Surface layer-hardened steel part and method of manufacturing the same

1. A surface layer-hardened steel part treated with a carburizing treatment or a carbonitriding treatment which includes a nitriding treatment at any given point in time during a total carburizing treatment time, a chemical composition of a steel of a non-carburized portion of the steel part comprising, by mass %:

C: 0.10% to 0.60%;

Si: 0.01% to 2.50%;

Mn: 0.20% to 2.00%;

S: 0.0001% to 0.100%;

Cr: 2.00% to 5.00%;

Al: 0.001% to 0.50%;

N: 0.0020% to 0.0200%;

P: 0.001% to 0.050%;

O: 0.0001% to 0.0030%; and

the balance consisting of iron and unavoidable impurities, wherein

a total amount of Cr, Si, and Mn of the non-carburized portion of the steel satisfies, by mass %, 2.0≦Cr+Si+Mn≦8.0;

in a surface layer portion from the topmost surface to 50 μm in depth of a carburized layer, an area ratio of a carbide in the surface layer portion is 15% or more;

the surface layer portion contains at least one precipitate of (Cr,Fe) 23 C 6 and (Cr,Fe) 3 C of 100 pieces or more per 100 μm 2 ; and

a reticular carbide of 10 μm or more along a prior γ austenite grain boundary is 2.5 pieces/mm 2 or less in the surface layer portion.

2. The surface layer-hardened steel part according to claim 1 , wherein the chemical composition of the non-carburized portion of the steel further comprises at least one of, by mass %:

Ca: 0.0005% to 0.0030%;

Mg: 0.0005% to 0.0030%; and

Zr: 0.0005% to 0.0030%.

3. The surface layer-hardened steel part according to claim 1 or 2 , wherein the chemical composition of the non-carburized portion of the steel further comprises at least one of, by mass %:

Mo: 0.01% to 1.00%;

B: 0.0005% to 0.0050%;

Cu: 0.05% to 1.00%; and

Ni: 0.05% to 2.00%.

4. The surface layer-hardened steel part according to claim 1 or 2 , wherein the chemical composition of the non-carburized portion of the steel further comprises at least one of, by mass %:

V: 0.005% to 0.50%;

Nb: 0.005% to 0.10%; and

Ti: 0.005% to 0.50%.

5. The surface layer-hardened steel part according to claim 3 , wherein the chemical composition of the non-carburized portion of the steel further comprises at least one of, by mass %:

V: 0.005% to 0.50%;

Nb: 0.005% to 0.10%; and

Ti: 0.005% to 0.50%.

6. A method for producing a surface layer-hardened steel part, the method comprising:

a shape forming process of forming the steel part out of a steel for a surface layer hardening treatment, wherein a chemical composition of the steel comprises, by mass %,

C: 0.10% to 0.60%,

Si: 0.01% to 2.50%,

Mn: 0.20% to 2.00%,

S: 0.0001% to 0.10%,

Cr: 2.00% to 5.00%,

Al: 0.001% to 0.50%,

N: 0.0020% to 0.020%,

P: 0.001% to 0.050%,

O: 0.0001% to 0.0030%,

the balance consisting of iron and unavoidable impurities, and

a total amount of Cr, Si, and Mn satisfies, by mass %, 2.0≦Cr+Si+Mn≦8.0;

a carburizing process of precipitating at least one precipitate of (Cr,Fe) 23 C 6 and (Cr,Fe) 3 C of 100 pieces or more per 100 μm 2 in a carburized layer of the steel part in a temperature range of 800° C. to 850° C. by a gas carburizing treatment in which a carbon potential of a carburizing atmosphere keeps 0.90% or more through 50% or more of a total carburizing time or by a gas carbonitriding treatment in which a process to penetrate a nitrogen actively into the surface layer is added at any given point in time during the gas carburizing treatment in which the carbon potential keeps 0.90% or more through 50% or more of the total carburizing treatment time; and

a quenching process of quenching from a temperature at which the gas carburizing treatment or the gas carbonitriding treatment are conducted or quenching just after holding at a temperature which is below the carburizing temperature, following the carburizing process.

7. The method for producing the surface layer-hardened steel part according to claim 6 , wherein the chemical composition further comprises at least one of, by mass %:

Ca: 0.0005% to 0.0030%;

Mg: 0.0005% to 0.0030%; and

Zr: 0.0005% to 0.0030%.

8. The method for producing the surface layer-hardened steel part according to claim 6 or 7 , wherein the chemical composition further comprises at least one of, by mass %:

Mo: 0.01% to 1.00%;

B: 0.0005% to 0.0050%;

Cu: 0.05% to 1.00%; and

Ni: 0.05% to 2.00%.

9. The method for producing the surface layer-hardened steel part according to claim 6 or 7 , wherein the chemical composition further comprises at least one of, by mass %:

V: 0.005% to 0.50%;

Nb: 0.005% to 0.10%; and

Ti: 0.005% to 0.50%.

10. The method for producing the surface layer-hardened steel part according to claim 6 or 7 , wherein the method further comprises: prior to the carburizing process,

a pre-carburizing process in which the gas carburizing treatment or the gas carbonitriding treatment is performed on the steel part in a temperature range of 800° C. to 1100° C. under a condition where the carbon potential of the carburizing atmosphere is 0.40% to 1.20%.

11. The method for producing the surface layer-hardened steel part according to claim 8 , wherein the chemical composition further comprises at least one of, by mass %:

V: 0.005% to 0.50%;

Nb: 0.005% to 0.10%; and

Ti: 0.005% to 0.50%.

12. The method for producing the surface layer-hardened steel part according to claim 8 , wherein the method further comprises: prior to the carburizing process,

a pre-carburizing process in which the gas carburizing treatment or the gas carbonitriding treatment is performed on the steel part in a temperature range of 800° C. to 1100° C. under a condition where the carbon potential of the carburizing atmosphere is 0.40% to 1.20%.

13. The method for producing the surface layer-hardened steel part according to claim 9 , wherein the method further comprises: prior to the carburizing process,

a pre-carburizing process in which the gas carburizing treatment or the gas carbonitriding treatment is performed on the steel part in a temperature range of 800° C. to 1100° C. under a condition where the carbon potential of the carburizing atmosphere is 0.40% to 1.20%.

14. The method for producing the surface layer-hardened steel part according to claim 11 , wherein the method further comprises: prior to the carburizing process,

a pre-carburizing process in which the gas carburizing treatment or the gas carbonitriding treatment is performed on the steel part in a temperature range of 800° C. to 1100° C. under a condition where the carbon potential of the carburizing atmosphere is 0.40% to 1.20%.