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WO2025127794A1 - Tôle d'acier et son procédé de fabrication - Google Patents

Tôle d'acier et son procédé de fabrication Download PDF

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Publication number
WO2025127794A1
WO2025127794A1 PCT/KR2024/096659 KR2024096659W WO2025127794A1 WO 2025127794 A1 WO2025127794 A1 WO 2025127794A1 KR 2024096659 W KR2024096659 W KR 2024096659W WO 2025127794 A1 WO2025127794 A1 WO 2025127794A1
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steel sheet
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steel
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Korean (ko)
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안연상
최용훈
박만영
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Posco Holdings Inc
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Posco Co Ltd
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    • 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • 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
    • C22C38/06—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
    • C22C38/18—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
    • 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
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • 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
    • C21D2211/005—Ferrite
    • 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

Definitions

  • the present invention relates to a steel plate suitable as a material for automobile structural members, and more specifically, to a high-strength composite steel plate having a tensile strength of 780 MPa or higher and a method for manufacturing the same.
  • high-strength automobile materials are classified into precipitation-strengthened steel, baking-hardened steel, solid solution-strengthened steel, and transformation-strengthened steel.
  • transformation-strengthened steel includes dual phase steel (DP steel), complex phase steel (CP steel), and transformation-induced plasticity steel (TRIP steel).
  • DP steel dual phase steel
  • CP steel complex phase steel
  • TRIP steel transformation-induced plasticity steel
  • Such transformation-strengthened steel is also called advanced high strength steel (AHSS).
  • DP steel is a steel that can secure high strength by finely and uniformly dispersing hard martensite within soft ferrite.
  • CP steel is a steel that contains two or more phases among ferrite, martensite, and bainite as its microstructure, and contains precipitation hardening elements such as Ti and Nb to improve strength.
  • TRIP steel is a steel that can secure high strength and high ductility by transforming finely and uniformly dispersed residual austenite phase into martensite during processing at room temperature.
  • hot-dip galvanized steel sheets with excellent corrosion resistance have been used as automobile materials from the past.
  • This hot-dip galvanized steel sheet can be manufactured through continuous hot-dip galvanizing equipment in which the recrystallization annealing process and the plating process are performed on the same line, so it has the advantage of being able to manufacture steel sheets with excellent corrosion resistance at low cost.
  • alloyed hot-dip galvanized steel sheets manufactured through alloying treatment in which reheating is performed after hot-dip galvanizing are widely used because they have excellent corrosion resistance as well as weldability and formability.
  • DP steel that has the characteristics of DP steel, such as low yield ratio and high ductility, while also having excellent burring and elongation flangeability.
  • hot-dip galvanized steel sheets with excellent corrosion resistance and weldability is also required.
  • Patent Document 1 for improving the workability of a high-strength steel plate proposes a composite grain steel plate mainly composed of martensite, and discloses a manufacturing method for dispersing fine copper precipitation particles having a particle size of 1 to 100 nm within the grain structure in order to improve the workability of the steel plate.
  • this document since Cu is excessively added at 2 to 5% for the precipitation of fine Cu particles, there is a concern that red-hot embrittlement due to Cu may occur, and there is a problem that the manufacturing cost increases excessively.
  • Patent document 2 proposes a high-strength hot-dip galvanized steel sheet with good hole expandability. Specifically, Patent document 2 discloses a steel sheet having a structure comprising ferrite as a matrix and pearlite in an amount of 2 to 10 area%, and in which carbonitride-forming elements such as Nb, Ti, and V are added for precipitation strengthening, thereby improving strength through precipitation strengthening and grain refinement.
  • carbonitride-forming elements such as Nb, Ti, and V are added for precipitation strengthening, thereby improving strength through precipitation strengthening and grain refinement.
  • the steel sheet according to this document has good hole expandability, there is a limit to improving the tensile strength, and there is a concern that cracks may occur during press forming due to high yield strength and low ductility.
  • Patent Document 3 discloses a method for manufacturing a composite structure steel plate with improved workability by utilizing a retained austenite phase.
  • the steel plate according to this document contains a large amount of Si and Al as an alloy composition, it is difficult to secure plating quality, and there is a problem that it is difficult to secure the surface quality of the steel during steelmaking and casting.
  • the initial yield strength (YS) due to transformation-induced plasticity is high, making it difficult to secure a low yield ratio required for automobiles, and there is a problem that processing cracks occur during press forming.
  • Patent Document 1 Japanese Patent Publication No. 2005-264176
  • Patent Document 2 Korean Patent Publication No. 10-2015-0073844
  • Patent Document 3 Japanese Patent Publication No. 2015-113504
  • One aspect of the present invention is to provide a steel sheet suitable for use as an automobile structural member, which has a high tensile strength of 780 MPa or higher and excellent work hardening rate and hole expandability (burring property).
  • a method for manufacturing the steel sheet is also provided.
  • a cold rolled steel sheet which includes, in wt%, carbon (C): 0.060 to 0.120%, silicon (Si): 1.20% or less (excluding 0%), manganese (Mn): 1.80 to 2.50%, molybdenum (Mo): 0.200% or less (excluding 0%), chromium (Cr): 0.800% or less (excluding 0%), phosphorus (P): 0.150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder iron and other unavoidable impurities.
  • the cold rolled steel sheet can satisfy the following relationship 1.
  • the cold rolled steel sheet may have a microstructure composed of an area fraction of 25 to 60% ferrite, 25 to 60% bainite, 20% or less martensite, and the remainder retained austenite.
  • the number of fine martensites having a size of 3 ⁇ m or less in terms of a circle-equivalent diameter may be 2.0 ⁇ 10 5 or more per unit area (1 mm 2 ).
  • Cold rolled steel sheets having such alloy composition and microstructure can have a low yield ratio and high ductility, and at the same time, can have excellent work hardening rate and burring resistance.
  • the cold rolled steel sheet may further include at least one of titanium (Ti): 0.040% or less and niobium (Nb): 0.040% or less.
  • a cold rolled steel sheet may have contents of C, Si, Cr and Mo in ferrite that satisfy the following relational expression 2, and in this case, the hardness of the ferrite may be increased, thereby obtaining the effect of reducing the hardness difference with the hard phase.
  • a cold rolled steel sheet may have a hardness relationship between microstructures that satisfies the following relational expression 3, and a relationship among a work hardening rate (n), an elongation (El), a hole expandability (HER), and a yield ratio (YR) in a strain range of 4 to 6% may satisfy the following relational expression 4.
  • H M represents the hardness of martensite
  • H F represents the hardness of ferrite
  • H B represents the hardness of bainite
  • a cold rolled steel sheet according to one embodiment of the present invention may have the characteristics of a yield strength of 450 MPa or more, a tensile strength of 780 MPa or more, a yield ratio of 0.70 or less, and an elongation of 15% or more.
  • the method comprises the steps of: preparing a steel slab; heating the steel slab to a temperature range of 1050 to 1300°C; finishing hot-rolling the heated steel slab at a temperature higher than the Ar3 transformation point to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a temperature range of 400 to 700°C; cold-rolling the coiled hot rolled steel sheet at a total reduction ratio of 40 to 80% to obtain a cold rolled steel sheet; continuously annealing the cold rolled steel sheet at a temperature range of 780 to 835°C; first cooling the continuously annealed cold rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 1 to 14°C/s; second cooling the first-cooled cold rolled steel sheet to a temperature range of 350 to 480°C at a cooling rate of 10°C/s or more;
  • the method may include a step of maintaining the secondarily cooled cold rolled
  • the steel slab has the above-described alloy composition and can satisfy relational expression 1.
  • the step of obtaining a galvanized steel sheet by galvanizing the maintained cold rolled steel sheet before final cooling may be further included.
  • the galvanized steel sheet may further be optionally subjected to alloying heat treatment.
  • the step of performing temper rolling at a reduction ratio of less than 1% after final cooling may be further included.
  • a cold rolled steel sheet (composite phase steel) having the characteristics of DP steel, such as low yield ratio and high ductility, as well as excellent work hardening rate and burring resistance.
  • the cold rolled steel sheet according to the present invention has an effect that makes it suitable for application as an automobile material, particularly as a structural member.
  • FIG. 1 is a drawing showing a graph of the relationship between relational expression 1 and mechanical properties (relational expression 3) according to one embodiment of the present invention.
  • FIG. 2 is a drawing showing a graph of the relationship between relational expression 1 and mechanical properties (relational expression 4) according to one embodiment of the present invention.
  • the inventors of the present invention have conducted in-depth research on a method for improving the work hardening rate as well as the hole expandability (burring property) of DP steel, which has been used as a material for conventional automobile structural members, while maintaining the low yield ratio and high elongation.
  • the inventors of the present invention have confirmed that it is possible to establish a structure composition advantageous for improving the work hardening rate and burring resistance by optimizing the alloy composition and manufacturing conditions, especially the annealing process, for a steel plate having a tensile strength of 780 MPa or higher. As a result, the present invention is provided.
  • the steel plate may contain, in wt%, carbon (C): 0.060 to 0.120%, silicon (Si): 1.20% or less (excluding 0%), manganese (Mn): 1.80 to 2.50%, molybdenum (Mo): 0.200% or less (excluding 0%), chromium (Cr): 0.800% or less (excluding 0%), phosphorus (P): 0.150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.100% or less (excluding 0%), and nitrogen (N): 0.010% or less (excluding 0%).
  • Carbon (C) is a very important element added to strengthen the transformation structure, and is effective in promoting the formation of martensite phase in composite structure steel while promoting the high strength of steel.
  • the content of C increases, the amount of martensite formed in the steel increases.
  • the content of C exceeds 0.120%, the strength is improved by the martensite formed in a large amount, but the difference in strength with respect to ferrite having a relatively low carbon concentration increases, and this difference in strength easily causes fracture at the interface between phases when stress is applied, so that the bending characteristics and elongation flangeability of the steel are inferior.
  • the weldability is inferior, so that welding defects occur when the customer processes the parts.
  • the content of C is less than 0.060%, it is very difficult to secure the desired strength.
  • C may be included in an amount of 0.060 to 0.120%.
  • C may be 0.070% or more, or 0.080% or more.
  • C may be 0.150% or less, or 0.110% or less.
  • Si is a useful element that can secure strength without lowering the ductility of steel.
  • Si is effective in promoting the formation of martensite by promoting the formation of ferrite and encouraging the enrichment of C into untransformed austenite.
  • Si is an effective element in increasing the strength of ferrite due to its high solid solution strengthening ability, thereby reducing the hardness difference between phases.
  • the content of Si exceeds 1.20%, the surface quality deteriorates during the plating process of the steel, making it impossible to secure the surface quality of the plated steel sheet.
  • there is no particular limitation on the lower limit content of Si and since the effect of Si can be obtained if it is at a level added during the steel manufacturing process, taking this into consideration, it can be limited to more than 0%.
  • Si may be included in an amount of more than 0% to 1.20%.
  • the Si may be 0.05% or more, or 0.11% or more.
  • the Si may be 1.10% or less, or 1.05% or less.
  • Manganese (Mn) is an element that is advantageous in strengthening steel by minimizing particles without damaging the ductility of steel and preventing hot embrittlement caused by the formation of FeS by completely precipitating sulfur (S) in steel as MnS. In addition, it plays a role in lowering the critical cooling rate at which the martensite phase is formed in composite phase steel, thereby contributing to the easier formation of the martensite phase.
  • the content of Mn is less than 1.80%, the strengthening effect of the steel is insufficient, making it difficult to secure the target level of strength.
  • the content exceeds 2.50% there is a high possibility that problems will occur in the weldability and hot-rollability of the steel, and there is a problem that the martensite phase is formed excessively, making the material unstable, and the Mn-Band (band of Mn oxide) is formed in the structure, increasing the risk of processing cracks and plate breakage.
  • Mn oxide is eluted onto the steel surface during the annealing process, which significantly inhibits the plating property during subsequent plating.
  • Mn may be included in an amount of 1.80 to 2.50%. In another embodiment of the present invention, the Mn may be included in an amount of 1.85% or more, or 1.90% or more. In yet another embodiment, the Mn may be included in an amount of 2.45% or less.
  • Molybdenum is an element useful for delaying the transformation of austenite into pearlite, and for improving the refinement and strength of ferrite. Mo has the advantage of controlling the yield ratio by increasing the hardenability of steel and forming martensite finely at grain boundaries.
  • Mo has the advantage of controlling the yield ratio by increasing the hardenability of steel and forming martensite finely at grain boundaries.
  • it is an expensive element, there is a problem that it becomes disadvantageous in manufacturing as the content increases, so it is useful to control the content appropriately.
  • the lower limit content of Mo is not particularly limited, and if it is added at a level during the steel manufacturing process, the effect of Mo can be obtained, and taking this into consideration, it can be limited to more than 0%.
  • Mo may be included in an amount of more than 0% to 0.200%. According to another embodiment of the present invention, Mo may be 0.001% or more. According to yet another embodiment, Mo may be 0.180% or less, or 0.170% or less.
  • Chromium (Cr) is an element with similar properties to the aforementioned Mn, and can be added to improve the hardenability of steel and secure high strength.
  • This Cr is effective in the formation of martensite, and is an element advantageous in the manufacture of composite phase steel with high ductility by minimizing the decrease in elongation compared to the increase in strength.
  • Cr-based carbides such as Cr 23 C 6 are formed, and some of these carbides are dissolved during the annealing process, and some remain without dissolving, so that the amount of solid solution C in martensite can be controlled below an appropriate level after subsequent cooling.
  • the occurrence of yield point elongation (YP-El) is suppressed, which is advantageous in the manufacture of composite phase steel with a low yield ratio.
  • the lower limit content of Cr is not particularly limited, and since the effect of Cr can be obtained if it is added at a level during the steel manufacturing process, it may be limited to more than 0% in consideration of this.
  • Cr may be included in an amount of more than 0% to 0.800%. In another embodiment of the present invention, Cr may be 0.010% or more, or 0.011% or more. In yet another embodiment, Cr may be 0.750% or less, or 0.700% or less.
  • Phosphorus (P) is the most advantageous element for securing strength without significantly damaging the formability of steel, but if added in excess, the possibility of brittle fracture increases significantly, increasing the possibility of slab plate breakage during hot rolling. In addition, there is a problem in that it acts as an element that impairs the surface properties of galvanized steel sheets.
  • the P since the above-mentioned problem may occur when the content of P exceeds 0.150%, the P may be included at 0.150% or less. Meanwhile, since the P may be inevitably added during the steel manufacturing process, 0% of the content may be excluded.
  • S Sulfur
  • S in steel is highly likely to cause red-hot embrittlement.
  • Aluminum (sol.Al) is an element added to refine the grain size and deoxidize steel.
  • the content of sol.Al is 0%, aluminum killed steel cannot be manufactured in a stable state, so 0% can be excluded for the content.
  • the content of sol.Al exceeds 0.100%, although it is advantageous for increasing the strength of the steel due to the grain refinement effect, there is an increased possibility that excessive inclusions will be formed during the steelmaking continuous casting operation, which will cause surface defects in the plated steel sheet. In addition, there is a concern that the manufacturing cost will increase, thereby reducing economic feasibility.
  • sol.Al may be included from more than 0% to 0.100%. According to another embodiment of the present invention, the sol.Al may be 0.005% or more, or 0.010% or more. According to yet another embodiment, the sol.Al may be 0.095% or less, or 0.090% or less.
  • Nitrogen (N) is an impurity that is inevitably added to steel, and it is effective to manage its content as low as possible.
  • the content may be limited to a range where operation is possible in consideration of this.
  • the content of N may be limited to 0.010% or less, but 0% may be excluded in consideration of the level that is inevitably added during the steel manufacturing process.
  • a steel plate according to one embodiment of the present invention may further include at least one of Ti and Nb in addition to the above-described alloy composition for the purpose of further improving the mechanical properties of the steel plate.
  • Titanium (Ti) and niobium (Nb) are effective elements for increasing the strength of steel and refining grains by forming fine precipitates.
  • the manufacturing cost increases and there is a problem in that excessive precipitates are formed, significantly reducing the ductility of the steel.
  • each when at least one of Ti and Nb is added, each may be included at 0.040% or less. According to another embodiment of the present invention, each of Ti and Nb may be 0.035% or less, or 0.030% or less.
  • the remaining component of the present invention is iron (Fe).
  • Fe iron
  • unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the normal manufacturing process, this cannot be ruled out. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of the contents are specifically mentioned in this specification.
  • a steel plate according to one embodiment of the present invention may be composed of the above-described alloy elements, and the relationship between some of the alloy elements may be limited to specific conditions.
  • the content relationship of C, Si, Cr, Mo, Ti and Nb in the alloy composition can be defined by the following relational expression 1.
  • the inventors of the present invention have repeatedly studied and found that reducing the difference in hardness between phases is effective in obtaining a low yield ratio and high ductility of composite structure steel while also obtaining excellent work hardening rate and burring resistance. Accordingly, a combination of specific alloying elements capable of increasing the solid solution strengthening of ferrite in composite structure has been factored into relational equation 1.
  • the concentrations of Si, Mo, and Cr in the ferrite increase, which causes a solid solution strengthening effect, thereby effectively reducing the hardness difference between phases with respect to a hard phase (e.g., martensite phase, etc.).
  • a hard phase e.g., martensite phase, etc.
  • Ti or Nb-based precipitates are formed in the ferrite grains, so that the effect of reducing the hardness difference between phases can be maximized.
  • the value of relation 1 may be 15.0 or greater, and according to another embodiment, it may be 16.0 or greater, 16.5 or greater.
  • the inventors of the present invention have found that, in order to improve the work hardening rate and hole expandability (burring property) of a steel plate, i.e., a composite phase steel according to one embodiment of the present invention, control of the fraction range of each phase is effective.
  • a steel plate according to one embodiment of the present invention may include ferrite of 25 to 60% in area fraction, bainite of 25 to 60%, martensite of 20% or less, and retained austenite as a residual structure.
  • the ferrite fraction of the steel plate according to one embodiment of the present invention is less than 25%, high ductility cannot be achieved and the work hardening rate cannot be improved. On the other hand, if the fraction exceeds 60%, the target level of strength cannot be secured.
  • the bainite phase is a structure that is advantageous in reducing the hardness difference between phases in a composite structure steel, and if the fraction is less than 25%, the effect of reducing the hardness difference between phases is insignificant, and thus burring property cannot be improved.
  • the fraction exceeds 60%, the fraction of martensite, which is a relatively hard phase, becomes low, making it impossible to secure the intended strength.
  • the bainite phase is formed between the ferrite and martensite phases, and thus can effectively reduce the hardness difference between the soft ferrite phase and the hard martensite phase.
  • the fraction of the martensite phase which is advantageous in securing the strength of the steel plate, exceeds 20%, the strength is excessively increased, resulting in reduced ductility, and the difference in hardness between the phases with respect to the soft ferrite phase becomes large, resulting in poor burring properties.
  • This martensite phase can be formed in an appropriate fraction finely and evenly dispersed in the steel.
  • the number of fine martensites having a size of 3 ⁇ m or less in terms of an equivalent circle diameter may be 2.0 ⁇ 10 5 or more per unit area (1 mm 2 ). When the number of the fine martensites is less than 2.0 ⁇ 10 5 per unit area (1 mm 2 ), the work hardening ability of the steel plate cannot be improved due to the presence of a large number of martensite phases having a relatively coarse size.
  • a steel plate according to one embodiment of the present invention may include a retained austenite phase as a residual structure, and the fraction thereof is not particularly limited.
  • the contents of C, Si, Cr, and Mo in the ferrite can satisfy the following relational expression 2 by controlling the relationships of specific elements in the alloy composition.
  • the content of elements advantageous to the solid solution strengthening effect in the ferrite phase increases, thereby generating the solid solution strengthening effect, and further, the hardness of the soft ferrite phase increases by forming precipitates in the ferrite.
  • the value of relational expression 2 is less than 500, the hardness difference between the ferrite and martensite phases is not effectively reduced, so that the burring property of the steel sheet becomes inferior.
  • a steel plate according to one embodiment of the present invention can have a characteristic of a low difference in hardness between phases of a composite structure by controlling the alloy composition and microstructure.
  • the steel plate according to one embodiment of the present invention can satisfy the hardness relationship between the ferrite, bainite, and martensite phases as expressed in Equation 3 below.
  • H M represents the hardness of martensite
  • H F represents the hardness of ferrite
  • H B represents the hardness of bainite
  • relational expression 3 if the value of relational expression 3 exceeds 2.0, it means that the difference in hardness between the phases of the composite structure is large, and in this case, the burring property of the steel plate cannot be secured.
  • the steel plate according to one embodiment of the present invention includes a certain fraction or more of ferrite and bainite phases as a microstructure, and a fine martensite phase is evenly dispersed and formed, so that deformation starts at a low stress in the initial stage of plastic deformation, thereby exhibiting characteristics such as a low yield ratio and a high work hardening rate.
  • This change in the microstructure alleviates local stress and deformation, thereby delaying the creation, growth, and coalescence of pores, and as a result, the effect of improving the ductility of the steel plate can be obtained.
  • the steel plate according to one embodiment of the present invention can satisfy the following relational expression 4 in the relationship between the work hardening rate (n), elongation (El), hole expandability (HER), and yield ratio (YR) in the 4 to 6% strain range.
  • a steel plate according to one embodiment of the present invention may have high strength, a low yield ratio and high ductility, and at the same time excellent work hardening rate and hole expandability.
  • the steel plate may have a yield strength of 450 MPa or more, a tensile strength of 780 MPa or more, a yield ratio of 0.70 or less, an elongation of 15% or more, and may satisfy the aforementioned relationship 4.
  • the steel sheet according to one embodiment of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.
  • the zinc-based plating layer may be, for example, a zinc-plated layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and/or magnesium in addition to zinc.
  • a steel plate can be manufactured by subjecting a prepared steel slab to the processes of [heating - hot rolling - cooling - coiling - cold rolling - annealing - cooling], and each process step is specifically described below.
  • the steel slab After preparing a steel slab according to one embodiment of the present invention, the steel slab can be heated.
  • the heating process of the steel slab is a process for smoothly performing the hot rolling process described below and sufficiently obtaining the target properties of the steel plate.
  • the steel slab can have the same alloy composition and alloy composition relationship (relationship 1) as the steel plate according to one embodiment of the present invention, and the description of each alloy element and the description of the composition relationship are replaced with the above-mentioned matters.
  • the heating of the steel slab can be performed at a temperature range of 1050 to 1300°C. If the heating temperature is lower than 1050°C, friction between the steel plate and the rolling mill increases, which causes a problem in that the load applied to the roller during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for temperature increase, but the amount of surface scale increases, which can lead to material loss.
  • the heating process of the steel slab can be performed in a temperature range of 1050 to 1300°C. According to another embodiment of the present invention, the heating process of the steel slab can be performed at 1100°C or higher, and according to yet another embodiment, it can be performed at 1250°C or lower.
  • the above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.
  • a hot rolled steel sheet can be manufactured by performing finishing hot rolling at a temperature higher than the Ar3 transformation point during the hot rolling. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature lower than the Ar3 transformation point, there is a concern that ferrite and austenite dual-phase rolling may be performed, resulting in material non-uniformity.
  • the above finishing hot rolling can be performed at a temperature range of 800 to 1000°C, and if the temperature exceeds 1000°C, there is a concern that material unevenness may occur due to the formation of abnormal coarse grains caused by high-temperature rolling, and this causes a problem of coil distortion occurring during subsequent cooling.
  • the hot-rolled steel sheet manufactured above can be coiled.
  • the coiling process can be performed at a temperature range of 400 to 700°C. If the coiling temperature is less than 400°C, the strength of the hot-rolled steel sheet may become excessively high, which may cause a rolling load during subsequent cold rolling. In addition, the cost and time required to cool the hot-rolled steel sheet to the coiling temperature are excessive, which causes an increase in the process cost. On the other hand, if the temperature exceeds 700°C, scale is excessively generated on the surface of the hot-rolled steel sheet, which is highly likely to cause surface defects and causes the plating property to deteriorate.
  • the above-mentioned hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet.
  • cold rolling can be performed at a cold reduction ratio (total reduction ratio) of 40 to 80%. If the cold reduction ratio is less than 40% during the cold rolling, it is not only difficult to secure the target thickness, but also difficult to correct the shape of the steel plate. On the other hand, if the cold reduction ratio exceeds 80%, there is a high possibility that cracks will occur at the edge of the steel plate, and there is a problem of generating a load during cold pressing.
  • the cold rolled steel sheet manufactured above can be subjected to continuous annealing treatment.
  • the continuous annealing treatment can be performed in a continuous alloying galvanizing furnace.
  • the continuous annealing treatment can be performed in a temperature range of 780 to 835°C. That is, by performing the continuous annealing treatment of the cold rolled steel sheet in a two-phase temperature range where ferrite and austenite coexist, not only austenite but also ferrite can be formed simultaneously with the recrystallization of the structure, while carbon can be distributed.
  • the temperature is less than 780°C, not only is recrystallization not sufficiently achieved, but also the austenite phase is not sufficiently formed, making it impossible to form the target microstructure phase composition after the continuous annealing treatment.
  • the temperature exceeds 835°C, the productivity decreases, and since the austenite phase is excessively formed, there is a problem that the fraction of the martensite phase becomes excessive after the subsequent cooling process. In this case, while the yield strength increases, the ductility decreases, so that the intended characteristics of low yield ratio and high ductility cannot be secured.
  • the surface concentration of elements such as Si, Mn, and B, which inhibit the wettability of the hot-dip galvanizing among the alloy compositions, may be excessive, which may deteriorate the plating surface quality.
  • the continuous annealing treatment can be performed at a temperature range of 780 to 835°C. According to another embodiment of the present invention, the continuous annealing treatment can be performed at 785°C or higher, and in another embodiment, at 830°C or lower, or at 820°C or lower.
  • an austenite phase can be formed at an area fraction of 70 to 95%, and a ferrite phase can be formed at an area fraction of 5 to 30%.
  • the cold rolled steel sheet that has been continuously annealed in the above two-phase temperature range can be cooled.
  • the cooling may be performed in steps, and as an example, the continuous annealing-treated cold rolled steel sheet may be first cooled at a cooling rate of 1 to 14°C/s to a temperature range of 630 to 690°C, and then the first-cooled cold rolled steel sheet may be secondarily cooled at a cooling rate of 10°C/s or more to a temperature range of 350 to 480°C. At this time, the cooling rate during the second cooling may be faster than the cooling rate during the first cooling.
  • the type and fraction of the microstructure formed can be controlled by performing stepwise cooling to a specific temperature range according to the cooling rate when cooling a cold rolled steel sheet in which a certain fraction of a ferrite phase is formed together with austenite.
  • a ferrite phase can be additionally introduced into the cold rolled steel sheet by performing primary cooling at a cooling rate of 1 to 14°C/s to a temperature range of 630 to 690°C on the continuously annealed cold rolled steel sheet.
  • the austenite phase formed during the continuous annealing process is transformed into an excessive fraction of ferrite, making it impossible to properly secure the bainite and martensite phases in the final microstructure.
  • the cooling rate exceeds 14°C/s, the additionally introduced ferrite phase is insufficient, which may reduce the ductility of the steel sheet.
  • the cooling end temperature during the first cooling is less than 630°C, the ferrite phase is not sufficiently formed.
  • the temperature exceeds 690°C, there is a problem in that the cooling rate must be excessively increased during the subsequent second cooling process, and there is concern that the fractions of the bainite phase and martensite phase in the final microstructure may not be sufficient.
  • a ferrite phase having an area fraction of 20 to 30% can be additionally formed, and at the same time, the remaining austenite phase has the effect of being dispersed.
  • secondary cooling can be performed at a cooling rate of 10°C/s or more to a temperature range of 350 to 480°C, and by maintaining the cold rolled steel sheet in this cooled state, a bainite phase can be introduced.
  • the cooling rate during the above secondary cooling is less than 10°C/s, pearlite may be generated during the cooling process, and the bainite phase may not be sufficiently formed.
  • the upper limit of the cooling rate during the above secondary cooling is not particularly limited, and a person skilled in the art may appropriately select it by considering the specifications of the cooling equipment. As an example, it may be performed at 100°C/s or less.
  • the cooling end temperature during the secondary cooling is less than 350°C, the martensite phase is excessively formed, and the martensite phase is tempered during the subsequent holding process, making it impossible to secure the intended yield strength ratio and high ductility.
  • the temperature exceeds 480°C, the bainite phase is not sufficiently formed, making it impossible to obtain the effect of reducing the hardness difference between phases by the bainite phase.
  • the second-cooled cold-rolled steel sheet may be maintained for 100 seconds or longer. If the maintenance time is less than 100 seconds, bainite cannot be obtained in a sufficient fraction.
  • the upper limit of the maintenance time is not particularly limited, and may be determined as the time for a bainite phase to be formed in a fraction intended by a person skilled in the art.
  • the maintenance process performed after the secondary cooling can be performed within the temperature range where the secondary cooling is completed, and therefore, there is no particular limitation on the temperature range. However, as one example, it can be performed at 400°C or lower.
  • a bainite phase having an area fraction of 25 to 60% can be formed during the maintenance process after the second cooling of the first-cooled cold-rolled steel sheet, and by forming the bainite phase in this way, the dispersion effect of the remaining austenite phase (approximately 10 to 20 fraction%) becomes greater. Accordingly, when the remaining austenite phase transforms into martensite during the subsequent cooling process, it can be formed with a fine and uniform distribution.
  • the cold rolled steel sheet which has undergone the above-mentioned stepwise (first and second) cooling and maintenance processes, can be finally cooled.
  • the final cooling of the cold rolled steel sheet that has undergone the stepwise cooling and holding process can be performed at a cooling rate of 3°C/s or more to a temperature of Ms-100°C or lower, and a martensite phase can be introduced during this process.
  • the martensite phase cannot be secured at the intended level.
  • the upper limit of the cooling rate and the lower limit of the cooling end temperature during the final cooling are not particularly limited, but in terms of forming a martensite phase of a certain fraction, the cooling may be performed at a cooling rate of, for example, 50°C/s or less, and as another example, the final cooling may be performed to room temperature.
  • the austenite that is transformed into a ferrite and bainite phases during the stepwise cooling and holding process and remains can be transformed into a martensite phase during the final cooling process.
  • the austenite phase transformed into a martensite phase during the final cooling process is uniformly and finely distributed inside the steel sheet due to the dispersion effect during the previous stepwise cooling and holding process, and thus the martensite phase formed during the final cooling process can also be formed while being finely and uniformly distributed.
  • the martensite phase formed during the final cooling process can be formed at an area fraction of 20% or less, and among these, the fine martensite phase can be formed at a ratio of 90% or more.
  • the fine martensite phase can refer to a martensite phase having an average crystal grain size of 3 ⁇ m or less.
  • the austenite phase remaining after forming the martensite phase during the final cooling process remains as it is and becomes the residual austenite phase of the final microstructure.
  • its fraction may be 2% or less (including 0%).
  • a steel sheet (cold rolled steel sheet) having a composite structure can be obtained by controlling the alloy composition and manufacturing conditions, and the composite structure of the steel sheet has a low difference in hardness between a soft phase and a hard phase, and further, by introducing an appropriate fraction of ferrite and bainite phases and forming a martensite phase that is finely and uniformly dispersed, the work hardening rate is improved due to relief of stress concentration, and burring property (hole expandability) can be improved.
  • a cold rolled steel sheet may be plated to obtain a plated steel sheet.
  • hot-dip galvanizing may be performed on a cold rolled steel sheet that has undergone a stepwise cooling and maintenance process prior to final cooling.
  • a cold rolled steel sheet that is, a cold rolled steel sheet that has undergone a stepwise cooling and maintenance process, can be immersed in a molten zinc-based plating bath to manufacture a molten zinc-based galvanized steel sheet.
  • an alloyed zinc-plated steel sheet can be obtained by subjecting a hot-dip galvanized steel sheet manufactured by hot-dip galvanizing according to one embodiment of the present invention to an alloying heat treatment.
  • the alloying heat treatment process conditions there is no particular limitation on the alloying heat treatment process conditions, and any normal conditions may be used.
  • the alloying heat treatment process may be performed in a temperature range of 480 to 600°C.
  • a temper rolling process can be further performed, and the temper rolling process can be performed not only on a cold rolled steel sheet that has undergone a final cooling process, but also on a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet that has undergone a final cooling process.
  • a large amount of dislocations are formed in the steel through the temper rolling process, thereby further improving the hardenability.
  • this can be performed at a reduction ratio of less than 1% (excluding 0%).
  • the reduction ratio is 1% or more during the temper rolling, it is advantageous in terms of dislocation formation, but side effects such as plate breakage may occur due to equipment capacity limitations.
  • each cold rolled steel plate was subjected to continuous annealing treatment under the conditions shown in Table 2 below, followed by step cooling (first - second cooling) and maintenance processes.
  • the concentrations of specific elements (C, Si, Mo, Cr) in the ferrite were measured using TEM (Transmission Electron Microscopy), EDS (Energy Dispersive Spectroscopy), and EELS (Electron Energy Loss Spectroscopy) analysis equipment.
  • the hardness of each phase was measured 10 times using a Vickers Micro Hardness Tester, and the average value was taken, and the equation 3 was calculated using the values.
  • the size of each particle was converted into an equivalent diameter and measured from the martensite area results measured using the image analyzer. At this time, the number of martensite having a size of 3 ⁇ m or less within a unit area of 1mm2 was calculated.
  • the cold rolled steel sheet according to one embodiment of the present invention has the characteristics of DP steel, such as high yield strength and high ductility, and, despite being a composite structure steel, has a low difference in hardness between phases, so it has excellent burring resistance and work hardening rate.
  • comparative steel 1 which satisfies the alloy composition system proposed in the present invention but does not satisfy the manufacturing conditions
  • comparative steels 2 to 6 which deviate from the relational expression 1 proposed in the present invention and do not satisfy the manufacturing conditions, did not form the intended microstructure, and thus at least one or more properties were inferior.
  • Comparative Steels 1 to 6 did not form the ferrite, bainite or martensite phases in the intended fraction, or the content relationship of specific elements in the ferrite did not satisfy the relational expression 2 according to one embodiment of the present invention, so that not only did they have inferior work hardening rates and fail to satisfy the relational expression 4, but also the difference in hardness between the phases was large.
  • Figure 1 is a graph showing the change in mechanical properties (Relationship 3) according to the content relationship of specific elements (Relationship 1).
  • Figure 2 is a graph showing the change in mechanical properties (Relationship 4) according to the content relationship of specific elements (Relationship 1).
  • the value of relational expression 4 can be secured as 150 or more. That is, according to one embodiment of the present invention, the work hardening rate and burring resistance of composite structure steel can be improved by controlling the content relationship of specific elements.
  • FIGS. 1 and 2 illustrate invention steels 1 to 5, which are steel plates according to one embodiment of the present invention, and comparative steels 2 to 6, which do not satisfy relational expression 1 according to one embodiment of the present invention.

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Abstract

La présente invention concerne une tôle d'acier appropriée en tant que matériau pour un élément structurel de véhicule et similaire, et, plus spécifiquement, une tôle d'acier à structure composite à haute résistance ayant une résistance à la traction égale ou supérieure à 780 MPa, et son procédé de fabrication.
PCT/KR2024/096659 2023-12-15 2024-12-11 Tôle d'acier et son procédé de fabrication Pending WO2025127794A1 (fr)

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