A steel sheet comprising: a component composition containing, on a percent by mass basis: C: 0.04% or more and 0.18% or less, Si: 0.6% or less, Mn: 1.5% or more and 3.2% or less, P: 0.05% or less, S: 0.015% or less, Al: 0.08% or less, N: 0.0100% or less, Ti: 0.010% or more and 0.035% or less, B: 0.0002% or more and 0.0030% or less, optionally one or two or more of: Cr: 0.001% or more and 0.8% or less, Mo: 0.001% or more and 0.5% or less, Sb: 0.001% or more and 0.2% or less, and Nb: 0.001% or more and 0.1% or less and optionally 1.0% or less in total of one or more of REM, Cu, Ni, V, Sn, Mg, Ca, and Co, the balance being Fe and incidental impurities; and a steel microstructure having an area percentage of a ferrite phase of 20% or more and 80% or less and an area percentage of a martensite phase of 20% or more and 80% or less, the area percentage being determined by microstructure observation, wherein a surface layer portion of the steel sheet has an average ferrite grain size of 5.0 μm or less and an inclusion density of 200 particles/mm 2 or less, and wherein the steel sheet has a surface hardness of 95% or more when the steel sheet has a hardness of 100% at a position 1/2t (where t represents a thickness of the steel sheet) away from a surface of the steel sheet in a thickness direction, and the steel sheet has a tensile strength of 780 MPa or more, measured in accordance with JIS Z 2241 on JIS No. 5 tensile test pieces sampled in a direction perpendicular to the rolling direction, and wherein the average ferrite grain size and the inclusion density are determined in accordance with the respective measurement methods described in the description. a component composition containing, on a percent by mass basis: C: 0.04% or more and 0.18% or less, Si: 0.6% or less, Mn: 1.5% or more and 3.2% or less, P: 0.05% or less, S: 0.015% or less, Al: 0.08% or less, N: 0.0100% or less, Ti: 0.010% or more and 0.035% or less, B: 0.0002% or more and 0.0030% or less, optionally one or two or more of: Cr: 0.001% or more and 0.8% or less, Mo: 0.001% or more and 0.5% or less, Sb: 0.001% or more and 0.2% or less, and Nb: 0.001% or more and 0.1% or less and optionally 1.0% or less in total of one or more of REM, Cu, Ni, V, Sn, Mg, Ca, and Co, the balance being Fe and incidental impurities; and C: 0.04% or more and 0.18% or less, Si: 0.6% or less, Mn: 1.5% or more and 3.2% or less, P: 0.05% or less, S: 0.015% or less, Al: 0.08% or less, N: 0.0100% or less, Ti: 0.010% or more and 0.035% or less, B: 0.0002% or more and 0.0030% or less, Cr: 0.001% or more and 0.8% or less, Mo: 0.001% or more and 0.5% or less, Sb: 0.001% or more and 0.2% or less, and Nb: 0.001% or more and 0.1% or less and a steel microstructure having an area percentage of a ferrite phase of 20% or more and 80% or less and an area percentage of a martensite phase of 20% or more and 80% or less, the area percentage being determined by microstructure observation, wherein a surface layer portion of the steel sheet has an average ferrite grain size of 5.0 μm or less and an inclusion density of 200 particles/mm 2 or less, and wherein the steel sheet has a surface hardness of 95% or more when the steel sheet has a hardness of 100% at a position 1/2t (where t represents a thickness of the steel sheet) away from a surface of the steel sheet in a thickness direction, and the steel sheet has a tensile strength of 780 MPa or more, measured in accordance with JIS Z 2241 on JIS No. 5 tensile test pieces sampled in a direction perpendicular to the rolling direction, and wherein the average ferrite grain size and the inclusion density are determined in accordance with the respective measurement methods described in the description.