US20250313926A1 - Steel sheet, member, and methods for producing same - Google Patents

Steel sheet, member, and methods for producing same

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Publication number
US20250313926A1
US20250313926A1 US18/863,170 US202318863170A US2025313926A1 US 20250313926 A1 US20250313926 A1 US 20250313926A1 US 202318863170 A US202318863170 A US 202318863170A US 2025313926 A1 US2025313926 A1 US 2025313926A1
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United States
Prior art keywords
less
steel sheet
annealing
cooling
bending
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
US18/863,170
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English (en)
Inventor
Yoshiyasu Kawasaki
Yusuke Wada
Hidekazu Minami
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, MINAMI, HIDEKAZU, NAKAGAITO, TATSUYA, WADA, YUSUKE
Publication of US20250313926A1 publication Critical patent/US20250313926A1/en
Pending legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/0205—
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
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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
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    • 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
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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
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    • C21D8/0252—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with application of tension
    • C—CHEMISTRY; METALLURGY
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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
    • C21D8/0247—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
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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
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    • 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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    • 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
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C2202/00—Physical properties
    • C22C2202/04—Hydrogen absorbing

Definitions

  • the present invention relates to a steel sheet, a member made of the steel sheet, and methods for producing them.
  • Automotive steel sheets have been reinforced to achieve both the reduction of CO 2 emissions due to an improvement of fuel efficiency by reducing the thickness and weight of steel sheets used in automobile bodies and an improvement of crash safety. Furthermore, new laws and regulations are continuously introduced. Thus, for the purpose of increasing the strength of an automobile body, high-strength steel sheets, particularly high-strength steel sheets with a tensile strength (hereinafter also referred to simply as TS) of 780 MPa or more, are increasingly applied to main structural members and reinforcing members (hereinafter also referred to as automobile frame structural members or the like) to be assembled to frames of automobile cabins. Furthermore, high-strength steel sheets used for frame structural members or the like of automobiles are required to have high member strength during press forming.
  • TS tensile strength
  • YR yield ratio
  • YS yield stress
  • 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 ⁇ CM1 2 +1676 ⁇ CM1+189) ⁇ 0.60, when martensite [2]: C concentration (CM2) is 0.8% or more, the hardness Hv2 satisfies Hv2/( ⁇ 982.1 ⁇ CM2 2 +1676 ⁇ CM2+189) ⁇ 0.60, and when martensite [3]: C concentration (CM3) is 0.8% or more, the hardness Hv3 satisfies Hv3/( ⁇ 982.1 ⁇ CM3 2 +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
  • Patent Literature 4 discloses a hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on the surface of the steel sheet, wherein the steel sheet has a chemical composition of, on a mass percent basis, C: 0.03% or more and 0.35% or less, Si: 0.005% or more and 2.0% or less, Mn: 1.0% or more and 4.0% or less, P: 0.0004% or more and 0.1% or less, S: 0.02% or less, sol.
  • the concentrated portion average interval is 1000 ⁇ m or less at a depth of 50 ⁇ m from the surface of the steel sheet, the concentrated portion average interval being an average interval in the direction perpendicular to the rolling direction of a concentrated portion in which Mn and/or Si spread in the rolling direction is concentrated, the number density of cracks with a depth of 3 ⁇ m or more and 10 ⁇ m or less on the surface of the steel sheet is 3/mm or more and 1000/mm or less, the steel sheet has a steel microstructure containing, on an area percent basis, bainite: 60% or more, retained austenite: 1% or more, martensite: 1% or more, and ferrite: 2% or more and less than 20%, and having a superhard phase average interval, which is the average closest distance of martensite and retained austenite, of 20 ⁇ m or less, and the hot-dip galvannealed steel sheet has mechanical characteristics with
  • a steel sheet with a tensile strength TS (hereinafter also referred to simply as TS) of more than 590 MPa has been applied to a structural member of an automobile exemplified by a center pillar, only a steel sheet with a TS of 590 MPa is applied to an impact energy absorbing member of an automobile exemplified by a front side member or a rear side member.
  • a steel sheet with higher YS and YR typically has lower press formability and, in particular, lower ductility, flangeability, bendability, and the like.
  • YS yield stress
  • YR yield ratio
  • a steel sheet with higher TS and YS typically has lower press formability and, in particular, lower ductility, flangeability, bendability, and the like.
  • Patent Literature 1 to Patent Literature 4 have a TS of 780 MPa or more, high YS and YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision.
  • aspects of the present invention have been developed in view of such circumstances and aim to provide a steel sheet with a tensile strength TS of 780 MPa or more, high yield stress YS and yield ratio YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision, and a method for producing the steel sheet.
  • aspects of the present invention also aim to provide a member made of the steel sheet and a method for producing the member.
  • steel sheet includes a galvanized steel sheet, and the galvanized steel sheet is a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheet
  • GA hot-dip galvannealed steel sheet
  • the tensile strength TS is measured in the tensile test according to JIS Z 2241 (2011).
  • high ductility means that the total elongation (El) 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 means that R (critical bending radius)/t (thickness) measured in the V-bending test according to JIS Z 2248 (2014) satisfies the following formulae (A) or (B) depending on TS.
  • the phrase “good axial compression characteristics” means that the critical spacer thickness (ST) in a U-bending+tight bending bending test satisfies the following formula (A) or (B) depending on TS.
  • the phrase “good axial compression characteristics” means that the stroke at the maximum load (SFmax) measured in a V-bending+orthogonal VDA bending test satisfies the following formulae (A) or (B) depending on TS.
  • good bending fracture characteristics means that the critical spacer thickness (ST) in the U-bending+tight bending bending test satisfies the formula (A) or (B) depending on TS, and the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test satisfies the formula (A) or (B) depending on TS.
  • the El (ductility), A (stretch flangeability), and R/t (bendability) are characteristics indicating the ease of forming a steel sheet during press forming (the degree of freedom of forming for press forming without cracking).
  • the U-bending+tight bending test is a test simulating the deformation and fracture behavior of a vertical wall portion in a collision test
  • the critical spacer thickness (ST) measured in the U-bending+tight bending test is a measure indicating the resistance to cracking of a steel sheet and a member of an automobile body in case of a collision (crashworthiness for absorbing impact energy without fracture).
  • Ductility (correlated with stretch formability, which is one mode of press formability) can be improved with specified components by controlling the area fraction of ferrite to 20.0% or more.
  • Flangeability correlated with stretch flangeability which is one mode of press formability, can be improved with specified components by controlling the area fraction of fresh martensite to 15.0% or less, the area fraction of retained austenite to 3.0% or less, and the area fraction of tempered martensite to 10.0% or more, and increasing the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain.
  • the critical spacer thickness (ST) measured in a U-bending+tight bending test simulating the deformation and fracture behavior of a vertical wall portion in a collision test, and the stroke at the maximum load (SFmax) measured in a V-bending+orthogonal VDA bending test simulating the deformation and fracture behavior of a bending ridge line portion in a collision test can be improved by controlling the area fraction of tempered martensite to 10.0% or more and increasing the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain.
  • the present disclosure is based on these findings.
  • the gist of the present disclosure is as follows:
  • a steel sheet including a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis,
  • [6] A member including the steel sheet according to any one of [1] to [5].
  • 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 [5] to at least one of forming and joining to produce a member.
  • aspects of the present invention provide a steel sheet with a tensile strength TS of 780 MPa or more, high yield stress YS and yield ratio YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision.
  • a member including a steel sheet according to aspects of the present invention as a material has high strength, high press formability, and enhanced crashworthiness, and can therefore be extremely advantageously applied to a structural member, an impact energy absorbing member, and the like of an automobile.
  • FIG. 1 is an example of a SEM image of aspects of the present invention (Inventive Example No. 13 in Examples).
  • FIG. 2 ( a ) is an explanatory view of U-bending (primary bending) in a U-bending+tight bending test in Examples.
  • FIG. 2 ( b ) is an explanatory view of tight bending (secondary bending) in a U-bending+tight bending test in Examples.
  • FIG. 3 ( a ) is an explanatory view of V-bending (primary bending) in a V-bending+orthogonal VDA bending test in Examples.
  • FIG. 3 ( b ) is an explanatory view of orthogonal VDA bending (secondary bending) in a V-bending+orthogonal VDA bending test in Examples.
  • FIG. 4 ( 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. 4 ( b ) is a perspective view of the test member illustrated in FIG. 4 ( a ) .
  • FIG. 4 ( 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: 0.030% or more and 0.250% or less, Si: 0.01% or more and 0.75% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 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, and has a steel microstructure, as a microstructure at a quarter thickness position of the base steel sheet, in which the area fraction of ferrite: 20.0% or more and 80.0% or less, an area fraction of fresh martensite: 15.0% or less, an area fraction of retained austenite: 3.0% or less, a value obtained by dividing a total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain
  • 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 an appropriate amount of tempered martensite, bainite, tempered bainite, or the like to ensure a TS of 780 MPa or more and high YS and YR.
  • a C content of less than 0.030% results in an increase in the area fraction of ferrite and makes it difficult to achieve a TS of 780 MPa or more. This also reduces YS and YR.
  • a C content of more than 0.250% results in an increase in the area fraction of fresh martensite, excessively high TS, and lower El. This also results in an increase in the area fraction of fresh martensite, lower bendability in a V-bending test, and undesired R/t (press formability).
  • the C content is 0.030% or more and 0.250% or less.
  • the C content is preferably 0.050% or more.
  • the C content is preferably 0.130% or less.
  • Si promotes ferrite transformation during annealing and in a cooling process after annealing.
  • Si is an element that affects the area fraction of ferrite.
  • a Si content of less than 0.01% results in a decrease in the area fraction of ferrite and lower ductility.
  • a Si content of more than 0.75% results in an increase in the volume fraction of retained austenite, the formation of hard fresh martensite by deformation-induced transformation of retained austenite when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, results in void formation and crack growth in a subsequent test, and results in undesired ⁇ , ST, and SFmax.
  • the Si content is 0.01% or more and 0.75% or less.
  • the Si content is preferably 0.10% or more.
  • the Si content is preferably 0.70% or less.
  • Mn 2.00% or More and Less than 3.50%
  • 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 void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax may not be achieved.
  • the V content is preferably 0.200% or less.
  • the V content is more preferably 0.060% or less.
  • 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.
  • the internal crack may act as a starting point of a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax may not be achieved.
  • the B content is preferably 0.0100% or less.
  • the B content is more preferably 0.0050% or less.
  • Cr is an element that enhances hardenability, and the addition of Cr forms an appropriate amount of tempered martensite and increases TS, YS, and YR. To produce such effects, 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.
  • a Cr content of more than 1.000% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired ⁇ and R/t.
  • 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 increases TS, YS, and YR. 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.
  • Ni content of more than 1.000% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired ⁇ and R/t.
  • 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 increases TS, YS, and YR. To produce such effects, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more.
  • a Mo content of more than 1.000% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired ⁇ and R/t.
  • 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 may make it difficult to achieve a TS of 780 MPa or more. This may also reduce 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 ⁇ , R/t, ST, and SFmax.
  • 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 may make it difficult to achieve a TS of 780 MPa or more. This may also reduce 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 ⁇ , R/t, ST, and SFmax.
  • 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 increases TS, YS, and YR. To produce such effects, 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.
  • a Cu content of more than 1.000% may result in an excessive increase in the area fraction of fresh martensite. Furthermore, a large number of coarse precipitates and inclusions may be formed. In such a case, excessively formed fresh martensite or a coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax may not be achieved.
  • the Cu content is preferably 1.000% or less.
  • the Cu content is more preferably 0.200% or less.
  • Ta forms fine carbide, nitride, or carbonitride during hot rolling or annealing and increases TS, YS, and YR. 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. To produce such effects, 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.
  • an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax 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 increases TS, YS, and YR. To produce such effects, the W content is preferably 0.001% or more. The W content is more preferably 0.030% or more.
  • a W content of more than 0.500% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired ⁇ and R/t.
  • 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 and bendability 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.
  • an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax 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 and bendability 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.
  • an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax 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 and bendability 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.
  • an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax 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 and bendability of a steel sheet.
  • the Zr content is preferably 0.0010% or more.
  • an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, and a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax may not be achieved.
  • the Zr content is preferably 0.1000% or less.
  • the Zr content is more preferably 0.0300% or less, even more preferably 0.0100% or less.
  • Ca is present as an inclusion in steel.
  • a Ca content of more than 0.0200% may result in a large number of coarse inclusions.
  • an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax may not be achieved.
  • the Ca content is preferably 0.0200% or less.
  • the Ca content is preferably 0.0020% or less.
  • the Ca content is more preferably 0.0019% or less, even more preferably 0.0018% or less.
  • the Ca content may have any lower limit but is preferably 0.0005% or more. Due to constraints on production technology, the Ca content is more preferably 0.0010% or more.
  • Se 0.0200% or less
  • Te 0.0200% or less
  • Ge 0.0200% or less
  • Sr 0.0200% or less
  • Cs 0.0200% or less
  • Hf 0.0200% or less
  • Pb 0.0200% or less
  • Bi 0.0200% or less
  • REM 0.0200% or less
  • Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements effective in improving the flangeability and bendability 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 void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired ⁇ , R/t, ST, and SFmax 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.
  • REM scandium
  • Y yttrium
  • Lu lutetium
  • REM concentration refers to the total content of one or two or more elements selected from the REM.
  • Average grain size of island-like fresh martensite and island-like retained austenite in ferrite grain 2.0 ⁇ m or less
  • the average grain size of island-like fresh martensite and island-like retained austenite (M′+RA′) in a ferrite grain is 2.0 ⁇ m or less.
  • the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain is preferably 1.0 ⁇ m or less.
  • the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain is preferably 0.1 ⁇ m or more, more preferably 0.2 ⁇ m or more.
  • a sample is cut out to form a thickness cross section (L cross section) parallel to the rolling direction of a steel sheet as an observation surface.
  • the observation surface of the sample is then polished with a diamond paste and is then subjected to final polishing with alumina.
  • the observation surface of the sample is then etched with 3% by volume nital to expose the microstructure.
  • the steel sheet is then observed at a quarter thickness position using a SEM at a magnification of 3000 times in five visual fields.
  • Ferrite a massive black region. Almost no carbide is contained. The area fraction of ferrite does not include isolated island-like fresh martensite and isolated island-like retained austenite in a ferrite grain.
  • Bainite and tempered bainite a black to dark gray region of a massive form, an indefinite form, or the like. A relatively small number of carbide particles are contained.
  • Tempered martensite a gray region of an indefinite form. A relatively large number of carbide particles are contained.
  • Hard second phase (retained austenite+fresh martensite): a white to light gray region of an indefinite form. No carbide is contained.
  • Carbide a dotted or linear white region. It is contained in bainite, tempered bainite, and tempered martensite.
  • Remaining microstructure the pearlite, cementite, and the like of known forms.
  • the total area of isolated island-like fresh martensite and isolated island-like retained austenite in a ferrite grain is divided by the number of isolated island-like fresh martensite grains and isolated island-like retained austenite grains in the ferrite grain to obtain an average area, and the average area is divided by the circumference ratio n, and the square root thereof is multiplied by 2 to obtain an equivalent circular diameter as the average grain size.
  • an island-like region with the outer periphery surrounded by ferrite and integrally formed without interruption is regarded as one to be measured.
  • the area fraction of retained austenite is measured as described below.
  • a base steel sheet is mechanically ground to a quarter thickness position in the thickness direction (depth direction) and is then chemically polished with oxalic acid to form an observation surface.
  • the observation surface is then observed by X-ray diffractometry.
  • a Moka radiation source is used for incident X-rays to determine the ratio of the diffraction intensity of each of (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of (200), (211), and (220) planes of bcc iron.
  • the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each plane. On the assumption that retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is defined as the area fraction of the retained austenite.
  • the area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard second phase determined as described above.
  • the area fraction of the remaining microstructure is determined by subtracting the area fraction of ferrite, the area fraction of bainite and tempered bainite, the area fraction of tempered martensite, and the area fraction of the hard second phase, which are determined as described above, from 100.0%.
  • the amount of diffusible hydrogen in a steel sheet is preferably 0.25 ppm by mass or less.
  • the amount of diffusible hydrogen in a steel sheet may have any lower limit and is preferably 0.01 ppm by mass or more due to constraints on production technology.
  • a base steel sheet in which the amount of diffusible hydrogen is measured may be, in addition to a high-strength steel sheet before coating treatment, a base steel sheet of a high-strength galvanized steel sheet after galvanizing treatment and before processing. It may also be a base steel sheet of a steel sheet subjected to processing, such as punching or stretch flange forming, after galvanizing treatment, or a base portion of a product produced by welding a steel sheet after processing.
  • the amount of diffusible hydrogen in a steel sheet is measured by the following method.
  • a test specimen with a length of 30 mm and a width of 5 mm is taken and, when a galvanized layer is formed on the steel sheet, the hot-dip galvanized layer or hot-dip galvannealed layer is removed with an alkali.
  • the amount of hydrogen released from the test specimen is then measured by a temperature-programmed desorption analysis method. More specifically, the test specimen is continuously heated from room temperature ( ⁇ 5° C. to 55° C.) to 300° C. at a heating rate of 200° C./h and is then cooled to room temperature.
  • the cumulative amount of hydrogen released from the test specimen from room temperature to 210° C. is measured as the amount of diffusible hydrogen in the steel sheet.
  • the amount of diffusible hydrogen is preferably measured after the completion of the production of the steel sheet.
  • the amount of hydrogen is more preferably measured within one week after the completion of the production of the steel sheet.
  • the room temperature should be within the range of annual temperature variations at the location in consideration of global production. Typically, it preferably ranges from 10° C. to 50° C.
  • 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 surface soft layer is formed in a region of 200 ⁇ m or less from the surface of the base steel sheet in the thickness direction.
  • the region where the surface soft layer is formed is preferably 150 ⁇ m or less, more preferably 120 ⁇ m or less, from the surface of the base steel sheet in the thickness direction.
  • the lower limit of the thickness of the surface soft layer is preferably, but not limited to, 8 ⁇ m or more, more preferably more than 17 ⁇ m.
  • the surface soft layer is preferably 30 ⁇ m or more, more preferably 40 ⁇ m or more.
  • 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.
  • 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 dew point can be more than ⁇ 20° 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 ⁇ 20° 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 steel sheet according to an embodiment of the present invention has a tensile strength of 780 MPa or more.
  • the reference values of the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio ( ⁇ ), the critical spacer thickness (ST) in a U-bending+tight bending test and the stroke at the maximum load (SFmax) in a V-bending+orthogonal VDA bending test, and the presence or absence of fracture (appearance crack) in the axial compression test of a steel sheet according to an embodiment of the present invention are as described above.
  • the tensile strength (TS), the yield stress (YS), the yield ratio (YR), 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 spacer thickness (ST) is measured in a U-bending+tight bending test described later in Examples.
  • the stroke at the maximum load (SFmax) in the V-bending+orthogonal VDA bending test is measured in a V-bending+orthogonal VDA bending test described later in Examples.
  • the presence or absence of fracture (appearance crack) in the axial compression test 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 with a galvanized layer may be a galvanized 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, an aluminum coated layer, or the like) 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, an aluminum coated layer, or the like) sequentially formed on the base 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.
  • a hot-dip galvannealed layer is preferably composed of, for example, 20% 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.
  • IBIT 700BK a corrosion inhibitor for Fe
  • 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.
  • 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 aspects of the present invention includes a hot rolling step of hot-rolling a steel slab with the chemical composition described above under a condition of a finish rolling temperature of 820° C. or more to produce a hot-rolled steel sheet; a heating step of heating the steel sheet after the hot rolling step in the temperature range of 350° C. or more and 600° C. or less at an average heating rate of 7° C./s or more; an annealing step of annealing under conditions of an annealing temperature: 750° C. or more and 900° C.
  • annealing time 20 seconds or more; after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7° C./s or more from (the annealing temperature ⁇ 30° C.) to 650° C. and an average cooling rate of 14° C./s or less from 650° C. to 500° C.; after the first cooling step, a second cooling step of applying a tension of 2.0 kgf/mm 2 or more to the steel sheet in the temperature range of 300° C. or more and 450° C.
  • a steel material can be melted by any method, for example, by a known melting method using a converter, an electric arc furnace, or the like.
  • a steel slab is preferably produced by continuous casting but may also be produced by ingot casting, thin slab casting, or the like. After a steel slab is produced, the steel slab may be temporarily cooled to room temperature and then reheated by a known method. Alternatively, without being cooled to room temperature, a steel slab may be subjected without problems to an energy-saving process, such as hot charge rolling or hot direct rolling, in which a hot slab is charged directly into a furnace or is immediately rolled after slight heat retention.
  • the slab heating temperature is preferably 1100° C. or more from the perspective of melting carbide and reducing rolling force.
  • the slab heating temperature is preferably 1300° C. or less to prevent an increase in scale loss.
  • the slab heating temperature is the surface temperature of the slab.
  • a slab is formed into a sheet bar by rough rolling under typical conditions.
  • the sheet bar is preferably heated with a bar heater or the like before finish rolling.
  • Finish rolling reduces the ductility, flangeability, and bendability of the final material as a result of an increase in rolling load or an increase in rolling reduction in an unrecrystallized state of austenite and development of an abnormal microstructure elongated in the rolling direction.
  • the finish rolling temperature is 820° C. or more.
  • the finish rolling temperature is preferably 830° C. or more, more preferably 850° C. or more.
  • the finish rolling temperature is preferably 1080° C. or less, more preferably 1050° C. or less.
  • the coiling temperature after hot rolling is not particularly limited, but it is necessary to consider the case where the ductility, flangeability, and bendability of the final material degrade.
  • the coiling temperature after hot rolling is preferably 300° C. or more.
  • the coiling temperature after hot rolling is preferably 700° C. or less.
  • Rough-rolled sheets may be joined together during hot rolling to continuously perform finish rolling.
  • a rough-rolled sheet may be temporarily coiled.
  • 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.
  • the friction coefficient in the rolling with lubrication is preferably 0.25 or less.
  • a metal electroplating treatment may be performed on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet thus formed to produce a metal electroplated steel sheet before annealing in which a metal electroplated layer before annealing is formed on at least one surface thereof.
  • metal coating excludes galvanizing (second coating).
  • the metal species of the electroplated layer may be any of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Pt, Au, Hg, Ti, Pb, and Bi and is preferably Fe.
  • Fe-based electroplating is described below as an example, the following conditions for the Fe-based electroplating can also be applied to another metal electroplating.
  • 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 average heating rate in the temperature range of 350° C. or more and 600° C. or less is preferably 100° C./s or less, more preferably 90° C./s or less.
  • the annealing temperature is 750° C. or more and 900° C. or less.
  • the annealing temperature is preferably 880° C. or less.
  • the annealing temperature is the highest temperature reached in the annealing step.
  • 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 TS, YS, and YR cannot be obtained.
  • the annealing time is 20 seconds or more.
  • the annealing time is preferably 30 seconds or more, more preferably 50 seconds or more.
  • 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 number of annealing processes may be two or more but is preferably one from the perspective of energy efficiency.
  • the dew point of the atmosphere in the annealing step is preferably ⁇ 30° C. or more.
  • 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 in the annealing atmosphere in the annealing step is more preferably ⁇ 25° C. or more, even more preferably more than ⁇ 20° C., even further more preferably ⁇ 15° C. or more, most preferably ⁇ 5° C. or more.
  • aspects of the present invention include, after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7° C./s or more from (the annealing temperature ⁇ 30° C.) to 650° C. and an average cooling rate of 14° C./s or less from 650° C. to 500° C.
  • the average cooling rate from (annealing temperature ⁇ 30° C.) to 650° C. is 7° C./s or more.
  • the average cooling rate from (annealing temperature ⁇ 30° C.) to 650° C. is preferably 9° C./s or more.
  • slow cooling in an intermediate-temperature region of 650° C. or less causes fine austenite at a ferrite grain boundary, after coalescence of adjacent ferrite with similar orientation resulting in one ferrite grain, to form isolated fine island-like austenite left in the ferrite grain, and finally an increase in the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in the ferrite grain.
  • the average cooling rate from 650° C. to 500° C. (first cooling stop temperature) is 14° C./s or less, preferably 12° C./s or less.
  • the average cooling rate from 650° C. to 500° C. is preferably 1° C./s or more, more preferably 2° C./s or more.
  • the average cooling rate (° C./s) is calculated by (650 (° C.) ⁇ 500 (° C.))/cooling time (s).
  • 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.
  • a hot-dip 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.
  • an alloying temperature of more than 600° C. results in transformation of non-transformed austenite into pearlite and makes it difficult to achieve a TS of 780 MPa or more.
  • the alloying temperature is more preferably 500° C. or more, even 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.
  • aspects of the present invention include, after the first cooling step (after a galvanizing step when the galvanizing step is performed), a second cooling step of applying a tension of 2.0 kgf/mm 2 or more to a steel sheet in the temperature range of 300° C. or more and 450° C. or less, subjecting the steel sheet to five 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 then cooling the steel sheet to a cooling stop temperature (second cooling stop temperature) of 250° C. or less.
  • a second cooling stop temperature of 250° C. or less.
  • applying a tension of 2.0 kgf/mm 2 or more to a steel sheet once or more can transform most of austenite into martensite by deformation-induced transformation (stress-strain-induced transformation), and subsequent tempering in the reheating step can reduce the area fraction of fresh martensite in the final microstructure and ensure an appropriate amount of tempered martensite. This can also reduce the amount of austenite immediately after the second cooling step and reduce the volume fraction of retained austenite in the final microstructure. Consequently, desired ⁇ , R/t, ST, and SFmax can be achieved.
  • 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.
  • subjecting a steel sheet to five or more passes can transform most of austenite into martensite by deformation-induced transformation (stress-strain-induced transformation), and subsequent tempering in the reheating step can reduce the area fraction of fresh martensite in the final microstructure and ensure an appropriate amount of tempered martensite. This can also reduce the amount of austenite immediately after the second cooling step and reduce the volume fraction of retained austenite in the final microstructure. Consequently, desired ⁇ , R/t, ST, and SFmax can be achieved.
  • the number of passes is preferably six or more passes, more preferably seven or more passes.
  • the steel sheet has a TS of 780 MPa or more, high YS and YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision.
  • 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 cold-rolled steel sheets or hot-rolled steel sheets were subjected to the treatments in the heating step, the annealing step, the first cooling step, the galvanizing step, the second cooling step, and the reheating step under the conditions shown in Table 2 to produce steel sheets (galvanized steel sheets).
  • Treatments in the heating step, the first coating step (metal coating step), the annealing step, the first cooling step, the second coating step (galvanizing step), the second cooling step, and the reheating step were performed under the conditions shown in Table 4 to produce steel sheets (galvanized steel sheets).
  • Treatments in the heating step, the first coating step (metal coating step), the annealing step, the first cooling step, the second cooling step, and the reheating step were performed under the conditions shown in Table 6 to produce steel sheets.
  • 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 hot-dip galvannealed steel sheet (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheet
  • GA hot-dip galvannealed steel sheet
  • the type in the coating step is also denoted by “GI” and “GA”.
  • no alloying treatment was performed, and the alloying temperature is indicated by “-”.
  • no galvanizing treatment was performed, and the results are indicated as CR (cold-rolled steel sheet (without coating)) or HR (hot-rolled steel sheet (without coating)).
  • 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 hot-dip 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.
  • 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 V-bending test, a U-bending+tight bending test, a V-bending+orthogonal VDA bending test, and an axial compression test were performed in the manner described below.
  • the tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio ( ⁇ ), R/t in the V-bending test, the critical spacer thickness (ST) in the U-bending+tight bending bending test, the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test, and the presence or absence of fracture (appearance crack) in the axial compression test 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, YR, and El of the test specimen were measured at a crosshead speed of 10 mm/min in the tensile test.
  • Tables 3, 5, and 7 show the results.
  • ⁇ ⁇ ( % ) ⁇ ( D f - D 0 ) / D 0 ⁇ ⁇ 100
  • a 100 mm ⁇ 35 mm test specimen was taken from the steel sheet by shearing and end grinding. The sides of 100 mm are parallel to the width (C) direction.
  • R/t was calculated by dividing the minimum bending radius (critical bending radius) R with no crack by the sheet thickness t.
  • a cleavage with a length of 200 ⁇ m or more was determined as a crack using a stereomicroscope manufactured by Leica at a magnification of 25 times.
  • a TS of 780 MPa or more and less than 980 MPa 2.0>R/t was determined to be good, and at a TS of 980 MPa or more, 2.5>R/t was determined to be good.
  • the U-bending+tight bending test was performed three times, and the critical spacer thickness (ST) without cracking in any of the three tests was determined.
  • ST critical spacer thickness
  • a cleavage with a length of 200 ⁇ m or more was determined as a crack using a stereomicroscope manufactured by Leica at a magnification of 25 times.
  • ST is a measure for evaluating fracture resistance characteristics (fracture resistance characteristics of a vertical wall portion in the axial compression test) in case of a collision. Tables 3, 5, and 7 show the results.
  • V-bending+orthogonal VDA bending test is performed as described below.
  • a 60 mm ⁇ 65 mm test specimen was taken from the steel sheet by shearing and end grinding.
  • the sides of 60 mm are parallel to the rolling (L) direction.
  • 90-degree bending (primary 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 3 was pressed against a steel sheet on a die A 3 with a V-groove to prepare a test specimen T 1 .
  • V-bending conditions in the V-bending+orthogonal VDA bending test are as follows:
  • VDA bending conditions in the V-bending+orthogonal VDA bending test are as follows:
  • the stroke at the maximum load was determined in a stroke-load curve of the VDA bending.
  • the average value of the stroke at the maximum load when the V-bending+orthogonal VDA bending test was performed three times was defined as SFmax (mm).
  • SFmax is a measure for evaluating fracture resistance characteristics (fracture resistance characteristics of a bending ridge line portion in the axial compression test) in case of a collision. Tables 3, 5, and 7 show the results.
  • the U-bending+tight bending test, the V-bending+orthogonal 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 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 comparative examples were not satisfactory in at least one of the tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio ( ⁇ ), R/t in the V-bending test, the critical spacer thickness (ST) in the U-bending+tight bending bending test, the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test, and the presence or absence of fracture (appearance crack) in the axial compression test.
  • TS tensile strength
  • YiS yield stress
  • YR yield ratio
  • El total elongation
  • ⁇ limiting hole expansion ratio
  • R/t in the V-bending test
  • the critical spacer thickness (ST) in the U-bending+tight bending bending test the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test
  • SFmax stroke at the maximum load measured in the V-bending+orthogonal VDA

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