High strength - high ductility cold rolled recovery annealed steel and process for manufacture thereof

Abstract

A high strength-high ductility cold rolled steel sheet is provided. The steel sheet has a recovery annealed microstructure, a yield strength greater than 820 megapascals (MPa) and a percent elongation to failure greater than 3.5%. In some instances, the steel alloy sheet has a Rockwell B hardness greater than 100 and may or may not exhibit a yield strength-to-tensile strength ratio between 0.25 and 1.00.

We claim: 
     
         1 . A process for making a high strength steel comprising:
 providing a steel alloy having a chemical composition in weight percent within a range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.02-0.07 Nb, 0.03 max Ti, 0.003 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, and 0.015-0.065 Al;   hot rolling the steel alloy to produce a hot rolled steel strip having a thickness of less than 10 mm;   cold rolling the hot rolled steel strip to produce a cold rolled steel sheet having a thickness less than 50% of the hot rolled steel strip thickness; and   recovery annealing the cold rolled steel sheet, the recovery annealed cold rolled steel sheet having a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%.   
     
     
         2 . The process of  claim 1 , wherein the steel alloy is hot rolled using a roughing treatment at temperatures between 1000 and 1200° C. and a finishing treatment having an entry temperature between 900 and 1100° C. and an exit temperature between 780 and 930° C. 
     
     
         3 . The process of  claim 2 , wherein the hot rolled strip has a thickness between 1.5 and 6.0 mm. 
     
     
         4 . The process of  claim 3 , further including coiling of the hot rolled strip at temperatures between 500 and 730° C. 
     
     
         5 . The process of  claim 4 , wherein the cold rolled steel sheet has a thickness between 0.3 and 2.3 mm. 
     
     
         6 . The process of  claim 5 , wherein the cold rolled sheet is recovery annealed at temperatures between 580 and 660° C. 
     
     
         7 . The process of  claim 6 , further including temper rolling the recovery annealed cold rolled sheet with between 0.0 and 3.50 percent deformation. 
     
     
         8 . The process of  claim 7 , wherein the cold rolled sheet is tempered rolled between 0.02 and 2.0 percent deformation and the recovery annealed cold rolled sheet has a percent elongation to failure greater than 8.0%. 
     
     
         9 . The process of  claim 8 , wherein the cold rolled sheet is tempered rolled between 0.02 and 1.0 percent deformation. 
     
     
         10 . The process of  claim 9 , wherein the microstructure of the cold rolled steel sheet has less than 5 volume percent recrystallized grains. 
     
     
         11 . A process for making a high strength steel sheet comprising:
 providing a steel slab with a thickness between 50 and 280 mm, the steel slab having a chemical composition in weight percent within a range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.10 max Nb, 0.03 max Ti, 0.10 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, and 0.015-0.065 Al;   soaking the steel slab at temperatures between 1150 and 1320° C.;   hot rolling the steel slab at temperatures between 780 and 1200° C. and producing a hot rolled sheet with a thickness between 1.5 and 6.0 mm;   coiling the hot rolled sheet at temperatures between 500 and 730° C.;   cold rolling the hot rolled steel strip to produce a cold rolled steel sheet having a thickness between 0.30 and 2.3 mm; and   recovery annealing the cold rolled steel sheet at temperatures between 580 and 660° C., the recovery annealed cold rolled steel sheet having a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%.   
     
     
         12 . The process of  claim 11 , wherein the hot rolling includes hot rolling the steel slab at temperatures between 1000 and 1200° C. and producing a transfer bar with a thickness between 45 and 70 mm. 
     
     
         13 . The process of  claim 12 , further including hot rolling the transfer bar at temperatures between 780 and 1100° C. and producing said hot rolled sheet with said thickness between 1.5 and 6.0 mm. 
     
     
         14 . The process of  claim 13 , wherein the cold rolled steel sheet has a microstructure with less 10 volume percent recrystallized grains. 
     
     
         15 . A high strength-high ductility cold rolled steel sheet comprising:
 a steel alloy sheet having a chemical composition in weight percent within a range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.10 max Nb, 0.03 max Ti, 0.10 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, and 0.015-0.065 Al;   said steel alloy sheet having a recovery annealed microstructure, a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%.   
     
     
         16 . The high strength-high ductility cold rolled steel of  claim 15 , wherein said steel alloy sheet has Rockwell B hardness greater than 100. 
     
     
         17 . The high strength-high ductility cold rolled steel of  claim 16 , wherein said steel alloy exhibits a yield strength-to-tensile strength ratio between 0.25 and 1.00. 
     
     
         18 . The high strength-high ductility cold rolled steel of  claim 17 , wherein said recovery annealed microstructure has less than 10 volume percent of recrystallized grains. 
     
     
         19 . The high strength-high ductility cold rolled steel of  claim 18 , wherein said recovery annealed microstructure has less than 5 volume percent of recrystallized grains. 
     
     
         20 . The high strength-high ductility cold rolled steel of  claim 19 , wherein said recovery annealed microstructure has less than 2 volume percent of recrystallized grains.