DESCRIPTION
Title of Invention: Nitrided or Soft Nitrided Part with
Excellent Wear Resistance and Pitting Resistance, and
5 Nitriding and Soft Nitriding Method
Technical Field
[OOOl] The present invention relates to a part
produced by gas nitriding or gas soft nitriding, in
10 particular a part in which wear resistance and pitting
resistance are demanded such as a CVT pulley or gear, and
a method of gas nitriding and gas soft nitriding used in
production of these parts.
15 Background Art
[0002] Steel parts used in automobiles and various
industrial machinery etc. are sometimes required to have
fatigue strength at their surfaces. For example, in CVT
pulleys for transmissions, wear resistance is demanded,
2 0 while in gears, the fatigue characteristic of pitting
resistance is demanded. For improvement of these
characteristics, improvement of the surface hardness of
the steel parts is considered effective. For steel
materials, nitriding and soft nitriding are being
2 5 increasingly applied. Nitriding and soft nitriding of
steel materials are advantageous in that a high surface
hardness is obtained and heat treatment strain is small.
[0003] Nitriding is a method of treatment that
diffuses nitrogen into the surface of a steel material,
3 0 'while soft nitriding is treatment that diffuses nitrogen
and carbon into the surface of the steel material. As the
medium used for the nitriding and soft nitriding, there
are gases, salt baths, plasma, etc. The transmission
parts of automobiles are mainly treated by the excellent
3 5 productivity gas nitriding and gas soft nitriding.
[0004] The hardened layer formed by the gas nitriding
and gas soft nitriding is comprised of a nitrogen
diffusion layer and a compound layer formed at the
surface side from the nitrogen diffusion layer and of a
thickness of several pm to several tens of pm. The
nitrogen diffusion layer is a layer hardened by diffused
5 nitrogen, solid-solution strengthening by carbon, and the
particle dispersion strengthening mechanism of nitrides.
It is known that improvement of the hardness and depth of
the nitrogen diffusion layer gives rise to an improvement
in the pitting resistance. In the past, therefore much
10 research has been conducted into improvement of the
hardness and depth of the diffusion layer. The compound
layer is comprised of an E phase mainly made of Fez-;N and
also containing carbon or a y' phase mainly made of Fe4N.
Compared with a steel material, the hardness is extremely
15 high. When the compound layer is formed, the wear
resistance is improved.
[OOOS] As conventional findings relating to the
compound layer and wear resistance, the following may be
mentioned. PLT 1 proposes a gear part which has been
2 0 nitrided or carbonitrided, has a content of nitrogen from
at least the surface down to a depth of 150 pm of 0.2 to
0.8%, has a quenched hardened layer of a mixed structure
of martensite and 10 to 40% of residual austenite, and
has excellent pitting resistance and wear resistance. PLT
25 1 has a description relating to the nitrogen content at
the steel surface, but has no description relating to the
components, composition, and properties of a compound
layer formed by nitriding.
[0006] Further, PLT 2 proposes a method of treatment
3 0 using a mixed gas with a resldual concentration of NH; of
45 to 65 vol% for soft nitriding at a gas temperature of
530 to 565'C for 2 hours to thereby form a compound layer
of a thickness of 2 to 12 pm containing pores and improve
the pitting resistance, wear resistance, etc. The
35 compound layer described in PLT 2 is comprised of Fe3N
( E ) , Fe4N ' 1, etc.
Citation List
Patent Literature
[0007] PLT 1: Japanese Patent Publication No. 7-
5 190173A
PLT 2: Japanese Patent Publication No. 11-72159A
Summary of Invention
Technical Problem
10 [0008] In the above-mentioned PLT 1, a part with
excellent pitting resistance and wear resistance is
proposed. However, surface hardening by quenching is
utilized, so compared with a normal nitrided and soft
nitrided part, the heat treatment strain is large and the
15 cost of the later grinding process swells.
[0009] In PLT 2, the thickness of the compound layer
was considered, but the pores were not optimized. For
this reason, sometimes this cannot be applied to parts
where high pitting strength is required.
2 0 [OOlO] The arts disclosed on the above-mentioned PLTs
1 and 2, as shown in the examples, are arts able to
improve the wear resistance, pitting resistance, and
other fatigue characteristics. However, the effects of
the components, composition, and properties of the
2 5 compound layer on the wear resistance and pitting
resistance have not been studied.
[OOll] The object of the present invention is to
provide a part with excellent wear resistance and pitting
resistance which enables demands for reducing the size
3 0 and lightening the weight of parts and high load capacity
to be met. Furthermore, as the means for the same, it
also provides the methods of gas nitriding and gas soft
nitriding optimally controlling the components and
composition of the compound layer.
35
Solution to Problem
[0012] The components, composition, and thickness of
the compound layer can be controlled by the treatment
temperature and the nitriding potential (K,) defined by
the following formula:
KN = (NH3 partial pressure) / [(Hz partial pressure)3'2]
5 . . . (formula 1)
However, the art of controlling the NH3 and N2 atmosphere
in a production scale nitriding furnace has only been
established in recent years, so there are still few
findings regarding the components, composition, and
10 properties of compound layers of actually produced parts.
[0013] Therefore, the inventors controlled the K, to
change the compound layer in various ways and investigate
the relationship of the compound layer and the wear
resistance. As a result, they discovered that the
15 improvement of the wear resistance is affected by the
components, composition, thickness, and hardness of the
compound layer and further is affected by the volume
ratio of the cavities formed by the atomic state nitrogen
diffusing into the steel during the nitriding becoming N2
2 0 molecules and being released from the steel (below,
called "pores").
[0014] Details of the obtained discoveries are
summarized in the following (a) to (e):
[OOlS] (a) The compound layer formed by gas nitriding
2 5 or gas soft nitriding is either of a y' single phase, E
single phase, and y ' t ~ph ase. The F phase is higher in
hardness than the y' phase, so to raise the wear
resistance, it is effective to make the compound layer
which is formed a single phase of the E phase. The & phase
3 0 is formed in the higher KN region than the y' phase, so
there is a need to set a lower limit of K,. Further, by
raising the amount of carbon in the steel or performing
soft nitriding, an E single phase is easily obtained.
[00161 (b) The E phase becomes harder the greater the
35 carbon and nitrogen contents. For this reason, to raise
the wear resistance of the F phase, raising the amounts of
carbon and nitrogen in the E phase is effective. For this
reason, it is necessary to raise the amount of carbon of
the steel serving as the source of supply of the carbon
and employ soft nitriding diffusing carbon so as to
5 further perform nitriding/soft nitriding in the high K,
region and raise the amount of nitrogen in the E phase.
[0017] (c) If the thickness of the compound layer
increases, pores are formed and the wear resistance and
pitting strength fall. For this reason, it is necessary
10 to suitably control the thickness of the compound layer.
Specifically, the thickness of the compound layer becomes
greater the higher the KN, SO it is necessary to provide
an upper limit of the K,.
[0018] (d) In actual gas nitriding, it is difficult to
15 continue to hold the furnace gas atmosphere constant. For
this reason, it is necessary to set a range of the K,
value where a compound layer satisfying the above (a) to
(c) is obtained. On the other hand, right after the start
of treatment, the atmosphere becomes particularly
2 0 unstable. It tends to take about 50 minutes until it
stabilizes. For this reason, at minutes 0 to 50 after
start of treatment, it is necessary to satisfy the above
(a) to (c) and, considering the fact that the atmosphere
is unstable, set the range of control of the K, value
2 5 broader.
[0019] Furthermore, the following findings were
obtained regarding the effect of the nitrogen diffusion
layer on the pitting resistance and the wear resistance.
[0020] (e) If there are Mn, Cr, or other nitride
3 0 forming elements in the steel, the nitrogen diffusion
layer changes in hardness and diffusion layer depth. The
pitting resistance is improved the higher the diffusion
layer hardness and, further, the deeper the diffusion
layer, so it becomes necessary to set the optimum ranges
3 5 of the components of the steel material.
[0021] (f) The nitrogen diffusion layer is lower in
wear resistance than the compound layer, so if the
compound layer is worn away, wear proceeds faster.
[0022] Therefore, to improve the wear resistance and
pitting resistance of a part utilizing gas nitriding and
gas soft nitriding, it is necessary to control the K, and
5 amount of C in the steel to control the amount of carbon
and nitrogen in the compound layer and form a compound
layer having few pores and having an & single phase of a
suitable thickness and hardness and adjust the steel
components to increase the thickness of the nitrogen
10 diffusion layer.
100231 Note that, to evaluate the pores
quantitatively, a SEM image of the compound layer was
used, 50 pm line segments parallel to the surface were
drawn every 2 pm from the surfacemost part to the
15 bottommost part of the compound layer, the average value
of the rates of the lengths of the pore parts in the line
segments was calculated, and this was defined as the
"pore volume ratio ( % ) " . Further, the evaluated value of
the compound layer hardness was made the average value of
2 0 10 random points of the compound layer measured using a
Microvicker's hardness meter at a load of 9.8x10-'~.
[0024] The present invention was completed based on
the above discoveries and has as its gist the gas
nitrided part and gas soft nitrided part shown in the
2 5 following (1) to (4) :
100251 (1) A nitrided part or soft nitrided part made
of a steel material comprising, by mass%,
C: 0.05 to 0.3%,
Si: 0.05 to 1.5%,
30 Mn: 0.2 to l.5%,
P: 0.025% or less,
S: 0.003 to 0.05%,
Cr: 0.5 to 2.0%,
Al: 0.01 to 0.05%, and
3 5 N: 0.003 to 0.025% and
having a balance of Fe and impurities,
wherein,
the surface layer comprises a compound layer containing
iron, nitrogen, and carbon and a nitrogen diffusion layer
positioned below the compound layer,
5 the compound layer comprises an E single phase,
the E single phase has a thickness of 8 to 30 pn and a
Vicker's hardness of 680HV or more, and
the E single phase has a volume ratio of pores of less
than 10%.
10 [0026] (2) The nitrided part or soft nitrided part
according to (I), further containing, by mass%, one or
both of Mo: 0.01 to less than 0.50% and V: 0.01 to less
than 0.50%.
I00271 (3) The nitrided part or soft nitrided part
15 according to (1) or (2), further containing, by mass%,
one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01
to less than 0.50%.
[0028] (4) The nitrided part or soft nitrided part
according to any one of (1) to (3) wherein the compound
2 0 layer includes, by atm%, (C+N) = 22% or more.
(5) A method of nitriding a part comprising a steel
material having the components according to any one of
(1) to (31, comprising heating the part in a gas
atmosphere comprising NH3, HZ, and NZ to 550 to 620°C for
2 5 1.0 to 10 hours, wherein a nitriding potential K, obtained
by the following (formula 1) is 0.3 to 2.0 in minute 0 to
50 in the nitriding time and is 0.70 to 1.50 from minute
50 on:
KN = (NH3 partial pressure) / [ (Hz partial press~re)~'~]
30 . . . (formula 1)
[0029] ( 6 ) A method of soft nitriding a part
comprising a steel material having the components
according to any one of (1) to (31, comprising heating
the part in a gas atmosphere comprising NH3, HZ, Nz, and
35 C02 to 550 to 620°C for 1.0 to 10 hours, wherein a
nitriding potential K, obtained by the following (formula
1) is 0.3 to 2.0 in minute 0 to 50 in the soft nitriding
time and is 0.70 to 1.50 from minute 50 on: KN = (NH3
partial pressure) / [ ( H Z partial pressure) 3/21 . . . (formula
1)
5
Advantageous Effects of Invention
[0030] The nitrided part and soft nitrided part of the
present invention are excellent in wear resistance and
pitting resistance, so can be utilized for the gears, CVT
10 pulleys, transmission parts, etc. of automobiles and
industrial machines.
Brief Description of Drawings
[0031] FIG. 1 is a view showing the shape of a small
15 roller used for a roller pitting test. Note that the
units of the dimensions $26, 28, and 130 in the figure are
"mm" .
FIG. 2 is a view showing the shape of a large roller used
for a roller pitting test. Note that the units of the
2 0 dimensions 4130 and R150 in the figure are "mm".
Description of Embodiments
[0032] Below, the requirements of the present
invention will be explained in detail. Note that the " % "
2 5 showing the contents of the components of the elements in
the steel material used as the material and the
concentration of elements at the surfaces of the parts
means "mass%".
[0033] (A) Regarding Chemical Composition of Steel
3 0 Material Used as Material
C: 0.05 to 0.3%
C is an element required for securing the core strength
of the part and the hardness of the compound layer. If
the content of C is less than 0.05%, the result does not
35 become the E phase single phase harder than the y ' phase
and excellent in wear resistance. Further, if the content
of C is over 0.3%, the steel rod or wire used as the
material or the steel rod or wire after hot forging
becomes too high in strength, so the machineability
greatly falls. The preferable range of the content of C
5 is 0.08 to 0.25%.
[0034] Si: 0.05 to 1.5%
Si raises the core hardness of a part by solid-solution
strengthening. Further, the quenching softening
resistance is raised and the pitting strength of the part
10 surface becoming a high temperature under wear conditions
is raised. To obtain these effects, 0.05% or more is
included. On the other hand, if the content of Si is over
1.5%, the steel rod or wire used as the material or the
steel rod or wire after hot forging becomes too high in
15 strength, so the machineability greatly falls. The
preferable range of the content of Si is 0.08 to 1.2%.
[0035] Mn: 0.2 to 1.5%
Mn raises the core hardness of the part by solid-solution
strengthening. Furthermore, Mn forms fine nitrides (Mn3N2)
2 0 at the time of nitriding and improves the wear resistance
and pitting resistance by precipitation strengthening. To
obtain these effects, the Mn has to be 0.2% or more. On
the other hand, if the content of Mn is over 1.5%, not
only does the effect of raising the pitting strength
2 5 become saturated, but also the steel rod or wire used as
the material or the steel rod or wire after hot forging
becomes too high in hardness, so the machineability
greatly falls. The preferable range of the Mn content is
0.4 to 1.2%.
3 0 [0036] P: 0.025% or less
The impurity P segregates at the grain boundaries and
causes the parts to become brittle. For this reason, if
the content of P exceeds 0.025%, sometimes the bending
fatigue strength falls. The preferable upper limit of the
3 5 P content for preventing a drop in the bending fatigue
strength is 0.018%.
100371 S: 0.003 to 0.05%
S bonds with Mn to form MnS and improve the
machineability. However, if the content is less than
0.003%, the effect of improvement of the machineability
is difficult to obtain. On the other hand, if the content
5 of S increases, coarse MnS becomes easier to form. In
particular, if the content is over 0.05%, the fall in
surface fatigue strength becomes remarkable. The
preferable range of the S content is 0.01 to 0.03%.
[0038] Cr: 0.5 to 2.0%
10 Cr forms fine nitrides (CrN) at the time of nitriding and
improves the wear resistance and pitting resistance by
precipitation strengthening. To obtain these effects, Cr
has to be 0.5% or more. On the other hand, if the content
of Cr exceeds 2.0%, not only does the effect of raising
15 the pitting strength become saturated, but also the steel
rod or wire used as the material or the steel rod or wire
after hot forging becomes too high in hardness, so the
machineability remarkably falls. The preferable range of
the Cr content is 0.7 to 1.8%.
2 0 [0039] Al: 0.01 to 0.05%
A1 is a deoxidizing element. For sufficient deoxidation,
0.01% or more is necessary. On the other hand, A1 easily
forms hard oxide-type inclusions. If the content of Al
exceeds 0.05%, the drop in the bending fatigue strength
2 5 becomes remarkable, so even if other requirements are
satisfied, the desired bending fatigue strength can no
longer be obtained. The preferable range of the A1
content is 0.02 to 0.04%.
[0040] N: 0.003 to 0.025%
3 0 N bonds with A1 and V to form A1N and VN. A1N and VN have
the effect of suppressing the formation of coarse
particles due to the pinning action and reduces the
variation in mechanical properties. If the content of N
is less than 0.003%, the effect cannot be obtained. On
3 5 the other hand, if the content of N is over 0.025%,
coarse A1N more easily forms, so the above effect cannot
be obtained. The preferable range of the N content is
0.005 to 0.020%.
[00411 The following are optional elements.
Mo: 0.01 to less than 0.50%
Mo forms fine nitrides (MozN) at the time of nitriding and
5 soft nitriding and improves the wear resistance and
pitting resistance by precipitation strengthening.
Further, Mo has the action of age hardening at the time
of nitriding to improve the core hardness of a part. To
obtain these effects, the Mo content is preferably 0.01%
10 or more. On the other hand, if the content of Mo is 0.50%
or more, the steel rod or wire used as the material or
the steel rod or wire after hot forging becomes too high
in hardness, so the machineability remarkably falls.
Further, the alloy cost increases. The preferable upper
15 limit of the Mo content for securing machineability is
less than 0.40%.
[00421 V: 0.01 to less than 0.50%
V forms fine nitrides (VN) at the time of nitriding and
soft nitriding and improves the wear resistance and
2 0 pitting resistance by precipitation strengthening.
Further, V has the action of age hardening at the time of
nitriding to improve the core hardness of a part. To
obtain these actions, the V content is preferably 0.01%
or more. On the other hand, if the content of V is 0.50%
2 5 or more, the steel rod or wire used as the material or
the steel rod or wire after hot forging becomes too high
in hardness, so the machineability remarkably falls.
Further, the alloy cost increases. The preferable range
of the V content for securing machineability is less than
30 0.40%.
100431 Cu: 0.01 to 0.50%
Cu acts as a solid-solution strengthening element to
improve the core hardness of a part and the hardness of
the nitrogen diffusion layer. To obtain the action of
35 solid-solution strengthening of Cu, a content of 0.01% or
more is preferable. On the other hand, if the content of
Cu is over 0.50%, the steel rod or wire used as the
material or the steel rod or wire after hot forging
becomes too high in hardness, so the machineability
remarkably falls. Further, the hot ductility falls, so
causes the formation of surface defects at the time of
5 hot rolling and the time of hot forging. The preferable
range of the Cu content for maintaining the hot ductility
is less than 0.40%.
[0044] Ni: 0.01 to 0.50%
Ni improves the core hardness and surface layer hardness
10 of a part by solid-solution strengthening. To obtain the
action of solid-solution strengthening by Ni, a content
of 0.01% or more is preferable. On the other hand, if the
content of Ni exceeds 0.50%, the steel rod or wire used
as the material or the steel rod or wire after hot
15 forging becomes too high in hardness, so the
machineability remarkably falls. Further, the alloy cost
increases. To obtain sufficient machineability, the
preferable range of the Ni content is less than 0.40%.
I00451 (B) Gas Nitriding and Gas Soft Nitriding
2 0 Temperature
When making the temperature of the gas nitriding
(nitriding temperature) less than 550°C, the speed of
nitrogen diffusion in the steel becomes smaller, so a
sufficient thickness of the hardened layer (nitrogen
2 5 diffusion layer or compound layer) cannot be obtained.
Further, if performing gas nitriding at a temperature of
over 620°C, the material transforms to an austenite phase
(y phase) with a smaller speed of diffusion of nitrogen
than a ferrite phase (a phase), so it becomes difficult
3 0 to obtain the thickness of the nitrogen diffusion layer.
For this reason, in the present invention, the treatment
temperature of the gas nitriding is made 550 to 620°C.
[0046] (C) Gas Nitriding and Gas Soft Nitriding Time
The time from the start to the end of the nitriding
3 5 (nitriding time) has an effect on the thickness of the
compound layer and depth of the nitrogen diffusion layer.
If the treatment time is shorter than 1.0 hour, the
diffusion layer becomes smaller in depth and the pitting
resistance falls. If over 10 hours, not only does the
pore ratio increase and the wear resistance fall, but
5 also an increase in the manufacturing cost is incurred.
For this reason, the treatment time is made 1.0 to 10
hours.
[0047] (D) KN Control During Gas Nitriding and Gas Soft
Nitriding
10 In the present invention, gas nitriding uses an
atmosphere comprised of NH3, HZ, and Nz, while gas soft
nitriding uses an atmosphere comprised of NH3, HZ, NZ, and
COz. The nitriding potential KN controls the flow rate of
NH3 and flow rate of N2 to adjust this. To form a compound
15 layer comprised of only the E phase, the range of KN
during the treatment is adjusted to become 0.3 to 2.0 at
minute 0 to 50 in the treatment time and to become 0.70
to 1.50 from minute 50 on. If K, is smaller than 0.3 at
minute 0 to 50 in the treatment time or if it is smaller
2 0 than 0.70 after minute 50, the thickness of the compound
layer becomes less than 8 pm or the concentration of
(C+N) in the compound layer becomes less than 22 atm%,
and the y ' phase is mixed in. As a result, the wear
resistance falls. On the other hand, if KN exceeds the
2 5 prescribed upper limit value of 1.50, the thickness of
the E phase becomes larger than 30 pm. Further, the
porosity sometimes becomes 10% or more.
[0048] To control the KN for nitriding, for example,
there is the method of seasoning the part, before
3 0 nitriding, by holding the inside of the furnace in a high
NH3 atmosphere, then adjusting the flows of NH3, HZ, and NZ
to give the target KN, while for gas soft nitriding,
further adjusting the flow of CO;., then introducing the
part into a furnace. However, the method of control of KN
3 5 of the present invention is not limited to this.
[0049] Note that, the atmosphere for performing gas
nitriding and gas soft nitriding sometimes includes
oxygen or other unavoidable impurities. In gas nitriding,
the total of NH3, HZ, and N2, while in gas soft nitriding,
the total of NH3, HZ, Nz, and C02 is preferably made 99.5%
5 (~01%)o r more.
[0050] (E) Identification of Compound Layer
The compound layer of the gas nitrided part and gas soft
nitrided part according to the present invention is an E
single phase. To discriminate among the phases, for
10 example, EBSD (Electron Backscatter Diffraction) attached
to an SEM (scan type electron microscope) can be used. In
the present invention, the crystal orientation is
measured by EBSD. The case where the region where the
confidence index (CI value) of FeZ-3N in the compound
15 layer is less than 0.05 is less than 10% is deemed as the
E single phase.
[00511 (F) Hardness of Compound Layer
The gas nitrided part and gas soft nitrided part
according to the present invention have average
2 0 hardnesses of the compound layers of 680HV or more.
[00521 It is known that the wear resistance greatly
depends on the hardness of the part from the surface down
to several tens of pm. The inventors measured the
Vicker's hardness of the compound layer based on
2 5 "Vicker's Hardness Test-Test Method" described in JIS Z
2244 (2003).
100531 The inventors compared and studied the results
of a wear test using a roller pitting test machine. As a
result, it became clear that to make the depth of wear
3 0 after a repeated 2x10~ cycles at a surface pressure of
1600 MPa 15 pm or less, the compound layer has to be 680
HV or more in hardness.
[00541 (G) Volume Ratio of Pores in Compound Layer
The gas nitrided part and gas soft nitrided part
3 5 according to the present invention have volume ratios of
pores in the compound layers of less than 10%. Test
pieces formed with various compound layers were evaluated
for wear resistance characteristics by a roller pitting
test. As a result, with a volume ratio of pores of 10% or
more, the amount of wear exceeded the target value of 15
5 w .
(H) Ratios of Components in Compound Layer
The gas nitrided part and gas soft nitrided part
according to the present invention have (C+N)
concentrations in the compound layer of 22 atm% or more.
10 Test pieces formed with various compound layers were
evaluated for wear resistance characteristics by a roller
pitting test. As a result, with a concentration of (C+N)
of less than 22 atm%, the amount of wear failed to
satisfy the target value of 15 pm or less.
15 Example 1
[0055] Steels "a" to "z" having the chemical
components shown in Table 1 were melted in a 50 kg vacuum
melting furnace, then were cast to form ingots. Note
that, in Table 1, "a" to "q" are steels having the
2 0 chemical components prescribed in the present invention.
On the other hand, the steels "s" to "2" are steels of
comparative examples with at least one or more elements
outside the chemical components prescribed in the present
invention.
- 16 -
[0056] Table 1
*l. Balance of chemical components is Fe and impurities.
*2. Empty fields show no alloy elements intentionally added.
*3. Underlines indicate outside scope of present invention.
5 [0057] Each ingot was hot forged to a diameter 35 mm
rod. Next, each rod was annealed, then machined to
fabricate a plate-shaped test piece for evaluation of the
type, thickness, hardness, and volume ratios of pores of
the compound layer. The plate-shaped test piece was made
10 a vertical 20 mm, horizontal 20 mm, and depth 2 mm one.
Further, a small roller for roller pitting test use was
fabricated for evaluating the wear depth and pitting
strength. The small roller had a diameter of 26 mm and a
length of 130 mm.
15 [0058] Next, gases of NH3, HZ, NZ (and, in case of gas
soft nitriding, C02) were introduced into the gas
nitriding furnace. The part was gas nitrided and gas soft
nitrided under the conditions shown in Table 2, then was
oil cooled using 80°C oil. In the gas nitriding and gas
soft nitriding, the Hz partial pressure in the atmosphere
was measured using a heat conducting type Hz sensor
directly attached to the gas nitriding furnace. The
5 difference in heat conductivity between the standard gas
and measured gas was measured converted to the gas
concentration. The Hz partial pressure was measured
continuously during the gas nitriding. Further, the NH3
partial pressure was measured with a manual glass tube
10 type NH3 analysis meter attached to the outside of the
furnace. At the same time as measuring the partial
pressure of the residual NH3 every 10 minutes, the
nitrlding potential K, was calculated and the flow rate of
NH3 and flow rate of N2 were adjusted to make it converge
15 to the target value. The nitriding potential KN was
calculated every 10 minutes of measurement of the NH3
partial pressure and the flow rate of NH3 and flow rate of
NL were adjusted to make it converge to the target value.

[0060] Test Nos. 1 to 25 are examples of the nitriding
and soft nitriding of the present invention. After the
nitriding and soft nitriding, the C-cross-section of each
plate shaped test piece (drawing direction) was polished
5 to a mirror finish, etched by a 3% Nital solution for 20
to 30 seconds, then measured for thickness of the
compound layer and the volume ratio of the pores by SEM.
[0061] The compound layer was photographed at 2000X.
From five fields of the photograph of the structure
10 (field area: 2.4~10' pm2), the thicknesses of five points
of the compound layer were measured at 10 pm intervals.
The average value of the total 25 points was obtained as
the compound thickness. Furthermore, 50 pm line segments
parallel to the surface were drawn every 2 pm from the
15 surfacemost part to the bottommost part of the compound
layer, the ratios of length including the pores in the
line segments were calculated using the following formula
(2), and the average value of the five fields was used as
the volume ratio of the pores.
2 0 Volume ratio of pores ( % ) = Length including pores (pm) /
50 (pm) x 100.. ,formula (2)
[0062] Further, a cross-section polisher was used to
polish the C-cross-section and an SEM (scan type electron
microscope) was used to photograph the structure. The
2 5 EBSD attached to the SEM was used to judge the phases
formed in the compound layer. The compound layer was
photographed at 2000X. Using five fields in the
photograph of structure (field area: 2.4x10'pm2), 50 pm
line segments parallel to the surface were drawn every 2
3 0 pm from the surface most part to the bottommost part of
the compound layer, and the ratios of the length in the
line segments where the CI value of F ~ z -w~aNs 0.05 or
less were calculated using the following formula (3). The
case where the average value of five fields was less than
3 5 10% was judged to be the E single phase.
Length where CI value of F~Z.~Ni s 0.05 or less (p)/5 0
(pm) ~ 1 0 0.. . formula (3)
LO0631 Next, the Vicker's hardness was measured by the
following method based on the "Vicker's Hardness Test -
5 Test Method" in JIS Z 2244 (2003). That is, the average
value of 10 points of Vicker's hardness at positions near
the center of the compound layer in the thickness
direction was defined as the hardness of the compound
layer. The hardness of the compound layer was measured
10 with a test load of 9.8x10-'N. The Vicker's hardness (HV)
was measured at 10 points of each field and the average
of the total 50 points was obtained.
[0064] Next, a small roller for roller pitting test
use was finally worked at the grip part for the purpose
15 of relieving the heat treatment strain, then was used as
a roller pitting test piece. The shape after the final
processing is shown in FIG. 1. The roller pitting test
was performed under the conditions shown in Table 3 by a
combination of the above small roller for roller pitting
2 0 test use and a large roller for roller pitting test use
of the shape shown in FIG. 2. Note that, the units of the
dimensions in FIGS. 1 and 2 are "rnrn". The large roller
for roller pitting test use was prepared using steel
satisfying the standard of SCM420 of JIS and the general
2 5 production process, that is, "normalizing+formation of
test piece+eutectoid carburization by a gas carburizing
furnace+low temperature tempering+polishingW. The
Vicker's hardness Hv at a position of 0.05 mm from the
surface, that is, a position of 0.05 mm depth, was 740 to
3 0 760, while the depth with a Vicker's hardness Hv of 550
or more was a range of 0.8 to 1.0 mm.
[0065] Table 3 shows the test conditions when
evaluating the wear depth. The test was stopped after a
repeated 2x10~ cycles. A roughness meter was used to run
3 5 the wear part of the small roller along the main shaft
direction then measure the maximum wear depth. The number
N was made 5 to calculate the average value of the wear
depth. The parts of the present invention were formed
targeting a wear depth of 15 pm or less.
[0066] Table 3
5
100671 Further, Table 4 shows the test conditions for
evaluation of the pitting strength. The test cutoff was
made 10' showing the fatigue limit of general steel. The
maximum surface pressure when the number of tests reached
10 lo7 without pitting occurring in the small roller test
piece was defined as the fatigue limit of the small
roller test piece. Pitting was detected by a vibration
meter attached to the test machine. After vibration
occurred, the rotations of both the small roller test
15 piece and large roller test piece were made to stop. The
occurrence of pitting and speed were confirmed. In the
parts of the present invention, a maximum surface
pressure at the fatigue limit of 1800 MPa or more was
targeted.
- 22 -
[0068] Table 4
Test piece size
Circumferential speed
[0069] The results are shown in Table 2. From Table 2,
in Test Nos. 1 to 25 satisfying all of the conditions
5 prescribed in the present invention, it is clear that the
amount of wear and the pitting strength both reach the
targets and good wear resistance and pitting resistance
were obtained. Further, in the tests. using steel
containing at least one of Mo, V, Cu, and Ni as well,
10 both the amounts of wear and pitting strengths reached
the targets and it is clear that both excellent wear
resistance and pitting resistance were obtained. On the
other hand, Test Nos. 26 to 40 outside the conditions
prescribed in the present invention are comparative
15 examples. It is clear that either or both of the wear
resistance and pitting resistance do not reach the
target. Test Nos. 26, 27, 30, 36, and 40 are examples
where E single phases are not formed, but this is because
the amount of C in the steel was not satisfied or the KN
2 0 value was low or both were not satisfied. Test Nos. 28
and 29 are examples where the upper limit of the KN value
during treatment became too high, so the E phase became
too large in thickness or cavity volume ratio. Test No.
31 is an example of a E single phase material satisfying
2 5 the above thickness and cavity volume ratio, but where
the KN value during the treatment was too low, so the
amount of (CtN) in the E phase was low and the hardness
was insufficient. Test Nos. 32 to 39 are examples where
the chemical components of the steel are not optimized.
Industrial Applicability
[0070] The gas nitrided part and gas soft nitrided
5 part of the present invention are excellent in wear
resistance and pitting resistance, so can be utilized for
the transmission parts of automobiles or industrial
machines etc.

CLAIMS
Claim 1. A nitrided part or soft nitrided part made
of a steel material comprising, by mass%,
C: 0.05 to 0.3%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 1.5%,
P: 0.025% or less,
S: 0.003 to 0.05%,
Cr: 0.5 to 2.0%,
Al: 0.01 to 0.05%, and
N: 0.003 to 0.025% and
having a balance of Fe and impurities,
wherein,
the surface layer comprises a compound layer
15 containing iron, nitrogen, and carbon and a nitrogen
diffusion layer positioned below the compound layer,
said compound layer comprises an E single phase,
said E single phase has a thickness of 8 to 30
pm and a Vicker's hardness of 680HV or more, and
2 0 said E single phase has a volume ratio of pores
of less than 10%.
Claim 2. The nitrided part or soft nitrided part
according to claim 1, further containing, by mass%, one
or both of Mo: 0.01 to less than 0.50% and V: 0.01 to
2 5 less than 0.50%.
Claim 3. The nitrided part or soft nitrided part
according to claim 1 or 2, further containing, by mass%,
one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01
to less than 0.50%.
3 0 Claim 4. The nitrided part or soft nitrided part
according to any one of claims 1 to 3, wherein said
compound layer includes, by atm%, (C+N) = 22% or more.
Claim 5. A method of nitriding a part comprising a
steel material having the components according to any one
35 of claims 1 to 3, comprising heating the part in a gas
atmosphere comprising NH3, HZ, and NZ to 550 to 620°C for
1.0 to 10 hours, wherein a nitriding potential K, obtained
by the following (formula 1) is 0.3 to 2.0 in minute 0 to
50 in said nitriding time and is 0.70 to 1.50 from minute
50 on:
5 K, = (NH3 partial pressure) / [(HZ partial pre~sure)~'~]
. . . (formula 1)
Claim 6. A method of soft nitriding a part
comprising a steel material having the components
according to any one of claims 1 to 3, comprising heating
10 the part in a gas atmosphere comprising NH3, HZ, Nz, and
COZ to 550 to 620°C for 1.0 to 10 hours, wherein a
ni-triding potential KN obtained by the following (formula
1) is 0.3 to 2.0 in minute 0 -to 50 in said soft nitriding
time and is 0.70 to 1.50 from minute 50 on:
15 KN = (NH3 partial pressure) / [(HZ partial pressure) 3 / Z 1
. . . (formula 1)