EP4176092A1 - Heat treated cold rolled steel sheet and a method of manufacturing thereof - Google Patents
Heat treated cold rolled steel sheet and a method of manufacturing thereofInfo
- Publication number
- EP4176092A1 EP4176092A1 EP21736390.2A EP21736390A EP4176092A1 EP 4176092 A1 EP4176092 A1 EP 4176092A1 EP 21736390 A EP21736390 A EP 21736390A EP 4176092 A1 EP4176092 A1 EP 4176092A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- steel sheet
- cold rolled
- temperature
- anyone
- rolled steel
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
- 239000010960 cold rolled steel Substances 0.000 title claims abstract description 23
- 238000004519 manufacturing process Methods 0.000 title claims description 10
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 102
- 239000010959 steel Substances 0.000 claims abstract description 102
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 43
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 19
- 239000000203 mixture Substances 0.000 claims abstract description 17
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 16
- 239000011572 manganese Substances 0.000 claims abstract description 16
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 14
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims abstract description 14
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 14
- 229910052710 silicon Inorganic materials 0.000 claims abstract description 11
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 10
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 10
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 10
- 239000010703 silicon Substances 0.000 claims abstract description 10
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims abstract description 9
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims abstract description 8
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims abstract description 8
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims abstract description 8
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 claims abstract description 7
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 7
- 239000011733 molybdenum Substances 0.000 claims abstract description 7
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 7
- 239000010955 niobium Substances 0.000 claims abstract description 7
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims abstract description 7
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 claims abstract description 6
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims abstract description 6
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims abstract description 6
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims abstract description 6
- 229910052796 boron Inorganic materials 0.000 claims abstract description 6
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 6
- 239000011651 chromium Substances 0.000 claims abstract description 6
- 230000001955 cumulated effect Effects 0.000 claims abstract description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 6
- 239000011574 phosphorus Substances 0.000 claims abstract description 6
- 239000010936 titanium Substances 0.000 claims abstract description 6
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 claims abstract description 5
- 239000011575 calcium Substances 0.000 claims abstract description 5
- 229910052791 calcium Inorganic materials 0.000 claims abstract description 5
- 239000012535 impurity Substances 0.000 claims abstract description 5
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 5
- 239000011593 sulfur Substances 0.000 claims abstract description 5
- 229910052719 titanium Inorganic materials 0.000 claims abstract description 5
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims abstract description 4
- 229910052742 iron Inorganic materials 0.000 claims abstract description 4
- 229910052749 magnesium Inorganic materials 0.000 claims abstract description 4
- 239000011777 magnesium Substances 0.000 claims abstract description 4
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 4
- 238000012545 processing Methods 0.000 claims abstract description 4
- 229910052720 vanadium Inorganic materials 0.000 claims abstract description 4
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 claims abstract description 4
- 229910052726 zirconium Inorganic materials 0.000 claims abstract description 4
- 229910052684 Cerium Inorganic materials 0.000 claims abstract description 3
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims abstract description 3
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 claims abstract description 3
- QCWXUUIWCKQGHC-UHFFFAOYSA-N Zirconium Chemical compound [Zr] QCWXUUIWCKQGHC-UHFFFAOYSA-N 0.000 claims abstract description 3
- GWXLDORMOJMVQZ-UHFFFAOYSA-N cerium Chemical compound [Ce] GWXLDORMOJMVQZ-UHFFFAOYSA-N 0.000 claims abstract description 3
- 229910052802 copper Inorganic materials 0.000 claims abstract description 3
- 239000010949 copper Substances 0.000 claims abstract description 3
- 238000001816 cooling Methods 0.000 claims description 24
- 238000000034 method Methods 0.000 claims description 20
- 238000000137 annealing Methods 0.000 claims description 17
- 238000002791 soaking Methods 0.000 claims description 10
- 230000008569 process Effects 0.000 claims description 9
- 238000005096 rolling process Methods 0.000 claims description 9
- 238000005098 hot rolling Methods 0.000 claims description 6
- 230000009467 reduction Effects 0.000 claims description 6
- 239000011248 coating agent Substances 0.000 claims description 4
- 238000000576 coating method Methods 0.000 claims description 4
- 238000010438 heat treatment Methods 0.000 claims description 4
- 238000005097 cold rolling Methods 0.000 claims description 3
- 238000003303 reheating Methods 0.000 claims description 2
- 239000011265 semifinished product Substances 0.000 claims 2
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 abstract description 7
- 230000000694 effects Effects 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 7
- 230000001627 detrimental effect Effects 0.000 description 5
- 239000000463 material Substances 0.000 description 5
- 238000009749 continuous casting Methods 0.000 description 4
- 238000005204 segregation Methods 0.000 description 4
- 238000007711 solidification Methods 0.000 description 4
- 230000008023 solidification Effects 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- 230000002349 favourable effect Effects 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
- 230000002411 adverse Effects 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000001934 delay Effects 0.000 description 2
- 238000003618 dip coating Methods 0.000 description 2
- 238000005530 etching Methods 0.000 description 2
- 239000000446 fuel Substances 0.000 description 2
- 238000001000 micrograph Methods 0.000 description 2
- 238000001953 recrystallisation Methods 0.000 description 2
- 238000012827 research and development Methods 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- -1 Aluminum nitrides Chemical class 0.000 description 1
- 238000012935 Averaging Methods 0.000 description 1
- 241001484259 Lacuna Species 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 239000005864 Sulphur Substances 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- PMHQVHHXPFUNSP-UHFFFAOYSA-M copper(1+);methylsulfanylmethane;bromide Chemical compound Br[Cu].CSC PMHQVHHXPFUNSP-UHFFFAOYSA-M 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 230000018109 developmental process Effects 0.000 description 1
- 238000007571 dilatometry Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 230000001131 transforming effect Effects 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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
- B21B3/02—Rolling special iron alloys, e.g. stainless steel
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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
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
- C21D1/20—Isothermal quenching, e.g. bainitic hardening
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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
-
- 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
- C21D6/005—Heat treatment of ferrous alloys containing Mn
-
- 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
- C21D6/008—Heat treatment of ferrous alloys containing Si
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- 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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
- C22C38/002—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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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—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
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0224—Two or more thermal pretreatments
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
- B21B2001/221—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by cold-rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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
Definitions
- the present invention relates to cold rolled steel sheet with high strength and high formability having tensile strength of 980 MPa or more and a total elongation of more than 14% which is suitable for use as a steel sheet for vehicles.
- Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and high strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns.
- automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency.
- US 9074 272 describes steels that have the chemical composition: 0.1 - 0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn and the remainder consisting of iron and the inevitable impurities.
- the microstructure includes residual austenite between 5 to 20%, bainitic ferrite 40 to 65%, polygonal ferrite 30 to 50% and less than 5% martensite.
- US 9074272 refers to a cold rolled steel sheet with excellent elongation but the invention described in it fails to achieve the strength of 900 MPa which is a mandate for reducing the weight while keeping the complex automotive part robust.
- the known prior art related to the manufacture of high strength and high formability steel sheets is inflicted by one or the other lacuna : hence there lies a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same.
- the purpose of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have:
- the steel sheet according to the invention may have a yield strength value greater than or above 540 MPa or even better above 550 MPa.
- such steel can also have a good suitability for forming, in particular for rolling with good weldability and coatability.
- Another objective of the present invention is also to make available a method for the manufacturing of these sheets that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.
- Carbon is present in the steel from 0.05% to 0.15%. Carbon is an element necessary for increasing the strength of a steel sheet by producing a low- temperature transformation phase such as martensite. Further carbon also plays a pivotal role in austenite stabilization. A content less than 0.05% would not securing the formation of martensite, thereby decreasing strength. On the other hand, at a carbon content exceeding 0.15%, a weld zone and a heat-affected zone are significantly hardened, and thus the mechanical properties of the weld zone are impaired. Hence the preferable limit is from 0.07% to 0.12% and more preferably from 0.08% to 0.11%.
- Manganese content of the steel of present invention is from 1.8% to 2.7%.
- Manganese is an element that imparts strength to the steel by solid solution strengthening. An amount of at least about 1.8 % by weight of manganese is needed in order to provide the strength and hardenability of the steel sheet as well as to form ferrite. Thus, a higher percentage of Manganese such as 1.9% to 2.5% is preferred and more preferably 2.1 % to 2.5%. But when manganese is more than 2.7%, this produces adverse effects such as slowing down the transformation of austenite during cooling after annealing, leading to a reduction of ductility. Moreover, a manganese content above 2.7% would also reduce the weldability of the present steel.
- Silicon content of the steel of present invention is from 0.1% to 1%. Silicon imparts the strength to the steel of present invention by solid solution strengthening. Silicon promotes the ferrite transformation. However, adding more than 1% of silicon does not improve the mentioned effect and leads to problems such as hot rolling embrittlement. Therefore, the concentration is controlled within an upper limit of 1%. A preferable limit for the presence of silicon is kept from 0.2% to 0.9% and more preferably from 0.3% to 0.7%.
- the content of aluminum of the steel of the present invention is from 0.01 to 0.8%. Within such range, aluminum bounds nitrogen in the steel to form aluminum nitride so as to reduce the size of the grains. But, whenever the content of aluminum exceeds 0.8% in the present invention, it will increase the Ac3 point, thereby lowering the productivity.
- the preferable range for aluminum is kept from 0.01% to 0.7 % and more preferably from 0.01% to 0.6%.
- the cumulated amounts of Silicon and Aluminum is at least 0.6% because both the elements are ferrite phase-generating element, thereby participating to the formation of ferrite that is favorable for both the elongation and ductility.
- Chromium content of the steel of present invention is from 0.1% to 0.9%. Chromium is an essential element that provide strength and hardening to the steel, but when used above 0.9% impairs surface finish of the steel. Hence to achieve the effects of chromium optimally the preferred limit is between 0.2% and 0.8% and more preferably from 0.2% to 0.7%.
- Titanium is an essential element which may be added to the steel of the present invention from 0.0001% to 0.1% and preferably from 0.01% to 0.08%, Similarly to niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form TiN appearing during solidification of the cast product. The amount of Ti is so limited to 0.1 % to avoid coarse TiN detrimental for hole expansion. In case the titanium content is below 0.0001% it does not impart any effect on the steel of present invention.
- Boron is an essential element for the present invention and is added in very small amount and is added from 0.0005% to 0.003%. Boron imparts hardenability and strength to the steel of present invention. However, when boron is added more than 0.003% the reliability of the steel sheet is found to be significantly lowered. Further boron the segregation may happen at grain boundaries which is detrimental for the formability.
- Niobium is an essential element that can be added to the steel from 0.01% to 0.1%, preferably from 0.01% to 0.06%. It is suitable for forming carbonitrides to impart strength to the steel according to the invention by precipitation hardening. Because niobium delays the recrystallization during the heating, the microstructure formed at the end of the holding temperature and as a consequence after the complete annealing is finer, this leads to the hardening of the product. But, when the niobium content is above 0.1% the amount of carbo-nitrides is not favorable for the present invention as large amount of carbo-nitrides tend to reduce the ductility of the steel.
- Vanadium is an optional element which may be added to the steel of the present invention up to 0.2%, preferably from 0.001% to 0.01%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form VN appearing during solidification of the cast product. The amount of V is so limited to 0.2% to avoid coarse VN detrimental for hole expansion. In case the vanadium content is below 0.001 % it does not impart any effect on the steel of present invention.
- Phosphorus content of the steel of present invention is limited to 0.09%.
- Phosphorus is an element which hardens in solid solution and also interferes with formation of carbides. Therefore, a small amount of phosphorus, of at least 0.002% can be advantageous, but phosphorus has its adverse effects also, such as a reduction of the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content is preferably limited a maximum of 0.02%.
- Sulfur is not an essential element but may be contained as an impurity in steel.
- the sulfur content is preferably as low as possible but is 0.09% or less and preferably less than 0.03%, from the viewpoint of manufacturing cost. Further if higher sulfur is present in steel it combines to form sulfide especially with Mn and Ti and reduces their beneficial impact on the present invention.
- Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the precipitation of Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel.
- Molybdenum is an optional element that constitutes from 0% to 0.2% of the Steel of present invention; Molybdenum improves hardenability and hardness, delays the appearance of Bainite hence promote the formation of Martensite, when added in an amount of at least 0.01%. Molybdenum also facilitate the formation of Ferrite. Flowever, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.2%. The preferable limit for Molybdenum is from 0.01% to 0.2%
- Nickel may be added as an optional element in an amount of 0% to 2% to increase the strength of the steel present invention and to improve its toughness. A minimum of 0.01% is preferred to get such effects. Flowever, when its content is above 2%, Nickel causes ductility deterioration. Copper may be added as an optional element in an amount of 0% to 2% to increase the strength of the of Steel of present invention and to improve its corrosion resistance. A minimum of 0.01% is preferred to get such effects. However, when its content is above 2%, it can degrade the surface aspects.
- Calcium is an optional element which may be added to the steel of present invention up to 0.005%, preferably from 0.0001% to 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the steel by arresting the detrimental sulphur content in globularizing it.
- Ce ⁇ 0.1%, Mg ⁇ 0.05% and Zr ⁇ 0.05% can be added individually or in combination in the following proportions: Ce ⁇ 0.1%, Mg ⁇ 0.05% and Zr ⁇ 0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.
- the remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.
- the microstructure of the steel sheet according to the invention comprises in area fraction, 40% to 60% of martensite, 5% to 40% of Inter-critical Ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.
- Martensite constitutes 40% to 60% of microstructure by area fraction. Martensite can notably be formed during the cooling after annealing and particularly after crossing the Ms temperature and particularly between Ms-10°C and 20°C or during cooling after overaging. Martensite imparts strength to the present invention. Preferable limit for Martensite is between 42% and 58% and more preferably between 43% and 56%.
- Inter-critical ferrite constitutes between 15% and 40% of microstructure by area fraction of the steel of present invention.
- This inter-critical ferrite imparts the steel of present invention with a total elongation of at least 14%.
- the intercritical ferrite results from the annealing at a temperature below Ac3.
- the intercritical ferrite is different from the ferrite that could be created after the annealing, named hereinafter “transformed ferrite”, that will be described below.
- the intercritical ferrite is polygonal.
- the transformed ferrite is enriched in carbon and manganese, i.e. has carbon and manganese contents which are higher than the carbon and manganese contents of the intercritical ferrite.
- the intercritical ferrite and the transformed ferrite can therefore be differentiated by observing a micrograph with a SEM microscope using secondary electrons, after etching with 2% Nital etching agent. On such micrograph, the intercritical ferrite appears in medium grey, whereas the transformed ferrite appears in dark grey, owing to its higher carbon and manganese contents. It is preferable to have inter-critical ferrite from 20% to 40% and more preferably from 25% to 38%.
- the total amount of transformed ferrite and bainite constitutes from 10% to 35% of microstructure by area fraction for the steel of present invention.
- Transformed Ferrite of present invention constitutes of Ferrite formed during the cooling after annealing and the steel according to the present invention always contains transformed ferrite that is the presence of transformed ferrite is always more than 0%.
- Transformed Ferrite imparts high strength as well as elongation to the steel of present invention.
- Transformed Ferrite of the present steel is rich in carbon and Manganese as compared to the inter-critical ferrite and it is mandatory to have transformed ferrite in the steel. Bainite forms during the averaging holding especially between 400°C and 480°C. To ensure an elongation of 14% it is necessary to have 10% of transformed ferrite and bainite. But whenever the total amount is present above 35% in steel of present invention it is not possible to have both tensile strength and the total elongation at the same time.
- the preferred limit for transformed ferrite and bainite for the present invention is between 15% and 30%.
- Residual Austenite is an optional microstructure and can be present between 0% and 5% in the steel.
- the microstructure of the cold rolled and heat treated steel sheet is free from microstructural components, such as pearlite, tempered martensite and cementite without impairing the mechanical properties of the steel sheets.
- a steel sheet according to the invention can be produced by any suitable method.
- a preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip.
- a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10.
- the slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.
- the temperature of the slab which is subjected to hot rolling, is at least 1000° C and must be below 1280°C.
- the temperature of the slab is lower than 1000° C, excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive. Therefore, the finish rolling temperature of the slab is above Ac3 and preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac3 + 150°C to Ac3+250°C
- a final rolling temperature range between Ac3 to Ac3+200°C is necessary to have a structure that is favorable to recrystallization and rolling. It is preferred that the final rolling pass to be performed at a temperature greater than 850°c and better of at least 950°C.
- the hot rolled steel obtained in this manner is then cooled at a cooling rate of at least 30°C/s to the coiling temperature.
- the cooling rate will be less than or equal to 200° C/s.
- the hot rolled steel is then coiled at a temperature between 475°C and 650°C to avoid ovalization and preferably between 475°C and 625°C to avoid scale formation. A more preferred range for such coiling temperature is between 500°C and 625°C.
- the coiled hot rolled steel is then cooled down to room temperature before subjecting it to optional hot band annealing.
- the hot rolled steel may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing.
- the hot rolled sheet may then subjected to an optional Hot Band Annealing at, for example, temperatures between 400°C and 750°C for preferably at least 12 hours and not more than 96 hours, the temperature remaining below 750°C to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity.
- an optional scale removal step of this hot rolled steel may performed through, for example, pickling of such sheet.
- This hot rolled steel is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%.
- the cold rolled steel sheet obtained from cold rolling process is then subjected to annealing to impart the steel of present invention with microstructure and mechanical properties.
- the said cold rolled steel sheet it is heated up to the soaking temperature between Ac1 +60°C and Ac3, preferably at a heating rate of at least 3°C/s, then the annealing is performed at that temperature during 5 to 500 seconds, preferably during 50 to 250 seconds. In a preferred embodiment, the heating is at least 10°C/s and more preferably at least 15°C/s. During this annealing Inter-critical ferrite forms.
- the preferred annealing soaking temperature is between Ac1 + 70°C and Ac3 and more preferably between Ac1 + 80°C and Ac3 - 30°C.
- the time and temperature of soaking are selected to ensure that the microstructure of the steel sheet at the end of the soaking contains at least 50% of Austenite and more preferably at least 60% of austenite.
- the cold rolled steel is cooled in a two-step cooling process wherein the first step starts from soaking temperature to a temperature T1 between 550°C and 650°C, at a cooling rate CR1 which is least 3°C/s, preferably at least 5°C/s and more preferably at least 10°C/s. During this step the transformed ferrite forms.
- the cold rolled steel is then held at T 1 during 1 s to 20 s and preferably between 2 s and 15 s and more preferably between 5 s and 12 s.
- the second step starts from cooling further the cold rolled steel sheet from T 1 to overaging temperature T2 between 400°C and 480°C, at a cooling rate CR2 of at least 3°C/s, preferably at least 5°C/s and more preferably at least 7°C/s.
- overaging is being performed at T2 during 5 to 100 seconds. During overaging, some bainite gets formed.
- the preferred temperature T2 for overaging is between 420°C and 475°C.
- the preferred time for overaging temperature during 15 to 75 seconds and more preferably 20 and 75 seconds.
- the cold rolled steel sheet can then either be cooled down to room temperature or can be brought to the temperature of a hot dip coating bath between 420°C and 680°C, depending on the nature of the coating, to facilitate hot dip coating of the cold rolled steel sheet.
- the final cooling down to room temperature is done at a cooling rate of at least 5°C/s and preferably at least 9°C/s to ensure the formation of fresh martensite in the steel of present invention.
- the cold rolled steel sheet can also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, etc, which may not require bringing it to the above-mentioned temperature range before coating.
- Samples of the steel sheets according to the invention and to some 10 comparative grades were prepared with the compositions gathered in table 1 and the processing parameters gathered in table 2.
- the corresponding microstructures of those steel sheets were gathered in table 3 and the properties in table 4.
- Table 1 depicts the steels with the compositions expressed in percentages by is weight.
- Table 1 also shows Ac1 and Ac3 temperature points which are calculated by dilatometry.
- Table 2 Process parameters
- Table 2 gathers the annealing process parameters implemented on steel samples of Table 1 , that were all reheated at 1230°C, hot-rolled with a finish rolling temperature of 875°C, coiled at 550°C and cold rolled with a 50% reduction rate 5 before undergoing annealing and two-steps cooling scheme including overaging: underlined values: not according to the invention.
- Table 3 gathers the results of test conducted in accordance of standards on 10 different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials.
- Table 3 microstructures of the trials underlined values: not according to the invention.
- Table 4 gathers the mechanical properties of both the inventive steel and reference steel. The tensile strength, yield strength and total elongation tests are conducted in accordance with JIS Z2241 standards.
- Table 4 mechanical properties of the trials underlined values: not according to the invention.
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Abstract
A cold rolled steel sheet having a composition comprising of 0.05 % ≤ Carbon ≤ 0.15 %, 1.8% ≤ Manganese ≤ 2.7%, 0.1% ≤ Silicon ≤ 1%,0.01% ≤ Aluminum ≤ 0.8%,0.1% ≤ Chromium ≤ 0.9%,0% ≤ Phosphorus ≤ 0.09%,0.0001% ≤ Titanium ≤ 0.1%,0.0005% ≤ Boron ≤ 0.003%, 0.01% ≤ Niobium ≤ 0.1%, 0 % ≤ Sulfur ≤ 0.09 %, 0 % ≤ Nitrogen ≤ 0.09%, 0% ≤ Vanadium ≤ 0.2%, 0%≤Molybdenum ≤0.2%, 0%≤ Nickel ≤2%, 0% ≤ Copper ≤ 2%, 0% ≤ Calcium ≤ 0.005%, 0% ≤ Cerium ≤ 0.1%, 0% ≤ Magnesium ≦ 0.05%, 0% ≤ Zirconium ≦ 0.05%, the remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.
Description
HEAT TREATED COLD ROLLED STEEL SHEET AND A METHOD OF
MANUFACTURING THEREOF
The present invention relates to cold rolled steel sheet with high strength and high formability having tensile strength of 980 MPa or more and a total elongation of more than 14% which is suitable for use as a steel sheet for vehicles.
Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and high strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns. Thus, now automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency.
Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated.
Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:
US 9074 272 describes steels that have the chemical composition: 0.1 - 0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn and the remainder consisting of iron and the inevitable impurities. The microstructure includes residual austenite between 5 to 20%, bainitic ferrite 40 to 65%, polygonal ferrite 30 to 50% and less than 5% martensite. US 9074272 refers to a cold rolled steel sheet with excellent elongation but the invention described in it fails to achieve the strength of 900 MPa which is a mandate for reducing the weight while keeping the complex automotive part robust.
The known prior art related to the manufacture of high strength and high formability steel sheets is inflicted by one or the other lacuna : hence there lies a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same. The purpose of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have:
- an ultimate tensile strength greater than or equal to 950 MPa and preferably above 980 or even above 1000 MPa,
- a total elongation greater than or equal to 14% and preferably greater than or equal to 15%.
In a preferred embodiment, the steel sheet according to the invention may have a yield strength value greater than or above 540 MPa or even better above 550 MPa.
Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and coatability.
Another objective of the present invention is also to make available a method for the manufacturing of these sheets that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.
Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.
Carbon is present in the steel from 0.05% to 0.15%. Carbon is an element necessary for increasing the strength of a steel sheet by producing a low- temperature transformation phase such as martensite. Further carbon also plays a pivotal role in austenite stabilization. A content less than 0.05% would not securing the formation of martensite, thereby decreasing strength. On the other hand, at a carbon content exceeding 0.15%, a weld zone and a heat-affected zone are significantly hardened, and thus the mechanical properties of the weld zone are
impaired. Hence the preferable limit is from 0.07% to 0.12% and more preferably from 0.08% to 0.11%.
Manganese content of the steel of present invention is from 1.8% to 2.7%. Manganese is an element that imparts strength to the steel by solid solution strengthening. An amount of at least about 1.8 % by weight of manganese is needed in order to provide the strength and hardenability of the steel sheet as well as to form ferrite. Thus, a higher percentage of Manganese such as 1.9% to 2.5% is preferred and more preferably 2.1 % to 2.5%. But when manganese is more than 2.7%, this produces adverse effects such as slowing down the transformation of austenite during cooling after annealing, leading to a reduction of ductility. Moreover, a manganese content above 2.7% would also reduce the weldability of the present steel.
Silicon content of the steel of present invention is from 0.1% to 1%. Silicon imparts the strength to the steel of present invention by solid solution strengthening. Silicon promotes the ferrite transformation. However, adding more than 1% of silicon does not improve the mentioned effect and leads to problems such as hot rolling embrittlement. Therefore, the concentration is controlled within an upper limit of 1%. A preferable limit for the presence of silicon is kept from 0.2% to 0.9% and more preferably from 0.3% to 0.7%.
The content of aluminum of the steel of the present invention is from 0.01 to 0.8%. Within such range, aluminum bounds nitrogen in the steel to form aluminum nitride so as to reduce the size of the grains. But, whenever the content of aluminum exceeds 0.8% in the present invention, it will increase the Ac3 point, thereby lowering the productivity. Hence the preferable range for aluminum is kept from 0.01% to 0.7 % and more preferably from 0.01% to 0.6%.
In a preferred embodiment, the cumulated amounts of Silicon and Aluminum is at least 0.6% because both the elements are ferrite phase-generating element, thereby participating to the formation of ferrite that is favorable for both the elongation and ductility.
Chromium content of the steel of present invention is from 0.1% to 0.9%. Chromium is an essential element that provide strength and hardening to the steel, but when used above 0.9% impairs surface finish of the steel. Hence to achieve the effects of chromium optimally the preferred limit is between 0.2% and 0.8% and more preferably from 0.2% to 0.7%.
Titanium is an essential element which may be added to the steel of the present invention from 0.0001% to 0.1% and preferably from 0.01% to 0.08%, Similarly to niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form TiN appearing during solidification of the cast product. The amount of Ti is so limited to 0.1 % to avoid coarse TiN detrimental for hole expansion. In case the titanium content is below 0.0001% it does not impart any effect on the steel of present invention.
Boron is an essential element for the present invention and is added in very small amount and is added from 0.0005% to 0.003%. Boron imparts hardenability and strength to the steel of present invention. However, when boron is added more than 0.003% the reliability of the steel sheet is found to be significantly lowered. Further boron the segregation may happen at grain boundaries which is detrimental for the formability.
Niobium is an essential element that can be added to the steel from 0.01% to 0.1%, preferably from 0.01% to 0.06%. It is suitable for forming carbonitrides to impart strength to the steel according to the invention by precipitation hardening. Because niobium delays the recrystallization during the heating, the microstructure formed at the end of the holding temperature and as a consequence after the complete annealing is finer, this leads to the hardening of the product. But, when the niobium content is above 0.1% the amount of carbo-nitrides is not favorable for the present invention as large amount of carbo-nitrides tend to reduce the ductility of the steel.
Vanadium is an optional element which may be added to the steel of the present invention up to 0.2%, preferably from 0.001% to 0.01%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form VN appearing during solidification of the cast product. The amount of V is so limited
to 0.2% to avoid coarse VN detrimental for hole expansion. In case the vanadium content is below 0.001 % it does not impart any effect on the steel of present invention.
Phosphorus content of the steel of present invention is limited to 0.09%. Phosphorus is an element which hardens in solid solution and also interferes with formation of carbides. Therefore, a small amount of phosphorus, of at least 0.002% can be advantageous, but phosphorus has its adverse effects also, such as a reduction of the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content is preferably limited a maximum of 0.02%.
Sulfur is not an essential element but may be contained as an impurity in steel. The sulfur content is preferably as low as possible but is 0.09% or less and preferably less than 0.03%, from the viewpoint of manufacturing cost. Further if higher sulfur is present in steel it combines to form sulfide especially with Mn and Ti and reduces their beneficial impact on the present invention.
Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the precipitation of Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel.
Molybdenum is an optional element that constitutes from 0% to 0.2% of the Steel of present invention; Molybdenum improves hardenability and hardness, delays the appearance of Bainite hence promote the formation of Martensite, when added in an amount of at least 0.01%. Molybdenum also facilitate the formation of Ferrite. Flowever, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.2%. The preferable limit for Molybdenum is from 0.01% to 0.2%
Nickel may be added as an optional element in an amount of 0% to 2% to increase the strength of the steel present invention and to improve its toughness. A minimum of 0.01% is preferred to get such effects. Flowever, when its content is above 2%, Nickel causes ductility deterioration.
Copper may be added as an optional element in an amount of 0% to 2% to increase the strength of the of Steel of present invention and to improve its corrosion resistance. A minimum of 0.01% is preferred to get such effects. However, when its content is above 2%, it can degrade the surface aspects.
Calcium is an optional element which may be added to the steel of present invention up to 0.005%, preferably from 0.0001% to 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the steel by arresting the detrimental sulphur content in globularizing it.
Other elements such as cerium, magnesium or zirconium can be added individually or in combination in the following proportions: Ce < 0.1%, Mg < 0.05% and Zr < 0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.
The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.
The microstructure of the steel sheet according to the invention comprises in area fraction, 40% to 60% of martensite, 5% to 40% of Inter-critical Ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.
Martensite constitutes 40% to 60% of microstructure by area fraction. Martensite can notably be formed during the cooling after annealing and particularly after crossing the Ms temperature and particularly between Ms-10°C and 20°C or during cooling after overaging. Martensite imparts strength to the present invention. Preferable limit for Martensite is between 42% and 58% and more preferably between 43% and 56%.
Inter-critical ferrite constitutes between 15% and 40% of microstructure by area fraction of the steel of present invention. This inter-critical ferrite imparts the steel of present invention with a total elongation of at least 14%. The intercritical
ferrite results from the annealing at a temperature below Ac3. The intercritical ferrite is different from the ferrite that could be created after the annealing, named hereinafter “transformed ferrite”, that will be described below. Contrarily to the transformed ferrite, the intercritical ferrite is polygonal. Besides, the transformed ferrite is enriched in carbon and manganese, i.e. has carbon and manganese contents which are higher than the carbon and manganese contents of the intercritical ferrite. The intercritical ferrite and the transformed ferrite can therefore be differentiated by observing a micrograph with a SEM microscope using secondary electrons, after etching with 2% Nital etching agent. On such micrograph, the intercritical ferrite appears in medium grey, whereas the transformed ferrite appears in dark grey, owing to its higher carbon and manganese contents. It is preferable to have inter-critical ferrite from 20% to 40% and more preferably from 25% to 38%.
The total amount of transformed ferrite and bainite constitutes from 10% to 35% of microstructure by area fraction for the steel of present invention.
Transformed Ferrite of present invention constitutes of Ferrite formed during the cooling after annealing and the steel according to the present invention always contains transformed ferrite that is the presence of transformed ferrite is always more than 0%. Transformed Ferrite imparts high strength as well as elongation to the steel of present invention. Transformed Ferrite of the present steel is rich in carbon and Manganese as compared to the inter-critical ferrite and it is mandatory to have transformed ferrite in the steel. Bainite forms during the averaging holding especially between 400°C and 480°C. To ensure an elongation of 14% it is necessary to have 10% of transformed ferrite and bainite. But whenever the total amount is present above 35% in steel of present invention it is not possible to have both tensile strength and the total elongation at the same time. The preferred limit for transformed ferrite and bainite for the present invention is between 15% and 30%.
Residual Austenite is an optional microstructure and can be present between 0% and 5% in the steel.
In addition to the above-mentioned microstructure, the microstructure of the cold rolled and heat treated steel sheet is free from microstructural components, such as pearlite, tempered martensite and cementite without impairing the mechanical properties of the steel sheets. A steel sheet according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip.
For example, a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.
The temperature of the slab, which is subjected to hot rolling, is at least 1000° C and must be below 1280°C. In case the temperature of the slab is lower than 1000° C, excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive. Therefore, the finish rolling temperature of the slab is above Ac3 and preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac3 + 150°C to Ac3+250°C
A final rolling temperature range between Ac3 to Ac3+200°C is necessary to have a structure that is favorable to recrystallization and rolling. It is preferred
that the final rolling pass to be performed at a temperature greater than 850°c and better of at least 950°C.
The hot rolled steel obtained in this manner is then cooled at a cooling rate of at least 30°C/s to the coiling temperature. Preferably, the cooling rate will be less than or equal to 200° C/s.
The hot rolled steel is then coiled at a temperature between 475°C and 650°C to avoid ovalization and preferably between 475°C and 625°C to avoid scale formation. A more preferred range for such coiling temperature is between 500°C and 625°C. The coiled hot rolled steel is then cooled down to room temperature before subjecting it to optional hot band annealing.
The hot rolled steel may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing. The hot rolled sheet may then subjected to an optional Hot Band Annealing at, for example, temperatures between 400°C and 750°C for preferably at least 12 hours and not more than 96 hours, the temperature remaining below 750°C to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel may performed through, for example, pickling of such sheet.
This hot rolled steel is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%. The cold rolled steel sheet obtained from cold rolling process is then subjected to annealing to impart the steel of present invention with microstructure and mechanical properties.
To anneal the said cold rolled steel sheet, it is heated up to the soaking temperature between Ac1 +60°C and Ac3, preferably at a heating rate of at least 3°C/s, then the annealing is performed at that temperature during 5 to 500 seconds, preferably during 50 to 250 seconds. In a preferred embodiment, the heating is at least 10°C/s and more preferably at least 15°C/s. During this annealing Inter-critical ferrite forms.
The preferred annealing soaking temperature is between Ac1 + 70°C and Ac3 and more preferably between Ac1 + 80°C and Ac3 - 30°C.
In a preferred embodiment, the time and temperature of soaking are selected to ensure that the microstructure of the steel sheet at the end of the soaking contains at least 50% of Austenite and more preferably at least 60% of austenite.
Then the cold rolled steel is cooled in a two-step cooling process wherein the first step starts from soaking temperature to a temperature T1 between 550°C and 650°C, at a cooling rate CR1 which is least 3°C/s, preferably at least 5°C/s and more preferably at least 10°C/s. During this step the transformed ferrite forms. The cold rolled steel is then held at T 1 during 1 s to 20 s and preferably between 2 s and 15 s and more preferably between 5 s and 12 s.
Thereafter the second step starts from cooling further the cold rolled steel sheet from T 1 to overaging temperature T2 between 400°C and 480°C, at a cooling rate CR2 of at least 3°C/s, preferably at least 5°C/s and more preferably at least 7°C/s..
Then overaging is being performed at T2 during 5 to 100 seconds. During overaging, some bainite gets formed. The preferred temperature T2 for overaging is between 420°C and 475°C. The preferred time for overaging temperature during 15 to 75 seconds and more preferably 20 and 75 seconds.
The cold rolled steel sheet can then either be cooled down to room temperature or can be brought to the temperature of a hot dip coating bath between 420°C and 680°C, depending on the nature of the coating, to facilitate hot dip coating of the cold rolled steel sheet. In either case, the final cooling down to room temperature is done at a cooling rate of at least 5°C/s and preferably at least 9°C/s to ensure the formation of fresh martensite in the steel of present invention.
The cold rolled steel sheet can also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, etc, which may not require bringing it to the above-mentioned temperature range before coating.
EXAMPLES
The following tests and examples presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the 5 advantageous features of the present invention and expound the significance of the parameters chosen by inventors after extensive experiments and further establish the properties that can be achieved by the steel according to the invention.
Samples of the steel sheets according to the invention and to some 10 comparative grades were prepared with the compositions gathered in table 1 and the processing parameters gathered in table 2. The corresponding microstructures of those steel sheets were gathered in table 3 and the properties in table 4.
Table 1 : composition of the trials
Table 1 depicts the steels with the compositions expressed in percentages by is weight.
Table 1 also shows Ac1 and Ac3 temperature points which are calculated by dilatometry.
Table 2 : Process parameters
Table 2 gathers the annealing process parameters implemented on steel samples of Table 1 , that were all reheated at 1230°C, hot-rolled with a finish rolling temperature of 875°C, coiled at 550°C and cold rolled with a 50% reduction rate 5 before undergoing annealing and two-steps cooling scheme including overaging:
underlined values: not according to the invention.
Table 3 gathers the results of test conducted in accordance of standards on 10 different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials.
Table 3 : microstructures of the trials
underlined values: not according to the invention.
Table 4 gathers the mechanical properties of both the inventive steel and reference steel. The tensile strength, yield strength and total elongation tests are conducted in accordance with JIS Z2241 standards. Table 4 : mechanical properties of the trials
underlined values: not according to the invention.
The examples show that the steel sheets according to the invention are the only one to show all the targeted properties thanks to their specific composition and microstructures.
Claims
1. A cold rolled steel sheet having a composition comprising of the following elements, expressed in percentage by weight:
0.05 % < Carbon < 0.15 %
1.8% < Manganese < 2.7%
0.1 % < Silicon < 1%
0.01 % < Aluminum < 0.8%
0.1 % < Chromium < 0.9 %
0 % < Phosphorus < 0.09 %
0.0001% < Titanium < 0.1%
0.0005% < Boron < 0.003%
0.01% < Niobium < 0.1%
0 % < Sulfur < 0.09 %
0 % < Nitrogen < 0.09% and can contain one or more of the following optional elements 0% < Vanadium < 0.2%
0% < Molybdenum < 0. 2%
0% < Nickel < 2%
0% < Copper < 2%
0% < Calcium < 0.005%
0% < Cerium < 0.1%
0% < Magnesium £ 0.05%
0% < Zirconium £ 0.05% the remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.
2. Cold rolled and coated steel sheet according to claim 1 , wherein the composition includes 0.2% to 0.9% of Silicon.
3. Cold rolled and coated steel sheet according to claim 1 or 2, wherein the composition includes 0.07% to 0.12% of Carbon.
4. Cold rolled and coated steel sheet according anyone of claims 1 to 3, wherein the composition includes 0.01% to 0.7% of Aluminum.
5. Cold rolled and coated steel sheet according to anyone of claims 1 to 4, wherein the composition includes 1.9% to 2.5% of Manganese.
6. Cold rolled and coated steel sheet according to anyone of claims 1 to 5, wherein the composition includes 0.2% to 0.8% of Chromium.
7. Cold rolled and coated steel sheet according to anyone of claims 1 to 6, wherein, the cumulated amounts of Silicon and Aluminum is more than 0.6%.
8. Cold rolled and coated steel sheet according to claim 1 to 7, wherein the cumulated amount of transformed ferrite and bainite is between 15% and 30%
9. Cold rolled and coated steel sheet according to anyone of claims 1 to 8, wherein the martensite amount is between 42% and 58%.
10. Cold rolled and coated steel sheet according to anyone of claims 1 to 9, wherein said steel sheet has an ultimate tensile strength of 950 MPa or more, and a total elongation of 14% or more.
11. Cold rolled and coated steel sheet according to claim 10, wherein said steel sheet has a yield strength of 540 MPa or more.
12. A method of production of a cold rolled and coated steel sheet comprising the following successive steps:
- providing a steel composition according to anyone of claims 1 to 7;
- reheating said semi-finished product to a temperature between 1000°C and 1280°C;
- rolling the said semi-finished product wherein the hot rolling finishing temperature shall be above Ac3 to obtain a hot rolled steel;
- cooling the hot rolled steel at a cooling rate of at least 30°C/s to a coiling temperature which is between 475°C and 650°C; and coiling the said hot rolled steel;
- cooling the said hot rolled steel to room temperature;
- optionally performing scale removal process on said hot rolled steel sheet;
- optionally annealing is performed on hot rolled steel sheet;
- optionally performing scale removal process on said hot rolled steel sheet;
- cold rolling the said hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;
- heating the cold rolled steel sheet from room temperature to soaking temperature between Ac1 +60°C and Ac3,
- then perform annealing at soaking temperature during 5 to 500 seconds
- then the cold rolled steel is cooled in a two-step cooling process wherein: o the first step starts from the soaking temperature with a cooling down toto a temperature T1 between 550°C and 650°C, at a cooling rate CR1 of least 3°C/s, o the cold rolled steel is then held at T1 during 1 s to 20 s o the second step starts then by cooling further the cold rolled steel sheet down from T1 to an overaging temperature T2 between 400°C and 480°C, at a cooling rate CR2 of at least 3°C/s, then perform overaging at T2 during 5 to 100 seconds,
- it is then optionally brought to a temperature range between 420°C and 680°C to facilitate coating and optionally coated the cold rolled sheet,
- thereafter cooling the cold rolled steel sheet to room temperature at a cooling rate of at least 5°C/s to obtain a cold rolled coated steel sheet.
13. A method according to claim 12, wherein the coiling temperature is between 475°C and 625°C.
14. A method according to anyone of claims 12 or 13, wherein the soaking temperature is selected so as to ensure the presence of at least 50% of austenite at the end of the soaking.
15. A method according to anyone of claims 12 to 14, wherein the temperature for overaging is between 420°C and 475°C.
16. A method according to anyone of claims 12 to 15, wherein the cooling rate after coating is at least 9°C/s.
17. Use of a steel sheet according to anyone of claims 1 to 11 or of a steel sheet produced according to the method of claims 12 to 16, for the manufacture of structural or safety parts of a vehicle.
18. Vehicle comprising a part obtained according to claim 17.
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| Application Number | Priority Date | Filing Date | Title |
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| PCT/IB2020/056330 WO2022008949A1 (en) | 2020-07-06 | 2020-07-06 | Heat treated cold rolled steel sheet and a method of manufacturing thereof |
| PCT/IB2021/055898 WO2022009032A1 (en) | 2020-07-06 | 2021-07-01 | Heat treated cold rolled steel sheet and a method of manufacturing thereof |
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| EP (1) | EP4176092A1 (en) |
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| JP4772497B2 (en) * | 2005-12-27 | 2011-09-14 | 新日本製鐵株式会社 | High-strength cold-rolled thin steel sheet excellent in hole expansibility and manufacturing method thereof |
| KR101561007B1 (en) * | 2014-12-19 | 2015-10-16 | 주식회사 포스코 | High strength cold rolled, hot dip galvanized steel sheet with excellent formability and less deviation of mechanical properties in steel strip, and method for production thereof |
| WO2016198906A1 (en) * | 2015-06-10 | 2016-12-15 | Arcelormittal | High-strength steel and method for producing same |
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| KR102153197B1 (en) * | 2018-12-18 | 2020-09-08 | 주식회사 포스코 | Cold rolled steel sheet with excellent formability, galvanized steel sheet, and manufacturing method thereof |
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