High strength steel sheet having excellent formability

Abstract

A high strength steel sheet contains, in percent by mass, 0.03 to 0.2% of C, 0.5 to 2.5% of Si, 1 to 3.0% of Mn, 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.02 to 0.15% of Al, 0.15% or less of Ti, 0.15% or less of No, and 0.15% or less of V; wherein the remainder includes Fe and inevitable impurities, and the content of Si satisfies the following formula (1), α−4.1≦[Si]≦α−2.4,   (1) provided, α=6.9×([C]+[Mn]/6+[Cr]/5+[Mo]/4+[Ti]/15+[Nb]/17+[V]/14) 1/2 is given, wherein [ ] shows the quantity (mass percent) of each element contained in the steel sheet. The high strength steel sheet is improved in formability (particularly, elongation), and excellent in balance between strength and elongation.

1 . A high strength steel sheet comprising, in percent by mass:
 C: 0.03 to 0.2%;   Si: 0.5 to 2.5%;   Mn: 1 to 3.0%;   Cr: 0.01 to 0.5%;   Mo: 0.01 to 0.5%;   Al: 0.02 to 0.15%;   Ti: 0.15% or less;   Nb: 0.15% or less; and   V: 0.15% or less,   wherein a metal structure of the steel sheet includes ferrite and a second phase, a fraction of ferrite is 10% to 90%, and a fraction of the second phase is 10% to 90%, and   the content of Si satisfies the following formula (1),
   α−4.1≦[Si]≦α−2.4  (1), 
   provided,
   α=6.9×([C]+[Mn]/6+[Cr]/5+[Mo]/4+[Ti]/15+[Nb]/17+[V]/14) 1/2    
   
       is given,
 wherein [ ] shows the quantity (mass percent) of each element contained in the steel sheet. 
 
     
     
         2 . The high strength steel sheet according to  claim 1 ,
 wherein the steel sheet contains at least one selected from a group including Ti: 0.01 to 0.15%, Nb: 0.01 to 0.15%, and V: 0.01 to 0.15%.   
     
     
         3 . The high strength steel sheet according to  claim 1 ,
 wherein the steel sheet contains B: 0.0005 to 0.01%.   
     
     
         4 . The high strength steel sheet according to  claim 1 ,
 wherein the steel sheet contains Ca: 0.0005 to 0.01%.   
     
     
         5 . The high strength steel sheet according to  claim 1 ,
 wherein the steel sheet contains at least one of Cu: 0.0005 to 0.5% and Ni: 0.0005 to 0.5%.   
     
     
         6 . The high strength steel sheet according to  claim 1 ,
 wherein a surface of the high strength steel sheet has a galvanizing layer formed thereon.   
     
     
         7 . The high strength steel sheet according to  claim 1 ,
 wherein a surface of the high strength steel sheet has an alloyed galvanizing layer formed thereon.   
     
     
         8 . The high strength steel sheet according to  claim 1 ,
 wherein a metal structure of the high strength steel sheet includes a mixed structure of ferrite and a second phase, and an area ratio (S c ) of an intermediate transformation structure in the second phase is 0.3 or less.