Steel for surface hardening for machine structural use and part for machine structural use

A steel part for machine structural use, said steel part being nitrided, then induction hardened at less than 900° C. and tempered at 300° C., wherein

• the steel part has a steel composition that comprises, in mass %,
• C:
• 0.3 to 0.6%,
• Si:
• 0.02 to 2.0%,
• Mn:
• 0.35 to less than 1.5%, and
• Al:
• 0.01 to 0.5%,
• is restricted to
• B:
• less than 0.0002%,
• S:
• 0.0001 to 0.021%,
• N:
• 0.003 to 0.0055%,
• P:
• 0.0001 to 0.03%, and
• O:
• 0.0001 to 0.0050%,
• Fe:
• balance and
• unavoidable impurities,
• wherein a ratio Mn/S is 70 to 30,000;
• and,
• said steel part has a core of non-nitrided steel and a nitrided layer on the steel part surface,
• wherein said nitrided layer has a thickness of 0.2 mm to 0.42 mm, a martensite microstructure, a Vicker's hardness of 650 to 994, and holes with 0.1 to 1 μm in an equivalent circle diameter on a scale of 5,133 to 47,635/mm2;
• and,
• wherein said core has a ferrite-pearlite microstructure.