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

A steel part for machine structure use obtained by machining steel for machine structure use for surface hardening into a steel part, said steel 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%,
• Al:
• 0.001 to 0.5%,
• N:
• 0.003 to 0.02%,
• S:
• 0.0001 to 0.025%,
• P:
• 0.0001 to 0.03%, and
• O:
• 0.0001 to 0.005%,
• having an Mn/S of 70 to 30000, and
• having a balance of substantially Fe and unavoidable impurities,
• nitriding the steel part, then induction hardening the steel part,
• wherein said steel part for machine structure use is characterized in that the surface layer 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 is made a Vicker's hardness of 650 or more if subject to tempering at 300° C.