STEEL SHEET

1 . A steel sheet comprising as a chemical composition, by mass %,

C: 0.18% to 0.275%;

Si: 0.02% to 0.15%;

Mn: 1.85% to 2.75%;

sol.Al: 0 . 0002 % to 0.5%;

Cr: 0.05% to 1.00%;

B: 0.0005% to 0.01%;

P: 0.1% or less;

S: 0.0035% or less;

N: 0.01% or less;

Ni: 0% to 0.15%;

Cu: 0% to 0.05%;

Ti: 0% to 0.1%;

Nb: 0% to 0.2%; and

a remainder including Fe and impurities,

wherein a cleanliness of a metal structure is 0.08% or less,

α which is an Mn segregation degree expressed by the following expression 1 is 1.6 or less, and

in a hot forming, a difference ΔHv in an average hardness after a hot forming between a low strain formed portion that undergoes a plastic strain of 5% or less and a high strain formed portion that undergoes a plastic strain of 20% or higher is 40 or less.

α=(a maximum Mn concentration, by mass %, in a thickness center portion of the steel sheet)/(an average Mn concentration, by mass %, at a position at a depth of ¼ of a sheet thickness from a surface of the steel sheet) . . . expression 1.

2 . The steel sheet according to claim 1 , wherein the chemical composition further includes, instead of a portion of Fe, by mass %, one or two selected from the group consisting of Ni: 0.02% to 0.15%, and Cu: 0.003% to 0.05%.

3 . The steel sheet according to claim 1 or 2 , wherein the chemical composition further includes, instead of a portion of Fe, by mass %, one or two selected from the group consisting of Ti: 0.005% to 0.1%, and Nb: 0.005% to 0.2%.

4 . The steel sheet according to claim 1 or 2 , wherein the surface of the steel sheet further includes a coated layer.

5 . The steel sheet according to claim 3 , wherein the surface of the steel sheet further includes a coated layer.