Steel for surface hardening for machine structural use and part for machine structural use
View Patent ↗The present invention is steel for surface hardening for machine structural use which contains, by 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 0.0003%, 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%, has a ratio Mn/S of Mn and S satisfying 70 to 30,000, has a balance of Fe and unavoidable impurities, and, when nitrided, then induction hardened, has a surface hardenability of a Vicker's hardness when tempered at 300° C. of 650 or more.
1. 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.
2. The steel part for machine structural use as claimed in claim 1 , wherein the hardened nitrided layer has a thickness of 0.4 mm or more.
3. The steel part for machine structural use as claimed in claim 1 , wherein the steel part is nitrided at a temperature in the range of 500 to 600° C.