Steel sheet

1. A steel sheet in which chemical components of a steel include, by mass %:

0.5% to 0.8% of C;

0.15% to 0.60% of Si;

0.40% to 0.90% of Mn;

0.010% to 0.070% of Al;

0.001% to 0.010% of Ti;

0.30% to 0.70% of Cr;

0.0005% to 0.0030% of Ca;

0.0003% to 0.0050% of REM;

0.020% or less of P;

0.0070% or less of S;

0.0040% or less of O; and

0.0075% or less of N, the balance composed of Fe and unavoidable impurities,

wherein the amounts of the respective elements in the chemical components, which are represented by mass %, satisfy the following Expression 1 and Expression 2, and

the steel contains a Ti-included-carbonitride as an inclusion, and a number density of the Ti-included-carbonitride having a long side of 5 μm or more is 3 pieces/mm 2 or less;

0.3≦{Ca/40.88+(REM/140)/2}(S/32.07)  (Expression 1)

Ca≦0.005−0.0035×C  (Expression 2).

2. The sheet according to claim 1 ,

wherein the chemical components further include at least one selected from a group consisting of, by mass %,

0% to 0.05% of Cu,

0% to 0.05% of Nb,

0% to 0.05% of V,

0% to 0.05% of Mo,

0% to 0.05% of Ni, and

0% to 0.0050% of B.

3. The steel sheet according to claim 1 or 2 ,

wherein the steel further contains a composite inclusion including Al, Ca, O, S, and REM, and an inclusion in which the Ti-included-carbonitride is attached to a surface of the composite inclusion.

4. The steel sheet according to claim 3 ,

wherein the amounts of the respective elements in the chemical components, which are represented by mass %, satisfy the following Expression 3;

18×(REM/140)−O/16≧0  (Expression 3).

5. The steel sheet according to claim 1 or 2 ,

wherein the amounts of the respective elements in the chemical components, which are represented by mass %, satisfy the following Expression 4;

18×(REM/140)−O/16≧0  (Expression 4).