High-strength steel sheet and method for producing the same

A high-strength steel sheet having a tensile strength of 1180 MPa or more, the steel sheet comprising in a chemical composition:

• C in a content of 0.10% (in mass percent, hereinafter the same for chemical compositions) to 0.30%;
• Si in a content of 1.40% to 3.0%;
• Mn in a content of 0.5% to 3.0%;
• P in a content of 0.1% or less (excluding 0%);
• S in a content of 0.05% or less (excluding 0%);
• Al in a content of 0.005% to 0.20%;
• N in a content of 0.01% or less (excluding 0%);
• and
• O in a content of 0.01% or less (excluding 0%),
• with the remainder consisting of iron and inevitable impurities,
• wherein a microstructure at a position corresponding to one-fourth the thickness of the steel sheet meets all of the following conditions (1) to (4):
• (1) the microstructure comprises ferrite in a volume fraction of 5% to 35% and bainitic ferrite and/or tempered martensite in a volume fraction of 50% or more relative to the entire microstructure, where the volume fractions of ferrite and bainitic ferrite and/or tempered martensite are determined by scanning electron microscopic observation;
• (2) the microstructure comprises MA constituent in a volume fraction of 20% or less (excluding 0%) relative to the entire microstructure, where the MA constituent is a mixed structure of fresh martensite and retained austenite, and where the volume fraction of MA constituent is determined by optical microscopic observation;
• (3) the microstructure comprises retained austenite in a volume fraction of 5% or more relative to the entire microstructure, where the volume fraction of retained austenite is determined by X-ray diffractometry;
• and
• (4) the microstructure has an amount of stacking faults in the retained austenite of 10.0×10−3 (nm/nm2) or less, where the amount is determined by transmission electron microscopic observation.