High-strength steel sheet and method for producing the same

A high-strength steel sheet comprising:

• a component composition including:
• C:
• 0.15% to 0.40%, by mass %,
• Si:
• 0.5% to 2.5%, by mass %,
• Mn:
• 0.5% to 2.4%, by mass %,
• P:
• 0.1% or lower, by mass %,
• S:
• 0.01% or lower, by mass %,
• Al:
• 0.01% to 0.5%, by mass %,
• N:
• 0.010% or lower, by mass %, and
• Fe and incidental impurities;
• and
• a steel microstructure containing, on an area-percentage basis with respect to the entire steel microstructure, 40% or more and less than 85% of a lower bainite, 5% or more and less than 40% martensite including tempered martensite, 10% or more and 30% or less retained austenite, and 10% or less (including 0%) polygonal ferrite, the retained austenite having an average C content of 0.60% by mass or more,
• wherein:
• a Mn segregation value at a surface of the steel sheet is 0.8% or less, the Mn segregation value being defined as a difference between maximum and minimum values of a Mn concentration at the surface as measured by line analysis in a 1-mm-long portion of the surface perpendicular to a rolling direction using electron probe microanalysis (EPMA),
• a tensile strength of the steel sheet is 1,320 MPa or more,
• a ratio R/t of a limit bending radius (R) to a thickness (t) of the steel sheet is 2.0 or less,
• tensile strength×total elongation of the steel sheet is 15,000 MPa·% or more, and
• tensile strength×hole expansion ratio of the steel sheet is 50,000 MPa·% or more.