Carburized shaft part

A carburized shaft part comprising, at a 3 mm depth from an outer circumferential surface or inside deeper than that, by mass %,

• C:
• 0.10 to 0.30%,
• Si:
• 0.01 to 0.30%,
• Mn:
• 0.4 to 2.0%,
• P:
• 0.050% or less,
• S:
• 0.005 to 0.020%,
• Cr:
• 0.4 to 3.5%,
• Al:
• 0.010 to 0.050%,
• N:
• 0.005 to 0.025%,
• O:
• 0.003% or less, and
• a balance of Fe and impurities,
• optionally further comprising, by mass %,
• Pb:
• 0.5% or less,
• one or more elements selected from the group consisting of V, Nb, and Ti:
• 0.1% or less in total content,
• one or more elements selected from the group consisting of Mo:
• 3.0% or less and Ni:
• 2.5% or less,
• Cu:
• 0 to 0.50%, and
• B:
• 0 to 0.020%,
• satisfying formula (1) and formula (2),
• having a C content of a surface layer part (Cs) by mass % of 0.60 to 1.00%,
• having at least one hole at said outer circumferential surface,
• having a total volume ratio (α′+γ) of martensite and retained austenite of 97% or more at a structure at a position of a 1 mm depth from said outer circumferential surface in an axial direction of said hole and at a position of a 20 μm depth from the surface of the hole,
• having a maximum retained austenite volume ratio (R1) of 10.0 to 30.0% at a position of a 1 mm depth from said outer circumferential surface in an axial direction of said hole and in a range of up to a 200 μm depth from the surface of the hole, and
• having a retained austenite reduction rate (Δγ) of 20% or more found by a formula (A) from said R1 and a volume ratio (R2) of retained austenite at a position of a 1 mm depth from said outer circumferential surface in an axial direction of said hole and at a position of a 20 μm depth from the surface of said hole:
• 1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Ni≥2.35 (1)
• 11.3≤−0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Ni≤33.8 (2)
• where, the contents (mass %) of the elements are entered in the notations of the elements in formula (1) and formula (2) and 0 is entered in the case where the elements are not included.
• Δγ=(R1−R2)/R1×100 (A)