US20250313912A1 - Steel sheet, member, and methods for producing same - Google Patents

Steel sheet, member, and methods for producing same

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Publication number
US20250313912A1
US20250313912A1 US18/863,180 US202318863180A US2025313912A1 US 20250313912 A1 US20250313912 A1 US 20250313912A1 US 202318863180 A US202318863180 A US 202318863180A US 2025313912 A1 US2025313912 A1 US 2025313912A1
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steel sheet
annealing
cooling
thickness direction
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US18/863,180
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English (en)
Inventor
Fangyi WANG
Yoshiyasu Kawasaki
Tatsuya Nakagaito
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KAWASAKI, YOSHIYASU, NAKAGAITO, TATSUYA, WANG, FANGYI
Publication of US20250313912A1 publication Critical patent/US20250313912A1/en
Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNOR'S INTEREST Assignors: KAWASAKI, YOSHIYASU, NAKAGAITO, TATSUYA, WANG, FANGYI
Pending legal-status Critical Current

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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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0252—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with application of tension
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface 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
    • C22C38/002—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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—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
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • 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 sheet, a member made of the steel sheet, and methods for producing them.
  • crashworthiness a steel sheet with high strength and enhanced crashworthiness when a vehicle collides while traveling
  • Patent Literature 1 discloses, as such a steel sheet serving as a material of an automobile body part, a high-strength steel sheet with high stretch flangeability and enhanced crashworthiness, which has a chemical composition containing, on a mass percent basis, C: 0.04% to 0.22%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% to 0.1%, and N: 0.001% to 0.005%, the remainder being Fe and incidental impurities, and which is composed of a ferrite phase as a main phase and a martensite phase as a second phase, the martensite phase having a maximum grain size of 2 ⁇ m or less and an area fraction of 5% or more.
  • Patent Literature 2 discloses a high-strength hot-dip galvanized steel sheet with high coating adhesion and formability having a hot-dip galvanized layer on the surface of a cold-rolled steel sheet, which has a surface layer ground off with a thickness of 0.1 ⁇ m or more and is pre-coated with 0.2 g/m 2 or more and 2.0 g/m 2 or less of Ni, wherein the cold-rolled steel sheet contains, on a mass percent basis, C: 0.05% or more and 0.4% or less, Si: 0.01% or more and 3.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more and 2.0% or less, Si+Al>0.5%, the remainder being Fe and incidental impurities, has a microstructure, on a volume fraction basis, 40% or more ferrite as a main phase, 8% or more retained austenite, two or more of three types of
  • Patent Literature 3 discloses a high-strength hot-dip galvanized steel sheet that has a chemical composition composed of, on a mass percent basis, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, and Al: 0.01% or more and 2.5% or less, the remainder being Fe and incidental impurities, and that has a steel sheet microstructure having, on an area fraction, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases being a martensite phase: 3% or less (including 0%) and bainitic ferrite phase: less than 5% (including 0%), the tempered martensite phase having an average grain size of 8 ⁇ m or
  • a steel sheet with higher TS and YS typically has lower press formability and, in particular, lower ductility, flangeability, bendability, and the like.
  • a steel sheet with higher TS and YS is applied to the impact energy absorbing members of automobiles, not only press forming is difficult, but also the member cracks in an axial compression test simulating a collision test. In other words, the actual impact absorbed energy is not increased as expected from the value of YS.
  • the impact energy absorbing members are currently limited to steel sheets with a TS of 590 MPa.
  • Patent Literature 1 to Patent Literature 3 have a TS of 1180 MPa or more, high YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression.
  • aspects of the present invention have been developed in view of such circumstances and aim to provide a steel sheet with a tensile strength TS of 1180 MPa or more, high yield stress YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression, together with an advantageous method for producing the steel sheet.
  • aspects of the present invention also aim to provide a member made of the steel sheet and a method for producing the member.
  • steel sheet includes a galvanized steel sheet, and the galvanized steel sheet is a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheet
  • GA hot-dip galvannealed steel sheet
  • the tensile strength TS is measured in the tensile test according to JIS Z 2241 (2011).
  • high yield stress YS means that YS measured in the tensile test according to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on TS measured in the tensile test.
  • high ductility means that the total elongation (E1) measured in the tensile test according to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on TS measured in the tensile test.
  • high flangeability refers to a limiting hole expansion ratio ( ⁇ ) of 30% or more as measured in the hole expansion test according to JIS Z 2256 (2020).
  • high bendability refers to a bending angle ( ⁇ ) of 80 degrees or more at the maximum load measured in a bending test according to the VDA standard (VDA 238-100) defined by German Association of the Automotive Industry.
  • good bending fracture characteristics refers to a stroke (S Fmax ) of 26.0 mm or more at the maximum load measured in a V-VDA bending test.
  • [7]A method for producing a steel sheet including:
  • [11]A method for producing a member including a step of subjecting the steel sheet according to any one of [1] to [5] to at least one of forming and joining to produce a member.
  • aspects of the present invention provide a steel sheet with a tensile strength TS of 1180 MPa or more, high yield stress YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression.
  • a member including a steel sheet according to aspects of the present invention as a material has high strength and enhanced crashworthiness and can therefore be extremely advantageously applied to an impact energy absorbing member or the like of an automobile.
  • FIG. 1 is a SEM microstructure image for explaining identification of a microstructure.
  • FIG. 2 - 3 ( e ) is a perspective view of a test specimen subjected to VDA bending (secondary bending) in V-VDA and an L cross-sectional observation surface.
  • FIG. 2 - 3 ( f ) is a cross-sectional view of a measurement point of a change in the grain size of bainitic ferrite in the thickness direction due to processing in an L cross-sectional observation surface of a test specimen subjected to VDA bending (secondary bending) in V-VDA.
  • FIG. 2 - 4 is a schematic view for explaining an AB region.
  • FIG. 3 is a schematic view of a stroke-load curve obtained in a V-VDA test.
  • FIG. 4 is a SEM microstructure image for explaining the measurement of the length of a crack specified in accordance with aspects of the present invention (Inventive Example No. 36 in Examples).
  • FIG. 5 ( a ) is a SEM microstructure image for explaining a method for measuring the grain size of bainitic ferrite before deformation by processing specified in accordance with aspects of the present invention (Inventive Example No. 35 in Examples).
  • FIG. 5 ( b ) is a SEM microstructure image for explaining a method for measuring the grain size of bainitic ferrite after deformation by processing specified in accordance with aspects of the present invention (Inventive Example No. 35 in Examples).
  • FIG. 6 - 1 ( a ) is a front view of a test member composed of a hat-shaped member and a steel sheet spot-welded together for an axial compression test in Examples.
  • FIG. 6 - 1 ( b ) is a perspective view of the test member illustrated in FIG. 6 - 1 ( a ).
  • FIG. 6 - 2 ( c ) is a schematic explanatory view of an axial compression test in Examples.
  • a steel sheet according to aspects of the present invention is a steel sheet including a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis, C: 0.050% or more and 0.400% or less, Si: more than 0.75% and 3.00% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the remainder being Fe and incidental impurities, the base steel sheet has a steel microstructure in which an area fraction of bainitic ferrite: 3.0% or more and 20.0% or less, an area fraction of tempered martensite: 40.0% or more and 90.0% or less, an area fraction of retained austenite: more than 3.0% and 15.0% or less, a concentration of carbon in retained austenite: 0.60% by mass or more and 1.30% by mass or less, an area
  • the steel sheet may have a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet.
  • a steel sheet with a galvanized layer may be a galvanized steel sheet.
  • the unit in the chemical composition is “% by mass” and is hereinafter expressed simply in “%” unless otherwise specified.
  • C is an element effective in forming appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite and ensuring a tensile strength TS of 1180 MPa or more and high YS.
  • a C content of less than 0.050% results in an increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS.
  • a C content of more than 0.400% results in an excessive increase in the concentration of carbon in retained austenite. This greatly increases the hardness of fresh martensite formed by deformation-induced transformation when a steel sheet is punched in a hole expansion test or is subjected to V-bending in a V-VDA test, and subsequently promotes void formation and crack growth, resulting in undesired ⁇ and S Fmax .
  • the C content is 0.050% or more and 0.400% or less.
  • the C content is preferably 0.100% or more.
  • the C content is preferably 0.300% or less.
  • Si suppresses the formation of carbides and promotes the formation of retained austenite during cooling and holding after annealing.
  • Si is an element that has an influence on the volume fraction of retained austenite and the concentration of carbon in retained austenite.
  • a Si content of 0.75% or less results in a decrease in the volume fraction of retained austenite and lower ductility.
  • a Si content of more than 3.00% results in an excessive increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS. This also excessively increases the concentration of carbon in austenite during annealing and results in undesired ⁇ and S Fmax .
  • the Si content is more than 0.75% and 3.00% or less.
  • the Si content is preferably 2.00% or less.
  • Mn 2.00% or More and Less than 3.50%
  • Mn is an element that adjusts the area fraction of bainitic ferrite, tempered martensite, or the like.
  • a Mn content of less than 2.00% results in an excessive increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS.
  • a Mn content of 3.50% or more results in a decrease in martensite start temperature Ms (hereinafter also referred to simply as an Ms temperature or Ms) and a decrease in martensite formed in a second cooling step.
  • Ms temperature a decrease in martensite start temperature
  • Ms temperature a decrease in martensite formed in a second cooling step.
  • This increases martensite formed during final cooling, does not sufficiently temper martensite formed at that time, and increases the area fraction of hard fresh martensite.
  • Fresh martensite acts as a starting point of void formation in a hole expansion test, a VDA bending test, or a V-VDA bending test.
  • An area fraction of fresh martensite exceeding 10% results in undesired ⁇ , ⁇ , and S Fmax .
  • the Mn content is 2.00% or more and less than 3.50%.
  • the Mn content is preferably 2.50% or more.
  • the Mn content is preferably 3.20% or less.
  • P is an element that has a solid-solution strengthening effect and increases TS and YS of a steel sheet.
  • the P content is 0.001% or more.
  • a P content of more than 0.100% results in segregation of P at a prior-austenite grain boundary and embrittlement of the grain boundary.
  • the P content is 0.001% or more and 0.100% or less.
  • the P content is preferably 0.030% or less.
  • S is present as a sulfide in steel.
  • S content of more than 0.0200%, after the steel sheet is punched or is subjected to V-bending in a V-VDA bending test, the number of voids formed increases, and desired ⁇ and S Fmax cannot be achieved.
  • the S content is 0.0200% or less.
  • the S content is preferably 0.0080% or less.
  • the lower limit of the S content is 0.0001% or more due to constraints on production technology.
  • Al suppresses the formation of carbides and promotes the formation of retained austenite during cooling and holding after annealing.
  • Al is an element that has an influence on the volume fraction of retained austenite and the concentration of carbon in retained austenite.
  • the Al content is preferably 0.010% or more.
  • an Al content of more than 2.000% results in an excessive increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS. This also excessively increases the C concentration of austenite during annealing and results in undesired ⁇ and S Fmax .
  • the Al content is 0.010% or more and 2.000% or less.
  • the Al content is preferably 0.015% or more.
  • the Al content is preferably 1.000% or less.
  • N is present as a nitride in steel.
  • N content of more than 0.0100%, after the steel sheet is punched or is subjected to V-bending in a V-VDA bending test, the number of voids formed increases, and desired ⁇ and S Fmax cannot be achieved.
  • the N content is 0.0100% or less.
  • the N content is preferably 0.0050% or less.
  • the N content may have any lower limit but is preferably 0.0005% or more due to constraints on production technology.
  • the B content is even more preferably 0.0005% or more, and even further more preferably 0.0007% or more.
  • the Ta content is more preferably 0.090% or less, even more preferably 0.080% or less.
  • W is an element that enhances hardenability, and the addition of W forms a large amount of tempered martensite and ensures a TS of 1180 MPa or more and high YS.
  • the W content is preferably 0.001% or more.
  • the W content is more preferably 0.030% or more.
  • the W content is preferably 0.500% or less.
  • the W content is more preferably 0.450% or less, even more preferably 0.400% or less.
  • the W content is even further more preferably 0.300% or less.
  • a Zn content of more than 0.0200% may result in a large number of coarse precipitates or inclusions.
  • a coarse precipitate or inclusion may act as a starting point of a crack in a hole expansion test, a VDA bending test, or a V-VDA bending test, and desired ⁇ , ⁇ , and S Fmax may not be achieved.
  • the Zn content is preferably 0.0200% or less.
  • the Zn content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • Zr is an element effective in spheroidizing the shape of an inclusion and improving the flangeability of a steel sheet.
  • the Zr content is preferably 0.0010% or more.
  • a Zr content of more than 0.1000% may result in a large number of coarse precipitates or inclusions.
  • a coarse precipitate or inclusion may act as a starting point of a crack in a hole expansion test, a VDA bending test, or a V-VDA bending test, and desired ⁇ , ⁇ , and S Fmax may not be achieved.
  • the Zr content is preferably 0.1000% or less.
  • the Zr content is more preferably 0.0300% or less, even more preferably 0.0100% or less.
  • Ca is present as an inclusion in steel.
  • a Ca content of more than 0.0200% may result in a large number of coarse inclusions.
  • a coarse precipitate or inclusion may act as a starting point of a crack in a hole expansion test, a VDA bending test, or a V-VDA bending test, and desired ⁇ , ⁇ , and S Fmax may not be achieved.
  • the Ca content is preferably 0.0200% or less.
  • the Ca content is preferably 0.0020% or less.
  • the Ca content is more preferably 0.0019% or less, even more preferably 0.0018% or less.
  • the Ca content may have any lower limit but is preferably 0.0005% or more. Due to constraints on production technology, the Ca content is more preferably 0.0010% or more.
  • Se 0.0200% or less
  • Te 0.0200% or less
  • Ge 0.0200% or less
  • Sr 0.0200% or less
  • Cs 0.0200% or less
  • Hf 0.0200% or less
  • Pb 0.0200% or less
  • Bi 0.0200% or less
  • REM 0.0200% or less
  • Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements effective in improving the flangeability of a steel sheet.
  • each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0001% or more.
  • a Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, or REM content of more than 0.0200% or an As content of more than 0.0500% may result in a large number of coarse precipitates or inclusions.
  • a coarse precipitate or inclusion may act as a starting point of a crack in a hole expansion test, a VDA bending test, or a V-VDA bending test, and desired ⁇ , ⁇ , and S Fmax may not be achieved.
  • each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0200% or less, and the As content is preferably 0.0500% or less.
  • the Se content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Se content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the Te content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Te content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the Ge content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Ge content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the As content is more preferably 0.0010% or more, even more preferably 0.0015% or more.
  • the As content is more preferably 0.0400% or less, even more preferably 0.0300% or less.
  • the Sr content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Sr content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the Cs content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Cs content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the Pb content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Pb content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the Bi content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Bi content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • the REM content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the REM content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • REM scandium
  • Y yttrium
  • Lu lutetium
  • REM concentration refers to the total content of one or two or more elements selected from the REM.
  • REM is preferably, but not limited to, Sc, Y, Ce, or La.
  • a base steel sheet of a steel sheet according to an embodiment of the present invention has a chemical composition containing, on a mass percent basis, C: 0.050% or more and 0.400% or less, Si: more than 0.75% and 3.00% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, and optionally containing at least one selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0
  • a base steel sheet of a steel sheet according to an embodiment of the present invention has a steel microstructure in which the area fraction of bainitic ferrite: 3.0% or more and 20.0% or less, the area fraction of tempered martensite (excluding retained austenite): 40.0% or more and 90.0% or less, the volume fraction of retained austenite: more than 3.0% and 15.0% or less, the concentration of carbon in retained austenite: 0.60% by mass or more and 1.30% by mass or less, the area fraction of fresh martensite: 10.0% or less (including 0.0%), and the density of carbides in tempered martensite: 8.0 particles/ ⁇ m 2 or less, the amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less, a V-VDA bending test is performed to a maximum load point, in an L cross section, a crack has a length of 400 ⁇ m or less, in a region formed from each position on a starting line present from a starting point of a bending peak on an outside of
  • the area fraction of bainitic ferrite is preferably 5.0% or more, more preferably 8.0% or more.
  • the area fraction of bainitic ferrite is preferably 18.0% or less, more preferably 15.0% or less.
  • the area fraction of tempered martensite is preferably 85.0% or less, more preferably 80.0% or less.
  • the area fraction of retained austenite is more than 3.0%.
  • the area fraction of retained austenite is preferably 5.0% or more.
  • an excessive increase in the area fraction of retained austenite results in fresh martensite formed by deformation-induced transformation acting as a starting point of void formation when a steel sheet is punched in a hole expansion test or is subjected to V-bending in a V-VDA test, and desired ⁇ and S Fmax cannot be achieved.
  • the area fraction of retained austenite is 15.0% or less.
  • the area fraction of retained austenite is preferably 12.0% or less, more preferably 10.0% or less.
  • tension in a second cooling step in a production method described later can be controlled to suppress the area fraction of retained austenite to 15.0% or less.
  • Applying a tension of 2.0 kgf/mm 2 or more once or more after a first cooling step (after a galvanizing treatment when the galvanizing treatment is performed (when necessary, after an alloying treatment)) then subjecting a steel sheet to four or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and subjecting the steel sheet to two or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for half the circumference of the roll cause deformation-induced transformation of unstable retained austenite to fresh martensite, temper the fresh martensite during subsequent cooling, and finally form tempered martensite.
  • the concentration of carbon in retained austenite is an indicator that has an influence on stability with which retained austenite transforms to martensite during deformation.
  • concentration of carbon in retained austenite is less than 0.60% by mass, the retained austenite is unstable, and deformation-induced martensite transformation occurs after stress application and before plastic deformation, so that required elongation cannot be achieved.
  • concentration of carbon in retained austenite is more than 1.30% by mass, when a steel sheet is punched in a hole expansion test or is subjected to V-bending in a V-VDA test, the hardness of fresh martensite formed from the retained austenite greatly increases, the formation and connection of voids are promoted, and desired ⁇ and S Fmax cannot be achieved.
  • the concentration of carbon in retained austenite is 0.60% by mass or more and 1.30% by mass or less.
  • the concentration of carbon in retained austenite is preferably 0.80% by mass or more.
  • the concentration of carbon in retained austenite is preferably 1.20% by mass or less.
  • an excessive increase in the area fraction of fresh martensite results in fresh martensite acting as a starting point of void formation in a hole expansion test, a VDA bending test, or a V-VDA bending test, and desired ⁇ , ⁇ , and S Fmax cannot be achieved.
  • the area fraction of fresh martensite increases, the amount of diffusible hydrogen in a steel sheet increases, and flangeability and bendability further decrease. From the perspective of ensuring high flangeability and bendability, the area fraction of fresh martensite is 10.0% or less, preferably 5.0% or less.
  • the area fraction of fresh martensite may have any lower limit and may be 0.0%.
  • fresh martensite refers to as-quenched (untempered) martensite.
  • the density of carbides in tempered martensite when the density of carbides in tempered martensite is more than 8.0 particles/ ⁇ m 2 , the number of voids caused by carbides increases in a hole expansion test, a VDA bending test, or a V-VDA bending test, which promotes the formation and growth of a crack, and desired ⁇ , ⁇ , and S Fmax cannot be achieved.
  • the density of carbides in tempered martensite is 8.0 particles/ ⁇ m 2 or less.
  • the density of carbides in tempered martensite is preferably 7.0 particles/ ⁇ m 2 or less, more preferably 6.0 particles/ ⁇ m 2 or less.
  • the density of carbides in tempered martensite is preferably 1.0 particles/ ⁇ m 2 or more, more preferably 2.0 particles/ ⁇ m 2 or more.
  • the remaining microstructure other than those described above is, for example, ferrite, lower bainite, pearlite, or carbide such as cementite.
  • the area fraction of pearlite is preferably 5.0% or less.
  • the type of the remaining microstructure can be determined, for example, by scanning electron microscope (SEM) observation.
  • a sample is cut out from a base steel sheet to form a thickness cross section parallel to the rolling direction of the base steel sheet as an observation surface.
  • the observation surface of the sample is then mirror-polished with a diamond paste.
  • the observation surface of the sample is then subjected to final polishing with colloidal silica and is then etched with 3% by volume nital to expose the microstructure.
  • bainitic ferrite From a microstructure image thus photographed, bainitic ferrite, tempered martensite, the hard second phase (retained austenite+fresh martensite), and the remaining microstructure are identified as described below.
  • Bainitic ferrite a black to dark gray region of a massive form, an indefinite form, or the like. No or a relatively small number of iron-based carbides is contained.
  • Tempered martensite a gray region of an indefinite form. A relatively large number of iron-based carbides is contained.
  • Hard second phase (retained austenite+fresh martensite): a white to light gray region of an indefinite form. No iron-based carbide is contained. One with a relatively large size has a gradually darker color with the distance from the interface with another microstructure and may have a dark gray interior.
  • Ferrite a massive black region. Almost no iron-based carbide is contained. When an iron-based carbide is contained, however, the area of ferrite includes the area of the iron-based carbide. The same applies to the bainitic ferrite and tempered martensite.
  • Cementite a dotted or linear white region. It is contained in tempered martensite, bainitic ferrite, and ferrite.
  • the region of each phase identified in the microstructure image is subjected to calculation by the following method.
  • a 20 ⁇ 20 grid spaced at regular intervals is placed on a region with an actual length of 23.1 ⁇ m ⁇ 17.6 ⁇ m, and the area fractions of bainitic ferrite, tempered martensite, and the hard second phase are calculated by a point counting method of counting the number of points on each phase.
  • Each area fraction is the average value of three area fractions determined from different 5000 ⁇ SEM images.
  • the area fraction of retained austenite is measured as described below.
  • a base steel sheet is mechanically ground to a quarter thickness position in the thickness direction (depth direction) and is then chemically polished with oxalic acid to form an observation surface.
  • the observation surface is then observed by X-ray diffractometry.
  • a MoKa radiation source is used for incident X-rays to determine the ratio of the diffraction intensity of each of (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of (200), (211), and (220) planes of bcc iron.
  • the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each plane. On the assumption that retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is defined as the area fraction of the retained austenite.
  • the lattice constant of the retained austenite is determined using a diffraction peak of the (220) plane of fcc iron (austenite) measured by the X-ray diffractometry.
  • the concentration of carbon in retained austenite is then determined using the following formula:
  • the area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard second phase determined as described above.
  • the area fraction of the remaining microstructure is determined by subtracting the area fraction of bainitic ferrite, the area fraction of tempered martensite, and the area fraction of the hard second phase, which are determined as described above, from 100.0%.
  • the dew point can be more than ⁇ 5° C. to further increase the thickness of the soft layer and significantly improve axial compression characteristics.
  • the dew point due to a metal coated layer, even when the dew point is ⁇ 5° C. or less and the soft layer has a small thickness, axial compression characteristics equivalent to those in the case where the soft layer has a large thickness can be achieved.
  • the coating weight of the Fe-based electroplated layer is more than 0 g/m 2 , preferably 2.0 g/m 2 or more.
  • the upper limit of the coating weight per side of the Fe-based electroplated layer is not particularly limited, and from the perspective of cost, the coating weight per side of the Fe-based electroplated layer is preferably 60 g/m 2 or less.
  • the coating weight of the Fe-based electroplated layer is preferably 50 g/m 2 or less, more preferably 40 g/m 2 or less, even more preferably 30 g/m 2 or less.
  • the coating weight of the Fe-based electroplated layer is measured as described below.
  • a sample with a size of 10 ⁇ 15 mm is taken from the Fe-based electroplated steel sheet and is embedded in a resin to prepare a cross-section embedded sample.
  • Three arbitrary places on the cross section are observed with a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and at a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based coated layer.
  • SEM scanning electron microscope
  • the average thickness of the three visual fields is multiplied by the specific gravity of iron to convert it into the coating weight per side of the Fe-based electroplated layer.
  • a surface soft layer is more preferably provided under an Fe-based electroplated layer, and this can significantly improve bending fracture resistance characteristics.
  • the Vickers hardness distribution is measured by the method described above from the interface between the Fe-based electroplated layer and the base steel sheet in the thickness direction, and the depth of the surface soft layer in the thickness direction is evaluated.
  • a V-VDA bending test is performed to the maximum load point; in an L cross section, in a region formed from each position on a starting line present from a starting point of a bending peak on the outside of a VDA bend to a position of 50 ⁇ m in a thickness direction up to a position of 50 ⁇ m on each side of the starting line perpendicular to the starting line, with respect to the average grain size of bainitic ferrite in the thickness direction, the ratio of the average grain size before processing to the average grain size after the processing (a change in average grain size due to processing): 5.0 or less
  • the symbol BF indicates bainitic ferrite
  • the symbol F indicates ferrite
  • the symbol TM indicates tempered martensite.
  • ⁇ (TM) indicates carbide in tempered martensite
  • H 1 indicates a hard second phase
  • X 1 (BF) indicates an island-like second phase in bainitic ferrite.
  • the bainitic ferrite BF in the steel sheet microstructure forms internal island-like retained austenite due to carbon partitioning.
  • a void is likely to be formed at the boundary between the bainitic ferrite BF and hard fresh martensite formed by the deformation-induced transformation of the island-like retained austenite.
  • the change in the average grain size of the bainitic ferrite BF in the thickness direction due to processing is more than 5.0, the bainitic ferrite BF is subjected to tensile stress in the rolling direction, and an increase in the number of voids promotes the formation and growth of a crack, thus impairing the bending fracture resistance characteristics.
  • the change in the average grain size of the bainitic ferrite in the thickness direction due to processing is 5.0 or less.
  • the change is preferably 4.8 or less, more preferably 4.5 or less.
  • V-VDA bending test is performed as described below.
  • a 60 mm ⁇ 65 mm test specimen is taken from the steel sheet by shearing.
  • the sides of 60 mm are parallel to the rolling (L) direction.
  • 90-degree bending (primary bending) is performed at a radius of curvature/thickness ratio of 4.2 in the rolling (L) direction with respect to an axis extending in the width (C) direction to prepare a test specimen.
  • a punch B 1 is pressed against a steel sheet on a die A 1 with a V-groove to prepare a test specimen T 1 .
  • the test specimen T 1 on support rolls A 2 is subjected to orthogonal bending (secondary bending) by pressing a punch B 2 against the test specimen T 1 in the direction perpendicular to the rolling direction.
  • the symbol D 1 denotes the width (C) direction
  • the symbol D 2 denotes the rolling (L) direction.
  • FIG. 3 is a schematic view of a stroke-load curve obtained in a V-VDA test.
  • a sample obtained by performing the V-VDA test to the maximum load point P and then removing the load when the load reaches 94.9% to 99.9% of the maximum load (see the symbol R in FIG. 3 ) is used as an evaluation sample in the V-VDA bending test.
  • FIG. 2 - 2 ( c ) illustrates the test specimen T 1 prepared by subjecting the steel sheet to V-bending (primary bending) in the V-VDA bending test.
  • FIG. 2 - 2 ( d ) illustrates a test specimen T 2 obtained by subjecting the test specimen T 1 to VDA bending (secondary bending).
  • the position indicated by the broken line in the test specimen T 2 in FIG. 2 - 2 ( d ) is the V-bending ridge line portion and corresponds to the position indicated by the broken line in the test specimen T 1 in FIG. 2 - 2 ( c ) before the VDA bending is performed.
  • VDA bending ridge line portion refers to the region within 5 mm on both sides of a VDA bending corner portion (peak) that is subjected to VDA bending and extends in the rolling direction.
  • FIG. 2 - 3 ( e ) shows the positional relationship between an L cross section AL of the V-bending ridge line portion and the VDA bending ridge line portion and the test specimen T 2 .
  • FIG. 2 - 3 ( f ) shows the L cross section AL with the D 2 direction being perpendicular to the drawing and the D 1 direction being parallel to the drawing.
  • the V-VDA bending test is performed to the maximum load point, and the length of a crack in the L cross section (hereinafter also referred to as the AL surface) in the overlap region of the V-bending ridge line portion and the VDA bending ridge line portion is determined as described below.
  • a sample is cut out from the base steel sheet such that the AL surface of the steel sheet subjected to the V-VDA bending test to the maximum load point is an observation surface.
  • the observation surface of the sample is then mirror-polished with a diamond paste.
  • the observation surface of the sample is then subjected to final polishing with colloidal silica and is then etched with 3% by volume nital to expose the microstructure.
  • a 25.6 ⁇ m ⁇ 17.6 ⁇ m visual field is photographed with a scanning electron microscope (SEM) under the conditions of an acceleration voltage of 15 kV and a magnification of 200 times such that the symmetry axis of the AL surface is perpendicular to the bending peak of the observation surface of the sample, and a crack is observed as a whole.
  • SEM scanning electron microscope
  • FIG. 4 shows an example of an image of an actually measured crack.
  • the symbol D 2 denotes the rolling (L) direction
  • the symbol D 4 denotes the thickness direction.
  • the symbol L indicates the length of the crack.
  • the V-VDA bending test is performed to the maximum load point.
  • a method for measuring the change in the average grain size of bainitic ferrite in the thickness direction due to processing is described below in a region (an AB region indicated by the dotted line in FIG. 2 - 3 ( f ), hereinafter also referred to as the AB region) of 0 to 50 ⁇ m from the surface of the steel sheet on the outside of a VDA bend and 50 ⁇ m on the left and right sides of the bending peak of the VDA bend.
  • FIG. 2 - 4 is a schematic view for explaining the AB region.
  • the term “AB region” refers to a region formed from each position of a starting line L 0 , which extends from a starting point of a bending peak t 0 on the outside of a VDA bend to a position of 50 ⁇ m in the thickness direction, to a position of 50 ⁇ m on each side of the starting line L 0 perpendicular to the starting line L 0 .
  • a method for measuring the change first, five 25.6 ⁇ m ⁇ 17.6 ⁇ m visual fields are photographed for each sample with a scanning electron microscope (SEM) under the conditions of an acceleration voltage of 15 kV and a magnification of 3000 times using a sample having the AL surface as an observation surface after the V-VDA bending test performed to the maximum load point (hereinafter also referred to as a sample after deformation) and using a sample used to measure the area fraction of the steel sheet microstructure (hereinafter also referred to as a sample before deformation).
  • the sample after deformation the AB region is photographed to observe bainitic ferrite deformed by processing (hereinafter also referred to as bainitic ferrite after deformation).
  • the sample before deformation is photographed from the surface of the base steel sheet to a position of 50 ⁇ m in the thickness direction to observe bainitic ferrite not deformed (hereinafter also referred to as bainitic ferrite before deformation).
  • a steel sheet according to an embodiment of the present invention has a tensile strength TS of 1180 MPa or more.
  • the tensile strength TS may have any upper limit but is preferably less than 1470 MPa.
  • the yield stress (YS), the total elongation (E1), the limiting hole expansion ratio ( ⁇ ), the reference values of the critical bending angle ( ⁇ ) in the VDA bending test and the stroke at the maximum load (S Fmax ) in the V-VDA bending test, and the presence or absence of axial compression fracture of a steel sheet according to an embodiment of the present invention are as described above.
  • galvanized layer refers to a coated layer containing Zn as a main component (Zn content: 50.0% or more), for example, a hot-dip galvanized layer or a hot-dip galvannealed layer.
  • the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al.
  • the hot-dip galvanized layer may optionally contain one or two or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less.
  • the hot-dip galvanized layer more preferably has an Fe content of less than 7.0% by mass. The remainder other than these elements is incidental impurities.
  • the hot-dip galvannealed layer is preferably composed of, for example, Zn, 20.0% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al.
  • the hot-dip galvannealed layer may optionally contain one or two or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less.
  • the hot-dip galvannealed layer more preferably has an Fe content of 7.0% by mass or more, even more preferably 8.0% by mass or more.
  • the hot-dip galvannealed layer more preferably has an Fe content of 15.0% by mass or less, even more preferably 12.0% by mass or less. The remainder other than these elements is incidental impurities.
  • the coating weight per side of the galvanized layer is preferably, but not limited to, 20 g/m 2 or more.
  • the coating weight per side of the galvanized layer is preferably 80 g/m 2 or less.
  • the thickness of a steel sheet according to an embodiment of the present invention is preferably, but not limited to, 0.5 mm or more, more preferably 0.6 mm or more.
  • the thickness is more preferably more than 0.8 mm.
  • the thickness is even more preferably 0.9 mm or more.
  • the thickness is more preferably 1.0 mm or more.
  • the thickness is even more preferably 1.2 mm or more.
  • the steel sheet preferably has a thickness of 3.5 mm or less.
  • the thickness is more preferably 2.3 mm or less.
  • the width of a steel sheet according to aspects of the present invention is preferably, but not limited to, 500 mm or more, more preferably 750 mm or more.
  • the steel sheet preferably has a width of 1600 mm or less, more preferably 1450 mm or less.
  • a method for producing a steel sheet according to an embodiment of the present invention includes: a hot rolling step of hot-rolling a steel slab with the chemical composition described above to produce a hot-rolled steel sheet; a pickling step of pickling the hot-rolled steel sheet; an annealing step of annealing the steel sheet after the pickling step at an annealing temperature of (Ac 1 +0.4 ⁇ (Ac 3 ⁇ Ac 1 ))° C. or more and 900° C. or less for an annealing time of 20 seconds or more; a first cooling step of cooling the steel sheet after the annealing step to a first cooling stop temperature of 400° C. or more and 600° C.
  • temperatures described above mean the surface temperatures of a steel slab and a steel sheet.
  • a steel slab with the chemical composition described above is prepared.
  • a steel material is melted to produce a molten steel with the chemical composition described above.
  • the melting method may be, but is not limited to, any known melting method using a converter, an electric arc furnace, or the like.
  • the resulting molten steel is then solidified into a steel slab.
  • the steel slab may be produced from the molten steel by any method, for example, a continuous casting method, an ingot casting method, a thin slab casting method, or the like. From the perspective of preventing macrosegregation, a continuous casting method is preferred.
  • the steel slab is hot-rolled to produce a hot-rolled steel sheet.
  • the hot-rolling may be performed in an energy-saving process.
  • the energy-saving process may be hot charge rolling (a method of charging a furnace with the steel slab as a hot piece not cooled to room temperature and hot-rolling the steel slab), hot direct rolling (a method of keeping the steel slab slightly warm and then immediately rolling the steel slab), or the like.
  • the hot-rolled steel sheet after the hot rolling step is pickled.
  • the pickling can remove an oxide from the surface of the steel sheet and ensure high chemical convertibility and coating quality.
  • the pickling may be performed once or multiple times.
  • the pickling may be performed under any conditions and may be performed in the usual manner.
  • the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet.
  • the cold rolling is, for example, multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
  • a cold-rolled steel sheet after the cold rolling may be pickled.
  • An embodiment of the present invention may include a first coating step of performing metal coating on one or both surfaces of the steel sheet after the hot rolling step (after the pickling step or after the cold rolling step after the pickling step when cold rolling is performed) and before the annealing step to form a metal coated layer (first coated layer).
  • a metal electroplating treatment may be performed on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet thus formed to produce a metal electroplated steel sheet before annealing in which a metal electroplated layer before annealing is formed on at least one surface thereof.
  • metal coating excludes galvanizing (second coating).
  • the metal electroplating treatment method is not particularly limited, as described above, the metal coated layer formed on the base steel sheet is preferably a metal electroplated layer, and the metal electroplating treatment is therefore preferably performed.
  • a sulfuric acid bath, a hydrochloric acid bath, a mixture of both, or the like can be used as an Fe-based electroplating bath.
  • the coating weight of the metal electroplated layer before annealing can be adjusted by the energization time or the like.
  • the phrase “metal electroplated steel sheet before annealing” means that the metal electroplated layer is not subjected to an annealing step, and does not exclude a hot-rolled steel sheet, a pickled sheet after hot rolling, or a cold-rolled steel sheet, each annealed in advance before a metal electroplating treatment.
  • a metal species of the electroplated layer may be any of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Pt, Au, Hg, Ti, Pb, and Bi and is preferably Fe.
  • Fe-based electroplating a production method using Fe-based electroplating is described below. However, the following conditions for the Fe-based electroplating can be applied to another metal electroplating as well.
  • the Fe ion content of an Fe-based electroplating bath before the start of energization is preferably 0.5 mol/L or more in terms of Fe 2+ .
  • the Fe ion content of an Fe-based electroplating bath before the start of energization is preferably 2.0 mol/L or less.
  • the Fe-based electroplating bath may contain an Fe ion and at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co.
  • the total content of these elements in the Fe-based electroplating bath is preferably such that the total content of these elements in an Fe-based electroplated layer before annealing is 10% by mass or less.
  • a metal element may be contained as a metal ion, and a non-metal element can be contained as part of boric acid, phosphoric acid, nitric acid, an organic acid, or the like.
  • An iron sulfate coating solution may contain a conductive aid, such as sodium sulfate or potassium sulfate, a chelating agent, or a pH buffer.
  • the temperature of an Fe-based electroplating solution is preferably 30° C. or more and 85° C. or less in view of constant temperature retention ability.
  • the pH of the Fe-based electroplating bath is also not particularly limited, is preferably 1.0 or more from the perspective of preventing a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or less in consideration of the electrical conductivity of the Fe-based electroplating bath.
  • the electric current density is preferably 10 A/dm 2 or more from the perspective of productivity and is preferably 150 A/dm 2 or less from the perspective of facilitating the control of the coating weight of an Fe-based electroplated layer.
  • the line speed is preferably 5 mpm or more from the perspective of productivity and is preferably 150 mpm or less from the perspective of stably controlling the coating weight.
  • a degreasing treatment and water washing for cleaning the surface of a steel sheet and also a pickling treatment and water washing for activating the surface of a steel sheet can be performed as a treatment before Fe-based electroplating treatment. These pretreatments are followed by an Fe-based electroplating treatment.
  • the degreasing treatment and water washing may be performed by any method, for example, by a usual method.
  • various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid, or a mixture thereof is preferred.
  • the acid concentration is not particularly limited and preferably ranges from approximately 1% to 20% by mass in consideration of the capability of removing an oxide film, prevention of a rough surface (surface defect) due to overpickling, and the like.
  • a pickling treatment liquid may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, or the like.
  • the steel sheet thus produced is annealed at an annealing temperature of (Ac 1 +0.4 ⁇ (Ac 3 ⁇ Ac 1 ))° C. or more and 900° C. or less for an annealing time of 20 seconds or more.
  • the number of annealing processes may be two or more but is preferably one from the perspective of energy efficiency.
  • Annealing Temperature (Ac 1 +0.4 ⁇ (Ac 3 ⁇ Ac 1 ))° C. or More and 900° C. Or Less
  • An annealing temperature lower than (Ac 1 +0.4 ⁇ (Ac 3 ⁇ Ac 1 ))° C. results in an insufficient proportion of austenite formed during heating in a two-phase region of ferrite and austenite. This results in an excessive increase in the area fraction of ferrite after annealing and lower YS. This may also excessively increase the C concentration in austenite during annealing and result in undesired ⁇ and S Fmax . This also makes it difficult to achieve a TS of 1180 MPa or more.
  • an annealing temperature of more than 900° C. results in excessive grain growth of austenite, a higher MS temperature, and a large amount of tempered martensite containing carbides, makes it difficult to form more than 3.0% of retained austenite, and results in lower ductility.
  • the annealing temperature is (Ac 1 +0.4 ⁇ (Ac 3 ⁇ Ac 1 ))° C. or more and 900° C. or less.
  • the annealing temperature is preferably 880° C. or less.
  • the annealing temperature is more preferably 870° C. or less.
  • the annealing temperature is preferably (Ac 1 +0.5 ⁇ (Ac 3 ⁇ Ac 1 ))° C. or more, more preferably (Ac 1 +0.6 ⁇ (Ac 3 ⁇ Ac 1 ))° C. or more.
  • the annealing temperature is the highest temperature reached in the annealing step.
  • the Ac 1 point (° C.) and the Ac 3 point (° C.) are calculated using the following formula:
  • annealing time refers to the holding time in the temperature range of (annealing temperature—40° C.) or more and the annealing temperature or less.
  • the annealing time includes, in addition to the holding time at the annealing temperature, the residence time in the temperature range of (annealing temperature—40° C.) or more and the annealing temperature or less in heating and cooling before and after reaching the annealing temperature.
  • bainitic ferrite is formed, and C diffuses from the formed bainitic ferrite to non-transformed austenite adjacent to the bainitic ferrite. This ensures a predetermined area fraction of retained austenite.
  • a holding time of 80 seconds or more in the holding temperature range may result in an excessive increase in the area fraction of bainitic ferrite and lower YS. This may also result in excessive diffusion of C from bainitic ferrite to non-transformed austenite, retained austenite with an area fraction of more than 15.0%, and undesired ⁇ and S Fmax .
  • the holding time in the holding temperature range is preferably less than 80 seconds.
  • the holding time in the holding temperature range is more preferably less than 60 seconds.
  • the holding time in the holding temperature range does not include the residence time in the temperature range after the galvanizing treatment in the coating step.
  • the load cells should be arranged parallel to the direction of the tension.
  • the tension is preferably 2.2 kgf/mm 2 or more, more preferably 2.4 kgf/mm 2 or more.
  • the tension is preferably 15.0 kgf/mm 2 or less, more preferably 10.0 kgf/mm 2 or less.
  • the tension is even more preferably 7.0 kgf/mm 2 or less, even further more preferably 4.0 kgf/mm 2 or less.
  • the application of the tension twice means that a first tension of 2.0 kgf/mm 2 or more is applied once, and after the tension becomes less than 2.0 kgf/mm 2 a second tension of 2.0 kgf/mm 2 or more is applied.
  • the application of the tension three times means that a first tension of 2.0 kgf/mm 2 or more is applied once, after the tension becomes less than 2.0 kgf/mm 2 a second tension of 2.0 kgf/mm 2 or more is applied, and after the tension becomes less than 2.0 kgf/mm 2 a third tension of 2.0 kgf/mm 2 or more is applied.
  • the steel sheet is then reheated in the temperature range of more than 300° C. and 500° C. or less (hereinafter also referred to as a reheating temperature range) and is held in the temperature range of more than 300° C. and 500° C. or less for 20 seconds or more and 900 seconds or less.
  • a reheating temperature (tempering temperature) of 300° C. or less results in insufficient tempering of martensite present in the steel at the end of the second cooling step, an excessive increase in fresh martensite, insufficient coarsening of carbides in tempered martensite, a density of carbides in the tempered martensite higher than a predetermined level, and consequently undesired ⁇ , ⁇ , and S Fmax .
  • This also results in insufficient release of hydrogen contained in the base steel sheet to the outside and increases the amount of diffusible hydrogen in the base steel sheet. This further reduces the flangeability and bendability.
  • a reheating temperature (tempering temperature) of more than 500° C. results in excessive tempering of martensite present in the steel at the end of the second cooling step and makes it difficult to achieve a TS of 1180 MPa or more. This also results in lower ductility because non-transformed austenite present in the steel at the end of the second cooling step is decomposed as carbide (pearlite). This may also result in insufficient release of hydrogen contained in the base steel sheet to the outside and increase the amount of diffusible hydrogen in the base steel sheet. This reduces the flangeability.
  • the reheating temperature is more than 300° C. and 500° C. or less.
  • the reheating temperature is the highest temperature reached in the reheating step.
  • the reheating temperature is preferably 340° C. or more, more preferably 360° C. or more.
  • the reheating temperature is preferably 460° C. or less, more preferably 440° C. or less.
  • a holding time (tempering time) of more than 900 seconds in the reheating temperature range results in excessive tempering of martensite present in the steel at the end of the second cooling step and makes it difficult to achieve a TS of 1180 MPa or more. This also results in lower ductility because non-transformed austenite present in the steel at the end of the second cooling step is decomposed as carbide (pearlite).
  • the holding time in the reheating temperature range is 20 seconds or more and 900 seconds or less.
  • the holding time is preferably 30 seconds or more, more preferably 40 seconds or more.
  • the holding time is preferably 500 seconds or less, more preferably 100 seconds or less.
  • the holding time in the reheating temperature range includes, in addition to the holding time at the reheating temperature, the residence time in the temperature range during heating and cooling before and after the reheating temperature is reached.
  • Carbide Control Parameter CP During Reheating 10,000 or More and 15,000 or Less
  • a carbide control parameter CP of less than 10,000 during reheating results in insufficient tempering of martensite present in the steel at the end of the second cooling step, an excessive increase in fresh martensite, insufficient coarsening of carbides in tempered martensite, a density of carbides in the tempered martensite higher than a predetermined level, and consequently undesired ⁇ , ⁇ , and S Fmax .
  • This also results in insufficient release of hydrogen contained in the base steel sheet to the outside and increases the amount of diffusible hydrogen in the base steel sheet. This further reduces the flangeability and bendability.
  • a carbide control parameter CP of more than 15,000 during reheating results in excessive tempering of martensite present in the steel at the end of the second cooling step and makes it difficult to achieve a TS of 1180 MPa or more. This also results in lower ductility because non-transformed austenite present in the steel at the end of the second cooling step is decomposed as carbide (pearlite).
  • the carbide control parameter CP during reheating is 10,000 or more and 15,000 or less.
  • the carbide control parameter CP during reheating is preferably 11,000 or more, more preferably 12,000 or more.
  • the carbide control parameter CP during reheating is preferably 14,500 or less, more preferably 14,000 or less.
  • the carbide control parameter during reheating is calculated using the following formula:
  • the material constant k is calculated using the following formula:
  • the carbon concentration of martensite formed in the second cooling step can be measured as described below.
  • V ⁇ 1 ⁇ C ⁇ 1 + V F ⁇ C F + V BF ⁇ C BF C T formula ⁇ ( 2 )
  • V ⁇ 1 1 - V F - V BF formula ⁇ ( 3 )
  • the steel sheet has a TS of 1180 MPa or more, high YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression.
  • a member according to an embodiment of the present invention has high strength and enhanced crashworthiness.
  • a member according to an embodiment of the present invention is suitable for an impact energy absorbing member used in the automotive field.
  • the sheet thickness has a small influence, and the test was performed without the grinding treatment.
  • “1*1” and “1*2” in Tables 4, 7, and 10 refer to the length of a crack formed in the L cross section of the V-bending ridge line portion and the VDA bending ridge line portion when the V-VDA bending test is performed to the maximum load point, and the change in the grain size of bainitic ferrite in the thickness direction due to processing in a region of 50 ⁇ m from the surface of the steel sheet on the outside of a VDA bend and 50 ⁇ m on the left and right sides of the bending peak of the VDA bend (a region formed from each position on a starting line present from a starting point of a bending peak on the outside of a VDA bend to a position of 50 ⁇ m in the thickness direction up to a position of 50 ⁇ m on each side of the starting line perpendicular to the starting line), respectively.
  • the coated layer to be peeled off is a galvanized layer when the galvanized layer is formed, is a metal coated layer when the metal coated layer is formed, or is a galvanized layer and a metal coated layer when the galvanized layer and the metal coated layer are formed.

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