Post Annealed High Tensile Strength Coated Steel Sheet having Improved Yield Strength and Hole Expansion

Abstract

A cold rolled, coated and post annealed steel sheet. The cold rolled steel sheet may comprise (in wt. %): C—0.1-0.3 %; Mn—1-3%; Si—0.5-3.5%; Al—0.05-1.5%; Mo+Cr is between 0-1.0%; and Mo+Cr is between 0.2-0.5%. The steel sheet may be coated with a zinc or zinc alloy coating. The coated steel sheet may be formed by cold rolling, zinc coating the cold rolled sheet and annealing said steel sheet after application of said zinc coating. The annealing is performed at a temperature and for a time sufficient to significantly increase the yield strength and hole expansion of the annealed coated cold rolled steel sheet as compared with the as coated sheet.

1 . A cold rolled, coated and post annealed steel sheet comprising:
 a cold rolled steel sheet comprising (in wt. %):
 C—0.1-0.3%; Mn—1-3%; Si—0.5-3.5%; Al—0.05-1.5%; Mo+Cr is between 0-1.0%; and Mo+Cr is between 0.2-0.5%; and 
   a zinc or zinc alloy coating on said cold rolled steel sheet;   said coated steel sheet having been formed by cold rolling, zinc coating the cold rolled sheet and annealing said steel sheet after application of said zinc coating, said annealing being performed at a temperature between 150-650° C. for a period of time sufficient to increase the yield strength of the annealed cold rolled coated steel sheet by at least 30% compared to the as coated cold rolled steel sheet and to increase the hole expansion of the annealed cold rolled coated steel sheet by at least 80% compared to the as coated cold rolled steel sheet.   
     
     
         2 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said annealing is performed at a temperature between 150-450° C. 
     
     
         3 . The cold rolled, coated and post annealed steel sheet of  claim 2 , wherein said annealing is performed at a temperature between 200-400° C. 
     
     
         4 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said annealing being performed for a period of time sufficient to increase the yield strength of the annealed cold rolled coated steel sheet by at least 40% compared to the as coated cold rolled steel sheet. 
     
     
         5 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said annealing being performed for a period of time sufficient to increase the hole expansion of the annealed cold rolled coated steel sheet by at least 95% compared to the as coated cold rolled steel sheet. 
     
     
         6 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said annealing being performed for a period of time sufficient to increase the total elongation of the annealed cold rolled coated steel sheet by at least 25% compared with the as coated sheet. 
     
     
         7 . The cold rolled, coated and post annealed steel sheet of  claim 6 , wherein said annealing being performed for a period of time sufficient to increase the total elongation of the annealed cold rolled coated steel sheet by at least 40% compared with the as coated sheet. 
     
     
         8 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said cold rolled steel sheet comprises C—0.15-0.25%. 
     
     
         9 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said cold rolled steel sheet comprises Mn—2-2.5%. 
     
     
         10 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said cold rolled steel sheet comprises Si—1.5-2.5%. 
     
     
         11 . The cold rolled, coated and post annealed steel sheet of  claim 1 , wherein said cold rolled steel sheet comprises Al—0.05-1.0%.