WO2020162556A1 - 溶融亜鉛めっき鋼板およびその製造方法 - Google Patents
溶融亜鉛めっき鋼板およびその製造方法 Download PDFInfo
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- WO2020162556A1 WO2020162556A1 PCT/JP2020/004628 JP2020004628W WO2020162556A1 WO 2020162556 A1 WO2020162556 A1 WO 2020162556A1 JP 2020004628 W JP2020004628 W JP 2020004628W WO 2020162556 A1 WO2020162556 A1 WO 2020162556A1
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- steel sheet
- hot
- less
- dip galvanized
- rolling
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- 229910001335 Galvanized steel Inorganic materials 0.000 title claims abstract description 38
- 239000008397 galvanized steel Substances 0.000 title claims abstract description 38
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 21
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 212
- 239000010959 steel Substances 0.000 claims abstract description 212
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 61
- 239000000463 material Substances 0.000 claims abstract description 34
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 30
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 18
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 17
- 239000000203 mixture Substances 0.000 claims abstract description 16
- 229910001567 cementite Inorganic materials 0.000 claims abstract description 15
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 claims abstract description 14
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 12
- 238000011282 treatment Methods 0.000 claims description 72
- 238000002791 soaking Methods 0.000 claims description 59
- 238000005096 rolling process Methods 0.000 claims description 55
- 238000010438 heat treatment Methods 0.000 claims description 53
- 238000001816 cooling Methods 0.000 claims description 46
- 238000005246 galvanizing Methods 0.000 claims description 39
- 238000005097 cold rolling Methods 0.000 claims description 29
- 230000009467 reduction Effects 0.000 claims description 26
- 230000000717 retained effect Effects 0.000 claims description 21
- 238000005098 hot rolling Methods 0.000 claims description 15
- 239000000126 substance Substances 0.000 claims description 12
- 239000012535 impurity Substances 0.000 claims description 11
- 230000036961 partial effect Effects 0.000 claims description 10
- 229910052739 hydrogen Inorganic materials 0.000 claims description 8
- 239000001257 hydrogen Substances 0.000 claims description 8
- 229910052684 Cerium Inorganic materials 0.000 claims description 7
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 7
- 229910052746 lanthanum Inorganic materials 0.000 claims description 7
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 6
- 239000010960 cold rolled steel Substances 0.000 claims description 5
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 4
- 238000007598 dipping method Methods 0.000 claims description 3
- 239000010410 layer Substances 0.000 description 44
- 238000000034 method Methods 0.000 description 39
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- 238000005275 alloying Methods 0.000 description 21
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- 230000000694 effects Effects 0.000 description 9
- 239000011572 manganese Substances 0.000 description 9
- 239000013078 crystal Substances 0.000 description 8
- 125000004432 carbon atom Chemical group C* 0.000 description 7
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 7
- 239000010936 titanium Substances 0.000 description 7
- 239000010949 copper Substances 0.000 description 6
- 238000001887 electron backscatter diffraction Methods 0.000 description 6
- 239000011777 magnesium Substances 0.000 description 6
- 239000010955 niobium Substances 0.000 description 6
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 5
- 239000011575 calcium Substances 0.000 description 5
- 229910052760 oxygen Inorganic materials 0.000 description 5
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- 238000003303 reheating Methods 0.000 description 5
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- 230000006866 deterioration Effects 0.000 description 4
- 238000009792 diffusion process Methods 0.000 description 4
- 229910052742 iron Inorganic materials 0.000 description 4
- 229910052748 manganese Inorganic materials 0.000 description 4
- 229910052719 titanium Inorganic materials 0.000 description 4
- 238000007545 Vickers hardness test Methods 0.000 description 3
- 238000002441 X-ray diffraction Methods 0.000 description 3
- 238000000137 annealing Methods 0.000 description 3
- 229910052787 antimony Inorganic materials 0.000 description 3
- 229910052796 boron Inorganic materials 0.000 description 3
- 229910052799 carbon Inorganic materials 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 229910052804 chromium Inorganic materials 0.000 description 3
- 229910052802 copper Inorganic materials 0.000 description 3
- 238000005261 decarburization Methods 0.000 description 3
- 230000007547 defect Effects 0.000 description 3
- 239000012467 final product Substances 0.000 description 3
- 230000014509 gene expression Effects 0.000 description 3
- 239000011133 lead Substances 0.000 description 3
- 229910052749 magnesium Inorganic materials 0.000 description 3
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- 229910052750 molybdenum Inorganic materials 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 238000005554 pickling Methods 0.000 description 3
- 238000001953 recrystallisation Methods 0.000 description 3
- 229910052710 silicon Inorganic materials 0.000 description 3
- 229910052721 tungsten Inorganic materials 0.000 description 3
- 238000004804 winding Methods 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- 229910000794 TRIP steel Inorganic materials 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 229910052797 bismuth Inorganic materials 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
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- 238000007654 immersion Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
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- 230000000670 limiting effect Effects 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 238000010791 quenching Methods 0.000 description 2
- 230000000171 quenching effect Effects 0.000 description 2
- 230000002441 reversible effect Effects 0.000 description 2
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- 238000010998 test method Methods 0.000 description 2
- 229910052718 tin Inorganic materials 0.000 description 2
- 239000011701 zinc Substances 0.000 description 2
- 229910052726 zirconium Inorganic materials 0.000 description 2
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 1
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 229910000797 Ultra-high-strength steel Inorganic materials 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- QCWXUUIWCKQGHC-UHFFFAOYSA-N Zirconium Chemical compound [Zr] QCWXUUIWCKQGHC-UHFFFAOYSA-N 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- WATWJIUSRGPENY-UHFFFAOYSA-N antimony atom Chemical compound [Sb] WATWJIUSRGPENY-UHFFFAOYSA-N 0.000 description 1
- 238000005279 austempering Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- JCXGWMGPZLAOME-UHFFFAOYSA-N bismuth atom Chemical compound [Bi] JCXGWMGPZLAOME-UHFFFAOYSA-N 0.000 description 1
- 229910052791 calcium Inorganic materials 0.000 description 1
- GWXLDORMOJMVQZ-UHFFFAOYSA-N cerium Chemical compound [Ce] GWXLDORMOJMVQZ-UHFFFAOYSA-N 0.000 description 1
- 239000003610 charcoal Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 239000011651 chromium Substances 0.000 description 1
- 239000010941 cobalt Substances 0.000 description 1
- 229910017052 cobalt Inorganic materials 0.000 description 1
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 description 1
- 239000008119 colloidal silica Substances 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000006477 desulfuration reaction Methods 0.000 description 1
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- 238000002050 diffraction method Methods 0.000 description 1
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- 238000010894 electron beam technology Methods 0.000 description 1
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- 230000002349 favourable effect Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 239000005431 greenhouse gas Substances 0.000 description 1
- 229910052735 hafnium Inorganic materials 0.000 description 1
- VBJZVLUMGGDVMO-UHFFFAOYSA-N hafnium atom Chemical compound [Hf] VBJZVLUMGGDVMO-UHFFFAOYSA-N 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
- FZLIPJUXYLNCLC-UHFFFAOYSA-N lanthanum atom Chemical compound [La] FZLIPJUXYLNCLC-UHFFFAOYSA-N 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
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- 239000002994 raw material Substances 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 230000001131 transforming effect Effects 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 1
- 238000010792 warming Methods 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—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
- C23C2/12—Aluminium or alloys based thereon
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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- C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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- 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
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- 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
- C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
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- 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
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- 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
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0436—Cold rolling
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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Definitions
- the present invention relates to a hot-dip galvanized steel sheet and a manufacturing method thereof, and mainly relates to a high-strength hot-dip galvanized steel sheet which is formed into various shapes as a steel sheet for automobiles by press working and the like and a manufacturing method thereof.
- the hot-dip galvanized steel sheet used for automobile parts is required to have not only strength but also various workability necessary for forming parts such as press formability and weldability. Specifically, from the viewpoint of press formability, a steel sheet is required to have excellent elongation (total elongation in tensile test: El), stretch flangeability (hole expansion ratio: ⁇ ), and bendability.
- TRIP steel sheets Transformation Induced Plasticity
- Patent Documents 1 to 3 disclose a technology relating to a high-strength TRIP steel sheet in which elongation and hole expansion rate are improved by controlling the structural composition fraction within a predetermined range.
- the steel sheet is heated and soaked in the reverse transformation temperature range (>Ac1) and then cooled to room temperature, and the temperature is about 460°C. It needs to be immersed in a hot dip galvanizing bath. Alternatively, after heating and soaking, it is necessary to cool the steel sheet to room temperature, then heat the steel sheet again to the hot dip galvanizing bath temperature and immerse it in the bath. Further, in order to produce an alloyed hot-dip galvanized steel sheet, it is usually necessary to reheat the steel sheet to a temperature range of 460° C. or higher because an alloying treatment is performed after immersion in the plating bath.
- Patent Document 4 after heating the steel plate to Ac1 or higher, quenching the steel plate to a temperature below the martensitic transformation start temperature (Ms), reheating to the bainite transformation temperature range, and holding the temperature range to stabilize austenite ( After austempering), reheating to the plating bath temperature or alloying treatment temperature for the plating alloying treatment is described.
- Ms martensitic transformation start temperature
- reheating to the bainite transformation temperature range and holding the temperature range to stabilize austenite
- After austempering After austempering
- reheating to the plating bath temperature or alloying treatment temperature for the plating alloying treatment is described.
- such a manufacturing method has a problem that martensite and bainite are excessively tempered in the plating alloying treatment step, so that the material is deteriorated.
- Patent Documents 5 to 9 disclose a method for manufacturing a hot-dip galvanized steel sheet, which includes cooling the steel sheet after plating alloying treatment and then reheating the martensite.
- Patent Document 10 describes a high-strength cold-rolled steel sheet whose surface layer portion is mainly composed of ferrite, which is manufactured by decarburizing the steel sheet.
- Patent Document 11 describes an ultra-high-strength cold-rolled steel sheet having a soft layer on the surface layer, which is manufactured by decarburizing and annealing a steel sheet.
- the bending deformation load of the member is originally expected from the steel plate strength depending on the deformation mode of the member at the time of collision deformation.
- Deformation load that is, the deformation load when the steel plate surface layer is not softened.
- the plastic strain that occurs increases toward the surface of the steel sheet. That is, the contribution to the deformation load is greater on the surface of the steel sheet than inside the steel sheet. Therefore, when the deformation of the member at the time of collision deformation is bending deformation, the deformation load of the member may decrease due to the softening of the steel plate surface.
- the present invention has been made in view of the above background, and an object of the present invention is to provide a hot-dip galvanized steel sheet that is excellent in press formability and that suppresses a decrease in load during bending deformation, and a manufacturing method thereof.
- the steel sheet is heated to a high temperature range of 650°C or higher, and the atmosphere in the furnace is deoxidized to the surface layer as a high oxygen potential. Form a charcoal region.
- the steel sheet is cooled to a low temperature region of 600° C. or lower, and the atmosphere in the furnace is set to have a low oxygen potential, and isothermal holding is performed for a predetermined time or longer. Due to this isothermal holding, carbon atoms inside the steel sheet are appropriately diffused in the decarburized region of the surface layer.
- the shear strain applied to the surface layer of the steel sheet is increased by limiting the cold rolling condition to a predetermined range.
- the steel sheet surface layer structure is refined. That is, the area of the crystal grain boundary increases in the surface layer of the steel sheet. Since the crystal grain boundaries act as diffusion paths for carbon atoms, the area of the crystal grain boundaries increases, and as a result, carbon atoms are likely to re-diffuse into the surface layer during isothermal holding at 600° C. or lower.
- a method for producing a hot-dip galvanized steel sheet which includes a hot-dip galvanizing step of applying galvanization, (A) Conditions of the following cold rolling steps (A1) and (A2): (A1) Performing cold rolling once or more once with a rolling line load satisfying the following formula (1) and a rolling reduction of 6% or more: 13 ⁇ A/B ⁇ 35 (1) (In the formula, A is the rolling line load (kgf/mm), and B is the tensile strength (kgf/mm 2 ) of the hot rolled steel sheet.) (A2) Satisfying the total cold reduction ratio of 30 to 80%, (B) In the hot dip galvanizing step, a steel sheet is heated to be subjected to a first soaking treatment, a first soaked steel sheet is first cooled and then a second soaking treatment, and a second soaking steel sheet is obtained.
- a hot-dip galvanized steel sheet that is excellent in press formability, specifically ductility, hole expandability and bendability, and that suppresses load reduction during bending deformation.
- the reference figure of a SEM secondary electron image is shown.
- 3 is a temperature-thermal expansion curve when a heat cycle corresponding to the hot dip galvanizing treatment according to the embodiment of the present invention is simulated by a thermal expansion measuring device. It is a figure which shows typically the test method for evaluating a bending deformation load.
- the hot-dip galvanized steel sheet according to the embodiment of the present invention has a hot-dip galvanized layer on at least one surface of the base material steel sheet, the base material steel sheet, in mass%, C: 0.050% to 0.350%, Si: 0.10% to 2.50%, Mn: 1.00% to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001% to 1.500%, N: 0.0100% or less, O: 0.0100% or less, Ti: 0% to 0.200%, B: 0% to 0.0100%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100%
- the steel structure in the range of 1 ⁇ 8 to 3 ⁇ 8 thickness centering on the position of 1 ⁇ 4 thickness from the surface of the base steel plate is the area%, Ferrite: 0% to 50%, Retained austenite: 0% to 30%, Tempered martensite: 5% or more, Fresh martensite: 0% to 10%, and total pearlite and cementite: 0% to 5% If the residual structure is present, the residual structure is composed of bainite, When a region having a hardness of 90% or less with respect to the hardness at a position of 1/4 thickness from the interface between the base material steel plate and the hot-dip galvanized layer to the base material steel plate side is a soft layer, There is a soft layer with a thickness of 10 ⁇ m or more on the base steel plate side, The soft layer includes tempered martensite, and The increase rate of the area% of tempered martensite from the interface in the soft layer into the base steel sheet is 5.0%/ ⁇ m or less.
- C is an essential element for ensuring the strength of the steel sheet. If less than 0.050%, the required high strength cannot be obtained, so the C content is set to 0.050% or more.
- the C content may be 0.070% or more, 0.080% or more, or 0.100% or more. On the other hand, if it exceeds 0.350%, the workability and weldability deteriorate, so the C content is made 0.350% or less.
- the C content may be 0.340% or less, 0.320% or less, or 0.300% or less.
- Si 0.10% to 2.50%
- Si is an element that suppresses the formation of iron carbide and contributes to the improvement of strength and formability, but excessive addition deteriorates the weldability of the steel sheet. Therefore, its content is set to 0.10 to 2.50%.
- the Si content may be 0.20% or more, 0.30% or more, 0.40% or more or 0.50% or more, and/or 2.20% or less, 2.00% or less or 1. It may be 90% or less.
- Mn 1.00% to 3.50%
- Mn manganese
- Mn is a strong austenite stabilizing element and is an element effective for increasing the strength of a steel sheet. Excessive addition deteriorates weldability and low temperature toughness. Therefore, its content is set to 1.00 to 3.50%.
- the Mn content may be 1.10% or higher, 1.30% or higher or 1.50% or higher, and/or 3.30% or lower, 3.10% or lower or 3.00% or lower. Good.
- P 0.050% or less
- P phosphorus
- the P content is limited to 0.050% or less.
- it is 0.045% or less, 0.035% or less, or 0.020% or less.
- the P removal cost becomes high, so from the viewpoint of economy, it is preferable to set the lower limit to 0.001%.
- S sulfur
- S is an element contained as an impurity and forms MnS in steel to deteriorate toughness and hole expandability. Therefore, the S content is limited to 0.0100% or less as a range in which the deterioration of the toughness and the hole expandability is not remarkable. It is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. However, in order to extremely reduce the S content, the desulfurization cost becomes high, and therefore the lower limit is preferably 0.0001% from the viewpoint of economy.
- Al 0.001% to 1.500%
- Al aluminum
- the upper limit of the amount of Al is 1.500%. It is preferably 1.200% or less, 1.000% or less, or 0.800% or less.
- N nitrogen
- nitrogen is an element contained as an impurity, and if its content exceeds 0.0100%, coarse nitride is formed in the steel to deteriorate bendability and hole expandability. Therefore, the N content is limited to 0.0100% or less. It is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. However, in order to extremely reduce the N content, the cost for removing N becomes high, and therefore the lower limit is preferably 0.0001% from the viewpoint of economy.
- O oxygen
- oxygen is an element contained as an impurity, and when the content thereof exceeds 0.0100%, a coarse oxide is formed in the steel to cause bendability and hole expansion. Therefore, the O content is limited to 0.0100% or less. It is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. However, from the viewpoint of manufacturing cost, the lower limit is preferably 0.0001%.
- the basic chemical composition of the base steel sheet according to the embodiment of the present invention is as described above. Further, the base steel sheet may contain the following elements, if necessary.
- V 0% to 1.00%, Nb: 0% to 0.100%, Ti: 0% to 0.200%, B: 0% to 0.0100%, Cr: 0% to 2.00% , Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00% , Sn: 0% to 1.00% and Sb: 0% to 1.00%]
- V vanadium), Nb (niobium), Ti (titanium), B (boron), Cr (chrome), Ni (nickel), Cu (copper), Co (cobalt), Mo (molybdenum), W (tungsten), Both Sn (tin) and Sb (antimony) are effective elements for increasing the strength of the steel sheet.
- the contents are V:0% to 1.00%, Nb:0% to 0.100%, Ti:0% to 0.200%, B:0% to 0.0100%, Cr:0%. ⁇ 2.00%, Ni:0% ⁇ 1.00%, Cu:0% ⁇ 1.00%, Co:0% ⁇ 1.00%, Mo:0% ⁇ 1.00%, W:0% To 1.00%, Sn: 0% to 1.00%, and Sb: 0% to 1.00%.
- Each element may be 0.005% or more or 0.010% or more.
- the B content may be 0.0001% or more or 0.0005% or more.
- Ca 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100% , Hf: 0% to 0.0100%, Bi: 0% to 0.0100% and REM other than Ce and La: 0% to 0.0100%
- Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium) and REM (rare earth elements) other than Ce and La are used for fine dispersion of inclusions in steel.
- Bi bismuth
- Mn and Si substitutional alloying elements
- the balance other than the above elements is Fe and impurities.
- Impurities are components that are mixed by various factors of the manufacturing process, including raw materials such as ores and scraps when industrially manufacturing the base steel sheet, and the mother according to the embodiment of the present invention. It includes those that are not intentionally added to the steel sheet.
- the impurities are elements other than the components described above, and are contained in the base material steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the hot-dip galvanized steel sheet according to the embodiment of the present invention. It also includes elements.
- Ferrite has excellent ductility but has a soft structure. In order to improve the elongation of the steel sheet, it may be contained depending on the required strength or ductility. However, if it is contained excessively, it becomes difficult to secure a desired steel plate strength. Therefore, its content may be 45% or less, 40% or less, or 35% or less, with the upper limit being 50% in area %.
- the ferrite content may be 0% in area %, for example, 3% or more, 5% or more, or 10% or more.
- Tempered martensite has a high-strength and tough structure and is an essential metal structure in the present invention. In order to balance strength, ductility and hole expandability at a high level, at least 5% or more in area% is contained. The area% is preferably 10% or more, and may be 15% or more or 20% or more. For example, the tempered martensite content may be 95% or less, 90% or less, 85% or less, 80% or less or 70% or less in area %.
- fresh martensite means martensite that has not been tempered, that is, martensite that does not contain carbide. Since this fresh martensite has a brittle structure, it becomes a starting point of fracture during plastic deformation and deteriorates the local ductility of the steel sheet. Therefore, the content is 0 to 10% in area %. It is more preferably 0 to 8% or 0 to 5%. The fresh martensite content may be 1% or more or 2% or more in area %.
- Retained austenite improves the ductility of the steel sheet by the TRIP effect of transforming to martensite by the work-induced transformation during the deformation of the steel sheet.
- the upper limit of the retained austenite is 30% in area %, and may be 25% or less or 20% or less.
- the content thereof is preferably 6% or more in area %, and may be 8% or more or 10% or more.
- the Si content in the base steel sheet is preferably 0.50% or more by mass %.
- total of pearlite and cementite 0-5%
- pearlite contains hard and coarse cementite and serves as a starting point of fracture during plastic deformation, it deteriorates the local ductility of the steel sheet. Therefore, the content thereof together with cementite is 0 to 5% in area %, and may be 0 to 3% or 0 to 2%.
- the remaining structure other than the above structure may be 0%, but if it exists, it is bainite.
- the bainite having the remaining structure may be any of upper bainite, lower bainite, or a mixed structure thereof.
- the base material steel sheet according to the present embodiment has a soft layer on its surface.
- the soft layer is a region in the base material steel sheet having a hardness of 90% or less with respect to the hardness at a position of 1 ⁇ 4 thickness from the interface between the base material steel sheet and the hot dip galvanized layer to the base material steel sheet side. Is to say.
- the thickness of the soft layer is 10 ⁇ m or more. When the thickness of the soft layer is less than 10 ⁇ m, bendability deteriorates.
- the thickness of the soft layer may be, for example, 15 ⁇ m or more, 18 ⁇ m or more, 20 ⁇ m or more or 30 ⁇ m or more, and/or 120 ⁇ m or less, 100 ⁇ m or less or 80 ⁇ m or less.
- the hardness (Vickers hardness) at a position of 1/4 thickness from the interface between the base steel plate and the hot-dip galvanized layer to the base steel plate side is generally 200 to 600 HV, for example 250 HV or higher or 300 HV or higher. And/or 550 HV or less or 500 HV or less.
- the Vickers hardness (HV) is usually about 1/3.2 of the tensile strength (MPa).
- the soft layer includes tempered martensite, the plate of the area% of tempered martensite from the interface between the base material steel sheet and the hot-dip galvanized layer to the inside of the base material steel sheet.
- the increase rate in the thickness direction is 5.0%/ ⁇ m or less. If it exceeds 5.0%/ ⁇ m, the decrease in load during bending deformation becomes apparent.
- the rate of increase in the plate thickness direction is 4.5%/ ⁇ m or less, 4.0%/ ⁇ m or less, 3.0%/ ⁇ m or less, 2.0%/ ⁇ m or less, or 1.0%/ ⁇ m or less. May be
- the lower limit of the rate of increase in the plate thickness direction is not particularly limited, but may be 0.1%/ ⁇ m or 0.2%/ ⁇ m, for example.
- the steel structure fraction of the hot dip galvanized steel sheet is evaluated by the SEM-EBSD method (electron beam backscattering diffraction method) and SEM secondary electron image observation.
- a sample is taken with the plate thickness cross section parallel to the rolling direction of the steel plate as the observation surface at the plate thickness cross section at the center position in the width direction, and the observation surface is mechanically polished to a mirror surface and then electrolytically polished. .. Then, in one or a plurality of observation fields in the range of 1/8th to 3/8ths centering on the 1/4th thickness from the surface of the base material steel plate on the observation side, a total of 2.0 ⁇ 10 ⁇ 9 m
- the crystal structure and orientation of two or more areas are analyzed by the SEM-EBSD method.
- "OIM Analysis 6.0" manufactured by TSL is used for the analysis of the data obtained by the EBSD method.
- the distance between the scores (step) is 0.03 to 0.20 ⁇ m.
- the area judged to be FCC iron from the observation results is defined as retained austenite. Further, a crystal grain boundary map is obtained with the boundaries where the crystal orientation difference is 15 degrees or more as grain boundaries.
- tempered martensite is a region that has a substructure in the grain and cementite precipitates with multiple variants, more specifically with two or more variants (for example, refer to FIG. 1). See figure).
- the area where cementite is lamellarly precipitated is judged to be pearlite (or the sum of pearlite and cementite).
- a region having low brightness and no substructure is judged to be ferrite (for example, see the reference diagram of FIG. 1 ).
- a region where the brightness is high and the underlying structure is not exposed by etching is determined as fresh martensite and retained austenite (for example, see the reference diagram of FIG. 1 ).
- An area that does not correspond to any of the above areas is determined to be bainite.
- the area ratio of each tissue is calculated by the point counting method.
- the area ratio of fresh martensite can be calculated by subtracting the area ratio of retained austenite obtained by the X-ray diffraction method.
- the area ratio of retained austenite is measured by the X-ray diffraction method. Measure the area ratio of FCC iron by the X-ray diffraction method by finishing the surface parallel to the plate surface to a mirror surface in the range of 1/8th to 3/8thth centering on the 1/4th thickness from the surface of the base steel sheet Then, the area ratio of retained austenite is defined as that.
- the increase rate of the area% of tempered martensite in the plate thickness direction is determined by the following method.
- a structure photograph is taken of a region including a soft layer.
- the area fraction of tempered martensite was calculated by the point counting method for each region of 10 ⁇ m in thickness and 100 ⁇ m in width at intervals of 10 ⁇ m from the interface between the base material steel plate and the hot-dip galvanized layer to calculate the area fraction of tempered martensite every 10 ⁇ m.
- the area percentages of the tempered martensite in the plate thickness direction are determined based on the values of the maximum slopes in the soft layer obtained by plotting the respective area fractions obtained in the above. For example, the slope between two points obtained by plotting the area fraction obtained in one region in the soft layer and the area fraction obtained in the region including the outside of the soft layer adjacent to the region is the maximum slope. In such a case, the inclination is determined as "a rate of increase in area% of tempered martensite in the thickness direction from the interface in the soft layer into the base steel sheet".
- the hardness from the surface of the steel plate to the inside of the steel plate is measured by the following method.
- a cross-section parallel to the rolling direction of the steel sheet, the cross-section at the center position in the width direction is taken as the observation surface, a sample is taken, the observation surface is polished to a mirror finish, and colloidal silica is used to remove the surface processing layer. Is used for chemical polishing.
- a microhardness measuring device with a position of 5 ⁇ m depth from the outermost layer as a starting point, from the surface to a position 1/4 of the plate thickness, a pitch of 10 ⁇ m in the thickness direction of the steel plate.
- a square pyramidal Vickers indenter with an apex angle of 136° is pushed in with a load of 2 g.
- the Vickers indentations may interfere with each other depending on the size of the Vickers indentations.
- the Vickers indenters are pushed in a zigzag manner to avoid mutual interference.
- the Vickers hardness is measured at 5 points at each thickness position, and the average value is taken as the hardness at that thickness position.
- the hardness profile in the depth direction is obtained by interpolating each data with a straight line.
- the thickness of the soft layer is obtained by reading the depth position where the hardness is 90% or less of the hardness at the position of 1 ⁇ 4 thickness from the hardness profile.
- the base material steel sheet according to the embodiment of the present invention has a hot-dip galvanized layer on at least one surface, preferably both surfaces.
- the plating layer may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer having any composition known to those skilled in the art, and may contain an additive element such as Al in addition to Zn.
- the amount of the plated layer attached is not particularly limited and may be a general amount.
- the hot-dip galvanized steel sheet manufacturing method includes a hot rolling step of hot rolling a slab having the same chemical composition as described above for the base material steel sheet, and a cold rolling step of cold rolling the obtained hot rolled steel sheet. Including a step, and a hot dip galvanizing step of applying hot dip galvanizing to the obtained cold rolled steel sheet, (A) Conditions of the following cold rolling steps (A1) and (A2): (A1) Performing cold rolling once or more once with a rolling line load satisfying the following formula (1) and a rolling reduction of 6% or more: 13 ⁇ A/B ⁇ 35 (1) (In the formula, A is the rolling line load (kgf/mm), and B is the tensile strength (kgf/mm 2 ) of the hot rolled steel sheet.) (A2) Satisfying the total cold reduction ratio of 30 to 80%, (B) In the hot dip galvanizing step, a steel sheet is heated to be subjected to a first soaking treatment, a first soaked steel sheet is first cooled and then a second
- the hot rolling step is not particularly limited and can be carried out under any appropriate conditions. Accordingly, the following description of the hot rolling process is intended to be exemplary only, and is intended to limit the hot rolling process in the present method to those performed under specific conditions as described below. Not something to do.
- a slab having the same chemical composition as described above for the base steel sheet is heated before hot rolling.
- the heating temperature of the slab is not particularly limited, it is generally preferably 1150° C. or higher because it sufficiently dissolves borides and carbides.
- the steel slab to be used is preferably cast by the continuous casting method from the viewpoint of manufacturability, but may be manufactured by the ingot casting method or the thin slab casting method.
- the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness.
- rough rolling is not particularly limited, but it is preferable to perform it so that the total rolling reduction at 1050° C. or higher is 60% or higher.
- the total rolling reduction may be 90% or less, for example.
- the finish rolling is preferably carried out in a range satisfying the conditions of a finish rolling inlet temperature of 900 to 1050° C., a finish rolling outlet temperature of 850° C. to 1000° C., and a total rolling reduction of 70 to 95%.
- a finish rolling inlet temperature 900 to 1050° C.
- a finish rolling outlet temperature 850° C. to 1000° C.
- a total rolling reduction of 70 to 95% When the finish rolling inlet side temperature is lower than 900° C., the finish rolling outlet side temperature is lower than 850° C., or the total reduction ratio is higher than 95%, the texture of the hot rolled steel sheet develops, so that in the final product sheet. Anisotropy may become apparent.
- the finish rolling inlet temperature is higher than 1050°C
- the finish rolling outlet temperature is higher than 1000°C, or the total rolling reduction is lower than 70%
- the crystal grain size of the hot rolled steel sheet becomes coarse, and the final product is produced. This may lead to coarsening of the plate structure and deterioration of workability.
- the finish rolling entrance temperature may be 950° C. or higher.
- the finish rolling outlet temperature may be 900° C. or higher.
- the total rolling reduction may be 75% or more, or 80% or more.
- Winding temperature 450-680°C
- the winding temperature is 450 to 680°C. If the coiling temperature is lower than 450°C, the strength of the hot-rolled sheet becomes excessive, which may impair the cold rolling property. On the other hand, when the coiling temperature is higher than 680° C., the cementite becomes coarse, and unmelted cementite remains, which may impair the workability.
- the winding temperature may be 500°C or higher and/or 650°C or lower.
- the obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to a treatment such as pickling if necessary.
- the pickling method of the hot rolled coil may be a conventional method.
- skin pass rolling may be performed to correct the shape of the hot rolled coil and improve pickling performance.
- (A) Cold rolling process [Cold rolling that satisfies the formula (1) and the rolling reduction is 6% or more is performed once or more]
- the obtained hot-rolled steel sheet is subjected to a cold rolling step, and in the cold rolling step, the rolling line load satisfies the following formula (1) and the rolling reduction is 6% or more. This includes performing hot rolling once or more. 13 ⁇ A/B ⁇ 35 (1)
- A is the rolling line load (kgf/mm)
- B is the tensile strength (kgf/mm 2 ) of the hot rolled steel sheet.
- the cold rolling may be either a tandem system in which a plurality of rolling stands are connected in series, or a reverse mill system in which one rolling stand is reciprocated.
- the rolling line load is not only the strength of the steel plate before cold rolling but also the roughness of the steel plate before cold rolling, the diameter of the work roll, the surface roughness of the work roll, the rotation speed of the work roll, the tension, the supply amount/temperature of the emulsion. It varies depending on various factors such as viscosity. However, the higher rolling line load means that the frictional force generated at the interface between the steel plate and the work roll becomes larger.
- the larger the frictional force, the larger the shear strain applied to the surface layer of the steel sheet, and the recrystallization in the surface layer portion of the steel sheet is promoted during heating in the subsequent hot dip galvanizing process, and the microstructure of the steel sheet surface layer becomes finer.
- the refinement of the structure means that the area of the crystal grain boundary serving as the diffusion path of carbon becomes large. As a result, re-diffusion of carbon atoms from the inside of the steel sheet to the surface layer is promoted during the second soaking treatment. In order to obtain this effect, it is necessary to control the rolling line load so that A/B is 13 or more and the rolling reduction is 6% or more.
- the upper limit of A/B is set to 35.
- A/B may be 20 or more and/or 30 or less.
- the reduction rate may be 10% or more, and/or 25% or less.
- the rolling wire load changes depending on the capacity of the cold rolling mill, and the tensile strength of the hot rolled steel sheet also changes depending on the chemical composition and the steel structure. This is because it is not easy to control the tensile strength of the rolled steel sheet within a desired range.
- JIS No. 5 tensile test pieces were taken from the vicinity of the width center of the hot-rolled steel sheet with the plate width direction as the test piece longitudinal direction, and the tensile test was performed according to JIS Z2241: 2011. To do.
- the rolling line load is normally measured as an operation control index, but a measuring instrument such as a load cell equipped in the rolling mill may be used.
- Total cold reduction 30-80%
- the total cold reduction is limited to 30 to 80%. If it is less than 30%, the accumulation of strain becomes insufficient, and the above-mentioned surface layer structure refining effect cannot be obtained. On the other hand, if the rolling reduction is excessive, the rolling load becomes excessive and the load of the cold rolling mill increases, so the upper limit is preferably 80%.
- the total cold reduction may be 40% or more, and/or 70% or less or 60% or less.
- (B) Hot dip galvanizing process [Average heating rate from 650° C. to the maximum heating temperature of Ac1+30° C. or more and 950° C. or less under an atmosphere satisfying the expressions (2) and (3): 0.5 to 10.0° C./sec]
- the obtained steel sheet is plated in the hot dip galvanizing step.
- the hot dip galvanizing step first, the steel sheet is heated and exposed to the first soaking treatment in an atmosphere satisfying the following expressions (2) and (3).
- the average heating rate from 650° C. to the maximum heating temperature of Ac1+30° C. or more and 950° C. or less is limited to 0.5 to 10.0° C./sec.
- the heating rate exceeds 10.0° C./sec, recrystallization of ferrite does not proceed sufficiently and the elongation of the steel sheet may deteriorate.
- the average heating rate is less than 0.5°C/sec, austenite is coarsened, and thus the steel structure finally obtained may be coarse.
- This average heating rate may be 1.0° C./sec or more, and/or 8.0° C./sec or less or 5.0° C./sec or less.
- the "average heating rate" is a value obtained by dividing the difference between 650°C and the maximum heating temperature by the elapsed time from 650°C to the maximum heating temperature.
- log(PH 2 O/PH 2 ) in equation (2) is the logarithm of the ratio of the partial pressure of water vapor (PH 2 O) to the partial pressure of hydrogen (PH 2 ) in the atmosphere, and is also called the oxygen potential. ..
- log(PH 2 O/PH 2 ) is less than ⁇ 1.10, a soft layer of 10 ⁇ m or more is not formed in the surface layer of the steel sheet in the final structure.
- log(PH 2 O/PH 2 ) exceeds ⁇ 0.07, the decarburization reaction proceeds excessively and the strength is lowered.
- the wettability with plating may be deteriorated to cause defects such as non-plating. If PH 2 is less than 0.010, an oxide is formed outside the steel sheet, the wettability with the plating deteriorates, and defects such as non-plating may occur.
- the upper limit of PH 2 is 0.150 from the viewpoint of the danger of hydrogen explosion.
- log(PH 2 O/PH 2 ) may be ⁇ 1.00 or higher and/or ⁇ 0.10 or lower.
- PH 2 may be 0.020 or more and/or 0.120 or less. ⁇ 1.10 ⁇ log(PH 2 O/PH 2 ) ⁇ 0.07 (2) 0.010 ⁇ PH 2 ⁇ 0.150 (3)
- First soaking treatment Hold at the maximum heating temperature of Ac1+30°C or higher and 950°C or lower for 1 second to 1000 seconds]
- the steel sheet is heated to at least Ac1+30° C. or higher, and soaking is performed at the temperature (maximum heating temperature).
- the upper limit is 950°C, preferably 900°C. If the soaking time is short, the austenitization does not proceed sufficiently, so the time is at least 1 second or longer. It is preferably 30 seconds or longer or 60 seconds or longer.
- the upper limit is made 1000 seconds, preferably 500 seconds. It is not always necessary to keep the steel plate at a constant temperature during soaking, and the steel plate may be changed within a range satisfying the above conditions. “Holding” in the first soaking treatment and the second soaking treatment and the third soaking treatment described later means that the temperature is a predetermined temperature ⁇ 20° C., preferably ⁇ 20° C., within a range not exceeding the upper and lower limits specified in each soaking treatment. It means maintaining within the range of 10°C. Therefore, for example, a heating or cooling operation which fluctuates by more than 40° C., preferably more than 20° C. within the temperature range specified in each soaking treatment by, for example, gradually heating or gradually cooling is carried out by the embodiment of the present invention. It is not included in the first, second and third soaking treatments.
- the cooling stop temperature is 300° C. to 600° C. which is the temperature of the subsequent second soaking treatment.
- the average cooling rate in the temperature range of 700° C. to 600° C. is 10 to 100° C./sec. If the average cooling rate is lower than 10° C./sec, the desired ferrite fraction may not be obtained.
- the average cooling rate may be 15°C/sec or higher, or 20°C/sec or higher. Further, the average cooling rate may be 80° C./sec or less or 60° C./sec or less.
- the "average cooling rate” is a value obtained by dividing 100°C, which is the difference between 700°C and 600, by the elapsed time from 700°C to 600°C.
- PH 2 is less than 0.0010, an oxide may be formed on the outside of the steel sheet, the wettability with the plating may deteriorate, and defects such as non-plating may occur.
- the upper limit of PH 2 is 0.1500 from the viewpoint of the danger of hydrogen explosion.
- log(PH 2 O/PH 2 ) may be ⁇ 1.00 or less. Further, PH 2 may be 0.0050 or more and/or 0.1000 or less. log(PH 2 O/PH 2 ) ⁇ -1.10 (4) 0.0010 ⁇ PH 2 ⁇ 0.1500 (5)
- the steel sheet is dipped in hot dip galvanizing.
- the steel plate temperature at this time has a small effect on the steel plate performance, but if the difference between the steel plate temperature and the plating bath temperature is too large, the plating bath temperature may change and operation may be hindered. It is desirable to provide a step of cooling the steel sheet within the range of 20°C to the plating bath temperature +20°C.
- Hot-dip galvanizing may be performed according to a conventional method.
- the plating bath temperature may be 440 to 460° C.
- the immersion time may be 5 seconds or less.
- the plating bath is preferably a plating bath containing 0.08 to 0.2% of Al, but may further contain Fe, Si, Mg, Mn, Cr, Ti and Pb as impurities. Further, it is preferable to control the basis weight of plating by a known method such as gas wiping. The basis weight is preferably 25 to 75 g/m 2 per side.
- the hot dip galvanized steel sheet on which the hot dip galvanized layer is formed may be subjected to an alloying treatment, if necessary.
- the alloying treatment temperature is lower than 460° C., not only the alloying speed becomes slower and productivity is impaired but also uneven alloying treatment occurs, so the alloying treatment temperature is set to 460° C. or higher.
- the alloying treatment temperature exceeds 600° C., alloying may proceed excessively and the plating adhesion of the steel sheet may deteriorate.
- the pearlite transformation may proceed and the desired metallographic structure may not be obtained. Therefore, the alloying treatment temperature is 600° C. or lower.
- the martensitic transformation in the present invention occurs after the ferrite transformation and the bainite transformation. C is distributed to austenite along with the ferrite transformation and the bainite transformation. Therefore, Ms at the time of heating to the austenite single phase and quenching does not match. Ms in the present invention is obtained by measuring the thermal expansion temperature in the second cooling.
- Ms in the present invention is a molten zinc from the start of hot dip galvanizing heat treatment (corresponding to room temperature) to the second cooling using an apparatus capable of measuring the amount of thermal expansion during continuous heat treatment such as a Formaster tester. It can be determined by reproducing the heat cycle of the plating line and measuring the thermal expansion temperature in the second cooling.
- FIG. 2 is a temperature-thermal expansion curve when a heat cycle corresponding to the hot dip galvanizing process according to the embodiment of the present invention is simulated by a thermal expansion measuring device.
- the steel sheet linearly shrinks in the second cooling step, but deviates from the linear relationship at a certain temperature.
- the temperature at this time is Ms in the present invention.
- the temperature of the third soaking treatment may be 240° C. or higher, or 400° C. or lower.
- the holding time may be 15 seconds or longer, 100 seconds or longer, and 400 seconds or shorter.
- the third soaking treatment After the third soaking treatment, cool to room temperature to obtain the final product. Temper rolling may be performed in order to flatten the steel sheet and adjust the surface roughness. In this case, the elongation rate is preferably 2% or less in order to avoid deterioration of ductility.
- the conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention.
- the present invention is not limited to this one condition example.
- the present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
- a steel having the chemical composition shown in Table 1 was cast to make a slab.
- the balance other than the components shown in Table 1 is Fe and impurities.
- These slabs were hot rolled under the conditions shown in Table 2 to produce hot rolled steel sheets. Then, the hot rolled steel sheet was pickled to remove the scale on the surface. Then, it cold-rolled. The plate thickness after cold rolling was 1.4 mm. Further, the obtained steel sheet was subjected to a continuous hot dip galvanizing treatment under the conditions shown in Table 2 and an appropriate alloying treatment. In each soaking treatment shown in Table 2, the temperature was maintained within the temperature range of ⁇ 10°C shown in Table 2.
- the component composition of the base steel sheet obtained by analyzing the sample collected from the manufactured hot-dip galvanized steel sheet was the same as that of the steel shown in Table 1.
- JIS No. 5 tensile test pieces are taken from the steel plate thus obtained in a direction perpendicular to the rolling direction, and a tensile test is performed in accordance with JIS Z2241:2011 to measure tensile strength (TS) and total elongation (El). did. Further, the “JFS T 1001 hole expansion test method” of the Japan Iron and Steel Federation standard was performed to measure the hole expansion ratio ( ⁇ ). Those having TS of 980 MPa or more, TS ⁇ El ⁇ 0.5 /1000 of 80 or more and passing the following bending tests were judged to have good mechanical properties and have favorable press formability for use as a member for automobiles. did.
- the maximum bending angle was measured by performing a bending test according to the method specified in the German Automobile Manufacturers Association (VDA) standard 238-100. Bending property of 90° or more for tensile strength less than 1180 MPa, 80° or more for tensile strength of 1180 MPa or more, less than 1470 MPa, and bending angle of 70° or more for those exceeding 1470 MPa. It was judged to be acceptable and passed (“ ⁇ ” in Table 3).
- a hat-shaped member having a closed cross-sectional shape as shown in Fig. 2 was produced and a static three-point bending test was conducted. The maximum load at that time was measured. If the value obtained by dividing the maximum load [kN] by the tensile strength [MPa] was 0.015 or more, the load reduction during bending deformation was sufficiently suppressed, and the result was determined as "pass" ("A" in Table 3).
- Comparative Example 4 the atmosphere in the furnace during the second soaking treatment in the hot dip galvanizing process did not satisfy the formula (4). As a result, the desired surface layer structure was not obtained, and the maximum load in the three-point bending test was inferior.
- Comparative Example 5 the atmosphere during heating in the hot dip galvanizing process did not satisfy the formula (2). As a result, the soft layer was not formed and the bendability was poor.
- the stop temperature of the second cooling in the hot dip galvanizing process was more than Ms-50°C. As a result, tempered martensite was not obtained and the tensile strength was less than 980 MPa. Also, the maximum load during the three-point bending test was inferior.
- Comparative Example 8 the temperature of the third soaking treatment in the hot dip galvanizing process was less than 200°C. As a result, the desired metallographic structure could not be obtained, and the press formability was inferior.
- A/B (rolling line load/tensile strength) in the cold rolling step was less than 13.
- the rolling reduction in the cold rolling step was less than 6%.
- the increase rate of the area% of the tempered martensite in the surface layer structure in the plate thickness direction was more than 5.0%/ ⁇ m, and the maximum load in the three-point bending test was inferior.
- Comparative Example 14 the temperature of the first soaking treatment in the hot dip galvanizing process was less than Ac1°C+30°C, and the stop temperature of the second cooling was more than Ms-50°C. As a result, the desired metallographic structure was not obtained, and the press formability and the maximum load in the three-point bending test were inferior.
- Comparative Example 15 the average cooling rate of the first cooling was less than 10°C/sec. As a result, ferrite was more than 50%, and the total amount of pearlite and cementite was more than 5%, and the press formability was inferior.
- the holding time of the second soaking treatment was more than 500 seconds, and the stop temperature of the second cooling was more than Ms-50°C. As a result, the desired metallographic structure could not be obtained, and the press formability was inferior.
- the temperature of the second soaking treatment was higher than 600°C. As a result, ferrite was more than 50%, and the total amount of pearlite and cementite was more than 5%, and the press formability was inferior.
- the temperature of the second soaking treatment in the hot dip galvanizing step was less than 300°C. As a result, the desired surface layer structure was not obtained, and the maximum load in the three-point bending test was inferior.
- the stop temperature of the second cooling in the hot dip galvanizing process was more than Ms-50°C. As a result, the desired metallographic structure was not obtained, and the press formability and the maximum load in the three-point bending test were inferior.
- the holding time of the second soaking treatment was less than 80 seconds. As a result, the increase rate of the area% of the tempered martensite in the surface layer structure in the plate thickness direction was more than 5.0%/ ⁇ m, and the maximum load in the three-point bending test was inferior.
- the holding time of the third soaking treatment in the hot dip galvanizing process was less than 5 seconds. As a result, the fresh martensite exceeded 10% and the press formability was inferior.
- Comparative Example 33 the atmosphere during heating in the hot dip galvanizing process did not satisfy the formula (2).
- Comparative Example 34 the hydrogen partial pressure during heating did not satisfy the formula (3).
- Comparative Example 35 the hydrogen partial pressure at the time of the second soaking treatment did not satisfy the formula (5).
- non-plating occurred in these comparative examples.
- Comparative Examples 57 to 62 since the chemical composition was not controlled within the predetermined range, the desired metallographic structure could not be obtained, and the press formability was poor. Further, in Comparative Examples 59 to 61, the toughness of the steel sheet was insufficient because the contents of C, Si and Mn were excessive, and the specimens brittlely fractured during the three-point bending test.
- the hot-dip galvanized steel sheets of the examples had a tensile strength of 980 MPa or more and a TS ⁇ El ⁇ 0.5 /1000 of 80 or more, and the results of the three-point bending test were good. From this, it can be seen that the press formability is excellent and the load reduction during bending deformation is suppressed. Further, with respect to the hot-dip galvanized steel sheets of Examples 10, 24, 31 and 39, the hardness at a position of 1 ⁇ 4 thickness from the interface between the base material steel sheet and the hot-dip galvanized layer to the base material steel sheet side was examined. 394 HV, 390 HV and 487 HV.
Abstract
Description
(i)連続溶融亜鉛めっき熱処理工程において、めっき処理またはめっき合金化処理の後に、Ms以下まで冷却することでマルテンサイトを生成させる。さらにその後、再加熱および等温保持を施すことでマルテンサイトを適度に焼き戻すとともに、残留オーステナイトを含む鋼板の場合には、さらに当該残留オーステナイトを安定化させることもできる。このような熱処理により、マルテンサイトがめっき処理またはめっき合金化処理により過剰に焼き戻されなくなるため、強度と延性のバランスが改善する。
(ii)高強度鋼板の曲げ性を改善するには、脱炭処理を施し表層部を軟質化することが有効であることはよく知られている。しかしながら、表層部を軟質化すると、場合により、曲げ変形荷重が、その鋼板強度から期待される変形荷重よりも低下してしまうことがあった。この課題を解決するため、本発明者らは、硬質組織であるマルテンサイトの面積率の鋼板表面から鋼板内部にかけての板厚方向変化率(増加率)を、所定の値以下に制限すれば、上記課題を克服できることを見出した。また、このような金属組織制御を実現するには、連続溶融亜鉛めっき熱処理工程において、まず、鋼板を650℃以上の高温域に加熱し、かつ、炉内の雰囲気を高酸素ポテンシャルとして表層に脱炭領域を形成させる。その後、600℃以下の低温域に鋼板を冷却し、かつ、炉内の雰囲気を低酸素ポテンシャルとして一定時間以上の等温保持を行う。この等温保持により鋼板内部の炭素原子が表層の脱炭領域に適度に拡散する。その結果、最終的に形成されるマルテンサイトの面積率の板厚方向変化率が、前記等温保持を行わない場合と比較して緩やかになることを見出した。但し、この等温保持工程は(i)で説明したMs以下まで冷却する工程の前に実施する必要がある。オーステナイトがマルテンサイトへ変態してしまうと、固溶炭素は炭化物としてマルテンサイト中に析出するため、鋼板内部から鋼板表層への炭素原子の再拡散が起こらないためである。
(iii)さらに、上記(ii)の効果は、連続溶融亜鉛めっき熱処理工程の前の冷間圧延条件が、所定の範囲内の場合により顕在化することを見出した。その詳細は明らかでないが、冷間圧延条件を所定の範囲に制限することにより、鋼板表層に付与されるせん断ひずみが大きくなると考えられる。このような表層ひずみを有する鋼板を連続溶融亜鉛めっき熱処理工程にて焼鈍すると、鋼板表層組織が微細化する。すなわち、鋼板表層部で結晶粒界の面積が増大する。結晶粒界は炭素原子の拡散パスとして作用するため、結晶粒界の面積が増大する結果、600℃以下での等温保持時に炭素原子が表層に再拡散しやすくなると考えられる。
(1)母材鋼板の少なくとも一方の表面に溶融亜鉛めっき層を有する溶融亜鉛めっき鋼板であって、前記母材鋼板が、質量%で、
C:0.050%~0.350%、
Si:0.10%~2.50%、
Mn:1.00%~3.50%、
P:0.050%以下、
S:0.0100%以下、
Al:0.001%~1.500%、
N:0.0100%以下、
O:0.0100%以下、
Ti:0%~0.200%、
B:0%~0.0100%、
V:0%~1.00%、
Nb:0%~0.100%、
Cr:0%~2.00%、
Ni:0%~1.00%、
Cu:0%~1.00%、
Co:0%~1.00%、
Mo:0%~1.00%、
W:0%~1.00%、
Sn:0%~1.00%、
Sb:0%~1.00%、
Ca:0%~0.0100%、
Mg:0%~0.0100%、
Ce:0%~0.0100%、
Zr:0%~0.0100%、
La:0%~0.0100%、
Hf:0%~0.0100%、
Bi:0%~0.0100%、および
Ce、La以外のREM:0%~0.0100%
を含有し、残部がFeおよび不純物からなる化学組成を有し、
前記母材鋼板の表面から1/4厚の位置を中心とした1/8厚~3/8厚の範囲における鋼組織が、面積%で、
フェライト:0%~50%、
残留オーステナイト:0%~30%、
焼き戻しマルテンサイト:5%以上、
フレッシュマルテンサイト:0%~10%、および
パーライトとセメンタイトの合計:0%~5%
を含有し、残部組織が存在する場合には、前記残部組織がベイナイトからなり、
前記母材鋼板と前記溶融亜鉛めっき層の界面から前記母材鋼板側に1/4厚の位置における硬度に対して90%以下の硬度を有する領域を軟質層としたときに、前記界面から前記母材鋼板側に厚さ10μm以上の軟質層が存在し、
前記軟質層が焼き戻しマルテンサイトを含み、かつ、
前記軟質層内における前記界面から前記母材鋼板内部への焼き戻しマルテンサイトの面積%の板厚方向増加率が5.0%/μm以下であることを特徴とする、溶融亜鉛めっき鋼板。
(2)前記鋼組織が、さらに、面積%で、残留オーステナイト:6%~30%を含有することを特徴とする、上記(1)に記載の溶融亜鉛めっき鋼板。
(3)上記(1)に記載の化学組成を有するスラブを熱間圧延する熱間圧延工程、得られた熱延鋼板を冷間圧延する冷間圧延工程、および得られた冷延鋼板に溶融亜鉛めっきを施す溶融亜鉛めっき工程を含む溶融亜鉛めっき鋼板の製造方法であって、
(A)前記冷間圧延工程が以下の(A1)および(A2)の条件:
(A1)圧延線荷重が下記式(1)を満足し、かつ、圧下率が6%以上である冷間圧延を1回以上施すこと、
13≦A/B≦35 ・・・(1)
(式中、Aは圧延線荷重(kgf/mm)であり、Bは熱延鋼板の引張強度(kgf/mm2)である。)
(A2)総冷間圧下率が30~80%であること
を満足し、
(B)前記溶融亜鉛めっき工程が、鋼板を加熱して第一均熱処理すること、第一均熱処理された鋼板を第一冷却し次いで第二均熱処理すること、第二均熱処理された鋼板を溶融亜鉛めっき浴に浸漬すること、めっきを施された鋼板を第二冷却すること、および第二冷却された鋼板を加熱し次いで第三均熱処理することを含み、さらに以下の(B1)~(B6)の条件:
(B1)第一均熱処理前の鋼板加熱時において、下記式(2)および(3)を満足する雰囲気下で、650℃~Ac1℃+30℃以上950℃以下の最高加熱温度までの平均加熱速度が0.5℃/秒~10.0℃/秒であること、
(B2)前記鋼板を前記最高加熱温度で1秒~1000秒間保持すること(第一均熱処理)、
(B3)第一冷却における700~600℃までの温度範囲の平均冷却速度が10~100℃/秒であること、
(B4)下記式(4)および(5)を満足する雰囲気下において、第一冷却された鋼板を300~600℃の範囲で80秒~500秒間保持すること(第二均熱処理)、
(B5)第二冷却がMs-50℃以下まで行われること、
(B6)第二冷却された鋼板を200~420℃の温度域に加熱し、次いで前記温度域で5~500秒間保持すること(第三均熱処理)
を満足することを特徴とする、上記(1)または(2)に記載の溶融亜鉛めっき鋼板の製造方法。
-1.10≦log(PH2O/PH2)≦-0.07 ・・・(2)
0.010≦PH2≦0.150 ・・・(3)
log(PH2O/PH2)<-1.10 ・・・(4)
0.0010≦PH2≦0.1500 ・・・(5)
(式中、PH2Oは水蒸気の分圧を示し、PH2は水素の分圧を示す。)
本発明の実施形態に係る溶融亜鉛めっき鋼板は、母材鋼板の少なくとも一方の表面に溶融亜鉛めっき層を有し、前記母材鋼板が、質量%で、
C:0.050%~0.350%、
Si:0.10%~2.50%、
Mn:1.00%~3.50%、
P:0.050%以下、
S:0.0100%以下、
Al:0.001%~1.500%、
N:0.0100%以下、
O:0.0100%以下、
Ti:0%~0.200%、
B:0%~0.0100%、
V:0%~1.00%、
Nb:0%~0.100%、
Cr:0%~2.00%、
Ni:0%~1.00%、
Cu:0%~1.00%、
Co:0%~1.00%、
Mo:0%~1.00%、
W:0%~1.00%、
Sn:0%~1.00%、
Sb:0%~1.00%、
Ca:0%~0.0100%、
Mg:0%~0.0100%、
Ce:0%~0.0100%、
Zr:0%~0.0100%、
La:0%~0.0100%、
Hf:0%~0.0100%、
Bi:0%~0.0100%、および
Ce、La以外のREM:0%~0.0100%
を含有し、残部がFeおよび不純物からなる化学組成を有し、
前記母材鋼板の表面から1/4厚の位置を中心とした1/8厚~3/8厚の範囲における鋼組織が、面積%で、
フェライト:0%~50%、
残留オーステナイト:0%~30%、
焼き戻しマルテンサイト:5%以上、
フレッシュマルテンサイト:0%~10%、および
パーライトとセメンタイトの合計:0%~5%
を含有し、残部組織が存在する場合には、前記残部組織がベイナイトからなり、
前記母材鋼板と前記溶融亜鉛めっき層の界面から前記母材鋼板側に1/4厚の位置における硬度に対して90%以下の硬度を有する領域を軟質層としたときに、前記界面から前記母材鋼板側に厚さ10μm以上の軟質層が存在し、
前記軟質層が焼き戻しマルテンサイトを含み、かつ、
前記軟質層内における前記界面から前記母材鋼板内部への焼き戻しマルテンサイトの面積%の板厚方向増加率が5.0%/μm以下であることを特徴としている。
まず、本発明の実施形態に係る母材鋼板(以下、単に鋼板とも称する)の化学組成を上述のように限定した理由について説明する。なお、本明細書において化学組成を規定する「%」は特に断りのない限り全て「質量%」である。また、本明細書において、数値範囲を示す「~」とは、特に断りがない場合、その前後に記載される数値を下限値および上限値として含む意味で使用される。
Cは、鋼板強度確保のために必須の元素である。0.050%未満では所要の高強度が得られないので、C含有量は0.050%以上とする。C含有量は0.070%以上、0.080%以上または0.100%以上であってもよい。一方、0.350%を超えると、加工性や溶接性が低下するので、C含有量は0.350%以下とする。C含有量は0.340%以下、0.320%以下または0.300%以下であってもよい。
Siは、鉄炭化物の生成を抑制し、強度と成形性の向上に寄与する元素であるが、過度の添加は鋼板の溶接性を劣化させる。従って、その含有量は0.10~2.50%とする。Si含有量は0.20%以上、0.30%以上、0.40%以上もしくは0.50%以上であってもよく、および/または2.20%以下、2.00%以下もしくは1.90%以下であってもよい。
Mn(マンガン)は強力なオーステナイト安定化元素であり、鋼板の高強度化に有効な元素である。過度の添加は溶接性や低温靭性を劣化させる。従って、その含有量は1.00~3.50%とする。Mn含有量は1.10%以上、1.30%以上もしくは1.50%以上であってもよく、および/または3.30%以下、3.10%以下もしくは3.00%以下であってもよい。
P(リン)は固溶強化元素であり、鋼板の高強度化に有効な元素であるが、過度の添加は溶接性および靱性を劣化させる。従って、P含有量は0.050%以下と制限する。好ましくは0.045%以下、0.035%以下または0.020%以下である。ただし、P含有量を極度に低減させるには、脱Pコストが高くなるため、経済性の観点から下限を0.001%とすることが好ましい。
S(硫黄)は不純物として含有される元素であり、鋼中でMnSを形成して靱性や穴広げ性を劣化させる。したがって、靱性や穴広げ性の劣化が顕著でない範囲として、S含有量を0.0100%以下と制限する。好ましくは0.0050%以下、0.0040%以下または0.0030%以下である。ただし、S含有量を極度に低減させるには、脱硫コストが高くなるため、経済性の観点から下限を0.0001%とすることが好ましい。
Al(アルミニウム)は、鋼の脱酸のため少なくとも0.001%を添加する。しかし、過剰に添加しても効果が飽和し徒にコスト上昇を招くばかりか、鋼の変態温度を上昇させ熱間圧延時の負荷を増大させる。従ってAl量は1.500%を上限とする。好ましくは1.200%以下、1.000%以下または0.800%以下である。
N(窒素)は不純物として含有される元素であり、その含有量が0.0100%を超えると鋼中に粗大な窒化物を形成して曲げ性や穴広げ性を劣化させる。したがって、N含有量は0.0100%以下と制限する。好ましくは0.0080%以下、0.0060%以下または0.0050%以下である。ただし、N含有量を極度に低減させるには、脱Nコストが高くなるため、経済性の観点から下限を0.0001%とすることが好ましい。
O(酸素)は不純物として含有される元素であり、その含有量が0.0100%を超えると鋼中に粗大な酸化物を形成して曲げ性や穴広げさせる。従って、O含有量は0.0100%以下と制限する。好ましくは0.0080%以下、0.0060%以下または0.0050%以下である。ただし、製造コストの観点から、下限を0.0001%とすることが好ましい。
V(バナジウム)、Nb(ニオブ)、Ti(チタン)、B(ボロン)、Cr(クロム)、Ni(ニッケル)、Cu(銅)、Co(コバルト)、Mo(モリブデン)、W(タングステン)、Sn(錫)およびSb(アンチモン)はいずれも鋼板の高強度化に有効な元素である。このため、必要に応じてこれらの元素のうち1種または2種以上を添加してもよい。しかしこれらの元素を過度に添加すると効果が飽和し徒にコストの増大を招く。従って、その含有量はV:0%~1.00%、Nb:0%~0.100%、Ti:0%~0.200%、B:0%~0.0100%、Cr:0%~2.00%、Ni:0%~1.00%、Cu:0%~1.00%、Co:0%~1.00%、Mo:0%~1.00%、W:0%~1.00%、Sn:0%~1.00%およびSb:0%~1.00%とする。各元素は0.005%以上または0.010%以上であってもよい。とりわけ、B含有量は0.0001%以上または0.0005%以上であってもよい。
Ca(カルシウム)、Mg(マグネシウム)、Ce(セリウム)、Zr(ジルコニウム)、La(ランタン)、Hf(ハフニウム)およびCe、La以外のREM(希土類元素)は鋼中介在物の微細分散化に寄与する元素であり、Bi(ビスマス)は鋼中におけるMn、Si等の置換型合金元素のミクロ偏析を軽減する元素である。それぞれ鋼板の加工性向上に寄与することから、必要に応じてこれらの元素のうち1種または2種以上を添加してもよい。ただし過度の添加は延性の劣化を引き起こす。従ってその含有量は0.0100%を上限とする。また、各元素は0.0005%以上または0.0010%以上であってもよい。
次に、本発明の実施形態に係る母材鋼板の内部組織の限定理由について説明する。
フェライトは延性に優れるが軟質な組織である。鋼板の伸びを向上させるために、要求される強度または延性に応じて含有させてもよい。但し、過度に含有すると所望の鋼板強度を確保することが困難となる。従って、その含有量は面積%で50%を上限とし、45%以下、40%以下または35%以下であってもよい。フェライト含有量は面積%で0%であってもよく、例えば、3%以上、5%以上または10%以上であってもよい。
焼戻しマルテンサイトは高強度かつ強靭な組織であり、本発明において必須となる金属組織である。強度、延性、穴広げ性を高い水準でバランスさせるために面積%で少なくとも5%以上を含有させる。好ましくは面積%で10%以上であり、15%以上または20%以上であってもよい。例えば、焼戻しマルテンサイト含有量は面積%で95%以下、90%以下、85%以下、80%以下または70%以下であってもよい。
本発明において、フレッシュマルテンサイトとは、焼き戻されていないマルテンサイトすなわち炭化物を含まないマルテンサイトを言うものである。このフレッシュマルテンサイトは脆い組織であるため、塑性変形時に破壊の起点となり、鋼板の局部延性を劣化させる。従って、その含有量は面積%で0~10%とする。より好ましくは0~8%または0~5%である。フレッシュマルテンサイト含有量は面積%で1%以上または2%以上であってもよい。
残留オーステナイトは、鋼板の変形中に加工誘起変態によりマルテンサイトへと変態するTRIP効果により鋼板の延性を改善する。一方、多量の残留オーステナイトを得るにはC等の合金元素を多量に含有させる必要がある。そのため、残留オーステナイトの上限値は面積%で30%とし、25%以下または20%以下であってもよい。但し、鋼板の延性を向上させたい場合は、その含有量は面積%で6%以上とすることが好ましく、8%以上または10%以上であってもよい。また、残留オーステナイトの含有量を6%以上とする場合には、母材鋼板中のSi含有量は質量%で0.50%以上とすることが好ましい。
パーライトは硬質かつ粗大なセメンタイトを含み、塑性変形時に破壊の起点となるため、鋼板の局部延性を劣化させる。従って、その含有量はセメンタイトと合わせて面積%で0~5%とし、0~3%または0~2%であってもよい。
本実施形態に係る母材鋼板は、その表面に軟質層を有する。本発明において、軟質層とは、母材鋼板と溶融亜鉛めっき層の界面から当該母材鋼板側に1/4厚の位置における硬度に対して90%以下の硬度を有する母材鋼板中の領域を言うものである。軟質層の厚みは10μm以上である。軟質層の厚みが10μmを下回る場合、曲げ性が劣化する。軟質層の厚みは、例えば、15μm以上、18μm以上、20μm以上もしくは30μm以上であってもよく、および/または120μm以下、100μm以下もしくは80μm以下であってもよい。また、母材鋼板と溶融亜鉛めっき層の界面から当該母材鋼板側に1/4厚の位置における硬度(ビッカース硬さ)は、一般的には200~600HVであり、例えば250HV以上もしくは300HV以上であってもよく、および/または550HV以下もしくは500HV以下であってもよい。なお通常ビッカース硬さ(HV)は引張強度(MPa)の1/3.2程度である。
本発明の実施形態に係る溶融亜鉛めっき鋼板では、軟質層は焼き戻しマルテンサイトを含み、母材鋼板と溶融亜鉛めっき層の界面から当該母材鋼板内部への焼き戻しマルテンサイトの面積%の板厚方向増加率は5.0%/μm以下である。5.0%/μmを上回ると、曲げ変形時の荷重低下が顕在化する。例えば、この板厚方向増加率は、4.5%/μm以下、4.0%/μm以下、3.0%/μm以下、2.0%/μm以下、または1.0%/μm以下であってもよい。板厚方向増加率の下限値は、特に限定されないが、例えば0.1%/μmまたは0.2%/μmであってもよい。
本発明の実施形態に係る母材鋼板は、少なくとも一方の表面、好ましくは両方の表面に溶融亜鉛めっき層を有する。当該めっき層は、当業者に公知の任意の組成を有する溶融亜鉛めっき層または合金化溶融亜鉛めっき層であってよく、Zn以外にもAl等の添加元素を含んでいてよい。また、当該めっき層の付着量は、特に制限されず一般的な付着量であってよい。
次に、本発明の実施形態に係る溶融亜鉛めっき鋼板の製造方法について説明する。以下の説明は、本発明の実施形態に係る溶融亜鉛めっき鋼板を製造するための特徴的な方法の例示を意図するものであって、当該溶融亜鉛めっき鋼板を以下に説明するような製造方法によって製造されるものに限定することを意図するものではない。
(A)前記冷間圧延工程が以下の(A1)および(A2)の条件:
(A1)圧延線荷重が下記式(1)を満足し、かつ、圧下率が6%以上である冷間圧延を1回以上施すこと、
13≦A/B≦35 ・・・(1)
(式中、Aは圧延線荷重(kgf/mm)であり、Bは熱延鋼板の引張強度(kgf/mm2)である。)
(A2)総冷間圧下率が30~80%であること
を満足し、
(B)前記溶融亜鉛めっき工程が、鋼板を加熱して第一均熱処理すること、第一均熱処理された鋼板を第一冷却し次いで第二均熱処理すること、第二均熱処理された鋼板を溶融亜鉛めっき浴に浸漬すること、めっきを施された鋼板を第二冷却すること、および第二冷却された鋼板を加熱し次いで第三均熱処理することを含み、さらに以下の(B1)~(B6)の条件:
(B1)第一均熱処理前の鋼板加熱時において、下記式(2)および(3)を満足する雰囲気下で、650℃~Ac1℃+30℃以上950℃以下の最高加熱温度までの平均加熱速度が0.5℃/秒~10.0℃/秒であること、
(B2)前記鋼板を前記最高加熱温度で1秒~1000秒間保持すること(第一均熱処理)、
(B3)第一冷却における700~600℃までの温度範囲の平均冷却速度が10~100℃/秒であること、
(B4)下記式(4)および(5)を満足する雰囲気下において、第一冷却された鋼板を300~600℃の範囲で80秒~500秒間保持すること(第二均熱処理)、
(B5)第二冷却がMs-50℃以下まで行われること、
(B6)第二冷却された鋼板を200~420℃の温度域に加熱し、次いで前記温度域で5~500秒間保持すること(第三均熱処理)
を満足することを特徴としている。
-1.10≦log(PH2O/PH2)≦-0.07 ・・・(2)
0.010≦PH2≦0.150 ・・・(3)
log(PH2O/PH2)<-1.10 ・・・(4)
0.0010≦PH2≦0.1500 ・・・(5)
(式中、PH2Oは水蒸気の分圧を示し、PH2は水素の分圧を示す。)
本方法においては、熱間圧延工程は、特に限定されず、任意の適切な条件下で実施することが可能である。したがって、熱間圧延工程に関する以下の説明は、単なる例示を意図するものであって、本方法における熱間圧延工程を以下に説明するような特定の条件下で行われるものに限定することを意図するものではない。
本方法では、例えば、加熱されたスラブに対し、板厚調整等のために、仕上げ圧延の前に粗圧延を施してもよい。このような粗圧延は、特に限定されないが、1050℃以上での総圧下率が60%以上となるように実施することが好ましい。総圧下率が60%未満であると、熱間圧延中の再結晶が不十分となるため、熱延板組織の不均質化につながる場合がある。上記の総圧下率は、例えば、90%以下であってもよい。
仕上げ圧延は、仕上げ圧延入側温度が900~1050℃、仕上げ圧延出側温度が850℃~1000℃、および総圧下率が70~95%の条件を満足する範囲で実施することが好ましい。仕上げ圧延入側温度が900℃を下回るか、仕上げ圧延出側温度が850℃を下回るか、または総圧下率が95%を上回ると、熱延鋼板の集合組織が発達するため、最終製品板における異方性が顕在化する場合がある。一方、仕上げ圧延入側温度が1050℃を上回るか、仕上げ圧延出側温度が1000℃を上回るか、または総圧下率が70%を下回ると、熱延鋼板の結晶粒径が粗大化し、最終製品板組織の粗大化ひいては加工性の劣化に繋がる場合がある。例えば、仕上げ圧延入側温度は950℃以上であってもよい。仕上げ圧延出側温度は900℃以上であってもよい。総圧下率は75%以上または80%以上であってもよい。
巻取温度は450~680℃とする。巻取温度は450℃を下回ると、熱延板強度が過大となり、冷間圧延性を損なう場合がある。一方、巻取温度が680℃を上回ると、セメンタイトが粗大化し、未溶解のセメンタイトが残存するために加工性を損なう場合がある。巻取温度は500℃以上であってよく、および/または650℃以下であってもよい。
[圧延線荷重が式(1)を満足し、かつ、圧下率が6%以上である冷間圧延を1回以上施す]
本方法では、得られた熱延鋼板は冷間圧延工程に供され、当該冷間圧延工程は、圧延線荷重が下記式(1)を満足し、かつ、圧下率が6%以上である冷間圧延を1回以上施すことを含む。
13≦A/B≦35 ・・・(1)
式中、Aは圧延線荷重(kgf/mm)であり、Bは熱延鋼板の引張強度(kgf/mm2)である。
なお、熱延鋼板の引張強度については、熱延鋼板の幅中央近傍から板幅方向を試験片長手方向としてJIS5号引張試験片を採取し、JIS Z2241:2011に準拠して引張試験を行い測定する。圧延線荷重の測定については、通常、操業管理指標として定常的に測定しているものであるが、例えば、圧延機に装備されているロードセル等の測定計を用いればよい。
冷間圧下率はトータルで30~80%の間に制限する。30%を下回るとひずみの蓄積が不十分となり上記表層組織微細化効果が得られない。一方、過度の圧下は圧延加重が過大となり冷延ミルの負荷増大を招くため、その上限は80%とすることが好ましい。例えば、総冷間圧下率は40%以上であってもよく、および/または70%以下もしくは60%以下であってもよい。
[式(2)および(3)を満足する雰囲気下で、650℃~Ac1+30℃以上950℃以下の最高加熱温度までの平均加熱速度:0.5~10.0℃/秒]
本方法においては、冷間圧延工程後、得られた鋼板は、溶融亜鉛めっき工程においてめっき処理を施される。当該溶融亜鉛めっき工程では、まず、下記式(2)および(3)を満足する雰囲気下で、鋼板が加熱され、第一均熱処理にさらされる。この鋼板加熱時において、650℃~Ac1+30℃以上950℃以下の最高加熱温度までの平均加熱速度は0.5~10.0℃/秒に制限される。加熱速度が10.0℃/秒を超えると、フェライトの再結晶が十分進行せず、鋼板の伸びが劣化する場合がある。一方、平均加熱速度が0.5℃/秒を下回ると、オーステナイトが粗大化するため、最終的に得られる鋼組織が粗大なものとなる場合がある。この平均加熱速度は1.0℃/秒以上であってもよく、および/または8.0℃/秒以下もしくは5.0℃/秒以下であってもよい。本発明において、「平均加熱速度」とは、650℃と最高加熱温度との差を650℃から最高加熱温度に至るまでの経過時間で割ることにより得られた値をいうものである。
-1.10≦log(PH2O/PH2)≦-0.07 ・・・(2)
0.010≦PH2≦0.150 ・・・(3)
十分にオーステナイト化を進行させるため、鋼板を少なくともAc1+30℃以上に加熱し、当該温度(最高加熱温度)で均熱処理を行う。但し、過剰に加熱温度を上げると、オーステナイト粒径の粗大化による靭性の劣化を招くばかりか、焼鈍設備の損傷にも繋がる。そのため上限は950℃、好ましくは900℃とする。均熱時間が短いとオーステナイト化が十分進行しないため、少なくとも1秒以上とする。好ましくは30秒以上または60秒以上である。一方、均熱時間が長すぎると生産性を阻害することから上限は1000秒、好ましくは500秒とする。均熱中は鋼板を必ずしも一定温度に保持する必要はなく、上記条件を満足する範囲で変動しても構わない。第一均熱処理ならびに後述する第二均熱処理および第三均熱処理における「保持」とは、各均熱処理において規定される上下限値を超えない範囲で温度を所定の温度±20℃、好ましくは±10℃の範囲内に維持することを意味するものである。したがって、例えば、徐々に加熱しまたは徐々に冷却することで、各均熱処理において規定される温度範囲内を40℃、好ましくは20℃を超えて変動する加熱または冷却操作は、本発明の実施形態に係る第一、第二および第三均熱処理には包含されない。
最高加熱温度で保持した後は第一冷却を行う。冷却停止温度は、続く第二均熱処理の温度である300℃~600℃である。700℃~600℃の温度範囲の平均冷却速度は10~100℃/秒とする。平均冷却速度が10℃/秒を下回ると所望のフェライト分率が得られない場合がある。平均冷却速度は15℃/秒以上または20℃/秒以上であってもよい。また、平均冷却速度は80℃/秒以下または60℃/秒以下であってもよい。本発明において、「平均冷却速度」とは、700℃と600との差である100℃を700℃から600℃に至るまでの経過時間で割ることにより得られた値をいうものである。
300~600℃の範囲で80~500秒間保持する第二均熱処理は、炉内の雰囲気を低酸素ポテンシャルにして、鋼板内部の炭素原子を先の加熱時に形成された脱炭領域に向かって適度に再拡散させるために行う。第二均熱処理の温度が300℃を下回るか、または保持時間が80秒を下回ると、炭素原子の再拡散が不十分となるため所望の表層組織が得られない。一方、第二均熱処理の温度が600℃を上回ると、フェライト変態が進行してしまい所望のフェライト分率が得られない。保持時間が500秒を上回ると、ベイナイト変態が過剰に進行するため、本発明の実施形態に係る金属組織を得ることができない。log(PH2O/PH2)が-1.10を上回ると、脱炭が進行してしまい所望の表層組織が得られない。また、PH2が0.0010を下回ると、鋼板外部に酸化物が形成され、めっきとの濡れ性が劣化し不めっきなどの欠陥を引き起こす場合がある。PH2の上限については、水素爆発の危険性の観点から0.1500とする。例えば、log(PH2O/PH2)は-1.00以下であってもよい。また、PH2は0.0050以上であってもよく、および/または0.1000以下であってもよい。
log(PH2O/PH2)<-1.10 ・・・(4)
0.0010≦PH2≦0.1500 ・・・(5)
例えば、溶融亜鉛めっき層を形成した溶融亜鉛めっき鋼板に対して、必要に応じて合金化処理を行ってもよい。その場合、合金化処理温度が460℃未満であると、合金化速度が遅くなり生産性を損なうばかりでなく、合金化処理むらが発生するので、合金化処理温度は460℃以上とする。一方、合金化処理温度が600℃を超えると、合金化が過度に進行して、鋼板のめっき密着性が劣化する場合がある。また、パーライト変態が進み所望の金属組織を得られない場合がある。したがって、合金化処理温度は600℃以下とする。
めっき処理またはめっき合金化処理後の鋼板にオーステナイトの一部ないしは大部分をマルテンサイトに変態させるため、マルテンサイト変態開始温度(Ms)-50℃以下まで冷却する第二冷却を行う。ここで生成したマルテンサイトは後の再加熱および第三均熱処理により焼戻され、焼戻しマルテンサイトとなる。冷却停止温度がMs-50℃を超えると、焼戻しマルテンサイトが十分形成されないため、所望の金属組織が得られない。鋼板の延性を改善するために残留オーステナイトを活用したい場合には、冷却停止温度に下限を設けることが望ましい。具体的には、冷却停止温度はMs-50℃~Ms-130℃の範囲に制御することが望ましい。
第二冷却の後、200℃~420℃の範囲に再加熱し第三均熱処理を行う。この工程では、第二冷却時に生成したマルテンサイトを焼き戻す。保持温度が200℃未満または保持時間が5秒未満の場合、焼き戻しが十分に進行しない。一方、ベイナイト変態が十分進行しないため、所望の残留オーステナイト量を得ることが困難となる。一方、保持温度が420℃を超えるか、あるいは保持時間が500秒を超えると、マルテンサイトが過剰に焼き戻されるとともに、ベイナイト変態が過剰に進行するために所望の強度および金属組織を得ることが困難となる。第三均熱処理の温度は240℃以上であってもよく、400℃以下であってもよい。また、保持時間は15秒以上または100秒以上であってもよく、400秒以下であってもよい。
Claims (3)
- 母材鋼板の少なくとも一方の表面に溶融亜鉛めっき層を有する溶融亜鉛めっき鋼板であって、前記母材鋼板が、質量%で、
C:0.050%~0.350%、
Si:0.10%~2.50%、
Mn:1.00%~3.50%、
P:0.050%以下、
S:0.0100%以下、
Al:0.001%~1.500%、
N:0.0100%以下、
O:0.0100%以下、
Ti:0%~0.200%、
B:0%~0.0100%、
V:0%~1.00%、
Nb:0%~0.100%、
Cr:0%~2.00%、
Ni:0%~1.00%、
Cu:0%~1.00%、
Co:0%~1.00%、
Mo:0%~1.00%、
W:0%~1.00%、
Sn:0%~1.00%、
Sb:0%~1.00%、
Ca:0%~0.0100%、
Mg:0%~0.0100%、
Ce:0%~0.0100%、
Zr:0%~0.0100%、
La:0%~0.0100%、
Hf:0%~0.0100%、
Bi:0%~0.0100%、および
Ce、La以外のREM:0%~0.0100%
を含有し、残部がFeおよび不純物からなる化学組成を有し、
前記母材鋼板の表面から1/4厚の位置を中心とした1/8厚~3/8厚の範囲における鋼組織が、面積%で、
フェライト:0%~50%、
残留オーステナイト:0%~30%、
焼き戻しマルテンサイト:5%以上、
フレッシュマルテンサイト:0%~10%、および
パーライトとセメンタイトの合計:0%~5%
を含有し、残部組織が存在する場合には、前記残部組織がベイナイトからなり、
前記母材鋼板と前記溶融亜鉛めっき層の界面から前記母材鋼板側に1/4厚の位置における硬度に対して90%以下の硬度を有する領域を軟質層としたときに、前記界面から前記母材鋼板側に厚さ10μm以上の軟質層が存在し、
前記軟質層が焼き戻しマルテンサイトを含み、かつ、
前記軟質層内における前記界面から前記母材鋼板内部への焼き戻しマルテンサイトの面積%の板厚方向増加率が5.0%/μm以下であることを特徴とする、溶融亜鉛めっき鋼板。 - 前記鋼組織が、さらに、面積%で、残留オーステナイト:6%~30%を含有することを特徴とする、請求項1に記載の溶融亜鉛めっき鋼板。
- 請求項1に記載の化学組成を有するスラブを熱間圧延する熱間圧延工程、得られた熱延鋼板を冷間圧延する冷間圧延工程、および得られた冷延鋼板に溶融亜鉛めっきを施す溶融亜鉛めっき工程を含む溶融亜鉛めっき鋼板の製造方法であって、
(A)前記冷間圧延工程が以下の(A1)および(A2)の条件:
(A1)圧延線荷重が下記式(1)を満足し、かつ、圧下率が6%以上である冷間圧延を1回以上施すこと、
13≦A/B≦35 ・・・(1)
(式中、Aは圧延線荷重(kgf/mm)であり、Bは熱延鋼板の引張強度(kgf/mm2)である。)
(A2)総冷間圧下率が30~80%であること
を満足し、
(B)前記溶融亜鉛めっき工程が、鋼板を加熱して第一均熱処理すること、第一均熱処理された鋼板を第一冷却し次いで第二均熱処理すること、第二均熱処理された鋼板を溶融亜鉛めっき浴に浸漬すること、めっきを施された鋼板を第二冷却すること、および第二冷却された鋼板を加熱し次いで第三均熱処理することを含み、さらに以下の(B1)~(B6)の条件:
(B1)第一均熱処理前の鋼板加熱時において、下記式(2)および(3)を満足する雰囲気下で、650℃~Ac1℃+30℃以上950℃以下の最高加熱温度までの平均加熱速度が0.5℃/秒~10.0℃/秒であること、
(B2)前記鋼板を前記最高加熱温度で1秒~1000秒間保持すること(第一均熱処理)、
(B3)第一冷却における700~600℃までの温度範囲の平均冷却速度が10~100℃/秒であること、
(B4)下記式(4)および(5)を満足する雰囲気下において、第一冷却された鋼板を300~600℃の範囲で80秒~500秒間保持すること(第二均熱処理)、
(B5)第二冷却がMs-50℃以下まで行われること、
(B6)第二冷却された鋼板を200~420℃の温度域に加熱し、次いで前記温度域で5~500秒間保持すること(第三均熱処理)
を満足することを特徴とする、請求項1または2に記載の溶融亜鉛めっき鋼板の製造方法。
-1.10≦log(PH2O/PH2)≦-0.07 ・・・(2)
0.010≦PH2≦0.150 ・・・(3)
log(PH2O/PH2)<-1.10 ・・・(4)
0.0010≦PH2≦0.1500 ・・・(5)
(式中、PH2Oは水蒸気の分圧を示し、PH2は水素の分圧を示す。)
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