Steel sheet and member, and production methods therefor

A steel sheet comprising:

• a chemical composition containing, in mass %,
• C:
• 0.05% or more and 0.20% or less,
• Si:
• 0.1% or more and 1.8% or less,
• Mn:
• 1.5% or more and 3.0% or less,
• P:
• 0.001% or more and 0.100% or less,
• S:
• 0.0500% or less,
• Al:
• 0.010% or more and 1.000% or less,
• N:
• 0.0100% or less, and
• one or both of Nb and Ti:
• 0.005% or more and 0.200% or less in total,
• with a balance consisting of Fe and inevitable impurities;
• a steel microstructure in which an area ratio of one or both of ferrite and bainite is 10% or more and 87% or less in total, an area ratio of martensite is 10% or more and 87% or less, an area ratio of retained austenite is 3% or more, a number density of grains constituting the retained austenite is 0.05 grains/μm2 or more, grains with an aspect ratio of 3 or less among the grains constituting the retained austenite satisfy the following (A), (B), (C), and (D), and [Mn]C/[Mn] is 1.05 or more and 2.00 or less, where [M]C is an average Mn concentration in mass % in a Mn-enriched region in steel and [Mn] is an average Mn concentration in mass % in the steel;
• and
• a tensile strength of 780 MPa or more,
• (A) a ratio of the grains with an aspect ratio of 3 or less to all of the grains constituting the retained austenite is 60% or more in area ratio,
• (B) an average C concentration of the grains with an aspect ratio of 3 or less is 0.3 mass % or more,
• (C) an average value of a shortest distance between the grains with an aspect ratio of 3 or less is 5 μm or less,
• (D) a maximum value of the shortest distance between the grains with an aspect ratio of 3 or less is 15 μm or less.