U.S. patent application number 11/319871 was filed with the patent office on 2006-06-29 for case-hardening steel superior in tooth surface fatigue strength, gear using the same, and method of production of the same.
This patent application is currently assigned to Nippon Steel Corporation and Honda Motor Co., Ltd.. Invention is credited to Hideo Kanisawa, Shuji Kozawa, Koki Mizuno, Tatsuro Ochi, Tomoko Serikawa.
Application Number | 20060137766 11/319871 |
Document ID | / |
Family ID | 36610015 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060137766 |
Kind Code |
A1 |
Kozawa; Shuji ; et
al. |
June 29, 2006 |
Case-hardening steel superior in tooth surface fatigue strength,
gear using the same, and method of production of the same
Abstract
The present invention provides case-hardening steel superior in
tooth surface fatigue strength and a gear using the same used for
parts of automobiles, construction machines, industrial machines,
etc., that is case-hardening steel superior in tooth surface
fatigue strength containing, by wt %, C: 0.1 to 0.3%, Si: 1.0 to
2.0%, Mn: 0.3 to 2.0%, S: 0.005 to 0.05%, Cr: 1.0 to 2.6%, Mo: 0.8
to 4.0%, V: 0.1 to 0.3%, Al: 0.001 to 0.2%, and N: 0.003 to 0.03%,
limiting P to 0.03% or less, and having as a balance iron and
unavoidable impurities, and satisfies the following expression,
31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) is 100 or more, and a
gear comprised of case-hardening steel and having an X-ray
diffraction half width at a depth of 50 mm from the surface of the
gear of 6.4 degrees or more.
Inventors: |
Kozawa; Shuji; (Muroran-shi,
JP) ; Ochi; Tatsuro; (Muroran-shi, JP) ;
Kanisawa; Hideo; (Muroran-shi, JP) ; Serikawa;
Tomoko; (Wako-shi, JP) ; Mizuno; Koki;
(Wako-shi, JP) |
Correspondence
Address: |
DORSEY & WHITNEY LLP;INTELLECTUAL PROPERTY DEPARTMENT
250 PARK AVENUE
NEW YORK
NY
10177
US
|
Assignee: |
Nippon Steel Corporation and Honda
Motor Co., Ltd.
|
Family ID: |
36610015 |
Appl. No.: |
11/319871 |
Filed: |
December 27, 2005 |
Current U.S.
Class: |
148/218 ;
148/223; 148/319; 420/111 |
Current CPC
Class: |
C23C 8/32 20130101; C23C
8/22 20130101 |
Class at
Publication: |
148/218 ;
148/223; 148/319; 420/111 |
International
Class: |
C22C 38/22 20060101
C22C038/22; C22C 38/24 20060101 C22C038/24; C23C 8/22 20060101
C23C008/22; C23C 8/32 20060101 C23C008/32; C23C 8/32 20060101
C23C008/32 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2004 |
JP |
2004-377855 (PAT. |
Dec 27, 2004 |
JP |
2004-377856 (PAT. |
Claims
1-8. (canceled)
9. Case-hardening steel superior in tooth surface fatigue strength
characterized by containing, by wt %, C: 0.1 to 0.3%, Si: 1.0 to
2.0%, Mn: 0.3 to 2.0%, S: 0.005 to 0.05%, Cr: 1.0 to 2.6%, Mo: 0.8
to 4.0%, V: 0.1 to 0.3%, Al: 0.001 to 0.2%, and N: 0.003 to 0.03%,
limiting P to 0.03% or less, and having a balance of iron and
unavoidable impurities, wherein satisfying the following expression
(1). 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%).gtoreq.100
(1)
10. Case-hardening steel superior in tooth surface fatigue strength
as set forth in claim 9 wherein said steel further includes, by wt
%, one or two of Nb: 0.2% or less and Ti: 0.2% or less
11. A gear superior in tooth surface fatigue strength characterized
in that it comprises steel as set forth in claim 9 and has an X-ray
diffraction half width at a depth of 50 .mu.m from the gear surface
of 6.4 degrees or more when forming the steel to a gear shape and
carburizing or carbonitriding the same, the "X-ray diffraction half
width" referred to here meaning the half width of the peak when
using a micro-area X-ray residual stress measurement system (Cr
lamp) to measure the .alpha.-Fe (211) plane over 60 seconds.
12. A gear superior in tooth surface fatigue strength as set forth
in claim 11, wherein said gear further includes, by wt %, one or
two of Nb: 0.2% or less and Ti: 0.2% or less
13. A gear superior in tooth surface fatigue strength as set forth
in claim 11, characterized in that the amount of Si, Cr, Mo and V
are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and
V: 0.10 to 0.25%, and satisfies the following expression (2)
instead of the expression (1): 37Si (%)+18Mn (%)+10Cr (%)+31Mo
(%)+201V (%)=100.about.150 (2)
14. A gear superior in tooth surface fatigue strength as set forth
in claim 12, characterized in that the amount of Si, Cr, Mo and V
are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and
V: 0.10 to 0.25%, and satisfies the following expression (2)
instead of the expression (1): 37Si (%)+18Mn (%)+10Cr (%)+31Mo
(%)+201V (%)=100.about.150 (2)
15. A method of production of a gear superior in tooth surface
fatigue strength characterized by forming the steel as set forth in
claim 14 to a gear shape, then subjecting it to vacuum
carburization or vacuum carbonitridation at a heating temperature
of 900 to 1050.degree. C. in range.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] The present application claims priority from Japanese Patent
Application Nos. 2004-377855 and 2004-377855, both filed Dec. 27,
2004 under 35 U.S.C. .sctn. 119. The entire disclosures and content
of these patent applications are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to case-hardening steel
superior in tooth surface fatigue strength and a gear using the
same used for parts of automobiles, construction machines,
industrial machines, etc. and a method of production of the
same.
BACKGROUND ART
[0003] In automobile transmissions etc., gears comprised of mainly
JIS SCr420, SCM420, and other case-hardening steels formed into
gear shapes, then subjected to surface hardening by carburization
quenching and tempering, etc. are used. In such gears, to increase
the output of the automobiles and improve the fuel efficiency etc.,
lighter weight and greater gear strength have been strongly
demanded. In the past, to improve the strength of the gears,
technology for improving the bending fatigue strength of the tooth
bases of the gears has been developed. Recently, however, along
with the development of practical hard shot peening, the emphasis
in increasing the strength of gears has been shifting from the
bending fatigue strength of the tooth bases of gears to the tooth
surface fatigue strength.
[0004] However, for improvement of the tooth surface fatigue
strength, improvement of the temper softening resistance has been
considered effective. In the past, as the means for improving the
temper softening resistance, several technologies improving the
composition of the steel materials of the gears have been proposed.
For example, Japanese Unexamined Patent Publication No. 7-242994
discloses steel containing Si in an amount of 1% or less and Cr in
1.5 to 5.0%. Further, Japanese Unexamined Patent Publication No.
2001-329337 discloses steel containing Si in an amount of 0.40 to
1.50%, Mn in 0.30 to 2.00%, and Cr in 0.50 to 3.00%. Further,
Japanese Patent Publication (A) No. 2003-231943 discloses steel
containing Si in an amount of 0.7 to 1.5%, Cr in 0.1 to 3.0%, and
Mo in 0.05 to 1.5%.
[0005] As explained above, as ingredients of steel for improving
the temper softening resistance, it is known that Si, Cr, Mn, Mo,
and other elements are effective, but at the present,
case-hardening steel superior in tooth surface fatigue strength and
gears of the same are being demanded by further improvement of the
temper softening resistance.
DISCLOSURE OF THE INVENTION
[0006] In consideration of the above, an object of the present
invention is to provide case-hardening steel superior in tooth
surface fatigue strength and a gear using the same by more
effectively improving the temper softening resistance.
[0007] As explained above, it is known that by increasing the
amounts of Si, Cr, Mn, Mo, etc. in steel, it is possible to improve
the temper softening resistance. The inventors discovered the
following matters to further improve the temper softening
resistance and thereby perfected the present invention:
[0008] (1) That in addition to the Si, Cr, Mn, and Mo, V also has
an effect of improving the temper softening resistance.
[0009] (2) That the total of the five elements (Si, Cr, Mn, Mo, and
V) having the effect of improving the temper softening resistance
in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) is 100 or more.
[0010] Further, even if suppressing the amounts of addition of Cr,
Mo, and V, solid solution hardening in the martensite structure is
achieved, the required temper softening resistance is secured, and
the production costs can be reduced, so
37Si(%)+18Mn(%)+10Cr(%)+31Mo(%)+21V(%) should be 100 to 150 in
range.
[0011] (3) That the improvement of the temper softening resistance
by the precipitation hardening by carbides of the above five
elements is insufficient and that the solid solution hardening of
the above five added elements in the martensite structure enables
more effective improvement of the temper softening resistance.
[0012] (4) That the temper softening resistance is improved by the
quenching in the carburization quenching etc. at a high
temperature, causing the carbides precipitated in the material
(steel) to dissolve, and making the interval from the surface of
the treated gear to a depth of 50 .mu.m have an X-ray diffraction
half width of 6.4 degrees or more.
[0013] The present invention was made to achieve the above
treatments and has as its gist the following:
[0014] (1) Case-hardening steel superior in tooth surface fatigue
strength characterized by containing, by wt %, [0015] C: 0.1 to
0.3%, [0016] Si: 1.0 to 2.0%, [0017] Mn: 0.3 to 2.0%, [0018] S:
0.005 to 0.05%, [0019] Cr: 1.0 to 2.6%, [0020] Mo: 0.8 to 4.0%,
[0021] V: 0.1 to 0.3%, [0022] Al: 0.001 to 0.2%, and [0023] N:
0.003 to 0.03%, [0024] limiting P to 0.03% or less, and [0025]
having a balance of iron and unavoidable impurities, and satisfys
the following expression (1). 31Si(%)+15Mn (%)+23Cr (%)+26Mo
(%)+100V(%).gtoreq.100 (1)
[0026] (2) A gear superior in tooth surface fatigue strength
characterized in that it comprises steel as set forth in (1) and
has an X-ray diffraction half width at a depth of 50 .mu.m from the
gear surface of 6.4 degrees or more when forming the steel to a
gear shape and carburizing or carbonitriding the same. The "X-ray
diffraction half width" referred to here means the half width of
the peak when using a micro-area X-ray residual stress measurement
system (Cr lamp) to measure the .alpha.-Fe (211) plane over 60
seconds.
[0027] (3) Case-hardening steel superior in tooth surface fatigue
strength as set forth in (1) wherein said steel further includes,
by wt %, one or two of [0028] Nb: 0.2% or less and [0029] Ti: 0.2%
or less.
[0030] (4) A gear superior in tooth surface fatigue strength as set
forth in (2), wherein said gear further includes, by wt %, one or
two of [0031] Nb: 0.2% or less and [0032] Ti: 0.2% or less.
[0033] (5) A gear superior in tooth surface fatigue strength as set
forth in (2) or (4), characterized in that the amount of Si, Cr, Mo
and V are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to
1.2%, and V: 0.10 to 0.25%, and satisfys the following expression
(2) instead of the expression (1). 37Si (%)+18Mn (%)+10Cr (%)+31Mo
(%)+201V (%)=100.about.150 (2)
[0034] (6) A method of production of a gear superior in tooth
surface fatigue strength characterized by forming the steel as set
forth in (5) to a gear shape, then subjecting it to vacuum
carburization or vacuum carbonitridation at a heating temperature
of 900 to 1050.degree. C. in range.
BEST MODE FOR WORKING THE INVENTION
[0035] In the past, it has been known that increasing the amount of
Si, Cr, Mn, Mo, and other elements in steel improves the temper
softening resistance. The inventors however believed that if
excessively adding these elements, large amounts of carbides would
precipitate and the average size of the carbides would increase and
therefore the temper softening resistance would conversely
deteriorate. Therefore, the inventors thought that by dissolving
Si, Cr, Mn, Mo, and other added elements in the steel, it might be
possible to effectively improve the tooth surface fatigue strength
of a gear.
[0036] Further, they thought that by similarly adding V as well to
the steel and dissolving it in the steel, it would be possible to
increase the temper softening resistance.
[0037] Therefore, the inventors postulated that by using a steel
containing suitable amounts of Si, Cr, Mn, Mo, V, and other
elements to make a gear and then making the added elements dissolve
by high temperature carburization quenching or other quenching, it
might be possible to further improve the temper softening
resistance. They therefore used different steels with different
amounts of addition of Si, Cr, Mn, Mo, V, and other elements to
form gear shapes, then hardened the surfaces of the gears by high
temperature carburization quenching and tempering so as to produce
different gears and investigated the fatigue life of the tooth
faces of the gears. Further, they confirmed whether the solid
solution hardening by the above-mentioned added elements in the
martensite structure improved the fatigue life of the tooth faces
by using the X-ray diffraction half width at a depth of 50 .mu.m
from the surface of the gear as an indicator of the amount of solid
solution hardening in the martensite structure and measuring the
X-ray diffraction half width at a depth of 50 .mu.m from the
surface of the produced gears by a micro-area X-ray residual stress
measurement system.
[0038] As a result, the following matters became clear. First, it
became clear that to achieve an improvement of the tooth surface
fatigue strength of a gear, just using steel increased in amounts
of addition of Si, Cr, Mn, Mo, etc. is insufficient. That is, the
inventors found that for improvement of the temper softening
resistance, addition of V in addition to the conventional Si, Cr,
Mn, or Mo is also effective, that just causing precipitation of
these added elements as carbides is insufficient for improvement of
the tooth surface fatigue strength of a gear, and that dissolution
of the added elements into the steel effectively leads to an
improvement of the tooth surface fatigue strength of a gear. From
this, they guessed that metallurgically, the increase in the temper
softening resistance through the precipitation hardening of the
added elements is insufficient for improving the tooth surface
fatigue strength of a gear and that the increase in the tempering
softening resistance through solid solution hardening by the added
elements in the martensite structure effectively may contribute to
improvement of the tooth surface fatigue strength of a gear.
[0039] Further, they found that in steel containing C, Si, Mn, S,
Cr, Mo, V, Al, N, and P in predetermined amounts and comprised of a
balance of iron, unavoidable impurities, etc., a total amount of
Si, Mn, Cr, Mo, and V in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V
(%) of 100 or more can more effectively improve the temper
softening resistance
[0040] Further, even if suppressing the amounts of addition of Cr,
Mo, and V and making 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+21V (%)
100 to 150 in range, solid solution hardening of the martensite
structure is achieved, the required temper softening resistance is
secured, and production costs can be reduced.
[0041] Further, it became clear that when using this steel as a
material, forming it to a gear shape, then subjecting the surface
of the gear to vacuum carburization, carbonitridation, and other
surface hardening, a gear having an X-ray diffraction half width at
a depth of 50 .mu.m from the surface of the gear of 6.4 degrees or
more is further improved in temper softening resistance, that is,
has a superior tooth surface fatigue strength.
[0042] From the above, steel containing C, Si, Mn, S, Cr, Mo, V,
Al, N, and P in predetermined amounts and comprised of a balance of
iron, unavoidable impurities, etc., having a total of Si, Mn, Cr,
Mo, and V in 31Si(%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) of 100 or
more, and further having a total of Si, Mn, Cr, Mo and V in 37Si
(%)+18Mn (%)+10Cr (%)+31Mo (%)+21V (%) of 100 to 150 can be said to
be useful as case-hardening steel superior in tooth surface fatigue
strength.
[0043] Further, by selecting gears having an X-ray diffraction half
width at a depth of 50 .mu.m from the surface of the gear of 6.4
degrees or more from gears obtained by using the above-mentioned
case-hardening steel as a material for shaping gears, then
subjecting the surfaces of the gears to vacuum carburization,
carbonitridation, and other surface hardening, it is suggested that
gears superior in tooth surface fatigue strength can be obtained.
Therefore, it is believed that the thus obtained gears rise in
temperature near the surface of the tooth faces to about
300.degree. C. due to the heat of friction generated due to contact
of the drive faces and driven faces of the gears at a high facial
pressure accompanied with sliding and have resistance even with the
temper softening arising as a result and that further this can
greatly contribute to higher output, improved fuel efficiency, etc.
in automobiles, construction machines, industrial machines, etc.
Note that while the gears superior in tooth surface fatigue
strength according to the present invention can be obtained in the
above-mentioned way, they may also be obtained by carburization or
carbonitridation, then shot peening, subzero cooling, WPC, WJP,
etc. Due to this, it becomes possible to make the residual
austenite at the surface of the gear transform to martensite and
increase the temper softening resistance.
[0044] Next, the ranges of the wt % of the chemical ingredients
included in the steel of the present invention (case-hardening
steel) will be explained. [0045] C: 0.1 to 0.3%
[0046] C is an element essential for maintaining the strength of
the steel. Its content determines the strength of the core part and
also affects the effective hardened layer depth. Therefore, in the
present invention, the lower limit of the amount of C was made 0.1%
to secure the core strength. However, if the content is too great,
the toughness falls, so 0.3% was made the upper limit. [0047] Si:
1.0 to 2.0%
[0048] Si is an element effective for improving the temper
softening resistance. Addition of 1.0% gives this effect.
Therefore, in the present invention, the lower limit of the amount
of Si was made 1.0%. However, if the content is over 2.0%, the
carburization ability deteriorates, so 2.0% was made the upper
limit. [0049] P Mn: 0.3 to 2.0%
[0050] Mn is an element effective for improving the hardenability
and further is an element effective for improving the temper
softening resistance. Further, it also has the action of
immobilizing the impurity element S unavoidably contained in the
steel as MnS and thereby rendering it harmless. Therefore, as the
amount of Mn, 0.3% or more is believed necessary. Therefore, in the
present invention, the lower limit of the amount of Mn was made
0.3%. However, if the content is over 2.0%, this ends up increasing
and stabilizing the residual austenite in the carburized layer to
an extent unable to be prevented even if performing subzero cooling
and the temper softening resistance conversely deteriorates, so
2.0% was made the upper limit. [0051] S: 0.005 to 0.05%
[0052] S is an impurity element unavoidably included, but from the
viewpoint of the machinability must be included in an amount of
0.005% or more. Therefore, in the present invention, the lower
limit of the amount of S was made 0.005%. However, if the content
is over 0.05%, the forgeability is inhibited, so 0.05% was made the
upper limit. [0053] Cr: 1.0 to 2.6%
[0054] Cr is an element effective for improving the hardenability
and is an element effective for improving the temper softening
resistance. Addition in 1.0% or more gives this effect. Therefore,
in the present invention, the lower limit of the amount of Cr was
made 1.0%. However, if the content exceeds 2.6%, the Cr carbides
present in the material will not completely dissolve even with high
temperature carburization and the temper softening resistance will
conversely deteriorate, so 2.6% was made the upper limit. Note that
to completely prevent the occurrence of coarse grains in the
carburization, Cr is preferably 1.0 to 1.8%. [0055] Mo: 0.8 to
4.0%
[0056] Mo is an element effective for improving the hardenability
and is an element effective for improving the temper softening
resistance. Addition in 0.8% or more gives this effect. Therefore,
in the present invention, the lower limit of the amount of Mn was
made 0.8%. However, if the content is over 4.0%, the Mo carbides
present in the material cannot completely dissolve even with high
temperature carburization and the temper softening resistance
conversely deteriorates, so 4.0% was made the upper limit. Note
that to completely prevent the occurrence of coarse grains in the
carburization, Mo is preferably 1.0 to 1.2%. [0057] V: 0.1 to
0.3%
[0058] V is an element effective for improving the temper softening
resistance. Addition of 0.1% or more gives this effect. Therefore,
in the present invention, the lower limit of the amount of V was
made 0.1%. However, if the content is over 0.3%, the V carbides
present in the material cannot completely dissolve in solid
solution even by high temperature carburization and the temper
softening resistance conversely deteriorates, so 0.3% was made the
upper limit. Note that to completely prevent the occurrence of
coarse grains in the carburization, V is preferably 0.1 to 0.25%.
[0059] Al: 0.001 to 0.2%
[0060] Al has the effect of refinement of the crystal grains due to
the formation of compounds with N, so 0.001% or more is considered
necessary. Therefore, in the present invention, the lower limit of
the amount of Al was made 0.001%. However, if over 0.2%, the
machineability is remarkably inhibited, so 0.2% was made the upper
limit. [0061] N: 0.003 to 0.03%
[0062] N is an unavoidably included element, but also has the
effect of refinement of the crystal grains by formation of
compounds with Al and N, so 0.003% or more is considered necessary.
Therefore, in the present invention, the lower limit of the amount
of N was made 0.003%. However, if the content is over 0.03%, the
forgeability is remarkably inhibited, so 0.03% was made the upper
limit. [0063] P: limited to 0.03% or less
[0064] P is an unavoidably included impurity element. It
precipitates at the grain boundaries and lowers the toughness, so
has to be limited to 0.03% or less. Therefore, in the present
invention, the amount of P was limited to 0.03% or less.
[0065] In addition, for the purpose of further refinement of the
crystal grains or preventing coarsening of the crystal grains in
the steel of the present invention, it is also possible to further
add Nb, Ti, etc. in addition to the above-mentioned chemical
ingredients. In this case, these elements are preferably included
in the following ranges not inhibiting the productivity of the hot
rolling, hot forging, cutting, etc. [0066] Nb: 0.2% or less and Ti:
0.2% or less, one or both
[0067] Nb and Ti have the effect of refinement of the crystal
grains due to the formation of compounds with N, so inclusion of
one or both of Nb and Ti is preferable. However, even if each
element is included in an amount of over 0.2%, the effect of
refinement of the crystal grains becomes saturated and the
economicalness is impaired, so 0.2% was made the upper limit.
[0068] Next, the total amount of the Si, Mn, Cr, Mo, and V in the
steel of the present invention will be explained. In the present
invention, the total amount of Si, Mn, Cr, Mo, and V in the
following formula being 100 or more is an essential condition.
[0069] This, as explained above, is based on intensive research and
development by the inventors and as a result the discovery that
when the total amount of the Si, Mn, Cr, Mo, and V in the following
expression is 100 or more, a gear superior in tooth surface fatigue
strength can be obtained. Note that at the left side in the
expression, the coefficients of the elements of Si, Mn, Cr, Mo, and
V differ because the extents by which the elements contribute to
improvement of the temper softening resistance differ.
31Si(%)+15Mn(%)+23Cr(%)+26Mo(%)+100V(%).gtoreq.100
[0070] Further, in the present invention, a total amount of Si, Mn,
Cr, Mo, and V in the following expression of 100 to 150 in range is
an essential condition. This is because, as explained above, the
inventors engaged in intensive research and as result found that
when the total amount of the Si, Mn, Cr, Mo, and V in the above
expression is 100 or more, a gear superior in tooth surface fatigue
strength can be obtained. If using a steel material of a high alloy
composition of over 150 in the above expression, the starting point
of the martensite transformation falls. Due to this, the amount of
residual austenite after vacuum carburization exceeds 20%. Compared
with martensite, residual austenite is softer. Due to this, a
remarkable drop in the strength of the surface of the gear is
caused. Therefore, in the present invention, a total amount of Si,
Mn, Cr, Mo, and V in the following expression of 150 or less was
made a condition. Note that at the left side in the following
expression, the coefficients of the elements of Si, Mn, Cr, Mo, and
V differ because the extents by which the elements contribute to
the improvement of the temper softening resistance differ. 37Si
(%)+18Mn (%)+10Cr(%)+31Mo (%)+201V (%)=100 to 150
[0071] Next, the reason for the gear according to the present
invention having an X-ray diffraction half width at a depth of 50
.mu.m from the surface of the gear of 6.4 degrees or more being
made a condition will be explained.
[0072] By satisfying the above formula and securing an X-ray
diffraction half width at a depth of 50 .mu.m from the surface of
the gear of 6.4 degrees or more, a gear superior in tooth surface
fatigue strength was realized. Even if using steel satisfying just
the above expression to form a gear shape and subjecting this to
carburization quenching and tempering at a general 930.degree. C.,
the X-ray diffraction half width at a depth of 50 .mu.m from the
surface of the gear will not necessarily become 6.4 degrees or
more. The inventors believed that therefore selection of a surface
hardening giving an X-ray diffraction half width of 6.4 degrees or
more was crucial. Further, at a stage before the surface hardening,
some of the Mn, Cr, Mo, and V remains as carbides, but as the
contents of Mo, V, etc. become greater, with carburization at the
general 930.degree. C., the dissolution of the carbides becomes
insufficient and an X-ray diffraction half width of 6.4 degrees or
more becomes impossible to secure. Therefore, it is believed
necessary to cause the carbides to dissolve at a carburization
temperature of preferably 950.degree. C. or more, in some cases
1000.degree. C. or more. Further, as the value at the left side in
the above expression becomes greater, the amount of residual
austenite tends to gradually become greater. Along with this, the
X-ray diffraction half width tends to become smaller. For this
reason, when the value of the above expression is 130 or more, it
is considered effect to further perform subzero cooling or shot
peening to transform the residual austenite to martensite and make
the X-ray half width 6.4 degrees or more.
[0073] Therefore, in the present invention, an X-ray diffraction
half width at a depth of 50 .mu.m from the gear surface of 6.4
degrees or more was made a condition. Note that the above-mentioned
X-ray diffraction half width means the half width of the peak when
using a micro-area X-ray residual stress measurement system (Cr
lamp) to measure the .alpha.-Fe (211) plane over 60 seconds.
[0074] Next, the reason for vacuum carburizing the tooth surfaces
of the gear at a heating temperature in a range of 900 to
1050.degree. C. after using above-mentioned steel material to form
a gear shape will be explained.
[0075] A carburization temperature of less than 900.degree. C. is
insufficient for making the carbides dissolve in the material
containing 100 or more of the elements in the above formula
(steel). 900.degree. C. or more, preferably 950.degree. C. or more,
is necessary. Therefore, in the present invention, the lower limit
of the carburization temperature was made 900.degree. C. However,
if the carburization temperature exceeds 1050.degree. C., the
problem of coarse grains arises, so 1050.degree. C. was made the
upper limit.
[0076] However, in general, as the method of carburization, gas
carburization and vacuum carburization are broadly used. The
inventors investigated this and found that with gas carburization,
the fine amount of oxygen contained in the carrier gas causes grain
boundary oxidation of about 10 .mu.m at the surface of the gear
resulting in a drop in the strength, so vacuum carburization must
be applied. Therefore, in the present invention, treating the tooth
surfaces of the gear shape by vacuum carburization was made a
condition.
[0077] Further, in the present embodiment, the inventors used the
above-mentioned steel material as a material to form a gear shape,
then subjected this to vacuum carburization at a heating
temperature of 900 to 1050.degree. C. in range so as to produce a
gear superior in tooth surface fatigue strength, but even if
performing broadly used treatment after the above vacuum
carburization, for example, shot peening, WPC, WJP, subzero
cooling, etc., the effect of the present invention will not be
inhibited, so these treatments may be performed after the vacuum
carburization.
EXAMPLES
Example 1
[0078] Below, the present invention will be explained in more
detail by examples. Note that these examples are for explaining the
present invention and do not limit the scope of the present
invention.
[0079] Hot rolled steel materials having the chemical compositions
shown in Table 1 were spheroidally annealed to secure
machineability, then were used to fabricate drive gears and driven
gears with pitch circle diameters of 65.8 mm, modules of 1.5, and
35 teeth (Test Nos. 1 to 15). TABLE-US-00001 TABLE 1 31 Si + Test
Chemical composition (wt %) 15 Mn + 23 Cr + No. C Si Mn P S Cr Mo V
Al N Others 26 Mo + 100 V 1 Inv. ex. 0.21 1.30 0.35 0.008 0.012
1.53 1.01 0.20 0.038 0.015 127 2 Inv. ex. 0.21 1.31 0.36 0.028
0.049 1.53 1.02 0.11 0.035 0.014 119 3 Inv. ex. 0.10 1.40 0.35
0.006 0.013 2.53 1.01 0.10 0.001 0.010 Ti:0.028 143 4 Inv. ex. 0.29
1.41 0.34 0.007 0.014 2.50 1.01 0.15 0.036 0.018 148 5 Inv. ex.
0.20 1.98 0.36 0.007 0.013 2.01 1.05 0.14 0.002 0.009 Ti:0.025 154
6 Inv. ex. 0.20 1.40 0.31 0.006 0.005 2.54 1.01 0.11 0.035 0.016
144 7 Inv. ex. 0.20 1.41 1.98 0.008 0.014 2.53 1.02 0.29 0.197
0.016 187 8 Inv. ex. 0.21 1.30 0.35 0.008 0.012 2.60 1.00 0.20
0.038 0.262 151 9 Inv. ex. 0.20 1.05 0.35 0.007 0.012 2.43 3.95
0.16 0.036 0.015 212 10 Inv. ex. 0.21 1.03 0.36 0.009 0.012 2.52
0.99 0.50 0.033 0.016 Nb:0.031 171 11 Comp. 0.21 2.54 0.36 0.007
0.014 1.51 1.01 0.15 0.035 0.013 160 ex. 12 Comp. 0.20 2.02 0.36
0.007 0.013 2.01 1.01 0.16 0.035 0.013 157 ex. 13 Comp. 0.21 0.25
0.78 0.014 0.018 1.16 1.02 0.10 0.035 0.013 83 ex. 14 Comp. 0.20
0.25 0.78 0.015 0.015 1.23 1.02 0.22 0.035 0.013 96 ex. 15 Comp.
0.21 1.03 0.36 0.009 0.012 2.52 1.51 0.10 0.033 0.016 145 ex.
[0080] Next, the surface hardening explained below was performed
under working conditions giving an effective hardened layer depth
of the gear of 0.6 mm. In Test Nos. 1 to 3, 5, 6, 11 to 15, vacuum
carburization quenching was performed at 1000.degree. C., then
tempering was performed at 200.degree. C. for 90 minutes. In Test
No. 7, vacuum carburization quenching was performed at 1000C,
subzero cooling was performed by liquid nitrogen for 60 minutes,
then finally tempering was performed at 200.degree. C. for 90
minutes. In Test No. 4, gas carburization at 950.degree. C. for 120
minutes and carbonitridation at 860.degree. C. for 30 minutes were
successively performed, then quenching was performed, then
tempering was performed at 200.degree. C. for 90 minutes, then shot
peening was performed at an arc height of 1.0. In Test Nos. 8 and
9, vacuum carburization quenching was performed at 1050.degree. C.,
subzero cooling was performed by liquid nitrogen for 60 minutes,
and finally tempering was performed at 200.degree. C. for 90
minutes. In Test No. 10, vacuum carburization was performed at
1050.degree. C., then tempering was performed at 200.degree. C. for
90 minutes, and finally shot peening was performed at an arc height
of 1.0.
[0081] Then, the inventors evaluated the amounts of increase of the
temper softening resistance due to the solution hardening by the
Si, Cr, Mn, Mo, and other added elements for the above-mentioned
treated Test Nos. 1 to 15. Note that temper softening resistance is
usually evaluated by using a microvicker's hardness meter etc. to
measure the hardness in a micro area, but with this method of
evaluation, the amount of hardening due to precipitation and the
amount of hardening due to solid solution cannot be differentiated,
so it is not possible to measure only the amount of hardening due
to solid solution. Therefore, in this embodiment, based on the
discovery that the amount of increase due to the solid solution
hardening in the martensite structure is important for improving
the tooth surface fatigue strength of a gear, the inventors
measured, the X-ray diffraction half width at a depth of 50 .mu.m
from the gear surface of the gear as an indicator of the amount of
increase due to the solid solution hardening in the martensite
structure by a micro-area X-ray residual stress measurement system
so as to evaluate the amount of increase of the temper softening
resistance. Note that the X-ray diffraction half width at a depth
of 50 .mu.m from the gear surface of Test Nos. 1 to 15 was found by
using a micro-area X-ray residual stress measurement system (Cr
lamp) to measure the half width of the peak for the .alpha.-Fe
(211) plane over 60 seconds.
[0082] Further, the inventors investigated the fatigue life of the
tooth surfaces of Test Nos. 1 to 15 by using a power circulating
type gear fatigue tester to investigate the lifetime (X) at a test
load of 200Nm. Note that the lifetime was measured by detecting the
vibration accompanying chipping of the tooth face. The above test
results are shown in Table 2. TABLE-US-00002 TABLE 2 X-ray
diffraction Test Surface half width Test results No. hardening
(degree) Lifetime (X) 1 Inv. ex. [1] 6.44 1,394,645 2 Inv. ex. [1]
6.48 1,275,430 3 Inv. ex. [1] 6.82 1,421,972 4 Inv. ex. [2], [3]
6.63 1,381,593 5 Inv. ex. [1] 6.51 1,291,377 6 Inv. ex. [1] 6.79
1,124,314 7 Inv. ex. [1], [4] 7.13 1,571,850 8 Inv. ex. [1], [4]
7.57 1,948,836 9 Inv. ex. [1], [4] 7.63 2,451,598 10 Inv. ex. [1],
[3] 6.55 2,022,445 11 Comp. ex. [1] 3.50 11,582 12 Comp. ex. [1]
5.09 700,228 13 Comp. ex. [1] 7.02 527,288 14 Comp. ex. [1] 6.61
922,487 15 Comp. ex. [1] 5.27 4,993 [1] Vacuum carburization
quenching and tempering [2] Carbonitridation quenching and
tempering [3] Shot peening [4] Subzero cooling
[0083] From these results, in the Invention Test Nos. 1 to 10, it
was learned that the gears had lifetimes of 1,000,000 or more, so
had superior tooth surface fatigue strengths. This was believed due
to the facts that the wt % of the chemical ingredients included in
the steel were in the predetermined ranges (C of 0.1 to 0.3% in
range, Si of 1.0 to 2.0% in range, Mn of 0.3 to 2.0% in range, S of
0.005 to 0.05% in range, Cr of 1.0 to 2.6% in range, Mo of 0.8 to
4.0% in range, V of 0.1 to 0.3% in range, Al of 0.001 to 0.2% in
range, N of 0.003 to 0.03% in range, and P of 0.03% or less), the
totals of the Si, Mn, Cr, Mo, and V in the steel in 31Si (%)+15Mn
(%)+23Cr (%)+26Mo (%)+100V(%) were 100 or more, and the X-ray
diffraction half widths at a depth of 50 .mu.m from the gear
surface of the gear were 6.4 or more.
[0084] As opposed to this, in Comparative Example Test Nos. 11, 12,
the gears had total amounts of Si, Mn, Cr, Mo, and V in the steel
in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) of 100 or more, but
had insufficient lifetimes of less than 1,000,000. This was
believed due to the high Si contents causing poor carburization
which in turned caused the concentration of C at the gear surfaces
to drop to 0.3 to 0.4% and therefore cause the X-ray diffraction
half widths to become less than 6.4 degrees.
[0085] In Comparative Example Test Nos. 13 and No. 14, the gears
had X-ray diffraction half widths of 6.4 or more, but had
insufficient lifetimes of less than 1,000,000. This was believed to
be probably due to the total amounts of the Si, Mn, Cr, Mo, and V
in the steel in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) being
less than 100 and therefore causing the temper softening
resistances to drop.
[0086] In Comparative Example Test No. 15, the gear had a total of
the Si, Mn, Cr, Mo, and V in the steel in 31Si (%)+15Mn (%)+23Cr
(%)+26Mo (%)+100V (%) of 100 or more, but had an insufficient
lifetime of less than 1,000,000. This was examined after the test.
As a result, it was confirmed that in Comparative Example Test No.
15, a large amount of residual austenite remained. Therefore, in
Test No. 15, it was believed that the large amount of residual
austenite present resulted in the X-ray diffraction half width
becoming less than 6.4 degrees and caused a drop in the temper
softening resistance. Therefore, in this comparative example, it
was believed that by further performing subzero cooling, shot
peening, or other treatment, it would be possible to make the
residual austenite transform to martensite, treat the residual
austenite, and make the X-ray half width 6.4 degrees or more.
Example 2
[0087] Hot rolled steel materials having the chemical compositions
shown in Table 3 were spheroidally annealed to secure
machineability, then were used to fabricate drive gears and driven
gears with pitch circle diameters of 65.8 mm, modules of 1.5, and
35 teeth (Test Nos. 1 to 17). TABLE-US-00003 TABLE 3 37 Si + Test
Chemical composition (wt %) 18 Mn + 10 Cr + No. C Si Mn P S Cr Mo V
Al N Others 31 Mo + 201 V 16 Inv. ex. 0.21 1.30 0.35 0.008 0.012
1.53 1.01 0.20 0.038 0.015 141 17 Inv. ex. 0.20 1.02 0.35 0.020
0.019 1.32 0.80 0.10 0.035 0.012 102 18 Inv. ex. 0.15 1.40 0.35
0.006 0.013 1.54 1.01 0.10 0.001 0.011 Ti:0.029 125 19 Inv. ex.
0.25 1.39 0.34 0.007 0.014 1.50 1.01 0.15 0.038 0.018 134 20 Inv.
ex. 0.20 1.49 0.36 0.007 0.013 1.10 0.80 0.10 0.001 0.009 Ti:0.023
118 21 Inv. ex. 0.20 1.40 0.31 0.006 0.005 1.02 0.81 0.10 0.033
0.015 113 22 Inv. ex. 0.19 1.01 1.95 0.008 0.014 1.01 0.85 0.20
0.039 0.015 149 23 Inv. ex. 0.21 1.30 0.35 0.008 0.012 1.74 0.85
0.21 0.035 0.019 149 24 Inv. ex. 0.20 1.04 0.36 0.009 0.015 1.51
1.19 0.15 0.034 0.014 127 25 Inv. ex. 0.20 1.12 0.36 0.008 0.011
1.49 0.99 0.25 0.033 0.016 Nb:0.031 144 26 Comp. 0.19 1.55 0.35
0.008 0.016 1.50 1.00 0.15 0.033 0.013 140 ex. 27 Comp. 0.21 0.25
0.78 0.014 0.018 1.16 1.02 0.10 0.035 0.013 87 ex. 28 Comp. 0.20
0.25 0.78 0.015 0.015 1.23 0.95 0.17 0.035 0.013 99 ex. 29 Comp.
0.19 1.01 1.90 0.009 0.015 1.55 1.20 0.20 0.033 0.016 164 ex. 30
Comp. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 141 ex.
31 Comp. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 141
ex. 32 Comp. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015
141 ex.
[0088] Next, the surface hardening treatment explained below was
performed under working conditions giving an effective hardened
layer depth of the gear of 0.6 mm. In Test Nos. 16, 18, 20 to 22,
24, 26, and 29, vacuum carburization quenching was performed at
1000.degree. C., then tempering was performed at 200.degree. C.
over 90 minutes. In Test No. 17, vacuum carburization quenching was
performed at 900.degree. C., then tempering was performed at
200.degree. C. over 90 minutes. In Test Nos. 19, 23, 27, and 28,
vacuum carburization quenching was performed at 950.degree. C.,
then tempering was performed at 200.degree. C. over 90 minutes. In
Test No. 25, vacuum carburization quenching was performed at
1050.degree. C., then tempering was performed at 200.degree. C.
over 90 minutes. In Test No. 30, vacuum carburization quenching was
performed at 950.degree. C., then tempering was performed at
200.degree. C. over 90 minutes. In Test No. 31, high carbon
carburization treatment by gas carburization quenching at
950.degree. C. and a carbon potential of 1.3 followed by gas
carburization quenching by a carbon potential of 0.95 was
performed, then tempering was performed at 200.degree. C. over 90
minutes. In Test No. 32, vacuum carburization quenching was
performed at 890.degree. C., then tempering was performed at
200.degree. C. over 90 minutes.
[0089] After the tempering, the inventors investigated the fatigue
life of the tooth faces in Test Nos. 16 to 32 by using a power
circulating type gear fatigue tester to investigate the lifetime
(X) at a test load of 200Nm. Note that the lifetime was measured by
detecting the vibration accompanying chipping of the tooth
face.
[0090] Further, the inventors evaluated the amount of increase of
the temper softening resistance due to the solid solution hardening
of Si, Cr, Mn, Mo, and other added elements for Test Nos. 16 to 32.
Note that the temper softening resistance was evaluated normally by
using a microvicker's hardness meter etc. to measure the hardness
in a micro area, but with this method of evaluation, the increase
in hardness due to precipitation hardening also ends up being
included and therefore only the increase in hardness due to solid
solution hardening cannot be measured. Therefore, in this example,
based on the discovery that the increase in hardness due to
solution hardening in the martensite structure is important in
improving the tooth surface fatigue strength of a gear, the
inventors used an optical microscope, scan type electron
microscope, etc. to examine the microstructure and investigate if
there were coarse carbides in the interval at a depth of 50 .mu.m
from the surface of the gears produced as an indicator of the
amount of increase of hardness due to the solid solution hardening
in the martensite structure, more specifically, if the average size
of the carbides was less than 1 .mu.m. Note that the average size
of the carbides was measured as explained next. First, after the
test, the gear was cut and buried in a resin to prepare a sample
which was then mirror polished. Then, the polished surface of the
sample was etched by a Nytal corrosive solution, a scan type
electron microscope was used to randomly observe carbides up to a
depth of 50 .mu.m from the gear surface, and the values of the
sizes of the carbides observed were arithmetically averaged.
[0091] Further, it is known that if the starting point of the
martensite transformation falls at the quenching stage, the amount
of residual austenite increases and a drop in the strength is
caused. Therefore, the ratio of the amount of the residual
austenite at a depth of 50 .mu.m from the surface of the gear in
each of Test Nos. 16 to 32 was found by observation of the
structure by a microscope. The above test results are shown in
Table 4. TABLE-US-00004 TABLE 4 Test results Amount of residual
Average size of austenite from carbides in interval tooth surface
Test Carburization from gear surface to to 50 .mu.m No. temperature
50 .mu.m depth depth Lifetime (X) 16 Inv. ex. 1000.degree. C.
Average size less 10% 1,394,645 than 1 .mu.m 17 Inv. ex.
900.degree. C. Average size less 10% 1,284,625 than 1 .mu.m 18 Inv.
ex. 1000.degree. C. Average size less 15% 1,226,956 than 1 .mu.m 19
Inv. ex. 950.degree. C. Average size less 16% 1,364,485 than 1
.mu.m 20 Inv. ex. 1000.degree. C. Average size less 13% 1,248,652
than 1 .mu.m 21 Inv. ex. 1000.degree. C. Average size less 13%
1,052,363 than 1 .mu.m 22 Inv. ex. 1000.degree. C. Average size
less 15% 1,327,421 than 1 .mu.m 23 Inv. ex. 950.degree. C. Average
size less 20% 1,462,147 than 1 .mu.m 24 Inv. ex. 1000.degree. C.
Average size less 18% 1,322,574 than 1 .mu.m 25 Inv. ex.
1050.degree. C. Average size less 14% 1,311,667 than 1 .mu.m 26
Comp. 1000.degree. C. Average size less 0% 10,285 ex. than 1 .mu.m
27 Comp. 950.degree. C. Average size less 15% 527,288 ex. than 1
.mu.m 28 Comp. 950.degree. C. Average size less 13% 922,487 ex.
than 1 .mu.m 29 Comp. 1000.degree. C. Average size less 30% 965,477
ex. than 1 .mu.m 30 Comp. Gas carburization Average size less 18%
501,448 ex. than 1 .mu.m 31 Comp. High carbon Average size more 7%
653,211 ex. carburization than 15 .mu.m 32 Comp. 890.degree. C.
Average size more 15% 844,856 ex. than 3 .mu.m
[0092] From these results, it became clear that since Test Nos. 16
to 25 of the examples of the present invention have lifetimes of
1,000,000 or more, they have superior tooth surface fatigue
strength. This is believed to be due to the facts that the wt % of
the chemical ingredients included in the steel material are in the
predetermined ranges (C of 0.15 to 0.25% in range, Si of 1.0 to
1.5% in range, Mn of 0.3 to 2.0% in range, S of 0.005 to 0.02% in
range, Cr of 1.0 to 1.8% in range, Mo of 0.8 to 1.2% in range, V of
0.10 to 0.25% in range, Al of 0.001 to 0.04% in range, N of 0.003
to 0.02% in range, and P of 0.02% or less), the total amount of the
Si, Mn, Cr, Mo, and V in the steel material in 37Si (%)+18Mn
(%)+10Cr (%)+3 1Mo (%)+201V (%) is 100 to 150 in range, vacuum
carburization is performed in a temperature range of 900 to
1050.degree. C., and other conditions are satisfied and thereby the
amount of precipitation of carbides at the surface of the gear is
reduced and the amount of residual austenite can be suppressed to
within 20%.
[0093] As opposed to this, in Comparative Example Test No. 26,
despite the small amount of precipitation of carbides and the
residual austenite being 20% or less, the gear had an insufficient
lifetime of less than 1,000,000. The inventors investigated this
after the test and as a result learned that the poor carburization
caused the concentration of C at the gear surface to become a low
0.3%. From this, it was believed that if the Si content of the
steel material is over 1.5%, the carburization ability
deteriorates.
[0094] In Comparative Example Test No. 27 and No. 28 as well, the
gears had small amounts of precipitation of carbides and amounts of
residual austenite of 20% or less, yet had insufficient lifetimes
of less than 1,000,000. This was believed to be possibly due to the
fact that the total amounts of the Si, Mn, Cr, Mo, and V in the
steel materials in 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%)
were less than 100. In Comparative Example Test No. 29, the gear
had an insufficient lifetime of less than 1,000,000 and did not
have tooth surface fatigue strength. This was believed because the
total amount of the Si, Mn, Cr, Mo, and V in the steel material in
37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%) exceeded 150 and the
amount of residual austenite was a large 30% and therefore the
strength fell.
[0095] In Comparative Example Test No. 30, the gear had a small
amount of precipitation of carbides and an amount of residual
austenite of 20% or less, yet had an insufficient lifetime of less
than 1,000,000. It was learned that this was due to granular
boundary oxidation of about 10 .mu.m at the gear surface of the
gear and that this formed starting points of fracture. From this,
it was believed that with gas carburization quenching at
950.degree. C., the fine amount of oxygen contained in the carrier
gas causes grain boundary oxidation at the tooth surfaces of the
gear and invites a drop in strength, so superior tooth surface
fatigue strength cannot be obtained.
[0096] In Comparative Example Test No. 31, the gear had an
insufficient lifetime of less than 1,000,000 and did not have
superior tooth surface fatigue strength. The inventors investigated
this after the test. As a result, they found that a troostite
structure was observed and the quenching was insufficient. This
insufficient quenching was believed due to the Cr, Mo, and V
dissolving in solid solution in the carbides of an average size of
15 .mu.m or so formed by the high carbon carburization and thereby
those elements becoming insufficient in the steel material matrix.
Due to this, it was believed that with high carbon carburization by
gas carburization quenching at 950.degree. C., a superior tooth
surface fatigue strength cannot be obtained.
[0097] In Comparative Example Test No. 32 subjected to vacuum
carburization quenching at 890.degree. C., it became clear that the
gear had an insufficient lifetime of less than 1,000,000 and that a
large number of carbides with an average size of 3 .mu.m or more
remained at the interval from the surface of the gear to a depth of
50 .mu.m. From this, it was believed that with vacuum carburization
quenching at 890.degree. C., a large number of carbides with an
average size of 3 .mu.m or more are formed and due to this superior
tooth surface fatigue strength cannot be obtained.
[0098] As explained above, it is possible to more effectively
improve the temper softening resistance and thereby provide
case-hardening steel superior in tooth surface fatigue strength and
a gear using the same and possible to use these to greatly
contribute to the higher output and improved fuel efficiency of
automobiles, construction machines, industrial machines, etc.
* * * * *