Cold Rolled Steel Sheet Having High Yield Ratio And Less Anisotropy, 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 hardenability.

1 . A cold rolled steel sheet with high yield ratio and low in-plane anisotropy index, the cold rolled steel sheet having a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities,
 wherein the composition satisfies the following relationship: 1≦(Cu/63.5)/(S/32)≦30, 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 to 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 to 0.02%, and the composition satisfies the following relationship: 1≦(Al/27)/(N/14)≦10, 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 comprising 0.01 to 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 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 high yield ratio and low in-plane anisotropy index, the cold rolled sheet having a composition comprising: 0.01% or less C, 0.08% or less S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.0001 to 0.002% of B, 0.002 to 0.04% of Nb, at least one kind selected from 0.01 to 0.2% of Cu, 0.01 to 0.3% of Mn and 0.004 to 0.2% of N, by weight, and the balance 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, and   wherein the steel sheet comprises at least one kind selected from (Mn,Cu)S precipitates 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 and Nb contents satisfy the following relationship, by weight: 0.8≦(Nb/93)/(C/12)≦5.0. 
     
     
         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) is from 5 to 30, where Cs=(C−Nb×12/93)×10,000. 
     
     
         9 . The cold rolled steel sheet according to  claim 8 , wherein the C content is from 0.001 to 0.01%. 
     
     
         10 . The cold rolled steel sheet according to one of preceding  claims 1  to  5 , wherein the cold rolled steel satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher. 
     
     
         11 . The cold rolled steel sheet according to one of preceding  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 at least one kind of 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr. 
     
     
         15 . The cold rolled steel sheet according to  claim 1  or  5 , wherein the composition further comprises 0.01 to 0.2% of Mo. 
     
     
         16 . The cold rolled steel sheet according to  claim 14 , wherein the composition further comprises 0.01 to 0.2% of Mo. 
     
     
         17 . The cold rolled steel sheet according to  claim 2 ,  4  or  5 , wherein the sum of Mn and Cu is from 0.08 to 0.4%. 
     
     
         18 . The cold rolled steel sheet according to  claim 2 ,  4  or  5 , wherein the Mn content is from 0.01 to 0.12%. 
     
     
         19 . The cold rolled steel sheet according to  claim 2 ,  4  or  5 , wherein the value of (Mn/55+Cu/63.5)/(S/32) is from 1 to 9. 
     
     
         20 . The cold rolled steel sheet according to  claim 3 ,  4  or  5 , wherein the value of (Al/27)/(N/14) is from 1 to 5. 
     
     
         21 . A method of producing a cold roller steel sheet with high yield ratio and low in-plane anisotropy index, the method comprising steps of:
 reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, the composition satisfying the following relationship: 1≦(Cu/63.5)/(S/32)≦30;   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 steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.   
     
     
         22 . The method according to  claim 21 , wherein the composition further comprising 0.01 to 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 to 0.02%, and the composition satisfies the following relationship: 1≦(Al/27)/(N/14)≦10, 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 to 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 the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less. 
     
     
         25 . A method of producing a cold roller steel sheet with high yield ratio and low in-plane anisotropy index, the method comprising steps of:
 reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less C, 0.08% of less S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, 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 Fe and other unavoidable impurities, 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,;   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 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.   
     
     
         26 . The method according to  claim 21  or  25 , wherein the C and Nb contents satisfy the following relationship, by weight: 0.8≦(Nb/93)/(C/12)≦5.0. 
     
     
         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) is from 5 to 30, where Cs=(C−Nb×12/93)×10,000. 
     
     
         29 . The method according to  claim 28 , wherein the C content is from 0.001 to 0.01%. 
     
     
         30 . The method according to one of preceding  claims 21  to  25 , wherein the cold rolled steel satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher. 
     
     
         31 . The method according to one of preceding  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 at least one kind of 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr. 
     
     
         35 . The method according to  claim 21  or  25 , wherein the composition further comprises 0.01 to 0.2% of Mo. 
     
     
         36 . The method according to  claim 34 , wherein the composition further comprises 0.01 to 0.2% of Mo. 
     
     
         37 . The method according to  claim 22 ,  24  or  25 ; wherein the sum of Mn and Cu is from 0.08 to 0.4%. 
     
     
         38 . The method according to  claim 22 ,  24  or  25 , wherein the Mn content is from 0.01 to 0.12%. 
     
     
         39 . The method according to  claim 22 ,  24  or  25 , wherein the value of (Mn/55+Cu/63.5)/(S/32) is from 1 to 9. 
     
     
         40 . The method according to  claim 23 ,  24  or  25 , wherein the value of (Al/27)/(N/14) is from 1 to 5.