| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet having a TS of at least 1,180 MPa, and excellent in elongation, stretch flange formability, delayed fracture resistance and collision resistance, and to provide a method of producing the same. <P>SOLUTION: The high-strength steel sheet comprises, by mass, C: 0.15 to 0.35%, Si: not more than 3.0%, Mn: 2.00 to 3.5%, P: not more than 0.10%, S: not more than 0.010%, Al: not more than 2.0%, N: not more than 0.010%, and Cu: 0.05 to 0.5%, while satisfying Si+Al≥0.7%, with the balance being Fe and inevitable impurities. The steel sheet structure includes, by area ratio, upper bainite: 50 to 90%, tempered martensite: 5 to 45%, residual austenite: 5 to 30%, untempered martensite: not more than 5%, (including 0%), and polygonal ferrite: not more than 5% (including 0%), provided that a coarse bulk structure, of the residual austenite and untempered martensite, having a circle equivalent diameter of at least 1 μm and a mean axis ratio (long axis/short axis) of not greater than 3 is present therein in an amount of not more than 7% (including 0%). <P>COPYRIGHT: (C)2013,JPO&INPIT |
