CN117280065A - Hot rolled steel sheet - Google Patents
Hot rolled steel sheet Download PDFInfo
- Publication number
- CN117280065A CN117280065A CN202280034063.1A CN202280034063A CN117280065A CN 117280065 A CN117280065 A CN 117280065A CN 202280034063 A CN202280034063 A CN 202280034063A CN 117280065 A CN117280065 A CN 117280065A
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- steel sheet
- hot
- rolled steel
- rolling
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 170
- 239000010959 steel Substances 0.000 title claims abstract description 170
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 42
- 230000000717 retained effect Effects 0.000 claims abstract description 21
- 239000000126 substance Substances 0.000 claims abstract description 18
- 239000000203 mixture Substances 0.000 claims abstract description 16
- 229910000734 martensite Inorganic materials 0.000 claims description 43
- 229910000859 α-Fe Inorganic materials 0.000 claims description 34
- 229910001563 bainite Inorganic materials 0.000 claims description 24
- 239000012535 impurity Substances 0.000 claims description 10
- 229910052721 tungsten Inorganic materials 0.000 claims description 6
- 229910052726 zirconium Inorganic materials 0.000 claims description 5
- 238000001816 cooling Methods 0.000 description 73
- 238000005096 rolling process Methods 0.000 description 52
- 230000000694 effects Effects 0.000 description 47
- 238000005098 hot rolling Methods 0.000 description 42
- 235000013339 cereals Nutrition 0.000 description 24
- 238000000034 method Methods 0.000 description 20
- 229910001562 pearlite Inorganic materials 0.000 description 19
- 230000009467 reduction Effects 0.000 description 19
- 238000012360 testing method Methods 0.000 description 19
- 238000010583 slow cooling Methods 0.000 description 18
- 230000009471 action Effects 0.000 description 14
- 239000010410 layer Substances 0.000 description 14
- 229910052751 metal Inorganic materials 0.000 description 14
- 239000002184 metal Substances 0.000 description 14
- 230000006866 deterioration Effects 0.000 description 12
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 11
- 239000013078 crystal Substances 0.000 description 11
- 238000005259 measurement Methods 0.000 description 10
- 238000007747 plating Methods 0.000 description 9
- 229910052761 rare earth metal Inorganic materials 0.000 description 9
- 238000001953 recrystallisation Methods 0.000 description 9
- 230000007423 decrease Effects 0.000 description 8
- 238000004519 manufacturing process Methods 0.000 description 8
- 229910052720 vanadium Inorganic materials 0.000 description 8
- 239000000523 sample Substances 0.000 description 7
- 239000006104 solid solution Substances 0.000 description 7
- 230000009466 transformation Effects 0.000 description 6
- 238000005336 cracking Methods 0.000 description 5
- 238000009863 impact test Methods 0.000 description 5
- 238000005482 strain hardening Methods 0.000 description 5
- 238000004458 analytical method Methods 0.000 description 4
- 229910052799 carbon Inorganic materials 0.000 description 4
- 229910001567 cementite Inorganic materials 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 4
- 238000001887 electron backscatter diffraction Methods 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 238000005728 strengthening Methods 0.000 description 4
- 238000004804 winding Methods 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 3
- 238000010521 absorption reaction Methods 0.000 description 3
- 230000001133 acceleration Effects 0.000 description 3
- 238000003776 cleavage reaction Methods 0.000 description 3
- 230000007547 defect Effects 0.000 description 3
- 238000011161 development Methods 0.000 description 3
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 3
- 230000003287 optical effect Effects 0.000 description 3
- 230000008520 organization Effects 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
- 238000001878 scanning electron micrograph Methods 0.000 description 3
- 230000007017 scission Effects 0.000 description 3
- 239000002436 steel type Substances 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 229910000746 Structural steel Inorganic materials 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 239000011248 coating agent Substances 0.000 description 2
- 238000000576 coating method Methods 0.000 description 2
- 239000000498 cooling water Substances 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000002950 deficient Effects 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 238000005530 etching Methods 0.000 description 2
- 238000010191 image analysis Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- 229910052742 iron Inorganic materials 0.000 description 2
- 229910052747 lanthanoid Inorganic materials 0.000 description 2
- 150000002602 lanthanoids Chemical class 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000005498 polishing Methods 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 238000005507 spraying Methods 0.000 description 2
- 229910052717 sulfur Inorganic materials 0.000 description 2
- 239000002344 surface layer Substances 0.000 description 2
- 239000013585 weight reducing agent Substances 0.000 description 2
- 229910000838 Al alloy Inorganic materials 0.000 description 1
- 229910021365 Al-Mg-Si alloy Inorganic materials 0.000 description 1
- 238000012935 Averaging Methods 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 229910000861 Mg 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
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 240000008042 Zea mays Species 0.000 description 1
- 235000016383 Zea mays subsp huehuetenangensis Nutrition 0.000 description 1
- 235000002017 Zea mays subsp mays Nutrition 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- 229910007567 Zn-Ni Inorganic materials 0.000 description 1
- 229910007614 Zn—Ni Inorganic materials 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000012670 alkaline solution Substances 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011247 coating layer Substances 0.000 description 1
- 239000008119 colloidal silica Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 230000008021 deposition Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000003618 dip coating Methods 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 238000010894 electron beam technology Methods 0.000 description 1
- HQFCOGRKGVGYBB-UHFFFAOYSA-N ethanol;nitric acid Chemical compound CCO.O[N+]([O-])=O HQFCOGRKGVGYBB-UHFFFAOYSA-N 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 239000000706 filtrate Substances 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 239000005431 greenhouse gas Substances 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 239000012770 industrial material Substances 0.000 description 1
- 235000009973 maize Nutrition 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 238000007711 solidification Methods 0.000 description 1
- 230000008023 solidification Effects 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 241000894007 species Species 0.000 description 1
- 238000010998 test method Methods 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
- 239000011701 zinc Substances 0.000 description 1
Classifications
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- 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
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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
- 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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/84—Controlled slow cooling
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/001—Heat treatment of ferrous alloys containing Ni
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/007—Heat treatment of ferrous alloys containing Co
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- C21D6/00—Heat treatment of ferrous alloys
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- 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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- 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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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- 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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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- 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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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- 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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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- 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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- 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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Abstract
The hot rolled steel sheet has a prescribed chemical composition; for metallic tissue, in area%: the retained austenite is less than 3.00%, the Rcf value representing the ratio of average section units before and after plastic deformation is 2.00 or more, and the tensile strength of the hot-rolled steel sheet is 980MPa or more.
Description
Technical Field
The present invention relates to a hot rolled steel sheet. More specifically, the present invention relates to a hot-rolled steel sheet which is formed into various shapes by press working or the like and is used, in particular, a hot-rolled steel sheet which has high strength and little deterioration in crack propagation stopping characteristics after plastic deformation.
The present application claims priority based on japanese patent application No. 2021-113549 at 7/8 of 2021 and incorporated herein by reference.
Background
In recent years, reduction of carbon dioxide emissions has been pursued in many fields from the viewpoint of global environment protection. Technology development for reducing the weight of a vehicle body for the purpose of reducing fuel consumption is also actively being conducted in automobile factories. However, in order to ensure the safety of passengers, emphasis is placed on improvement of collision resistance, and therefore, it is not easy to reduce the weight of the vehicle body.
In order to reduce emission of greenhouse gases by weight reduction of a vehicle body, it has been studied to use a high-strength steel sheet to thin a member. Accordingly, a steel sheet having both high strength and excellent formability has been strongly desired, and in order to cope with these demands, some techniques have been proposed. On the other hand, non-patent document 1 describes that plastic deformation becomes difficult with the increase in strength of the steel sheet, and in general, crack propagation stopping characteristics are degraded.
Further, in the forced working portion such as a bent portion, since the forced working portion is subjected to large plastic deformation at the time of press forming, and the strength is increased by work hardening, the crack propagation stop characteristic is further lowered, and press cracking may occur in the portion subjected to large plastic deformation. The deterioration of crack propagation stopping properties after plastic deformation has been a problem of thick plate materials used for ship or structural steel, but with recent increases in strength, studies have been required for forming hot rolled steel plates as automotive materials.
Regarding the technique of improving toughness after plastic deformation, for example, patent document 1 discloses a steel sheet for a large-sized structure, which is excellent in crack propagation stopping characteristics after plastic deformation by setting the ferrite grain size of the surface layer to 3 μm or less in addition to strict control of impurity elements.
Patent document 2 discloses a large-sized structural steel sheet having excellent crack propagation stopping characteristics after plastic deformation by forming ferrite grains having a flatness ratio of 2 or more and a short axial diameter of 5 μm or less and a structure including subgrain grains having an equivalent circle diameter of 3 μm or less in the ferrite crystals.
Prior art literature
Patent literature
Patent document 1: japanese patent No. 3499085
Patent document 2: japanese patent No. 3467767
Non-patent literature
Non-patent document 1: gao Qiaoxiong three, river field treatment, tide Tian Hao, maize original Zhou: iron and steel, 99, (2013), 4, 312-321
Disclosure of Invention
Problems to be solved by the invention
The techniques disclosed in patent documents 1 and 2 are related to steel sheets for large structures, and do not refer to hot-rolled steel sheets. Further, since the steel sheet has a strength of 450 to 700MPa, it is difficult to apply the techniques disclosed in patent documents 1 and 2 to a hot-rolled steel sheet having a high strength of 980MPa or more mainly composed of bainite or martensite.
The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and less deterioration in crack propagation stopping characteristics after plastic deformation.
Means for solving the problems
In view of the above problems, the present inventors have repeatedly studied the relationship between the chemical composition and the metal structure of a hot-rolled steel sheet and the mechanical properties, and as a result, have obtained the following findings (a) to (e), and have completed the present invention.
(a) In order to obtain excellent tensile (maximum) strength, it is preferable to effectively use a hard structure. That is, it is preferable to include martensite or bainite in the metallic structure.
(b) However, since the hard structure is a structure having insufficient crack propagation stopping properties, when only a metal structure mainly composed of these structures is produced, excellent crack propagation stopping properties cannot be ensured.
(c) Further, since work hardening occurs by receiving plastic deformation, crack propagation stopping characteristics after plastic deformation are further lowered.
(d) In order to provide a high-strength hot-rolled steel sheet having excellent crack propagation stopping properties and to suppress deterioration of the crack propagation stopping properties after plastic deformation, it is effective to obtain a microstructure such as a microstructure that is fine and has a curved propagation path of cracks after plastic deformation.
Specifically, the miniaturization of the cross-section unit after plastic deformation is effective for suppressing deterioration of crack propagation stopping characteristics after plastic deformation.
(e) In order to miniaturize the section unit after plastic deformation, it is effective to control the heating conditions of the slab, the hot rolling conditions, and the cooling conditions after hot rolling. This can produce austenite grains which are fine and have a large difference in orientation between the tissues generated during bcc transformation, and can miniaturize the section unit after plastic deformation.
The gist of the present invention based on the above findings is as follows.
(1) The hot rolled steel sheet according to an embodiment of the present invention has a chemical composition comprising, in mass%:
C：0.040～0.400％、
Si：0.05～3.00％、
Mn：1.00～4.00％、
sol.Al：0.001～0.500％、
p:0.100% or less,
S:0.0300% or less,
N: less than 0.1000 percent,
O:0.0100% or less,
Ti：0～1.000％、
V：0～1.000％、
Nb：0～1.000％、
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％、
1 or more than 2 of Zr, co, zn and W: 0 to 1.00% in total
Sn：0～0.05％，
The rest part contains Fe and impurities;
in the case of a metallic structure,
in area-%: the retained austenite is less than 3.00%,
an Rcf value representing a ratio of average cross-section units before and after plastic deformation of 2.00 or more;
the tensile strength of the hot-rolled steel sheet is 980MPa or more.
(2) The hot-rolled steel sheet according to the above (1), wherein the chemical composition may contain 1 or 2 or more kinds of elements selected from the group consisting of:
Ti：0.010～1.000％、
V：0.010～1.000％、
Nb：0.010～1.000％、
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 to 0.1000 percent
Bi：0.0005～0.020％。
(3) The hot rolled steel sheet according to the above (1) or (2), wherein the above metal structure may be represented by area%: ferrite 15.00-60.00% and martensite 40.00-85.00%.
(4) The hot rolled steel sheet according to the above (1) or (2), wherein the above metal structure may be represented by area%: the bainite is 50.00% or more.
(5) The hot rolled steel sheet according to the above (1) or (2), wherein the above metal structure may be represented by area%: martensite exceeds 85.00%.
Effects of the invention
According to the above aspect of the present invention, a hot-rolled steel sheet having high strength and little deterioration in crack propagation stopping characteristics after plastic deformation can be obtained.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material for use in automobile parts, machine structural parts, and building parts.
Detailed Description
The chemical composition and the microstructure of the hot-rolled steel sheet according to the present embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications may be made without departing from the scope of the present invention.
The numerical values described below with "to" are defined, and the lower limit value and the upper limit value are included in the range. With respect to values expressed as "below" or "above," the value is not included in the numerical range. In the following description, the% of the chemical composition of the steel sheet is mass% unless otherwise specified.
1. Chemical composition
The hot-rolled steel sheet according to the present embodiment contains C: 0.040-0.400%, si:0.05 to 3.00 percent of Mn: 1.00-4.00%, sol.Al:0.001 to 0.500 percent, P:0.100% or less, S:0.0300% or less, N: less than 0.1000%, O: less than 0.0100%, the remainder: fe and impurities. The elements are described in detail below.
(1-1)C：0.040～0.400％
C increases the fraction of the hard phase and decreases the transformation point of the hard phase, thereby increasing the strength of the hot-rolled steel sheet. When the C content is less than 0.040%, it becomes difficult to obtain a desired strength. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.
On the other hand, when the C content exceeds 0.400%, a large amount of carbide is formed in the tissue, and the occurrence of internal defects during plastic deformation is promoted, and deterioration of crack propagation stopping characteristics after plastic deformation becomes large. As a result, a desired Rcf value cannot be obtained. Therefore, the C content is set to 0.400% or less. The C content is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.150% or less.
(1-2)Si：0.05～3.00％
Si has the effect of increasing the strength of a hot-rolled steel sheet by solid solution strengthening at normal temperature and the effect of increasing the toughness of a hot-rolled steel sheet by solid solution softening at low temperature. Si also has a function of strengthening steel (suppressing defects such as occurrence of voids in steel) by deoxidizing. If the Si content is less than 0.05%, the effect due to the above-mentioned action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, more preferably 0.80% or more.
However, when the Si content exceeds 3.00%, the surface properties and chemical conversion treatability, and further ductility and weldability of the steel sheet are significantly deteriorated, and the surface energy of fracture is lowered. This facilitates the generation and propagation of cracks during plastic deformation, and increases deterioration of crack propagation stopping characteristics after plastic deformation. As a result, a desired Rcf value cannot be obtained. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, more preferably 2.50% or less.
(1-3)Mn：1.00～4.00％
Mn has an effect of suppressing ferrite transformation to increase the strength of a hot-rolled steel sheet and an effect of improving the toughness of a hot-rolled steel sheet by solid solution softening at a low temperature. When 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 1.30% or more, more preferably 1.50% or more.
On the other hand, when the Mn content exceeds 4.00%, the effect of reducing the surface energy of fracture increases, and the occurrence and propagation of cracks during plastic deformation become easier, and deterioration of crack propagation stopping characteristics after plastic deformation increases. As a result, a desired Rcf value cannot be obtained. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less.
(1-4)sol.Al：0.001～0.500％
Al has the function of deoxidizing the steel to strengthen the steel, and also has the function of improving the toughness of the hot-rolled steel sheet by exhibiting solid solution softening at low temperature, similarly to Si. When the al content is less than 0.001%, the effect due to the above-mentioned action cannot be obtained. Further, when the sol.al content is less than 0.001%, a desired Rcf value cannot be obtained. Therefore, the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.010% or more.
On the other hand, when the sol.al content exceeds 0.500%, the above effect is saturated and economically unfavorable, and therefore the sol.al content is set to 0.500% or less. The sol.al content is preferably 0.300% or less, more preferably 0.100% or less.
In addition, sol.Al refers to acid-soluble Al, and indicates solid-solution Al existing in steel in a solid-solution state.
(1-5) P: less than 0.100%
P is an element generally contained as an impurity, but is also an element having an effect of improving the strength of a hot-rolled steel sheet by solid solution strengthening. Therefore, P may be positively contained, but P is an element that is liable to segregate, and if the P content exceeds 0.100%, the decrease in grain boundary strength due to grain boundary segregation becomes remarkable, and grain boundary fracture tends to occur. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less.
The lower limit of the P content is not particularly limited, but is preferably set to 0.001% or more from the viewpoint of refining cost.
(1-6) S:0.0300% or less
S is an element contained as an impurity, and forms sulfide-based inclusions in steel to promote the generation of cracks. If the S content exceeds 0.0300%, crack generation during plastic deformation becomes remarkable, and crack propagation stop characteristics after plastic deformation are remarkably lowered. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less.
The lower limit of the S content is not particularly limited, but is preferably set to 0.0001% or more from the viewpoint of refining cost.
(1-7) N: less than 0.1000%
N is an element contained in steel as an impurity, and has an effect of promoting the generation of cracks starting from the impurity. When the N content exceeds 0.1000%, the occurrence of cracks during plastic deformation becomes remarkable, and the crack propagation stopping characteristics after plastic deformation are remarkably lowered. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less.
The lower limit of the N content is not particularly limited, but may be set to 0.0001% or more. In the case where the metal structure is further refined by containing 1 or 2 or more of Ti, nb, and V, the N content is preferably set to 0.0010% or more, and more preferably set to 0.0020% or more, in order to promote precipitation of carbonitrides.
(1-8) O:0.0100% or less
If O is contained in a large amount in steel, coarse oxides serving as starting points of fracture are formed, causing 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, more preferably 0.0050% or less.
In order to disperse a large amount of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.
The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities are components mixed from raw materials such as ores, scrap iron, and manufacturing environments, and/or components which are allowed within a range that does not adversely affect the hot-rolled steel sheet of the present embodiment.
The hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in place of part of Fe. The lower limit of the content in the case of not containing an optional element is 0%. Hereinafter, optional elements will be described in detail.
(1-9) Ti:0.010 to 1.000 percent of Nb: 0.010-1.000% and V:0.010 to 1.000 percent
Ti, nb, and V are elements that are finely precipitated as carbide and nitride in steel and enhance the strength of the steel by precipitation strengthening. Therefore, 1 or 2 or more of these elements may be contained. In order to obtain this effect more reliably, the contents of Ti, nb, and V are preferably set to 0.010% or more, respectively. It is not necessary to contain all of Ti, nb, and V, and the content of any 1 species may be 0.010% or more. The content of Ti, nb, and V is preferably 0.060% or more, more preferably 0.080% or more, respectively.
On the other hand, if the content of any one of Ti, nb and V exceeds 1.000%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the contents of Ti, nb and V are each set to 1.000% or less. Preferably 0.800% or less, more preferably 0.500% or less.
(1-10) Cu:0.01 to 2.00 percent of Cr:0.01 to 2.00 percent of Mo:0.01 to 1.00 percent of Ni:0.02 to 2.00 percent and B: 0.0001-0.0100%
Cu, cr, mo, ni and B each have an effect of improving hardenability of a hot-rolled steel sheet. Further, ni has an effect of effectively suppressing grain boundary cracking of a slab due to Cu when Cu is contained. Therefore, 1 or 2 or more of these elements may be contained.
As described above, cu has an effect of improving hardenability of the hot-rolled steel sheet. In order to obtain the effect by the above action more reliably, the Cu content is preferably set to 0.01% or more, more preferably to 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less.
As described above, cr has an effect of improving hardenability of the hot-rolled steel sheet. In order to obtain the effect by the above action more reliably, the Cr content is preferably set to 0.01% or more, more preferably to 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly lowered. Therefore, the Cr content is set to 2.00% or less.
As described above, mo has an effect of improving hardenability of a hot-rolled steel sheet and an effect of improving strength of the hot-rolled steel sheet by precipitating carbide in the steel. In order to obtain the effect by the above action more reliably, the Mo content is preferably set to 0.01% or more, more preferably to 0.02% or more. However, even if the Mo content is set to more than 1.00%, the effect due to the above action is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less, more preferably 0.20% or less.
As described above, ni has an effect of improving hardenability of the hot-rolled steel sheet. In addition, ni has an effect of effectively suppressing grain boundary cracking of a slab due to Cu when Cu is contained. In order to obtain the effect by the above action more reliably, the Ni content is preferably set to 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain Ni in a large amount. Therefore, the Ni content is set to 2.00% or less.
As described above, B has an effect of improving hardenability of the hot-rolled steel sheet. In order to obtain the effect by this action more reliably, the B content is preferably set to 0.0001% or more, more preferably 0.0002% or more. However, when the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, and therefore the B content is set to 0.0100% or less. The B content is preferably set to 0.0050% or less.
(1-11) Ca:0.0005 to 0.0200 percent, mg: 0.0005-0.0200%, REM:0.0005 to 0.1000 percent of Bi:0.0005 to 0.020%
Ca. Mg and REM each have an effect of improving crack propagation stopping characteristics of a hot-rolled steel sheet by adjusting the shape of inclusions in the steel to a preferable shape. In addition, bi has an effect of improving crack propagation stopping characteristics of the hot-rolled steel sheet by refining the solidification structure. Therefore, 1 or 2 or more of these elements may be contained. In order to obtain the effect by the above action more reliably, it is preferable to set at least 0.0005% of any one of Ca, mg, REM and Bi. However, if the Ca content or Mg content exceeds 0.0200% or the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, and the crack propagation stopping properties of the hot-rolled steel sheet may be deteriorated instead. In addition, even if the Bi content is set to more than 0.020%, the effect due to the above action is saturated, which is not economically preferable. Therefore, the Ca content and Mg content were set to 0.0200% or less, the REM content was set to 0.1000% or less, and the Bi content was set to 0.020% or less. The Bi content is preferably 0.010% or less.
Here, REM means 17 elements in total including Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of lanthanoids, it is industrially added in the form of misch metals.
(1-12) 1 or more than 2 of Zr, co, zn and W: 0 to 1.00% by weight of Sn:0 to 0.05 percent
Regarding Zr, co, zn, and W, the present inventors confirmed that: even if these elements are contained in an amount of 1.00% or less in total, the effect of the hot-rolled steel sheet of the present embodiment is not impaired. Accordingly, 1 or 2 or more of Zr, co, zn, and W may be contained in total at most 1.00%.
Furthermore, the present inventors confirmed that: even if Sn is contained in a small amount, the effect of the hot rolled steel sheet of the present embodiment is not impaired. However, if a large amount of Sn is contained, defects may occur during hot rolling, and thus the Sn content is set to 0.05% or less.
The chemical composition of the hot-rolled steel sheet may be measured by a general analytical method. For example, measurement can be performed by using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), inductively coupled plasma-atomic emission spectrometry. The sol.Al may be measured by ICP-AES using a filtrate obtained by thermally decomposing a sample with an acid. The C and S may be measured by a combustion-infrared absorption method, N by an inert gas melting-thermal conductivity method, and O by an inert gas melting-non-dispersive infrared absorption method.
2. Metal structure of hot rolled steel sheet
Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
The microstructure of the hot-rolled steel sheet according to the present embodiment is expressed in area%: the residual austenite is less than 3.00%, and the Rcf value representing the ratio of average section units before and after plastic deformation is 2.0 or more.
Therefore, the hot-rolled steel sheet according to the present embodiment can obtain high strength and excellent crack propagation stopping characteristics even after plastic deformation.
In the present embodiment, the tissue fraction and Rcf value in the metal structure at the center position in the sheet width direction are defined by a depth of 1/4 of the sheet thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface) in the cross section parallel to the rolling direction. The reason for this is that the microstructure at this position represents a representative microstructure of the steel sheet.
(2-1) area ratio of retained austenite: less than 3.00%
Retained austenite is a metallic structure that exists as fcc even at room temperature. The carbon concentration in the structure around the retained austenite may become a starting point of crack generation by transformation into hard martensite during plastic deformation. When the area ratio of retained austenite is 3.00% or more, the above effect becomes remarkable, and crack propagation stopping characteristics after plastic deformation are remarkably lowered. Therefore, the area ratio of the retained austenite is set to be less than 3.00%. The area ratio of the retained austenite is preferably 2.00% or less, less than 1.50% or 1.00% or less, more preferably less than 1.00% or less than 0.50%. The smaller the retained austenite is, the more preferable, and thus the area ratio of the retained austenite may be 0.00%.
Methods for measuring the area ratio of retained austenite include methods such as X-ray diffraction, EBSP (electron back scattering diffraction image, electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. In the present embodiment, the area ratio of the retained austenite is measured by X-ray diffraction.
In the measurement of the retained austenite area ratio by X-ray diffraction in the present embodiment, first, the integrated intensities of the total 7 peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220) are obtained by using Co-ka rays in a cross section parallel to the rolling direction at the center position in the widthwise direction of the hot-rolled steel sheet at a depth of 1/4 of the sheet thickness (a region from 1/8 depth to 3/8 depth from the surface), and calculated by using an intensity averaging method, thereby obtaining the area ratio of retained austenite.
The microstructure of the hot-rolled steel sheet according to the present embodiment may contain ferrite, martensite, bainite, and pearlite in addition to retained austenite.
The area ratio of ferrite may be set to 60.00% or less, 50.00% or less, or 45.00% or less. The area ratio of ferrite may be set to 0.00% or more or 0.05% or more.
The area ratio of martensite may be set to 100.00% or less or 99.00% or less. The area ratio of martensite may be set to 0.00% or more, 1.00% or more, or 1.50% or more.
The area ratio of bainite may be set to 100.00% or less or 96.00% or less. The area ratio of bainite may be set to 0.00% or more or 0.01% or more.
The area ratio of the retained austenite, ferrite, bainite, and martensite described above is an area ratio applicable to all of the 3 steel types (DP steel, bainitic steel, and martensitic steel) described later.
Hereinafter, preferred area ratios of each structure in each of the 3 steel types (DP steel, bainitic steel, and martensitic steel) will be described.
(2-2) DP steel
In the case where the metal structure contains less than 3.00% of retained austenite, and ferrite, martensite, and a trace amount of the remaining structure in terms of area ratio, it is preferable that: the area ratio of ferrite is 15.00-60.00%, the area ratio of martensite is 40.00-85.00%, and the area ratio of the residual structure is lower than 45.00%. By setting the area ratio of each structure as described above, strength and ductility can be improved in a balanced manner.
The following description is made for each organization in this scheme.
Area ratio of ferrite: 15.00-60.00%
Ferrite is a structure generated when fcc changes to bcc at a relatively high temperature. Ferrite has a high work hardening rate, and thus has an effect of improving the strength-ductility balance of the hot-rolled steel sheet. By setting the area ratio of ferrite to 15.00% or more, the effects due to the above-described actions can be sufficiently obtained. Therefore, the area ratio of ferrite is preferably set to 15.00% or more. The area ratio of ferrite is more preferably 20.00% or more, 25.00% or more, or 30.00% or more.
On the other hand, ferrite has a low strength, and therefore if the area ratio is excessive, a desired tensile strength may not be obtained. Therefore, the area ratio of ferrite is preferably set to 60.00% or less. The area ratio of ferrite is more preferably 55.00% or less, and still more preferably 50.00% or less.
Area ratio of martensite: 40.00-85.00%
Martensite is a structure that is generated when fcc changes to bcc at a relatively low temperature. Martensite is a structure containing fine and high dislocation density grains, and has an effect of improving the strength of a hot-rolled steel sheet. The effect due to the above-described action can be sufficiently obtained by setting the area ratio of martensite to 40.00% or more. Therefore, the area ratio of martensite is preferably set to 40.00% or more. The area ratio of martensite is preferably 50.00% or more.
On the other hand, the martensite is insufficient in ductility, and if the area ratio is excessive, the ductility of the hot-rolled steel sheet may be lowered. Therefore, the area ratio of martensite is preferably set to 85.00% or less. The area ratio of martensite is more preferably 80.00% or less, still more preferably 75.00% or less or 70.00% or less.
Area ratio of remaining tissue: less than 45.00%
In this embodiment, pearlite and bainite may be contained in an amount of less than 45.00% in total as the remainder of the structure. The area ratio of the remaining tissue may be 10.00% or less or 5.00% or less. The remaining tissue may not be contained, and the area ratio may be 0.00% in total.
(2-3) Bainite Steel
In the case where the metal structure contains less than 3.00% of retained austenite, and bainite, and a trace amount of the remaining structure in terms of area ratio, it is preferable that: the area ratio of the bainite is more than 50.00%, and the area ratio of the residual structure is less than 50.00%. By setting the area ratio of each structure as described above, strength, ductility, and hole expansibility can be simultaneously improved.
The following description is made for each organization in this scheme.
Area ratio of bainite: 50.00% or more
Bainite is a structure that is generated when fcc changes to bcc at low temperatures. Bainite is a microstructure including fine grains and carbides, and has an effect of uniformly improving strength, ductility, and hole expansibility of a hot-rolled steel sheet. The effect due to the above-described action can be sufficiently obtained by setting the area ratio of bainite to 50.00% or more. Therefore, the area ratio of bainite is preferably set to 50.00% or more. The area ratio of bainite is more preferably 80.00% or more, 85.00% or more, or 90.00% or more.
The upper limit is not particularly limited, but may be set to 100.00% or less.
Residual tissue: less than 50.00%
In this embodiment, pearlite, ferrite, and martensite may be contained in an amount of less than 50.00% in total as the remaining structure. The area ratio of the remaining tissue is more preferably 20.00% or less, 15.00% or less, or 10.00% or less. The remaining tissue may not be contained, and the area ratio may be 0.00% in total.
(2-4) martensitic Steel
In the case where the metal structure contains less than 3.00% of retained austenite, martensite, and a trace amount of the remaining structure in terms of area ratio, it is preferable that: the total area ratio of martensite is 85.00% or more, and the area ratio of the remaining structure is less than 15.00%. By setting the area ratio of each structure as described above, the strength and hole expansibility can be improved at the same time.
The following description is made for each organization in this scheme.
Aggregate of area ratios of martensite: more than 85.00%
As described above, martensite has an effect of improving the strength of the hot rolled steel sheet. Further, martensite has a random crystal orientation of the structure, and thus has an effect of improving hole expansibility of the hot-rolled steel sheet. By setting the area ratio of martensite to be more than 85.00%, the effect due to the above-described action can be sufficiently obtained. Therefore, the area ratio of martensite is preferably set to be more than 85.00%. The area ratio of martensite is more preferably 90.00% or more, 93.00% or more, or 95.00% or more. The upper limit is not particularly limited, but may be set to 100.00% or less.
Residual tissue: below 15.00%
In this embodiment, pearlite, ferrite, and bainite may be contained in an amount of less than 15.00% in total as the remainder. The area ratio of the remaining tissue is more preferably 10.00% or less, 7.00% or less, or 5.00% or less. The remaining tissue may not be contained, and the area ratio may be 0.00% in total.
(2-5) area ratio of pearlite: less than 5.00%
Pearlite is a lamellar metallic structure in which cementite precipitates as a layer between ferrite and ferrite, and is soft when compared with bainite or martensite. If the area ratio of pearlite is 5.00% or more, carbon is consumed by cementite contained in pearlite, and the strength of martensite and bainite is lowered, and a tensile strength of 980MPa or more may not be obtained. Therefore, in any of the above embodiments, the area ratio of pearlite can be set to less than 5.00%. The area ratio of pearlite is more preferably 3.00% or less. In order to improve stretch flangeability of the hot-rolled steel sheet, the area ratio of pearlite is preferably reduced as much as possible, and the area ratio of pearlite is more preferably 0.00%.
The area ratio of pearlite here is an area ratio applicable to all of the above-mentioned 3 steel types (DP steel, bainitic steel, and martensitic steel).
The structure other than the retained austenite was measured by the following method.
The area ratio of ferrite and pearlite was measured by the following method. In a section parallel to the rolling direction, samples were collected so that a depth of 1/4 of the plate thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface) and a central position in the plate width direction were observed. The cross section of the sample parallel to the rolling direction was finished to a mirror surface, and the sample was polished at room temperature for 8 minutes using colloidal silica containing no alkaline solution, to remove strain introduced into the surface layer of the sample.
The crystal orientation was obtained by measuring a region having a length of 50 μm and a depth of 1/4 of the plate thickness from the surface (a region having a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface) and a central position in the width direction of the plate at a measurement interval of 0.1 μm at any position in the longitudinal direction of the sample cross section by an electron back scattering diffraction methodInformation. The number of measured points is set to at least 500 points. For the measurement, an EBSD device composed of a thermal field emission type scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector) was used. At this time, the vacuum degree in the EBSD device was set to 9.6X10 -5 Pa or less, the acceleration voltage was set to 15kV, the irradiation current level was set to 13, and the irradiation level of the electron beam was set to 62.
Further, the reflected electron image is captured in the same field of view. First, from the reflected electron image, crystal grains in which ferrite and cementite are precipitated in a layered state are specified, and the area ratio of the crystal grains is calculated to obtain the area ratio of pearlite. Then, the obtained crystal orientation information was determined to be ferrite in a region having a Grain Average Misorientation value of 1.0 ° or less by using a function "Grain Average Misorientation" mounted in software "OIM Analysis (registered trademark)" attached to the EBSD Analysis apparatus, for the crystal grains other than those determined to be pearlite. The area ratio of ferrite is obtained by obtaining the area ratio of the area determined to be ferrite.
The same observation surface as in the above measurement was polished, and then subjected to nitric acid-ethanol etching, and at least 3 regions were observed in a region of 30 μm×30 μm at a position 1/4 of the plate thickness from the surface (a region 1/8 depth from the surface in the plate thickness direction to a region 3/8 depth from the surface in the plate thickness direction) using an optical microscope and a Scanning Electron Microscope (SEM). The microstructure photograph obtained by the microstructure observation was subjected to image analysis to obtain the area ratio of bainite. Then, after the Lepera etching was performed at the same observation position, the structure was observed with an optical microscope and a scanning electron microscope, and the obtained structure photograph was subjected to image analysis, whereby the area ratio of martensite was calculated.
In the above-described tissue observation, each tissue was identified by the following method.
Since martensite is a structure having a high dislocation density and a lower structure such as a block (block) and a packet (packet) in a crystal grain, it can be distinguished from other metal structures by using an electron channel contrast image using a scanning electron microscope.
A structure which is a collection of lath-shaped crystal grains and which is not martensitic in a structure which does not contain Fe-based carbide having a length of 20nm or more in the interior of the structure, and a structure which contains Fe-based carbide having a length of 20nm or more in the interior of the structure, and which has a single modification, that is, fe-based carbide elongated in the same direction, are regarded as bainite. Here, the Fe-based carbide extending in the same direction means a carbide in which the difference in the extending direction of the Fe-based carbide is within 5 °.
The rolling direction of the hot rolled steel sheet was determined by the following method.
First, test pieces were collected so that the thickness and cross section of the hot-rolled steel sheet could be observed. The plate thickness cross section of the test piece thus collected was finished by mirror polishing, and then observed with an optical microscope. The observation range was set to the total thickness of the sheet thickness, and the direction parallel to the extending direction of the crystal grains was determined as the rolling direction.
Rcf value representing the ratio of average section units before and after plastic deformation: 2.00 or more
In general, since the strength is increased by the introduction of dislocations during plastic deformation, the crack propagation stopping characteristics of the hot rolled steel sheet after plastic deformation are reduced. Since a crack is formed in a split section called a section unit, it is important to suppress propagation of the crack to miniaturize the section unit and bend a propagation path. In the present embodiment, the reduction of crack propagation stopping characteristics after plastic deformation is suppressed by controlling the value of Rcf (Ratio of cleavage facet: ratio of cleavage planes, which may also be referred to as ratio of cleavage planes) indicating the ratio of cross-sectional units before and after plastic deformation.
The Rcf value represents the ratio of average cross-section units before and after plastic deformation, and is expressed by the following formula using the Cf1 value, which is the average cross-section unit before plastic deformation, and the Cf2 value, which is the average cross-section unit after plastic deformation.
Rcf＝Cf1/Cf2
In a hot-rolled steel sheet subjected to plastic deformation, the strength increases due to work hardening by dislocation introduction, and thus crack propagation stopping characteristics decrease. On the other hand, crack propagation stop characteristics are also affected by the cross-sectional units, and the finer the cross-sectional units, the more the propagation path is curved, and the more propagation of cracks can be suppressed. Therefore, in order to exhibit high crack propagation stopping characteristics also after plastic deformation, it is necessary to increase the Rcf value.
If the Rcf value is less than 2.00, the effect of increasing the strength by dislocation introduction is greater than the effect by miniaturization of the cross-section unit, and thus it is estimated that the crack propagation stop characteristic is lowered. Thus, the Rcf value is set to 2.00 or more. Preferably 2.20 or more, more preferably 2.30 or more.
The higher the Rcf value, the more preferable, and the upper limit is not particularly specified, but may be set to 5.00 or less, 4.00 or less, or 3.00 or less.
The Cf1 value and the Cf2 value can be obtained by the following methods.
In this embodiment, in order to calculate the Cf1 value and the Cf2 value, brittle fracture needs to occur. As a test method for generating brittle fracture, for example, it is only required to use a test according to JIS Z2242: 2018, a small-sized V-notch test piece of 2.5mm was produced in a direction in which the width direction (C direction) of the hot-rolled steel sheet became the longitudinal direction of the test piece, and a Charpy impact test was performed at-196 ℃. The test piece having a thickness of the hot-rolled steel sheet of less than 2.5mm may be a full thickness.
The rolling direction of the hot-rolled steel sheet was determined by the above method, and the direction perpendicular to the rolling direction was determined as the width direction of the hot-rolled steel sheet.
The image region of the SEM image taken for calculating the Cf1 value and the Cf2 value was set to a 1/4 depth position (a region from 1/8 depth to 3/8 depth of the plate thickness) from the surface of the steel plate in the cross section parallel to the rolling direction, and the center position in the width direction. For SEM image photographing, SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies, inc., was used to set the emitter to tungsten at 9.6X10 -5 In a vacuum of Pa or less, the acceleration voltage was set to 1.5kV. The shooting magnification is set to 1000 times, and the number of shooting fields is set to 3Above the field of view.
In the SEM images taken, ductile fracture parts, called tear ridges, are taken as bright contrast. The area surrounded by the tearing ridge is set as one splitting micro-surface, the equivalent circle diameter is obtained from the area of each splitting micro-surface, and the equivalent circle diameter is set as a section unit of each splitting micro-surface. From the obtained cross-section unit, the area average diameter weighted by the area of each split micro-surface was obtained, and this was set as the average cross-section unit.
The above-described processing was performed on the hot-rolled steel sheet before plastic deformation and the hot-rolled steel sheet after plastic deformation, to calculate the Cf1 value and the Cf2 value, respectively.
In the charpy impact test after plastic deformation, JIS No. 5 tensile test pieces were produced in a direction in which the width direction (C direction) of the hot-rolled steel sheet became the test piece length direction, and after 10% compressive pre-strain was applied to the steel material in the test piece length direction, various test pieces were collected.
Standard deviation of Mn concentration: 0.60 mass% or less
In the hot-rolled steel sheet of the present embodiment, the standard deviation of the Mn concentration at the center position in the sheet width direction may be set to 0.60 mass% or less with a depth of 1/4 of the sheet thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface). This can suppress the development of a region where Mn is locally concentrated and the fracture energy is reduced, and further suppress the generation of local cracks during plastic deformation and the reduction of crack propagation stopping characteristics after plastic deformation.
The standard deviation of the Mn concentration is preferably 0.50 mass% or less, more preferably 0.47 mass% or less. The lower limit of the standard deviation of the Mn concentration is preferably smaller from the viewpoint of suppressing the decrease in fracture energy, but the substantial lower limit is 0.10 mass% from the viewpoint of the restrictions of the manufacturing process.
After mirror polishing a cross section (L-section) parallel to the rolling direction of the hot rolled steel sheet, the surface of the steel sheet was measured at a depth of 1/4 of the thickness (a region from 1/8 of the depth of the thickness to 3/8 of the depth of the thickness), and the center position in the width direction of the sheet was measured by an Electron Probe Microanalyzer (EPMA), and the standard deviation of the Mn concentration was measured. The measurement conditions are as follows: the acceleration voltage was set at 15kV, the magnification was set at 5000 times, and a distribution image was measured (region from 1/8 depth to 3/8 depth from the surface) and was in the range of 20 μm in the thickness direction of the sample plate. More specifically, the Mn concentration at 40000 or more was measured with the measurement interval set to 0.1. Mu.m. Next, the standard deviation of the Mn concentration was obtained by calculating the standard deviation based on the Mn concentrations obtained from all the measurement points.
3. Tensile characteristics
Tensile characteristics (tensile strength) among mechanical properties of hot rolled steel sheet according to JIS Z2241: 2011. Test piece was set as JIS Z2241: 2011 test piece No. 5. The collecting position of the tensile test piece may be set to 1/4 of the distance from the end in the width direction of the sheet, and the direction perpendicular to the rolling direction may be set to the longitudinal direction.
The hot-rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 980MPa or more. Preferably 1000MPa or more. If the tensile strength is less than 980MPa, the applicable components are limited, and the contribution to weight reduction of the vehicle body is small. The upper limit is not particularly limited, but may be set to 1780MPa or less, 1500MPa or less, or 1300MPa or less from the viewpoint of suppressing die wear.
4. Plate thickness
The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be set to 1.20 to 8.00mm. When the plate thickness of the hot rolled steel sheet is less than 1.20mm, there are cases where it becomes difficult to secure the rolling completion temperature, and the rolling load becomes excessive and hot rolling becomes difficult. Therefore, the thickness of the hot-rolled steel sheet according to the present embodiment may be 1.20mm or more. Preferably 1.40mm or more. On the other hand, when the plate thickness exceeds 8.00mm, the influence of the standard deviation of the Mn concentration becomes remarkable in some cases, and it becomes difficult to obtain a desired Rcf value. Therefore, the plate thickness may be set to 8.00mm or less. Preferably 6.00mm or less or 3.00mm or less.
5. Others
(5-1) coating
The hot-rolled steel sheet of the present embodiment having the above-described chemical composition and metallic structure may be produced into a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer may be a plating layer or a hot dip plating layer. Examples of the plating layer include a zinc plating layer and a Zn-Ni alloy plating layer. Examples of the hot dip coating layer include a hot dip galvanized layer, an alloyed hot dip galvanized layer, a hot dip aluminized layer, a hot dip zn—al alloy layer, a hot dip zn—al—mg alloy layer, and a hot dip zn—al—mg—si alloy layer. The plating deposition amount is not particularly limited, and may be set as in the conventional case. Further, the corrosion resistance can be further improved by performing an appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.
6. Production conditions
A preferred method for producing a hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallic structure is as follows.
In order to obtain the hot-rolled steel sheet of the present embodiment, it is effective to heat the slab under predetermined conditions, then hot-roll the slab, accelerate the cooling to a predetermined temperature range, then slowly cool the slab if necessary, and control the cooling process until coiling.
In a preferred method for producing a hot-rolled steel sheet according to the present embodiment, the following steps (1) to (7) are performed in this order. The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
(1) The slab is heated and maintained at a temperature of 1100 ℃ or higher for 6000 seconds or longer. In addition, when heating, more preferably: after being maintained in a temperature range of 700 to 850 ℃ for 900 seconds or more, the heating is further performed, and the temperature range of 1100 ℃ or more is maintained for 6000 seconds or more.
(2) Hot rolling is performed in a temperature range of 850 to 1100 ℃ so that the total sheet thickness is reduced by 90% or more.
(3) The initial temperature of hot rolling is 850 ℃ or higher and lower than 930 ℃, the rolling temperature of the first two stages of the 1 st to final stages of hot rolling is 850 ℃ or higher and lower than 950 ℃, and the rolling reduction is lower than 30%.
(4) The rolling temperature of the final stage of the hot rolling and the stage preceding the final stage is set to 930 ℃ or higher and lower than 1010 ℃, the rolling reduction is set to 50% or higher, and the rolling completion temperature is set to 950 ℃ or higher and lower than 1010 ℃.
(5) Cooling is performed at an average cooling rate of 50 ℃ per second or more within 1.0 second after completion of hot rolling, and the cooling start temperature is set to 850 ℃ or more and less than 960 ℃.
(6) Cooling to a temperature range of 600-730 ℃ at an average cooling rate of 50 ℃/s or more, and slowly cooling at an average cooling rate of less than 5 ℃/s for 2.0 seconds or more in the temperature range of 600-730 ℃. Then, the mixture is cooled to a temperature range of 350 ℃ or lower at an average cooling rate of 50 ℃ per second or higher.
It should be noted that the slow cooling may be omitted, and the cooling may be performed at an average cooling rate of 50 ℃/s or more to a temperature range of 350 ℃ or less without the slow cooling.
(7) Coiling in a temperature region below 350 ℃.
(6-1) slab, slab temperature at the time of Hot Rolling, and holding time
As the slab to be hot-rolled, a slab obtained by continuous casting, a slab obtained by casting and cogging, or the like can be used, and a slab obtained by hot working or cold working the above may be used as needed. The slab to be hot-rolled is preferably heated and maintained at a temperature of 1100 ℃ or higher for 6000 seconds or longer. In the case of holding at 1100 ℃ or higher, the steel sheet temperature may be varied in a temperature range of 1100 ℃ or higher, or may be set to be constant. By holding the temperature in the region of 1100 ℃ or higher for 6000 seconds or longer, austenite grains can be made uniform when the slab is heated. By making austenite grains uniform, recrystallization of austenite in a hot rolling stage (first two stages of the 1 st to final stages of hot rolling) described later can be suppressed, and as a result, a desired Rcf value can be obtained. When the holding temperature is lower than 1100 ℃ or the holding time is lower than 6000 seconds, it becomes difficult to make austenite grains uniform, and recrystallization of austenite in the hot rolling stage described later cannot be suppressed, and as a result, a desired Rcf value may not be obtained.
In the heating of the slab, the slab may be further heated after being held at a temperature of 700 to 850 ℃ for 900 seconds or more and held at a temperature of 1100 ℃ or more for 6000 seconds or more. In the case of holding in a temperature range of 700 to 850 ℃, the temperature of the steel sheet may be varied in the temperature range or may be set to be constant. In the austenite transformation in the temperature range of 700 to 850 ℃, mn is distributed between ferrite and austenite, and Mn can be diffused in the ferrite region by extending the transformation time. Thus, mn micro-segregation existing unevenly in the slab can be eliminated, and the standard deviation of Mn concentration can be significantly reduced. If the standard deviation of Mn concentration is large, the Mn is locally concentrated to develop a region having a reduced fracture energy, which promotes the occurrence of cracks during plastic deformation, and thus a desired Rcf value may not be obtained.
The hot rolling preferably uses a reversing mill or a tandem mill as the multipass rolling. In particular, from the viewpoint of industrial productivity and stress load on steel sheets during rolling, it is more preferable that at least the final 2 stages are set as hot rolling using a tandem mill.
(6-2) reduction ratio of hot rolling: the total thickness reduction of 90% or more in a temperature range of 850-1100 DEG C
By hot rolling in a temperature range of 850 to 1100 ℃ so that the total thickness is reduced by 90% or more, it is possible to mainly refine the recrystallized austenite grains and to promote the accumulation of strain energy into the unrecrystallized austenite grains. Further, recrystallization of austenite can be promoted, and atomic diffusion of Mn can be promoted, so that the standard deviation of Mn concentration can be reduced. As a result, the occurrence of cracks during plastic deformation can be promoted, and a desired Rcf value can be obtained. Therefore, it is preferable to perform hot rolling in a temperature range of 850 to 1100 ℃ so that the total sheet thickness is reduced by 90% or more. If the total rolling reduction in the temperature range of 850 to 1100 ℃ is less than 90%, the standard deviation of Mn concentration may become high, and development of the region where Mn is locally concentrated and the fracture energy is lowered may not be suppressed, and crack generation during plastic deformation may be promoted. As a result, a desired Rcf value may not be obtained.
The reduction in plate thickness in the temperature range of 850 to 1100 ℃ can be expressed as { (t 0-t 1)/t 0} ×100 (%) when the inlet plate thickness before the initial rolling in the temperature range is set to t0 and the outlet plate thickness after the rolling in the final stage in the rolling in the temperature range is set to t 1.
(6-3) starting temperature of hot rolling: the rolling temperature of the first two stages of the 1 st to final stages of hot rolling is 850 ℃ or more and less than 930 ℃): a rolling reduction of 850 ℃ or more and less than 950 ℃): below 30%
Preferably, it is: the initial temperature of the hot rolling is 850 ℃ or higher and lower than 930 ℃, the rolling temperature of the first two stages of the 1 st to final stages of the hot rolling is 850 ℃ or higher and lower than 950 ℃, and the rolling reduction of the first two stages of the 1 st to final stages of the hot rolling is lower than 30%. By setting the start temperature of hot rolling to a relatively low temperature and by performing the hot rolling in the hot rolling stage at a low temperature and a low reduction rate, recrystallization in the hot rolling stage can be suppressed and strain can be accumulated in austenite grains. As a result, unrecrystallized austenite having a high dislocation density in the grains can be maintained to the rolling subsequent stage. Thus, a desired Rcf value can be obtained. When the rolling start temperature is 930 ℃ or higher, the rolling temperature of the first two stages of the 1 st to final stages of the hot rolling is 950 ℃ or higher, or the rolling reduction is 30% or higher, recrystallization of austenite in the first two stages of the 1 st to final stages of the hot rolling cannot be suppressed, and as a result, a desired Rcf value may not be obtained. Further, when the start temperature of hot rolling is lower than 850 ℃, or when the rolling temperature of the first two stages of the 1 st to final stages of hot rolling is lower than 850 ℃, it becomes difficult to set the rolling temperature of the final stage of hot rolling and the rolling temperature of the preceding stage of the final stage to 930 ℃ or higher, and as a result, a desired Rcf value may not be obtained.
(6-4) Rolling temperature of the final stage of hot rolling and the stage preceding the final stage: 930 ℃ or higher and lower than 1010 ℃, the rolling reduction: more than 50 percent of rolling finishing temperature: 950 ℃ to below 1010 DEG C
Preferably, it is: the rolling temperatures of the final stage and the stage preceding the final stage of the hot rolling are set to 930 ℃ or higher and lower than 1010 ℃, the rolling reduction rates of the final stage and the stage preceding the final stage are set to 50% or higher, and the rolling completion temperature (temperature after rolling of the final stage) is set to 950 ℃ or higher. The recrystallization of austenite in the subsequent stage of rolling can be promoted by setting the rolling reduction of the final stage of hot rolling and the stage immediately preceding the final stage to 50% or more and the rolling completion temperature to 950 ℃. By forming grains having a high dislocation density by suppressing recrystallization in the rolling front stage, recrystallization occurs in the rolling rear stage, and the difference in orientation between the structures formed from recrystallized austenite becomes large. Thus, the Rcf value can be increased, and a desired Rcf value can be obtained. It can be assumed that: in the case of a structure having a large difference in orientation, rotation of crystals occurs during plastic deformation, thereby improving crack propagation stop characteristics at the interface and increasing Rcf value. When the rolling temperature of the final stage and the stage preceding the final stage of hot rolling is lower than 930 ℃, the rolling reduction of the final stage and the stage preceding the final stage is lower than 50%, or the rolling completion temperature is lower than 950 ℃, austenite recrystallization may be insufficient, and a desired Rcf value may not be obtained.
Further, by setting the rolling temperature of the final stage of hot rolling and the rolling completion temperature of the stage preceding the final stage to be lower than 1010 ℃, it is possible to refine the structure by suppressing coarsening of the austenite grain diameter. This suppresses occurrence of cracks during plastic deformation, and improves the Rcf value.
(6-5) an average cooling rate of cooling within 1.0 second after completion of hot rolling: 50 ℃/s or more, cooling start temperature: 850 ℃ above and below 960 DEG C
In order to suppress growth of austenite grains refined by hot rolling, it is preferable that: cooling is performed at an average cooling rate of 50 ℃/sec or more within 1.0 sec after completion of hot rolling, and a cooling start temperature is set to 850 ℃ or more and lower than 960 ℃. In order to cool the steel sheet at an average cooling rate of 50 ℃/sec or more within 1.0 sec after completion of hot rolling, the steel sheet may be cooled at a high average cooling rate immediately after completion of hot rolling, for example, by spraying cooling water onto the surface of the steel sheet. By setting the cooling start temperature to 850 ℃ or higher and lower than 960 ℃, and cooling at an average cooling rate of 50 ℃/sec or higher within 1.0 sec after completion of hot rolling, austenite grains and the structure formed thereafter can be made finer. This suppresses occurrence of cracks during plastic deformation, and improves the Rcf value.
The cooling start temperature is a temperature immediately before cooling at an average cooling rate of 50 ℃/sec or more, for example, immediately before spraying cooling water onto the surface of the steel sheet.
The average cooling rate is a value obtained by dividing the temperature decrease 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 introduced from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.
(6-6) cooling to a temperature range of 600 to 730 ℃ at an average cooling rate of 50 ℃/sec or more, and slowly cooling at an average cooling rate of less than 5 ℃/sec for 2.0 seconds or more in a temperature range of 600 to 730 ℃. Then, the mixture is cooled to a temperature range of 350 ℃ or lower at an average cooling rate of 50 ℃ per second or higher.
It should be noted that the slow cooling may be omitted, and the cooling may be performed at an average cooling rate of 50 ℃/s or more to a temperature range of 350 ℃ or less without the slow cooling.
After the cooling, the steel sheet is cooled to a temperature range of 600 to 730 ℃ at an average cooling rate of 50 ℃/sec or more, whereby the production of ferrite and pearlite having low strength can be suppressed. This improves the strength of the hot-rolled steel sheet.
In the cooling, the bainite can be sufficiently precipitated by performing slow cooling at an average cooling rate of less than 5 ℃/s for 2.0 seconds or more in a temperature range of 600 to 730 ℃. This makes it possible to achieve both the strength and crack propagation stopping characteristics of the hot-rolled steel sheet. The average cooling rate here is a value obtained by dividing the temperature decrease width of the steel sheet from the cooling stop temperature of the accelerated cooling to the stop temperature of the slow cooling by the time required from the time of stopping the accelerated cooling to the time of stopping the slow cooling.
The DP steel can be stably produced by slowly cooling in a high temperature range (660 to 730 ℃) in a temperature range of 600 to 730 ℃. Further, by slowly cooling in a low temperature range (600 ℃ or more and less than 660 ℃) in a temperature range of 600 to 730 ℃, the bainitic steel can be stably produced.
In order to suppress the area ratio of pearlite and obtain a desired tensile strength, it is preferable to set the average cooling rate from the cooling stop temperature of slow cooling to the winding temperature to 50 ℃/sec or more. This makes it possible to harden the parent phase structure.
The average cooling rate here is a value obtained by dividing the temperature decrease width of the steel sheet from the cooling stop temperature of the slow cooling at an average cooling rate of less than 5 ℃/s to the coiling temperature by the time required from the time when the slow cooling at an average cooling rate of less than 5 ℃/s is stopped to the time when the steel sheet is coiled.
The upper limit of the time for performing slow cooling is determined according to the design of the apparatus, but may be set to be approximately less than 10.0 seconds. The lower limit of the average cooling rate of slow cooling is not particularly set, but the temperature rise without cooling may be set to 0 ℃/s or more, which is accompanied by a large investment in equipment.
Note that slow cooling may not be performed. By performing accelerated cooling to a temperature range of 350 ℃ or lower at an average cooling rate of 50 ℃/sec or more without slow cooling, the formation of ferrite and pearlite having low strength can be suppressed, and the formation of martensite can be promoted. This makes it possible to stably produce the martensitic steel, while achieving both the strength and crack propagation stopping characteristics of the hot-rolled steel sheet.
The average cooling rate is a value obtained by dividing the temperature decrease 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 introduced from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.
The upper limit value of the cooling rate is not particularly limited, but if the cooling rate is increased, the cooling equipment becomes large-scale, and the equipment cost increases. Therefore, considering the equipment cost, it is preferably 300 ℃/sec or less.
(6-7) winding temperature: 350 ℃ below
The winding temperature is set to be below 350 ℃. By setting the winding temperature to 350 ℃ or lower, the amount of iron carbide precipitated can be reduced, and the variation in hardness distribution in the hard phase can be reduced. As a result, the starting point and propagation path of the crack are reduced, and occurrence of the crack during plastic deformation can be suppressed, and a desired Rcf value can be obtained.
Examples
Next, the effects of an embodiment of the present invention will be described more specifically by way of examples, but the conditions in the examples are one condition example employed for confirming the operability and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Steels having chemical compositions shown in tables 1 and 2 were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. Using the obtained slab, hot-rolled steel sheets shown in tables 5A and 5B were obtained under the manufacturing conditions shown in tables 3A to 4.
The martensitic steel was obtained by cooling to a temperature range of 350 ℃ or less at a desired average cooling rate without slow cooling. Further, as for an example in which slow cooling is performed, DP steel is obtained by performing slow cooling in a high temperature region (660 to 730 ℃) in a temperature region of 600 to 730 ℃, and bainitic steel is obtained by performing slow cooling in a low temperature region (600 ℃ or more and less than 660 ℃) in a temperature region of 600 to 730 ℃.
The area ratio of the metal structure, the Rcf value, the standard deviation of the Mn concentration, and the tensile strength TS were obtained for the obtained hot-rolled steel sheet by the above-described method. The measurement results obtained are shown in tables 5A and 5B.
Method for evaluating characteristics of hot-rolled steel sheet
(1) Tensile characteristics
When the tensile strength TS is 980MPa or more, a hot-rolled steel sheet having high strength is set and judged to be acceptable. On the other hand, when the tensile strength TS is lower than 980MPa, the steel sheet is set to be not a hot-rolled steel sheet having high strength and is determined to be defective.
(2) Crack propagation stop characteristics
Crack propagation stopping properties were evaluated by the Charpy impact test. According to JIS Z2242: 2018, a small-sized V-notch test piece of 2.5mm was manufactured in a direction in which the width direction (C direction) of the hot-rolled steel sheet became the longitudinal direction of the test piece, and a Charpy impact test was performed at-196 ℃. The test piece having a thickness of less than 2.5mm was subjected to the test in full thickness.
Further, a JIS No. 5B tensile test piece was produced in a direction in which the width direction (C direction) of the hot-rolled steel sheet became the longitudinal direction of the test piece, and after a compressive pre-strain of 10% was applied to the steel material in the longitudinal direction of the test piece, the V-notch test piece was collected. The test piece was subjected to a Charpy impact test at-196℃by the above method to obtain an absorption energy after plastic deformation.
When the reduction ratio of the absorbed energy after plastic deformation (("absorbed energy before plastic deformation" - "absorbed energy after plastic deformation")/"absorbed energy before plastic deformation") is 30.00% or less, the deterioration of crack propagation stop characteristics before and after plastic deformation is set to be small and judged to be acceptable. On the other hand, when the reduction rate of the absorbed energy after plastic deformation exceeds 30.00%, the deterioration of crack propagation stop characteristics before and after plastic deformation is set to be large and determined to be defective.
TABLE 1
Underlined indicates outside the scope of the present invention.
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TABLE 4 Table 4
The underline indicates that the manufacturing conditions are not preferable.
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From an examination of Table 5A and Table 5B, it is found that: the hot-rolled steel sheet of the example of the present invention has high strength and little deterioration of crack propagation stopping characteristics after plastic deformation. On the other hand, it is known that: the hot-rolled steel sheet of the comparative example does not have 1 or more of the above characteristics.
Claims (5)
1. A hot-rolled steel sheet characterized by comprising, in mass%, the chemical composition:
C：0.040～0.400％、
Si：0.05～3.00％、
Mn：1.00～4.00％、
sol.Al：0.001～0.500％、
p:0.100% or less,
S:0.0300% or less,
N: less than 0.1000 percent,
O:0.0100% or less,
Ti：0～1.000％、
V：0～1.000％、
Nb：0～1.000％、
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％、
1 or more than 2 of Zr, co, zn and W: 0 to 1.00% by weight of Sn:0 to 0.05 percent,
the rest part contains Fe and impurities;
in the case of a metallic structure,
in area-%: the retained austenite is less than 3.00%,
an Rcf value representing a ratio of average cross-section units before and after plastic deformation of 2.00 or more;
the tensile strength of the hot-rolled steel sheet is 980MPa or more.
2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains 1 or 2 or more selected from the group consisting of the following elements in mass%:
Ti：0.010～1.000％、
V：0.010～1.000％、
Nb：0.010～1.000％、
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 to 0.1000 percent
Bi：0.0005～0.020％。
3. The hot rolled steel sheet as claimed in claim 1 or 2 wherein the metallic structure is in area%: ferrite 15.00-60.00% and martensite 40.00-85.00%.
4. The hot rolled steel sheet as claimed in claim 1 or 2 wherein the metallic structure is in area%: the bainite is 50.00% or more.
5. The hot rolled steel sheet as claimed in claim 1 or 2 wherein the metallic structure is in area%: martensite exceeds 85.00%.
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| JP3499085B2 (en) | 1996-06-28 | 2004-02-23 | 新日本製鐵株式会社 | Low Yield Ratio High Tensile Steel for Construction Excellent in Fracture Resistance and Manufacturing Method Thereof |
| JP3467767B2 (en) | 1998-03-13 | 2003-11-17 | Ｊｆｅスチール株式会社 | Steel with excellent brittle crack arrestability and method of manufacturing the same |
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