Steel part for machine structural use and manufacturing method thereof
View Patent ↗The present invention provides a steel part for machine structural use whose fatigue strength and toughness are improved and a manufacturing method thereof. A steel part made of a steel containing, in mass %, C: 0.05 to 0.20%, Si: 0.10 to 1.00%, Mn: 0.75 to 3.00%, P: 0.001 to 0.050%, S: 0.001 to 0.200%, V: exceeding 0.20 to 0.25%, Cr: 0.01 to 1.00%, Al: 0.001 to 0.500%, and N: 0.0080 to 0.0200%, and a balance being composed of Fe and inevitable impurities, in which a steel structure contains a bainite structure having an area ratio of 95% or more, a bainite lath width is 5 μm or less, V carbide having an average grain diameter of not less than 4 nm nor more than 7 nm dispersedly exists in the bainite structure, and an area ratio of V carbide in the bainite structure is 0.18% or more.
1. A steel part for machine structural use made of a steel consisting of,
in mass %,
C: 0.05 to 0.20%,
Si: 0.10 to 0.40%,
Mn: 0.75 to 3.00%,
P: 0.001 to 0.050%,
S: 0.001 to 0.200%,
V: exceeding 0.20 to 0.25%,
Cr: 0.01 to 0.35%,
Al: 0.001 to 0.500%, and
N: 0.0080 to 0.0200%, and
optionally Ca: 0.0003 to 0.0100%,
optionally Mg: 0.0003 to 0.0100%,
optionally Zr: 0.0005 to 0.1000%,
optionally Mo: 0.01 to 0.18%, and
optionally Nb: 0.001 to 0.200%,
a balance being composed of Fe and inevitable impurities, wherein
a steel structure contains a bainite structure having an area ratio of 95% or more,
a bainite lath width is 5 μM or less,
V carbide having an average grain diameter of not less than 4 nm nor more than 7 nm dispersedly exists in the bainite structure, and
an area ratio of V carbide in the bainite structure is 0.18% or more.
2. The steel part for machine structural use according to claim 1 , wherein Charpy absorbed energy at 20° C. is 80 J/cm 2 or more and an endurance ratio is 0.60 or more.
3. A manufacturing method of a steel part for machine structural use, comprising: heating a steel product consisting of, in mass %, C: 0.05 to 0.20%, Si: 0.10 to 0.40% Mn: 0.75 to 3.00%, P: 0.001 to 0.050%, S: 0.001 to 0.200%, V: exceeding 0.20 to 0.25%, Cr: 0.01 to 0.35%, Al: 0.001 to 0.500%, and N: 0.0080 to 0.0200%, and optionally Ca: 0.0003 to 0.0100%, optionally Mg: 0.0003 to 0.0100%, optionally Zr: 0.0005 to 0.1000%, optionally Mo: 0.01 to 0.18%, and optionally Nb: 0.001 to 0.200%, a balance being composed of Fe and inevitable impurities to not lower than 1100° C. nor higher than 1300° C. and hot forging the steel product; after said hot forging, cooling the hot-forged steel product at an average cooling rate down to 300° C. set to be not less than 3° C./second nor more than 120° C./second; and after said cooling, performing an aging treatment within a temperature range of not lower than 550° C. nor higher than 700° C., wherein the steel part contains a bainite structure having an area ratio of 95% or more, a bainite lath width is 5 μm or less, V carbide having an average grain diameter of not less than 4 nm nor more than 7 nm dispersedly exists in the bainite structure, and an area ratio of V carbide in the bainite structure is 0.18% or more.