Classifications

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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
  • C23C2/34—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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  • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • 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
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • 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
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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  • 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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  • 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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  • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
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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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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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
  • C23C2/04—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
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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
  • C23C2/04—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
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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
• • 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
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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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
  • C21D2211/003—Cementite
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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
  • C21D2211/004—Dispersions; Precipitations
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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
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite
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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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  • 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
  • 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

Definitions

• the present invention relates to a steel sheet.
• steel sheets to be applied to vehicle components are formed into component shapes, and, normally, the formability deteriorates as the strengths of the steel sheets increase. Therefore, there is a strong desire for steel sheets to be applied to vehicle components to have both a high strength and excellent formability.
• stretch flanging (hole expansion) or bending is often used, and thus the steel sheets need to have a high strength and to be excellent in terms of elongation, stretch flangeability and bending workability.
• a dual-phase steel sheet (hereinafter, DP steel) composed of a composite structure of soft ferrite and hard martensite is known.
• DP steel a dual-phase steel sheet
• the DP steel sheet is excellent in terms of elongation, but cracks occur in some cases due to the formation of voids in the interface between ferrite and martensite, which have significantly different hardness, and thus there is a case where the DP steel sheet is poor in terms of stretch flangeability or bending workability.
• Patent Document 2 proposes a high strength hot-rolled steel sheet that is obtained by setting the cooling rate in a temperature range from the solidification of a slab to 1300°C to 10 to 300 °C/min and, after finish rolling, coiling the slab at 500°C or higher and 700°C or lower and has a steel structure composed of a ferrite single phase and a tensile strength of 1180 MPa or more.
• Patent Document 2 discloses that the high strength hot-rolled steel sheet is excellent in terms of the bending workability.
• the high strength hot-rolled steel sheet described in Patent Document 2 is manufactured by reheating a slab without cooling the slab to lower than 900°C where ferrite begins to be formed and hot-rolling the slab. Therefore, there is a problem in that segregation formed during solidification is not sufficiently reduced and there is a case where the bending workability is not stable.
• the stretch flangeability is not taken into account.
• Patent Document 3 proposes a method for manufacturing a steel sheet having a ferrite area fraction of 80% or more and a tensile strength of 980 MPa or more by completing hot rolling within five hours after continuous casting to form a solid solution of Ti exceeding the solubility in ⁇ and precipitating fine TiC together with ferritic transformation during coiling at 550°C or higher and 700°C or lower and a high strength hot-rolled steel sheet that is obtained by the manufacturing method.
• Patent Document 3 since continuous casting through the completion of hot finish rolling is performed in an austenite region to suppress the precipitation of coarse TiC, there has been a case where the bending workability deteriorates due to Mn segregation.
• Patent Document 3 as well, similar to Patent Document 2, the stretch flangeability is not taken into account.
• Patent Document 4 discloses a high-strength hot-rolled steel sheet having both excellent strength and excellent workability (particularly, bending workability), and a method of producing the same.
• This steel sheet has a certain composition as well as microstructures such that an area ratio of ferrite phase is 95% or more, an average grain size of the ferrite phase is 8 ⁇ m or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm.
• This steel sheet also has a tensile strength of 980 MPa or more.
• the present invention has been made in consideration of the above-described problems, and an object of the present invention is to provide a steel sheet having a high strength and being excellent in terms of elongation, stretch flangeability and bending workability.
• the steel sheet of the present invention also includes steel sheets having a cover such as a plating layer on the surface.
• the present inventors studied steel sheets that are favorable in all of the strength, the elongation, the stretch flangeability and the bending workability. As a result, it was found that a steel sheet having a high strength and being excellent in terms of elongation, stretch flangeability and bending workability can be manufactured by optimizing the chemical composition and manufacturing conditions to control the microstructure of the steel sheet and Mn segregation and controlling the precipitation form of a Ti-based carbide.
• the steel sheet of the present invention is preferable as a material that is used in uses for automobiles, home appliances, mechanical structures, construction and the like, and, in particular, when the steel sheet is used as a material for components such as inner sheet members, structural members, suspension members, and the like of automobiles, not only is a contribution made to weight reduction in vehicle bodies and improvement in impact resistance but the steel sheet is also easily worked into component shapes.
• Numerical value limiting ranges expressed below using “to” include the values at both ends as the lower limit and the upper limit in the ranges. However, numerical values expressed with 'less than' or ⁇ more than' are not included in numerical value ranges. In the following description, “%” regarding the chemical composition of the steel sheet indicates “mass%” in all cases.
• C is an element that bonds to Ti or the like to form a carbide, thereby increasing the tensile strength of steel.
• the C content is set to 0.050% or more.
• the C content is preferably set to 0.070% or more.
• the C content is set to 0.250% or less.
• the C content is preferably 0.220% or less, more preferably 0.200% or less and still more preferably 0.180% or less.
• Si is an element having an action of increasing the tensile strength of steel by solid solution strengthening and the enhancement of hardenability.
• Si is an element that also has an action of suppressing the precipitation of cementite.
• the Si content is set to 0.005% or more.
• the Si content is preferably 0.010% or more.
• the Si content is set to 2.000% or less.
• the Si content is preferably 1.500% or less and more preferably 1.300% or less.
• Mn is an element having an action of increasing the tensile strength of steel by solid solution strengthening and the enhancement of hardenability.
• the Mn content is set to 0.10% or more.
• the Mn content is preferably 0.30% or more and more preferably 0.50% or more.
• the Mn content is set to 3.00% or less.
• the Mn content is preferably 2.50% or less, more preferably 2.00% or less and still more preferably 1.50% or less.
• Al is an element having an action of cleaning steel by deoxidation in a steelmaking stage.
• the sol. Al content is set to 0.001% or more.
• the sol. Al content is preferably 0.01% or more, more preferably 0.02% or more and still more preferably 0.03% or more.
• the sol. Al content is set to 1.00% or less.
• the sol. Al content is preferably 0.80% or less and more preferably 0.60% or less.
• sol. Al refers to acid-soluble Al.
• Ti is an element that bonds to C to form a Ti-based carbide and contributes to increase in the tensile strength of the steel sheet.
• Ti is an element having an action of refining the microstructure by forming a Ti nitride to suppress the coarsening of austenite during the reheating and hot rolling of a slab.
• the Ti content is set to 0.150% or more.
• the Ti content is preferably 0.170% or more, more preferably 0.190% or more and still more preferably 0.210% or more.
• the Ti content is set to 0.400% or less.
• the Ti content is preferably 0.380% or less and more preferably 0.350% or less.
• N is an element having an action of refining the microstructure by forming a Ti nitride to suppress the coarsening of austenite during the reheating and hot rolling of a slab.
• the N content is set to 0.0010% or more.
• the N content is preferably 0.0015% or more and more preferably 0.0020% or more.
• the N content is set to 0.0100% or less.
• the N content is preferably 0.0060% or less and more preferably 0.0050% or less.
• the P content is an element that is contained in steel as an impurity and has an action of degrading the stretch flangeability or bending workability of the steel sheet. Therefore, the P content is set to 0.100% or less.
• the P content is preferably 0.060% or less, more preferably 0.040% or less and still more preferably 0.020% or less.
• P is mixed from a raw material as an impurity, and the lower limit thereof is not particularly limited, but the P content is preferably as small as possible from the viewpoint of ensuring the bending workability.
• the P content is preferably 0.001% or more and more preferably 0.005% or more.
• the S content is an element that is contained in steel as an impurity and has an action of degrading the stretch flangeability or bending workability of the steel sheet. Therefore, the S content is set to 0.0100% or less.
• the S content is preferably 0.0080% or less, more preferably 0.0060% or less and still more preferably 0.0030% or less.
• S is mixed from the raw material as an impurity, and the lower limit thereof is not particularly limited, but the S content is preferably as small as possible from the viewpoint of ensuring the bending workability. However, when the S content is excessively decreased, the manufacturing cost increases. From the viewpoint of the manufacturing cost, the S content is preferably 0.0001% or more, more preferably 0.0005% or more and still more preferably 0.0010% or more.
• the remainder of the chemical composition of the steel sheet according to the present embodiment includes Fe and impurities.
• the impurity means a substance that is mixed from ore as a raw material, a scrap, the manufacturing environment or the like and is allowed to an extent that the steel sheet according to the present embodiment is not adversely affected.
• the steel sheet according to the present embodiment may contain the following optional elements instead of some of Fe. Since the steel sheet according to the present embodiment is capable of solving the problems even when the optional elements are not contained, the lower limit of the amount of the optional elements is 0%.
• Nb is an optional element.
• Nb is an element having effects on the suppression of the coarsening of the crystal grain diameters of the steel sheet and an increase in the tensile strength of the steel sheet by the refinement of the ferrite grain diameters or precipitation hardening attributed to the precipitation of Nb as NbC.
• the Nb content is preferably set to 0.001% or more.
• the Nb content is more preferably 0.005% or more and still more preferably 0.010% or more.
• the Nb content is set to 0.100% or less.
• the Nb content is preferably 0.060% or less and more preferably 0.030% or less.
• V is an optional element.
• V is an element having effects on an increase in the tensile strength of the steel sheet by the formation of a solid solution in steel and increase in the tensile strength of the steel sheet by precipitation hardening attributed to the precipitation of V as a carbide, a nitride, a carbonitride or the like in steel.
• the V content is preferably set to 0.005% or more.
• the V content is more preferably 0.010% or more and still more preferably 0.050% or more.
• the V content is set to 1.000% or less.
• the V content is preferably 0.800% or less and more preferably 0.600% or less.
• Mo is an optional element. Mo is an element having effects on the high-strengthening of the steel sheet by the enhancement of the hardenability of steel and the formation of a carbide or a carbonitride. In order to obtain these effects, the Mo content is preferably set to 0.001% or more. The Mo content is more preferably 0.005% or more, still more preferably 0.010% or more and far still more preferably 0.050% or more.
• the Mo content exceeds 1.000%, there is a case where the cracking sensitivity of a steel material such as a slab is enhanced. Therefore, in a case where Mo is contained, the Mo content is set to 1.000% or less.
• the Mo content is more preferably 0.800% or less and still more preferably 0.600% or less.
• Cu is an optional element.
• Cu is an element having an effect on improvement in the toughness of steel and an effect on an increase in the tensile strength. In order to obtain these effects, the Cu content is preferably set to 0.02% or more.
• the Cu content is set to 1.00% or less.
• the Cu content is preferably 0.50% or less and more preferably 0.30% or less.
• Ni is an optional element.
• Ni is an element having an effect on improvement in the toughness of steel and an effect on an increase in the tensile strength. In order to obtain these effects, the Ni content is preferably set to 0.02% or more.
• the Ni content is set to 1.00% or less.
• the Ni content is preferably 0.50% or less and more preferably 0.30% or less.
• Cr is an optional element. Cr is an element having an effect on an increase in the tensile strength by the enhancement of the hardenability of steel. In order to obtain this effect, the Cr content is preferably set to 0.02% or more. The Cr content is more preferably 0.05% or more and still more preferably 0.10% or more.
• the Cr content is set to 2.00% or less.
• the Cr content is preferably 1.50% or less, more preferably 1.00% or less and still more preferably 0.50% or less.
• W is an optional element.
• W is an element having an effect on an increase in the tensile strength by the formation of a carbide or a carbonitride. In order to obtain this effect, the W content is preferably set to 0.020% or more.
• the W content is set to 1.000% or less.
• the W content is preferably 0.800% or less.
• B is an optional element.
• B is an element having an effect on an increase in the tensile strength of the steel sheet by grain boundary strengthening or solid solution strengthening.
• the B content is preferably set to 0.0001% or more.
• the B content is more preferably 0.0002% or more.
• the B content is set to 0.0020% or less.
• the B content is more preferably 0.0015% or less.
• Ca is an optional element.
• Ca is an element having an effect on the refinement of the microstructure of the steel sheet by the dispersion of a number of fine oxides in molten steel.
• Ca is an element having an effect on improvement in the stretch flangeability of the steel sheet by fixing S in molten steel as spherical CaS to suppress the formation of an elongated inclusion such as MnS.
• the Ca content is preferably set to 0.0002% or more.
• the Ca content is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• the Ca content is set to 0.0100% or less.
• the Ca content is preferably 0.0050% or less and more preferably 0.0030% or less.
• Mg is an optional element. Similar to Ca, Mg is an element having effects on the suppression of the formation of coarse MnS by the formation of an oxide or a sulfide in molten steel and the refinement of the microstructure of the steel sheet by the dispersion of a number of fine oxides. In order to obtain these effects, the Mg content is preferably set to 0.0002% or more. The Mg content is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• the Mg content exceeds 0.0100%, an oxide in steel increases, and there is a case where the toughness of the steel sheet deteriorates. Therefore, in a case where Mg is contained, the Mg content is set to 0.0100% or less.
• the Mg content is preferably 0.0050% or less and more preferably 0.0030% or less.
• REM is an optional element. Similar to Ca, REM is also an element having effects on the suppression of the formation of coarse MnS by the formation of an oxide or a sulfide in molten steel and the refinement of the microstructure of the steel sheet by the dispersion of a number of fine oxides. In the case of obtaining these effects, the REM content is preferably set to 0.0002% or more. The REM content is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• the REM content when the REM content exceeds 0.0100%, an oxide in steel increases, and there is a case where the toughness of the steel sheet deteriorates. Therefore, in a case where REM is contained, the REM content is set to 0.0100% or less.
• the REM content is preferably 0.0050% or less and more preferably 0.0030% or less.
• REM rare earth metal
• the REM content refers to the total amount of these elements.
• Bi is an optional element.
• Bi is an element having an effect on improvement in the formability of the steel sheet by the refinement of the solidification structure.
• the Bi content is preferably set to 0.0001% or more.
• the Bi content is more preferably 0.0005% or more.
• the Bi content is set to 0.0200% or less.
• the Bi content is preferably 0.0100% or less and more preferably 0.0070% or less.
• C is precipitated as a Ti-based carbide and contributes to the high-strengthening of the steel sheet.
• excess C forms pearlite, cementite, MA or the like and consequently degrades the stretch flangeability or the bending workability.
• %Ti* in the formula (1) is obtained from the following formula (2).
• %Ti * %Ti ⁇ 48 ⁇ ⁇ %N / 14 + %S / 32 )
• %C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%, respectively.
• the microstructure of the steel sheet will be described.
• the microstructure at a 1/4 depth position of the sheet thickness from the surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder including bainite.
• the average crystal grain diameter is 10.0 ⁇ m or less
• the average aspect ratio of crystal grains is 0.30 or more
• the standard deviation of the Mn concentration is 0.60 mass% or less.
• a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state.
• the reason for regulating the microstructure at the 1/4 depth position of the sheet thickness in the sheet thickness direction from the surface of the steel sheet is that the microstructure at this position is a typical microstructure of the steel sheet.
• Total area fraction of pearlite and cementite 0% to 5%
• the area fraction of ferrite is set to 60% or more.
• the area fraction of ferrite is preferably 70% or more, more preferably 80% or more and may be 100% (that is, a ferrite single phase).
• the microstructure contains, in addition to ferrite, a small amount of MA, which is allowed as long as the area fraction is 5% or less.
• the area fraction is preferably 4% or less, more preferably 3% or less and most preferably 2% or less.
• pearlite and cementite are precipitated, which is allowed as long as the total area fraction is 5% or less.
• the total area fraction is preferably 4% or less, more preferably 3% or less and most preferably 2% or less.
• the remainder other than the above-described structures includes bainite.
• the hardness difference is small between bainite and ferrite that has been precipitation-hardened by a Ti-based carbide. Therefore, bainite has a small effect on the degradation of the hole expandability compared with MA (Martensite-Austenite constituents), pearlite and cementite. Therefore, bainite is contained as the remainder in microstructure.
• the average crystal grain diameter is set to 10.0 ⁇ m or less.
• the average crystal grain diameter is preferably 8.0 ⁇ m or less. Since the average crystal grain diameter is preferably as small as possible, the lower limit is not particularly limited. However, it is technically difficult to refine crystal grains by ordinary hot rolling such that the average crystal grain diameter becomes less than 1.0 ⁇ m. Therefore, the average crystal grain diameter may be set to 1.0 ⁇ m or more.
• the average crystal grain diameter in the present embodiment refers to the average value of crystal grain diameters for which a region that is surrounded by grain boundaries having a crystal orientation difference of 15° or more and has a circle equivalent diameter of 0.3 ⁇ m or more in a material having a bcc crystal structure, that is, ferrite, bainite, martensite, and pearlite is defined as a crystal grain, and the crystal grain diameters of residual austenite are not included in the average crystal grain diameter.
• the average aspect ratio of bcc crystal grains is 0.30 or more.
• the aspect ratio is a value obtained by dividing the length of the minor axis of a crystal grain by the length of the major axis and has a value of 0 to 1.00.
• the average aspect ratio of crystal grains becomes smaller, the crystal grains become flatter, and, as the average aspect ratio becomes closer to 1.00, it is indicated that a crystal grain becomes more equiaxial.
• the average aspect ratio of the crystal grains is less than 0.30, there are a number of flat crystal grains, the anisotropy of the material becomes large, and the stretch flangeability and the bending workability deteriorate. Therefore, the average aspect ratio of the crystal grains excluding residual austenite is set to 0.30 or more. As the crystal grains become more equiaxial, the anisotropy becomes smaller, and the workability becomes superior, and thus the average aspect ratio of the crystal grains excluding residual austenite is preferably as close to 1.00 as possible.
• the average crystal grain diameter, the average aspect ratio of the crystal grains and the area fractions of the microstructure are obtained by the scanning electron microscopic (SEM) observation and the electron back scattering diffraction (EBSD) analysis of the microstructure at the 1/4 depth position of the steel thickness from the surface of the steel sheet of a cross section of the steel sheet parallel to a rolling direction and the sheet thickness direction using an EBSD analyzer composed of a thermal field emission scanning electron microscope and an EBSD detector.
• SEM scanning electron microscopic
• EBSD electron back scattering diffraction
• crystal orientation information is acquired at 0.2 ⁇ m intervals while differentiating fcc and bcc.
• Crystal grain boundaries having a crystal orientation difference of 15° or more are specified using the software attached to the EBSD analyzer ("OIM Analysis (registered trademark)" manufactured byAMETEK, Inc.).
• the average crystal grain diameter of bcc is obtained by defining a region that is surrounded by crystal grain boundaries having a crystal orientation difference of 15° or more and has a circle equivalent diameter of 0.3 ⁇ m or more as a crystal grain.
• a crystal grain boundary having a crystal orientation difference of 15° or more is mainly a ferrite grain boundary or a block boundary of martensite and bainite.
• a method for measuring ferrite grain diameters according to JIS G 0552: 2013, there is a case where a grain diameter is calculated even for a ferrite grain having a crystal orientation difference of less than 15°, and furthermore, a block of martensite or bainite is not calculated. Therefore, as the average crystal grain diameter in the present embodiment, a value obtained by EBSD analysis as described above is adopted. In the EBSD analysis, since the length of the major axis and the length of the minor axis of each crystal grain are also obtained at the same time, the average aspect ratio of bcc crystal grains is also obtained by adopting the present method.
• the area fraction of ferrite is measured by the following method.
• a region that is surrounded by grain boundaries having a crystal orientation difference of 5° or more and has a circle equivalent diameter of 0.3 ⁇ m or more is defined as a crystal grain.
• the area fraction is calculated for crystal grains for which a value that is obtained by an analysis with Grain Average Misorientation analysis equipped in OIM Analysis (GAM value) is 0.6° or less.
• GEM value Grain Average Misorientation analysis equipped in OIM Analysis
• the reason for defining a boundary having a crystal orientation difference of 5° or more as a grain boundary at the time of obtaining the area fraction of ferrite is that there is a case where it is not possible to differentiate different microstructures formed as close variants from the same prior austenite grain.
• the area fraction of pearlite and cementite is obtained by observing microstructures revealed by Nital etching with a SEM.
• the area fraction of MA is obtained by observing a microstructure revealed by LePera etching with an optical microscope.
• the area fraction may be obtained by an image analysis or may be obtained by a point counting method.
• the area fractions may be obtained by the point counting method with lattice spacings of 5 ⁇ m after three or more visual fields (100 ⁇ m ⁇ 100 ⁇ m/visual field) in a region at the 1/4 depth position of the sheet thickness from the surface of the steel sheet are observed at a magnification of 1000 times.
• the area fraction of MA may be obtained by the point counting method with lattice spacings of 5 ⁇ m after two or more visual fields (200 ⁇ m ⁇ 200 ⁇ m/visual field) in a region at the 1/4 depth position of the sheet thickness from the surface of the steel sheet are observed at a magnification of 500 times.
• the standard deviation of the Mn concentration at the 1/4 depth position of the sheet thickness from the surface of the steel sheet according to the present embodiment is 0.60 mass% or less. In such a case, a local unevenness in the tensile strength attributed to Mn segregation is reduced, and it is possible to stably obtain favorable bending workability.
• the value of the standard deviation of the Mn concentration is desirably as small as possible, but the substantial lower limit is 0.10 mass% due to restrictions in the manufacturing process.
• the standard deviation of the Mn concentration can be obtained by collecting a sample such that a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet becomes an observed section, mirror-polishing the observed section and then measuring the 1/4 depth position of the sheet thickness from the surface of the steel sheet with an electron probe microanalyzer (EPMA).
• the acceleration voltage is set to 15 kV
• the magnification is set to 5000 times
• a distribution image in a range that is 20 ⁇ m long in the rolling direction of the sample and 20 ⁇ m long in the sheet thickness direction of the sample is measured. More specifically, the measurement intervals are set to 0.1 ⁇ m, and the Mn concentrations at 40000 or more sites are measured.
• the standard deviation is calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentration.
• a carbide containing Ti (Ti-based carbide) is precipitated in ferrite.
• Ti is an element having a high driving force for the precipitation of a carbide in ferrite, and the control of the content and a heat treatment make it easy to control the precipitation state of a carbide.
• the Ti-based carbide also has a high precipitation hardening capability.
• the Ti-based carbide refers to a carbide having a NaCl-type crystal structure containing Ti. In a case where such a carbide contains Ti, even when a small amount of other carbide-forming alloying elements are contained, the above-described driving force is not significantly weakened, and thus the effect can be obtained.
• the Ti-based carbide may contain other carbide-forming alloying elements, for example, Mo, Nb, V, Cr and W. Furthermore, even when the Ti-based carbide is a carbonitride in which some of carbon atoms have been substituted with nitrogen atoms, the precipitation state does not change, and thus the effect can be obtained.
• the stretch flangeability of the steel sheet becomes stably favorable.
• the state where "the Ti-based carbides are precipitated in a semi-coherent state" mentioned in the present embodiment refers to such case.
• the hole expandability deteriorates.
• Whether or not the Ti-based carbides having the Baker-Nutting orientation relationship are in a semi-coherent state is determined as described below. That is, an annular dark-field scanning transmission electron microscopic image, for which the detection angle of an annular detector is set to 60 mrad or more and 200 mrad or less in the scanning transmission electron microscopy (magnification: 910,000 times to 5,100,000 times), is captured by injecting electron beams into a thin film sample for a transmission electron microscope produced from the 1/4 depth position of the sheet thickness from the surface along a [001] orientation of ferrite.
• a particle forming a plate-like form having a (100) plane of ferrite in the matrix as a habit plane and a particle forming a plate-like form having a (010) plane of ferrite as a habit plane are regarded as the Ti-based carbides having the Baker-Nutting orientation relationship
• a case where the numbers of the crystal planes of a ⁇ 010 ⁇ plane of ferrite and a ⁇ 01-1 ⁇ plane of the Ti-based carbides that sandwich the habit plane of the (100) plane of the particle forming a plate-like form having a (100) plane of ferrite in the matrix as a habit plane and the habit plane of the (100) plane of the particle forming a plate-like form having a (010) plane of ferrite as a habit plane coincide with each other is determined as a coherent state
• a case where the numbers of the crystal planes do not coincide with each other is determined as the semi-coherent state.
• the Ti-based carbides having the Baker-Nutting orientation relationship in steel from which the observed thin film sample for a transmission electron microscope has been collected are determined to be in the semi-coherent state.
• the thickness of the Ti-based carbide may be 1 nm or more and 5 nm or less.
• the thickness of the Ti-based carbide is measured by the following method.
• a thin film sample for a transmission electron microscope is produced from the 1/4 depth position in the sheet thickness direction from the surface of the steel sheet and observed with a scanning transmission electron microscope (hereinafter, also referred to as "STEM").
• STEM scanning transmission electron microscope
• the thickness In a Ti-based carbide forming sheet surfaces on the (100) plane and the (010) plane of ferrite observed in a STEM image captured by injecting electron beams along the [001] orientation of ferrite, the length of a small side between the sizes of the Ti-based carbide measured along the [100] and [010] orientations of ferrite is regarded as the thickness.
• a scale is corrected such that the interatomic distance, which is as long as 10 unit lattices, becomes 2.866 nm in each of the [100] orientation and the [010] orientation of ferrite in a site where no precipitates are shown in the image.
• the steel sheet according to the present embodiment has a high strength and is excellent in terms of the elongation, the stretch flangeability, and the bending workability by the control of the microstructure, the precipitation form of the Ti-based carbide and Mn segregation.
• the tensile strength (TS) of the steel sheet according to the present embodiment is set to 980 MPa or more.
• the tensile strength is preferably 1080 MPa or more.
• the upper limit is not particularly regulated; however, as the tensile strength increases, press forming becomes more difficult. Therefore, the tensile strength may be set to 1800 MPa or less.
• the steel sheet according to the present embodiment aims at TS ⁇ ⁇ , which serves as an index of the balance between the strength and the stretch flangeability, of 50000 MPa ⁇ % or more and aims at TS ⁇ El, which serves as an index of the balance between the strength and the elongation, of 14000 MPa % or more.
• TS ⁇ El is more preferably 15000 MPa ⁇ % or more.
• TS ⁇ ⁇ is more preferably 55000 MPa % or more, still more preferably 60000 MPa ⁇ % or more and far still more preferably 65000 MPa ⁇ % or more.
• the tensile strength and elongation of the steel sheet are evaluated by the tensile strength and the total elongation at fracture (El) using a No. 5 test piece regulated in JIS Z 2241: 2011.
• the stretch flangeability of the steel sheet is evaluated with the limiting hole expansion ratio ( ⁇ ) regulated in JIS Z 2256: 2010.
• the present inventors are confirming that the steel sheet according to the present embodiment can be obtained by a manufacturing method including a heating step, a hot rolling step, a cooling step and a coiling step as described below.
• the slab or steel piece may be a slab or steel piece obtained by continuous casting or casting and blooming or may be also a slab or steel piece obtained by additionally performing hot working or cold working on the above-described slab or steel piece.
• the slab or steel piece that is to be subjected to hot rolling is heated, the slab or steel piece is caused to retain in a temperature range of 700°C to 850°C for 900 seconds or longer.
• Mn is distributed to ferrite and austenite. Therefore, when the transformation time is extended by extending the retention time, it is possible to diffuse Mn in the ferrite region. This eliminates Mn microsegregation that is unevenly distributed in the slab and significantly reduces the standard deviation of the Mn concentration.
• Heating temperature 1280°C or higher and SRT (°C) or higher
• the heating temperature of the slab or steel piece that is to be subjected to hot rolling is set to 1280°C or higher and a temperature SRT (°C) represented by the following formula (3) or higher.
• a temperature SRT (°C) represented by the following formula (3) or higher.
• the heating temperature is lower than 1280°C, there is a case where the reduction in the standard deviation of the Mn concentration due to the diffusion of Mn during heating becomes insufficient.
• the heating temperature is lower than the SRT (°C), the solutionizing of a Ti carbonitride becomes insufficient, and, in any cases, the tensile strength or bending workability of the steel sheet deteriorates. Therefore, the temperature of the slab or steel piece that is to be subjected to hot rolling is set to 1280°C or higher and the SRT (°C) or higher.
• the fact that "the temperature of the slab or steel piece is 1280°C or higher and the SRT (°C) or higher” means that the temperature of the slab or steel piece is higher than the higher temperature of 1280°C and the SRT (°C) or the higher temperature of 1280°C and the SRT (°C) is the same as the temperature of the slab or steel piece.
• [element symbol] in the formula (3) indicates the amount of each element by mass%.
• multi-pass hot rolling is performed on the slab or steel piece after the heating step using a plurality of rolling stands to produce a hot-rolled steel sheet.
• the hot rolling step is divided into rough rolling and finish rolling that is performed after the rough rolling.
• the multi-pass hot rolling can be performed using a reverse mill or a tandem mill; however, at least several stages from the end are preferably performed using a tandem mill from the viewpoint of the industrial productivity.
• the time from the beginning of the rough rolling to the completion of the finish rolling is set to 600 seconds or shorter.
• the time is preferably 500 seconds or shorter, more preferably 400 seconds or shorter and most preferably 320 seconds or shorter.
• the rolling reduction and the rolling temperature are controlled depending on the specification of a roller, the sheet thickness and sheet width of a coil to be manufactured and a desired material, but the time from the beginning of rough rolling to the completion of finish rolling is not comprehensively controlled.
• the present inventors newly found that the time from the beginning of the rough rolling to the completion of the finish rolling affects the precipitation state of the Ti-based carbide.
• the total rolling reduction within the temperature range of 850°C to 1100°C can be represented by (t0 - t1)/t0 ⁇ 100 (%) where the inlet sheet thickness before the first pass in rolling within this temperature range is indicated by t0 and the outlet sheet thickness after the final pass in the rolling within this temperature range is indicated by t1.
• the FT (°C) is lower than TR (°C) represented by the following formula (4), significantly flat austenite is formed before cooling after the finish rolling, the microstructure is elongated in the rolling direction in the final product steel sheet, the average aspect ratio of crystal grains excluding residual austenite and having a bcc structure becomes smaller, and the plastic anisotropy becomes large. In this case, the elongation, stretch flangeability and/or bending workability of the steel sheet deteriorates. Therefore, the FT (°C) is set to the TR (°C) or higher.
• the FT (°C) exceeds 1080°C, the structure becomes coarse, and the bending workability of the steel sheet deteriorates. Therefore, the FT (°C) is set to 1080°C or lower.
• the FT (°C) is preferably 1060°C or lower.
• the temperature during the finish rolling refers to the surface temperature of steel and can be measured with a radiation-type thermometer or the like.
• TR ⁇ C 805 + 385 ⁇ Ti + 584 ⁇ Nb
• [element symbol] in the formula (4) indicates the amount of each element by mass%, and zero is assigned in a case where the corresponding element is not contained.
• the method for manufacturing the steel sheet according to the present embodiment has, as the next step of the hot rolling step, a cooling step of cooling the hot-rolled steel sheet with water to a temperature range of 650°C to 800°C at an average cooling rate of 45 °C/second or faster.
• the cooling step is begun within 3.0 seconds after the end of the hot rolling step (after the completion of the finish rolling).
• the water cooling is begun within 3.0 seconds after the completion of the finish rolling.
• the time is preferably 2.0 seconds or shorter and more preferably 1.5 seconds or shorter.
• the average cooling rate is set to 45 °C/second or faster.
• the average cooling rate is preferably 50 °C/second or faster and more preferably 55 °C/second or faster.
• the upper limit is not particularly limited, but is preferably 300 °C/second or slower from the viewpoint of the facility cost.
• the average cooling rate is a value obtained by dividing the amount of temperature dropped from the beginning of the water cooling after the completion of the hot rolling to the stopping of the water cooling by the required time.
• the steel sheet After cooled to 650°C to 800°C at an average cooling rate of 45 °C/second or faster, the steel sheet is caused to retain in the corresponding temperature range.
• the retention time at 650°C to 800°C is short, since it becomes difficult to obtain a desired ferrite area fraction, the retention time needs to be five seconds or longer.
• the retention time is preferably seven seconds or longer.
• the retention time within this temperature range is set to 50 seconds or shorter.
• the retention time is preferably 40 seconds or shorter.
• the steel sheet is cooled to a temperature of 550°C or lower (coiling temperature) in a manner that the average cooling rate within a temperature range of 550°C to 650°C becomes 45 °C/second or faster.
• the upper limit of the average cooling rate is not particularly limited, but is preferably 300 °C/second or slower from the viewpoint of the facility cost.
• the steel sheet is coiled at 350°C or higher and lower than 550°C.
• the coiling temperature is lower than 350°C, non-transformed austenite transforms into martensite, and the hole expandability or the bending workability deteriorates.
• the coiling temperature becomes 550°C or higher, a Ti-based carbide having a coherent interface is formed after the coiling, and the hole expandability deteriorates.
• the coiling temperature is preferably 400°C or higher and lower than 500°C.
• a plated steel sheet having a plating layer may be produced by performing plating on the surface of the steel sheet after the coiling step. Even in a case where plating is performed, there is no problem in performing the plating as long as the conditions for the method for manufacturing the steel sheet according to the present embodiment are satisfied.
• the plating may be any of electroplating and hot-dip plating, and the plating type is also not particularly limited, but is ordinarily zinc-based plating including zinc plating and zinc alloy plating.
• an electrolytic zinc-plated steel sheet As examples of the plated steel sheet, an electrolytic zinc-plated steel sheet, an electrolytic zinc-nickel alloy-coated steel sheet, a hot-dip galvanized steel sheet, a galvannealed steel sheet, a hot-dip zinc-aluminum alloy-coated steel sheet and the like are exemplary examples.
• the plating adhesion amount may be an ordinary amount. Before the plating, Ni or the like may be applied to the surface as pre-plating.
• well-known temper rolling may be performed as appropriate for the purpose of shape correction.
• the sheet thickness of the steel sheet according to the present embodiment is not particularly limited, but is preferably 8.0 mm or less since, in a case where the sheet thickness is too thick, microstructures formed in the surface layer and the inside of the steel sheet significantly differ.
• the sheet thickness is more preferably 6.0 mm or less.
• the sheet thickness is preferably 1.0 mm or more.
• the sheet thickness is more preferably 1.2 mm or more.
• the microstructures at the 1/4 depth positions of the sheet thicknesses from the surfaces of the steel sheets were observed, and the area fractions of individual structures, the average crystal grain diameters and average aspect ratios of the crystal grains having a bcc structure and the standard deviations of the Mn concentrations were obtained.
• the area fractions of the microstructure at the 1/4 depth position of the sheet thickness from the surface of the steel sheet, the average crystal grain diameter and average aspect ratio of the crystal grains having a bcc structure were obtained by the scanning electron microscopic (SEM) observation and electron back scattering diffraction (EBSD) analysis of the microstructure at the 1/4 depth position of the sheet thickness from the surface of the steel sheet of a cross section of the steel sheet parallel to a rolling direction and the sheet thickness direction using an EBSD analyzer composed of a thermal field emission scanning electron microscope and an EBSD detector.
• SEM scanning electron microscopic
• EBSD electron back scattering diffraction
• crystal orientation information was acquired at 0.2 ⁇ m intervals while differentiating fcc and bcc. Crystal grain boundaries having a crystal orientation difference of 15° or more were specified using the software attached to the EBSD analyzer ("OIM Analysis (registered trademark)" manufactured by AMETEK, Inc.).
• the average crystal grain diameter of bcc was obtained by defining a region that was surrounded by crystal grain boundaries having a crystal orientation difference of 15° or more, was identified as bcc and had a circle equivalent diameter of 0.3 ⁇ m or more as a crystal grain.
• the area fraction of ferrite was measured by the following method.
• the area fraction was calculated for crystal grains for which a value that was obtained by an analysis with Grain Average Misorientation analysis equipped in OIM Analysis (GAM value) was 0.6° or less.
• the area fraction of pearlite and cementite was obtained by the point counting method with lattice spacings of 5 ⁇ m after the microstructure revealed by Nital etching in a region at the 1/4 depth position of the sheet thickness from the surface of the steel sheet was observed at three visual fields using a SEM at a magnification of 1000 times.
• the area fraction of MA was obtained by the point counting method with lattice spacings of 5 ⁇ m after the structure revealed by LePera etching in the region at the 1/4 depth position of the sheet thickness from the surface of the steel sheet was observed at two visual fields using an optical microscope at a magnification of 500 times.
• the standard deviation of the Mn concentration was obtained by mirror-polishing a cross section of the steel sheet parallel to the rolling direction and the sheet thickness direction and then measuring the 1/4 depth position of the sheet thickness from the surface of the steel sheet with an electron probe microanalyzer (EPMA).
• the acceleration voltage was set to 15 kV
• the magnification was set to 5000 times
• a distribution image in a range that was 20 ⁇ m long in the sample rolling direction and 20 ⁇ m long in the sample sheet thickness direction was measured. More specifically, the measurement intervals were set to 0.1 ⁇ m, and the Mn concentrations at 40000 or more sites were measured.
• the standard deviation was calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentration.
• the tensile strengths TS (MPa) and the total elongations at fracture El (%) were measured according to JIS Z 2241: 2011.
• the limiting hole expansion ratios ( ⁇ ) were measured according to JIS Z 2256: 2010.
• the bending workability was evaluated by a 90° V bend test in which the bend radius was set to twice the sheet thickness.
• Table 3A and Table 3B show the microstructures and the test results of the mechanical properties.
• the tensile strength was evaluated as a high strength in a case where the tensile strength was 980 MPa or more.
• the elongation was evaluated as excellent in a case where the product of the tensile strength and the total elongation at fracture (TS ⁇ El) was 14000 MPa ⁇ % or more. In addition, in a case where TS ⁇ ⁇ was 50000 MPa % or more, the stretch flangeability was evaluated as excellent.
• the bending workability was evaluated as excellent bending workability (OK) when cracking did not occur in all test pieces during the bend test performed three times and evaluated as insufficient bending workability (NG) when cracking occurred in one or more test pieces. [Table 3A] Test No.
• the steel sheet of the present invention is preferable as a material that is used in uses for automobiles, home appliances, mechanical structures, construction and the like, and, in particular, when the steel sheet is used as a material for components such as inner sheet members, structural members, suspension members, and the like of automobiles, not only is a contribution made to weight reduction in vehicle bodies and improvement in impact resistance but the steel sheet is also easily worked into component shapes. Therefore, the steel sheet of the present invention makes an extreme industrial contribution.

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