presence Absence
    12 0 885 105 Presence Presence
    13 -7 851 180 Presence Presence
    14 3 830 105 Absence Absence
    15 -6 820 147 Absence Absence
    16 -5 822 136 Presence Presence
    17 -28 806 280 Presence Absence
    18 18 865 123 Presence Absence
    19 6 755 110 Presence Presence
    20 -16 835 220 Absence Absence
    [Table 3B]
    Test No. Annealing Step Remarks
    Dew Point (°C) Annealing Temperature (°C) Holding Time (s) Presence or Absence of Plating Presence or Absence of Alloying
    21 6 799 89 Absence Absence Comparative Example
    22 -2 832 105 Presence Presence
    23 Fractured in Cold Rolling Step
    24 -1 760 136 Absence Absence
    25 5 766 136 Absence Absence
    26 -9 820 147 Presence Presence
    27 6 790 136 Presence Absence
    28 -16 755 123 Presence Presence
    29 -20 820 155 Presence Presence
    30 -9 800 136 Presence Absence
    31 Fractured in Cold Rolling Step
    32 0 880 105 Absence Absence
    33 -12 840 123 Presence Absence
    34 2 890 136 Presence Presence
    35 -2 888 123 Presence Presence
    36 -45 820 100 Presence Presence
    37 14 730 155 Presence Absence
    38 -13 830 20 Presence Presence
    39 -10 850 80 Presence Presence Invention Example
    40 Fractured in Cold Rolling Step Comparative Example
    The underline indicates that the underlined item falls outside the scope of the present invention.
  • From the obtained cold-rolled steel sheets, the microstructure of the 1/4 depth position and the microstructure of the surface layer soft part were evaluated by the method described above. The direction of rolling of the steel sheet was known from the manufacturing process, and therefore the determination of the rolling direction was not performed.
  • The tensile strength (TS), the critical bending angle (C-axis bending angle) θC, and the anisotropy of the critical bending angle were evaluated in the following manner.
  • The tensile strength (TS) was obtained by performing a tensile test according to JIS Z 2241: 2022 with a JIS No. 5 tensile test piece collected from the cold-rolled steel sheet in a direction perpendicular to a rolling direction. In a case where the tensile strength was 1180 MPa or more, the steel sheet was determined as having a high strength.
  • The critical bending angle was determined by the bending test described above in accordance with VDA (Verband der Automobilindustrie) 238-100. In a case where the tensile strength was 1180 to 1470 MPa, it was determined as being acceptable when the critical bending angle θC was 90° or more. In a case where the tensile strength was more than 1470 MPa, it was determined as being acceptable when the critical bending angle θC was 80° or more. When the above criteria were not satisfied, it was determined as not having excellent bendability and being unacceptable.
  • In the present example, the reason why the criteria of the critical bending angle θC is changed depending on the tensile strength as described above is to take the following background into consideration. The absolute value of the critical bending angle increases with decreasing tensile strength. For example, in the examples of No. 21 and No. 24, the absolute value of the critical bending angle is at a level comparable to that of the steel material of the invention example. However, the critical bending angles of Nos. 21 and 24 are only high because they are steel materials having relatively low tensile strength. In other words, in a case where the steel sheet has the same chemical composition and tensile strength as Nos. 21 and 24 and satisfies Hs/Hc, which is the feature of the present application, the critical bending angle is likely to be further increased.
  • In a case where the θL/θC, which is the ratio of the critical bending angle θL in the direction along the rolling direction to the critical bending angle θC in the direction perpendicular to the rolling direction and the sheet thickness direction, was more than 0.80, it was determined as having the excellent bendability anisotropy and being acceptable (OK). On the other hand, when the θL/θC was 0.80 or less, it was determined as not having the excellent bendability anisotropy and being unacceptable (NG). The obtained results are shown in Tables 5A and 5B.
  • As can be seen from Tables 5A and 5B, all the invention examples had excellent bendability and small bending anisotropy. [Table 4A]
    Test No. Component 1/4 Depth Position Microstructural ratio (area %) Surface Layer Soft Part Microstructural ratio (area %) Surface Layer Soft Part
    Ferrite and Bainite Martensite Pearlite and Residual γ Ferrite Martensite, Bainite, Pearlite, and Residual γ ML/Mc (-) Aspect Ratio of Island-Shaped Hard Phase (-) Major Axis of Island-Shaped Hard Phase (µm)
    1 a 13 87 0 74 26 0.98 1.6 2.3
    2 a 20 80 0 78 22 0.97 2.0 3.0
    3 a 8 92 0 67 33 0.96 2.1 3.3
    4 b 17 83 0 75 25 0.97 2.0 3.2
    5 b 28 72 0 76 24 0.97 1.9 2.9
    6 b 16 84 0 72 28 0.97 2.0 3.0
    7 c 0 100 0 76 24 0.97 2.0 3.0
    8 c 0 100 0 62 38 0.98 1.6 2.2
    9 c 2 98 0 68 32 0.98 1.4 1.7
    10 d 0 100 0 60 40 0.96 2.3 3.7
    11 e 0 100 0 65 35 0.96 2.3 3.7
    12 e 2 98 0 69 31 0.96 2.2 3.4
    13 e 10 82 8 70 30 0.96 2.1 3.5
    14 f 0 100 0 72 28 0.98 1.4 1.7
    15 g 10 90 0 59 41 0.95 2.7 4.7
    16 h 19 81 0 82 18 0.96 2.5 4.2
    17 i 37 55 8 89 11 0.98 1.6 2.1
    18 j 5 95 0 96 4 0.97 1.8 2.7
    19 k 0 100 0 84 16 0.96 2.1 3.4
    20 l 0 100 0 60 40 0.98 1.5 1.9
    [Table 4B]
    Test No. Component 1/4 Depth Position Microstructural ratio (area %) Surface Layer Soft Part Microstructural ratio (area %) Surface Layer Soft Part
    Ferrite and Bainite Martensite Pearlite and Residual γ Ferrite Martensite, Bainite, Pearlite, and Residual γ ML/Mc (-) Aspect Ratio of Island-Shaped Hard Phase (-) Major Axis of Island-Shaped Hard Phase (µm)
    21 m 19 81 0 85 15 0.97 1.8 2.5
    22 n 0 100 0 68 32 0.97 2.0 3.1
    23 o Fractured in Cold Rolling Step
    24 p 70 30 0 85 15 0.97 1.8 2.7
    25 q 14 86 0 25 75 0.98 1.6 2.0
    26 r 0 100 0 65 35 0.97 1.8 2.6
    27 s 32 68 0 87 13 0.97 1.8 2.6
    28 u 34 66 0 85 15 0.98 1.6 2.3
    29 v 0 100 0 65 35 0.97 2.0 3.1
    30 a 12 88 0 80 20 0.90 3.5 5.3
    31 a Fractured in Cold Rolling Step
    32 c 2 98 0 68 32 0.92 3.2 5.6
    33 c 3 97 0 60 40 0.90 3.5 6.0
    34 e 0 100 0 75 25 0.89 3.8 6.1
    35 e 2 98 0 85 15 0.88 4.0 7.2
    36 f 0 100 0 25 75 0.96 2.4 4.0
    37 f 56 44 0 90 10 0.88 4.1 6.5
    38 f 0 100 0 32 68 0.97 1.9 2.9
    39 w 5 95 0 60 40 0.95 2.8 3.2
    40 c Fractured in Cold Rolling Step
    The underline indicates that the underlined item falls outside the scope of the present invention.
    [Table 5A]
    Test No. Tensile Strength (MPa) Hs/Hc C-axis Bending Angle θC (°) θL/θC Sheet Thickness (mm) Remarks
    1 1261 0.63 97 OK 1.8 Invention Example
    2 1193 0.64 100 OK 1.6
    3 1259 0.63 97 OK 1.4
    4 1257 0.61 96 OK 2.0
    5 1201 0.64 96 OK 2.0
    6 1253 0.62 100 OK 1.4
    7 1499 0.57 95 OK 1.4
    8 1510 0.62 89 OK 1.2
    9 1496 0.60 93 OK 1.0
    10 1520 0.60 90 OK 1.4
    11 1574 0.61 87 OK 1.0
    12 1578 0.60 88 OK 1.2
    13 1527 0.61 90 OK 1.0
    14 1553 0.59 90 OK 1.2
    15 1217 0.64 96 OK 1.8
    16 1335 0.63 90 OK 1.6
    17 1302 0.65 90 OK 1.6
    18 1355 0.57 100 OK 1.4
    19 1511 0.59 92 OK 1.4
    20 1519 0.63 87 OK 1.2
    [Table 5B]
    Test No. Tensile Strength (MPa) Hs/Hc C-axis Bending Angle θC (°) θL/θC Sheet Thickness (mm) Remarks
    21 1082 0.63 95 OK 1.8 Comparative Example
    22 1759 0.65 55 OK 1.2
    23 Fractured in Cold Rolling Step
    24 1080 0.74 86 OK 1.6
    25 1323 0.85 85 NG 1.6
    26 1598 0.62 40 OK 1.8
    27 1426 0.65 87 NG 1.6
    28 1334 0.63 66 OK 1.4
    29 1688 0.64 58 OK 2.0
    30 1271 0.62 97 NG 1.6
    31 Fractured in Cold Rolling Step
    32 1521 0.60 91 NG 1.2
    33 1541 0.62 86 NG 1.4
    34 1636 0.60 82 NG 1.6
    35 1622 0.57 88 NG 1.4
    36 1611 0.78 60 OK 1.4
    37 1311 0.57 98 NG 2.0
    38 1617 0.76 68 OK 1.8
    39 1580 0.64 82 OK 1.2 Invention Example
    40 Fractured in Cold Rolling Step Comparative Example
    The underline indicates that the underlined item falls outside the scope of the present invention.
    • INDUSTRIAL APPLICABILITY
  • According to the aspect of the present invention, it is possible to provide a steel sheet having excellent bendability and small bending anisotropy, and a manufacturing method of the steel sheet.

Claims (9)

  1. A steel sheet comprising, as a chemical composition, by mass%:
    C: 0.09% to 0.25%;
    Si: 0.01% to 2.00%;
    Mn: 1.5% to 3.5%;
    P: 0.100% or less;
    S: 0.0500% or less;
    N: 0.0100% or less;
    O: 0.0060% or less;
    Al: 0% to 1.000%;
    Cr: 0% to 2.000%;
    Mo: 0% to 1.000%;
    W: 0% to 1.000%;
    Co: 0% to 0.500%;
    B: 0% to 0.0100%;
    Nb: 0% to 0.50%;
    Ti: 0% to 0.5000%;
    V: 0% to 0.500%;
    Ta: 0% to 0.100%;
    Sn: 0% to 0.050%;
    Sb: 0% to 0.050%;
    As: 0% to 0.050%;
    Ni: 0% to 1.000%;
    Cu: 0% to 1.000%;
    Ca: 0% to 0.050%;
    Zr: 0% to 0.050%;
    Mg: 0% to 0.050%;
    REM: 0% to 0.100%, and
    a remainder comprising Fe and impurities,
    in a microstructure at a 1/4 depth position that is a position away from a surface by 1/4 of a sheet thickness along a sheet thickness direction, by area ratio,
    a total of ferrite and bainite is 0% to 40%,
    martensite is 50% to 100%, and
    a total of pearlite and residual austenite is 0% to 10%, and
    in a microstructure of a surface layer soft part that is a region from the surface to 20 µm along the sheet thickness direction, by area ratio,
    ferrite is 50% or more, and
    a total of one or two or more of martensite, bainite, pearlite, and residual austenite is 0% to 50%,
    Hs/Hc, which is a ratio of Vickers hardness Hs of the surface layer soft part to Vickers hardness He of the 1/4 depth position, is 0.65 or less,
    in the ferrite of the surface layer soft part, ML/Mc, which is a ratio of Taylor factor M value ML of a L cross section to Taylor factor M value Mc of a C cross section, is 0.95 or more, and
    an aspect ratio is 3.0 or less and a major axis is 5.0 µm or shorter of island-shaped hard phase of the surface layer soft part.
  2. The steel sheet according to Claim 1,
    wherein θL/θC, which is a ratio of a critical bending angle θL in a direction along a rolling direction to a critical bending angle θC in a direction perpendicular to the rolling direction and the sheet thickness direction, is more than 0.80.
  3. The steel sheet according to Claim 1 or 2,
    wherein a tensile strength is 1180 MPa or more.
  4. The steel sheet according to Claim 2,
    wherein the critical bending angle θC is 90° or more in a case where the tensile strength is 1180 to 1470 MPa, and
    the critical bending angle θC is 80° or more in a case where the tensile strength is more than 1470 MPa.
  5. The steel sheet according to Claim 1 or 2,
    wherein a sheet thickness is 1.0 mm or more.
  6. The steel sheet according to Claim 1 or 2,
    wherein the chemical composition comprises, by mass%, one or two or more of
    Al: 0.001% to 1.000%,
    Cr: 0.001% to 2.000%,
    Mo: 0.010% to 1.000%,
    W: 0.001% to 1.000%,
    Co: 0.010% to 0.500%,
    B: 0.0001% to 0.0100%,
    Nb: 0.01% to 0.50%,
    Ti: 0.0001% to 0.5000%,
    V: 0.001% to 0.500%,
    Ta: 0.001% to 0.100%,
    Sn: 0.001% to 0.050%,
    Sb: 0.001% to 0.050%,
    As: 0.001% to 0.050%,
    Ni: 0.010% to 1.000%,
    Cu: 0.001% to 1.000%,
    Ca: 0.001% to 0.050%,
    Zr: 0.001% to 0.050%,
    Mg: 0.0001% to 0.050%, and
    REM: 0.001% to 0.100%.
  7. The steel sheet according to Claim 1 or 2,
    wherein a hot-dip galvanized layer is provided on the surface.
  8. The steel sheet according to Claim 1 or 2,
    wherein a hot-dip galvannealed layer is provided on the surface.
  9. A manufacturing method of a steel sheet comprising:
    a hot rolling step of heating a slab comprising, as a chemical composition, by mass%,
    C: 0.09% to 0.25%;
    Si: 0.01% to 2.00%;
    Mn: 1.5% to 3.5%;
    P: 0.100% or less;
    S: 0.0500% or less;
    N: 0.0100% or less;
    O: 0.0060% or less;
    Al: 0% to 1.000%;
    Cr: 0% to 2.000%;
    Mo: 0% to 1.000%;
    W: 0% to 1.000%;
    Co: 0% to 0.500%;
    B: 0% to 0.0100%;
    Nb: 0% to 0.50%;
    Ti: 0% to 0.5000%;
    V: 0% to 0.500%;
    Ta: 0% to 0.100%;
    Sn: 0% to 0.050%;
    Sb: 0% to 0.050%;
    As: 0% to 0.050%;
    Ni: 0% to 1.000%;
    Cu: 0% to 1.000%;
    Ca: 0% to 0.050%;
    Zr: 0% to 0.050%;
    Mg: 0% to 0.050%;
    REM: 0% to 0.100%, and
    a remainder comprising Fe and impurities, and performing hot rolling that starts finish rolling under a condition that an average temperature from a surface to 10% of a thickness is 850°C or lower at a first finishing stand to obtain a hot-rolled steel sheet;
    a coiling step of coiling the hot-rolled steel sheet at a coiling temperature of 550°C to 450°C after the hot rolling step;
    a holding step of holding the hot-rolled steel sheet at a temperature higher than the coiling temperature and 650°C to 500°C for 1 hour or longer,
    a cold rolling step of performing cold rolling on the hot-rolled steel sheet under conditions that a sheet thickness reduction is 20% to 80% and an average dynamic friction coefficient of last two passes is 0.025 or more after the holding step to obtain a cold-rolled steel sheet; and
    an annealing step of annealing the cold-rolled steel sheet by holding in a temperature range of 740°C to 900°C at an atmosphere having a dew point of -30°C to 20°C for 60 seconds or longer after the cold rolling step.
EP23903529.8A 2022-12-15 2023-12-13 Steel sheet and method for manufacturing steel sheet Pending EP4636109A1 (en)

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JP2015145521A (en) 2014-02-03 2015-08-13 Ｊｆｅスチール株式会社 High strength cold rolled steel sheet and method for producing the same
WO2020196060A1 (en) 2019-03-28 2020-10-01 日本製鉄株式会社 High-strength steel sheet

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PL3162908T3 (en) * 2014-07-14 2019-10-31 Nippon Steel & Sumitomo Metal Corp Hot-rolled steel sheet
JP6304455B2 (en) * 2016-03-31 2018-04-04 Ｊｆｅスチール株式会社 Thin steel plate and plated steel plate, hot rolled steel plate manufacturing method, cold rolled full hard steel plate manufacturing method, heat treatment plate manufacturing method, thin steel plate manufacturing method and plated steel plate manufacturing method
US11492677B2 (en) * 2017-12-27 2022-11-08 Jfe Steel Corporation High-strength steel sheet and method for producing the same

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JP2015145521A (en) 2014-02-03 2015-08-13 Ｊｆｅスチール株式会社 High strength cold rolled steel sheet and method for producing the same
WO2020196060A1 (en) 2019-03-28 2020-10-01 日本製鉄株式会社 High-strength steel sheet

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