TECHNICAL FIELD
[0001] The present invention relates to a crankshaft.
BACKGROUND ART
5 [0002] Some crankshafts are subjected to surface hardening by
high-frequency quenching or soft nitriding to improve fatigue strength
and wear resistance.
[0003] JP Hei1(1989)-303309 A discloses a method of manufacturing a
crankshaft including: separately producing a shaft, a bush, a flywheel,
10 a pin and a web; welding them together to an integral product;
quenching and tempering; and machining.
[0004] JP Hei7(1995)-34134 A discloses a method of surface-treating a
crankshaft including: forming a crankshaft using a non-heat-treated
steel with reduced carbon content, performing high-frequency
15 quenching and tempering to achieve a hardness of HV 400 to below 500
after hardening; and performing surface plastic working such that the
hardness of the surface-hardened layer is HV 500 to 700.
[0005] Japanese Patent No. 4332070 discloses a high-strength steel for
a large forging and a large crankshaft made of such a steel. This
20 publication states that the steel for a forging is to be heated to 870 °C,
cooled at a cooling rate of 20 °C/min (quenching), and then tempered at
580 to 630 °C.
[0006] WO 2017/56896 A1 discloses a roughly shaped crankshaft, a
nitrided crankshaft and a method of manufacturing them. This
25 publication describes quenching at a temperature not lower than the
temperature 10 °C lower than the A3 point, and tempering at a
temperature not lower than 550 °C and not higher than the A1 point.
PRIOR ART DOCUMENTS
30 PATENT DOCUMENTS
[0007] [Patent Document 1] JP Hei1(1989)-303309 A
[Patent Document 2] JP Hei7(1995)-34134 A
[Patent Document 3] Japanese Patent No. 4332070
[Patent Document 4] WO 2017/56896 A1
35

3
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0008] A crankshaft is not only required to have fatigue strength and
wear resistance, but also seizure resistance. In recent years,
5 lubricating oils with lower viscosities and crankshafts with frictional
portions constituted by thinner shafts have been developed to improve
fuel efficiency; accordingly, even better seizure resistance is required of
a crankshaft.

We claim:
1. A crankshaft comprising a journal and a pin, having a
chemical composition of, in mass %:
5 0.35 to 0.40 % C;
up to 0.70 % Si;
1.00 to 2.00 % Mn;
up to 0.50 % Cr;
up to 0.050 % Al;
10 up to 0.020 % N;
up to 0.020 % P;
0.005 to 0.200 % S; and
balance Fe and impurities,
for each of the journal and the pin, a microstructure of a surface
15 containing tempered martensite in not less than 80 volume % and a
hardness of the surface being not higher than HV 450.
2. The crankshaft according to claim 1, further comprising a
fillet,
20 wherein a hardness of the fillet from a surface down to a depth
of 2.0 mm is not lower than HV 580.
3. The crankshaft according to claim 2, wherein a
microstructure of the fillet from the surface down to the depth of 2.0
25 mm contains martensite in not less than 80 volume %.
4. The crankshaft according to any one of claims 1 to 3, wherein
the hardness of the surface of each of the journal and the pin is not
higher than HV 250.
30
5. The crankshaft according to any one of claims 1 to 4, wherein
the microstructure of the surface of each of the journal and the pin
further contains cementite in not less than 10 volume %.
35 6. The crankshaft according to any one of claims 1 to 5, wherein

17
a surface roughness Ra of each of the journal and the pin is not higher
than 0.100 μm.