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

Claim 12:
13. The method according to claim 9 for manufacturing 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 the 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,
an average of a number density per unit area (hereinafter also simply referred to as “the number density”) 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, and

[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,
the high-strength steel sheet comprises a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet, the method comprising:
immersing the high-strength steel sheet dwelling in the temperature region in the range from 550 degrees C. to 300 degrees C. in a plating bath comprising zinc as a main component to form the galvanized layer or the zinc alloy plated layer on one surface or both surfaces of the steel sheet.