Patent ID: 11885025
Assignee: NIPPON STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C | IPC C

Claim 0:
1. 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.