Steel sheet and plated steel sheet

This steel sheet has a specific chemical composition and is provided with a structure represented by, in terms of area ratio, 0–30% ferrite and 70–100% bainite. When a crystal grain is defined as a region which is surrounded by grain boundaries having a misorientation of 15° or higher and for which the equivalent circle diameter is 0.3 μm or larger, the proportion of crystal grains having an intragranular misorientation of 5–14° relative to all of the crystal grains is 20–100% in terms of area ratio. The grain boundary number density of a solid solution of C, or the total grain boundary number density of a solid solution of C and a solid solution of B is 1 particle/nm 2 to 4.5 particles/nm 2 inclusive. The average particle size of cementite precipitated in the grain boundaries is no larger than 2 μm.