Low carbon-high manganese steel and manufacturing process thereof

Abstract

A low carbon-high manganese steel sheet and process for manufacturing the sheet is provided. The process includes soaking a steel slab with a desired chemical composition within a temperature range of 1200-1350° C., followed by hot rolling of the slab into hot strip. The cold rolled sheet is continuously annealed within a temperature range of 730-850° C. and temper rolled between 1.0-2.0%. The temper rolled sheet has a yield strength greater than 280 megapascals (MPa), a tensile strength greater than 400 MPa, an elongation to fracture greater than 30%, an n-value greater than 0.15, and a bakehard index between 15-35 MPa.

We claim: 
     
         1 . A process for making a low carbon-high manganese steel sheet, the process comprising:
 providing a steel slab having a chemical composition within a range in weight percent of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40 max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being Fe and incidental impurities;   soaking the steel slab within a temperature range of 1200-1350° C.;   hot rolling the soaked steel slab using a roughing treatment and producing a transfer bar;   hot rolling the transfer bar using a finishing treatment and producing hot rolled strip;   cold rolling the hot rolled strip and producing cold rolled sheet with a reduction in thickness ranging between 50-80%;   continuous annealing the cold rolled sheet within a temperature range of 730-850° C.; and   temper rolling the annealed cold rolled sheet between 1.0-2.0%, the temper rolled sheet having a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa and an elongation to fracture greater than 30%.   
     
     
         2 . The process of  claim 1 , wherein the temper rolled sheet has an n-value greater than 0.15. 
     
     
         3 . The process of  claim 2 , wherein the finishing treatment has an entry temperature between 950-1150° C. and an exit temperature between 800-950° C. 
     
     
         4 . The process of  claim 3 , further including coiling the hot rolled strip within a temperature range of 550-730° C. 
     
     
         5 . The process of  claim 4 , further including cooling the continuous annealed sheet during a first cooling treatment using an average cooling rate between 4-100° C./s. 
     
     
         6 . The process of  claim 5 , further including cooling the continuous annealed sheet during a second cooling treatment using an average cooling rate between 3-20° C./s. 
     
     
         7 . The process of  claim 6 , wherein the continuous annealed sheet is cooled to a temperature equal to or less than 200° C. before being temper rolled. 
     
     
         8 . The process of  claim 7 , wherein the temper rolled sheet has a thickness between 0.30-2.5 mm. 
     
     
         9 . The process of  claim 7 , further including bake hardening the continuous annealed sheet, the continuous annealed sheet having a BH2 of between 15-35 MPa. 
     
     
         10 . A process for making a dent resistant low carbon-high manganese steel sheet, the process comprising:
 providing a steel slab having a chemical composition with the range in weight percent of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40 max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being Fe and incidental impurities;   soaking the steel slab within a temperature of 1200-1350° C.;   hot rolling the soaked steel slab using a roughing treatment and producing a transfer bar;   hot rolling the transfer bar using a finishing treatment and producing hot rolled strip, the finishing treatment having an entry temperature between 950-1150° C. and an exit temperature between 800-950° C.;   coiling the hot rolled strip within a temperature range of 550-730° C.;   cold rolling the hot rolled strip and producing cold rolled sheet with a reduction in thickness between 50-80%;   continuous annealing the cold rolled sheet within a temperature range of 730-850° C.; and   temper rolling the annealed cold rolled sheet between 1.0-2.0%, the temper rolled sheet having a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa, an elongation to fracture greater than 30% and an n-value greater than 0.15.   
     
     
         11 . The process of  claim 10 , wherein the continuous annealed cold rolled sheet is cooled to a temperature equal to or less than 200° C. using a first cooling treatment having an average cooling rate between 4-100° C./s and a second cooling treatment having an average cooling rate between 3-20° C./s. 
     
     
         12 . The process of  claim 11 , wherein the temper rolled sheet has a thickness between 0.30-2.5 mm. 
     
     
         13 . A dent resistant low carbon-high manganese steel sheet comprising:
 a cold rolled steel sheet having a thickness between 0.20-2.5 mm and a chemical composition within a range in weight percent of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40 max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being Fe and incidental impurities;   a completely recovered and recrystallized microstructure; and   a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa and an elongation to fracture greater than 30%.   
     
     
         14 . The steel sheet of  claim 13 , wherein said cold rolled steel sheet has been temper rolled between 1.0-2.0% and has no yield point elongation. 
     
     
         15 . The steel sheet of  claim 14 , wherein said cold rolled steel sheet has an n-value greater than 0.15. 
     
     
         16 . The steel sheet of  claim 15 , wherein said cold rolled steel sheet has at least 0.015 wt % free Al. 
     
     
         17 . The steel sheet of  claim 16 , wherein said cold rolled steel sheet has a thickness between 0.30-2.5 mm. 
     
     
         18 . The steel sheet of  claim 17 , wherein said cold rolled steel has a BH2 of between 15-35 MPa.