Nippon Steel Corporation

Koji Watari

Tatsuya Iwasaki

Hidekazu Sueno

A carburized shaft part having a predetermined composition, a C content at a surface layer part of a mass % of 0.60 to 1.00%, at least one hole at an outer circumferential surface, a total volume ratio of martensite and retained austenite of 97% or more at a structure at a position of a 1 mm depth from the outer circumferential surface in an axial direction of the hole and a position of a 20 &#x3bc;m depth from the surface of the hole, a maximum retained austenite volume ratio (R1) of 10.0 to 30.0% at a position of a 1 mm depth from the outer circumferential surface in the axial direction of the hole and a range up to a 200 &#x3bc;m depth from the surface of the hole, and a retained austenite reduction ratio of 20% or more found from R1 and the retained austenite volume ratio (R2) at a position of a 1 mm depth from the outer circumferential surface in the axial direction of the hole and a position of a 20 &#x3bc;m depth from the surface of the hole by the formula (A): &#x394;&#x3b3;=(R1&#x2212;R)/R1&#xd7;100.

Carburized shaft part

A shaft part excellent in static torsional strength and torsional fatigue strength containing, by mass %, essential elements of C: 0.35 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.5 to 2.6%, P: 0.050% or less, S: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, and O: 0.003% or less, further containing optional elements, having a balance of Fe and impurities, having a chemical composition satisfying formula (1), having at least one hole at an outer circumferential surface, having a volume ratio (R1) of 4 to 20% of retained austenite at a position of a 2 mm depth from the outer circumferential surface, having a volume ratio of retained austenite at a position of a 2 mm depth from the outer circumferential surface in an axial direction of the hole and at a position of a 20 &#x3bc;m depth from the surface of the hole as R2, and having a reduction rate &#x394;&#x3b3; of 40% or more of retained austenite found by the formula (A): &#x394;&#x3b3;=[(R1&#x2212;R2)/R1]&#xd7;100: Formula (1): 15.0&#x2264;25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni&#x2264;27.2 (Notations of elements in formula are contents of the elements).

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Tokyo, Japan

Koji Watari