Case hardening steel with excellent fatigue properties

1. A case hardening steel comprising as a chemical composition, by mass %:

C: 0.10% to 0.40%;

Si: 0.01% to 0.80%;

Mn: 0.1% to 1.5%;

Cr: 0.35% to 2.0%;

Al: 0.01% to 0.05%;

REM: 0.0001% to 0.050%;

O: 0.0001% to 0.0030%;

Ti: less than 0.005%;

N: 0.015% or less;

P: 0.03% or less;

S: 0.01% or less; and

the remainder comprising Fe and impurities,

wherein the case hardening steel includes a composite inclusion which is an inclusion containing REM, O, S, and Al, to which TiN is adhered, and

the sum of a number density of TiN having a maximum diameter of 1 μm or more which independently exists without adhesion to the inclusion, and a number density of MnS having a maximum diameter of 10 μm or more is 5 pieces/mm 2 or less.

2. A case hardening steel comprising as a chemical composition, by mass %:

C: 0.10% to 0.40%;

Si: 0.01% to 0.80%;

Mn: 0.1% to 1.5%;

Cr: 0.35% to 2.0%;

Al: 0.01% to 0.05%;

Ca: 0.0050% or less;

REM: 0.0001% to 0.050%;

O: 0.0001% to 0.0030%;

Ti: less than 0.005%;

N: 0.015% or less;

P: 0.03% or less;

S: 0.01% or less; and

the remainder comprising Fe and impurities,

wherein the case hardening steel includes a composite inclusion which is an inclusion containing REM, Ca, O, S, and Al, to which TiN is adhered, and

the sum of a number density of TiN having a maximum diameter of 1 μm or more which independently exists without adhesion to the inclusion, and a number density of MnS having a maximum diameter of 10 μm or more is 5 pieces/mm 2 or less.

3. The case hardening steel according to claim 1 or 2 , further comprising as the chemical composition, one or more kinds of elements selected from the group consisting of, by mass %;

V: 0.70% or less;

Mo: 1.00% or less;

W: 1.00% or less;

Ni: 3.50% or less;

Cu: 0.50% or less;

Nb: less than 0.050%; and

B: 0.0050% or less.