Patent ID: 7507307

Claim:
A method for manufacturing an ultra high strength cold-rolled steel sheet, said steel being a ferrite martensite dual-phase steel, the method comprising continuously annealing a cold-rolled steel sheet consisting essentially of, in terms of weight percentages, 0.07 to 0.15% C, 1.4 to 2% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol.Al, 0.005% or less N, 0.0003 to 0.003% B, optionally at least one element selected from the group consisting of Ti and Mo, and the balance being Fe, wherein said continuous annealing comprises the steps of: (a) heating the cold-rolled steel sheet at from 800° C. to 870° C. for 10 seconds or more to provide a heated steel sheet; (b) slowly cooling the heated steel sheet down to from 670° C. to 750° C. to provide a slowly cooled steel sheet; (c) rapidly cooling the slowly cooled steel sheet down to 100° C. or less at a cooling speed of over 500° C./sec to provide a rapidly cooled steel sheet; (d) reheating the rapidly cooled steel sheet at from 325° C. to 425° C. for from 5 minutes to 20 minutes to provide a reheated steel sheet; (e) cooling the reheated steel sheet down to room temperature to provide a cooled steel sheet; and (f) coiling the cooled steel sheet, thereby obtaining a steel sheet having a tensile strength of 980 MPa or more, a hole-expanding ratio of 60% or more and an elongation of 15% or more.