| 摘要 |
<p>A high-strength steel sheet which has a tensile strength in, e.g., the 980-MPa class or above, is satisfactory in stretchability, stretch flangeability, and spot weldability, and has excellent delayed-fracture resistance. The steel sheet comprises a steel whose chemical composition contains 0.12-0.25% C, 1.0-3.0% Si, 1.5-3.0% Mn, up to 0.15% P, up to 0.02% S, and up to 0.4% Al, the remainder being iron and incidental impurities, and the ratio of the content of Si to that of C, Si/C, being 7-14 by mass. A vertical section of the steel sheet has microstructures which comprise 1) at least 50% bainitic ferrite, 2) at least 3% lathy residual austenite, and 3) blocky residual austenite contained in an amount of 1% to a half of the proportion of the lathy residual austenite, each proportion in % being based on all structures, and 4) the blocky second phase has an average size of 10 mum or smaller.</p> |
