Methods for production of highly formable extra deep draw enameling steel -- product and process for manufacture thereof

Abstract

A new alloy and process for making highly formable enamel steel substrates. The new alloy is manufactured from a vacuum degassed steel on a Continuous Annealing Line (CAL) with in-line temper equipment and possesses the ductility and enameling properties that make the steel suitable for the manufacture of deep draw and extra deep draw enameling parts. Target applications for the new alloy and process include but are not limited to deep draw and extra deep draw parts such as bathtubs, kitchen sinks, kitchen oven liners and general parts which are press-formed from flat sheets into complicated shapes, which are subsequently vitreously enameled.

We claim: 
     
         1 . A process for producing high strength steel, the process comprising:
 providing a steel slab having a chemical composition in weight percent (wt %) with a range of 0.0100 max C, 0.0600 max Si, 0.17 max Mn, 0.0100 max P, 0.0200-0.0500 S, 0.0150-0.0800 Al; 0.10 max Cr, 0.1000 max Cu, 0.0100 max Nb, 0.0500 max Mo, 0.0100 max N, 0.0600 max Ni, 0.0650-1500 Ti, 0.0004 max B, 0.0150 max Sn, with the balance being Fe and incidental impurities;   the chemical composition in wt % of the steel slab obeying the following formulas:
   14.0≧[Ti/(C+N)]≧8.0  (1)
 
   and 
   Ti free =[Ti (addition) −(4·C)−(3.4·N)−(1.5·S)]≧0.020  (2)
 
   soaking the steel slab at a temperature of at least 1200° C.;   hot rolling the 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;   pickling and cold rolling the hot rolled sheet and producing cold rolled sheet;   annealing the cold rolled sheet in a continuous annealing line (CAL), the annealed cold rolled sheet having a surface roughness between 2.0-3.5 Ra, a lower yield strength of at least 110 MPa, a tensile strength of at least 280 MPa, an elongation to failure of at least 40%, an N-value of at least 0.200, and an R-value of at least 1.80.   
     
     
         2 . The process of  claim 1 , wherein soaked steel slab is hot rolled using the roughing treatment within a temperature range of 900-1200° C. 
     
     
         3 . The process of  claim 2 , wherein the transfer bar has a thickness between 20-70 mm, inclusive. 
     
     
         4 . The process of  claim 3 , wherein the transfer bar hot rolled in the finishing treatment has an entry temperature between 900-1100° C., inclusive, and the hot rolled strip exiting the finishing treatment has an exit temperature between 760-960° C., inclusive, and a thickness between 1.5-6.0 mm. 
     
     
         5 . The process of  claim 4 , further including coiling the hot rolled strip at a temperature between 560-790° C., inclusive. 
     
     
         6 . The process of  claim 5 , wherein the cold rolled sheet has a reduction in thickness compared to the hot rolled strip of between 50-90%, inclusive. 
     
     
         7 . The process of  claim 6 , wherein the cold rolled sheet is annealed in the CAL at a soak temperature between 760-880° C. 
     
     
         8 . The process of  claim 7 , wherein the cold rolled sheet is held at the soak temperature for a soak time up to 180 seconds and then cooled to an ambient temperature. 
     
     
         9 . The process of  claim 8 , further including interrupting the cooling of the cold rolled sheet to the ambient temperature and holding the cooling cold rolled sheet in a temperature range between 250-600° C. for a predetermined time after the cold rolled sheet has annealed at the soak temperature for the soak time. 
     
     
         10 . The process of  claim 8 , further including rapidly cooling the cold rolled sheet from the soak temperature to the ambient temperature at an average cooling rate of up to 100 K/s and then reheating the cooled cold rolled sheet to a temperature range between 250-600° C. for a predetermined time. 
     
     
         11 . The process of  claim 8 , further including temper rolling the cold rolled sheet between 0.3-2.0%. 
     
     
         12 . A process for producing highly formable steel, the process comprising:
 providing a steel slab having a chemical composition in weight percent (wt %) with a range of 0.0100 max C, 0.0600 max Si, 0.17 max Mn, 0.0100 max P, 0.0200-0.0500 S, 0.0150-0.0800 Al; 0.10 max Cr, 0.1000 max Cu, 0.0100 max Nb, 0.0500 max Mo, 0.0100 max N, 0.0600 max Ni, 0.0650-1500 Ti, 0.0004 max B, 0.0150 max Sn, with the balance being Fe and incidental impurities;   the chemical composition in wt % of the steel slab obeying the following formulas:
   14.0≧[Ti/(C+N)]≧8.0  (1)
 
   and 
   Ti free =[Ti (addition) −(4·C)−(3.4·N)−(1.5·S)]≧0.020  (2)
 
   soaking the steel slab at a temperature of at least 1200° C.;   hot rolling the steel slab using a roughing treatment within a temperature range between 900-1200° C., inclusive, and producing a transfer bar having a thickness between 20-70 mm, inclusive;   hot rolling the transfer bar using a finishing treatment with an entry temperature between 900-1100° C., inclusive, and an exit temperature between 760-960° C., inclusive, and producing hot rolled strip with a thickness between 1.5-6.0 mm, inclusive;   coiling the hot rolled strip at a temperature between 560-790° C., inclusive;   pickling and cold rolling the hot rolled sheet and producing cold rolled sheet; and   annealing the cold rolled sheet in a continuous annealing line (CAL), the annealed cold rolled sheet having a surface roughness between 2.0-3.5 Ra, a lower yield strength of at least 110 MPa, a tensile strength of at least 280 MPa, an elongation to failure of at least 40%, an N-value of at least 0.200, and an R-value of at least 1.80.   
     
     
         13 . The process of  claim 12 , wherein the cold rolled sheet has a reduction in thickness compared to the hot rolled strip of between 50-90%, inclusive. 
     
     
         14 . The process of  claim 13 , wherein the cold rolled sheet is annealed in the CAL at a soak temperature between 760-880° C. 
     
     
         15 . The process of  claim 14 , wherein the cold rolled sheet is held at the soak temperature for a soak time up to 180 seconds and then cooled to an ambient temperature. 
     
     
         16 . The process of  claim 15 , further including interrupting the cooling of the cold rolled sheet to the ambient temperature and holding the cooling cold rolled sheet in a temperature range between 250-600° C. for a predetermined time after the cold rolled sheet has annealed at the soak temperature for the soak temperature. 
     
     
         17 . The process of  claim 15 , further including rapidly cooling the cold rolled sheet from the soak temperature to the ambient temperature and then reheating the cooled cold rolled sheet to a temperature range between 250-600° C. for a predetermined time. 
     
     
         18 . The process of  claim 15 , further including temper rolling the cold rolled sheet between 0.3-2.0%.