High-strength cold rolled steel sheet having excellent formability, and plated steel sheet

Abstract

To provide a high-strength cold rolled steel sheet that has well-balanced tensile strength and elongation as well as well-balanced tensile strength and stretch-flangeability, and a plated steel sheet manufactured by plating the steel sheet. The high-strength cold rolled steel sheet contains: 0.10 to 0.28% of C, 1.0 to 2.0% of Si, 1.0 to 3.0% of Mn, and 0.03 to 0.10% of Nb in terms of % by mass, wherein the content of Al is controlled to 0.5 or less, the content of P is controlled to 0.15% or less, and the content of S is controlled to 0.02% or less, and wherein residual austenite accounts for 5 to 20%, bainitic ferrite accounts for 50% or more, and polygonal ferrite accounts for 30% or less (containing 0%), of the entire structure, and wherein a mean number of residual austenite blocks is 20 or more as determined when the random area (15 μm×15 μμm) is observed by EBSP (electron back scatter diffraction pattern).

1 . A high-strength cold rolled steel sheet, which contains: 
 0.10 to 0.28% of C,    1.0 to 2.0% of Si,    1.0 to 3.0% of Mn, and    0.10% or less (containing 0%) of Nb in terms of % by mass,    wherein the content of Al is controlled to 0.5 or less,    the content of P is controlled to 0.15% or less, and    the content of S is controlled to 0.02% or less,    and wherein residual austenite accounts for 5 to 20%,    bainitic ferrite accounts for 50% or more, and    polygonal ferrite accounts for 30% or less (containing 0%), of the entire structure,    and wherein a mean number of residual austenite blocks is 20 or more as determined when the random area (15 μm×15 μm) is observed by EBSP (electron back scatter diffraction pattern).    
   
   
       2 . The high-strength cold rolled steel sheet according to  claim 1 , which contains 0.03 to 0.10% of Nb.  
   
   
       3 . The high-strength cold rolled steel sheet according to  claim 2 , which contains 0.05 to 0.10% of Nb.  
   
   
       4 . The high-strength cold rolled steel sheet according to  claim 1 , further containing: at least one of the group consisting of 
 1.0% or less (more than 0%) of Mo,    0.5% or less (more than 0%) of Ni, and    0.5% or less (more than 0%) of Cu.    
   
   
       5 . The high-strength cold rolled steel sheet according to  claim 1 , further containing: at least one of the group consisting of 
 0.003% or less (more than 0%) of Ca, and    0.003% or less (more than 0%) of REM.    
   
   
       6 . The high-strength cold rolled steel sheet according to  claim 1 , further containing: at least one of the group consisting of 
 0.1% or less (more than 0%) of Ti or    0.1% or less (more than 0%) of V.    
   
   
       7 . The high-strength cold rolled steel sheet according to  claim 1 , wherein the content of C in the residual austenite is 0.8% or more.  
   
   
       8 . The high-strength cold rolled steel sheet according to  claim 1 , wherein the bainitic ferrite accounts for 70% or more of the entire structure.  
   
   
       9 . A plated steel sheet manufactured by plating the cold rolled steel sheet according to  claim 1.