High strength steel sheet, high strength member, and methods for manufacturing the same

A high strength steel sheet having a chemical composition in mass % containing:

• C:
• 0.06% or more and 0.14% or less,
• Si:
• 0.1% or more and 1.5% or less,
• Mn:
• 1.4% or more and 2.2% or less,
• P:
• 0.05% or less,
• S:
• 0.0050% or less,
• Al:
• 0.01% or more and 0.20% or less,
• Nb:
• 0.015% or more and 0.060% or less,
• further containing one of the following (I) to (III);
• (I) N:
• 0.10% or less,
• Ti:
• 0.001% or more and 0.025% or less, and
• optionally containing B:
• 0.0001% or more and less than 0.002%,
• (II) N:
• 0.0035% or less,
• Ti:
• 0.001% or more and 0.030% or less, and
• optionally containing B:
• 0.0001% or more and less than 0.002%, and,
• (III) N:
• 0.10% or less,
• Ti:
• 0.001% or more and 0.030% or less, and
• B:
• 0.0001% or more and less than 0.002%,
• optionally containing one or more of following (A) to (B);
• (A) one of, or two or more of
• Cr:
• 0.01% or more and 0.15% or less,
• Mo:
• 0.01% or more and less than 0.10%, and
• V:
• 0.001% or more and 0.065% or less,
• (B) one of or two of,
• Cu:
• 0.001% or more and 0.2% or less, and
• Ni:
• 0.001% or more and 0.1% or less,
• contents of S, N and Ti satisfying Formula (1) below,
• a balance being Fe and an inevitable impurity,
• comprising, in terms of area fraction relative to an entire steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total of pearlite, bainite and retained austenite,
• a total content of Nb and Ti contained in a precipitate having a particle size of smaller than 20 nm being 25 mass ppm or more and 220 mass ppm or less, and
• a difference between a maximum value and a minimum value of the total content of Nb and Ti contained in the precipitate having a particle size of smaller than 20 nm, in a longitudinal direction of the steel sheet, being smaller than 20 mass ppm,
• [% Ti]−(48/14)[% N]−(48/32)[% S]≤0 Formula (1):
• in Formula (1), [% Ti] represents content (mass %) of component element Ti, [% N] represents content (mass %) of component element N, and [% S] represents content (mass %) of component element S.