Nitrided steel part and method of production of same

1. A nitrided steel part comprising a steel material as a material, the steel material consisting of, by mass %,

C: 0.2 to 0.6%,

Si: 0.05 to 1.5%,

Mn: 0.2 to 2.5%,

P: 0.025% or less,

S: 0.003 to 0.05%,

Cr: 0.05 to 0.5%,

Al: 0.01 to 0.05%,

N: 0.003 to 0.025%,

optionally one or more of

Mo: 0.01 to less than 0.50%,

V: 0.01 to less than 0.50%,

Cu: 0.01 to less than 0.50%,

Ni: 0.01 to less than 0.50%, and

Ti: 0.005 to less than 0.05%, and

a balance of Fe and impurities,

the surface structure of the nitrided steel part is mainly composed of ferrite phase,

the nitrided steel part comprising a compound layer of a thickness of 3 μm or less comprising iron, nitrogen, and carbon formed on the steel surface and a hardened layer formed under the compound layer,

an effective hardened layer depth of the nitrided steel part being 160 to 410 μm,

wherein

the effective hardened layer depth is a depth where a Vickers hardness measured in the depth direction from the surface of the nitrided steel part, using a hardness distribution in the depth direction of the Vickers hardness measured in accordance with JIS Z 2244, is 250HV or more; and

a ratio of voids in an area of 25 μm 2 in a range of 5 μm depth from an outermost surface of the steel part is less than 10%.

2. A method of nitriding production of a nitrided steel part, comprising

using as a material a steel material consisting of, by mass %,

C: 0.2 to 0.6%,

Si: 0.05 to 1.5%,

Mn: 0.2 to 2.5%,

P: 0.025% or less,

S: 0.003 to 0.05%,

Cr: 0.05 to 0.5%,

Al: 0.01 to 0.05%,

N: 0.003 to 0.025%,

optionally one or more of

Mo: 0.01 to less than 0.50%,

V: 0.01 to less than 0.50%,

Cu: 0.01 to less than 0.50%,

Ni: 0.01 to less than 0.50%, and

Ti: 0.005 to less than 0.05%, and

a balance of Fe and impurities,

gas nitriding by heating the steel material in a gas atmosphere containing NH 3 , H 2 , and N 2 to 550 to 620° C., and making the overall treatment time A 1.5 to 10 hours,

the gas nitriding comprised of high K N value treatment having a treatment time of X hours and a low K N value treatment after the high K N value treatment having a treatment time of Y hours,

the high K N value treatment having a nitriding potential K NX determined by formula (1) of 0.15 to 1.50 and having an average value K NXave of the nitriding potential K NX determined by formula (2) of 0.30 to 0.80, and

the low K N value treatment having a nitriding potential K NY determined by formula (3) of 0.02 to 0.25, having an average value K NYave of the nitriding potential K NY determined by formula (4) of 0.03 to 0.20 and having an average value K Nave of the nitriding potential determined by formula (5) of 0.07 to 0.30:

K NX =( NH 3 partial pressure) X /[( H 2 partial pressure) 3/2 ] X   (1)

K NXave =Σ i=1 n ( X 0 ×K NXi )/ X   (2)

K NY =( NH 3 partial pressure) Y /[( H 2 partial pressure) 3/2 ] Y   (3)

K NYave =Σ i=1 n ( Y 0 ×K NYi )/ Y   (4)

K Nave =( X×K NXave +Y×K NYave )/ A   (5)

wherein, in formula (2) and formula (4), the subscript “i” is a number indicating the number of measurements for each constant time interval, X 0 indicates the measurement interval (hours) of the nitriding potential K NX , Y 0 indicates the measurement interval (hours) of the nitriding potential K NY , K NXi indicates the nitriding potential at the i-th measurement during the high K N value treatment, and K Nyi indicates the nitriding potential at the i-th measurement during the low K N value treatment,

wherein, the surface structure of the nitrided steel part is mainly composed of ferrite phase.

3. The method of production of the nitrided steel part of claim 2 wherein the gas atmosphere includes a total of 99.5 vol % of NH 3 , H 2 , and N 2 .