High-strength steel sheet and method for producing the same

A high-strength steel sheet that has a component composition comprising, on a mass percent basis:

• C:
• 0.07% to 0.25%,
• Si:
• 0.01% to 1.80%,
• Mn:
• 1.8% to 3.2%,
• P:
• 0.05% or less,
• S:
• 0.02% or less,
• Al:
• 0.01% to 2.0%, and
• N:
• 0.01% or less;
• at least one of
• B:
• 0.0001% to 0.005%,
• Ti:
• 0.005% to 0.04%, and
• Nb:
• 0.005% to 0.06%;
• and
• a balance being Fe and incidental impurities,
• that has a steel microstructure in which an area percentage of ferrite ranges from 5% to 50% in a thickness cross-section in a rolling direction, a total area percentage of fresh martensite and retained γ ranges from 2% to 30%, each of the fresh martensite and the retained γ has an average grain size of 5 μm or less, and a ratio of the fresh martensite and the retained γ adjacent only to ferrite with respect to the fresh martensite and the retained γ from a surface to 200 μm in the thickness direction is 30% or less in total area percentage, and
• that has a yield strength of 550 MPa or more.