WO2024202803A1 - 鋼板、部材およびそれらの製造方法 - Google Patents

鋼板、部材およびそれらの製造方法 Download PDF

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Publication number
WO2024202803A1
WO2024202803A1 PCT/JP2024/006906 JP2024006906W WO2024202803A1 WO 2024202803 A1 WO2024202803 A1 WO 2024202803A1 JP 2024006906 W JP2024006906 W JP 2024006906W WO 2024202803 A1 WO2024202803 A1 WO 2024202803A1
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Prior art keywords
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steel sheet
steel
ferrite
content
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
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PCT/JP2024/006906
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English (en)
French (fr)
Japanese (ja)
Inventor
芳怡 王
大起 山岸
由康 川崎
達也 中垣内
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
JFE Steel Corp
Original Assignee
JFE Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
Priority to CN202480021833.8A priority Critical patent/CN121002205A/zh
Priority to KR1020257032187A priority patent/KR20250159682A/ko
Priority to EP24778945.6A priority patent/EP4663800A1/en
Priority to JP2024534744A priority patent/JP7616490B1/ja
Publication of WO2024202803A1 publication Critical patent/WO2024202803A1/ja
Priority to MX2025011410A priority patent/MX2025011410A/es
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/10—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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—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
    • C23C2/04—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
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    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00—Electroplating: Baths therefor
    • C25D3/02—Electroplating: Baths therefor from solutions
    • C25D3/22—Electroplating: Baths therefor from solutions of zinc
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
    • C25D5/34—Pretreatment of metallic surfaces to be electroplated
    • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
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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
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to steel plates, components made from the steel plates, and methods for manufacturing them.
  • Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet as a material for such automotive components, which has a composition containing, by mass%, C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, Al: 0.010-1.5%, N: 0.007% or less, with the balance being Fe and unavoidable impurities, and which has a microstructure containing, by area ratio, 20-87% ferrite, 3-10% martensite and retained austenite in total, and 10-60% tempered martensite, with a high TS-El balance, excellent stretch flangeability, and excellent workability with a low YR.
  • Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, the steel sheet structure containing a ferrite phase of 40 to 90% by volume and a retained austenite phase of 3 to 25%, the retained austenite phase having a solute carbon amount of 0.70 to 1.00%, an average particle size of 2.0 ⁇ m or less, an average distance between particles of 0.1 to 5.0 ⁇ m, a decarburized layer thickness in the steel sheet surface layer of 0.01 to 10.0 ⁇ m, an average particle size of oxides contained in the steel sheet surface layer of 30 to 120 nm, an average density of 1.0 ⁇ 10 12 particles/m 2 or more, and a work hardening coefficient (n value) at the time of 3 to 7% plastic deformation of 0.080 or more on average, which is characterized in that the high-strength hot-dip galvanized steel sheet has high ductility while ensuring a high strength
  • Patent Document 3 discloses a high-strength hot-dip galvanized steel sheet with excellent delayed fracture resistance, which has, by volume fraction, 40 to 90% ferrite phase and 5% or less retained austenite phase, with the proportion of unrecrystallized ferrite in the entire ferrite phase being 50% or less, a grain size ratio, which is the value obtained by dividing the average grain size in the rolling direction of the ferrite phase by the average grain size in the sheet width direction, of 0.75 to 1.33, a length ratio, which is the value obtained by dividing the average length in the rolling direction of the hard structure dispersed in island shapes by the average length in the sheet width direction, of 0.75 to 1.33, and an average aspect ratio of inclusions of 5.0 or less.
  • YS yield stress
  • impact absorption energy energy absorbed during impact
  • increasing the TS and YS of a steel sheet generally reduces press formability, particularly properties such as ductility, hole expandability, and bendability. Therefore, when it is assumed that such steel sheets with increased TS and YS are used in the aforementioned automobile impact energy absorbing components, press forming becomes difficult, and variation during forming reduces the yield. In particular, a decrease in press formability at the ends of the steel sheet leads to the occurrence of end cracks in the actual component.
  • Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet that has improved both ductility, which is the press formability of the inside of the steel sheet, and stretch flangeability, which is the press formability of the ends of the steel sheet, but it cannot be said that the bendability and energy absorption characteristics are sufficient.
  • Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet that has improved ductility through the formation of retained austenite inside the steel sheet and improved mechanical cutting properties through the formation of a decarburized layer on the surface of the steel sheet, but the bendability and energy absorption properties are not sufficient.
  • Patent Document 3 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and the amount of unrecrystallized ferrite is limited to a small amount to improve ductility, and delayed fracture resistance and anisotropy are improved by forming a decarburized layer on the surface of the steel sheet, but the bendability and energy absorption characteristics cannot be said to be sufficient.
  • the steel sheets disclosed in Patent Documents 1 to 3 have a TS of 1180 MPa or more, and also have a high YS, a high yield ratio YR, excellent press formability (bendability and ductility of the steel sheet), and excellent energy absorption properties.
  • the present invention has been made in consideration of the above problems, and has an object to provide a steel sheet having a tensile strength TS of 1180 MPa or more and less than 1470 MPa, and also having a high yield stress YS, a high yield ratio YR, excellent press formability (bendability and ductility of the steel sheet), and excellent energy absorption characteristics, together with an advantageous manufacturing method thereof.
  • Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.
  • the steel sheet referred to here includes galvanized steel sheet, which is a hot-dip galvanized steel sheet (hereinafter also referred to as GI), a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA), or an electrolytic galvanized steel sheet (hereinafter also referred to as EG).
  • galvanized steel sheet which is a hot-dip galvanized steel sheet (hereinafter also referred to as GI), a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA), or an electrolytic galvanized steel sheet (hereinafter also referred to as EG).
  • the tensile strength TS is measured by a tensile test conforming to JIS Z 2241 (2011).
  • having a high yield stress YS, a high yield ratio YR, excellent press formability (bendability and ductility of the steel sheet), and excellent energy absorption characteristics means satisfying the following:
  • a high yield stress YS means that the YS measured by a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured by the tensile test.
  • (A) In the case of 1180 MPa ⁇ TS ⁇ 1320 MPa, 750 MPa ⁇ YS (B) 1320 MPa ⁇ TS, 850 MPa ⁇ YS
  • a high yield ratio YR means that the YR calculated based on the yield stress YS and tensile strength TS measured in a tensile test conforming to JIS Z 2241 (2011) is 0.70 or more.
  • excellent bendability refers to a bending angle ( ⁇ ) of 80° or more at maximum load measured in a bending test conforming to the VDA standard (VDA238-100) set forth by the German Association of the Automotive Industry.
  • excellent ductility excellent stretch formability inside the steel sheet
  • El total elongation measured in a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test.
  • A When 1180 MPa ⁇ TS ⁇ 1320 MPa, 11.0% ⁇ El
  • B 1320 MPa ⁇ TS, 9.0% ⁇ El
  • excellent energy absorption characteristics means that the integral value AE of the load-stroke curve at maximum load, measured in a bending test in accordance with the VDA standard (VDA238-100) set by the German Association of the Automotive Industry, is 70,000 N/mm or more.
  • the present inventors have conducted extensive research in order to achieve the above object.
  • the steel sheet has a surface soft layer having a thickness of 20 ⁇ m or more in the sheet thickness direction from the surface of the base steel sheet
  • the structure at the 1/4 position of the sheet thickness of the base steel sheet has an area ratio of ferrite: less than 20.0% (including 0.0%), a volume ratio of retained austenite: 3.0% or more and 15.0% or less, an area ratio of fresh martensite: 10.0% or less (including 0.0%), a total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 45.0% or more and 90.0% or less
  • a steel plate having a base steel plate comprising: In mass percent, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%; P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, A composition comprising Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities;
  • the structure at 1/4 of the sheet thickness of the base steel sheet is as follows: Ferrite area ratio: less than 20.0% (including 0.0%), Area ratio of retained austenite: 3.0% or more and 15.0% or less, Area ratio of fresh martensite: 10.0% or less (including 0.0%), Total area ratio of bainitic ferrite and tempered martensite: 45.0% or more and 90.0% or less; a steel
  • the composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, The steel plate according to the above [1], containing at least one selected from Bi:
  • [4] A member made using the steel plate according to any one of [1] to [3] above.
  • [5] A hot rolling process in which a steel slab having the composition according to [1] or [2] is hot rolled to obtain a hot rolled steel sheet; A pickling process of pickling the hot-rolled steel sheet; An annealing process in which the steel sheet after the pickling process is heated and annealed under the conditions of an annealing temperature of 720°C to 860°C, a holding time of 20 seconds or more, and a dew point of -10°C or more; A rapid heating step of rapidly heating the annealing temperature to the annealing temperature + 10 ° C.
  • the method for producing a steel sheet further includes a cold rolling step of cold rolling the steel sheet after the pickling step and before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet.
  • the method for producing a steel sheet according to the above [5] or [6] further comprises an alloying treatment step of subjecting the steel sheet on which the zinc plating layer has been formed to an alloying treatment.
  • the method for producing a steel sheet according to [5] or [6] further comprising, after the reheating and holding step, an electrogalvanizing step of immersing the steel sheet in an electrogalvanizing bath to form a zinc plating layer on the steel sheet.
  • a method for manufacturing a component comprising the step of subjecting the steel plate according to any one of [1] to [3] above to at least one of forming and joining to form a component.
  • a steel sheet can be obtained which has a tensile strength TS of 1180 MPa or more and less than 1470 MPa, as well as a high yield stress YS, a high yield ratio YR, excellent press formability (bendability and ductility of the steel sheet), and excellent energy absorption characteristics.
  • members made from the steel plate of the present invention have high strength, high yield stress YS, high yield ratio YR, excellent press formability (bendability and ductility of the steel plate), and excellent energy absorption properties, and can be extremely advantageously used in automobile impact energy absorbing members, etc.
  • the steel sheet of the present invention is a steel sheet having a base steel sheet, the base steel sheet containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities, and
  • the steel sheet has a structure in which the area ratio of ferrite is less than 20.0% (including 0.0%), the area ratio of retained austenite is 3.0% or more and 15.0% or less, the area ratio of fresh martensite is 10.0% or less (including 0.0%), and the total area ratio of bainitic ferrite and tempered martensite is 45.0% or more and 90.0% or less, and where the grain boundary length L F-MA in contact
  • the steel plate of the present invention has a tensile strength TS (hereinafter, tensile strength TS may be simply referred to as TS) of 1180 MPa or more and less than 1470 MPa, as well as a high yield stress YS (hereinafter, yield stress YS may be simply referred to as YS), a high yield ratio YR (hereinafter, yield ratio YR may be simply referred to as YR), excellent press formability (bendability and ductility of the steel plate), and excellent energy absorption characteristics.
  • the steel sheet may have a zinc plating layer as the outermost layer on one or both sides of the steel sheet.
  • the steel sheet having a zinc plating layer may be a zinc-plated steel sheet.
  • a steel sheet having a hot-dip galvanized layer among the zinc-coated layers may be referred to as a hot-dip galvanized steel sheet.
  • a steel sheet having a galvannealed layer among the galvanized layers may be referred to as a galvannealed steel sheet.
  • the steel sheet having an electrolytic zinc-plated layer may be referred to as an electrolytic zinc-plated steel sheet.
  • composition of the steel sheet according to the embodiment of the present invention will be described. Note that the unit of the composition is always “mass%”, but hereinafter, unless otherwise specified, it will be simply represented as "%”.
  • C 0.050% or more and 0.400% or less C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite and retained austenite to ensure a TS of 1180 MPa or more and less than 1470 MPa and a high YS.
  • the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. In addition, this also leads to a decrease in YS.
  • the C content exceeds 0.400%, the area ratio of fresh martensite increases excessively, making it difficult to make TS less than 1470 MPa.
  • the increase in fresh martensite may introduce mobile dislocations into ferrite and bainitic ferrite, resulting in a decrease in YR.
  • the C content is set to 0.050% or more and 0.400% or less, preferably 0.100% or more, and more preferably 0.300% or less.
  • Si 0.20% or more and 3.00% or less Si suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area ratio of retained austenite.
  • Si content if the Si content is less than 0.20%, the area ratio of retained austenite decreases, and ductility decreases.
  • the Si content exceeds 3.00%, the area ratio of ferrite increases excessively, which may result in a decrease in YS. Furthermore, the YR decreases. Therefore, the Si content is set to 0.20% or more and 3.00% or less, preferably 2.00% or less, and more preferably 0.50% or more.
  • Mn 1.00% or more and less than 3.50%
  • Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc.
  • the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 1180 MPa or more. In addition, this also leads to a decrease in YS.
  • Ms point or Ms the martensite transformation start temperature
  • the martensite generated during final cooling increases, and the martensite generated at that time is not sufficiently tempered, and the area ratio of hard fresh martensite increases.
  • the increase in fresh martensite may introduce mobile dislocations into ferrite and bainitic ferrite, decreasing YS. Furthermore, YR decreases.
  • the Mn content is set to 1.00% or more and less than 3.50%, preferably 2.00% or more, and preferably 3.00% or less.
  • P 0.001% or more and 0.100% or less
  • P is an element that has a solid solution strengthening effect and increases the TS and YS of a steel sheet.
  • the P content is set to 0.001% or more.
  • the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, which increases the amount of voids generated during a VDA bending test, and there is a risk that the desired bendability cannot be achieved. Therefore, the P content is set to 0.001% or more and 0.100% or less, and preferably 0.030% or less.
  • S 0.0001% or more and 0.0200% or less S exists as sulfides in steel.
  • the S content is set to 0.0200% or less, preferably 0.0080% or less, and more preferably 0.0040% or less. Due to restrictions in production technology, the S content is set to 0.0001% or more, and preferably 0.0003% or more.
  • Al 0.010% or more and 2.000% or less
  • Al suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite.
  • Al is an element that affects the volume fraction of retained austenite.
  • the Al content is set to 0.010% or more.
  • the Al content is preferably 0.015% or more.
  • the Al content is preferably 1.000% or less, and more preferably 0.100% or less.
  • N 0.0100% or less N exists as nitrides in steel.
  • the N content is set to 0.0100% or less, and preferably to 0.0050% or less.
  • the N content is preferably 0.0005% or more, and more preferably 0.0010% or more.
  • the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the basic components, with the balance other than the basic components including Fe (iron) and unavoidable impurities.
  • the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the basic components, with the balance consisting of Fe and unavoidable impurities.
  • the base steel sheet of the steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained so long as the optional components shown below are contained in amounts below the upper limit amounts, so no lower limit is set. Note that when the optional elements shown below are contained in amounts below the preferred lower limit values described below, the elements are considered to be included as unavoidable impurities.
  • Nb 0.200% or less
  • Ti 0.200% or less
  • V 0.200% or less
  • B 0.0100% or less
  • Cr 1.000% or less
  • Ni 1.000% or less
  • Mo 1.000% or less
  • Sb 0.200% or less
  • Sn 0.200% or less
  • Cu 1.000% or less
  • Ta 0.100% or less
  • W 0.500% or less
  • Mg 0.200% or less
  • Nb 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
  • the Nb content is preferably 0.001% or more.
  • the Nb content is more preferably 0.005% or more.
  • the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and there is a risk that the desired bendability cannot be achieved. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less.
  • the Nb content is more preferably 0.060% or less.
  • Ti 0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. In order to obtain such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and there is a risk that the desired bendability cannot be achieved. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.
  • V 0.200% or less
  • V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
  • the V content is preferably 0.001% or more.
  • the V content is more preferably 0.005% or more.
  • the V content is further preferably 0.010% or more, and even more preferably 0.030% or more.
  • the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and the desired bendability may not be achieved. Therefore, when V is contained, the V content is preferably 0.200% or less.
  • the V content is more preferably 0.060% or less.
  • B 0.0100% or less
  • B is an element that enhances hardenability by segregating at the austenite grain boundaries.
  • B is an element that suppresses the formation and grain growth of ferrite during cooling after annealing.
  • the B content is preferably 0.0001% or more.
  • the B content is more preferably 0.0002% or more.
  • the B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more.
  • the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling, and the amount of voids generated during a VDA bending test may increase, making it difficult to achieve the desired bendability. Therefore, when B is contained, the B content is preferably 0.0100% or less, and more preferably 0.0050% or less.
  • Cr 1.000% or less
  • Cr is an element that enhances hardenability, so the addition of Cr produces a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS.
  • the Cr content is preferably 0.0005% or more.
  • the Cr content is more preferably 0.010% or more.
  • the Cr content is further preferably 0.030% or more, and even more preferably 0.050% or more.
  • the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and mobile dislocations are introduced into ferrite and bainitic ferrite, which may reduce the YS.
  • the Cr content is preferably 1.000% or less. Moreover, the Cr content is more preferably 0.800% or less, even more preferably 0.700% or less, and even more preferably 0.200% or less.
  • Ni 1.000% or less
  • Ni is an element that enhances hardenability, so the addition of Ni produces a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS.
  • the Ni content is more preferably 0.020% or more.
  • the Ni content is even more preferably 0.040% or more, and even more preferably 0.060% or more.
  • the Ni content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and mobile dislocations are introduced into ferrite and bainitic ferrite, which may decrease the YS.
  • the Ni content is preferably 1.000% or less.
  • the Ni content is more preferably 0.800% or less.
  • the Ni content is more preferably 0.600% or less, and even more preferably 0.400% or less.
  • Mo 1.000% or less
  • Mo is an element that enhances hardenability, so the addition of Mo produces a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS.
  • the Mo content is preferably 0.010% or more.
  • the Mo content is more preferably 0.030% or more.
  • the Mo content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and mobile dislocations are introduced into ferrite and bainitic ferrite, which may decrease the YS.
  • fresh martensite becomes the origin of void generation during the VDA bending test, the desired bendability of the steel sheet may not be achieved.
  • the Mo content is preferably 1.000% or less.
  • the Mo content is more preferably 0.500% or less, further preferably 0.450% or less, and further preferably 0.400% or less.
  • the Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less.
  • the Mo content is more preferably 0.100% or less.
  • Sb 0.200% or less
  • Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it becomes difficult to achieve a TS of 1180 MPa or more. It also leads to a decrease in YS. Therefore, it is preferable that the Sb content is 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, which may result in a decrease in energy absorption properties. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.
  • Sn 0.200% or less
  • Sn is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it becomes difficult to achieve a TS of 1180 MPa or more. It also leads to a decrease in YS. Therefore, it is preferable that the Sn content is 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, which may lead to a decrease in energy absorption properties. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.
  • Cu 1.000% or less
  • Cu is an element that enhances hardenability, so the addition of Cu produces a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS.
  • the Cu content is 0.005% or more.
  • the Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more.
  • the Cu content is more preferably 0.020% or more, and even more preferably 0.050% or more.
  • the area ratio of fresh martensite increases excessively, and a large amount of coarse precipitates and inclusions may be generated.
  • the fresh martensite and the coarse precipitates and inclusions become the starting points for void generation during the VDA bending test, so that the desired bendability of the steel sheet may not be achieved.
  • the increase in the area ratio of fresh martensite may introduce mobile dislocations into ferrite and bainitic ferrite, which may decrease the YS. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.
  • Ta 0.100% or less Ta, like Ti, Nb and V, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing.
  • Ta partially dissolves in Nb carbides or Nb carbonitrides to generate composite precipitates such as (Nb, Ta) (C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS.
  • the Ta content is 0.001% or more. It is more preferable that the Ta content is 0.002% or more, and even more preferable that it is 0.004% or more.
  • the Ta content is preferably 0.100% or less.
  • the Ta content is more preferably 0.090% or less, and even more preferably 0.080% or less.
  • the Ta content is more preferably 0.020% or less.
  • W 0.500% or less W is an element that enhances hardenability, so the addition of W produces a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS.
  • the W content is preferably 0.001% or more.
  • the W content is more preferably 0.030% or more.
  • the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, and mobile dislocations are introduced into ferrite and bainitic ferrite, which may decrease the YS.
  • fresh martensite becomes the origin of void generation during the VDA bending test, the desired bendability of the steel sheet may not be achieved.
  • the W content is preferably 0.500% or less.
  • the W content is more preferably 0.450% or less, and even more preferably 0.400% or less. It is even more preferable that the W content is 0.300% or less. It is even more preferable that the W content is 0.100% or less.
  • Mg 0.0200% or less
  • Mg is an element that is effective in making the shape of inclusions such as sulfides and oxides spheroidal and improving the ultimate deformability and further the bendability of the steel sheet.
  • the Mg content is preferably 0.0001% or more.
  • the Mg content is more preferably 0.0005% or more, even more preferably 0.0010% or more, and even more preferably 0.0020% or more.
  • the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and the desired bendability may not be achieved. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less.
  • the Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
  • Zn 0.0200% or less
  • Zn is an element that is effective in making the shape of inclusions spheroidal and improving the ultimate deformability and further the bendability of the steel sheet.
  • the Zn content is preferably 0.0010% or more.
  • the Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
  • the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and the desired bendability may not be achieved. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.
  • the Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
  • Co 0.0200% or less
  • Co is an element that is effective in making the shape of inclusions spheroidal and improving the ultimate deformability and further the bendability of the steel sheet.
  • the Co content is preferably 0.0010% or more.
  • the Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
  • the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and the desired bendability may not be achieved. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.
  • the Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
  • Zr 0.1000% or less
  • Zr is an element that is effective in making the shape of inclusions spheroidal and improving the ultimate deformability and further the bendability of the steel sheet.
  • the Zr content is preferably 0.0010% or more.
  • the Zr content exceeds 0.1000%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and the desired bendability may not be achieved. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less.
  • the Zr content is more preferably 0.0300% or less, and even more preferably 0.0100% or less.
  • Ca 0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, the amount of voids generated during the VDA bending test increases, and the desired bendability may not be achieved. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. Although the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. In addition, due to restrictions on production technology, the Ca content is more preferably 0.0010% or more.
  • Se 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all elements effective for improving the ultimate deformability and further the bendability of the steel sheet. In order to obtain such an effect, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each 0.0001% or more.
  • the Se content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Se content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the Te content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Te content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the Ge content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Ge content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the As content is more preferably 0.0010% or more, and further preferably 0.0015% or more.
  • the As content is more preferably 0.0400% or less, and further preferably 0.0300% or less.
  • the Sr content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Sr content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the Cs content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Cs content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the Hf content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Hf content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the Pb content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Pb content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • the Bi content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the Bi content is more preferably 0.0180% or less, further preferably 0.0150% or less, and even more preferably 0.0100% or less.
  • the REM content is more preferably 0.0005% or more, and further preferably 0.0008% or more.
  • the REM content is more preferably 0.0180% or less, and further preferably 0.0150% or less.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
  • the REM content in the present invention is the total content of one or more elements selected from the above-mentioned REM. There are no particular limitations on the REM, but it is preferable that they are La and/or Ce.
  • the base steel sheet of the steel sheet of the present invention has, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, and optionally Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.00 It has a composition containing at least one selected from the following: 0% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less,
  • the structure at the 1/4 position of the plate thickness of the base steel plate is as follows: area ratio of ferrite: less than 20.0% (including 0.0%), area ratio of retained austenite: 3.0% to 15.0%, area ratio of fresh martensite: 10.0% or less (including 0.0%), total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 45.0% to 90.0%, and L F-MA /L F : 0.70 or less.
  • Ferrite area ratio less than 20.0% (including 0.0%)
  • Soft ferrite is a phase that improves ductility.
  • the area ratio of ferrite may increase excessively, making it difficult to achieve a TS of 1180 MPa or more.
  • it may also lead to a decrease in YR. Therefore, the area ratio of ferrite is less than 20.0%.
  • the area ratio of ferrite is preferably 15.0% or less.
  • the lower limit of the area ratio of ferrite is not particularly limited and may be 0.0%.
  • Area fraction of retained austenite 3.0% or more and 15.0% or less From the viewpoint of obtaining good ductility, the area fraction of retained austenite is set to 3.0% or more.
  • the area fraction of retained austenite is preferably 3.5% or more.
  • the area ratio of the retained austenite is set to 15.0% or less.
  • the area ratio of the retained austenite is preferably 12.0% or less, and more preferably 10.0% or less.
  • Area ratio of fresh martensite 10.0% or less (including 0.0%) If the area ratio of fresh martensite is excessively increased, mobile dislocations are introduced into ferrite or bainitic ferrite, and the desired YS may not be achieved. In addition, the desired YR cannot be obtained. In addition, since fresh martensite becomes the origin of void generation during the VDA bending test, the desired bendability of the steel sheet may not be achieved. In order to ensure high YS and good bendability of the steel sheet, the area ratio of fresh martensite is 10.0% or less, preferably 5.0% or less. The lower limit of the area ratio of fresh martensite is not particularly limited and may be 0.0%. It should be noted that fresh martensite is martensite that has not been quenched (i.e., has not been tempered).
  • Total area ratio of bainitic ferrite and tempered martensite 45.0% or more and 90.0% or less
  • Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bending properties of steel sheets.
  • Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite.
  • Tempered martensite is effective for improving TS and YS. Therefore, the total area ratio of bainitic ferrite and tempered martensite is 45.0% or more. It is preferably 50.0% or more.
  • the total area ratio of bainitic ferrite and tempered martensite is set to 90.0% or less.
  • the total area ratio of bainitic ferrite and tempered martensite is preferably 85.0% or less.
  • Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.
  • L F-MA /L F 0.70 or less
  • the steel structure of the base steel sheet of the steel sheet according to one embodiment of the present invention further satisfies L F-MA /L F : 0.70 or less.
  • L F-MA grain boundary length on the ferrite grain boundary in contact with the MA structure (fresh martensite and/or retained austenite)
  • L F grain boundary length of ferrite. Note that L F-MA and L F are grain boundary lengths measured in the same unit.
  • the presence of fresh martensite introduces mobile dislocations into ferrite, which makes the ferrite more likely to yield, and reduces the YS.
  • L F-MA /L F exceeds 0.70, the desired YS may not be achieved. Also, the desired YR cannot be obtained. Therefore, L F-MA /L F is set to 0.70 or less. L F-MA /L F is preferably 0.68 or less, more preferably 0.65 or less. The lower limit of L F-MA /L F is not particularly limited, and may be 0.00. In order to make L F-MA /L F 0.70 or less, for example, by raising the temperature from the annealing temperature at a rate of 10°C/sec or more in the rapid heating step described below, austenite can be generated around the ferrite only by the diffusion of C.
  • Such austenite has a high M S point and becomes bainitic ferrite or martensite during cooling.
  • the martensite becomes tempered martensite in the reheating and holding step.
  • a structure other than the hard phase (hard second phase) is generated from the austenite generated only by the diffusion of C.
  • the area ratio of the remaining structure other than the above is preferably 10.0% or less.
  • the area ratio of the remaining structure is more preferably 5.0% or less.
  • the area ratio of the remaining structure may be 0.0%.
  • the remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of the remaining structure can be confirmed, for example, by observation with a scanning electron microscope (SEM).
  • Soft surface layer total area ratio of structures other than ferrite is 1/2 or less at 1/4 position of sheet thickness.
  • the base steel sheet of the steel sheet according to one embodiment of the present invention has a soft surface layer on the surface of the base steel sheet.
  • the soft surface layer contributes to suppressing the progression of bending cracks during press forming and vehicle body collisions, thereby improving the bendability and energy absorption characteristics of the steel sheet.
  • the soft surface layer means a decarburized layer, and is a surface region in which the total area ratio of structures other than ferrite in a cross section at 1/4 of the sheet thickness is S A /2 or less, where S A is the total area ratio of structures other than ferrite.
  • the soft surface layer is formed in a region of 20 ⁇ m or more from the surface of the base steel sheet in the sheet thickness direction.
  • the region in which the soft surface layer is formed is preferably 200 ⁇ m or less, more preferably 150 ⁇ m or less, and even more preferably 120 ⁇ m or less from the surface of the base steel sheet in the sheet thickness direction.
  • the thickness of the soft surface layer is preferably 25 ⁇ m or more, and more preferably 30 ⁇ m or more.
  • the 1/4 sheet thickness position of the steel sheet where the total area ratio SA of the structures other than ferrite is measured is a non-surface soft layer (a layer that does not satisfy the conditions of the surface soft layer defined in the present invention). It is.
  • the area ratios of ferrite, bainitic ferrite, tempered martensite and hard phase (hard second phase (retained austenite + fresh martensite)) at the 1/4 sheet thickness position of the base steel sheet are measured as follows. That is, a sample is cut out from the base steel sheet so that the plate thickness cross section parallel to the rolling direction of the base steel sheet becomes the observation surface. The observation surface of the sample is then mirror-polished using diamond paste. The observation surface of the sample is then finish-polished using colloidal silica, and etched with 3 vol. % nital to reveal the structure.
  • Ferrite A black region with a blocky shape. It contains almost no iron-based carbides. However, if it does contain iron-based carbides, the area of the ferrite includes the area of the iron-based carbides. The same applies to bainitic ferrite and tempered martensite, which will be described later.
  • Bainitic ferrite This is a region that is black to dark gray in color and has a massive or amorphous shape. It also contains no iron carbides or contains a relatively small amount of them.
  • Tempered martensite This is a gray area with an amorphous morphology. It also contains a relatively large number of iron-based carbides.
  • Hard phase (hard second phase (retained austenite + fresh martensite)): This is a region that is white to light gray in color and has an amorphous form. It does not contain iron-based carbides. If the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray.
  • Carbides These are white areas that are dot-like or linear in shape and are included in tempered martensite, bainitic ferrite, and ferrite.
  • Remaining structure Examples include lower bainite, pearlite, and internal oxides, and the forms thereof are as known in the art.
  • each phase identified in the structural image is color-coded (four-value image) using Adobe Photoshop from Adobe Systems, Inc., and the area of each phase is calculated.
  • the area of each phase (total area of each phase) is divided by the area of the observation region (25.6 ⁇ m ⁇ 17.6 ⁇ m) and multiplied by 100 to calculate the value for the three fields of view. The average of these values is then used as the area ratio of each phase (ferrite, bainitic ferrite, tempered martensite, and hard second phase).
  • the area ratio of retained austenite is measured as follows.
  • the base steel sheet is mechanically ground in the thickness direction (depth direction) to a position of 1/4 of the thickness, and then chemically polished with oxalic acid to obtain an observation surface.
  • the observation surface is then observed by X-ray diffraction.
  • MoK ⁇ rays are used as the incident X-rays, and the ratio of the diffraction intensity of each of the (200), (220), and (311) faces of fcc iron (austenite) to the diffraction intensity of each of the (200), (211), and (220) faces of bcc iron is obtained, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face.
  • the retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.
  • the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard phase (hard second phase) determined as described above.
  • [Area ratio of fresh martensite (%)] [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]
  • the area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard phase (hard second phase) determined as described above from 100.0%.
  • [Area ratio of remaining structure (%)] 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]
  • a zinc-plated layer is formed on the steel sheet, first peel off the zinc-plated layer and measure the structure in the same way as at the 1/4 position in the sheet thickness direction at 1 ⁇ m intervals from a position 1 ⁇ m in the sheet thickness direction from the surface of the base steel sheet to a position 100 ⁇ m in the sheet thickness direction. After that, measurements are made at 20 ⁇ m intervals up to the center of the sheet thickness.
  • L F-MA and L F are measured as follows.
  • the ferrite and the hard phase (hard second phase) are identified as described above.
  • the ferrite and the hard phase (hard second phase) are manually extracted, and L F-MA and L F are calculated, respectively, using the open source ImageJ.
  • the object function of Adobe Illustrator is used to draw a curve along the ferrite grain boundary L F and a curve along the grain boundary L F-MA that contacts the ferrite and martensite structure (MA structure: fresh martensite + retained austenite), and the length of the curve is measured.
  • the value of L F-MA divided by L F is calculated, and the average value is taken as L F-MA /L F.
  • the tensile strength TS of a steel plate according to one embodiment of the present invention is 1180 MPa or more and less than 1470 MPa.
  • the yield stress (YS), yield ratio (YR), total elongation (El), bendability and energy absorption characteristics of the steel plate according to one embodiment of the present invention are as described above.
  • TS tensile strength
  • Yi yield stress
  • YiR yield ratio
  • El total elongation
  • Zinc-plated layer A steel sheet according to one embodiment of the present invention may have a zinc-plated layer formed on a base steel sheet, and this zinc-plated layer may be provided on only one surface of the base steel sheet, or on both surfaces.
  • the zinc-plated layer referred to here refers to a plating layer whose main component is Zn (Zn content: 50.0 mass% or more), and examples of this include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrolytic zinc-plated layer.
  • the hot-dip galvanized layer is composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al.
  • the hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%.
  • the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.
  • the galvannealed layer is preferably composed of, for example, Zn, 20% or less by mass of Fe, and 0.001% to 1.0% by mass of Al.
  • the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% to 3.5% by mass.
  • the Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more.
  • the Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.
  • the electrolytic zinc plating layer is preferably composed of, for example, Zn and 9.0% to 25.0% by mass of Ni. The remainder other than the above elements are unavoidable impurities.
  • the coating weight per side of the hot-dip galvanized layer and the alloyed hot-dip galvanized layer is not particularly limited, but is preferably 20 g/m2 or more . Also, the coating weight per side of the hot-dip galvanized layer and the alloyed hot-dip galvanized layer is preferably 80 g/ m2 or less. The coating weight per side of the electrogalvanized layer is not particularly limited, but is preferably 10 g/m2 or more . Also, the coating weight per side of the electrogalvanized layer is preferably 100 g/m2 or less .
  • the coating weight of the zinc-plated layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% hydrochloric acid aqueous solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the test material before and after dissolution is then measured, and the value is divided by the surface area of the base steel sheet (the surface area of the part that was covered with plating) to calculate the plating coverage (g/ m2 ).
  • a corrosion inhibitor for Fe Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.
  • the thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably more than 0.8 mm, more preferably 0.9 mm or more, further preferably 1.0 mm or more, and most preferably 1.2 mm or more.
  • the thickness of the steel plate is preferably 3.5 mm or less, and more preferably 2.3 mm or less.
  • the width of the steel plate of the present invention is not particularly limited, but is preferably 500 mm or more, more preferably 750 mm or more, and is preferably 1600 mm or less, more preferably 1450 mm or less.
  • a method for producing a steel sheet according to an embodiment of the present invention includes a hot rolling process in which a steel slab having the above-mentioned component composition is hot-rolled to produce a hot-rolled steel sheet, a pickling process in which the hot-rolled steel sheet is pickled, an annealing process in which the steel sheet after the pickling process or the steel sheet after the cold rolling process is heated and annealed under conditions of an annealing temperature of 720° C. or more and 860° C. or less, a holding time of 20 seconds or more, and a dew point of ⁇ 10° C. or more, and a rapid heating process in which the temperature is raised from the annealing temperature to an annealing temperature +10° C.
  • the steel sheet after the rapid heating step is heated to a cooling stop temperature of 100°C or more and 300°C or less; and the reheating and holding step of heating the steel sheet after the cooling step to a tempering temperature of 460°C or less and holding the steel sheet in the above temperature range for a tempering time of 10 seconds or more and 2000 seconds or less, or further including a cold rolling step of cold rolling the steel sheet after the pickling step and before the annealing step with a reduction ratio of 20% or more and 80% or less to obtain a cold rolled steel sheet.
  • the steel sheet passing speed LS (m/min) in the annealing process satisfies the following formula (1).
  • t sheet thickness (mm) after the pickling process and before the annealing process
  • W sheet thickness (mm) after the pickling process and before the annealing process.
  • the above temperatures refer to the surface temperatures of the steel slab and the steel plate.
  • a steel slab having the above-mentioned composition is prepared.
  • a steel material is melted to obtain molten steel having the above-mentioned composition.
  • the melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used.
  • the obtained molten steel is then solidified to obtain a steel slab.
  • the method for obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, it is preferable to adopt a continuous casting method as a method for obtaining a steel slab from molten steel.
  • the hot rolling process Next, in the hot rolling process, the steel slab is hot rolled to produce a hot rolled steel sheet.
  • the hot rolling may be performed by applying an energy-saving process, such as direct rolling (a method in which a steel slab is not cooled to room temperature, but is charged as a hot piece into a heating furnace and hot rolled), direct rolling (a method in which a steel slab is briefly kept at a certain temperature and then immediately rolled), etc.
  • the hot rolling conditions are not particularly limited, and the hot rolling can be performed under the following conditions, for example. That is, the steel slab is once cooled to room temperature, and then reheated and rolled.
  • the slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoints of dissolving carbides and reducing the rolling load.
  • the slab heating temperature is preferably 1300°C or lower.
  • the slab heating temperature is based on the temperature of the steel slab surface.
  • the steel slab is subjected to rough rolling according to a conventional method to obtain a rough rolled plate (hereinafter also referred to as a sheet bar).
  • the sheet bar is subjected to finish rolling to obtain a hot rolled steel plate.
  • the slab heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling in order to prevent problems during the finish rolling.
  • the finish rolling temperature is preferably 800 ° C or higher.
  • the finish rolling temperature is in the range of 950°C or less.
  • the finish rolling temperature is in the range of 800°C or more and 950°C or less.
  • the hot-rolled steel sheet is coiled.
  • the coiling temperature is preferably 450°C or higher. It is also preferable that the coiling temperature is 750°C or lower.
  • the sheet bars may be joined together during hot rolling and continuous finish rolling may be performed.
  • the sheet bar may be wound once before the finish rolling.
  • a part or all of the finish rolling may be lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material.
  • the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.
  • hot rolling process including rough rolling and finish rolling
  • steel slabs are made into sheet bars by rough rolling and then made into hot rolled steel sheets by finish rolling.
  • such division is not important and it is not a problem as long as the specified size is achieved.
  • the hot-rolled steel sheet after the hot rolling process is pickled.
  • pickling oxides on the surface of the steel sheet can be removed, and good chemical conversion treatment properties and plating quality are ensured.
  • Pickling may be performed once or multiple times. There are no particular limitations on the pickling conditions, and the usual methods may be followed.
  • the cold rolling process Next, the hot-rolled steel sheet is cold-rolled as necessary to obtain a cold-rolled steel sheet.
  • the cold rolling is performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
  • the cold rolling reduction (cumulative reduction) is not particularly limited, but is preferably 20% or more.
  • the cold rolling reduction is preferably 80% or less. If the cold rolling reduction is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and the TS and bendability of the final product may be reduced. On the other hand, if the cold rolling reduction exceeds 80%, the steel sheet is likely to have a defective shape and the amount of zinc coating may be non-uniform. Furthermore, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.
  • the steel sheet obtained as described above is heated and annealed at an annealing temperature of 720°C to 860°C and a holding time (annealing time) of 20 seconds or more.
  • the number of annealing steps may be two or more, but one is preferable from the viewpoint of energy efficiency.
  • the annealing step is a step including the temperature increase treatment and the soaking treatment.
  • Annealing temperature 720°C or more and 860°C or less
  • the rate of austenite generation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and YS decreases.
  • the amount of fresh martensite increases due to excessive concentration of carbon in austenite during annealing, L F-MA /L F exceeds 0.70, and the desired YS cannot be achieved. Furthermore, there is a risk that the bendability of the steel sheet cannot be achieved. Furthermore, it becomes difficult to achieve TS of 1180 MPa or more.
  • the annealing temperature is set to 720°C or higher and 860°C or lower.
  • the annealing temperature is preferably 850°C or lower.
  • the annealing temperature is the maximum temperature reached in the annealing process.
  • Holding time 20 seconds or more If the holding time (annealing time) is less than 20 seconds, the generation rate of austenite during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite may increase excessively after annealing, and YS may decrease. Also, YR may decrease. In addition, the amount of fresh martensite increases due to excessive concentration of carbon in austenite during annealing, and L F-MA /L F may exceed 0.70, and the desired YS may not be obtained. Also, YR may decrease. Furthermore, the bendability of the steel sheet may not be achieved. Furthermore, it becomes difficult to make TS 1180 MPa or more.
  • the holding time is set to 20 seconds or more.
  • the holding time is preferably 30 seconds or more, and more preferably 50 seconds or more.
  • the retention time is preferably 900 seconds or less, and more preferably 800 seconds or less.
  • the holding time is the holding time in the temperature range of (annealing temperature -40°C) or more and the annealing temperature or less.
  • the holding time includes not only the holding time at the annealing temperature, but also the residence time in the temperature range of (annealing temperature -40°C) or more and the annealing temperature or less during heating and cooling before and after reaching the annealing temperature.
  • Dew point of the atmosphere in the annealing step -10°C or higher
  • the dew point of the atmosphere in the annealing step is -10°C or higher.
  • the dew point of the annealing atmosphere in the annealing step is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably more than 5°C.
  • the dew point of the annealing atmosphere in the annealing step is preferably 30° C. or lower.
  • the steel sheet threading speed LS (m/min) in the annealing process satisfies formula (1).
  • t sheet thickness (mm) after pickling process and before annealing process
  • W sheet width (mm) after pickling process and before annealing process
  • E (ton/h) LS x t x W x ⁇ x 60 x 10 -6 ...Formula (2)
  • t plate thickness (mm)
  • W plate width (mm)
  • ⁇ specific gravity of iron (ton/m 3 )
  • ⁇ 7.86 (ton/m 3 ).
  • E is preferably more than 3.0 tons/h and less than 390.0 tons/h.
  • the above formula (1) can be derived. If E is 3.0 ton/h or less, a soft surface layer is excessively formed during annealing, which may make it difficult to achieve a TS of 1180 MPa or more. Also, this may result in a decrease in YS.
  • E is preferably more than 3.0 tons/h and less than 390.0 tons/h.
  • E is more preferably 16.0 ton/h or more.
  • E is more preferably 190.0 ton/h or less.
  • LS (m/min) is more than 3.0/(t ⁇ W ⁇ 4.716 ⁇ 10 ⁇ 4 ). W ⁇ 4.716 ⁇ 10 ⁇ 4 ) or more is more preferable.
  • LS (m/min) is preferably less than 390.0/(t ⁇ W ⁇ 4.716 ⁇ 10 ⁇ 4 ). It is more preferable that LS (m/min) is 190.0/(t ⁇ W ⁇ 4.716 ⁇ 10 ⁇ 4 ) or less.
  • the sheet passing speed LS (m/min) is calculated by dividing the distance (m) the steel sheet traveled in the annealing process by the time (min) required to transport the steel sheet in the annealing process.
  • a radiant tube furnace is generally used as the heat treatment furnace for the annealing process.
  • the annealing temperature is obtained by a thermometer that measures the surface temperature of the steel plate.
  • a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel plate is suitable.
  • a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel plate.
  • a multiple reflection measurement method that utilizes the wedge-shaped space between the transport roll in the furnace and the steel plate may be used.
  • the steel sheet obtained as described above is rapidly heated from the annealing temperature to the annealing temperature + 10°C or higher at a heating rate of 10°C/sec or higher.
  • the treatment in the rapid heating step performed after the annealing may also be performed two or more times, but the treatment in the rapid heating step may be performed only once.
  • the heating rate from the annealing temperature to the temperature reached in the rapid heating step described below is 10° C./sec or more.
  • the heating rate is 10° C./sec or more.
  • the diffusion of Mn from ferrite to austenite is suppressed, and austenite can be generated only by the diffusion of C.
  • Such austenite has a high M S point, and becomes bainitic ferrite or martensite during cooling.
  • the martensite becomes tempered martensite in the reheating and holding process.
  • L F-MA /L F can be made 0.70 or less, and YS can be improved.
  • the heating rate from the annealing temperature to the rapid heating process reaching temperature is set to 10° C./s or more.
  • the heating rate is more preferably 30° C./s or more.
  • the heating rate in the rapid heating step is preferably 300°C/sec or less, and more preferably 150°C/sec or less.
  • Rapid heating process temperature annealing temperature + 10°C or more If the rapid heating process temperature is less than the annealing temperature + 10°C, the generation of austenite is insufficient only by the diffusion of C, L F-MA /L F exceeds 0.70, and YS may decrease. Also, YR decreases. Therefore, the rapid heating process temperature is set to be the annealing temperature + 10°C or more.
  • the temperature reached in the rapid heating step is preferably the annealing temperature + 30° C. or more, and more preferably the annealing temperature + 40° C. or more.
  • the upper limit of the temperature reached in the rapid heating step is not particularly set, but from the viewpoint of production efficiency, it is preferably less than the annealing temperature + 100°C, and more preferably not more than the annealing temperature + 90°C.
  • the induction heating (IH) device in the rapid heating step adjusts the output to rapidly heat the steel sheet so that the temperature of the steel sheet is within the range of annealing temperature + 10°C or more (preferably, 740°C or more and 940°C or less).
  • the induction heating (IH) device is preferably of a transverse type.
  • the appropriate rapid heating temperature varies depending on the composition of the steel sheet. Therefore, it is preferable that the suitable temperature range is predicted in advance by measurement, calculation or simulation, and set in consideration of the temperature of the steel sheet in the annealing step.
  • the temperature reached in the rapid heating process can be measured by a thermometer that measures the surface temperature of the steel sheet.
  • the method of temperature measurement is not particularly limited, but for example, a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel sheet is suitable.
  • a radiation thermometer since it may be affected by reflected light of infrared rays emitted by the surrounding furnace body, a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel sheet.
  • a multiple reflection type measurement method using a wedge-shaped space between the transport roll in the furnace and the steel sheet may be adopted.
  • a holding step may be performed in which the steel sheet is held in a temperature range of 400°C or more and 600°C or less (hereinafter also referred to as the holding temperature range) for less than 80 seconds.
  • Holding time in holding temperature range less than 80 seconds
  • bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite, thereby ensuring a predetermined area ratio of retained austenite.
  • the holding time in the holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite may increase excessively, and the YS may decrease.
  • excessive diffusion of C from bainitic ferrite to untransformed austenite may occur, and the area ratio of retained austenite may exceed 15.0%, making it difficult to achieve the desired YS and bendability. Therefore, the holding time in the holding temperature range is preferably less than 80 seconds.
  • the holding time in the holding temperature range is more preferably less than 60 seconds.
  • Cooling stop temperature 100°C to 300°C
  • the cooling step is a step necessary for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the subsequent reheating and holding step within a predetermined range. If the cooling stop temperature is less than 100°C, the untransformed austenite present in the steel is transformed into martensite in the cooling step. As a result, the area ratio of tempered martensite ultimately increases excessively, making it difficult to obtain an area ratio of retained austenite of 3.0% or more, and reducing ductility. On the other hand, when the cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. As a result, the desired YS may not be obtained.
  • the cooling stop temperature is set to 100° C. or higher and 300° C. or lower.
  • the cooling stop temperature is preferably 120° C. or higher.
  • the cooling stop temperature is preferably 280° C. or lower.
  • the steel sheet may be subjected to a galvanizing treatment (hot-dip galvanizing, or further, alloyed hot-dip galvanizing treatment).
  • a galvanized steel sheet can be obtained.
  • the galvanizing treatment include hot-dip galvanizing treatment and alloyed galvanizing treatment (hot-dip galvanizing treatment and alloying treatment).
  • a hot-dip galvanized steel sheet can be obtained by subjecting a steel sheet to a hot-dip galvanizing treatment, and a galvannealed steel sheet can be obtained by further subjecting the steel sheet to an alloying treatment.
  • the hot-dip galvanizing treatment and the alloying treatment will be collectively referred to as a galvannealed treatment.
  • hot-dip galvanizing it is preferable to immerse the steel sheet in a hot-dip galvanizing bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like.
  • the hot-dip galvanizing bath There are no particular limitations on the hot-dip galvanizing bath as long as it has the composition of the zinc plating layer described above, but it is preferable to use, for example, a plating bath with an Al content of 0.10 mass% or more, with the remainder consisting of Zn and unavoidable impurities.
  • the above-mentioned Al content is preferably 0.23 mass% or less.
  • alloying hot-dip galvanizing treatment it is preferable to carry out alloying treatment by heating the galvanized steel sheet to an alloying temperature of 450° C. or more after carrying out the hot-dip galvanizing treatment as described above.
  • the alloying temperature is preferably 600° C. or less. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate is slow, and alloying may be difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite is transformed into pearlite, making it difficult to achieve a TS of 1180 MPa or more, and ductility is reduced.
  • the alloying temperature is more preferably 470°C or more.
  • the alloying temperature is more preferably 570°C or less.
  • the coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 20 g/ m2 or more per side.
  • the coating weight of the zinc coating layer per side is preferably 80 g/ m2 or less.
  • the coating weight can be adjusted by gas wiping or the like.
  • the steel sheet is reheated to a tempering temperature of 460°C or lower (hereinafter also referred to as a reheating temperature range), and the steel sheet is held in the tempering temperature range of 460°C or lower for a tempering time of 10 seconds to 2000 seconds.
  • Tempering temperature 460° C. or less If the tempering temperature (reheating temperature) exceeds 460° C., the martensite present in the steel at the end of the cooling process is excessively tempered, making it difficult to achieve a TS of 1180 MPa or more. In addition, the untransformed austenite present in the steel at the end of the cooling process decomposes as carbide (pearlite), resulting in a decrease in ductility. Therefore, the tempering temperature (reheating temperature) is set to 460° C. or less. The tempering temperature is the maximum temperature reached in the reheating and holding step. The tempering temperature is preferably 450° C. or less, and more preferably 440° C. or less.
  • the tempering temperature is preferably above 300°C, more preferably above 320°C.
  • Tempering time (holding time) in the reheating temperature range 10 seconds or more and 2000 seconds or less If the tempering time (holding time) in the reheating temperature range is less than 10 seconds, the tempering of the martensite present in the steel at the end of the cooling process does not proceed sufficiently, and fresh martensite increases excessively. In addition, the coarsening of the carbides in the tempered martensite does not proceed sufficiently, and the density of the carbides in the tempered martensite may become a predetermined amount or more. As a result, the desired YS may not be achieved. In addition, the YR decreases. Furthermore, there is a risk that the bendability may not be achieved.
  • the tempering time (holding time) in the reheating temperature range exceeds 2000 seconds, the tempering of martensite present in the steel at the end of the cooling process proceeds excessively, making it difficult to achieve a TS of 1180 MPa or more.
  • the untransformed austenite present in the steel at the end of the second cooling process decomposes as carbide (pearlite), resulting in a decrease in ductility. Therefore, the tempering time (holding time) in the reheating temperature range is set to 10 seconds or more and 2000 seconds or less.
  • the tempering time is preferably 20 seconds or more, more preferably 30 seconds or more.
  • the tempering time is preferably 1000 seconds or less, more preferably 900 seconds or less.
  • the tempering time (holding time) in the reheating temperature range includes not only the holding time at the reheating temperature, but also the residence time in that temperature range during heating and cooling before and after reaching the reheating temperature.
  • the cooling conditions after holding in the reheating temperature range are not particularly limited and may be conventional. Cooling methods that can be used include, for example, gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling. From the viewpoint of preventing surface oxidation, it is preferable to cool to 50°C or less after holding in the reheating temperature range, and more preferably to room temperature.
  • the average cooling rate for cooling after holding in the reheating temperature range is preferably, for example, 1°C/sec or more. It is also preferable that this average cooling rate is 50°C/sec or less.
  • an electrogalvanizing treatment After cooling to room temperature, an electrogalvanizing treatment may be performed. By performing an electrogalvanizing treatment on the steel sheet, an electrogalvanized steel sheet can be obtained.
  • the treatment conditions for the electrogalvanizing treatment are not particularly limited, and may be performed according to a conventional method.
  • the steel sheet obtained as described above may be further subjected to temper rolling. If the reduction rate of temper rolling exceeds 2.00%, the yield stress increases, and there is a risk of the dimensional accuracy decreasing when the steel sheet is formed into a component. Therefore, the reduction rate of temper rolling is preferably 2.00% or less.
  • the lower limit of the reduction rate of temper rolling is not particularly limited, but from the viewpoint of productivity, it is preferably 0.05% or more.
  • Temper rolling may be performed on a device continuous with the annealing device for performing each of the above-mentioned steps (online), or on a device discontinuous with the annealing device for performing each of the steps (offline).
  • the number of rolling times of temper rolling may be one or two or more. As long as the same elongation rate as that of temper rolling can be imparted, rolling using a leveler or the like may be used.
  • a member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material).
  • the raw material steel plate is subjected to at least one of forming and joining to form a member.
  • the above steel plate has a TS of 1180 MPa or more, a high YS, a high YR, and excellent ductility, bendability, and energy absorption properties. Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance properties. Therefore, the member according to one embodiment of the present invention is particularly suitable for application to impact energy absorbing members used in the automotive field.
  • a method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (e.g., a steel plate manufactured by the above-mentioned steel plate manufacturing method) to at least one of forming and joining to form a component.
  • the molding method is not particularly limited, and for example, a general processing method such as press processing can be used.
  • the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc.
  • the molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.
  • the obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling at a finish rolling temperature of 900°C to obtain a hot-rolled steel sheet.
  • the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (reduction rate: 50%) to obtain a cold-rolled steel sheet having a sheet thickness shown in Table 3.
  • the obtained cold-rolled steel sheet was then subjected to an annealing step, a rapid heating step, a cooling step, a galvanizing step, and a reheating and holding step under the conditions shown in Table 2 to obtain a steel sheet (galvanized steel sheet).
  • a holding step was performed after the rapid heating step and before the cooling step.
  • the holding step conditions were a holding temperature of 480° C. and a holding time of 60 seconds.
  • a hot-dip galvanizing treatment, a galvannealed plating treatment, or an electrogalvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI), a galvannealed steel sheet (hereinafter also referred to as GA), or an electrogalvanized steel sheet (hereinafter also referred to as EG).
  • the hot-dip galvanizing treatment and the galvannealing treatment were carried out after the cooling step and before the reheating and holding step, and the electrolytic galvanizing treatment was carried out after the reheating and holding step.
  • the type of plating process is also indicated as "GI", "GA” and "EG”.
  • the alloying temperature is indicated as "-" for the GI and EG steel sheets, since no alloying treatment is performed.
  • the cold-rolled steel sheets obtained without performing the zinc plating treatment in the zinc plating process are indicated as "CR".
  • the zinc plating bath temperature was 470° C. for all of the production of EG, GI and GA.
  • the amount of zinc plating applied was 10-100 g/ m2 when producing EG, 45-72 g/ m2 per side when producing GI, and 45 g/ m2 per side when producing GA.
  • the composition of the zinc plating layer of the finally obtained zinc-plated steel sheet was as follows: GI: 0.1-1.0 mass% Fe, 0.2-0.33 mass% Al, and the balance being Zn and unavoidable impurities; GA: 8.0-12.0 mass% Fe, 0.1-0.23 mass% Al, and the balance being Zn and unavoidable impurities; EG: 9.0-25.0 mass% Ni, and the balance being Zn and unavoidable impurities.
  • the zinc plating layers were formed on both sides of the base steel sheet in each case.
  • the steel structure of the obtained steel sheet was identified in the manner described above.
  • the measurement results are shown in Table 3.
  • F is ferrite
  • BF bainitic ferrite
  • TM tempered martensite
  • RA retained austenite
  • FM fresh martensite
  • LB lower bainite
  • ⁇ carbide.
  • the surface layer in which the total area ratio of structures other than ferrite was 1 ⁇ 2 or less at the 1 ⁇ 4 position of the sheet thickness was defined as the soft surface layer.
  • tensile tests and VDA bending tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), limit bending angle ( ⁇ ) in the VDA bending test, bending deformation absorbed energy amount, and production efficiency (E) were evaluated according to the following criteria.
  • ⁇ TS ⁇ (Pass) 1180 MPa or more and less than 1470 MPa ⁇ (Fail): Less than 1180 MPa or 1470 MPa or more
  • ⁇ AE ⁇ (Pass) The integral value of the load-stroke curve is 70,000 N ⁇ mm or more ⁇ (Pass): The integral value of the load-stroke curve is less than 70,000 N ⁇ mm
  • VDA Bending Test The VDA bending test was performed according to the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry. Specifically, a test piece of 70 mm x 60 mm was taken from the obtained steel plate by shearing, where the 60 mm side was parallel to the rolling (L) direction. The test piece was subjected to a VDA bending test under the following conditions. Test method: Roll support, punch pressing roll diameter: ⁇ 30 mm Punch tip R: 0.4 mm Distance between rolls: (sheet thickness x 2) + 0.5 mm Stroke speed: 20 mm/min Bending direction: direction perpendicular to rolling (C).
  • the angle of the outer bend of the center of the plate-shaped test piece when the load F from the pushing bending jig from above is maximum is measured as the bending angle (limit bending angle) (°).
  • the integral value (absorbed energy) of the load-stroke curve up to the maximum load is calculated using the obtained load-stroke curve.
  • the average values of the limit bending angle and the integral value (absorbed energy) of the load-stroke curve at the maximum load when the above VDA bending test is performed three times are designated as ⁇ (°) and AE, respectively. The results are shown in Table 4.
  • the components obtained by forming or joining using the steel plate of the present invention had the excellent characteristics characteristic of the present invention in terms of tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), limit bending angle ( ⁇ ) in the VDA bending test, and integral value (absorbed energy) (AE) of the load-stroke curve in the VDA bending test. It was also found that the production efficiency (E) was good.

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WO2021200580A1 (ja) * 2020-03-31 2021-10-07 Ｊｆｅスチール株式会社 鋼板、部材及びそれらの製造方法
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