High-strength steel sheet with excellent ductility and hole-expandability

A steel sheet including, in mass %, C: 0.05% or more and 0.30% or less, Si: 0.05% or more and 6.00% or less, Mn: 1.50% or more and 10.00% or less, and the balance: Fe and impurities, a steel sheet structure is composed of, in area ratio, 15% or more and 80% or less of ferrite and 20% or more and 85% or less in total of a hard structure composed of any one of bainite, martensite, or retained austenite, or any combination thereof, and to a steel sheet thickness t, an area ratio of a maximum coupled ferrite region in a region from a t/2 position at the steel sheet thickness center to a position at a depth of 3t/8 from a surface is 80% or more in area ratio to a total ferrite area, and a two-dimensional isoperimetric constant of the maximum coupled ferrite region is 0.35 or less.