Case hardening steel and manufacturing method thereof

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

C: 0.1% to 0.5%,

Si: 0.01% to 1.5%

Mn: 0.3% to 1.8%,

S: 0.001% to 0.15%,

Cr: 0.4% to 2.0%,

Ti: 0.05% to 0.2%,

Al: limited to 0.2% or less,

N: limited to 0.0050% or less,

P: limited to 0.025% or less,

O: limited to 0.0025% or less, and

a balance of Fe and inevitable impurities,

wherein a number d of a sulfide having an equivalent circle diameter more than 5 μm per 1 mm 2 and a mass percentage [S] of S satisfy: d≦500×[S]+1.

2. The case hardening steel according to claim 1 , further comprising, by mass %, as the chemical composition, at least one selected from:

Nb: less than 0.04%,

Mo: 1.5% or less,

Ni: 3.5% or less,

V: 0.5% or less,

B: 0.005% or less,

Ca: 0.005% or less,

Mg: 0.003% or less, and

Zr: 0.005% or less.

3. The case hardening steel according to claim 2 , wherein an [Al]/[Ca] which is a ratio of a mass percentage [Al] of Al to a mass percentage [Ca] of Ca is 1 or more and 100 or less.

4. The case hardening steel according to claim 1 or 2 , wherein a maximum equivalent circle diameter D μm of the sulfide and the mass percentage [S] of S satisfy: D≦250×[S]+10.

5. The case hardening steel according to claim 1 or 2 , wherein an amount of Mn is 1.0% or less, and a [Mn]/[S] which is a ratio of a mass percentage [S] of S to a mass percentage [Mn] of Mn is 100 or less.

6. The case hardening steel according to claim 1 or 2 , wherein a ratio of bainite is 30% or less in a microstructure.

7. The case hardening steel according to claim 1 or 2 , wherein a maximum equivalent circle diameter of Ti-based precipitates is 40 μm or less.

8. A method of manufacturing a case hardening steel in which a number d of a sulfide having an equivalent circle diameter more than 5 μm per 1 mm 2 and a mass percentage [S] of S satisfy: d≦500×[S]+1, the method comprising:

casting a steel having a chemical composition which contains: by mass %, C: 0.1% to 0.5%, Si: 0.01% to 1.5%, Mn: 0.3% to 1.8%, S: 0.001% to 0.15%, Cr: 0.4% to 2.0%, Ti: 0.05% to 0.2%, Al: limited to 0.2% or less, N: limited to 0.0050% or less, P: limited to 0.025% or less, O: limited to 0.0025% or less, and the balance of iron and inevitable impurities, at an average cooling rate of 12 to 100° C./min;

soaking the steel for 3 to 180 min in a soaking temperature range of 1250° C. to 1320° C.;

hot-rolling the steel so that a finish rolling is performed in a finishing temperature range of 840° C. to 1000° C. after heating the steel in a temperature range of 1150° C. to 1320° C.; and

cooling the steel so that an average cooling rate in a temperature range of 800° C. to 500° C. is 1° C./s or less.

9. The method of manufacturing the case hardening steel according to claim 8 , wherein the chemical composition further contains: by mass %, at least one selected from: Nb: less than 0.04%, Mo: 1.5% or less, Ni: 3.5% or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, Mg: 0.003% or less, and Zr: 0.005% or less.

10. The method of manufacturing the case hardening steel according to claim 9 , wherein an [Al]/[Ca] which is a ratio of a mass percentage [Al] of Al to a mass percentage [Ca] of Ca is 1 or more and 100 or less.

11. The method of manufacturing the case hardening steel according to claim 8 or 9 , wherein an amount of Mn is 1.0% or less, and a [Mn]/[S] which is a ratio of a mass percentage [S] of S to a mass percentage [Mn] of Mn is 100 or less.

12. The case hardening steel according to claim 1 , wherein S: 0.010% to 0.15%.

13. The method of manufacturing the case hardening steel according to claim 8 , wherein S: 0.010% to 0.15%.