Patent ID: 7468109

Claim:
A steel sheet comprising: 0.10-0.20 wt. % C; 0.8-2.5 wt. % Si; 1.5-2.5 wt. % Mn; 0.01-0.10 wt. % Al; 0≦P≦0.1 wt. %; and 0<S≦0.002 wt. %, wherein the steel sheet has a structure comprising ≧70 area % bainitic ferrite, 2-20 area % total residual austenite, and 0-15 area % polygonal ferrite and quasi-polygonal ferrite, relative to a total area percentage of the structure, wherein a proportion of minute residual austenite having an average particle size of ≦5 μm relative to the total residual austenite is ≧60%, wherein the steel sheet exhibits a tensile strength (TS) of ≧980 MPa and a total elongation (El) of ≧18%, and wherein a relationship of the tensile strength (TS) and a stretch flangeability (λ) within the steel sheet satisfies the following equation: TS ( MPa )×λ(%)≧50,000.