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

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

• C:
• 0.030% or more and 0.500% or less,
• Si:
• 0.01% or more and 2.50% or less,
• Mn:
• 0.10% or more and 5.00% or less,
• P:
• 0.100% or less,
• S:
• 0.0200% or less,
• Al:
• 0.100% or less,
• N:
• 0.0100% or less,
• O:
• 0.0100% or less, and
• Ti:
• 0.002% or more and 0.200% or less,
• with the balance being Fe and incidental impurities,
• wherein an effective Ti mole fraction (xTi,eff) determined by formula (1) below is 0.001 or more,
• wherein the high strength steel sheet has, at a position ¼ of a thickness, a steel microstructure
• in which an area fraction of martensite is 80% or more and 99% or less and
• in which a sum of an area fraction of ferrite and/or a volume fraction of retained austenite is more than 0% and 20% or less,
• wherein, at a position 5 μm from a surface of the steel sheet in a thickness direction, a concentration of Si is 0.10% or more and 0.60% or less,
• wherein a number density of MnS present in Mn-segregated regions on the surface of the steel sheet is 5.0 pieces/mm2 or less,
• wherein a standard deviation of Vickers hardness of the surface of the steel sheet is 15 or less, and
• wherein a hardness variation frequency per 1100 μm in a sheet width direction on the surface of the steel sheet is 7 or less:
• x
• Ti
• ,
• eff
• =
• x
• Ti
• -
• x
• N
• -
• x
• S
• ,
• (
• 1
• )
• where xTi, xN, and xS are contents (mole fractions) of respective elements in the steel sheet.