High-strength steel sheet and method for producing same

A high-strength steel sheet comprising:

• a chemical composition containing, in mass %,
• C:
• 0.120% or more and 0.250% or less,
• Si:
• 0.80% or more and 2.00% or less,
• Mn:
• 1.50% or more and 2.45% or less,
• P:
• 0.001% or more and 0.100% or less,
• S:
• 0.0200% or less,
• Al:
• 0.010% or more and 1.000% or less,
• N:
• 0.0100% or less,
• Mo:
• 0.500% or less,
• Cr:
• 0.300% or less,
• Ca:
• 0.0200% or less, and
• Sb:
• 0.200% or less,
• with a balance being Fe and inevitable impurities, wherein Mneq calculated according to the following formula (1) is 2.40% or more and 3.40% or less,
• Mneq=0.26×[% Si]+[% Mn]+3.5×[% P]+2.68×[% Mo]+1.29×[% Cr] (1)
• where [% Si], [% Mn], [% P], [% Mo], and [% Cr] denotes content in mass % of Si, Mn, P, Mo, and Cr in steel, respectively, and is 0 in the case where each of Si, Mn, P, Mo, and Cr is not contained;
• a steel microstructure in which an area ratio of ferrite is 15% or more and 55% or less, an area ratio of hard phase which is any of bainitic ferrite, bainite, tempered martensite, and quenched martensite is 40% or more and 85% or less, a volume fraction of retained austenite is 4% or more and 20% or less, a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less, where the high-temperature tensile test is performed such that after heating a test piece to 900° C. at 100° C./s, the test piece immediately gas-cooled at 40° C./s and after reaching 700° C., a tensile test immediately conducted at a crosshead rate of 50 mm/s until the test piece fractured;
• and
• a tensile strength of 980 MPa or more.