ORIGINAL
DESCRIPTION
STEEL FOR NITRIDING AND NITRIDED COMPONENT
TECHNICAL FIELD
[00011
The present invention relates to a steel for nitriding and a component
having been nitrided (hereinafter, referred to as a nitrided component). More
particularly, it relates to a steel for nitriding that is suitable for being used as
a material for a nitrided component such as an automobile ring gear, which
steel is easily subjected to cutting before nitriding, has a high bending fatigue
strength and surface fatigue strength after nitriding, and further can suppress
expansion (heat treatment distortion) caused by nitriding, and a nitrided
component produced by using the steel.
BACKGROUND ART
[00021
A component used for an automobile transmission is usually subjected to
casehardening treatment such as carburizing-quenching, induction hardening,
or nitriding from the viewpoint of improvement in bending fatigue strength
and surface fatigue strength.
[00031
Among these treatments, the "carburizing-quenching" is a treatment in
which a low carbon steel is generally used, and after C has intruded and
diffused in an austenite zone of a high temperature of Ac3 point or higher,
quenching is performed. This treatment has an advantage of being capable of
obtaining a high surface hardness and a large case depth, but has a problem of
a large heat treatment distortion because this treatment is associated with
r(
ansformation. Therefore, in the case where a high component accuracy is
required, finishing such as grinding or honing is needed after carburizingquenching.
Also, this treatment has a problem that the fatigue strength is
decreased with a so-called "nonmartensitic layer" such as a grain boundary
oxidized layer or an incompletely quenched layer, which is formed on an outer
layer, being a fracture starting point of bending fatigue and the like.
[0004]
The "induction hardening" is a treatment in which quenching is
performed by rapidly heating a steel to an austenite zone of a high
temperature of ACQ point or higher and by cooling it. This treatment has an
advantage that the case depth can be regulated with relative ease, but is not a
casehardening treatment in which C is intruded and diffused as in
carburization. Therefore, to obtain a necessary surface hardness, case depth,
and core hardness, a medium carbon steel having a C content higher than that
of a steel for carburizing is generally used. However, the medium carbon steel
has a problem of decreased machinability because the hardness thereof is
higher than that of the low carbon steel. Also, this treatment has a problem
that a high-frequency heating coil must be prepared for each component.
[00051
The "nitriding" is a treatment in which a high surface hardness and a
proper case depth are obtained by intrusion and diffusion of N at a
temperature of about 450 to 650°C that is not higher than the Acl point. The
nitriding treatment has an advantage that the heat treatment distortion is
small even if a steel is, for example, oil-cooled because the treatment
temperature of nitriding is lower than the treatment temperatures of
carburizing-quenching and induction hardening.
[0006]
h Especially, of the "nitriding", "nitrocarburizing" is a treatment in which
l
a high surface hardness is obtained by intrusion and diffusion of N and C at a
tgmperature of about 500 to 600°C that is not higher than the Acl point. This
treatment is suitable for mass production because not only the heat treatment
distortion is small but also the treatment time is several hours, being shorter
than that in the case where only N is intruded and diffused.
[00071
However, the conventional steel for nitriding has the problems described
in the following (1) to (4).
[0008l
(1) Since nitriding is a treatment in which quenching treatment from a
high-temperature austenite zone is not performed, strengthening associated
with martensitic transformation cannot be applied. Therefore, in order to
provide a nitrided component with a desired strength, it is necessary to
increase the hardness before nitriding. However, in the case where the
hardness is increased by containing a large amount of alloying element, the
cutting becomes difficult to perform.
[OOO~]
(2) The aluminum chromium molybdenum steel (SACM645) specified in
JIS G 4053 (2008), which is a typical steel for nitriding, can provide a high
surface hardness because Cr, Al, and the like form nitrides near the surface.
However, since the case depth is shallow, a high surface fatigue strength
cannot be provided. Also, if the surface hardness is too high, the damage
against a pair-gear becomes undesirably high.
[oo 101
(3) Mo (molybdenum) is an element that combines with C in steel at the
nitriding temperature to form carbides, and thereby improves the core
hd ardness after nitriding. However, since Mo is an expensive element, the use
of a large amount of Mo is unfavorable in terms of economy.
1001 11
(4) Also, although the heat treatment distortion of nitriding is smaller
than that of carburizing-quenching and induction hardening, in the case where
an alloying element is contained to provide a nitrided component with a
desired strength, large amounts of alloy nitrides are formed by nitriding, and
the surface of the nitrided component expands. Therefore, even in nitriding,
the amount of heat treatment distortion undesirably increases. In particular,
an automobile ring gear poses a problem even if being subjected to slight heat
treatment distortion because the automobile ring gear is nitrided after having
been machined into a thin-wall final shape and having been subjected to gear
cutting.
[00121
Concerning a material for nitrided component, the techniques described
in, for example, Patent Documents 1 and 2 have been proposed.
[OO13 1
Patent Document 1 discloses a "material for nitrided component
excellent in broaching workability" consisting, by mass percent, of C: 0.10 to
0.40%, Si: 0.50% or less, Mn: 0.30 to 1.50%, Cr: 0.30 to 2.00%, V: more than
0.15% to 0.50%, and Al: 0.02 to 0.50%, further containing, as necessary, one
element or two or more elements of Ni: 2.00% or less, Mo: 0.50% or less, S:
0.20% or less, Bi: 0.30% or less, Se: 0.30% or less, Ca: 0.10% or less, Te: 0.30%
or less, Nb: 0.50% or less, and Ti: 1.00% or less, the balance of Fe and
impurities, and consisting of a ferritic-pearlitic structure having a ferrite
hardness of HV190 or higher, and a "method for producing nitrided
component" using the material.
[00141
llA Patent Document 2 discloses a "material for nitrided component
excellent in broaching workability" consisting, by mass percent, of C: 0.10 to
0.40%, Si: 0.50% or less, Mn: 0.30 to less than 1.50%, Cr: 0.30 to 2.00%, and Al:
0.02 to 0.50%, further containing, as necessary, one element or two or more
elements of Ni: 2.00% or less, Mo: 0.50% or less, S: 0.20% or less, Bi: 0.30% or
less, Se: 0.30% or less, Ca: 0.10% or less, Te: 0.30% or less, Nb: 0.50% or less,
Ti: 1.00% or less, and V: 0.50% or less, the balance of Fe and impurities, and
consisting of a bainitic structure having a hardness of HV210 or higher, and a
"method for producing nitrided component" using the material.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
100 151
Patent Document 1: JP2005-281857A
Patent Document 2: JP2006-249504A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[OOI~I
In the material for a nitrided component proposed in Patent Document 1,
the ferrite hardness before nitriding treatment is as high as 192 or higher in
Vickers hardness (hereinafter, the "Vickers hardness" is sometimes referred to
as an "HV") as shown in Example of Patent Document 1. Therefore, this
material is not excellent in machinability in the case where the cutting speed
is high.
[OO 1 71
In the material for a nitrided component proposed in Patent Document 2
as well, the bainite hardness before nitriding treatment is as high as 218 or
hkk igher in Vickers hardness as shown in Example of Patent Document 2, so
that it is difficult to say that this material is excellent in machinability in the
case where the cutting speed is high.
I [00181
The present invention has been made in view of the above present
situation, and accordingly an objective thereof is to provide a steel for nitriding
that is suitable for being used as a material for a nitrided component, which
steel is easily subjected to cutting before nitriding, and moreover, has a high
bending fatigue strength and surface fatigue strength after nitriding and
further is configured so that expansion (heat treatment distortion) caused by
nitriding can be suppressed even if the content of Mo, which is an expensive
element, is restricted to 0.05 mass% or less, and a nitrided component
produced by using the steel.
MEANS FOR SOLVING THE PROBLEMS
[00191
To solve the problems, the present inventors conducted various studies.
As the result, the findings of (a) to (d) described below have been obtained.
[00201
(a) The machinability before nitriding treatment is improved by reducing
the C content as much as possible and by keeping the Mo content low.
[002 11
(b) The decrease in strength caused by the decrease in the C content can
be compensated by increasing the Mn content and/or the Cr content and by
containing V.
[00221
,t (c) The formation of hard inclusions (TiN) exerting an adverse influence
on the bending fatigue strength and surface fatigue strength can be
suppressed by restricting the Ti content and the N content.
[00231
(d) The crystal lattice is distorted by the alloy nitrides formed by
nitriding, and the component surface expands, thereby producing heat
treatment distortion. This expansion (heat treatment distortion) caused by
nitriding can be suppressed by properly regulating the contents of Mn, Cr, Mo
and V that form alloy nitrides when nitriding is performed.
[00241
The present invention has been completed based on the above-described
findings, and involves steels for nitriding described in (1) and (2), and a
nitrided component described in (3).
[00251
(1) A steel for nitriding having a chemical composition consisting of, by
mass percent, C: 0.07 to 0.14%, Si: 0.10 to 0.30%, Mn: 0.4 to 1.0%, S: 0.005 to
0.030%, Cr: 1.0 to 1.5%, Mo: 0.05% or less (including o%), Al: 0.010% or more
to less than 0.10%, and V: 0.10 to 0.25%, Fnl expressed by Formula (1) is 2.30
or less, and the balance of Fe and impurities, wherein P, N, Ti and 0 among
the impurities are P: 0.030% or less, N: 0.008% or less, Ti: 0.005% or less, and
0: 0.0030% or less:
Fnl = 0.61Mn + 1.11Cr + 0.35Mo + 0.47V ... (1)
where, the symbol of each element in Formula (1) represents the content
thereof in mass percent.
[0026]
(2) The steel for nitriding according to (1) having a chemical composition
containing, in lieu of a part of Fe, at least one element selected from, by mass
percent, Cu: 0.30% or less and Ni: 0.25% or less.
(3) A nitrided component having the chemical composition according to
(1) or (21, in which the surface hardness is 650 to 900 in Vickers hardness, the
core hardness is 150 or higher in Vickers hardness, and the effective case
depth is 0.15 mm or larger.
[00281
In the present invention, the "nitriding" is not only a treatment in which
only N is intruded and diffused, but includes "nitrocarburizing" that is a
treatment in which N and C are intruded and diffused. That is, the
"nitriding" in the present invention includes not only "2411 nitriding" specified
in JIS B 6905 (1995) but also "2421 nitrocarburizing" specified therein.
[00291
The "impurities" in the "Fe and impurities" described as the balance
mean elements that mixedly enter from raw materials, such as ore or scrap, or
production environments when steel materials are produced on an industrial
scale.
[00301
Also, the "surface hardness" means an arithmetic mean value of the
values obtained by measuring Vickers hardnesses at optional ten points at a
position 0.03 mm deep from the surface of a test specimen by using a Vickers
hardness tester with the test force being 0.98 N in conformity to "Vickers
hardness test - test method" described in JIS Z 2244 (2009).
[003 11
The "effective case depth" means a distance from the surface to a
position at which the Vickers hardness is 420, which distance is determined by
using a distribution chart of Vickers hardness (that is, a transition curve of
Vickers hardness) at the time when measurement is made at predetermined
intervals from the test specimen surface with the test force being 1.96 N.
ADVANTAGEOUS EFFECTS OF THE INVENTION
For the steel for nitriding of the present invention, cutting before
nitriding is easy to perform, and also the amount of expansion caused by
nitriding is small. Moreover, the nitrided component produced by using this
steel as a material is provided with a high bending fatigue strength and
surface fatigue strength although the content of Mo, which is an expensive
element, is as low as 0.05 mass% or less.
BRIEF DESCRIPTION OF THE DRAWINGS
Lo0331
[Figure 11 Figure 1 is views showing the shape of an expansion measuring test
specimen that is used in Example. The unit of each dimension in the figure is
"mm".
[Figure 21 Figure 2 is views showing the rough shape, in a state of being cut
out of a steel bar, of a notched Ono type rotating bending fatigue test specimen
that is used in Example. The unit of each dimension in the figure is "mm".
[Figure 31 Figure 3 is a view showing the rough shape, in a state of being cut
out of a steel bar, of a roller pitching small roller test specimen that is used in
Example. The unit of each dimension in the figure is "mm".
[Figure 41 Figure 4 is views showing the rough shape, in a state of being cut
out of a steel bar, of a roller pitching large roller test specimen that is used in
Example. Figure 4 (a) is a front view in the case where the roller pitching
large roller test specimen having the rough shape is cut in half on the
centerline, and Figure 4 (b) is a sectional view taken on the centerline. The
unit of each dimension in the figure is "mm".
- t i g u r e 51 Figure 5 is a diagram showing a heat pattern of 'gas
nitrocarburizing" and the subsequent cooling performed on the test specimens
shown in Figures 1 to 3 using Steels 1 to 12 as materials in Example.
[Figure 61 Figure 6 is a diagram showing a heat pattern of "carburizingquenching-
tempering" performed on the test specimens shown in Figures 1 to 3
using Steel 13 as a material in Example.
[Figure 71 Figure 7 is a diagram showing a heat pattern of "carburizingquenching-
tempering" performed on the test specimen shown in Figure 4 using
Steel 13 as a material in Example.
[Figure 81 Figure 8 is views showing the finished shape of a notched Ono type
rotating bending fatigue test specimen that is used in Example. The unit of
each dimension in the figure is "mm".
[Figure 91 Figure 9 is a view showing the finished shape of a roller pitching
small roller test specimen that is used in Example. The unit of each
dimension in the figure is "mm".
[Figure 101 Figure 10 is views showing the finished shape of a roller pitching
large roller test specimen that is used in Example. Figure 10 (a) is a front
view in the case where the roller pitching large roller test specimen is cut in
half on the centerline, and Figure 10 (b) is a sectional view taken on the
centerline. The unit of each dimension in the figure is "mm".
[Figure 111 Figure 11 is a view and diagrams for explaining a method for
examination conducted to measure the amount of expansion caused by "gas
nitrocarburizing" or "carburizing-quenching-tempering". Figure 11 (a) shows
a state before "gas nitrocarburizing" or "carburizing-quenching-tempering",
and Figure 11 (b) shows a state during the time from "gas nitrocarburizing" to
oil cooling or a state after "carburizing-quenching-tempering".
MODE FOR CARRYING OUT THE INVENTION
11
In the following, the requirements of the present invention are explained
in detail. In the explanation below, symbol "%" concerning the content of each
element means "percent by mass".
LO0351
(A) Chemical composition of steel
C: 0.07 to 0.14%
C (carbon) is an element essential for ensuring the strength of nitrided
component, and 0.07% or more of C must be contained. However, if the C
content increases and exceeds 0.14%, the hardness before nitriding increases,
resulting in a decrease in machinability. Therefore, the C content is 0.07 to
0.14%. In order to ensure the strength of nitrided component more stably, the
C content is preferably 0.09% or more. Also, when importance is attached to
the machinability, the C content is preferably 0.12% or less.
Lo0361
Si: 0.10 to 0.30%
Si (silicon) is a deoxidizing element. In order to achieve this effect,
0.10% or more of Si must be contained. However, if the Si content increases
and exceeds 0.30%, the hardness before nitriding increases, resulting in a
decrease in machinability. Therefore, the Si content is 0.10 to 0.30%. The Si
content is preferably 0.12% or more, and is preferably 0.25% or less.
LO0371
Mn: 0.4 to 1.0%
Mn (manganese) has an action for ensuring the bending fatigue strength
and surface fatigue strength of nitrided component, and also is a deoxidizing
element. In order to achieve these effects, 0.4% or more of Mn must be
contained. However, if the Mn content increases and exceeds 1.0%, the
hardness before nitriding increases excessively, resulting in a decrease in
l b achinability. Therefore, the Mn content is 0.4 to 1.0%. In order to ensure
the strength of nitrided component more stably, the Mn content is preferably
0.5% or more. Also, when importance is more attached to the machinability,
the Mn content is preferably 0.6% or less.
[00381
S: 0.005 to 0.030%
S (sulfur) combines with Mn to form MnS, so that S has an action for
improving the machinability. However, if the S content is less than 0.005%,
the above-described effect cannot be achieved. On the other hand, if the S
content exceeds 0.030%, coarse MnS is formed, so that the hot forgeability and
bending fatigue strength decrease. Therefore, the S content is 0.005 to
0.030%. In order to ensure the machinability more stably, the S content is
preferably 0.010% or more. Also, when importance is more attached to the
hot forgeability and bending fatigue strength, the S content is preferably
0.025% or less.
Lo0391
Cr: 1.0 to 1.5%
Cr (chromium) has actions for increasing the surface hardness and core
hardness in nitriding and for ensuring the bending fatigue strength and
surface fatigue strength of component. However, if the Cr content is less than
1.0%, the above-described effects cannot be achieved. On the other hand, if
the Cr content increases and exceeds 1.5%, the hardness before nitriding
increases, resulting in a decrease in machinability. Therefore, the Cr content
is 1.0 to 1.5%. In order to increase the surface hardness and core hardness in
nitriding more stably, the Cr content is preferably 1.1% or more. Also, when
importance is more attached to the machinability, the Cr content is preferably
1.4% or less.
[OO~O]
1 - t Mo: 0.05% or less (including 0%)
Mo (molybdenum) need not necessarily be contained. If Mo is contained,
Mo combines with C in steel at the nitriding temperature to form carbides, so
that the core hardness after nitriding is improved. However, if the Mo
1 content increases and exceeds 0.05%, not only the raw material cost goes up
but also the hardness before nitriding increases, resulting in a decrease in
machinability. Therefore, the Mo content is 0.05% or less. When importance
is attached to the machinability, the Mo content is preferably 0.03% or less.
[00411
Al: 0.010% or more to less than 0.10%
A1 (aluminum) is a deoxidizing element. Also, Al combines with N that
intrudes and diffuses from the surface at the time of nitriding to form AlN, so
that Al has an action for improving the surface hardness. In order to achieve
these effects, 0.010% or more of Al must be contained. However, if the Al
content increases and becomes 0.10% or more, not only the machinability is
decreased by the formation of hard A1203, but also there arises a problem that
the nitrided case depth becomes shallow and thereby the bending fatigue
strength and surface fatigue strength are decreased. Therefore, the A1
content is 0.010% or more to less than 0.10%. The preferable lower limit of Al
content is 0.020%, and also the preferable upper limit thereof is 0.070%.
[00421
V: 0.10 to 0.25%
V (vanadium), like Mo, combines with C in steel at the nitriding
temperature to form carbides, so that V has an action for improving the core
hardness after nitriding. Also, V combines with N and/or C, which intrude
and diffuse from the surface at the time of nitriding, to form nitrides and/or
carbo-nitrides, so that V also has an action for improving the surface hardness.
! In order to achieve these effects, 0.10% or more of V must be contained.
Z owever, if the V content increases and exceeds 0.25%, the hardness before
nitriding becomes too high, so that not only the machinability decreases, but
also the above-described effects saturate because V does not dissolve in a
matrix in hot forging and the subsequent normalizing. Therefore, the V
content is 0.10 to 0.25%. The V content is preferably 0.15% or more and
0.20% or less.
Lo0431
Fnl: 2.30 or less
The alloying element having a strong affinity for nitrogen combines with
nitrogen when nitriding is performed, and forms alloy nitrides in a nearsurface
portion. Since the alloy nitrides distort the crystal lattice, the
component surface expands, and the heat treatment distortion occurs.
Especially for Mn, Cr, Mo and V, the alloy nitrides are easily precipitated in
the near-surface portion. In some cases, therefore, the expansion (heat
treatment distortion) caused by nitriding cannot be suppressed even though
the contents of these elements are within the above-described ranges.
However, if Fnl expressed by Formula (1) is 2.30 or less, the excessive
precipitation of alloy nitrides in nitriding is suppressed, and thus, the amount
of expansion in nitriding becomes small and the heat treatment distortion can
be suppressed.
Fnl = 0.61Mn + 1.11Cr + 0.35Mo + 0.47V ... (1)
where, the symbol of each element in Formula (1) represents the content
thereof by mass percent.
Lo0441
Therefore, for Mn, Cr, Mo and V, the contents are made within the
already-described ranges, and additionally are made such that the Fnl is 2.30
or less. The Fnl is preferably 1.50 or more and 2.20 or less.
Lo0451
Z In one of the steels for nitriding of the present invention, besides the
containing of the above-described elements, the balance is Fe and impurities,
wherein P, N, Ti and 0 among the impurities are P: 0.030% or less, N: 0.008%
or less, Ti: 0.005% or less, and 0: 0.0030% or less.
[00461
In the following, P, N, Ti and 0 among the impurities are explained.
loo471
P: 0.030% or less
P (phosphorus) is an impurity contained in a steel, and segregates at the
crystal grain boundaries and embrittles, the steel. In particular, if the P
content exceeds 0.030%, the degree of embrittlement becomes sometimes
remarkable. Therefore, in the present invention, the content of P in the
impurities is 0.030% or less. The content of P in the impurities is preferably
0.020% or less.
[00481
N: 0.008% or less
N (nitrogen) in a steel combines with elements such as C and V and
easily forms carbo-nitrides. If a carbo-nitride such as VCN is formed before
nitriding, the hardness increases, and the machinability decreases. Therefore,
in the present invention, N is an unfavorable element. Also, since this carbonitride
has a high solid solution temperature, V is less liable to be dissolved in
a matrix by the heating in hot forging and the subsequent normalizing, and if
the content of N in steel is high, the above-described effects of V due to
nitriding cannot be achieved sufficiently. Therefore, in the present invention,
the content of N in the impurities is 0.008% or less. The content of N in the
impurities is preferably 0.006% or less.
[00491
Ti: 0.005% or less
L Ti (titanium) has a high affinity for N, and combines with N in steel to
easily form TIN, which is a hard nitride. If the Ti content exceeds 0.005%, the
formed coarse TiN undesirably decreases the bending fatigue strength and
surface fatigue strength. Therefore, in the present invention, the content of
Ti in the impurities is 0.005% or less. The content of Ti in the impurities is
preferably 0.003% or less.
[00501
0: 0.0030% or less
0 (oxygen) forms oxide system inclusions, which are a cause for fatigue
fracture occurring with the inclusion being a starting point, and undesirably
decreases the bending fatigue strength and surface fatigue strength. In
particular, if the 0 content exceeds 0.0030%, the fatigue strengths decrease
remarkably. Therefore, in the present invention, the content of 0 in the
impurities is 0.0030% or less. The content of 0 in the impurities is preferably
0.0020% or less.
[00511
As already described, the "impurities" mean elements that mixedly enter
from raw materials, such as ore or scrap, or production environments when
steel materials are produced on an industrial scale.
100521
In another one of the steels for nitriding of the present invention, in lieu
of a part of Fe, at least one element selected from Cu and Ni are contained.
[00531
In the following, explanation is given of the operational advantages and
the reasons for restricting the contents of Cu and Ni, which are optional
elements.
100541
Cu: 0.30% or less
h Cu (copper) has an action for improving the core hardness. Therefore,
to achieve this effect, Cu may be contained. However, if the Cu content
increases, the machinability decreases. Therefore, the content of Cu, if
contained, is provided with an upper limit, and is 0.30% or less. The content
of Cu, if contained, is preferably 0.20% or less.
[00551
On the other hand, to achieve the above-described effect of Cu stably, the
content of Cu, if contained, is preferably 0.10% or more, further preferably
0.15% or more.
Lo0561
Ni: 0.25% or less
Ni (nickel) has an action for improving the core hardness. Therefore, to
achieve this effect, Ni may be contained. ow ever, if the Ni content increases,
the machinability decreases. Therefore, the content of Ni, if contained, is
provided with an upper limit, and is 0.25% or less. The content of Ni, if
contained, is preferably 0.20% or less.
Loo571
On the other hand, to achieve the above-described effect of Ni stably, the
content of Ni, if contained, is preferably 0.05% or more, further preferably
0.10% or more.
[00581
For Cu and Ni, only either one element of them may be contained, or two
elements of them may be contained compositely. The total content of these
elements may be 0.55%, and is preferably 0.50% or less.
[00591
(B) Surface hardness of nitrided component
For a nitrided component, that is, a component having been subjected to
nitriding, if the surface hardness thereof is low, the bending fatigue strength,
/ L r f a c e fatigue strength, and wear resistance undesirably decrease. However,
if the surface hardness is 650 or higher in HV, the nitrided component can be
provided with a desired strength. On the other hand, if the surface hardness
increases and especially exceeds 900 in HV, the attack ability against a mating
gear becomes undesirably high. Therefore, the surface hardness of nitrided
component is 650 to 900 in HV. The preferable lower limit of surface
hardness is 700 in HV, and the preferable upper limit thereof is 800 in HV.
[OO~O]
(C) Core hardness of nitrided component
If the core hardness of a nitrided component is low, plastic deformation
occurs in the nitrided component when a load is applied to the component,
pitting occurs on account of a crack generated in the component, and the
surface fatigue strength undesirably decreases. In order to suppress the
plastic deformation in the nitrided component, the core hardness must be 150
or higher in HV. Therefore, the core hardness of the nitrided component of
the present invention is 150 or higher in HV. The preferable lower limit of
the core hardness is 170 in HV.
1006 11
The upper limit of core hardness need not be defined especially; however,
the upper limit of the core hardness that can be attained when the steel for
nitriding of the present invention is nitrided without being quenched is about
250 in HV.
LO0621
(D) Effective case depth of nitrided component
If the effective case depth of a nitrided component is shallow, a fracture
is generated with an internal portion being a starting point, and thereby the
bending fatigue strength and surface fatigue strength are undesirably
decreased. In order to suppress the fracture occurring with an internal
kb" rtion being a starting point, the effective case depth must be 0.15 mm or
larger. Therefore, the effective case depth of the nitrided component of the
present invention is 0.15 mm or larger. The preferable lower limit of the
effective case depth is 0.20 mm.
100631
The upper limit of the effective case depth need not be defined especially.
However, in order to increase the effective case depth, the nitriding treatment
time must be prolonged, which results in an increase in cost. Therefore, the
effective case depth is preferably 0.50 mm or less, further preferably 0.45 mm
or less.
100641
(E) Method for producing nitrided component
The nitrided component of the present invention can be produced by
subjecting the steel having the chemical composition described in (A) to
working, heat treatment, and nitriding treatment under the conditions, for
example, described below.
100651
(E-1) Hot forging
A billet, steel bar, or the like of the steel having the chemical
composition described in (A) is cut, and thereafter is hot-forged into a rough
shape by being heated to 1000 to 1270°C.
[00661
(E-2) Normalizing
The nitrided component of the present invention may be produced by
being cut in a state of being hot-forged and by being subjected to nitriding
treatment. However, if the component is normalized as necessary, the crystal
grains thereof can be made finer. In this case, the normalizing treatment is
preferably performed at a temperature of 850 to 970°C.
If slow cooling such as furnace cooling is performed in the cooling after
normalizing, a carbo-nitride such as VCN precipitates in the cooling process,
and thereby the hardness is increased, which sometimes results in a decrease
in machinability. Therefore, in the cooling after normalizing, it is preferable
that the precipitation of a carbo-nitride such as VCN in the cooling process be
suppressed by taking a proper measure, for example, by performing cooling by
wind.
[OO~BI
In order to suppress the precipitation of a carbo-nitride such as VCN in
the cooling process and to maintain the machinability, it is preferable that the
lower limit of cooling rate be 0.5OC/sec, and the upper limit thereof be 5OCtsec.
[00691
(E-3) Cutting
The normalized component having a rough shape is cut by using a lathe
or the like, and thereafter is worked into a finished shape of the nitrided
component by using a broaching machine or a gear shaper.
[00701
(E-4) Nitriding
The method of nitriding treatment for obtaining the nitrided component
of the present invention is not defined specifically, and gas nitriding treatment,
salt bath nitriding .treatment, ion nitriding treatment, or the like can be
employed. The treatment temperature in nitriding treatment is preferably
500 to 650°C. In the case of nitrocarburizing treatment, for example, RX gas
is used in addition to NH3, and the treatment has only to be performed in an
atmosphere in which the ratio of NH3 to RX gas is 1:l.
[00711
- t The treatment time is different depending on the treatment temperature.
I
I In the case where the nitriding treatment is performed at 560°C, a desired
surface hardness, core hardness, and effective case depth can be obtained in
nine hours.
[00721
In the case where it is desired to suppress the formation of a brittle
compound, it is preferable that fluorine gas be used as the preparation of
nitriding treatment using NH3, or a gaseous mixture of NH3 and H2 be used for
nitriding treatment.
[00731
For the cooling after the nitriding treatment, an appropriate method
such as furnace cooling or oil cooling may be used.
[00741
In the following, the present invention is explained more specifically by
referring to Example in which gas nitrocarburizing is performed. The present
invention is not limited to this Example.
EXAMPLES
[00751
Steels 1 to 13 having the chemical compositions given in Table 1 were
melted by using a vacuum furnace, an atmospheric melting furnace, or a
converter to prepare ingots or a cast piece.
[00761
Specifically, for Steels 1 to 9, 11, and 12, the steels were melted by using
a 180-kg vacuum furnace, and thereafter ingots were prepared by ingot
making.
[00771
*
For Steel 10, the steel was melted by using a 180-kg atmospheric
melting furnace, and thereafter an ingot was prepared by ingot making.
[00781
For Steel 13, the steel was melted by using a 70-ton converter, and
thereafter a cast piece was prepared by continuous casting.
[00791
Steels 1 to 5 in Table 1 are steels of inventive examples whose chemical
compositions are within the range defined in the present invention, and on the
other hand, Steels 6 to 13 are steels of comparative examples whose chemical
compositions fall outside the range defined in the present invention.
[00801
Among the steels of comparative examples, Steel 13 is a steel
corresponding to SCr420H specified in JIS G 4052 (2008).
[00811
[Table 11

The ingots of Steels 1 to 12 were subjected to homogenizing treatment in
which the steels were held at 1250°C for 5 hours, and thereafter were hotforged
by being heated to 1200°C, whereby steel bars having diameters of 25
mm, 35 mm, and 60 mm, with a length of 1000 mm were prepared.
[00831
Also, the cast piece of Steel 13 was bloomed into a billet by being heated
to 1250°C for 3 hours, and thereafter was hot-forged by being heated to 1200°C,
whereby steel bars having diameters of 25 mm, 35 mm, 60 mm, and 140 mm,
with a length of 1000 mm were prepared.
[00841
Among the steel bars, the steel bars of Steels 3 to 13 having diameters of
25 mm, 35 mm, and 60 mm were subjected to "normalizing" in which the steel
bars were held at 920°C for 1 hour, and thereafter were cooled by wind.
[00851
Also, the steel bar of Steel 13 having a diameter of 140 mm was
subjected to "normalizing" in which the steel bar was held at 900°C for 4 hours,
and thereafter was allowed to cool.
[00861
From the steel bars of Steels 1 and 2 in a state of being hot-forged and
the steel bars of Steels 3 to 13 having been normalized, various test specimens
were sampled. The surface fatigue strength was evaluated by the roller
pitting test.
[00871
Specifically, first, the steel bar having a diameter of 25 mm was
subjected to so-called "transverse cutting", that is, was cut perpendicularly to
the axial direction (longitudinal direction). After a cut specimen had been
embedded in a resin so that the cut surface was a surface to be examined, the
- 'k cut surface was polished so as to be of mirror finish to prepare Vickers
I hardness test specimens and micro-structure observation specimens in a state
of being hot-forged or having been normalized.
COO881
Also, from the steel bar having a diameter of 60 mm, a lathe turning test
specimen with a diameter of 50 mm and a length of 490 mm was sampled.
[00891
Further, from the central portion of the steel bar having a diameter of 25
mm, an expansion measuring test specimen shown in Figure 1 and a notched
Ono type rotating bending fatigue test specimen having a rough shape shown
in Figure 2 were cut out in parallel with the axial direction. Similarly, from
the central portion of the steel bar having a diameter of 35 mm, a roller pitting
small roller test specimen having a rough shape shown in Figure 3 was cut out
in parallel with the axial direction.
[00901
Also, from the central portion of the steel bar having a diameter of 140
mm, a roller pitting large roller test specimen having a rough shape shown in
Figure 4 was cut out in parallel with the axial direction. In Figure 4, Figure 4
(a) is a front view in the case where the rough shaped roller pitching large
roller test specimen is cut in half on the centerline, and Figure 4 (b) is a
sectional view taken on the centerline.
[00911
The units of all the dimensions of cut-out test specimens shown in
Figures 1 to 4 are "mm". The finishing symbols of three kinds shown in
Figures 1 to 4 are the "triangle symbols" representing surface roughness
described in Explanation table 1 of JIS B 0601 (1982).
[00921
2L Also, letter "G" attached to the finishing symbol is an abbreviation of
working method showing "grinding" specified in JIS B 0122 (1978).
[00931
Among the test specimens having been prepared as described above, the
rough shaped notched Ono type rotating bending fatigue test specimens and
the rough shaped roller pitting small roller test specimens of Steels 1 to 12
were subjected to "gas nitrocarburizing" and "oil cooling" (hereinafter, referred
to as "gas nitrocarburizing/oil cooling") in the heat pattern shown in Figure 5.
In Figure 5, "120°C OIL COOLING" indicates that cooling was performed by
plunging the test specimen into oil with an oil temperature of 120°C.
[00941
Also, the expansion measuring test specimens of Steels 1 to 12 were
subjected to "gas nitrocarburizing/oil cooling" in the heat pattern shown in
Figure 5 after indentations had been formed at a total of 32 places by using a
Vickers hardness tester as described later.
[00951
On the other hand, the rough shaped notched Ono type rotating bending
fatigue test specimen and the rough shaped roller pitting small roller test
specimen of Steel 13 were subjected to "carburizing-quenching-tempering" in
the heat pattern shown in Figure 6. In Figure 6, "Cp" represents carbon
potential. Also, "120°C OIL QUENCHING" indicates that quenching was
performed by plunging the test specimen into oil with an oil temperature of
120°C. Further, "AC" represents air cooling.
[00961
Also, the expansion measuring test specimen of Steel 13 was subjected to
"carburizing-quenching-tempering" in the heat pattern shown in Figure 6 after
indentations have been formed at a total of 32 places by using a Vickers
hardness tester as described later.
Further, the rough shaped roller pitting large roller test specimen of
Steel 13 was subjected to "carburizing-quenching-tempering" in the heat
pattern shown in Figure 7. In Figure 7 as well, as in Figure 6, "Cp"
represents carbon potential. Also, "50°C OIL QUENCHING" indicates that
quenching was performed by plunging the test specimen into oil with an oil
temperature of 50°C. Further, "AC" represents air cooling.
[00981
The rough shaped test specimens that have been subjected to "gas
nitrocarburizing/oil cooling" or "carburizing-quenching-tempering" were finishworked
to prepare the notched Ono type rotating bending fatigue test
specimen shown in Figure 8, the roller pitting small roller test specimen shown
in Figure 9, and the roller pitting large roller test specimen shown in Figure 10.
In Figure 10, Figure 10 (a) is a front view in the case where the roller pitching
large roller test specimen is cut in half on the centerline, and Figure 10 (b) is a
sectional view taken on the centerline.
[00991
The units of all the dimensions of the test specimens shown in Figures 8
to 10 are "mm". The finishing symbols of two kinds shown in Figures 8 to 10
are, as in Figures 1 to 4, the "triangle symbols" representing surface roughness
described in Explanation table 1 of JIS B 0601 (1982).
[01001
Also, letter "G" attached to the finishing symbol is an abbreviation of
working method showing "grinding" specified in JIS B 0122 (1978).
[01011
Further, "-" is a "waveform symbol" indicating that the surface is a basemetal
one, that is, a surface in a state of being subjected to "gas
nitrocarburizing/oil cooling" or "carburizing-quenching-tempering".
By using the test specimens having been prepared as described above,
tests described in the following <> to <<7>> were conducted.
[01031
<> Vickers hardness test on test specimen in state of being hotforged
or having been normalized
The HV hardness was measured at a total of five points, consisting of
one point in a central portion and four points in an R/2 portion ("R" represents
the radius of steel bar) of the Vickers hardness test specimen, which is in a
state of being hot-forged or having been normalized, by using a Vickers
hardness tester with the test force being 9.8 N in conformity to "Vickers
hardness test - test method" described in JIS Z 2244 (2009). The arithmetic
mean value of HV hardness values at the five points was made the HV
hardness in a state of being hot-forged or having been normalized.
[o 1041
<<2>> Micro-structure observation in state of being hot-forged or having
been normalized
The micro-structure observation specimen in a state of being hot-forged
or having been normalized was etched with nital, and the R/2 portion was
observed under an optical microscope with the magnification being x400.
[01051
As the result, the micro-structure was any of bainite, a two-phase mixed
structure consisting of ferrite and bainite, a two-phase mixed structure
consisting of ferrite and pearlite, and a three-phase mixed structure consisting
of ferrite, pearlite, and bainite.
[OIO~I
<<3>> Lathe turning test
t By using the lathe turning test specimen, a lathe turning test was
I conducted under the conditions described below.
Tool: Cemented carbide tool (material symbol: CA5525)
Circumferential speed: 360 mlmin
Feed: 0.4 mmlrev
Depth of cut: 1 mm
Lubricant: Water-soluble lubricant
The cutting resistance at the time of lathe turning was measured. When the
cutting resistance was 750 N or less, it was evaluated that the machinability is
good.
[01071
Further, the chips formed at the time of lathe turning were also observed
to evaluate the chip disposal ability. When the chips were cut in pieces and
there did not occur a trouble such that the chips twined around the material
being tested, it was judged that "the chip disposal ability is good", and on the
other hand, when the chips are long and there occurred a trouble such that the
chips twined around the material being tested, it was judged that "the chip
disposal ability is poor".
[O 1081
<<4>> Measurement of amount of expansion caused "gas
nitrocarburizing/oil cooling" or "carburizing-quenching-tempering"
First, indentations were formed by using a Vickers hardness tester with
the test force being 0.98 N at a total of 32 places including 16 places of position
Nos. 1A to 16A that were 50 pm deep from the reference surface and 200 pm
spaced in the expansion measuring test specimen shown in Figure 1 and 16
places of position Nos. 1B to 16B that were further 200 pm deep from the
position Nos. 1A to 16A and 200 pm spaced as shown in Figure 11 (a). In
' L gure 11, only "1 to 16" that are position numbers are shown, and symbols "A
and "B" showing the depth position are omitted.
[01091
Next, the test specimens of Steels 1 to 12 on which the indentations had
been formed were subjected to the "gas nitrocarburizing/oil cooling" in the heat
pattern shown in Figure 5, and also the test specimen of Steel 13 on which the
indentations had been formed were subjected to the "carburizing-quenchingtempering"
in the heat pattern shown in Figure 6.
[01101
After the "gas nitrocarburizing/oil cooling" or "carburizing-quenchingtempering"
had been performed, on each of the test specimens, the distance
d(n) at 16 places between the indentations formed at position No. nA and
position No. nB (n represents an integer of 1 to 16) was measured. In the case
where the indentations after the "gas nitrocarburizing/oil cooling" or
"carburizing-quenching-tempering" were obscure, the distance d(n) between
the indentations was measured after the surface to be examined had been
buffed lightly.
[01111
The amount of expansion was calculated by the following Formula:
[{d(l) + d(2) + . . . + d(n)) - 16 x 2001 / 16
[01121
<<5>> Measurement of surface hardness, core hardness, and effective
case depth after "gas nitrocarburizing/oil cooling" or "carburizing-quenchingtempering"
By using the roller pitting small roller test specimen before testing,
which had been finish-worked after "gas nitrocarburizing/oil cooling" or
"carburizing-quenching-tempering", a portion thereof having a diameter of 26
mm was transversely cut. After a cut specimen had been embedded in a resin
L that the cut surface was a surface to be examined, the cut surface was
polished so as to be of mirror finish, and the surface hardness, core hardness,
and effective case depth were examined by using a Vickers hardness tester.
[01131
Specifically, the HV hardnesses were measured at optional 10 points at a
position 0.03 mm deep from the surface of test specimen by using a Vickers
hardness tester with the test force being 0.98 N in conformity to "Vickers
hardness test - test method" described in JIS Z 2244 (2009). The arithmetic
mean value of the measurement values was made the "surface hardness".
101 141
Also, by using the same resin-embedded specimen, as in the abovedescribed
case, the HV hardnesses were measured at optional 10 points at a
position 2 mm deep from the surface of test specimen by using a Vickers
hardness tester with the test force being 1.96 N. The arithmetic mean value
of the measurement values was made the "core hardness".
[01151
Further, by using the same resin-embedded specimen, as in the abovedescribed
case, the HV hardnesses were measured at predetermined intervals
in the direction directed from the surface of test specimen toward the center
thereof by using a Vickers hardness tester with the test force being 1.96 N, and
thereby an HV harness distribution chart was prepared. The distance from
the surface to a position at which the HV hardness is 420 was made the
effective case depth.
[01161
<<6>> Ono type rotating bending fatigue test
By using the Ono type rotating bending fatigue test specimen having
been finish-worked, an Ono type rotating bending fatigue test was conducted
under the conditions described below, and the "rotating bending fatigue
' hLdf ength" was evaluated by the maximum strength at which rupture did not
1 occur at a number of cycles of 107.
In the case where a steel had a rotating bending fatigue strength
equivalent to or higher than that of Test No. 13 in which "carburizingquenching-
tempering" was performed by using Steel 13 corresponding to
SCr420H specified in JIS G 4052 (20081, the bending fatigue strength was
made excellent.
[0118]
Temperature: Room temperature
Atmosphere: In the air
Rotating speed: 3000 rpm
[01191
<<7>> Roller pitting test
By using the roller pitting small roller test specimen and the roller
pitting large roller test specimen, which had been finish-worked, a roller
pitting test was conducted under the conditions described below, and the life
duration at the time when pitting with a size on the major axis of 1 mm or
larger occurred was measured. The above-described test was conducted three
times, and the average life duration of three times was made a "pitting life".
The evaluated number of cycles was 1 x 107 at the maximum.
[01201
In the case where a steel had a pitting life exceeding 1 x 107 cycles
equivalent to or longer than that of test No. 13 in which "carburizingquenching-
tempering" was performed by using Steel 13 corresponding to
SCr420H specified in JIS G 4052 (2008), it was evaluated that the steel had a
high surface fatigue strength.
[01211
Slip factor: 40%
Interfacial pressure: 1600 MPa
Rotating speed of small roller test specimen: 1000 rpm
Lubrication: Performed by spraying lubricating oil for automatic
transmission having an oil temperature of 100°C onto the contact portion of
the roller pitting small roller test specimen and the roller pitting large roller
test specimen at a rate of 2 litters per minute
101221
The "slip factor" is a value calculated by the following Formula:
((v2 - v1) 1 Vll x 100
where, "Vl" is the tangential speed of the surface of the roller pitting small
rolling test specimen, and "V2" is the tangential speed of the surface of the
roller pitting large rolling test specimen.
101231
Table 2 summarizes the test results obtained from the examinations
using the test specimens sampled from the state of being hot-forged or the test
specimens sampled after having been "normalized".
Lo1241
Symbols "B", "F, and "P" in the "Micro-structure" column in Table 2
mean bainite, ferrite, and pearlite, respectively. Also, in a column of "chip
disposal ability ", symbol 0 indicates that chips are cut in pieces and there
does not occur a trouble such that the chips "twine" around the material being
tested, that is, "the chip disposal ability is good, and symbol x indicates that
the chips are long and there occurs a trouble such that the chips twine around
the material being tested, that is, "the chip disposal ability is poor".
[01251
L Table 3 summarizes the test results obtained from the tests using the
test specimens that are finish-worked after the "gas nitrocarburizing/oil
cooling" or "carburizing-quenching-tempering".
[o 1261
[Table 21
Table 2
[01271
[Table 31
Inventive
Examples
Comparative
Examples
Symbols "F", "P" and "B" in the "Micro-structure" column mean ferrite,
pearlite and bainite, respectively.
* indicates that chemical compositions fall outside the range defined
in the present invention.
Test
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
Steel
1
2
3
4
5
*6
* 7
* 8
* 9
*10
*11
*12
*I3
Hardness
(HV)
165
167
184
170
210
240
320
85
158
245
160
285
165
Microstructure
F+P
F+P
F+P+B
F+P
F+P+B
F+B
B
F+P
F+P
F+B
F+P
B
F+P
Cutting
resistance
)
680
690
696
684
71 1
720
825
650
675
775
690
805
690
Chip
disposal
ability
o
o
o
o
o
0
o
x
o
o
o
o
o
I-' n
0
8 I-'
I9
U1 u00
Table 3
Inventive
Examples
Comparative
Examples
Test Nos. 1-12 are results obtained from the tests using the test specimens that are finish-worked after the
"gas nitrocarburizingloil cooling" and Test No. 13 is result obtained from the test using the test specimen that is
finish-worked after the "carburizing-quenching-tempering".
* indicates that chemical compositions fall outside the range defined in the present invention.
# indicates that surface hardness, core hardness or effective case depth of the nitrided component does not
satisfy the condition defined in the present invention.
$ indicates the criteria for assessment.
Test
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
Steel
1
2
3
4
5
* 6
* 7
* 8
* 9
* l o
*11
*12
*13
Amount of
expansion
1.8
1.8
2.0
2.0
2.1
2.6
3 -0
0.9
1.6
1.8
2.0
2.5
4.1
Surface
hardness
( H v )
715
725
745
778
750
78 1
778
# 615
# 640
7 10
# 649
745
658
Core
hardness
( H v )
168
170
189
175
220
253
345
# 80
165
255
170
298
3 05
Effective
case depth
(mm)
0.21
0.25
0.24
0 -24
0.29
0.22
0.2 1
# 0.07
# 0.12
0.2 1
# 0.11
0.32
0.75
Ono type
rotating bending
fatigue strength
(MPa)
450
460
470
500
510
5 00
530
3 50
3 90
420
400
460
$ 430
Pitting life
(cycles)
>1.0xl0~
>1.0xl0~
>1.0xl0~
>1.0xl0~
>1.0xl0~
>l.0xlo7
>1.0xl0~
1.5~10'
2 . 0 ~ 1 0 ~
5 . 8 ~ 1 0 ~
6 . 1 ~ 1 0 ~
>l.0xlo7
$ >l.0xlo7
L e steels have a good machinability before nitrocarburizing, has a rotating
bending fatigue strength exceeding 430 MPa that is the rotating bending
fatigue strength of Test No. 13 subjected to "carburizing-quenching-tempering"
by using Steel 13 corresponding to SCr420H specified in JIS G 4052 (2008),
has a pitting life equivalent to that of Test No. 13, has a high bending fatigue
strength after nitrocarburizing, and is excellent in pitting resistance.
[O 1291
In contrast, for Test Nos. 6 to 12 of comparative examples that do not
satisfy the conditions defined in the present invention, the machinability
decreases, the amount of expansion caused in nitriding is large, or the rotating
bending fatigue strength and pitting life are poorer than those of Test No. 13
using the Steel 13.
[01301
Specifically, for Test No. 6, the Fnl of Steel 6 used is as large as 2.38,
exceeding the value defined in the present invention, so that the amount of
expansion in nitriding is as large as 2.6 ym.
[01311
For Test No. 7, the contents of C and Mn of Steel 7 used are higher than
the values defined in the present invention, and the HV hardness after
normalizing is high. Therefore, the cutting resistance is 825 N, and the
machinability is poor. Further, the Fnl of Steel 7 is as large as 2.82,
exceeding the value defined in the present invention, so that the amount of
expansion in nitriding is as large as 3.0 ym.
[o 1321
For Test No. 8, since the contents of C and Cr of Steel 8 used are lower
than the values defined in the present invention, the rotating bending fatigue
strength and the pitting life are 350 MPa and 1.5 x 105 cycles, respectively,
being poorer than those of Test No. 13 using Steel 13. Also, since the S
tent of Steel 8 is lower than the range defined in the present invention, the
chip disposal ability is poor.
LO1331
For Test No. 9, since the Cr content of Steel 9 used is lower than the
value defined in the present invention, the rotating bending fatigue strength
and the pitting life are 390 MPa and 2.0 x 106 cycles, respectively, being poorer
than those of Test No. 13 using Steel 13.
[O 1341
For Test No. 10, since the contents of Ti, N, and 0 of Steel 10 used are
higher than the values defined in the present invention, the bending fatigue
strength and the pitting life are 420 MPa and 5.8 x 106 cycles, respectively,
being poorer than those of Test No. 13 using Steel 13. Also, since the N
content is higher than the value defined in the present invention, the cutting
resistance is 775 N, so that the machinability is also poor.
Lo1351
For Test No. 11, since the V content of Steel 11 used is lower than the
value defined in the present invention, the rotating bending fatigue strength
and the pitting life are 400 MPa and 6.1 x 106 cycles, respectively, being poorer
than those of Test No. 13 using Steel 13.
Lo1361
For Test No. 12, the contents of Mn and Mo of Steel 12 used are higher
than the values defined in the present invention, and the W hardness after
normalizing is high. Therefore, the cutting resistance is 805 N, so that the
machinability is poor.
INDUSTRIAL APPLICABILITY
[01371
\ The steel for nitriding of the present invention is easily subjected to
cutting before nitriding, and moreover, the nitrided component produced by
using this steel for nitriding as a material has a high bending fatigue strength
and surface fatigue strength although the content of Mo, which is an expensive
element, is as low as 0.05 mass% or less. Therefore, the steel for nitriding of
the present invention is suitable for being used as a material for a nitrided
component required to have a high bending fatigue strength and surface
fatigue strength. Further, the steel for nitriding of the present invention is
suitable as a material for a thin-wall nitrided component such as an
automobile ring gear because the amount of expansion caused by nitriding is
small.

h\We claim:
1. A steel for nitriding having a chemical composition consisting of, by
mass percent, C: 0.07 to 0.14%, Si: 0.10 to 0.30%, Mn: 0.4 to 1.0%, S: 0.005 to
0.030%, Cr: 1.0 to 1.5%, Mo: 0.05% or less (including O%), Al: 0.010% or more
to less than 0.10%, and V: 0.10 to 0.25%, Fnl expressed by Formula (1) is 2.30
or less, and the balance of Fe and impurities, wherein P, N, Ti and 0 among
the impurities are P: 0.030% or less, N: 0.008% or less, Ti: 0.005% or less, and
0: 0.0030% or less:
Fnl = 0.61Mn + 1.11Cr + 0.35Mo + 0.47V ... (1)
where, the symbol of each element in Formula (1) represents the content
thereof in mass percent.
2. The steel for nitriding according to claim 1 having a chemical
composition containing, in lieu of a part of Fe, at least one element selected
from, by mass percent, Cu: 0.30% or less and Ni: 0.25% or less.
3. A nitrided component having the chemical composition according to
claim 1 or 2, in which the surface hardness is 650 to 900 in Vickers hardness,
the core hardness is 150 or higher in Vickers hardness, and the effective case
depth is 0.15 mm or larger.
Dated this 18th day of June, 2013.
Nippon Steel & Sumitomo Metal Corporation; and
Honda Motor Co., Ltd.
L L (Varun Sharma)
of Amarchand & Mangaldas &
Suresh A. Shroff & Co.
Attorneys for the Applicants