Process for manufacturing ferritic hot rolled steel strip

Abstract

A process for manufacturing a ferritic hot rolled steel strip is provided. The process includes providing a steel slab, hot rolling the slab to produce a transfer bar and ferritic hot rolling the transfer bar to produce hot rolled strip. The ferritic hot rolled strip is coiled at temperatures between 580-780° C. and has a yield strength between 130-210 MPa, a tensile strength greater than 260 MPa, a uniform elongation greater than 15%, a total elongation to failure greater than 30%, and an n-value greater than 0.2.

We claim: 
     
         1 . A process for manufacturing a ferritic hot rolled steel strip comprising:
 providing a steel slab having a chemical composition in weight percent within a range of 0.10 maximum (max) C, 0.15-0.60 Mn, 0.20 max Si, 0.04 max Ti, 0.008 max V, 0.006 max Mo, 0.1 max Ni, 0.05 max Cr, 0.08 max Cu, 0.015 max S, 0.04 max P, 0.01 max N, 0.006 max B, 0.06 max Al, balance Fe and melting impurities;   soaking the steel slab within a temperature range between 1100 to 1400° C.;   hot rolling the soaked steel slab in a roughing treatment at temperatures between 900 to 1400° C. and producing a transfer bar;   hot rolling the transfer bar in a finishing treatment using a finishing treatment at entry temperatures between 900-1100° C. and exit temperatures between 720-850° C. and producing hot rolled strip; and   coiling the hot rolled strip at a coiling station at temperatures between 580-780° C.;   the coiled hot rolled steel strip having a yield strength between 130-210 MPa, a tensile strength greater than 260 MPa, a uniform elongation greater than 15%, a total elongation to failure greater than 30% and an n-value greater than 0.2.   
     
     
         2 . The process of  claim 1 , wherein the slab is hot rolled in the roughing treatment at temperatures between 1200-1300° C. 
     
     
         3 . The process of  claim 2 , wherein the slab is hot rolled in the roughing treatment at temperatures between 1220-1280° C. 
     
     
         4 . The process of  claim 3 , wherein the slab is hot rolled in the roughing treatment at temperatures between 1230-1270° C. 
     
     
         5 . The process of  claim 1 , wherein the finishing treatment has an entry temperature between 1000-1080° C. and an exit temperature between 750-825° C. 
     
     
         6 . The process of  claim 5 , wherein the finishing treatment has an entry temperature between 1035-1065° C. and an exit temperature between 760-800° C. 
     
     
         7 . The process of  claim 1 , wherein the hot rolled strip is coiled at temperatures between 640-750° C. 
     
     
         8 . The process of  claim 7 , wherein the hot rolled strip is coiled at temperatures between 660-700° C. 
     
     
         9 . The process of  claim 1 , wherein the hot rolled strip has a thickness between 1.5-6.5 mm. 
     
     
         10 . The process of  claim 1 , wherein the hot rolled strip is only air cooled on a run-out table between the finishing treatment and the coiling station. 
     
     
         11 . The process of  claim 10 , wherein the hot rolled strip is only static air cooled on the run-out table between the finishing treatment and the coiling station. 
     
     
         12 . The process of  claim 1 , wherein the coiled hot rolled strip has a uniform elongation of at least 17.5% and a total elongation of at least 35%. 
     
     
         13 . The process of  claim 12 , wherein the coiled hot rolled strip has a uniform elongation of at least 20% and a total elongation of at least 38%. 
     
     
         14 . The process of  claim 1 , wherein the steel slab has at least 0.025 wt % C. 
     
     
         15 . The process of  claim 14 , wherein the steel slab has between 0.050-0.080% C and is the coiled hot rolled steel strip is a bake hardenable steel with a yield strength between 180-210 MPa. 
     
     
         16 . The process of  claim 15 , further including the steps of cold rolling the hot rolled strip to produce cold rolled sheet followed by hot dip galvanizing or continuous annealing of the cold rolled sheet without an over-aging step. 
     
     
         17 . The process of  claim 1 , wherein the ratio of Mn to S ratio (Mn/S) in the steel slab is at least 15. 
     
     
         18 . The process of  claim 1 , wherein the Ti and N content in the steel slab obeys the relationship −0.001 ≦(Ti−3.42N)≦0.002. 
     
     
         19 . The process of  claim 1 , wherein the B and N content in the steel slab obeys the relationship −0.001≦(B−0.78N)≦0.002. 
     
     
         20 . The process of  claim 1 , wherein the Mn/S ratio in steel slab obeys the relationship Mn/S >15.