Steel sheet and plated steel sheet

A steel sheet, comprising:

• a chemical composition represented by, in mass %,
• C:
• 0.008 to 0.150%,
• Si:
• 0.01 to 1.70%,
• Mn:
• 0.60 to 2.50%,
• Al:
• 0.010 to 0.60%,
• Ti:
• 0 to 0.200%,
• Nb:
• 0 to 0.200%,
• Ti+Nb:
• 0.015 to 0.200%,
• Cr:
• 0 to 1.0%,
• B:
• 0 to 0.10%,
• Mo:
• 0 to 1.0%,
• Cu:
• 0 to 2.0%,
• Ni:
• 0 to 2.0%,
• Mg:
• 0 to 0.05%,
• REM:
• 0 to 0.05%,
• Ca:
• 0 to 0.05%,
• Zr:
• 0 to 0.05%,
• P:
• 0.05% or less,
• S:
• 0.0200% or less,
• N:
• 0.0060% or less, and
• balance:
• Fe and impurities;
• and
• a structure represented by, by area ratio,
• ferrite:
• 0 to 30%, and
• bainite:
• 70 to 100%, wherein
• a region that is surrounded by a grain boundary having a misorientation of 15° or more and has a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, and wherein the proportion of crystal grains each having an intragranular misorientation of 5 to 14° to all crystal grains is 20 to 100% by area ratio,
• a grain boundary number density of solid-solution C or a grain boundary number density of the total of solid-solution C and solid-solution B is 1 piece/nm2 or more and 4.5 pieces/nm2 or less, and
• an average grain size of cementite precipitated at grain boundaries is 2 μm or less.