US20250290185A1 - Steel sheet, member, and methods for producing same - Google Patents

Steel sheet, member, and methods for producing same

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Publication number
US20250290185A1
US20250290185A1 US18/863,164 US202318863164A US2025290185A1 US 20250290185 A1 US20250290185 A1 US 20250290185A1 US 202318863164 A US202318863164 A US 202318863164A US 2025290185 A1 US2025290185 A1 US 2025290185A1
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United States
Prior art keywords
less
steel sheet
annealing
layer
cooling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
US18/863,164
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English (en)
Inventor
Fangyi WANG
Yoshiyasu Kawasaki
Tatsuya Nakagaito
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
JFE Steel Corp
Original Assignee
JFE Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
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 US20250290185A1 publication Critical patent/US20250290185A1/en
Pending legal-status Critical Current

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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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • 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
    • 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
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties 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
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    • 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
    • C21D8/0242—Flattening; Dressing; Flexing
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    • 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
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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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
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    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • 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
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments

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 automobile body parts, 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 containing, on a volume fraction basis, 40% or more ferrite as a main phase, 8% or more retained austenite, two or more of three
  • TS denotes tensile strength (MPa)
  • EL denotes total elongation percentage (%)
  • ⁇ denotes hole expansion ratio (%)
  • a tensile strength of 980 MPa or more when martensite [1]:C concentration (CM1) is less than 0.8%, hardness Hv1 satisfies Hv1/( ⁇ 982.1 ⁇ CM12+1676 ⁇ CM1+189) ⁇ 0.60, when martensite [2]:C concentration (CM2) is 0.8% or more, the hardness Hv2 satisfies Hv2/( ⁇ 982.1 ⁇ CM22+1676 ⁇ CM2+189) ⁇ 0.60, and when martensite [3]:C concentration (CM3) is 0.8% or more, the hardness Hv3 satisfies Hv3/( ⁇ 982.1 ⁇ CM32+1676 ⁇ CM3+189) ⁇ 0.80).
  • 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.
  • 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 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.
  • El, ⁇ , and ⁇ described above are characteristics indicating formability at the time of press forming of a steel sheet.
  • the V-VDA bending test is a test simulating the deformation and fracture behavior of a bending ridge line portion in a collision test
  • the stroke at the maximum load (S Fmax ) measured in the V-VDA bending test is a characteristic indicating the resistance to cracking of a member.
  • the base steel sheet of the steel sheet has a steel microstructure in which the area fraction of ferrite: 57.0% or less, the total area fraction of bainitic ferrite and tempered martensite: 40.0% or more and 90.0% or less, the area fraction of retained austenite: 3.0% or more and 10.0% or less, the area fraction of fresh martensite: 10.0% or less, and the value obtained by dividing the area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite is 0.70 or more, a V-VDA bending test is performed to a maximum load point, in a V-bending ridge line portion and a VDA
  • a steel sheet including a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis,
  • [7] A member including the steel sheet according to any one of [1] to [6].
  • a method for producing a steel sheet including:
  • a method for producing a member including a step of subjecting the steel sheet according to any one of [1] to [6] 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 used to identify a microstructure.
  • FIG. 2 - 1 ( a ) is an explanatory view of V-bending (primary bending) in a V-VDA bending test in Examples.
  • FIG. 2 - 1 ( b ) is an explanatory view of VDA bending (secondary bending) in the V-VDA bending test in Examples.
  • FIG. 2 - 2 ( c ) is a perspective view of a test specimen subjected to V-bending (primary bending) in V-VDA.
  • FIG. 2 - 2 ( d ) is a perspective view of a test specimen subjected to VDA bending (secondary bending) in V-VDA.
  • FIG. 2 - 3 ( e ) 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. 3 is a schematic view of a stroke-load curve obtained in a V-VDA test.
  • FIG. 4 ( a ) is an example of a SEM microstructure image showing a void at a boundary between a hard phase and a soft phase.
  • FIG. 4 ( b ) is an example of a SEM microstructure image showing a void due to fracture of a hard phase.
  • FIG. 4 ( c ) is an example of a SEM microstructure image showing a void due to carbide.
  • FIG. 5 - 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. 5 - 1 ( b ) is a perspective view of the test member illustrated in FIG. 5 - 1 ( a ).
  • FIG. 5 - 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 ferrite: 57.0% or less, a total area fraction of bainitic ferrite and tempered martensite: 40.0% or more and 90.0% or less, an area fraction of retained austenite: 3.0% or more and 10.0% or less, an area fraction of fresh martensite: 10.0% or less, and a value obtained by dividing an area fraction of
  • 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.
  • TS tensile strength
  • 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 carbide and promotes the formation of retained austenite during cooling and holding after annealing.
  • Si is an element that affects the volume fraction of retained austenite.
  • a Si content of 0.75% or less results in a decrease in the volume fraction of retained austenite and lower ductility.
  • 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 first cooling step.
  • Ms temperature or Ms martensite start temperature
  • 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 10.0% results in undesired ⁇ and SFmax. Furthermore, desired ⁇ may not be achieved.
  • 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. Due to constraints on production technology, the S content is 0.0001% or more.
  • Al suppresses the formation of carbide and promotes the formation of retained austenite during cooling and holding after annealing.
  • Al is an element that affects the volume fraction of 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 in 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.
  • a base chemical composition of a base steel sheet of a steel sheet according to an embodiment of the present invention has been described above.
  • a base steel sheet of a steel sheet according to an embodiment of the present invention has a chemical composition that contains the base components and the remainder other than the base components including Fe (iron) and incidental impurities.
  • a base steel sheet of a steel sheet according to an embodiment of the present invention preferably has a chemical composition that contains the base components and the remainder composed of Fe and incidental impurities.
  • a base steel sheet of a steel sheet according to an embodiment of the present invention may contain, in addition to the base components, at least one selected from the following optional components.
  • the following optional components are contained in an amount equal to or less than their respective upper limits described below, the advantages according to aspects of the present invention can be achieved. Thus, there is no particular lower limit. Any of the following optional elements contained in amounts below the following appropriate lower limits is considered to be an incidental impurity.
  • Nb forms fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases TS and YS.
  • the Nb content is preferably 0.001% or more.
  • the Nb content is more preferably 0.005% or more.
  • a Nb content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, 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 Nb content is preferably 0.200% or less.
  • the Nb content is more preferably 0.060% or less.
  • Ti forms fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases TS and YS.
  • the Ti content is preferably 0.001% or more.
  • the Ti content is more preferably 0.005% or more.
  • a Ti content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, 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 A, a, and S Fmax may not be achieved.
  • the Ti content is preferably 0.200% or less.
  • the Ti content is more preferably 0.060% or less.
  • V forms fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases TS and YS.
  • the V content is preferably 0.001% or more.
  • the V content is more preferably 0.005% or more.
  • the V content is even more preferably 0.010% or more, even further more preferably 0.030% or more.
  • a V content of more than 0.200% 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.
  • V content is preferably 0.200% or less.
  • the V content is more preferably 0.060% or less.
  • B is an element that segregates at an austenite grain boundary and enhances hardenability. B is also an element that suppresses the formation and grain growth of ferrite during cooling after annealing. To produce such effects, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more.
  • the B content is even more preferably 0.0005% or more, even further more preferably 0.0007% or more.
  • a B content of more than 0.0100% may result in a crack in a steel sheet during hot rolling. 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 may not be achieved.
  • Cr is an element that enhances hardenability, and the addition of Cr forms a large amount of tempered martensite and ensures a TS of 1180 MPa or more and high YS.
  • the Cr content is preferably 0.0005% or more.
  • the Cr content is more preferably 0.010% or more.
  • Cr is even more preferably 0.030% or more, even further more preferably 0.050% or more.
  • the Cr content is preferably 1.000% or less.
  • the Cr content is more preferably 0.800% or less, even more preferably 0.700% or less.
  • Ni is an element that enhances hardenability, and the addition of Ni forms a large amount of tempered martensite and ensures a TS of 1180 MPa or more and high YS. To produce such effects, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, even further more preferably 0.060% or more.
  • the Ni content is preferably 1.000% or less.
  • the Ni content is more preferably 0.800% or less.
  • the Ni content is even more preferably 0.600% or less, even further more preferably 0.400% or less.
  • Mo is an element that enhances hardenability, and the addition of Mo forms a large amount of tempered martensite and ensures a TS of 1180 MPa or more and high YS.
  • the Mo content is preferably 0.010% or more.
  • the Mo content is more preferably 0.030% or more.
  • the Mo content is preferably 1.000% or less.
  • the Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, even further more preferably 0.400% or less.
  • the Mo content is even more preferably 0.350% or less, even further more preferably 0.300% or less.
  • Sb is an element effective in suppressing the diffusion of C near the surface of a steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet.
  • An excessive increase of a soft layer near the surface of a steel sheet makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS.
  • the Sb content is preferably 0.002% or more.
  • the Sb content is more preferably 0.005% or more.
  • an Sb content of more than 0.200% may result in no soft layer near the surface of a steel sheet and lower flangeability and bendability.
  • the Sb content is preferably 0.200% or less.
  • the Sb content is more preferably 0.020% or less.
  • Sn is an element effective in suppressing the diffusion of C near the surface of a steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet.
  • An excessive increase of a soft layer near the surface of a steel sheet makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS.
  • the Sn content is preferably 0.002% or more.
  • the Sn content is more preferably 0.005% or more.
  • a Sn content of more than 0.200% may result in no soft layer near the surface of a steel sheet and lower flangeability and bendability.
  • the Sn content is preferably 0.200% or less.
  • the Sn content is more preferably 0.020% or less.
  • Cu is an element that enhances hardenability, and the addition of Cu forms a large amount of tempered martensite and ensures a TS of 1180 MPa or more and high YS.
  • the Cu content is preferably 0.005% or more.
  • the Cu content is more preferably 0.008% or more, even more preferably 0.010% or more.
  • the Cu content is even more preferably 0.020% or more.
  • Ta forms fine carbide, nitride, or carbonitride during hot rolling or annealing and increases TS and YS. Furthermore, Ta partially dissolves in Nb carbide or Nb carbonitride and forms a complex precipitate, such as (Nb, Ta) (C, N). This suppresses coarsening of a precipitate and stabilizes precipitation strengthening. This further improves TS and YS.
  • the Ta content is preferably 0.001% or more.
  • the Ta content is more preferably 0.002% or more, even more preferably 0.004% or more.
  • a Ta content of more than 0.100% 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 Ta content is preferably 0.100% or less.
  • 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.
  • Mg is an element effective in spheroidizing the shape of an inclusion of sulfide, oxide, or the like and improving the flangeability of a steel sheet.
  • the Mg content is preferably 0.0001% or more.
  • the Mg content is more preferably 0.0005% or more, even more preferably 0.0010% or more.
  • a Mg 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 Mg content is preferably 0.0200% or less.
  • the Mg content is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • Zn is an element effective in spheroidizing the shape of an inclusion and improving the flangeability of a steel sheet. To produce such effects, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, even more preferably 0.0030% or more.
  • 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.
  • Co is an element effective in spheroidizing the shape of an inclusion and improving the flangeability of a steel sheet.
  • the Co content is preferably 0.0010% or more.
  • the Co content is more preferably 0.0020% or more, even more preferably 0.0030% or more.
  • a Co 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 Co content is preferably 0.0200% or less.
  • the Co 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.
  • the Ca content is preferably 0.0020% 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 Hf content is more preferably 0.0005% or more, even more preferably 0.0008% or more.
  • the Hf 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.
  • Bi is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • REM is more preferably 0.0005% or more, even more preferably 0.0008% or more. REM is more preferably 0.0180% or less, even more preferably 0.0150% or less.
  • REM refers to scandium (Sc) with atomic number 21 , yttrium (Y) with atomic number 39 , and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 .
  • 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 ferrite: 57.0% or less, the total area fraction of bainitic ferrite and tempered martensite: 40.0% or more and 90.0% or less, the area fraction of retained austenite: 3.0% or more and 10.0% or less, the area fraction of fresh martensite: 10.0% or less, and the value obtained by dividing the area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite is 0.70 or more, a V-VDA bending test is performed to a maximum load point, in a V-bending ridge line portion and a VDA bending ridge line portion, the value obtained by dividing the number of voids in contact with a hard phase among all voids by the total number of voids is 0.60 or less, in a V-bending flat portion and the VDA bending ridge line portion, the value obtained by dividing the number of voids in contact
  • Soft ferrite is a phase that improves ductility.
  • an excessive increase in the area fraction of ferrite makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS. This also excessively increases the C concentration in austenite during annealing and results in undesired ⁇ and S Fmax .
  • the area fraction of ferrite is 57.0% or less.
  • the area fraction of ferrite is preferably 30.0% or less, more preferably 20.0% or less.
  • the area fraction of ferrite may have any lower limit and may be 0.0%.
  • Bainitic ferrite and tempered martensite have intermediate hardness as compared with soft ferrite, hard fresh martensite, and the like and is an important phase for ensuring high flangeability and bendability and good bending fracture characteristics and axial compression characteristics.
  • Bainitic ferrite is also a phase useful for utilizing the diffusion of C from bainitic ferrite to non-transformed austenite to form an appropriate amount of retained austenite.
  • Tempered martensite is also effective in improving TS.
  • the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is 40.0% or more.
  • the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 60.0% or more.
  • the total area fraction of bainitic ferrite and tempered martensite is 90.0% or less.
  • the total area fraction of bainitic ferrite and tempered martensite is preferably 87.0% or less, more preferably 85.0% or less.
  • bainitic ferrite refers to upper bainite that is formed in a relatively high temperature region and has a small amount of carbide.
  • the area fraction of retained austenite is 3.0% or more.
  • the area fraction of retained austenite is preferably 5.0% or more.
  • the area fraction of retained austenite is 10.0% or less.
  • the area fraction of retained austenite is preferably 9.0% or less, more preferably 8.0% 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 10.0% or less.
  • Applying a tension of 2.0 kgf/mm 2 or more once or more after a holding 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 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 diffusion of C from bainitic ferrite to non-transformed austenite increases the area fraction of retained austenite.
  • the value obtained by dividing the area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite is 0.70 or more.
  • the value obtained by dividing the area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite is preferably 0.75 or more.
  • the upper limit is not particularly limited, and the value obtained by dividing the area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite may be 1.00.
  • the area fraction of the remaining microstructure other than those described above is preferably 10.0% or less.
  • the area fraction of the remaining microstructure is more preferably 5.0% or less.
  • the area fraction of the remaining microstructure may be 0.0%.
  • the remaining microstructure is, for example, but not limited to, lower bainite, pearlite, carbide such as cementite, or the like.
  • the type of the remaining microstructure can be determined, for example, by scanning electron microscope (SEM) observation.
  • the area fractions of ferrite, bainitic ferrite, tempered martensite, and a hard phase are measured at a quarter thickness position of a base steel sheet as described below.
  • 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.
  • the symbol BF indicates bainitic ferrite
  • the symbol F indicates ferrite
  • the symbol TM indicates tempered martensite.
  • ⁇ denotes carbide
  • H 1 denotes a hard phase.
  • Tempered martensite a gray region of an indefinite form. A relatively large number of iron-based carbide particles is contained.
  • Hard phase (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.
  • Carbide a dotted or linear white region. It is contained in tempered martensite, bainitic ferrite, and ferrite.
  • Remaining microstructure the lower bainite, pearlite, and the like of known forms.
  • 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 ferrite, bainitic ferrite, tempered martensite, and the hard phase (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 Mok ⁇ 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 area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard phase (hard second phase) determined as described above.
  • a base steel sheet of a steel sheet according to an embodiment of the present invention preferably has a surface soft layer on the surface of the base steel sheet.
  • the surface soft layer contributes to the suppression of the development of flex cracking during press forming and in case of a collision of an automobile body and therefore further improves bending fracture resistance characteristics.
  • the term “surface soft layer” means a decarburized layer and refers to a surface layer region with a Vickers hardness of 85% or less with respect to the Vickers hardness of a cross section at a quarter thickness position.
  • the quarter thickness position of the base steel sheet where the Vickers hardness is measured is a non-surface-soft layer (a layer that does not satisfy the condition of the hardness of the surface soft layer defined in accordance with aspects of the present invention).
  • the Vickers hardness is measured at a load of 10 gf in accordance with JIS Z 2244-1 (2020).
  • the ratio of the number of measurements in which the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet is 7.0 GPa or more is 0.10 or less with respect to the total number of measurements at the quarter depth position in the thickness direction of the surface soft layer.
  • the ratio of the number of measurements in which the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet is 7.0 GPa or more is preferably 0.10 or less with respect to the total number of measurements at the quarter depth position in the thickness direction of the surface soft layer.
  • the ratio of the nanohardness of 7.0 GPa or more is 0.10 or less, it means a low ratio of a hard microstructure (martensite or the like), an inclusion, or the like, and this can further suppress the formation and connection of voids and crack growth in the hard microstructure (martensite and the like), inclusion, or the like during press forming and in case of a collision, thus resulting in good R/t, ⁇ , and SFmax.
  • the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the steel sheet has a standard deviation ⁇ of 1.8 GPa or less
  • the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the steel sheet has a standard deviation ⁇ of 2.2 GPa or less.
  • the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the steel sheet preferably has a standard deviation ⁇ of 1.8 GPa or less
  • the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the steel sheet preferably has a standard deviation ⁇ of 2.2 GPa or less.
  • the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet preferably has a standard deviation ⁇ of 1.7 GPa or less.
  • the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet more preferably has a standard deviation ⁇ of 1.3 GPa or less.
  • the standard deviation ⁇ of the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet may have any lower limit and may be 0.5 GPa or more.
  • the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet more preferably has a standard deviation ⁇ of 2.1 GPa or less.
  • the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet more preferably has a standard deviation ⁇ of 1.7 GPa or less.
  • the standard deviation ⁇ of the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet may have any lower limit and may be 0.6 GPa or more.
  • nanohardness of a sheet surface at a quarter depth position and at a half depth position in the thickness direction refers to a hardness measured by the following method.
  • 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.
  • the nanohardness is measured with Hysitron tribo-950 and a Berkovich diamond indenter under the conditions of a load of 500 ⁇ N, a measurement area of 50 ⁇ m ⁇ 50 ⁇ m, and a dot-to-dot distance of 2 ⁇ m.
  • the nanohardness is measured with Hysitron tribo-950 and a Berkovich diamond indenter under the conditions of a load of 500 ⁇ N, a measurement area of 50 ⁇ m ⁇ 50 ⁇ m, and a dot-to-dot distance of 2 ⁇ m.
  • the quarter position is a position of 25 ⁇ m from the surface of the surface soft layer
  • the half position is a position of 50 ⁇ m from the surface of the surface soft layer.
  • the nanohardness is measured at 300 points or more at the position of 25 ⁇ m, and the nanohardness is also measured at 300 points or more at the position of 50 ⁇ m.
  • a steel sheet according to an embodiment of the present invention preferably has a metal coated layer (first coated layer, precoated layer) on one or both surfaces of a base steel sheet (the metal coated layer (first coated layer) excludes a hot-dip galvanized layer and a galvanized layer of a hot-dip galvannealed layer).
  • the metal coated layer is preferably a metal electroplated layer, and the metal electroplated layer is described below as an example.
  • the metal electroplated layer When the metal electroplated layer is formed on the surface of a steel sheet, the metal electroplated layer as the outermost surface layer contributes to the suppression of the occurrence of flex cracking during press forming and in case of a collision of an automobile body and therefore further improves the bending fracture resistance characteristics.
  • 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 metal species of the metal 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.
  • an Fe-based electroplated layer is described below as an example, the following conditions for Fe can also be applied to other metal species.
  • 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.
  • the Fe-based electroplated layer may be, in addition to pure Fe, an alloy coated layer, such as an Fe—B alloy, an Fe—C alloy, an Fe—P alloy, an Fe—N alloy, an Fe—O alloy, an Fe—Ni alloy, an Fe—Mn alloy, an Fe—Mo alloy, or an Fe—W alloy.
  • the Fe-based electroplated layer may have any chemical composition and preferably has a chemical composition containing 10% by mass or less in total of one or two or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder being Fe and incidental impurities.
  • the C content is preferably 0.08% by mass or less.
  • 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.
  • high bending fracture characteristics in a V-VDA bending test can be achieved by void control as described above.
  • the void is likely to develop along a boundary between the hard phase and a soft phase and finally causes a crack.
  • void control for a void not adjacent to a hard phase, for example, for a void formed adjacent to carbide, it is thought that the connection and development of the void are less likely to occur.
  • the value obtained by dividing the number of voids in contact with a hard phase (the number of voids at a boundary between the hard phase and a soft phase and the number of voids formed by fracture of the hard phase) among all voids by the total number of voids is 0.60 or less. This value is preferably 0.59 or less, more preferably 0.58 or less. The lower limit is not particularly limited, and the value may be 0.00.
  • the area fraction of a hard phase is relatively low.
  • the value obtained by dividing the number of voids in contact with a hard phase among all voids by the total number of voids is 0.20 or less. This value is preferably 0.19 or less, more preferably 0.18 or less.
  • the lower limit is not particularly limited, and the value may be 0.00.
  • soft phase refers to a phase other than the hard phase.
  • 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 indicates the width (C) direction
  • the symbol D 2 indicates 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.
  • a V-bending ridge line portion and a VDA bending ridge line portion “a” (an overlap region “a” of the V-bending ridge line portion and the VDA bending ridge line portion), and a V-bending flat portion (unprocessed portion) and the VDA bending ridge line portion “b” are shown in FIG. 2 - 2 ( d ).
  • V-bending ridge line portion refers to the region within 5 mm on both sides of a V-bending corner portion (peak) that is subjected to V-bending and extends in the width direction.
  • V-bending flat portion refers to a region other than the V-bending ridge line portion in a steel sheet.
  • 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 L cross section AL with the D 2 direction being perpendicular to the drawing and the D 1 direction being parallel to the drawing.
  • a void in the V-bending ridge line portion and the VDA bending ridge line portion and a void in the V-bending flat portion and the VDA bending ridge line portion are measured as described below.
  • a thickness cross section obtained by cutting a steel sheet after a V-VDA bending test in a V-bending ridge line portion and a VDA bending ridge line portion “a” and in a V-bending flat portion and the VDA bending ridge line portion “b” in a direction perpendicular to the rolling direction is polished, and three visual fields in a C cross section in a region of 0 to 100 ⁇ m from the surface of the steel sheet at a bending peak portion on the outside of a VDA bend (an AB region indicated by the dotted line in FIG.
  • the number of voids in which more than 0% of the circumferential length is in contact with a hard phase is the sum of the number of voids at a boundary between the hard phase and a soft phase and the number of voids formed by fracture of the hard phase.
  • the value obtained by dividing the number of voids in contact with a hard phase by the total number of voids specified in accordance with aspects of the present invention is calculated by averaging in three visual fields the values obtained by dividing the number of voids in which more than 0% of the circumferential length is in contact with the hard phase (the sum of the number of voids at a boundary between the hard phase and the soft phase and the number of voids formed by fracture of the hard phase) by the total number of voids.
  • This measurement is performed on a test specimen prepared by performing the V-VDA bending test to the maximum load and then removing the load when the load reaches 94.9% to 99.9% (for example, 95%) of the maximum load (see FIG. 3 again).
  • an island-like region with the outer periphery surrounded by a microstructure and integrally formed without interruption is regarded as one to be measured.
  • FIGS. 4 ( a ) to 4 ( c ) show examples of a microstructure image for explaining a void.
  • the symbol H 1 indicates a hard phase
  • the symbol S 1 indicates a soft phase.
  • the symbol V 1 in FIG. 4 ( a ) indicates a void at a boundary between a hard phase and a soft phase
  • the symbol V 2 in FIG. 4 ( b ) indicates a void formed by fracture of a hard phase
  • the symbol V 3 in FIG. 4 ( c ) indicates a void due to carbide
  • the symbol ⁇ indicates carbide.
  • void formation in a steel sheet microstructure may be caused by carbide (see the symbols V 3 and ⁇ ).
  • carbide has a mean free path of less than 0.20 ⁇ m, the distance between voids due to the carbide increases, stress is concentrated on a portion where a void is formed, and voids are easily connected. Consequently, desired S Fmax cannot be achieved.
  • carbide has a mean free path L M of 0.20 ⁇ m or more.
  • L M is preferably 0.25 ⁇ m or more, more preferably 0.30 ⁇ m or more.
  • L M is preferably 0.50 ⁇ m or less, more preferably 0.45 ⁇ m or less.
  • the mean free path L M and the standard deviation of carbide are measured as described below.
  • carbide is extracted by manual color-coding from the SEM microstructure image used for the microstructure fraction measurement to obtain an image of only the carbide.
  • the area fraction of all carbide particles and the coordinate of the center of gravity and the equivalent circular diameter of each carbide particle are determined using ImageJ from an open source. Assuming that carbide is three-dimensionally homogeneous, the area fraction of the carbide is defined as the volume fraction of the carbide.
  • the mean free path L M of carbide is calculated using the following formula:
  • n, i, j, d ij , and d iave are as follows:
  • n the number of all carbide particles in the visual field (25.6 ⁇ m ⁇ 17.6 ⁇ m).
  • i the number of a carbide particle (one carbide particle A arbitrarily selected from all carbide particles) to measure the distance from another carbide particle, and i is an integer in the range of 1 to n.
  • j the number of a carbide particle other than the carbide particle A, and j is an integer in the range of 1 to n other than i.
  • d ij the distance ( ⁇ m) between the i-th carbide particle (the carbide particle A) and the j-th carbide particle.
  • d iave the average distance ( ⁇ m) between all carbide particles (excluding the i-th carbide particle) and the i-th carbide particle in the visual field.
  • 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 (El), the limiting hole expansion ratio ( ⁇ ), the critical bending angle ( ⁇ ) in the VDA bending test, 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.
  • the tensile strength (TS), the yield stress (YS), and the total elongation (El) are measured in the tensile test according to JIS Z 2241 (2011) described later in Examples.
  • the limiting hole expansion ratio ( ⁇ ) is measured in the hole expansion test according to JIS Z 2256 (2020) described later in Examples.
  • the critical bending angle ( ⁇ ) in the VDA bending test is measured in the VDA bending test according to VDA 238-100 described later in Examples.
  • the stroke at the maximum load (S Fmax ) in the V-VDA bending test is measured in a V-VDA bending test described later in Examples.
  • the presence or absence of axial compression fracture is measured in an axial compression test described later in Examples.
  • a steel sheet according to an embodiment of the present invention may have a galvanized layer formed on a base steel sheet (on the surface of the base steel sheet or on the surface of a metal coated layer when the metal coated layer is formed) as the outermost surface layer, and the galvanized layer may be provided on only one surface or both surfaces of the base steel sheet.
  • a steel sheet according to aspects of the present invention may have a base steel sheet and a second coated layer (a galvanized layer) formed on the base steel sheet or may have a base steel sheet and a metal coated layer (a first coated layer (excluding a second coated layer of a galvanized layer) and a second coated layer (a galvanized layer) sequentially formed on the base steel sheet.
  • a steel sheet with a galvanized layer may be a galvanized steel sheet.
  • 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 coating weight of the galvanized layer is measured as described below.
  • a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor for Fe (“IBIT 700BK” (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of 10% by mass aqueous hydrochloric acid.
  • a steel sheet as a sample is immersed in the treatment liquid to dissolve a galvanized layer.
  • the mass loss of the sample due to the dissolution is measured and is divided by the surface area of a base steel sheet (the surface area of a coated portion) to calculate the coating weight (g/m 2 ).
  • 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 + (Ac 3 ⁇ Ac 1 ) ⁇ 3 ⁇ 4) ° 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 100° C. or more and 300° 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 rolling may be performed under any conditions, for example, under the following conditions.
  • the steel slab is then rough-rolled in the usual manner to form a rough-rolled sheet (hereinafter also referred to as a sheet bar).
  • the sheet bar is then finish-rolled to form a hot-rolled steel sheet.
  • the sheet bar is preferably heated with a bar heater or the like before finish rolling to prevent trouble in the finish rolling.
  • the finish rolling temperature is preferably 800° C. or more to reduce the rolling load.
  • an abnormal microstructure elongated in the rolling direction may be developed and impair the workability of an annealed sheet.
  • a finish rolling temperature of 800° C. or more not only the steel microstructure of the hot-rolled steel sheet but also the steel microstructure of the final product is likely to be uniform. A nonuniform steel microstructure tends to result in lower bendability.
  • the finish rolling temperature is preferably 950° C. or less.
  • the finish rolling temperature is preferably 800° C. or more and 950° C. or less.
  • the hot-rolled steel sheet is coiled.
  • the coiling temperature is preferably 450° C. or more.
  • the coiling temperature is preferably 750° C. or less.
  • Sheet bars may be joined together during hot rolling to continuously perform the finish rolling.
  • the sheet bar may be temporarily coiled before the finish rolling.
  • the finish rolling may be partly or entirely rolling with lubrication.
  • the rolling with lubrication is also effective in making the shape and the material quality of a steel sheet uniform.
  • the friction coefficient in the rolling with lubrication is preferably 0.10 or more and 0.25 or less.
  • the steel slab is typically formed into a sheet bar by the rough rolling and then into a hot-rolled steel sheet by the finish rolling.
  • the finish rolling Depending on the mill capacity or the like, however, such classification is not concerned, provided that a predetermined size is obtained.
  • 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.
  • the rolling reduction (cumulative rolling reduction ratio) in the cold rolling is preferably, but not limited to, 20% or more.
  • the rolling reduction in the cold rolling is preferably 80% or less.
  • a rolling reduction of less than 20% in the cold rolling tends to result in coarsening or a lack of uniformity of the steel microstructure in the annealing step and may result in the final product with lower TS or bendability.
  • a rolling reduction of more than 80% in the cold rolling tends to result in a steel sheet with a poor shape and may result in an uneven galvanizing coating weight.
  • 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).
  • 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.
  • 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 + (Ac 3 ⁇ Ac 1 ) ⁇ 3 ⁇ 4° 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)+ (Ac 3 ⁇ Ac 1 ) ⁇ 3 ⁇ 4° C.) or More And 900° C. Or Less
  • An annealing temperature lower than (Ac 1 + (Ac 3 ⁇ Ac 1 ) ⁇ 3 ⁇ 4° 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 also excessively increases the C concentration in austenite during annealing and results 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 carbide, makes it difficult to form 3.0% or more of retained austenite, and results in lower ductility.
  • the annealing temperature is (Ac 1 + (Ac 3 ⁇ Ac 1 ) ⁇ 3 ⁇ 4) ° 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 + (Ac 3 ⁇ Ac 1 ) ⁇ 4 ⁇ 5) ° C. or more, more preferably (Ac 1 + (Ac 3 ⁇ Ac 1 ) ⁇ 5 ⁇ 6° 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:
  • an annealing time of less than 20 seconds 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 also excessively increases the C concentration in austenite during annealing and results in undesired ⁇ and S Fmax . This also makes it difficult to achieve a TS of 1180 MPa or more.
  • the annealing time is 20 seconds or more.
  • the annealing time is preferably 30 seconds or more, more preferably 50 seconds or more.
  • the annealing time may have any upper limit and is preferably 900 seconds or less, more preferably 800 seconds or less.
  • the annealing time is even more preferably 300 seconds or less, even further more preferably 220 seconds or less.
  • 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.
  • the dew point of the atmosphere in the annealing step is preferably ⁇ 30° C.
  • Annealing at a dew point of ⁇ 30° C. or more in the annealing atmosphere in the annealing step can promote a decarburization reaction and more deeply form a surface soft layer.
  • the dew point of the annealing atmosphere in the annealing step is more preferably ⁇ 25° C. or more, even more preferably ⁇ 15° C. or more, most preferably more than ⁇ 5° C.
  • the dew point of the annealing atmosphere in the annealing step may have any upper limit and is preferably 30° C. or less in order to suitably prevent oxidation of the surface of an Fe-based electroplated layer and to improve the coating adhesion when a galvanized layer is provided.
  • the steel sheet annealed as described above is then cooled to a first cooling stop temperature of 100° C. or more and 300° C. or less.
  • the first cooling step is a step necessary to control the area fraction of tempered martensite and the volume fraction of retained austenite formed in the subsequent reheating step within predetermined ranges.
  • a first cooling stop temperature of less than 100° C. almost all the non-transformed austenite present in the steel is transformed into martensite in the first cooling step. This finally results in an excessive increase in the area fraction of tempered martensite, makes it difficult to form 3.0% or more by area of retained austenite, and results in lower ductility.
  • a second cooling stop temperature of more than 300° C. results in a decrease in the area fraction of tempered martensite and an increase in the area fraction of fresh martensite.
  • the first cooling stop temperature is 100° C. or more and 300° C. or less.
  • the first cooling stop temperature is preferably 120° C. or more.
  • the first cooling stop temperature is preferably 280° C. or less.
  • the steel sheet is held in the temperature range of 350° C. or more and 550° C. or less (hereinafter also referred to as a holding temperature range) for 3 seconds or more and less than 80 seconds.
  • Holding time in holding temperature range: 3 seconds or more and less than 80 seconds
  • 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.
  • the value obtained by dividing the area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite cannot be in the desired range, and the desired ⁇ and S Fmax cannot be achieved. Good axial compression characteristics also cannot be achieved.
  • the area fraction of retained austenite is less than 3.0%
  • the area fraction of fresh martensite is more than 10.0%
  • desired ductility cannot be achieved
  • desired ⁇ and S Fmax also cannot be achieved.
  • Good axial compression characteristics also cannot be achieved.
  • the holding temperature range is 350° C. or more and 550° C. or less.
  • the holding temperature range is preferably 360° C. or more, more preferably 370° C. or more.
  • the holding temperature range is preferably 530° C. or less, more preferably 510° C. or less.
  • a holding time of less than 3 seconds in the holding temperature range makes it difficult to form 3.0% or more of retained austenite and results in lower ductility.
  • a holding time of 80 seconds or more in the holding temperature range results in an excessive increase in the area fraction of bainitic ferrite and lower YS. This also results in excessive diffusion of C from bainitic ferrite to non-transformed austenite, retained austenite with an area fraction of more than 10.0%, and undesired S Fmax . Furthermore, desired ⁇ may not be achieved.
  • the holding time in the holding temperature range is preferably 3 seconds or more and less than 80 seconds.
  • the holding time in the holding temperature range is preferably 5 seconds or more.
  • the holding time in the holding temperature range is 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 hot-dip galvanizing treatment in the coating step.
  • the steel sheet may be subjected to a galvanizing treatment.
  • a galvanized steel sheet can be produced by the galvanizing treatment.
  • the galvanizing treatment is, for example, a hot-dip galvanizing treatment or a galvannealing treatment.
  • the steel sheet is immersed in a galvanizing bath at 440° C. or more and 500° C. or less, and the coating weight is then adjusted by gas wiping or the like.
  • the hot-dip galvanizing bath is not particularly limited as long as the galvanized layer has the composition described above.
  • the galvanizing bath preferably has a composition with an Al content of 0.10% by mass or more, the remainder being Zn and incidental impurities.
  • the Al content is preferably 0.23% by mass or less.
  • the galvanized steel sheet is preferably heated to an alloying temperature of 450° C. or more to perform an alloying treatment.
  • the alloying temperature is preferably 600° C. or less.
  • An alloying temperature of less than 450° C. may result in a low Zn—Fe alloying speed and make alloying difficult.
  • martensite formed in the first cooling step is not sufficiently tempered, the area fraction of fresh martensite increases excessively, and desired ⁇ , ⁇ , and S Fmax may not be achieved.
  • an alloying temperature of more than 600° C. results in transformation of non-transformed austenite into pearlite, makes it difficult to achieve a TS of 1180 MPa or more, and results in lower ductility.
  • the alloying temperature is more preferably 510° C. or more.
  • the alloying temperature is more preferably 570° C. or less.
  • the coating weight of each of the hot-dip galvanized steel sheet (GI) and the hot-dip galvannealed steel sheet (GA) is preferably 20 g/m 2 or more per side.
  • the coating weight per side of the galvanized layer is preferably 80 g/m 2 or less.
  • the coating weight can be adjusted by gas wiping or the like.
  • the steel sheet after the holding step is then cooled to a second cooling stop temperature of 50° C. or less.
  • the number of passes to which the steel sheet is subjected during contact with the roll for a quarter circumference of the roll is preferably five or more passes, more preferably six or more passes.
  • the upper limit is not particularly limited, but the number of passes to which the steel sheet is subjected during contact with the roll for a quarter circumference of the roll is preferably 12 or less passes, more preferably 10 or less passes.
  • the number of passes to which the steel sheet is subjected during contact with the roll for half a circumference of the roll is preferably three or more passes, more preferably four or more passes.
  • the upper limit is not particularly limited, but the number of passes to which the steel sheet is subjected during contact with the roll for half a circumference of the roll is preferably six or less passes, more preferably five or less passes.
  • the load cells should be arranged parallel to the direction of the tension.
  • the load cells are preferably disposed at a position of 200 mm from both ends of the roll.
  • the length of the roll to be used is preferably 1500 mm or more.
  • the length of the roll to be used is preferably 2500 mm or less.
  • 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.
  • Second Cooling Stop Temperature 50° C. Or Less
  • the cooling conditions in the second cooling step are not particularly limited and may be based on a usual method.
  • the cooling method is, for example, gas jet cooling, mist cooling, roll cooling, water cooling, natural cooling, or the like.
  • the average cooling rate is preferably, for example, 1° C./s or more and 50° C./s or less.
  • the average cooling rate can be calculated by “(cooling start temperature (° C.) ⁇ second cooling stop temperature (° C.))/cooling time(s)”.
  • the steel sheet thus produced may be further subjected to temper rolling.
  • a rolling reduction of more than 2.00% in the temper rolling may result in an increase in yield stress and a decrease in dimensional accuracy when the steel sheet is formed into a member.
  • the rolling reduction in the temper rolling is preferably 2.00% or less.
  • the lower limit of the rolling reduction in the temper rolling is preferably, but not limited to, 0.05% or more from the perspective of productivity.
  • the temper rolling may be performed with an apparatus coupled to an annealing apparatus for each step (on-line) or with an apparatus separated from the annealing apparatus for each step (off-line).
  • the number of temper rolling processes may be one or two or more.
  • the rolling may be performed with a leveler or the like, provided that the elongation can be equivalent to that in the temper rolling.
  • Conditions other than those described above are not particularly limited and may be based on a usual method.
  • a member according to an embodiment of the present invention is a member produced by using the steel sheet described above (as a material).
  • the steel sheet as a material is subjected to at least one of forming and joining to produce a member.
  • 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.
  • a method for producing a member according to an embodiment of the present invention includes a step of subjecting the steel sheet (for example, a steel sheet produced by the method for producing a steel sheet) to at least one of forming and joining to produce a member.
  • the steel sheet for example, a steel sheet produced by the method for producing a steel sheet
  • the forming method is, for example, but not limited to, a typical processing method, such as press working.
  • the joining method is also, for example, but not limited to, typical welding, such as spot welding, laser welding, or arc welding, riveting, caulking, or the like.
  • the forming conditions and the joining conditions are not particularly limited and may be based on a usual method.
  • a steel material with the chemical composition (the remainder being Fe and incidental impurities) listed in Table 1 was produced by steelmaking in a converter and was formed into a steel slab in a continuous casting method.
  • “ ⁇ ” indicates the content at the level of incidental impurities.
  • Hot-rolled steel sheets No. 1 to No. 57, No. 60 to No. 74, No. 80 to 93, and No. 100 to No. 105 thus produced were pickled and cold-rolled (rolling reduction: 50%) to produce cold-rolled steel sheets with thicknesses shown in Tables 3, 6, and 9.
  • Hot-rolled steel sheets No. 58 and No. 59, No. 75 to No. 79, and No. 95 to 99 were pickled to produce hot-rolled steel sheets (pickled) with thicknesses shown in Tables 3, 6, and 9.
  • the cold-rolled steel sheets or hot-rolled steel sheets were subjected to the annealing step, the first cooling step, the holding step, the coating step, the second cooling step, and the reheating step under the conditions shown in Table 2 and were subjected to treatments in the first coating step (metal coating step), the annealing step, the first cooling step, the holding step, the second coating step (galvanizing step), the second cooling step, and the reheating step under the conditions shown in Tables 5 and 8 to produce steel sheets (galvanized steel sheets).
  • Tables 5 and 8 show the presence or absence of the first coating step (metal coating step) and the coating type in the treatment in the metal coating step for the steel sheets No. 60 to No. 105.
  • Tables 6 and 9 show the thickness of the surface soft layer, the metal coating weight, and the hardness distribution of the surface soft layer for the steel sheets No. 60 to No. 105.
  • the hot-dip galvanizing treatment or the galvannealing treatment was performed to produce a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed steel sheet (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheet
  • GA galvannealed steel sheet
  • Tables 2, 5, and 8 the type in the coating step is also denoted by “GI” and “GA”.
  • the alloying temperature is indicated by “ ⁇ ”.
  • a cold-rolled steel sheet formed without the galvanizing treatment in the galvanizing step is denoted by “CR”, and a hot-rolled steel sheet formed without the galvanizing treatment in the galvanizing step is denoted by “HR”.
  • the galvanizing bath temperature was 470° C. in the production of GI and GA.
  • the galvanizing coating weight ranged from 45 to 72 g/m 2 per side to produce GI and was 45 g/m 2 per side to produce GA.
  • composition of the galvanized layer of the final galvanized steel sheet in GI contained Fe: 0.1% to 1.0% by mass and Al: 0.2% to 0.33% by mass, the remainder being Zn and incidental impurities.
  • GA contained Fe: 8.0% to 12.0% by mass and Al: 0.1% to 0.23% by mass, the remainder being Zn and incidental impurities.
  • the galvanized layer was formed on both surfaces of the base steel sheet.
  • the term “pass 1 ” refers to the number of passes to which the steel sheet is subjected, 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, after an average tension of 2.0 kgf/mm 2 or more is applied once or more in the temperature range of 300° C. or more and 450° C. or less in the second cooling step, and the phrase “the number of passes 2” refers to the number of passes to which the steel sheet is subsequently subjected, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for half a circumference of the roll.
  • Tables 3, 6, and 9 show the measurement results.
  • F denotes ferrite
  • BF denotes bainitic ferrite
  • TM denotes tempered martensite
  • RA denotes retained austenite
  • FM denotes fresh martensite
  • LB denotes lower bainite
  • P denotes pearlite
  • ⁇ denotes carbide.
  • L M denotes the mean free path of the center of gravity of carbide
  • ⁇ C denotes the average value of the standard deviation of the distance between carbide particles.
  • *1 is the value obtained by dividing the number of voids in contact with a hard phase (the number of voids at a boundary between the hard phase and a soft phase and the number of voids formed by fracture of the hard phase) among all voids by the total number of voids in an overlap region of a V-bending ridge line portion and a VDA bending ridge line portion
  • *2 is the value obtained by dividing the number of voids in contact with a hard phase (the number of voids at a boundary between the hard phase and a soft phase and the number of voids formed by fracture of the hard phase) among all voids by the total number of voids in an overlap region of a V-bending flat portion and the VDA bending ridge line portion.
  • Measurement is performed on the surface soft layer as described below. After smoothing a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet by wet grinding, measurement was performed in accordance with JIS Z 2244-1 (2020) using a Vickers hardness tester at a load of 10 gf from a 1- ⁇ m position to a 100- ⁇ m position in the thickness direction from the surface of the steel sheet at intervals of 1 ⁇ m. Measurement was then performed at intervals of 20 ⁇ m to the central portion in the thickness direction. A region with hardness corresponding to 85% or less of the hardness at the quarter thickness position is defined as a soft layer (surface soft layer), and the thickness of the region in the thickness direction is defined as the thickness of the soft layer.
  • a tensile test, a hole expansion test, a VDA bending test, a V-VDA bending test, and an axial compression test were performed in the manner described below.
  • the tensile strength (TS), the yield stress (YS), the total elongation (El), the limiting hole expansion ratio ( ⁇ ), the critical bending angle ( ⁇ ) in the VDA bending test, the stroke at the maximum load (S Fmax ) in the V-VDA bending test, and the presence or absence of axial compression fracture were evaluated in accordance with the following criteria.
  • the tensile test was performed in accordance with JIS Z 2241 (2011).
  • a JIS No. 5 test specimen was taken from the steel sheet such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet.
  • TS, YS, and El of the test specimen were measured at a crosshead speed of 10 mm/min in the tensile test.
  • Tables 4, 7, and 10 show the results.
  • ⁇ ⁇ ( % ) ⁇ ( D f - D 0 ) / D 0 ⁇ ⁇ 100
  • VDA bending test was performed in a bending test according to the VDA standard (VDA 238-100) defined by German Association of the Automotive Industry.
  • a 70 mm ⁇ 60 mm test specimen was taken from the steel sheet by shearing.
  • the sides of 60 mm are parallel to the rolling (L) direction.
  • test specimen was subjected to the VDA bending test under the following conditions.
  • V-VDA Bending Test V-Bending+Orthogonal VDA Bending Test
  • V-VDA bending test was performed as described below.
  • a 60 mm ⁇ 65 mm test specimen was taken from the steel sheet by shearing.
  • the sides of 60 mm are parallel to the rolling (L) direction.
  • 90-degree bending was 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 was 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 was 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 indicates the width (C) direction
  • the symbol D 2 indicates the rolling (L) direction.
  • V-bending conditions in the V-VDA bending test are as follows:
  • the stroke at the maximum load is determined in a stroke-load curve of the VDA bending.
  • the average value of the stroke at the maximum load in the V-VDA bending test performed three times is defined as S Fmax (mm). Tables 4, 7, and 10 show the results.
  • a 160 mm ⁇ 200 mm test specimen was taken from the steel sheet by shearing. The sides of 160 mm are parallel to the rolling (L) direction.
  • a hat-shaped member 10 with a depth of 40 mm illustrated in FIGS. 5 - 1 ( a ) and 5 - 1 ( b ) was produced by forming (bending) with a die having a punch corner radius of 5.0 mm and a die corner radius of 5.0 mm.
  • the steel sheet used as the material of the hat-shaped member was separately cut into a size of 80 mm ⁇ 200 mm. Next, the cut-out steel sheet 20 and the hat-shaped member 10 were spot-welded together to produce a test member 30 as illustrated in FIGS.
  • FIG. 5 - 1 ( a ) is a front view of the test member 30 produced by spot-welding the hat-shaped member 10 and the steel sheet 20 .
  • FIG. 5 - 1 ( b ) is a perspective view of the test member 30 .
  • spot welds 40 were positioned such that the distance between an end portion of the steel sheet and a weld was 10 mm and the distance between the welds was 45 mm.
  • the test member 30 was joined to a base plate 50 by TIG welding to prepare an axial compression test sample.
  • the axial compression test sample was collided with an impactor 60 at a constant collision speed of 10 mm/min to compress the axial compression test sample by 70 mm.
  • the compression direction D 3 was a direction parallel to the longitudinal direction of the test member 30 .
  • the VDA bending test, the V-VDA bending test, and the axial compression test of a steel sheet with a thickness of more than 1.2 mm were all performed on a steel sheet with a thickness of 1.2 mm in consideration of the influence of the sheet thickness.
  • a steel sheet with a thickness of more than 1.2 mm was ground on one side to have a thickness of 1.2 mm.
  • the ground surface in the VDA bending test was the inside of the bend (the side in contact with the punch), and the ground surface in the V-VDA bending test was the outside of the bend (the side in contact with the die) in the V-bending test and was the inside of the bend (the side in contact with the punch) in the subsequent VDA bending test.
  • the sheet thickness has a small influence, and the test was performed without the grinding treatment.
  • the ratio of the number of measurements in which the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet is 7.0 GPa or more is more preferably 0.10 or less with respect to the total number of measurements at the quarter depth position in the thickness direction.
  • the ratio of the nanohardness of 7.0 GPa or more is 0.10 or less, it means a low ratio of a hard microstructure (martensite or the like), an inclusion, or the like, and this could further suppress the formation and connection of voids and crack growth in the hard microstructure (martensite and the like), inclusion, or the like during press forming and in case of a collision, thus resulting in good R/t and SFmax.
  • the comparative examples were not satisfactory in at least one of the tensile strength (TS), the yield stress (YS), the total elongation (El), the limiting hole expansion ratio ( ⁇ ), the critical bending angle ( ⁇ ) in the VDA bending test, the stroke at the maximum load (S Fmax ) in the V-VDA bending test, and the presence or absence of fracture in the axial compression test.

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