Steel sheet, member, and methods for producing the same

A steel sheet comprising a chemical composition containing, in mass %,

• C:
• 0.10% or more and 0.30% or less,
• Si:
• 0.20% or more and 1.20% or less,
• Mn:
• 2.5% or more and 4.0% or less,
• P:
• 0.050% or less,
• S:
• 0.020% or less,
• Al:
• 0.10% or less,
• N:
• 0.01% or less,
• Ti:
• 0.100% or less,
• Nb:
• 0.002% or more and 0.050% or less,
• B:
• 0.0015% or more and 0.0040% or less, and
• the balance being Fe and incidental impurities,
• wherein the chemical composition satisfies formula (1) below,
• a total area fraction of martensite and bainite is 95% or more,
• a retained austenite area fraction is 5% or less,
• a ferrite area fraction is 1% or less,
• a prior-austenite grain size is 10 μm or less,
• a C concentration in a prior-austenite grain boundary is equal to or more than 1.5 times a C content in steel,
• a B concentration in the prior-austenite grain boundary is 0.05% or more in mass %, and
• a variation in B concentration within the same prior-austenite grain boundary is less than 0.010% in mass %,
• (
• [
• %
• ⁢
• N
• ]
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• 14
• )
• /
• (
• [
• %
• ⁢
• Ti
• ]
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• 47.9
• )
• <
• 1.
• formula
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• (
• 1
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• in formula (1), [% N] and [% Ti] respectively represent a N content and a Ti content (mass %) in steel.