Patent ID: 8876987

Claim:
A high strength steel sheet comprising a chemical composition including, in mass %, C: 0.10% or more and 0.59% or less, Si: 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, N: 0.010% or less, and the balance being Fe and incidental impurities, wherein a relation [Si %]+[Al %]=0.7% or more is satisfied (where [X %] indicates mass % of element X), wherein the steel sheet has a microstructure such that: martensite has an area ratio of 5% or more and 70% or less to the entire microstructure of the steel sheet, retained austenite is contained in an amount of 5% or more and 40% or less, and bainitic ferrite in upper bainite has an area ratio of 5% or more to the entire microstructure of the steel sheet, where a total of the area ratio of the martensite, the amount of the retained austenite and the area ratio of the bainitic ferrite is 40% or more, 25% or more of the martensite is tempered martensite, polygonal ferrite has an area ratio of more than 10% and less than 50% to the entire microstructure of the steel sheet and an average grain size of 8 μm or less, and an average diameter of a group of polygonal ferrite grains is 15 μm or less, where the group of polygonal ferrite grains is represented by a group of ferrite grains composed of adjacent polygonal ferrite grains, an average carbon content in the retained austenite is 0.70 mass % or more, and the steel sheet has a tensile strength of 780 MPa or more.