Nano-precipitation strengthened cold-rolled batch annealed high strength low alloy steel sheet

Abstract

A high strength low alloy steel sheet product produced by a continuous strip process. The sheet product comprises from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, and from 0.075 to 0.12% weight percent Titanium and an Fe balance. The steel sheet product has been subjected to cold rolling and batch annealing to form a steel sheet product having a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%.

1 . A high strength low alloy steel sheet product produced by a continuous strip process, the sheet product comprising from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, and from 0.075 to 0.12% weight percent Titanium and an Fe balance, wherein the steel sheet product has been subjected to cold rolling and batch annealing to form a steel sheet product having a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%. 
     
     
         2 . The high strength low alloy steel sheet product of  claim 1 , wherein the TiC precipitates are approximately 3.0 to 7.0 nm. 
     
     
         3 . The high strength low alloy steel sheet product of  claim 1 , wherein the steel sheet comprises 0.06 weight percent Carbon, 1.1 weight percent Manganese and 0.12 weight percent Titanium. 
     
     
         4 . The high strength low alloy steel sheet product of  claim 1 , wherein the steel sheet comprises a hole expansion ratio of at least 70%. 
     
     
         5 . The high strength low alloy steel sheet product of  claim 1  further comprising a tensile strength of at least 570 MPa. 
     
     
         6 . The high strength low alloy steel sheet product of  claim 1  further comprising less than 0.04 weight percent Niobium. 
     
     
         7 . The high strength low alloy steel sheet product of  claim 1  wherein the steel sheet is produced from a slab of approximately 55 mm to 85 mm thick. 
     
     
         8 . The high strength low alloy steel sheet product of  claim 1 , wherein the steel sheet is produced with a coiling temperature of 600 to 675C. 
     
     
         9 . The high strength low alloy steel sheet product of  claim 1 , wherein the steel sheet is produced by cold rolling to 40 to 75%. 
     
     
         10 . The high strength low alloy steel sheet product of  claim 1 , wherein the steel sheet is produced hot rolling the slabs to a 2.2-3.3 mm thickness using a six-pass reduction schedule. 
     
     
         11 . The high strength low alloy steel sheet product of  claim 1  wherein the steel sheet is produced by hot rolling the steel slab at a hot rolling finishing temperature of 875 to 950C. 
     
     
         12 . The high strength low alloy steel sheet product of  claim 11 , wherein the hot rolled strip is cooled at a cooling rate of at least 30° C./s to a coiling temperature of 600-675° C. 
     
     
         13 . A method for producing a high strength low alloy steel sheet product comprising from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, and from 0.075 to 0.12% weight percent Titanium and an Fe balance, a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%, comprising the steps of:
 continuously casting a steel slab approximately 55 mm-85 mm thick;   maintaining a temperature of the steel slab;   hot rolling the steel slab to a steel sheet at a finishing temperature of 875-950C;   cooling the steel slab;   coiling the steel sheet at a temperature of 600-675C;   cold rolling the steel sheet to 40-75%; and   hydrogen batch annealing the steel sheet at temperature of 600-650C to achieve a fully recrystallized ferritic microstructure with nano precipitates of TiC.   
     
     
         14 . The method for producing a high strength low alloy steel sheet product of  claim 13 , wherein the step of hot rolling includes hot rolling the slabs to a 2.2-3.3 mm thickness using a six-pass reduction schedule. 
     
     
         15 . The method for producing a high strength low alloy steel sheet product of  claim 13 , wherein the step of cooing the steel slab includes a cooling rate of more than 30° C./s. 
     
     
         16 . The method for producing a high strength low alloy steel sheet product of  claim 13 , wherein the steel sheet includes a tensile strength of at least 570 MPa. 
     
     
         17 . The method for producing a high strength low alloy steel sheet product of  claim 13 , wherein the steel sheet includes a hole expansion ratio of least 70%. 
     
     
         18 . A high strength low alloy steel sheet product produced by a continuous strip process, the sheet product comprising from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, from 0.075 to weight percent Titanium, 0.04 weight percent Niobium and an Fe balance, wherein the steel sheet product has been subjected to cold rolling and batch annealing to form a steel sheet product having a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%. 
     
     
         19 . The high strength low alloy steel sheet product of  claim 18 , wherein the TiC precipitates are approximately 3.0 to 7.0 nm. 
     
     
         20 . The high strength low alloy steel sheet product of  claim 18 , wherein the steel sheet comprises 0.06 weight percent Carbon, 1.1 weight percent Manganese and 0.12 weight percent Titanium. 
     
     
         21 . A high strength low alloy steel sheet product produced by a continuous strip process, the sheet product comprising from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, and from 0.075 to 0.12% weight percent Titanium and an Fe balance, wherein the steel sheet product has been subjected to cold rolling and batch annealing to form a steel sheet product having a yield strength of at least 550 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%. 
     
     
         22 . The high strength low alloy steel sheet product of  claim 21 , wherein the TiC precipitates are approximately 3.0 to 7.0 nm. 
     
     
         23 . The high strength low alloy steel sheet product of  claim 21 , wherein the steel sheet comprises 0.06 weight percent Carbon, 1.1 weight percent Manganese and 0.12 weight percent Titanium. 
     
     
         24 . The high strength low alloy steel sheet product of  claim 21 , wherein the steel sheet comprises a hole expansion ratio of at least 70%.