Patent ID: 7922835

Claim:
A high strength steel sheet excellent in formability, resistant to delayed fracture and compatible with chemical conversion coating treatment and hot-dip galvanizing, said steel sheet consisting essentially of, in mass, 0.03 to 0.20% C, 0.107 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; the amounts of Si and Al in mass % and the target strength (TS) of said steel sheet satisfy the following expression (1); and the metallographic structure of said steel sheet contains ferrite and martensite without containing retained austenite and has a tensile strength of 980 MPa or more and a value of TS×E1 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 the designed strength of said steel sheet in terms of MPa and [Si] is the amount of Si in terms of mass %.