High-strength steel sheet and method for producing same

A high-strength steel sheet having a chemical composition comprising, by mass %:

• C:
• 0.05% to 0.15%,
• Si:
• 1.6% or less,
• Mn:
• 1.3% to 2.4%,
• P:
• 0.05% or less,
• S:
• 0.0050% or less,
• Al:
• 0.01% to 0.10%,
• N:
• 0.010% or less,
• Ti:
• 0.005% to 0.10%,
• B:
• 0.0002% to 0.004%, and
• at least one of Mo:
• 0.005% to 0.50% and Cr:
• 0.005% to 1.0%,
• the balance being Fe and unavoidable impurities;
• and
• a multi-phase structure comprising, by volume fraction, ferrite in a range of 35% to 65%, residual austenite in a range of 7% or less (including 0%), martensite in a range of 20% to 60%, and bainite in a range of 20% or less (including 0%),
• wherein the ferrite has an average crystal grain size of 8 μm or less and an average aspect ratio of 3.5 or less,
• the martensite has an average crystal grain size of 3 μm or less,
• the bainite has an average crystal grain size of 3 μm or less when contained, and
• a standard deviation of Vickers hardness of the steel sheet is 30 or less when measured at 200 μm intervals from a position of 100 μm from a surface to a central portion in a thickness direction of the steel sheet.