Classifications

• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
  • C23C2/29—Cooling or quenching
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
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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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high-strength steel sheet and a method for producing the same.
• the weight of the vehicle body is reduced while maintaining the strength of the vehicle body.
• a high-strength steel sheet having a tensile strength (TS) of 1320 MPa or more as a frame component around a cabin of the vehicle body.
• the ductility of the steel sheet tends to decrease as the strength of the steel sheet increases. In this case, the formability of the steel sheet becomes insufficient, and it is difficult to press the steel sheet into a complicated shape.
• Patent Literatures 1 and 2 disclose a technique for achieving both strength and ductility of a steel sheet.
• delayed fracture may occur.
• the delayed fracture is a phenomenon in which, when a component to which stress is applied is placed in a hydrogen intrusion environment, hydrogen intrudes into the component to reduce an interatomic bonding force or to cause local deformation, so that a microcrack is generated, and the component is broken as the microcrack develops.
• the high-strength steel sheet is required to have not only sufficient formability (ductility and hole expandability) but also favorable delayed fracture resistance characteristics.
• aspects of the present invention have been made in view of the above points, and an object thereof is to provide a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent in formability (ductility and hole expandability) and delayed fracture resistance characteristics.
• the present inventors have conducted intensive studies, and as a result, have found that the above object is achieved by adopting the following configuration, thereby completing aspects of the present invention.
• aspects of the present invention include the following [1] to [8].
• a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent in formability (ductility and hole expandability) and delayed fracture resistance characteristics.
• a high-strength steel sheet according to aspects of the present invention has a component composition and a microstructure described below, and satisfies a diffusible hydrogen amount in steel described below.
• the “high-strength steel sheet” is also simply referred to as “steel sheet”.
• the sheet thickness of the steel sheet is not particularly limited, and is, for example, 0.3 to 3.0 mm and preferably 0.5 to 2.8 mm.
• the high strength means that a tensile strength (TS) is 1320 MPa or more.
• the high-strength steel sheet according to aspects of the present invention has a tensile strength of 1320 MPa or more, and is also excellent in formability (ductility and hole expandability) and delayed fracture resistance characteristics. Therefore, the high-strength steel sheet according to aspects of the present invention has a very high utility value in industrial fields such as automobiles and electric equipment, and is particularly extremely useful for weight reduction of a frame component of a vehicle body of an automobile.
• component composition of the high-strength steel sheet according to aspects of the present invention (hereinafter, also referred to as “component composition according to aspects of the present invention” for convenience) will be described.
• % in the component composition according to aspects of the present invention means “mass %” unless otherwise specified.
• C increases the strength of tempered martensite, bainite, and fresh martensite.
• C also improves the stability of retained austenite and improves the ductility of the steel sheet.
• the amount of C is 0.130% or more, preferably 0.150% or more, more preferably 0.160% or more, and still more preferably 0.170% or more.
• the amount of C is 0.350% or less, preferably 0.330% or less, and more preferably 0.310% or less.
• Si suppresses the formation of a carbide, so that a decrease in hole expandability due to a difference in hardness between the carbide and each structure is suppressed. Si provides stable retained austenite and ensures favorable ductility.
• the amount of Si is 0.50% or more, preferably 0.55% or more, and more preferably 0.60% or more.
• the amount of Si is 2.50% or less, preferably 2.30% or less, and more preferably 2.00% or less.
• Mn forms a microstructure mainly including tempered martensite and bainite, thereby suppressing a difference in hardness between respective structures and improving the hole expandability.
• Mn is an element contributing to the stabilization of retained austenite and is effective for ensuring favorable ductility.
• the amount of Mn is 2.00% or more, preferably 2.20% or more, and more preferably 2.50% or more.
• the amount of Mn is 4.00% or less, preferably 3.70% or less, and more preferably 3.50% or less.
• the amount of P is 0.100% or less, preferably 0.070% or less, more preferably 0.050% or less, still more preferably 0.030% or less, and particularly preferably 0.010% or less.
• the amount of S is 0.0500% or less, preferably 0.0100% or less, and more preferably 0.0050% or less.
• Al acts as a deoxidizer to reduce an inclusion in the steel sheet. Therefore, the amount of Al is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more.
• the amount of Al is 2.000% or less, preferably 1.500% or less, more preferably 1.000% or less, still more preferably 0.500% or less, and particularly preferably 0.100% or less.
• the amount of N is preferably smaller. Specifically, the amount of N is 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.
• Ti, Nb, and V contribute to precipitation strengthening, and are thus effective in increasing the strength of the steel sheet.
• Ti, Nb, and V make the delayed fracture resistance characteristics better by making the grain size of prior austenite grains finer, accordingly making tempered martensite and bainite finer, or forming a fine precipitate (precipitate A S described below) that becomes a trap site of hydrogen.
• the amount of Ti, the amount of Nb, and the amount of V are each 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.
• the amount of Ti and the amount of Nb are each 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.
• the amount of V is 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.
• the component composition according to aspects of the present invention may further include, by mass %, at least one element selected from the group consisting of elements described below.
• W improves the hardenability of the steel sheet. W makes the delayed fracture resistance characteristics better by generating a fine carbide containing W and becoming a trap site of hydrogen, or making tempered martensite and bainite finer.
• the amount of W is preferably 0.500% or less, more preferably 0.300% or less, and still more preferably 0.150% or less.
• the lower limit of the amount of W is not particularly limited, but is, for example, 0.010% and preferably 0.050% from the viewpoint of obtaining the addition effect of W.
• B is effective for improving hardenability.
• B forms a microstructure mainly including tempered martensite and bainite and prevents deterioration of hole expandability.
• the amount of B is preferably 0.0100% or less, more preferably 0.0070% or less, and still more preferably 0.0050% or less.
• the lower limit of the amount of B is not particularly limited, but is, for example, 0.0005% and preferably 0.0010% from the viewpoint of obtaining the addition effect of B.
• Ni is an element stabilizing retained austenite and is effective for ensuring favorable ductility. Ni increases the strength of the steel by solid-solution strengthening.
• the amount of Ni is preferably 2.000% or less, more preferably 1.000% or less, and still more preferably 0.500% or less.
• the lower limit of the amount of Ni is not particularly limited, but is, for example, 0.010% and preferably 0.050% from the viewpoint of obtaining the addition effect of Ni.
• Co is an element effective for improving hardenability, and is effective for strengthening the steel sheet.
• the amount of Co is preferably 2.000% or less, more preferably 1.000% or less, and still more preferably 0.500% or less.
• the lower limit of the amount of Co is not particularly limited, but is, for example, 0.010% and preferably 0.050% from the viewpoint of obtaining the addition effect of Co.
• the amount of Cr is preferably 1.000% or less, more preferably 0.800% or less, and still more preferably 0.500% or less.
• the lower limit of the amount of Cr is not particularly limited, but is, for example, 0.030% and preferably 0.050% from the viewpoint of obtaining the addition effect of Cr.
• Mo improves the balance between strength and ductility. Mo makes the delayed fracture resistance characteristics better by generating a fine carbide containing Mo and becoming a trap site of hydrogen, or making tempered martensite and bainite finer.
• the amount of Mo is preferably 1.000% or less, more preferably 0.800% or less, and still more preferably 0.500% or less.
• the lower limit of the amount of Mo is not particularly limited, but is, for example, 0.010% and preferably 0.050% from the viewpoint of obtaining the addition effect of Mo.
• Cu is an element effective for strengthening steel. Cu suppresses intrusion of hydrogen into the steel sheet, so that Cu is more excellent in delayed fracture resistance characteristics.
• the amount of Cu is preferably 1.000% or less, more preferably 0.500% or less, and still more preferably 0.200% or less.
• the lower limit of the amount of Cu is not particularly limited, but is, for example, 0.010% and preferably 0.050% from the viewpoint of obtaining the addition effect of Cu.
• Sn and Sb suppress decarburization of a surface layer region of the steel sheet (a region having a depth of about several tens ⁇ m from the surface of the steel sheet) caused by nitriding or oxidation of the surface of the steel sheet, and prevent a decrease in the area fraction of tempered martensite on the surface of the steel sheet.
• the amount of Sn and the amount of Sb are each preferably 0.500% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.
• the lower limits of the amount of Sn and the amount of Sb are not particularly limited, but are each, for example, 0.001% and preferably 0.003% from the viewpoint of obtaining the addition effect of Sn and Sb.
• Ta generates an alloy carbide or an alloy carbonitride and contributes to high strength. Ta is partially dissolved in Nb carbide or Nb carbonitride to form a composite precipitate such as (Nb, Ta) (C, N), thereby significantly suppressing coarsening of the precipitate and stabilizing contribution to strength due to precipitation strengthening.
• the amount of Ta is preferably 0.100% or less, more preferably 0.080% or less, and still more preferably 0.070% or less.
• the lower limit of the amount of Ta is not particularly limited, but is, for example, 0.005% and preferably 0.010% from the viewpoint of obtaining the addition effect of Ta.
• Zr improves the hardenability of the steel sheet.
• Zr makes the delayed fracture resistance characteristics better by generating a fine carbide containing Zr and becoming a trap site of hydrogen, or making tempered martensite and bainite finer.
• the amount of Zr is preferably 0.200% or less, more preferably 0.150% or less, and still more preferably 0.100% or less.
• the lower limit of the amount of Zr is not particularly limited, but is, for example, 0.005% and preferably 0.010% from the viewpoint of obtaining the addition effect of Zr.
• Hf affects the distribution state of an oxide and makes the delayed fracture resistance characteristics better.
• the amount of Hf is preferably 0.020% or less, more preferably 0.015% or less, and still more preferably 0.010% or less.
• the lower limit of the amount of Hf is not particularly limited, but is, for example, 0.001% and preferably 0.003% from the viewpoint of obtaining the addition effect of Hf.
• Ca, Mg, and REM rare earth metal
• the amount of Ca, the amount of Mg, and the amount of REM are each preferably 0.0090% or less, more preferably 0.0080% or less, and still more preferably 0.0070% or less.
• the lower limits of the amount of Ca, the amount of Mg, and the amount of REM are not particularly limited, but are each, for example, 0.0005% and preferably 0.0010% from the viewpoint of obtaining the addition effect of Ca, Mg, and REM.
• the balance in the component composition according to aspects of the present invention consists of Fe and inevitable impurities.
• microstructure according to aspects of the present invention (hereinafter, also referred to as “microstructure according to aspects of the present invention” for convenience) will be described.
• the area fraction is an area fraction with respect to the entire microstructure.
• the area fraction of each structure is determined by the method described in EXAMPLES described below.
• Tempered martensite and bainite contribute to tensile strength.
• tempered martensite and bainite By mainly including tempered martensite and bainite, it is effective for enhancing hole expandability while maintaining high strength.
• the total area fraction of bainite and tempered martensite is 70.0% or more, preferably 72.0% or more, and more preferably 74.0% or more.
• the total area fraction of bainite and tempered martensite is 95.0% or less, preferably 93.0% or less, and more preferably 90.0% or less.
• Fresh martensite causes a large difference in hardness between tempered martensite and bainite, thus reducing the hole expandability during punching due to the difference in hardness. Therefore, it is necessary to avoid excessive presence of fresh martensite in the steel sheet.
• the area fraction of fresh martensite is 15.0% or less, preferably 14.0% or less, and more preferably 13.0% or less.
• the lower limit is not particularly limited, but from the viewpoint of tensile strength, the area fraction of fresh martensite is preferably 1.0% or more, more preferably 3.0% or more, and still more preferably 5.0% or more.
• the area fraction of retained austenite is 5.0% or more, preferably 6.0% or more, and more preferably 7.0% or more.
• the area fraction of retained austenite is 15.0% or less, preferably 14.0% or less, more preferably 13.0% or less, and still more preferably 12.0% or less.
• the structure excluding tempered martensite, bainite, fresh martensite, and retained austenite
• the area fraction of the balance structure in the microstructure according to aspects of the present invention is preferably 5.0% or less.
• the precipitate A is a carbide, nitride, or carbonitride containing at least one element selected from the group consisting of Ti, Nb, and V.
• the average grain size of the precipitate A is 0.001 ⁇ m or more, preferably 0.005 ⁇ m or more, and more preferably 0.010 ⁇ m or more.
• the average grain size of the precipitate A is 0.050 ⁇ m or less, preferably 0.040 ⁇ m or less, more preferably less than 0.040 ⁇ m, still more preferably 0.035 ⁇ m or less, particularly preferably 0.030 ⁇ m or less, and most preferably 0.020 ⁇ m or less.
• the average grain size of the precipitate A is determined by the method described in EXAMPLES described below.
• the precipitate A S is the precipitate A having a major axis of 0.050 ⁇ m or less.
• a number density (number per unit area) N S of the precipitate A S is 10/ ⁇ m 2 or more.
• the fine precipitate A S act as a trap site of hydrogen, thereby improving the delayed fracture resistance characteristics.
• N S is preferably more than 125/ ⁇ m 2 , more preferably 200/ ⁇ m 2 or more, and still more preferably 310/ ⁇ m 2 or more.
• N S is not particularly limited. However, when the absolute amount of the fine precipitate A S increases, the rolling force increases, and it may be difficult to produce the steel sheet. Therefore, N S is preferably 1,000/ ⁇ m 2 or less and more preferably 800/ ⁇ m 2 or less.
• the precipitate A L is the precipitate A having a major axis of more than 0.050 ⁇ m.
• a ratio (N S /N L ) of the number density N S (unit: number/ ⁇ m 2 ) of the precipitate A S and a number density N L (unit: number/ ⁇ m 2 ) of the precipitate A L is 10.0 or more.
• the fine precipitate A S Since the fine precipitate A S has a small grain size, it is considered that it is difficult to accumulate strain and stress. Since the fine precipitate A S has a circular shape, the surface thereof is understood to be a curved surface, and it is considered that strain and stress easily escape along the curved surface.
• the coarse precipitate A L includes the precipitate A having a quadrangular shape, and the surface thereof is considered to be flat, and it is considered that strain and stress are more likely to be accumulated.
• an initial crack is likely to occur in the sheared end face, and the delayed fracture resistance characteristics of the sheared end face are deteriorated.
• the delayed fracture resistance characteristics of the steel sheet can be improved by reducing the abundance ratio of the coarse precipitate A L .
• N S /N L is preferably 11.0 or more, more preferably 12.0 or more, still more preferably more than 12.1, particularly preferably 12.2 or more, and most preferably 13.0 or more.
• the upper limit of N S /N L is not particularly limited, but is preferably 100.0 or less, more preferably 80.0 or less, still more preferably 50.0 or less, and particularly preferably 30.0 or less.
• Ni is preferably 50/ ⁇ m 2 or less and more preferably 35/ ⁇ m 2 or less.
• N S and N L are determined by the method described in EXAMPLES described below.
• the diffusible hydrogen amount in steel is 0.50 ppm by mass or less, preferably 0.40 ppm by mass or less, more preferably 0.30 ppm by mass or less, and still more preferably 0.25 ppm by mass or less.
• the lower limit of the diffusible hydrogen amount in steel is not particularly limited, but is, for example, 0.01 ppm by mass due to restrictions on production technology.
• the diffusible hydrogen amount in steel is determined by the method described in EXAMPLES described below.
• the high-strength steel sheet according to aspects of the present invention may include a plating layer on a surface thereof.
• the plating layer is formed by a plating treatment described below.
• the plating layer examples include a zinc plating layer (Zn plating layer) and an Al plating layer, and among them, a zinc plating layer is preferable.
• the zinc plating layer may contain elements such as Al and Mg.
• the plating layer may be a plating layer subjected to alloying (alloyed plating layer).
• a coating weight (coating weight per one surface) of the plating layer is preferably 20 g/m 2 or more, more preferably 25 g/m 2 or more, and still more preferably 30 g/m 2 or more, from the viewpoint of controlling the coating weight of the plating layer and the viewpoint of corrosion resistance.
• the coating weight of the plating layer is preferably 120 g/m 2 or less, more preferably 100 g/m 2 or less, and still more preferably 70 g/m 2 or less.
• a steel slab (steel material) having the above-described component composition according to aspects of the present invention is prepared.
• the steel slab is cast from molten steel, for example, by a known method such as a continuous casting method.
• the method for producing molten steel is not particularly limited, and a known method using a converter furnace, an electric furnace, or the like can be adopted.
• the steel slab may be cooled, for example, by being placed after casting and before being subjected to hot rolling described below.
• an average cooling rate v1 at 700 to 600° C. is preferably 5.0° C./h or more, more preferably 10.0° C./h or more, and still more preferably 15.0° C./h or more.
• An average cooling rate v2 at 600 to 500° C. is preferably 2.5° C./h or more, more preferably 5.0° C./h or more, and still more preferably 10.0° C./h or more.
• the coarse precipitate A L may be precipitated during casting.
• the average cooling rate v1 and the average cooling rate v2 satisfy the above ranges, the distribution state of the precipitate in the steel slab becomes uniform, and the coarse precipitate A L precipitated during casting is easily redissolved when the steel slab is heated in hot rolling described below. That is, the value of N S /N L tends to be large.
• the upper limit of the average cooling rate v1 is not particularly limited, but is, for example, 150.0° C./h and preferably 100.0° C./h.
• the upper limit of the average cooling rate v2 is not particularly limited, but is, for example, 200.0° C./h or less and preferably 150.0° C./h.
• the prepared steel slab is subjected to hot rolling under the conditions (heating temperature and finish rolling finishing temperature) described below to obtain a hot-rolled steel sheet.
• the steel slab is heated.
• the heating temperature of the steel slab is 1100° C. or higher, and preferably 1150° C. or higher.
• the heating temperature of the steel slab is preferably within the above range.
• the upper limit of the heating temperature of the steel slab is not particularly limited, but when the heating temperature is too high, scale loss increases as the oxidation amount increases. Therefore, the heating temperature of the steel slab is preferably 1400° C. or lower and more preferably 1350° C. or lower.
• the steel slab heated to the heating temperature described above is subjected to hot rolling including finish rolling to form a hot-rolled steel sheet.
• each structure of the hot-rolled steel sheet to be obtained may become coarse, and each structure during the subsequent heat treatment may also become coarse. In this case, for example, when cooling is stopped, it becomes difficult to stably obtain fine retained austenite that is mechanically stable and is less likely to undergo martensite transformation, and sufficient retained austenite cannot be obtained, and ductility is reduced.
• the finish rolling finishing temperature is 850° C. or higher, preferably 855° C. or higher, and more preferably 860° C. or higher.
• the crystal grain size becomes excessively coarse, and surface roughness may occur during press working.
• the finish rolling finishing temperature is 950° C. or lower, preferably 940° C. or lower, and more preferably 930° C. or lower.
• the hot-rolled steel sheet obtained by hot rolling is coiled under the conditions (coiling temperature T) described below.
• the coiling temperature T is 400° C. or higher, preferably 420° C. or higher, and more preferably 430° C. or higher.
• the coiling temperature T is 700° C. or lower, preferably 680° C. or lower, and more preferably 670° C. or lower.
• the coiling temperature T is an end face temperature of the coiled hot-rolled steel sheet (that is, coil).
• the coiled hot-rolled steel sheet (coil) is retained until cold rolling described below is performed.
• the method of controlling the thermal history of the coiled hot-rolled steel sheet (coil) is not particularly limited, and examples thereof include a method of covering the coil, and a method of applying hot air and/or cold air to the coil.
• the temperature of the coiled hot-rolled steel sheet (coil) is the temperature of the surface of the coil measured using a radiation thermometer when there is no cover, and is the temperature inside the cover measured using a thermocouple when there is a cover.
• the coil retained under the condition satisfying the above Formula 1 may be subjected to pickling as necessary before cold rolling described below.
• the pickling method may be performed according to a conventional method.
• skin pass rolling may be performed.
• the coiled hot-rolled steel sheet is retained under the condition satisfying the above Formula 1, subjected to pickling as necessary, and then subjected to cold rolling to obtain a cold-rolled steel sheet.
• a reduction ratio in the cold rolling is preferably 25% or more and more preferably 30% or more.
• the reduction ratio is preferably 75% or less and more preferably 70% or less.
• the cold-rolled steel sheet obtained by cold rolling is subjected to the heat treatment under the conditions described below.
• the cold-rolled steel sheet is held (heated) in a temperature region T1, then cooled to a cooling stop temperature T2, and then held (reheated) in a temperature region T3.
• the temperature in the temperature region T1 is 800° C. or higher, preferably 830° C. or higher, and more preferably 850° C. or higher.
• the temperature in the temperature region T1 is 950° C. or lower, preferably 940° C. or lower, and more preferably 930° C. or lower.
• the retention time in the temperature region T1 is 30 seconds or more, preferably 65 seconds or more, and more preferably 100 seconds or more.
• the upper limit of the retention time in the temperature region T1 is not particularly limited, but is, for example, 800 seconds, preferably 500 seconds, and more preferably 200 seconds.
• the cooling stop temperature T2 is 150° C. or higher, preferably 160° C. or higher, and more preferably 170° C. or higher.
• the cooling stop temperature T2 is 250° C. or lower, preferably 240° C. or lower, and more preferably 230° C. or lower.
• the temperature in the temperature region T3 is 250° C. or higher, preferably 260° C. or higher, and more preferably 270° C. or higher.
• the temperature in the temperature region T3 is 400° C. or lower, preferably 380° C. or lower, and more preferably 360° C. or lower.
• the upper limit of the retention time in the temperature region T3 is not particularly limited, but is, for example, 800 seconds, preferably 500 seconds, and more preferably 300 seconds.
• the cold-rolled steel sheet subjected to the heat treatment described above may be subjected to a plating treatment for forming a plating layer.
• the plating treatment include a hot-dip galvanizing treatment.
• a zinc plating layer is formed as the plating layer.
• the hot-dip galvanizing treatment for example, the cold-rolled steel sheet subjected to the heat treatment described above is immersed in a hot-dip galvanizing bath at 440 to 500° C. After the immersion, the coating weight of the plating layer is adjusted by gas wiping or the like.
• elements such as Al, Mg, and Si may be mixed, and further elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn may be mixed.
• the amount of Al in the hot-dip galvanizing bath is preferably 0.08 to 0.30%.
• the plating treatment may include an alloying treatment for alloying the formed plating layer.
• the zinc plating layer is alloyed at a temperature (alloying temperature) of 450 to 600° C.
• alloying temperature is too high, untransformed austenite transforms into pearlite, and the area fraction of retained austenite becomes too small.
• the concentration of Fe in the alloyed zinc plating layer is preferably 8 to 17 mass %.
• the cold-rolled steel sheet subjected to the heat treatment and the plating treatment corresponds to the high-strength steel sheet according to aspects of the present invention.
• the cold-rolled steel sheet subjected to the heat treatment corresponds to the high-strength steel sheet according to aspects of the present invention.
• Molten steel having the component composition shown in Table 1 below and the balance consisting of Fe and inevitable impurities was produced by a converter furnace and a steel slab was obtained by a continuous casting method.
• the obtained steel slab was cooled under the conditions shown in Table 2 below.
• the cooled steel slab was subjected to hot rolling, coiling, retention, cold rolling, and the heat treatment under the conditions shown in Table 2 below to obtain a cold-rolled steel sheet (CR) having a sheet thickness of 1.4 mm.
• the reduction ratio of the cold rolling was set to 50%.
• Some cold-rolled steel sheets were subjected to a hot-dip galvanizing treatment to form zinc plating layers on both surfaces, thereby obtaining hot-dip galvanized steel sheets (GI).
• the coating weight (coating weight per one surface) of the zinc plating layer was set to 45 g/m 2 .
• Some hot-dip galvanized steel sheets were subjected to an alloying treatment to alloy the formed zinc plating layer, thereby obtaining a galvannealed steel sheet (GA).
• the concentration of Fe in the alloyed zinc plating layer was adjusted to fall within a range of 9 to 12 mass %.
• a hot-dip galvanizing bath having an amount of Al of 0.19 mass % was used.
• a hot-dip galvanizing bath having an amount of Al of 0.14 mass % was used. The bath temperature was set to 465° C. in both cases.
• the obtained steel sheet was polished such that a cross section (L cross section) at a position of 1 ⁇ 4 of the sheet thickness parallel to the rolling direction (a position corresponding to 1 ⁇ 4 of the sheet thickness in the depth direction from the surface of the steel sheet) was an observation surface, and an observation sample was prepared.
• TM tempered martensite
• B bainite
• FM fresh martensite
• ⁇ R retained austenite
• the observation surface of the observation sample was corroded using nital, and then an SEM image was obtained by observing 10 fields of view at a magnification of 2000 times using a scanning electron microscope (SEM).
• SEM scanning electron microscope
• the area fraction (unit: %) of each structure was determined.
• the average area fraction of the 10 fields of view was taken as the area fraction of each structure.
• the light gray region was determined as fresh martensite, and the dark gray region where a carbide was precipitated was determined as tempered martensite and bainite.
• the area fraction of fresh martensite was set to a value obtained by subtracting the area fraction of retained austenite obtained by the method described below from the area fraction of the light gray region.
• the area fraction (unit: %) of retained austenite was determined by an X-ray diffraction method.
• the observation surface of the observation sample was polished by 0.1 mm in the sheet thickness direction and further polished by 0.1 mm by chemical polishing to obtain a polished surface.
• the integrated intensity of diffraction peaks of each of planes of ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ of fcc iron and each of planes of ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ of bcc iron was measured using CoK ⁇ ray.
• a ratio (integrated intensity) of the integrated intensity of each of planes of ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ of fcc iron to the integrated intensity of each of planes of ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ of bcc iron was determined.
• a replica sample was collected from the observation surface of the observation sample by a replica method.
• EDS Energy dispersive X-ray spectroscopy
• a precipitate containing at least one selected from the group consisting of Ti, Nb, and V was identified as the precipitate A.
• the circle equivalent diameter of each precipitate identified as the precipitate A was determined, and the average value for 10 fields of view was taken as the average grain size (unit: ⁇ m) of the precipitate A.
• the major axis of the precipitate A was measured.
• the longest length through the particles was measured, and this was taken as the major axis of the precipitate A.
• the number of precipitates A (that is, the precipitates A S ) having a major axis of 0.050 ⁇ m or less was measured, and the measured number was divided by the area of 10 fields of view to obtain the number density N S (unit: number/ ⁇ m 2 ) of the precipitate A S .
• the number of precipitates A (that is, the precipitates A L ) having a major axis of more than 0.050 ⁇ m was measured, and the measured number was divided by the area of 10 fields of view to obtain the number density N L (unit: number/ ⁇ m 2 ) of the precipitate A L .
• N S /N L The ratio (N S /N L ) of N S and N L was determined.
• a test piece having a size of 5 mm ⁇ 30 mm was cut out from the obtained steel sheet.
• a plating layer (zinc plating layer or alloyed zinc plating layer) was formed, the plating layer was removed using a router (precision grinder).
• the test piece was placed in a quartz tube, and the inside of the quartz tube was replaced with argon gas (Ar). Thereafter, the temperature in the quartz tube was raised to 400° C. at a rate of 200° C./hr, and the amount of hydrogen generated from the inside of the quartz tube during the temperature rise was measured by a temperature rising analysis method using a gas chromatograph.
• Ar argon gas
• the obtained steel sheet was evaluated by the following test. The results are shown in Table 3 below.
• a JIS No. 5 test piece in which a direction perpendicular to the rolling direction was a tensile direction was collected.
• a tensile test was performed in accordance with JIS Z 2241 (2011) to measure the tensile strength (TS) and the total elongation (EL).
• the obtained steel sheet was subjected to a hole expansion test in accordance with JIS Z 2256 (2010).
• the obtained steel sheet was cut to collect a test piece having a size of 100 mm ⁇ 100 mm.
• a hole having a diameter of 10 mm was punched into the collected test piece with a clearance of 12 ⁇ 1%.
• a conical punch having an apex angle of 60° was pushed into the hole in a state of being pressed at a wrinkle pressing force of 9 ton, and a hole diameter D f (unit: mm) at a crack generation limit was measured.
• D f unit: mm
• the hole expansion ratio ⁇ (unit: %) was determined from the following formula. When ⁇ was 25% or more, it was evaluated that the hole expandability was excellent.
• a test piece was collected from the obtained steel sheet.
• the plating layer was dissolved and removed using diluted hydrochloric acid, stored (dehydrogenated) at room temperature for 1 day, and then a test piece was collected.
• the length of the long side (the length in a direction perpendicular to the rolling direction) was set to 100 mm, and the length of the short side (the length in the rolling direction) was set to 30 mm.
• the end face on the long side was defined as an evaluation end face
• the end face on the short side was defined as a non-evaluation end face
• Cutting of the evaluation end face was performed by shearing.
• the clearance for shearing was 10%, and the rake angle was 0.5 degrees.
• the evaluation end face was in a state of being subjected to shearing. That is, machining for removing burrs was not performed. On the other hand, machining for removing burrs was performed on the non-evaluation end face.
• Such a test piece was subjected to bending.
• the bending was performed under the condition that a ratio (R/t) of a bending radius R and a sheet thickness t of the test piece was 4.0, and a bending angle was 90 degrees (V-shaped bending).
• a punch having a tip radius of 8.0 mm was used. More specifically, a punch having the above-described tip radius and having a U-shape (the tip portion has a semicircular shape, and the thickness of the body portion is 2R) was used.
• a die having a corner bending radius of 30 mm was used for the bending.
• a bent portion having a bending angle of 90 degrees was formed on the test piece by adjusting a depth at which the punch pushes the test piece.
• test piece on which the bent portion was formed was sandwiched and clamped using a hydraulic jack, and bolted in a state where the following residual stress S1, S2, or S3 was loaded on the outermost layer of the bent portion.
• test pieces The number of test pieces was two for each of the loaded residual stresses S1, S2, and S3.
• the necessary clamping degree was calculated by CAE (Computer Aided Engineering) analysis.
• Bolting was performed in advance by passing a bolt through an elliptical (minor axis: 10 mm, major axis: 15 mm) hole provided 10 mm inside from the non-evaluation end face of the test piece.
• test piece after bolting was immersed in hydrochloric acid (aqueous hydrogen chloride solution) having a pH of 4, and the pH was controlled to be constant under the condition of 25° C.
• hydrochloric acid aqueous hydrogen chloride solution
• the amount of hydrochloric acid was 1 L or more per test piece.
• microcracks After a lapse of 48 hours from the immersion, the presence or absence of visible (having a length of about 1 mm) microcracks was confirmed for the test piece in hydrochloric acid. This microcrack indicates the initial state of the delayed fracture.
• the steel sheet of No. 44 (steel symbol: T) satisfies all of the above (i) to (v), but the amount of C is slightly small, so that the evaluation result of delayed fracture resistance characteristics is estimated to be “Good”.

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