TECHNICAL FIELD
[0001] The present invention relates to a nitrided steel part, more particularly a crankshaft or
other nitrided steel part excellent in bending straightening ability and bending fatigue
characteristic, and a method of production of the same.
BACKGROUND ART
l 0 [0002] Steel parts used in automobiles and various industrial machinery etc. are improved in
fatigue strength, wear resistance, seizing resistance, and other mechanical properties by
carburizing hardening, high-frequency hardening, nitriding, soft nitriding, and other surface
hardening heat treatment.
[0003] Nitriding and soft nitriding are performed in the ferrite region of the A1 point or less.
15 During treatment, there is no phase transformation, so it is possible to reduce the heat treatment
strain. For this reason, nitriding and soft nitriding are often used for parts requiring high
dimensional precision and large sized parts. For example, they are applied to the gears used for
transmission parts in automobiles and the crankshafts used for engines.
[ 0004) Nitriding is a method of treatment diffusing nitrogen into the surface of a steel material.
20 For the medium used for the nitriding, there are a gas, salt bath, plasma, etc. For the transmission
parts of an automobile, gas nitriding is mainly being used since it is excellent in productivity.
Due to gas nitriding, the surface of the steel material is formed with a compound layer of a
thickness of 10 ftm or more. Furthermore, the surface layer of a steel material at the lower side of
the compound layer is formed with a nitrogen diffused layer forming a hardened layer. The
25 compound layer is mainly comprised ofFe2_3N and Fe4N. The hardness of the compound layer
is extremely high compared with the steel of the base material. For this reason, the compound
layer improves the wear resistance and pitting resistance of a steel part in the initial stage of use.
[0005] However, a compound layer is low in touglmess and low in deformability, so
sometimes the compound layer and the base layer peel apart at their interface during use and the
30 strength of the part falls. For this reason, it is difficult to use a gas nitrided part as a part
subjected to impact stress and large bending stress.
[0006] Therefore, for use as a part subjected to impact stress and large bending stress,
reduction of the thickness of the compound layer and, furthermore, elimination of the compound
layer are sought. In this regard, it is known that the thickness of the compound layer can be
35 controlled by the treatment temperature of the nitriding and the nitriding potential KN found
from the NH 3 partial pressure and H2 partial pressure by the following formula:
l
[0007] KN=(NH3 partial pressure)/((H2 partial pressure)
312
]
[0008] !flowering the nitriding potential Kn it is also possible to make the compound layer
thinner and even eliminate the compound layer. However, iflowering the nitriding potential KN,
it becomes hard for nitrogen to diffuse into the steel. In this case, the hardness of the hardened
5 layer becomes lower and the depth becomes shallower. As a result, the nitrided part falls in
fatigue strength, wear resistance, and seizing resistance. To deal with such a drop in
performance, there is the method of mechanically polishing or shot blasting etc. the nitride part
after gas nitriding to remove the compound layer. However, with this method, the production
costs become higher.
I 0 [0009] PLT 1 proposes the method of dealing with such a problem by controlling the
atmosphere of the gas nitriding by a nitriding parameter KN=(NH3 partial pressure)/[(H2 partial
pressure)
1 12
] different from the nitriding potential and reducing the variation in depth of the
hardened layer.
[0010] PLT 2 proposes a gas nitriding method enabling formation of a hardened layer (nitrided
15 layer) without forming a compound layer. The method of PL T 2 first removes the oxide film of a
part by fluoride treatment then nitrides the part. A non-nitriding material is necessary as a fixture
for placing the treated part in a treatment furnace.
[0011] However, the nitriding parameter proposed in PL T 1 may be useful for control of the
depth of the hardened layer, but does not improve the functions of a part.
20 [0012] As proposed in PLT 2, in the case of the method of preparing a non-nitriding fixture
and first performing fluoride treatment, the problems arise of the selection of the fixture and the
increase in the number of work steps.
CITATION LIST
PATENT LITERATURE
25 [0013] PLT 1: Japanese Patent Publication No. 2006-28588A
PLT 2: Japanese Patent Publication No. 2007-31759A
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0014] An object of the present invention is to provide a nitrided steel part excellent in bending
30 straightening ability and bending fatigue characteristic solving the two simultaneously difficult
to solve problems of reduction of the thickness of a low toughness and low deformability
compound layer and increase of the depth of the hardened layer and able to answer the demands
for reduction of the size and decrease of the weight of a part or a higher load capacity ar1d to
provide a nitriding method of the same.
35 SOLUTION TO PROBLEM
[0015] The inventors studied the method of making the compound layer fanned on the surface
2
of the steel material by nitriding thinner and obtaining a deep hardened layer. Furthermore, they
simultaneously studied methods of keeping the nitrogen fi·om fanning a gas and creating voids
near the surface of a steel material at the time of nitriding (in particular, at the time of treatment
by a high KN value). In addition, they investigated the relationship between the nitriding
5 conditions and the bending straightening ability and bending fatigue characteristic. As a result,
the inventors obtained the following findings (a) to (d):
[0016] (a) Regarding KN value in gas nitriding
In general, the KN value is defined by the following formula using the NH3 partial pressure and
the H2 partial pressure in the atmosphere in the furnace performing the gas nitriding (below,
10 referred to as the "nitriding atmosphere" or simply the "atmosphere").
[0017] KN=(NH3 partial pressure)/[(H2 partial pressure)
312
]
[0018] The KN value can be controlled by the gas flow rates. However, a certain time is
required after setting the gas flow rates until the nitriding atmosphere reaches the equilibrium
state. For this reason, the KN value changes with each instant even before the KN value reaches
15 the equilibrinm state. Further, even if changing the KN value in the middle of the gas nitriding,
the KN value fluctuates until reaching the equilibrinm state.
[0019) The above such fluctuation of the KN value has an effect on the compound layer,
surface hardness, and depth of the hardened layer. For this reason, not only the target value of the
KN value, but also the range of variation of the KN value during gas nitriding have to be
20 controlled to within a predetermined range.
[0020] (b) Regarding realization of both suppression offormation of compound layer and
securing surface hardness and depth of hardened layer
In the various experiments conducted by the inventors, the thickness of the compound layer,
voids in the compound layer, surface hardness, and depth of the hardened layer were related to
25 the bending straightening ability and bending fatigue characteristic ofthe nitrided part. If the
compound layer is thick and, further, there are many voids in the compound layer, cracks easily
form starting from the compound layer and the bending straightening ability and bending fatigue
strength fall.
[0021] Further, the lower the surface hardness and the shallower the depth of the hardened
3 0 layer, the more cracks and fractures occur starting from the diffused layer and the more the
bending fatigue strength fails. Furthermore, if the surface hardness is too high, the bending
straightening ability deteriorates. That is, the inventors discovered that if the compound layer is
thin, there arc few voids in the compound layer, and the surface hardness is in a certain range,
and as the depth of the hardened layer increases, the bending straightening ability and the
3 5 bending fatigue characteristic become better.
[0022] From the above, to achieve both a bending straightening ability and bending fatigue
3
characteristic, it is important to prevent the formation of a compound layer as much as possible,
to control the surface hardness to a certain range, and increase the depth of the hardened layer,
[0023] To finally suppress the formation ofthe compound layer and secure the depth of the
hardened layer, it is efficient to form a compound layer once, then break down the formed
5 compound layer and utilize it as a source of supply of nitrogen to the hardened layer.
Specifically, in the first half of the gas nitriding, gas nitriding raising the nitriding potential (high
KN value treatment) is performed to form the compound layer. Further, in the second half of the
gas nitriding, gas nitriding lowered in nitriding potential than the high KN value treatment (low
KN value treatment) is performed, As a result, the compound layer formed in the high KN value
10 treatment is broken down into Fe and N. TheN diffuses, thereby promoting the formation of a
nitrogen diffused layer (hardened layer). Finally, at the nitrided part, it is possible to make the
compound layer thinner, raise the surface hardness, and increase the depth of the hardened layer.
[0024] (c) Regarding suppression offonnation of voids
When nitriding by the high KN value in the first half of the gas nitriding, sometimes a layer
15 including voids (porous layer) is formed in the compound layer (FIG. lA). In this case, even
after the nitrides break down and the nitrogen diffused layer (hardened layer) is formed, voids
remain as they are inside the nitrogen diffused layer. If voids remain inside the nitrogen diffused
layer, the nitrided part falls in fatigue strength. If restricting the upper limit of the KN value when
forming the compound layer in the high KN value treatment, it is possible to suppress the
20 formation of the porous layer and voids (FIG. lB).
[0025] (d) Regarding relationship of components of steel material and compound layer and
nitrogen diffused layer
If Cis present in the steel material, the compound layer easily becomes thicker. Further, ifMn,
Cr, and other nitride compound forming elements are present, the hardness ofthe nitrogen
25 diffused layer and the depth of the diffused layer changes. The bending straightening ability is
improved the thinner the thickness of the compound layer or the lower the surface hardness and
the bending fatigue characteristic is improved the higher the surface hardness or the deeper the
diffused layer, so it becomes necessary to set the optimal range of the steel material components.
[0026] The present:invention was made based on the above discoveries and has as its gist the
30 following:
[0027] [I] 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%, AI: 0.01 to 0.05%, N: 0:003 to 0.025% and a balance of Fe and
impurities, the nitrided steel part comprising a compound layer of a thickness of 3 J..lm or less
35 comprising iron, nitrogen, and carbon formed on the steel surface and a hardened layer formed
under the compoundlayer, an effective hardened layer depth of the nitrided steel part being 160
4
to 410 1-1m.
[0028] [2] The nitrided steel part of [1] wherein the steel material contains, in place of part of
Fe, one or both ofMo: 0.01 to less than 0.50% and V: 0.01 to less than 0.50%.
[0029] [3] The nitrided steel part of [1] or [2] wherein the steel material contains, in place of
5 part of Fe, one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01 to less than 0.50%.
[0030] [4] The nitrided part of any one of [1] to [3] wherein the steel material contains, in place
of part of Fe, Ti1 0.005 to less than 0.05%.
[0031] [5] A method ofnitriding 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
10 0.05%, Cr: 0.05 to 0.5%, AI: 0.01 to 0.05%, N: 0.003 to 0.025% and a balance of Fe and
impmities and gas nitriding by heating the steel material in a gas atmosphere containing NH 3,
H2, and N2 to 550 to 620°C, and making the overall treatment time A 1.5 to 10 hours, the gas
nitriding comprised of high KN value treatment having a treatment time of X hours and a low
KN value treatment after the high KN value treatment having a treatment time of Y hours, the
15 high KN value tJ;eatment having a nitriding potential KNx determined by formula (1) of 0.15 to
1.50 and having an average value KNxave of the nitriding potential KNx determined by formula
(2) of 0.30 to 0.80, the low KN value treatment having a nitriding potential KNY determined by
fonnula (3) of0.02 to 0.25, having an average value KNYave of the nitriding potential KNY
determined by formula ( 4) of0.03 to 0.20 and having an average value KNave of the nitriding
20 potential determined by formula (5) of 0.07 to 0.30:
3/2
KNJF(NH3 partial pressure)x/[(H2 partial pressure) Jx ··· (I)
KNxa"' = L7=,(XoxKNxi)/ X··· (2)
312
KNy=(NH3 partial pressure)y/{(H2 partial pressure) ]y ··· (3)
25 KNave=(XxKNXave+YxKNYaveJIA··· (5)
where, in formula (2) and formula (4), the subscript "i" is a number indicating the number of
measurements for each constant time interval, Xo indicates the measurement interval (hours) of
the nitriding potential KNX, Yo indicates the measurement interval (hours) of the nitriding
potential KNy, KNxi indicates the nitriding potential at the i-th measurement during the high KN
30 value treatment, and KNYi indicates the nitriding potential at the i-th measurement during the
low KN value treatment.
[0032] [6] The method of production of the nitrided steel part of [5] wherein the gas
atmosphere includes a total of99.5 vol% ofNH3, H2, and N2.
[0033] [7] The method of production of the nitrided steel part of [5] or [6] wherein the steel
5
material contains, in place of part of the Fe, one or both ofMo: 0.01 to less than 0.50% and V:
0.01 to less than 0.50%.
(0034] [8] The method of production of the nitrided steel part of any one of [ 5] to [7] wherein
the steel material contains, in place of part of the Fe, one or both ofCu: 0.01 to less than 0.50%
5 and Ni: 0.1)1 to less than 0.50%.
[0035] [9] The method of production of the nitrided part of any one of[5] to [8] wherein the
steel material contains, in place of part of the Fe, Ti: 0.005 to less than 0.05%.
ADVANTAGEOUS EFFECTS OF INVENTION
[0036] According to the present invention, it is possible to obtain a nitrided steel part having a
10 thin compound layer, suppressed fonnation of voids (porous layer), furthermore, certain surface
hardness and a deep hardened layer, and an excellent bending straightening ability and bending
fatigue characteristic.
BRIEF DESCRIPTION OF DRAWINGS
[0037] [FIGS. 1] Views showing a compound layer after nitriding, wherein FIG. lA shows an
15 example of formation of a porous layer containing voids in the compound layer and FIG. IB
shows an example where formation of a porous layer and voids is suppressed.
[FIG. 2] A view showing a relationship of an average value KNxave of a nitriding potential of a
high KN value treatment and a surface hardness and compound layer thickness.
[FIG. 3] A view showing a relationship of an average value KNYave of a nitriding potential of a
20 low KN value treatment and a surface hardness and compound layer thickness.
[FIG. 4] A view showing a relationship of an average value KNave of a nitriding potential and a
surface hardness and compound layer thickness.
[FIG. 5] The shape of a block shaped test piece for static bending test use used for evaluating a
bending straightening ability.
25 [FIG. 6] The shape of a columnar test piece for evaluating a bending fatigue characteristic.
DESCRIPTION OF EMBODIMENTS
[0038] Below, the requirements of the present invention will be explained in detail. First, the
chemical composition of the steel material used as a material win be explained. Below, the "%"
showing the contents of the component elements and concentrations of elements at the part
3 0 surface mean "mass%".
[0039] C: 0.2 to 0.6%
Cis an element required for securing the core hardness of a part. If the content of Cis less than
0.2%, the core strength becomes too low, so the bending fatigue strength greatly falls. Further, if
the content of C exceeds 0.6%, during high KN value treatment, the compound layer thickness
35 easily becomes larger. Further, during low KN value treatment, the compound layer becomes
resistant to brealcdown. For this reason, it becomes difficult to reduce the compound layer
6
thickness after nitriding and the bending straightening ability and bending fatigue strength
greatly fall. The preferable range of the C content is 0.25 to 0.55%.
[0040] Si: 0.05 to 1.5%
Si raises the core hardness by solution strengthening. Further, it is a deoxidizing element. To
5 obtain these effects, 0.05% or more is included. On the other hand, if the content of Si exceeds
1.5%, in bars and wire rods, the strength after hot forging becomes too high, so the machinability
greatly falls. In addition, the bending straightening ability falls. The preferable range of the Si
content is 0.08 to 1.3%.
[0041] Mn: 0.2 to 2.5%
I 0 Mn raises the core hardness by solution strengthening. Furthermore, Mn forms fine nitrides
(Mn3N2) in the hardened layer at the time of nitriding and improves the bending fatigue strength
by precipitation strengthening. To obtain these effects, Mn has to be 0.2% or more. On the other
hand, if the content of Mn exceeds 2.5%, the effect of raising the bending fatigue strength
becomes saturated. Furthermore, the effective hardened layer depth becomes shallower, so the
15 pitting strength and the bending fatigue strength fall. Further, the bars and wire rods used as
materials become too high in hardness after hot forging, so the bending straightening ability and
the machinability greatly fall. The preferable range of the Mn content is 0.4 to 2.3%.
(0042) P: 0.025%or less
P is .an impurity and precipitates at the grain boundaries to make a part brittle, so the content is
20 preferably small. If the content ofP is over 0.025%, sometimes the bending straightening ability
and bending fatigue strength fall. The preferable upper limit of the content of P for preventing a
drop in the bending straightening ability and the bending fatigue strength is 0.018%. It is difficult
to make the content completely zero. The practical lower limit is 0.00 I%.
[004.3] S: 0.003 to 0.05%
25 S bonds with Mn to form MnS and raise the machinability .• J:o obtain this effect, S has to be
0.003% or more. However, if the content ofS exceeds 0.05%, coarse MnS easily forms and the
bending straightening ability and bending fatigue strength greatly fall. The preferable range of
the S content is 0.005 to 0.03%.
[0044] Cr: 0.05 to 0.5%
30 Cr forms fine nitrides (CrN) in the hardened layer during nitriding and improves the bending
fatigne strength by precipitation strengthening. To obtain the effects, Cr has to be 0.5% or more.
On the other hand, if the content of Cr is over 0.5%, the precipitation strengthening ability
becomes saturated. Furthermore, the effective hardened layer depth becomes shallower, so the
pitting strength and bending fatigue strength fall. Further, the bars and wire rods used as
3 5 materials become too high in hardness after hot forging, so the bending straightening ability and
machinability remarkably fall. The preferable range oftheCr content is 0.07 to 0.4%.
7
[0045] AI: 0.01 to 0.05%
' AI is a deoxidizing element. For sufficient deoxidation, 0.01% or more is necessary. On the other
• hand, AI easily forms hard oxide inclusions. If the content of AI exceeds 0.05%, the bending
: fatigue strength remarkably falls. Even if other requirements are met, the desired bending fatigue
5 strength can no longer be obtained. The preferable range of the Al content is 0.02 to 0.04%.
[0046] N: 0.003 to 0.025%
N bonds with AI, V, and Ti to form AIN, VN, and TiN. Due to their actions of pinning austenite
grains, AlN, VN, and TiN have the effect of refining the structure of the steel material before
. nitriding and reducing the variation in mechanical,characteristics of the nitrided steel part. If the
10. content ofN is less than 0.003%, this effect is difficultto obtain. On the other hand, if the
content ofN exceeds 0.025%, coarse AlN easily forms, so the above effect becomes difficult to
obtain. The preferable range of the content ofN is•0.005 to 0.020%.
[0047] The steel used as the material for the nitrided steel part of the present invention may
also contain the elements shown below in addition to the above elements.
15 [0048] Mo: 0.01 to less than 0.50%
Mo forms fine nitrides (Mo2N) in the hardened layer during nitriding and improves the bending
fatigue strength by precipitation strengthening. Further, Mo has the action of age hardening and
improves the core hardness at the time of nitriding. The content of Mo for obtaining these effects
has to be 0.01% or more. On the other hand, if the content ofMo is 0.50% or more, the bars and
20 wire rods used as materials become too high in hardness after hot forging, so the bending
straightening ability and machinability remarkably fall. In addition, the alloy costs increase. The
preferable upper limit of the Mo content is less than 0.40%.
[0049] V: 0.01 to less than 0.50%
· , V forms fine nitrides (VN) at the time of nitriding and improves the bending fatigue strength by
25 precipitation strengthening. Further, V has the action of age hardening to improve the core
hardness at the time of nitriding. Furthermore, due to the action of pinning austenite grains, it
also has the effect of refining the structure of the steel.materia1 before nitriding. To obtain these
actions, V has to be 0.01% or more. On the other hand, if the content ofV is 0.50% or more, the
bars and wire rods used for materials become too high in hardness after hot forging, so the
30 ·. bending straightening ability and machinability remarkably fall. In addition, the alloy costs
increase. The preferable range of content ofV is less than 0.40%.
[0050] Cu: 0.01 to 0.50%
Cu improves the core hardness of the part and the hardness of the nitrogen diffused layer as a
solution strengthening element. To obtain the action of solution strengthening of Cu, inclusion of
3 5 0.0 I% or more is necessary. On the other hand, if the• content of Cu exceeds 0.50%, the bars and
wire rods used as materials become too high in hardness after hot forging, so the bending
8
straightening ability and machinability remarkably fall. In addition, the hot ductility falls.
Therefore, this becomes a cause of surface scratches at the time of hot rolling and at the time of
hot forging. The preferable range of the content of Cu is less than 0.40%.
[0051] Ni: 0.01 to 0.50%
5 Ni improves the core hardness and surface layer hardness by solution strengthening. To obtain
the action of solution strengthening ofNi, inclusion of 0.01% or more is necessary. On the other
hand, if the content ofNi exceeds 0.50%, the bais and wire rods used as materials become too
high in hardness after hot forging, so the bending straightening ability and machinability
remarkably fall. In addition, the alloy costs increase. The preferable range of the Ni content is
l 0 less than 0.40%.
[0052] Ti: 0.005 to 0.05%
Ti bonds with N to form TiN and improve the core hardness and surface layer hardness. To
obtain this action, Ti has to be 0.005% or more. On the other hand, if the content ofTi is 0.05%
or more, the effect of improving the core hardness and surface layer hardness becomes saturated.
15 In addition, the alloy costs increase. The preferable range of content of Ti is 0.007 to less than
0.04%.
[0053] The balance of the steel is Fe and impurities. "Impurities" mean components which are
contained in the starting materials or mixed in dUring the process of production and not
components which are intentionally included in the steeL The above optional added elements of
20 Mo, V, Cu, Ni, and Ti are sometimes included in amounts of less than the above lower limits, but
in this case, just the effects of the elements explained above are not sufficiently obtained. The
effect of improvement of the pitting resistance and bending fatigue characteristic of the present
invention is obtained, so this is not a problem.
[0054] Below, the method of production of the nitrided steel part of the present invention will
25 be explained. The method of production explained below is just one example. The nitrided steel
part of the present invention need only have a thickness of the compound layer of 3 J.tm or less
and an effective hardened layer depth of 160 to 410 J.tm. It is not limited to the following method
of production.
[0055] In the method of production of the nitrided steel part of the present invention, steel
30 having the above-mentioned components is gas nitrided. The treatment temperature of the gas
nitriding is 550 to 620°C, while the treatment time A ofthe gas nitriding as a whole is 1.5 to 10
hours.
[0056] Treatment Temperature: 550 to 620°C
The temperature of the gas nitriding (nitriding temperature) is mainly correlated with the rate of
35 diffusion of nitrogen and affects the surface hardness and depth of the hardened layer. If the
nitriding temperature is too low, the rate of diffusion of nitrogen is slow, the surface hardness
9
becomes low, and the depth of the hardened layer becomes shallower. On the other hand, if the
nitriding temperature is over the Ac1 point, austenite phases (y phases) with a smaller rate of
diffusion of nitrogen than ferrite phases (a phases) are formed in the steel, the surface hardness
becomes lower, and the depth of the hardened layer becomes shallower. Therefore, in the present
5 embodiment, the nitriding temperature is 550 to 620°C around the ferrite temperature region. In
this case, the surface hardness can be kept from becoming lower and the depth of the hardened
layer can be kept from becoming shallower.
(0057] Treatment Time A of Gas Nitriding as a Whole: 1.5 to I 0 Hours
The gas nitriding is performed in an atmosphere including NH3, H2, and N2. The time of the
10 nitriding as a whole, that is, the time from the start to end of the nitriding (treatment time A), is
correlated with the formation and breakdown of the compound layer and the diffusion of
nitrogen and affects the surface hardness and depth of the hardened layer. If the treatment time A
is too short, the surface hardness becomes lower and the depth of the hardened layer becomes .
shallower. On the other hand, if the treatment time A is too long, the nitrogen is removed and the
15 surface hardness of the steel falls. If the treatment time A is too long, further, the manufacturing
costs rise. Therefore, the treatment time A of the nitriding as a whole is 1.5 to 10 hours.
[0058J Note that, the atmosphere ofthe gas nitriding of the present embodiment includes not
only NH3, H2, and N2 but also unavoidable impurities such as oxygen and carbon dioxide. The
preferable atmosphere is NH3, H2, and N2 in a total of99.5% (vol%) or more. The later
20 explained KN value is calculated from the ratio of the NH3 and H2 partial pressures in the
atmosphere, so is not affected by the magnitude of the N2 partial pressure. However, to raise the
stability ofKN control, the N2 partial pressure is preferably 0.2 to 0.5 atm.
[0059] High K~ Value Treatment and Low KN Value Treatment
The above-mentioned gas nitriding includes a step of performing high KN value treatment and a
25 step of performing low KN value treatment. In high KN value treatment, gas nitriding is
performed by a nitriding potential KNx higher than the low KN value treatment. Furthermore,
after high KN value treatment, low KN value treatment is performed. In the low KN value
treatment, gas nitriding is performed by a nitriding potential KNY lower than the high KN value
treatment.
30 [0060] In this way, in the present nitriding method, two-stage gas nitriding (high KN value
treatment and low KN value treatment) is performed. By raising the nitriding potential KN value
in the first half of the gas nitriding (high KN value treatment), a compound layer is formed at the
surface of the steel. After that, by lowering the nitriding potential KN value in the second half of
the gas nitriding (low KN value treatment), the compound layer formed at the surface of the steel
35 is broken down into Fe and Nand the nitrogen (N) is made to penetrate and diffuse in the steel.
By the two-stage gas nitriding, the thickness of the compound layer formed by the high KN value
10
treatment is reduced while the nitrogen obtained by breakdown of the compound layer is used to
obtain a sufficient depth ofthe hardened layer.
[0061] The nitriding potential of the high KN value treatment is denoted as KNx, while the
nitriding potential of the low KN value treatment is denoted as KNY· At this time, the nitriding
5 potentials KNx and KNY are defined by the following formula:
[0062] KNx~(NH3 partial pressure)x/[(H2 partial pressure)
312
]x
KNy~(NH3 partial pressure)y/[(H2 partial pressure)
312
]y
[0063] The partial pressures ofthe NH3 and H2 in the atmosphere of the gas nitriding can be
controlled by adjusting the flow rates of the gases.
10 [0064] When shifting from the high KN value treatment to the low KN value treatment, if
adjusting the flow rates of the gases to lower the KN value, a certain extent of time is required
until the partial pressures ofNH3 and Hz in the furnace stabilize. The gas flow rates can'be
adjusted for changing the KN value one time or if necessary several times. To increase the
amount of drop of the KN value more, the method oflowering the NH3.flow rate and raising the
15 H2 flow rate is effective. The point of time when the KNi value after high KN value treatment
finally becomes 0.25 or less is defmed as the start timing of the low KN value treatment.
[0065] The treatment time of the high KN value treatment is denoted as "X"(hours ), while the
treatment time of the low KN value treatment is denoted as "Y" (hours). The total of the
treatment time X and the treatment time Y is within the treatment time A of the nitriding overall,
20 preferably is the treatment time A.
[0066] Various Conditions at High K!'! Value Treatment and Low K!'! Value Treatment
As explained above, the nitriding potential during the high KN value treatment is denoted as
KNx, while the nitriding potential during the low KN value treatment is denoted by KNY·
Furthermore, the average value of the nitriding potential during high KN value treatment is
25 denoted by "KNxave", while the average value of the nitriding potential during low KN value
treatment is denoted by "KNYave"· KNxave and KNYave are defmed by the following formulas:
[0067]
30 [0068] Here, the subscript "i" is a.number expressing the number of times of measurement
every certain time interval. Xo indicates the measurement interval of the nitriding potential KNx
(hours), Yo indicates the measurement interval of the nitriding potential KNY (hours), KNxi
indicates the nitriding potential at the i-th measurement during the high KN value treatment, and
KNYi indicates the nitriding potential at the i-th measurement during the low KN value
11
treatment.
[0069] For example, Xo is made 15 minutes. 15 minutes after the start of treatment,
measurement is conducted the first time (i= I). Each 15 minutes after that, measurement is
conducted the second time (i=2) and the third time (i=3). KNxave is calculated by measurement
5 of the "n" number of times measurable up to the treatment time. KNYave is calculated in the
same way.
[0070] Furthermore, the average value ofthe nitriding potential of the nitriding as a whole is
denoted as "KNave". The average value KNave is defined by the following formula:
[0071] KNave=(XxKNxave+YxKNYave)/A
1 0 [0072] In the nitriding method of the present invention, the nitriding potential KNx, average
value KNxave' and treatment time X of the high KN value treatment and the nitriding potential
KNx, average value KNYave' treatment time Y, and average value KNave of the low KN value
treatment satisfy the following conditions (I) to (IV):
(I) Average value KNXave: 0.30 to 0.80
15 (II) Average value KNYave: 0.03 to 0.20
(III) KNx: 0.15 to 1.50, and KNy: 0.02 to 0.25
(IV) Average value KNave: 0.07 to 0.30
Below, the Conditions (I) to (IV) will be explained.
(0073] (I) Average Value KNXave ofNitriding Potential in High KN Treatment
20 In the high KN value treatment, the average value KNxave of the nitriding potential has to be
0.30 to 0.80 to form a compound layer of a sufficient thickness.
[0074] FIG. 2 is a view showing the relationship of the average value KNxave and the surface
hardness and compound layer thickness. FIG. 2 is obtained from the following experiments.
[0075] The steel "a" having the chemical composition prescribed in the present invention (see
25 Table l, below, called the "test material") was gas nitrided in a gas atmosphere containing NH3,
H2, and N2. In the gas nitriding, the test material was inserted into a heat treatment furnace
heated to a predetermined temperature and able to be controlled in atmosphere then NH3, N2,
and H2 gases were introduced .. At this time, the partial pressures of the NH3 and H2 in the
atmosphere of the gas nitriding were measured while adjusting the flow rates of the gases to
30 control the nitriding potential KN value. The KN value was found by the NH 3 partial pressure
and H2 partial pressure.
[0076] The H2 partial pressure during gas nitriding was measured by using a heat conduction
type H2 sensor directly attached to the gas nitriding furnace body and converting the difference
in standard gas and measured .gas to the gas concentration. The H 2 partial pressure was measured
35 continuously during the gas nitriding. The NH3 partial pressure during the gas nitriding was
measured by attachment of a manual glass tube type NH3 analysis meter outside ofthe fumace.
12
The partial pressure of the residual NH 3 was calculated and found every 15 minutes. Every 15
minutes of measurement of the NH3 partial pressure, the nitriding potential KN v'alue was
calculated. The NH3 flow rate and N2 flow rate were adjusted to converge to the;target values.
10077] The gas nitriding was performed with a temperature of the atmosphere of 590°C, a
5 treatment time X of 1.0 hour, a treatment time Y of2.0 hours, a KNYave of a constant 0.05, and a
KNxave changed from 0.10 to 1.00. The overall treatment time A was made 3.0 hours.
10078] Test materials gas nitrided by various average values KNxave were measured and tested
as follows.
[0079] Measurement of Thickness of Compound Layer
10 After gas nitriding, the cross-section of the test material was polished, etched, and examined
under an optical microscope. The etching was performed by a 3% Nita! solution for 20 to 30
seconds. A compound layer was present at the surface layer of the steel and was observed as a
white uncorroded layer. From five fields of the photographed structure taken by an optical
microscope at 500X (field area: 2.2x10
4
)lm\ the thicknesses of the compound layer at four
15 points were respectively :measured every 30 )liD. The average value of the values of the 20 points
measured was defined as the compound thickness ()lm). When the compound layer thickness
was 3 )lm or less, peeling and cracking were largely suppressed. Accordingly, in the present
invention, the compound layer thickness has to be made 3 )lm or less. The compound layer
thickness may also be 0 ..
20 [0080) Phase Structure of Compound Layer
The phase structure of the compound layer is preferably one where, by area ratio, y' (Fe4N)
becomes 50% or more. The balance is£ (Fe2_3N). With general soft nitriding,. the compound
layer becomes mainly£ (Fe2_3N), but with the nitriding of the present invention, the ratio ofy'
(F e4N) become larger. The phase structure of the compound layer can be investigated by the
25 SEM-EBSD method.
[0081] Measurement of Void Area Ratio
Furthermore, the area ra\io of the voids in the surface layer structure at a cross-section of the test
material was measured by observation under an optical microscope. The ratio of voids in an area
of 25 )liD
2
in a range of 5 ·).!Ill depth from the outermost surface (below, referred to as the "void
30 area ratio") was calculate,d for each field in measurement of five fields at a power of lOOOX
(field area: 5.6xl0
3
)lm\ If the void area ratio is 10% or more, the surface roughness ofthe
nitrided part after gas nitriding becomes coarser. Furthermore, the compound layer becomes
brittle, so the nitrided part falls in fatigue strength. Therefore, in the present invention, the void
area ratio has to be less than 10%. The void area ratio is preferably less than 8%, more preferably
3 5 less than 6%.
[0082] Measurement of Surface Hardness
13
Furthermore, the smface hardness and effective hardened layer depth of the test material after
gas nitriding were found by the following method. The Vickers hardness in the depth direction
from the sample surface was measured based on JIS Z 2244 by a test force. of 1.96N. Further, the
average value of three points of the Vickers hardness at a position of 50 f.tm depth from the
5 surface was defined as the surface hardness (HV). In the present invention, 350HV to SOOHV is
targeted as a surface hardness equal to the case of general gas nitriding where over 3 f.tm of a
compound layer remains.
[0083] Measurement of Effective Hardened Layer Depth
In the present invention, the effective hardened layer depth (f.tm) is defined as the depth in a
I 0 range where the Vickers hardness in the distribution measured in the depth direction from the
surface of the test material using the hardness distribution in the depth direction obtained by the
above Vickers hardness test is 250HV or more.
[0084] At the treatment temperature of 570 to 590°C, in the case of general gas nitriding where
a compound layer of 10 f.!m or more is formed, if the treatment time of the gas nitriding as a
15 whole is A (hours), the effective hardened layer depth becomes the value found by the following
formula (A)±20 f.!m.
[0085] Effective hardened layer depth (J.lm)=l30x{treatment time A (hours)}
112
···(A)
{0086] In the nitrided steel part of the present invention, the effective hardened layer depth was
made 130x{treatment time A (hours)}
112
. In the present embodiment, the treatment time A of the
20 gas nitriding as a whole, as explained above, was 1.5 to 10 hours, so the effective hardened layer
depth was targeted as 160 to 410 f.!m.
[0087] As a result of the above-mentioned measurement test, if the average value KNYave is
0.20 or more, the effective hardened layer depth was 160 to 410 f.!m (when A=3, effective
hardened layer depth 225 f.lm). Furthermore, in the results of the measurement tests, the surface
25 hardnesses and thicknesses of the compound layers of the test materials obtained by gas nitriding
at the different average values KNxave were used to prepare FIG. 2.
[0088] The solid line in FIG. 2 is a graph showing the relationship of the average value
KNxave and surface hardness (HV). The broken line in FIG. 2 is a graph showing the
relationship of the average value KNxave and the thickness of the compound layer (f.lm).
30 [0089] Referring to the solid line graph of FIG. 2, if the average value K'NYave at the low KN
value treatment is constant, as the average value KNxave at the high KN value treatment becomes
higher, the snrface hardness of the nitrided part remarhbly increases. Further, when the average
value KNxave becomes 0.30 or more, the surface hardness becomes the targeted 350HV or more.
On the other hand, if the average valne KNxave is higher than 0.30, even if the average value
35 KNxave becomes further higher, the snrface hardness remains substantially constant. That is, in
the graph of the average value KNxave and surface hardness (solid line in FIG. 2), there is an
14
inflection point near KNxave=0.30.
[0090] Furthermore, referring to the broken line graph of FIG. 2, as the average value KNxave
falls from !.00, the compound thickness remarkably decreases. Further, when the average value
KNxave becomes 0.80, the thickness of the compound layer becomes 3 f.im or less. On the other
5 hand, with an average value KNxave of 0.80 or less, as the average value KNxave falls, the
thickness of the compound layer is decreased, but compared with when the average value
KNxave is higher than 0.80, the amount of reduction of the thickness of the compound layer is
small. That is, in the graph of the average value KNxave and surface hardness (solid line in FIG.
2), there is an inflection point near KNxave=0.80.
10 [0091] From the above results, in the present invention, the average value KNxave of the
nitriding potential of the high KN value treatment is made 0.30 to 0.80. By controlling it to this
range, the nitrided steel can be raised in surface hardness and the thickness of the compound
layer can be suppressed. Furthermore, a sufficient effective hardened layer depth can be
obtained. If the average value KNxave is less than 0.30, the compound is insufficiently formed,
15 the surface hardness falls, and a sufficient effective hardened layer depth cannot be obtained. If
the average value KNxave exceeds 0.80, sometimes the thickness of the compound layer exceeds
3 J.Lm and, furthermore, the void area ratio becomes 10% or more. The preferable lower limit of
the average value KNXave is 0.35. Further, the preferable upper limit of the average value
KNxave is 0,70.
20 [0092] (II) Average Value KNYave ofNitriding Potential at Low Kl'! Value Treatment
The average value KNYave of the nitriding potential of the low KN value treatment is 0.03 to
0.20.
[0093] FIG. 3 is a view showing the relationship of the average value KNY ave and the surface
hardness and compound layer thickness. FIG. 3 was obtained by the following test.
25 [0094] Steel "a" having the chemical composition prescribed in the present invention was gas
nitrided by a temperature of the )litriding atmosphere of 590°C, a tre(itrnent time X of 1.0 hour, a
treatment time Y of 2.0 hours, an average value KNxave of a constant 0.40, and an average value
KNYave chm1ged from 0.01 to 0.30. The overall treatment time A was 3.0 hours.
[0095] After the nitriding, the above-mentioned methods were used to measure the surface
30 hardness (HV), effective hardened layer depth (!lm), and compoundlayer thickness ().lm) at the
different average values KNYave· As a result of measurement of the effective hardened layer
depth, if the average value KNYave is 0.02 or more, the effective hardened layer depth became
225 ).lm or more. Furthermore, the surface hardnesses and the compound thicknesses obtained by
the measurement tests were plotted to prepare FIG. 3.
35 [0096] The.solid line in FIG. 3 is a graph showing the relationship of the average value
KNYave and the surface hardness, while the broken line is a graph showing the relationship of
15
the average value KNYave and the depth of the compound layer. Referring to the solid line graph
of FIG; 3, as the average value KNYave becomes higher from 0, the surface hardness remarkably
increases. Further, when KNYave becomes 0.03, the surface hardness becomes 570HV or more.
Furthe!'more, when KNYave is 0.03 or more, even ifKNYave becomes higher, the surface
5 hardness is substantially constant. Due to the above, in the graph of the average value KNYave
and the surface hardness, there is an inflection point near the average value KNYave=0.03.
[0097] , On the other hand, if referring to the broken line graph in FIG. 3, the thickness of the
compound layer is substantially constant until the average value KNYave falls from 0.30 to 0.25.
However, as the average value KNYave falls from 0.25, the thickness of the compound layer
10 remarkably decreases. Further, when the average value KNYave becomes 0.20, the thickness of
the compound layer becomes 3 [.lm or less. Furthermore, when the average value KNYave is 0.20
or less, as the average value KNYave falls, the thickness of the compound layer decreases, but
compared with when the average value KNYave is higher than 0.20, the amount of decrease of
the thickness of the compound layer is small. Due to this, in the graph of the average value
15 KNYave and the thickness of the compound layer, there is an inflection point near the average
value KNYave=0.20.
[00981 From the above results, in the present invention, the average value KNYave of the low
KN value treatment is limited to 0.03 to 0.20. In this case, the gas nitrided steel becomes higher
in surface hardness and the thickness of the compound layer can be suppressed. Furthermore, it
20 is possible to obtain a sufficient effective hardened layer depth. If the average value KNYave is
less than 0.03, nitrogen is removed from the surface and the surface hardness falls. On the other
hand, if the average value KNYave exceeds 0.20, the compound insufficiently breaks down, the
effective hardened layer depth is shallow, and the surface hardness falls. The preferable lower
limit of the average value KNYave is 0.05. The preferable upper limit of the average value
25 KNYave is 0.18.
[0099] (III) Scope ofNitriding Potentials KNX and KNY During Nitriding
In gas .nitriding, a certain time is required after setting the gas flow rates until the KNi value in
the atmosphere reaches the equilibrium state. For this reason, the KNi value changes with each
instant until the KNi value reaches the equilibrium state. Furthermore, when shifting from the
30 high KN value treatment to low KN value treatment, the setting of the KNi value is changed in
the middle ofthe gas nitriding. In this case as well, the KNi value fluctuates until reaching the
equilibrium state.
[0100]· Such fluctuations in the KNi value have an effect on the compound layer and depth of
the hardened layer. Therefore, in the high KN value treatment and low KN value treatment, not
35 only are the average value KNxave and average value KNYave made the above ranges, but also
the nitriding potential KNx during the high KN value treatment and the nitriding potential KNY
16
during the low KN value treatment aTe controlled to predetermined ranges.
[0101] Specifically, in the present invention, to form a,sufficient compound layer, the nitriding
potential KNx during the high KN value treatment is macle 0.15 to 1.50. To make the compound
layer thin and the depth of the hardened layer larger, the nitriding potential KNY during the low
5 KN value treatment is made 0.02 to 0.25.
[0102] Table I shows the compound layer thickness (11m), void area ratio(%), effective
hardened layer depth (11m), and surface hardness (HV) ofthe nitrided part in the case ofnitriding
steel containing C: 0.45%, Si: 0.70%, Mn: 1.01 %, P: 0.015%, S: 0.015%, Cr: 0.25%, AI:
0.028%, and N: 0.0009% and having a balance of Fe and impurities (below, referred to as "steel
10 'a"') by various nitriding potentials KNx and KNY· Table I was obtained by the following tests.
17
-00
[0103] Table 1
Test Temp. Time
no. X
(OC) (h)
1 590 1.0
2 590 1.0
3 590 1.0
4 590 1.0
5 590 1.0
6 590 1.0
7 590 1.0
8 590 1.0
9 590 1.0
10 590 1.0
11 590 1.0
12 590 1.0
13 590 1.0
14 590 1.0
15 590 1.0
16 590 1.0
High Kn value treatment
Nitriding potential
Min. Max. Aver.
value value value
Knxmin Knxmax Knxave
0.12 0.50 0.40
0.14 0.50 0.40
0.15 0.50 0.40
0.25 0.50 0.40
0.25 1.40 0.40
0.25 1.50 0.40
0.30 1.55 0.40
0.30 1.60 0.40
0.30 0.50 0.40
. 0.30 0.50 0.40
0.30 0.50 0.40
0.30 0.50 0.40
0.30 0.50 0.40
0.30 0.50 0.40
0.30 0.50 0.40
0.30 0.50 0.40
Low Kn value treatment
Time
Nitriding potential
y Min. Max. Aver.
value value value
(h) Knymin Knymax KnYave
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.01 0.15 0.10
2.0 0.02 0.15 0.10
2.0 0.03 0.15 0.10
2.0 0.05 0.15 0.10
2.0 0.05 0.20 0.10
2.0 0.05 0.22 0.10
2.0 0.05 0.25 0.10
2.0 0.05 '- 0.27~ 0.10 ------- -----······----------·-···---
Nitriding Effective
Nitriding Compound Void hardened
Surface
Time potential layer area layer
hardness
A Aver. thickness ratio depth
value (actual)
(h) Knave (f.!m) (%) (f.!m) (Hv)
3.0 0.20 None 2 195 310
3.0 0.20 None 2 243 335
3.0 0.20 1 4 241 391
3.0 0.20 I 4 240 394
3.0 0.20 2 8 238 400
3.0 0.20 2 9 241 403
3.0 0.20 3 14 242 408
3.0 0.20 6 16 250 401
3.0 0.20 None 3 242 283
3.0 0.20 None 3 •. ·243 . 390
3.0 0.20 None 3 247 390
3.0 0.20 1 3 241 396
3.0 0.20 2 4 240 400
3.0 0.20 2 4 242 399
3.0 0.20 3 5 244 402
3.0 0.20 5 5 252 409
[0104] Using the steel "a" as a test material, the gas nitriding shown in Table 1 (high KN value
treatment and low KN value treatment) was performed to produce a nitrided pmt. Specifically,
the atmospheric temperature of the gas nitriding in the different tests was made 590°C, the
treatment time X was made 1.0 hour, the treatment time. Y was made 2.0 hours, KNxave was
5 made a constant 0.40, and KNYave was made a constant 0.1 0. Further, during gas nitriding, the
minimum values KNXmin and KNYmin and the maximum values KNXmax and KNYmax of KNx
and KNY were changed to perform high KN value treatment and low KN value treatment. The
treatment time A of the nitriding as a whole was made 3.0 hours.
[0105] In the case of general gas nitriding where a compound layer of 10 Jlm or more is formed
10 at a treatment temperature of 570 to 590°C, if making the treatment time of the gas nitriding as a
whole 3.0 hours, the effective hardened layer depth became 225 J.lm±20 J.lm. The nitride part
after gas nitriding was measured for compound layer thickness, void mea ratio, effective
hmdened layer depth, and surface hmdness by the above measurement methods to obtain Table
1.
15 '[0106] Referring to Table 1, in Test Nos. 3 to 6 and 10 to 15, the minimum value KNXmin and
maximum value KNxmax were 0.15 to 1.50 and the minimum value KNYmin and maximum
value KNYmax were 0.02 to 0.25. As a result, the compound thickness was a thin 3 Jlm or less
and voids were kept down to less than I 0%. Furthermore, the effective hardened layer depth was
.225 J.lm or more, while the surface hardness was 350HV or more.
20 [0107] On the other hand, in Test Nos. 1 and 2, KNXmin was less than 0.15, so the surface
hardness was less than 570HV. In Test No. I, furthermore, KNXmin was less than 0.14, so the
effective hmdened layer depth was less than 225 Jlm.
[0108] In Test Nos. 7 and 8, KNxmax exceeded 1.5, so the voids in the compound layer
became 10% or more. In Test No.8, furthermore, KNxmax exceeded 1.55, so the thickness of the
25 compound layer exceeded 3 J.llll.
[0109] In Test No. 9, KNYmin was less than 0.02, so the surface hardness was less than
.350HV. This is believed because not only was the compound layer eliminated by the low KN
·value treatment, but also denitration occurred from the surface layer. Furthermore, in Test No.
16, KNvmax exceeded 0.25. For this reason, the thickness ofthe compound layer exceeded 3
30 'f.im. KNYmax exceeded 0.25, so it is believed that the compound layer did not sufficiently break
down .
. [0110] From the above results, the nitriding potential KNx in the high KN value treatment is
, made 0.15 to 1.50 and the nitriding potential KNY in the low KN value treatment is made 0.02 to
. 0.25. In this case, in the part after nitriding, the thickness ofthe compound layer can be made
35 sufficiently thin and voids can be suppressed. Furthermore, the effective hardened layer depth
can be made sufficiently deep and a high surface hardness is obtained.
19
(0111] If the nitriding potential KNx is less than 0.15, the effective hardened layer becomes
too shallow and the surface hardness becomes too: low. If the nitriding potential KNx exceeds
1.50, the compound layer becomes too thick and voids excessively remain.
[0112] Further, if the nitriding potential KNY is less than 0.02, denitration occurs and the
5' surface hardness falls. On the other hand, if the nitriding potential KNY is over 0.20, the
compound layer becomes too thick. Therefore, in the present embodiment, the nitriding potential
KNx during the high KN value treatment is 0.15 to 1.50, and the nitriding potential KNY in the
low KN value treatment is 0.02 to 0.25.
[0113] The preferable lower limit of the nitriding potential KNx is 0.25. The preferable upper
10. limit ofKNx is 1.40. The preferable lower limit ofKNY is 0.03. The preferable upper limit of
KNY is 0.22.
[0114] (IV) Average Value KNave ofNitriding Potential During Nitriding
In gas nitriding of the present embodiment, furthermore, the average value KNave of the nitriding
potential defined by formula (2) is 0.07 to 0.30.
15 [0115] KNave=(XxKNxave+YxKNYave)/A ··· (2)
[0116] FIG. 4 is a view showing the relationship between the average value KNave• surface
hardness (HV), and depth of the compound layer (JJ.m). FIG. 4 was obtained by conducting the
following tests. TI1e steel "a" was gas nitrided as a test material. The atmospheric temperature in
the gas nitriding was made 590°C. Further, the treatment time X, treatment time Y, and range
20 and average value of the nitriding potential (KNx. KNY, KNxave• KNYavel were changed to
perform gas nitriding (high KN value treatment and low KN value treatment).
[0117] The test materials after gas nitriding under the various test conditions were measured
for the compound layer thicknesses and surface hardnesses by the above methods. The obtained
compound layer thicknesses and surface hardnesses were measured and FIG. 4 was prepared.
25 [0118] The solid line in FIG. 4 is a graph showing the relationship between the average value
KNave of the nitriding potential and the. surface hardness (HV). The broken line in FIG. 4 is a
graph showing the relationship between the average value KNave and the thickness of the
compound layer (JJ.m).

CLAIMS
Claim I. 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%,
AI: 0.01 to 0.05%,
N: 0.003 to 0.025% and
a balance ofF e and impurities,
the nitrided steel part comprising a compound layer of a thickness of 3 J.Lm or less
comprising iron, nitrogen, and carbon formed on the steel surface and a hardened layer formed
15 under the compound layer,
20
an effective hardened layer depth of the nitrided steel part being 160 to 410 J.Lm.
Claim 2. The nitrided steel part of claim 1 wherein the steel material contains, in place of
part of Fe, one or both ofMo: 0.01 to less than 0.50% and V: 0.01 to less than 0.50%.
Claim 3. The nitrided steel part of claim l or 2 wherein the steel material contains, in
place of part of Fe, one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01 to less than 0.50%.
Claim 4. The nitrided part of any one of claims I to 3 wherein the steeln:~aterial contains,
25 in place of part of Fe, Ti: 0.005 to less than 0.05%.
Claim 5. A method of nitriding 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%,
AI: 0.01 to 0.05%,
N: 0.003 to 0.025% and