Patent ID: 8262818

Claim:
A method for producing a high strength steel sheet excellent in formability, compatible with chemical conversion coating treatment and hot-dip galvanizing, wherein said steel sheet comprises, in mass, 0.03 to 0.20% C, 0.005 to 0.3% Si, 1.0 to 3.1% Mn, 0.001 to 0.06% P, 0.001 to 0.01% S, 0.0005 to 0.01% N, 0.2 to 1.2% Al, and not more than 0.5% Mo, with the balance consisting of Fe and unavoidable impurities; wherein the amounts of Si and Al in mass % and a target strength (TS) of said steel sheet satisfy the following expression (1); and a metallographic structure of said steel sheet contains ferrite and martensite without containing retained austenite and the steel sheet has a tensile strength of 980 MPa or more and a value of TS×El of 16,000 or more; (0.0012×[target strength TS]−0.29−[Si])/2.45<Al<1.5−3×[Si] (1) where [target strength TS] is a target strength of said steel sheet in terms of MPa and [Si] is an amount of Si in terms of mass %, said method comprising: hot rolling at a finishing temperature of the Ar 3 transformation temperature or higher; coiling at 400° C. to 550° C.; successively applying ordinary pickling; thereafter primary cold rolling at a reduction ratio of 30 to 70%; then recrystallization annealing in a continuous annealing process; and successively skin-pass rolling.