| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength steel sheet which is excellent in bendability and fatigue strength and has a tensile strength in the 780 MPa class. SOLUTION: The steel sheet has (1) a composition containing 0.05 to 0.20% C, 0.6 to 2.0% Si, 1.6 to 3.0% Mn, up to 0.05% P, up to 0.01% S, up to 0.1% Al, and up to 0.01% N, the remainder being iron and incidental impurities, and has (2) a microstructure comprising a polygonal ferrite structure and a structure formed by low-temperature transformation. When a plane located in the range from a surface of the steel sheet to a depth of 0.1 mm therefrom is examined with a scanning electron microscope with respect to twenty sights in total in different positions in the sheet-width direction, then the maximum areal proportion of the polygonal ferrite (Fmax) and the minimum areal proportion of the ferrite (Fmin) in a 50μm×50μm area in the sights satisfy the relationships Fmax≤80%, Fmin≥10%, and Fmax-Fmin≤40%. COPYRIGHT: (C)2009,JPO&INPIT
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