High Strength Thin-Gauge Steel Sheet Excellent in Elongation and Hole Expandability and Method of Production of Same

Abstract

The present invention provides high strength thin-gauge steel sheet with excellent elongation and hole expandability having a tensile strength of 500 MPa or more and a method of production of high strength thin-gauge steel sheet with excellent elongation and hole expandability enabling production of this on an industrial scale, that is, high strength thin-gauge steel sheet comprised of, by mass %, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%, P≦0.02%, S≦0.02%, Al≦2.0%, N≦0.01%, and a balance of Fe and unavoidable impurities and having a microstructure comprised of ferrite with an area fraction of 10 to 85% and residual austenite with a volume fraction of 1 to 10%, an area fraction of 10% to 60% of tempered martensite, and a balance of bainite.

1 . High strength thin-gauge steel sheet with excellent elongation and hole expandability characterized by being comprised of, by mass %, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%, P≦0.02%, S≦0.02%, Al≦2.0%, N≦0.01%, and a balance of Fe and unavoidable impurities and having a microstructure comprised of ferrite with an area fraction of 10 to 85% and residual austenite with a volume fraction of 1 to 10%, an area fraction of 10% to 60% of tempered martensite, and a balance of bainite.  
     
     
         2 . High strength thin-gauge steel sheet with excellent elongation and hole expandability according to  claim 1  characterized by further including as chemical ingredients one or more of V: 0.005 to 1%, Ti: 0.002 to 1%, Nb: 0.002 to 1%, Cr: 0.005 to 2%, Mo: 0.005 to 1%, B: 0.0002 to 0.1%, Mg: 0.0005 to 0.01%, REM: 0.0005 to 0.01%, and Ca: 0.0005 to 0.01%.  
     
     
         3 . High strength thin-gauge steel sheet with excellent elongation and hole expandability according to  claim 1  characterized by further satisfying the following formula (A):  
         (0.0012 ×[TS  target value]−0.29)/3<[Al]+0.7[Si]<1.0   (A)  TS target value is design value of strength of steel sheet in units of MPa, [Al] is mass % of Al, and [Si] is mass % of Si,    
     
     
         4 . A method of production of high strength thin-gauge steel sheet with excellent elongation and hole expandability characterized by producing a slab comprised of, by mass %, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%, P≦0.02%, S≦0.02%, Al≦2.0%, and N≦0.01% and, further, when necessary, one or more types of V: 0.005 to 1%, Ti: 0.002 to 1%, Nb: 0.002 to 1%, Cr: 0.005 to 2%, Mo: 0.005 to 1%, B: 0.0002 to 0.1%, Mg: 0.0005 to 0.01%, REM: 0.0005 to 0.01%, and Ca: 0.0005 to 0.01%, and a balance of Fe and unavoidable impurities, heating it in a range of 1150 to 1250° C., then hot rolling it in a temperature range of 800 to 950° C., coiling it at 700° C. or less, then pickling it as normal, then cold rolling by a reduction rate of 30 to 80%, then, in a continuous annealing process, soaking it at 600° C. to the Ac 3  point+50° C. for recrystallization annealing, cooling to 600° C. to the Ar3 point by an average cooling rate of 30° C./s or less, then cooling to 400° C. or less by an average cooling rate of 10 to 150° C./s, then holding at higher than a cooling end temperature of said cooling and 150 to 400° C. for 1 to 20 minutes, then cooling to thereby obtain a metal structure having a microstructure comprised of ferrite with an area fraction of 10 to 85% and residual austenite with a volume fraction of 1 to 10%, an area fraction of 10% to 60% of tempered martensite, and a balance of bainite.  
     
     
         5 . A method of production of high strength thin-gauge steel sheet with excellent elongation and hole expandability according to  claim 4  characterized by, in the continuous annealing process, soaking at 600° C. to the Ac 3  point+50° C. for recrystallization annealing, cooling by an average cooling rate of 10 to 150° C./s to 400° C. or less, then heating and holding a first time at 150 to 400° C. for 1 to 20 minutes, then heating and holding a second time at a temperature 30 to 300° C. higher than the first heating and holding temperature to 500° C. for 1 to 100 seconds, then cooling.  
     
     
         6 . A method of production of high strength thin-gauge steel sheet with excellent elongation and hole expandability according to  claim 4  characterized by, in the continuous annealing process, soaking at 600° C. to the Ac 3  point+50° C. for recrystallization annealing, cooling by an average cooling rate of 10 to 1 50° C./s to 400° C. or less, then heating and holding a first time at 150 to 400° C. for 1 to 20 minutes, cooling to the martensitic transformation point or less, heating and holding a second time at the cooling end temperature to 500° C. for 1 to 100 seconds, then cooling.