Cold rolled steel sheet having superior formability and high yield ratio, process for producing the same

Abstract

Disclosed herein is a Nb—Ti composite IF steel in which fine precipitates, such as CuS precipitates, having a size of 0.2 μm or less are distributed. The distribution of fine precipitates in the Nb—Ti composite IF steel enhances the yield strength and lowers the in-plane anisotropy index. The nanometer-sized precipitates allow the formation of minute crystal grains. As a result, dissolved carbon is present in a larger amount in the crystal grain boundaries than within the crystal grains, which is advantageous in terms of room-temperature non-aging properties and bake handenability.

1 . A cold rolled steel sheet with superior formability and high yield ratio, the cold rolled steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities,
 wherein the composition satisfies the following relationships: 1≦(Cu/63.5)/(S*/32)≦30 and S*═S−0.8×(Ti−0.8×(48/14)×N)×(32/48), and   wherein the steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.   
   
   
       2 . The cold rolled steel sheet according to  claim 1 , wherein the composition further comprises 0.01-0.3% of Mn and satisfies the following relationship: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less. 
   
   
       3 . The cold rolled steel sheet according to  claim 1 , wherein the N content is 0.004-0.02% and the composition satisfies the following relationships: 1≦(Al/27)/(N*/14)≦10 and N*═N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less. 
   
   
       4 . The cold rolled steel sheet according to  claim 1 , wherein the composition further comprises 0.01-0.3% of Mn, and 0.004 to 0.02% of N, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10 and N*═N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less. 
   
   
       5 . A cold rolled steel sheet with superior formability and high yield ratio, the cold rolled steel sheet having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, by weight, and the balance of Fe and other unavoidable impurities,
 wherein the composition satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10, where the N content is 0.004% or more, S═S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N═N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and   wherein the steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.   
   
   
       6 . The cold rolled steel sheet according to  claim 1  or  5 , wherein the C, Ti, Nb, N and S contents satisfy the following relationships: 0.8≦(Ti*/48+Nb/93)/(C/12)≦5.0 and Ti*═Ti−0.8×((48/14)×N+(48/32)×3). 
   
   
       7 . The cold rolled steel sheet according to  claim 6 , wherein the C content is 0.005% or less. 
   
   
       8 . The cold rolled steel sheet according to  claim 1  or  5 , wherein solute carbon (Cs) [Cs═(C—Nb—12/93−Ti*×12/48)×10000 in which Ti*═Ti−0.8×((48/14)×N+(48/32)×S), provided that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and Ti contents, is from 5 to 30. 
   
   
       9 . The cold rolled steel sheet according to  claim 8 , wherein the C content is 0.001-0.01%. 
   
   
       10 . The cold rolled steel sheet according to any one of  claims 1  to  5 , wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher. 
   
   
       11 . The cold rolled steel sheet according to any one of  claims 1  to  5 , wherein the number of the precipitates is 1×10 6 /mm 2  or more. 
   
   
       12 . The cold rolled steel sheet according to  claim 1  or  5 , wherein the P content is 0.015% or less. 
   
   
       13 . The cold rolled steel sheet according to  claim 1  or  5 , wherein the P content is from 0.03% to 0.2%. 
   
   
       14 . The cold rolled steel sheet according to  claim 1  or  5 , wherein the composition further comprises one or two kinds selected from 0.1-0.8% of Si and 0.2-1.2% of Cr. 
   
   
       15 . The cold rolled steel sheet according to  claim 1  or  5 , wherein the composition further comprises 0.01-0.2% of Mo. 
   
   
       16 . The cold rolled steel sheet according to  claim 14 , wherein the composition further comprises 0.01-0.2% of Mo. 
   
   
       17 . The cold rolled steel sheet according to any one of  claims 2 ,  4  and  5 , wherein the sum of Mn and Cu is from 0.05% to 0.4%. 
   
   
       18 . The cold rolled steel sheet according to any one of  claims 2 ,  4  and  5 , wherein the Mn content is 0.01-0.12%. 
   
   
       19 . The cold rolled steel sheet according to any one of  claims 2 ,  4  and  5 , wherein the value of (Mn/55+Cu/63.5)/(S*/32) is in the range of 1 to 9. 
   
   
       20 . The cold rolled steel sheet according to any one of  claims 3  to  5 , wherein the value of (Al/27)/(N*/14) is in the range of 1 to 6. 
   
   
       21 . A method for producing a cold rolled steel sheet with superior formability and high yield ratio, the method comprising the steps of:
 reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities and the composition satisfying the following relationships: 1≦(Cu/63.5)/(S*/32)≦30 and S*═S−0.8×(Ti−0.8×(48/14)×N)×(32/48);   hot rolling the reheated slab at a finish rolling temperature of the Ar 3  transformation point or higher to provide a hot rolled steel sheet;   cooling the hot rolled steel sheet at a rate of 300° C./min or higher;   winding the cooled steel sheet at 700° C. or lower;   cold rolling the wound steel sheet; and   continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprising CuS precipitates having an average size of 0.2 μm or less.   
   
   
       22 . The method according to  claim 21 , wherein the composition further comprises 0.01-0.3% of Mn and satisfies the following relationship: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less. 
   
   
       23 . The method according to  claim 21 , wherein the N content is 0.004-0.02% and the composition satisfies the following relationships: 1≦(Al/27)/(N*/14)≦10 and N*═N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less. 
   
   
       24 . The method according to  claim 21 , wherein the composition further comprises 0.01-0.3% of Mn, the N content is 0.004-0.02%, and the composition satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10 and N*═N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less. 
   
   
       25 . A method for producing a cold rolled steel sheet with superior formability and high yield ratio, the method comprising the steps of:
 reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, by weight, and the balance of Fe and other unavoidable impurities and the composition satisfying the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10, where the N content is 0.004% or more, S*═S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N═N−0.8×(Ti−0.8×(48/32)×S))×(14/48);   hot rolling the reheated slab at a finish rolling temperature of the Ar 3  transformation point or higher to provide a hot rolled steel sheet;   cooling the hot rolled steel sheet at a rate of 300° C./min or higher;   winding the cooled steel sheet at 700° C. or lower;   cold rolling the wound steel sheet; and   continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprising at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.   
   
   
       26 . The method according to  claim 21  or  25 , wherein the C, Ti, Nb, N and S contents satisfy the following relationships: 0.8≦(Ti*/48+Nb/93)/(C/12)≦5.0 and Ti*═Ti−0.8×((48/14)×N+(48/32)×S) 
   
   
       27 . The method according to  claim 26 , wherein the C content is 0.005% or less. 
   
   
       28 . The method according to  claim 21  or  25 , wherein solute carbon (Cs) [Cs═(C—Nb×12/93−Ti*×12/48)×10000 in which Ti*═Ti−0.8×((48/14)×N+(48/32)×S), provided that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and Ti contents, is from 5 to 30. 
   
   
       29 . The method according to  claim 28 , wherein the C content is 0.001-0.01%. 
   
   
       30 . The method according to any one of  claims 21  to  25 , wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher. 
   
   
       31 . The method according to any one of  claims 21  to  25 , wherein the number of the precipitates is 1×10 6 /mm 2  or more. 
   
   
       32 . The method according to  claim 21  or  25 , wherein the P content is 0.015% or less. 
   
   
       33 . The method according to  claim 21  or  25 , wherein the P content is from 0.03% to 0.2%. 
   
   
       34 . The method according to  claim 21  or  25 , wherein the composition further comprises one or two kinds selected from 0.1-0.8% of Si and 0.2-1.2% of Cr. 
   
   
       35 . The method according to  claim 21  or  25 , wherein the composition further comprises 0.01-0.2% of Mo. 
   
   
       36 . The method according to  claim 34 , wherein the composition further comprises 0.01-0.2% of Mo. 
   
   
       37 . The method according to any one of  claims 22 ,  24  and  25 , wherein the sum of Mn and Cu is from 0.08% to 0.4%. 
   
   
       38 . The method according to any one of  claims 22 ,  24  and  25 , wherein the Mn content is 0.01-0.12%. 
   
   
       39 . The method according to any one of  claims 22 ,  24  and  25 , wherein the value of (Mn/55+Cu/63.5)/(S*/32) is in the range of 1 to 9. 
   
   
       40 . The method according to any one of  claims 23  to  25 , wherein the value of (Al/27)/(N*/14) is in the range of 1 to 6.