High Strength Cold Rolled Steel Sheet Having Excellent Shape Freezability, and Method for Manufacturing the Same

Abstract

A high-strength isotropic cold-rolled steel sheet having excellent shape-fixability, and a method for manufacturing the same are disclosed. The steel sheet is made using a low-carbon steel comprising a little amounts of Ti to have an r of 1.3 or less, r approaching 1, and a low in-plane anisotropy index (Dr) of 0.15 or less, ensuring excellent shape-fixability so as to be suitable for automotive outer panels which are essentially subjected to deformation in a stretching mode.

1 . A high-strength cold-rolled steel sheet having excellent shape-fixability, comprising in percent by weight: 0.01˜0.05% of C; 0.005˜0.06% of Ti; 0.1˜1% of Mn; 0.1% or less of Si; 0.03% or less of P; 0.03% or less of S; 0.08% or less of Sol. Al; 0.01% or less of N; and the balance of Fe and other unavoidable impurities in terms of weight %, wherein a composition of Ti and N satisfies the relationship: Ti/N>5, and a composition of Ti and C satisfies the relationship: (48/12)C−Ti*>0.03% [where Ti*=Ti−(48/14)N], the steel sheet having an Aging Index (AI) of 30 MPa or less.  
   
   
       2 . The cold-rolled steel sheet according to  claim 1 , wherein the steel sheet comprises 0.015˜0.035% of C.  
   
   
       3 . The cold-rolled steel sheet according to  claim 1 , wherein the steel sheet comprises 0.01˜0.04% of Ti.  
   
   
       4 . The cold-rolled steel sheet according to  claim 1 , wherein the relationship (48/12)C−Ti* is in the range of 0.06˜0.11%.  
   
   
       5 . A method for manufacturing a high-strength cold-rolled steel sheet having excellent shape-fixability, the method comprising the steps of: 
 finish rolling steel at an Ar 3  temperature or more to provide a hot rolled steel sheet, the steel comprising in percent by weight: 0.01˜0.05% of C; 0.005˜0.06% of Ti; 0.1˜1% of Mn; 0.1% or less of Si; 0.03% or less of P; 0.03% or less of S; 0.08% or less of Sol. Al; 0.01% or less of N; and the balance of Fe and other unavoidable impurities in terms of weight %, wherein a composition of Ti and N satisfies the relationship: Ti/N<5, and a composition of Ti and C satisfies the relationship: (48/12)C−Ti*<0.03% [where Ti*=Ti−(48/14)N], the steel having an aging index of 30 MPa or less;    rapid quenching the hot rolled steel sheet at a rate of 50° C./sec or more, followed by coiling the steel sheet at a temperature of 650° C. or less;    acid pickling the coiled steel sheet, followed by cold rolling the steel sheet at a reduction rate of 50˜80%;    annealing the cold-rolled steel sheet after heating to a temperature of recrystallization temperature˜AC 3 ;    primary cooling of the annealed steel sheet to a temperature of 600˜700° C. at a rate of 3° C./sec or more, followed by secondary cooling of the cooled steel sheet to a temperature of 100˜500° C. at a rate of 30° C./sec or more;    overaging the cooled steel sheet at a temperature of 200˜500° C. within 10 minutes; and    skin pass rolling the steel sheet at a reduction rate of 0.5% or more.    
   
   
       6 . The method according to  claim 5 , wherein the steel comprises 0.015˜0.035% of C.  
   
   
       7 . The method according to  claim 5 , wherein the steel comprises 0.01˜0.04% of Ti.  
   
   
       8 . The method according to  claim 5 , wherein the relationship (48/12)C−Ti* is the range of 0.06˜0.11%.  
   
   
       9 . The method according to  claim 5 , wherein rapid quenching of the hot rolled steel sheet is performed within 1 second of completion of finish rolling.  
   
   
       10 . The method according to  claim 5 , wherein annealing of the steel sheet is performed at a temperature of 760˜820° C. for 5 minutes or less.  
   
   
       11 . The method according to  claim 5 , wherein the steel sheet is heated at a rate of 3° C./sec or more for annealing.  
   
   
       12 . The cold-rolled steel sheet according to  claim 2 , wherein the relationship (48/12)C−Ti* is in the range of 0.06˜0.11%.  
   
   
       13 . The cold-rolled steel sheet according to  claim 3 , wherein the relationship (48/12)C−Ti* is in the range of 0.06˜0.11%.  
   
   
       14 . The method according to  claim 6 , wherein the relationship (48/12)C−Ti* is the range of 0.06˜0.11%.  
   
   
       15 . The method according to  claim 7 , wherein the relationship (48/12)C−Ti* is the range of 0.06˜0.11%.  
   
   
       16 . The method according to  claim 10 , wherein the steel sheet is heated at a rate of 3° C./sec or more for annealing.