High-strength steel sheet having excellent workability and production process therefor

摘要

A high-strength steel sheet comprises carbon: 0.06 to 0.25 mass %, Si: 0.5 to 3.5 mass % and Mn: 0.7 to 4 mass %. Its mother structure is ferrite, its second phase structure comprises martensite and the residual austenite and the second phase structure measured by image analysis has an area fraction of 25% or less based on the total structure. The steel sheet satisfies the following requirements (1) to (3): (1) the volume fraction (Vtgammahd R) of the residual austenite is 5% or more; (2) the ratio (SFgamma_R/Vtgamma_R) of the area fraction (SFgamma_R) of the residual austenite within ferrite to Vtgamma_R is 0.65 or more; and (3) the ratio [alpha2/(alpha1+gamma_R)] of the space factor (alpha2) of martensite to the second phase structure (alpha1+gamma_R) is 0.25 to 0.60. The steel sheet has excellent balance between strength and local elongation, and a low yield ratio.