High-strength steel sheet and method for manufacturing same

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.