Cold rolled recovery annealed mild steel and process for manufacture thereof

Abstract

A high strength mild steel alloy is provided. In addition, a process for making the high strength steel alloy is also provided. The process includes providing a mild steel alloy with a chemical composition in weight percent within a range of 0.12-0.25 carbon, 0.30-1.70 manganese, 0.50 max silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, and 0.015-0.065 aluminum. Hot rolled steel strip with a thickness of less than 10 millimeters is cold rolled to produce a cold rolled steel sheet that has a thickness that is less than 50% of the hot rolled steel strip thickness which is subsequently recovery annealed to provide sheet material having a yield strength greater than 550 megapascals (MPa) and a percent elongation to failure greater than 3.5%.

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 0.12-0.25 carbon, 0.30-1.70 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, 0.015-0.065 aluminum, balance iron and incidental impurities;   hot rolling the steel alloy to produce a hot rolled strip having a thickness of less than 10 mm;   cold rolling the hot rolled strip to produce a cold rolled sheet having a thickness less than 50% of the hot rolled strip thickness; and   recovery annealing the cold rolled sheet, the recovery annealed cold rolled sheet having a yield strength greater than 550 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 950 and 1350° C. and a finishing treatment having an entry temperature between 950 and 1100° C. and an exit temperature between 780 and 920° 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 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 500 and 620° C. 
     
     
         7 . The process of  claim 6 , wherein a microstructure of the recovery annealed cold rolled sheet has less than 10 volume percent recrystallized grains. 
     
     
         8 . The process of  claim 7 , wherein the microstructure of the recovery annealed cold rolled sheet has less than 5 volume percent recrystallized grains. 
     
     
         9 . The process of  claim 8 , wherein the microstructure of the recovery annealed cold rolled sheet has less than 2 volume percent recrystallized grains. 
     
     
         10 . The process of  claim 1 , further including temper rolling the recovery annealed cold rolled sheet between 0.0 and 5.0%. 
     
     
         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 0.12-0.25 carbon, 0.30-1.70 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, 0.015-0.065 aluminum, balance iron and incidental impurities;   soaking the steel slab at a temperatures between 1100 and 1350° C.;   hot rolling the steel at temperatures between 780 and 1100° C. and producing a hot rolled strip with a thickness between 1.5 and 6.0 mm;   coiling the hot rolled strip at temperatures between 500 and 730° C.;   cold rolling the hot rolled strip to produce a cold rolled sheet having a thickness between 0.30 and 2.3 mm; and   recovery annealing the cold rolled sheet at temperatures between 500 and 620° C., the recovery annealed cold rolled sheet having a yield strength greater than 550 MPa and a percent elongation to failure greater than 3.5%.   
     
     
         12 . The process of  claim 11 , wherein the hot rolling includes a roughing treatment that produces a transfer bar with a thickness between 45 and 70 mm. 
     
     
         13 . The process of  claim 12 , wherein the hot rolling includes a finishing treatment at temperatures between 780 and 920° C. and produces the hot rolled strip with a thickness between 1.5 and 6.0 mm. 
     
     
         14 . The process of  claim 11 , further including temper rolling the recovery annealed cold rolled sheet between 0.0 and 5.0%. 
     
     
         15 . A high strength-high ductility cold rolled steel comprising:
 a steel alloy having a chemical composition in weight percent within a range of 00.12-0.25 carbon, 0.30-1.70 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, 0.015-0.065 aluminum, balance iron and incidental impurities;   said steel alloy having a recovery annealed microstructure, a yield strength greater than 550 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 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 15 , wherein said recovery annealed microstructure has less than 5 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 2 volume percent of recrystallized grains.