| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength steel sheet which is superior in workability and has a tensile strength (TS) of 980 MPa or more. SOLUTION: This steel sheet includes 0.16% to 0.72% C, 3.0% or less Si, 0.5% to 3.0% Mn, 0.1% or less P, 0.07% or less S, 3.0% or less Al and 0.010% or less N, while Si+Al is controlled to satisfy 0.7% or more; and has a metallurgical structure in which martensite occupies 10% to 90% by an area rate with respect to the whole metallurgical structure, retained austenite occupies 5% to 50% and bainitic ferrite in upper bainite occupies 5% or more by an area rate with respect to the whole metallurgical structure, and in which the sum of the area rate of the martensite with respect to the whole metallurgical structure, and the amount of the retained austenite and the area rate of the bainitic ferrite in the upper bainite with respect to the whole metallurgical structure satisfies 60% or more, the area rate of polygonal ferrite with respect to the whole metallurgical structure satisfies 10% or less (including 0%), and an average C content in the retained austenite is controlled to 0.70% to 2.00%. COPYRIGHT: (C)2010,JPO&INPIT
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