EP1735474A1 - Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheets - Google Patents
Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheetsInfo
- Publication number
- EP1735474A1 EP1735474A1 EP05789750A EP05789750A EP1735474A1 EP 1735474 A1 EP1735474 A1 EP 1735474A1 EP 05789750 A EP05789750 A EP 05789750A EP 05789750 A EP05789750 A EP 05789750A EP 1735474 A1 EP1735474 A1 EP 1735474A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- steel sheet
- less
- hot
- steel
- grain
- 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.)
- Granted
Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 389
- 239000010959 steel Substances 0.000 title claims abstract description 389
- 239000010960 cold rolled steel Substances 0.000 title claims abstract description 47
- 238000000034 method Methods 0.000 title claims abstract description 42
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 36
- 239000002244 precipitate Substances 0.000 claims abstract description 72
- 230000032683 aging Effects 0.000 claims abstract description 67
- 239000012535 impurity Substances 0.000 claims abstract description 21
- 229910052799 carbon Inorganic materials 0.000 claims description 87
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 86
- 238000000137 annealing Methods 0.000 claims description 56
- 238000005098 hot rolling Methods 0.000 claims description 37
- 238000005096 rolling process Methods 0.000 claims description 29
- 238000005097 cold rolling Methods 0.000 claims description 18
- 238000007598 dipping method Methods 0.000 claims description 17
- 230000009467 reduction Effects 0.000 claims description 14
- 230000004888 barrier function Effects 0.000 claims description 9
- 238000010438 heat treatment Methods 0.000 claims description 8
- 230000009466 transformation Effects 0.000 claims description 8
- DBIMSKIDWWYXJV-UHFFFAOYSA-L [dibutyl(trifluoromethylsulfonyloxy)stannyl] trifluoromethanesulfonate Chemical compound CCCC[Sn](CCCC)(OS(=O)(=O)C(F)(F)F)OS(=O)(=O)C(F)(F)F DBIMSKIDWWYXJV-UHFFFAOYSA-L 0.000 claims 4
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 46
- 239000010955 niobium Substances 0.000 description 44
- 239000010936 titanium Substances 0.000 description 32
- 238000007792 addition Methods 0.000 description 31
- 230000000052 comparative effect Effects 0.000 description 28
- 230000000694 effects Effects 0.000 description 25
- 229910052757 nitrogen Inorganic materials 0.000 description 25
- 230000001965 increasing effect Effects 0.000 description 24
- 230000002708 enhancing effect Effects 0.000 description 20
- 230000006866 deterioration Effects 0.000 description 16
- 238000007670 refining Methods 0.000 description 15
- 229910052782 aluminium Inorganic materials 0.000 description 14
- 239000011572 manganese Substances 0.000 description 12
- 229910052758 niobium Inorganic materials 0.000 description 11
- 229910052719 titanium Inorganic materials 0.000 description 11
- 229910052750 molybdenum Inorganic materials 0.000 description 10
- 239000000203 mixture Substances 0.000 description 9
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 8
- 229910052796 boron Inorganic materials 0.000 description 8
- 230000008569 process Effects 0.000 description 8
- 230000002542 deteriorative effect Effects 0.000 description 7
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 6
- 239000011248 coating agent Substances 0.000 description 6
- 238000000576 coating method Methods 0.000 description 6
- 229910052710 silicon Inorganic materials 0.000 description 6
- 239000010703 silicon Substances 0.000 description 6
- 238000005728 strengthening Methods 0.000 description 6
- 230000008878 coupling Effects 0.000 description 5
- 238000010168 coupling process Methods 0.000 description 5
- 238000005859 coupling reaction Methods 0.000 description 5
- BHEPBYXIRTUNPN-UHFFFAOYSA-N hydridophosphorus(.) (triplet) Chemical compound [PH] BHEPBYXIRTUNPN-UHFFFAOYSA-N 0.000 description 5
- 238000011835 investigation Methods 0.000 description 5
- 238000001953 recrystallisation Methods 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 4
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 4
- 150000004767 nitrides Chemical class 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- 229910052717 sulfur Inorganic materials 0.000 description 4
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 3
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 3
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 238000009792 diffusion process Methods 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- -1 particularly Substances 0.000 description 3
- 229910052698 phosphorus Inorganic materials 0.000 description 3
- 239000011593 sulfur Substances 0.000 description 3
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 2
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 2
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 2
- 238000005275 alloying Methods 0.000 description 2
- 229910001566 austenite Inorganic materials 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 230000007774 longterm Effects 0.000 description 2
- 239000011733 molybdenum Substances 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 238000003860 storage Methods 0.000 description 2
- 238000005482 strain hardening Methods 0.000 description 2
- 230000007704 transition Effects 0.000 description 2
- 241000905957 Channa melasoma Species 0.000 description 1
- 229910000655 Killed steel Inorganic materials 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- 229910004688 Ti-V Inorganic materials 0.000 description 1
- 229910010968 Ti—V Inorganic materials 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000003466 anti-cipated effect Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000018109 developmental process Effects 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 238000005324 grain boundary diffusion Methods 0.000 description 1
- 229910052748 manganese Inorganic materials 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
- 239000000463 material Substances 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000001000 micrograph Methods 0.000 description 1
- 239000003973 paint Substances 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- 238000007747 plating Methods 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 230000000979 retarding effect Effects 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 230000035882 stress Effects 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 150000004763 sulfides Chemical class 0.000 description 1
- 238000009827 uniform distribution Methods 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
Classifications
-
- 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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- 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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- 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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
Definitions
- the present invention relates to a bake-hardenable cold-rolled steel sheet for automobile bodies, a hot-dipped steel sheet manufactured using the same, and a method for manufacturing the same. More particularly, the present invention relates to a bake-hardenable high-strength cold-rolled steel sheet having excellent bake hard- enability, aging resistance at room temperature, and secondary work embrittlement resistance, a hot-dipped steel sheet manufactured using the same, and a method for manufacturing the same.
- bake-hardenable cold-rolled steel sheets typically have a tensile strength of 390 MPa, and exhibit excellent ductility due t o the fact that their yield strength is approximately the same as that of mild steel upon press forming. Additionally, the bake-hardenable cold-rolled steel sheets have a char ⁇ acteristic in which the yield strength thereof is increased upon paint baking or coating treatment after press forming.
- steel sheets are mainly used, which are manufactured by batch annealing low carbon, P-added, Al-killed steels, and have a bake hardening value of about 40 to 50 MPa.
- Japanese Unexamined Patent Publication No. (Sho) 57-89437 discloses a bake- hardenable cold-rolled steel sheet, which uses a Ti-added steel comprising 0.010 wt% or less of C, and has a bake hardening value of about 40 MPa.
- the methods of the disclosures impart bake hardenability to the steel sheets while preventing deterioration of the other properties of the steel sheets by appropriately controlling the amount of solute elements in the steel through appropriate control of the added amount of Ti and Nb or the cooling rate during annealing.
- it is necessary to strictly control the amount of Ti, N, and S in the manufacturing process in order to ensure appropriate bake hardenability there is a problem of increasing manufacturing costs.
- U.S. Patent Nos. 5,556,485 and 5,656,102 disclose methods of manufacturing a bake-hardenable cold-rolled steel sheet using a Ti-V based ultra low carbon steel, which comprises 0.0005 - 0.1 wt% of C; 0 ⁇ 2.5 wt% of Mn; 0 ⁇ 0.5 wt% of Al; 0 ⁇ 0.04 wt% of N; 0 ⁇ 0.5 wt% of Ti; and 0.005 ⁇ 0.6 % of V.
- Hei 9-249936 discloses a method for enhancing the ductility of the steel by relieving stress concentration on grain boundaries through addition of V and Nb.
- Japanese Unexamined Patent Publication No. (Hei) 8-49038 discloses a method for enhancing the formability through addition of Zr
- Japanese Unexamined Patent Publication No. (Hei) 7-278654 discloses a method for enhancing the formability by increasing the strength while minimizing deterioration of work hardening index (N- value) through addition of Cr.
- the present invention has been made in view of the above problems, and it is an object of the invention to provide bake-hardenable high-strength cold-rolled steel sheets having bake hardenability, aging resistance, and secondary work em- brittlement resistance by adding a small amount of Nb together with Al, Mo and B, while appropriately controlling of the Nb/C ratio and the grain size, hot-dipped steel sheets manufactured using the same, and a method for manufacturing the same.
- a bake-hardenable cold-rolled steel sheet manufactured through hot rolling, cold rolling and continuous annealing of a steel, the steel sheet comprising: 0.0016 ⁇ 0.01 % of C; 0.1 % or less of Si; 0.2 ⁇ 1.5 % of Mn; 0.05 ⁇ 0.15 % of P; 0.01 % or less of S; 0.08 ⁇ 0.5 % of (soluble) Al; 0.0025 % or less of N; 0.003 ⁇ 0.1 % of Nb; 0 ⁇ 0.003 % of Ti; 0.01 ⁇ 0.4 % of Mo; 0.0005 ⁇ 0.005 % of B; and the balance of Fe and other unavoidable impurities, in terms of weight%, while satisfying an Nb/C ratio of 0.3 ⁇ 0.7, wherein the steel sheet has fine AlN pre ⁇ cipitates formed upon hot rolling the steel, and a grain
- a hot-dipped steel sheet man ⁇ ufactured through hot rolling, cold rolling, continuous annealing, and hot dipping of a steel comprising: 0.0016-0.01 %of C; 0.1% or less of Si; 0.2 ⁇ 1.5% of Mn; 0.05 ⁇ 0.15 of P; 0.01 % or less of S; 0.08 ⁇ 0.5% of (soluble) Al; 0.0025% or less of N; 0.003 ⁇ 0.1% of Nb; 0 ⁇ 0.003 % of Ti; 0.01 ⁇ 0.4% of Mo; 0.0005 ⁇ 0.005% of B; and the balance of Fe and other unavoidable impurities, in terms of weight%, while satisfying an Nb/C ratio of 0.3 ⁇ 0.7, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size(ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for
- a method for manu ⁇ facturing a bake-hardenable high-strength cold-rolled steel sheet comprising the steps of: hot-rolling a steel slab with finish rolling at or above the Ar transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200 °C or more, the steel slab comprising: 0.0016 ⁇ 0.01 % of C; 0.1 % or less of Si; 0.2 ⁇ 1.5 % of Mn; 0.05 ⁇ 0.15 % of P; 0.01 % or less of S; 0.08 ⁇ 0.5 % of (soluble) Al; 0.0025 % or less of N; 0.003 ⁇ 0.1 % of Nb; 0 ⁇ 0.003 % of Ti; 0.01 ⁇ 0.4 % of Mo; 0.0005 ⁇ 0.005 % of B; and the balance of Fe and other un- avoidable impurities, in terms of weight%, while satisfying an Nb/C
- a method for manufacturing a hot-dipped steel sheet comprising the steps of: hot-rolling a steel slab with finish rolling at or above an Ar transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200 °C or more, the steel slab comprising: 0.0016 ⁇ 0.01 % of C; 0.1 % or less of Si; 0.2 ⁇ 1.5 % of Mn; 0.05 ⁇ 0.15 % of P; 0.01 % or less of S; 0.08 ⁇ 0.5 % of (soluble) Al; 0.0025 % or less of N; 0.003 ⁇ 0.1 % of Nb; 0 ⁇ 0.003 % of Ti; 0.01 ⁇ 0.4 % of Mo; 0.0005 ⁇ 0.005 % of B; and the balance of Fe and other unavoidable impurities, in terms of weight%, while satisfying an Nb/C ratio of 0.3 ⁇ 0.7; coiling the hot-
- Fig. 1 is a graph illustrating the influence of grain size upon bake hardening value and aging index
- Fig. 2 is a graph illustrating the influence of Al content upon mechanical properties
- Fig. 3 is a graph illustrating the influence of Mo content upon the bake hardening value and the aging index
- Fig. 4 is a micrograph of microstructure of Inventive steel No. 8 after annealing
- Fig. 5 is a graph illustrating the influence of drawing ratio upon DBTT for the Inventive steel No.
- Fig. 6 is a graph illustrating stress-strain curves before and after aging the Inventive steel No. 8; and [47] Fig. 7 is a graph illustrating the influence of the Nb/C ratio upon the bake hardening value and the aging index for steels of the invention.
- Fig. 7 is a graph illustrating the influence of the Nb/C ratio upon the bake hardening value and the aging index for steels of the invention.
- Best Mode for Carrying Out the Invention [49] Preferred embodiments of the invention will now be described in detail.
- Requirements for a bake-hardenable cold-rolled steel sheet include a high bake hardening value, a low aging index at room temperature, and excellent secondary work embrittlement resistance.
- Bake hardening or natural aging is a phenomenon occurring when interstitial elements exiting as solute elements in a steel, particularly, nitrogen or carbon, are fixed to dislocations generated during deformation.
- contents of solute nitrogen and carbon are increased in the steel, a bake hardening value is also increased, with ac ⁇ companying natural aging due to the excessive amount of solute elements, thereby causing deterioration of formability.
- it is very important to optimize the amount of solute nitrogen and carbon in the steel.
- nitrogen Since nitrogen has a higher diffusion speed in the steel than that of carbon, nitrogen affects the natural aging rather than carbon.
- the amount of solute nitrogen is reduced in order to secure excellent aging resistance.
- the bake hardenability and the aging resistance may be differently exhibited according to a position of the solute carbon in the steel, that is, according to whether the solute carbon is located in grain boundaries or in grains, which will be described hereinafter.
- the reason for the varying influence on the bake hardenability and the aging resistance according to the position of the solute carbon in the steel is related to the mobility of the solute carbon.
- the solute carbon When the solute carbon is located in grain, it can move relatively freely, thereby in ⁇ fluencing not only the bake hardenability but also the aging resistance.
- the solute carbon in the grain boundary is located in a relatively stable position, it has minimal affects upon low temperature aging, but during high temperature baking, it is activated and affects the bake hardenability.
- the solute carbon in grain influences the aging resistance and the bake hardenability simultaneously, whereas the solute carbon in the grain boundary influences only the bake hardenability.
- appropriate control of the position of the solute carbon in the steel that is, controlling the solute carbon to be located in the grain boundaries rather than the grains can secure the aging resistance and the bake hardenability at the same time.
- Fig. 1 is a graph illustrating the relationship between variation in grain size and bake hardening (BH) value and an aging index (AI), as one of the results of studies performed by the inventors.
- BH bake hardening
- AI aging index
- the fine AlN precipitates are formed upon hot rolling, by addition of large quantities of Al to the steel, and suppress grain growth when annealing a cold-rolled steel sheet, thereby allowing the grains to become finer.
- the Al content is appropriately controlled so as to allow the fine AlN precipitates to be formed upon hot rolling.
- fine grains are more securely achieved by appropriately controlling the size of the AlN precipitates.
- Al is known as a deoxidizer and/or a component added for fixing nitrogen by use of coarse AlN precipitates in a bake-hardenable steel.
- Secondary work embrittlement means that cracks are formed during a secondary working process after a primary working process.
- the cracks are caused by a phenomenon in which fractures are formed along the grain boundaries due to phosphorous (P), which is present in the grain boundaries of the steel and weakens coupling force between the grains.
- P phosphorous
- P is the solute element which provides the least reduction of elongation in comparison to an increase of strength, and, in particular, has an advantage in that it is low in price.
- Fig. 3 is a graph showing one of the results obtained by analyzing the effect of enhancing the aging resistance through addition of Mo, and it can be seen from Fig. 3 that when Mo content is increased, the BH value is not significantly changed, whereas the AI is lowered, thereby enhancing the aging resistance.
- Carbon (C) is an element for solid solution strengthening and bake hardening. If carbon content is excessively low, the tensile strength of the steel is significantly lowered, sufficient bake hardening effect is not secured due to a low absolute amount of carbon in the steel, and the secondary work embrittlement resistance is also de ⁇ teriorated since the site competition effect between the solute carbon and P is removed.
- the carbon content is preferably 0.0016 % or more in order to ensure an effect of carbon addition.
- the upper limit of the carbon content is set to 0.01 %, and preferably to 0.0025 %.
- the carbon content is in the range of 0.0016 ⁇ 0.0025 %, the ability to control the carbon content to be in this range is realized in a level of 95 % or more in practice.
- Mn is an element for preventing hot embrittlement caused by formation of FeS by completely precipitating sulfur in the steel into MnS while allowing the grains to be reduced in size without deteriorating the ductility, as well as strengthening the steel.
- Mn is preferably added in the range of 0.2 % or more.
- MnO oxides
- coating defects such as a stripe pattern
- the Mn content is preferably in the range of 0.2 ⁇ 1.5 %, and more particularly, in the range of 0.2 ⁇ 1.2 %.
- Phosphorus (P) P is a substitutional alloying element, which provides the highest solid solution strengthening effect, and serves to enhance the in-plane anisotropy index while improving the strength of the steel.
- P causes the grains of a hot- rolled steel sheet to become finer, and then serves to promote development of the mi- crostructure having (111) plane, which is advantageous in that it enhances the average r-value (plasticity-anisotropy index), during a subsequent annealing process.
- Al is generally used for deoxidizing and fixing the solute nitrogen in the steel. However, in the present invention, Al is combined with N during hot rolling to form fine AlN precipitates in the steel, thereby suppressing the grain growth and thus promoting grain refining during the annealing process, thereby enhancing the aging resistance and bake hardenability.
- grain refining is mainly achieved by NbC precipitates.
- the fine AlN precipitates suppress the grain growth together with the NbC precipitates during the annealing process, the grains can be further reduced in size, thereby providing excellent aging resistance and bake hardenability.
- an upper limit of the Al content is preferably 0.5 %, and more preferably 0.12 %.
- the AlN precipitates It is necessary for the AlN precipitates to have a size, which can serve as a barrier to suppress the grain growth during annealing of the cold-rolled steel sheets, and it is preferable that the AlN precipitates have an average size of 20 D or less. [139] When the AlN precipitates have the average size of 20 D or less as described above, finer grains are obtained. [140] [141] Nitrogen (N): Nitrogen exists in a solute state before or after annealing, and de ⁇ teriorates the formability of the steel. Furthermore, since nitrogen has a higher aging ability than other interstitial solute elements, it is necessary to fix nitrogen by use of Ti or Al.
- Titanium is typically added to the steel as a carbonitride-forming element, and forms nitrides such as TiN, sulfides such as TiS or Ti 4 C 2 S 2 , and carbides such as TiC, in the steel.
- Niobium (Nb) Niobium is a very important element together with Al and Mo in the present invention.
- Nb is an intensive carbonitride-forming element, and fixes carbon in the steel as NbC precipitates.
- the NbC precipitates are very fine in comparison to other precipitates, it acts as an intensive barrier to impede the grain growth during recrystallization annealing, thereby causing the grains to become finer.
- the bake hardenability and the secondary work embrittlement resistance are enhanced.
- the grains are refined by means of the NbC precipitates, and the amount of Nb is controlled so as to secure the bake hard ⁇ enability by allowing the predetermined amount of solute carbon to remain in the steel.
- the Nb content is preferably in the range of 0.003 - 0.1 %, and more preferably in the range of 0.003 - 0.011 %.
- It is desirable that the size of NbC precipitates is 30 nm or less.
- Mo serves to enhance the strength of the steel or to form an Mo-based carbide in the solute state in the steel.
- Mo serves to increase the coupling force of the grain boundaries in the solute state in the steel, thereby preventing fracture of the grain boundaries caused by P, that is, enhancing the secondary work embrittlement resistance, and suppresses the diffusion of carbon by virtue of affinity with carbon, thereby enhancing the aging resistance.
- addition of too much Mo will cause an increase of manufacturing costs.
- the Mo content is preferably in the range of 0.01 - 0.4 %, and more preferably in the range of 0.01 - 0.1 %.
- B Boron is an interstitial element, and is located within the steel. B is in the solute state in the grain boundaries or combined with nitrogen to form the nitride of BN. [163] Since B has a significant influence upon the properties of the steel according to an added amount, it is necessary to precisely control the amount added. [164] That is, when a small quantity of B is added, B is segregated in the grain boundaries, and enhances the secondary work embrittlement resistance. [165] However, when B is added to the steel above a predetermined amount, the steel can be remarkably deteriorated in ductility while being subjected to an increase of the strength, and thus it is necessary to add an appropriate amount of B.
- the amount of B is preferably in the range of 0.0005 ⁇ 0.005 %, and more preferably in the range of 0.0005 ⁇ 0.0015 %.
- the steel sheet of the invention must have a grain size(ASTM No.) of 9 or more while satisfying the compositions as described above.
- ASTM number of 9 or more
- the amount of solute carbon in the grain boundaries is increased, thereby allowing the bake hardenability to be further enhanced while maintaining excellent aging resistance at room temperature.
- the steel sheet of the invention has a greater amount of solute carbon in the grain boundaries rather than in the grains.
- a preferable amount of solute carbon in grain is ap ⁇ proximately 3 - 6 ppm.
- the size of AlN precipitates formed during hot rolling is preferably 20 D or less.
- the size of AlN precipitates is 20 D or less, the grains become finer, resulting in further enhancement of the bake hardenability.
- the steel sheet of the invention has excellent bake hardenability, aging resistance, secondary work embrittlement resistance, and a tensile strength of 300 MPa or more.
- a preferable embodiment of a method for manufacturing steel of the invention will now be described. [176] After manufacturing a molten steel having the composition as described above, a steel slab is manufactured through continuous casting of the molten steel, and is then heated to a temperature of 1,200 °C or more.
- the heated steel slab is subjected to hot- rolling with finish rolling at or above the Ar transformation temperature, preferably at a temperature of 900 ⁇ 950 °C, which is immediately above the Ar transformation temperature, thereby providing a hot rolled steel sheet
- Heating of the steel slab to the temperature of 1,200 °C or more is for the purpose of providing sufficiently homogeneous austenite structure to the steel slab before hot rolling.
- the structure of the steel may have combined grains instead of homogeneous austenite grains, causing deterioration of the properties of the steel.
- finish hot rolling temperature If the finish hot rolling temperature is too low, the top and tail portions, and edges of a hot coil become single-phase regions, respectively, thereby increasing the in-plane anisotropy while deteriorating the formability of the steel. If the finish hot rolling temperature is excessively high, remarkably coarse grains are formed in the steel, so that defects such as orange peels can be easily formed on the surface of the steel sheet after working. [180] Thus, the finish hot rolling temperature must be established on the basis of these viewpoints as described above.
- the hot coiling temperature is preferably in the range of 550 ⁇ 650 °C, and more preferably in the range of 600 ⁇ 650 °C.
- cold rolling is performed, thereby manufacturing a cold-rolled steel sheet.
- a reduction rate is preferably 70 ⁇ 80 %.
- the reduction rate is maintained at 70% or more for the purpose of enhancing the formability of the steel sheet, in particular, the r- value, together with the aging resistance by virtue of grain refining.
- the cold-rolled steel sheet is subjected to continuous annealing by a typical method.
- continuous annealing of the invention may be performed at about a temperature of 770 ⁇ 830°C.
- annealing is preferably performed at a temperature of 770 °C or more.
- the cold-rolled steel sheet is cooled from the continuous annealing temperature to the hot dipping temperature, and is then subjected to hot dipping.
- hot dipping is not limited to particular conditions, and is performed by a typical method.
- a hot dipping temperature is typically in the rage of about 450 ⁇ 500 °C.
- the hot dipped steel sheet is subjected to temper rolling.
- temper rolling is performed in order to ensure the aging resistance at room temperature together with appropriate bake hardenability.
- the temper rolling ratio is a little higher than typical temper rolling, and preferably in the range of 1.2 ⁇ 1.5 %.
- Example 1 Steel slabs having compositions as shown in Table 1 were heated to a temperature of 1,220 °C, and subjected to finish hot rolling at a temperature of 920 °C in order to provide hot rolled steel sheets. [205] The hot rolled steel sheets were coiled under the conditions shown in Table 2, and subjected to cold rolling and annealing. Then, the annealed cold-rolled steel sheets were subjected to hot dipping at a temperature of 450 °C, followed by temper rolling at a temper rolling ratio of about 1.5 %.
- BH bake hardening
- AI aging index
- ASTM No. grain size
- DBTT ductility- brittleness transition temperature
- the conventional steel is a steel comprising 0.0019 wt% of C; 0.63 wt% of Mn; 0.056 wt% of P; 0.03 wt% of soluble Al; 0.005 wt% of Ti; 0.006 wt% of Nb, and 0.014 wt% of N.
- tensile strengths of the Inventive steel No. 8 before and after annealing were measured, and stress-strain curves thereof are shown in Fig. 6.
- Fig. 6 In Fig.
- an as-received curve is a stress-strain curve for the tensile strength of the steel measured immediately after being manufactured, and a 180-days aged curve is a stress-strain curve for the tensile strength of the steel measured after transportation of the steel to Thailand and storage for 6 months.
- the Inventive steel No. 8 was maintained at an average temperature of 32 °C and a humidity of about 70 %.
- Inventive steel Nos. 1 to 10 have a grain size(ASTM No.) of 9.8 ⁇ 12.5, and have AlN precipitates with an average size of 20 D or less.
- Fig. 4 It can be appreciated from Fig. 4 that the Inventive steel No. 8 has very fine grains while having a very uniform distribution of grains over the microstructure.
- the grains of the inventive steels are very fine, and this is because the inventive steels have a greater content of Al than the comparative steel so that fine AlN precipitates are formed in the inventive steels, and suppress grain growth upon recrystallization annealing together with NbC precipitates.
- the bake hardening value is in the range of 38.1 ⁇ 58.4 MPa, and the AI for evaluating the aging resistance at room temperature is in the range of 8.0 ⁇ 29.1 MPa. Accordingly, it can be appreciated that balance between the BH value and the aging resistance is excellent.
- a relatively low AI compared with a relatively high bake hardening value is based on retarding effect of solute carbon in the steel caused by addition of Mo in addition to grain refining effect caused by the AlN precipitates.
- the DBTT at the drawing ratio of 2.0 is in the range of -40 ⁇ -70 °C.
- Comparative steel No. 1 has a carbon content of 0.0054 wt% higher than the carbon content of the invention in the range of 0.0016 ⁇ 0.0025 %.
- Comparative steel No. 1 has an excellent DBTT and a significantly high BH value, it has an AI of 30 MPa, and thus, it can be seen that the aging resistance is significantly deteriorated. That is, although the Comparative steel No. 1 has a grain size of ASTM No. 11.2, and has very fine grains that satisfy the requirement for grains size of the invention, the high content of solute carbon in the steel causes the aging resistance of the Comparative steel No. 1 to be deteriorated. [227] Additionally, Comparative steel No.
- the Comparative steel No. 2 has a soluble Al content of 0.04 wt%, which is lower than the Al content of the invention in the range of 0.08 ⁇ 0.12 %, and a Ti content of 0.025 %, which is higher than the Ti content of the invention.
- the Comparative steel No. 2 has large grains, a low BH value, and a deteriorated DBTT. [228] That is, for the Comparative steel No. 2, the grain refining effect by virtue of the AlN precipitates, and the effect of enhancing the BH value were not achieved.
- Comparative steel No. 3 satisfies the composition of the invention, but has a carbon content of 0.0012 wt%, which is lower than the carbon content of the invention.
- Comparative steel No. 3 has coarse grains, low BH value and AI, and a deteriorated DBTT of 20 °C.
- Comparative steel No. 4 has a soluble Al content lower than that of the invention, and an Nb content higher than that of the invention.
- the grain refining effect by virtue of Al, and the effect of enhancing the BH value were not achieved.
- the Comparative steel No. 4 has a grain size of ASTM No. 9.1 that satisfies the re ⁇ quirement for grain size of the invention, it can be seen that the excessive addition of Nb causes excessive generation of NbC precipitates, and lack of the solute carbon in the steel, so that the BH value is not obtained at all, and the DBTT is significantly de- teriorated.
- Comparative steel No. 5 does not comprise Mo and B at all, and the effect of enhancing the secondary work em- brittlement resistance by virtue of Mo and B is thus not expected in the Comparative steel No. 5.
- Comparative steel No. 5 has excellent BH and AI values, it has a significantly deteriorated DBTT due to non-addition of Mo and B.
- Comparative steel No. 6 has a soluble Al content lower than that of invention, and does not comprise Mo at all. It can be seen that although the Comparative steel No. 6 has excellent BH value and AI, it has a deteriorated DBTT due to decrease of the coupling force between the grains caused by non-addition of Mo compared with a high content of P.
- Comparative steel No. 7 has a soluble Al content lower than that of invention, and does not comprise Mo and M at all.
- the Comparative steel No. 7 has a BH of 34.1, and an AI of 22.8. It has a deteriorated DBTT due to non-addition of Mo and B.
- Comparative steel No. 8 has a P content 0.12 % higher than that of the invention in the range of 0.05 ⁇ 0.11 %, and does not comprise B. It can be seen that, although the DBTT of the Comparative steel No. 8 is enhanced by virtue of Mo, the effect of enhancing the DBTT is limited due to high content addition of P, and in particular, the DBTT is still negative due to non-addition of B.
- Comparative steel No. 9 does not comprise Mo, and has an AI of 33.4 MPa. It can be seen that the aging resistance is deteriorated.
- Comparative steel No. 10 has an Nb/C ratio lower than that of the invention. Although it has excellent bake hardenability due to a high BH value, it also has de ⁇ teriorated aging resistance due to a high AI.
- Example 2 [242] Samples were manufactured following the same compositions and manufacturing conditions as those of the Inventive steel No. 8 as shown in Tables 1 and 2 except that the ratio of Nb/C was changed as shown in Fig. 7.
- the BH value and AI were measured according to variation of the ratio of Nb/C, and results thereof are shown in Fig. 7.
- the Nb/C ratio must be in the range of 0.3 ⁇ 0.7 in order to secure a BH value of 30 MPa or more and an AI of 30 MPa or less.
- Industrial Applicability As apparent from the above description, according to the present invention, the bake-hardenable high-strength cold-rolled steel sheets having excellent bake hard- enability, aging resistance, and secondary work embrittlement resistance, and the hot- dipped steel sheets using the same may be provided.
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Abstract
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KR20040020204 | 2004-03-25 | ||
KR1020040103610A KR20050095537A (en) | 2004-03-25 | 2004-12-09 | Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheets |
PCT/KR2005/000828 WO2006001583A1 (en) | 2004-03-25 | 2005-03-22 | Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheets |
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EP1735474A1 true EP1735474A1 (en) | 2006-12-27 |
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US (2) | US20070181232A1 (en) |
EP (1) | EP1735474B1 (en) |
JP (1) | JP5127444B2 (en) |
WO (1) | WO2006001583A1 (en) |
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KR100685030B1 (en) * | 2005-07-08 | 2007-02-20 | 주식회사 포스코 | Steel sheet for deep drawing having excellent resistance to secondary work embrittlement, fatigue property and coatability, and method for manufacturing the same |
WO2007035060A1 (en) * | 2005-09-23 | 2007-03-29 | Posco | Bake-hardenable cold rolled steel sheet with superior strength, galvannealed steel sheet using the cold rolled steel sheet and method for manufacturing the cold rolled steel sheet |
KR100685037B1 (en) * | 2005-09-23 | 2007-02-20 | 주식회사 포스코 | Bake-hardenable cold rolled steel sheet with superior strength and aging resistance, galvannealed steel sheet using the cold rolled steel sheet and method for manufacturing the cold rolled steel sheet |
KR100775339B1 (en) * | 2006-11-21 | 2007-11-08 | 주식회사 포스코 | Cold rolled steel sheet having excellent in-plane anisotropy and workability and the method for manufacturing the same |
KR20080061855A (en) * | 2006-12-28 | 2008-07-03 | 주식회사 포스코 | Dual phase steel having superior deep drawing, and method for manufacturing of it |
KR101105040B1 (en) * | 2008-06-23 | 2012-01-16 | 주식회사 포스코 | Bake Hardened Steel with Excellent Surface Properties and Secondary Working Embrittlement Resistance and Manufacturing Method Thereof |
CN103228808B (en) | 2010-11-29 | 2014-05-28 | 新日铁住金株式会社 | High-strength bake-hardening cold-rolled steel sheet and method for manufacturing same |
JP5984800B2 (en) * | 2011-04-28 | 2016-09-06 | 東洋鋼鈑株式会社 | Surface-treated steel sheet for battery container, battery container and battery |
WO2016195456A1 (en) * | 2015-06-05 | 2016-12-08 | 주식회사 포스코 | High-strength thin steel sheet with excellent drawability and bake hardenability, and method for manufacturing same |
KR101758557B1 (en) | 2015-06-05 | 2017-07-18 | 주식회사 포스코 | High-strength thin steel sheet having excellent drawability and bake hardenability and method for manufacturing the same |
WO2018073116A2 (en) * | 2016-10-17 | 2018-04-26 | Tata Steel Ijmuiden B.V. | Method for producing a steel strip for painted parts |
CN109844159B (en) | 2016-10-17 | 2021-09-07 | 塔塔钢铁艾默伊登有限责任公司 | Steel for painted parts |
MX2019004458A (en) * | 2016-10-17 | 2019-06-17 | Tata Steel Ijmuiden Bv | Steel substrate for painted parts. |
CN109283053A (en) * | 2018-11-01 | 2019-01-29 | 唐山钢铁集团有限责任公司 | The ultra-low carbon baking hardening timeliness evaluation method of steel |
US20210319098A1 (en) * | 2018-12-31 | 2021-10-14 | Intel Corporation | Securing systems employing artificial intelligence |
CN114262844A (en) * | 2021-12-03 | 2022-04-01 | 本钢板材股份有限公司 | Low-cost 220BH cold-rolled baking hardened steel and preparation method thereof |
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US20090272468A1 (en) | 2009-11-05 |
EP1735474A4 (en) | 2010-09-29 |
JP5127444B2 (en) | 2013-01-23 |
EP1735474B1 (en) | 2015-10-21 |
WO2006001583A1 (en) | 2006-01-05 |
US20070181232A1 (en) | 2007-08-09 |
JP2007530783A (en) | 2007-11-01 |
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