Dual phase steel sheet with good bake-hardening properties

Abstract

A dual phase steel sheet with good bake-hardening properties is provided. The steel sheet is characterized in containing (in terms of percent by mass) C: no less than 0.06% and less than 0.25%; Si+Al: 0.5 to 3%; Mn: 0.5 to 3%; P: no more than 0.15%; and S: no more than 0.02%; and also meeting the following condition (in terms of space factor) that retained austenite is at least 3%, bainite is at least 30%, and ferrite is no more than 50%, and further characterized in differing in stress larger than 50 MPa before and after application of 2% pre-strain and ensuing heat treatment for paint baking at 170° C. for 20 minutes. The steel sheet has well-balanced strength and workability, exhibits good bake-hardening properties at the time of paint baking, and offers good resistance to natural aging.

What is claimed is:  
     
         1 . A dual phase steel sheet with good bake-hardening properties containing (in terms of percent by mass): 
 C: no less than 0.06% and less than 0.25%;    Si+Al: 0.5 to 3%;    Mn: 0.5 to 3%;    P: no more than 0.15% (excluding 0%); and    S: no more than 0.02% (excluding 0%),    wherein said steel sheet comprising (in terms of space factor):    retained austenite: at least 3%;    bainite: at least 30%; and    ferrite: no more than 50% (including 0%),    and wherein said steel sheet has difference in stress larger than 50 MPa before and after ensuing heat treatment for paint baking at 170° C. for 20 minutes, after application of 2% pre-strain.    
     
     
         2 . The dual phase steel sheet as defined in  claim 1 , wherein said difference in stress is no less than 100 MPa.  
     
     
         3 . The dual phase steel sheet as defined in  claim 1 , wherein the space factor of bainite is no less than 60%.  
     
     
         4 . The dual phase steel sheet as defined in  claim 1 , further containing at least one of the following constituents (in terms of percent by mass): 
 Mo: no less than 0.05% and no more than 1%;    Ni: no less than 0.05% and no more than 0.5%;    Cu: no less than 0.05% and no more than 0.5%; and    Cr: no less than 0.05% and no more than 1%.    
     
     
         5 . The dual phase steel sheet as defined in  claim 1 , further containing at least one of the following constituents (in terms of percent by mass): 
 Ti: no less than 0.01% and no more than 0.1%;    Nb: no less than 0.01% and no more than 0.1%; and    V: no less than 0.01% and no more than 0.1%.    
     
     
         6 . The dual phase steel sheet as defined in  claim 1 , further containing (in terms of percent by mass) at least one of 
 Ca: no less than 3 ppm and no more than 30 ppm and    REM: no less than 3 ppm and no more than 30 ppm.