High strength cold rolled steel sheet and method for manufacturing the same

Abstract

A high strength cold rolled steel sheet consists essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, optionally 0.05% or less Ti, optionally 0.05% or less B, by weight, and a balance of substantially Fe and inevitable impurities, and satisfying the following formulae (6) and (7): (12/93)×Nb*/C≧1.2  (6), TS −4050× Ceq ≧−0.75× TS +380  (7), wherein Nb*=Nb−(93/14)×N, Ceq=C+(1/50)×Si+(1/25)×Mn+(1/2)×P, wherein TS is the tensile strength in MPa, and C, Si, Mn, P, N and Nb denote the content in % by weight of carbon, silicon, manganese, phosphorus, nitrogen, and niobium, respectively. The high strength cold rolled steel sheet has excellent combined formability, resistance to embrittlement during secondary operation, formability at welded portions, and anti-burring performance, and has a desirable surface appearance and uniformity of material in a coil, and thus can be desirably used for automobile exterior panels.

What is claimed is:  
     
         1 . A high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, and optionally 0.05% or less Ti, optionally 0.002% or less B, by weight, and a balance of substantially Fe and inevitable impurities; and satisfying the following formulae (6) and (7):  
       (12/93)×Nb*/C≧1.2  (6)  TS− 4050× Ceq≧− 0.75× TS+ 380  (7),  
       wherein Nb*=Nb−(93/14)×N, Ceq=C+(1/50)×Si+(1/25)×Mn+(1/2)×P, 
 TS denotes the tensile strength in MPa, and C, Si, Mn, P, N, and Nb denote the content in % by weight of carbon, silicon, manganese, phosphorus, nitrogen and niobium, respectively.  
 
     
     
         2 . The high strength steel sheet of  claim 1 , further containing 0.05% or less Ti, by weight.  
     
     
         3 . The high strength steel sheet of  claim 1 , further containing 0.002% or less B, by weight.  
     
     
         4 . A high strength cold rolled steel sheet consisting essentially of: 0.0040 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb in an amount satisfying the following formula (8):  
       1≦(93/12)×(Nb/C)≦2.5  (8),  
       wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; the high strength cold rolled steel sheet having a volumetric proportion of NbC of 0.03 to 0.10%; and 70% or more thereof being in a size of 10 to 40 nm.  
     
     
         5 . A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: 
 (a) preparing a continuous casting slab of a steel which consists essentially of 0.0040 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb in an amount satisfying the following formula (8):    1≦(93/12)×(Nb/C)≦2.5  (8),     wherein C and Nb denote the content in % by weight of carbon and niobium, respectively;    (b) preparing a hot rolled steel sheet by finish rolling the slab from step (a) at reduction ratios satisfying the following formulae (9) through (11):    10≦HR1  (9) 2≦HR2≦30  (10)  HR 1+ HR 2− HR 1× HR 2/100≦60  (11),     wherein HR1 and HR2 denote the reduction ratio in % in the finish rolling at the pass just before the final pass and the final pass, respectively;    (c) cold rolling the hot rolled sheet from step (b) and    (d) annealing the sheet from step (c).    
     
     
         6 . A high strength steel sheet consisting essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02 or less S, 0.01% to 0.1% sol.Al, 0.00100% or less N, 0.036 to 0.14% Nb, and optionally containing 0.0015% or less B, by weight, and satisfying the following formula (12):  
       1.1<(Nb×12)/(C×93)<2.5  (12),  
       wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; the high strength steel sheet having an average grain size of 10 μm or less and an r value of 1.8 or more.  
     
     
         7 . The high strength steel sheet of  claim 6 , further containing 0.0015% or less B, by weight.  
     
     
         8 . The high strength steel sheet of  claim 6 , further containing 0.019% or less Ti, by weight, and satisfying the following formula (13):  
       Ti≦(48/14)×N+(48/32)×S  (13),  
       wherein N, S, and Ti denote the content in % by weight of nitrogen, sulfur, and titanium, respectively.  
     
     
         9 . The high strength steel sheet of  claim 8 , further containing 0.0015% or less B, by weight.  
     
     
         10 . A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: 
 (a) preparing a continuous casting slab of a steel which consists essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.0100% or less N, 0.036 to 0.14% Nb, by weight, and which satisfies the following formula (12):    1.1<(Nb×12)/(C×93)<2.5  (12),     wherein C and Nb denote the content in % by weight of carbon and niobium, respectively;    (b) preparing a sheet bar by direct rolling or heating the slab from step (a) to a temperature of from 1100 to 1250° C. followed by rough rolling;    (c) finish rolling the sheet bar from step (b) to a total reduction ratio of the pass just before the final pass and the final pass to produce a hot rolled steel sheet of 10 to 40%;    (d) coiling the hot rolled steel sheet from step (c) at a cooling speed of 15° C./sec or more to a temperature below 700° C., followed by coiling at a temperature of from 620 to 610° C.;    (e) cold rolling the coiled hot rolled steel sheet from step (d) at a reduction ratio of 50% or more, followed by heating the steel sheet at a heating speed of 20° C./sec or more;    (f) annealing the steel sheet from step (e) at a temperature between 860° C. and an Ar3 transformation temperature, and    (g) temper rolling the annealed steel sheet from step (f) at a reduction ratio of 0.4 to 1.0%.    
     
     
         11 . A high strength cold rolled steel sheet consisting essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight, and satisfying the following formulae (3), (4) and (14):  
       11.0 ≦r+ 50.0× n   (3) 2.9≦ r+ 5.00× n   (4) 1.98−66.3×C≦(Nb×12)/(C×93)≦3.24−80.0×C  (14),  
       wherein r denotes the r value, n denotes the n value at 1 to 5% strain, and C and Nb denote the content in % by weight of carbon and niobium, respectively.  
     
     
         12 . The high strength steel sheet of  claim 11 , further containing 0.002% or less B, by weight.  
     
     
         13 . A high strength cold rolled steel sheet containing essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, 0.05% or less Ti, and optionally 0.002% or less B, by weight, and satisfying the following formulae (3), (4) and (15):  
       11.0≦ r+ 50.0× n   (3) 2.9≦ r+ 5.00× n   (4) 1.98−66.3×C≦(Nb×12)/(C×93)+(Ti*×12)/(C×48)≦3.24−80.0×C  (15),  
       wherein r denotes the r value, n denotes the n value at 1 to 5% strain, Ti* is not more than 0, and C, S, N, Nb, and Ti denote the content in % by weight of carbon, nitrogen, niobium, and titanium, respectively.  
     
     
         14 . The high strength steel sheet of  claim 13 , further containing 0.002% or less B, by weight.  
     
     
         15 . A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: 
 (a) preparing a continuous casting slab of a steel which consists essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight, and which satisfies the following formula (14):    1.98×66.3×C≦(Nb×12)/(C×93)≦3.24−80.0×C  (14),     wherein C and Nb denote the content in % by weight of carbon and niobium, respectively;    (b) preparing a coiled hot rolled steel sheet by finish rolling the slab from step (a) at a total reduction ratio of the pass just before the final pass and the final pass of 60% or less;    (c) cold rolling the hot rolled steel sheet from step (b) and    (d) annealing the sheet from step (c).    
     
     
         16 . The method for manufacturing a high strength steel sheet of  claim 15 , wherein the finish rolling is conducted at a temperature of 870° C. or higher, the coiling is conducted at a temperature of 550° C. or higher, the cold rolling is conducted at a rolling reduction ratio of 50 to 85%, and the continuous annealing is conducted at a temperature of 780 to 880° C.  
     
     
         17 . A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: 
 (a) preparing a continuous casting slab of a steel which consists essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1 sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, 0.05% or less Ti, by weight, and which satisfies the following formula (15):    1.98×66.3×C≦(Nb×12)/(C×93)+(Ti*×12)/(C×48)≦3.24−80.0×C  (15),    wherein Ti*=Ti−(48/14)×N−(48/32)×S, Ti*=0 when Ti* is not more than 0, and C, S, N, Nb, and Ti denote the content in % by weight of carbon, nitrogen, niobium, and titanium, respectively;    (b) preparing a coiled hot rolled steel sheet by finish rolling the slab from step (a) to a total reduction ratio of the pass just before the final pass and the final pass of 60% or less;    (c) cold rolling the hot rolled steel sheet from step (b) and    (d) followed by annealing the sheet from step (c).    
     
     
         18 . The method of manufacturing a high strength steel sheet of  claim 17 , wherein the finish rolling is conducted at a temperature of 870° C. or higher, the coiling is conducted at a temperature of 550° C. or higher, the cold rolling is conducted at a rolling reduction ratio of 50 to 85%, and the continuous annealing is conducted at a temperature of 780 to 880° C.