Classifications

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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
  • 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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • 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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
• • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
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  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • 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
• • 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
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F17/00—Multi-step processes for surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by subclass C21D or C22F or class C25

Definitions

• This disclosure relates to a high-strength steel sheet with excellent formability which is mainly suitable for automobile structural members and a method for manufacturing the same, and in particular, to provision of a high-strength steel sheet with high productivity that has a tensile strength (TS) of 780 MPa or more and that is excellent in ductility as well as in stretch flangeability and fatigue properties.
• TS tensile strength
• JP2004218025A (PTL 1) describes "a high-strength steel sheet with excellent workability and shape fixability comprising: a chemical composition containing, in mass%, C: 0.06 % to 0.6 %, Si + Al: 0.5 % to 3 %, Mn: 0.5 % to 3 %, P: 0.15 % or less (exclusive of 0 %), and S: 0.02 % or less (inclusive of 0 %); and a structure that contains tempered martensite: 15 % or more by area to the entire structure, ferrite: 5 % to 60 % by area to the entire structure, and retained austenite: 5 % or more by volume to the entire structure, and that may contain bainite and/or martensite, wherein a ratio of the retained austenite transforming to martensite upon application of a 2 % strain is 20 % to 50 %.
• JP2011195956A (PTL 2) describes "a high-strength thin steel sheet with excellent elongation and hole expansion formability, comprising: a chemical composition containing, in mass%, C : 0.05 % or more and 0.35 % or less, Si: 0.05 % or more and 2.0 % or less, Mn: 0.8 % or more and 3.0 % or less, P : 0.0010 % or more and 0.1 % or less, S : 0.0005 % or more and 0.05 % or less, N : 0.0010 % or more and 0.010 % or less, and Al: 0.01 % or more and 2.0 % or less, and the balance consisting of iron and incidental impurities; and a metallographic structure that includes a dominant phase of ferrite, bainite, or tempered martensite, and a retained austenite phase in an amount of 3 % or more and 30 % or less, wherein at a phase interface at which the austenite phase comes in contact with
• JP201090475A (PTL 3) describes "a high-strength steel sheet comprising a chemical composition containing, in mass%, C : 0.17 % or more and 0.73 % or less, Si: 3.0 % or less, Mn: 0.5 % or more and 3.0 % or less, P: 0.1 % or less, S: 0.07 % or less, Al: 3.0 % or less, and N: 0.010 % or less, where Si + Al is 0.7 % or more, and the balance consisting of Fe and incidental impurities; and a structure that contains martensite: 10 % or more and 90 % or less by area to the entire steel sheet structure, retained austenite content: 5 % or more and 50 % or less, and bainitic ferrite in upper bainite: 5 % or more by area to the entire steel sheet structure, wherein the steel sheet satisfies conditions that 25 % or more of the martensite is tempered martensite, a total of the area ratio
• JP2008174802A (PTL 4) describes "a high-strength cold-rolled steel sheet with a high yield ratio and having a tensile strength of 980 MPa or more, the steel sheet comprising, on average, a chemical composition that contains, by mass%, C : more than 0.06 % and 0.24 % or less, Si ⁇ 0.3 %, Mn: 0.5 % to 2.0 %, P ⁇ 0.06 %, S ⁇ 0.005 %, Al ⁇ 0.06 %, N ⁇ 0.006 %, Mo: 0.05 % to 0.5 %, Ti: 0.03 % to 0.2 %, and V: more than 0.15 % and 1.2 % or less, and the balance consisting of Fe and incidental impurities, wherein the contents of C, Ti, Mo, and V satisfy 0.8 ⁇ (C/12)/ ⁇ (Ti/48) + (Mo/96) + (V/51) ⁇ ⁇ 1.5, and wherein an area ratio of ferrite phase is 95
• JP2010275627A (PTL 5) describes "a high-strength steel sheet with excellent workability comprising a chemical composition containing C : 0.05 mass% to 0.3 mass%, Si: 0.01 mass% to 2.5 mass%, Mn: 0.5 mass% to 3.5 mass%, P: 0.003 mass% to 0.100 mass%, S: 0.02 mass% or less, and Al: 0.010 mass% to 1.5 mass%, where a total of the Si and Al contents is 0.5 mass% to 3.0 mass%, and the balance consisting of Fe and incidental impurities; and a metallic structure that contains, by area, ferrite: 20 % or more, tempered martensite: 10 % to 60 %, and martensite: 0 % to 10 %, and that contains, by volume, retained austenite: 3 % to 10 %, where a ratio (m)/(f) of a Vickers hardness (m) of the tempered martensite to a Vickers hardness (f) of the ferrite
• JP4268079B (PTL 6) describes "an ultra-high-strength steel sheet exhibiting an excellent elongation in an ultra-high-strength range with a tensile strength of 1180 MPa or more, and having excellent hydrogen embrittlement resistance, the steel sheet comprising a chemical composition containing, in mass%, C : 0.06 % to 0.6 %, Si + Al: 0.5 % to 3 %, Mn: 0.5 % to 3 %, P : 0.15 % or less (exclusive of 0 %), S: 0.02 % or less (inclusive of 0 %), and the balance: Fe and incidental impurities; and a structure that contains tempered martensite: 15 % to 60 % by area to the entire structure, ferrite: 5 % to 50 % by area to the entire structure, retained austenite: 5 % or more by area to the entire structure, and massive martensite with an aspect ratio of 3 or less: 15 % to 45 %, where an
• PTL 6 also describes a method for manufacturing the ultra-high-strength steel sheet comprising: heating and retaining a steel satisfying the aforementioned composition at a temperature from A 3 to 1100 °C for 10 s or more, and then cooling the steel at a mean cooling rate of 30 °C/s or higher to a temperature at or below Ms, and repeating this cycle at least twice; and heating and retaining the steel at a temperature from (A 3 - 25 °C) to A 3 for 120 s to 600 s, and then cooling the steel at a mean cooling rate of 3 °C/s or higher to a temperature at or above Ms and at or below Bs, at which the steel is retained for at least one second.
• PTL 1 teaches the high-strength steel sheet has excellent workability and shape fixability
• PTL 2 teaches the high-strength thin steel sheet has excellent elongation and hole expansion formability
• PTL 3 teaches the high-strength steel sheet has excellent workability, in particular, excellent ductility and stretch flangeability. None of them however takes into account fatigue properties.
• the high-strength cold-rolled steel sheet with a high yield ratio described in PTL 4 uses expensive elements, Mo and V, which results in increased costs and a low elongation (EL), which is as low as approximately 19 %.
• the high-strength steel sheet described in PTL 5 exhibits, for example, TS of 980 MPa or more and TS x EL of approximately 24000 MPa ⁇ %, which remain, although may be relatively high when compared to general-use material, insufficient to meet the ongoing requirements for steel sheets.
• the ultra-high tensile-strength steel sheet described in PTL 6 requires performing annealing treatment at least three times during its manufacture, resulting in low productivity in actual facilities.
• TS tensile strength
• high-strength steel sheet is intended to include high-strength galvanized steel sheets having a galvanized surface.
• a steel sheet obtained according to the disclosure has the following target properties:
• a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, stretch flangeability, and fatigue properties with high productivity.
• a high-strength steel sheet manufactured by the method according to the disclosure is highly beneficial in industrial terms, because it can improve fuel efficiency when applied to, e.g., automobile structural members by a reduction in the weight of automotive bodies.
• a steel slab with a predetermined chemical composition is heated and hot rolled.
• CT mean coiling temperature
• the hot-rolled steel sheet is provided with a structure in which a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present.
• cold rolling is performed after hot rolling to set as low a rolling reduction as possible so that the resulting structure of the hot-rolled steel sheet will remain intact as much as possible.
• a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present in the structure of the steel sheet before subjection to annealing treatment. Consequently, even when annealing treatment is performed just once at a ferrite-austenite dual phase region, it becomes possible to form a structure that contains appropriate amounts of ferrite, bainitic ferrite, and retained austenite, and in which fine retained austenite and fine bainitic ferrite are distributed.
• the structure may contain an appropriate amount of tempered martensite. As a result, it becomes possible to manufacture a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, stretch flangeability, and fatigue properties with high productivity.
• TS tensile strength
• C is an element that is important for increasing the strength of steel, has a high solid solution strengthening ability, and is essential for guaranteeing the presence of a desired amount of retained austenite to improve ductility.
• the steel sheet is made brittle or susceptible to delayed fracture.
• the C content is 0.10 % or more and 0.35 % or less, preferably 0.15 % or more and 0.30 % or less, and more preferably 0.18 % or more and 0.26 % or less.
• Si 0.50 % or more and 2.50 % or less
• Si is an element that is effective in suppressing decomposition of retained austenite to carbides. Si also exhibits a high solid solution strengthening ability in ferrite, and has the property of purifying ferrite by facilitating solute C diffusion from ferrite to austenite to improve ductility. Moreover, Si dissolved in ferrite improves strain hardenability and increases the ductility of ferrite itself. To obtain this effect, the Si content needs to be 0.50 % or more. If the Si content exceeds 2.50 %, however, an abnormal structure grows, causing ductility to deteriorate.
• the Si content is 0.50 % or more and 2.50 % or less, preferably 0.80 % or more and 2.00 % or less, and more preferably 1.20 % or more and 1.80 % or less.
• Mn 2.00 % or more and less than 3.50 %
• Mn is effective in guaranteeing strength. Mn also improves hardenability to facilitate formation of a multi-phase structure. Moreover, Mn acts to suppress formation of ferrite and pearlite during a cooling process after hot rolling, and thus is an effective element in causing the hot-rolled sheet to have a structure in which a low temperature transformation phase (bainite or martensite) is dominantly present. To obtain this effect, the Mn content needs to be 2.00 % or more. If the Mn content is 3.50 % or more, however, Mn segregation becomes significant in the sheet thickness direction, leading to deterioration of fatigue properties.
• the Mn content is 2.00 % or more and less than 3.50 %, preferably 2.00 % or more and 3.00 % or less, and more preferably 2.00 % or more and 2.80 % or less.
• P is an element that has a solid solution strengthening effect and can be added depending on a desired strength. P also facilitates transformation to ferrite, and thus is an effective element in forming a multi-phase structure. To obtain this effect, the P content needs to be 0.001 % or more. If the P content exceeds 0.100 %, however, weldability degrades and, when a galvanized layer is subjected to alloying treatment, the alloying rate decreases, impairing galvanizing quality.
• the P content is 0.001 % or more and 0.100 % or less, and preferably 0.005 % or more and 0.050 % or less.
• the S content needs to be 0.0200 % or less. Under manufacturing constraints, however, the S content is necessarily 0.0001 % or more.
• the S content is 0.0001 % or more and 0.0200% or less, and preferably 0.0001 % or more and 0.0050 % or less.
• N 0.0005 % or more and 0.0100 % or less
• N is an element that deteriorates the anti-aging property of steel. Deterioration of the anti-aging property becomes more pronounced, particularly when the N content exceeds 0.0100 %. Under manufacturing constraints, the N content is necessarily 0.0005 % or more, although smaller N contents are more preferable.
• the N content is 0.0005 % or more and 0.0100 % or less, and preferably 0.0005 % or more and 0.0070 % or less.
• At least one element selected from the group consisting of Ti and B may also be included.
• the resulting hot-rolled sheet may be provided more advantageously with a structure in which a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present.
• Ti forms fine precipitates during hot rolling or annealing to increase strength.
• Ti precipitates as TiN with N, and may thus suppress precipitation of BN when B is added to the steel, thereby effectively bringing out the effect of B as described below.
• the Ti content needs to be 0.005 % or more. If the Ti content exceeds 0.100 %, however, strengthening by precipitation works excessively, leading to deterioration of ductility. Therefore, the Ti content is preferably 0.005 % or more and 0.100 % or less, and more preferably 0.010 % or more and 0.080 % or less.
• B has the effect of suppressing ferrite-pearlite transformation during a cooling process after hot rolling so that the hot-rolled sheet has a structure in which a low temperature transformation phase (bainite or martensite), in particular martensite is dominantly present.
• B is also effective in increasing the strength of steel. To obtain this effect, the B content needs to be 0.0001 % or more. However, excessively adding B beyond 0.0050 % forms excessive martensite, raising a concern that ductility might decrease due to a rise in strength.
• the B content is preferably 0.0001 % or more and 0.0050 % or less, and more preferably 0.0005 % or more and 0.0030 % or less.
• Mn content/B content the Mn content divided by the B content (Mn content/B content) equals 2100 or less, and more preferably 2000 or less. No lower limit is particularly placed on the Mn content/B content, yet a preferred lower limit is approximately 300.
• At least one element selected from the group consisting of the following may also be included:
• A1 0.01 % or more and 1.00 % or less
• Al is an element that is effective in forming ferrite and improving the balance between strength and ductility. To obtain this effect, the Al content needs to be 0.01 % or more. On the other hand, an A1 content exceeding 1.00 % leads to deterioration of surface characteristics.
• the Al content is 0.01 % or more and 1.00 % or less, and preferably 0.03 % or more and 0.50 % or less.
• Nb 0.005 % or more and 0.100 % or less
• Nb forms fine precipitates during hot rolling or annealing to increase strength. To obtain this effect, the Nb content needs to be 0.005 % or more. If the Nb content exceeds 0.100 %, however, formability deteriorates.
• the Nb content is 0.005 % or more and 0.100 % or less.
• Cr and Cu not only serve as solid-solution-strengthening elements, but also act to stabilize austenite in a cooling process during annealing, facilitating formation of a multi-phase structure.
• the Cr and Cu contents each need to be 0.05 % or more. If the Cr and Cu contents both exceed 1.00 %, formability deteriorates.
• Sb and Sn may be added as necessary for suppressing decarbonization of a region extending from the surface layer of the steel sheet to a depth of about several tens of micrometers, which is caused by nitriding and/or oxidation of the steel sheet surface. Suppressing such nitriding or oxidation is effective in preventing a reduction in the amount of martensite formed in the steel sheet surface and guaranteeing strength. To obtain this effect, the Sb and Sn contents each need to be 0.002 % or more. However, excessively adding any of these elements beyond 0.200 % leads to deterioration of toughness. Therefore, when Sb and Sn are added to steel, respective contents are 0.002 % or more and 0.200 % or less.
• Ta 0.001 % or more and 0.100 % or less
• Ta forms alloy carbides or alloy carbonitrides, and contributes to increasing the strength of steel. It is also believed that Ta has the effect of significantly suppressing coarsening of precipitates when partially dissolved in Nb carbides or Nb carbonitrides to form complex precipitates, such as (Nb, Ta) (C, N), and providing a stable contribution to increasing strength through strengthening by precipitation.
• This precipitate-stabilizing effect can be obtained when the Ta content is 0.001 % or more.
• excessively adding Ta beyond 0.100 % fails to further increase the precipitate-stabilizing effect, but instead increases alloy costs. Therefore, when Ta is added to steel, the Ta content is 0.001 % or more and 0.100 % or less.
• Ca, Mg, and REM are elements that are used for deoxidation, and are effective in causing spheroidization of sulfides and mitigating the adverse effect of sulfides on local ductility and stretch flangeability.
• Ca, Mg, and REM each need to be added to steel in an amount of 0.0005 % or more.
• excessively adding Ca, Mg, and REM beyond 0.0050 % leads to increased inclusions and the like, causing defects on the steel sheet surface and internal defects.
• the balance other than the above components consists of Fe and incidental impurities.
• the method for manufacturing a high-strength steel sheet according to the disclosure comprises: preparing a steel slab with the aforementioned chemical composition; subjecting the steel slab to hot rolling by heating the steel slab to a temperature of 1100 °C or higher and 1300 °C or lower, hot rolling the steel slab with a finisher delivery temperature of 800 °C or higher and 1000 °C or lower to form a hot-rolled steel sheet, and coiling the hot-rolled steel sheet at a mean coiling temperature of 200 °C or higher and 500 °C or lower; subjecting the hot-rolled steel sheet to pickling treatment; optionally cold rolling the hot-rolled steel sheet at a rolling reduction below 30 % to form a cold-rolled steel sheet; subjecting the hot-rolled or cold-rolled steel sheet to annealing by retaining the steel sheet at a temperature of 740 °C or higher and 840 °C or lower for 10 s or more and 900 s or less, and then cooling the
• the temperatures such as the finisher delivery temperature, the mean coiling temperature, and the like, all represent temperatures measured at the steel sheet surface.
• the mean cooling rate is also calculated from temperatures measured at the steel sheet surface. The following explains the reasons for the limitations placed on the manufacturing conditions.
• Precipitates that are present at the time of heating of a steel slab will remain as coarse precipitates in the resulting steel sheet, making no contribution to strength. Thus, remelting of any Ti- and Nb-based precipitates precipitated during casting is required.
• the steel slab heating temperature needs to be 1100 °C or higher.
• the steel slab heating temperature needs to be 1300 °C or lower.
• the steel slab heating temperature is 1100 °C or higher and 1300 °C or lower, and preferably 1150 °C or higher and 1250 °C or lower.
• a steel slab is preferably made with continuous casting to prevent macro segregation, yet may be produced with other methods such as ingot casting or thin slab casting.
• the steel slab thus produced may be cooled to room temperature and then heated again according to the conventional method.
• energy-saving processes such as hot direct rolling or direct rolling in which either a warm steel slab without being fully cooled to room temperature is charged into a heating furnace, or a steel slab undergoes heat retaining for a short period and immediately hot rolled.
• a steel slab is subjected to rough rolling under normal conditions and formed into a sheet bar. When the heating temperature is low, the sheet bar is preferably heated using a bar heater or the like prior to finish rolling from the viewpoint of preventing troubles during hot rolling.
• Finisher delivery temperature in hot rolling 800 °C or higher and 1000 °C or lower
• the heated steel slab is hot rolled through rough rolling and finish rolling to form a hot-rolled steel sheet.
• finisher delivery temperature exceeds 1000 °C
• the amount of oxides (scales) generated suddenly increases and the interface between the steel substrate and oxides becomes rough, which tends to impair the surface quality after pickling and cold rolling.
• any hot-rolling scales remaining after pickling adversely affect ductility. Further, grain size increases excessively and fatigue properties deteriorate.
• the finisher delivery temperature is below 800 °C, rolling load and burden increase, rolling is performed more often in a state in which recrystallization of austenite does not occur, and an abnormal texture develops.
• the final product has a significant planar anisotropy, and not only does the material properties become less uniform, but also the ductility itself deteriorate.
• the finisher delivery temperature in hot rolling needs to be 800 °C or higher and 1000 °C or lower, and preferably 820 °C or higher and 950 °C or lower.
• Mean coiling temperature after hot rolling 200 °C or higher and 500 °C or lower
• the mean coiling temperature after the hot rolling is above 500 °C
• ferrite and pearlite form during cooling and retaining processes after the hot rolling. Consequently, it becomes difficult to provide the hot-rolled sheet with a structure in which a single phase structure of martensite, a single phase structure of bainite, or a martensite-bainite mixed structure is dominantly present, making it difficult to impart a desired ductility to the steel sheet obtained after annealing or to balance its strength and ductility.
• the mean coiling temperature after the hot rolling is below 200 °C, the hot-rolled steel sheet is degraded in terms of shape, deteriorating productivity.
• the mean coiling temperature after the hot rolling needs to be 200 °C or higher and 500 °C or lower, preferably 300 °C or higher and 450 °C or lower, and more preferably 350 °C or higher and 450 °C or lower.
• Finish rolling may be performed continuously by joining rough-rolled sheets during the hot rolling.
• Rough-rolled sheets may be coiled on a temporary basis.
• At least part of finish rolling may be conducted as lubrication rolling to reduce rolling load in hot rolling.
• Conducting lubrication rolling in such a manner is effective from the perspective of making the shape and material properties of a steel sheet uniform.
• the coefficient of friction is preferably 0.10 or more and 0.25 or less.
• the hot-rolled steel sheet thus produced is subjected to pickling.
• Pickling enables removal of oxides from the steel sheet surface, and is thus important to ensure that the high-strength steel sheet as the final product has good chemical convertibility and a sufficient quality of coating.
• Pickling may be performed in one or more batches.
• the hot-rolled steel sheet may be subjected to cold rolling to form a cold-rolled steel sheet.
• cold rolling is performed, rolling reduction in cold rolling is of great importance.
• the rolling reduction in cold rolling is less than 30 %, preferably 25 % or less, and more preferably 20 % or less. No lower limit is particularly placed on the rolling reduction in cold rolling. It may be greater than 0 %.
• the number of rolling passes and the rolling reduction per pass are not particularly limited, and the effect of the disclosure may be obtained with any number of rolling passes and any rolling reduction per pass.
• Annealing temperature 740 °C or higher and 840 °C or lower
• An annealing temperature below 740 °C cannot ensure formation of a sufficient amount of austenite during the annealing. Consequently, a desired amount of retained austenite cannot be obtained in the end, making it difficult to yield good ductility and to balance strength and ductility.
• an annealing temperature above 840 °C is within a temperature range of austenite single phase, and a desired amount of fine retained austenite cannot be produced in the end, which makes it difficult again to ensure good ductility and to balance strength and ductility.
• the annealing temperature is 740 °C or higher and 840 °C or lower, and preferably 750 °C or higher and 830 °C or lower.
• Annealing treatment holding time 10 s or more and 900 s or less
• a annealing treatment holding time shorter than 10 s cannot ensure formation of a sufficient amount of austenite during the annealing. Consequently, a desired amount of retained austenite cannot be obtained in the end, making it difficult to yield good ductility and to balance strength and ductility.
• an annealing treatment holding time longer than 900 s causes grain coarsening, a desired amount of fine retained austenite cannot be produced in the end, making it difficult to ensure good ductility and to balance strength and ductility. This also inhibits productivity.
• the annealing treatment holding time is 10 s or more and 900 s or less, preferably 30 s or more and 750 s or less, and more preferably 60 s or more and 600 s or less.
• the mean cooling rate to a cooling stop temperature of 150 °C or higher and 350 °C or lower is below 5 °C/s, a large amount of ferrite is produced during cooling, making it difficult to guarantee a desired strength.
• the mean cooling rate is above 30 °C/s, a low temperature transformation phase forms excessively, degrading ductility.
• the mean cooling rate to a cooling stop temperature of 150 °C or higher and 350 °C or lower is 5 °C/s or higher and 30 °C/s or lower, and preferably 10 °C/s or higher and 30 °C/s or lower.
• the cooling in the annealing is preferably performed by gas cooling; however, furnace cooling, mist cooling, roll cooling, water cooling, and the like can also be employed in combination.
• the cooling stop temperature is above 350 °C, it is higher than the martensite transformation starting temperature (Ms), with the result that tempered martensite is not produced when reheating treatment is performed subsequently, hard and fresh martensite (martensite not tempered) remains in the resulting structure, and hole expansion formability (stretch flangeability) ends up deteriorating.
• Ms martensite transformation starting temperature
• the cooling stop temperature is below 150 °C, austenite transforms to martensite in a large amount, and a desired amount of retained austenite cannot be obtained in the end, making it difficult to obtain good ductility and to balance strength and ductility. Therefore, the cooling stop temperature is 150 °C or higher and 350 °C or lower, and preferably 180 °C or higher and 320 °C or lower.
• Reheating temperature higher than 350 °C and 550 °C or lower
• the reheating temperature is above 550 °C, decomposition of retained austenite occurs, and a desired amount of retained austenite cannot be obtained in the end, making it difficult to yield good ductility and balance strength and ductility.
• the heating temperature is 350 °C or lower, a desired amount of tempered martensite cannot be obtained, making it difficult to ensure hole expansion formability (stretch flangeability).
• the reheating temperature is higher than 350 °C and 550 °C or lower, and preferably 370 °C or higher and 530 °C or lower.
• Holding time at reheating temperature 10 s or more
• the holding time at the reheating temperature is 10 s or more.
• a holding time longer than 600 s does not increase the amount of retained austenite and ductility does not significantly improve, where the effect reaches a plateau. Therefore, the holding time at the reheating temperature is preferably 600 s or less, more preferably 30 s or more and 500 s or less, and still more preferably 60 s or more and 400 s or less.
• Cooling after the holding is not particularly limited, and any method may be used to implement cooling to a desired temperature.
• the steel sheet thus obtained may be subjected to galvanizing treatment such as hot-dip galvanizing.
• the above-described steel sheet subjected to the annealing treatment is immersed in a galvanizing bath at 440 °C or higher and 500 °C or lower for hot-dip galvanizing, after which coating weight adjustment is performed using gas wiping or the like.
• a galvanizing bath with an Al content of 0.10 % or more and 0.22 % or less is preferably used.
• the alloying treatment is performed in a temperature range of 470 °C to 600 °C after hot-dip galvanizing.
• alloying treatment is performed at a temperature above 600 °C, untransformed austenite transforms to pearlite, where the presence of a desired volume fraction of retained austenite cannot be ensured and ductility may degrade. Therefore, when a galvanized layer is subjected to alloying treatment, the alloying treatment is preferably performed in a temperature range of 470 °C to 600 °C. Electrogalvanized plating may also be performed.
• the skin pass rolling is preferably performed with a rolling reduction of 0.1 % or more and 1.0 % or less.
• a rolling reduction below 0.1 % provides only a small effect and complicates control, and hence 0.1 % is the lower limit of the favorable range.
• a rolling reduction above 1.0 % significantly degrades productivity, and thus 1.0 % is the upper limit of the favorable range.
• the skin pass rolling may be performed on-line or off-line. Skin pass may be performed in one or more batches with a target rolling reduction. No particular limitations are placed on other manufacturing conditions, yet from the perspective of productivity, the aforementioned series of processes such as annealing, hot-dip galvanizing, and alloying treatment on a galvanized layer are preferably carried out on a CGL (Continuous Galvanizing Line) as the hot-dip galvanizing line. After the hot-dip galvanizing, wiping may be performed for adjusting the coating amounts.
• CGL Continuous Galvanizing Line
• the following describes the microstructure of a steel sheet manufactured by the method according to the disclosure.
• Total area ratio of ferrite and bainitic ferrite 30 % or more and 75 % or less
• a high-strength steel sheet manufactured by the method according to the disclosure comprises a multi-phase structure in which retained austenite having an influence mainly on ductility and, more preferably, a small amount of martensite affecting strength are diffused in a structure in which soft ferrite with high ductility is dominantly present.
• bainitic ferrite forms adjacent to ferrite and retained austenite/martensite, and reduces the difference in hardness between ferrite and retained austenite and between ferrite and martensite to suppress the occurrence of cracking during hole expansion test and of fatigue cracking during fatigue test.
• the total area ratio of ferrite and bainitic ferrite needs to be 30 % or more.
• the total area ratio of ferrite and bainitic ferrite needs to be 75 % or less.
• the total area ratio of ferrite and bainitic ferrite is preferably 35 % or more and 70 % or less.
• Bainitic ferrite is effective in ensuring better hole expansion formability and better fatigue properties since, as described above, it forms adjacent to ferrite and retained austenite/martensite and has the effect of reducing the difference in hardness between ferrite and retained austenite and between ferrite and martensite to suppress the occurrence of cracking during hole expansion test and of fatigue cracking during fatigue test. Therefore, the area ratio of bainitic ferrite is preferably 5 % or more. On the other hand, to secure stable strength, the area ratio of bainitic ferrite is preferably 25 % or less.
• the term "bainitic ferrite” means such ferrite that is produced during the process of annealing at a temperature of 740 °C or higher and 840 °C or lower, followed by cooling to and holding at a temperature of 600 °C or lower, and that has a high dislocation density as compared to normal ferrite.
• ferrite is acicular ferrite
• ferrite may include polygonal ferrite and non-recrystallized ferrite. To ensure good ductility, however, it is preferred that the area ratio of polygonal ferrite is 20 % or less and the area ratio of non-recrystallized ferrite is 5 % or less. The area ratios of polygonal ferrite and of non-recrystallized ferrite may be 0 %.
• the area ratios of ferrite and bainitic ferrite can be determined by polishing a cross section of a steel sheet taken in the sheet thickness direction to be parallel to the rolling direction (L-cross section), etching the cross section with 3 vol.% nital, and averaging the results from observing ten locations at 2000 times magnification under an SEM (scanning electron microscope) at a position of sheet thickness x 1/4 (a position at a depth of one-fourth of the sheet thickness from the steel sheet surface) and calculating the area ratios of ferrite and bainitic ferrite for the ten locations with Image-Pro, available from Media Cybernetics, Inc., using the structure micrographs imaged with the SEM. In the structure micrographs, ferrite and bainitic ferrite appear as a gray structure (base steel structure), while retained austenite and martensite as a white structure.
• Identification of ferrite and bainitic ferrite is made by EBSD (Electron Back Scatter Diffraction) measurement. Specifically, a crystal grain (phase) that includes a sub-boundary with a grain boundary angle of smaller than 15° is identified as bainitic ferrite, for which the area ratio is calculated and used as the area ratio of bainitic ferrite.
• the area ratio of ferrite can be calculated by subtracting the area ratio of bainitic ferrite from the area ratio of the above-described gray structure.
• the area ratio of tempered martensite needs to be 5 % or more.
• the area ratio of tempered martensite is 8 % or more. If the area ratio of tempered martensite exceeds 15 %, however, it becomes difficult to obtain a sufficient amount of retained austenite. This results in a difficulty in obtaining good ductility and balancing strength and ductility. Therefore, the area ratio of tempered martensite needs to be 15 % or less.
• tempered martensite can be identified by determining whether cementite or retained austenite is included in martensite (tempered martensite is martensite containing cementite or retained austenite).
• the area ratio of tempered martensite can be determined by polishing an L-cross section of a steel sheet, etching the cross section with 3 vol.% nital, and averaging the results from observing ten locations at 2000 times magnification under an SEM (scanning electron microscope) at a position of sheet thickness x 1/4 and calculating the area ratios of ferrite and bainitic ferrite for the ten locations with Image-Pro, available from Media Cybernetics, Inc., using the structure micrographs imaged with the SEM.
• volume fraction of retained austenite 8 % or more
• the volume fraction of retained austenite needs to be 8 % or more.
• the volume fraction of retained austenite is 10 % or more. No upper limit is particularly placed on the volume fraction of retained austenite, yet it is around 35 %.
• the volume fraction of retained austenite is calculated by determining the x-ray diffraction intensity of a plane of sheet thickness x 1/4, which is exposed by polishing the steel sheet surface to a depth of one-fourth of the sheet thickness.
• the intensity ratio of the peak integrated intensity of the ⁇ 111 ⁇ , ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of retained austenite to the peak integrated intensity of the ⁇ 110 ⁇ , ⁇ 200 ⁇ , and ⁇ 211 ⁇ planes of ferrite is calculated for all of the twelve combinations, the results are averaged, and the average is used as the volume fraction of retained austenite.
• Mean grain size of retained austenite 2 ⁇ m or less
• retained austenite contributes to improving the ductility and fatigue properties of the steel sheet. Accordingly, to ensure good ductility and fatigue properties, retained austenite needs to have a mean grain size of 2 ⁇ m or less. For better ductility and fatigue properties, it is preferred that retained austenite has a mean grain size of 1.5 ⁇ m or less. No lower limit is particularly placed on the mean grain size, yet it is around 0.1 ⁇ m.
• the mean grain size of retained austenite can be determined by averaging the results from observing twenty locations at 15000 times magnification under a TEM (transmission electron microscope) and averaging the equivalent circular diameters calculated from the areas of retained austenite grains identified with Image-Pro, as mentioned above, using the structure micrographs imaged with the TEM.
• bainitic ferrite forms in the process of cooling to and holding at a temperature of 600 °C or lower following the annealing in a temperature range of 740 °C to 840 °C.
• bainitic ferrite forms adjacent to ferrite and retained austenite, and has the effect of reducing the difference in hardness between ferrite and retained austenite to suppress the occurrence of fatigue cracking and propagation of cracking. It is thus more advantageous if bainitic ferrite is densely distributed, in other words, if bainitic ferrite has a small mean free path.
• bainitic ferrite needs to have a mean free path of 3 ⁇ m or less.
• bainitic ferrite has a mean free path of 2.5 ⁇ m or less. No lower limit is particularly placed on the mean free path, yet it is around 0.5 ⁇ m.
• the mean grain size of bainitic ferrite can be determined by averaging the areas of grains by dividing the area of bainitic ferrite in the measured region calculated by EBSD (Electron Back Scatter Diffraction) measurement by the number of bainitic ferrite grains in the measured region to identify an equivalent circle diameter.
• EBSD Electro Back Scatter Diffraction
• the microstructures according to the disclosure may include carbides such as martensite, pearlite, cementite, and the like, as well as other microstructures well known as steel sheet microstructures. Any microstructure that has an area ratio of 15 % or less may be used without detracting from the effect of the disclosure.
• hot rolling was not performed, and annealing treatment was conducted under the conditions presented in Table 2 to produce high-strength hot-rolled steel sheets (HR).
• HR high-strength hot-rolled steel sheets
• Table 2 Table 2 No. Steel ID Slab heating temp. Hot-rolling, conditions Cold-rolling conditions Annealing treatment conditions Reheating treatment conditions Type* Remarks Finisher delivery temp. Mean coiling temp. Rolling reduction Annealing temp. Annealing holding time Mean cooling rate Cooling stop temp. Reheating temp.
• the high-strength hot-rolled steel sheets (HR), high-strength cold-rolled steel sheets (CR), hot-dip galvanizing steel sheets (GI), galvannealed steel sheets (GA), and electrogalvanized steel sheets (EG) thus obtained were subjected to structure observation, tensile test, hole expansion test, and fatigue test.
• tensile test was performed in accordance with JIS Z 2241 (2011 ) to measure TS (tensile strength) and EL (total elongation), using JIS No. 5 test pieces that were sampled such that the longitudinal direction of each test piece coincides with a direction perpendicular to the rolling direction of the steel sheet (the C direction).
• TS and EL were determined to be good when EL ⁇ 34 % for TS 780 MPa grade, EL ⁇ 27 % for TS 980 MPa grade, and EL ⁇ 23 % for TS 1180 MPa grade, and TS x EL ⁇ 27000 MPa ⁇ %.
• D f is a hole diameter at the time of occurrence of cracking (mm) and Do is an initial hole diameter (mm).
• TS and EL were determined to be good when ⁇ ⁇ 40 % for TS 780 MPa grade, ⁇ ⁇ 30 % for TS 980 MPa grade, and ⁇ ⁇ 20 % TS 1180 MPa grade.
• productivity was evaluated according to the lead time costs, including:
• the sheet passage ability during hot rolling was determined to be low when the risk of trouble during rolling increased with increasing rolling load.
• the sheet passage ability during cold rolling was determined to be low when the risk of trouble during rolling increased with increasing rolling load.
• each final-annealed sheet were determined to be poor when defects such as blow hole generation and segregation on the surface layer of the slab could not be scaled-off, cracks and irregularities on the steel sheet surface increased, and a smooth steel sheet surface could not be obtained.
• the surface characteristics were also determined to be poor when the amount of oxides (scales) generated suddenly increased, the interface between the steel substrate and oxides was roughened, and the surface quality after pickling and cold rolling degraded, or when some hot-rolling scales remained after pickling. Structure observation was performed following the above-described procedure. The evaluation results are shown in Tables 3 and 4.
• each of our examples has TS of 780 MPa or more, and the present disclosure enables manufacture of high-strength steel sheets with high productivity that are excellent not only in ductility but also in hole expansion formability (stretch flangeability) and fatigue properties. It can also be appreciated that each of our examples exhibits excellent sheet passage ability during hot rolling and cold rolling, as well as excellent surface characteristics of the final-annealed sheet. In contrast, comparative examples are inferior in terms of one or more of tensile strength, ductility, balance between strength and ductility, hole expansion formability (stretch flangeability), fatigue properties, and productivity.

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• 2015
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