Gas-carburized steel part excellent in surface fatigue strength, steel product for gas carburizing, and manufacturing method of gas-carburized steel part

1. A gas-carburized steel part comprising:

a gas-carburized layer containing C: 0.50 mass % to 0.87 mass % on a surface thereof and

a base metal, wherein the base metal contains, in mass %:

C: 0.1 to 0.29%,

Si: greater than 1.5% to 4.0%,

Mn: 0.2 to 3.0%,

Cr: 0.5 to 5.0%,

Al: 0.005 to 0.1%,

S: 0.001 to 0.3%,

N: 0.003 to 0.03%,

O: limited to 0.0050% or less,

P: limited to 0.025% or less, and

the balance being Fe and unavoidable impurities;

wherein:

Expression (1) is satisfied in the base metal, and

the gas-carburized steel part has an alloy shortage layer satisfying Expression (2) below in a range from the surface to 2 to 50 μm in depth;

32≧3.5[Si %]+[Mn %]+3[Cr %]>9;  Expression (1):

3.5[Si %]+[Mn %]+3[Cr %]≦9;  Expression (2):

wherein [Si %], [Mn %], and [Cr %] in Expressions (1) and (2) represent contents of Si, Mn, and Cr, respectively, in mass %.

2. The gas-carburized steel part according to claim 1 , wherein the base metal further contains, in mass %, one or more selected from the group consisting of:

Nb: 0.01 to 0.3%,

Ti: 0.01 to 0.3%, and

V: 0.01 to 0.3%.

3. The gas-carburized steel part according to claim 1 , wherein the base metal further contains, in mass %, one or more selected from the group consisting of:

Ni: 0.2 to 3.0%,

Cu: 0.2 to 3.0%,

Co: 0.2 to 3.0%,

Mo: 0.05 to 0.4%,

W: 0.05 to 0.4%, and

B: 0.0006 to 0.005%.

4. The gas-carburized steel part according to claim 1 , wherein the base metal further contains, in mass %:

one or more selected from the group consisting of:

Nb: 0.01 to 0.3%,

Ti: 0.01 to 0.3%, and

V: 0.01 to 0.3%,

and one or more selected from the group consisting of:

Ni: 0.2 to 3.0%,

Cu: 0.2 to 3.0%,

Co: 0.2 to 3.0%,

Mo: 0.05 to 0.4%,

W: 0.05 to 0.4%, and

B: 0.0006 to 0.005%.

5. The gas-carburized steel part according to claim 1 , wherein the base metal contains, in mass %, Si: 1.70 to 4.0%.

6. A method for manufacturing the gas-carburized steel part of claim 1 ,

the manufacturing method comprising:

performing primary carburizing in which a steel product for gas carburizing is subjected to a heat treatment under an atmosphere where an oxide layer is formed;

removing the oxide layer formed on a surface; and

then, performing secondary carburizing in a carburizing atmosphere.

7. A method for manufacturing the gas-carburized steel part of claim 2 , the manufacturing method comprising:

performing primary carburizing in which a steel product for gas carburizing is subjected to a heat treatment under an atmosphere where an oxide layer is formed;

removing the oxide layer formed on a surface; and

then, performing secondary carburizing in a carburizing atmosphere.

8. A method for manufacturing the gas-carburized steel part of claim 3 , the manufacturing method comprising:

performing primary carburizing in which a steel product for gas carburizing is subjected to a heat treatment under an atmosphere where an oxide layer is formed;

removing the oxide layer formed on a surface; and

then, performing secondary carburizing in a carburizing atmosphere.

9. A method for manufacturing the gas-carburized steel part of claim 4 , the manufacturing method comprising:

performing primary carburizing in which a steel product for gas carburizing is subjected to a heat treatment under an atmosphere where an oxide layer is formed;

removing the oxide layer formed on a surface; and

then, performing secondary carburizing in a carburizing atmosphere.