EP4656758A1 - High-strength steel sheet and method for manufacturing same - Google Patents

High-strength steel sheet and method for manufacturing same

Info

Publication number
EP4656758A1
EP4656758A1 EP24767131.6A EP24767131A EP4656758A1 EP 4656758 A1 EP4656758 A1 EP 4656758A1 EP 24767131 A EP24767131 A EP 24767131A EP 4656758 A1 EP4656758 A1 EP 4656758A1
Authority
EP
European Patent Office
Prior art keywords
amount
steel sheet
less
heat treatment
rolled steel
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP24767131.6A
Other languages
German (de)
English (en)
French (fr)
Inventor
Ryusuke ISHITO
Junya TOBATA
Hidekazu Minami
Yuki Toji
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
Publication of EP4656758A1 publication Critical patent/EP4656758A1/en
Pending legal-status Critical Current

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Classifications

    • 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
    • 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
    • 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
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C22C38/06—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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—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
    • C23C2/04—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
    • 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
    • C23C2/29—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—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 a high strength steel sheet and a method for manufacturing the same.
  • delayed fracture may occur due to an increase in residual stress in the component or a deterioration of delayed fracture resistance of the steel sheet itself.
  • the delayed fracture is a phenomenon in which when a formed component is placed in a hydrogen entering environment, hydrogen enters a steel sheet constituting the component, whereby interatomic bonding strength decreases or local deformation occurs, thus causing fine cracks, and development of the fine cracks leads to fracturing.
  • high strength steel sheets used in automobiles are required to be resistant to cracking at a bent ridge portion (i.e., required to have excellent bendability) during bending from a formability viewpoint.
  • skeletal parts of automobiles have many end surfaces formed through shearing, and such shear end surfaces are required to be free from cracking caused by bending forming (bending).
  • shear end surface depends on the shear clearance, and occurrence of cracking at a shear end surface also depends on the shear clearance.
  • high strength steel sheets used in automobiles need to have a wide optimal clearance range with respect to bending forming of shear end surfaces.
  • an object of the present invention is to provide a high strength steel sheet having a tensile strength (TS) of not less than 1,180 MPa, having excellent bendability and excellent delayed fracture resistance, and having a wide optimal clearance range with respect to bending forming of a shear end surface, as well as a method for manufacturing the same.
  • TS tensile strength
  • the present invention provides the following [1] to [5].
  • the present invention can provide a high strength steel sheet having a tensile strength (TS) of not less than 1,180 MPa, having excellent bendability and excellent delayed fracture resistance, and having a wide optimal clearance range with respect to bending forming of a shear end surface, as well as a method for manufacturing the same.
  • TS tensile strength
  • a high strength steel sheet of the present embodiment (hereinafter also referred to as the "present high strength steel sheet”) includes a steel sheet, and may further include a plating layer on a surface of the steel sheet as described later.
  • the steel sheet included in the present high strength steel sheet has the chemical composition and microstructure (steel structure) which are to be described later, and satisfies an instability index k and an instability index d which are to be described later.
  • high strength means having a tensile strength (TS) of not less than 1,180 MPa.
  • the high strength steel sheet has a tensile strength (TS) of not less than 1,180 MPa, has excellent bendability and excellent delayed fracture resistance, and has a wide optimal clearance range with respect to bending forming of a shear end surface.
  • TS tensile strength
  • optimal clearance range with respect to bending forming of a shear end surface is also simply called “optimal clearance range.”
  • the thickness of the steel sheet is not particularly limited and is, for example, not less than 0.5 mm and not more than 3.0 mm.
  • present chemical composition The chemical composition of the steel sheet included in the present high strength steel sheet (hereinafter, also referred to as “present chemical composition”) is described.
  • the unit "%" used for the chemical composition means “mass%” unless otherwise noted.
  • C is an element that is one of important basic components of steel and that influences the amount of martensite and the total amount of ferrite and bainitic ferrite.
  • the C content is not less than 0.030%, preferably not less than 0.050%, and more preferably not less than 0.100%.
  • the C content is not more than 0.500%, preferably not more than 0.400%, and more preferably not more than 0.350%.
  • Si is an element that is one of important basic components of steel and that influences the TS and the amount of retained austenite.
  • the Si content is not less than 0.50%, preferably not less than 0.55%, and more preferably not less than 0.60%.
  • the Si content is not more than 2.50%, preferably not more than 2.00%, and more preferably not more than 1.80%.
  • Mn is an element that is one of important basic components of steel and that influences the amount of martensite and the total amount of ferrite and bainitic ferrite.
  • the Mn content is not less than 1.50%, preferably not less than 2.00%, and more preferably not less than 2.20%.
  • the Mn content is not more than 5.00%, preferably not more than 4.50%, and more preferably not more than 4.00%.
  • the P content is not more than 0.100%, preferably not more than 0.070%, and more preferably not more than 0.030%.
  • the lower limit thereof is not particularly limited.
  • P is a solid solution strengthening element and is capable of increasing the strength of the steel sheet.
  • the P content is preferably not less than 0.001%, more preferably not less than 0.003%, and even more preferably not less than 0.005%.
  • the S content is not more than 0.0200%, preferably not more than 0.0050%, and more preferably not more than 0.0025%.
  • the S content is preferably not less than 0.0001%, more preferably not less than 0.0003%, and even more preferably not less than 0.0005% due to production engineering constraints.
  • Al is present as an oxide and embrittles the steel sheet, resulting in lower delayed fracture resistance.
  • the Al content is not more than 1.000%, preferably not more than 0.500%, more preferably not more than 0.150%, and even more preferably not more than 0.070%.
  • the lower limit thereof is not particularly limited.
  • Al is capable of suppressing the generation of a carbide during heat treatments, which will be described later, and promoting the generation of retained austenite.
  • the Al content is preferably not less than 0.001%, more preferably not less than 0.005%, and even more preferably not less than 0.010%.
  • N is present as a nitride and embrittles the steel sheet, resulting in lower delayed fracture resistance.
  • the N content is not more than 0.0100%, preferably not more than 0.0080%, and more preferably not more than 0.0050%.
  • the N content is preferably not less than 0.0001%, more preferably not less than 0.0005%, and even more preferably not less than 0.0010% due to production engineering constraints.
  • the O content is not more than 0.0100%, preferably not more than 0.0080%, and more preferably not more than 0.0050%.
  • the O content is preferably not less than 0.0001%, more preferably not less than 0.0007%, and even more preferably not less than 0.0015% due to production engineering constraints.
  • the present inventors found, through an earnest study, that Nb influences the instability index k of retained austenite.
  • the Nb content is not less than 0.005%, preferably not less than 0.008%, and more preferably not less than 0.010%.
  • the Nb content is not more than 0.100%, preferably not more than 0.080%, and more preferably not more than 0.050%.
  • the present chemical composition may further include at least one element (another element) selected from the group consisting of the elements described below, in percentage by mass.
  • the contents of Ti and V are each not more than 0.200%, preferably not more than 0.150%, and more preferably not more than 0.100%.
  • Ti and V form fine carbides, nitrides, or carbonitrides during hot rolling or heat treatments, which will be described later, thereby increasing the strength of the steel sheet.
  • the contents of Ti and V are each preferably not less than 0.001%, more preferably not less than 0.005%, and even more preferably not less than 0.015%.
  • the contents of Ta and W are each not more than 0.10%, preferably not more than 0.09%, and more preferably not more than 0.08%.
  • Ta and W form fine carbides, nitrides, or carbonitrides during hot rolling or heat treatments, which will be described later, thereby increasing the strength of the steel sheet.
  • the contents of Ta and W are each preferably not less than 0.01%, more preferably not less than 0.03%, and even more preferably not less than 0.05%.
  • the B content is not more than 0.0100%, preferably not more than 0.0080%, and more preferably not more than 0.0060%.
  • the lower limit thereof is not particularly limited.
  • B segregates in a prior austenite grain boundary during heat treatments to be described later, thus improving the hardenability.
  • the B content is preferably not less than 0.0003%, more preferably not less than 0.0005%, and even more preferably not less than 0.0010%.
  • the contents of Cr, Mo, and Ni are each not more than 1.00%, preferably not more than 0.80%, and more preferably not more than 0.50%.
  • the lower limits thereof are not particularly limited.
  • Cr, Mo, and Ni are elements that improve the hardenability
  • the contents of Cr, Mo, and Ni are each preferably not less than 0.01%, more preferably not less than 0.04%, and even more preferably not less than 0.08%.
  • the Co content is not more than 0.010%, preferably not more than 0.008%, and more preferably not more than 0.006%.
  • the lower limit thereof is not particularly limited.
  • Co is an element that improves the hardenability
  • the Co content is preferably not less than 0.001%, more preferably not less than 0.003%, and even more preferably not less than 0.005%.
  • the Cu content is not more than 1.00%, preferably not more than 0.80%, and more preferably not more than 0.60%.
  • the lower limit thereof is not particularly limited.
  • the Cu content is preferably not less than 0.01%, more preferably not less than 0.03%, and even more preferably not less than 0.05%.
  • the Sn content is not more than 0.200%, preferably not more than 0.150%, and more preferably not more than 0.100%.
  • the lower limit thereof is not particularly limited.
  • Sn is an element that improves the hardenability
  • the Sn content is preferably not less than 0.001%, more preferably not less than 0.010%, and even more preferably not less than 0.020%.
  • the Sb content is not more than 0.200%, preferably not more than 0.100%, and more preferably not more than 0.050%.
  • the lower limit thereof is not particularly limited.
  • Sb is an element that controls the surface layer softening thickness and allows for adjustment of the strength
  • the Sb content is preferably not less than 0.001%, more preferably not less than 0.003%, and even more preferably not less than 0.005%.
  • the contents of Ca, Mg, and REM are each not more than 0.0100%, preferably not more than 0.0080%, and more preferably not more than 0.0050%.
  • Ca, Mg, and REM are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformability of the steel sheet.
  • the contents of Ca, Mg, and REM are each preferably not less than 0.0005%, more preferably not less than 0.0010%, and even more preferably not less than 0.0015%.
  • the contents of Zr and Te are each not more than 0.100%, preferably not more than 0.080%, and more preferably not more than 0.060%.
  • Zr and Te are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformability of the steel sheet.
  • the contents of Zr and Te are each preferably not less than 0.001%, more preferably not less than 0.008%, and even more preferably not less than 0.015%.
  • the Hf content is not more than 0.10%, preferably not more than 0.09%, and more preferably not more than 0.08%.
  • the lower limit thereof is not particularly limited.
  • Hf is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformability of the steel sheet.
  • the Hf content is preferably not less than 0.01%, more preferably not less than 0.02%, and even more preferably not less than 0.03%.
  • the Bi content is not more than 0.200%, preferably not more than 0.150%, and more preferably not more than 0.100%.
  • the lower limit thereof is not particularly limited.
  • the Bi content is preferably not less than 0.001%, more preferably not less than 0.020%, even more preferably not less than 0.050%, and particularly preferably not less than 0.090%.
  • the balance in the present chemical composition consists of Fe and inevitable impurities.
  • present microstructure the microstructure of the steel sheet included in the present high strength steel sheet.
  • Total amount of ferrite and bainitic ferrite not more than 10%
  • the total amount of ferrite and bainitic ferrite is not more than 10%, preferably not more than 9%, and more preferably not more than 8%.
  • the lower limit thereof is not particularly limited.
  • the amounts of ferrite and bainitic ferrite are obtained in the following manner.
  • the steel sheet is polished to expose, as an observation surface, an L cross section at the 1/4 sheet thickness position (the position corresponding to 1/4 of the sheet thickness from the surface of the steel sheet in the depth direction).
  • the observation surface is etched using 3 vol% Nital and then observed in 10 fields with a scanning electron microscope (SEM) at a magnification of 2,000X, whereby an SEM image of each field is obtained.
  • SEM scanning electron microscope
  • ferrite and bainitic ferrite are observed as recessed structures with their inside portions being flat.
  • the area fraction (unit: %) of ferrite and bainitic ferrite in each SEM image is obtained, and the average of the obtained area fractions of the 10 fields is defined as the total amount of ferrite and bainitic ferrite.
  • the amount of retained austenite is not less than 3%, preferably not less than 5%, more preferably not less than 7%, and even more preferably not less than 8% because excellent bendability can be achieved.
  • the amount of retained austenite is not more than 20%, preferably not more than 15%, and more preferably not more than 13% because excellent delayed fracture resistance can be achieved.
  • the amount of retained austenite is determined in the following manner.
  • the steel sheet is polished to expose an L cross section at a position 0.1 mm deeper than the 1/4 sheet thickness position.
  • This L cross section is further polished by 0.1 mm in the depth direction by chemical polishing to obtain an observation surface.
  • integral intensity ratios of diffraction peaks are determined using CoK ⁇ radiation in an X-ray diffraction (XRD) instrument. More specifically, integral intensity ratios between diffraction peaks of respective planes ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ of fcc iron and those of respective planes ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ of bcc iron are determined.
  • the average of nine integral intensity ratios is defined as the volume fraction (unit: %) of the amount of retained austenite, and the obtained value is specified as the amount of retained austenite.
  • the amount of martensite is not less than 70%, preferably not less than 75%, and more preferably not less than 80% because a TS of not less than 1,180 MPa can be achieved.
  • tempered martensite is important in terms of the contribution to achievement of a high TS, and the amount of tempered martensite is preferably not less than 80%.
  • the amount of martensite is determined in the following manner.
  • the amount of retained austenite and the total amount of ferrite and bainitic ferrite are obtained by the foregoing methods. Next, the sum of those amounts is subtracted from 100%, and the obtained value (unit: %) is defined as the amount of martensite.
  • the amount of martensite herein includes both amounts of hardened martensite and tempered martensite.
  • the amount of retained austenite is the volume fraction as described above, and this is substantially equivalent to the area fraction. Accordingly, the amount of retained austenite is, together with the total amount of ferrite and bainitic ferrite which is the area fraction, subtracted from 100%.
  • the present inventors found, through an earnest study, that the instability index k of retained austenite (also simply referred to as "instability index k”) influences the optimal clearance range.
  • the instability index k is less than 6.1, preferably not more than 5.0, more preferably not more than 4.0, and even more preferably not more than 3.5.
  • the instability index k is for example not less than 1.0, preferably not less than 1.5, and more preferably not less than 2.0.
  • the present inventors found, through an earnest study, that the instability index d of retained austenite in an initial stage of working (also simply referred to as "instability index d”) influences the delayed fracture resistance and the optimal clearance range.
  • the instability index d When the instability index d is too high, the stability of retained austenite in an initial stage of working is low, and the retained austenite excessively transforms to hard martensite in the initial stage of working and would act as starting points of generation of delayed fractures in a hydrogen entering environment, thus lowering the delayed fracture resistance.
  • the instability index d is less than 5.7, preferably not more than 5.0, more preferably not more than 4.0, and even more preferably not more than 3.5.
  • the instability index d is for example not less than -15.0, preferably not less than -10.0, and more preferably not less than -5.0.
  • the instability index k and the instability index d are determined in the following manner.
  • FIG. 1 is a schematic view showing a specimen used in the tensile test.
  • FIG. 2 is a graph schematically showing the relation between tensile stress applied to a specimen 1 and tensile strain during the tensile test.
  • tensile stress is applied to a specimen to impart tensile strain (tensile plasticity strain).
  • tensile strain tensile plasticity strain
  • a plurality of specimens to each of which given tensile strain ⁇ has been imparted in the range of 0% to 10% (0 to 0.10) are obtained in this manner.
  • the tensile strain ⁇ before working (before impartment of tensile strain) is 0%.
  • the obtained approximation formula is applied to Formula (1) below, and the slope a of the approximation formula is obtained as the instability index k of retained austenite.
  • log f ⁇ ⁇ k ⁇ ⁇ + log f ⁇ 0
  • fy represents the amount of retained austenite when the tensile strain ⁇ is imparted
  • f ⁇ 0 represents the amount of retained austenite before working.
  • f ⁇ 0 represents the actual measurement value of the amount of retained austenite before working
  • f ⁇ 0 represents the estimate value of retained austenite before working
  • FIG. 3 is an exemplary graph showing the relation between the tensile strain ⁇ and the amount of retained austenite.
  • FIG. 4 is another example of the graph.
  • FIG. 5 is still another example of the graph.
  • the slope a (instability index k) of the approximation formula in FIG. 3 is 2.3 and is smaller than the slope a (instability index k) of the approximation formula in FIG. 4 , that is, 11.9.
  • the slope a (instability index k) of the approximation formula being small indicates that the change in the amount of retained austenite is small during working, and the stability of retained austenite is good.
  • the slope a (instability index k) of the approximation formula is as small as 4.0.
  • the slope of the approximation formula is large as far as an initial stage of working (initial stage of the tensile test) is concerned. That is, in this case, the stability of retained austenite is insufficient in the initial stage of working.
  • the present high strength steel sheet may further have a plating layer on a surface of the steel sheet for the purpose of improving corrosion resistance and other properties.
  • Examples of the plating layer include a galvanizing layer, a galvannealing layer, and an electrogalvanizing layer.
  • the plating layer is formed by a plating treatment to be described later.
  • the coating weight of the plating layer is not particularly limited and is preferably 20 to 80 g/m 2 per one side.
  • present manufacturing method a method for manufacturing a high strength steel sheet according to the present embodiment.
  • present manufacturing method is also a method for manufacturing the present high strength steel sheet described above.
  • the temperature at which a steel slab, the steel sheet, or the like is heated or cooled which is described below, means a surface temperature of the steel slab, the steel sheet, or the like, unless otherwise specified.
  • a method of manufacturing molten steel which becomes a steel slab is not particularly limited, and known methods using a converter, an electric furnace, or the like are applicable. It is preferable to obtain a steel slab from molten steel by a continuous casting method for the sake of preventing macro-segregation.
  • a steel slab having the present chemical composition described above is retained at a slab heating temperature described below and then hot-rolled to obtain a hot rolled steel sheet.
  • the slab heating temperature is preferably not lower than 1,220°C, more preferably not lower than 1,230°C, and even more preferably higher than 1,230°C.
  • the slab heating temperature is preferably not higher than 1,300°C, more preferably not higher than 1,290°C, and even more preferably not higher than 1,280°C.
  • the hot rolled steel sheet obtained through the hot rolling is cooled.
  • an average cooling rate v 1 from 800°C to 600°C satisfies the range described below.
  • the present inventors found, through an earnest study, that the average cooling rate v 1 from 800°C to 600°C (also simply referred to as "average cooling rate v 1 ”) influences the instability index k of retained austenite.
  • the average cooling rate v 1 is not lower than 30°C/s, preferably not lower than 35°C/s, and more preferably not lower than 40°C/s.
  • the hot rolled steel sheet thus cooled is subjected to pickling and cold rolling to obtain a cold rolled steel sheet.
  • pickling is capable of removing oxides on a surface of the hot rolled steel sheet, pickling is important to ensure good chemical convertibility and good plating quality of the final product, i.e., a high strength steel sheet. Pickling may be performed only once, or may be divided into plural steps.
  • the hot rolled steel sheet is subjected to pickling, thereby obtaining a pickled sheet. Subsequently, the pickled sheet is dried as appropriate.
  • Cold rolling may be performed on the pickled sheet before being dried or after being dried.
  • the rolling reduction in the cold rolling and the sheet thickness after the rolling are not particularly limited.
  • the number of rolling passes and the rolling reduction for each pass are also not particularly limited.
  • the cold rolled steel sheet obtained by the cold rolling is subjected to a heat treatment A, a heat treatment B, and a heat treatment C, as described below, in this order.
  • the heat treatment C is preceded by working described below.
  • the cold rolled steel sheet obtained by the cold rolling is subjected to the heat treatment A.
  • the cold rolled steel sheet is retained (heated) at a temperature T1 described below and then cooled to a cooling stop temperature Ta described below.
  • the cold rolled steel sheet is retained (heated) at the temperature T1.
  • the temperature T1 is too low, the amount of martensite decreases, while the total amount of ferrite and bainitic ferrite increases, and this makes it difficult to achieve a TS of not less than 1,180 MPa.
  • the temperature T1 is not lower than 800°C, preferably not lower than 820°C, and more preferably not lower than 840°C.
  • the temperature T1 is for instance not higher than 940°C, preferably not higher than 920°C, and more preferably not higher than 900°C.
  • the retaining time t 1 is 10 seconds or more, preferably 30 seconds or more, and more preferably 50 seconds or more.
  • the retaining time t 1 is for instance 300 seconds or less, preferably 250 seconds or less, and more preferably 200 seconds or less.
  • the cold rolled steel sheet having been retained at the temperature T1 is cooled to the cooling stop temperature Ta.
  • the cooling stop temperature Ta is not lower than 100°C, preferably not lower than 120°C, and more preferably not lower than 140°C.
  • the cooling stop temperature Ta is not higher than (Ms point - 80°C), preferably not higher than (Ms point - 90°C), and more preferably not higher than (Ms point - 100°C).
  • the average cooling rate v 2 is not lower than 20°C/s, preferably not lower than 22°C/s, and more preferably not lower than 24°C/s.
  • the average cooling rate v 2 is for instance not higher than 65°C/s, preferably not higher than 55°C/s, and more preferably not higher than 45°C/s.
  • an average cooling rate v 3 from the Ms point to the cooling stop temperature Ta influences the instability index d of retained austenite in an initial stage of working.
  • the average cooling rate v 3 is not higher than 150°C/s, preferably not higher than 120°C/s, and more preferably not higher than 90°C/s.
  • the average cooling rate v 3 is for instance not lower than 5°C/s, preferably not lower than 8°C/s, and more preferably not lower than 10°C/s.
  • tension F imparted to the cold rolled steel sheet from the Ms point to the cooling stop temperature Ta (also simply referred to as "tension F") influences the instability index d of retained austenite in an initial stage of working.
  • the tension F is not less than 5 MPa, preferably not less than 6 MPa, and more preferably not less than 8 MPa.
  • the tension F is not more than 100 MPa, preferably not more than 50 MPa, and more preferably not more than 25 MPa.
  • Ms 519 - 474 ⁇ [%C] - 30.4 ⁇ [%Mn] - 12.1 ⁇ [%Cr] - 7.5 ⁇ [%Mo] - 17.7 ⁇ [%Ni]
  • [%M] represents the content of an element M in the chemical composition, and when the element M is not contained, [%M] is 0.
  • the cold rolled steel sheet having been cooled to the cooling stop temperature Ta is subjected to the heat treatment B.
  • the cold rolled steel sheet is retained (heated) at a temperature T2 described below and then cooled to a temperature (e.g., room temperature) lower than the temperature T2.
  • the room temperature is 25 ⁇ 5°C, for instance.
  • the cold rolled steel sheet is retained (heated) at the temperature T2. This stabilizes retained austenite.
  • the temperature T2 is not lower than the cooling stop temperature Ta, preferably not lower than (Ta + 10°C), and more preferably not lower than (Ta + 20°C) .
  • the temperature T2 is not higher than 450°C, preferably not higher than 420°C, and more preferably not higher than 400°C.
  • the retaining time t 2 is 5 seconds or more, preferably 50 seconds or more, and more preferably 80 seconds or more.
  • the retaining time t 2 is 1,000 seconds or less, preferably 800 seconds or less, and more preferably 400 seconds or less.
  • the cold rolled steel sheet is wrought to impart equivalent plastic strain to the cold rolled steel sheet after the heat treatment A described above and before the heat treatment C described below.
  • the temperature during the working is not particularly limited.
  • the working may be carried out while the cold rolled steel sheet is retained at the temperature T2 or after the cold rolled steel sheet is cooled to, for example, the room temperature subsequent to the retention at the temperature T2.
  • equivalent plastic strain imparted to the cold rolled steel sheet through the working also simply referred to as "equivalent plastic strain" influences the instability index d of retained austenite in an initial stage of working.
  • the equivalent plastic strain is not less than 0.10%, preferably not less than 0.15%, and more preferably not less than 0.30%.
  • the equivalent plastic strain is not more than 5.00%, preferably not more than 4.00%, and more preferably not more than 3.00%.
  • the number of times the cold rolled steel sheet is wrought is not particularly limited.
  • the working may be divided into plural steps as long as the total equivalent plastic strain imparted to the cold rolled steel sheet through the whole working is within the foregoing range.
  • Examples of the method of working the cold rolled steel sheet include: a method in which the cold rolled steel sheet is subjected to temper rolling; and a method in which the cold rolled steel sheet is wrought using a tension leveler.
  • Examples of the leveler include a tension leveler, a continuous stretcher leveler, and a roller leveler, and a tension leveler is favorable.
  • the equivalent plastic strain is an elongation rate of the steel sheet (cold rolled steel sheet) and is determined from the change in length of the steel sheet before and after working.
  • the cold rolled steel sheet having been cooled to, for example, the room temperature is subjected to the heat treatment C.
  • the cold rolled steel sheet is heated to a temperature T3 described below and then cooled to a temperature (e.g., room temperature) lower than the temperature T3 without being retained at the temperature T3.
  • a temperature e.g., room temperature
  • the temperature T3 is not lower than 150°C, preferably not lower than 160°C, and more preferably not lower than 170°C.
  • the temperature T3 is not higher than 400°C, preferably not higher than 350°C, and more preferably not higher than 300°C.
  • the cold rolled steel sheet having been heated to the temperature T3 is immediately cooled without being retained at the temperature T3 as described above.
  • the average cooling rate in the temperature range described below is controlled to the range described below.
  • the average cooling rate v 4 is not higher than 50.0°C/h, preferably not higher than 48.0°C/h, and more preferably not higher than 45.0°C/h.
  • the average cooling rate v 4 is not lower than 1.0°C/h, preferably not lower than 1.2°C/h, and more preferably not lower than 1.4°C/h due to production engineering constraints.
  • the cold rolled steel sheet having undergone the heat treatment C corresponds to the steel sheet included in the present high strength steel sheet described above.
  • the cold rolled steel sheet may be wrought to again impart an equivalent plastic strain of not less than 0.10% and not more than 5.00%. After the working, the cold rolled steel sheet may be heated at a temperature of not lower than 100°C and not higher than 400°C.
  • the cold rolled steel sheet may be subjected to a plating treatment. A plating layer is thereby formed.
  • the plating treatment is performed, for instance, during or after the heat treatment A described above.
  • a galvanizing treatment or a galvannealing treatment treatment in which alloying is performed after galvanizing treatment
  • the cold rolled steel sheet is cooled from 750°C to 600°C at the average cooling rate v 2 , for example.
  • an electrogalvanizing treatment is performed after the heat treatment B, for example.
  • the electrogalvanizing treatment include a Zn-Ni electric alloy plating treatment, and a pure Zn electroplating treatment.
  • the plating treatment is not limited to the aforementioned galvanizing treatment, galvannealing treatment, or electrogalvanizing treatment.
  • the metal species used in the plating treatment is not limited to Zn and may be other metals (e.g., Al).
  • the coating weight of the plating layer to be formed can be adjusted by performing wiping during the plating treatment.
  • plating treatment can be carried out according to an ordinary method.
  • a series of treatments including the heat treatments A to C and the plating treatment as described above is preferably carried out in a continuous galvanizing line (CGL) for the sake of productivity.
  • CGL continuous galvanizing line
  • the obtained steel slab was retained at a slab heating temperature as shown in Table 2 below and then hot rolled to obtain a hot rolled steel sheet.
  • the hot rolled steel sheet obtained was cooled. In this process, the hot rolled steel sheet was cooled from 800°C to 600°C at the average cooling rate v 1 as shown in Table 2 below.
  • the hot rolled steel sheet thus cooled was pickled and then cold rolled to obtain a cold rolled steel sheet.
  • the cold rolled steel sheet obtained was subjected to the heat treatments A to C under the conditions shown in Table 2 below. Before the heat treatment C, the cold rolled steel sheet was wrought under the conditions shown in Table 2 below.
  • the cold rolled steel sheet (CR) was subjected to a plating treatment (galvanizing treatment, galvannealing treatment, or electrogalvanizing treatment) to obtain a galvanized steel sheet (GI), a galvannealed steel sheet (GA), or an electrogalvanized steel sheet (EG).
  • a plating treatment galvanizing treatment, galvannealing treatment, or electrogalvanizing treatment
  • the galvanizing treatment and the galvannealing treatment were each performed during cooling in the heat treatment A.
  • the electrogalvanizing treatment was performed after the heat treatment B (before the heat treatment C).
  • the bath temperature was set to 470°C for both cases of manufacturing GI and GA.
  • the coating weight of the plating layer was 45 to 72 g/m 2 per one side when GI was manufactured and 45 g/m 2 per one side when GA was manufactured.
  • the alloying temperature was set to 500°C.
  • the composition of the plating layer of GI was the composition including 0.1 to 1.0 mass% of Fe and 0.2 to 1.0 mass% of Al with the balance being Zn and inevitable impurities.
  • the composition of the plating layer of GA was the composition including 7 to 15 mass% of Fe and 0.1 to 1.0 mass% of Al with the balance being Zn and inevitable impurities.
  • the electrogalvanizing treatment was performed such that the plating layer had a coating weight of 30 g/m 2 per one side.
  • each of the cold rolled steel sheet (CR), the galvanized steel sheet (GI), the galvannealed steel sheet (GA), and the electrogalvanized steel sheet (EG) is also simply referred to as "steel sheet.”
  • a JIS No. 5 specimen (gauge length: 50 mm; parallel width: 25 mm) with its longitudinal direction (tensile direction) being perpendicular to the rolling direction was sampled.
  • a tensile test was performed in accordance with JIS Z 2241 under the condition of a crosshead speed of 1.67 x 10 -1 mm/s; thus, the tensile strength (TS) was determined.
  • the strength was determined to be high when the TS was not less than 1,180 MPa.
  • a specimen (width: 30 mm; length: 100 mm) with its longitudinal direction being perpendicular to the rolling direction was sampled.
  • a bending test was performed in accordance with a V-block method stated in JIS Z 2248 to measure a minimum bending radius R with which cracking did not occur at a bent ridge portion.
  • the bendability was determined to be excellent when a value obtained by dividing the minimum bending radius R by the sheet thickness t (R/t) was not more than 6.0.
  • the specimen was immersed in an aqueous solution containing 3 mass% of NaCl and 3 g/L of NH 4 SCN and retained for 24 hours with an applied current density of 0 or 0.05 mA/cm 2 . Thereafter, a tensile test (SSRT test) was performed at a tensile rate of 5 ⁇ m/min to break the specimen, thus determining the tensile strength (TS).
  • SSRT test tensile test
  • the ratio of the TS when the applied current density was 0.05 mA/cm 2 to the TS when the applied current density was 0 mA/cm 2 was obtained as a stress ratio.
  • the optimal clearance range with respect to bending forming of a shear end surface was determined as follows.
  • each of the obtained steel sheets was sheared to sample a specimen (width: 30 mm; length: 100 mm) with its longitudinal direction being perpendicular to the rolling direction.
  • the rake angle in shearing was set to 0° in all examples, and the shear clearance was varied to 5%, 10%, 15%, 20%, 25%, 30%, and 35%.
  • Occurrence or non-occurrence of cracking was checked by observing the shear end surface of the specimen with a digital microscope (RH-2000, manufactured by HIROX Co., Ltd.) at a magnification of 40X.
  • Type of steel Hot rolling Cooling Heat treatment A Heat treatment B Working Heat treatment C Final sheet thickness [mm]
  • Type Remarks Slab heating temp. [°C] Average cooling rate v 1 [°C/s] Temp. T1 [°C] Retaining time t 1 [s] Average cooling rate v 2 [°C/s] Average cooling rate v 3 [°C/s]
  • Tension F [MPa] Ms point [°C] Cooling stop temp. Ta [°C] Temp. T2 [°C] Retaining time t 2 [s] Equivalent plastic strain [%] Number of times of working Temp.
  • Type of steel Hot rolling Cooling Heat treatment A Heat treatment B Working Heat treatment C Final sheet thickness [mm]
  • Type Remarks Slab heating temp. [°C] Average cooling rate v 1 [°C/s] Temp. T1 [°C] Retaining time t 1 [s] Average cooling rate v 2 [°C/s] Average cooling rate v 3 [°C/s]
  • Tension F [MPa] Ms point [°C] Cooling stop temp. Ta [°C] Temp. T2 [°C] Retaining time t 2 [s] Equivalent plastic strain [%] Number of times of working Temp.
  • Type of steel Hot rolling Cooling Heat treatment A Heat treatment B Working Heat treatment C Final sheet thickness [mm]
  • Type Remarks Slab heating temp. [°C] Average cooling rate v 1 [°C/s] Temp. T1 [°C] Retaining time t 1 [s] Average cooling rate v 2 [°C/s] Average cooling rate v 3 [°C/s]
  • Tension F [MPa] Ms point [°C] Cooling stop temp. Ta [°C] Temp. T2 [°C] Retaining time t 2 [s] Equivalent plastic strain [%] Number of times of working Temp.
  • Type of steel Hot rolling Cooling Heat treatment A Heat treatment B Working Heat treatment C Final sheet thickness [mm]
  • Type Remarks Slab heating temp. [°C] Average cooling rate v 1 [°C/s] Temp. T1 [°C] Retaining time t 1 [s] Average cooling rate v 2 [°C/s] Average cooling rate v 3 [°C/s]
  • Tension F [MPa] Ms point [°C] Cooling stop temp. Ta [°C] Temp. T2 [°C] Retaining time t 2 [s] Equivalent plastic strain [%] Number of times of working Temp.
  • Type of steel Hot rolling Cooling Heat treatment A Heat treatment B Working Heat treatment C Final sheet thickness [mm]
  • Type Remarks Slab heating temp. [°C] Average cooling rate v 1 [°C/s] Temp. T1 [°C] Retaining time t 1 [s] Average cooling rate v 2 [°C/s] Average cooling rate v 3 [°C/s]
  • Tension F [MPa] Ms point [°C] Cooling stop temp. Ta [°C] Temp. T2 [°C] Retaining time t 2 [s] Equivalent plastic strain [%] Number of times of working Temp.
  • Type of steel Hot rolling Cooling Heat treatment A Heat treatment B Working Heat treatment C Final sheet thickness [mm]
  • Type Remarks Slab heating temp. [°C] Average cooling rate v 1 [°C/s] Temp. T1 [°C] Retaining time t 1 [s] Average cooling rate v 2 [°C/s] Average cooling rate v 3 [°C/s]
  • Tension F [MPa] Ms point [°C] Cooling stop temp. Ta [°C] Temp. T2 [°C] Retaining time t 2 [s] Equivalent plastic strain [%] Number of times of working Temp.
  • the steel sheets of Nos. 1 to 8, 10 to 12, 14 to 16, 18 to 20, 22, 24, 26, 28 to 32, 34 to 36, 38, 40, 42 to 44, 46 to 48, 50 to 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 85, 87, 88, 90, 91, and 93 to 113 had a TS of not less than 1,180 MPa, excellent bendability and excellent delayed fracture resistance, and a wide optimal clearance range.

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
EP24767131.6A 2023-03-06 2024-03-05 High-strength steel sheet and method for manufacturing same Pending EP4656758A1 (en)

Applications Claiming Priority (2)

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JP2023033678 2023-03-06
PCT/JP2024/008225 WO2024185764A1 (ja) 2023-03-06 2024-03-05 高強度鋼板およびその製造方法

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2015193897A (ja) 2014-03-17 2015-11-05 株式会社神戸製鋼所 延性及び曲げ性に優れた高強度冷延鋼板および高強度溶融亜鉛めっき鋼板、並びにそれらの製造方法
WO2019187090A1 (ja) 2018-03-30 2019-10-03 日本製鉄株式会社 鋼板およびその製造方法
WO2020174805A1 (ja) 2019-02-25 2020-09-03 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法

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Publication number Priority date Publication date Assignee Title
CN112004955B (zh) * 2018-04-23 2022-03-04 日本制铁株式会社 钢构件及其制造方法
US12428700B2 (en) * 2021-06-11 2025-09-30 Jfe-Steel Corporation High strength steel sheet and method for manufacturing the same

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2015193897A (ja) 2014-03-17 2015-11-05 株式会社神戸製鋼所 延性及び曲げ性に優れた高強度冷延鋼板および高強度溶融亜鉛めっき鋼板、並びにそれらの製造方法
WO2019187090A1 (ja) 2018-03-30 2019-10-03 日本製鉄株式会社 鋼板およびその製造方法
WO2020174805A1 (ja) 2019-02-25 2020-09-03 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
See also references of WO2024185764A1
YOSHISUKE MISAKATAKESHI MASUI, SOSEI TO KAKOU, vol. 17, 1976, pages 988 - 994

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CN120693418A (zh) 2025-09-23
JP7666749B2 (ja) 2025-04-22
WO2024185764A1 (ja) 2024-09-12
JPWO2024185764A1 (https=) 2024-09-12
KR20250137156A (ko) 2025-09-17

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