High-strength steel sheet, high-strength galvanized steel sheet, method for manufacturing high-strength steel sheet, and method for manufacturing high-strength galvanized steel sheet

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

• C:
• 0.07% to 0.30%,
• Si:
• 0.10% to 2.5%,
• Mn:
• 1.8% to 3.7%,
• P:
• 0.03% or less,
• S:
• 0.0020% or less,
• Sol. Al:
• 0.01% to 1.0%,
• N:
• 0.0006% to 0.0055%,
• O:
• 0.0008% to 0.0025%, and the balance being Fe and inevitable impurities,
• wherein a Mn-segregation degree in a region within 100 μm from a surface of the steel sheet in a thickness direction is 1.5 or less,
• in a plane parallel to the surface of the steel sheet in a region within 100 μm from the surface of the steel sheet in the thickness direction, the number of oxide-based inclusion grains having a grain long diameter of 5 μm or more is 1000 or less per 100 mm2,
• a proportion of the number of oxide-based inclusion grains having a chemical composition containing alumina in an amount of 50 mass % or more, silica in an amount of 20 mass % or less, and calcia in an amount of 40 mass % or less to the total number of oxide-based inclusion grains having a grain long diameter of 5 μm or more is 80% or more,
• a metallographic structure including, in terms of volume fraction, a martensite phase and a bainite phase in an amount of 25% to 100% in total, a ferrite phase in an amount of less than 75% (including 0%), and an austenite phase in an amount of less than 15% (including 0%), and
• a tensile strength of 980 MPa or more.