Steel for machine structure use for surface hardening and steel part for machine structure use

1. A steel part for machine structure use, containing, by mass %,

C: 0.30 to 0.6%,

Si: 0.02 to 2.0%,

Mn: 1.5% to 3.0%,

W: 0.0025 to 0.5%,

Cr: 0.01 to 2.0%,

Mo: 0.01 to 1.0%,

V: 0.01 to 1.0%,

Al: 0.001 to 0.5%,

N: 0.003 to 0.02%,

S: 0.0001 to 0.025%,

P: 0.0001 to 0.03%,

O: 0.0001 to 0.005%,

Fe: balance, and

unavoidable impurities,

wherein an Mn/S is 70 to 30000,

wherein said steel part is machined, nitrided, and then induction hardened, and

wherein the surface layer of the steel part from the surface down to a depth of 0.4 mm or more is a nitrided layer and the hardness of the nitrided layer from the surface down to a depth of 0.2 mm has a Vicker's hardness of 650 or more if subject to tempering at 300° C.

2. The steel part for machine structure use as set forth in claim 1 , characterized in that said steel is nitrided by soft nitriding.

3. The steel part for machine structure use as set forth in claim 1 , characterized in that said nitrided layer from the surface down to a depth of 5 μm or more includes pores of a circle equivalent diameter of 0.1 to 1 μm in an amount of 10000/mm 2 or more.

4. The steel part for machine structure use as set forth in claim 1 , characterized by said steel further containing, by mass %, one or more of

B: 0.0005 to 0.005%,

Nb: 0.005 to 0.3%,

Ti: 0.005 to 0.2%,

Ni: 0.05 to 2.0%,

Cu: 0.01 to 2.0%,

Ca: 0.0005 to 0.01%,

Mg: 0.0005 to 0.01%,

Zr: 0.0005 to 0.05%, and

Te: 0.0005 to 0.1%.

5. The steel part for machine structure use as set forth in claim 4 , characterized in that said steel is nitrided by soft nitriding.

6. The steel part for machine structure use as set forth in claim 4 , characterized in that said nitrided layer from the surface down to a depth of 5 μm or more includes pores of a circle equivalent diameter of 0.1 to 1 μm in an amount of 10000/mm 2 or more.