Patent ID: 8802005

Claim:
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/mm 2 ; and, wherein said core has a ferrite-pearlite microstructure.