EP3279362A1 - Hochfestes kaltgewalztes stahlblech mit hervorragenden bearbeitbarkeits- und kollisionseigenschaften und einer zugfestigkeit von 980 mpa oder mehr sowie verfahren zur herstellung davon - Google Patents

Hochfestes kaltgewalztes stahlblech mit hervorragenden bearbeitbarkeits- und kollisionseigenschaften und einer zugfestigkeit von 980 mpa oder mehr sowie verfahren zur herstellung davon Download PDF

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Publication number
EP3279362A1
EP3279362A1 EP16772043.2A EP16772043A EP3279362A1 EP 3279362 A1 EP3279362 A1 EP 3279362A1 EP 16772043 A EP16772043 A EP 16772043A EP 3279362 A1 EP3279362 A1 EP 3279362A1
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European Patent Office
Prior art keywords
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steel
temperature
steel sheet
strength
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EP16772043.2A
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English (en)
French (fr)
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EP3279362A4 (de
EP3279362B1 (de
Inventor
Tadao Murata
Yuichi Futamura
Koji Kasuya
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Kobe Steel Ltd
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Kobe Steel Ltd
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Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Priority claimed from PCT/JP2016/056168 external-priority patent/WO2016158159A1/ja
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/02—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
    • C22C38/12—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with 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
    • C22C38/34—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
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • 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
    • 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
    • C21D2211/005—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 cold-rolled steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness and to a method for producing the same.
  • the present invention relates to the high-strength cold-rolled steel sheet described above, a high-strength electrogalvanized steel sheet having an electrogalvanized layer formed on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed layer formed on a surface of the high-strength cold-rolled steel sheet, and to a method for producing the same.
  • a steel sheet whose surface has been subjected to galvanization such as electrogalvanization (which may hereafter be denoted as EG), hot-dip galvanizing (which may hereafter be denoted as GI), or hot-dip galvannealing (which may hereafter be denoted as GA), which may hereafter be comprehensively referred to as galvanized steel sheet, is often used from the viewpoint of corrosion resistance.
  • galvanized steel sheets as well, increase in strength and formability is demanded in the same manner as in the above high-strength steel sheet.
  • Patent Literature 1 discloses a hot-dip galvannealed steel sheet having a metal structure in which martensite and retained austenite are mixedly present in ferrite and having a tensile strength TS of 490 to 880 MPa by reinforcement of the complex structure thereof, thus having a good press formability.
  • Patent Literature 2 discloses a high-strength steel sheet being excellent in stretch-flangeability in which the steel sheet structure is made of 10 to 50% of a ferrite phase and 10 to 50% of a tempered martensite phase in a volume fraction, with the balance being a hard phase, and in which the average crystal grain size in the steel sheet structure is 10 ⁇ m or less.
  • Patent Literature 3 discloses a high-strength galvanized steel sheet having a maximum tensile strength of 900 MPa or more and being excellent in collision absorption energy in which a dynamic/static ratio as large as that of a steel sheet of 590 MPa class and a maximum tensile strength of 900 MPa or more are compatible with each other, as well as a method for producing the same.
  • This production method is characterized in that, after performing galvanization, cooling is performed, and rolling is performed with use of a roll having a roughness (Ra) of 3.0 or less.
  • Patent Literatures 1 and 2 the formability of a steel sheet can be improved. However, no consideration is made on the crashworthiness. In contrast, according to the technique disclosed in Patent Literature 3, the crashworthiness of the steel sheet can be improved. However, no consideration is made on the formability as evaluated by ductility and stretch-flangeability.
  • the present invention has been made in view of the aforementioned circumstances, and an object thereof is to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, having good formability as evaluated by ductility and stretch-flangeability, and having excellent crashworthiness.
  • Another object of the present invention is to provide a high-strength electrogalvanized steel sheet having an electro galvanized layer on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface of the high-strength cold-rolled steel sheet.
  • Still another object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed steel sheet having the above properties in combination.
  • a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more according to the present invention that has solved the aforementioned problems is a steel sheet containing, in mass%, C: 0.10% or more to 0.5% or less, Si: 1.0% or more to 3% or less, Mn: 1.5% or more to 7% or less, P: more than 0% to 0.1% or less, S: more than 0% to 0.05% or less, Al: 0.005% or more to 1% or less, N: more than 0% to 0.01% or less, and O: more than 0% to 0.01% or less, with a balance being iron and inevitable impurities.
  • the gist lies in that a metal structure at a position of 1/4 of a sheet thickness satisfies (1) to (4) below.
  • the term "MA" is an abbreviation for Martensite-Austenite Constituent.
  • the steel sheet may further contain, as other elements, in mass%:
  • a high-strength electrogalvanized steel sheet having an electrogalvanized layer on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface of the high-strength cold-rolled steel sheet are also comprised within the scope of the present invention.
  • the high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10°C/second or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T°C to 550°C or lower for 50 seconds or more, and thereafter cooling to room temperature.
  • a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10°C/second or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T°C to 550°C or lower for 50 seconds or more, and after performing hot
  • a high-strength hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10°C/second or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T°C to 550°C or lower for 50 seconds or more, and after
  • the component composition and the metal structure are suitably controlled, so that a high-strength cold-rolled steel sheet, a high-strength electrogalvanized steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more and being excellent both in formability as evaluated by ductility and stretch-flangeability and in crashworthiness can be provided.
  • the high-strength cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet according to the present invention is particularly excellent in stretch-flangeability among the formability properties.
  • the present invention can also provide a method for producing the high-strength cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet described above.
  • the high-strength cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet according to the present invention are extremely useful in the fields of industry such as automobiles.
  • FIG. 1 is a schematic descriptive view showing one example of a heat treatment pattern performed in the Examples.
  • the present inventors have repeatedly made eager studies in order to improve all of ductility, stretch-flangeability, and crashworthiness in a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more.
  • the present inventors have found out that, in order to improve the ductility while ensuring the tensile strength by setting a ferrite fraction in the metal structure to be a predetermined amount or less and setting the balance structure to be a hard phase, it is effective to appropriately control a ratio V MA /V ⁇ of an area ratio V MA of an MA structure, in which quenched martensite and retained austenite are combined, to a volume ratio V ⁇ of retained austenite relative to the whole of the metal structure and that, in order to improve the stretch-flangeability, it is effective to set the ferrite fraction in the metal structure to be a predetermined amount or less and to make the MA structure finer and, in order to improve the crashworthiness, it is effective to make the MA structure finer and to
  • the high-strength cold-rolled steel sheet according to the present invention is characterized in that the metal structure at a position of 1/4 of the sheet thickness satisfies (1) to (4) below.
  • Methods of measuring the fractions in the metal structure as defined in the above (1) to (3) may differ from each other, so that a sum of the fractions may exceed 100%.
  • the metal structure is observed with a scanning electron microscope, so that the measured area ratio is a ratio obtained when the whole of the metal structure is assumed to be 100%.
  • the area ratio measured with use of a scanning electron microscope includes that of quenched martensite and retained austenite as an area ratio of the hard phase.
  • the retained austenite fraction in the metal structure is calculated by X-ray diffractometry, while in the above (3), the area ratio of the MA structure in which quenched martensite and retained austenite are combined is observed with an optical microscope.
  • the fraction of retained austenite and quenched martensite is measured in a duplicated manner by a plurality of methods. Accordingly, a sum of the fractions in the metal structure as defined in the above (1) to (3) may exceed 100%. Also, hereafter, the retained austenite may be denoted as retained ⁇ . Further, the structure in which quenched martensite and retained ⁇ are combined may be denoted as MA structure.
  • the ductility and the crashworthiness are rendered compatible with each other when the value of the above ratio V MA /V ⁇ is controlled to satisfy the above formula (i).
  • the retained ⁇ is positively generated in the present invention in order to enhance the strength - elongation balance that constitutes an index of ductility.
  • the MA structure is inevitably formed in the steel sheet.
  • the value of the above ratio V MA /V ⁇ is preferably 0.55 or more, more preferably 0.60 or more.
  • the value of the above ratio V MA /V ⁇ becomes excessively large, the MA structure is excessively generated.
  • the quenched martensite that exists in the MA structure is a very hard structure, so that, when the MA structure is excessively generated, cracks are liable to be generated at the interface to other structures at the time of collision, and accordingly, the crashworthiness is rather deteriorated. Therefore, in the present invention, the value of the above ratio V MA /V ⁇ is set to be 1.50 or less in order to reduce the area ratio of quenched martensite in the MA structure to ensure the crashworthiness.
  • the value of the above ratio V MA /V ⁇ is preferably 1.40 or less, more preferably 1.30 or less.
  • % with regard to the component composition of a steel sheet means “mass%”.
  • the C amount is an element that is necessary for ensuring the tensile strength of 980 MPa or more and for enhancing the stability of retained ⁇ to ensure a predetermined amount of the retained ⁇ .
  • the C amount is set to be 0.10% or more.
  • the C amount is preferably 0.12% or more, more preferably 0.15% or more.
  • the C amount is set to be 0.5% or less.
  • the C amount is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.25% or less.
  • the Si is an element that acts as a solute-strengthening element and contributes to a higher strength of the steel. Also, Si suppresses generation of carbide and effectively acts for generation of retained ⁇ , so that Si is an element that is necessary for ensuring an excellent strength - elongation balance.
  • the Si amount is set to be 1.0% or more.
  • the Si amount is preferably 1.2% or more, more preferably 1.35% or more, and still more preferably 1.5% or more.
  • the Si amount is set to be 3% or less.
  • the Si amount is preferably 2.8% or less, more preferably 2.6% or less.
  • Mn is an element that contributes to a higher strength of the steel sheet by enhancing the hardenability and suppressing the generation of ferrite. Further, Mn is an element that is necessary for stabilizing ⁇ to generate retained ⁇ . In the present invention, the Mn amount is set to be 1.5% or more. The Mn amount is preferably 1.6% or more, more preferably 1.7% or more, still more preferably 1.8% or more, and furthermore preferably 2.0% or more. However, when the Mn amount is excessively large, the strength after hot rolling increases, so that cracks may be generated during the cold rolling, or the weldability of the final product may decrease. Also, when Mn is added in an excessively large amount, Mn is segregated to deteriorate the ductility and the stretch-flangeability. Accordingly, the Mn amount is set to be 7% or less. The Mn amount is preferably 5.0% or less, more preferably 4.0% or less, and still more preferably 3.0% or less.
  • the P amount is an impurity element that is inevitably contained and, when contained in an excessively large amount, deteriorates the weldability of the final product. Accordingly, the P amount is set to be 0.1% or less in the present invention.
  • the P amount is preferably 0.08% or less, more preferably 0.05% or less. The smaller the P amount is, the better it is. However, it is industrially difficult to set the P amount to be 0%. A lower limit of the P amount is 0.0005% from the industrial point of view.
  • the S amount is set to be 0.05% or less in the present invention.
  • the S amount is preferably 0.01% or less, more preferably 0.005% or less. The smaller the S amount is, the better it is. However, it is industrially difficult to set the S amount to be 0%. A lower limit of the S amount is 0.0001% from the industrial point of view.
  • the Al amount is an element that acts as a deoxidizer. In order that such an action may be exhibited, the Al amount is set to be 0.005% or more in the present invention.
  • the Al amount is more preferably 0.01% or more.
  • the Al amount is set to be 1% or less in the present invention.
  • the Al amount is preferably 0.8% or less, more preferably 0.6% or less.
  • N is an impurity element that is inevitably contained and, when N is contained in an excessively large amount, nitride is deposited in a large amount to deteriorate the ductility, stretch-flangeability, and crashworthiness. Accordingly, the N amount is set to be 0.01% or less in the present invention. The N amount is preferably 0.008% or less, more preferably 0.005% or less. Since nitride in a small amount contributes to a higher strength of the steel sheet, the N amount may be 0.001% or more.
  • the O amount is an impurity element that is inevitably contained and, when contained in an excessively large amount, deteriorates the ductility and the crashworthiness. Accordingly, the O amount is set to be 0.01% or less in the present invention.
  • the O amount is preferably 0.005% or less, more preferably 0.003% or less. The smaller the O amount is, the better it is. However, it is industrially difficult to set the O amount to be 0%. A lower limit of the O amount is 0.0001% from the industrial point of view.
  • the cold-rolled steel sheet according to the present invention satisfies the aforementioned component composition, and the balance is made of iron and inevitable impurities.
  • the inevitable impurities may include the above-mentioned elements such as P, S, N, and O, which may be brought into the steel depending on the circumstances of raw materials, facility materials, production equipment, and the like, and may also include tramp elements such as Pb, Bi, Sb, and Sn.
  • the cold-rolled steel sheet of the present invention may further contain, as other elements,
  • Cr and Mo are each an element that acts to improve the strength of the steel sheet by enhancing hardenability.
  • the amount of each of Cr and Mo is preferably set to be 0.1% or more, more preferably 0.3% or more.
  • the amount is preferably 1% or less, more preferably 0.8% or less, still more preferably 0.5% or less.
  • Cr and Mo may be used either alone or in combination. When Cr and Mo are used in combination, it is preferable that each amount is within the above range of the content when used alone, and a sum of the contents of Cr and Mo is 1.5% or less.
  • Ti, Nb, and V are each an element that acts to improve the strength of the steel sheet by forming carbide and nitride in the steel sheet and to make prior ⁇ grains finer.
  • the amount of each of Ti, Nb, and V is preferably set to be 0.005% or more, more preferably 0.010% or more.
  • carbide is deposited at the grain boundary, so that the stretch-flangeability and the crashworthiness of the steel sheet are deteriorated.
  • the amount of each of Ti, Nb, and V is preferably set to be 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less. These elements may be used either alone or in combination of two or more that are arbitrarily selected.
  • Cu and Ni are each an element that acts effectively for generation and stabilization of retained ⁇ . Also, Cu and Ni act to improve the corrosion resistance of the steel sheet. In order that such an action may be effectively exhibited, the amount of each of Cu and Ni is preferably set to be 0.05% or more, more preferably 0.10% or more. However, when Cu is contained in an excessively large amount, the hot formability is deteriorated. Accordingly, when Cu is added alone, the amount of Cu is preferably set to be 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less.
  • the amount of Ni is preferably set to be 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less.
  • Cu and Ni may be used either alone or in combination. When Cu and Ni are used in combination, the above action is more likely to be exhibited, and also, by incorporation of Ni, the deterioration of hot formability caused by addition of Cu is more likely to be suppressed. When Cu and Ni are used in combination, a sum of the amounts of Cu and Ni is preferably set to be 1.5% or less, more preferably 1.0% or less.
  • B is an element that improves hardenability and is an element that acts to allow austenite to exist stably down to room temperature.
  • the amount of B is preferably set to be 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more.
  • the amount of B is preferably set to be 0.005% or less.
  • the amount of B is more preferably 0.004% or less, still more preferably 0.0035% or less.
  • Ca, Mg, and REM are elements that act to finely disperse the inclusions in the steel sheet.
  • the amount of each of Ca, Mg, and REM is preferably set to be 0.0005% or more, more preferably 0.0010% or more.
  • the amount of each of Ca, Mg, and REM is preferably set to be 0.01% or less, more preferably 0.008% or less, and still more preferably 0.007% or less.
  • REM is an abbreviation for Rare earth metal (rare earth element), and is meant to include lanthanoid elements which are fifteen elements from La to Lu, and Sc and Y.
  • An electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip galvannealed layer may be formed on a surface of the high-strength cold-rolled steel sheet.
  • the scope of the present invention includes a high-strength electrogalvanized steel sheet (which may hereafter be referred to as EG steel sheet) having an electrogalvanized layer formed on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet (which may hereafter be referred to as GI steel sheet) having a hot-dip galvanized layer formed on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet (which may hereafter be referred to as GA steel sheet) having a hot-dip galvannealed layer formed on a surface of the high-strength cold-rolled steel sheet.
  • EG steel sheet high-strength electrogalvanized
  • the high-strength cold-rolled steel can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10°C/second or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T°C to 550°C or lower for 50 seconds or more, and thereafter cooling to room temperature.
  • a heating temperature is not particularly limited; however, the heating temperature is preferably set to be, for example, 1000 to 1300°C.
  • the heating temperature is lower than 1000°C, solid solution of carbide is insufficiently formed, and a sufficient strength is hardly obtained.
  • the heating temperature is higher than 1300°C, the structure of the hot-rolled steel sheet becomes coarse, and also the MA structure of the cold-rolled steel sheet is liable to become coarse. As a result, the crashworthiness tends to decrease.
  • the rolling rate at a final stand of finish rolling is set to be 5 to 25%.
  • the rolling rate is less than 5%, the austenite grain size after hot rolling becomes coarse, and the average circle-equivalent diameter of the MA structure in the cold-rolled steel sheet after annealing becomes large. As a result, the stretch-flangeability decreases. Accordingly, in the present invention, it is necessary that the rolling rate is set to be 5% or more.
  • the rolling rate is preferably 6% or more, more preferably 7% or more, and still more preferably 8% or more.
  • the rolling rate exceeds 25%, the average circle-equivalent diameter of the MA structure also becomes large, leading to deterioration of the stretch-flangeability and crashworthiness.
  • the mechanism therefor is not clear; however, this seems to be because the structure after hot rolling is made non-homogeneous.
  • the rolling rate is set to be 25% or less.
  • the rolling rate is preferably 23% or less, more preferably 20% or less.
  • the finish rolling end temperature is set to be 900°C or lower.
  • the finish rolling end temperature is preferably 890°C or lower, more preferably 880°C or lower.
  • the temperature of the Ar 3 point was calculated on the basis of the following formula (ii).
  • brackets [ ] indicate the content of each element (mass%), and calculation may be made by assuming that the content of an element that is not contained in the steel sheet is 0 mass%.
  • Ar 3 point ° C 910 ⁇ 310 ⁇ C ⁇ 80 ⁇ Mn ⁇ 20 ⁇ Cu ⁇ 15 ⁇ Cr ⁇ 55 ⁇ Ni ⁇ 80 ⁇ Mo
  • the coiling temperature is set to be 600°C or lower.
  • the coiling temperature is preferably 580°C or lower, more preferably 570°C or lower, and still more preferably 550°C or lower.
  • the steel sheet After the hot rolling, the steel sheet may be coiled, cooled to room temperature, pickled by a conventional method in accordance with the needs, and subsequently cold-rolled by a conventional method.
  • the cold rolling rate in the cold rolling may be set to be, for example, 30 to 80%.
  • annealing is carried out by heating, at an average heating rate of 10°C/sec or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more.
  • the average heating rate of heating to the above temperature region after the cold rolling is lower than 10°C/sec, the austenite grains grow and become coarse during the heating, so that the average circle-equivalent diameter of the MA structure in the cold-rolled steel sheet becomes large, and the stretch-flangeability decreases.
  • the average heating rate is set to be 10°C/sec or more.
  • the average heating rate is preferably 12°C/sec or more, more preferably 15°C/sec or more.
  • An upper limit of the above average heating rate is not particularly limited; however, the average heating rate is typically about 100°C/sec at the maximum.
  • the soaking temperature is set to be the Ac 3 point or higher in the present invention.
  • the soaking temperature is preferably (Ac 3 point + 10°C) or higher, more preferably (Ac 3 point + 20°C) or higher.
  • An upper limit of the soaking temperature is not particularly limited. However, when the soaking temperature is too high, the austenite may be coarsened, so that the soaking temperature is preferably (Ac 3 point + 100°C) or lower, more preferably (Ac 3 point + 50°C) or lower.
  • the soaking time is set to be 50 seconds or more.
  • the soaking time is preferably 60 seconds or more.
  • An upper limit of the soaking time is not particularly limited; however, when the soaking time is too long, concentration of Mn into the austenite phase proceeds, and the Ms point may decrease, leading to increase or coarsening of the MA structure. Accordingly, the soaking time is preferably set to be 3600 seconds or less, more preferably 3000 seconds or less.
  • the steel sheet need not be thermostatically held at the same temperature, so that the steel sheet may be heated and cooled in a fluctuating manner within the above temperature region.
  • the temperature of the aforementioned Ac 3 point can be calculated on the basis of the following formula (iii) disclosed in " The Physical Metallurgy of Steels" (William C. Leslie, published by Maruzen Co., Ltd. on May 31, 1985, page 273 ).
  • brackets [ ] indicate the content of each element (mass %), and calculation may be made by assuming that the content of an element that is not contained in the steel sheet is 0 mass%.
  • the steel sheet is cooled to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower.
  • T°C arbitrary cooling stop temperature
  • untransformed austenite can be transformed to martensite and hard bainite phase, and the MA structure also can be made finer.
  • martensite exists as quenched martensite immediately after the transformation; however, the martensite is tempered while being reheated and held in a later step and remains as tempered martensite. This tempered martensite does not give adverse effects on any of the ductility, stretch-flangeability, and crashworthiness of the steel sheet.
  • the cooling stop temperature T is set to be equal to or lower than the temperature of the Ms point.
  • the cooling stop temperature T is preferably (Ms point - 20°C) or lower, more preferably (Ms point - 50°C) or lower.
  • a lower limit of the cooling stop temperature T is set to be 100°C or higher.
  • the cooling stop temperature T is preferably 110°C or higher, more preferably 120°C or higher.
  • the temperature of the aforementioned Ms point can be calculated on the basis of the following formula (iv).
  • brackets [ ] indicate the content of each element (mass%), and calculation may be made by assuming that the content of an element that is not contained in the steel sheet is 0 mass%.
  • Ms point ° C 561 ⁇ 474 ⁇ C ⁇ 33 ⁇ Mn ⁇ 17 ⁇ Ni ⁇ 17 ⁇ Cr ⁇ 21 ⁇ Mo
  • an average cooling rate down to the cooling stop temperature T that lies in the above temperature range is set to be 10°C/sec or more. Excessive generation of ferrite can be suppressed by appropriately controlling the cooling rate down to the cooling stop temperature T after soaking and holding. In other words, when the average cooling rate is lower than 10°C/sec, ferrite is excessively generated during the cooling, and the stretch-flangeability decreases. Accordingly, in the present invention, the average cooling rate is set to be 10°C/sec or more. The average cooling rate is preferably 15°C/sec or more, more preferably 20°C/sec or more. An upper limit of the above average cooling rate is not particularly limited, and the steel sheet may be cooled by cooling with water or cooling with oil.
  • the steel sheet After the steel sheet is cooled down to an arbitrary cooling stop temperature T°C that lies in the temperature range of 100°C or higher and the Ms point or lower, it is important that the steel sheet is reheated to a temperature region of higher than the cooling stop temperature T°C to 550°C or lower, and the steel sheet is held in this temperature region for 50 seconds or more.
  • the hard phase such as martensite can be tempered, and untransformed austenite can be transformed to bainitic ferrite or bainite.
  • the steel sheet is reheated to a temperature exceeding the cooling stop temperature T after the steel sheet is cooled to the cooling stop temperature T.
  • the reheating temperature is preferably (T + 20°C) or higher, more preferably (T + 30°C) or higher, and still more preferably (T + 50°C) or higher.
  • the reheating temperature is set to be 550°C or lower.
  • the reheating temperature is preferably 520°C or lower, more preferably 500°C or lower, and still more preferably 450°C or lower.
  • reheating means, as it is stated, heating, that is, raising the temperature from the above cooling stop temperature T. Accordingly, the reheating temperature is a temperature higher than the above cooling stop temperature T. Therefore, even if the reheating temperature is, for example, within a temperature region of 100°C or higher and 550°C or lower, this does not fall under the category of the reheating of the present invention if the cooling stop temperature T and the reheating temperature are the same as each other or if the reheating temperature is lower than the cooling stop temperature T.
  • the steel sheet After the steel sheet is reheated to the temperature region of higher than the cooling stop temperature T°C to 550°C or lower, the steel sheet is held in the temperature region for 50 seconds or more.
  • the reheating holding time is less than 50 seconds, the MA structure is excessively generated, and the ductility cannot be improved. Further, the MA structure becomes coarse, and the average circle-equivalent diameter cannot be appropriately controlled, so that the stretch-flangeability cannot be improved either.
  • the ratio V MA /V ⁇ of the area ratio V MA of the MA structure to the volume ratio V ⁇ of the retained ⁇ cannot be appropriately controlled, so that the crashworthiness cannot be improved either.
  • the reheating holding time is set to be 50 seconds or more.
  • the reheating holding time is preferably 80 seconds or more, more preferably 100 seconds or more, and still more preferably 200 seconds or more.
  • An upper limit of the reheating holding time is not particularly limited.
  • the reheating holding time is preferably set to be 1500 seconds or less, more preferably 1000 seconds or less.
  • An average cooling rate during the cooling is not particularly limited; however, the average cooling rate is preferably, for example, 0.1°C/sec or more, more preferably 0.4°C/sec or more. Further, the average cooling rate is preferably, for example, 200°C/sec or less, more preferably 150°C/sec or less.
  • the high-strength cold-rolled steel sheet according to the present invention obtained by cooling to room temperature may be subjected to electro galvanization, hot-dip galvanizing, or hot-dip galvannealing in accordance with a conventional method.
  • the electro galvanization may be carried out, for example, by subjecting the above high-strength cold-rolled steel sheet to energization while immersing the steel sheet into a zinc solution of 50 to 60°C (particularly 55°C) so as to perform an electrogalvanization treatment.
  • the plating adhesion amount is not particularly limited and may be, for example, about 10 to 100 g/m 2 per one surface.
  • the hot-dip galvanizing may be carried out, for example, by immersing the above high-strength cold-rolled steel sheet into a hot-dip galvanizing bath of 300°C or higher and 550°C or lower, so as to perform a hot-dip galvanizing treatment.
  • the plating time may be suitably adjusted so that a desired plating adhesion amount can be ensured.
  • the plating time is preferably set to be, for example, 1 to 10 seconds.
  • the hot-dip galvannealing may be carried out by performing an alloying treatment after the above hot-dip galvanizing.
  • the alloying treatment temperature is not particularly limited; however, when the alloying treatment temperature is too low, the alloying does not proceed sufficiently, so that the alloying treatment temperature is preferably 450°C or higher, more preferably 460°C or higher, and still more preferably 480°C or higher. However, when the alloying treatment temperature is too high, the alloying proceeds too much, and the Fe concentration in the plating layer becomes high, thereby deteriorating the plating adhesion property. From such a viewpoint, the alloying treatment temperature is preferably 550°C or lower, more preferably 540°C or lower, and still more preferably 530°C or lower.
  • the alloying treatment time is not particularly limited and may be adjusted so that the hot-dip galvanized layer may be alloyed. The alloying treatment time is preferably, for example, 10 to 60 seconds.
  • a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can also be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3 point or higher and 900°C or lower, coiling with a coiling temperature being 600°C or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10°C/second or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T°C to 550°C or lower for 50 seconds or more, and after performing
  • the steps until heating to the temperature region of higher than the cooling stop temperature T°C to 550°C or lower are the same as those of the above-described method for producing a high-strength cold-rolled steel sheet according to the present invention, so that the hot-dip galvanizing and the holding for 50 seconds or more that is carried out in the above temperature region of higher than the cooling stop temperature T°C to 550°C or lower may be simultaneously carried out in the same step.
  • the hot-dip galvanizing may be carried out within the holding time in the reheating temperature region, that is, in the temperature region of higher than the cooling stop temperature T°C to 550°C or lower, and a conventional method can be adopted as a specific plating method.
  • the steel sheet heated to the temperature region of higher than the cooling stop temperature T°C to 550°C or lower may be immersed into a plating bath adjusted to have a temperature within the range of higher than the cooling stop temperature T°C to 550°C or lower, so as to perform a hot-dip galvanizing treatment.
  • the plating time may be suitably adjusted so that a desired plating amount can be ensured within the time of the reheating holding.
  • the plating time is preferably set to be, for example, 1 to 10 seconds.
  • the reheating temperature at which only the heating is carried out and the temperature of the plating bath used for performing the hot-dip galvanizing may be different from each other.
  • heating or cooling may be carried out from one temperature to the other temperature.
  • Furnace heating, induction heating, or the like may be adopted as a method for the heating.
  • a high-strength hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can also be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10°C/second or more, to a temperature region of the Ac 3 point or higher, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature T°C that lies in a temperature range of 100°C or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T°C to 550°C or lower for 50 seconds or more, and
  • the steps until heating to the temperature region of higher than the cooling stop temperature T°C to 550°C or lower are the same as those of the above-described method for producing a high-strength cold-rolled steel sheet according to the present invention, so that the hot-dip galvanizing and the holding for 50 seconds or more that is carried out in the above temperature region of higher than the cooling stop temperature T°C to 550°C or lower may be simultaneously carried out in the same step, and thereafter the hot-dip galvanized layer may be alloyed, followed by cooling down to room temperature.
  • the alloying treatment temperature is not particularly limited; however, when the alloying treatment temperature is too low, the alloying does not proceed sufficiently, so that the alloying treatment temperature is preferably 450°C or higher, more preferably 460°C or higher, and still more preferably 480°C or higher. However, when the alloying treatment temperature is too high, the alloying proceeds too much, and the Fe concentration in the plating layer becomes high, thereby deteriorating the plating adhesion property. From such a viewpoint, the alloying treatment temperature is preferably 550°C or lower, more preferably 540°C or lower, and still more preferably 530°C or lower.
  • the alloying treatment time is not particularly limited and may be adjusted so that the hot-dip galvanized layer may be alloyed.
  • the alloying treatment time is preferably, for example, 10 to 60 seconds.
  • the alloying treatment is carried out after performing the hot-dip galvanizing treatment for a predetermined period of time within the temperature region of higher than the cooling stop temperature T°C to 550°C or lower, so that the time needed for the alloying treatment is not included in the holding time within the temperature region of higher than the cooling stop temperature T°C to 550°C or lower.
  • the steel sheet After performing the hot-dip galvanizing within the holding time in the temperature region of higher than the cooling stop temperature T°C to 550°C or lower and performing the alloying treatment in accordance with the needs, the steel sheet may be cooled down to room temperature.
  • the average cooling rate during the cooling is not particularly limited; however, the average cooling rate is preferably, for example, 0.1°C/sec or more, more preferably 0.4°C/sec or more. Further, the average cooling rate is preferably, for example, 200°C/sec or less, more preferably 150°C/sec or less.
  • the high-strength cold-rolled steel sheet according to the present invention has a tensile strength of 980 MPa or more.
  • the tensile strength is preferably 1000 MPa or more, more preferably 1010 MPa or more.
  • the above high-strength cold-rolled steel sheet is excellent in formability as evaluated by ductility and stretch-flangeability, and also is excellent in crashworthiness.
  • the ductility can be evaluated by strength - elongation balance.
  • a product of the tensile strength TS (MPa) and the elongation EL (%) is 13000 MPa ⁇ % or more are rated as acceptable.
  • the value of TS ⁇ EL is preferably 13100 MPa ⁇ % or more, more preferably 13200 MPa ⁇ % or more.
  • the stretch-flangeability can be evaluated by strength - hole expansion ratio balance.
  • a product of the tensile strength TS (MPa) and the hole expansion ratio ⁇ (%) is 40000 MPa ⁇ % or more are rated as acceptable.
  • the value of TS ⁇ ⁇ is preferably 41000 MPa ⁇ % or more, more preferably 42000 MPa ⁇ % or more.
  • the crashworthiness can be evaluated by strength-VDA bending angle balance.
  • a product of the tensile strength TS (MPa) and the VDA bending angle (°) is 90000 MPa ⁇ ° or more are rated as acceptable.
  • the value of TS ⁇ VDA bending angle is preferably 90500 MPa ⁇ ° or more, more preferably 91000 MPa ⁇ ° or more.
  • the thickness of the high-strength cold-rolled steel sheet according to the present invention is not particularly limited; however, the steel sheet is preferably a thin steel sheet having a thickness of, for example, 6 mm or less.
  • FIG. 1 shows one example of a heat treatment pattern that was carried out in the continuous annealing.
  • the reference sign 1 denotes a heating step, 2 a soaking step, 3 a cooling step, 4 a reheating holding step, and 5 a cooling stop temperature.
  • a slab obtained by ingot-making was heated to 1250°C, and hot rolling was carried out to a sheet thickness of 2.3 mm with the rolling reduction in the final stand of finish rolling being set to be a rolling reduction shown in the following Table 2-1 or 2-2 and with the finish rolling end temperature being set to be a temperature shown in the following Table 2-1 or 2-2.
  • the steel sheet was cooled down to a coiling temperature shown in the following Table 2-1 or 2-2 at an average cooling rate of 30°C/sec, followed by coiling. After the coiling, the steel sheet was cooled in air to room temperature, so as to produce a hot-rolled steel sheet.
  • cold rolling was carried out to produce a cold-rolled steel sheet having a thickness of 1.2 mm.
  • the obtained cold-rolled steel sheet was subjected to continuous annealing based on the heat treatment pattern shown in FIG. 1 . That is, the obtained cold-rolled steel sheet was heated as a heating step at an average heating rate shown in the following Table 2-1 or 2-2 up to the soaking temperature shown in the following Table 2-1 or 2-2, and was held at the soaking temperature as a soaking step.
  • the following Table 2-1 or 2-2 shows the soaking time. Further, the following Table 2-1 or 2-2 shows a value calculated by subtracting the temperature of the Ac 3 point from the soaking temperature.
  • the steel sheet was cooled as a cooling step at an average cooling rate shown in the following Table 2-1 or 2-2 down to the cooling stop temperature T°C shown in the following Table 2-1 or 2-2.
  • the steel sheet was heated to the reheating temperature shown in the following Table 2-1 or 2-2 and was held at the reheating temperature as a reheating holding step, followed by cooling down to room temperature to produce a test sample material.
  • the following Table 2-1 or 2-2 shows the reheating holding time. Also, the following Table 2-1 or 2-2 shows a value calculated by subtracting the cooling stop temperature T from the reheating temperature.
  • the Ms point was calculated in accordance with the above formula (iv) based on the component composition shown in the following Table 1.
  • the results are shown in the following Tables 2-1 and 2-2.
  • the following Tables 2-1 and 2-2 also show a value obtained by subtracting the temperature of the Ms point from the cooling stop temperature T.
  • No. 11 shown in the following Table 2-1 and No. 29 shown in the following Table 2-2 are samples in which the reheating holding step was not carried out after the cooling was stopped at the cooling stop temperature T shown in the following Table 2-1 or 2-2. That is, in No. 11, the steel sheet was cooled with the cooling stop temperature T set to be 440°C, and thereafter cooled to 350°C, which was lower than that temperature, and held at 350°C for 600 seconds.
  • the following Table 2-1 gives 350°C in the section of the reheating temperature and gives 600 seconds in the section of the reheating holding time.
  • the steel sheet was cooled with the cooling stop temperature T set to be 350°C, and thereafter cooled to 330°C, which was lower than that temperature, and held at 330°C for 300 seconds.
  • the following Table 2-2 gives 330°C in the section of the reheating temperature and gives 300 seconds in the section of the reheating holding time.
  • No. 15 shown in the following Table 2-1 is a sample in which the above test sample material was immersed into a galvanizing bath of 55°C to perform an electrogalvanization treatment and thereafter washed with water and dried to produce an electrogalvanized steel sheet.
  • the electrogalvanization treatment was carried out with an electric current density set to be 40 A/dm 2 .
  • the galvanizing adhesion amount was 40 g/m 2 per one surface.
  • washing treatments such as degreasing with alkaline aqueous solution immersion, washing with water, and pickling or the like were carried out as appropriate, so as to produce a test sample material having an electrogalvanized layer on the surface of the cold-rolled steel sheet.
  • Table 2-1 the section of classification for No. 15 gives "EG".
  • No. 36 shown in the following Table 2-2 is a sample in which the above test sample material was immersed into a hot-dip galvanizing bath of 460°C to perform a hot-dip galvanizing treatment, thereby to produce a hot-dip galvanized steel sheet.
  • the hot-dip galvanizing adhesion amount was 30 g/m 2 per one surface.
  • Table 2-2 the section of classification for No. 36 gives "GI".
  • No. 6 shown in the following Table 2-1 is a sample in which the above test sample material was immersed into a hot-dip galvanizing bath of 460°C to perform a hot-dip galvanizing treatment, followed by heating to 500°C to perform an alloying treatment, thereby to produce a hot-dip galvannealed steel sheet.
  • the hot-dip galvannealing adhesion amount was 30 g/m 2 per one surface.
  • the section of classification for No. 6 gives "GA".
  • Test sample materials in which none of the electrogalvanization treatment, hot-dip galvanizing treatment, and hot-dip galvannealing treatment was carried out are denoted as "cold-rolled" in the section of classification in the following Tables 2-1 and 2-2.
  • test sample material was subjected to nital corrosion, followed by performing observation at the position of 1/4 of the sheet thickness in three fields of view at a magnification of 1000 times with a scanning electron microscope, so as to capture a photomicrograph image.
  • the observation field of view was such that one field of view had a size of 100 ⁇ m ⁇ 100 ⁇ m.
  • the lattice interval set to be 5 ⁇ m the area ratio of ferrite was measured by the point counting method with the number of lattice points being 20 ⁇ 20, and an average value of the three fields of view was calculated. The calculation results are shown in the following Tables 3-1 and 3-2.
  • the area ratio of ferrite was calculated by excluding the area ratio of the hard phase that existed in the ferrite phase.
  • the structure other than ferrite, pearlite, and cementite calculated by the point counting method was assumed to be a hard phase.
  • a value obtained by subtracting the area ratio of ferrite and the sum area ratio of pearlite and cementite from 100% was calculated as an area ratio of the hard phase.
  • the hard phase included quenched martensite and retained ⁇ and included at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.
  • test sample material was polished down to the position of 1/4 of the sheet thickness with use of a sandpaper of #1000 to #1500, and further the surface was subjected to electrolytic polishing down to the depth of 10 to 20 ⁇ m, followed by measuring the volume ratio V ⁇ of retained ⁇ with use of an X-ray diffractometer.
  • "RINT 1500" manufactured by Rigaku Corporation was used as the X-ray diffractometer and, with use of a Co target, a power of 40 kV - 200 mA was output to measure the range of 40° to 130° in terms of 2 ⁇ .
  • test sample material was subjected to LePera corrosion, followed by performing observation at the position of 1/4 of the sheet thickness in three fields of view at a magnification of 1000 times with an optical microscope, so as to capture a photomicrograph image.
  • the observation field of view was such that one field of view had a size of 100 ⁇ m ⁇ 100 ⁇ m.
  • the portion whitened by LePera corrosion was regarded as the MA structure.
  • the lattice interval set to be 5 ⁇ m the area ratio of the MA structure was measured by the point counting method with the number of lattice points being 20 ⁇ 20, and an average value of the three fields of view was calculated. The calculation results are shown in the following Tables 3-1 and 3-2.
  • the ratio V MA /V ⁇ of the area ratio V MA of the MA structure to the volume ratio V ⁇ of the retained ⁇ was calculated on the basis of the volume ratio V ⁇ of the retained ⁇ and the area ratio V MA of the MA structure calculated by the above-described procedure.
  • the calculation results are shown in the following Tables 3-1 and 3-2.
  • test piece defined in JIS Z2201 was cut out so that the direction perpendicular to the rolling direction of the obtained test sample material would be a longitudinal direction.
  • a tensile test was carried out so as to measure the tensile strength TS and the elongation EL. The measurement results are shown in the following Tables 3-1 and 3-2.
  • the samples in which the tensile strength was 980 MPa or more were evaluated as having a high strength and being acceptable, whereas the samples in which the tensile strength was less than 980 MPa were evaluated as having an insufficient strength and being a reject.
  • TS ⁇ elongation EL was calculated on the basis of the measured values of tensile strength TS and elongation EL. The calculation results are shown in the following Tables 3-1 and 3-2.
  • the value of TS ⁇ EL indicates a strength - elongation balance and serves as an index for evaluating the ductility.
  • the samples in which the value of TS ⁇ EL was 13000 MPa ⁇ % or more were evaluated as having an excellent ductility and being acceptable, whereas the samples in which the value of TS ⁇ EL was less than 13000 MPa ⁇ % were evaluated as having a poor ductility and being a reject.
  • TS ⁇ hole expansion ratio ⁇ was calculated on the basis of the measured values of tensile strength TS and hole expansion ratio ⁇ . The calculation results are shown in the following Tables 3-1 and 3-2.
  • the value of TS ⁇ ⁇ indicates a strength - hole expansion ratio balance and serves as an index for evaluating the stretch-flangeability.
  • the samples in which the value of TS ⁇ ⁇ was 40000 MPa ⁇ % or more were evaluated as having an excellent stretch-flangeability and being acceptable, whereas the samples in which the value of TS ⁇ ⁇ was less than 40000 MPa ⁇ % were evaluated as having a poor stretch-flangeability and being a reject.
  • the samples in which the value of TS ⁇ VDA was 90000 MPa ⁇ ° or more were evaluated as having an excellent crashworthiness and being acceptable, whereas the samples in which the value of TS ⁇ VDA was less than 90000 MPa ⁇ ° were evaluated as having a poor crashworthiness and being a reject.
  • the samples rated as "reject" in the total evaluation section are steel sheets that do not satisfy one or more of the requirements defined in the present invention, and at least one of ductility, stretch-flangeability, and crashworthiness could not be improved.
  • the details are as follows.
  • No. 2 is a sample in which a predetermined amount of retained ⁇ and the MA structure could not be ensured because the cooling stop temperature T after the soaking was an extremely low temperature of 25°C which was lower than 100°C, so that the value of V MA /V ⁇ was below the defined range. As a result, the value of TS ⁇ EL was small, so that the ductility could not be improved.
  • Nos. 3 and 38 are samples in which the MA structure was coarsened because the average heating rate after the coiling was too small. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • No. 4 is a sample in which the MA structure was coarsened because the cooling stop temperature T after the soaking was too high and exceeded the temperature region of 100°C or higher and the Ms point or lower.
  • the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • the value of TS ⁇ VDA was small, so that the crashworthiness could not be improved.
  • No. 7 is a sample in which the MA structure was coarsened because the finish rolling end temperature was too high. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • No. 8 is a sample in which the MA structure was coarsened because the coiling temperature was too high. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • Nos. 9 and 39 are samples in which ferrite was excessively generated because the average cooling rate after the soaking was too small. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • No. 11 is a sample in which the value of V MA /V ⁇ was too large because the cooling stop temperature T after the soaking was too high and exceeded the temperature region of 100°C or higher and the Ms point or lower and because the reheating holding was not carried out after the cooling. As a result, the value of TS ⁇ VDA was small, so that the crashworthiness could not be improved.
  • No. 13 is a sample in which the MA structure was coarsened because the rolling reduction at the final stand during the finish rolling was too high and exceeded the range defined in the present invention.
  • the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • the value of TS ⁇ VDA was small, so that the crashworthiness could not be improved.
  • No. 14 is a sample in which the MA structure was coarsened because the rolling reduction at the final stand during the finish rolling was too low and was below the range defined in the present invention. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • No. 16 is a sample in which ferrite was excessively generated because the soaking was carried out at a temperature below the AC 3 point. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • No. 23 is a sample in which the MA structure was coarsened because the reheating holding time was too short.
  • the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • the MA structure was generated excessively.
  • the value of TS ⁇ EL was small, so that the ductility could not be improved.
  • the value of V MA /V ⁇ was too large.
  • the value of TS ⁇ VDA was small, so that the crashworthiness was deteriorated.
  • No. 24 is a sample in which decomposition of austenite occurred and a predetermined amount of retained ⁇ and the MA structure could not be ensured because the reheating temperature carried out after the cooling was too high. As a result, TS was small.
  • No. 29 is a sample in which the MA structure was coarsened and the value of V MA /Y ⁇ was too large because the rolling reduction at the final stand during the finish rolling was too high and exceeded the range defined in the present invention and because the reheating holding was not carried out after the cooling.
  • the value of TS ⁇ ⁇ was small, so that the stretch-flangeability could not be improved.
  • the value of TS ⁇ VDA was small, so that the crashworthiness could not be improved.
  • No. 33 is a sample in which the C amount was too small, so that a retained ⁇ amount within the range defined in the present invention could not be ensured. As a result, the value of TS ⁇ EL was small, so that the ductility was deteriorated.
  • No. 34 is a sample in which the Si amount was too small, so that a retained ⁇ amount within the range defined in the present invention could not be ensured. As a result, the value of TS ⁇ EL was small, so that the ductility was deteriorated.
  • No. 35 is a sample in which the Mn amount was too small, so that the hardenability was insufficient, and ferrite was excessively generated. As a result, the value of TS ⁇ ⁇ was small, so that the stretch-flangeability was deteriorated.
  • No. 41 is a sample in which a predetermined amount of retained ⁇ could not be ensured because the cooling stop temperature T after the soaking was below 100°C. As a result, the value of TS ⁇ EL was small, so that the ductility could not be improved.
EP16772043.2A 2015-03-31 2016-03-01 Hochfestes kaltgewalztes stahlblech mit hervorragenden bearbeitbarkeits- und kollisionseigenschaften und einer zugfestigkeit von 980 mpa oder mehr sowie verfahren zur herstellung davon Active EP3279362B1 (de)

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