Patent ID: 7780799

Claim:
A high-strength cold-rolled steel sheet 780 MPa or more in tensile strength, said steel sheet having excellent local formability, 60% or more hole expandability and suppressed weld hardness increase, characterized by: consisting of, in weight, C: 0.05 to 0.09%, Si: 0.4 to 1.3%, Mn: 2.5 to 3.2%, P: 0.001 to 0.05%, N: 0.0005 to 0.004%, Al: 0.005 to 0.1%, Ti: 0.001 to 0.045%, and one or more of: Nb: 0.001 to 0.04%, B: 0.0002 to 0.0015%, Mo: 0.05 to 0.50%, Ca: 0.0003 to 0.01%, REM: 0.0002 to 0.01% and S in the range stipulated by the following expression (A), with a balance of Fe and unavoidable impurities; the microstructures of said steel sheet being composed of bainite of 7% or more in terms of area percentage and the balance consisting of one or more of ferrite, martensite, tempered martensite and retained austenite; and said components in said steel sheet satisfying the following expressions (C) and (D) when Mneq. is defined by the following expression (B); S≦0.08×(Ti(%)−3.43×N(%))+0.004 (A), where, when a value of the member Ti(%)−3.43×N(%) of said expression (A) is negative, the value is regarded as zero, and S is precipitated as Ti type sulfide, Mn eq .=Mn(%)−0.29−Si(%)+6.24×C(%) (B), 950≧(Mn eq ./(C(%)−(Si(%)/75)))×bainite area percentage (%) (C), C(%)+(Si(%)/20)+(Mn(%)/18)≦0.30 (D).