High strength steel sheet and method for manufacturing the same

A high strength steel sheet having a tensile strength of at least 1470 MPa and (tensile strength (TS)×total elongation (T. EL)) of at least 29000 MPa·%, comprising a composition including by mass %,

• C:
• 0.55% to 0.73%,
• Si:
• 3.0% or less,
• Al:
• 3.0% or less,
• Si+Al:
• at least 0.7%,
• Cr:
• 0.2% to 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 having a microstructure satisfying conditions:
• area ratio of martensite with respect to the entire microstructure of 15% to 90%;
• content of retained austenite of 10% to 50%, wherein at least 50% of said martensite is constituted of tempered martensite and an area ratio of said tempered martensite with respect to the entire microstructure of the steel sheet is at least 10%;
• and an area ratio of polygonal ferrite with respect to the entire microstructure of the steel sheet is 10% or less,
• wherein average carbon concentration in said retained austenite is at least 0.7mass %.