U.S. patent application number 12/734813 was filed with the patent office on 2011-02-24 for steel for machine structure use for surface hardening and steel part for machine structure use.
Invention is credited to Masayuki Hashimura, Shuji Kozawa, Kei Miyanishi, Atsushi Mizuno, Hajime Saitoh.
Application Number | 20110041959 12/734813 |
Document ID | / |
Family ID | 42268634 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110041959 |
Kind Code |
A1 |
Mizuno; Atsushi ; et
al. |
February 24, 2011 |
STEEL FOR MACHINE STRUCTURE USE FOR SURFACE HARDENING AND STEEL
PART FOR MACHINE STRUCTURE USE
Abstract
Steel for machine structure use for surface hardening
containing, by mass %, C: 0.3 to 0.6%, Si: 0.02 to 2.0%, Mn: 1.5%
to 3.0%, W: 0.0025 to 0.5%, Al: 0.001 to 0.5%, N: 0.003 to 0.02%,
S: 0.0001 to 0.025%, P: 0.0001 to 0.03%, and O: 0.0001 to 0.0050%,
having an Mn/S of 70 to 30000, and having a balance of
substantially Fe and unavoidable impurities.
Inventors: |
Mizuno; Atsushi; (Tokyo,
JP) ; Hashimura; Masayuki; (Tokyo, JP) ;
Saitoh; Hajime; (Tokyo, JP) ; Kozawa; Shuji;
(Tokyo, JP) ; Miyanishi; Kei; (Tokyo, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
42268634 |
Appl. No.: |
12/734813 |
Filed: |
September 11, 2009 |
PCT Filed: |
September 11, 2009 |
PCT NO: |
PCT/JP2009/066326 |
371 Date: |
May 24, 2010 |
Current U.S.
Class: |
148/318 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/38 20130101; C22C 38/04 20130101; C22C 38/22 20130101; C21D
2211/005 20130101; C21D 2211/009 20130101; C22C 38/02 20130101;
C21D 1/06 20130101; B32B 15/04 20130101; C21D 9/32 20130101; C22C
38/24 20130101; C23C 8/32 20130101; Y02P 10/253 20151101; Y02P
10/25 20151101; C22C 38/18 20130101; C22C 38/06 20130101; C23C 8/26
20130101; C21D 2211/004 20130101; C21D 1/10 20130101; C22C 38/12
20130101; C21D 2211/008 20130101 |
Class at
Publication: |
148/318 |
International
Class: |
C23C 8/26 20060101
C23C008/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 19, 2008 |
JP |
2008324643 |
Claims
1. Steel for machine structure use for surface hardening
containing, by mass %, C: 0.3 to 0.6%, Si: 0.02 to 2.0%, Mn: 1.5%
to 3.0%, W: 0.0025 to 0.5%, Al: 0.001 to 0.5%, N: 0.003 to 0.02%,
S: 0.0001 to 0.025%, P: 0.0001 to 0.03%, and O: 0.0001 to 0.005%,
having an Mn/S of 70 to 30000, and having a balance of
substantially Fe and unavoidable impurities.
2. Steel for machine structure use for surface hardening as set
forth in claim 1, characterized by further containing, by mass %,
one or more of Cr: 0.01 to 2.0%, Mo: 0.01 to 1.0%, and V: 0.01 to
1.0%.
3. Steel for machine structure use for surface hardening as set
forth in claim 1, characterized by further containing, by mass %,
B: 0.0005 to 0.005%.
4. Steel for machine structure use for surface hardening as set
forth in claim 1, characterized by further containing, by mass %,
one or more of Nb: 0.005 to 0.3%, Ti: 0.005 to 0.2%, Ni: 0.05 to
2.0%, and Cu: 0.01 to 2.0%.
5. Steel for machine structure use for surface hardening as set
forth in claim 1, characterized by further containing, by mass %,
one or more of Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005
to 0.05%, and Te: 0.0005 to 0.1%.
6. Steel for machine structure use for surface hardening as set
forth in claim 1, characterized in that said steel for machine
structure use for surface hardening is steel which is nitrided,
then induction hardened.
7. Steel for machine structure use for surface hardening as set
forth in claim 6 characterized in that said nitriding is soft
nitriding.
8. A steel part for machine structure use obtained by machining
steel for machine structure use for surface hardening as as set
forth in claim 1, nitriding it, then induction hardening it, said
steel part for machine structure use characterized in that the
surface layer from the surface down to a depth of 0.4 mm or more is
a nitrided layer and the hardness of the nitrided layer from the
surface down to a depth of 0.2 mm is a Vicker's hardness at the
time of tempering at 300.degree. C. of 650 or more.
9. A steel part for machine structure use as set forth in claim 8,
characterized in that said nitriding is soft nitriding.
10. A steel part for machine structure use as set forth in claim 8,
characterized in that said nitrided layer from the surface down to
a depth of 5 .mu.m or more includes pores of a circle equivalent
diameter of 0.1 to 1 .mu.m in an amount of 10000/mm.sup.2 or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to steel for machine structure
use for surface hardening and in particular a steel part for
machine structure use having a high contact fatigue strength
applied to a gear, continuously variable transmission, constant
velocity joint, hub, or other power transmission part for an
automobile etc.
BACKGROUND ART
[0002] Steel parts for machine structure use, for example, gears of
automatic transmissions, sheaves of continuously variable
transmissions, constant velocity joints, hubs, bearings and other
power transmission parts require high contact fatigue strength.
[0003] In the past, in general, the above parts have been obtained
by working materials such as JIS SCr420, SCM420, or other C: 0.2%
or so case hardened steel into parts, then treating them by
carburized quenching to form C: 0.8% or so martensite structure
hardened layers on the surfaces of the parts and improve the
contact fatigue strength.
[0004] However, carburized quenching is accompanied with austenite
transformation at a 950.degree. C. or so high temperature and is
treated for a long period of 5 to 10 hours, in some cases, 10 hours
or more, so larger heat treatment deformation due to coarsening of
the crystal grains (distortion during quenching) is difficult to
avoid.
[0005] For this reason, when a steel part requires high precision,
after carburized quenching, the steel part has to be ground, honed,
or otherwise finished.
[0006] In addition to this, in recent years, there has been a
rising demand for reduction of the noise of automobile engines
etc., so surface hardening methods with smaller heat distortion
compared with carburized quenching such as induction hardening and
soft nitriding have come into focus.
[0007] Induction hardening is a method of austenizing and quenching
only the required part of the surface layer part of a steel
material by heating for a short time and enables a surface hardened
part with small distortion during quenching to be obtained with
good precision. However, if trying to use just induction hardening
to obtain a hardness equivalent to that of a carburized quenched
material, 0.8% or more of C has to be added.
[0008] If the amount of C in the steel becomes 0.8% or more, the
internal hardness, which is unnecessary for improvement of the
contact fatigue strength, also rises and the machineability
remarkably deteriorates, so it is not possible to just increase the
amount of C in the steel. There are limits to improving the contact
fatigue strength by just induction hardening.
[0009] Soft nitriding is a surface hardening method mainly causing
nitrogen and carbon to simultaneously diffuse in and permeate the
surface of a steel material to form a hardened layer in the 500 to
600.degree. C. temperature range, which is less than the
transformation point, and improve the wear resistance, seize
resistance, fatigue resistance, etc.
[0010] The surface of the steel material is formed with nitrides by
the diffused nitrogen. Usually, at the surfacemost layer of the
steel material, a compound layer mainly comprised of Fe.sub.3N,
Fe.sub.4N, or other Fe nitrides is formed. Inside, a nitrided layer
with N diffused in it is formed.
[0011] Soft nitriding can be performed at a low temperature.
Further, compared with carburization, the treatment time is a short
one of 2 to 4 hours or so, therefore this is often used for the
production of steel parts requiring low distortion. However, with
just soft nitriding, the hardened layer depth is shallow, so this
cannot be used for transmission gears etc. to which a high contact
pressure is applied.
[0012] In recent years, as a technique making up for the defects of
induction hardening and soft nitriding and giving better mechanical
properties, in particular better contact fatigue strength,
performing soft nitriding, then induction hardening has been
experimented with (see PLT's 1 to 7).
[0013] For example, PLT 1 discloses the method of combining
induction hardening and gas soft nitriding to make up for their
respective defects and obtain excellent mechanical properties, in
particular, high contact fatigue strength by improvement of the
softening resistance.
[0014] However, with the method of PLT 1, the surface hardness is
high, but the concentration of N in the nitrided layer is low, so
the high temperature hardness is low, sufficient softening
resistance cannot be obtained at the surface of the gear etc.
becoming high in temperature during operation, and in the final
analysis a high contact fatigue strength cannot be obtained.
[0015] PLT 2 also discloses the method of combining induction
hardening and soft nitriding to produce parts for machine structure
use excellent in mechanical strength. With the method of PLT 2, to
enable the nitrides to form a solid solution, 900.degree. C. to
1200.degree. C. high temperature induction heating is
necessary.
[0016] However, the amounts of addition of elements with a high
affinity with N which promote the breakdown and dispersion of
nitrides are insufficient, so high temperature heating is required.
Therefore, the surface of the steel material is formed with an
oxide layer to a remarkable extent and the mechanical properties
end up greatly deteriorating.
[0017] Further, with the method of PLT 2, no consideration is given
to the method of forming a thick compound layer, so good contact
fatigue strength cannot be obtained under a high contact
pressure.
[0018] PLT 3 discloses a method of production of a part for machine
structure use excellent in mechanical strength characterized by
treating steel comprised of, by wt %, C: 0.35 to 0.65%, Si: 0.03 to
1.50%, Mn: 0.3 to 1.0%, Cr: 0.1 to 3.0%, and a balance of Fe and
impurities by soft nitriding under conditions giving a nitrided
layer depth of 150 .mu.m or more, then by induction hardening under
conditions where the nitrided layer austenizes.
[0019] However, in the method of production of PLT 3, no
consideration is given to raising the contact fatigue strength by
the formation of a required thickness of a nitrided layer.
[0020] PLT 4 discloses a method of heat treatment of a machine part
characterized by soft nitriding an iron-based material worked into
the shape of a part so as to make nitrogen diffuse in and permeate
the surface layer and form a compound layer, then induction
hardening the part under conditions where the compound layer is
consumed, the diffusion layer of the newly formed surface layer is
denitrided, and a porous layer is formed at the surfacemost
part.
[0021] However, in the heat treatment method of PLT 4, no
consideration is given to raising the contact fatigue strength by
the formation of a required thickness of a nitrided layer.
[0022] PLT 5 discloses a roller support shaft used for a cam
follower device made of an iron-based alloy containing Cr, Mo, V,
and W in a total of 1.0 to 20.0 wt % and C and N in a total of 0.5
to 1.2 wt % and having a balance of unavoidable impurities and Fe,
nitrided at its surface, then induction quenched at the outer
peripheral parts other than the two ends.
[0023] However, in the roller support shaft of PLT 5, no
consideration is given to raising the contact fatigue strength by
the formation of a required thickness of a nitrided layer.
[0024] PLT 6 also discloses a method of combining induction
hardening and nitriding to obtain excellent mechanical properties.
However, the nitriding in the method of PLT 6 is performed at a
high temperature of 600.degree. C. or more, so the compound layer
is thin. Furthermore, the N concentration in it is low, so the
amount of N diffusing due to decomposition at the time of induction
hardening is also small.
[0025] In the end, with the nitriding of PLT 6, while a compound
layer can be formed, formation of a thick, high N concentration
nitrided layer is difficult, so even if combined with induction
hardening, formation of a high softening resistance, good contact
fatigue strength nitrided layer is not possible.
[0026] PLT 7 discloses steel for machine structure use excellent in
strength, ductility, toughness, and wear resistance characterized
by containing, by mass %, C: over 0.30%, 0.50% or less, Si: 1.0% or
less, Mn: 1.5% or less, Mo: 0.3% to 0.5%, Ti: 0.1% or less, and B:
0.0005% to 0.01%, having a balance of Fe and unavoidable
impurities, having at its surface a hardened layer of a thickness
50 .mu.m or less and a Vicker's hardness of 750 or more, and having
structures other than said hardened layer with an old austenite
grain size of 10 .mu.m or less, a martensite percentage of 90% or
more, and a Vicker's hardness of 450 to less than 750.
[0027] However, the steel for machine structure use of PLT 7 does
not form the required thickness of nitrided layer and raise the
contact fatigue strength, so even if this can be applied to a metal
belt of a continuously variable transmission, it is difficult to
apply this to gears of automatic transmissions, sheaves of
continuously variable transmissions, constant velocity joints,
hubs, and other power transmission parts subjected to high contact
pressures.
[0028] Whatever the case, steel for structural use for surface
hardening able to be used for power transmission parts subjected to
high contact pressures has not been provided up to now.
CITATION LIST
Patent Literature
[0029] PLT 1: Japanese Patent Publication (A) No. 06-172961
[0030] PLT 2: Japanese Patent Publication (A) No. 07-090363
[0031] PLT 3: Japanese Patent Publication (A) No. 07-090364
[0032] PLT 4: Japanese Patent Publication (A) No. 10-259421
[0033] PLT 5: Japanese Patent Publication (A) No. 2004-183589
[0034] PLT 6: Japanese Patent Publication (A) No. 2007-077411
[0035] PLT 7: Japanese Patent Publication (A) No. 2007-177317
SUMMARY OF INVENTION
Technical Problem
[0036] The present invention, in view of this situation, has as its
task making up for the defects of the low surface hardness or
internal hardness resulting from just induction hardening or soft
nitriding by combining induction hardening and soft nitriding and
providing a steel part for machine structure use excellent in
contact fatigue strength (i) provided with a high surface hardness,
internal hardness, and temper softening resistance unable to be
obtained by a conventional soft nitrided and induction hardened
steel part and, furthermore, (ii) formed with a sufficient
lubricating film at its operating surface and a steel for machine
structure use for surface hardening use used for said steel
part.
Solution to Problem
[0037] To raise the contact fatigue strength of the steel part, (i)
improvement of the surface hardness, (ii) increase of the depth of
the hardened layer, and (iii) improvement of the softening
resistance for maintaining high temperature strength at an
operating surface becoming high in temperature (around 300.degree.
C.) are effective. Furthermore, to prevent seizure of operating
surfaces and sticking, it is effective to form a sufficient
lubricating film.
[0038] Based on this, the inventors engaged in intensive research
on the surface hardening of steel parts by combination of soft
nitriding and induction heat treatment and obtained the following
discoveries:
[0039] (a) To increase the softening resistance, forming a nitrided
layer with a high N concentration is effective. With just
nitriding, even if a compound layer can be formed, formation of a
high N concentration, thick nitrided layer is difficult and
increasing the softening resistance is impossible.
[0040] To increase the softening resistance, it is necessary to use
the compound layer formed at the time of soft nitriding (layer
mainly comprised of Fe.sub.3N, Fe.sub.4N, or other Fe nitrides) as
a source of supply of N and use the later performed induction
heating to break down the compound and cause a sufficient amount of
N to diffuse in the steel.
[0041] Here, FIG. 1 shows an example of the cross-sectional
distribution of hardness from the surface to the core direction in
a soft nitrided material and a soft nitrided and induction hardened
material.
[0042] In a soft nitrided material, the surfacemost layer of the
nitrided layer (see FIG. 2(a), FIG. 2(a) explained later) is formed
with a compound layer and, as, shown in FIG. 1, exhibits an
extremely high hardness, but the compound layer is thin.
[0043] Further, with induction heating, it is learned that the
compound layer at the surfacemost layer breaks down, N diffuses
inside, and the surfacemost layer falls somewhat in hardness, but
the hardened layer (nitrided layer) effective for improving the
contact fatigue strength increases.
[0044] Note that, the surface layer structure of a quenched, soft
nitrided material is martensite and the core is a ferrite-pearlite
structure.
[0045] If making the thickness of the compound layer breaking down
by induction hardening 10 .mu.m or more, it is possible to increase
the thickness of the high N concentration nitrided layer. The
compound layer formed by nitridation becomes a brittle compound
layer and is degraded in mechanical properties in some cases
depending on the nitridation conditions, so usually effort is made
to reduce the thickness of the compound layer.
[0046] As opposed to this, the present invention is characterized
by deliberately making the thickness of the compound layer greater
to deliberately utilize the properties of the compound layer. That
is, the present invention increases the thickness of the compound
layer to form martensite containing a large amount of N at the time
of induction hardening and obtain a structure with a high softening
resistance.
[0047] In the present invention, due to the formation of a
structure with a high softening resistance, the softening
resistance at the time of a high temperature is strikingly
increased.
[0048] (b) At the time of soft nitriding, to form a thick compound
layer, it is necessary to reduce the amount of S inhibiting the
formation of Fe--N compounds. If S forms a solid solution in the
steel material on its own, it will concentrate at the surface of
the steel material and inhibit nitriding. To suppress this action
of S, a certain amount or more of M is added to immobilize the S as
MnS and render it harmless.
[0049] Mn, if added in an amount satisfying Mn/S.gtoreq.70,
suppresses the action of S and exhibits a remarkable effect for
formation of a compound layer. Provided, however, that Mn/S is
preferably 30000 or less.
[0050] (c) At the time of induction heating, to promote the
breakdown of the compound layer and diffusion of N into the steel
and increase the nitrided layer depth, it is necessary to add the
required amount of W with a high affinity with N. Due to the
addition of W, the N concentration in the nitrided layer remarkably
increases and, furthermore, the nitrided layer increases in depth
and the softening resistance is improved.
[0051] Due to the addition of W, even at a low temperature of less
than 900.degree. C., it is possible to sufficiently promote the
diffusion of N. In this way, it is possible to lower the heating
temperature, so it is possible to prevent the deterioration of the
mechanical properties due to the effect of refining the size of the
crystal grains and the effect of reducing the oxide layer.
[0052] (d) To prevent seizure or sticking of the operating
surfaces, it is effective to provide oil reservoirs so that a film
of a lubricant is formed without break. The present invention is
characterized by the formation of a compound layer at the surface
layer of the steel material by soft nitriding and then using the
subsequent induction heating for austenization for hardening and
thereby form a nitrided layer.
[0053] FIG. 2 show an example of a nitride layer observed by an
optical microscope and a scan type electron microscope. FIG. 2(a)
shows a nitride layer observed by an optical microscope, while FIG.
2(b) shows a nitride layer observed by a scan type electron
microscope (enlargement of layer part).
[0054] From FIG. 2(b), it will be understood that the nitride layer
is a hard, porous layer with a large number of pores functioning as
oil reservoirs due to breakdown of the compound layer. Due to the
nitride layer including a large number of pores, the lubricating
effect is improved and the wear resistance and durability are
greatly improved.
[0055] By controlling the soft nitriding and induction heating
conditions, it is possible to form pores of a circle equivalent
diameter of 0.1 .mu.m to 1 .mu.m in dimensions in a density of
10000/mm.sup.2 or more in the nitrided layer from the surface down
to a depth of 5 .mu.m or more. The pores in the nitrided layer
effectively function as oil reservoirs.
[0056] The present invention was completed based on the above
findings and has as its gist the following:
[0057] (1) Steel for machine structure use for surface hardening
containing, by mass %, [0058] C: 0.3 to 0.6%, [0059] Si: 0.02 to
2.0%, [0060] Mn: 1.5% to 3.0%, [0061] W: 0.0025 to 0.5%, [0062] Al:
0.001 to 0.5%, [0063] N: 0.003 to 0.02%, [0064] S: 0.0001 to
0.025%, [0065] P: 0.0001 to 0.03%, and [0066] O: 0.0001 to 0.005%,
[0067] having an Mn/S of 70 to 30000, and [0068] having a balance
of substantially Fe and unavoidable impurities.
[0069] (2) Steel for machine structure use for surface hardening as
set forth in (1), characterized by further containing, by mass %,
one or more of [0070] Cr: 0.01 to 2.0%, [0071] Mo: 0.01 to 1.0%,
and [0072] V: 0.01 to 1.0%.
[0073] (3) Steel for machine structure use for surface hardening as
set forth in (1) or (2), characterized by further containing, by
mass %, [0074] B: 0.0005 to 0.005%.
[0075] (4) Steel for machine structure use for surface hardening as
set forth in any one of (1) to (3), characterized by further
containing, by mass %, one or more of [0076] Nb: 0.005 to 0.3%,
[0077] Ti: 0.005 to 0.2%, [0078] Ni: 0.05 to 2.0%, and [0079] Cu:
0.01 to 2.0%.
[0080] (5) Steel for machine structure use for surface hardening as
set forth in any one of (1) to (4), characterized by further
containing, by mass %, one or more of [0081] Ca: 0.0005 to 0.01%,
[0082] Mg: 0.0005 to 0.01%, [0083] Zr: 0.0005 to 0.05%, and [0084]
Te: 0.0005 to 0.1%.
[0085] (6) Steel for machine structure use for surface hardening as
set forth in any one of (1) to (5), characterized in that said
steel for machine structure use for surface hardening is steel
which is nitrided, then induction hardened.
[0086] (7) Steel for machine structure use for surface hardening as
set forth in (6) characterized in that said nitriding is soft
nitriding.
[0087] (8) A steel part for machine structure use obtained by
machining steel for machine structure use for surface hardening as
set forth in any one of (1) to (7), nitriding it, then induction
hardening it, said steel part for machine structure use
characterized in that the surface layer from the surface down to a
depth of 0.4 mm or more is a nitrided layer and the hardness of the
nitrided layer from the surface down to a depth of 0.2 mm is a
Vicker's hardness at the time of tempering at 300.degree. C. of 650
or more.
[0088] (9) A steel part for machine structure use as set forth in
(8), characterized in that said nitriding is soft nitriding.
[0089] (10) A steel part for machine structure use as set forth in
(8) or (9), characterized in that said nitrided layer from the
surface down to a depth of 5 .mu.m or more includes pores of a
circle equivalent diameter of 0.1 to 1 .mu.m in an amount of
10000/mm.sup.2 or more.
Advantageous Effects of Invention
[0090] According to the present invention, it is possible to
provide steel for structural use for surface hardening able to be
applied to power transmission parts of automobiles etc. and
possible to provide steel parts having high contact fatigue
strength, in particular, gears, continuously variable transmission,
constant velocity joints, hubs, and other steel parts for machine
structures.
BRIEF DESCRIPTION OF DRAWINGS
[0091] FIG. 1 is a view showing an example of the distribution of
hardness in the cross-section from the surface in the direction of
the core in a soft nitrided material and in a soft nitrided and
induction hardened material.
[0092] FIG. 2 are views showing an example of a nitrided layer when
observed under an optical microscope and a scan type electron
microscope. (a) shows a nitrided layer observed by an optical
micrograph, while (b) shows a nitrided layer (layer part enlarged)
observed by a scan type electron microscope.
[0093] FIG. 3 is a view showing the relationship between the Mn/S
and compound layer thickness (.mu.m).
[0094] FIG. 4 is a view showing the relationship between the N
concentration (mass %) at a position of 0.2 mm from the surface
layer after induction hardening and the 300.degree. C. tempered
hardness (Hv).
DESCRIPTION OF EMBODIMENTS
[0095] The present invention has as its basic idea to produce a
steel part from which a high contact fatigue strength is demanded
by nitriding or soft nitriding steel to which suitable amounts of
Mn and W are added, then induction hardening it to deeply form a
nitrided layer with a high N concentration and improve the hardness
and softening resistance.
[0096] First, the reasons for defining the composition of
ingredients forming the basis of the present invention will be
explained. Here, the % means the mass %.
[0097] C: 0.3 to 0.6%
[0098] C is an important element for obtaining strength of the
steel. In particular, it is an element required for reducing the
ferrite percentage of the structure before induction hardening,
improving the hardening ability at the time of induction hardening,
and increasing the depth of the hardened layer.
[0099] If less than 0.3%, the ferrite percentage is high and the
hardening at the time of induction hardening is insufficient, so
the lower limit was made 0.3%. The preferable lower limit is
0.36%.
[0100] On the other hand, if too great, the machineability and
forgeability at the time of fabrication of the steel part
remarkably fall and, furthermore, the possibility of occurrence of
quenching cracks at the time of induction hardening becomes
greater, so the upper limit was made 0.6%. The preferable upper
limit is 0.53%.
[0101] Si: 0.02 to 2.0%
[0102] Si is an element having the effect of raising the softening
resistance of the quenched layer and increasing the contact fatigue
strength. To obtain this effect, 0.02% or more has to be added.
Preferably, 0.1% or more, more preferably 0.25% or more is
added.
[0103] However, if over 2.0%, the decarburization at the time of
forging becomes remarkable, so 2.0% was made the upper limit. The
preferable upper limit was 1.44%.
[0104] Mn: 1.5 to 3.0%
[0105] Mn is an element effective for improving the quenchability
and increasing the softening resistance and thereby raising the
contact fatigue strength. Furthermore, Mn has the effect of
immobilizing the S in the steel as MnS and suppressing the action
of S in concentrating at the surface of the steel material to
inhibit entry of N and of promoting the formation of a thick
compound layer due to nitriding or soft nitriding.
[0106] To immobilize S as MnS to render it harmless, Mn has to be
added to satisfy Mn/S.gtoreq.70.
[0107] Further, Mn is an element having the effect of lowering the
ferrite percentage of the structure before induction hardening and
raising the hardening ability at the time of induction hardening.
To obtain the effect of addition, 1.5% or more has to be added.
Preferably, it is 1.55% or more, more preferably 1.6% or more.
[0108] However, if over 3.0%, when producing a steel material, it
becomes too hard and the cuttability of steel bars etc. is
obstructed. Furthermore, Mn segregates between dendrites at the
solidification stage at the time of steelmaking and makes the steel
material locally harden and become brittle, so 3.0% was made the
upper limit. The preferable upper limit is 2.59%, while the more
preferable upper limit is 2.29%.
[0109] S: 0.0001 to 0.025%
[0110] S is a soft nitriding inhibiting element which has the
action of improving the machineability, but concentrating at the
surface of the steel material and obstructing entry of N into the
steel material at the time of soft nitriding.
[0111] If over 0.025%, the action of inhibiting nitriding becomes
remarkable and, furthermore, the forgeability also remarkably
degrades, so even if added for improving the machineability, it
should be kept at 0.025% or less. Preferably, it is 0.019% or less,
more preferably 0.009% or less. The lower limit was made the
industrial limit of 0.0001%.
[0112] To immobilize the S in the steel as MnS and render it
harmless, it is necessary that 30000.gtoreq.Mn/S.gtoreq.70.
[0113] Mn/S: 70 to 30000
[0114] As explained above, this prevents the S from concentrating
at the surface of the steel material, so a certain ratio or more of
Mn to the S has to be added to immobilize the S as MnS and render
the S harmless.
[0115] If the ratio Mn/S of the amounts of addition of Mn and S is
less than 70, S concentrates at the surface of the steel material
and inhibits the formation of the compound layer at the time of
nitriding or soft nitriding, so Mn/S was made 70 or more. If Mn/S
is 70 or more, the effect of addition is remarkable.
[0116] The upper limit of Mn and the lower limit of S are set, so
there is no particular need to set the upper limit of Mn/S, but
when Mn/S.apprxeq.30000, the effect of addition of Mn becomes
saturated, so the upper limit was made 30000.
[0117] FIG. 3 is a view showing the relationship between the Mn/S
obtained by soft nitriding the steel material under the later
explained conditions and the thickness (.mu.m) of the compound
layer. From FIG. 3, it is understood that if making Mn/S 70 or
more, at the time of soft nitriding, it is possible to obtain a
compound layer of a thickness of 10 .mu.m or more.
[0118] W: 0.0025 to 0.5%
[0119] W is an element with a good affinity with N and having the
action of raising the quenchability and promoting the diffusion of
N at the time of induction heating to raise the contact fatigue
strength. Further, W is an element having the action of lowering
the ferrite percentage of the structure before induction hardening
and raising the hardening ability at the time of induction
hardening.
[0120] Furthermore, W is an element which does not easily segregate
in the steel and has the action of uniformly raising the
quenchability of steel and thereby making up for the drop in the
quenchability due to the segregation of Mn. To obtain the effect of
addition of W, 0.0025% or more has to be added. Preferably, it is
0.01% or more, more preferably 0.05% or more.
[0121] Provided, however, if over 0.5%, the machineability
deteriorates and, furthermore, the effect of addition is saturated
and the economy is impaired, so 0.5% was made the upper limit. The
preferable upper limit is 0.40%, more preferably 0.25%.
[0122] Al: 0.001 to 0.5%
[0123] Al precipitates and disperses as Al nitrides in the steel
and effectively acts for refining the austenite structure at the
time of induction hardening.
[0124] Furthermore, it is an element having the action of raising
the quenchability and increasing the depth of the hardened layer.
Further, Al is an element effective for improvement of the
machineability.
[0125] To obtain the effect of addition, 0.001% or more has to be
added. Preferably, it is 0.005% or more, more preferably 0.010% or
more.
[0126] However, if over 0.5% is added, the precipitate coarsens and
the steel is made brittle, so the upper limit was made 0.5%. The
preferable upper limit is 0.31%, more preferably 0.14%.
[0127] N: 0.003 to 0.02%
[0128] N is an element forming various types of nitrides and
effectively acting to refine the austenite structure at the time of
induction hardening. To obtain the effect of addition, 0.003% or
more has to be added. Preferably, it is 0.005% or more.
[0129] However, if over 0.02% is added, the forgeability
deteriorates, so 0.02% was made the upper limit. The preferable
upper limit is 0.01%.
[0130] P: 0.0001 to 0.03%
[0131] P is an element segregating at the grain boundaries and
acting to reduce the toughness. For this reason, it has to be
reduced as much as possible. It is limited to 0.03% or less.
Preferably, it is limited to 0.01% or less. The lower limit is made
the industrial limit of 0.0001%.
[0132] O: 0.0001 to 0.0050%
[0133] O is present in the steel as Al.sub.2O.sub.3, SiO.sub.2, and
other oxide-based inclusions, but if O is too great, said oxides
end up becoming large in size and form starting points leading to
fracture of the power transmission parts, so has to be limited to
0.0050% or less.
[0134] The less, the better, so 0.0020% or less is preferable.
Furthermore, when aiming at longer life, 0.0015% or less is
preferable. The lower limit is the industrial limit of 0.0001%.
[0135] Next, the composition of ingredients of the optional
elements of the present invention will be explained.
[0136] [Contact Fatigue Strength Improving Elements]
[0137] Cr: 0.01 to 2.0%
[0138] Cr is an element having the effect of raising the nitrided
properties of the steel and of raising the softening resistance of
the quenched layer to improve the contact pressure fatigue
strength. To obtain the effect of addition, 0.01% or more is added.
Preferably, it is 0.1% or more, more preferably 0.52% or more.
[0139] Provided, however, that if over 2.0% is added, the
machineability deteriorates, so 2.0% was made the upper limit. The
preferable upper limit is 1.74%, while the more preferable upper
limit is 1.30%.
[0140] Mo: 0.01 to 1.0%
[0141] Mo is an element which has the effect of raising the
softening resistance of the quenched layer to improve the contact
fatigue strength and the effect or strengthening and toughening the
quenched layer to improve the bending fatigue strength. To obtain
the effect of addition, 0.01% or more has to be added. Preferably,
it is 0.05% or more, more preferably 0.12% or more.
[0142] Provided, however, even if over 1.0% is added, the effect of
addition becomes saturated and the economy is impaired, so 1.0% was
made the upper limit. The preferable upper limit is 0.80%, the more
preferable upper limit is 0.69%.
[0143] V: 0.01 to 1.0%
[0144] V is an element precipitating and dispersing as nitrides in
the steel and effectively acting to refine the austenite structure
at the time of induction hardening. To obtain the effect of
addition, 0.01% or more has to be added. Preferably, it is 0.10% or
more, more preferably 0.25% or more.
[0145] However, even if over 1.0% is added, the effect of addition
becomes saturated and the economy is impaired, so the upper limit
was made 1.0%. The preferable upper limit is 0.80%, the more
preferable upper limit is 0.68%.
[0146] In the present invention, to raise the contact fatigue
strength, one or more of Cr: 0.01 to 2.0%, Mo: 0.01 to 1.0%, and V:
0.01 to 1.0% are added.
[0147] [Quenchability Improving Elements]
[0148] B: 0.0005 to 0.005%
[0149] B is an element contributing to the improvement of the
quenchability. To obtain the effect of addition, 0.0005% or more
has to be added. Preferably, it is 0.001% or more. Provided,
however, even if over 0.0050% is added, the effect of addition is
saturated, so 0.005% was made the upper limit. The preferable upper
limit is 0.003%.
[0150] [Steel Reinforcing Elements]
[0151] Nb: 0.005 to 0.3%
[0152] Nb is an element precipitating and dispersing as nitrides in
the steel and effectively acting to refine the austenite structure
at the time of induction hardening. To obtain the effect of
addition, 0.005% or more has to be added. Preferably, it is 0.01%
or more, more preferably 0.04% or more.
[0153] However, if over 0.3% is added, the effect of addition
becomes saturated and the economy is impaired, so the upper limit
was made 0.3%. The preferable upper limit is 0.2%, while the more
preferable upper limit is 0.16%.
[0154] Ti: 0.005 to 0.2%
[0155] Ti is an element precipitating and dispersing as nitrides in
the steel and effectively acting to refine the austenite structure
at the time of induction hardening. To obtain the effect of
addition, 0.005% or more has to be added. Preferably it is 0.02% or
more, more preferably 0.05% or more.
[0156] However, if over 0.2% is added, the precipitate coarsens and
the steel is made brittle, so the upper limit was made 0.2%. The
preferable upper limit is 0.15%, the more preferable upper limit is
0.11%.
[0157] Ni: 0.05 to 2.0%
[0158] Ni is an element further improving the toughness. To obtain
the effect of addition, 0.05% or more has to be added. Preferably,
it is 0.10% or more, more preferably 0.21% or more.
[0159] However, if over 2.0% is added, the machineability
deteriorates, so 2.0% was made the upper limit. The preferable
upper limit is 1.5%, the more preferable upper limit is 0.96%.
[0160] Cu: 0.01 to 2.0%
[0161] Cu is an element strengthening ferrite and effective for
improvement of the quenchability and improvement of the corrosion
resistance. To obtain the effect of addition, 0.01% or more has to
be added. Preferably, it is 0.09% or more, more preferably 0.14% or
more.
[0162] However, even if over 2.0% is added, the effect of
improvement of the mechanical properties becomes saturated, so 2.0%
was made the upper limit. The preferable upper limit is 1.5%, while
the more preferable upper limit is 0.95%. Note that, Cu
particularly lowers the hot rollability and easily becomes a cause
of flaws at the time of rolling, so is preferably added
simultaneously with the Ni.
[0163] In the present invention, to reinforce the steel material,
one or more of Nb: 0.005 to 0.3%, Ti: 0.005 to 0.2%, Ni: 0.05 to
2.0%, and Cu: 0.01 to 2.0% is added.
[0164] [Bending Strength Improving Elements]
[0165] When seeking an improvement of the bending fatigue strength
at the steel part, one or more of Ca: 0.0005 to 0.01%, Mg: 0.0005
to 0.01%, Zr: 0.0005 to 0.05%, and Te: 0.0005 to 0.1% is added to
the steel material.
[0166] The above elements are elements suppressing bending fatigue
fracture of gears and fatigue fracture of the bottom of the splines
of shaft parts caused by flattening of MnS and thereby further
improving the bending fatigue strength.
[0167] To obtain the effect of addition, Ca: 0.0005% or more, Mg:
0.0005% or more, Zr: 0.0005% or more, or Te: 0.0005% or more has to
be added. Preferably, Ca: 0.0010% or more, Mg: 0.0010% or more, Zr:
0.0010% or more, or Te: 0.0010% or more is added.
[0168] However, even if adding Ca: over 0.01%, Mg: over 0.01%, Zr:
over 0.05%, and Te: over 0.1%, the effect of addition becomes
saturated and the economy is impaired, so Ca: 0.01%, Mg: 0.01%, Zr:
0.05%, and Te: 0.1% were made the upper limits. The preferable
upper limits are Ca: 0.005%, Mg: 0.005%, Zr: 0.005%, and Te:
0.07%.
[0169] That is, to obtain an MnS flattening suppressing effect, one
or more of Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005 to
0.05%, and Te: 0.0005 to 0.1% may be added.
[0170] Further, in the present invention, in addition to the above
elements, it is possible to include Pb, Bi, Sn, Zn, REM, and Sb to
an extent not impairing the effect of the present invention.
[0171] Next, the thickness and hardness of the nitrided layer at
the surface layer of a steel part will be explained.
[0172] The steel part of the present invention is a steel part
obtained by machining steel of the present invention, nitriding or
soft nitriding it, then induction hardening it, characterized in
that the surface layer from the surface down to a depth of 0.4 mm
or more is a nitrided layer and the hardness of the nitrided layer
from the surface down to a depth of 0.2 mm is a Vicker's hardness
at the time of tempering at 300.degree. C. of 650 or more.
[0173] If the nitrided layer has a thickness of less than 0.4 mm,
the surface layer having a sufficient hardness becomes thinner.
Before fracture of a surface starting point occurs, internal
fracture, that is, "spalling", occurs and the lifetime becomes
short, so the surface layer from the surface down to a depth of
0.4mm or more is designated as the "nitrided layer".
[0174] Contact fatigue fracture is fracture of a surface starting
point formed at a sliding surface raised in temperature (to around
300.degree. C.), so maintaining the high temperature strength, that
is, increasing the temper softening resistance, is effective for
improving the contact fatigue strength.
[0175] In the nitrided layer of a depth from the surface of 0.2 mm,
if the Vicker's hardness when tempering at 300.degree. C. is less
than 650, the nitrided layer cannot withstand a high contact
pressure, so at the nitrided layer from the surface down to a depth
of 0.2 mm, the Vicker's hardness when tempering at 300.degree. C.
is made 650 or more.
[0176] At an actual steel part, whether it is a steel part obtained
by soft nitriding, then induction hardening can be judged by (a)
the distribution of structures observed under an optical microscope
after obtaining a micro sample from the steel part and corroding it
by a Nital corrosive solution, (b) the distribution of hardness
measured from the surface to the core, and, furthermore, (c) the N
concentration from the surface to the core measured by EPMA.
[0177] To form a nitrided layer with a high N concentration to
obtain a high contact fatigue strength, it is necessary to form a
compound layer breaking down and supplying N at the time of high
frequency heat treatment (layer mainly comprised of Fe.sub.3N,
Fe.sub.4N, or other Fe nitrides) at the steel surface. For this
reason, the nitriding or soft nitriding is necessary and important
treatment.
[0178] To make a sufficient amount of N diffuse in the steel, make
the surface layer of the steel hard, and deeply form a nitrided
layer with a high temper softening resistance, it is necessary to
make the thickness of the compound layer after nitriding or soft
nitriding 10 .mu.m or more.
[0179] If soft nitriding at a high temperature of over 600.degree.
C., the compound layer becomes thin and, furthermore, the N
concentration in the compound layer becomes lower, so the soft
nitriding temperature is made less than 600.degree. C. If the soft
nitriding temperature is a low temperature, it is possible to
prevent heat treatment deformation, grain boundary oxidation, etc.
of the steel material, so from this point as well, the soft
nitriding temperature is made less than 600.degree. C.
[0180] To form a thick compound layer, the soft nitriding
temperature is preferably 500.degree. C. or more. The depth of the
nitrided layer reaches the saturated state even if soft nitriding
for a long time, so the soft nitriding time is preferably 1 to 3
hours.
[0181] The cooling after the soft nitriding may be performed by any
method of air cooling, N.sub.2 gas cooling, oil cooling, etc. As
the soft nitriding, gas soft nitriding or salt bath soft nitriding
may be used.
[0182] As the method for supplying nitrogen to the surface of the
steel material and forming a 10 .mu.m or more compound layer at the
surface layer of the steel material, not only soft nitriding, but
also nitriding may be used. The "nitriding" referred to here is not
a method, like soft nitriding, of treatment in a mixed atmosphere
of NH.sub.3 and CO.sub.2 (and sometimes also N.sub.2), but a
surface hardening method of treatment by NH.sub.3 for a long time
and is an industrially different method.
[0183] To break down the compound layer formed at the surface of
the steel material by soft nitriding and, furthermore, make the N
diffuse in the steel to form a nitrided layer with a high N
concentration deep from the surface down to a depth of 0.4 mm or
more and obtain a high hardness of a Vicker's hardness at the time
of quenching at 300.degree. C. of 650 or more at the nitrided layer
from the surface down to a depth of 0.2 mm, it is necessary to soft
nitride, then induction heat the steel to heat it to the austenite
region and quench it.
[0184] The heating conditions at the time of the induction
hardening have to be set considering the breakdown of the compound
layer. The heating temperature has to be made the austenization
temperature to less than 900.degree. C. and the holding time 0.05
to 5 seconds.
[0185] Here, FIG. 4 shows the relationship of the N concentration
at a position 0.2 mm from the surface after induction hardening and
the Vicker's hardness at the time of tempering at 300.degree. C.
From FIG. 4, it will be understood that to make the Vicker's
hardness 650 or more, the N concentration has to be made 0.35% or
more.
[0186] If the heating temperature is 900.degree. C. or more, N
unnecessarily diffuses to the inside and the N concentration at the
nitrided layer from the surface down to 0.2 mm required for
improvement of the contact fatigue strength becomes less than
0.35%.
[0187] As a result, the Vicker's hardness when tempering at
300.degree. C. becomes less than 650 and, furthermore, the increase
in the oxide layer causes deterioration of the mechanical
properties. If the heating temperature is less than the
austenization temperature, martensite transformation does not occur
and a high surface hardness cannot be obtained.
[0188] If the holding time is less than 0.05 second, the breakdown
of the compound layer and diffusion of the N produced become
insufficient. On the other hand, if over 5 seconds, N is
unnecessarily diffused inward and the N concentration at the
nitrided layer from the surface down to 0.2 mm required for
improvement of the contact fatigue strength becomes less than
0.35%.
[0189] As a result, the Vicker's hardness when tempering at
300.degree. C. becomes less than 650.
[0190] The frequency of the induction is around 400 kHz if a small
sized steel part and around 5 kHz if a large sized steel part.
[0191] The coolant used for the quenching is preferably water, a
polymer quenching agent, or other water-based one with a large
cooling ability. After induction hardening, usually it is
preferable to temper the steel part at a low temperature of around
150.degree. C. based on a carburized quenched part so as to secure
the toughness of the part.
[0192] Next, the surface layer structure of a steel part will be
explained.
[0193] The steel part of the present invention is a steel part
obtained by soft nitriding, then induction hardening, characterized
in that a surface layer from the surface down to a depth of 5 .mu.m
or more includes pores of a circle equivalent diameter of 0.1 to 1
.mu.m in an amount of 10000/mm.sup.2 or more.
[0194] At a steel member such as a gear which fractures due to
contact fatigue due to rolling, lubrication of the operating
surfaces is important. If the lubrication is insufficient, the
steel materials will contact each other, seizing and sticking will
occur, and the contact fatigue strength falls.
[0195] To form a lubricating film giving sufficient lubrication, it
is effective to provide oil reservoirs so that a lubricant oil film
is formed without break on the operating surfaces. The present
invention is characterized by soft nitriding the surface layer of
the steel material to form a compound layer mainly comprised of
Fe.sub.3N, Fe.sub.4N, and Fe nitrides, then induction heating it to
austenize and quench it to form a nitrided layer.
[0196] The nitrided layer is formed by the breakdown of the
compound layer, but at this time, the N in the compound layer
diffuses inside to form a nitrided layer and the old compound layer
becomes a hard porous layer in which a large number of pores are
dispersed. These pores function as oil reservoirs resulting in an
improvement in the lubricating effect and a greater improvement in
the wear resistance and durability. This is a discovery which the
inventors learned as a result of intensive research.
[0197] The fact that if pores of a circle equivalent diameter of
0.1 to 1 .mu.m are present in an amount of 10000/mm.sup.2 or more
in a nitrided layer from surface down to 5 .mu.m or more in depth,
they effectively function as oil reservoirs was discovered by the
inventors after intensive research.
[0198] To ensure that the size and distribution of the pores fall
in the required range, the soft nitriding conditions and induction
heating conditions have to be suitably controlled. The compound
layer formed by the soft nitriding also has some pores, so an oil
reservoir effect is expressed, but said compound layer is extremely
fragile and cannot withstand a large load, so a high contact
fatigue strength cannot be obtained.
[0199] If the pores are coarse, the surface roughness deteriorates
and becomes the starting points of pitching and contact fatigue and
therefore inhibits the contact fatigue strength, so the size of the
pores was limited to a circle equivalent diameter of 1 .mu.m or
less. On the other hand, if the pores are too small, the pores do
not sufficiently function as oil reservoirs, so the size of the
pores has to be a circle equivalent diameter of 0.1 .mu.m or
more.
[0200] If the number of pores is too small, the pores do not
effectively function as oil reservoirs, so there have to be
10000/mm.sup.2 or more at the nitrided layer from the surface down
to 5 [2m or more in depth.
[0201] The gear faces of gears and other sliding members, in normal
operation, are worn down by 5 .mu.m or more before the end of their
lifetime, so it is necessary that there be pores in the amount of
10000/mm.sup.2 or more at the surface down to a depth of 5 .mu.m or
more. If the pores are large in size, the density depends on the
soft nitriding conditions and induction heating conditions.
[0202] To make the nitrided layer and effective porous layer, heat
treatment under soft nitriding conditions and induction hardening
conditions enabling a high contact fatigue strength to be obtained
is essential. Preferably, soft nitriding is performed at
580.degree. C. to less than 600.degree. C., then induction heating
is performed at 880.degree. C. to less than 900.degree. C. for 1
second to 4 seconds.
[0203] The structure of the steel member has to be made one with a
surface layer of martensite and a core remaining as a
ferrite-pearlite structure. If quenching only the surface layer to
cause martensite transformation and giving the surface layer
compressive residual stress, the contact fatigue strength is
improved. If the core as well is made to transform to martensite,
the surface layer falls in compressive residual stress and the
contact fatigue strength falls.
Examples
[0204] Next, examples of the present invention will be explained,
but the conditions of the examples are illustrations of the
conditions employed for confirming the workability and effect of
the present invention. The present invention is not limited to
these illustrations of conditions. The present invention may employ
various conditions so long as not deviating from the gist of the
present invention and achieving the object of the present
invention.
Examples
[0205] Various steel materials with the chemical compositions shown
in Table 1 and Table 2 (continuation of Table 1) were forged, then
annealed, then machined to fabricate test pieces for use for roller
pitching fatigue tests, that is, (a) small roller test pieces
having cylindrical parts of diameters of 26 mm and widths of 28 mm
and (b) large roller test piece having cylindrical parts of
diameters of 130 mm and widths of 18 mm.
[0206] Furthermore, test pieces of a diameter of 26 mm and a length
of 100 mm were fabricated for use for hardness tests for
investigating the softening resistance by tempering.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Example Class
C Si Mn P S Cr Mo W V Al O 1 Inv. ex. 0.31 0.70 2.12 0.025 0.013
1.23 0.79 0.39 0.28 0.051 0.002 2 Inv. ex. 0.39 0.60 1.92 0.028
0.009 1.51 0.58 0.31 0.37 0.038 0.001 3 Inv. ex. 0.43 0.78 2.39
0.015 0.009 0.68 0.15 0.07 0.40 0.083 0.001 4 Inv. ex. 0.48 0.53
1.98 0.012 0.021 1.66 0.12 0.07 0.08 0.077 0.005 5 Inv. ex. 0.50
1.13 1.81 0.014 0.018 0.67 0.76 0.32 0.68 0.160 0.002 6 Inv. ex.
0.53 1.39 2.54 0.017 0.014 1.41 0.80 0.10 0.31 0.098 0.001 7 Inv.
ex. 0.54 0.92 2.00 0.024 0.016 1.43 0.59 0.12 0.36 0.130 0.003 8
Inv. ex. 0.55 1.25 1.75 0.025 0.014 1.89 0.69 0.06 0.49 0.101 0.001
9 Inv. ex. 0.56 0.41 1.55 0.010 0.017 2.00 0.66 0.15 0.56 0.075
0.002 10 Inv. ex. 0.58 0.50 2.18 0.017 0.007 0.95 0.57 0.17 0.43
0.171 0.003 11 Inv. ex. 0.50 1.12 2.23 0.019 0.025 0.82 0.63 0.07
0.67 0.098 0.005 12 Inv. ex. 0.55 1.44 1.62 0.015 0.011 1.61 0.79
0.25 0.43 0.022 0.002 13 Inv. ex. 0.47 1.05 2.36 0.024 0.014 1.30
0.42 0.18 0.25 0.039 0.003 14 Inv. ex. 0.49 0.76 1.60 0.025 0.021
1.24 0.17 0.40 0.59 0.021 0.001 15 Inv. ex. 0.43 1.03 2.38 0.027
0.016 1.93 0.21 0.25 0.29 0.081 0.001 16 Inv. ex. 0.48 0.41 2.05
0.016 0.006 1.09 0.42 0.34 0.53 0.012 0.003 17 Inv. ex. 0.36 0.31
1.55 0.025 0.019 0.54 0.23 0.27 0.30 0.029 0.004 18 Inv. ex. 0.42
1.13 1.87 0.016 0.006 0.60 0.51 0.38 0.51 0.136 0.003 19 Inv. ex.
0.45 0.51 2.29 0.028 0.016 1.84 0.12 0.14 0.44 0.014 0.002 20 Inv.
ex. 0.58 0.43 2.07 0.010 0.022 0.61 0.15 0.25 0.22 0.099 0.002 21
Inv. ex. 0.35 1.35 1.70 0.016 0.012 0.70 0.77 0.40 0.07 0.081 0.005
22 Inv. ex. 0.52 1.06 2.12 0.026 0.016 1.60 0.05 0.30 0.12 0.034
0.004 23 Inv. ex. 0.49 1.60 1.76 0.020 0.008 1.74 0.38 0.12 0.55
0.080 0.004 24 Inv. ex. 0.42 0.26 2.27 0.014 0.007 1.63 0.69 0.05
0.25 0.291 0.005 25 Inv. ex. 0.47 0.79 2.59 0.017 0.013 1.22 0.05
0.13 0.22 0.130 0.002 Chemical composition (mass %) Example N B Nb
Ti Ni Cu Ca Mg Zr Te Mn/S 1 0.003 167 2 0.018 220 3 0.019 257 4
0.004 95 5 0.014 101 6 0.005 176 7 0.005 125 8 0.018 127 9 0.018 89
10 0.015 319 11 0.004 0.003 90 12 0.006 0.005 0.02 150 13 0.004
0.09 0.06 172 14 0.004 0.68 0.15 76 15 0.009 0.001 0.22 0.79 151 16
0.006 0.05 0.12 0.95 367 17 0.005 0.003 0.16 82 18 0.004 0.003 0.04
0.11 0.21 0.57 315 19 0.003 0.001 0.0025 145 20 0.010 0.66 0.48
0.0014 94 21 0.008 0.0016 144 22 0.007 0.001 0.10 0.80 0.75 0.065
134 23 0.005 0.002 0.48 0.11 0.0019 0.0044 209 24 0.003 0.12 0.14
0.0020 0.0046 316 25 0.004 0.004 0.05 0.08 0.96 0.09 0.0005 0.0007
0.0027 0.010 196
TABLE-US-00002 TABLE 2 Chemical composition (mass %) Example Class
C Si Mn P S Cr Mo W V Al O 26 Inv. ex. 0.41 0.50 1.62 0.015 0.009
0.68 6.15 0.07 0.40 0.083 0.001 27 Inv. ex. 0.45 0.49 1.63 0.011
0.011 0.52 0.01 0.06 0.10 0.093 0.001 28 Inv. ex. 0.53 0.70 1.65
0.012 0.010 1.10 0.07 0.06 0.10 0.025 0.002 29 Inv. ex. 0.50 0.64
1.60 0.013 0.013 0.95 0.09 0.06 0.11 0.036 0.001 30 Inv. ex. 0.45
0.32 1.75 0.009 0.012 0.85 0.11 0.07 0.12 0.025 0.001 31 Inv. ex.
0.42 0.25 1.89 0.012 0.010 0.94 0.05 0.12 0.13 0.026 0.002 32 Comp.
ex. 0.15 0.83 1.54 0.022 0.016 0.82 1.01 0.38 0.31 0.267 0.002 33
Comp. ex. 0.44 1.33 0.51 0.015 0.025 0.51 1.43 0.12 0.12 0.074
0.002 34 Comp. ex. 0.44 0.03 0.80 0.027 0.055 1.60 1.25 0.12 0.11
0.056 0.001 35 Comp. ex. 0.52 0.01 0.92 0.010 0.011 1.52 0.06 0.600
0.002 36 Comp. ex. 0.21 0.50 1.52 0.012 0.025 0.69 0.16 0.35 0.094
0.002 37 Comp. ex. 0.55 0.28 0.81 0.010 0.025 0.60 0.11 0.37 0.020
0.005 38 Comp. ex. 0.55 0.28 1.75 0.010 0.100 0.60 0.11 0.23 0.020
0.004 39 Comp. ex. 0.35 0.80 1.65 0.013 0.023 0.80 0.47 0.03 0.33
0.029 0.001 40 Comp. ex. 0.40 1.32 1.68 0.018 0.016 1.89 0.41 0.12
0.39 0.017 0.002 41 Comp. ex. 0.44 1.26 1.68 0.029 0.021 0.91 0.12
0.07 0.15 0.013 0.003 42 Comp. ex. 0.45 1.19 2.00 0.014 0.017 0.72
0.48 0.05 0.31 0.025 0.003 43 Comp. ex. 0.47 0.72 1.69 0.023 0.022
1.34 0.43 0.02 0.22 0.026 0.004 44 Comp. ex. 0.55 0.45 1.89 0.021
0.018 1.52 0.25 0.03 0.15 0.052 0.003 45 Comp. ex. 0.44 0.86 0.58
0.022 0.024 0.54 0.09 0.30 0.027 0.002 46 Comp. ex. 0.45 0.25 0.55
0.005 0.015 0.50 0.60 0.50 0.030 0.001 47 Comp. ex. 0.55 0.25 0.35
0.005 0.013 0.51 0.61 0.49 0.029 0.001 48 Comp. ex. 0.55 0.25 0.55
0.003 0.010 2.90 0.61 0.51 0.033 0.001 49 Comp. ex. 0.54 0.12 0.79
0.009 0.010 0.30 0.020 0.001 50 Comp. ex. 0.55 0.10 0.41 0.009
0.022 1.01 0.05 0.10 0.440 0.001 51 Comp. ex. 0.58 1.00 0.51 0.008
0.010 0.98 0.30 0.100 0.001 52 Comp. ex. 0.20 0.25 1.00 0.010 0.017
0.90 0.50 0.015 0.001 53 Comp. ex. 0.40 0.50 1.20 0.009 0.013 2.00
0.015 0.001 Chemical composition (mass %) Example N B Nb Ti Ni Cu
Ca Mg Zr Te Mn/S 26 0.005 174 27 0.004 148 28 0.006 0.001 0.02 165
29 0.004 6.62 0.14 123 30 0.004 0.001 0.02 0.25 0.11 146 31 0.004
0.002 0.06 0.04 0.36 0.09 0.0005 0.0006 0.0026 0.010 189 32 0.028
96 33 0.026 20 34 0.013 15 35 0.021 84 36 0.021 0.002 61 37 0.009
32 38 0.005 18 39 0.027 0.002 0.050 72 40 0.014 105 41 0.013 0.03
81 42 0.020 0.002 115 43 0.014 0.0020 77 44 0.012 105 45 0.003 0.10
24 46 0.006 0.0016 37 47 0.006 0.0017 27 48 0.006 0.0019 55 49
0.006 79 50 0.006 19 51 0.006 51 52 0.006 0.50 59 53 0.006 92
[0207] The small roller test pieces and large roller test pieces
were soft nitrided (nitrided in N.sub.2(0.45
Nm.sup.3/h)+NH.sub.3(0.5 Nm.sup.3/h)+CO.sub.2(0.05 Nm.sup.3/h)
atmosphere at a predetermined temperature for 2 hours and cooled in
N.sub.2 gas), then induction hardened (frequency 100 kHz).
[0208] For the coolant at the time of induction hardening, tap
water or a polymer quenching agent was used. After this, the pieces
were tempered at 150.degree. C. for 60 minutes for use for the
fatigue test.
[0209] The test pieces of the large rollers and small rollers were
used for a standard contact fatigue test, that is, roller pitching
fatigue test. The roller pitching fatigue test was conducted by
applying various Hertz stresses (contact pressures) to the small
rollers and pushing the large rollers, making the rotating
directions of the two test pieces the same at the contact parts,
and making the slip rate -40% (at the contact parts, the peripheral
speeds of the large rollers being 40% larger than the peripheral
speeds of the small roller test pieces).
[0210] The oil temperature of the gear oil fed to the above contact
parts was made 90.degree. C. The test was usually ended at
10,000,000.times. (10.sup.7.times.) showing the fatigue limit of
steel. The maximum Hertz stress where pitching did not occur at the
small roller and a speed of 10,000,000 was reached was made the
fatigue limit of the small roller.
[0211] The occurrence of pitching was detected by a vibration meter
attached to the tester. After detecting vibration, the two rollers
were stopped from rotating and the occurrence of pitching and the
speed were confirmed.
[0212] The test pieces for measurement of hardness were soft
nitrided and induction hardened under the same conditions as the
case of the small roller test pieces and large roller test pieces.
After this, the pieces were quenched at 300.degree. C. for 60
minutes and cut. The cut cross-sections were measured for the
distribution of hardness from the surfaces to the cores by a
Vicker's hardness meter. Note that the quenched surface layer
structure was martensite and the non-quenched core remained a
ferrite-pearlite structure.
[0213] Further, the N concentration at a position 0.2 m from the
surface was measured by EPMA.
[0214] The density of pores of a circle equivalent diameter of 0.1
to 1 .mu.m was found by taking test pieces soft nitrided and
induction hardened under the same conditions as the small roller
test pieces and large roller test pieces, cutting them at surfaces
perpendicular to the rolling direction, embedding these in a resin,
mirror polishing them, and quantifying the resultant surface layer
structures by image processing.
[0215] The measurement was performed by a power of 3000.times. for
a field of 50 .mu.m.sup.2 for 40 fields. The measurement value was
converted to the number of pores per mm.sup.2 to calculate the
density.
[0216] The test results are shown in Table 3 and Table 4
(continuation of Table 3)
TABLE-US-00003 TABLE 3 After induction hardening Soft nitriding
Induction Position 0.2 mm Compound heating Nitrided from surface
Fatigue test layer conditions layer 300.degree. C. Pore maximum
Temp. thickness Temp. Time thick. N conc. tempered density hertz
stress Example Class (.degree. C.) .mu.m (.degree. C.) (second) mm
(%) hardness (/mm.sup.2) (MPa) Remarks 1 Inv. ex. 552 28 897 1.6
0.48 0.89 726 8119 3800 2 Inv. ex. 582 30 877 2.5 0.45 0.97 725
5930 3800 3 Inv. ex. 582 25 898 1.5 0.42 0.64 740 57209 4000 4 Inv.
ex. 566 23 837 4.2 0.41 0.54 730 2424 3800 5 Inv. ex. 575 21 894
2.9 0.45 0.86 733 7799 3800 6 Inv. ex. 579 28 846 1.4 0.42 0.60 693
1491 3800 7 Inv. ex. 583 24 881 4.9 0.42 0.70 690 8731 3800 8 Inv.
ex. 577 23 832 4.4 0.41 0.38 679 2661 3800 9 Inv. ex. 590 19 854
4.9 0.42 0.67 704 8603 3800 10 Inv. ex. 555 28 868 4.2 0.43 0.84
758 2485 3800 11 Inv. ex. 553 21 893 2.4 0.41 0.44 684 6117 3800 B
addition 12 Inv. ex. 598 27 898 3.0 0.44 0.97 750 41245 4000 B, Ti
addition 13 Inv. ex. 552 25 861 0.08 0.44 0.93 718 2375 3800 Nb, Ti
addition 14 Inv. ex. 587 20 859 3.2 0.45 0.91 798 5635 3800 Ni, Cu
addition 15 Inv. ex. 588 22 801 2.4 0.44 0.94 778 7704 3800 B, Ni,
Cu addition 16 Inv. ex. 595 29 855 3.4 0.52 0.73 794 6120 3800 Ti,
Ni, Cu addition 17 Inv. ex. 570 17 885 2.2 0.44 0.89 774 8915 3800
B, Nb addition 18 Inv. ex. 565 33 826 2.6 0.48 0.91 734 2607 3800
B, Nb, Ti, Ni, Cu addition 19 Inv. ex. 569 25 893 4.5 0.43 0.77 747
6110 3800 B, Ca addition 20 Inv. ex. 583 22 898 1.1 0.43 0.98 768
82387 4000 Ni, Cu, Mg addition 21 Inv. ex. 590 24 882 4.4 0.47 0.97
750 8516 3800 Zr addition 22 Inv. ex. 571 22 881 2.5 0.46 0.89 716
6656 3800 B, Ti, Ni, Cu, Te addition 23 Inv. ex. 580 31 872 4.5
0.44 0.89 720 2130 3800 B, Ni, Cu, Ca, Mg addition 24 Inv. ex. 574
27 898 0.5 0.42 0.58 755 5916 3800 Nb, Ti, Mg, Zr addition 25 Inv.
ex. 588 28 822 3.2 0.42 0.73 714 7884 3800 B, Nb, Ti, Ni, Cu, Ca,
Mg, Zr, Te addition
TABLE-US-00004 TABLE 4 Soft After induction hardening Fatigue
nitridation Induction Position 0.2 mm test Compound heating
Nitrided from surface maximum layer conditions layer 300.degree. C.
Pore Hertz Temp. thick. Temp. Time thick. N conc. temper density
stress Example Class (.degree. C.) .mu.m (.degree. C.) (sec) mm (%)
hardness (/mm.sup.2) (MPa) Remarks 26 Inv. ex. 593 21 895 1.3 0.41
0.49 731 70083 4000 27 Inv. ex. 591 28 893 1.9 0.46 0.44 723 79563
4000 28 Inv. ex. 591 29 891 2.3 0.48 0.66 761 14633 3900 B, Ti
addition 29 Inv. ex. 595 28 897 3.1 0.44 0.60 755 76575 4000 Ni, Cu
addition 30 Inv. ex. 594 26 898 1.8 0.45 0.57 731 14999 3900 B, Ti,
Ni, Cu addition 31 Inv. ex. 597 27 894 1.0 0.43 0.50 714 74858 4000
B, Nb, Ti, Ni, Cu, Ca, Mg, Zr, Te addition 32 Comp. ex. 593 15 876
1.4 0.33 0.57 595 2376 2900 C lower limit exceeded 33 Comp. ex. 590
2 850 3.2 0.13 0.07 465 4934 2600 Mn lower limit exceeded, Mn/S
exceeded 34 Comp. ex. 571 1 833 4.5 0.10 0.08 473 2674 2500 Mn
lower limit exceeded, S upper limit exceeded, Mn/S exceeded 35
Comp. ex. 564 10 802 1.0 0.32 0.29 590 2465 2800 Si, Mn lower limit
exceeded, W, Mo non-addition, Al upper limit exceeded 36 Comp. ex.
599 5 804 1.0 0.22 0.14 582 4523 2900 C lower limit exceeded, W
non-addition 37 Comp. ex. 559 4 884 5.0 0.29 0.25 570 2754 2800 Mn
lower limit exceeded, W non-addition, Mn/S exceeded 38 Comp. ex.
585 1 879 3.2 0.09 0.18 467 2038 2600 S upper limit exceeded, Mn/S
exceeded 39 Comp. ex. 587 12 1200 4.7 0.65 0.20 575 4271 3000
Induction heating temp. upper limit exceeded 40 Comp. ex. 595 25
700 3.5 0.20 0.03 398 2816 2900 Induction heating temp. lower limit
exceeded 41 Comp. ex. 577 20 890 8.0 0.70 0.27 471 1865 2900
Induction heating time upper limit exceeded 42 Comp. ex. 580 21 845
0.03 0.07 0.04 425 880 2600 Induction heating time lower limit
exceeded 43 Comp. ex. 680 5 837 4.9 0.10 0.05 401 3082 2600 Soft
nitriding temp. upper limit exceeded 44 Comp. ex. 490 9 880 3.0
0.25 0.36 651 1155 2700 Soft nitriding temp. lower limit exceeded
45 Comp. ex. 700 3 886 2.1 0.15 0.06 388 4670 2800 Mn lower limit
exceeded, W non-addition, Mn/S exceeded, soft nitriding temp. upper
limit exceeded 46 Comp. ex. 570 2 910 2.5 0.14 0.07 507 575 2600 Mn
lower limit exceeded, W non-addition, Mn/S exceeded, induction
heating temp. upper limit exceeded 47 Comp. ex. 570 3 910 1.2 0.10
0.06 556 4366 2600 Mn lower limit exceeded, W non-addition, Mn/S
exceeded, induction heating temp. upper limit exceeded 48 Comp. ex.
570 3 910 4.2 0.12 0.11 580 2865 2700 Mn lower limit exceeded, W
non-addition, Mn/S exceeded, induction heating temp. upper limit
exceeded 49 Comp. ex. 590 10 1000 4.0 0.32 0.20 572 3482 2900 Mn
lower limit exceeded, W, Mo, V non-addition induction heating temp.
upper limit exceeded 50 Comp. ex. 590 2 1150 2.0 0.36 0.19 544 3337
2900 Mn lower limit exceeded, W non-addition, Mn/S exceeded,
induction heating temp. upper limit exceeded 51 Comp. ex. 590 4 900
6.0 0.32 0.21 575 1639 2700 Mn lower limit exceeded, W, Mo
non-addition, Mn/S exceeded, induction heating temp. upper limit
exceeded 52 Comp. ex. 650 4 1000 2.0 0.30 0.10 518 1293 2800 C, Mn
lower limit exceeded, W, Mo non-addition, nitriding temp.,
induction heating temp. upper limit exceeded 53 Comp. ex. 650 4
1000 2.0 0.30 0.08 535 3757 2900 Mn lower limit exceeded, W, Mo, V
non-addition nitriding temp., induction heating temp. upper limit
exceeded
[0217] As shown in Table 3 and Table 4, the invention examples of
Examples 1 to 31 all have a lifetime of the roller pitching fatigue
test of 10,000,000 (10.sup.7) or more and an excellent contact
fatigue strength (high fatigue test life) and give good results
compared with the comparative examples of Examples 32 to 53.
[0218] In the invention examples of Examples 1, 2, and 4 to 10
using the steel materials to which suitable amounts of Mn and W are
added, (a) by less than 600.degree. C. soft nitriding, a compound
layer of a thickness of 10 .mu.m is formed, (b) by quenching after
an austenization temperature of less than 900.degree. C. and 0.08
to 4.9 seconds of induction heating, a nitrided layer of a
thickness of 0.4 mm or more is formed, and (c) by a high N
concentration, a Vicker's hardness of 650 or more is obtained by
300.degree. C. tempering at a position 0.2 mm from the surface.
[0219] As a result, in the invention examples of Examples 1, 2, and
4 to 10, excellent contact fatigue strength is obtained.
[0220] In the invention examples of Examples 11, 13 to 19, and 21
to 25 using steel materials to which optional elements are added as
well, the lifetime in roller pitching fatigue tests is 10,000,000
or more and a good contact fatigue strength is obtained.
[0221] Furthermore, in the invention examples of Examples 3, 12,
20, and 26 to 31 (pore density of 10000/mm.sup.2 or more) as well,
the lifetime in roller pitching fatigue tests is 10,000,000 or more
and a good contact fatigue strength is obtained.
[0222] As opposed to this, in the comparative examples of Examples
32 to 38 where steel types with chemical compositions outside the
scope defined by the present invention are soft nitrided, then
induction hardened, the fatigue test life does not reach 10,000,000
in each case.
[0223] In particular, in the comparative examples of Examples 33,
34, and 38, the Mn/S is low and the surface concentration of S
cannot be prevented. As a result, the compound layer formed by the
soft nitriding is thin, the thickness of the nitrided layer of the
steel part is a small one of less than 0.4 mm after induction
hardening, the N concentration from the surface down to a depth of
0.2 mm is low, and the Vicker's hardness with 300.degree. C.
tempering is a value less than 650.
[0224] In the comparative examples of Examples 39 to 42 where the
chemical compositions are in the scope defined by the present
invention, but the induction heating conditions exceed the
conditions of the present invention, in each case, a sufficiently
thick compound layer is formed by the soft nitriding, but the
induction heating conditions are not sufficient, so the fatigue
test life does not reach 10,000,000.
[0225] For example, in the comparative example of Example 39, the
induction heating temperature is too high, N unnecessarily diffuses
inside, and the thickness of the nitrided layer is 0.65 mm or in
the scope of the present invention, but the N concentration of the
steel part from the surface down to a depth of 0.2 mm is a low
0.20%, the 300.degree. C. tempered Vicker's hardness is also 575,
that is, does not reach 650, and furthermore an oxide layer is
formed at the surface layer and the fatigue life is short.
[0226] Further, in the comparative example of Example 41, the
induction heating time is too long and N unnecessarily diffuses
inside and the thickness of the nitrided layer is 0.70 mm or in the
scope of the present invention, but the N concentration of the
steel part from the surface down to a depth of 0.2 mm is a low
0.27%, the 300.degree. C. tempered Vicker's hardness is a low 471,
and the fatigue life is short.
[0227] In the comparative examples of Examples 43 to 44 with soft
nitriding conditions outside the scope of the conditions of the
present invention, in each case, the compound layer is also thin,
so the nitrided layer of the steel part is thin and the fatigue
life does not reach 10,000,000.
[0228] For example, in the comparative example of Example 44, the
chemical composition is in the range defined by the present
invention, the N concentration from the surface down to a depth of
0.2 mm is a high 0.36%, and the Vicker's hardness when tempering at
300.degree. C. is also a high value of 651, but the soft nitriding
temperature is too low, so the compound layer is thin, the nitrided
layer of the steel part is also a thin 0.25 mm, and the fatigue
life is short.
[0229] In the comparative example of Example 45 where the chemical
composition and the soft nitriding conditions exceed the scope of
the present invention, the Mn/S is small and the soft nitriding
temperature is too high, so the compound layer is thin.
Furthermore, W is not added, so the nitrided layer of the steel
part is also a thin 0.15 mm, the N concentration from the surface
down to a depth of 0.2 mm is a low 0.06%, the Vicker's hardness
when tempering at 300.degree. C. is also 388, that is, does not
reach 650, and the fatigue life is short.
[0230] In the comparative examples of Examples 46 to 51 where the
chemical compositions and the induction heating conditions exceed
the scope of the present invention, in each case, the compound
layer is thin, so the nitrided layer of the steel part is thin, the
300.degree. C. tempered hardness from the surface down to a depth
of 0.2 mm is low, and the fatigue life does not reach
10,000,000.
[0231] For example, in the comparative example of Example 48, the
Mn/S is small, so the compound layer is thin. Further, W is not
added and the induction heating temperature is high, so the
thickness of the nitrided layer of the steel part is a small 0.12
mm, the N concentration from the surface down to a depth of 0.2 mm
is 0.18%, the 300.degree. C. tempered hardness is a low 580, and
the fatigue life is short.
[0232] In the comparative examples of Examples 52 to 53 with
chemical compositions outside the scope defined by the present
invention and, furthermore, soft nitriding conditions and induction
heating conditions outside the scope of the present invention as
well, the compound layer is thin, so the nitrided layer of the
steel part is thin, the 300.degree. C. tempered hardness from the
surface down to a depth of 0.2 mm is low, and the fatigue life does
not reach 10,000,000.
[0233] For example, in the comparative example of Example 53, the
Mn/S is 70 or more, but the soft nitriding temperature is high, so
the compound layer is thin. Further, W is not added and the
induction heating temperature is high, so the nitrided layer of the
steel part is a thin 0.30 mm, the N concentration from the surface
down to 0.2 mm is 0.08%, the 300.degree. C. tempered hardness is a
low 535, and the fatigue life is extremely short.
[0234] From the above results, it is understood that in the
invention examples where (a) suitable amounts of Mn and W are added
and (b) soft nitriding and later induction hardening are performed
under suitable conditions to obtain suitable nitrided layer
thickness and hardness, an excellent contact fatigue strength is
obtained.
INDUSTRIAL APPLICABILITY
[0235] According to the present invention, it is possible to
provide steel for structural use for surface hardening able to be
applied to power transmission parts of automobiles etc. and provide
steel parts having a high contact fatigue strength, in particular
gears, continuously variable transmissions, constant velocity
joints, hubs, bearings and other steel parts for machine structure
use. As a result, the present invention greatly contributes to the
higher output and lower cost of automobiles, so the industrial
applicability is large.
* * * * *