Patent ID: 8257513

Claim:
A high strength steel sheet with 780 MPa or above tensile strength excellent in bending workability and fatigue strength, wherein (1) a composition of the steel sheet comprises: C: 0.05-0.20% (in mass % with respect to chemical composition, hereinafter the same); Si: 0.6-2.0%; Mn: 1.6-3.0%; P: 0.05% or below; S: 0.01% or below; Al: 0.1% or below; N: 0.01% or below; and iron and inevitable impurities, and (2) a microstructure of the steel sheet comprises a polygonal ferrite structure and a structure formed by low-temperature transformation, in which when a plane located at a depth of 0.1 mm from a surface of the steel sheet is in the observation under a scanning electron microscope (SEM) with respect to twenty sights in total in different positions in the sheet-width direction, the maximum value of areal proportion of the polygonal ferrite (Fmax) and the minimum value of areal proportion of the polygonal ferrite (Fmin) in a 50 μm×50 μm area in each sight satisfy all of Fmax≦80%, Fmin≧10%, and Fmax−Fmin≦40%.