Steel sheet

A steel sheet comprising:

• a base iron;
• a scale of 10.0 μm or less in thickness on a surface of the base iron;
• and
• a subscale between the base iron and the scale,
• wherein the base iron comprises a chemical composition represented by, in mass %,
• C:
• 0.05% to 0.20%,
• Si:
• 0.01% to 1.50%,
• Mn:
• 1.50% to 2.50%,
• P:
• 0.05% or less,
• S:
• 0.03% or less,
• Al:
• 0.005% to 0.10%,
• N:
• 0.008% or less,
• Cr:
• 0.30% to 1.00%,
• Ti:
• 0.06% to 0.20%,
• Nb:
• 0.00% to 0.10%,
• V:
• 0.00% to 0.20%,
• B:
• 0.0000% to 0.0050%,
• Cu:
• 0.00% to 0.50%,
• Ni:
• 0.00% to 0.50%,
• Mo:
• 0.00% to 0.50%,
• W:
• 0.00% to 0.50%,
• Ca:
• 0.0000% to 0.0050%,
• Mg:
• 0.0000% to 0.0050%,
• REM:
• 0.000% to 0.010%, and
• the balance:
• Fe and impurities,
• wherein, in the base iron, a percentage of an amount of Ti contained in carbide or carbonitride of 100 nm or more and 1 μm or less in grain diameter to a parameter Tieff represented by a following formula 1 is 30% or less, [Ti] denoting a Ti content (mass %) and [N] denoting a N content (mass %) in the following formula 1,
• wherein, in the subscale,
• an average value of Cr concentrations is 1.50 mass % to 5.00 mass %, wherein the average value of Cr concentrations is an average value Ave of maximum values Cmax among 50 or more measurement regions, each of the 50 or more measurement regions is made of 10 measurement points of Cr concentration continually lining up in a rolling direction, and an interval between the measurement points is 0.1 μm, and
• one part or more exist(s) where a ratio of one's maximum value Cmax to the other's maximum value Cmax is 0.90 or less or 1.11 or more between two adjacent measurement regions among the 50 or more measurement regions,
• Tieff=[Ti]−48/14[N] (formula 1).