WO2013118679A1 - 高強度冷延鋼板及びその製造方法 - Google Patents
高強度冷延鋼板及びその製造方法 Download PDFInfo
- Publication number
- WO2013118679A1 WO2013118679A1 PCT/JP2013/052468 JP2013052468W WO2013118679A1 WO 2013118679 A1 WO2013118679 A1 WO 2013118679A1 JP 2013052468 W JP2013052468 W JP 2013052468W WO 2013118679 A1 WO2013118679 A1 WO 2013118679A1
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- Prior art keywords
- steel sheet
- less
- rolled steel
- cold
- limited
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- 239000010960 cold rolled steel Substances 0.000 title claims abstract description 85
- 238000004519 manufacturing process Methods 0.000 title claims description 30
- 238000000034 method Methods 0.000 title claims description 17
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 181
- 239000010959 steel Substances 0.000 claims abstract description 181
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 52
- 239000002344 surface layer Substances 0.000 claims abstract description 49
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 42
- 239000000203 mixture Substances 0.000 claims abstract description 15
- 229910000734 martensite Inorganic materials 0.000 claims description 61
- 239000010410 layer Substances 0.000 claims description 38
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 36
- 238000005096 rolling process Methods 0.000 claims description 32
- 229910052698 phosphorus Inorganic materials 0.000 claims description 28
- 238000000137 annealing Methods 0.000 claims description 27
- 238000001816 cooling Methods 0.000 claims description 27
- 230000009466 transformation Effects 0.000 claims description 27
- 230000001965 increasing effect Effects 0.000 claims description 24
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- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 19
- 239000011574 phosphorus Substances 0.000 claims description 19
- 238000005097 cold rolling Methods 0.000 claims description 17
- 239000012535 impurity Substances 0.000 claims description 17
- 229910001392 phosphorus oxide Inorganic materials 0.000 claims description 17
- VSAISIQCTGDGPU-UHFFFAOYSA-N tetraphosphorus hexaoxide Chemical compound O1P(O2)OP3OP1OP2O3 VSAISIQCTGDGPU-UHFFFAOYSA-N 0.000 claims description 17
- 239000002131 composite material Substances 0.000 claims description 15
- 238000010438 heat treatment Methods 0.000 claims description 15
- 229910052742 iron Inorganic materials 0.000 claims description 14
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 14
- 229910052684 Cerium Inorganic materials 0.000 claims description 13
- 229910052710 silicon Inorganic materials 0.000 claims description 13
- 238000005098 hot rolling Methods 0.000 claims description 12
- 229910052749 magnesium Inorganic materials 0.000 claims description 12
- 229910052748 manganese Inorganic materials 0.000 claims description 12
- 229910052791 calcium Inorganic materials 0.000 claims description 11
- 229910052757 nitrogen Inorganic materials 0.000 claims description 11
- 238000005275 alloying Methods 0.000 claims description 10
- 229910052782 aluminium Inorganic materials 0.000 claims description 10
- 229910052735 hafnium Inorganic materials 0.000 claims description 10
- 229910052739 hydrogen Inorganic materials 0.000 claims description 10
- 239000001257 hydrogen Substances 0.000 claims description 10
- 229910052717 sulfur Inorganic materials 0.000 claims description 10
- 229910052726 zirconium Inorganic materials 0.000 claims description 10
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 9
- 229910052804 chromium Inorganic materials 0.000 claims description 7
- 229910052799 carbon Inorganic materials 0.000 claims description 5
- 229910052759 nickel Inorganic materials 0.000 claims description 5
- 229910052719 titanium Inorganic materials 0.000 claims description 5
- BHEPBYXIRTUNPN-UHFFFAOYSA-N hydridophosphorus(.) (triplet) Chemical compound [PH] BHEPBYXIRTUNPN-UHFFFAOYSA-N 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052721 tungsten Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 238000004804 winding Methods 0.000 claims description 4
- 229910052802 copper Inorganic materials 0.000 claims description 3
- 230000000717 retained effect Effects 0.000 abstract description 33
- 230000000694 effects Effects 0.000 description 39
- 238000005452 bending Methods 0.000 description 32
- 238000007747 plating Methods 0.000 description 28
- 238000005261 decarburization Methods 0.000 description 16
- 229910001563 bainite Inorganic materials 0.000 description 15
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- 239000008397 galvanized steel Substances 0.000 description 13
- 230000015572 biosynthetic process Effects 0.000 description 12
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 12
- 238000005728 strengthening Methods 0.000 description 12
- 239000011701 zinc Substances 0.000 description 12
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- 150000001247 metal acetylides Chemical class 0.000 description 9
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- 229910001562 pearlite Inorganic materials 0.000 description 9
- 238000005336 cracking Methods 0.000 description 8
- 239000000463 material Substances 0.000 description 8
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 7
- 230000001771 impaired effect Effects 0.000 description 7
- 238000005554 pickling Methods 0.000 description 7
- 238000012360 testing method Methods 0.000 description 7
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 6
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- 210000001519 tissue Anatomy 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 239000013078 crystal Substances 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
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- 229910052747 lanthanoid Inorganic materials 0.000 description 3
- 150000002602 lanthanoids Chemical class 0.000 description 3
- 229910052746 lanthanum Inorganic materials 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 238000001953 recrystallisation Methods 0.000 description 3
- 238000005496 tempering Methods 0.000 description 3
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 238000002441 X-ray diffraction Methods 0.000 description 2
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- 238000006243 chemical reaction Methods 0.000 description 2
- 238000004090 dissolution Methods 0.000 description 2
- 238000000445 field-emission scanning electron microscopy Methods 0.000 description 2
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- 229910052718 tin Inorganic materials 0.000 description 2
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- 229910001122 Mischmetal Inorganic materials 0.000 description 1
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 229910007567 Zn-Ni Inorganic materials 0.000 description 1
- 229910007564 Zn—Co Inorganic materials 0.000 description 1
- 229910007614 Zn—Ni Inorganic materials 0.000 description 1
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- 229910052787 antimony Inorganic materials 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
- 229910052790 beryllium Inorganic materials 0.000 description 1
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- 229910052738 indium Inorganic materials 0.000 description 1
- 229910052740 iodine Inorganic materials 0.000 description 1
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- 239000000314 lubricant Substances 0.000 description 1
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- 150000002910 rare earth metals Chemical class 0.000 description 1
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Classifications
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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
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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- C—CHEMISTRY; METALLURGY
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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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
- C21D8/0247—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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- C—CHEMISTRY; METALLURGY
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
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- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C—ALLOYS
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
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- 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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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- 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
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- 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
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- 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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- 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
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
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- 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
- C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
- C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
- C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
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- 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
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
- C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
- C23C28/321—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer with at least one metal alloy layer
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- 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
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
- C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
- C23C28/322—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer only coatings of metal elements only
- C23C28/3225—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer only coatings of metal elements only with at least one zinc-based layer
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- 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
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
- C23C28/34—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates
- C23C28/345—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates with at least one oxide layer
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- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/34—Pretreatment of metallic surfaces to be electroplated
- C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D7/00—Electroplating characterised by the article coated
- C25D7/06—Wires; Strips; Foils
- C25D7/0614—Strips or foils
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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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
- C21D8/0226—Hot rolling
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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
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/22—Electroplating: Baths therefor from solutions of zinc
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/56—Electroplating: Baths therefor from solutions of alloys
- C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12535—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.] with additional, spatially distinct nonmetal component
- Y10T428/12611—Oxide-containing component
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12785—Group IIB metal-base component
- Y10T428/12792—Zn-base component
- Y10T428/12799—Next to Fe-base component [e.g., galvanized]
Definitions
- the present invention relates to a high-strength cold-rolled steel sheet having excellent bendability and a method for producing the same.
- This high-strength cold-rolled steel sheet includes those having a film, galvanizing or the like formed on the surface.
- Non-Patent Document 1 As factors governing the bendability of high-strength steel sheets, it is known that (a) the difficulty of necking and (b) the difficulty of occurrence of cracks (voids) inside the steel sheet are important (for example, Non-Patent Document 1). For example, in a steel sheet having low elongation, necking is likely to occur during bending, and the deformation is localized, so that bending workability deteriorates. In addition, steel made of ferrite and martensite has inferior bendability due to martensite cracking and void formation at the interface. As a result, increasing the strength leads to deterioration of elongation, so the bendability is poor. In addition, since the increase in strength may be accompanied by an increase in the martensite volume fraction, the increase in strength tends to cause deterioration in bendability.
- Patent Document 1 discloses that the component composition is, by mass%, C: more than 0.02% to 0.20%, Si: 0.01 to 2.0%, Mn: 0.1 to 3.0%, P: 0.003 to 0.10%, S: 0.020% or less, Al: 0.001 to 1.0%, N: 0.0004 to 0.015%, A steel sheet containing Ti: 0.03 to 0.2% and the balance being Fe and impurities has been proposed.
- the metal structure of this steel sheet contains ferrite in an area ratio of 30 to 95%, and the remaining second phase is composed of one or more of martensite, bainite, pearlite, cementite and retained austenite, and contains martensite.
- this steel sheet contains Ti carbonitride precipitates having a particle size of 2 to 30 nm with an average interparticle distance of 30 to 300 nm and crystallized TiN particles with a particle size of 3 ⁇ m or more with an average interparticle distance of 50 to 500 ⁇ m. To do. According to such a steel plate, good bendability can be obtained, but since precipitation strengthening is used, it is not easy to ensure a high balance between strength and elongation.
- Patent Document 2 as a steel sheet having excellent bendability, C: 0.03 to 0.11%, Si: 0.005 to 0.5%, Mn: 2.0 to 4.0 as mass%. %, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0%, N: 0.01% or less, and further Ti or 0.51% or less and Nb: 0.50% or less of Ti + (Nb / 2)
- a steel sheet is described that has a composition that contains ⁇ 0.03, the balance being Fe and impurities, and a tensile strength of 540 MPa or more.
- the average interval in the plate width direction of the Mn concentrated portion expanded in the rolling direction at a t / 20 depth position (t: plate thickness of the steel plate) from the surface is 300 ⁇ m or less, and the area ratio of ferrite is 60 %,
- the average particle diameter of ferrite is 1.0 to 6.0 ⁇ m, and precipitates with a particle diameter of 1 to 10 nm are contained in ferrite at 100 pieces / ⁇ m 2 or more. According to such a steel plate, good bendability can be obtained, but since the main phase is ferrite and the retained austenite volume fraction is limited to less than 3%, it is applied to a high strength steel plate of 700 MPa or more. Is not easy.
- Patent Document 3 as a steel sheet having both ductility and bendability, C: 0.08 to 0.25%, Si: 0.7% or less, Mn: 1.0 to 2. 6%, Al: 1.5% or less, P: 0.03% or less, S: 0.02% or less, and N: 0.01% or less, and the relationship between Si and Al is 1.0
- a steel sheet is described which satisfies% ⁇ Si + Al ⁇ 1.8% and has a component composition comprising the balance Fe and impurities.
- This steel sheet has TS ⁇ 590 (TS: tensile strength (MPa)), TS ⁇ El ⁇ 17500 (El: total elongation (%)), and ⁇ ⁇ 1.5 ⁇ t ( ⁇ : critical bending radius (mm), t: plate thickness (mm)).
- TS tensile strength
- El total elongation
- ⁇ 1.5 ⁇ t
- ⁇ critical bending radius
- t plate thickness
- Patent Document 4 as a steel sheet having good ductility and bendability, C: 0.08 to 0.20%, Si: 1.0% or less, Mn: 1.8 to 3. 0%, P: 0.1% or less, S: 0.01% or less, sol.
- a steel sheet containing Al: 0.005 to 0.5%, N: 0.01% or less and Ti: 0.02 to 0.2% and having a component composition consisting of the remainder Fe and impurities is described.
- This steel sheet is, by volume, composed of ferrite: 10% or more, bainite: 20-70%, retained austenite: 3-20%, and martensite: 0-20%, and the average grain size of the ferrite is 10 ⁇ m or less.
- the bainite has an average particle size of 10 ⁇ m or less
- the retained austenite has an average particle size of 3 ⁇ m or less
- the martensite has an average particle size of 3 ⁇ m or less.
- the steel sheet has a tensile strength (TS) of 780 MPa or more, a product of the tensile strength (TS) and total elongation (El) (TS ⁇ El value) of 14000 MPa ⁇ % or more, and a minimum bending radius in a bending test of 1 It has mechanical properties of 5 t or less (t: plate thickness), and the plate thickness is 2.0 mm or more. According to the technique described in Document 4, it is possible to ensure good ductility and bendability, but it is not easy to achieve both strength and bendability at a high level.
- Patent Document 5 as a steel sheet having excellent bendability, C: 0.03-0.12%, Si: 0.02-0.50%, Mn: 2.0-4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0% and N: 0.01% or less, and Ti: 0.50% or less and Nb: 0.50% or less of Ti + (Nb / 2)
- a steel sheet containing a composition satisfying ⁇ 0.03 and having the balance of Fe and impurities and having a tensile strength of 540 MPa or more is described.
- This steel sheet has a structure in which the area ratio of ferrite is 60% or more and the average grain size of ferrite is 1.0 to 6.0 ⁇ m.
- the alloyed hot-dip galvanized layer contains, by mass%, Fe: 8 to 15% and Al: 0.08 to 0.50%, with the balance being Zn and impurities.
- the addition amount of C is limited to a low range of 0.12% or less, it can be applied to a steel plate of 780 MPa or less, but it is not easy to apply to a further high strength steel plate. Further, since the area ratio of retained austenite is less than 3%, it is not easy to obtain excellent ductility.
- Patent Document 6 as a steel sheet having excellent workability, C: 0.03 to 0.17%, Si: 0.01 to 0.75%, Mn: 1.5 to 2.5% in mass% , P: 0.080% or less, S: 0.010% or less, sol.
- a steel sheet containing Al: 0.01 to 1.20%, Cr: 0.3 to 1.3%, the balance being Fe and inevitable impurities is described.
- This steel sheet is composed of 30 to 70% ferrite by volume, less than 3% retained austenite, and the remaining martensite, and has a structure in which 20% or more of the martensite is tempered martensite.
- the volume ratio of retained austenite is limited to less than 3%, it has a problem that it has excellent bendability but low uniform elongation. As a result, also in bending, when a thick plate is bent, there is a concern that cracks due to necking may occur on the surface of the steel plate.
- Patent Document 7 as a steel sheet excellent in bending workability, in wt%, C: 0.12 to 0.30%, Si: 1.2% or less, Mn: 1 to 3%, P: 0.020 % Or less, S: 0.010% or less, sol.
- a steel sheet containing Al: 0.01 to 0.06%, the balance being Fe and inevitable impurities is described.
- This steel sheet has a soft layer of C: 0.1 wt% or less on the surface layer on one side and 3-15 vol% on both sides, and the balance consists of a composite structure of residual austenite with less than 10 vol% and a low-temperature transformation phase or further ferrite. .
- the decarburization annealing must be performed twice in total after hot rolling and after cold rolling, resulting in poor productivity.
- the present invention provides a high-strength cold-rolled steel sheet having excellent bendability and a method for producing the same.
- the gist of the present invention is as follows.
- the first aspect of the present invention is, in mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.50%, Mn: 1.30 to 3.50%, P : 0.001 to 0.050%, S: 0.0001 to 0.0100%, Al: 0.001 to 1.500%, and N: 0.0001 to 0.0100%, and Ti is 0.00.
- Nb is limited to 0.150% or less
- V is limited to 0.150% or less
- Cr is limited to 2.00% or less
- Ni is limited to 2.00% or less
- Cu is limited to 2.00% or less
- Mo is limited to 1.00% or less
- W is limited to 1.00% or less
- at least one of Ca, Ce, Mg, Zr, Hf, and REM The total content of which is limited to 0.5000% or less, and the balance is composed of iron and inevitable impurities.
- the surface microstructure in the steel sheet surface layer contains 3 to 10% retained austenite and 90% or less ferrite in volume fraction, and the internal microstructure at the t / 4 depth position from the surface where the sheet thickness is t, It contains 3-30% residual austenite in volume fraction, and the ratio Hvs / Hvb between the hardness Hvs of the steel sheet surface layer and the hardness Hvb at the t / 4 depth position is more than 0.75 to 0.90, It is a high-strength cold-rolled steel sheet having a maximum tensile strength of 700 MPa or more.
- the surface layer microstructure further has a volume fraction of 10 to 87% ferrite, 10 to 50% tempered martensite, and 15%. You may contain the fresh martensite restrict
- the internal microstructure In the high-strength cold-rolled steel sheet according to the above (1) or (2), the internal microstructure further has a volume fraction of 10 to 87% ferrite and 10 to 50% tempered martensite. And fresh martensite limited to 15% or less.
- a film containing at least one of a phosphorus oxide and a composite oxide containing phosphorus is formed on at least one side. May be.
- an electrogalvanized layer may be formed on at least one side.
- a film containing at least one of a phosphorus oxide and a composite oxide containing phosphorus may be formed on the electrogalvanized layer.
- a hot-dip galvanized layer may be formed on at least one side.
- a film containing at least one of a phosphorus oxide and a composite oxide containing phosphorus may be formed on the hot-dip galvanized layer.
- an alloyed hot-dip galvanized layer may be formed on at least one side.
- a film containing at least one of phosphorous oxide and phosphorus-containing composite oxide may be formed on the alloyed hot-dip galvanized layer.
- the second aspect of the present invention is, in mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.50%, Mn: 1.30 to 3.50%, P : 0.001 to 0.050%, S: 0.0001 to 0.0100%, Al: 0.001 to 1.500%, and N: 0.0001 to 0.0100%, and Ti is 0.00.
- Nb is limited to 0.150% or less
- V is limited to 0.150% or less
- Cr is limited to 2.00% or less
- Ni is limited to 2.00% or less
- Cu is limited to 2.00% or less
- Mo is limited to 1.00% or less
- W is limited to 1.00% or less
- a method of manufacturing a cold-rolled steel sheet (12) In the cold-rolled steel sheet manufacturing method according to (11) above, a film containing at least one of a phosphorus oxide and a complex oxide containing phosphorus is formed on at least one surface of the high-strength cold-rolled steel sheet. Also good. (13) In the cold rolled steel sheet manufacturing method according to (11) above, an electrogalvanized layer may be formed on at least one surface of the high strength cold rolled steel sheet. (14) In the cold-rolled steel sheet manufacturing method according to (13), a film containing at least one of a phosphorus oxide and a composite oxide containing phosphorus may be formed on the electrogalvanized layer.
- a hot-dip galvanized layer may be formed on at least one surface of the high-strength cold-rolled steel sheet.
- the rolled steel sheet may be formed by dipping in a galvanizing bath and cooling in a state of being heated or cooled to a temperature range of (zinc plating bath temperature ⁇ 40) ° C. to (zinc plating bath temperature + 50) ° C.
- a coating containing at least one of a phosphorus oxide and a composite oxide containing phosphorus may be formed on the hot-dip galvanized layer.
- an alloyed hot-dip galvanized layer may be formed on at least one surface of the high-strength cold-rolled steel sheet,
- the high-strength cold-rolled steel sheet is immersed in a galvanizing bath while being heated or cooled to a temperature range of (zinc plating bath temperature ⁇ 40) ° C. to (zinc plating bath temperature +50) ° C. at a temperature of 460 ° C. or higher. You may form by cooling after giving an alloying process.
- a film including at least one of a phosphorus oxide and a composite oxide containing phosphorus is formed on the alloyed hot-dip galvanized layer. Good.
- the inventors of the present invention have provided a high-strength cold-rolled steel sheet having a maximum tensile strength of 700 MPa or more that can obtain excellent bendability by preventing cracks in the steel sheet generated in the deformed portion by bending and necking of the steel sheet surface.
- the present inventors have a predetermined component composition, and after controlling the microstructure to a predetermined structure, the steel plate surface layer can be softened by performing a decarburization process, and the maximum tensile strength is increased. It was clarified that even a high strength cold-rolled steel sheet of 700 MPa or more can obtain excellent bendability as if it were a low-strength steel sheet.
- the ratio “(surface hardness) / (t / 4 depth position hardness)” of the hardness of the steel sheet surface layer and the hardness at the t / 4 depth position is achieved by setting the ratio “(surface hardness) / (t / 4 depth position hardness)” of the hardness of the steel sheet surface layer and the hardness at the t / 4 depth position to more than 0.75 to 0.90. can get.
- the microstructure in the surface layer portion of the steel sheet contains 3-10% residual austenite and 90% or less ferrite in volume fraction, and the internal microstructure at the t / 4 depth position of the steel sheet is By containing 3 to 30% of retained austenite as a fraction, cracking due to necking can be suppressed, and further improvement in bendability can be obtained.
- the steel sheet of the present invention has not only a necking suppression effect during bending but also a necking suppression effect during a tensile test and press working due to the inclusion of retained austenite, and therefore has good elongation.
- a steel sheet excellent in bendability is a steel sheet having a bending radius R of 1.0 mm or less and causing no cracking or necking in a 90-degree V bending test based on JIS Z 2248 (2006), or It means that the bending radius R is 0.5 mm or less and no cracking occurs.
- C: 0.075-0.300% C is contained in order to increase the strength of the base steel sheet.
- the C content is preferably 0.280% or less, and more preferably 0.260% or less.
- the C content is less than 0.075%, the strength is lowered, and the maximum tensile strength of 700 MPa or more cannot be ensured.
- the C content is preferably 0.090% or more, and more preferably 0.100% or more.
- Si: 0.30-2.50% Si is the most important element because it promotes the decarburization reaction and softens the steel sheet surface layer. If the Si content exceeds 2.50%, the base steel sheet becomes brittle and the ductility deteriorates, so the upper limit is made 2.50%. From the viewpoint of ensuring ductility, the Si content is preferably 2.20% or less, and more preferably 2.00% or less. On the other hand, if the Si content is less than 0.30%, a large amount of coarse iron-based carbides are formed, the residual austenite structure fraction of the internal microstructure cannot be made 3 to 30%, and the elongation decreases. End up.
- the lower limit value of Si is preferably 0.50% or more, and more preferably 0.70% or more.
- Si is an element necessary for suppressing the coarsening of iron-based carbides in the base steel sheet and increasing the strength and formability.
- the lower limit value of Si is preferably 1% or more, and more preferably 1.2% or more.
- Mn: 1.30 to 3.50% Mn is contained to increase the strength of the base steel sheet. However, if the Mn content exceeds 3.50%, a coarse Mn-concentrated portion is generated at the center of the thickness of the base steel sheet, and brittleness is likely to occur, and troubles such as cracking of the cast slab are likely to occur. . Further, when the Mn content exceeds 3.50%, the weldability is also deteriorated. Therefore, the Mn content is 3.50% or less. From the viewpoint of weldability, the Mn content is preferably 3.20% or less, and more preferably 3.00% or less.
- the Mn content is set to 1.30% or more.
- the Mn content is preferably 1.50% or more, and more preferably 1.70% or more.
- P 0.001 to 0.050%
- P tends to segregate in the central part of the thickness of the base steel sheet, and causes the weld to become brittle. If the P content exceeds 0.050%, the welded portion is significantly embrittled, so the P content is 0.050% or less.
- the lower limit of the content of P is not particularly defined, the effect of the present invention is exhibited. However, since the content of P is less than 0.001% is accompanied by a significant increase in production cost, 0.001 % Is the lower limit.
- S 0.0001 to 0.0100% S adversely affects weldability and manufacturability during casting and hot rolling. For this reason, the upper limit of the S content is set to 0.0100% or less. Further, since S is combined with Mn to form coarse MnS to lower the ductility and stretch flangeability, it is preferably 0.0050% or less, and more preferably 0.0025% or less. The lower limit of the content of S is not particularly defined, and the effect of the present invention is exhibited. However, if the content of S is less than 0.0001%, a significant increase in production cost is caused, so 0.0001% Is the lower limit.
- Al: 0.001-1.500% Al is an important element because it promotes the decarburization reaction and softens the surface layer of the steel sheet.
- the Al content exceeds 1.500%, weldability deteriorates, so the upper limit of the Al content is 1.500%.
- the Al content is preferably 1.200% or less, and more preferably 0.900% or less.
- Al is an element effective as a deoxidizing material, but if the Al content is less than 0.001%, the effect as the deoxidizing material cannot be obtained sufficiently, so the lower limit of the Al content is 0. 0.001% or more. In order to obtain a sufficient deoxidation effect, the Al content is preferably 0.003% or more.
- N 0.0001 to 0.0100% N forms coarse nitrides and degrades ductility and stretch flangeability, so it is necessary to suppress the addition amount. If the N content exceeds 0.0100%, this tendency becomes remarkable, so the upper limit of the N content is set to 0.0100% or less. N is preferably set to 0.0080% or less because it causes blowholes during welding. The lower limit of the content of N is not particularly defined, and the effect of the present invention is exhibited. However, if the content of N is less than 0.0001%, a significant increase in manufacturing cost is caused, so 0.0001% Is the lower limit.
- the base steel sheet of the high-strength cold-rolled steel sheet according to the present embodiment is based on a composition containing the above elements, the balance being iron and inevitable impurities, and other components may not be included.
- the steel sheet may further contain Ti, Nb, V, Cr, Ni, Cu, Mo, W, Ca, Ce, Mg, Zr, Hf, and REM in the following content ranges as necessary. Good.
- the lower limit of these elements is 0%, in order to acquire a desired effect, it is good also as the lower limit shown below, respectively.
- the content of unavoidable impurities is permissible as long as the effect of the present invention is not significantly deteriorated, but is preferably reduced as much as possible.
- Ti 0.005 to 0.150%
- Ti is an element that contributes to increasing the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization.
- the Ti content is preferably 0.150% or less.
- the Ti content is more preferably 0.120% or less, and further preferably 0.100% or less.
- the lower limit of the Ti content is not particularly defined, and the effects of the present invention are exhibited.
- the Ti content is preferably 0.005% or more.
- the Ti content is more preferably 0.010% or more, and further preferably 0.015% or more.
- Nb 0.005 to 0.150%
- Nb is an element that contributes to an increase in the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization.
- the Nb content is preferably 0.150% or less.
- the Nb content is more preferably 0.120% or less, and further preferably 0.100% or less.
- the lower limit of the Nb content is not particularly defined, and the effects of the present invention are exhibited.
- the Nb content is preferably 0.005% or more.
- the Nb content is more preferably 0.010% or more, and further preferably 0.015% or more.
- V 0.005-0.150%
- V is an element that contributes to increasing the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization.
- the V content is preferably 0.150% or less.
- the lower limit of the content of V is not particularly limited, and the effect of the present invention is exhibited.
- the content of V is preferably 0.005% or more.
- Cr: 0.01-2.00% Cr is an element that suppresses phase transformation at high temperature and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Cr content exceeds 2.00%, hot workability is impaired and productivity is lowered. Therefore, the Cr content is preferably 2.00% or less. Although the lower limit of the Cr content is not particularly defined, the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cr, the Cr content may be 0.01% or more. preferable.
- Ni 0.01-2.00%
- Ni is an element that suppresses phase transformation at high temperature and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Ni content exceeds 2.00%, weldability is impaired, so the Ni content is preferably 2.00% or less.
- the lower limit of the Ni content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Ni, the Ni content should be 0.01% or more. preferable.
- Cu: 0.01-2.00% is an element that increases the strength by being present in the steel as fine particles, and can be added instead of a part of C and / or Mn. If the Cu content exceeds 2.00%, weldability is impaired, so the Cu content is preferably 2.00% or less. The lower limit of the Cu content is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cu, the Cu content should be 0.01% or more. preferable.
- Mo 0.01-1.00%
- Mo is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Mo content exceeds 1.00%, hot workability is impaired and productivity is lowered. For this reason, the Mo content is preferably 1.00% or less.
- the lower limit of the content of Mo is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Mo, the content of Mo is 0.01% or more. preferable.
- W 0.01-1.00%
- W is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the W content exceeds 1.00%, hot workability is impaired and productivity is lowered. Therefore, the W content is preferably 1.00% or less.
- the lower limit of the W content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by W, the W content may be 0.01% or more. preferable.
- a total of 0.0001 to 0.5000% of at least one of Ca, Ce, Mg, Zr, Hf, and REM Ca, Ce, Mg, Zr, Hf, and REM are effective elements for improving formability, and one or more of them can be added. However, if the total content of at least one of Ca, Ce, Mg, Zr, Hf, and REM exceeds 0.5000%, the ductility may be impaired. For this reason, the total content of each element is preferably 0.5000% or less.
- the lower limit of the content of at least one of Ca, Ce, Mg, Zr, Hf, and REM is not particularly defined, and the effect of the present invention is exhibited. However, the effect of improving the formability of the base steel sheet is sufficiently obtained.
- the total content of each element is preferably 0.0001% or more. From the viewpoint of moldability, the total content of one or more of Ca, Ce, Mg, Zr, Hf, and REM is more preferably 0.0005% or more, and 0.0010% or more. Is more preferable.
- REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. REM and Ce are often added by misch metal, and may contain a lanthanoid series element in combination with La and Ce. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. Even if the metal La or Ce is added, the effect of the present invention is exhibited.
- the internal microstructure means a microstructure at a t / 4 depth position, where t is the thickness of the base steel plate.
- the surface layer microstructure described later means a microstructure on the surface of the base steel plate, strictly speaking, in a plane parallel to the plate surface of the base steel plate and 20 ⁇ m deep from the surface.
- the internal microstructure of the steel sheet contains 3-30% residual austenite by volume fraction in the range of t / 8 to 3t / 8 depth centered on the t / 4 depth position. Residual austenite is effective in suppressing necking that occurs during bending by greatly improving ductility. On the other hand, retained austenite becomes a starting point of fracture and deteriorates bendability. For this reason, it is preferable that the retained austenite contained in the microstructure of the base steel sheet is 3 to 20% in terms of volume fraction.
- the lower limit of retained austenite in the internal microstructure is preferably 5% or 8% or more.
- the structural fraction of retained austenite in the surface layer portion of the steel sheet is limited to 3 to 10%, and the structural fraction of ferrite is limited to 90% or less. If the retained austenite fraction in the surface layer is less than 3%, for example, in the 90-degree V bending test, if the bending radius is 1.0 mm or less, necking occurs in the surface layer portion and the bendability deteriorates. For this reason, it is necessary to make the retained austenite fraction of the steel sheet surface layer 3% or more.
- the retained austenite fraction of the surface layer portion is set to 10% or less, preferably 8% or less, more preferably 5.8% or less.
- the hardness ratio between the steel sheet surface layer and the steel sheet interior (t / 4 depth position) described later is more than 0.75 to 0 .90 or less, and excellent bendability can be achieved. If the ferrite fraction of the surface microstructure exceeds 90%, it becomes difficult to secure a predetermined retained austenite microstructure fraction, and excellent bendability cannot be secured. And
- the surface microstructure and the internal microstructure of the high-strength cold-rolled steel sheet according to this embodiment may each include one or more of tempered martensite, ferrite, pearlite, and cementite in addition to the residual austenite. If it is the range demonstrated below, the objective of this invention can be achieved.
- the volume fraction is 10 to 87 in the range of t / 8 to 3t / 8 with the t / 4 depth position as the center.
- Tempered martensite greatly improves the tensile strength. For this reason, tempered martensite may be contained in the structure of the base steel sheet in a volume fraction of 50% or less. Tempered martensite is martensite in which iron-based carbides such as ⁇ , ⁇ , and ⁇ are precipitated by holding martensite at 200 to 500 ° C, and causes cracking compared to fresh martensite. It ’s hard to be. From the viewpoint of tensile strength, the volume fraction of tempered martensite is preferably 1% or more, and more preferably 10% or more. On the other hand, if the volume fraction of tempered martensite contained in the microstructure of the base steel plate exceeds 50%, it is not preferable because the yield stress is excessively increased and the shape freezeability is deteriorated.
- Ferrite 10-87% Ferrite is effective in improving ductility. For this reason, the ferrite may be contained in the structure of the base steel sheet in a volume fraction of 10% or more. Moreover, since ferrite is a soft structure, the upper limit may be 87% in volume fraction in order to ensure sufficient strength.
- Total of bainitic ferrite and bainite 10-50%
- Bainitic ferrite and bainite are structures with an excellent balance between strength and ductility, and are structures having intermediate strengths between soft ferrite and hard martensite, tempered martensite, and retained austenite. It also contributes to improving the balance of sex. Therefore, the total volume may include 10 to 50%.
- the volume fraction of pearlite contained in the base steel sheet structure is preferably 5% or less, more preferably 3% or less.
- the volume fraction of coarse cementite contained in the base steel sheet structure is preferably 10% or less, and more preferably 5% or less.
- Coarse cementite means cementite having a nominal particle size of 2 ⁇ m or more. Cementite is fragile compared to iron, and the interfacial strength between iron and cementite is also small. Therefore, it becomes a starting point of crack formation and void formation during bending, and deteriorates bendability. For this reason, it is necessary to reduce the volume ratio of coarse cementite.
- fine iron-based carbides contained in the bainite structure or tempered martensite may be contained because the bendability is not deteriorated.
- the volume fraction of each tissue as described above can be measured by the following method, for example.
- the volume fraction of retained austenite is calculated by performing X-ray diffraction using the plane parallel to the plate surface of the base steel sheet and the t / 4 depth position as the observation plane, and calculating the area fraction. Can be considered.
- the volume fractions of ferrite, pearlite, bainite, cementite, tempered martensite and fresh martensite were collected by taking a sample with the thickness cross section parallel to the rolling direction of the base steel sheet as the observation surface, and polishing the observation surface.
- the volume fraction of retained austenite in the surface layer is calculated by performing X-ray diffraction using a plane parallel to the plate surface of the base steel sheet and a depth of 20 ⁇ m from the surface as the observation plane, and calculating the area fraction. It can be regarded as a fraction.
- the volume fractions of ferrite, pearlite, bainite, cementite, tempered martensite and fresh martensite were collected by taking a sample with the thickness cross section parallel to the rolling direction of the base steel sheet as the observation surface, and polishing the observation surface.
- the area fraction is measured by observing with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), and can be regarded as the volume fraction.
- the present inventors have found that excellent bendability can be obtained by applying a decarburizing treatment to a steel sheet having the above component composition and structure to soften the steel sheet surface layer. That is, when the ratio “Hvs / Hvb” of the hardness Hvs of the surface layer of the steel sheet to the hardness Hvb at the t / 4 depth position of the base steel sheet is more than 0.75 to 0.90, excellent bendability is obtained. It is done.
- the reason why the hardness ratio exceeds 0.75 is that if the hardness ratio is 0.75 or less, the steel sheet is too soft and it is difficult to ensure a maximum tensile strength of 700 MPa or more. Preferably it is 0.8 or more.
- it exceeds 0.90 since it contains a large amount of retained austenite, it is possible to suppress necking at the time of bending deformation, but micro cracks may occur and the bendability is poor.
- the “hardness” used here is the hardness of 10 points each at a load of 10 g indentation using a Vickers hardness tester at the position of t / 4 in the sheet thickness section parallel to the rolling direction of the steel sheet surface layer and the steel sheet. The average value is taken as the hardness of each.
- the present inventors investigated the relationship between bendability and steel sheet properties as a preliminary test, and found that the average was within the range of t / 8 to 3t / 8 depth positions.
- the hardness does not depend on the position, and the steel sheet structure differs at the center of the plate thickness (t / 2 depth position) due to Mn center segregation.
- the average hardness is between t / 8 and 3t / 8 depth positions. I also found it different. From this, the hardness at the t / 4 depth position that can represent the hardness of the steel plate base material is defined as the hardness (Hvb) of the base material.
- the hardness of the steel sheet surface layer decreases and the softened area expands in the sheet thickness direction. It was found that the thickness and the degree of softening of the softened layer can be represented by measuring the hardness at a certain depth position. From this, the hardness at the position of 20 ⁇ m from the surface of the steel sheet is measured, and if it is a plated steel sheet, the hardness at the position of 20 ⁇ m from the plating layer / base metal interface is measured to obtain the hardness (Hvs) of the steel sheet surface layer.
- the measurement position was set to 20 ⁇ m from the surface for the following reason.
- the steel sheet hardness was Hv 100 to 400
- the indentation size was about 8 to 13 ⁇ m, and when the measurement position was too close to the steel sheet surface, accurate hardness measurement was difficult.
- the measurement position was set to a position of 20 ⁇ m from the surface. In measuring the hardness of the steel sheet surface layer, in order to prevent sagging of the surface of the steel sheet during polishing, it is preferable to perform polishing and hardness measurement after applying a plate to the steel sheet and embedding resin.
- the high-strength cold-rolled steel sheet of the present invention may be any of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and an electrogalvanized steel sheet as long as the hardness of the steel sheet surface layer satisfies the above range.
- the galvanized layer is not particularly limited.
- an alloyed galvanized layer containing less than 7% by mass of Fe and the balance consisting of Zn, Al and unavoidable impurities is used.
- a material containing 7 to 15% by mass of Fe and the balance of Zn, Al and inevitable impurities can be used. Also.
- the galvanized layer contains at least one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs, and REM, or It may be mixed. Even if the alloyed galvanized layer contains or mixes at least one of the above-mentioned elements, the effects of the present invention are not impaired, and depending on the content, the corrosion resistance and workability are preferably improved. There is also.
- the high-strength cold-rolled steel sheet of the present invention has a film containing at least one of a phosphorus oxide and a complex oxide containing phosphorus on the surface of the cold-rolled steel sheet or the surface of the galvanized steel sheet. May be.
- the film containing at least one of phosphorus oxide and phosphorus-containing composite oxide can function as a lubricant when processing the steel sheet, and can protect the surface of the steel sheet and the alloyed galvanized layer.
- Step plate manufacturing method Next, a method for producing the above-described high-strength cold-rolled steel sheet will be described in detail.
- a slab having the above-described component composition is cast.
- a slab produced by a continuous casting slab, a thin slab caster or the like can be used.
- a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting may be used.
- the slab heating temperature is to secure a finish rolling temperature equal to or higher than the Ar3 transformation point, and the decrease in the slab heating temperature leads to an excessive increase in rolling load, making rolling difficult. Since there is a concern of causing a shape defect of the base steel plate after rolling, it is necessary to set the temperature to 1050 ° C. or higher.
- the upper limit of the slab heating temperature is not particularly defined, and the effect of the present invention is exhibited. However, since it is not economically preferable to make the heating temperature excessively high, the upper limit of the slab heating temperature is 1350 ° C. or less. It is desirable.
- Hot rolling needs to be completed at a finish rolling temperature not lower than the Ar3 transformation point temperature. If the finish rolling temperature is lower than the Ar3 transformation point, it becomes a two-phase rolling of ferrite and austenite, and the hot-rolled sheet structure becomes a heterogeneous mixed grain structure, even if it undergoes a cold rolling process and a continuous annealing process. Is not eliminated, and the steel sheet is inferior in ductility and bendability.
- the upper limit of the finish rolling temperature is not particularly defined, and the effect of the present invention is exhibited. However, when the finish rolling temperature is excessively high, the slab heating temperature must be excessively high in order to secure the temperature. I must. For this reason, the upper limit temperature of the finish rolling temperature is desirably 1100 ° C. or lower.
- Ar3 901-325 ⁇ C + 33 ⁇ Si-92 ⁇ (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2) + 52 ⁇ Al
- the hot rolling coiling temperature is set to 750 ° C. or lower in order to prevent the thickness of the oxide formed on the surface of the hot-rolled steel sheet from increasing excessively and deteriorating the pickling property.
- the winding temperature is preferably 720 ° C. or lower, and more preferably 700 ° C. or lower.
- the coiling temperature is preferably 400 ° C. or higher.
- the winding temperature is preferably 420 ° C. or higher.
- pickling removes oxides on the surface of the hot-rolled steel sheet, and is therefore important for improving the plateability of the base steel sheet. Moreover, pickling may be performed once or may be performed in a plurality of times.
- the hot-rolled steel sheet after pickling is cold-rolled for the purpose of plate thickness adjustment and shape correction.
- the rolling reduction is preferably in the range of 30 to 80%. If the rolling reduction is less than 30%, it is difficult to keep the shape flat, and the ductility of the final product may be deteriorated.
- the rolling reduction in cold rolling is preferably 35% or more, and more preferably 40% or more. On the other hand, when the rolling reduction ratio exceeds 80%, the cold rolling load becomes too large and cold rolling becomes difficult. Therefore, the rolling reduction is preferably 80% or less.
- the effect of the present invention is exhibited without particularly defining the number of rolling passes and the rolling reduction for each rolling pass.
- the obtained cold-rolled steel sheet is passed through an annealing line and annealed in a temperature range of (Ac1 transformation point + 40) ° C. to (Ac3 transformation point + 50) ° C.
- the decarburization treatment means that the atmosphere in the furnace during annealing is in the following range to diffuse C contained in the steel sheet surface layer into the atmosphere, lower the C concentration of the steel sheet surface layer, and reduce the fraction of hard structure It is a process to reduce.
- decarburization is performed by setting the atmosphere in the furnace during annealing to a log (water pressure / hydrogen partial pressure) in the range of ⁇ 3.0 to 0.0.
- a log water pressure / hydrogen partial pressure
- the logarithm of the ratio of the moisture pressure of the atmospheric gas to the hydrogen partial pressure is -3.0 to 0.0.
- decarburization from the surface layer of the cold-rolled steel sheet by annealing can be appropriately promoted.
- the logarithm of the ratio of the moisture pressure and the hydrogen partial pressure is less than ⁇ 3.0, decarburization from the surface layer of the cold-rolled steel sheet by annealing is insufficient.
- the logarithm of the ratio of moisture pressure to hydrogen partial pressure is preferably ⁇ 2.5 or more.
- the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is more than 0.0, decarburization from the surface layer of the cold-rolled steel sheet by annealing is excessively promoted, and the strength of the steel sheet may be insufficient.
- the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is preferably ⁇ 0.3 or less.
- the atmosphere at the time of annealing contains nitrogen, water vapor
- the temperature range during annealing is (Ac1 transformation point + 40) ° C. to (Ac3 transformation point + 50) ° C. because austenite is formed during annealing, and this austenite is martensite, bainite or retained austenite. This is to increase the strength of the steel sheet.
- the annealing temperature is less than (Ac1 transformation point + 40) ° C., the volume fraction of austenite formed during annealing is small, and it is difficult to ensure a strength of 700 MPa or more. For this reason, the lower limit of the annealing temperature is set to (Ac1 transformation point + 40) ° C.
- the annealing temperature is set to (Ac3 transformation point +50) ° C. or lower. It is desirable. However, although it is an effect excluding economical efficiency, excellent bendability can be obtained.
- Ac1 and Ac3 transformation points are calculated by the following formula using the content (mass%) of each element.
- Ac1 723-10.7 ⁇ Mn ⁇ 16.9 ⁇ Ni + 29.1 ⁇ Si + 16.9 ⁇ Cr + 6.38 ⁇ W
- Ac3 910 ⁇ 203 ⁇ (C) 0.5 ⁇ 15.2 ⁇ Ni + 44.7 ⁇ Si + 104 ⁇ V + 31.5 ⁇ Mo-30 ⁇ Mn-11 ⁇ Cr ⁇ 20 ⁇ Cu + 700 ⁇ P + 400 ⁇ Al + 400 ⁇ Ti
- the residence time in the above-described annealing temperature and atmosphere is 20 seconds to 600 seconds. If the residence time is less than 20 seconds, the hard tissue fraction becomes too small and it is difficult to ensure a high strength of 700 MPa or more. That is, austenite is formed by dissolution of carbides, but it takes some time for dissolution. When annealing is performed for less than 20 seconds, a sufficient amount of austenite cannot be secured due to insufficient time for the carbide to dissolve. As a result, it is difficult to ensure a strength of 700 MPa or more. Therefore, the lower limit of the annealing temperature time was set to 20 seconds. On the other hand, staying longer than 600 seconds is not preferable because not only the effect is saturated but also productivity is deteriorated. For this reason, the upper limit of the annealing temperature was set to 600 seconds.
- the average cooling rate in the temperature range of 700 ° C. to 500 ° C. is set to 0.5 ° C./second or more and 500 ° C./second or less, and the cooling is stopped in the temperature range of 100 to 330 ° C.
- the average cooling rate in the above temperature range is less than 0.5 ° C./second, the residence time in this temperature range is long, and a large amount of ferrite and pearlite is generated. For this reason, it becomes difficult to ensure the strength of 700 MPa or more.
- a cooling rate exceeding 500 ° C./second not only an excessive facility investment is required, but there is a concern that the temperature variation in the plate increases.
- the cooling stop temperature is set to 330 ° C. or lower, preferably 300 ° C. or lower, more preferably 250 ° C. or lower. Thereby, martensite is formed at the time of cooling, and the intensity
- the variation in the cooling stop temperature and the variation in the material will be increased.
- the lower limit of the cooling stop temperature is preferably set to 100 ° C. or higher.
- the temperature is desirably 130 ° C. or higher, and more desirably 160 ° C. or higher.
- temper martensite formed during cooling or promote bainite transformation to achieve both high strength and bendability.
- Tempering is a treatment for precipitating iron carbide or recovering dislocations by holding martensite in a temperature range of 350 to 500 ° C. By performing tempering, the characteristics of martensite can be greatly improved, and the bendability can be greatly improved.
- the reason why the holding time is set to 10 to 1000 seconds is to cause precipitation of a sufficient amount of carbide and recovery of dislocations. If the holding time is less than 10 seconds, the tempering effect that is the effect of the present invention cannot be obtained. On the other hand, the reason for setting it to 1000 seconds or less is not preferable because excessive retention reduces productivity. In addition, bainite transformation may occur during holding, which often contributes to stabilization of retained austenite.
- the holding said by this invention means that a steel plate stays for said time in said temperature range. Therefore, it does not mean only the case where it is kept isothermal in this temperature range, but includes gradual heating and gradual cooling in this temperature range.
- Plating is performed by heating to °C or cooling and immersing in hot dip galvanizing bath.
- the plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature.
- the bath immersion plate temperature is lower than (hot dip galvanizing bath temperature -40) ° C, the heat removal at the time of immersion in the plating bath is large, and some of the molten zinc may solidify and deteriorate the plating appearance.
- the lower limit is (hot dip galvanizing bath temperature ⁇ 40) ° C.
- the plate temperature before immersion is lower than (hot dip galvanizing bath temperature ⁇ 40) ° C.
- reheating is performed before immersion in the plating bath and the plate temperature is set to (hot dip galvanizing bath temperature ⁇ 40) ° C. or higher. It may be immersed in.
- the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature + 50) ° C., operational problems accompanying the rise of the plating bath temperature are induced.
- the plating bath may contain Mg, Mn, Si, Cr, etc. in addition to pure zinc and Fe, Al.
- alloying the plating layer when alloying the plating layer, it is performed at 460 ° C. or higher.
- the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. If it exceeds 600 ° C., carbide precipitates in the austenite and austenite is decomposed, so that it is difficult to ensure a strength of 700 MPa or more and good bendability, so this is the upper limit.
- the galvanization of the surface of the cold rolled steel sheet is not limited to the one performed by hot dip galvanization described above, and may be performed by electroplating. In that case, it may be carried out according to a conventional method.
- a film containing at least one of phosphorus oxide and complex oxide containing phosphorus is applied to the surface of the cold-rolled steel sheet of the present invention or the surface of the plated layer of the galvanized steel sheet. It doesn't matter.
- skin pass rolling can be performed after the above-described annealing.
- the rolling reduction is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. Since productivity will fall remarkably when it exceeds 1.5%, this is made an upper limit.
- the skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.
- Plating bath temperature 50 ⁇ 2 ° C., current density: 60 A / dm 2 , plating solution flow rate: 1 m / sec.
- the evaluation of bendability is based on JIS Z 2248 (2006).
- the obtained steel sheet was cut out in a direction perpendicular to the rolling direction, the end face was mechanically ground, and a 35 mm ⁇ 100 mm test piece was prepared.
- the 90-degree V bending test was performed using a 90 ° die and punch with R of 0.5 to 6 mm.
- the surface of the sample after the bending test was observed with a magnifying glass, and the minimum bending radius without cracks was defined as the critical bending radius.
- a steel plate having a limit bending radius of 1 mm or less and no necking or a steel plate having a limit bending radius of 0.5 mm or less was defined as a steel plate having excellent bendability.
- the column of the steel sheet type indicates the form of the steel sheet, and indicates CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: hot-dip galvanized steel sheet, and EG: electrogalvanized steel sheet.
- + P was added to the steel sheet on which the phosphorous oxide-based inorganic film was formed.
- Those satisfying the conditions of the present invention have both a maximum tensile strength of 700 MPa or more and good bendability.
- the balance (TS ⁇ El) between the strength (TS) and the total elongation (El) was also as good as 18000 (MPa ⁇ %) or more.
- the present invention provides a high-strength cold-rolled steel sheet excellent in bendability having a maximum tensile strength of 700 MPa or more suitable for automobile structural members, reinforcing members, and suspension members at low cost. It can be expected to make a significant contribution to the weight reduction, and the industrial effect is extremely high.
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Abstract
Description
このような鋼板によれば、良好な曲げ性を得ることが出来るが、析出強化を利用しているため、強度と伸びとのバランスを高レベルで確保することは容易ではない。
(2)上記(1)に記載の高強度冷延鋼板では、前記表層ミクロ組織が、さらに、体積分率で、10~87%のフェライト、10~50%の焼き戻しマルテンサイト、及び15%以下に制限されたフレッシュマルテンサイトを含有してもよい。
(3)上記(1)又は(2)に記載の高強度冷延鋼板では、前記内部ミクロ組織が、更に、体積分率で、10~87%のフェライト、10~50%の焼き戻しマルテンサイト、及び15%以下に制限されたフレッシュマルテンサイトを含有してもよい。
(4)上記(1)~(3)のいずれか一項に記載の高強度冷延鋼板では、少なくとも片面に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜が形成されてもよい。
(5)上記(1)~(3)のいずれか一項に記載の高強度冷延鋼板では、少なくとも片面に、電気亜鉛めっき層が形成されてもよい。
(6)上記(5)に記載の高強度冷延鋼板では、前記電気亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜が形成されてもよい。
(7)上記(1)~(3)のいずれか一項に記載の高強度冷延鋼板では、少なくとも片面に、溶融亜鉛めっき層が形成されてもよい。
(8)上記(7)に記載の高強度冷延鋼板では、前記溶融亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜が形成されてもよい。
(9)上記(1)~(3)のいずれか一項に記載の高強度冷延鋼板では、少なくとも片面に、合金化溶融亜鉛めっき層が形成されてもよい。
(10)上記(9)に記載の高強度冷延鋼板では、前記合金化溶融亜鉛めっき層の上に、リン酸化物およびリン含む複合酸化物の少なくとも1種を含む皮膜が形成されてもよい。
(11)本発明の第二の態様は、質量%で、C:0.075~0.300%、Si:0.30~2.50%、Mn:1.30~3.50%、P:0.001~0.050%、S:0.0001~0.0100%、Al:0.001~1.500%、及びN:0.0001~0.0100%であり、Tiが0.150%以下に制限され、Nbが0.150%以下に制限され、Vが0.150%以下に制限され、Crが2.00%以下に制限され、Niが2.00%以下に制限され、Cuが2.00%以下に制限され、Moが1.00%以下に制限され、Wが1.00%以下に制限され、Ca、Ce、Mg、Zr、Hf、及びREMの少なくとも1種の合計が0.5000%以下に制限され、残部が鉄および不可避的不純物からなる成分組成を有し、1050℃以上の状態とされたスラブに対し、仕上げ圧延温度をAr3変態点以上に設定された熱間圧延を行い、その後750℃以下の温度域にて巻き取ることにより熱延鋼板を得る熱間圧延工程と、前記熱延鋼板に対し、30~80%の圧下率で冷間圧延を行うことにより冷延鋼板を得る冷間圧延工程と、前記冷延鋼板に対し、(Ac1変態点+40)℃~(Ac3変態点+50)℃の温度域で、かつlog(水分圧/水素分圧)が-3.0~0.0の雰囲気で20秒~600秒焼鈍を行い、ついで、700~500℃間を0.5~500℃/秒の冷却速度として、100~330℃に冷却した後、350~500℃で10~1000秒間の保持を行うことにより高強度冷延鋼板を得る熱処理工程と、を備える冷延鋼板製造方法である。
(12)上記(11)に記載の冷延鋼板製造方法では、前記高強度冷延鋼板の少なくとも片面には、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成してもよい。
(13)上記(11)に記載の冷延鋼板製造方法では、前記高強度冷延鋼板の少なくとも片面に、電気亜鉛めっき層を形成してもよい。
(14)上記(13)に記載の冷延鋼板製造方法では、前記電気亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成してもよい。
(15)上記(11)に記載の冷延鋼板製造方法では、前記高強度冷延鋼板の少なくとも片面に、溶融亜鉛めっき層を形成してもよく、前記溶融亜鉛めっき層は、前記高強度冷延鋼板を、(亜鉛めっき浴温度―40)℃~(亜鉛めっき浴温度+50)℃の温度範囲に加熱又は冷却した状態で、亜鉛めっき浴に浸漬し、冷却することにより形成されてもよい。
(16)上記(15)に記載の冷延鋼板製造方法では、前記溶融亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成してもよい。
(17)上記(11)に記載の冷延鋼板製造方法では、前記高強度冷延鋼板の少なくとも片面に、合金化溶融亜鉛めっき層を形成してもよく、前記合金化溶融亜鉛めっき層は、前記高強度冷延鋼板を、(亜鉛めっき浴温度―40)℃~(亜鉛めっき浴温度+50)℃の温度範囲に加熱又は冷却した状態で、亜鉛めっき浴に浸漬し、460℃以上の温度で合金化処理を施した後、冷却することにより形成してもよい。
(18)上記(17)に記載の冷延鋼板製造方法では、前記合金化溶融亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成してもよい。
その結果、本発明者らは、所定の成分組成を有し、ミクロ組織を所定の組織へと制御した上で、脱炭処理を施すことで鋼板表層を軟化することが出来、引張最大強度が700MPa以上の高強度冷延鋼板であっても、あたかも、低強度の鋼板であるかのような優れた曲げ性を得ることが出来ることを明らかにした。この効果は、鋼板表層の硬度とt/4深さ位置の硬度の比「(表層の硬度)/(t/4深さ位置の硬度)」を0.75超~0.90とすることで得られる。
加えて、鋼板の表層部におけるミクロ組織が、体積分率で3~10%の残留オーステナイト及び90%以下のフェライトを含有し、且つ、鋼板のt/4深さ位置における内部ミクロ組織が、体積分率で3~30%の残留オーステナイトを含有することで、ネッキング起因の割れも抑制でき、更なる曲げ性の向上が得られる。特に、曲げ加工は、表層ほど歪が大きくなることから、表層と鋼板内部の硬さを表記範囲内にすることで大きな曲げ性の改善効果が得られる。
また、本発明の鋼板は、残留オーステナイトの含有により、曲げ加工時のネッキング抑制効果だけでなく、引張試験やプレス加工時のネッキング抑制効果も得られるため、伸びも良好である。
以下の説明において、曲げ性に優れた鋼板とは、JIS Z 2248(2006年)に基づく90度V曲げ試験にて、曲げ半径Rが1.0mm以下で割れ及びネッキングが発生しないもの、又は、曲げ半径Rが0.5mm以下で割れが発生しないものを意味する。
まず、本実施形態に係る冷延鋼板または亜鉛めっき鋼板を構成する鋼の成分組成について説明する。以下の説明における%は、質量%を表す。
Cは、母材鋼板の強度を高めるために含有される。しかし、Cの含有量が0.300%を超えると伸び性が及び溶接性が不十分となり、高い曲げ性を確保することが困難となる。Cの含有量は0.280%以下であることが好ましく、0.260%以下であることがより好ましい。一方、Cの含有量が0.075%未満であると強度が低下し、700MPa以上の引張最大強度を確保することが出来ない。強度を高めるため、Cの含有量は0.090%以上であることが好ましく、0.100%以上であることがより好ましい。
Siは、脱炭反応を促進させ、鋼板表層の軟化を招くことから最も重要な元素である。Siの含有量が2.50%を超えると母材鋼板が脆化し、延性が劣化するため、上限を2.50%とする。延性確保の観点から、Siの含有量は2.20%以下であることが好ましく、2.00%以下であることがより好ましい。一方、Siの含有量が0.30%未満では粗大な鉄系炭化物が多量に生成し、内部ミクロ組織の残留オーステナイト組織分率を3~30%とすることが出来ず、伸びが低下してしまう。この観点から、Siの下限値は0.50%以上であることが好ましく、0.70%以上がより好ましい。加えて、Siは、母材鋼板における鉄系炭化物の粗大化を抑制し、強度と成形性を高めるために必要な元素である。また、固溶強化元素として、鋼板の高強度化に寄与するため添加する必要がある。この観点から、Siの下限値は1%以上であることが好ましく、1.2%以上がより好ましい。
Mnは、母材鋼板の強度を高めるために含有される。しかし、Mnの含有量が3.50%を超えると母材鋼板の板厚中央部に粗大なMn濃化部が生じ、脆化が起こりやすくなり、鋳造したスラブが割れるなどのトラブルが起こりやすい。また、Mnの含有量が3.50%を超えると溶接性も劣化する。したがって、Mnの含有量は、3.50%以下とする。溶接性の観点から、Mnの含有量は3.20%以下であることが好ましく、3.00%以下であることがより好ましい。一方、Mnの含有量が1.30%未満であると、焼鈍後の冷却中に軟質な組織が多量に形成されるため、700MPa以上の引張最大強度を確保することが難しくなる。このことから、Mnの含有量を1.30%以上とする。Mnの含有量は、さらに強度を高めるために、1.50%以上であることが好ましく、1.70%以上であることがより好ましい。
Pは母材鋼板の板厚中央部に偏析する傾向があり、溶接部を脆化させる。Pの含有量が0.050%を超えると溶接部が大幅に脆化するため、Pの含有量を0.050%以下とする。Pの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Pの含有量を0.001%未満とすることは製造コストの大幅な増加を伴うことから、0.001%を下限値とする。
Sは、溶接性ならびに鋳造時および熱延時の製造性に悪影響を及ぼす。このことから、Sの含有量の上限値を0.0100%以下とする。また、SはMnと結びついて粗大なMnSを形成して延性や伸びフランジ性を低下させるため、0.0050%以下とすることが好ましく、0.0025%以下とすることがより好ましい。Sの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Sの含有量を0.0001%未満とすることは製造コストの大幅な増加を伴うため、0.0001%を下限値とする。
Alは、脱炭反応を促進させ、鋼板表層の軟化を招くことから重要な元素である。Alの含有量が1.500%を超えると溶接性が悪化するため、Alの含有量の上限を1.500%とする。この観点から、Alの含有量は1.200%以下とすることが好ましく、0.900%以下とすることがより好ましい。また、Alは脱酸材としても有効な元素であるが、Alの含有量が0.001%未満では脱酸材としての効果が十分に得られないことから、Alの含有量の下限を0.001%以上とする。脱酸の効果を十分に得るにはAl量は0.003%以上とすることが好ましい。
Nは、粗大な窒化物を形成し、延性および伸びフランジ性を劣化させることから、添加量を抑える必要がある。Nの含有量が0.0100%を超えると、この傾向が顕著となることから、N含有量の上限値を0.0100%以下とする。また、Nは、溶接時のブローホール発生の原因になることから、好ましくは0.0080%以下とする。Nの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Nの含有量を0.0001%未満にすると、製造コストの大幅な増加を招くことから、0.0001%を下限値とする。
Tiは、析出物強化、フェライト結晶粒の成長抑制による細粒強化および再結晶の抑制を通じた転位強化にて、母材鋼板の強度上昇に寄与する元素である。しかし、Tiの含有量が0.150%を超えると、炭窒化物の析出が多くなり成形性が劣化するため、Tiの含有量は0.150%以下であることが好ましい。成形性の観点から、Tiの含有量は0.120%以下であることがより好ましく、0.100%以下であることがさらに好ましい。Tiの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Tiによる強度上昇効果を十分に得るにはTiの含有量は0.005%以上であることが好ましい。母材鋼板の高強度化には、Tiの含有量は0.010%以上であることがより好ましく、0.015%以上であることがさらに好ましい。
Nbは、析出物強化、フェライト結晶粒の成長抑制による細粒強化および再結晶の抑制を通じた転位強化にて、母材鋼板の強度上昇に寄与する元素である。しかし、Nbの含有量が0.150%を超えると、炭窒化物の析出が多くなり成形性が劣化するため、Nbの含有量は0.150%以下であることが好ましい。成形性の観点から、Nbの含有量は0.120%以下であることがより好ましく、0.100%以下であることがさらに好ましい。Nbの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Nbによる強度上昇効果を十分に得るにはNbの含有量は0.005%以上であることが好ましい。母材鋼板の高強度化には、Nbの含有量は0.010%以上であることがより好ましく、0.015%以上であることがさらに好ましい。
Vは、析出物強化、フェライト結晶粒の成長抑制による細粒強化および再結晶の抑制を通じた転位強化にて、母材鋼板の強度上昇に寄与する元素である。しかし、Vの含有量が0.150%を超えると、炭窒化物の析出が多くなり成形性が劣化するため、Vの含有量は0.150%以下であることが好ましい。Vの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Vによる強度上昇効果を十分に得るにはVの含有量は0.005%以上であることが好ましい。
Crは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Crの含有量が2.00%を超えると、熱間での加工性が損なわれ、生産性が低下することから、Crの含有量は2.00%以下であることが好ましい。Crの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Crによる高強度化の効果を十分に得るには、Crの含有量は0.01%以上であることが好ましい。
Niは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Niの含有量が2.00%を超えると、溶接性が損なわれることから、Niの含有量は2.00%以下であることが好ましい。Niの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Niによる高強度化の効果を十分に得るには、Niの含有量は0.01%以上であることが好ましい。
Cuは微細な粒子として鋼中に存在することで強度を高める元素であり、Cおよび/またはMnの一部に替えて添加することができる。Cuの含有量が2.00%を超えると、溶接性が損なわれることから、Cuの含有量は2.00%以下であることが好ましい。Cuの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Cuによる高強度化の効果を十分に得るには、Cuの含有量は0.01%以上であることが好ましい。
Moは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Moの含有量が1.00%を超えると、熱間での加工性が損なわれ、生産性が低下する。このことから、Moの含有量は1.00%以下であることが好ましい。Moの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Moによる高強度化の効果を十分に得るには、Moの含有量は0.01%以上であることが好ましい。
Wは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Wの含有量が1.00%を超えると、熱間での加工性が損なわれ、生産性が低下することから、Wの含有量は1.00%以下であることが好ましい。Wの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Wによる高強度化の効果を十分に得るには、Wの含有量は0.01%以上であることが好ましい。
Ca、Ce、Mg、Zr、Hf、REMは、成形性の改善に有効な元素であり、1種又は2種以上を添加することができる。しかし、Ca、Ce、Mg、Zr、Hf、REMの少なくとも1種の含有量の合計が0.5000%を超えると、却って延性を損なう恐れがある。このため、各元素の含有量の合計は0.5000%以下であることが好ましい。Ca、Ce、Mg、Zr、Hf、REMの少なくとも1種の含有量の下限は、特に定めることなく本発明の効果は発揮されるが、母材鋼板の成形性を改善する効果を十分に得るには、各元素の含有量の合計が0.0001%以上であることが好ましい。成形性の観点から、Ca、Ce、Mg、Zr、Hf、REMの1種または2種以上の含有量の合計が0.0005%以上であることがより好ましく、0.0010%以上であることがさらに好ましい。
なお、REMとは、Rare Earth Metalの略であり、ランタノイド系列に属する元素をさす。REMやCeはミッシュメタルにて添加されることが多く、LaやCeの他にランタノイド系列の元素を複合で含有する場合がある。不可避不純物として、これらLaやCe以外のランタノイド系列の元素を含んだとしても本発明の効果は発揮される。また、金属LaやCeを添加したとしても本発明の効果は発揮される。
次に、内部ミクロ組織について説明する。ここで、内部ミクロ組織とは、母材鋼板の板厚をtとして、t/4深さ位置におけるミクロ組織を意味する。尚、後述する表層ミクロ組織とは、母材鋼板の表面、厳密には、母材鋼板の板面に平行かつ表面から20μm深さの面におけるミクロ組織を意味する。
鋼板の内部ミクロ組織は、t/4深さ位置を中心としたt/8~3t/8深さの範囲において、体積分率で3~30%の残留オーステナイトを含有する。残留オーステナイトは、延性を大きく向上させることで、曲げ加工時に発生するネッキング抑制に効果がある。一方で、残留オーステナイトは、破壊の起点となって曲げ性を劣化させる。このため、母材鋼板のミクロ組織に含まれる残留オーステナイトを体積分率で3~20%とすることが好ましい。内部ミクロ組織の残留オーステナイトの下限は5%もしくは8%以上が好ましい。
「表層ミクロ組織のフェライト:90%以下」
更に優れた曲げ性を具備するためには、鋼板表層部における残留オーステナイトの組織分率を3~10%に制限し、且つ、フェライトの組織分率を90%以下に制限する。表層における残留オーステナイト分率が3%未満では、例えば、90度V曲げ試験において、曲げ半径1.0mm以下では、表層部にネッキングが生じ、曲げ性を劣化させる。このため、鋼板表層の残留オーステナイト分率を3%以上にする必要がある。一方で、残留オーステナイトは、曲げ成形中にマルテンサイトへと変態し、割れの起点となることから、脱炭処理を行うことで鋼板表層のオーステナイト分率を低下させる必要がある。フレッシュマルテンサイトの分率を15%以下に低下させたとしても、残留オーステナイトがマルテンサイトに変態することにより生じたマルテンサイトを起点とした曲げ性劣化は避けがたい。このことから、表層部の残留オーステナイト分率は、10%以下とし、好ましくは8%以下、より好ましくは5.8%以下とする。
鋼板の内部ミクロ組織と表層部ミクロ組織の残留オーステナイト分率を上記範囲とすることで、後述する鋼板表層部と鋼板内部(t/4深さ位置)との硬度比を0.75超~0.90以下とすることが可能となり、優れた曲げ性を具備することが出来る。
尚、表層ミクロ組織のフェライトの組織分率を90%超とする場合には、所定の残留オーステナイト組織分率を確保することが困難となり、優れた曲げ性を確保出来ないため、90%を上限とする。
さらに、ベイニティックフェライト、ベイナイト、パーライトの1種以上を含んでもよい。以下に説明する範囲であれば、本発明の目的を達成することができる。
焼戻しマルテンサイトは、引張強度を大きく向上させる。このため、焼戻しマルテンサイトは、母材鋼板の組織に体積分率で50%以下含まれていてもよい。焼き戻しマルテンサイトとは、マルテンサイトを200~500℃で保持することで、θ、ε、η等の鉄基炭化物を析出させたマルテンサイトであり、フレッシュマルテンサイトに比べ、割れの発生の原因となり難い。引張強度の観点から、焼戻しマルテンサイトの体積分率は1%以上とすることが好ましく、10%以上とすることがより好ましい。一方、母材鋼板のミクロ組織に含まれる焼戻しマルテンサイトの体積分率が50%を超えると、降伏応力が過度に高まり、形状凍結性が劣化することが懸念されるため好ましくない。
フェライトは、延性の向上に有効である。このため、フェライトは、母材鋼板の組織に体積分率で10%以上含まれていても良い。また、フェライトは軟質な組織であるため、十分な強度を確保するために体積分率で87%を上限としてもよい。
フレッシュマルテンサイトは、引張強度を大きく向上させるが、一方で破壊の起点となって曲げ性を大きく劣化させるため、母材鋼板の組織に体積分率で15%以下に制限することが好ましい。曲げ性を高めるにはフレッシュマルテンサイトの体積分率を10%以下とすることがより好ましく、5%以下とすることが更に好ましい。
フレッシュマルテンサイトとは、鉄基炭化物を含まないマルテンサイトであり、非常に硬くて脆い。この結果、曲げ加工を行った場合、割れの起点となり曲げ性を大幅に劣化させてしまう。このことから、体積率は出来るだけ小さくすることが望ましい。
ベイニティックフェライトおよびベイナイトは、強度と延性のバランスに優れた組織であり、また、軟質なフェライトと硬質なマルテンサイト、焼戻しマルテンサイトおよび残留オーステナイトの中間の強度を有する組織であり、強度と曲げ性のバランス向上にも寄与する。このため体積分率で、合計10~50%含んでいてもよい。
パーライトが多くなると、延性が劣化する。このことから、母材鋼板の組織に含まれるパーライトの体積分率は、5%以下であることが好ましく、3%以下であることがより好ましい。
その他の組織として、粗大なセメンタイトなど上記以外の組織が含まれていてもよい。しかし、母材鋼板の組織中に粗大なセメンタイトが多くなると、曲げ性が劣化する。このことから、母材鋼板の組織に含まれる粗大なセメンタイトの体積分率は、10%以下であることが好ましく、5%以下であることがより好ましい。粗大なセメンタイトとは、公称粒径で2μm以上のセメンタイトを意味する。セメンタイトは、鉄に比べてもろく、鉄とセメンタイトの界面強度も小さいことから、曲げ成形中に割れやボイド形成の起点となり、曲げ性を劣化させる。このことから、粗大なセメンタイトの体積率は小さくする必要がある。一方、ベイナイト組織や焼き戻しマルテンサイト中に含まれる微細な鉄基炭化物は、曲げ性を劣化させないことから含有しても良い。
(内部ミクロ組織)
残留オーステナイトの体積分率は、母材鋼板の板面に平行かつt/4深さ位置の面を観察面としてX線回折を行い、面積分率を算出し、それを持って体積分率と見なすことができる。また、フェライト、パーライト、ベイナイト、セメンタイト、焼戻しマルテンサイトおよびフレッシュマルテンサイトの体積分率は、母材鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、t/4深さ位置を中心としたt/8~3t/8深さの範囲を電界放射型走査型電子顕微鏡(FE-SEM:Field Emission Scanning Electron Microscope)で観察して面積分率を測定し、それを持って体積分率と見なすことができる。
なお、各組織の体積分率の測定位置を、表面からt/4深さ位置を中心としたt/8~3t/8深さの範囲としたのは、鋼板表層は脱炭が原因で鋼板組織がt/8~3t/8深さの範囲の鋼板組織と異なっており、板厚中心もMn偏析が原因でマルテンサイトを多く含む組織となり、他の位置と鋼板組織が大きく異なるためである。
(表層ミクロ組織)
一方、表層における残留オーステナイトの体積分率は、母材鋼板の板面に平行かつ表面から20μm深さの面を観察面としてX線回折を行い、面積分率を算出し、それを持って体積分率と見なすことができる。また、フェライト、パーライト、ベイナイト、セメンタイト、焼戻しマルテンサイトおよびフレッシュマルテンサイトの体積分率は、母材鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、電界放射型走査型電子顕微鏡(FE-SEM:Field Emission Scanning Electron Microscope)で観察して面積分率を測定し、それを持って体積分率と見なすことができる。
次に、鋼板表層の硬度と前記母材鋼板のt/4深さ位置の硬度との比を規定した理由について説明する。
軟化位置では、鋼板硬度がHv100~400であり、圧痕サイズが8~13μm程度となり、測定位置が鋼板表面に近すぎる場合は、正確な硬度測定が難しかった。一方、測定位置が鋼板表面から離れすぎると、軟化層が含まれないことから、曲げ性と鋼板表層の硬度の間の関係を正確に求めることが出来なかった。このことから、測定位置を表面から20μm位置とした。
なお、鋼板表層の硬度測定にあたっては、研磨時の鋼板表面のダレを防止するため、鋼板に当て板を行い樹脂埋め込みした後、研磨、硬度測定を行うとよい。
本発明の高強度冷延鋼板は、鋼板表層の硬さが上記範囲を満たす限り、冷延鋼板、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板並びに電気亜鉛めっき鋼板のいずれであってもよい。
亜鉛めっき層としては、特に限定されず、例えば、溶融亜鉛めっき層としては、Feを7質量%未満含有し、残部がZn、Alおよび不可避的不純物からなるものなどが、合金化溶融亜鉛めっき層としては、Feを7~15質量%含有し、残部がZn、Alおよび不可避的不純物からなるものなどが使用できる。
また。亜鉛めっき層は、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、Bi、Sr、I、Cs、REMの少なくとも1種を含有、あるいは混入するものであってもよい。合金化亜鉛めっき層が、上記の元素の少なくとも1種を含有、あるいは混入するものであっても、本発明の効果は損なわれず、その含有量によっては耐食性や加工性が改善される等好ましい場合もある。
リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜は、鋼板を加工する際に潤滑剤として機能させることができ、鋼板の表面や合金化亜鉛めっき層を保護することができる。
次に、上述の高強度冷延鋼板を製造する方法について詳細に説明する。
鋼板を製造するには、まず、上述した成分組成を有するスラブを鋳造する。熱間圧延に供するスラブは、連続鋳造スラブや薄スラブキャスターなどで製造したものを用いることができる。さらに、鋳造後に直ちに熱間圧延を行う連続鋳造-直接圧延(CC-DR)のようなプロセスを用いてもよい。
一方、仕上げ圧延温度の上限は特に定めることなく、本発明の効果は発揮されるが、仕上げ圧延温度を過度に高温とした場合、その温度を確保するためにスラブ加熱温度を過度に高温にしなければならない。このことから、仕上げ圧延温度の上限温度は、1100℃以下とすることが望ましい。
Ar3=901-325×C+33×Si-92×(Mn+Ni/2+
Cr/2+Cu/2+Mo/2)+52×Al
一方、巻き取り温度が400℃未満となると熱延鋼板の強度が過度に高まり、冷間圧延が困難となるため、巻き取り温度は400℃以上とすることが望ましい。冷間圧延の負荷を軽減するため、巻き取り温度は420℃以上とすることが好ましい。ただし、400℃未満で巻き取ったとしても、その後、箱型炉にて焼鈍を行い、熱延板の軟化処理を行うことで、冷間圧延が可能となることから、400℃未満で巻き取っても構わない。
なお、冷間圧延工程において、圧延パスの回数、各圧延パス毎の圧下率については特に規定することなく本発明の効果は発揮される。
水分圧と水素分圧の比の対数が-3.0未満であると、焼鈍を行うことによる冷延鋼板表層からの脱炭が不十分となる。脱炭を促進するために、水分圧と水素分圧の比の対数は、-2.5以上であることが好ましい。一方、水分圧と水素分圧の比の対数が0.0超であると、焼鈍を行うことによる冷延鋼板表層からの脱炭が過度に促進されて、鋼板の強度が不十分となる恐れがある。鋼板の強度を確保するために、水分圧と水素分圧の比の対数は、-0.3以下であることが好ましい。また、焼鈍を行う際の雰囲気は、窒素と水蒸気と水素とを含み、窒素を主体とするものであることが好ましく、窒素と水蒸気と水素の他に、酸素が含まれていてもよい。
焼鈍温度が(Ac1変態点+40)℃未満では、焼鈍時に形成したオーステナイトの体積率が小さく、700MPa以上の強度を確保することが難しい。このことから、焼鈍温度の下限を(Ac1変態点+40)℃とする。
一方で、焼鈍温度が過度に高温になりすぎると、経済的に好ましくないばかりでなく、ロールや製造設備の劣化が顕著となるので、焼鈍温度は、(Ac3変態点+50)℃以下とすることが望ましい。ただし、経済性を除く効果であるが、優れた曲げ性を得ることができる。
Ac1=723-10.7×Mn-16.9×Ni+29.1×Si
+16.9×Cr+6.38×W
Ac3=910-203×(C)0.5-15.2×Ni+44.7×Si
+104×V+31.5×Mo-30×Mn-11×Cr
-20×Cu+700×P+400×Al+400×Ti
上記温度範囲での平均冷却速度が0.5℃/秒未満であると、この温度範囲での滞在時間が長時間となってフェライトやパーライトが多量に生成される。このため、700MPa以上の強度を確保することが難しくなる。一方、500℃/秒を上回る冷却速度では、過度の設備投資を必要とするばかりでなく、板内の温度バラツキの増大等を招く懸念がある。
保持の温度範囲を350~500℃とするのは、冷却中に形成したマルテンサイトを焼き戻す、あるいは、ベイナイト変態を促進させ、高強度と曲げ性の両立を図るためである。焼き戻しとは、マルテンサイトを350~500℃の温度域で保持を行うことで、鉄系炭化物を析出させたり、転位の回復を行う処理である。焼き戻しを行うことで、マルテンサイトの特性を大きく向上出来、曲げ性を大きく向上させることが出来る。
めっき浴浸漬板温度は、溶融亜鉛めっき浴温度より40℃低い温度から溶融亜鉛めっき浴温度より50℃高い温度までの温度範囲とすることが望ましい。浴浸漬板温度が(溶融亜鉛めっき浴温度-40)℃を下回ると、めっき浴浸漬進入時の抜熱が大きく、溶融亜鉛の一部が凝固してしまいめっき外観を劣化させる場合があることから、下限を(溶融亜鉛めっき浴温度-40)℃とする。
実験例1~85として、表1、表2に示す成分組成を有するスラブを1230℃に加熱し、表3~6に示す製造条件に基づき熱間圧延、冷間圧延、及び熱処理を行い、板厚1.2mmの冷延鋼板を製造した。幾つかの実験例においては、表5、表6に示すめっき条件に基づきめっき処理を施した。
表1、2は、実験例1~85で用いたスラブの鋼種A~Y、a~dの成分組成を示す。表3、4には、スラブ特性、熱間圧延条件、冷間圧延条件を示す。表5、6には、熱処理条件、及びめっき条件を示す。
尚、表1~6において、本発明の範囲から逸している数値に下線を付与している。また、表5、6において、合金化温度の列におけるハイフンは合金化処理を施していないことを意味する。
また、実験例1の冷延鋼板(CR)、実験例54、71の溶融亜鉛めっき鋼板(GI)、及び実験例15の合金化溶融亜鉛めっき鋼板(GA)の表面には、リン酸および過酸化水素をリン酸/H2O2重量比=0.1~10の範囲で含有するpH1~7の水溶液を塗布し、水洗せずに400℃の温度で焼付乾燥して、P量に換算して10~500mg/m2の付着量で、リン酸化物系無機皮膜を形成させた。
また、得られた鋼板から試料を作成して、先述の方法で鋼板の表層(すなわち、母材鋼板の板面に平行かつ表面から20μm深さの面)及びt/4深さ位置における鋼板組織を測定した。その結果を表7、8に示す。表7、8において、Fはフェライト、γRは残留オーステナイト、TMは焼き戻しマルテンサイト、Mはフレッシュマルテンサイト、Bはベイナイト、Pはパーライトを意味する。尚、炭化物は、フェライトの面積率にカウントしている。
更に、表9、10に、鋼板の表層硬さ(Hvs)、t/4深さ位置の硬さ(Hvb)、硬さ比(Hvs/Hvb)、TS、EL、TS×EL、最小曲げ半径、めっき中Fe(ハイフンは、合金化処理を施していないことを示す)、及び鋼板種を示す。
尚、表7~10において、本発明の範囲から逸している数値に下線を付与している。TSは、引張り試験をJIS Z 2241(2011年)に準拠して測定した。
本発明の条件を満たすものは、700MPa以上の引張最大強度と良好な曲げ性を両立している。強度(TS)と全伸び(El)とのバランス(TS×El)も18000(MPa・%)以上と良好であった。
Claims (18)
- 質量%で、
C:0.075~0.300%、
Si:0.30~2.50%、
Mn:1.30~3.50%、
P:0.001~0.050%、
S:0.0001~0.0100%、
Al:0.001~1.500%、及び
N:0.0001~0.0100%
であり、
Tiが0.150%以下に制限され、
Nbが0.150%以下に制限され、
Vが0.150%以下に制限され、
Crが2.00%以下に制限され、
Niが2.00%以下に制限され、
Cuが2.00%以下に制限され、
Moが1.00%以下に制限され、
Wが1.00%以下に制限され、
Ca、Ce、Mg、Zr、Hf、及びREMの少なくとも1種の合計が0.5000%以下に制限され、
残部が鉄および不可避的不純物からなる成分組成を有し、
鋼板表層における表層ミクロ組織が、体積分率で、3~10%の残留オーステナイト及び90%以下のフェライトを含有し、
板厚をtとして前記表面からt/4深さ位置における内部ミクロ組織が、体積分率で3~30%の残留オーステナイトを含有し、
前記鋼板表層の硬度Hvsと前記t/4深さ位置における硬度Hvbとの比Hvs/Hvbが0.75超~0.90であり、
引張最大強度が700MPa以上である
ことを特徴とする高強度冷延鋼板。 - 前記表層ミクロ組織が、さらに、体積分率で、10~87%のフェライト、10~50%の焼き戻しマルテンサイト、及び15%以下に制限されたフレッシュマルテンサイトを含有する
ことを特徴とする請求項1に記載の高強度冷延鋼板。 - 前記内部ミクロ組織が、更に、体積分率で、10~87%のフェライト、10~50%の焼き戻しマルテンサイト、及び15%以下に制限されたフレッシュマルテンサイトを含有する
ことを特徴とする請求項1に記載の高強度冷延鋼板。 - 少なくとも片面に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜が形成されている
ことを特徴とする、請求項1~3のいずれか一項に記載の高強度冷延鋼板。 - 少なくとも片面に、電気亜鉛めっき層が形成されている
ことを特徴とする、請求項1~3のいずれか一項に記載の強度冷延鋼板。 - 前記電気亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜が形成されている
ことを特徴とする、請求項5に記載の高強度冷延鋼板。 - 少なくとも片面に、溶融亜鉛めっき層が形成されている
ことを特徴とする、請求項1~3のいずれか一項に記載の高強度冷延鋼板。 - 前記溶融亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜が形成されている
ことを特徴とする請求項7に記載の高強度冷延鋼板。 - 少なくとも片面に、合金化溶融亜鉛めっき層が形成されている
ことを特徴とする請求項1~3のいずれか一項に記載の高強度冷延鋼板。 - 前記合金化溶融亜鉛めっき層の上に、リン酸化物およびリン含む複合酸化物の少なくとも1種を含む皮膜が形成されている
ことを特徴とする請求項9に記載の高強度冷延鋼板。 - 質量%で、C:0.075~0.300%、Si:0.30~2.50%、Mn:1.30~3.50%、P:0.001~0.050%、S:0.0001~0.0100%、Al:0.001~1.500%、及びN:0.0001~0.0100%であり、Tiが0.150%以下に制限され、Nbが0.150%以下に制限され、Vが0.150%以下に制限され、Crが2.00%以下に制限され、Niが2.00%以下に制限され、Cuが2.00%以下に制限され、Moが1.00%以下に制限され、Wが1.00%以下に制限され、Ca、Ce、Mg、Zr、Hf、及びREMの少なくとも1種の合計が0.5000%以下に制限され、残部が鉄および不可避的不純物からなる成分組成を有し、1050℃以上の状態とされたスラブに対し、仕上げ圧延温度をAr3変態点以上に設定された熱間圧延を行い、その後750℃以下の温度域にて巻き取ることにより熱延鋼板を得る熱間圧延工程と、
前記熱延鋼板に対し、30~80%の圧下率で冷間圧延を行うことにより冷延鋼板を得る冷間圧延工程と、
前記冷延鋼板に対し、Ac1変態点+40℃~Ac3変態点+50℃の温度域で、かつlog(水分圧/水素分圧)が-3.0~0.0の雰囲気で20秒~600秒焼鈍を行い、ついで、700~500℃間を0.5~500℃/秒の冷却速度として、100~330℃に冷却した後、350~500℃で10~1000秒間の保持を行うことにより高強度冷延鋼板を得る熱処理工程と、
を備えることを特徴とする冷延鋼板製造方法。 - 前記高強度冷延鋼板の少なくとも片面に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成する皮膜形成工程
を更に備えることを特徴とする、請求項11に記載の冷延鋼板製造方法。 - 前記高強度冷延鋼板の少なくとも片面に、電気亜鉛めっき層を形成する電気亜鉛めっき工程
を更に備えることを特徴とする、請求項11に記載の冷延鋼板製造方法。 - 前記電気亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成する皮膜形成工程
を更に備えることを特徴とする、請求項13に記載の冷延鋼板製造方法。 - 前記高強度冷延鋼板の少なくとも片面に、溶融亜鉛めっき層を形成する溶融亜鉛めっき工程
を更に備え、
前記溶融亜鉛めっき工程では、
前記高強度冷延鋼板を、亜鉛めっき浴温度―40℃~亜鉛めっき浴温度+50℃の温度範囲に加熱又は冷却した状態で、亜鉛めっき浴に浸漬し、冷却する
ことを特徴とする請求項11に記載の冷延鋼板製造方法。 - 前記溶融亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成する皮膜形成工程
を更に備えることを特徴とする、請求項15に記載の冷延鋼板製造方法。 - 前記高強度冷延鋼板の少なくとも片面に、合金化溶融亜鉛めっき層を形成する合金化溶融亜鉛めっき工程
を更に備え、
前記合金化溶融亜鉛めっき工程では、
前記高強度冷延鋼板を、亜鉛めっき浴温度―40℃~亜鉛めっき浴温度+50℃の温度範囲に加熱又は冷却した状態で、亜鉛めっき浴に浸漬し、460℃以上の温度で合金化処理を施した後、冷却する
ことを特徴とする請求項11に記載の冷延鋼板製造方法。 - 前記合金化溶融亜鉛めっき層の上に、リン酸化物およびリンを含む複合酸化物の少なくとも1種を含む皮膜を形成する工程
を更に備えることを特徴とする、請求項17に記載の冷延鋼板製造方法。
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US10544474B2 (en) | 2020-01-28 |
JP5454746B2 (ja) | 2014-03-26 |
TWI468534B (zh) | 2015-01-11 |
EP2813595A1 (en) | 2014-12-17 |
EP2813595A4 (en) | 2016-03-23 |
MX2014009471A (es) | 2014-09-25 |
KR101622063B1 (ko) | 2016-05-17 |
KR20140117477A (ko) | 2014-10-07 |
BR112014019206A8 (pt) | 2017-07-11 |
JPWO2013118679A1 (ja) | 2015-05-11 |
PL2813595T3 (pl) | 2020-07-13 |
EP2813595B1 (en) | 2020-01-01 |
CN104105807B (zh) | 2017-05-31 |
CN104105807A (zh) | 2014-10-15 |
TW201343934A (zh) | 2013-11-01 |
BR112014019206A2 (ja) | 2017-06-20 |
ES2768598T3 (es) | 2020-06-23 |
US20140377582A1 (en) | 2014-12-25 |
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