Cold rolled and galvanized or galvannealed dual phase high strength steel and method of its production

Abstract

A method of producing cold rolled and annealed dual phase high strength steel sheets, and sheets produced by the method, including hot dip galvanized and galvannealed steel sheets having a tensile strength of at least about 750 MPa and a superior balance of strength and ductility using conventional thermomechanical processing parameters robust to the processing conditions. The synergistic effect on hardenability of Cr and V, within a controlled range, facilitates production using robust processing conditions, enabling production of a high strength product having a very low yield ratio.

We claim:  
     
         1 . A cold rolled and annealed dual phase galvanized or galvannealed steel sheet having a tensile strength of at least about 750 MPa and a superior balance of strength and ductility, the steel sheet essentially comprising, by weight: 
 C=0.07 to 0.2%    Si=0.1 to 0.5%    Mn=1.75 to 3.5%    Cr=0.15 to 1.0%    V=0.02 to 0.1 0%    P=no more than about 0.05%    S=no more than about 0.01%    Nb=no more than about 0.1%    Ti=no more than about 0.1%    Al=no more than about 0.01%    N=no more than about 0.01%    Balance Fe and unavoidable impurities    
     
     
         2 . The steel sheet defined in  claim 1 , wherein the YR is within the range of about 50 to 54%.  
     
     
         3 . The steel sheet defined in  claim 1 , wherein the Cr content is within the range of about 0.20 to about 0.55%.  
     
     
         4 . The steel sheet defined in  claim 1 , wherein the V content is within the range of about 0.03 to about 0.08%.  
     
     
         5 . The steel sheet defined in  claim 1 , wherein the Cr content is within the range of about 0.20 to about 0.55% and the V content is within the range of about 0.03 to about 0.08%.  
     
     
         6 . The steel sheet defined in  claim 5 , wherein the YR is within the range of about 50 to 54%.  
     
     
         7 . The steel sheet as defined in  claim 1  having the following composition, by weight: 
 C=0.08 to 0.16%  
 Si=0.1 to 0.35%  
 Mn=2.0 to 2.75%  
 Cr=0.20 to 0.55%  
 V=0.03 to 0.08%  
 P=0.005 to 0.010%  
 S=0.002 to 0.008%  
 Nb=0.02 to 0.05%  
 Ti=0.02 to 0.05%  
 Al=0.02 to 0.07%  
 N=0.0010 to 0.0050%  
 Balance Fe and unavoidable impurities  
 
     
     
         8 . A method for producing a cold rolled and annealed high strength dual phase steel sheet having a tensile strength of at least about 750 MPa and a yield ratio within the range of about 50% to about 54%, comprising the steps of: 
 rough rolling a steel containing, by weight, about 0.07% to about 0.2% of C, about 0.1% to about 0.5% of Si, about 1.75% to about 3.5% of Mn, about 0.15% to about 1.0% of Cr, about 0.02% to about 0.10% V, no more than about 0.05% of P, no more than about 0.01% of S, no more than about 0.1% of Nb, no more than about 0.1% of Ti, no more than about 0.10% of Al, no more than about 0.01% of N, and the balance Fe and incidental impurities,    finish rolling the rough rolled steel band at a temperature not lower than the Ar3 point,    coiling the finished rolled steel band at a temperature lower than the Ar1 point,    pickling the hot rolled steel band, and    cold rolling the steel to form a strip having the desired thickness, producing a total reduction of not less than 40%.    
     
     
         9 . The method defined in  claim 8  further comprising the step of hot dip galvanizing or galvannealing the cold rolled strip.  
     
     
         10 . The method defined in  claim 9 , wherein the step of hot dip galvanizing or galvannealing the cold rolled strip comprises 
 soaking the strip at a temperature between Ac1 and Ac3,    cooling the strip to a coating bath temperature of about 470° C. at a conventional cooling rate depending on line speed,    hot dip coating the strip, and    cooling the coated strip to ambient temperature at a conventional cooling rate.    
     
     
         11 . The method defined in  claim 10 , wherein the coated strip is galvannealed at a conventional galvannealing temperature of about 520° C.  
     
     
         12 . The method defined in  claim 8 , wherein the Cr content of the steel is within the range of about 0.02 to about 0.55%.  
     
     
         13 . The method defined in  claim 8 , wherein the V content is within the range of about 0.03 to about 0.08%.  
     
     
         14 . The method defined in  claim 8 , wherein the Cr content is within the range of about 0.20 to about 0.55% and the V content is within the range of about 0.03 to about 0.08%.  
     
     
         15 . The method defined in  claim 8  further comprises the step of temper rolling or tension leveling the strip to increase the yield strength to a desired level up to that approaching the tensile strength.  
     
     
         16 . The method defined in  claim 8 , wherein the cold rolling reduction is within the range of about 60 to 70%.  
     
     
         17 . The method defined in  claim 8 , wherein the steel sheet has the following composition, by weight: 
 C=0.08 to 0.16%    Si=0.1 to 0.35%    Mn=2.0 to 2.75%    Cr=0.20 to 0.55%    V=0.03 to 0.08%    P=0.005 to 0.010%    S=0.002 to 0.008%    Nb=0.02 to 0.05%    Ti=0.02 to 0.05%    Al=0.02 to 0.07%    N=0.0010 to 0.0050%    Balance Fe and unavoidable impurities