High-strength steel sheet having excellent moldability and impact resistance, and method for manufacturing high-strength steel sheet having excellent moldability and impact resistance

A high-strength steel sheet excellent in formability and impact resistance, the high-strength steel sheet comprising a chemical composition comprising:

• by mass %,
• C in a range from 0.080 to 0.500%;
• Si of 2.50% or less;
• Mn in a range from 0.50 to 5.00%;
• P of 0.100% or less;
• S of 0.0100% or less;
• Al in a range from 0.001 to 2.000%;
• N of 0.0150% or less;
• O of 0.0050% or less;
• and
• a balance comprising Fe and inevitable impurities,
• and in a steel sheet satisfying a formula (1),
• the high-strength steel sheet comprising a microstructure in a region from ⅛t (t:
• sheet thickness) to ⅜t (t:
• sheet thickness) from a steel sheet surface, the microstructure comprising:
• by volume %,
• 20% or more of acicular ferrite;
• 20% or more of an island-shaped hard structure comprising one or more of martensite, tempered martensite, and residual austenite,
• 2% to 25% of residual austenite;
• 20% or less of aggregated ferrite;
• and
• 5% or less of pearlite and/or cementite in total, wherein
• in the island-shaped hard structure, an average aspect ratio of a hard region having an equivalent circle diameter of 1.5 μm or more is 2.0 or more, and an average aspect ratio of a hard region having an equivalent circle diameter of less than 1.5 μm is less than 2.0, and
• an average of a number density per unit area of the hard region having the equivalent circle diameter of less than 1.5 μm is equal to or more than 1.0×1010 pieces·m−2, and when the number density of the island-shaped hard structure in an area of at least 5.0×10−10 m2 in each of three view fields is obtained, a ratio between a maximum number density and a minimum number density thereof is 2.5 or less,
• [Si]+0.35[Mn]+0.15[Al]+2.80[Cr]+0.84[Mo]+0.50[Nb]+0.30[Ti]≥1.00 (1)
• [element]:
• mass % of each element.