High-strength steel sheet and a manufacturing method thereof

According to the present invention, it is possible to obtain a high strength steel sheet having tensile strength of at least 1470 MPa and (tensile strength × total elongation) of at least 29000 MPa · % by: specifying a composition of the steel sheet to include, by mass %, C: 0.30% to 0.73% (inclusive of 0.30% and 0.73%), Si: 3.0% or less, Al: 3.0% or less, Si + Al: at least 0.7%, Cr: 0.2% to 8.0% (inclusive of 0.2% and 8.0%), Mn: 10.0% or less, Cr + Mn: at least 1.0%, P: 0.1 % or less, S: 0.07% or less, N: 0.010% or less, and remainder as Fe and incidental impurities; and processing the steel sheet such that microstructure of the steel sheet satisfies following conditions: area ratio of martensite with respect to the entire microstructure of the steel sheet is in the range of 15% to 90% (inclusive of 15% and 90%); content of retained austenite is in the range of 10% to 50% (inclusive of 10% and 50%); at least 50% of said martensite is constituted of tempered martensite and area ratio of said tempered martensite with respect to the entire microstructure of the steel sheet is at least 10%; and area ratio of polygonal ferrite with respect to the entire microstructure of the steel sheet is 10% or less (inclusive of 0%).