Steel sheet for heat treatment

1. A steel sheet for heat treatment having a chemical composition comprising, by mass %:

C: 0.05 to 0.50%;

Si: 0.50 to 5.0%;

Mn: 1.5 to 4.0%;

P: 0.05% or less;

S: 0.05% or less;

N: 0.01% or less;

Ti: 0.01 to 0.10%;

B: 0.0005 to 0.010%;

Cr: 0 to 1.0%;

Ni: 0 to 2.0%;

Cu: 0 to 1.0%;

Mo: 0 to 1.0%;

V: 0 to 1.0%;

Ca: 0 to 0.01%;

Al: 0 to 1.0%;

Nb: 0 to 1.0%;

REM: 0 to 0.1%; and

the balance: Fe and impurities, wherein

a maximum height roughness Rz on a surface of the steel sheet is 3.0 to 10.0 μm,

carbides are present in the steel sheet, and a number density of the carbides, each having a circle-equivalent diameter of 0.1 μm or larger, is 8.0×10 3 /mm 2 or lower, and

a Mn segregation degree α expressed by a following formula (1) is 1.6 or lower:

α=[Maximum Mn concentration (mass %) at sheet-thickness center portion]/[Average Mn concentration (mass %) in ¼ sheet-thickness depth position from surface]  (i).

2. The steel sheet for heat treatment according to claim 1 , wherein the chemical composition contains, by mass %, one or more elements selected from:

Cr: 0.01 to 1.0%;

Ni: 0.1 to 2.0%;

Cu: 0.1 to 1.0%;

Mo: 0.1 to 1.0%;

V: 0.1 to 1.0%;

Ca: 0.001 to 0.01%;

Al: 0.01 to 1.0%;

Nb: 0.01 to 1.0%; and

REM: 0.001 to 0.1%.

3. The steel sheet for heat treatment according to claim 1 , wherein an index of cleanliness of steel specified in JIS G 0555(2003) is 0.10% or lower.

4. The steel sheet for heat treatment according to claim 2 , wherein an index of cleanliness of steel specified in JIS G 0555(2003) is 0.10% or lower.