WO2022254847A1 - 鋼板 - Google Patents
鋼板 Download PDFInfo
- Publication number
- WO2022254847A1 WO2022254847A1 PCT/JP2022/009304 JP2022009304W WO2022254847A1 WO 2022254847 A1 WO2022254847 A1 WO 2022254847A1 JP 2022009304 W JP2022009304 W JP 2022009304W WO 2022254847 A1 WO2022254847 A1 WO 2022254847A1
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- WIPO (PCT)
- Prior art keywords
- less
- steel sheet
- content
- rolling
- vickers hardness
- Prior art date
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 194
- 239000010959 steel Substances 0.000 title claims abstract description 194
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 39
- 239000000203 mixture Substances 0.000 claims abstract description 15
- 239000000126 substance Substances 0.000 claims abstract description 15
- 238000005096 rolling process Methods 0.000 claims description 108
- 238000000034 method Methods 0.000 claims description 51
- 238000012360 testing method Methods 0.000 claims description 27
- 239000013078 crystal Substances 0.000 claims description 23
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 18
- 229910000734 martensite Inorganic materials 0.000 claims description 17
- 238000009864 tensile test Methods 0.000 claims description 14
- 229910052751 metal Inorganic materials 0.000 claims description 9
- 239000002184 metal Substances 0.000 claims description 9
- 239000012535 impurity Substances 0.000 claims description 8
- 229910001562 pearlite Inorganic materials 0.000 claims description 6
- 229910001563 bainite Inorganic materials 0.000 claims description 5
- 229910052742 iron Inorganic materials 0.000 claims description 3
- 229910052782 aluminium Inorganic materials 0.000 abstract description 5
- 229910052799 carbon Inorganic materials 0.000 abstract description 5
- 229910052710 silicon Inorganic materials 0.000 abstract description 4
- 229910052748 manganese Inorganic materials 0.000 abstract description 3
- 229910052757 nitrogen Inorganic materials 0.000 abstract description 3
- 229910052717 sulfur Inorganic materials 0.000 abstract description 3
- 229910052698 phosphorus Inorganic materials 0.000 abstract description 2
- 239000000047 product Substances 0.000 description 61
- 239000010410 layer Substances 0.000 description 46
- 238000007747 plating Methods 0.000 description 30
- 230000009467 reduction Effects 0.000 description 28
- 238000005097 cold rolling Methods 0.000 description 22
- 230000000694 effects Effects 0.000 description 21
- 238000005098 hot rolling Methods 0.000 description 18
- 238000004519 manufacturing process Methods 0.000 description 18
- 238000000465 moulding Methods 0.000 description 17
- 230000006872 improvement Effects 0.000 description 16
- 230000008569 process Effects 0.000 description 16
- 239000002245 particle Substances 0.000 description 14
- 229910052761 rare earth metal Inorganic materials 0.000 description 14
- 230000000052 comparative effect Effects 0.000 description 13
- 235000019587 texture Nutrition 0.000 description 12
- 238000005275 alloying Methods 0.000 description 11
- 238000000137 annealing Methods 0.000 description 10
- 238000001816 cooling Methods 0.000 description 10
- 229910052749 magnesium Inorganic materials 0.000 description 10
- 230000003746 surface roughness Effects 0.000 description 10
- 229910052726 zirconium Inorganic materials 0.000 description 10
- 229910001297 Zn alloy Inorganic materials 0.000 description 9
- 230000015572 biosynthetic process Effects 0.000 description 9
- 229910052725 zinc Inorganic materials 0.000 description 9
- 239000011701 zinc Substances 0.000 description 9
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 8
- 229910052791 calcium Inorganic materials 0.000 description 8
- 238000010438 heat treatment Methods 0.000 description 8
- 238000005259 measurement Methods 0.000 description 8
- 230000001186 cumulative effect Effects 0.000 description 7
- 239000010960 cold rolled steel Substances 0.000 description 6
- 230000006866 deterioration Effects 0.000 description 6
- 238000009826 distribution Methods 0.000 description 6
- 238000002791 soaking Methods 0.000 description 6
- 238000005728 strengthening Methods 0.000 description 6
- 230000009466 transformation Effects 0.000 description 6
- 238000010191 image analysis Methods 0.000 description 5
- 229910052787 antimony Inorganic materials 0.000 description 4
- 239000003153 chemical reaction reagent Substances 0.000 description 4
- 229910052804 chromium Inorganic materials 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- 230000007797 corrosion Effects 0.000 description 4
- 238000005260 corrosion Methods 0.000 description 4
- 230000007547 defect Effects 0.000 description 4
- 239000012467 final product Substances 0.000 description 4
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 4
- 238000001953 recrystallisation Methods 0.000 description 4
- 238000005204 segregation Methods 0.000 description 4
- 229910052718 tin Inorganic materials 0.000 description 4
- 229910052721 tungsten Inorganic materials 0.000 description 4
- 230000002776 aggregation Effects 0.000 description 3
- 239000011247 coating layer Substances 0.000 description 3
- 238000009749 continuous casting Methods 0.000 description 3
- 238000012937 correction Methods 0.000 description 3
- 229910052760 oxygen Inorganic materials 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 238000007711 solidification Methods 0.000 description 3
- 230000008023 solidification Effects 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- 229910001335 Galvanized steel Inorganic materials 0.000 description 2
- 238000007545 Vickers hardness test Methods 0.000 description 2
- 238000005054 agglomeration Methods 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 229910052793 cadmium Inorganic materials 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 230000000994 depressogenic effect Effects 0.000 description 2
- 230000009977 dual effect Effects 0.000 description 2
- 239000008397 galvanized steel Substances 0.000 description 2
- 229910052732 germanium Inorganic materials 0.000 description 2
- 229910052747 lanthanoid Inorganic materials 0.000 description 2
- 150000002602 lanthanoids Chemical class 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 238000000879 optical micrograph Methods 0.000 description 2
- 238000005554 pickling Methods 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 229910052709 silver Inorganic materials 0.000 description 2
- 239000000758 substrate Substances 0.000 description 2
- 229910052715 tantalum Inorganic materials 0.000 description 2
- 150000003568 thioethers Chemical class 0.000 description 2
- 238000003466 welding Methods 0.000 description 2
- 238000004804 winding Methods 0.000 description 2
- 229910052765 Lutetium Inorganic materials 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 238000004220 aggregation Methods 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 229910052785 arsenic Inorganic materials 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 229910052790 beryllium Inorganic materials 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- 229910052792 caesium Inorganic materials 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 229910052801 chlorine Inorganic materials 0.000 description 1
- 239000012141 concentrate Substances 0.000 description 1
- 230000010485 coping Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 229910052733 gallium Inorganic materials 0.000 description 1
- 238000000227 grinding Methods 0.000 description 1
- 229910052735 hafnium Inorganic materials 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 238000003703 image analysis method Methods 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 239000003112 inhibitor Substances 0.000 description 1
- 229910052740 iodine Inorganic materials 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 229910052744 lithium Inorganic materials 0.000 description 1
- OHSVLFRHMCKCQY-UHFFFAOYSA-N lutetium atom Chemical compound [Lu] OHSVLFRHMCKCQY-UHFFFAOYSA-N 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 239000007769 metal material Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 229910052762 osmium Inorganic materials 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 229910052699 polonium Inorganic materials 0.000 description 1
- 229910052700 potassium Inorganic materials 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052702 rhenium Inorganic materials 0.000 description 1
- 229910052703 rhodium Inorganic materials 0.000 description 1
- 229910052701 rubidium Inorganic materials 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 229910052708 sodium Inorganic materials 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- 238000010301 surface-oxidation reaction Methods 0.000 description 1
- 230000008961 swelling Effects 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
- 238000007740 vapor deposition Methods 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
Classifications
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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
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- 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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
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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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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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
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
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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
- C21D2211/002—Bainite
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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
- C21D2211/005—Ferrite
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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
Definitions
- the present invention relates to steel sheets.
- Ghost lines are caused by preferential deformation around the soft phase when a steel plate having a hard phase and a soft phase such as DP (Dual Phase) steel is press-formed, resulting in minute irregularities on the surface of the order of 1 mm. That is. Since the unevenness forms a striped pattern on the surface, a press-molded product with ghost lines has poor appearance quality.
- An object of the present invention is to provide a steel sheet capable of realizing excellent appearance quality in a molded product.
- the gist of the present invention is the following steel plate.
- Chemical composition is mass %, C: 0.030% to 0.145%, Si: 0% to 0.500%, Mn: 0.50% to 2.50%, P: 0% to 0.100%, S: 0% to 0.020%, Al: 0% to 1.000%, N: 0% to 0.0100%, B: 0% to 0.0050%, Mo: 0% to 0.80%, Ti: 0% to 0.200%, Nb: 0% to 0.10%, V: 0% to 0.20%, Cr: 0% to 0.80%, Ni: 0% to 0.25% O: 0% to 0.0100%, Cu: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 0.20%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr: 0% to 0.0100%, REM: 0% to 0.0100%, the remainder being iron and impurities,
- the metal structure consists of ferrite with a volume fraction of 70 to
- the area of the hard phase connected in the rolling direction by 100 ⁇ m or more is 30% or less of the total area of the hard phase.
- the steel plate according to (2) In the region of 1/4 to 1/2 in the plate thickness direction, the area of the hard phase connected in the rolling direction by 100 ⁇ m or more is 30% or less of the total area of the hard phase.
- the average value of the Vickers hardness H 1/4 at the 1/4 position in the plate thickness direction is 150 to 300, The steel sheet according to any one of the above (1) to (4), wherein the average value of the Vickers hardness H 1/2 at the 1/2 position in the thickness direction is 155 to 305.
- the present inventors have studied a method for suppressing the generation of ghost lines after press-forming a high-strength steel sheet.
- a steel sheet such as DP (Dual Phase) steel in which a hard phase and a soft phase coexist
- the area around the soft phase is mainly deformed during forming, and fine unevenness is generated on the surface of the steel sheet, resulting in ghost lines.
- a so-called appearance defect may occur.
- a ghost line is formed in a band shape (stripe shape) by swelling and deformation so that a soft phase is depressed during press forming of a steel sheet while a hard phase is not depressed or rather becomes convex.
- a band-like structure is formed in a hard phase such as martensite.
- the inventor found that it is possible to suppress the band-like hard phase in the final product by controlling the hot-rolled structure and suppressing the band-like structure during the production of the steel sheet.
- the steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.030% to 0.145%, Si: 0% to 0.500%, Mn: 0.50% to 2.50%, P: 0% to 0.100%, S: 0% to 0.020%, Al: 0% to 1.000%, N: 0% to 0.0100%, B: 0% to 0.0050%, Mo: 0% to 0.80%, Ti: 0% to 0.200%, Nb: 0% to 0.10%, V: 0% to 0.20%, Cr: 0% to 0.80%, Ni: 0% to 0.25% O: 0% to 0.0100%, Cu: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 0.20%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr: 0% to 0.0100%, REM: 0% to 0.0100%, The balance is iron and impurities. Each element will be described below.
- C is an element that increases the strength of the steel sheet.
- the C content should be 0.030% or more.
- the C content is preferably 0.035% or more, more preferably 0.040% or more, still more preferably 0.050% or more, still more preferably 0.060% That's it.
- the C content is made 0.145% or less.
- the C content is preferably 0.110% or less, more preferably 0.090% or less.
- Si is a deoxidizing element for steel, and is an effective element for increasing the strength without impairing the ductility of the steel sheet.
- the Si content is set to 0.500% or less.
- the Si content is preferably 0.450% or less, more preferably 0.250% or less, even more preferably 0.100% or less.
- the lower limit of the Si content includes 0%, but in order to improve the strength-formability balance of the steel sheet, the Si content may be 0.0005% or more or 0.0010% or more, more preferably 0.090 %, more preferably 0.100% or more.
- Mn is an element that enhances the hardenability of steel and contributes to the improvement of strength.
- the Mn content should be 0.50% or more.
- the Mn content is preferably 1.20% or more, more preferably 1.40% or more, still more preferably over 1.60%, and even more preferably 1.65% or more.
- the Mn content is set to 2.50% or less.
- the Mn content is preferably 2.25% or less, more preferably 2.00% or less, even more preferably 1.80% or less.
- P is an element that embrittles steel.
- the P content is preferably 0.080% or less, more preferably 0.050% or less.
- the lower limit of the P content includes 0%, the production cost can be further reduced by setting the P content to 0.001% or more. Therefore, the P content may be 0.001% or more.
- S is an element that forms Mn sulfides and deteriorates formability such as ductility, hole expandability, stretch flangeability and bendability of the steel sheet.
- S content is set to 0.020% or less.
- the S content is preferably 0.010% or less, more preferably 0.008% or less.
- the lower limit of the S content includes 0%, the production cost can be further reduced by setting the S content to 0.0001% or more. Therefore, the S content may be 0.0001% or more.
- Al 0% to 1.000%
- Al is an element that functions as a deoxidizer and is an element that is effective in increasing the strength of steel.
- the Al content is set to 1.000% or less.
- the Al content is preferably 0.650% or less, more preferably 0.600% or less, even more preferably 0.500% or less.
- the lower limit of the Al content includes 0%, the Al content may be 0.005% or more in order to sufficiently obtain the deoxidizing effect of Al.
- N is an element that forms nitrides and deteriorates formability such as ductility, hole expandability, stretch flangeability and bendability of the steel sheet.
- the N content is set to 0.0100% or less.
- N is also an element that causes welding defects during welding and hinders productivity. Therefore, the N content is preferably 0.0080% or less, more preferably 0.0070% or less, and even more preferably 0.0040% or less.
- the lower limit of the N content includes 0%, the production cost can be further reduced by setting the N content to 0.0005% or more. Therefore, the N content may be 0.0005% or more.
- the steel sheet according to this embodiment may contain the following elements as optional elements.
- the content is 0% when the following optional elements are not contained.
- B is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since B does not necessarily have to be contained, the lower limit of the B content includes 0%. In order to sufficiently obtain the strength-improving effect of B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. Further, when the B content is 0.0050% or less, it is possible to suppress the formation of B precipitates and the decrease in the strength of the steel sheet. Therefore, the B content is 0.0050% or less, preferably 0.0030% or less. The B content may be between 0.0001% and 0.0050%.
- Mo is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since Mo does not necessarily have to be contained, the lower limit of the Mo content includes 0%. In order to sufficiently obtain the strength improvement effect of Mo, the Mo content is preferably 0.001% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. In addition, when the Mo content is 0.80% or less, it is possible to suppress a decrease in hot workability and a decrease in productivity. Therefore, the Mo content is 0.80% or less, preferably 0.40% or less, and more preferably 0.20% or less. The Mo content may be between 0.001% and 0.80%, or between 0% and 0.40%. In addition, by including both Cr and Mo, the content of Cr: 0.20% to 0.80% and Mo: 0.05% to 0.80%, the strength of the steel sheet is more reliably improved. preferred because it can
- Ti is an element that has the effect of reducing the amounts of S, N, and O that generate coarse inclusions that act as starting points for fracture.
- Ti has the effect of refining the structure and improving the strength-formability balance of the steel sheet.
- the lower limit of the Ti content includes 0%.
- the Ti content is preferably 0.001% or more, more preferably 0.010% or more.
- the Ti content is set to 0.200% or less.
- the Ti content is preferably 0.080% or less, more preferably 0.060% or less.
- the Ti content may be from 0% to 0.100% or from 0.001% to 0.200%.
- Nb is an element that contributes to the improvement of the strength of a steel sheet through strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. Since Nb does not necessarily have to be contained, the lower limit of the Nb content includes 0%. In order to sufficiently obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more. Further, when the Nb content is 0.10% or less, it is possible to promote recrystallization and suppress the remaining non-recrystallized ferrite, thereby ensuring the formability of the steel sheet. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.05% or less, more preferably 0.04% or less. The Nb content may be between 0.001% and 0.10%.
- V is an element that contributes to the improvement of the strength of the steel sheet through strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. Since V does not necessarily have to be contained, the lower limit of the V content includes 0%. In order to sufficiently obtain the strength improvement effect of V, the V content is preferably 0.001% or more, more preferably 0.01% or more, and even more preferably 0.03% or more. Further, when the V content is 0.20% or less, it is possible to suppress the deterioration of the formability of the steel sheet due to the precipitation of a large amount of carbonitrides. Therefore, the V content is set to 0.20% or less. The V content is preferably 0.10% or less. The V content may be 0% to 0.10%, or may be 0.001% to 0.20%.
- Cr 0% to 0.80%
- Cr is an element that increases the hardenability of steel and contributes to the improvement of the strength of the steel sheet. Since Cr does not necessarily have to be contained, the lower limit of the Cr content includes 0%. In order to sufficiently obtain the strength improvement effect of Cr, the Cr content is preferably 0.001% or more, more preferably 0.20% or more, and particularly preferably 0.30% or more. In addition, when the Cr content is 0.80% or less, it is possible to suppress the formation of coarse Cr carbides that may serve as starting points for fracture. Therefore, the Cr content is set to 0.80% or less.
- the Cr content is preferably 0.70% or less, more preferably 0.50% or less. The Cr content may be between 0% and 0.70%, or between 0.001% and 0.80%.
- Ni is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since Ni does not necessarily have to be contained, the lower limit of the Ni content includes 0%. In order to sufficiently obtain the strength improvement effect of Ni, the Ni content is preferably 0.001% or more, more preferably 0.05% or more. Moreover, it can suppress that the weldability of a steel plate falls as Ni content is 0.25% or less. Therefore, the Ni content is set to 0.25% or less. The Ni content is preferably 0.20% or less, more preferably 0.15% or less. The Ni content may be between 0.001% and 0.20%.
- O is an element mixed in during the manufacturing process.
- the O content may be 0%.
- the refining time can be shortened and the productivity can be increased. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
- the O content should be 0.0100% or less.
- the O content may be 0.0070% or less, 0.0040% or less, or 0.0020% or less.
- Cu is an element that exists in steel in the form of fine particles and contributes to the improvement of the strength of the steel sheet.
- the Cu content may be 0%, the Cu content is preferably 0.001% or more in order to obtain such effects.
- the Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more.
- the Cu content is set to 1.00% or less.
- the Cu content may be 0.60% or less, 0.40% or less, or 0.20% or less.
- W is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet.
- the W content may be 0%, the W content is preferably 0.001% or more in order to obtain such effects.
- the W content may be 0.01% or more, 0.02% or more, or 0.10% or more.
- the W content should be 1.00% or less.
- the W content may be 0.80% or less, 0.50% or less, or 0.20% or less.
- Sn is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet.
- the Sn content may be 0%, the Sn content is preferably 0.001% or more in order to obtain such effects.
- the Sn content may be 0.01% or more, 0.05% or more, or 0.08% or more.
- the Sn content should be 1.00% or less.
- the Sn content may be 0.80% or less, 0.50% or less, or 0.20% or less.
- Sb is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet.
- the Sb content may be 0%, the Sb content is preferably 0.001% or more in order to obtain such effects.
- the Sb content may be 0.01% or more, 0.05% or more, or 0.08% or more.
- the Sb content should be 0.20% or less.
- the Sb content may be 0.18% or less, 0.15% or less, or 0.12% or less.
- Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets.
- the Ca, Mg, Zr and REM contents may be 0%, but in order to obtain such effects, the Ca, Mg, Zr and REM contents are each preferably 0.0001% or more. , 0.0005% or more, 0.0010% or more, or 0.0015% or more.
- the content of each of Ca, Mg, Zr and REM can be ensured.
- the Ca, Mg, Zr and REM contents are each 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0030% or less.
- REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoids.
- the REM content is the total content of these elements.
- the rest of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities.
- impurities include those that are mixed from steel raw materials or scrap and/or during the steelmaking process, or elements that are allowed within a range that does not impair the properties of the steel sheet according to the present embodiment.
- impurities H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au , Pb, Bi, and Po.
- the total amount of impurities may be 0.200% or less.
- the chemical composition of the steel sheet mentioned above can be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Incidentally, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
- C and S may be measured using the combustion-infrared absorption method
- N may be measured using the inert gas fusion-thermal conductivity method.
- the coating layer on the surface may be removed by mechanical grinding, and then the chemical composition may be analyzed.
- the metal structure consists of ferrite with a volume fraction of 70 to 95% and a hard phase with a volume fraction of 5 to 30%
- the volume fraction of the hard phase is set to 5% or more.
- the volume fraction of the hard phase is set to 30% or less, so that surface unevenness during molding can be reduced, and the appearance after molding can be improved.
- the remainder of the metallographic structure other than the hard phase is ferrite, and the volume fraction of ferrite is 70 to 95%.
- the volume fraction of ferrite is preferably 72% or more, more preferably 75% or more.
- the volume fraction of the hard phase is preferably 28% or less, more preferably 25% or less.
- the sum of the volume fractions of ferrite and hard phases in the metallographic structure is 100%.
- the hard phase is a hard structure that is harder than ferrite, and is composed of, for example, one or more of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of strength improvement, the hard phase preferably comprises one or more of martensite, bainite, and tempered martensite, and more preferably martensite.
- the volume fraction of the hard phase in the metallographic structure can be obtained by the following method.
- the metal structure (microstructure) from the W / 4 position or 3W / 4 position of the width W of the obtained steel plate (that is, the position of W / 4 in the width direction from either end of the steel plate in the width direction)
- a sample (approximately 20 mm in the rolling direction, 20 mm in the width direction, and the thickness of the steel sheet) is collected, and the metal structure (microstructure) is observed from the surface at half the thickness of the plate using an optical microscope.
- the plate thickness cross-section in the direction perpendicular to the rolling direction is polished as an observation surface and etched with a repeller reagent.
- Microstructure is classified from optical micrographs at a magnification of 500 or 1000. When observed with an optical microscope after repeller corrosion, each structure is color-coded, for example, black for bainite and pearlite, white for martensite (including tempered martensite), and gray for ferrite. It can be easily distinguished from hard tissue. In the optical micrograph, the non-gray areas showing ferrite are the hard phases.
- Image analysis is performed using software to determine the area fraction of the hard phase.
- the maximum brightness value L max and the minimum brightness value L min of the image are obtained from the image, and pixels with brightness from L max ⁇ 0.3 (L max ⁇ L min ) to L max
- a white region is defined as a white region
- a portion having pixels from L min to L min +0.3 (L max ⁇ L min ) is defined as a black region
- the other portion is defined as a gray region.
- the inventors of the present invention have found that if the Vickers hardness distribution of the steel sheet is highly biased, the hard phase is likely to be connected in a band shape, and as a result, ghost lines tend to occur in the molded product obtained by press-molding the steel sheet. . In particular, attention was paid to the bias of the Vickers hardness distribution in a region relatively close to the surface of the steel plate.
- the ghost line is formed as if it were interrupted in the middle, and it was found that the appearance defect caused by the long ghost line can be suppressed.
- the value X1 obtained by dividing the standard deviation ⁇ 1/4 of the Vickers hardness H 1/4 at the 1/4 position in the plate thickness direction by the average value H AVE 1/4 of the Vickers hardness H 1/4 was 0.025 or less. It was found that it is effective to improve the surface quality of the surface of the steel sheet and the molded product obtained by press-molding the steel sheet.
- Vickers hardness refers to hardness in accordance with JIS Z 2244:2009 Vickers hardness test.
- the Vickers hardness here is HV0.2, which is the Vickers hardness at a test force of 1.9614 N (0.2 kgf).
- the Vickers hardness is observed in a cross section parallel to the thickness direction and the rolling direction of the steel sheet (a cross section perpendicular to the width direction), which is the central cross section in the width direction of the steel sheet.
- Observation at the “1/4 position in the plate thickness direction” means that 50 points are measured at a pitch of 150 ⁇ m in the rolling direction at a position that is 1/4 in the plate thickness direction from the surface of the steel plate, and the back surface of the steel plate. 1/4 in the plate thickness direction from 150 ⁇ m pitch in the rolling direction with 50 measurement points.
- the pitch in the rolling direction of the object to be observed may be less than 150 ⁇ m or more than 150 ⁇ m, but the upper limit of the pitch in the rolling direction is 400 ⁇ m and the lower limit is 50 ⁇ m.
- the number of measurement points in the rolling direction may be less than 50 or may be more than 50, but the lower limit of the number of measurement points in the rolling direction is 30. It is preferable that the length of the observation target in the rolling direction is 5 mm or more, in order to perform more accurate surface quality determination considering the positions with and without ghost lines.
- At least one of cross sections in the middle in the width direction of the steel plate may have the same configuration as the configuration of the cross section.
- the present inventors reduce the bias in the Vickers hardness distribution in the rolling direction near the surface of the steel sheet.
- the value X1 is 0.025 or less.
- the value X1 is set to 0.025 or less.
- the value X1 is less than or equal to 0.020. Note that the lower limit of the value X1 is zero.
- the value X2 obtained by dividing the standard deviation ⁇ 1/2 of the Vickers hardness H 1/2 at the 1/2 position in the plate thickness direction by the average value H AVE 1/2 of the Vickers hardness H 1/2 is 0.030 or less
- the value X1 is 0.025 or less
- the present inventors have also paid attention to the deviation of the Vickers hardness distribution in the region deep from the surface of the steel sheet.
- the value X2 obtained by dividing the standard deviation ⁇ 1/2 of the Vickers hardness H 1/2 at the 1/2 position in the plate thickness direction by the average value H AVE 1/2 of the Vickers hardness H 1/2 was 0.030 or less. It was found that it is effective to further improve the surface quality of the surface of the steel sheet and the molded product obtained by press-molding the steel sheet.
- the observation at the "1/2 position in the plate thickness direction” refers to the observation of 50 measurement points at a pitch of 150 ⁇ m in the rolling direction at a position that is 1/2 from the surface of the steel plate in the plate thickness direction.
- Observation at the “1/2 position in the plate thickness direction” and observation at the “1/4 position in the plate thickness direction” are the same observation contents except that the positions to be observed in the plate thickness direction are different.
- the inventors of the present invention reduce the deviation of the Vickers hardness distribution in the rolling direction at the center of the steel sheet. It has been found that the occurrence of ghost lines can be suppressed by setting the average particle size to 0.030 or less. Therefore, in this embodiment, the value X2 is set to 0.030 or less. Preferably, the value X2 is less than or equal to 0.025. Note that the lower limit of the value X2 is zero.
- the average crystal grain size of ferrite is 5.0 to 30.0 ⁇ m
- the average grain size of ferrite is preferably 30.0 ⁇ m or less. More preferably, the thickness is 15.0 ⁇ m or less.
- the average crystal grain size of ferrite is 5.0 ⁇ m or more, it is possible to suppress the formation of agglomeration of ferrite grains having the ⁇ 001 ⁇ orientation. Even if the individual particles with the ⁇ 001 ⁇ orientation of ferrite are small, if these particles are aggregated and formed, deformation will concentrate on the aggregated parts.
- the preferable average crystal grain size of ferrite is 5.0 ⁇ m or more. It is more preferably 8.0 ⁇ m or more, still more preferably 10.0 ⁇ m or more, and even more preferably 15.0 ⁇ m or more.
- the average grain size of ferrite in steel sheets can be obtained by the following method. Specifically, 10 fields of view were observed at a magnification of 500 times in the area from the surface of the steel plate etched with a repeller reagent to the position of 1/2 of the plate thickness in the plate thickness direction. Image analysis is performed in the same manner as described above using image analysis software, and the area fraction occupied by ferrite and the number of ferrite particles are calculated. By summing them up and dividing the area fraction occupied by ferrite by the number of ferrite particles, the average area fraction per ferrite particle is calculated. The equivalent circle diameter is calculated from the average area fraction and the number of particles, and the obtained equivalent circle diameter is taken as the average crystal grain size of ferrite.
- the average crystal grain size of the hard phase is 1.0 to 5.0 ⁇ m
- the preferable average crystal grain size of the hard phase in the steel sheet is preferably 5.0 ⁇ m or less. It is more preferably 4.5 ⁇ m or less, still more preferably 4.0 ⁇ m or less.
- the average crystal grain size of the hard phase is 1.0 ⁇ m or more, it is possible to suppress the generation of hard phase particles by agglomeration. By reducing the size of individual particles of the hard phase and suppressing aggregation of these particles, deterioration of appearance after molding can be suppressed. Therefore, it is preferable to set the preferable average crystal grain size of the hard phase in the steel sheet to 1.0 ⁇ m or more. It is more preferably 1.5 ⁇ m or more, and still more preferably 2.0 ⁇ m or more.
- the average crystal grain size of the hard phase can be obtained by the following method. Specifically, 10 fields of view were observed at a magnification of 500 times in the area from the surface of the steel plate etched with a repeller reagent to the position of 1/2 of the plate thickness in the plate thickness direction. Image analysis is performed in the same manner as described above using image analysis software, and the area fraction occupied by the hard phase and the number of hard phase particles are calculated. By summing them up and dividing the area fraction occupied by the hard phase by the number of particles of the hard phase, the average area fraction per particle of the hard phase is calculated. The equivalent circle diameter is calculated from the average area fraction and the number of particles, and the obtained equivalent circle diameter is taken as the average crystal grain size of the hard phase.
- the area of the hard phase connected to 100 ⁇ m or more in the rolling direction is 30% or less of the total area of the hard phase.
- the area of the hard phase connected in the rolling direction by 100 ⁇ m or more is 30% or less of the total hard phase area, when the steel sheet is press-formed, the hard phase is deformed to bulge and the soft phase is recessed around the hard phase. The deformation is suppressed from continuing long in the rolling direction, and the occurrence of easily visible ghost lines can be suppressed. Therefore, in the present embodiment, it is preferable that the area of the hard phases connected in the rolling direction by 100 ⁇ m or more is 30% or less of the total area of the hard phases in the region of 1/4 to 1/2 in the plate thickness direction. More preferably, this ratio is 20% or less. The lower limit of this percentage is zero percent.
- the method for measuring the above ratio in this embodiment is as follows. First, in a cross section parallel to the thickness direction and the rolling direction of the steel sheet, and in the cross section at the center of the width direction of the steel sheet, a region of 1/4 to 1/2 in the thickness direction from the surface of the steel sheet and rolled A 400 ⁇ m observation range (connected hard phase observation range) is defined in the direction.
- the length of the connecting hard phase observation range in the rolling direction may be less than 400 ⁇ m (eg, 300 ⁇ m) or may be greater than 400 ⁇ m (eg, 500 ⁇ m). However, the lower limit of the length of the connected hard phase observation range in the rolling direction is up to 250 ⁇ m.
- the area AR1 of the hard phase connected by 100 ⁇ m or more in the rolling direction is measured.
- a hard phase connected by 100 ⁇ m or more in the rolling direction is extracted by image processing by the above-described hard phase measuring method.
- "connected” indicates that the grain boundaries of the hard phase are in contact.
- the area AR2 of all the hard phases is measured by the above-described hard phase measuring method. After that, AR1/AR2 is calculated.
- the surface texture aspect ratio Str (ISO25178) of the test piece after applying 5% strain by a tensile test is 0.28 or more
- the aspect ratio Str of the surface texture of the test piece after applying 5% strain by the tensile test (hereinafter referred to as the “post-tensile test piece”) is the surface of the molded product obtained by forming (for example, press forming) the steel plate It is an index showing the anisotropy of the unevenness of the surface.
- the aspect ratio Str is defined by ISO (International Organization for Standardization) 25178 and is a numerical value between zero and one. The closer the aspect ratio Str is to zero, the greater the anisotropy, and the more streaks are present on the surface of the observation range. On the other hand, the closer the aspect ratio Str is to 1, the less the surface shape of the observation range depends on the specific direction.
- the aspect ratio Str becomes a value close to 1 when there is no directionality in the uneven shape on the surface of the test piece after tension, and there is no convex shape or concave shape extending long in one direction.
- the surface aspect ratio Str of the test piece after tension is large and the anisotropy in the surface shape is small. Therefore, the aspect ratio Str of the surface properties of the test piece after tension is preferably 0.28 or more.
- the aspect ratio Str of the test piece after tension is 0.30 or more, more preferably 0.35 or more.
- the method of measuring the aspect ratio Str of the test piece after tension in this embodiment is as follows. Specifically, a JIS No. 5 test piece is cut in a direction (width direction) perpendicular to the rolling direction of the steel plate from a quarter position in the plate width direction from the edge of the steel plate, and the surface of this test piece is polished with abrasive paper. This makes the surface mirror-like. Next, the specimen is subjected to a tensile test to apply a strain of 5%. The unevenness of the surface of the test piece to which 5% strain is applied is measured with a laser microscope. The aspect ratio Str is calculated from the measurement results. The aspect ratio Str can be calculated by processing the coordinate data of the surface shape obtained with a laser microscope with analysis software in compliance with ISO25178. In the analysis, no S filter was used and the L filter was 0.8 mm.
- the average value H AVE1/4 of the Vickers hardness H 1/4 at the 1/4 position in the plate thickness direction is 150 to 300
- the tensile strength of the steel plate can be ensured to be 540 MPa or more.
- the average value HAVE1/4 of the Vickers hardness H 1/4 at the 1 ⁇ 4 position in the plate thickness direction is 300 or less, the steel plate does not become excessively hard at the 1 ⁇ 4 position in the plate thickness direction. As a result, the effect of leveling the unevenness of the surface during rolling of the steel sheet is sufficiently exhibited.
- the Vickers hardness in this embodiment refers to hardness according to JIS Z 2244:2009 Vickers hardness test.
- the average value H AVE1/4 of the Vickers hardness H 1/4 at the 1/4 position in the plate thickness direction is measured by the following method. A total of 100 points were measured at 50 points each at a pitch of 150 ⁇ m in the rolling direction at positions 1/4 in the plate thickness direction from the front and back surfaces of the steel plate, and the average value was defined as HAVE1/4 .
- the average value H AVE1/2 of the Vickers hardness H 1/2 at the 1/2 position in the plate thickness direction is 155 to 305
- the average value H AVE1/2 of the Vickers hardness H 1/2 at the 1/2 position in the sheet thickness direction is 155 or more
- the tensile strength of the steel sheet can be ensured to be 540 MPa or more.
- the average value H AVE1/2 of the Vickers hardness H 1/2 at the 1/2 position in the plate thickness direction is 305 or less, the steel plate does not become excessively hard at the 1/2 position in the plate thickness direction. As a result, the effect of leveling the unevenness of the surface during rolling of the steel sheet is sufficiently exhibited.
- the method of measuring the average value H AVE1/2 of the Vickers hardness H 1/2 at the 1/2 position in the plate thickness direction is the Vickers hardness at the 1/4 position in the plate thickness direction, except that the measurement position in the plate thickness direction is different. It is the same as the method for measuring the average value H AVE1/4 of H 1/4 .
- the molded product of the steel plate of this embodiment is suitable as an automobile panel.
- Automotive panels include panel system parts such as door outers. Examples of the panel system parts include a hood outer panel, a quarter panel such as a fender panel, a door outer panel, a roof panel, and the like.
- the strength of such automobile panels is also being increased in the same manner as automobile structural members, and the strength of hot-rolled steel sheets that are in the process of production of steel sheets to be automobile panels is also increasing. Furthermore, with the thinning of automobile panels, the rolling reduction in the cold rolling process during steel sheet production is also increasing.
- Automobile panel steel sheets particularly door panel steel sheets, have a width exceeding 1000 mm, and hood panel steel sheets have a width exceeding 1500 mm.
- Such a wide steel sheet tends to have a large reduction load (rolling mill load) in the cold rolling process.
- rolling mill load rolling mill load
- the rolling load during cold rolling becomes particularly large when the width is about 1500 mm or more
- a steel sheet with a tensile strength of 780 MPa when the width is about 1200 mm or more
- the rolling load during cold rolling increases. become particularly large.
- the precision of the steel sheet shape deteriorates.
- the plate thickness of the steel plate is 0.20 to 1.00 mm
- the plate thickness of the steel plate according to the present embodiment is not limited to a specific range, but is preferably 0.20 to 1.00 mm in consideration of versatility and manufacturability.
- the plate thickness is preferably 0.20 mm or more, preferably 0.35 mm or more, and more preferably 0.40 mm or more.
- the plate thickness is preferably 1.00 mm or less, preferably 0.70 mm or less, and more preferably 0.60 mm or less.
- the plate thickness of the steel plate can be measured with a micrometer.
- the steel plate has a tensile strength of 540 to 980 MPa
- the tensile strength of the steel sheet according to the present embodiment is not limited to a specific range, it is preferably 540-980 MPa.
- the steel sheet has a tensile strength of 980 MPa or less, it is easy to secure formability when the steel sheet is pressed.
- Tensile strength is measured by taking a JIS No. 5 tensile test piece with the longitudinal direction perpendicular to the rolling direction from the steel plate and performing a test in accordance with JIS (Japanese Industrial Standards) Z2241: 2011 Metal Material Tensile Test Method. be done.
- the steel sheet according to this embodiment may have a plating layer on at least one surface of the steel sheet.
- the plating layer includes a zinc plating layer, a zinc alloy plating layer, and an alloying zinc plating layer and an alloying zinc alloy plating layer obtained by subjecting these to an alloying treatment.
- the zinc plating layer and the zinc alloy plating layer are formed by a hot dip plating method, an electroplating method, or a vapor deposition plating method.
- the Al content of the galvanized layer is 0.5% by mass or less, the adhesion between the surface of the steel sheet and the galvanized layer can be sufficiently ensured, so the Al content of the galvanized layer is 0.5%. % by mass or less is preferable.
- the galvanized layer is a hot-dip galvanized layer
- the Fe content of the hot-dip galvanized layer is preferably 3.0% by mass or less in order to increase the adhesion between the steel sheet surface and the galvanized layer.
- the galvanized layer is an electrogalvanized layer
- the Fe content of the electrogalvanized layer is preferably 0.5% by mass or less from the viewpoint of improving corrosion resistance.
- the zinc plating layer and the zinc alloy plating layer include Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, Zr, I, K, La, Li, Mg, Mn, One or more of Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM, in a range that does not impair the corrosion resistance and formability of the steel sheet and may contain In particular, Ni, Al and Mg are effective in improving the corrosion resistance of steel sheets.
- the zinc plated layer or zinc alloy plated layer may be a zinc alloyed layer or a zinc alloy plated layer that has been alloyed.
- the hot-dip galvanized layer after the alloying treatment is used from the viewpoint of improving the adhesion between the steel sheet surface and the alloyed coating layer.
- the Fe content of the hot-dip zinc alloy plating layer is preferably 7.0% by mass to 13.0% by mass.
- the Fe content in the plating layer can be obtained by the following method. Only the plated layer is dissolved and removed using a 5% by volume HCl aqueous solution containing an inhibitor. By measuring the Fe content in the obtained solution using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), the Fe content (% by mass) in the plating layer is obtained.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
- the steel plate is an automobile outer panel
- a press-formed product that can be manufactured by press-forming the steel plate described above will be described.
- This press-formed product has the same chemical composition as the steel plate described above.
- the press-formed product may have the above-described plated layer on at least one surface. Since the press-molded product is obtained by press-molding the steel plate described above, the occurrence of ghost lines is suppressed and the appearance quality is excellent. As a result, it is possible to realize automobiles with high market value due to the superior appearance that is directly visible to consumers.
- Specific examples of the press-formed product include, as described above, panel system parts (automobile outer panel) such as door outers of automobile bodies. Examples of the panel system parts include a hood outer panel, a quarter panel such as a fender panel, a door outer panel, a roof panel, and the like.
- the steel plate according to the present embodiment can obtain the effect as long as it has the above characteristics regardless of the manufacturing method. However, the following method is preferable because it can be produced stably.
- the steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (i) to (iv). (i) a slab forming step of solidifying the molten steel having the above chemical composition to form a slab; (ii) a hot-rolling step of heating the slab and hot-rolling it so that the rolling end temperature is 950° C.
- slab molding process In the slab forming process, molten steel having a predetermined chemical composition is formed into a slab.
- the manufacturing method of the slab forming process is not limited. For example, it is possible to use a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace and producing it by a continuous casting method. An ingot casting method, a thin slab casting method, or the like may be employed instead of the continuous casting method.
- the slab is heated to 1100°C or higher prior to hot rolling.
- the heating temperature is preferably less than 1300° C. from an economical point of view.
- the steel slab heated to the above heating temperature is hot rolled.
- finish rolling is performed after rough rolling.
- reduction is performed multiple times. Finish rolling is performed in a plurality of consecutive rolling stands, and the rolling reduction in the latter half of the rolling stand is made larger than the rolling reduction in the first half of the rolling stand.
- the rolling reduction in the first half of finish rolling is set to less than 35%, and the rolling reduction in the second half of finish rolling is set to 35% or more.
- the ratio P2/P1 of the rolling reduction P1 in the first half of the rolling stand and the rolling reduction P2 in the latter half of the rolling stand is preferably more than 1.0 and 1.6 or less.
- P2/P1 exceeds 1.0, the hot-rolled sheet can be sufficiently softened, and formation of band-like hard phases in the structure of the molded product, which is the final product, can be suppressed.
- P2/P1 exceeds 1.0, the hot-rolled sheet can be sufficiently softened, and formation of band-like hard phases in the structure of the molded product, which is the final product, can be suppressed.
- the rolling reduction at the final rolling stand is preferably 40% or more. As a result, it is possible to more easily suppress the formation of bands of hard phases such as pearlite and martensite in the structure of the hot-rolled sheet. can be more easily suppressed.
- the first to third stands are the first half stands, and the fifth to seventh stands are the latter half stands.
- the number of rolling stands is not limited, and the rolling rate of the rolling stands in the latter half of the plurality of rolling stands may be set higher than the rolling rate of the rolling stands in the first half.
- the rolling end temperature shall be 950°C or less.
- the average grain size of the hot-rolled steel sheet can be prevented from becoming excessively large.
- the average crystal grain size of the final product sheet can be made small, and sufficient yield strength can be ensured and high surface quality after forming can be ensured.
- the coiling temperature in the hot rolling process is preferably 450-650°C.
- the coiling temperature in the hot rolling process is preferably 450-650°C.
- the crystal grain size can be made minute, and sufficient strength of the steel sheet can be ensured.
- the pickling property can be sufficiently secured.
- the strength of the hot-rolled steel sheet does not increase excessively, and the load on equipment for the cold rolling process can be suppressed to further increase productivity.
- Cold rolling process In the cold-rolling process, cold-rolling is performed at a cumulative reduction rate RCR of 50 to 90% to obtain a cold-rolled steel sheet.
- RCR cumulative reduction rate
- the cumulative reduction rate RCR is set to 50 to 90%.
- annealing process is performed by heating the cold-rolled steel sheet to a soaking temperature of 750 to 900° C. and holding it.
- the soaking temperature is 750° C. or higher, recrystallization of ferrite and reverse transformation from ferrite to austenite proceed sufficiently, and a desired texture can be obtained.
- the soaking temperature is 900° C. or less, the crystal grains are densified and sufficient strength can be obtained.
- the heating temperature is not excessively high, and productivity can be increased.
- the cold-rolled steel sheet after soaking in the annealing step is cooled. Cooling is performed so that the average cooling rate from the soaking temperature is 5.0 to 50° C./sec. When the average cooling rate is 5.0° C./second or more, the ferrite transformation is not excessively accelerated, and the amount of hard phases such as martensite produced can be increased to obtain the desired strength. . Moreover, the steel sheet can be cooled more uniformly in the width direction of the steel sheet by setting the average cooling rate to 50° C./sec or less.
- the cold-rolled steel sheet obtained by the above method may be further subjected to a plating step for forming a plating layer on the surface.
- the plating layer formed in the plating step may be alloyed.
- the alloying temperature is, for example, 450-600.degree.
- a steel sheet with less hard phase connected can be obtained by applying a post-stage large reduction that increases the reduction rate in the latter half of finish rolling in the hot rolling process.
- the anisotropy of the uneven shape on the surface is reduced, the occurrence of ghost lines can be suppressed, and excellent appearance quality can be obtained.
- the hot-rolled sheet can be moderately softened, and the cold-rolling workability can be improved without requiring softening annealing or double cold rolling.
- the steel sheet after hot rolling is not subjected to shape correction by a leveler as a shape correction device.
- the steel sheet of the present embodiment is required to have high surface properties in order to ensure high appearance quality. Therefore, a steel plate that requires shape correction by a leveler cannot be used in this embodiment.
- the steel sheet of the present embodiment is not expected to be manufactured by a manufacturing method including a special hot rolling process in which a leveler is arranged on the stand exit side of finish rolling. Therefore, a leveler is not used in combination with the steel sheet manufacturing method of the present embodiment.
- the coil was unwound, and the resulting hot-rolled sheet was cut into a test piece to measure the tensile strength.
- Tensile strength was evaluated according to JIS Z 2241:2011.
- the test piece was JIS Z 2241:2011 No. 5 test piece.
- the tensile test piece was sampled from the 1/4 part from the edge in the width direction, and the direction perpendicular to the rolling direction was taken as the longitudinal direction.
- annealing and cooling were performed under the conditions of the soaking temperature and the cooling rate after heating (average cooling rate) shown in Table 3.
- some of the steel sheets were subjected to various types of plating to form a plating layer on the surface, and then subjected to alloying treatment at the alloying temperature shown in Table 3.
- CR indicates no plating, GI hot-dip galvanized, GA galvannealed, and EG electro-galvanized.
- the product plate No. Tensile strength was measured for A1a to K1a. Tensile strength was evaluated according to JIS Z 2241:2011. The test piece was JIS Z 2241:2011 No. 5 test piece. The tensile test piece was sampled from the 1/4 part from the edge in the width direction, and the direction perpendicular to the rolling direction was taken as the longitudinal direction. When the obtained tensile strength was 540 MPa or more, it was determined to be high strength and passed. On the other hand, when the obtained tensile strength was less than 540 MPa, it was judged to be unacceptable because the strength was inferior.
- the obtained product plate No. The volume fractions of ferrite and hard phases in the metal structures of A1a to K1a were measured by the method described above.
- the Vickers hardness H 1/4 was measured at 50 points at 150 ⁇ m intervals in the rolling direction at 1/4 positions in the plate thickness direction from the surface by the method described above. Further, the Vickers hardness H 1/4 was measured at 50 points in the rolling direction at 150 ⁇ m intervals in the thickness direction from the back surface by the method described above. Then, a value X1 was calculated by dividing the standard deviation ⁇ 1/4 of the Vickers hardness H 1/4 at 100 points by the average value H AVE1/4 of the Vickers hardness H 1/4 at 100 points.
- the obtained product plate No. For A1a to K1a, the Vickers hardness H 1/2 was measured at 50 points in the rolling direction at a measurement interval of 150 ⁇ m in the thickness direction 1/2 position from the surface by the method described above. Then, the value X2 was calculated by dividing the standard deviation ⁇ 1/2 of the Vickers hardness H 1/2 at these 50 points by the average value H AVE1/2 of the Vickers hardness H 1/2 at 50 points.
- the obtained product plate No. With respect to A1a to K1a, the area ratio of the hard phase connected to 100 ⁇ m or more in the rolling direction was measured by the above-described method in the region of 1/4 to 1/2 in the plate thickness direction.
- the product board No. For each of A1a to K1a, the aspect ratio Str of the surface texture was measured by the above-described method after applying 5% strain by a tensile test to a tensile test piece whose surface was mirror-finished with abrasive paper or the like.
- the product plate No. For each of A1a to K1a, the surface roughness Wa (arithmetic mean waviness) after applying 5% strain by a tensile test to a tensile test piece whose surface was mirror-finished with abrasive paper or the like was measured by the following method. . Using a laser displacement measuring device (Keyence VK-X1000), 50 lines of the profile were measured along the direction perpendicular to the rolling direction. At this time, components with wavelengths of 0.8 mm or less and 2.5 mm or more were removed. From the obtained results, the arithmetic mean waviness is calculated according to JIS B 0601:2013, and the average value of a total of 50 lines is calculated. As a result, the surface roughness Wa of the product sheet was obtained.
- a laser displacement measuring device Keyence VK-X1000
- the product plate No The product of the tensile strength of each product sheet of A1a to K1a and the aspect ratio Str of the surface texture of the test piece after tension was calculated.
- Tensile strength TS ⁇ aspect ratio Str is an index indicating that the higher the tensile strength, the smaller the anisotropy of the uneven shape of the surface despite the higher strength and lower workability.
- the aspect ratio Str of the surface texture of the test piece after tension in the example tends to be clearly higher than the aspect ratio Str of the surface texture of the test piece after tension in the comparative example.
- the uneven shape of the surface has little anisotropy and is excellent in strength and surface quality. More specifically, all of the examples had a tensile strength exceeding 540 MPa, indicating high strength.
- the aspect ratio Str of the surface texture of the test piece after tension is 0.28 or more, the area of the connected hard phase of 100 ⁇ m or more is 30% or less with respect to the area of the total hard phase, and the ghost line was sufficiently suppressed.
- the tensile strength TS ⁇ aspect ratio Str exceeds 200 and is sufficiently high, and despite the high strength and low workability, the anisotropy of the uneven surface shape is small. It is shown. Furthermore, the average value of (tensile strength of product sheet - tensile strength of hot rolled sheet) in 10 examples was 77, whereas in eight comparative examples (tensile strength of product sheet - The average value of hot-rolled sheet tensile strength) was about 54. That is, in Examples, there was a sufficient difference between the tensile strength of the product sheet and the tensile strength of the hot-rolled sheet, and the hot-rolled sheet was softened. In particular, it was demonstrated that the load on the rolling mill in the cold rolling process was reduced for wide product sheets suitable for automobile hood panels and automobile door panels.
- the product plate No. 1 which is a comparative example, In A2a and B2a, the rolling reduction in the second half of finish rolling in hot rolling is small, so the streaky unevenness on the surface of the steel sheet cannot be sufficiently smoothed, and in the region of 1/4 to 1/2 in the rolling direction , The area ratio of the hard phase connected to 100 ⁇ m or more in the rolling direction exceeds 40%, the aspect ratio Str of the surface texture of the test piece after tension is less than 0.28, and the tensile strength TS ⁇ Since the aspect ratio Str was less than 180, the surface quality after molding was poor.
- product plate No. 1, which is a comparative example was used.
- the reduction ratio in the second half of finish rolling in hot rolling is small, so the streaky unevenness on the surface of the steel plate cannot be sufficiently smoothed, and in the region of 1/4 to 1/2 in the rolling direction ,
- the area ratio of the hard phase connected to 100 ⁇ m or more in the rolling direction exceeds 30%, the aspect ratio Str of the surface texture of the test piece after tension is less than 0.28, and the tensile strength TS ⁇ Since the aspect ratio Str was less than 170, the surface quality after molding was poor.
- product plate No. 1, which is a comparative example was used.
- the rolling reduction in the latter half is small.
- the streaky unevenness on the surface of the steel sheet cannot be sufficiently leveled, and the area ratio of the hard phase connected to the rolling direction by 100 ⁇ m or more in the region of 1/4 to 1/2 in the rolling direction exceeds 30%.
- the aspect ratio Str of the surface properties of the test piece after tension is less than 0.28, and the tensile strength TS x aspect ratio Str is less than 170, so the surface quality after molding was low. .
- No. 3 which is a comparative example.
- band-like Mn segregation was likely to occur because the carbon content exceeded the preferred range.
- the area ratio of the hard phase connected to 100 ⁇ m or more in the rolling direction exceeded 30%, and the tensile strength TS ⁇ aspect ratio Str was less than 180. Therefore, the surface quality after molding was low.
- product plate No. 1, which is a comparative example was used.
- F1a the carbon content did not reach the preferable range, the volume fraction of ferrite was excessive, and the volume fraction of the hard phase was small, so that the tensile strength of the product sheet did not reach 540 MPa and was low. .
- product plate No. 1 which is a comparative example, was used.
- G1a band-like Mn segregation occurred during solidification of the steel because the Mn content exceeded the preferred range.
- the area ratio of the hard phase connected to 100 ⁇ m or more in the rolling direction exceeded 40%, and the tensile strength TS ⁇ aspect ratio Str was less than 170. Therefore, the surface quality after molding was low.
- the product plate No. with the same plate thickness A1a and A2a, no. B1a and B2a, no. C1a and C2a, and No. Contrast D1a and D2a.
- the surface roughnesses Wa of A1a, B1a, C1a, and D1a are 0.058 ⁇ m, 0.055 ⁇ m, 0.058 ⁇ m, and 0.055 ⁇ m, respectively.
- the product sheet No. 1 which is a comparative example.
- the surface roughnesses Wa of A2a, B2a, C2a and D2a are 0.050 ⁇ m, 0.053 ⁇ m, 0.056 ⁇ m and 0.055 ⁇ m, respectively.
- the surface roughness Wa of A1a is the product sheet No. 1, which is a comparative example.
- the surface roughness Wa of A2a is greater than or equal to that of product sheet No. 1, which is an example.
- the surface roughnesses Wa of B1a, C1a, and D1a are also the same as those of product sheet No. 1, which is a comparative example. It is greater than or equal to the surface roughness Wa of B2a, C2a, and D2a.
- product plate No. Aspect ratios Str of A1a, B1a, C1a, and D1a are all the same as those of product sheet No. 1, which is a comparative example.
- the product plate No. 1 which is an example, was manufactured.
- the surface roughness Wa is equal to that of product sheet No. 1, which is a comparative example. It was demonstrated that the aspect ratio Str is high even though the surface roughness Wa of A2a, B2a, C2a, and D2a is greater than or equal to Wa, so that the anisotropy of the unevenness of the surface is small and the surface quality is excellent.
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Abstract
Description
C:0.030%~0.145%、
Si:0%~0.500%、
Mn:0.50%~2.50%、
P:0%~0.100%、
S:0%~0.020%、
Al:0%~1.000%、
N:0%~0.0100%、
B:0%~0.0050%、
Mo:0%~0.80%、
Ti:0%~0.200%、
Nb:0%~0.10%、
V:0%~0.20%、
Cr:0%~0.80%、
Ni:0%~0.25%
O:0%~0.0100%、
Cu:0%~1.00%、
W:0%~1.00%、
Sn:0%~1.00%、
Sb:0%~0.20%、
Ca:0%~0.0100%、
Mg:0%~0.0100%、
Zr:0%~0.0100%、
REM:0%~0.0100%、
残部が鉄および不純物であり、
金属組織が、体積分率が70~95%のフェライトと、体積分率が5~30%の硬質相とからなり、
板厚方向1/4位置におけるビッカース硬さH1/4の標準偏差を前記ビッカース硬さH1/4の平均値で除した値X1が0.025以下、
板厚方向1/2位置におけるビッカース硬さH1/2の標準偏差を前記ビッカース硬さH1/2の平均値で除した値X2が0.030以下、
である鋼板。
板厚方向1/2位置におけるビッカース硬さH1/2の平均値が155~305であることを特徴とする前記(1)~(4)のいずれか一項に記載の鋼板。
本発明者は、高強度の鋼板をプレス成形した後において、ゴーストラインの発生を抑制する方法について検討した。前述したように、DP(Dual Phase)鋼のような硬質相と軟質相が混在する鋼板では、成形時に主に軟質相周辺が変形し、鋼板表面に微小な凹凸が生じることで、ゴーストラインと呼ばれる外観不良が発生することがある。ゴーストラインは、鋼板のプレス成形時に軟質相が凹む一方で硬質相は凹まないかむしろ凸となるように盛り上がって変形することで、バンド状(縞状)に生じる。バンド状組織は、マルテンサイト等の硬質相に形成される。
C:0.030%~0.145%、
Si:0%~0.500%、
Mn:0.50%~2.50%、
P:0%~0.100%、
S:0%~0.020%、
Al:0%~1.000%、
N:0%~0.0100%、
B:0%~0.0050%、
Mo:0%~0.80%、
Ti:0%~0.200%、
Nb:0%~0.10%、
V:0%~0.20%、
Cr:0%~0.80%、
Ni:0%~0.25%
O:0%~0.0100%、
Cu:0%~1.00%、
W:0%~1.00%、
Sn:0%~1.00%、
Sb:0%~0.20%、
Ca:0%~0.0100%、
Mg:0%~0.0100%、
Zr:0%~0.0100%、
REM:0%~0.0100%、
残部が鉄および不純物である。以下、各元素について説明する。
Cは、鋼板の強度を高める元素である。所望の強度を得るために、C含有量は0.030%以上とする。強度をより高めるため、C含有量は、好ましくは0.035%以上であり、より好ましくは0.040%以上であり、さらに好ましくは0.050%以上であり、さらに好ましくは0.060%以上である。
また、C含有量を0.145%以下とすることで、凝固時のMnの拡散が助長され、これによりバンド状のMn偏析が生じやすくなることを抑制できる。その結果、鋼板のプレス成形後のゴーストラインの発生を抑制できる。そのため、C含有量は0.145%以下とする。C含有量は、0.110%以下が好ましく、0.090%以下がより好ましい。
Siは、鋼の脱酸元素であり、鋼板の延性を損なわずに強度を高めるのに有効な元素である。Si含有量を0.500%以下とすることで、スケール剥離性の低下による表面欠陥の発生を抑制できる。そのため、Si含有量は0.500%以下とする。Si含有量は0.450%以下が好ましく、0.250%以下がより好ましく、0.100%以下がさらに好ましい。
Si含有量の下限は0%を含むが、鋼板の強度-成形性バランスを向上するために、Si含有量は0.0005%以上または0.0010%以上としてもよく、より好ましくは0.090%超、さらに好ましくは0.100%以上である。
Mnは、鋼の焼入れ性を高めて、強度の向上に寄与する元素である。所望の強度を得るために、Mn含有量は0.50%以上とする。Mn含有量は、好ましくは1.20%以上、より好ましくは1.40%以上、さらに好ましくは1.60%超、さらに好ましくは1.65%以上である。
また、Mn含有量が2.50%以下であると、鋼の凝固時に縞状のMn偏析が生じることを抑制できる。そのため、Mn含有量は2.50%以下とする。Mn含有量は、2.25%以下が好ましく、2.00%以下がより好ましく、1.80%以下がさらに好ましい。
Pは、鋼を脆化する元素である。P含有量が0.100%以下であると、鋼板が脆化して生産工程において割れ易くなることを抑制できる。そのため、P含有量は0.100%以下とする。P含有量は0.080%以下が好ましく、0.050%以下がより好ましい。
P含有量の下限は0%を含むが、P含有量を0.001%以上とすることで、製造コストをより低減できる。そのため、P含有量は0.001%以上としてもよい。
Sは、Mn硫化物を形成し、鋼板の延性、穴拡げ性、伸びフランジ性および曲げ性などの成形性を劣化させる元素である。S含有量が0.020%以下であると、鋼板の成形性が著しく低下することを抑制できる。そのため、S含有量は0.020%以下とする。S含有量は0.010%以下が好ましく、0.008%以下がより好ましい。
S含有量の下限は0%を含むが、S含有量を0.0001%以上とすることで、製造コストをより低減できる。そのため、S含有量は0.0001%以上としてもよい。
Alは、脱酸材として機能する元素であり、鋼の強度を高めるのに有効な元素である。Al含有量を1.000%以下とすることで鋳造性を高くできるので生産性を高くできる。そのため、Al含有量は1.000%以下とする。Al含有量は0.650%以下が好ましく、0.600%以下がより好ましく、0.500%以下がさらに好ましい。
Al含有量の下限は0%を含むが、Alによる脱酸効果を十分に得るために、Al含有量は0.005%以上としてもよい。
Nは、窒化物を形成し、鋼板の延性、穴拡げ性、伸びフランジ性および曲げ性などの成形性を劣化させる元素である。N含有量が0.0100%以下であると、鋼板の成形性が低下することを抑制できる。そのため、N含有量は0.0100%以下とする。また、Nは、溶接時に溶接欠陥を発生させて生産性を阻害する元素でもある。そのため、N含有量は、好ましくは0.0080%以下であり、より好ましくは0.0070%以下であり、さらに好ましくは0.0040%以下である。
N含有量の下限は0%を含むが、N含有量を0.0005%以上とすることで、製造コストをより低減できる。そのため、N含有量は0.0005%以上としてもよい。
Bは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Bは必ずしも含有させなくてよいので、B含有量の下限は0%を含む。Bによる強度向上効果を十分に得るためには、B含有量は、0.0001%以上が好ましく、0.0005%以上がより好ましく、0.0010%以上がさらに好ましい。
また、B含有量が0.0050%以下であると、B析出物が生成して鋼板の強度が低下することを抑制できる。そのため、B含有量は0.0050%以下とし、好ましくは0.0030%以下である。B含有量は、0.0001%~0.0050%であってもよい。
Moは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Moは必ずしも含有させなくてよいので、Mo含有量の下限は0%を含む。Moによる強度向上効果を十分に得るためには、Mo含有量は、0.001%以上が好ましく、0.05%以上がより好ましく、0.10%以上がさらに好ましい。
また、Mo含有量が0.80%以下であると、熱間加工性が低下して生産性が低下することを抑制できる。そのため、Mo含有量は、0.80%以下とし、好ましくは0.40%以下であり、より好ましくは0.20%以下である。Mo含有量は、0.001%~0.80%であってもよいし、0%~0.40%であってもよい。
なお、CrおよびMoの両方を含み、その含有量をCr:0.20%~0.80%およびMo:0.05%~0.80%とすることで、鋼板の強度をより確実に向上することができるため、好ましい。
Tiは、破壊の起点として働く粗大な介在物を発生させるS量、N量およびO量を低減する効果を有する元素である。また、Tiは組織を微細化し、鋼板の強度-成形性バランスを高める効果がある。Tiは必ずしも含有させなくてよいので、Ti含有量の下限は0%を含む。上記効果を十分に得るためには、Ti含有量は0.001%以上とすることが好ましく、0.010%以上とすることがより好ましい。
また、Ti含有量が0.200%以下であると、粗大なTi硫化物、Ti窒化物およびTi酸化物の形成を抑制でき、鋼板の成形性を確保することができる。そのため、Ti含有量は0.200%以下とする。Ti含有量は0.080%以下とすることが好ましく、0.060%以下とすることがより好ましい。Ti含有量は、0%~0.100%であってもよいし、0.001%~0.200%であってもよい。
Nbは、析出物による強化、フェライト結晶粒の成長抑制による細粒化強化および再結晶の抑制による転位強化によって、鋼板の強度の向上に寄与する元素である。Nbは必ずしも含有させなくてよいので、Nb含有量の下限は0%を含む。上記効果を十分に得るためには、Nb含有量は0.001%以上とすることが好ましく、0.005%以上とすることがより好ましく、0.01%以上とすることがさらに好ましい。
また、Nb含有量が0.10%以下であると、再結晶を促進して未再結晶フェライトが残存することを抑制でき、鋼板の成形性を確保することができる。そのため、Nb含有量は0.10%以下とする。Nb含有量は好ましくは0.05%以下であり、より好ましくは0.04%以下である。Nb含有量は、0.001%~0.10%であってもよい。
Vは、析出物による強化、フェライト結晶粒の成長抑制による細粒化強化および再結晶の抑制による転位強化によって、鋼板の強度の向上に寄与する元素である。Vは必ずしも含有させなくてよいので、V含有量の下限は0%を含む。Vによる強度向上効果を十分に得るためには、V含有量は、0.001%以上が好ましく、0.01%以上がより好ましく、0.03%以上がさらに好ましい。
また、V含有量が0.20%以下であると、炭窒化物が多量に析出して鋼板の成形性が低下することを抑制できる。そのため、V含有量は、0.20%以下とする。V含有量は好ましくは0.10%以下である。V含有量は、0%~0.10%であってもよいし、0.001%~0.20%であってもよい。
Crは、鋼の焼入れ性を高め、鋼板の強度の向上に寄与する元素である。Crは必ずしも含有させなくてよいので、Cr含有量の下限は0%を含む。Crによる強度向上効果を十分に得るためには、Cr含有量は、0.001%以上が好ましく、0.20%以上がさらに好ましく、0.30%以上が特に好ましい。
また、Cr含有量が0.80%以下であると、破壊の起点となり得る粗大なCr炭化物が形成されることを抑制できる。そのため、Cr含有量は0.80%以下とする。Cr含有量は好ましくは0.70%以下であり、より好ましくは0.50%以下である。Cr含有量は、0%~0.70%であってもよいし、0.001%~0.80%であってもよい。
Niは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Niは必ずしも含有させなくてよいので、Ni含有量の下限は0%を含む。Niによる強度向上効果を十分に得るためには、Ni含有量は、0.001%以上が好ましく、0.05%以上がより好ましい。
また、Ni含有量が0.25%以下であると、鋼板の溶接性が低下することを抑制できる。そのため、Ni含有量は0.25%以下とする。Ni含有量は好ましくは0.20%以下であり、より好ましくは0.15%以下である。Ni含有量は、0.001%~0.20%であってもよい。
Oは、製造工程で混入する元素である。O含有量は0%であってもよい。なお、O含有量を0.0001%以上とすることで、精錬時間を短くして生産性を高くできる。したがって、O含有量は0.0001%以上、0.0005%以上又は0.0010%以上であってもよい。一方で、O含有量が0.0100%以下であると、粗大な酸化物の形成を抑えることができ、鋼板の延性、穴広げ性、伸びフランジ性及び/又は曲げ性などの成形性を高くできる。したがって、O含有量は0.0100%以下とする。O含有量は0.0070%以下、0.0040%以下又は0.0020%以下であってもよい。
Cuは、微細な粒子の形態で鋼中に存在し、鋼板の強度の向上に寄与する元素である。Cu含有量は0%であってもよいが、このような効果を得るためには、Cu含有量は0.001%以上であることが好ましい。Cu含有量は0.01%以上、0.03%以上又は0.05%以上であってもよい。一方で、Cu含有量を1.00%以下とすることで、鋼板の溶接性を良好にできる。したがって、Cu含有量は1.00%以下とする。Cu含有量は0.60%以下、0.40%以下又は0.20%以下であってもよい。
Wは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。W含有量は0%であってもよいが、このような効果を得るためには、W含有量は0.001%以上であることが好ましい。W含有量は0.01%以上、0.02%以上又は0.10%以上であってもよい。一方で、W含有量を1.00以下にすることで、熱間加工性を高くして生産性を高くできる。したがって、W含有量は1.00%以下とする。W含有量は0.80%以下、0.50%以下又は0.20%以下であってもよい。
Snは、結晶粒の粗大化を抑制し、鋼板の強度の向上に寄与する元素である。Sn含有量は0%であってもよいが、このような効果を得るためには、Sn含有量は0.001%以上であることが好ましい。Sn含有量は0.01%以上、0.05%以上又は0.08%以上であってもよい。一方で、Sn含有量を1.00%以下にすることで、鋼板の脆化を抑制できる。したがって、Sn含有量は1.00%以下とする。Sn含有量は0.80%以下、0.50%以下又は0.20%以下であってもよい。
Sbは、結晶粒の粗大化を抑制し、鋼板の強度の向上に寄与する元素である。Sb含有量は0%であってもよいが、このような効果を得るためには、Sb含有量は0.001%以上であることが好ましい。Sb含有量は0.01%以上、0.05%以上又は0.08%以上であってもよい。一方で、Sn含有量を0.20%以下にすることで、鋼板の脆化を抑制できる。したがって、Sb含有量は0.20%以下とする。Sb含有量は0.18%以下、0.15%以下又は0.12%以下であってもよい。
(Mg:0%~0.0100%)
(Zr:0%~0.0100%)
(REM:0%~0.0100%)
Ca、Mg、Zr及びREMは、鋼板の成形性の向上に寄与する元素である。Ca、Mg、Zr及びREM含有量は0%であってもよいが、このような効果を得るためには、Ca、Mg、Zr及びREM含有量はそれぞれ0.0001%以上であることが好ましく、0.0005%以上、0.0010%以上又は0.0015%以上であってもよい。一方で、Ca、Mg、Zr及びREMのそれぞれについて、含有量を0.0100%以下とすることで、鋼板の延性を確保できる。したがって、Ca、Mg、Zr及びREM含有量はそれぞれ0.0100%以下とし、0.0080%以下、0.0060%以下又は0.0030%以下であってもよい。本明細書におけるREMとは、原子番号21番のスカンジウム(Sc)、原子番号39番のイットリウム(Y)及びランタノイドである原子番号57番のランタン(La)~原子番号71番のルテチウム(Lu)の17元素の総称であり、REM含有量はこれら元素の合計含有量である。
金属組織における硬質相の体積分率を5%以上とすることで、鋼板の強度を十分に向上できる。そのため、硬質相の体積分率を5%以上とする。一方、硬質相の体積分率を30%以下とすることで、硬質相をより均一に分散させることができるので、成形時の表面凹凸を少なくでき、成形後の外観を向上できる。
また、金属組織における硬質相以外の残部はフェライトであり、該フェライトの体積分率は70~95%となる。なお、フェライトの体積分率は、72%以上が好ましく、75%以上がより好ましい。また、硬質相の体積分率は、28%以下が好ましく、25%以下がより好ましい。金属組織におけるフェライトと硬質相の体積分率の合計は、100%である。
得られた鋼板の板幅WのW/4位置もしくは3W/4位置(すなわち、鋼板のいずれかの幅方向端部から幅方向にW/4の位置)から金属組織(ミクロ組織)観察用の試料(サイズは、おおむね、圧延方向に20mm×幅方向に20mm×鋼板の厚さ)を採取し、光学顕微鏡を用いて表面から板厚1/2厚における金属組織(ミクロ組織)の観察を行い、鋼板の表面(めっきが存在する場合はめっき層を除いた表面)から板厚1/2厚までの硬質相の面積分率を算出する。試料の調整として、圧延直角方向の板厚断面を観察面として研磨し、レペラー試薬にてエッチングする。
本発明者は、鋼板のビッカース硬さ分布に偏りが大きいと、硬質相がバンド状に連結し易く、その結果、鋼板をプレス成形した成形品にゴーストラインが生じ易い傾向にあることを知見した。特に、鋼板の表面に比較的近い領域におけるビッカース硬さ分布の偏りに着目した。そして、鋼板の圧延方向において、ビッカース硬さ分布の偏りが小さい箇所では、ゴーストラインが途中で途切れたように形成され、ゴーストラインが長尺であることに起因する外観不良を抑制できることを発見した。結果、板厚方向1/4位置におけるビッカース硬さH1/4の標準偏差σ1/4をビッカース硬さH1/4の平均値HAVE1/4で除した値X1を0.025以下とすることが、鋼板およびこの鋼板をプレス成形した成形品の表面の面品質を高くするのに有効であることを発見した。
前述したように、値X1が0.025以下であることにより、鋼板をプレス成形した成形品におけるゴーストラインの発生を抑制できる。本発明者は、さらに、鋼板の表面から深い領域でのビッカース硬さ分布の偏りにも着目した。結果、板厚方向1/2位置におけるビッカース硬さH1/2の標準偏差σ1/2をビッカース硬さH1/2の平均値HAVE1/2で除した値X2を0.030以下とすることが、鋼板およびこの鋼板をプレス成形した成形品の表面の面品質をより一層高くするのに有効であることを発見した。
フェライトの平均結晶粒径が30.0μm以下であることで、成形後の外観の低下を抑制できる。そのため、フェライトの平均結晶粒径は、好ましくは30.0μm以下とすることが好ましい。より好ましくは15.0μm以下とする。
一方、フェライトの平均結晶粒径が5.0μm以上であることで、フェライトの{001}方位を持つ粒子が凝集して生成されることを抑制できる。フェライトの{001}方位を持つ個々の粒子が小さくても、これらの粒子が凝集して生成すると、凝集した部分に変形が集中するため、これらの粒子の凝集を抑制することで成形後の外観の低下を抑制できる。そのため、フェライトの好ましい平均結晶粒径を5.0μm以上とすることが好ましい。より好ましくは8.0μm以上、さらに好ましくは10.0μm以上、さらにより好ましくは15.0μm以上である。
硬質相の平均結晶粒径が5.0μm以下であることで、成形後の外観の低下を抑制できる。そのため、鋼板における硬質相の好ましい平均結晶粒径は、5.0μm以下とすることが好ましい。より好ましくは4.5μm以下、さらに好ましくは4.0μm以下とする。
一方、硬質相の平均結晶粒径が、1.0μm以上であることで、硬質相の粒子が凝集して生成されることを抑制できる。硬質相の個々の粒子を小さくし且つこれらの粒子の凝集を抑制することで成形後の外観の低下を抑制できる。そのため、鋼板における硬質相の好ましい平均結晶粒径を1.0μm以上とすることが好ましい。より好ましくは1.5μm以上であり、さらに好ましくは2.0μm以上である。
圧延方向に100μm以上連結した硬質相の面積が全硬質相の面積に対し30%以下であることで、鋼板をプレス成形したときに硬質相の盛り上がり変形と当該硬質相の周囲の軟質相の凹み変形とが圧延方向に長く連続することが抑制され、視認し易いゴーストラインの発生を抑制できる。よって、本実施形態では、板厚方向1/4~1/2の領域において、圧延方向に100μm以上連結した硬質相の面積が全硬質相の面積に対し30%以下とすることが好ましい。この割合が20%以下であることがより好ましい。この割合の下限はゼロ%である。
引張試験により5%ひずみを付与した後の試験片(以下、「引張後試験片」と称す)における表面性状のアスペクト比Strは、鋼板を成形(例えばプレス成形)して得られる成形品の表面の凹凸の異方性を示す指標である。なお、アスペクト比StrはISO(国際標準化機構)25178に規定されており、ゼロ~1の間の数値である。アスペクト比Strがゼロに近いほど、異方性が大であり、観察範囲の表面に筋目が存在することとなる。一方、アスペクト比Strが1に近いほど、観察範囲の表面形状が特定の方向に依存しないことを示す。
板厚方向1/4位置におけるビッカース硬さH1/4の平均値HAVE1/4が150以上であることにより、鋼板の引張強さ540MPa以上を確保できる。また、板厚方向1/4位置におけるビッカース硬さH1/4の平均値HAVE1/4が300以下であることにより、鋼板の板厚方向1/4位置において鋼板が過度に硬くならずに済み、鋼板の圧延時において表面の凹凸を均す効果が十分に発揮される。
板厚方向1/2位置におけるビッカース硬さH1/2の平均値HAVE1/2が155以上であることにより、鋼板の引張強さ540MPa以上を確保できる。また、板厚方向1/2位置におけるビッカース硬さH1/2の平均値HAVE1/2が305以下であることにより、鋼板の板厚方向1/2位置において鋼板が過度に硬くならずに済み、鋼板の圧延時において表面の凹凸を均す効果が十分に発揮される。
本実施形態の鋼板の成形品は、自動車パネルとして好適である。自動車パネルとして、ドアアウタ等のパネル系部品が挙げられる。パネル系部品として、フードのアウターパネル、フェンダーパネル等のクオーターパネル、ドアアウターパネル、ルーフパネル等を例示できる。
このような自動車パネルにおいても、自動車構造部材と同様に高強度化が進められており、自動車パネルとなる鋼板の製造途中の熱延板の強度も増加している。さらに、自動車パネルの薄肉化に伴い、鋼板製造途中の冷間圧延工程における圧下率も増加している。そして、自動車パネル鋼板、特に、ドアパネル用鋼板は、幅が1000mmを超えるものがあり、フードパネル用鋼板は、幅が1500mmを超えるものがある。このような幅広の鋼板は、冷間圧延工程における圧下負荷(圧延機の負荷)が大きくなる傾向にある。例えば、引張強さ540MPa級の鋼板では、幅が1500mm程度以上になると冷延時の圧下負荷が特に大きくなり、引張強さ780MPa級の鋼板では、幅が1200mm程度以上になると冷延時の圧下負荷が特に大きくなる。
このような冷間圧延時における圧下負荷の増大に対処しなければ、鋼板形状の精度が悪化する。また、このような冷間圧延時における圧下負荷の増大に対処する方法として、従来、冷間圧延前に軟質化焼鈍を行うことや、冷間圧延工程を2回に分けて行うこと等の対策を行っており、生産性が低く製造コストが増加していた。
一方で、本実施形態では、(i)本実施形態の化学組成および金属組織を有し、(ii)板厚方向1/4位置におけるビッカース硬さH1/4の標準偏差σ1/4をビッカース硬さH1/4の平均値HAVE1/4で除した値X1が0.025以下であり、且つ、(iii)板厚方向1/2位置におけるビッカース硬さH1/2の標準偏差σ1/2をビッカース硬さH1/2の平均値HAVE1/2で除した値X2が0.030以下である鋼板としている。これにより、前記のような幅広のパネルであっても、(a)熱延板組織をより軟質にすることで冷間圧延時の圧延負荷を低減しつつ、(b)成形品のゴーストライン低減を実現できる。
本実施形態に係る鋼板の板厚は、特定の範囲に限定されないが、汎用性や製造性を考慮すると、0.20~1.00mmが好ましい。板厚を0.20mm以上とすることで、成形品形状を平坦に維持することが容易になり、寸法精度および形状精度を向上することができる。そのため、板厚は0.20mm以上が好ましく、0.35mm以上が好ましく、より好ましくは0.40mm以上である。
一方、板厚が1.00mm以下とすることで部材の軽量化効果が大きくなる。そのため、板厚は1.00mm以下が好ましく、0.70mm以下が好ましく、より好ましくは0.60mm以下である。鋼板の板厚は、マイクロメータで測定できる。
本実施形態に係る鋼板の引張強さは、特定の範囲に限定されないが、540~980MPaであることが好ましい。鋼板の引張強さが540MPa以上であることにより、薄肉且つ高強度の鋼板を実現できる。また、鋼板の引張強さが980MPa以下であることにより、鋼板をプレス加工する際の成形性を確保し易い。
引張強さは、圧延方向に直角な方向を長手方向とするJIS5号引張試験片を鋼板から採取し、JIS(日本工業規格)Z2241:2011 金属材料引張試験方法に則った試験を行うことで測定される。
亜鉛めっき層が溶融亜鉛めっき層の場合、鋼板表面と亜鉛めっき層との密着性を高めるため、溶融亜鉛めっき層のFe含有量は3.0質量%以下が好ましい。
亜鉛めっき層が電気亜鉛めっき層の場合、電気亜鉛めっき層のFe含有量は、耐食性の向上の点で、0.5質量%以下が好ましい。
次に、上述した鋼板をプレス成形することで製造できるプレス成形品について説明する。このプレス成形品は、上述した鋼板と同じ化学組成を有する。また、上記プレス成形品は、少なくとも一方の表面に上述しためっき層を備えていてもよい。上記プレス成形品は、上述した鋼板をプレス成形して得られるものであるため、ゴーストラインの発生が抑制されており、外観品質に優れる。その結果、直接消費者の目に触れる外観が優れていることで商品性の高い自動車を実現できる。プレス成形品の具体例としては例えば、上述したように、自動車車体のドアアウタ等のパネル系部品(自動車外板パネル)が挙げられる。パネル系部品として、フードのアウターパネル、フェンダーパネル等のクオーターパネル、ドアアウターパネル、ルーフパネル等を例示できる。
次に、本実施形態に係る鋼板の好ましい製造方法について説明する。本実施形態に係る鋼板は、製造方法に関わらず上記の特徴を有していればその効果が得られる。しかしながら、以下の方法によれば安定して製造できるので好ましい。
(i)上記の化学組成を有する溶鋼を凝固させてスラブを成形するスラブ成形工程、
(ii)スラブを加熱し、圧延終了温度が950℃以下となるように熱間圧延して熱延鋼板を得た後、450~650℃で巻き取る熱間圧延工程、
(iii)巻き取った熱延鋼板を巻き戻して、累積圧下率であるRCRが50~90%である冷間圧延を行って冷延鋼板を得る冷間圧延工程、
(iv)冷延鋼板を焼鈍し、その後必要に応じて上述しためっき層を形成する工程、
以下、各工程について説明する。
スラブ成形工程では、所定の化学組成を有する溶鋼を、スラブに成形する。スラブ成形工程の製法については限定されない。例えば、転炉又は電気炉等を用いて上記化学組成の溶鋼を溶製し、連続鋳造法により製造したスラブを用いることができる。連続鋳造法に代えて、造塊法、薄スラブ鋳造法等を採用してもよい。
スラブを、熱間圧延に先立って、1100℃以上に加熱する。加熱温度を1100℃以上とすることで、続く熱間圧延において圧延反力が過度に大きくならず、目的とする製品厚を得やすい。また、板形状の精度を高くでき、巻き取りをスムーズに行うことができる。
加熱温度の上限については限定する必要はないが、経済上の観点から、鋼片加熱温度は1300℃未満とすることが好ましい。
仕上げ圧延は、複数の連続する圧延スタンドで行われ、後半の圧延スタンドでの圧下率を前半の圧延スタンドでの圧下率より大きくする。前半の仕上げ圧延の圧下率を35%未満とするとともに、後半の仕上げ圧延の圧下率を35%以上とする。これにより、後半の仕上げ圧延の圧下率を高くでき、その結果、熱延加工された板としての熱延板を適度に軟質化できる。よって、冷間圧延工程時における圧延機の負荷を低減できる。さらに、熱延板の組織においてパーライトやマルテンサイト等の硬質相がバンド状に生成するのを抑制でき、最終製品である成形品の組織においても、マルテンサイト等の硬質相がバンド状に生成するのを抑制できる。
前半の圧延スタンドでの圧下率P1と後半の圧延スタンドでの圧下率P2の比P2/P1は、1.0超1.6以下であることが好ましい。P2/P1を1.0超えとすることで、熱延板を十分に軟質化でき、かつ最終製品である成形品の組織において硬質相がバンド状に生成するのを抑制できる。また、P2/P1を1.6以下とすることで、後半の圧延スタンドへの負荷を軽減できる。
最終の圧延スタンドでの圧下率は、40%以上とすることが好ましい。これにより、熱延板の組織においてパーライトやマルテンサイト等の硬質相がバンド状に生成するのをより容易に抑制でき、最終製品である成形品の組織においても、マルテンサイト等の硬質相がバンド状に生成するのを、より容易に抑制できる。
冷間圧延工程では、累積圧下率であるRCRが50~90%である冷間圧延を行って冷延鋼板を得る。所定の残留応力が付与された熱延鋼板を上記の累積圧下率で冷間圧延することで、焼鈍、冷却後に、所望の集合組織を有するフェライトが得られる。
焼鈍工程では、750~900℃の均熱温度まで冷延鋼板を加熱して保持する焼鈍を行う。均熱温度が750℃以上であることにより、フェライトの再結晶およびフェライトからオーステナイトへの逆変態が十分に進行し、所望の集合組織を得ることができる。一方、均熱温度が900℃以下であることにより、結晶粒が緻密化し、十分な強度を得られる。さらに、加熱温度が過度に高くなく、生産性を高くできる。
冷却工程では、焼鈍工程での均熱後の冷延鋼板を冷却する。冷却に際しては、均熱温度からの平均冷却速度が5.0~50℃/秒となるように冷却する。上記平均冷却速度が5.0℃/秒以上であることにより、フェライト変態が過剰に促進されずに済み、マルテンサイト等の硬質相の生成量を多くして、所望の強度を得ることができる。また、平均冷却速度が50℃/秒以下であることにより、鋼板の幅方向において鋼板をより均一に冷却できる。
上記の方法で得られた冷延鋼板に、さらに、表面にめっき層を形成するめっき工程を行ってもよい。
前記めっき工程で形成されためっき層に対し合金化を行ってもよい。合金化工程では、合金化温度は、例えば450~600℃である。
Claims (8)
- 化学組成が質量%で、
C:0.030%~0.145%、
Si:0%~0.500%、
Mn:0.50%~2.50%、
P:0%~0.100%、
S:0%~0.020%、
Al:0%~1.000%、
N:0%~0.0100%、
B:0%~0.0050%、
Mo:0%~0.80%、
Ti:0%~0.200%、
Nb:0%~0.10%、
V:0%~0.20%、
Cr:0%~0.80%、
Ni:0%~0.25%
O:0%~0.0100%、
Cu:0%~1.00%、
W:0%~1.00%、
Sn:0%~1.00%、
Sb:0%~0.20%、
Ca:0%~0.0100%、
Mg:0%~0.0100%、
Zr:0%~0.0100%、
REM:0%~0.0100%、
残部が鉄および不純物であり、
金属組織が、体積分率が70~95%のフェライトと、体積分率が5~30%の硬質相とからなり、
板厚方向1/4位置におけるビッカース硬さH1/4の標準偏差を前記ビッカース硬さH1/4の平均値で除した値X1が0.025以下、
板厚方向1/2位置におけるビッカース硬さH1/2の標準偏差を前記ビッカース硬さH1/2の平均値で除した値X2が0.030以下、
である鋼板。 - 前記フェライトの平均結晶粒径が5.0~30.0μm、前記硬質相の平均結晶粒径が、1.0~5.0μmであることを特徴とする請求項1に記載の鋼板。
- 板厚方向1/4~1/2の領域において、圧延方向に100μm以上連結した硬質相の面積が全硬質相の面積に対し30%以下、であることを特徴とする請求項1または2に記載の鋼板。
- 引張試験により5%ひずみを付与した後の試験片における表面性状のアスペクト比Str(ISO25178)が0.28以上であ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鋼板。
- 板厚方向1/4位置におけるビッカース硬さH1/4の平均値が150~300、
板厚方向1/2位置におけるビッカース硬さH1/2の平均値が155~305であることを特徴とする請求項1~4のいずれか一項に記載の鋼板。 - 前記硬質相が、マルテンサイト、ベイナイト、焼き戻しマルテンサイト、およびパーライトのいずれか1種以上からなることを特徴とする請求項1~5のいずれか一項に記載の鋼板。
- 前記鋼板の板厚が0.20mm~1.00mmであることを特徴とする、請求項1~6の何れか一項に記載の鋼板。
- 前記鋼板が自動車外板パネルであることを特徴とする、請求項1~7の何れか一項に記載の鋼板。
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WO2017168957A1 (ja) * | 2016-03-31 | 2017-10-05 | Jfeスチール株式会社 | 薄鋼板およびめっき鋼板、並びに、熱延鋼板の製造方法、冷延フルハード鋼板の製造方法、薄鋼板の製造方法およびめっき鋼板の製造方法 |
WO2018151331A1 (ja) * | 2017-02-20 | 2018-08-23 | 新日鐵住金株式会社 | 高強度鋼板 |
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WO2017168957A1 (ja) * | 2016-03-31 | 2017-10-05 | Jfeスチール株式会社 | 薄鋼板およびめっき鋼板、並びに、熱延鋼板の製造方法、冷延フルハード鋼板の製造方法、薄鋼板の製造方法およびめっき鋼板の製造方法 |
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