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US20150090377A1 - Steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product - Google Patents

Steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product Download PDF

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US20150090377A1
US20150090377A1 US14/382,437 US201314382437A US2015090377A1 US 20150090377 A1 US20150090377 A1 US 20150090377A1 US 201314382437 A US201314382437 A US 201314382437A US 2015090377 A1 US2015090377 A1 US 2015090377A1
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steel sheet
formed product
amount
press
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US14/382,437
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Toshio Murakami
Junya Naitou
Keisuke Okita
Shushi Ikeda
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Kobe Steel Ltd
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Kobe Steel Ltd
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Assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) reassignment KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: IKEDA, SHUSHI, MURAKAMI, TOSHIO, NAITOU, JUNYA, OKITA, KEISUKE
Publication of US20150090377A1 publication Critical patent/US20150090377A1/en
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    • 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B21D22/208—Deep-drawing by heating the blank or deep-drawing associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D37/00—Tools as parts of machines covered by this subclass
    • B21D37/16—Heating or cooling
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • C—CHEMISTRY; METALLURGY
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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
    • 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
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
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    • 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
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    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/38—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
    • C22C38/50—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D2221/00—Treating localised areas of an article

Definitions

  • the present invention relates to a steel sheet for hot pressing use used in manufacturing structural components of an automobile and suitable for hot press forming, a press-formed product obtained from such a steel sheet for hot pressing use, and a method for manufacturing the press-formed product, and relates more specifically to a steel sheet for hot pressing use that is useful in being applied to a hot press forming method securing a predetermined strength by being subjected to heat treatment simultaneously with impartation of the shape in forming a pre-heated steel sheet (blank) into a predetermined shape, a press-formed product, and a useful method for manufacturing such a press-formed product.
  • a hot press forming method has been employed for manufacturing components in which a steel sheet is heated to a predetermined temperature (for example, a temperature at which a state of an austenitic phase is achieved), the strength is lowered, the steel sheet is thereafter formed using a tool of a temperature (room temperature for example) lower than the steel sheet, thereby impartation of a shape and rapid heat treatment (quenching) utilizing the temperature difference of the both are executed simultaneously, and the strength after forming is secured.
  • a hot-press forming method is referred to by various names such as a hot forming method, hot stamping method, hot stamp method, die quench method, and the like in addition to the hot press method.
  • FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming described above, 1 in the drawing is a punch, 2 is a die, 3 is a blank holder, 4 is a steel sheet (blank), BHF is a blank holding force, rp is punch shoulder radius, rd is die shoulder radius, and CL is punch/die clearance respectively. Also, out of these components, in the punch 1 and the die 2 , passages 1 a, 2 a through which a cooling medium (water for example) can pass are formed inside of each, and it is configured that these members are cooled by making the cooling medium pass through these passages.
  • a cooling medium water for example
  • forming is started in a state the steel sheet (blank) 4 is heated to a two-phase zone temperature (between Ac 1 transformation point and Ac 3 transformation point) or a single-phase zone temperature of Ac 3 transformation point or above and is softened. That is, in a state the steel sheet 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3 , the steel sheet 4 is pressed in to the inside of a hole of the die 2 by the punch 1 , and is formed into a shape corresponding to the shape of the outer shape of the punch 1 while reducing the outside diameter of the steel sheet 4 .
  • the steel sheet As a steel sheet for hot pressing use widely used at present, one using 22Mn—B5 steel as a raw material is known.
  • the steel sheet has the tensile strength of approximately 1,500 MPa and the elongation of approximately 6-8%, and is applied to a shock resistant member (a member not causing deformation as much as possible and not causing breakage in collision).
  • a shock resistant member a member not causing deformation as much as possible and not causing breakage in collision.
  • application to a component requiring deformation such as an energy absorption member is difficult because elongation (ductility) is low.
  • the tensile strength: 1,500 MPa class is achieved on the high strength side (shock resistant portion side) according to these technologies, the maximum tensile strength is 700 MPa and the elongation EL is approximately 17% on the low strength side (energy absorption portion side), and achievement of higher strength and higher ductility are required in order to further improve the energy absorption properties.
  • Non-Patent Literature 1 Hirosue et al. “Nippon Steel Technical Report” No. 378, pp. 15-20 (2003)
  • the present invention has been developed in view of such circumstances as described above, and its object is to provide a steel sheet for hot pressing use capable of obtaining a press-formed product that can achieve the balance of high strength and elongation with a high level when uniform property is required within a formed product, capable of achieving the balance of high strength and elongation with a high level according to each region when regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product, and useful in obtaining a press-formed product excellent in softening prevention property in a HAZ, a press-formed product exerting the properties described above, and a useful method for manufacturing such a press-formed product.
  • the steel sheet for hot pressing use of the present invention which could achieve the object described above contains:
  • the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1) shown below, and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more.
  • “equivalent circle diameter” is the diameter of an imaginary circle having an area same to the size (area) of Ti containing precipitates (TiC for example) (“the average equivalent circle diameter” is the average value thereof).
  • the steel sheet for hot pressing use of the present invention it is also useful to contain, as other elements, (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total, and the like, and the properties of the press-formed product is improved further according to the kind of the elements contained.
  • the method for manufacturing a press-formed product of the present invention which could achieve the object described above includes the steps of heating the steel sheet for hot pressing use as described above to a temperature of Ac 1 transformation point+20° C. or above and Ac 3 transformation point ⁇ 20° C. or below, thereafter starting press-forming of the steel sheet, and holding the steel sheet at the bottom dead point and executing cooling to a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., while securing the average cooling rate of 20° C./s or more within a tool.
  • the metal microstructure within the pressed steel includes retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, some of Ti-containing precipitates contained in pressed steel, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 10 nm or less, the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by the formula (1) shown below, and the balance of high strength and elongation can be achieved as the uniform property of a high level within the formed product.
  • the area ratio of annealed martensite and/or annealed bainite means the total area ratio of both microstructures when both microstructures are included, and means, when either one microstructure is included, the area ratio of the microstructure.
  • another method for manufacturing a press-formed product of the present invention which could achieve the object described above includes the steps of using the steel sheet for hot pressing use as described above, dividing a heating region of the steel sheet into at least two regions, heating one region thereof to a temperature of Ac 3 transformation point or above and 950° C. or below, heating another region to a temperature of Ac 1 transformation point+20° C. or above and Ac 3 transformation point ⁇ 20° C. or below, thereafter starting press-forming of the both regions, and holding the both regions at the bottom dead point and executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing the average cooling rate of 20° C./s or more within a tool.
  • Another press-formed product of the present invention is a press-formed product of a steel sheet having the chemical component composition as described above in which the pressed steel includes a first region whose metal microstructure includes retained austenite: 3 area % or more and 20 area % or less and martensite as quenched: 80 area % or more, and a second region whose metal microstructure includes retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, some of Ti-containing precipitates contained in steel of the second region, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 10 nm or less, and the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by the formula (1) shown below.
  • the balance of high strength and elongation can be achieved with a high level according to each region, the regions corresponding to the shock resistant portion and the energy absorption portion are present within a single formed product, and softening prevention property of a HAZ in spot welding in the second region becomes excellent.
  • the size of Ti-containing precipitates is controlled, the precipitation rate is controlled for Ti that does not form TiN, and the ratio of tempered hard phase (martensitic phase, bainitic phase and the like), hard phase (as-quenched martensite phase) and retained austenite phase is adjusted with respect to the metal microstructure, by hot-pressing the steel sheet under a predetermined condition, high strength-elongation balance of the press-formed product can be made a high level.
  • the shock resistant portion and the energy absorption portion can be formed within a single formed product, the balance of high strength and elongation can be achieved with a high level for each portion, and softening prevention property in a HAZ becomes excellent.
  • FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming.
  • the present inventors carried out studies from various aspects in order to achieve such a steel sheet for hot pressing use that can obtain a press-formed product exhibiting excellent ductility (elongation) also while securing high strength after press-forming in manufacturing the press-formed product by heating a steel sheet to a predetermined temperature and thereafter executing hot press forming.
  • C is an important element in achieving the balance of high strength and elongation of a case uniform properties are required within a formed product with a high level or in securing retained austenite particularly in the low strength/high ductility portion of a case the regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product. Also, by concentration of C to austenite in heating of hot press forming, retained austenite can be formed after quenching. Also, C contributes to increase of the amount of martensite, and increases the strength. In order to exert such effects, C content should be 0.15% or more.
  • C content becomes excessive and exceeds 0.5%, two phase zone heating range becomes narrow, and the balance of high strength and elongation of a case uniform properties are required within a formed product is not achieved with a high level, or it becomes hard to adjust the metal microstructure to that targeted particularly in the low strength/high ductility portion (a microstructure in which a predetermined amount of annealed martensite and/or annealed bainite is secured) of a case the regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product.
  • Preferable lower limit of C content is 0.17% or more (more preferably 0.20% or more), and more preferable upper limit is 0.45% or less (further more preferably 0.40% or less).
  • Si exerts an effect of forming retained austenite by suppressing that martensite is tempered during cooling of tool-quenching and cementite is formed, or that untransformed austenite is disintegrated.
  • Si content should be 0.2% or more.
  • ferrite is liable to be formed, formation of single-phase microstructure becomes hard in heating, and required fractions of bainite and martensite cannot be secured in a steel sheet for hot pressing use.
  • Preferable lower limit of Si content is 0.5% or more (more preferably 1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or less).
  • Mn is an element effective in enhancing quenchability and suppressing formation of a microstructure (ferrite, pearlite, bainite and the like) other than martensite and retained austenite during cooling of tool-quenching. Also, Mn is an element stabilizing austenite, and is an element contributing to increase of retained austenite amount. In order to exert such effects, Mn should be contained by 0.5% or more. Although Mn content is preferable to be as much as possible when only properties are considered, because the cost of adding alloy increases, Mn content is made 3% or less. Preferable lower limit of Mn content is 0.7% or more (more preferably 1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or less).
  • P content is made 0.05% or less (exclusive of 0%).
  • Preferable upper limit of P content is 0.045% or less (more preferably 0.040% or less).
  • S is also an element inevitably included in steel, S deteriorates ductility, and therefore S is preferable to be reduced as much as possible.
  • S content is made 0.05% or less (exclusive of 0%).
  • Preferable upper limit of S content is 0.045% or less (more preferably 0.040% or less).
  • Al is useful as a deoxidizing element, fixes solid-solution N present in steel as AIN, and is useful in improving ductility.
  • Al content should be 0.01% or more.
  • preferable lower limit of Al content is 0.02% or more (more preferably 0.03% or more), and preferable upper limit is 0.8% or less (more preferably 0.6% or less).
  • B is an element contributing to prevention of formation of ferrite, pearlite and bainite during cooling after heating to a two-phase zone temperature of (Ac 1 transformation point-Ac 3 transformation point) because B has an action of suppressing ferrite transformation, pearlite transformation and bainite transformation on the high strength portion side, and to secure retained austenite.
  • B should be contained by 0.0002% or more, however, even when B is contained excessively exceeding 0.01%, the effects saturate.
  • Preferable lower limit of B content is 0.0003% or more (more preferably 0.0005% or more), and preferable upper limit is 0.008% or less (more preferably 0.005% or less).
  • Ti develops improvement effect of quenchability by fixing N and holding B in a solid solution state.
  • it is important to contain Ti more than the stoichiometric ratio of Ti and N (3.4 times of N content) by 0.002% or more.
  • drop of the strength in the HAZ can be suppressed by precipitation strengthening caused by that Ti dissolved in welding the hot stamp formed product is formed as TiC and by the effect of delaying increase of the dislocation density and the like by the effect of preventing movement of dislocation by TiC.
  • Ti-containing precipitates TiN for example
  • TiN Ti-containing precipitates
  • More preferable lower limit of Ti content is 3.4[N]+0.005% or more (further more preferably 3.4[N]+0.01% or more), and more preferable upper limit is 3.4[N]+0.09% or less (further more preferably 3.4[N]+0.08% or less).
  • N is an element inevitably mixed in, and is preferable to be reduced as much as possible, however, because there is a limit in reducing N in an actual process, 0.001% is made the lower limit. Also, when N content becomes excessive, Ti-containing precipitates (TiN for example) formed is coarsened, these precipitates work as the fracture origin, ductility of the steel sheet is deteriorated, and therefore the upper limit is made 0.01%. More preferable upper limit of N content is 0.008% or less (further more preferably 0.006% or less).
  • the basic chemical composition in the steel sheet for hot pressing use of the present invention is as described above, and the remainder is iron and inevitable impurities other than P, S (O, H and the like for example). Further, in the steel sheet for hot pressing use of the present invention, according to the necessity, it is also useful to further contain (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total, and the like, and the properties of the steel sheet for hot pressing use are improved further according to the kind of the element contained. Preferable range when these elements are contained and reasons for limiting the range are as follows.
  • V, Nb and Zr have effects of forming fine carbide and miniaturizing the microstructure by a pinning effect. In order to exert such effects, it is preferable to contain them by 0.001% or more in total. However, when the content of these elements becomes excessive, coarse carbide is formed and becomes a start point of breakage, and ductility is deteriorated adversely. Therefore, it is preferable to contain these elements by 0.1% or less in total. More preferable lower limit of the content of these elements in total is 0.005% or more (further more preferably 0.008% or more), and more preferable upper limit in total is 0.08% or less (further more preferably 0.06% or less).
  • Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite transformation and bainite transformation, therefore prevent formation of ferrite, pearlite and bainite during cooling after heating, and act effectively in securing retained austenite.
  • the content is preferable to be as much as possible when only the properties are considered, because the cost for adding alloys increases, 1% or less in total is preferable.
  • More preferable lower limit of these elements in total is 0.05% or more (further more preferably 0.06% or more), and more preferable upper limit in total is 0.5% or less (further more preferably 0.3% or less).
  • these elements miniaturize inclusions, they act effectively in improving ductility. In order to exert such effects, it is preferable to contain them by 0.0001% or more in total. Although the content is preferable to be as much as possible when only the properties are considered, because the effects saturate, 0.01% or less in total is preferable. More preferable lower limit of these elements in total is 0.0002% or more (further more preferably 0.0005% or more), and more preferable upper limit in total is 0.005% or less (further more preferably 0.003% or less).
  • Control of Ti-containing precipitates and the formula (1) is for preventing softening of the HAZ and is the control required fundamentally for a formed product, however, variation of these values between before and after hot-press forming is small, and therefore it is necessary that they have already been controlled at the stage of before forming (the steel sheet for hot pressing use).
  • Ti that is excessive relative to N in the steel before forming be present in a solid solution state or a fine state
  • Ti-containing precipitates can be maintained in the solid solution state or the fine state in heating of hot press forming.
  • the amount of precipitated Ti in the press-formed product can be controlled to a predetermined amount or less, softening in the HAZ is prevented, and thereby the properties of the welded joint can be improved.
  • the equivalent circle diameter of the Ti-containing precipitates of the object is stipulated to be 30 nm or less is that it is necessary to control the Ti-containing precipitates and excluding TiN formed coarsely in the melting stage that does not affect microstructure change and properties thereafter.
  • the size of the Ti-containing precipitates (the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter is 30 nm or less) is preferably 5 nm or less, more preferably 3 nm or less.
  • the Ti-containing precipitates of the object of the present invention also include precipitates containing Ti such as TiVC, TiNbC, TiVCN, TiNbCN and the like in addition to TiC and TiN.
  • the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter in the press-formed product is 30 nm or less is stipulated to be 10 nm or less
  • the same before forming is stipulated to be 6 nm or less.
  • the reason the size of the precipitates of the formed product is stipulated to be larger than that of the steel sheet is that Ti is present in the steel sheet as fine precipitates or in a solid solution state, and, when heating of 15 min or more at near 800° C. is executed, the Ti-containing precipitates are slightly coarsened.
  • the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter is 30 nm or less is 10 nm or less.
  • the average equivalent circle diameter of fine precipitates of 30 nm or less is made 6 nm or less and that majority of Ti is present in a solid solution state in the stage of the steel sheet for hot stamp use.
  • the Ti amount present as the precipitates other than TiN is 0.5 times or below of the balance obtained by deducting Ti that forms TiN from total Ti (that is, 0.5 ⁇ [(total Ti amount (mass %)) ⁇ 3.4[N]] or less) (the requirement of (B) described above).
  • Precipitated Ti amount (mass %) ⁇ 3.4[N] is preferably 0.4 ⁇ [(total Ti amount (mass %)) ⁇ 3.4[N]] or less, more preferably 0.3 ⁇ [(total Ti amount (mass %)) ⁇ 3.4[N]] or less.
  • the metal microstructure cannot be controlled only by the hot pressing condition, and it is necessary to control the microstructure of the raw material steel thereof (the steel sheet for hot pressing use) beforehand.
  • the metal microstructure In order to secure the proper amount of annealed martensite and annealed bainite which are fine and largely contributing to ductility in the press-formed product, it is necessary to make the sum total of the fraction of bainite and the fraction of martensite in the steel sheet for hot pressing use 80 area % or more.
  • the sum total of the fraction of bainite and the fraction of martensite is less than 80 area %, the fraction of annealed martensite and/or annealed bainite targeted in the formed steel sheet is hardly secured, and the amount of other microstructure (ferrite for example) increases to deteriorate the strength-elongation balance.
  • the sum total of the fraction of bainite and the fraction of martensite is preferably 90 area % or more, more preferably 95 area % or more.
  • the remainder of the metal microstructure is not particularly limited, at least any of ferrite, pearlite or retained austenite can be cited for example.
  • the steel sheet (the steel sheet for hot pressing use) of the present invention as described above can be manufactured by that a billet obtained by melting steel having the chemical component composition as described above is subjected to hot rolling with heating temperature: 1,100° C. or above (preferably 1,150° C. or above) and 1,300° C. or below (preferably 1,250° C. or below) and the finish rolling temperature of 930° C. or above (preferably 950° C. or above) and 1,050° C. or below (preferably 1,000° C. or below), cooling (rapid cooling) is executed immediately thereafter to 450° C. or below (preferably 400° C. or below) with the average cooling rate of 20° C./s or more (preferably 30° C./s or more), and winding is executed at 100° C. or above (preferably 150° C. or above) and 450° C. or below (preferably 400° C. or below).
  • heating temperature 1,100° C. or above (preferably 1,150° C. or above) and 1,300° C. or below (preferably 1,250° C. or
  • the method described above is for executing control so that (1) rolling is finished at a temperature range where dislocation introduced by hot rolling remains within austenite, (2) Ti-containing precipitates such as TiC and the like are formed finely on the dislocation by rapid cooling immediately thereafter, and (3) bainite transformation or martensite transformation is caused by rapid cooling and winding thereafter.
  • the steel sheet for hot pressing use having the chemical component composition, metal microstructure and Ti-precipitation state as described above may be used for manufacturing by a hot press forming as it is, and may be used for manufacturing by hot press forming after being subjected to cold rolling with the draft: 10-80% (preferably 20-70%) after pickling. Further, it is also possible to use the steel sheet for hot pressing use or the material obtained by cold rolling thereof for manufacturing by hot press forming after being subjected to such heat treatment of heating to 830° C. or above (preferably 850° C. or above and 900° C. or below), rapid cooling thereafter to 450° C. or below (preferably 400° C.
  • the steel sheet subjected to such cold rolling and heat treatment is also included in the steel sheet for hot pressing use of the present invention as far as the required properties are fulfilled.
  • the steel sheet for hot pressing use of the present invention may be subjected to plating containing at least one element out of Al, Zn, Mg and Si on the surface thereof (the surface of the base steel sheet).
  • the press formed product having a single property (may be hereinafter referred to as “single region formed product”) can have an optimum microstructure of low strength and high ductility.
  • the reasons for stipulating each requirement in this forming method are as described below.
  • the heating temperature should be controlled to a predetermined range.
  • the heating temperature of the steel sheet is below Ac 1 transformation point+20° C., sufficient amount of austenite cannot be secured in heating, and a predetermined amount of retained austenite cannot be secured in the final microstructure (the microstructure of the formed product).
  • the transformation amount to austenite increases excessively in heating, and a predetermined amount of annealed martensite and annealed bainite cannot be secured in the final microstructure (the microstructure of the formed product).
  • the cooling finishing temperature becomes higher than the temperature that is lower than the bainite transformation starting temperature Bs by 100° C. and the average cooling rate is less than 20° C./s, the microstructure such as ferrite, pearlite and the like is formed, a predetermined amount of retained austenite cannot be secured, and elongation (ductility) in the formed product deteriorates.
  • the cooling finishing temperature is not particularly limited as far as it is a temperature or below, the temperature being lower than Bs by 100° C., and can be the martensite transformation starting point Ms or below for example.
  • control of the average cooling rate basically becomes unnecessary at the stage the temperature becomes equal to or below the temperature lower than the bainite transformation starting temperature Bs by 100° C.
  • cooling may be executed to the room temperature with the average cooling rate of 1° C./s or more and 100° C./s or less for example.
  • control of the average cooling rate within the tool while being held at the bottom dead point can be achieved by means such as (a) to control the temperature of the forming tool (the cooling medium shown in FIG. 1 above), and (b) to control the thermal conductivity of the tool.
  • the metal microstructure is formed of retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, and the balance of high strength and elongation can be achieved with a high level and as a uniform property within the formed product.
  • the reasons for setting the range of each requirement (basic microstructure) in such a hot press-formed product are as described below.
  • Retained austenite has an effect of increasing the work hardening ratio (transformation induced plasticity) and improving ductility of the press-formed product by being transformed to martensite during plastic deformation.
  • the fraction of retained austenite should be made 3 area % or more. Ductility becomes more excellent as the fraction of retained austenite is higher.
  • retained austenite that can be secured is limited, and approximately 20 area % becomes the upper limit.
  • Preferable lower limit of retained austenite is 5 area % or more (more preferably 7 area % or more).
  • the main microstructure annealed martensite and/or annealed bainite which is fine and has low dislocation density
  • ductility (elongation) of the press-formed product can be enhanced while securing a predetermined strength.
  • the fraction of annealed martensite and/or annealed bainite is made 30 area % or more. However, when this fraction exceeds 87 area %, the fraction of retained austenite becomes insufficient, and ductility (residual ductility) deteriorates.
  • Preferable lower limit of annealed martensite and/or annealed bainite is 40 area % or more (more preferably 50 area % or more), and preferable upper limit is less than 80 area % (more preferably less than 70 area %).
  • the fraction of martensite as quenched is made 10 area % or more.
  • the fraction of martensite as quenched increases excessively, strength increases excessively and elongation becomes insufficient, and therefore the fraction thereof should be 67 area % or less.
  • Preferable lower limit of the fraction of martensite as quenched is 20 area % or more (more preferably 30 area % or more), and preferable upper limit is 60 area % or less (more preferably 50 area % or less).
  • microstructure there is no specific limit other than the microstructure described above, and ferrite, pearlite, bainite and the like may be included as the remainder microstructure, however, these microstructures are inferior in contribution to strength and contribution to ductility compared to other microstructures, and it is basically preferable not to be contained (it may also be 0 area %). However, up to 20 area % is allowable.
  • the remainder microstructure is preferably 10 area % or less, more preferably 5 area % or less.
  • the press-formed product (single region formed product) described above, some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 10 nm or less.
  • the average equivalent circle diameter of Ti-containing precipitates having 30 nm or less equivalent circle diameter is preferably 8 nm or less, more preferably 6 nm or less.
  • the amount of Ti present as the precipitates other than TiN is 0.5 times or less of Ti of the balance obtained by deducting Ti that forms TiN from total Ti (that is 0.5 ⁇ [(total Ti amount (mass %)) ⁇ 3.4[N]] or less).
  • Precipitated Ti amount (mass %) ⁇ 3.4[N] is preferably 0.4 ⁇ [(total Ti amount (mass %)) ⁇ 3.4[N]] or less, more preferably 0.3 ⁇ [(total Ti amount (mass %)) ⁇ 3.4[N]] or less.
  • the press forming condition heat forming condition
  • the properties such as strength, elongation and the like of the press-formed product can be controlled, the press-formed product with high ductility (residual ductility) is obtained, and therefore application to a portion (energy absorption member for example) to which it has been difficult to apply conventional press-formed products becomes also possible which is very useful in expanding the application range of the press-formed product.
  • a press-formed product exerting strength-ductility balance according to each region (may be hereinafter referred to as “plural region formed product”) is obtained when the heating temperature and the condition of each region in forming are properly controlled and the microstructure of each region is adjusted in manufacturing the press-formed product by press forming of a steel sheet using a press-forming tool.
  • the plural region formed product can be manufactured as described above using the steel sheet for hot pressing use of the present invention by dividing a heating region of the steel sheet into at least two regions, heating one region thereof (hereinafter referred to as the first region) to a temperature of Ac 3 transformation point or above and 950° C. or below, heating another region (hereinafter referred to as the second region) to a temperature of Ac 1 transformation point+20° C. or above and Ac 3 transformation point ⁇ 20° C. or below, thereafter starting press forming of both of the first and second regions, and being held at the bottom dead point in both of the first and second regions and executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing the average cooling rate of 20° C./s or more within a tool.
  • the heating region of the steel sheet into at least two regions (high strength side region and low strength side region) and controlling the manufacturing condition according to each region, such a press-formed product that strength-ductility balance according to each region is exerted is obtained.
  • the second region out of two regions corresponds to the low strength side region, and the manufacturing condition, microstructure and properties in this region is basically same to those of the single region formed product described above.
  • the manufacturing condition for forming the other first region (corresponding to the high strength side region) will be described.
  • this manufacturing method it is required to form regions with different heating temperature by a single steel sheet, however, by using an existing heating furnace (for example, far infrared furnace, electric furnace+shield), controlling while making the boundary section of the temperature 50 mm or less is possible.
  • an existing heating furnace for example, far infrared furnace, electric furnace+shield
  • the heating temperature In order to properly adjust the microstructure of the press-formed product, it is necessary to control the heating temperature to a predetermined range. By properly controlling this heating temperature, transformation to a microstructure mainly of martensite is caused while securing a predetermined amount of retained austenite in the cooling step after heating, and a desired microstructure can be achieved within the range of the final hot press-formed product.
  • the steel sheet heating temperature in this region is below Ac 3 transformation point, a sufficient amount of austenite cannot be obtained in heating, and a predetermined amount of retained austenite cannot be secured in the final microstructure (the microstructure of the formed product).
  • the heating temperature of the steel sheet is preferably Ac 3 transformation point+50° C. or above and 930° C. or below.
  • the average cooling rate should be 20° C./s or more and the cooling finishing temperature should be martensite transformation starting temperature (Ms point) or below.
  • the average cooling rate at that time is preferably 30° C./s or more (more preferably 40° C./s or more).
  • the cooling finishing temperature is 400° C. or below, preferably 300° C. or below.
  • the metal microstructure, precipitates and the like are different between the first region and the second region.
  • the metal microstructure is of retained austenite: 3-20 area % (the action and effect of retained austenite are same to the above), and martensite as quenched: 80 area % or more.
  • the metal microstructure and Ti state (the average equivalent circle diameter of Ti-containing precipitates, the value of the precipitated Ti amount (mass %) ⁇ 3.4[N], and the like) same to those of the single region formed product described above are fulfilled.
  • the area fraction of martensite as quenched should be 80 area % or more.
  • the fraction of martensite as quenched is preferably 85 area % or more (more preferably 90 area % or more).
  • ferrite, pearlite, bainite and the like may be included in a part thereof.
  • Steel (steel Nos. 1-32) having the chemical component composition shown in Tables 1, 2 below was molten in vacuum, was made a slab for experiment, was thereafter made a steel sheet by hot rolling, was thereafter cooled, and was subjected to treatment that simulates winding (sheet thickness: 3.0 mm).
  • the winding simulated treatment method included cooling to the winding temperature, putting the sample thereafter into a furnace heated to the winding temperature, holding for 30 min, and cooling in the furnace.
  • the manufacturing condition for the steel sheet at that time is shown in Table 3 below.
  • Ms point (° C.) 550-361 ⁇ [C] ⁇ 39 ⁇ [Mn] ⁇ 10 ⁇ [Cu] ⁇ 17 ⁇ [Ni] ⁇ 20 ⁇ [Cr] ⁇ 5 ⁇ [Mo]+30 ⁇ [Al] (4)
  • Treatment (1) The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm).
  • Treatment (2) The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), was heated thereafter to 860° C. simulating continuous annealing, was cooled thereafter to 400° C. with the average cooling rate of 30° C./s, and was held.
  • Treatment (3) The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), was heated thereafter to 860° C. simulating continuous hot dip galvanizing line, was cooled thereafter to 400° C. with the average cooling rate of 30° C./s, was held, was thereafter heated further by (500° C. ⁇ 10 s), and was cooled thereafter.
  • An extraction replica sample was prepared, and a transmission electron microscope image (magnifications: 100,000 times) of Ti-containing precipitates was photographed using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • EDX energy dispersion type X-ray spectrometer
  • the area of the Ti-containing precipitates of at least 100 pieces was measured by image analysis, those having the equivalent circle diameter of 30 nm or less were extracted, and the average value thereof was made the size of the precipitates. Also, in the table, the size is shown as “average equivalent circle diameter of Ti-containing precipitates”.
  • precipitated Ti amount (mass %) ⁇ 3.4[N] the Ti amount present as the precipitates
  • extraction residue analysis in extraction treatment, the precipitates coagulate, and fine precipitates also can be measured
  • precipitated Ti amount (mass %) ⁇ 3.4[N] expressed as “precipitated Ti amount ⁇ 3.4[N]” in Tables 4, 5) was obtained.
  • the Ti-containing precipitates partly contained V and Nb, the contents of these precipitates were also measured.
  • Each steel sheet described above (1.6 mm t ⁇ 150 mm ⁇ 200 mm) (with respect to those other than the treatment of (1)-(3) described above, the thickness was adjusted to 1.6 mm by hot rolling) was heated to a predetermined temperature in a heating furnace, and was thereafter subjected to press forming and cooling treatment using the tool ( FIG. 1 above) of a hat shape to obtain the formed product.
  • the press forming conditions (heating temperature, average cooling rate, and rapid cooling finishing temperature in press forming) are shown in Table 6 below.
  • TS tensile strength
  • elongation total elongation EL
  • observation of the metal microstructure the fraction of each microstructure
  • hardness drop amount after heat treatment were measured by methods described below.
  • the tensile test was executed using JIS No. 5 test specimen, and the tensile strength (TS) and the elongation (EL) were measured. At this time, the strain rate of the tensile test was made 10 mm/s. In the present invention, the case 780-1,270 MPa of the tensile strength (TS) and 20% or more of the elongation (EL) were satisfied and the strength-elongation balance (TS ⁇ EL) was 20,000 (MPa ⁇ %) or more was evaluated to have passed.
  • heating was executed to 700° C. with the average heating rate of 50° C./s using a heat treatment simulator, cooling was thereafter executed with the average cooling rate of 50° C./s, and the hardness drop amount ( ⁇ Hv) relative to the original hardness (Vickers hardness) was measured.
  • the hardness drop amount ( ⁇ Hv) was 50 Hv or less, the softening prevention property in the HAZ was determined to be excellent.
  • those of the steel Nos. 3, 6, 7, 9, 13, 18, 22 are the comparative examples not satisfying any of the requirements stipulated in the present invention, and any of the properties is deteriorated. That is, that of the steel No. 3 uses a steel sheet with low Si content, the fraction of retained austenite in the formed product is not secured, the elongation is not enough, and the strength-elongation balance is deteriorated. In that of the steel No. 6, the finish rolling temperature in manufacturing the steel sheet is low, the relationship of the formula (1) is not fulfilled in either stage of the steel sheet for hot pressing use and the formed product, the elongation is not enough to deteriorate the strength-elongation balance, and the softening prevention property is also deteriorated.
  • the average cooling rate from the finish rolling temperature to winding in manufacturing the steel sheet is low, ferrite is formed in the stage of the formed product and the fraction of martensite as quenched cannot be secured, the strength and elongation drop, and the strength-elongation balance (TS ⁇ EL) is deteriorated.
  • the winding temperature in manufacturing the steel sheet is high, the precipitated Ti amount is excessive in the stage of the steel sheet for hot pressing use, and, when press forming is executed using such a steel sheet, even if the forming condition is appropriate, the precipitated Ti amount is excessive also in the formed product, and the softening prevention property is also deteriorated.
  • the cooling rate in press forming is slow, bainite and pearlite are formed in the stage of the formed product and the fraction of martensite as quenched cannot be secured, the strength and elongation drop, and the strength-elongation balance (TS ⁇ EL) is deteriorated.
  • the steel sheet with excessive C content is used, the fraction of martensite of the steel sheet is low, the heating temperature in press forming is high, the fraction of martensite and/or annealed bainite in the formed product cannot be secured, the strength becomes high, and only low elongation EL is obtained (the strength-elongation balance (TS ⁇ EL) is also deteriorated).
  • the steel sheet with excessive Ti content is used, the strength-elongation balance (TS ⁇ EL) is deteriorated, and the softening prevention property is also deteriorated.
  • Each steel sheet described above (3.0 mm t ⁇ 150 mm ⁇ 200 mm) was heated to a predetermined temperature in a heating furnace, and was subjected thereafter to press forming and cooling treatment using the tool ( FIG. 1 above) of a hat shape to obtain the formed product.
  • the steel sheet was put in an infrared furnace and the portion intended to be high-strengthened (the steel sheet portion corresponding to the first region) was configured so that infrared rays directly hit so as to allow high temperature heating, whereas the portion intended to be low-strengthened (the steel sheet portion corresponding to the second region) was shielded with a cover so that a part of the infrared rays was blocked so as to allow low temperature heating, and thereby the heating temperature was differentiated. Therefore, the formed product has the regions with different strength within a single component.
  • the press forming conditions heating temperature, average cooling rate, and rapid cooling finishing temperature of each region in press forming) are shown in Table 13 below.
  • TS tensile strength
  • elongation total elongation EL
  • observation of the metal microstructure the fraction of each microstructure in each region were obtained similarly to Example 1.
  • JP-A-No. 2012-053845 Japanese Patent Application
  • the steel sheet for hot pressing use of the present invention is suitable to structural components of an automobile.

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Abstract

A steel sheet for hot pressing use according to the present invention has a specified chemical component composition, wherein some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 6 nm or less, the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1) shown below, and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more.

Precipitated Ti amount (mass %)−3.4[N]≦0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
(In the formula (1), [N] represents the content (mass %) of N in the steel.)

Description

    TECHNICAL FIELD
  • The present invention relates to a steel sheet for hot pressing use used in manufacturing structural components of an automobile and suitable for hot press forming, a press-formed product obtained from such a steel sheet for hot pressing use, and a method for manufacturing the press-formed product, and relates more specifically to a steel sheet for hot pressing use that is useful in being applied to a hot press forming method securing a predetermined strength by being subjected to heat treatment simultaneously with impartation of the shape in forming a pre-heated steel sheet (blank) into a predetermined shape, a press-formed product, and a useful method for manufacturing such a press-formed product.
    • BACKGROUND ART
  • As one of the fuel economy improvement measures of an automobile triggered by global environment problems, weight reduction of the vehicle body is advancing, and it is necessary to high-strengthen a steel sheet used for an automobile as much as possible. On the other hand, when a steel sheet is high-strengthened, shape accuracy in press forming comes to deteriorate.
  • On this account, a hot press forming method has been employed for manufacturing components in which a steel sheet is heated to a predetermined temperature (for example, a temperature at which a state of an austenitic phase is achieved), the strength is lowered, the steel sheet is thereafter formed using a tool of a temperature (room temperature for example) lower than the steel sheet, thereby impartation of a shape and rapid heat treatment (quenching) utilizing the temperature difference of the both are executed simultaneously, and the strength after forming is secured. Also, such a hot-press forming method is referred to by various names such as a hot forming method, hot stamping method, hot stamp method, die quench method, and the like in addition to the hot press method.
  • FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming described above, 1 in the drawing is a punch, 2 is a die, 3 is a blank holder, 4 is a steel sheet (blank), BHF is a blank holding force, rp is punch shoulder radius, rd is die shoulder radius, and CL is punch/die clearance respectively. Also, out of these components, in the punch 1 and the die 2, passages 1 a, 2 a through which a cooling medium (water for example) can pass are formed inside of each, and it is configured that these members are cooled by making the cooling medium pass through these passages.
  • In hot press forming (hot deep drawing for example) using such a tool, forming is started in a state the steel sheet (blank) 4 is heated to a two-phase zone temperature (between Ac1 transformation point and Ac3 transformation point) or a single-phase zone temperature of Ac3 transformation point or above and is softened. That is, in a state the steel sheet 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pressed in to the inside of a hole of the die 2 by the punch 1, and is formed into a shape corresponding to the shape of the outer shape of the punch 1 while reducing the outside diameter of the steel sheet 4. Also, by cooling the punch 1 and the die 2 in parallel with forming, heat removal from the steel sheet 4 to the tools (the punch 1 and the die 2) is executed, holding and cooling are further executed at a forming bottom dead point (the temporal point the tip of the punch is positioned at the deepest point: the state shown in FIG. 1), and thereby quenching of the raw material is executed. By executing such a forming method, a formed product of 1,500 MPa class with excellent dimensional accuracy can be obtained, the forming load can be reduced compared with a case a component of a same strength class is cold-formed, and therefore less capacity of the press machine is needed.
  • As a steel sheet for hot pressing use widely used at present, one using 22Mn—B5 steel as a raw material is known. The steel sheet has the tensile strength of approximately 1,500 MPa and the elongation of approximately 6-8%, and is applied to a shock resistant member (a member not causing deformation as much as possible and not causing breakage in collision). However, application to a component requiring deformation such as an energy absorption member is difficult because elongation (ductility) is low.
  • As a steel sheet for hot pressing use exerting excellent elongation, technologies such as the patent literatures 1-4 for example have also been proposed. According to these technologies, the basic strength class of each steel sheet is adjusted by setting the carbon content in the steel sheet to various ranges, and elongation is improved by introducing ferrite with high deformability and reducing the average grain size of ferrite and martensite. Although these technologies are effective in improving elongation, they are still insufficient from the viewpoint of improving elongation matching the strength of the steel sheet. For example, those having 1,470 MPa or more of the tensile strength TS have the elongation EL of approximately 10.2% at the maximum, and further improvement is required.
  • On the other hand, even the formed product having a lower strength class compared to the hot stamp formed product, 980 MPa class or 1,180 MPa class of the tensile strength TS for example, having been studied until now has a problem in forming accuracy of cold press forming, and there are needs for low-strength hot press forming as an improvement measure therefor. At that time, it is necessary to largely improve energy absorption properties in the formed product.
  • Particularly, in recent years, development of the technology for differentiating the strength within a single component is proceeding. As such a technology, a technology has been proposed in which the portion that must be prevented from deforming has high strength (high strength side: shock resistant portion side), and the portion that needs energy absorption has low strength and high ductility (low strength side: energy absorption portion side). For example, in a passenger car of the middle class or above, there is a case that portions having both functions of shock resistant property and energy absorption property are provided within a component of a B-pillar and rear side member considering compatibility in a side collision and a rear collision (a function for protecting the counterpart side also when a small-sized car collides with). For the purpose of manufacturing such members, (a) a method of joining a steel sheet becoming of low strength even in being heated to a same temperature and tool-quenched to a normal steel sheet for hot pressing use (tailored weld blank: TWB), (b) a method for differentiating the strength for each region of a steel sheet by differentiating the cooling rate in the tool, (c) a method for differentiating the strength by differentiating the heating temperature for each region of a steel sheet, and the like have been proposed.
  • Although the tensile strength: 1,500 MPa class is achieved on the high strength side (shock resistant portion side) according to these technologies, the maximum tensile strength is 700 MPa and the elongation EL is approximately 17% on the low strength side (energy absorption portion side), and achievement of higher strength and higher ductility are required in order to further improve the energy absorption properties.
  • In the meantime, although automobile components are required to be joined mainly by spot welding, it is known that drop of the strength in the weld heat affected zone (HAZ) is extreme, and the strength of the welded joint drops (softening) (non-patent literature 1 for example).
    • CITATION LIST Patent Literature
  • [Patent Literature 1] JP-A 2010-065292
  • [Patent Literature 2] JP-A 2010-065293
  • [Patent Literature 3] JP-A 2010-065294
  • [Patent Literature 4] JP-A 2010-065295
    • Non-Patent Literature
  • [Non-Patent Literature 1] Hirosue et al. “Nippon Steel Technical Report” No. 378, pp. 15-20 (2003)
    • SUMMARY OF INVENTION Technical Problems
  • The present invention has been developed in view of such circumstances as described above, and its object is to provide a steel sheet for hot pressing use capable of obtaining a press-formed product that can achieve the balance of high strength and elongation with a high level when uniform property is required within a formed product, capable of achieving the balance of high strength and elongation with a high level according to each region when regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product, and useful in obtaining a press-formed product excellent in softening prevention property in a HAZ, a press-formed product exerting the properties described above, and a useful method for manufacturing such a press-formed product.
    • Solution to Problems
  • The steel sheet for hot pressing use of the present invention which could achieve the object described above contains:
  • C: 0.15-0.5% (means mass %, hereinafter the same with respect to the chemical component composition);
  • Si: 0.2-3%;
  • Mn: 0.5-3%;
  • P: 0.05% or less (exclusive of 0%);
  • S: 0.05% or less (exclusive of 0%);
  • Al: 0.01-1%;
  • B: 0.0002-0.01%;
  • Ti: 3.4[N]+0.002% or more and 3.4[N]+0.1% or less ([N] expresses N content (mass %)), and
  • N: 0.001-0.01% respectively, with the remainder consisting of iron and inevitable impurities, in which
  • some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 6 nm or less, the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1) shown below, and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more. Also, “equivalent circle diameter” is the diameter of an imaginary circle having an area same to the size (area) of Ti containing precipitates (TiC for example) (“the average equivalent circle diameter” is the average value thereof).

  • Precipitated Ti amount (mass %)−3.4[N]≦0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
  • (In the formula (1), [N] represents the content (mass %) of N in the steel.)
  • In the steel sheet for hot pressing use of the present invention, according to the necessity, it is also useful to contain, as other elements, (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total, and the like, and the properties of the press-formed product is improved further according to the kind of the elements contained.
  • The method for manufacturing a press-formed product of the present invention which could achieve the object described above includes the steps of heating the steel sheet for hot pressing use as described above to a temperature of Ac1 transformation point+20° C. or above and Ac3 transformation point−20° C. or below, thereafter starting press-forming of the steel sheet, and holding the steel sheet at the bottom dead point and executing cooling to a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., while securing the average cooling rate of 20° C./s or more within a tool.
  • In the press-formed product of the present invention, the metal microstructure within the pressed steel includes retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, some of Ti-containing precipitates contained in pressed steel, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 10 nm or less, the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by the formula (1) shown below, and the balance of high strength and elongation can be achieved as the uniform property of a high level within the formed product. Also, the area ratio of annealed martensite and/or annealed bainite means the total area ratio of both microstructures when both microstructures are included, and means, when either one microstructure is included, the area ratio of the microstructure.

  • Precipitated Ti amount (mass %)−3.4[N]≦0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
  • (In the formula (1), [N] represents the content (mass %) of N in the steel.)
  • On the other hand, another method for manufacturing a press-formed product of the present invention which could achieve the object described above includes the steps of using the steel sheet for hot pressing use as described above, dividing a heating region of the steel sheet into at least two regions, heating one region thereof to a temperature of Ac3 transformation point or above and 950° C. or below, heating another region to a temperature of Ac1 transformation point+20° C. or above and Ac3 transformation point−20° C. or below, thereafter starting press-forming of the both regions, and holding the both regions at the bottom dead point and executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing the average cooling rate of 20° C./s or more within a tool.
  • Another press-formed product of the present invention is a press-formed product of a steel sheet having the chemical component composition as described above in which the pressed steel includes a first region whose metal microstructure includes retained austenite: 3 area % or more and 20 area % or less and martensite as quenched: 80 area % or more, and a second region whose metal microstructure includes retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, some of Ti-containing precipitates contained in steel of the second region, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 10 nm or less, and the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by the formula (1) shown below. In such a press-formed product, the balance of high strength and elongation can be achieved with a high level according to each region, the regions corresponding to the shock resistant portion and the energy absorption portion are present within a single formed product, and softening prevention property of a HAZ in spot welding in the second region becomes excellent.

  • Precipitated Ti amount (mass %)−3.4[N]≦0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
  • (In the formula (1), [N] represents the content (mass %) of N in the steel.)
    • Advantageous Effects of Invention
  • According to the present invention, because a steel sheet is used in which the chemical component composition is strictly stipulated, the size of Ti-containing precipitates is controlled, the precipitation rate is controlled for Ti that does not form TiN, and the ratio of tempered hard phase (martensitic phase, bainitic phase and the like), hard phase (as-quenched martensite phase) and retained austenite phase is adjusted with respect to the metal microstructure, by hot-pressing the steel sheet under a predetermined condition, high strength-elongation balance of the press-formed product can be made a high level. Also, when hot-pressing is executed under different conditions in plural regions, the shock resistant portion and the energy absorption portion can be formed within a single formed product, the balance of high strength and elongation can be achieved with a high level for each portion, and softening prevention property in a HAZ becomes excellent.
    • BRIEF DESCRIPTION OF DRAWING
  • [FIG. 1] FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming.
    •  DESCRIPTION OF EMBODIMENTS
  • The present inventors carried out studies from various aspects in order to achieve such a steel sheet for hot pressing use that can obtain a press-formed product exhibiting excellent ductility (elongation) also while securing high strength after press-forming in manufacturing the press-formed product by heating a steel sheet to a predetermined temperature and thereafter executing hot press forming.
  • As a result of the studies, it was found out that, when the chemical component composition of the steel sheet for hot pressing use was strictly stipulated, the size of Ti-containing precipitates and precipitated Ti amount were controlled and the metal microstructure was made an appropriate one, by hot press forming of the steel sheet under a predetermined condition, a press-formed product in which retained austenite of a predetermined amount was secured after press forming and intrinsic ductility (residual ductility) was enhanced could be obtained, and the present invention was completed.
  • In the steel sheet for hot pressing use of the present invention, it is necessary to strictly stipulate the chemical component composition, and the reasons for limiting the range of each chemical component are as follows.
    • [C: 0.15-0.5%]
  • C is an important element in achieving the balance of high strength and elongation of a case uniform properties are required within a formed product with a high level or in securing retained austenite particularly in the low strength/high ductility portion of a case the regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product. Also, by concentration of C to austenite in heating of hot press forming, retained austenite can be formed after quenching. Also, C contributes to increase of the amount of martensite, and increases the strength. In order to exert such effects, C content should be 0.15% or more.
  • However, when C content becomes excessive and exceeds 0.5%, two phase zone heating range becomes narrow, and the balance of high strength and elongation of a case uniform properties are required within a formed product is not achieved with a high level, or it becomes hard to adjust the metal microstructure to that targeted particularly in the low strength/high ductility portion (a microstructure in which a predetermined amount of annealed martensite and/or annealed bainite is secured) of a case the regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product. Preferable lower limit of C content is 0.17% or more (more preferably 0.20% or more), and more preferable upper limit is 0.45% or less (further more preferably 0.40% or less).
    • [Si: 0.2-3%]
  • Si exerts an effect of forming retained austenite by suppressing that martensite is tempered during cooling of tool-quenching and cementite is formed, or that untransformed austenite is disintegrated. In order to exert such an effect, Si content should be 0.2% or more. Also, when Si content becomes excessive and exceeds 3%, ferrite is liable to be formed, formation of single-phase microstructure becomes hard in heating, and required fractions of bainite and martensite cannot be secured in a steel sheet for hot pressing use. Preferable lower limit of Si content is 0.5% or more (more preferably 1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or less).
    • [Mn: 0.5-3%]
  • Mn is an element effective in enhancing quenchability and suppressing formation of a microstructure (ferrite, pearlite, bainite and the like) other than martensite and retained austenite during cooling of tool-quenching. Also, Mn is an element stabilizing austenite, and is an element contributing to increase of retained austenite amount. In order to exert such effects, Mn should be contained by 0.5% or more. Although Mn content is preferable to be as much as possible when only properties are considered, because the cost of adding alloy increases, Mn content is made 3% or less. Preferable lower limit of Mn content is 0.7% or more (more preferably 1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or less).
    • [P: 0.05% or Less (Exclusive of 0%)]
  • Although P is an element inevitably included in steel, because P deteriorates ductility, P is preferable to be reduced as much as possible. However, because extreme reduction causes increase of the steel making cost and to make it 0% is difficult in manufacturing, P content is made 0.05% or less (exclusive of 0%). Preferable upper limit of P content is 0.045% or less (more preferably 0.040% or less).
  • [S: 0.05% or Less (Exclusive of 0%)]
  • Similar to P, S is also an element inevitably included in steel, S deteriorates ductility, and therefore S is preferable to be reduced as much as possible. However, because extreme reduction causes increase of the steel making cost and to make it 0% is difficult in manufacturing, S content is made 0.05% or less (exclusive of 0%). Preferable upper limit of S content is 0.045% or less (more preferably 0.040% or less).
    • [Al: 0.01-1%]
  • Al is useful as a deoxidizing element, fixes solid-solution N present in steel as AIN, and is useful in improving ductility. In order to effectively exert such an effect, Al content should be 0.01% or more. However, when Al content becomes excessive and exceeds 1%, Al2O3 is formed excessively, and ductility is deteriorated. Also, preferable lower limit of Al content is 0.02% or more (more preferably 0.03% or more), and preferable upper limit is 0.8% or less (more preferably 0.6% or less).
    • [B: 0.0002-0.01%]
  • B is an element contributing to prevention of formation of ferrite, pearlite and bainite during cooling after heating to a two-phase zone temperature of (Ac1 transformation point-Ac3 transformation point) because B has an action of suppressing ferrite transformation, pearlite transformation and bainite transformation on the high strength portion side, and to secure retained austenite. In order to exert such effects, B should be contained by 0.0002% or more, however, even when B is contained excessively exceeding 0.01%, the effects saturate. Preferable lower limit of B content is 0.0003% or more (more preferably 0.0005% or more), and preferable upper limit is 0.008% or less (more preferably 0.005% or less).
    • [Ti: 3.4[N]+0.002% or More and 3.4[N]+0.1% or Less: [N] Expresses N Content (Mass %)]
  • Ti develops improvement effect of quenchability by fixing N and holding B in a solid solution state. In order to exert such an effect, it is important to contain Ti more than the stoichiometric ratio of Ti and N (3.4 times of N content) by 0.002% or more. Also, by making Ti added excessively relative to N present in a solid solution state within the hot stamp formed product and finely dispersing the precipitated compound, drop of the strength in the HAZ can be suppressed by precipitation strengthening caused by that Ti dissolved in welding the hot stamp formed product is formed as TiC and by the effect of delaying increase of the dislocation density and the like by the effect of preventing movement of dislocation by TiC. However, when Ti content becomes excessive to be more than 3.4[N]+0.1%, Ti-containing precipitates (TiN for example) formed is coarsened, and ductility of the steel sheet deteriorates. More preferable lower limit of Ti content is 3.4[N]+0.005% or more (further more preferably 3.4[N]+0.01% or more), and more preferable upper limit is 3.4[N]+0.09% or less (further more preferably 3.4[N]+0.08% or less).
    • [N: 0.001-0.01%]
  • N is an element inevitably mixed in, and is preferable to be reduced as much as possible, however, because there is a limit in reducing N in an actual process, 0.001% is made the lower limit. Also, when N content becomes excessive, Ti-containing precipitates (TiN for example) formed is coarsened, these precipitates work as the fracture origin, ductility of the steel sheet is deteriorated, and therefore the upper limit is made 0.01%. More preferable upper limit of N content is 0.008% or less (further more preferably 0.006% or less).
  • The basic chemical composition in the steel sheet for hot pressing use of the present invention is as described above, and the remainder is iron and inevitable impurities other than P, S (O, H and the like for example). Further, in the steel sheet for hot pressing use of the present invention, according to the necessity, it is also useful to further contain (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total, and the like, and the properties of the steel sheet for hot pressing use are improved further according to the kind of the element contained. Preferable range when these elements are contained and reasons for limiting the range are as follows.
  • [At Least One Element Selected from the Group Consisting of V, Nb and Zr by 0.1% or Less (Exclusive of 0%) in Total]
  • V, Nb and Zr have effects of forming fine carbide and miniaturizing the microstructure by a pinning effect. In order to exert such effects, it is preferable to contain them by 0.001% or more in total. However, when the content of these elements becomes excessive, coarse carbide is formed and becomes a start point of breakage, and ductility is deteriorated adversely. Therefore, it is preferable to contain these elements by 0.1% or less in total. More preferable lower limit of the content of these elements in total is 0.005% or more (further more preferably 0.008% or more), and more preferable upper limit in total is 0.08% or less (further more preferably 0.06% or less).
  • [At Least One Element Selected from the Group Consisting of Cu, Ni, Cr and Mo: 1% or Less (Exclusive of 0%) in Total]
  • Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite transformation and bainite transformation, therefore prevent formation of ferrite, pearlite and bainite during cooling after heating, and act effectively in securing retained austenite. In order to exert such effects, it is preferable to contain them by 0.01% or more in total. Although the content is preferable to be as much as possible when only the properties are considered, because the cost for adding alloys increases, 1% or less in total is preferable. Also, because there is an action of largely increasing the strength of austenite, the load of hot rolling increases, manufacturing of the steel sheet becomes difficult, and therefore 1% or less is also preferable from the viewpoint of manufacturability. More preferable lower limit of these elements in total is 0.05% or more (further more preferably 0.06% or more), and more preferable upper limit in total is 0.5% or less (further more preferably 0.3% or less).
  • [At Least One Element Selected from the Group Consisting of Mg, Ca and REM by 0.01% or Less (Exclusive of 0%) in Total]
  • Because these elements miniaturize inclusions, they act effectively in improving ductility. In order to exert such effects, it is preferable to contain them by 0.0001% or more in total. Although the content is preferable to be as much as possible when only the properties are considered, because the effects saturate, 0.01% or less in total is preferable. More preferable lower limit of these elements in total is 0.0002% or more (further more preferably 0.0005% or more), and more preferable upper limit in total is 0.005% or less (further more preferably 0.003% or less).
  • In the steel sheet for hot pressing use of the present invention, (A) some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 6 nm or less, (B) relationship of precipitated Ti amount (mass %)−3.4[N]≦0.5×[(total Ti amount (mass %))−3.4[N]] (the relationship of the formula (1) described above) is fulfilled, and (C) the metal microstructure contains at least either one of bainite and martensite, and the sum total of the fraction of bainite and the fraction of martensite is 80 area % or more, are also important requirements.
  • Control of Ti-containing precipitates and the formula (1) is for preventing softening of the HAZ and is the control required fundamentally for a formed product, however, variation of these values between before and after hot-press forming is small, and therefore it is necessary that they have already been controlled at the stage of before forming (the steel sheet for hot pressing use). By making Ti that is excessive relative to N in the steel before forming be present in a solid solution state or a fine state, Ti-containing precipitates can be maintained in the solid solution state or the fine state in heating of hot press forming. Thus, the amount of precipitated Ti in the press-formed product can be controlled to a predetermined amount or less, softening in the HAZ is prevented, and thereby the properties of the welded joint can be improved.
  • From such a viewpoint, it is necessary to disperse the Ti-containing precipitates finely, and, for the purpose, it is necessary that some of the Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 6 nm or less (the requirement of (A) described above). Also, the reason the equivalent circle diameter of the Ti-containing precipitates of the object is stipulated to be 30 nm or less is that it is necessary to control the Ti-containing precipitates and excluding TiN formed coarsely in the melting stage that does not affect microstructure change and properties thereafter. The size of the Ti-containing precipitates (the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter is 30 nm or less) is preferably 5 nm or less, more preferably 3 nm or less. Further, the Ti-containing precipitates of the object of the present invention also include precipitates containing Ti such as TiVC, TiNbC, TiVCN, TiNbCN and the like in addition to TiC and TiN.
  • Further, as described below, although the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter in the press-formed product is 30 nm or less is stipulated to be 10 nm or less, the same before forming (the steel sheet for hot pressing use) is stipulated to be 6 nm or less. The reason the size of the precipitates of the formed product is stipulated to be larger than that of the steel sheet is that Ti is present in the steel sheet as fine precipitates or in a solid solution state, and, when heating of 15 min or more at near 800° C. is executed, the Ti-containing precipitates are slightly coarsened. In order to secure the properties as a formed product, it is necessary that the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter is 30 nm or less is 10 nm or less. In order to achieve the precipitation state in the hot stamp formed product, it is necessary that the average equivalent circle diameter of fine precipitates of 30 nm or less is made 6 nm or less and that majority of Ti is present in a solid solution state in the stage of the steel sheet for hot stamp use.
  • Also, in the steel sheet for hot pressing use, it is necessary that, out of Ti, majority of Ti other than that used for precipitating and fixing N is present in the solid solution state or the fine state. For that purpose, it is necessary that the Ti amount present as the precipitates other than TiN (that is, precipitated Ti amount (mass %)−3.4[N]) is 0.5 times or below of the balance obtained by deducting Ti that forms TiN from total Ti (that is, 0.5×[(total Ti amount (mass %))−3.4[N]] or less) (the requirement of (B) described above). Precipitated Ti amount (mass %)−3.4[N] is preferably 0.4×[(total Ti amount (mass %))−3.4[N]] or less, more preferably 0.3×[(total Ti amount (mass %))−3.4[N]] or less.
  • Although control of the metal microstructure is intrinsically necessary for achieving desired strength-elongation balance in the formed product, the metal microstructure cannot be controlled only by the hot pressing condition, and it is necessary to control the microstructure of the raw material steel thereof (the steel sheet for hot pressing use) beforehand. In order to secure the proper amount of annealed martensite and annealed bainite which are fine and largely contributing to ductility in the press-formed product, it is necessary to make the sum total of the fraction of bainite and the fraction of martensite in the steel sheet for hot pressing use 80 area % or more. When the sum total of the fraction of bainite and the fraction of martensite is less than 80 area %, the fraction of annealed martensite and/or annealed bainite targeted in the formed steel sheet is hardly secured, and the amount of other microstructure (ferrite for example) increases to deteriorate the strength-elongation balance. The sum total of the fraction of bainite and the fraction of martensite is preferably 90 area % or more, more preferably 95 area % or more.
  • Further, in the steel sheet for hot pressing use of the present invention, although the remainder of the metal microstructure is not particularly limited, at least any of ferrite, pearlite or retained austenite can be cited for example.
  • The steel sheet (the steel sheet for hot pressing use) of the present invention as described above can be manufactured by that a billet obtained by melting steel having the chemical component composition as described above is subjected to hot rolling with heating temperature: 1,100° C. or above (preferably 1,150° C. or above) and 1,300° C. or below (preferably 1,250° C. or below) and the finish rolling temperature of 930° C. or above (preferably 950° C. or above) and 1,050° C. or below (preferably 1,000° C. or below), cooling (rapid cooling) is executed immediately thereafter to 450° C. or below (preferably 400° C. or below) with the average cooling rate of 20° C./s or more (preferably 30° C./s or more), and winding is executed at 100° C. or above (preferably 150° C. or above) and 450° C. or below (preferably 400° C. or below).
  • The method described above is for executing control so that (1) rolling is finished at a temperature range where dislocation introduced by hot rolling remains within austenite, (2) Ti-containing precipitates such as TiC and the like are formed finely on the dislocation by rapid cooling immediately thereafter, and (3) bainite transformation or martensite transformation is caused by rapid cooling and winding thereafter.
  • The steel sheet for hot pressing use having the chemical component composition, metal microstructure and Ti-precipitation state as described above may be used for manufacturing by a hot press forming as it is, and may be used for manufacturing by hot press forming after being subjected to cold rolling with the draft: 10-80% (preferably 20-70%) after pickling. Further, it is also possible to use the steel sheet for hot pressing use or the material obtained by cold rolling thereof for manufacturing by hot press forming after being subjected to such heat treatment of heating to 830° C. or above (preferably 850° C. or above and 900° C. or below), rapid cooling thereafter to 450° C. or below (preferably 400° C. or below) at a cooling rate of 20° C./s or more (preferably 30° C./s or more), and thereafter holding at 450° C. or below for 10 s or more and 1,000 s or less, or tempering at a temperature of 450° C. or below. The steel sheet subjected to such cold rolling and heat treatment is also included in the steel sheet for hot pressing use of the present invention as far as the required properties are fulfilled. Also, the steel sheet for hot pressing use of the present invention may be subjected to plating containing at least one element out of Al, Zn, Mg and Si on the surface thereof (the surface of the base steel sheet).
  • By using the steel sheet for hot pressing use as described above, executing heating to a temperature of Ac1 transformation point+20° C. or above and Ac3 transformation point−20° C. or below, thereafter starting press-forming, and executing cooling to a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., while securing the average cooling rate of 20° C./s or more within the tool during forming and after completion of forming, the press formed product having a single property (may be hereinafter referred to as “single region formed product”) can have an optimum microstructure of low strength and high ductility. The reasons for stipulating each requirement in this forming method are as described below.
  • In order to form austenite between laths of martensite and bainite within the steel sheet and to form annealed martensite and annealed bainite excellent in ductility by annealing martensite and bainite, the heating temperature should be controlled to a predetermined range. When the heating temperature of the steel sheet is below Ac1 transformation point+20° C., sufficient amount of austenite cannot be secured in heating, and a predetermined amount of retained austenite cannot be secured in the final microstructure (the microstructure of the formed product). Also, when the heating temperature of the steel sheet exceeds Ac3 transformation point−20° C., the transformation amount to austenite increases excessively in heating, and a predetermined amount of annealed martensite and annealed bainite cannot be secured in the final microstructure (the microstructure of the formed product).
  • In order to make austenite formed in the heating step described above a desired microstructure while preventing formation of the microstructure such as ferrite or pearlite, it is necessary to hold the steel sheet at the bottom dead point and to properly control the average cooling rate and the cooling finishing temperature within the tool. From such a viewpoint, it is necessary to make the average cooling rate within the tool 20° C./s or more and to make the cooling finishing temperature a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C. The average cooling rate at that time is preferably 30° C./s or more (more preferably 40° C./s or more). By transforming austenite having been present in heating to bainite and martensite while preventing formation of the microstructure such as ferrite or pearlite by making the cooling finishing temperature a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., fine austenite is made remain between the laths of bainite and martensite, and a predetermined amount of retained austenite is secured while securing bainite and martensite.
  • When the cooling finishing temperature becomes higher than the temperature that is lower than the bainite transformation starting temperature Bs by 100° C. and the average cooling rate is less than 20° C./s, the microstructure such as ferrite, pearlite and the like is formed, a predetermined amount of retained austenite cannot be secured, and elongation (ductility) in the formed product deteriorates. Also, the cooling finishing temperature is not particularly limited as far as it is a temperature or below, the temperature being lower than Bs by 100° C., and can be the martensite transformation starting point Ms or below for example.
  • Although control of the average cooling rate basically becomes unnecessary at the stage the temperature becomes equal to or below the temperature lower than the bainite transformation starting temperature Bs by 100° C., cooling may be executed to the room temperature with the average cooling rate of 1° C./s or more and 100° C./s or less for example. Also, control of the average cooling rate within the tool while being held at the bottom dead point can be achieved by means such as (a) to control the temperature of the forming tool (the cooling medium shown in FIG. 1 above), and (b) to control the thermal conductivity of the tool.
  • In the press-formed product (single region formed product) manufactured by hot press forming as described above, the metal microstructure is formed of retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, and the balance of high strength and elongation can be achieved with a high level and as a uniform property within the formed product. The reasons for setting the range of each requirement (basic microstructure) in such a hot press-formed product are as described below.
  • Retained austenite has an effect of increasing the work hardening ratio (transformation induced plasticity) and improving ductility of the press-formed product by being transformed to martensite during plastic deformation. In order to exert such an effect, the fraction of retained austenite should be made 3 area % or more. Ductility becomes more excellent as the fraction of retained austenite is higher. In the composition used for a steel sheet for an automobile, retained austenite that can be secured is limited, and approximately 20 area % becomes the upper limit. Preferable lower limit of retained austenite is 5 area % or more (more preferably 7 area % or more).
  • By making the main microstructure annealed martensite and/or annealed bainite which is fine and has low dislocation density, ductility (elongation) of the press-formed product can be enhanced while securing a predetermined strength. From such a viewpoint, the fraction of annealed martensite and/or annealed bainite is made 30 area % or more. However, when this fraction exceeds 87 area %, the fraction of retained austenite becomes insufficient, and ductility (residual ductility) deteriorates. Preferable lower limit of annealed martensite and/or annealed bainite is 40 area % or more (more preferably 50 area % or more), and preferable upper limit is less than 80 area % (more preferably less than 70 area %).
  • Because martensite as quenched is a microstructure inferior in ductility, when much amount thereof is present, elongation is deteriorated, however, in order to achieve high strength of over 100 kg/mm2 class in a microstructure with low matrix strength such as annealed martensite, it is necessary to secure a predetermined amount of martensite as quenched. From such a viewpoint, the fraction of martensite as quenched is made 10 area % or more. However, when the fraction of martensite as quenched increases excessively, strength increases excessively and elongation becomes insufficient, and therefore the fraction thereof should be 67 area % or less. Preferable lower limit of the fraction of martensite as quenched is 20 area % or more (more preferably 30 area % or more), and preferable upper limit is 60 area % or less (more preferably 50 area % or less).
  • There is no specific limit other than the microstructure described above, and ferrite, pearlite, bainite and the like may be included as the remainder microstructure, however, these microstructures are inferior in contribution to strength and contribution to ductility compared to other microstructures, and it is basically preferable not to be contained (it may also be 0 area %). However, up to 20 area % is allowable. The remainder microstructure is preferably 10 area % or less, more preferably 5 area % or less.
  • In the press-formed product (single region formed product) described above, some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 10 nm or less. By fulfilling such a requirement, a press-formed product capable of achieving the balance of high strength and elongation with a high level can be obtained. The average equivalent circle diameter of Ti-containing precipitates having 30 nm or less equivalent circle diameter is preferably 8 nm or less, more preferably 6 nm or less.
  • Also, in the press-formed product (single region formed product), the amount of Ti present as the precipitates other than TiN (precipitated Ti amount (mass %)−3.4[N]) is 0.5 times or less of Ti of the balance obtained by deducting Ti that forms TiN from total Ti (that is 0.5×[(total Ti amount (mass %))−3.4[N]] or less). By fulfilling such a requirement, because Ti dissolved at the time of welding is finely precipitated in the HAZ and existing fine Ti-containing precipitates suppresses restoration of dislocation and the like, softening in the HAZ is prevented, and weldability becomes excellent. Precipitated Ti amount (mass %)−3.4[N] is preferably 0.4×[(total Ti amount (mass %))−3.4[N]] or less, more preferably 0.3×[(total Ti amount (mass %))−3.4[N]] or less.
  • When the steel sheet for hot pressing use of the present invention is used, by properly adjusting the press forming condition (heating temperature and cooling rate), the properties such as strength, elongation and the like of the press-formed product can be controlled, the press-formed product with high ductility (residual ductility) is obtained, and therefore application to a portion (energy absorption member for example) to which it has been difficult to apply conventional press-formed products becomes also possible which is very useful in expanding the application range of the press-formed product. Also, not only the single region formed product described above, a press-formed product exerting strength-ductility balance according to each region (may be hereinafter referred to as “plural region formed product”) is obtained when the heating temperature and the condition of each region in forming are properly controlled and the microstructure of each region is adjusted in manufacturing the press-formed product by press forming of a steel sheet using a press-forming tool.
  • The plural region formed product can be manufactured as described above using the steel sheet for hot pressing use of the present invention by dividing a heating region of the steel sheet into at least two regions, heating one region thereof (hereinafter referred to as the first region) to a temperature of Ac3 transformation point or above and 950° C. or below, heating another region (hereinafter referred to as the second region) to a temperature of Ac1 transformation point+20° C. or above and Ac3 transformation point−20° C. or below, thereafter starting press forming of both of the first and second regions, and being held at the bottom dead point in both of the first and second regions and executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing the average cooling rate of 20° C./s or more within a tool.
  • According to the method described above, by dividing the heating region of the steel sheet into at least two regions (high strength side region and low strength side region) and controlling the manufacturing condition according to each region, such a press-formed product that strength-ductility balance according to each region is exerted is obtained. The second region out of two regions corresponds to the low strength side region, and the manufacturing condition, microstructure and properties in this region is basically same to those of the single region formed product described above. Below, the manufacturing condition for forming the other first region (corresponding to the high strength side region) will be described. Also, in executing this manufacturing method, it is required to form regions with different heating temperature by a single steel sheet, however, by using an existing heating furnace (for example, far infrared furnace, electric furnace+shield), controlling while making the boundary section of the temperature 50 mm or less is possible.
    • (Manufacturing Condition of the First Region/High Strength Side Region)
  • In order to properly adjust the microstructure of the press-formed product, it is necessary to control the heating temperature to a predetermined range. By properly controlling this heating temperature, transformation to a microstructure mainly of martensite is caused while securing a predetermined amount of retained austenite in the cooling step after heating, and a desired microstructure can be achieved within the range of the final hot press-formed product. When the steel sheet heating temperature in this region is below Ac3 transformation point, a sufficient amount of austenite cannot be obtained in heating, and a predetermined amount of retained austenite cannot be secured in the final microstructure (the microstructure of the formed product). Also, when the heating temperature of the steel sheet exceeds 950° C., the grain size of austenite becomes large in heating, martensite transformation starting temperature (Ms point) and martensite transformation finishing temperature (Mf point) rise, retained austenite cannot be secured in quenching, and excellent formability is not achieved. The heating temperature of the steel sheet is preferably Ac3 transformation point+50° C. or above and 930° C. or below.
  • In order to make austenite formed in the heating step described above a desired microstructure while preventing formation of the microstructure such as ferrite or pearlite, it is necessary to be held at the bottom dead point and to properly control the average cooling rate and the cooling finishing temperature within the tool. From such a viewpoint, the average cooling rate should be 20° C./s or more and the cooling finishing temperature should be martensite transformation starting temperature (Ms point) or below. The average cooling rate at that time is preferably 30° C./s or more (more preferably 40° C./s or more). By transforming austenite having been present in heating to martensite while preventing formation of the microstructure such as ferrite or pearlite by making the cooling finishing temperature the martensite transformation starting temperature (Ms point) or below, martensite is secured. Specifically, the cooling finishing temperature is 400° C. or below, preferably 300° C. or below.
  • In the press-formed product obtained by such a method, the metal microstructure, precipitates and the like are different between the first region and the second region. In the first region, the metal microstructure is of retained austenite: 3-20 area % (the action and effect of retained austenite are same to the above), and martensite as quenched: 80 area % or more. In the second region, the metal microstructure and Ti state (the average equivalent circle diameter of Ti-containing precipitates, the value of the precipitated Ti amount (mass %)−3.4[N], and the like) same to those of the single region formed product described above are fulfilled.
  • By making the main microstructure of the first region martensite with high strength containing a predetermined amount of retained austenite, ductility and high strength in a specific region in the press-formed product can be secured. From such a viewpoint, the area fraction of martensite as quenched should be 80 area % or more. The fraction of martensite as quenched is preferably 85 area % or more (more preferably 90 area % or more). Also, as the microstructure in the first region, ferrite, pearlite, bainite and the like may be included in a part thereof.
  • Although the effect of the present invention will be shown below more specifically by examples, the examples described below do not limit the present invention, and any of the design alterations judging from the purposes described above and below is to be included in the technical range of the present invention.
    • EXAMPLES Example 1
  • Steel (steel Nos. 1-32) having the chemical component composition shown in Tables 1, 2 below was molten in vacuum, was made a slab for experiment, was thereafter made a steel sheet by hot rolling, was thereafter cooled, and was subjected to treatment that simulates winding (sheet thickness: 3.0 mm). The winding simulated treatment method included cooling to the winding temperature, putting the sample thereafter into a furnace heated to the winding temperature, holding for 30 min, and cooling in the furnace. The manufacturing condition for the steel sheet at that time is shown in Table 3 below. Also, Ac1 transformation point, Ac3 transformation point, Ms point, and Bs point in Tables 1, 2 were obtained using the formulae (2)-(5) below (refer to “The Physical Metallurgy of Steels”, Leslie, Maruzen Company, Limited (1985) for example). Also, the treatment (1)-(3) shown in the remarks column in Table 3 expresses that each treatment (rolling, cooling, alloying) shown below was executed.

  • Ac1 transformation point (° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni]  (2)

  • Ac3 transformation point (° C.)=910-203×[C]1/2+44.7×[Si]−30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]−11×[Cr]+31.5×[Mo]−20×[Cu]−15.2×[Ni]  (3)

  • Ms point (° C.)=550-361×[C]−39×[Mn]−10×[Cu]−17×[Ni]−20×[Cr]−5×[Mo]+30×[Al]  (4)

  • Bs point (° C.)=830-270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−83×[Mo]  (5)
  • wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] represent the content (mass %) of C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni respectively. Also, when the element shown in each term of the formulae (2)-(5) above is not contained, calculation is done assuming that the term is null.
  • Treatment (1): The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm).
  • Treatment (2): The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), was heated thereafter to 860° C. simulating continuous annealing, was cooled thereafter to 400° C. with the average cooling rate of 30° C./s, and was held.
  • Treatment (3): The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), was heated thereafter to 860° C. simulating continuous hot dip galvanizing line, was cooled thereafter to 400° C. with the average cooling rate of 30° C./s, was held, was thereafter heated further by (500° C.×10 s), and was cooled thereafter.
  • TABLE 1
    Steel Chemical component composition* (mass %)
    No. C Si Mn P S Al B Ti N V Nb Cu Ni
    1 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    2 0.150 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    3 0.220 0.05 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    4 0.220 0.25 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    5 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.018 0.0040 — — — —
    6 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    7 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    8 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    9 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    10  0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    11  0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    12  0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    13  0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    14  0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    15  0.220 2.00 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    16  0.350 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    Chemical component
    Steel composition* (mass %) Ac3 − Ac1 + Bs − Ms point
    No. Zr Mg Ca REM Cr Mo 20° C. (° C.) 20° C. (° C.) 100° C. (° C.) (° C.)
    1 — — — — — — 845 765 563 425
    2 — — — — 0.20 — 860 768 568 446
    3 — — — — 0.20 — 792 735 549 421
    4 — — — — 0.20 — 801 741 549 421
    5 — — — — 0.20 — 833 768 549 421
    6 — — — — 0.20 — 843 768 549 421
    7 — — — — 0.20 — 843 768 549 421
    8 — — — — 0.20 — 843 768 549 421
    9 — — — — 0.20 — 843 768 549 421
    10  — — — — 0.20 — 843 768 549 421
    11  — — — — 0.20 — 843 768 549 421
    12  — — — — 0.20 — 843 768 549 421
    13  — — — — 0.20 — 843 768 549 421
    14  — — — — 0.20 — 843 768 549 421
    15  — — — — 0.20 — 879 792 549 421
    16  — — — — 0.20 — 818 768 514 374
    *The remainder: iron and inevitable impurities other than P, S, N.
  • TABLE 2
    Steel Chemical component composition* (mass %)
    No. C Si Mn P S Al B Ti N V Nb Cu Ni
    17 0.450 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    18 0.720 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    19 0.220 1.20 0.80 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    20 0.220 1.20 2.40 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    21 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.100 0.0040 — — — —
    22 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.200 0.0040 — — — —
    23 0.220 0.50 1.20 0.0050 0.0020 0.40 0.0020 0.044 0.0040 — — — —
    24 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 0.030 — — —
    25 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — 0.020 — —
    26 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 0.20 —
    27 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — 0.20
    28 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    29 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    30 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    31 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    32 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — —
    Steel Chemical component composition* (mass %) Ac3 − Ac1 + Bs − Ms point
    No. Zr Mg Ca REM Cr Mo 20° C. (° C.) 20° C. (° C.) 100° C. (° C.) (° C.)
    17 — — — — 0.20 — 802 768 487 338
    18 — — — — 0.20 — 766 768 414 240
    19 — — — — 0.20 — 855 773 585 436
    20 — — — — 0.20 — 807 756 441 374
    21 — — — — 0.20 — 866 768 549 421
    22 — — — — 0.20 — 906 768 549 421
    23 — — — — 0.20 — 960 748 549 432
    24 — — — — 0.20 — 846 768 549 421
    25 — — — — 0.20 — 843 768 549 421
    26 — — — — 0.20 — 839 768 549 419
    27 — — — — 0.20 — 840 765 541 417
    28 — — — — 0.20 0.20 849 768 532 420
    29 0.015 — — — 0.20 — 843 768 549 421
    30 — 0.002 — — 0.20 — 843 768 549 421
    31 — — 0.002 — 0.20 — 843 768 549 421
    32 — — — 0.002 0.20 — 843 768 549 421
    *The remainder: iron and inevitable impurities other than P, S, N.
  • TABLE 3
    Steel sheet manufacturing condition
    Average cooling
    Finish rate of finish
    Heating rolling rolling temper- Winding
    temper- temper- ature - winding temper-
    Steel ature ature temperature ature
    No. (° C.) (° C.) (° C./s) (° C.) Remarks
    1 1200 950 30 200 —
    2 1200 950 30 200 —
    3 1200 950 30 200 —
    4 1200 950 30 200 —
    5 1200 950 30 200 —
    6 1200 820 30 200 —
    7 1200 950 10 200 —
    8 1200 950 30 400 —
    9 1200 950 30 580
    10 1200 950 30 400 Treatment (1)
    11 1200 950 30 400 Treatment (2)
    12 1200 950 30 400 Treatment (3)
    13 1200 950 30 200 —
    14 1200 950 30 200 —
    15 1200 950 30 200 —
    16 1200 950 30 200 —
    17 1200 950 30 200 —
    18 1200 950 30 200 —
    19 1200 950 30 200 —
    20 1200 950 30 200 —
    21 1200 950 30 200 —
    22 1200 950 30 200 —
    23 1200 950 30 200 —
    24 1200 950 30 200 —
    25 1200 950 30 200 —
    26 1200 950 30 200 —
    27 1200 950 30 200 —
    28 1200 950 30 200 —
    29 1200 950 30 200 —
    30 1200 950 30 200 —
    31 1200 950 30 200 —
    32 1200 950 30 200 —
  • With respect to the steel sheet obtained, analysis of the precipitation state of Ti and observation of the metal microstructure (the fraction of each microstructure) were executed by the procedure described below. The result is shown in Tables 4, 5 below along with the calculated value of 0.5×[total Ti amount(mass %)−3.4[N]] (shown as 0.5×(total Ti amount−3.4[N])).
    • [Analysis of Precipitation State of Ti of Steel Sheet]
  • An extraction replica sample was prepared, and a transmission electron microscope image (magnifications: 100,000 times) of Ti-containing precipitates was photographed using a transmission electron microscope (TEM). At this time, by composition analysis of the precipitates using an energy dispersion type X-ray spectrometer (EDX), Ti-containing precipitates were identified. The area of the Ti-containing precipitates of at least 100 pieces was measured by image analysis, those having the equivalent circle diameter of 30 nm or less were extracted, and the average value thereof was made the size of the precipitates. Also, in the table, the size is shown as “average equivalent circle diameter of Ti-containing precipitates”. Further, with respect to precipitated Ti amount (mass %)−3.4[N] (the Ti amount present as the precipitates), extraction residue analysis (in extraction treatment, the precipitates coagulate, and fine precipitates also can be measured) was executed using a mesh with mesh diameter: 0.1 μm, and precipitated Ti amount (mass %)−3.4[N] (expressed as “precipitated Ti amount−3.4[N]” in Tables 4, 5) was obtained. Also, when the Ti-containing precipitates partly contained V and Nb, the contents of these precipitates were also measured.
    • [Observation of Metal Microstructure (Fraction of Each Microstructure)]
  • (1) With respect to the microstructure of martensite and bainite in the steel sheet, the steel sheet was corroded by nital, martensite and bainite were distinguished from each other by SEM observation (magnifications: 1,000 times or 2,000 times), and each fraction (area ratio) was obtained.
  • (2) The retained austenite fraction in the steel sheet was measured by X-ray diffraction method after the steel sheet was ground up to ¼ thickness thereof and was thereafter subjected to chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7, P. 776).
  • TABLE 4
    Steel sheet for press forming use
    Precipitated Ti Average equivalent circle Fraction of
    Steel amount-3.4[N] 0.5 × (total Ti amount-3.4[N]) diameter of Ti-containing martensite Fraction of bainite
    No. (mass %) (mass %) precipitates (nm) (area %) (area %) Others
    1 0.003 0.015 2.5 100 0 —
    2 0.004 0.015 2.9 100 0 —
    3 0.004 0.015 2.8 100 0 —
    4 0.004 0.015 2.7 100 0 —
    5 0.001 0.003 2.3 100 0 —
    6 0.023 0.015 3.8 100 0 —
    7 0.004 0.015 2.4 0 65 Ferrite: 35%
    8 0.004 0.015 2.1 12 88 —
    9 0.020 0.015 2.0 0 40 Ferrite: 30%,
    Pearlite: 30%
    10 0.003 0.015 2.2 10 90 —
    11 0.004 0.015 2.7 10 90 —
    12 0.004 0.015 2.5 10 90 —
    13 0.003 0.015 2.0 100 0 —
    14 0.005 0.015 2.6 100 0
    15 0.005 0.015 2.2 100 0 —
    16 0.004 0.015 2.6 90 0 Retained
    austenite 10%
  • TABLE 5
    Steel sheet for press forming use
    Precipitated Ti Average equivalent circle Fraction of
    Steel amount-3.4[N] 0.5 × (total Ti amount-3.4[N]) diameter of Ti-containing martensite Fraction of bainite
    No. (mass %) (mass %) precipitates (nm) (area %) (area %) Others
    17 0.005 0.015 2.7 80 0 Retained
    austenite 20%
    18 0.003 0.015 2.2 60 0 Retained
    austenite 40%
    19 0.005 0.015 2.0 100 0 —
    20 0.003 0.015 3.0 100 0 —
    21 0.032 0.043 2.1 100 0 —
    22 0.130 0.093 12.0 100 0 —
    23 0.005 0.015 2.6 100 0 —
    24 0.003 0.015 2.6 100 0 —
    25 0.003 0.015 2.8 100 0 —
    26 0.003 0.015 2.6 100 0 —
    27 0.003 0.015 2.4 100 0 —
    28 0.003 0.015 2.8 100 0 —
    29 0.003 0.015 2.7 100 0 —
    30 0.003 0.015 2.6 100 0 —
    31 0.003 0.015 2.5 100 0 —
    32 0.003 0.015 2.7 100 0 —
  • Each steel sheet described above (1.6 mmt×150 mm×200 mm) (with respect to those other than the treatment of (1)-(3) described above, the thickness was adjusted to 1.6 mm by hot rolling) was heated to a predetermined temperature in a heating furnace, and was thereafter subjected to press forming and cooling treatment using the tool (FIG. 1 above) of a hat shape to obtain the formed product. The press forming conditions (heating temperature, average cooling rate, and rapid cooling finishing temperature in press forming) are shown in Table 6 below.
  • TABLE 6
    Press forming condition
    Rapid cooling
    Heating Average finishing
    Steel temperature cooling rate temperature
    No. (° C.) (° C./s) (° C.)
    1 810 40 300
    2 810 40 300
    3 760 40 300
    4 770 40 300
    5 800 40 300
    6 810 40 300
    7 810 40 300
    8 810 40 300
    9 810 40 300
    10 810 40 300
    11 810 40 300
    12 810 40 300
    13 810 5 300
    14 810 40 600
    15 830 40 300
    16 790 40 300
    17 790 40 300
    18 770 40 300
    19 820 40 300
    20 780 40 300
    21 830 40 300
    22 840 40 300
    23 830 40 300
    24 820 40 300
    25 810 40 300
    26 810 40 300
    27 800 40 300
    28 810 40 300
    29 810 40 300
    30 810 40 300
    31 800 40 300
    32 810 40 300
  • With respect to the formed product obtained, tensile strength (TS), elongation (total elongation EL), observation of the metal microstructure (the fraction of each microstructure), and hardness drop amount after heat treatment were measured by methods described below.
    • [Measurement of Tensile Strength (TS) and Elongation (Total Elongation EL)]
  • The tensile test was executed using JIS No. 5 test specimen, and the tensile strength (TS) and the elongation (EL) were measured. At this time, the strain rate of the tensile test was made 10 mm/s. In the present invention, the case 780-1,270 MPa of the tensile strength (TS) and 20% or more of the elongation (EL) were satisfied and the strength-elongation balance (TS×EL) was 20,000 (MPa·%) or more was evaluated to have passed.
    • [Observation of Metal Microstructure (Fraction of Each Microstructure)]
  • (1) With respect to the microstructure of annealed martensite, bainite and annealed bainite in the steel sheet, the steel sheet was corroded by nital, annealed martensite, bainite and annealed bainite were distinguished from each other by SEM observation (magnifications: 1,000 times or 2,000 times), and each fraction (area ratio) was obtained.
  • (2) The retained austenite fraction in the steel sheet was measured by X-ray diffraction method after the steel sheet was ground up to ¼ thickness thereof and was thereafter subjected to chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7, P. 776).
  • (3) With respect to the fraction of martensite as quenched, the steel sheet was LePera-corroded, the area ratio of the white contrast was measured as the mixture microstructure of martensite as quenched and retained austenite, the retained austenite fraction obtained by X-ray diffraction was deducted therefrom, and the fraction of martensite as quenched was calculated.
    • [Hardness Drop Amount After Heat Treatment]
  • As the thermal hysteresis based on spot welding, heating was executed to 700° C. with the average heating rate of 50° C./s using a heat treatment simulator, cooling was thereafter executed with the average cooling rate of 50° C./s, and the hardness drop amount (ΔHv) relative to the original hardness (Vickers hardness) was measured. When the hardness drop amount (ΔHv) was 50 Hv or less, the softening prevention property in the HAZ was determined to be excellent.
  • The observation results (fraction of each microstructure, precipitated Ti amount−3.4[N]) of the metal microstructure are shown in Tables 7, 8 below. Also, the mechanical properties (tensile strength TS, elongation EL, TS×EL, and hardness drop amount ΔHv) of the formed product are shown in Table 9 below. Further, although the value of precipitated Ti amount−3.4[N] in the formed product is slightly different compared to the value of precipitated Ti amount-3.4[N] in the steel sheet for press forming use, this is a measurement error.
  • TABLE 7
    Metal microstructure of formed product
    Fraction Average equivalent circle Precipitated Ti
    Steel Fraction of annealed martensite Fraction of martensite of retained diameter of Ti-containing amount-3.4[N]
    No. and/or annealed bainite (area %) as quenched (area %) austenite (area %) precipitates (nm) (mass %) Others
    1 70 23 7 2.1 0.005 —
    2 70 23 7 2.5 0.005 Ferrite 3%
    3 70 30 0 2.4 0.009 —
    4 70 25 5 2.5 0.008 —
    5 70 23 7 2.3 0.001 —
    6 70 23 7 13 0.025 —
    7 50 8 7 2.2 0.005 Ferrite 35%
    8 70 23 4 2.9 0.007 —
    9 70 23 7 3.2 0.031 —
    10 70 23 7 2.6 0.010 —
    11 70 23 7 2.0 0.010 —
    12 70 23 7 2.8 0.010 —
    13 70 0 0 2.3 0.005 Bainite 25%, Pearlite
    5%
    14 70 23 7 2.2 0.008 —
    15 70 23 9 2.4 0.005 —
    16 60 29 11 2.4 0.010 —
  • TABLE 8
    Metal microstructure of formed product
    Fraction Average equivalent circle Precipitated
    Steel Fraction of annealed martensite Fraction of martensite of retained diameter of Ti-containing Ti amount-3.4[N]
    No. and/or annealed bainite (area %) as quenched (area %) austenite (area %) precipitates (nm) (mass %) Others
    17 55 32 13 2.3 0.005 —
    18 16 62 22 2.1 0.006 —
    19 70 25 5 2.3 0.008 —
    20 70 21 9 2.5 0.010 —
    21 70 23 7 2.7 0.040 —
    22 70 23 7 12.0 0.180 —
    23 70 23 7 2.0 0.009 —
    24 70 23 7 2.6 0.010 —
    25 70 23 7 2.8 0.008 —
    26 70 23 7 2.1 0.006 —
    27 70 23 7 3.0 0.009 —
    28 70 23 7 2.7 0.006 —
    29 70 24 6 2.7 0.008 —
    30 70 22 8 2.5 0.007 —
    31 70 23 7 2.7 0.008 —
    32 70 24 6 2.8 0.007 —
  • TABLE 9
    Mechanical properties of formed product
    Hardness
    Tensile Elongation drop
    Steel strength EL TS × EL amount
    No. TS (MPa) (%) (MPa · %) ΔHv (Hv)
    1 1016 21.6 21919 21
    2 1027 20.9 21461 22
    3 1050 13.0 13650 23
    4 983 22.3 21906 22
    5 1022 22.4 22894 26
    6 1524 10.3 15697 52
    7 721 19.4 13987 22
    8 1011 21.8 22040 22
    9 1053 20.8 21908 62
    10 1064 21.3 22628 22
    11 1048 20.2 21178 21
    12 1061 21.2 22535 21
    13 876 16.2 14191 22
    14 1053 20.1 21120 22
    15 1047 23.0 24112 22
    16 1192 20.3 24188 21
    17 1260 21.4 26955 21
    18 1321 10.2 13474 22
    19 1016 23.6 23984 22
    20 1054 22.6 23811 21
    21 1047 22.6 23678 28
    22 1071 16.4 17564 51
    23 1060 22.8 24143 22
    24 1052 23.1 24343 21
    25 1045 22.3 23283 22
    26 1042 23.7 24648 21
    27 1062 22.0 23409 22
    28 1051 22.5 23603 22
    29 1020 22.6 23052 22
    30 1041 22.5 23423 21
    31 1033 23.1 23862 21
    32 1042 22.7 23653 22
  • From these results, following consideration can be made. Those of the steel Nos. 1, 2, 4, 5, 8, 10-12, 14-17, 19-21, 23-32 are examples fulfilling the requirements stipulated in the present invention, and it is known that components excellent in strength-elongation balance and excellent in softening prevention property have been obtained.
  • On the other hand, those of the steel Nos. 3, 6, 7, 9, 13, 18, 22 are the comparative examples not satisfying any of the requirements stipulated in the present invention, and any of the properties is deteriorated. That is, that of the steel No. 3 uses a steel sheet with low Si content, the fraction of retained austenite in the formed product is not secured, the elongation is not enough, and the strength-elongation balance is deteriorated. In that of the steel No. 6, the finish rolling temperature in manufacturing the steel sheet is low, the relationship of the formula (1) is not fulfilled in either stage of the steel sheet for hot pressing use and the formed product, the elongation is not enough to deteriorate the strength-elongation balance, and the softening prevention property is also deteriorated.
  • In that of the steel No. 7, the average cooling rate from the finish rolling temperature to winding in manufacturing the steel sheet is low, ferrite is formed in the stage of the formed product and the fraction of martensite as quenched cannot be secured, the strength and elongation drop, and the strength-elongation balance (TS×EL) is deteriorated. In that of the steel No. 9, the winding temperature in manufacturing the steel sheet is high, the precipitated Ti amount is excessive in the stage of the steel sheet for hot pressing use, and, when press forming is executed using such a steel sheet, even if the forming condition is appropriate, the precipitated Ti amount is excessive also in the formed product, and the softening prevention property is also deteriorated.
  • In that of the steel No. 13, the cooling rate in press forming is slow, bainite and pearlite are formed in the stage of the formed product and the fraction of martensite as quenched cannot be secured, the strength and elongation drop, and the strength-elongation balance (TS×EL) is deteriorated. In that of the steel No. 18, the steel sheet with excessive C content is used, the fraction of martensite of the steel sheet is low, the heating temperature in press forming is high, the fraction of martensite and/or annealed bainite in the formed product cannot be secured, the strength becomes high, and only low elongation EL is obtained (the strength-elongation balance (TS×EL) is also deteriorated). In that of the steel No. 22, the steel sheet with excessive Ti content is used, the strength-elongation balance (TS×EL) is deteriorated, and the softening prevention property is also deteriorated.
    • Example 2
  • Steel (steel Nos. 33-37) having the chemical component composition shown in Table 10 below was molten in vacuum, was made a slab for experiment, was thereafter hot-rolled, and was thereafter cooled and wound (sheet thickness: 3.0 mm). The manufacturing condition for the steel sheet at that time is shown in Table 11 below.
  • TABLE 10
    Steel Chemical component composition* (mass %)
    No. C Si Mn P S Al B Ti N V Nb
    33 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — —
    34 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — —
    35 0.350 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — —
    36 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — —
    37 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — —
    Chemical
    component
    composition*
    Steel (mass %) Ac3 − Ac1 + Bs − Ms point
    No. Cu Ni Cr Mo 20° C. (° C.) 20° C. (° C.) 100° C. (° C.) (° C.)
    33 — — 0.20 — 843 768 549 421
    34 — — 0.20 — 843 768 549 421
    35 — — 0.20 — 818 768 514 374
    36 — — 0.20 — 843 768 549 421
    37 — — — — 845 765 563 425
    *The remainder: iron and inevitable impurities other than P, S, N.
  • TABLE 11
    Steel sheet manufacturing condition
    Average cooling
    Finish rate of finish
    Heating rolling rolling temper- Winding
    temper- temper- ature - winding temper-
    Steel ature ature temperature ature
    No. (° C.) (° C.) (° C./s) (° C.) Remarks
    33 1200 950 30 200 —
    34 1200 950 30 200 —
    35 1200 950 30 200 —
    36 1200 950 30 200 Treatment (1)
    37 1200 950 30 200 —
  • With respect to the steel sheet obtained, analysis of the precipitation state of Ti-containing precipitates and observation of the metal microstructure (the fraction of each microstructure) were executed similarly to Example 1. The result is shown in Table 12 below.
  • TABLE 12
    Steel sheet for press forming use
    Precipitated Ti Average equivalent circle Fraction of Fraction of
    Steel amount-3.4[N] 0.5 × (total Ti amount-3.4[N]) diameter of Ti-containing martensite bainite
    No. (mass %) (mass %) precipitates (nm) (area %) (area %) Others
    33 0.002 0.015 2.7 100 0 —
    34 0.003 0.015 2.9 100 0 —
    35 0.004 0.015 2.3 90 0 Retained
    austenite 10%
    36 0.001 0.015 3.0 100 0 —
    37 0.002 0.015 2.5 100 0 —
  • Each steel sheet described above (3.0 mmt×150 mm×200 mm) was heated to a predetermined temperature in a heating furnace, and was subjected thereafter to press forming and cooling treatment using the tool (FIG. 1 above) of a hat shape to obtain the formed product. At this time, the steel sheet was put in an infrared furnace and the portion intended to be high-strengthened (the steel sheet portion corresponding to the first region) was configured so that infrared rays directly hit so as to allow high temperature heating, whereas the portion intended to be low-strengthened (the steel sheet portion corresponding to the second region) was shielded with a cover so that a part of the infrared rays was blocked so as to allow low temperature heating, and thereby the heating temperature was differentiated. Therefore, the formed product has the regions with different strength within a single component. The press forming conditions (heating temperature, average cooling rate, and rapid cooling finishing temperature of each region in press forming) are shown in Table 13 below.
  • TABLE 13
    Press forming condition
    Rapid cooling
    Heating Average finishing
    Steel temperature cooling rate temperature
    No. Region (° C.) (° C./s) (° C.)
    33 Low strength side 810 40 300
    High strength side 920 40 300
    34 Low strength side 800 40 300
    High strength side 840 40 300
    35 Low strength side 800 40 300
    High strength side 920 40 300
    36 Low strength side 790 40 300
    High strength side 920 40 300
    37 Low strength side 810 40 300
    High strength side 920 40 300
  • With respect to the formed product obtained, tensile strength (TS), elongation (total elongation EL), and observation of the metal microstructure (the fraction of each microstructure) in each region were obtained similarly to Example 1.
  • The observation results (fraction of each microstructure) of the metal microstructure are shown in Table 14 below. Also, the mechanical properties (tensile strength TS, elongation EL, TS×EL, and hardness drop amount) of the formed product are shown in Table 15 below.
  • Further, the case 1,470 MPa or more of the tensile strength (TS) and 8% or more of the elongation (EL) on the high strength side were fulfilled and the strength-elongation balance (TS×EL) was 14,000 (MPa·%) or more was evaluated to have passed (the evaluation criteria of the low strength side are same to those of Example 1).
  • TABLE 14
    Metal microstructure of formed product
    Fraction Fraction Average equivalent circle Precipitated
    Steel Fraction of annealed martensite of martensite as of retained diameter of Ti-containing Ti amount-3.4[N]
    No. Region and/or annealed bainite (area %) quenched (area %) austenite (area %) precipitates (nm) (mass %) Others
    33 Low 70 23 7 6.2 0.008 —
    strength
    side
    High 0 94 6 2.5 0.005 —
    strength
    side
    34 Low 70 23 7 6.0 0.007 —
    strength
    side
    High 0 61 6 2.3 0.008 Ferrite 33%
    strength
    side
    35 Low 60 29 11 6.0 0.009 —
    strength
    side
    High 0 95 5 2.4 0.008 —
    strength
    side
    36 Low 70 23 7 4.0 0.005 —
    strength
    side
    High 0 94 6 2.9 0.007 —
    strength
    side
    37 Low 70 24 6 5.8 0.009 —
    strength
    side
    High 0 94 6 2.7 0.008 —
    strength
    side
  • TABLE 15
    Mechanical properties of formed product
    Hardness
    Tensile Elongation drop
    Steel strength EL TS × EL amount
    No. Region TS (MPa) (%) (MPa · %) ΔHv(Hv)
    33 Low strength side 1010 21.4 21586 30
    High strength side 1511 10.1 15261 45
    34 Low strength side 1014 21.8 22117 30
    High strength side 1278 12.1 15464 42
    35 Low strength side 1192 21.6 25740 30
    High strength side 1820 9.7 17654 50
    36 Low strength side 1066 21.9 23345 30
    High strength side 1499 10.1 15140 43
    37 Low strength side 1007 20.3 20442 30
    High strength side 1501 10.8 16211 45
  • From this result, following consideration can be made. Those of the steel Nos. 33, 35-37 are examples fulfilling the requirements stipulated in the present invention, and it is known that components excellent in strength-elongation balance in each region have been obtained. On the other hand, in that of the steel No. 34, the heating temperature in press forming is low, and the strength on the high strength side drops (the strength difference relative to the low strength side is less than 300 MPa).
  • Although the present invention has been described in detail and referring to specific embodiments, it is obvious for a person with an ordinary skill in the art that various alterations and amendments can be effected without departing from the spirit and the range of the present invention.
  • The present application is based on Japanese Patent Application (JP-A-No. 2012-053845) applied on Mar. 9, 2012, and the contents thereof are hereby incorporated by reference.
    • INDUSTRIAL APPLICABILITY
  • The steel sheet for hot pressing use of the present invention is suitable to structural components of an automobile.
    • REFERENCE SIGNS LIST
  • 1 . . . punch
  • 2 . . . die
  • 3 . . . blank holder
  • 4 . . . steel sheet (blank)

Claims (6)

1. A steel sheet comprising, by mass %, iron and:
C: from 0.15-0.5%;
Si: from 0.2-3%;
Mn: from 0.5-3%;
P: from 0.05% or less (exclusive of 0%);
S: from 0.05% or less (exclusive of 0%);
Al: from 0.01-1%;
B: from 0.0002-0.01%;
Ti: from 3.4[N]+0.002% to 3.4[N]+0.1% ([N] expresses N content (mass %)), and
N: from 0.001-0.01% and
Ti-containing precipitates said precipitates have an equivalent circle diameter of 30 nm or less, and an average equivalent circle diameter of 6 nm or less, and wherein
the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1), and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more:

Precipitated Ti amount (mass %)−3.4[N]≦0.5×[(total Ti amount (mass %))−3.4[N]] (1)
where [N] represents the content (mass %) of N in the steel.
2. The steel sheet according to claim 1 further comprising:
(a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total;
(b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total; and/or
(c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total.
3. A method for manufacturing a press-formed product comprising:
heating the steel sheet of claim 1 to a temperature of Ac1 transformation point+20° C. or above and Ac3 transformation point−20° C. or below;
press-forming the heated steel sheet; and
holding the steel sheet at the bottom dead point and cooling to a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., with an average cooling rate of 20° C./s or more thereby producing the press-formed product.
4. A press-formed product of a steel sheet, said sheet comprising iron and
C: from 0.15-0.5%;
Si: from 0.2-3%;
Mn: from 0.5-3%;
P: from 0.05% or less (exclusive of 0%);
S: from 0.05% or less (exclusive of 0%);
Al: from 0.01-1%;
B: from 0.0002-0.01%;
Ti: from 3.4[N]+0.002% to 3.4[N]+0.1% ([N] expresses N content (mass %)), and
N: from 0.001-0.01%,
wherein a metal microstructure within the press-formed product comprises retained austenite from 3-20 area %, annealed martensite and/or annealed bainite from 30-87 area %, and martensite as quenched from 10-67 area %, Ti-containing precipitates, said precipitates have an equivalent circle diameter of 30 nm or less, and an average equivalent circle diameter of 10 nm or less,
and the amount of precipitated Ti and the total amount of Ti in the formed product fulfill the relationship represented by formula (1):

Precipitated Ti amount (mass %)−3.4[N]0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
where [N] represents the content (mass %) of N in the steel.
5. A method for manufacturing a press-formed product comprising:
subdividing a heating region of the steel sheet from claim 1 into at least two regions thereby producing divided regions,
heating one divided region to a temperature of from Ac3 transformation point to 950° C.,
heating another divided region not identical to the previous divided region to a temperature of from Ac1 transformation point+20° C. to Ac3 transformation point−20° C.,
press forming the heated subdivided regions; and
holding the subdivided regions at the bottom dead point and cooling to a temperature of martensite transformation starting temperature Ms or below at an average cooling rate of 20° C./s or more thereby producing the press-formed product.
6. A press-formed product of a steel sheet, said sheet comprising iron and
C: from 0.15-0.5%;
Si: from 0.2-3%;
Mn: from 0.5-3%;
P: from 0.05% or less (exclusive of 0%);
S: from 0.05% or less (exclusive of 0%);
Al: from 0.01-1%;
B: from 0.0002-0.01%;
Ti: from 3.4[N]+0.002% to 3.4[N]+0.1% ([N] expresses N content (mass %)), and
N: from 0.001-0.01%,
wherein
the press-formed product comprises a first region whose metal microstructure comprises retained austenite of from 3-20 area % and martensite as quenched of 80 area % or more, and a second region whose metal microstructure comprises retained austenite of from 3-20 area %, annealed martensite and/or annealed bainite of from 30-87 area %; and martensite as quenched of from 10-67 area %, and
some Ti-containing precipitates comprising the steel of the second region have an equivalent circle diameter of 30 nm or less, and an average equivalent circle diameter of 10 nm or less, and
the precipitated Ti amount and the total Ti amount in the formed product fulfill the relationship represented by formula (1):

Precipitated Ti amount (mass %)−3.4[N]0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
wherein [N] represents the content (mass %) of N in the steel.
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