Galvannealed Steel Sheet and Method for Producing the Same

Abstract

A galvannealed steel sheet contains, by % by mass, about 0.05 to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfies the relations, Si +Al >0.6% and Cr +V +Mo =0.1 to 2%, the balance being Fe and inevitable impurities.

1 . A galvannealed steel sheet including, by % by mass, about 0.05 to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6% and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities. 
     
     
         2 . A galvannealed steel sheet including, by % by mass, about 0.05 to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6%, N≦0.007%−(0.003×Al)%, and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities. 
     
     
         3 . The galvannealed steel sheet according to  claim 1 , further comprising, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni. 
     
     
         4 . The galvannealed steel sheet according to  claim 2 , further comprising, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni. 
     
     
         5 . The galvannealed steel sheet according to  claim 1 , further comprising, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         6 . The galvannealed steel sheet according to  claim 2 , further comprising, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         7 . The galvannealed steel sheet according to  claim 3 , further comprising, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         8 . The galvannealed steel sheet according to  claim 4 , further comprising, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         9 . The galvannealed steel sheet according to  claim 1 , including a residual austenite phase at a volume ratio of about 3 to about 20%. 
     
     
         10 . A method for producing a galvannealed steel sheet, comprising:
 annealing, in a temperature range of about 730° C. to about 900° C., a cold-rolled steel sheet containing, by % by mass, about 0.05 to about 0.25% of C, about 2% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6% and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities;   cooling the annealed cold-rolled steel sheet at a cooling rate of about 3 to about 100° C./second;   retaining the cooled cold-rolled steel sheet in a temperature range of about 350° C. to about 600° C. for about 30 to about 250 seconds;   hot-dip galvanizing the cold-rolled steel sheet after the retention; and   alloying the hot-dip galvanized cold-rolled steel sheet at a temperature of about 470° C. to about 600° C.   
     
     
         11 . A method for producing a galvannealed steel sheet, comprising:
 annealing, in a temperature range of about 730° C. to about 900° C., a cold-rolled steel sheet containing, by % by mass, about 0.05 to about 0.25% of C, about 2% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6%, N≦0.007%−(0.003×Al)%, and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities;   cooling the annealed cold-rolled steel sheet at a cooling rate of about 3 to about 100° C./second;   retaining the cooled cold-rolled steel sheet in a temperature range of about 350° C. to about 600° C. for about 30 to about 250 seconds;   hot-dip galvanizing the cold-rolled steel sheet after the retention; and   alloying the hot-dip galvanized cold-rolled steel sheet at a temperature of about 470° C. to about 600° C.   
     
     
         12 . The method according to  claim 10 , wherein the cold-rolled steel sheet further contains, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni. 
     
     
         13 . The method according to  claim 11 , wherein the cold-rolled steel sheet further contains, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni. 
     
     
         14 . The method according to  claim 10 , wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         15 . The method according to  claim 11 , wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         16 . The method according to  claim 12 , wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         17 . The method according to  claim 13 , wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less. 
     
     
         18 . The method according to  claim 10 , wherein the galvannealed steel sheet contains a residual austenite phase at a volume ratio of about 3 to about 20%.