WO2021182389A1 - 熱延鋼板 - Google Patents
熱延鋼板 Download PDFInfo
- Publication number
- WO2021182389A1 WO2021182389A1 PCT/JP2021/008970 JP2021008970W WO2021182389A1 WO 2021182389 A1 WO2021182389 A1 WO 2021182389A1 JP 2021008970 W JP2021008970 W JP 2021008970W WO 2021182389 A1 WO2021182389 A1 WO 2021182389A1
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- WIPO (PCT)
- Prior art keywords
- less
- hot
- steel sheet
- rolled steel
- retained austenite
- Prior art date
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 128
- 239000010959 steel Substances 0.000 title claims abstract description 128
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 106
- 230000000717 retained effect Effects 0.000 claims abstract description 95
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 64
- 239000013078 crystal Substances 0.000 claims abstract description 53
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 51
- 239000000126 substance Substances 0.000 claims abstract description 19
- 239000000203 mixture Substances 0.000 claims abstract description 16
- 229910052751 metal Inorganic materials 0.000 claims description 26
- 239000002184 metal Substances 0.000 claims description 26
- 229910000859 α-Fe Inorganic materials 0.000 claims description 17
- 229910001562 pearlite Inorganic materials 0.000 claims description 12
- 239000012535 impurity Substances 0.000 claims description 8
- 229910052725 zinc Inorganic materials 0.000 claims description 7
- 229910052721 tungsten Inorganic materials 0.000 claims description 5
- 229910052726 zirconium Inorganic materials 0.000 claims description 5
- 238000005496 tempering Methods 0.000 claims description 4
- 229910052797 bismuth Inorganic materials 0.000 claims description 3
- 230000000694 effects Effects 0.000 description 34
- 238000001816 cooling Methods 0.000 description 27
- 230000009471 action Effects 0.000 description 22
- 238000005098 hot rolling Methods 0.000 description 22
- 238000004804 winding Methods 0.000 description 20
- 238000005096 rolling process Methods 0.000 description 15
- 238000000034 method Methods 0.000 description 14
- 238000007747 plating Methods 0.000 description 14
- 238000012360 testing method Methods 0.000 description 12
- 238000001887 electron backscatter diffraction Methods 0.000 description 11
- 230000001965 increasing effect Effects 0.000 description 11
- 238000005259 measurement Methods 0.000 description 10
- 229910052761 rare earth metal Inorganic materials 0.000 description 9
- 238000004458 analytical method Methods 0.000 description 8
- 238000004519 manufacturing process Methods 0.000 description 8
- 239000010410 layer Substances 0.000 description 7
- 150000001247 metal acetylides Chemical class 0.000 description 7
- 230000002708 enhancing effect Effects 0.000 description 6
- 230000009466 transformation Effects 0.000 description 6
- 239000011701 zinc Substances 0.000 description 6
- 238000005452 bending Methods 0.000 description 5
- 238000005498 polishing Methods 0.000 description 5
- 239000006104 solid solution Substances 0.000 description 5
- 238000009864 tensile test Methods 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 4
- 229910001567 cementite Inorganic materials 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 4
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 4
- 230000003287 optical effect Effects 0.000 description 4
- 239000002245 particle Substances 0.000 description 4
- 238000001556 precipitation Methods 0.000 description 4
- 238000007670 refining Methods 0.000 description 4
- 229920006395 saturated elastomer Polymers 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 3
- 229910052804 chromium Inorganic materials 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 238000005336 cracking Methods 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 3
- 239000007788 liquid Substances 0.000 description 3
- 229910052750 molybdenum Inorganic materials 0.000 description 3
- 229910052759 nickel Inorganic materials 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 230000001376 precipitating effect Effects 0.000 description 3
- 239000000047 product Substances 0.000 description 3
- 229910052720 vanadium Inorganic materials 0.000 description 3
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 230000001133 acceleration Effects 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 229910052782 aluminium Inorganic materials 0.000 description 2
- 229910052796 boron Inorganic materials 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 229910003460 diamond Inorganic materials 0.000 description 2
- 239000010432 diamond Substances 0.000 description 2
- 238000002050 diffraction method Methods 0.000 description 2
- 238000010894 electron beam technology Methods 0.000 description 2
- 238000009713 electroplating Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- 229910052747 lanthanoid Inorganic materials 0.000 description 2
- 150000002602 lanthanoids Chemical class 0.000 description 2
- 239000000843 powder Substances 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 238000005070 sampling Methods 0.000 description 2
- 229910052710 silicon Inorganic materials 0.000 description 2
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 description 2
- 229910010271 silicon carbide Inorganic materials 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 229910052717 sulfur Inorganic materials 0.000 description 2
- 239000002344 surface layer Substances 0.000 description 2
- 229910052719 titanium Inorganic materials 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 229910018134 Al-Mg Inorganic materials 0.000 description 1
- 229910021365 Al-Mg-Si alloy Inorganic materials 0.000 description 1
- 229910018467 Al—Mg Inorganic materials 0.000 description 1
- 238000012935 Averaging Methods 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 229910001122 Mischmetal Inorganic materials 0.000 description 1
- 229910000990 Ni alloy Inorganic materials 0.000 description 1
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 229910007570 Zn-Al Inorganic materials 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 230000005465 channeling Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 239000000706 filtrate Substances 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 238000010191 image analysis Methods 0.000 description 1
- 239000012770 industrial material Substances 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 239000003595 mist Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 230000003746 surface roughness Effects 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 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
- C22C38/02—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
- C22C38/18—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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
-
- 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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- 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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- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
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- 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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- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- 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
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—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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- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—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 a hot-rolled steel sheet.
- the present application claims priority based on Japanese Patent Application No. 2020-041811 filed in Japan on March 11, 2020, the contents of which are incorporated herein by reference.
- Patent Document 1 discloses a hot-rolled steel sheet having excellent local deformability and excellent ductility with little orientation dependence of formability, and a method for producing the same. The present inventors have found that the hot-rolled steel sheet described in Patent Document 1 has a large amount of ferrite and therefore has excellent ductility, but the local ductility may be insufficient.
- An object of the present invention is to provide a hot-rolled steel sheet having excellent strength, ductility and local ductility. Further, more preferably, it is an object of the present invention to provide a hot-rolled steel sheet having the above-mentioned various properties and further having excellent local bendability.
- the gist of the present invention made based on the above findings is as follows.
- (1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition of mass%. C: 0.100 to 0.350%, Si: 1.00 to 3.00%, Mn: 1.00 to 4.00%, sol.
- the rest consists of Fe and impurities
- the metal structure is% of the area, Bainite: 40-92%, Tempering martensite: 5-40%, Residual austenite: 3-20%, Ferrite: 5% or less, Fresh martensite: 5% or less, and pearlite: 5% or less,
- the number% of the crystal grains of the bainite in contact with both the tempered martensite and the retained austenite is 80% or more of the total crystal grains of the bainite.
- the C concentration in the retained austenite is 0.80% by mass or more,
- the average crystal grain size of the retained austenite is 0.70 ⁇ m or less.
- the standard deviation of Vickers hardness is 25 HV 0.01 or less.
- the hot-rolled steel sheet according to (1) above may have a surface roughness Rz of 15.0 ⁇ m or less.
- (3) The hot-rolled steel sheet according to (1) or (2) above has a chemical composition of% by mass. Ti: 0.005 to 0.300%, Nb: 0.005 to 0.100%, V: 0.005 to 0.500%, Cu: 0.01-2.00%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, Ni: 0.02-2.00%, B: 0.0001 to 0.0100%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020% It may contain one or more selected from the group consisting of.
- the hot-rolled steel sheet according to this embodiment has a mass% of C: 0.100 to 0.350%, Si: 1.00 to 3.00%, Mn: 1.00 to 4.00%, sol. .. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities including.
- C 0.100 to 0.350% C is an element required to obtain the desired strength. If the C content is less than 0.100%, it becomes difficult to obtain the desired strength. Therefore, the C content is set to 0.100% or more.
- the C content is preferably 0.120% or more and 0.150% or more.
- the C content exceeds 0.350%, the transformation rate becomes slow and MA (mixed phase of martensite and retained austenite) is likely to be formed. As a result, it is not possible to obtain a structure having uniform strength, and it becomes difficult to obtain excellent local ductility. Therefore, the C content is set to 0.350% or less.
- the C content is preferably 0.330% or less and 0.310% or less.
- Si 1.00 to 3.00% Si has the effect of delaying the precipitation of cementite. By this action, the amount of austenite remaining untransformed, that is, the surface integral of retained austenite can be increased. Further, by the above action, it is possible to maintain a large amount of solid solution C in the hard phase and prevent coarsening of cementite, and as a result, the strength of the steel sheet can be increased. In addition, Si itself has the effect of increasing the strength of the hot-rolled steel sheet by strengthening the solid solution. Further, Si has an action of making the steel sound by deoxidation (suppressing the occurrence of defects such as blow holes in the steel). If the Si content is less than 1.00%, the effect of the above action cannot be obtained.
- the Si content is set to 1.00% or more.
- the Si content is preferably 1.20% or more and 1.50% or more.
- the Si content exceeds 3.00%, the precipitation of cementite is significantly delayed and the area ratio of retained austenite is excessively increased, which is not preferable.
- the Si content is 3.00% greater than the surface texture and chemical conversion of the hot-rolled steel sheet, more with ductility and weldability is significantly degraded, A 3 transformation point increases significantly. This may make it difficult to perform hot rolling in a stable manner. Therefore, the Si content is set to 3.00% or less.
- the Si content is preferably 2.70% or less and 2.50% or less.
- Mn 1.00 to 4.00% Mn has the effect of suppressing the ferrite transformation and increasing the strength of the hot-rolled steel sheet. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more and 1.80% or more. On the other hand, if the Mn content exceeds 4.00%, the local ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less and 3.50% or less.
- sol. Al 0.001 to 2.000% sol.
- Al has the effect of deoxidizing the steel to make the steel sheet sound and suppressing the precipitation of cementite from austenite to promote the formation of retained austenite.
- sol. If the Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, sol.
- the Al content is 0.001% or more.
- sol. The Al content is preferably 0.010% or more.
- sol. If the Al content exceeds 2.000%, the above effects are saturated and economically unfavorable.
- sol. The Al content is 2.000% greater, A 3 transformation point is greatly increased, stable becomes difficult to perform hot rolling. Therefore, sol.
- the Al content is 2.000% or less. sol.
- the Al content is preferably 1.500% or less and 1.300% or less.
- sol. Al means acid-soluble Al, and indicates solid solution Al existing in steel in a solid solution state.
- P 0.100% or less
- P is an element generally contained as an impurity, but has an effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P may be positively contained.
- P is also an element that easily segregates.
- the P content is preferably 0.030% or less.
- the lower limit of the P content does not need to be specified, but it is preferably 0.001% or more from the viewpoint of refining cost.
- S 0.0300% or less
- S is an element contained as an impurity and forms sulfide-based inclusions in the steel to reduce the ductility of the hot-rolled steel sheet.
- the S content exceeds 0.0300%, the ductility of the hot-rolled steel sheet is significantly reduced. Therefore, the S content is 0.0300% or less.
- the S content is preferably 0.0050% or less.
- the lower limit of the S content does not need to be specified, but it is preferably 0.0001% or more from the viewpoint of refining cost.
- N 0.1000% or less
- N is an element contained in steel as an impurity and has an effect of reducing the ductility of the hot-rolled steel sheet. If the N content exceeds 0.1000%, the ductility of the hot-rolled steel sheet is significantly reduced. Therefore, the N content is set to 0.1000% or less.
- the N content is preferably 0.0800% or less and 0.0700% or less.
- the lower limit of the N content does not need to be specified in particular, but as will be described later, when one or more of Ti, Nb and V are contained to refine the metal structure, the precipitation of carbonitride
- the N content is preferably 0.0010% or more, and more preferably 0.0020% or more in order to promote the above.
- O 0.0100% or less
- O forms a coarse oxide that is a starting point of fracture when it is contained in a large amount in steel, and causes brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less.
- the O content is preferably 0.0080% or less and 0.0050% or less.
- the O content may be 0.0005% or more and 0.0010% or more in order to disperse a large number of fine oxides when the molten steel is deoxidized.
- the rest of the chemical composition of the hot-rolled steel sheet according to this embodiment consists of Fe and impurities.
- the impurities are elements mixed from ore, scrap, manufacturing environment, etc. as raw materials, or elements intentionally added in a small amount, which adversely affect the hot-rolled steel sheet according to the present embodiment. Means something that is acceptable to the extent that it does not exist.
- the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in addition to the above elements.
- the lower limit of the content is 0%.
- Ti 0.005 to 0.300%, Nb: 0.005 to 0.100% and V: 0.005 to 0.500% Since Ti, Nb and V all precipitate as carbides or nitrides in steel and have an action of refining the metal structure by a pinning effect, one or more of these elements are contained. May be good. In order to obtain the effect of the above action more reliably, the Ti content should be 0.005% or more, the Nb content should be 0.005% or more, or the V content should be 0.005% or more. It is preferable to do so. However, even if these elements are excessively contained, the effect of the above action is saturated and it is economically unfavorable. Therefore, the Ti content is 0.300% or less, the Nb content is 0.100% or less, and the V content is 0.500% or less.
- Cu 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00% and B: 0.0001 to 0.0100%
- Cr, Mo, Ni and B all have the effect of enhancing the hardenability of the steel sheet.
- Cr and Ni have an action of stabilizing retained austenite
- Cu and Mo have an action of precipitating carbides in the steel to increase the strength of the hot-rolled steel sheet.
- Ni has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu when Cu is contained. Therefore, one or more of these elements may be contained.
- the Cu has the effect of enhancing the hardenability of the steel sheet and the effect of precipitating it as carbide in the steel at low temperature to increase the strength of the hot-rolled steel sheet.
- the Cu content is preferably 0.01% or more, and more preferably 0.05% or more.
- the Cu content is set to 2.00% or less.
- the Cu content is preferably 1.50% or less and 1.00% or less.
- the Cr content is preferably 0.01% or more and 0.05% or more.
- the Cr content is set to 2.00% or less.
- Mo has an action of enhancing the hardenability of the steel sheet and an action of precipitating carbides in the steel to increase the strength.
- the Mo content is preferably 0.01% or more and 0.02% or more.
- the Mo content is set to 1.00% or less.
- the Mo content is preferably 0.50% or less and 0.20% or less.
- Ni has the effect of enhancing the hardenability of the steel sheet. Further, when Ni contains Cu, it has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to obtain the effect of the above action more reliably, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain it in a large amount. Therefore, the Ni content is set to 2.00% or less.
- B has an effect of enhancing the hardenability of the steel sheet.
- the B content is preferably 0.0001% or more and 0.0002% or more.
- the B content is set to 0.0100% or less.
- the B content is preferably 0.0050% or less.
- Ca, Mg and REM all have an effect of improving the formability of the hot-rolled steel sheet by adjusting the shape of the inclusions to a preferable shape.
- Bi has an effect of improving the formability of the hot-rolled steel sheet by refining the solidified structure. Therefore, one or more of these elements may be contained. In order to obtain the effect of the above action more reliably, it is preferable that any one or more of Ca, Mg, REM and Bi is 0.0005% or more.
- the Ca content or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, which in turn reduces the ductility of the hot-rolled steel sheet. May cause you to. Further, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Ca content and Mg content are 0.0200% or less, the REM content is 0.1000% or less, and the Bi content is 0.020% or less.
- the Bi content is preferably 0.010% or less.
- REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements.
- lanthanoids they are industrially added in the form of misch metal.
- Zr, Co, Zn and W 0 to 1.00% in total and Sn: 0 to 0.050% Regarding Zr, Co, Zn and W
- the present inventors have confirmed that even if the total content of these elements is 1.00% or less, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. There is. Therefore, one or more of Zr, Co, Zn and W may be contained in a total of 1.00% or less. Further, the present inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if a small amount of Sn is contained, but flaws may occur during hot rolling.
- the Sn content is 0.050% or less.
- the chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
- sol. Al may be measured by ICP-AES using a filtrate obtained by heat-decomposing the sample with an acid.
- C and S may be measured by using the combustion-infrared absorption method
- N may be measured by using the inert gas melting-heat conductivity method
- O may be measured by using the inert gas melting-non-dispersion infrared absorption method.
- the metal structure of the hot-rolled steel sheet according to the present embodiment is bainite: 40 to 92%, tempered martensite: 5 to 40%, retained austenite: 3 to 20%, ferrite: 5% or less, fresh in area%. Martensite: 5% or less, and pearlite: 5% or less, and the number% of the bainite crystal grains in contact with both the tempered martensite and the retained austenite is 80% or more of the total crystal grains of the bainite.
- the C concentration in the retained austenite is 0.80% by mass or more, the average crystal grain size of the retained austenite is 0.70 ⁇ m or less, and the standard deviation of bainite hardness is 25 HV 0.01 or less.
- the metal structure of the plate thickness cross section parallel to the rolling direction at the position 1/4 of the plate thickness from the surface and the center position in the plate width direction is defined. The reason is that the metal structure at this position shows a typical metal structure of the hot-rolled steel sheet.
- the "1/4 position" of the plate thickness is an observation position for specifying the metal structure, and is not strictly limited to the 1/4 depth. The metal structure obtained by observing somewhere in the range of 1/8 to 3/8 depth of the plate thickness can be regarded as the metal structure at the 1/4 position.
- Bainite 40-92% Bainite is a structure that improves the strength and ductility of hot-rolled steel sheets. If the area ratio of bainite is less than 40%, the desired strength and ductility cannot be obtained. Therefore, the area ratio of bainite is set to 40% or more. Preferably, it is 50% or more, 55% or more, 65% or more, 70% or more. On the other hand, if the area ratio of bainite exceeds 92%, the desired ductility cannot be obtained. Therefore, the area ratio of bainite is 92% or less. Preferably, it is 90% or less and 85% or less.
- Tempering martensite 5-40% Tempering martensite is a structure that improves the strength of hot-rolled steel sheets. If the area ratio of tempered martensite is less than 5%, the desired strength cannot be obtained. Therefore, the area ratio of tempered martensite is set to 5% or more. Preferably, it is 10% or more and 15% or more. On the other hand, if the area ratio of tempered martensite exceeds 40%, the desired ductility cannot be obtained. Therefore, the area ratio of tempered martensite is 40% or less. Preferably, it is 35% or less and 30% or less.
- Residual austenite is a structure that improves the ductility of hot-rolled steel sheets. If the area ratio of retained austenite is less than 3%, the desired ductility cannot be obtained. Therefore, the area ratio of retained austenite is set to 3% or more. Preferably, it is 5% or more, 7% or more, and 10% or more. On the other hand, if the area ratio of retained austenite exceeds 20%, the desired strength cannot be obtained. Therefore, the area ratio of retained austenite is set to 20% or less. Preferably, it is 18% or less and 15% or less.
- the area ratio of ferrite is set to 5% or less. Preferably, it is 4% or less, 3% or less, and 2% or less. Since the smaller the area ratio of ferrite is, the more preferable it is, the area ratio of ferrite may be 0%.
- Fresh martensite 5% or less Since fresh martensite has a hard structure, it contributes to improving the strength of hot-rolled steel sheets. However, fresh martensite has poor ductility and is also an organization that reduces local ductility. If the area ratio of fresh martensite is too large, the desired ductility and local ductility cannot be obtained. Therefore, the area ratio of fresh martensite shall be 5% or less. Preferably, it is 4% or less, 3% or less, and 2% or less. Since the smaller the area ratio of fresh martensite is, the more preferable it is, the area ratio of fresh martensite may be 0%.
- the area ratio of pearlite is set to 5% or less. Preferably, it is 4% or less, 3% or less, and 2% or less. Since the smaller the area ratio of pearlite is, the more preferable it is, the area ratio of pearlite may be 0%.
- the number% of bainite grains in contact with both tempered martensite and retained austenite is 80% or more of the total bainite grains. We have 80% or more of the total grains of bainite. It was found that the local ductility of the hot-rolled steel sheet was improved by contacting the bainite crystal grains of bainite with both tempered martensite and retained austenite. The present inventors speculate that this mechanism is as follows.
- the tempered martensite When the retained austenite and the tempered martensite are in contact with each other, at the interface between the retained austenite and the tempered martensite, the tempered martensite at the time of deformation due to the difference in hardness between the soft retained austenite and the hard tempered martensite. Stress concentration occurs in. As a result, voids are likely to be formed at the interface between tempered martensite and retained austenite. Voids formed at the interface between tempered martensite and retained austenite cause deterioration of the local ductility of hot-rolled steel sheets. Therefore, by making sure that 80% or more of the total bainite grains are in contact with both tempered martensite and retained austenite, stress is concentrated on the tempered martensite during deformation. Is less likely to occur. As a result, the local ductility of the hot-rolled steel sheet can be improved.
- the number% of bainite crystal grains in contact with both tempered martensite and retained austenite is less than 80% of the total grain of bainite, the local ductility of the hot-rolled steel sheet cannot be improved. Therefore, the number% of bainite crystal grains in contact with both tempered martensite and retained austenite is 80% or more of the total bainite crystal grains. Preferably, it is 83% or more, 85% or more, and 87% or more.
- the upper limit of the number% of the number of bainite crystal grains in contact with both tempered martensite and retained austenite is not particularly specified, but may be 100%, 99%, or 98%.
- the crystal grains other than the bainite crystal grains that are in contact with both tempered martensite and retained austenite are bainite crystal grains that are not in contact with tempered martensite but are in contact with only retained austenite. And bainite crystals that are in contact with both tempered martensite and pearlite.
- the area ratio of the tissues other than retained austenite is measured by the following method.
- a test piece is collected from a hot-rolled steel sheet so that a metal structure can be observed at a plate thickness cross section parallel to the rolling direction, at a position 1/4 of the plate thickness from the surface and at a center position in the plate width direction.
- the polished surface is nital-corroded, and at least three regions of 30 ⁇ m ⁇ 30 ⁇ m are observed for structure using an optical microscope and a scanning electron microscope (SEM).
- SEM scanning electron microscope
- each tissue is identified by the following method. It is a collection of lath-shaped crystal grains, and a structure containing Fe-based carbides having a major axis of 20 nm or more and extending in different directions inside the structure is regarded as tempered martensite. Depending on the heat treatment conditions, a plurality of types of Fe-based carbides may be present inside the tempered martensite.
- Fresh martensite is a structure with a high dislocation density and substructures such as blocks and packets in the grain, so it should be distinguished from other metal structures by electron channeling contrast images using a scanning electron microscope. Is possible.
- It is a collection of lath-shaped crystal grains, and has a structure that is not fresh martensite among the structures that do not contain Fe-based carbides with a major axis of 20 nm or more inside the structure, or contains Fe-based carbides with a major axis of 20 nm or more inside the structure.
- a structure in which the Fe-based carbide has a single variant, that is, the Fe-based carbide extending in the same direction, is regarded as bainite.
- the Fe-based carbide elongated in the same direction means that the difference in the elongation direction of the Fe-based carbide is within 5 °.
- a structure that is a lumpy crystal grain and does not contain a substructure such as a lath inside the structure is regarded as ferrite.
- a structure in which plate-shaped ferrite and Fe-based carbide are layered is regarded as pearlite.
- the number% of bainite crystal grains in contact with both tempered martensite and retained austenite is obtained by performing the following measurement in the same region as that observed with the above-mentioned optical microscope and scanning electron microscope. After polishing the plate thickness cross section using silicon carbide paper of # 600 to # 1500, diamond powder having a particle size of 1 to 6 ⁇ m is mirror-finished using a diluted solution such as alcohol or a liquid dispersed in pure water. .. Next, the strain introduced into the surface layer of the sample is removed by electrolytic polishing. Electron backscattering in a region of 50 ⁇ m in length and 1/8 depth from the surface to 3/8 depth of the plate thickness at an arbitrary position in the longitudinal direction of the sample cross section at a measurement interval of 0.1 ⁇ m.
- Crystal orientation information is obtained by measuring by diffraction method.
- an EBSD analyzer composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the EBSD analyzer is 9.6 ⁇ 10-5 Pa or less
- the acceleration voltage is 15 kV
- the irradiation current level is 13
- the electron beam irradiation level is 62.
- bainite surrounded by grain boundaries having an average crystal orientation difference of 15 ° or more is regarded as one bainite crystal grain.
- the crystal orientation information obtained by EBSD analysis is used with the "Image Quality" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Can be identified.
- the area ratio of retained austenite is measured by the following method.
- the area ratio of retained austenite is measured by X-ray diffraction.
- the integrated intensity of a total of 6 peaks of (211), ⁇ (111), ⁇ (200), and ⁇ (220) is obtained and calculated using the intensity averaging method. As a result, the area ratio of retained austenite is obtained.
- C concentration in retained austenite 0.80% by mass or more
- the C concentration (carbon concentration) in retained austenite undergoes a large amount of martensitic transformation in the early stage of deformation, and then Since it acts as hard martensite in the subsequent deformation, it reduces local ductility.
- the C concentration in the retained austenite is set to 0.80% by mass or more.
- the C concentration in the retained austenite is more preferably 0.90% by mass or more, 1.00% by mass or more, and 1.20% by mass or more. Further, by setting the C concentration in the retained austenite to 2.00% by mass or less, excessive stabilization of the retained austenite can be suppressed, and transformation-induced plasticity (TRIP) can be more reliably expressed. Therefore, the C concentration in the retained austenite may be 2.00% by mass or less.
- the C concentration in the retained austenite is determined by X-ray diffraction. Specifically, X-ray diffraction with Cu—K ⁇ rays is performed on the metal structure at the position 1/4 of the plate thickness from the steel plate surface and the center position in the plate width direction in the plate thickness cross section parallel to the rolling direction to obtain retained austenite.
- the lattice constant a (unit: angstrom) is obtained from the reflection angles of the (200) plane, (220) plane, and (311) plane, and the C concentration (C ⁇ ) in the retained austenite is calculated according to the following formula (A). As a result, the C concentration (mass%) in the retained austenite is obtained.
- Average crystal grain size of retained austenite 0.70 ⁇ m or less
- the size of retained austenite has a great influence on the stability of retained austenite. If the average crystal grain size of the retained austenite is more than 0.70 ⁇ m, the retained austenite is not uniformly dispersed in the steel, and the TRIP effect of the retained austenite cannot be effectively exhibited. As a result, the local ductility of the hot-rolled steel sheet cannot be improved. Therefore, the average crystal grain size of retained austenite is 0.70 ⁇ m or less. Preferably, it is 0.60 ⁇ m or less and 0.50 ⁇ m or less. The average crystal grain size of retained austenite may be 0.10 ⁇ m or more.
- the average crystal grain size of retained austenite is measured by the following method. A test piece is taken from the hot-rolled steel sheet so that the metal structure of the sheet thickness cross section parallel to the rolling direction can be observed at the position 1/4 of the sheet thickness from the surface and the center position in the plate width direction.
- a diamond powder having a particle size of 1 to 6 ⁇ m is mirror-surfaced using a diluted solution such as alcohol or a liquid dispersed in pure water. Finish to.
- the strain introduced into the surface layer of the sample is removed by electrolytic polishing. Electron backscattering in a region of 50 ⁇ m in length and 1/8 depth from the surface to 3/8 depth of the plate thickness at an arbitrary position in the longitudinal direction of the sample cross section at a measurement interval of 0.1 ⁇ m. Crystal orientation information is obtained by measuring by diffraction method.
- an EBSD analyzer composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the EBSD analyzer is 9.6 ⁇ 10-5 Pa or less
- the acceleration voltage is 15 kV
- the irradiation current level is 13
- the electron beam irradiation level is 62.
- the average crystal grain size of retained austenite is calculated.
- the crystal particle size of each retained austenite is obtained by determining that the crystal structure is fcc as retained austenite and calculating the equivalent circle diameter of each retained austenite. By calculating the average value of the crystal grain size of retained austenite in the observation region, the average crystal grain size of retained austenite is obtained.
- Standard deviation of Vickers hardness 25 HV 0.01 or less If the standard deviation of Vickers hardness is more than 25 HV 0.01, the hardness difference between structures is large, and the local diffusivity of the hot-rolled steel sheet cannot be improved. Therefore, the standard deviation of Vickers hardness is 25 HV 0.01 or less. Preferably, it is 23 HV 0.01 or less, 20 HV 0.01 or less, and 18 HV 0.01 or less. The standard deviation of Vickers hardness may be 1 HV 0.01 or more. The standard deviation of Vickers hardness is preferably small from the viewpoint of improving the local ductility of the hot-rolled steel sheet. That is, the standard deviation of the Vickers hardness can be lowered by sufficiently softening the tempered martensite having a high hardness in the hot-rolled steel sheet.
- the standard deviation of Vickers hardness is obtained by the following method. Among the plate thickness cross sections parallel to the rolling direction, the Vickers hardness is measured at equal intervals of 300 or more measurement points within the range of plate thickness ⁇ 1 mm in the metal structure at the center position in the plate width direction. The measured load is 10 gf. Based on the measurement result, the standard deviation of Vickers hardness (HV0.01) is calculated.
- Maximum height roughness Rz 15.0 ⁇ m or less
- the maximum height roughness Rz of the surface of the hot-rolled steel sheet may be 15.0 ⁇ m or less. By setting the maximum height roughness Rz of the surface to 15.0 ⁇ m or less, the local bendability can be improved.
- the maximum height roughness Rz of the surface is preferably 14.0 ⁇ m or less and 13.0 ⁇ m or less.
- the lower limit of the maximum height roughness Rz of the surface is not particularly limited, but may be 1.0 ⁇ m or more.
- the maximum height roughness Rz is obtained by measuring in accordance with JIS B 0601: 2013.
- the hot-rolled steel sheet according to this embodiment may have a tensile (maximum) strength of 1180 MPa or more. By setting the tensile strength to 1180 MPa or more, it is possible to contribute to weight reduction of the vehicle body.
- the upper limit of the tensile strength is not particularly limited, but may be 1500 MPa or less.
- the total elongation may be 10.0% or more, and the product (TS ⁇ l-El) of the tensile strength TS and the local elongation l-El is 8400 MPa ⁇ % or more. May be good.
- the upper limit of the total elongation may be 30.0% or less, and the upper limit of TS ⁇ l-El may be 15000 MPa ⁇ % or less.
- Tensile strength, total elongation and local elongation are measured in accordance with JIS Z 2241: 2011 using JIS Z 2241: 2011 No. 5 test piece.
- the sampling position of the tensile test piece may be 1/4 from the end in the plate width direction, and the tensile test piece may be collected so that the direction perpendicular to the rolling direction is the longitudinal direction.
- the plate thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.5 to 8.0 mm.
- the thickness of the hot-rolled steel sheet according to the present embodiment may be 0.5 mm or more.
- the plate thickness is 1.2 mm or more and 1.4 mm or more.
- the plate thickness is 8.0 mm or less.
- the plate thickness is 6.0 mm or less.
- the hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like.
- the plating layer may be an electroplating layer or a hot-dip plating layer.
- the electroplating layer include electrogalvanization and electroZn—Ni alloy plating.
- the hot-dip plating layer include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, and the like.
- NS hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plat
- the amount of plating adhered is not particularly limited and may be the same as the conventional one. Further, the corrosion resistance can be further improved by subjecting an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
- an appropriate chemical conversion treatment for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid
- the temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
- the temperature of the hot-rolled steel sheet is measured by a contact-type or non-contact-type thermometer if it is at the end in the plate width direction. If it is not the end of the hot-rolled steel sheet in the plate width direction, it is measured by a thermocouple or calculated by heat transfer analysis.
- the slab is heated to 1100 ° C. or higher and held, and then hot-rolled.
- Hot rolling is performed in a temperature range of 850 to 1100 ° C.
- Hot rolling is completed at 850 ° C. or higher.
- the mixture is cooled to the winding temperature at an average cooling rate of 30 ° C./s or higher.
- the winding temperature is set to a temperature range of T1-100 ° C. or higher and lower than T1 (° C.).
- the take-up tension at the time of take-up shall be 3.6 kg / mm 2 or more.
- T1 (° C.) is represented by the following formula ⁇ 1>.
- the average cooling rate in the temperature range from the winding temperature to room temperature is set to 20 ° C./h or less.
- T1 (° C.) 591-474 x [C] -33 x [Mn] -17 x [Ni] -17 x [Cr] -21 x [Mo] ... ⁇ 1>
- the [element symbol] in each formula indicates the content (mass%) of each element in steel. If the element is not contained, 0 is substituted.
- slabs obtained by continuous casting slabs obtained by casting and slabs, and the like can be used. If necessary, those slabs that have been hot-worked or cold-worked can be used.
- the slab to be subjected to hot rolling is held at 1100 ° C. or higher in order to make the size of austenite crystal grains uniform when the slab is heated.
- the holding time (holding time) at 1100 ° C. or higher is preferably 6000 seconds or longer.
- the heating temperature of the slab is preferably 1300 ° C. or lower. Further, by setting the heating temperature of the slab to 1170 ° C. or lower, the maximum height roughness Rz of the surface of the hot-rolled steel sheet can be reduced. As a result, the local bendability of the hot-rolled steel sheet can be improved.
- the temperature of the steel sheet may be changed at 1100 ° C. or higher, or may be constant.
- Hot rolling By performing hot rolling in a temperature range of 850 to 1100 ° C., recrystallized austenite grains can be made finer. Hot rolling is preferably performed so that the total plate thickness is reduced by 90% or more in the temperature range of 850 to 1100 ° C.
- the plate thickness reduction in the temperature range of 850 to 1100 ° C. means that the inlet plate thickness before the first pass in rolling in this temperature range is t 0, and the outlet plate thickness after the final pass in rolling in this temperature range is t. When it is 1 , it can be expressed as (t 0 ⁇ t 1 ) / t 0 ⁇ 100 (%).
- Hot rolling completion temperature 850 ° C. or higher
- the hot rolling completion temperature is preferably 850 ° C. or higher.
- the upper limit of the completion temperature of hot rolling is not particularly limited, but may be 1100 ° C. or lower.
- Cooling after completion of hot rolling Average cooling rate of 30 ° C / s or higher An average cooling rate of 30 ° C / s or higher after completion of hot rolling in order to suppress the growth of austenite grains refined by hot rolling. It is preferable to cool down to a temperature range lower than T1 (° C.).
- the formation of ferrite and pearlite can be suppressed by cooling to a temperature range of less than T1 (° C) at an average cooling rate of 30 ° C./s or higher.
- the average cooling rate here refers to the range of temperature drop of the steel sheet from the start of accelerated cooling (when the steel sheet is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel sheet is taken out from the cooling equipment).
- the upper limit of the average cooling rate is not specified, but if the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, it is preferably 300 ° C./s or less.
- the cooling stop temperature is preferably T1-100 ° C. or higher in relation to the winding temperature described later.
- Winding temperature T1-100 ° C. or higher and lower than T1 (° C.)
- the winding temperature is T1-100 ° C. or higher and lower than T1 (° C.).
- T1-100 ° C. or higher and lower than T1 (° C.) By setting the winding temperature to T1-100 ° C. or higher and lower than T1 (° C.), a desired amount of tempered martensite can be obtained, and as a result, the desired amount of tempered martensite and retained austenite can be obtained. Crystal grains of bainite in contact can be obtained. Also, a desired size of retained austenite can be obtained.
- Winding tension 3.6 kg / mm 2 or more
- the steel plate tension (winding tension) at the time of winding shall be 3.6 kg / mm 2 or more.
- the take-up tension at the time of take-up is appropriately set according to the target strength class and the dimensions of the steel plate (plate thickness, plate width), but in general, the take-up is 3.0 kg / mm 2 or less. Often done with tension.
- the present inventors hot-rolled a slab having the above chemical composition under the above conditions, and then wound the slab at a winding tension of 3.6 kg / mm 2 or more to obtain tempered martensite.
- the upper limit of the take-up tension is not particularly specified, but may be 5.0 kg / mm 2 or less from the viewpoint of suppressing an increase in the equipment load.
- the take-up tension at the time of take-up should be controlled to be within a predetermined range by the output of the motor.
- the average cooling rate in the temperature range from winding temperature to room temperature is 20 ° C / h or less. After winding, the average cooling rate in the temperature range from winding temperature to room temperature is set to 20 ° C / h or less. , The tempered martensite can be sufficiently softened, and the difference in hardness between the structures can be sufficiently reduced. On the other hand, if the average cooling rate after winding exceeds 20 ° C./h, the tempered martensite cannot be sufficiently softened, and the standard deviation of Vickers hardness may increase. Therefore, the average cooling rate in the temperature range from the winding temperature to the room temperature is set to 20 ° C./h or less.
- the average cooling rate in the temperature range from the winding temperature to room temperature to 20 ° C./h or less, a sufficient amount of C can be concentrated in the retained austenite. As a result, the local ductility and ductility of the hot-rolled steel sheet can be improved.
- the lower limit of the average cooling rate for cooling after winding is not particularly specified, but may be 5 ° C./h or higher. Further, the average cooling rate after winding may be controlled by a heat insulating cover, an edge mask, mist cooling, or the like.
- the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention.
- the present invention is not limited to this one-condition example.
- various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
- the area ratio of each structure, the number% of bainite crystal grains in contact with both tempered martensite and retained austenite, the C concentration in retained austenite, and the average of retained austenite are obtained by the above method.
- the crystal grain size, the standard deviation of the Vickers hardness, and the maximum height roughness Rz of the surface were determined. The obtained measurement results are shown in Table 5.
- Tensile strength, ductility and local ductility Tensile tests were performed in accordance with JIS Z 2241: 2011.
- the test piece was JIS Z 2241: 2011 No. 5 test piece.
- the sampling position of the tensile test piece was 1/4 from the end in the plate width direction, and the tensile test piece was collected so that the direction perpendicular to the rolling direction was the longitudinal direction. This gave tensile (maximum) strength, total elongation and local elongation.
- the local elongation was set as the value obtained by subtracting the uniform elongation from the total elongation.
- the tensile (maximum) strength is 1180 MPa or more, it is judged to be acceptable as having excellent strength, and if the tensile (maximum) strength is less than 1180 MPa, it is judged to be unacceptable because it does not have excellent strength. Judged.
- TS ⁇ l-El the product (TS ⁇ l-El) of the tensile strength TS and the local elongation l-El is 8400 MPa ⁇ % or more, it is judged that the product has excellent local ductility, and TS ⁇ l-El is determined to be acceptable. If it was less than 8400 MPa ⁇ %, it was judged to be unacceptable because it did not have excellent local ductility.
- the local bendability was evaluated by the following method.
- the No. 1 test piece described in JIS Z 2204: 2014 was prepared, and the V bending test was performed using the V block method described in JIS Z 2248: 2014.
- the test piece was collected so that the direction perpendicular to the rolling direction was the longitudinal direction (the bending ridge line coincided with the rolling direction), and the test piece was bent so that the surface was bent outside.
- the radius at the bottom of the V block was changed from 1.0 mm to 6.0 mm in 0.5 mm increments, and the smallest radius at which the test piece did not crack was determined as the limit bending radius R (mm).
- R / t obtained by dividing the limit bending radius R (mm) by the test piece plate thickness t (mm) was 1.6 or less, it was judged to be acceptable as having excellent local bendability.
- Table 5 shows the obtained measurement results.
- the production No. whose chemical composition and / or metal structure is not within the range specified in the present invention. 4-10 and 25-29 were inferior in any one or more of the properties (tensile strength, ductility and local ductility).
- the hot-rolled steel sheet according to the present invention it is possible to provide a hot-rolled steel sheet having excellent strength, ductility and local ductility. Further, according to the above-mentioned preferable aspect of the present invention, it is possible to provide a hot-rolled steel sheet having the above-mentioned various characteristics and further having excellent local bendability.
- the hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
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Abstract
Description
本願は、2020年3月11日に、日本に出願された特願2020-041811号に基づき優先権を主張し、その内容をここに援用する。
(b)優れた延性を得るためには、金属組織中に所望量の残留オーステナイトを含ませることが必要である。しかしながら、残留オーステナイトを含ませると、熱延鋼板の局部延性が低下してしまう。
(c)残留オーステナイトを含ませた上で優れた局部延性を得るためには、残留オーステナイト中のC濃度、残留オーステナイトの平均粒径、焼き戻しマルテンサイトおよび残留オーステナイトの両方に接するベイナイトの結晶粒の個数%、並びに、ビッカース硬度の標準偏差を所望の範囲に制御することが必要である。
(d)優れた局部曲げ性を得るためには、熱延鋼板の表面の最大高さ粗さRzを制御することが必要である。
(1)本発明の一態様に係る熱延鋼板は、化学組成が、質量%で、
C :0.100~0.350%、
Si:1.00~3.00%、
Mn:1.00~4.00%、
sol.Al:0.001~2.000%、
P :0.100%以下、
S :0.0300%以下、
N :0.1000%以下、
O :0.0100%以下、
Ti:0~0.300%、
Nb:0~0.100%、
V :0~0.500%、
Cu:0~2.00%、
Cr:0~2.00%、
Mo:0~1.00%、
Ni:0~2.00%、
B :0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
REM:0~0.1000%、
Bi:0~0.020%、
Zr、Co、ZnおよびWのうち1種または2種以上:合計で0~1.00%、並びにSn:0~0.050%を含有し、
残部がFeおよび不純物からなり、
金属組織が、面積%で、
ベイナイト:40~92%、
焼き戻しマルテンサイト:5~40%、
残留オーステナイト:3~20%、
フェライト:5%以下、
フレッシュマルテンサイト:5%以下、および
パーライト:5%以下からなり、
前記焼き戻しマルテンサイトおよび前記残留オーステナイトの両方に接する前記ベイナイトの結晶粒の個数%が、前記ベイナイトの全結晶粒のうち80%以上であり、
前記残留オーステナイト中のC濃度が0.80質量%以上であり、
前記残留オーステナイトの平均結晶粒径が0.70μm以下であり、
ビッカース硬度の標準偏差が25HV0.01以下である。
(2)上記(1)に記載の熱延鋼板は、表面の最大高さ粗さRzが15.0μm以下であってもよい。
(3)上記(1)または(2)に記載の熱延鋼板は、前記化学組成が、質量%で、
Ti:0.005~0.300%、
Nb:0.005~0.100%、
V :0.005~0.500%、
Cu:0.01~2.00%、
Cr:0.01~2.00%、
Mo:0.01~1.00%、
Ni:0.02~2.00%、
B :0.0001~0.0100%、
Ca:0.0005~0.0200%、
Mg:0.0005~0.0200%、
REM:0.0005~0.1000%、および
Bi:0.0005~0.020%
からなる群から選択される1種または2種以上を含有してもよい。
本実施形態に係る熱延鋼板は、質量%で、C:0.100~0.350%、Si:1.00~3.00%、Mn:1.00~4.00%、sol.Al:0.001~2.000%、P:0.100%以下、S:0.0300%以下、N:0.1000%以下、O:0.0100%以下、並びに、残部:Feおよび不純物を含む。以下に各元素について詳細に説明する。
Cは、所望の強度を得るために必要な元素である。C含有量が0.100%未満では、所望の強度を得ることが困難となる。したがって、C含有量は0.100%以上とする。C含有量は、好ましくは0.120%以上、0.150%以上である。
一方、C含有量が0.350%超では、変態速度が遅くなることでMA(マルテンサイトおよび残留オーステナイトの混合相)が生成しやすくなる。その結果、強度の均一な組織を得ることができず、優れた局部延性を得ることが困難となる。したがって、C含有量は0.350%以下とする。C含有量は好ましくは0.330%以下、0.310%以下である。
Siは、セメンタイトの析出を遅延させる作用を有する。この作用により、オーステナイトが未変態で残留する量、すなわち残留オーステナイトの面積分率を高めることができる。また、上記作用により、硬質相中の固溶C量を多く保つこと、およびセメンタイトの粗大化を防ぐことができ、これらの結果、鋼板の強度を高めることができる。またSi自体も固溶強化により熱延鋼板の強度を高める効果がある。また、Siは脱酸により鋼を健全化する(鋼にブローホールなどの欠陥が生じることを抑制する)作用を有する。Si含有量が1.00%未満では、上記作用による効果を得ることができない。したがって、Si含有量は1.00%以上とする。Si含有量は、好ましくは1.20%以上、1.50%以上である。一方、Si含有量が3.00%超では、セメンタイトの析出を著しく遅延させ、残留オーステナイトの面積率が過剰に高まるため好ましくない。また、Si含有量が3.00%超では、熱延鋼板の表面性状および化成処理性、さらには延性および溶接性が著しく劣化するとともに、A3変態点が著しく上昇する。これにより、安定して熱間圧延を行うことが困難になる場合がある。したがって、Si含有量は3.00%以下とする。Si含有量は、好ましくは2.70%以下、2.50%以下である。
Mnは、フェライト変態を抑制して熱延鋼板を高強度化する作用を有する。Mn含有量が1.00%未満では、所望の引張強さを得ることができない。したがって、Mn含有量は1.00%以上とする。Mn含有量は、好ましくは1.50%以上、1.80%以上である。一方、Mn含有量が4.00%超では、熱延鋼板の局部延性が劣化する。したがって、Mn含有量は4.00%以下とする。Mn含有量は、好ましくは3.70%以下、3.50%以下である。
sol.Alは、Siと同様に、鋼を脱酸して鋼板を健全化するとともに、オーステナイトからのセメンタイトの析出を抑制することで、残留オーステナイトの生成を促進する作用を有する。sol.Al含有量が0.001%未満では上記作用による効果を得ることができない。したがって、sol.Al含有量は、0.001%以上とする。sol.Al含有量は、好ましくは0.010%以上である。一方、sol.Al含有量が2.000%超では、上記効果が飽和するとともに経済的に好ましくない。さらに、sol.Al含有量が2.000%超では、A3変態点が著しく上昇し、安定して熱間圧延を行うことが困難になる。そのため、sol.Al含有量は2.000%以下とする。sol.Al含有量は、好ましくは1.500%以下、1.300%以下である。
なお、本実施形態においてsol.Alとは、酸可溶性Alを意味し、固溶状態で鋼中に存在する固溶Alのことを示す。
Pは、一般的に不純物として含有される元素であるが、固溶強化により熱延鋼板の強度を高める作用を有する。したがって、Pを積極的に含有させてもよい。しかし、Pは偏析し易い元素でもある。P含有量が0.100%を超えると、粒界偏析に起因する延性の低下が顕著となる。したがって、P含有量は、0.100%以下とする。P含有量は、好ましくは0.030%以下である。P含有量の下限は特に規定する必要はないが、精錬コストの観点から、0.001%以上とすることが好ましい。
Sは、不純物として含有される元素であり、鋼中に硫化物系介在物を形成して熱延鋼板の延性を低下させる。S含有量が0.0300%を超えると、熱延鋼板の延性が著しく低下する。したがって、S含有量は0.0300%以下とする。S含有量は、好ましくは0.0050%以下である。S含有量の下限は特に規定する必要はないが、精錬コストの観点から、0.0001%以上とすることが好ましい。
Nは、不純物として鋼中に含有される元素であり、熱延鋼板の延性を低下させる作用を有する。N含有量が0.1000%超では、熱延鋼板の延性が著しく低下する。したがって、N含有量は0.1000%以下とする。N含有量は、好ましくは0.0800%以下、0.0700%以下である。N含有量の下限は特に規定する必要はないが、後述するようにTi、NbおよびVの1種または2種以上を含有させて金属組織の微細化を図る場合には、炭窒化物の析出を促進させるためにN含有量は0.0010%以上とすることが好ましく、0.0020%以上とすることがより好ましい。
Oは、鋼中に多く含まれると破壊の起点となる粗大な酸化物を形成し、脆性破壊や水素誘起割れを引き起こす。そのため、O含有量は0.0100%以下とする。O含有量は、0.0080%以下、0.0050%以下とすることが好ましい。溶鋼の脱酸時に微細な酸化物を多数分散させるために、O含有量は0.0005%以上、0.0010%以上としてもよい。
Ti、NbおよびVは、いずれも、鋼中に炭化物または窒化物として析出し、ピン止め効果によって金属組織を微細化する作用を有するため、これらの元素の1種または2種以上を含有させてもよい。上記作用による効果をより確実に得るためには、Ti含有量を0.005%以上とするか、Nb含有量を0.005%以上とするか、あるいはV含有量を0.005%以上とすることが好ましい。しかし、これらの元素を過剰に含有させても、上記作用による効果が飽和して経済的に好ましくない。したがって、Ti含有量は0.300%以下とし、Nb含有量は0.100%以下とし、V含有量は0.500%以下とする。
Cu、Cr、Mo、NiおよびBは、いずれも、鋼板の焼入性を高める作用を有する。また、CrおよびNiは残留オーステナイトを安定化させる作用を有し、CuおよびMoは鋼中に炭化物を析出して熱延鋼板の強度を高める作用を有する。さらに、Niは、Cuを含有させる場合においては、Cuに起因するスラブの粒界割れを効果的に抑制する作用を有する。したがって、これらの元素の1種または2種以上を含有させてもよい。
Ca、MgおよびREMは、いずれも、介在物の形状を好ましい形状に調整することにより、熱延鋼板の成形性を高める作用を有する。また、Biは、凝固組織を微細化することにより、熱延鋼板の成形性を高める作用を有する。したがって、これらの元素の1種または2種以上を含有させてもよい。上記作用による効果をより確実に得るためには、Ca、Mg、REMおよびBiのいずれか1種以上を0.0005%以上とすることが好ましい。しかし、Ca含有量またはMg含有量が0.0200%を超えると、あるいはREM含有量が0.1000%を超えると、鋼中に介在物が過剰に生成され、却って熱延鋼板の延性を低下させる場合がある。また、Bi含有量を0.020%超としても、上記作用による効果は飽和してしまい、経済的に好ましくない。したがって、Ca含有量、Mg含有量を0.0200%以下、REM含有量を0.1000%以下、並びにBi含有量を0.020%以下とする。Bi含有量は、好ましくは0.010%以下である。
Zr、Co、ZnおよびWについて、本発明者らは、これらの元素を合計で1.00%以下含有させても、本実施形態に係る熱延鋼板の効果は損なわれないことを確認している。そのため、Zr、Co、ZnおよびWのうち1種または2種以上を合計で1.00%以下含有させてもよい。
また、本発明者らは、Snを少量含有させても本実施形態に係る熱延鋼板の効果は損なわれないことを確認しているが、熱間圧延時に疵が発生する場合があるため、Sn含有量は0.050%以下とする。
次に、本実施形態に係る熱延鋼板の金属組織について説明する。
本実施形態に係る熱延鋼板では、金属組織が、面積%で、ベイナイト:40~92%、焼き戻しマルテンサイト:5~40%、残留オーステナイト:3~20%、フェライト:5%以下、フレッシュマルテンサイト:5%以下、およびパーライト:5%以下からなり、前記焼き戻しマルテンサイトおよび前記残留オーステナイトの両方に接する前記ベイナイトの結晶粒の個数%が、前記ベイナイトの全結晶粒のうち80%以上であり、前記残留オーステナイト中のC濃度が0.80質量%以上であり、前記残留オーステナイトの平均結晶粒径が0.70μm以下であり、ビッカース硬度の標準偏差が25HV0.01以下である。
なお、本実施形態では、圧延方向に平行な板厚断面の、表面から板厚の1/4位置、且つ板幅方向中央位置における金属組織を規定する。その理由は、この位置における金属組織が、熱延鋼板の代表的な金属組織を示すからである。なお、板厚の「1/4位置」とは、金属組織を特定するための観察位置であり、厳密に1/4深さに限定されない。板厚の1/8~3/8深さの範囲のどこかを観察して得られる金属組織を、1/4位置の金属組織とみなすことができる。
ベイナイトは、熱延鋼板の強度および延性を向上する組織である。ベイナイトの面積率が40%未満であると、所望の強度および延性を得ることができない。そのため、ベイナイトの面積率は40%以上とする。好ましくは、50%以上、55%以上、65%以上、70%以上である。
一方、ベイナイトの面積率が92%超であると、所望の延性を得ることができない。そのため、ベイナイトの面積率は92%以下とする。好ましくは、90%以下、85%以下である。
焼き戻しマルテンサイトは、熱延鋼板の強度を向上する組織である。焼き戻しマルテンサイトの面積率が5%未満であると、所望の強度を得ることができない。そのため、焼き戻しマルテンサイトの面積率は5%以上とする。好ましくは、10%以上、15%以上である。
一方、焼戻しマルテンサイトの面積率が40%を超えると、所望の延性を得ることができない。そのため、焼き戻しマルテンサイトの面積率は40%以下とする。好ましくは、35%以下、30%以下である。
残留オーステナイトは、熱延鋼板の延性を向上する組織である。残留オーステナイトの面積率が3%未満であると、所望の延性を得ることができない。そのため、残留オーステナイトの面積率は3%以上とする。好ましくは、5%以上、7%以上、10%以上である。
一方、残留オーステナイトの面積率が20%を超えると、所望の強度を得ることができない。そのため、残留オーステナイトの面積率は20%以下とする。好ましくは、18%以下、15%以下である。
フェライトは軟質な組織であるため、フェライトの面積率が多すぎると、所望の強度を得ることができない。そのため、フェライトの面積率は5%以下とする。好ましくは、4%以下、3%以下、2%以下である。フェライトの面積率は少ない程好ましいため、フェライトの面積率は0%であってもよい。
フレッシュマルテンサイトは硬質な組織であるため、熱延鋼板の強度の向上に寄与する。しかし、フレッシュマルテンサイトは延性に乏しく、さらに局部延性を低下させる組織でもある。フレッシュマルテンサイトの面積率が多すぎると、所望の延性および局部延性を得ることができない。そのため、フレッシュマルテンサイトの面積率は5%以下とする。好ましくは、4%以下、3%以下、2%以下である。フレッシュマルテンサイトの面積率は少ない程好ましいため、フレッシュマルテンサイトの面積率は0%であってもよい。
パーライトの面積率が多すぎると、残留オーステナイト量が減少し、十分な量の、焼き戻しマルテンサイトおよび残留オーステナイトの両方に接するベイナイトの結晶粒を確保することができない場合がある。そのため、パーライトの面積率は5%以下とする。好ましくは、4%以下、3%以下、2%以下である。パーライトの面積率は少ない程好ましいため、パーライトの面積率は0%であってもよい。
本発明者らは、ベイナイトの全結晶粒のうち、個数%で80%以上のベイナイトの結晶粒が、焼き戻しマルテンサイトおよび残留オーステナイトの両方に接することで、熱延鋼板の局部延性が向上することを知見した。このメカニズムは以下の通りであると本発明者らは推測する。
まず、熱延鋼板から、圧延方向に平行な板厚断面の、表面から板厚の1/4位置且つ板幅方向中央位置における金属組織が観察できるように試験片を採取する。次に、板厚断面を研磨した後、研磨面をナイタール腐食し、光学顕微鏡および走査型電子顕微鏡(SEM)を用いて、30μm×30μmの領域を少なくとも3領域組織観察する。この組織観察により得られた組織写真に対して画像解析を行うことによって、フェライト、パーライト、ベイナイトおよび焼き戻しマルテンサイトのそれぞれの面積率を得る。その後、同様の観察位置に対し、レペラー腐食をした後、光学顕微鏡および走査型電子顕微鏡を用いて組織観察を行い、得られた組織写真に対して画像解析を行うことによって、フレッシュマルテンサイトの面積率を算出する。
ラス状の結晶粒の集合であり、組織の内部に長径が20nm以上かつ異なる方向に伸長したFe系炭化物を含む組織を焼き戻しマルテンサイトとみなす。熱処理条件によっては、焼き戻しマルテンサイトの内部に複数種のFe系炭化物が存在する場合がある。
フレッシュマルテンサイトは転位密度が高く、かつ粒内にブロックやパケットといった下部組織を持つ組織であるので、走査型電子顕微鏡を用いた電子チャンネリングコントラスト像によれば、他の金属組織と区別することが可能である。
板状のフェライトとFe系炭化物とが層状に重なっている組織をパーライトとみなす。
板厚断面を#600から#1500の炭化珪素ペーパーを使用して研磨した後、粒度1~6μmのダイヤモンドパウダーを、アルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げる。次に、電解研磨によりサンプルの表層に導入されたひずみを除去する。サンプル断面の長手方向の任意の位置において、長さ50μm、表面から板厚の1/8深さ~表面から板厚の3/8深さの領域を、0.1μmの測定間隔で電子後方散乱回折法により測定して結晶方位情報を得る。測定には、サーマル電界放射型走査電子顕微鏡(JEOL製JSM-7001F)とEBSD検出器(TSL製DVC5型検出器)とで構成されたEBSD解析装置を用いる。この際、EBSD解析装置内の真空度は9.6×10-5Pa以下、加速電圧は15kV、照射電流レベルは13、電子線の照射レベルは62とする。
本実施形態では、残留オーステナイトの面積率はX線回折により測定する。まず、熱延鋼板の圧延方向に平行な板厚断面の、板厚の1/4位置且つ板幅方向中央位置において、Co-Kα線を用いて、α(110)、α(200)、α(211)、γ(111)、γ(200)、γ(220)の計6ピークの積分強度を求め、強度平均法を用いて算出する。これにより、残留オーステナイトの面積率を得る。
残留オーステナイト中のC濃度(炭素濃度)が0.80質量%未満の場合、残留オーステナイトは、変形の早期において多量にマルテンサイト変態し、そして、その後の変形にて硬質なマルテンサイトとして働くため、局部延性を低下させる。残留オーステナイト中のC濃度を0.80質量%以上とすることにより、残留オーステナイトが適度に安定化し、変形後期の高歪域まで残留オーステナイトを残すことが出来、その結果、熱延鋼板の局部延性を向上することができる。したがって、残留オーステナイト中のC濃度は0.80質量%以上とする。残留オーステナイト中のC濃度は、より好ましくは0.90質量%以上、1.00質量%以上、1.20質量%以上である。
また、残留オーステナイト中のC濃度を2.00質量%以下とすることにより、残留オーステナイトの過度な安定化を抑制し、変態誘起塑性(TRIP)をより確実に発現させることができる。したがって、残留オーステナイト中のC濃度は2.00質量%以下としてもよい。
残留オーステナイトの大きさは、残留オーステナイトの安定性に大きな影響を及ぼす。残留オーステナイトの平均結晶粒径が0.70μm超であると、残留オーステナイトが鋼中に均一に分散せず、残留オーステナイトのTRIP効果を効果的に発揮させることができない。その結果、熱延鋼板の局部延性を向上することができない。そのため、残留オーステナイトの平均結晶粒径は、0.70μm以下とする。好ましくは、0.60μm以下、0.50μm以下である。残留オーステナイトの平均結晶粒径は、0.10μm以上としてもよい。
熱延鋼板から、圧延方向に平行な板厚断面の、表面から板厚の1/4位置且つ板幅方向中央位置における金属組織が観察できるように試験片を採取する。
ビッカース硬度の標準偏差が25HV0.01超であると、組織間の硬度差が大きいため、熱延鋼板の局部延性を向上することができない。そのため、ビッカース硬度の標準偏差は25HV0.01以下とする。好ましくは、23HV0.01以下、20HV0.01以下、18HV0.01以下である。
ビッカース硬度の標準偏差は1HV0.01以上としてもよい。
ビッカース硬度の標準偏差は、熱延鋼板の局部延性を向上させる観点から、小さい方が望ましい。つまり、熱延鋼板において硬度の大きい焼き戻しマルテンサイトを十分に軟化することで、ビッカース硬度の標準偏差を下げることができる。
圧延方向に平行な板厚断面のうち、板幅方向中央位置における金属組織において、板厚×1mmの範囲で、300点以上の測定点を等間隔に、ビッカース硬度を測定する。測定荷重は10gfとする。測定結果に基づいて、ビッカース硬度(HV0.01)の標準偏差を算出する。
熱延鋼板の表面の最大高さ粗さRzは、15.0μm以下であってもよい。表面の最大高さ粗さRzを15.0μm以下とすることで、局部曲げ性を向上することができる。表面の最大高さ粗さRzは、好ましくは14.0μm以下、13.0μm以下である。表面の最大高さ粗さRzの下限は特に限定しないが、1.0μm以上としてもよい。
本実施形態に係る熱延鋼板は、引張(最大)強さが1180MPa以上であってもよい。引張強さを1180MPa以上とすることで、車体軽量化により寄与することができる。引張強さの上限は特に限定する必要は無いが、1500MPa以下としてもよい。
引張強さ、全伸びおよび局部伸びは、JIS Z 2241:2011の5号試験片を用いて、JIS Z 2241:2011に準拠して測定する。引張試験片の採取位置は、板幅方向の端部から1/4部分とし、圧延方向に直角な方向が長手方向となるように引張試験片を採取すればよい。
本実施形態に係る熱延鋼板の板厚は特に限定されないが、0.5~8.0mmとしてもよい。熱延鋼板の板厚を0.5mm以上とすることで、圧延完了温度の確保が容易になるとともに圧延荷重を低減できるため、熱間圧延を容易に行うことができる。したがって、本実施形態に係る熱延鋼板の板厚は0.5mm以上としてもよい。好ましくは、板厚は1.2mm以上、1.4mm以上である。また、板厚を8.0mm以下とすることで、金属組織の微細化が容易となり、上述した金属組織を容易に確保することができる。したがって、板厚は8.0mm以下としてもよい。好ましくは、板厚は6.0mm以下である。
上述した化学組成および金属組織を有する本実施形態に係る熱延鋼板は、耐食性の向上等を目的として、表面にめっき層を備えさせて表面処理鋼板としてもよい。めっき層は電気めっき層であってもよく溶融めっき層であってもよい。電気めっき層としては、電気亜鉛めっき、電気Zn-Ni合金めっき等が例示される。溶融めっき層としては、溶融亜鉛めっき、合金化溶融亜鉛めっき、溶融アルミニウムめっき、溶融Zn-Al合金めっき、溶融Zn-Al-Mg合金めっき、溶融Zn-Al-Mg-Si合金めっき等が例示される。めっき付着量は特に制限されず、従来と同様としてよい。また、めっき後に適当な化成処理(例えば、シリケート系のクロムフリー化成処理液の塗布と乾燥)を施すことで、耐食性をさらに高めることも可能である。
本実施形態に係る熱延鋼板の好適な製造方法では、以下の工程(1)~(6)を順次行う。なお、本実施形態におけるスラブの温度および鋼板の温度は、スラブの表面温度および鋼板の表面温度のことをいう。本実施形態において熱延鋼板の温度は、板幅方向最端部であれば接触式または非接触式温度計で測定する。熱延鋼板の板幅方向最端部以外であれば、熱電対により測定するか、伝熱解析により計算する。
(2)850~1100℃の温度域で熱間圧延を行う。
(3)850℃以上で熱間圧延を完了する。
(4)熱間圧延完了後、30℃/s以上の平均冷却速度で巻取り温度まで冷却する。
(5)巻取り温度をT1-100℃以上、T1(℃)未満の温度域として巻取る。また、巻取り時の巻取り張力を3.6kg/mm2以上とする。なお、T1(℃)は下記式<1>により表される。
(6)巻取り温度~室温の温度域における平均冷却速度を20℃/h以下とする。
ただし、各式中の[元素記号]は各元素の鋼中の含有量(質量%)を示す。当該元素を含有しない場合は0を代入する。
熱間圧延に供するスラブは、連続鋳造により得られたスラブ、鋳造および分塊により得られたスラブなどを用いることができる。必要によっては、それらスラブに熱間加工または冷間加工を加えたものを用いることができる。
熱間圧延の完了温度は850℃以上とすることが好ましい。熱間圧延の完了温度を850℃以上とすることで、オーステナイト中のフェライト核生成サイト数の過剰な増大を抑制することができる。さらにその結果、最終組織(製造された熱延鋼板の金属組織)におけるフェライトの生成を抑えられ、高強度の熱延鋼板を得ることができる。熱間圧延の完了温度の上限は特に限定されないが、1100℃以下としてもよい。
熱間圧延により細粒化したオーステナイト結晶粒の成長を抑制するため、熱間圧延完了後は、30℃/s以上の平均冷却速度でT1(℃)未満の温度域まで冷却を行うことが好ましい。
巻取り温度はT1-100℃以上、T1(℃)未満の温度域とする。巻取り温度をT1-100℃以上、T1(℃)未満とすることで、所望量の焼き戻しマルテンサイトを得ることができ、その結果、所望量の、焼き戻しマルテンサイトおよび残留オーステナイトの両方に接するベイナイトの結晶粒を得ることができる。また、所望のサイズの残留オーステナイトを得ることができる。
巻取り時の鋼板張力(巻取り張力)は3.6kg/mm2以上とする。
巻取り時の巻取り張力は、目標とする強度クラス、鋼板の寸法(板厚、板幅)によって適宜、設定されるが、一般的に、巻取りは3.0kg/mm2以下の巻取り張力で行われることが多い。しかし、本発明者らは、上記化学組成を有するスラブを上記条件にて熱間圧延した後、巻取り時の巻取り張力を3.6kg/mm2以上として巻き取ることで、焼き戻しマルテンサイトおよび残留オーステナイトの両方に接するベイナイトの結晶粒の個数%を多くすることができ、熱延鋼板の局部延性を向上できることを知見した。これは、巻取り時の巻取り張力を高めることによって、焼き戻しマルテンサイトと未変態のオーステナイトとの間にひずみが生じ、その結果、その位置でのベイナイトの生成が促進されたためだと考えている。
巻取り張力の上限は特に規定しないが、設備負荷の増大を抑制する観点から5.0kg/mm2以下としてもよい。
巻取り後、巻取り温度~室温の温度域における平均冷却速度を20℃/h以下とすることで、焼き戻しマルテンサイトを十分に軟化させることができ、組織間の硬度差を十分に低減することができる。一方で、巻取り後の平均冷却速度が20℃/hを超えると、焼き戻しマルテンサイトを十分に軟化させることができず、ビッカース硬度の標準偏差が増大してしまう場合がある。そのため、巻取り温度~室温の温度域における平均冷却速度は20℃/h以下とする。また、巻取り温度~室温の温度域における平均冷却速度を20℃/h以下とすることで、残留オーステナイトに十分な量のCを濃化させることができる。それらの結果、熱延鋼板の局部延性および延性を向上することができる。
巻取り後の冷却の平均冷却速度の下限は特に規定しないが、5℃/h以上としてもよい。また、巻取り後の平均冷却速度は、保温カバーやエッジマスク、ミスト冷却等によって制御するとよい。
JIS Z 2241:2011に準拠して引張試験を行った。試験片はJIS Z 2241:2011の5号試験片とした。引張試験片の採取位置は、板幅方向の端部から1/4部分とし、圧延方向に直角な方向が長手方向となるよう引張試験片を採取した。これにより、引張(最大)強さ、全伸びおよび局部伸びを得た。なお、局部伸びは、全伸びから均一伸びを引いた値とした。
局部曲げ性は、以下の方法により評価した。
JIS Z 2204:2014に記載の1号試験片を作成し、JIS Z 2248:2014に記載のVブロック法を用い、V曲げ試験を行った。試験片は圧延方向に対して垂直な方向が長手方向(曲げ稜線が圧延方向と一致)となるよう採取し、表面が曲げ外側になるように曲げた。Vブロックの底部における半径を1.0mmから6.0mmまで0.5mm刻みで変化させ、試験片に割れが発生しなかった最も小さい半径を限界曲げ半径R(mm)として求めた。限界曲げ半径R(mm)を試験片板厚t(mm)で除した値R/tが1.6以下である場合を局部曲げ性に優れるとして合格判定とした。
本発明に係る熱延鋼板は、自動車部材、機械構造部材さらには建築部材に用いられる工業用素材として好適である。
Claims (3)
- 化学組成が、質量%で、
C :0.100~0.350%、
Si:1.00~3.00%、
Mn:1.00~4.00%、
sol.Al:0.001~2.000%、
P :0.100%以下、
S :0.0300%以下、
N :0.1000%以下、
O :0.0100%以下、
Ti:0~0.300%、
Nb:0~0.100%、
V :0~0.500%、
Cu:0~2.00%、
Cr:0~2.00%、
Mo:0~1.00%、
Ni:0~2.00%、
B :0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
REM:0~0.1000%、
Bi:0~0.020%、
Zr、Co、ZnおよびWのうち1種または2種以上:合計で0~1.00%、並びにSn:0~0.050%を含有し、
残部がFeおよび不純物からなり、
金属組織が、面積%で、
ベイナイト:40~92%、
焼き戻しマルテンサイト:5~40%、
残留オーステナイト:3~20%、
フェライト:5%以下、
フレッシュマルテンサイト:5%以下、および
パーライト:5%以下からなり、
前記焼き戻しマルテンサイトおよび前記残留オーステナイトの両方に接する前記ベイナイトの結晶粒の個数%が、前記ベイナイトの全結晶粒のうち80%以上であり、
前記残留オーステナイト中のC濃度が0.80質量%以上であり、
前記残留オーステナイトの平均結晶粒径が0.70μm以下であり、
ビッカース硬度の標準偏差が25HV0.01以下であることを特徴とする熱延鋼板。 - 表面の最大高さ粗さRzが15.0μm以下であることを特徴とする請求項1に記載の熱延鋼板。
- 前記化学組成が、質量%で、
Ti:0.005~0.300%、
Nb:0.005~0.100%、
V :0.005~0.500%、
Cu:0.01~2.00%、
Cr:0.01~2.00%、
Mo:0.01~1.00%、
Ni:0.02~2.00%、
B :0.0001~0.0100%、
Ca:0.0005~0.0200%、
Mg:0.0005~0.0200%、
REM:0.0005~0.1000%、および
Bi:0.0005~0.020%
からなる群から選択される1種または2種以上を含有することを特徴とする請求項1または2に記載の熱延鋼板。
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JP2012172203A (ja) | 2011-02-22 | 2012-09-10 | Nippon Steel Corp | 局部変形能に優れ、成形性の方位依存性の少ない延性に優れた高強度熱延鋼板 |
JP2017524820A (ja) * | 2014-07-03 | 2017-08-31 | アルセロールミタル | 強度、延性および成形性が改善された高強度鋼板を製造する方法 |
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JP6152782B2 (ja) * | 2013-11-19 | 2017-06-28 | 新日鐵住金株式会社 | 熱延鋼板 |
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JP2012172203A (ja) | 2011-02-22 | 2012-09-10 | Nippon Steel Corp | 局部変形能に優れ、成形性の方位依存性の少ない延性に優れた高強度熱延鋼板 |
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