Steel sheet, member, and methods for manufacturing same

A steel sheet, comprising:

• a chemical composition containing, in mass %:
• C:
• 0.05% to 0.25%,
• Si:
• 0.30% to 1.50%,
• Mn:
• 1.5% to 4.5%,
• P:
• 0.005% to 0.050%,
• S:
• 0.01% or less,
• sol. Al:
• less than 1.0%, and
• N:
• less than 0.015%,
• formula (1) below being satisfied, and
• the balance being iron and incidental impurities;
• and
• a steel microstructure having:
• an area fraction of polygonal ferrite:
• 10% or more and 80% or less,
• a total area fraction of upper bainite, tempered martensite, and lower bainite:
• 10% or more and 70% or less,
• a volume fraction of retained austenite:
• 3% or more and 15% or less, and
• an area fraction of quenched martensite:
• 15% or less (including 0%),
• wherein a maximum concentration of P [Pm] within 1 μm from a surface of the steel sheet in a thickness direction is 0.025 mass % or more, and formula (2) is satisfied:
• [
• Si
• ]
• /
• [
• M
• ⁢
• n
• ]
• ≤
• 0.35
• ,
• formula
• ⁢
• (
• 1
• )
• [
• Pm
• ]
• /
• [
• P
• ]
• ≥
• 1.5
• formula
• ⁢
• (
• 2
• )
• where in formula (1), [Si] is a Si content (mass %), and [Mn] is a Mn content (mass %), and
• in formula (2), [P] is a P content (mass %).