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%, and
one or more of
W: 0.0025 to 0.5%,
Cr: 0.2 to 2.0%,
Mo: 0.05 to 1.0%,
V: 0.05 to 1.0%,
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%,
Te: 0.0005 to 0.1%, and
a balance of Fe 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 hardened nitrided layer on a surface of the steel part,
wherein said steel part has holes having an equivalent circle diameter of 0.1 to 1 μm down to a depth of at least 5 μm from a surface of said hardened nitrided layer present on a scale of 5,133 to 47,635/mm 2 ; and
wherein the hardened nitrided layer has a Vicker's hardness of 650 or more and a thickness of 0.2 mm or more at its surface.