Carburized shaft part
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 μm 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 μm 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 μm depth from the surface of the hole by the formula (A): Δγ=(R1−R)/R1×100.
1. 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=( R 1− R 2)/ R 1×100 (A)
2. The carburized shaft part according to claim 1 , wherein said R2 is 20% or less.
3. The carburized shaft part according to claim 1 , wherein the carburized shaft part has a plastic flow layer at the surface of said hole.
4. The carburized shaft part according to claim 3 , wherein said plastic flow layer has a thickness of 0.5 to 15 μm.
5. The carburized shaft part according to claim 2 , wherein the carburized shaft part has a plastic flow layer at the surface of said hole.