Patent ID: 7008488

Claim:
A high-strength steel sheet having excellent workability comprising: 0.06 to 0.25% by mass of carbon; 0.5 to 3.5% by mass of Si; and 0.7 to 4% by mass of Mn, wherein mother structure of said steel sheet is ferrite, second phase structure of said steel sheet comprises martensite and the residual austenite and said second phase structure (α1+γ R ) has an area fraction of 25% or less based on the total structure when it is measured by image analysis, and wherein said steel sheet satisfies the following requirements (1) to (3): (1) the volume fraction (Vtγ R ) of said residual austenite is 5% or more when a measurement specimen of said residual austenite is measured by saturation magnetization measurement, (2) the ratio (SFγ R /Vtγ R ) of the area fraction (SFγ R ) of said residual austenite within the ferrite particle to Vtγ R is 0.65 or more when the area fraction is measured by FE-SEM/EBSP, and (3) the ratio [α2/(α1+γ R )] of the space factor (α2) of said martensite to the second phase structure (α1+γ R ) satisfies the following expression: 0.25≦[α2/(α1+γ R )]≦0.60, wherein the space factor (α2) is calculated from a difference between the second phase structure (α1+γ R ) and the residual austenite (Vtγ R ).