U.S. patent application number 13/982594 was filed with the patent office on 2014-02-06 for steel for nitriding and nitrided component.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. The applicant listed for this patent is Yuya Gyotoku, Hideki Imataka, Atsushi Kobayashi, Susumu Maeda, Masato Yuya. Invention is credited to Yuya Gyotoku, Hideki Imataka, Atsushi Kobayashi, Susumu Maeda, Masato Yuya.
Application Number | 20140034194 13/982594 |
Document ID | / |
Family ID | 46602622 |
Filed Date | 2014-02-06 |
United States Patent
Application |
20140034194 |
Kind Code |
A1 |
Imataka; Hideki ; et
al. |
February 6, 2014 |
STEEL FOR NITRIDING AND NITRIDED COMPONENT
Abstract
A steel for nitriding having a chemical composition consisting
of, by mass percent, C: 0.07-0.14%, Si: 0.10-0.30%, Mn: 0.4-1.0%,
S: 0.005-0.030%, Cr: 1.0-1.5%, Mo: .ltoreq.0.05% (including 0%),
Al: 0.010% or more to less than 0.10%, V: 0.10-0.25%, optionally at
least one element selected from Cu: .ltoreq.0.30% and Ni:
.ltoreq.0.25% [0.61Mn+1.11Cr+0.35Mo+0.47.ltoreq.2.30], and the
balance of Fe and impurities. P, N, Ti and O among the impurities
are P: .ltoreq.0.030%, N: .ltoreq.0.008%, Ti: .ltoreq.0.005%, and
O: .ltoreq.0.0030%. The steel is easily subjected to cutting before
nitriding and suitable for use as an automobile ring gear. The
nitrided component having a surface hardness of 650-900 HV, core
hardness being .gtoreq.150 HV, and effective case depth of
.gtoreq.0.15 mm has excellent bending fatigue strength and surface
fatigue strength although the content of Mo is as low as
.ltoreq.0.05% and has a small amount of expansion caused by
nitriding.
Inventors: |
Imataka; Hideki;
(Kitakyushu-shi, JP) ; Yuya; Masato;
(Nishinomiya-shi, JP) ; Gyotoku; Yuya;
(Utsunomiya-shi, JP) ; Kobayashi; Atsushi;
(Utsunomiya-shi, JP) ; Maeda; Susumu;
(Utsunomiya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Imataka; Hideki
Yuya; Masato
Gyotoku; Yuya
Kobayashi; Atsushi
Maeda; Susumu |
Kitakyushu-shi
Nishinomiya-shi
Utsunomiya-shi
Utsunomiya-shi
Utsunomiya-shi |
|
JP
JP
JP
JP
JP |
|
|
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
NIPPON STEEL & SUMITOMO METAL CORPORATION
Tokyo
JP
|
Family ID: |
46602622 |
Appl. No.: |
13/982594 |
Filed: |
January 26, 2012 |
PCT Filed: |
January 26, 2012 |
PCT NO: |
PCT/JP2012/051650 |
371 Date: |
October 11, 2013 |
Current U.S.
Class: |
148/318 ;
420/109; 420/111; 420/90 |
Current CPC
Class: |
C22C 38/46 20130101;
C22C 38/22 20130101; C21D 1/06 20130101; C22C 38/50 20130101; C22C
38/44 20130101; C22C 38/06 20130101; C22C 38/02 20130101; C22C
38/24 20130101; C21D 9/32 20130101; C22C 38/001 20130101; C22C
38/04 20130101; C22C 38/20 20130101; C23C 8/26 20130101; C22C 38/28
20130101 |
Class at
Publication: |
148/318 ;
420/111; 420/109; 420/90 |
International
Class: |
C22C 38/50 20060101
C22C038/50; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/28 20060101 C22C038/28; C22C 38/00 20060101
C22C038/00; C22C 38/22 20060101 C22C038/22; C22C 38/20 20060101
C22C038/20; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C23C 8/26 20060101
C23C008/26; C22C 38/24 20060101 C22C038/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2011 |
JP |
2011-019868 |
Claims
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 0%), Al: 0.010% or more to less than 0.10%, and V: 0.10
to 0.25%, Fn1 expressed by Formula (1) is 2.30 or less, and the
balance of Fe and impurities, wherein P, N, Ti and O among the
impurities are P: 0.030% or less, N: 0.008% or less, Ti: 0.005% or
less, and O: 0.0030% or less: Fn1=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, 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.
Description
TECHNICAL FIELD
[0001] 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
[0002] 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.
[0003] 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 Ac.sub.3 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 transformation. Therefore, in the case where a high
component accuracy is required, finishing such as grinding or
honing is needed after carburizing-quenching. 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 Ac.sub.3 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.
[0005] 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.degree. C. that
is not higher than the Ac.sub.1 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] Especially, of the "nitriding", "nitrocarburizing" is a
treatment in which a high surface hardness is obtained by intrusion
and diffusion of N and C at a temperature of about 500 to
600.degree. C. that is not higher than the Ac.sub.1 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.
[0007] However, the conventional steel for nitriding has the
problems described in the following (1) to (4).
[0008] (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.
[0009] (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.
[0010] (3) Mo (molybdenum) is an element that combines with C in
steel at the nitriding temperature to form carbides, and thereby
improves the core hardness after nitriding. However, since Mo is an
expensive element, the use of a large amount of Mo is unfavorable
in terms of economy.
[0011] (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.
[0012] Concerning a material for nitrided component, the techniques
described in, for example, Patent Documents 1 and 2 have been
proposed.
[0013] 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.
[0014] 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
[0015] Patent Document 1: JP2005-281857A [0016] Patent Document 2:
JP2006-249504A
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0017] 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.
[0018] 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 higher 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.
[0019] 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
[0020] To solve the problems, the present inventors conducted
various studies. As the result, the findings of (a) to (d)
described below have been obtained.
[0021] (a) The machinability before nitriding treatment is improved
by reducing the C content as much as possible and by keeping the Mo
content low.
[0022] (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.
[0023] (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.
[0024] (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.
[0025] 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).
[0026] (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 0%), Al: 0.010% or more to less than
0.10%, and V: 0.10 to 0.25%, Fn1 expressed by Formula (1) is 2.30
or less, and the balance of Fe and impurities, wherein P, N, Ti and
O among the impurities are P: 0.030% or less, N: 0.008% or less,
Ti: 0.005% or less, and O: 0.0030% or less:
Fn1=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.
[0027] (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.
[0028] (3) A nitrided component having the chemical composition
according to (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.
[0029] 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.
[0030] 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.
[0031] 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).
[0032] 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
[0033] 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
[0034] FIG. 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
[0035] FIG. 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".
[0036] FIG. 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".
[0037] FIG. 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. FIG. 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 FIG. 4 (b) is
a sectional view taken on the centerline. The unit of each
dimension in the figure is "mm".
[0038] FIG. 5 is a diagram showing a heat pattern of "gas
nitrocarburizing" and the subsequent cooling performed on the test
specimens shown in FIGS. 1 to 3 using Steels 1 to 12 as materials
in Example.
[0039] FIG. 6 is a diagram showing a heat pattern of
"carburizing-quenching-tempering" performed on the test specimens
shown in FIGS. 1 to 3 using Steel 13 as a material in Example.
[0040] FIG. 7 is a diagram showing a heat pattern of
"carburizing-quenching-tempering" performed on the test specimen
shown in FIG. 4 using Steel 13 as a material in Example.
[0041] FIG. 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".
[0042] FIG. 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".
[0043] FIG. 10 is views showing the finished shape of a roller
pitching large roller test specimen that is used in Example. FIG.
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 FIG. 10
(b) is a sectional view taken on the centerline. The unit of each
dimension in the figure is "mm".
[0044] FIG. 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". FIG.
11 (a) shows a state before "gas nitrocarburizing" or
"carburizing-quenching-tempering", and FIG. 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
[0045] 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".
[0046] (A) Chemical Composition of Steel
[0047] C: 0.07 to 0.14%
[0048] 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.
[0049] Si: 0.10 to 0.30%
[0050] 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.
[0051] Mn: 0.4 to 1.0%
[0052] 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
machinability. 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.
[0053] S: 0.005 to 0.030%
[0054] 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.
[0055] Cr: 1.0 to 1.5%
[0056] 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.
[0057] Mo: 0.05% or Less (Including 0%)
[0058] 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 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.
[0059] Al: 0.010% or More to Less than 0.10%
[0060] Al (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 Al.sub.2O.sub.3, 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 Al 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%.
[0061] V: 0.10 to 0.25%
[0062] 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.
However, 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.
[0063] Fn1: 2.30 or Less
[0064] The alloying element having a strong affinity for nitrogen
combines with nitrogen when nitriding is performed, and forms alloy
nitrides in a near-surface 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 Fn1 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.
Fn1=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.
[0065] Therefore, for Mn, Cr, Mo and V, the contents are made
within the already-described ranges, and additionally are made such
that the Fn1 is 2.30 or less. The Fn1 is preferably 1.50 or more
and 2.20 or less.
[0066] 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 O among the impurities
are P: 0.030% or less, N: 0.008% or less, Ti: 0.005% or less, and
O: 0.0030% or less.
[0067] In the following, P, N, Ti and O among the impurities are
explained.
[0068] P: 0.030% or Less
[0069] 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.
[0070] N: 0.008% or Less
[0071] 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 carbo-nitride 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.
[0072] Ti: 0.005% or Less
[0073] 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.
[0074] O: 0.0030% or Less
[0075] O (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 O content exceeds
0.0030%, the fatigue strengths decrease remarkably. Therefore, in
the present invention, the content of O in the impurities is
0.0030% or less. The content of O in the impurities is preferably
0.0020% or less.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] Cu: 0.30% or Less
[0080] 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.
[0081] 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.
[0082] Ni: 0.25% or Less
[0083] Ni (nickel) has an action for improving the core hardness.
Therefore, to achieve this effect, Ni may be contained. However, 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.
[0084] 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.
[0085] 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.
[0086] (B) Surface Hardness of Nitrided Component
[0087] For a nitrided component, that is, a component having been
subjected to nitriding, if the surface hardness thereof is low, the
bending fatigue strength, surface 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.
[0088] (C) Core Hardness of Nitrided Component
[0089] 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.
[0090] 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.
[0091] (D) Effective Case Depth of Nitrided Component
[0092] 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 portion 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.
[0093] 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.
[0094] (E) Method for Producing Nitrided Component
[0095] 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.
[0096] (E-1) Hot Forging
[0097] 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.degree. C.
[0098] (E-2) Normalizing
[0099] 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.degree. C.
[0100] 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.
[0101] 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 0.5.degree. C./sec, and the upper limit thereof be
5.degree. C./sec.
[0102] (E-3) Cutting
[0103] 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.
[0104] (E-4) Nitriding
[0105] 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.degree.
C. In the case of nitrocarburizing treatment, for example, RX gas
is used in addition to NH.sub.3, and the treatment has only to be
performed in an atmosphere in which the ratio of NH.sub.3 to RX gas
is 1:1.
[0106] The treatment time is different depending on the treatment
temperature. In the case where the nitriding treatment is performed
at 560.degree. C., a desired surface hardness, core hardness, and
effective case depth can be obtained in nine hours.
[0107] 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 NH.sub.3, or a gaseous
mixture of NH.sub.3 and H.sub.2 be used for nitriding
treatment.
[0108] For the cooling after the nitriding treatment, an
appropriate method such as furnace cooling or oil cooling may be
used.
[0109] 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
[0110] 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.
[0111] 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.
[0112] For Steel 10, the steel was melted by using a 180-kg
atmospheric melting furnace, and thereafter an ingot was prepared
by ingot making.
[0113] For Steel 13, the steel was melted by using a 70-ton
converter, and thereafter a cast piece was prepared by continuous
casting.
[0114] 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.
[0115] Among the steels of comparative examples, Steel 13 is a
steel corresponding to SCr420H specified in JIS G 4052 (2008).
TABLE-US-00001 TABLE 1 Chemical composition (in mass %, balance: Fe
and impurities) Steel C Si Mn P S Cr Mo Al V Ti N O other Fn1
Inventive 1 0.09 0.10 0.54 0.012 0.022 1.20 -- 0.027 0.13 0.003
0.0047 0.0009 -- 1.72 Examples 2 0.10 0.11 0.49 0.010 0.015 1.24 --
0.026 0.15 0.001 0.0056 0.0010 -- 1.75 3 0.10 0.15 1.00 0.014 0.015
1.24 -- 0.030 0.15 0.002 0.0050 0.0010 -- 2.06 4 0.07 0.20 0.65
0.019 0.015 1.40 0.03 0.034 0.19 0.001 0.0070 0.0009 -- 2.05 5 0.12
0.19 0.95 0.014 0.023 1.15 0.04 0.028 0.22 0.002 0.0065 0.0011 Cu:
0.24, Ni: 0.21 1.97 Comparative 6 0.12 0.18 0.98 0.015 0.018 1.48
0.05 0.033 0.25 0.001 0.0060 0.0008 -- *2.38 Examples 7 *0.25 0.25
*1.80 0.015 0.025 1.45 -- 0.035 0.23 0.001 0.0065 0.0008 -- *2.82 8
*0.06 0.12 0.66 0.012 *0.002 *0.32 -- 0.030 0.11 0.002 0.0053
0.0009 -- 0.81 9 0.11 0.28 0.95 0.014 0.022 *0.58 -- 0.029 0.11
0.002 0.0060 0.0010 -- 1.28 10 0.12 0.23 0.92 0.016 0.018 1.10 0.04
0.028 0.13 *0.095 *0.0195 *0.0040 -- 1.86 11 0.08 0.11 0.62 0.014
0.015 1.02 -- 0.054 *0.03 0.001 0.0070 0.0010 -- 1.52 12 0.14 0.17
*1.25 0.013 0.018 1.10 *0.52 0.044 0.21 0.005 0.0054 0.0014 -- 2.26
13 *0.20 0.24 0.85 0.015 0.012 1.22 -- 0.029 *-- 0.003 *0.0120
0.0010 -- 1.87 Fn1 = 0.61Mn + 1.11Cr + 0.35Mo + 0.47V *indicates
that chemical compositions fall outside the range defined in the
present invention.
[0116] The ingots of Steels 1 to 12 were subjected to homogenizing
treatment in which the steels were held at 1250.degree. C. for 5
hours, and thereafter were hot-forged by being heated to
1200.degree. C., whereby steel bars having diameters of 25 mm, 35
mm, and 60 mm, with a length of 1000 mm were prepared.
[0117] Also, the cast piece of Steel 13 was bloomed into a billet
by being heated to 1250.degree. C. for 3 hours, and thereafter was
hot-forged by being heated to 1200.degree. C., whereby steel bars
having diameters of 25 mm, 35 mm, 60 mm, and 140 mm, with a length
of 1000 mm were prepared.
[0118] 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.degree. C.
for 1 hour, and thereafter were cooled by wind.
[0119] 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.degree. C. for 4 hours, and thereafter was allowed to cool.
[0120] 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.
[0121] 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 cut surface
was polished so as to be of mirror finish to prepare Vickers
hardness test specimens and micro-structure observation specimens
in a state of being hot-forged or having been normalized.
[0122] 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.
[0123] Further, from the central portion of the steel bar having a
diameter of 25 mm, an expansion measuring test specimen shown in
FIG. 1 and a notched Ono type rotating bending fatigue test
specimen having a rough shape shown in FIG. 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
FIG. 3 was cut out in parallel with the axial direction.
[0124] 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 FIG. 4 was cut out in parallel with
the axial direction. In FIG. 4, FIG. 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 FIG. 4 (b) is a
sectional view taken on the centerline.
[0125] The units of all the dimensions of cut-out test specimens
shown in FIGS. 1 to 4 are "mm". The finishing symbols of three
kinds shown in FIGS. 1 to 4 are the "triangle symbols" representing
surface roughness described in Explanation table 1 of JIS B 0601
(1982).
[0126] Also, letter "G" attached to the finishing symbol is an
abbreviation of working method showing "grinding" specified in JIS
B 0122 (1978).
[0127] 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
FIG. 5. In FIG. 5, "120.degree. C. OIL COOLING" indicates that
cooling was performed by plunging the test specimen into oil with
an oil temperature of 120.degree. C.
[0128] Also, the expansion measuring test specimens of Steels 1 to
12 were subjected to "gas nitrocarburizing/oil cooling" in the heat
pattern shown in FIG. 5 after indentations had been formed at a
total of 32 places by using a Vickers hardness tester as described
later.
[0129] 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 FIG.
6. In FIG. 6, "Cp" represents carbon potential. Also, "120.degree.
C. OIL QUENCHING" indicates that quenching was performed by
plunging the test specimen into oil with an oil temperature of
120.degree. C. Further, "AC" represents air cooling.
[0130] Also, the expansion measuring test specimen of Steel 13 was
subjected to "carburizing-quenching-tempering" in the heat pattern
shown in FIG. 6 after indentations have been formed at a total of
32 places by using a Vickers hardness tester as described
later.
[0131] 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 FIG.
7. In FIG. 7 as well, as in FIG. 6, "Cp" represents carbon
potential. Also, "50.degree. C. OIL QUENCHING" indicates that
quenching was performed by plunging the test specimen into oil with
an oil temperature of 50.degree. C. Further, "AC" represents air
cooling.
[0132] The rough shaped test specimens that have been subjected to
"gas nitrocarburizing/oil cooling" or
"carburizing-quenching-tempering" were finish-worked to prepare the
notched Ono type rotating bending fatigue test specimen shown in
FIG. 8, the roller pitting small roller test specimen shown in FIG.
9, and the roller pitting large roller test specimen shown in FIG.
10. In FIG. 10, FIG. 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 FIG. 10 (b) is a sectional view taken on the
centerline.
[0133] The units of all the dimensions of the test specimens shown
in FIGS. 8 to 10 are "mm". The finishing symbols of two kinds shown
in FIGS. 8 to 10 are, as in FIGS. 1 to 4, the "triangle symbols"
representing surface roughness described in Explanation table 1 of
JIS B 0601 (1982).
[0134] Also, letter "G" attached to the finishing symbol is an
abbreviation of working method showing "grinding" specified in JIS
B 0122 (1978).
[0135] Further, ".about." is a "waveform symbol" indicating that
the surface is a base-metal one, that is, a surface in a state of
being subjected to "gas nitrocarburizing/oil cooling" or
"carburizing-quenching-tempering".
[0136] By using the test specimens having been prepared as
described above, tests described in the following <<1>>
to <<7>> were conducted.
[0137] <<1>> Vickers Hardness Test on Test Specimen in
State of being Hot-Forged or Having been Normalized
[0138] 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.
[0139] <<2>> Micro-Structure Observation in State of
being Hot-Forged or Having been Normalized
[0140] 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 .times.400.
[0141] 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.
[0142] <<3>> Lathe Turning Test
[0143] By using the lathe turning test specimen, a lathe turning
test was conducted under the conditions described below.
[0144] Tool: Cemented carbide tool (material symbol: CA5525)
[0145] Circumferential speed: 360 m/min
[0146] Feed: 0.4 mm/rev
[0147] Depth of cut: 1 mm
[0148] 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.
[0149] 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".
[0150] <<4>> Measurement of Amount of Expansion Caused
"Gas Nitrocarburizing/Oil Cooling" or
"Carburizing-Quenching-Tempering"
[0151] 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 .mu.m
deep from the reference surface and 200 .mu.m spaced in the
expansion measuring test specimen shown in FIGS. 1 and 16 places of
position Nos. 1B to 16B that were further 200 .mu.m deep from the
position Nos. 1A to 16A and 200 .mu.m spaced as shown in FIG. 11
(a). In FIG. 11, only "1 to 16" that are position numbers are
shown, and symbols "A" and "B" showing the depth position are
omitted.
[0152] 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 FIG. 5,
and also the test specimen of Steel 13 on which the indentations
had been formed were subjected to the
"carburizing-quenching-tempering" in the heat pattern shown in FIG.
6.
[0153] After the "gas nitrocarburizing/oil cooling" or
"carburizing-quenching-tempering" 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.
[0154] The amount of expansion was calculated by the following
Formula:
[{d(1)+d(2)+ . . . +d(n)}-16.times.200]/16
[0155] <<5>> Measurement of Surface Hardness, Core
Hardness, and Effective Case Depth after "Gas Nitrocarburizing/Oil
Cooling" or "Carburizing-Quenching-Tempering"
[0156] 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 so 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.
[0157] 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".
[0158] Also, by using the same resin-embedded specimen, as in the
above-described 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".
[0159] Further, by using the same resin-embedded specimen, as in
the above-described 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.
<<6>> Ono Type Rotating Bending Fatigue Test
[0160] 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 strength" was evaluated by the maximum
strength at which rupture did not occur at a number of cycles of
10.sup.7.
[0161] In the case where a steel had a rotating bending fatigue
strength equivalent to or higher than that of Test No. 13 in which
"carburizing-quenching-tempering" was performed by using Steel 13
corresponding to SCr420H specified in JIS G 4052 (2008), the
bending fatigue strength was made excellent.
[0162] Temperature: Room temperature
[0163] Atmosphere: In the air
[0164] Rotating speed: 3000 rpm
[0165] <<7>> Roller Pitting Test
[0166] 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.times.10.sup.7 at the
maximum.
[0167] In the case where a steel had a pitting life exceeding
1.times.10.sup.7 cycles equivalent to or longer than that of test
No. 13 in which "carburizing-quenching-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.
[0168] Slip factor: 40%
[0169] Interfacial pressure: 1600 MPa
[0170] Rotating speed of small roller test specimen: 1000 rpm
[0171] Lubrication: Performed by spraying lubricating oil for
automatic transmission having an oil temperature of 100.degree. 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
[0172] The "slip factor" is a value calculated by the following
Formula:
{(V2-V1)/V1}.times.100
where, "V1" 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.
[0173] 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".
[0174] 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 .largecircle. 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".
[0175] 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".
TABLE-US-00002 TABLE 2 Cutting Chip Test Hardness Micro- resistance
disposal No. Steel (HV) structure (N) ability Inventive 1 1 165 F +
P 680 .smallcircle. Examples 2 2 167 F + P 690 .smallcircle. 3 3
184 F + P + B 696 .smallcircle. 4 4 170 F + P 684 .smallcircle. 5 5
210 F + P + B 711 .smallcircle. Comparative 6 *6 240 F + B 720
.smallcircle. Examples 7 *7 320 B 825 .smallcircle. 8 *8 85 F + P
650 x 9 *9 158 F + P 675 .smallcircle. 10 *10 245 F + B 775
.smallcircle. 11 *11 160 F + P 690 .smallcircle. 12 *12 285 B 805
.smallcircle. 13 *13 165 F + P 690 .smallcircle. 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.
TABLE-US-00003 TABLE 3 Ono type Amount of Surface Core Effective
rotating bending Test expansion hardness hardness case depth
fatigue strength Pitting life No. Steel (.mu.m) (HV) (HV) (mm)
(MPa) (cycles) Inventive 1 1 1.8 715 168 0.21 450 >1.0 .times.
10.sup.7 Examples 2 2 1.8 725 170 0.25 460 >1.0 .times. 10.sup.7
3 3 2.0 745 189 0.24 470 >1.0 .times. 10.sup.7 4 4 2.0 778 175
0.24 500 >1.0 .times. 10.sup.7 5 5 2.1 750 220 0.29 510 >1.0
.times. 10.sup.7 Comparative 6 *6 2.6 781 253 0.22 500 >1.0
.times. 10.sup.7 Examples 7 *7 3.0 778 345 0.21 530 >1.0 .times.
10.sup.7 8 *8 0.9 #615 #80 #0.07 350 1.5 .times. 10.sup.5 9 *9 1.6
#640 165 #0.12 390 2.0 .times. 10.sup.6 10 *10 1.8 710 255 0.21 420
5.8 .times. 10.sup.6 11 *11 2.0 #649 170 #0.11 400 6.1 .times.
10.sup.6 12 *12 2.5 745 298 0.32 460 >1.0 .times. 10.sup.7 13
*13 4.1 658 305 0.75 $430 $>1.0 .times. 10.sup.7 Test Nos. 1-12
are results obtained from the tests using the test specimens that
are finish-worked after the "gas nitrocarburizing/oil 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.
[0176] From Tables 2 and 3, it is apparent that, for Test Nos. 1 to
5 using Steels 1 to 5 satisfying the conditions defined in the
present invention as materials, the 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.
[0177] 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.
[0178] Specifically, for Test No. 6, the Fn1 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 .mu.m.
[0179] 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
Fn1 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 .mu.m.
[0180] 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.times.10.sup.5 cycles, respectively, being poorer than
those of Test No. 13 using Steel 13. Also, since the S content of
Steel 8 is lower than the range defined in the present invention,
the chip disposal ability is poor.
[0181] 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.times.10.sup.6 cycles, respectively, being poorer than those of
Test No. 13 using Steel 13.
[0182] For Test No. 10, since the contents of Ti, N, and O 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.times.10.sup.6 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.
[0183] 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.times.10.sup.6 cycles, respectively, being poorer than those of
Test No. 13 using Steel 13.
[0184] 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 HV hardness after normalizing is high. Therefore, the cutting
resistance is 805 N, so that the machinability is poor.
INDUSTRIAL APPLICABILITY
[0185] 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.
* * * * *