METHOD FOR PRODUCING COLD ROLLED STEEL PLATE OF SUPER HIGH STRENGTH

摘要

A method for producing a cold rolled steel plate of super high strength, which has a process of continuously annealing a cold rolled steel plate having a substantial chemical composition in wt %: C: 0.07 to 0.15 %, Si: 0.7 to 2 %, Mn: 1.8 to 3 %, P: 0.02 % or less, S: 0.01 % or less, Sol. Al: 0.01 to 0.1 %, N: 0.005 %, B: 0.0003 to 0.003 %, and balance: Fe, wherein the continuous annealing process comprises a step of heating the cold rolled steel plate before annealing at 800 to 870°C for 10 sec, a step of slowly cooling the heated steel plate to 650 to 750°C, a step of rapidly cooling the steel plate after the slow cooling to 100°C or lower at a cooling rate of more than 500°C/sec, a step of re-heating the steel plate after the rapid cooling at 325 to 425°C for 5 to 20 min, and a step of cooling the steel plate after the re-heating to room temperature and winding it up. A steel plate produced by the method has a tensile strength of 980 MPa or more and is excellent in stretch-flamging property and spot welding property, and thus can be suitably used as a cold rolled steel plate of super high strength for a structural member of an automobile.