Method of producing hot rolled high strength dual phase steels using room temperature water quenching

Abstract

A process for manufacturing hot rolled high strength dual phase steels. In some instances, hot rolled strip is continuously cooled to less than 100° C. prior to coiling using a cooling rate between 40-70° C./second. In other instances, hot rolled strip is cooled to less than 100° C. prior to coiling using a step cooling treatment. In yet other instances, hot rolled strip is continuously cooled to less than 100° C. prior to coiling using a cooling rate between 70-100° C./second. For hot rolled strip subjected to a cooling rate between 40-70° C./second, or subjected to the step cooling treatment, hot rolled steel sheet with a tensile strength greater than 590 MPa is produced. For hot rolled strip subjected to a cooling rate between 70-100° C./second, hot rolled steel sheet with a tensile strength greater than 690 MPa is produced. Also, the hot rolled steel sheet has a ferrite plus martensite microstructure free of pearlite and bainite.

We claim: 
     
         1 . A process for manufacturing a hot rolled high strength dual phase steel comprising:
 providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.09 C, 0.8-2.0 Mn, 0.4 min Cr, 0.08 max Si, 0.05 max P, 0.005-0.010 N, balance Fe and melting impurities, the steel slab chemical composition also having an Al/N ratio of 10.0 max and a Nb+Ti+Mo content of 0.015 max;   soaking the steel slab within a temperature range of 1150-1300° C.;   hot rolling the soaked steel slab in a roughing treatment and producing a transfer bar;   hot rolling the transfer bar in a finishing treatment and producing hot rolled strip, the finishing treatment having an entry temperature between 950-1120° C. and an exit temperature between 800-850° C.; and   cooling the hot rolled strip to less than 100° C. before coiling;   coiling the cooled hot rolled strip, the coiled hot rolled steel strip having a microstructure of ferrite plus martensite that is free of pearlite and bainite.   
     
     
         2 . The process of  claim 1 , wherein the hot rolled strip is continuously cooled from the finishing treatment exit temperature to RT. 
     
     
         3 . The process of  claim 2 , wherein the hot rolled strip is subjected to a cooling rate between 40-70° C./sec. 
     
     
         4 . The process of  claim 1 , wherein the RT hot rolled strip has a percent volume fraction of martensite between 10-25%. 
     
     
         5 . The process of  claim 4 , wherein the RT hot rolled strip has a tensile strength of at least 590 MPa and a percent elongation to failure of at least 17.5%. 
     
     
         6 . The process of  claim 5 , further including tension leveling the RT hot rolled strip, the tension leveled hot rolled strip giving an increase in the as rolled hot band yield strength of at least 40 MPa. 
     
     
         7 . The process of  claim 6 , further including bake hardening the tension leveled hot rolled strip, the bake hardened hot rolled strip having an additional increase in the yield strength of at least 30 MPa. 
     
     
         8 . The process of  claim 1 , wherein cooling the hot rolled strip to RT includes a step cooling treatment. 
     
     
         9 . The process of  claim 8 , wherein the step cooling treatment includes cooling the hot rolled strip from the exit temperature of the finishing treatment to a ferrite formation zone, holding the cooled hot rolled strip within the ferrite formation zone for at least 3 seconds and cooling the hot rolled strip to RT. 
     
     
         10 . The process of  claim 9 , wherein the ferrite formation zone is between 640-720° C. 
     
     
         11 . The process of  claim 10 , wherein the RT hot rolled strip has a percent volume fraction of martensite between 10-25%. 
     
     
         12 . The process of  claim 11 , wherein the RT hot rolled strip has a tensile strength of at least 590 MPa and a percent elongation to failure of at least 17.5%. 
     
     
         13 . The process of  claim 12 , further including tension leveling the RT hot rolled strip, the tension leveled hot rolled strip giving an increase in the as rolled hot band yield strength of at least 40 MPa. 
     
     
         14 . The process of  claim 13 , further including bake hardening the tension leveled hot rolled strip, the bake hardened hot rolled strip having an additional increase in the yield strength of at least 30 MPa. 
     
     
         15 . The process of  claim 1 , wherein the hot rolled strip is subjected to a cooling rate between 70-100° C./sec. 
     
     
         16 . The process of  claim 15 , wherein the RT hot rolled strip has a percent volume fraction of martensite between 20-35%. 
     
     
         17 . The process of  claim 16 , wherein the RT hot rolled strip has a tensile strength of at least 690 MPa and a percent elongation to failure of at least 17.5%. 
     
     
         18 . The process of  claim 17 , further including tension leveling the RT hot rolled strip, the tension leveled hot rolled strip giving an increase in the as rolled hot band yield strength of at least 40 MPa. 
     
     
         19 . The process of  claim 18 , further including bake hardening the tension leveled hot rolled strip, the bake hardened hot rolled strip having an additional increase in the yield strength of at least 30 MPa.