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 (Vt gamma R) of the residual austenite is 5 % or more; (2) the ratio (SF gamma R/ Vt gamma R) of the area fraction (SF gamma R) of the residual austenite within ferrite to Vt gamma R is 0.65 or more; and (3) the ratio Ä alpha 2/( alpha 1 + gamma R)Ü of the space factor ( alpha 2) of martensite to the second phase structure ( alpha 1 + gamma R) is 0.25 to 0.60. The steel sheet has excellent balance between strength and local elongation, and a low yield ratio.