Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
213774
-1-
STEEL FOR CARBURIZED GEAR
BACKGROUND OF THE INVENTION
r
1. Field of the Invention
The present invention relates to a steel for
carburized gear capable of realizing high fatigue
strength and long endurance life by the conventional
heat treatment comprising the steps of gas
carburization, quenching and tempering. The filed of
industrial application thereof covers a wide range of
industries including those of automobile, construction
vehicle and industrial machine wherein gears are used.
2. Description of the Prior Art
For improving the fatigue strength and endurance
life of gears treated by the gas carburization,
quenching and tempering, various techniques have been
proposed which include one in which, as.disclosed in
Japanese Patent Application Laid-Open No. 83848/1922,
the amounts of Si, Mn, Cr and the like which are
oxidized more easily than Fe are reduced in the steel
in order to reduce the intergranular oxidation or
incompletely hardened layer causing fatigue cracks,
while the hardenability and mechanical properties
thereof are regulated by the incorporation of Ni, Mo
or the like having a resistance to oxidation greater
than that of Fe. Such various techniques also include
one in which a fine spherical carbide is precipitated
in the surface part of the steel so as to increase the
hardness of the surface by enhancing the carbon
potential at the time of carburization, this technique
2~~~~~~
-2-
potential at the time of carburization, this technique
being generally known as the high-concentration
carburization, plasma carburization or excess
carburization. The above various techniques further
include one in which a residual surface compression
stress is imparted to the steel by shot peening so as
to retard the progress of fatigue cracks.
However, all the above techniques for
improvements are concerned with the properties of
gears prior to actual use, and do not contemplate the
gears in actual use, namely in a gearing state under
imposed load. Especially, when the driving and driven
faces of gears contact each other at a high contact
surface pressure, a surface fatigue phenomenon arises
which cannot be dealt with only by the contemplation
of the properties of the gears prior to use.
Additionally, in the recent failures of gears, the
contact surface fatigue is most predominant in
accordance with the demands for higher engine output
and promotion of gear miniaturization.
More specifically, in the actual use and gearing
state of the gears, it is conceivable that the
temperature of the contact surface of the gears is
raised to 200 - 300C by the friction under contact
surface pressure inclusive of slip. When exposed to
' such high temperatures, as generally admitted, the
' hardness of the carburized case is decreased as
compared with that prior to use.
Maintaining the hardness of the carburized case
is the most important factor against the surface
fatigue. There has been an unsolved problem that,
even if the hardness of the carburized case prior to
use is improved by the above techniques for
improvements, the decrease of the hardness of the
,~llrr
~1~'~"~44
-3-
I
carburized case attributed to the frictional heat
during use brings about the surface fatigue.
In order to solve the above problem easily at a
low cost, the present invention has developed a steel
for carburized gear capable of providing the gear with
softening resistance through the conventional steps
of
gas carburization, quenching and tempering without
resort to any special heat treatment, by regulating
the chemical composition of a steel as a material to
be carburized. The gist of the present invention
resides in utilizing Si which is an element having
effective softening resistance. It is believed that
Si acts to retard the diffusion of carbon owing to the
chemical repulsive force thereof to C to thereby
inhibit the formation and cohesion of a carbide which
is the cause of the softening of the steel. However,
Si is a strong ferrite stabilizing element, so that
there is a problem that it elevates the Y ~ a phase
transformation initiating temperature of the steel to
thereby induce a ferrite formation in a core part
structure having a less carbon content at the
customary quenching temperature after carburization.
The formation of a ferrite is very unfavorable from
the viewpoint of strength because it renders the
microstructure of the steel nonuniform to thereby
preferentially advance cracks. There is a further
problem that Si is an element in the presence of which
an intergranular oxidation is very likely to occur at
the time of carburization.
SUMMARY OF THE INVENTION
An object of the present invention is to solve
the above problems of the use of Si, and to provide a
~r
2I~~7~4
-4-
steel in which the effect of Si contributing to the
softening resistance of the steel is markedly
exhibited.
The present invention made with a view toward
solving the above problems is constituted of a steel
for carburized gear having softening resistance,
consisting essentially of, in weight percentages, 0.18
to 0.25 C, 0.45 to 1.008 Si, 0.40 to 0.70 Mn, 0.30
to 0.70 Ni, 1.00 to 1.50 Cr, 0.30 to 0.70 Mo, up to
0.50 Cu, 0.015 to 0.030 A1, 0.03 to 0.30$ V, 0.010
to 0.030 Nb, up to 0.0015 0, 0.0100 to 0.0200 N and
the balance consisting of Fe and inevitable impurity
elements, wherein quenching at 820°C or higher after
carburization does not cause any ferrite to be formed
in a hardened structure of the core part of the steel,
and wherein, while tempering is generally performed at
160 to 180°C after the quenching, reheating at any of
temperatures inclusive of the tempering temperature
and up to 300°C does not cause the hardness of a
carburized case of the steel to decrease by HV 50 or
more from the one after the carburization, quenching
and tempering.
Moreover, preferably, there is provided a steel
for carburized gear, which further includes, in its
material, at least one member selected from the group
- consisting of 0.005 to 0.0208 S, 0.03 to 0.09 Pb and
0.003 to 0.030$ Te, all by weight percentages, as an
element capable of improving the machinability of the
stee.t without marked detriment to the fatigue
properties thereof.
Throughout the specification, all percentages
specified are by weight unless otherwise indicated.
BRIEF DESCRIPTION OF THE DRAWINGS
-5-
FIG. 1 is an explanatory view of carburizing,
quenching and tempering conditions;
FIG. 2 is an explanatory view of heat treatment
conditions adopted in the reheating experiment;
FIG. 3 is a graph showing the relationship
between hardness decrease after repeating and Si
content;
FIG. 4 is an explanatory view of heat treatment
conditions adopted in the experiment simulating the
carburization and quenching at the core part of each
of the test materials of Table 1 and 2;
FIG. 5 is an explanatory view of the conditions
for carburization and quenching of a test piece;
FIG. 6 is a graph showing the relation ship
between intergranular oxidation depth and Si content;
FIG. 7(a) is a schematic diagram of a roller
pitting fatigue tester;
FIG. 7(b) is a schematic diagram of a test piece
for use in roller pitting fatigue test ;..
FIG.7(c) is a schematic diagram of a load roller
for use in roller pitting fatigue test;
FIG. 8 is a graph showing the pitting fatigue
lives of the steel of the present invention and
conventional steels;
~ FIG. 9 shows the changes of surface hardness
decrease during rolling with time of the steel of the
present invention and conventional steels;
DIGS. 10 - 15 are micrographs showing the
microstructures of metal test pieces carburized under
the conditions shown in FIG. 4; and
FIG. 15 is a micrograph showing the carburized
microstructure of a core part of a conventional steel
processed under the conditions shown in FIG. 1.
-6-
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the first place, the starting point of the
present invention was to develop a technique for
improving the fatigue strength of the carburized gear
steel. A first fruit of such development efforts was
disclosed in the above Japanese Patent Application
Laid-Open No. 83848/1992. However, in recent years,
the contact surface pressure applied to gears has so
increased that the occurrence of damages caused by the
contact surface fatigue has become frequent.
Therefore, besides the above invention, studies have
been made to investigate the effects of alloying
elements on the resistance to the lowering of the
hardness of the carburized case, i.e., the resistance
, to the softening of the carburized case, against the
heat buildup brought about by gear surface contact,
with a specified view toward improving the surface
fatigue strength of the gear steel.
For preparing test materials, test steel ingots
having chemical compositions (by weight ~) shown in
Tables 1 and 2 were produced by the use of a high-
frequency induction melting furnace, hot forged so as
to each have a diameter of 30 mm, and normalized at
920°C for 1 hr. Each of the resultant steels was
machined so as to obtain a test piece having a
diameter of 25 mm, and carburized, quenched and
tempered under the conditions as indicated in FIG. 1.
With respect to each of the carburized test pieces, a
reheating test was conducted under the conditions as
indicated in FIG. 2, and the hardness of the
carburized case at a depth of 50 pm from the surface
of the test piece was measured. Herein, this hardness
~~37'~~4
of the carburized case at a depth of 50 um from the
surface of the test piece is referred to simply as the
hardness after the repeating. In Table 3, the
- difference between the hardness after the repeating at
220 to 300°C and the hardness at 180°C as the
conventional temperature for tempering subsequent to
carburization and quenching, namely the degree of
softening, is indicated as the hardness decrease by
repeating. The softening resistance was evaluated on
the basis of the magnitude of the degree of softening,
presuming that the smaller the hardness decrease by
repeating, the greater the softening resistance. FIG.
3 shows the relationship between the above hardness
decrease by repeating and the Si content of the steel.
It is apparent therefrom that, in a region where the
Si content is low, the higher the repeating
temperature, the greater the hardness decrease. More
specifically, when the repeating temperature is 220°C,
the hardness decrease is only HV 50 on the maximum,
.20 and has scarcely any correlation with the Si content
of the steel. When the repeating temperature is
260°C, the hardness decrease exceeds HV 50 in a region
where the Si content is 0.25 wt.~ or lower. The
hardness decrease is more marked when the repeating
temperature is 300°C. Provided that any material, the
hardness decrease by repeating of which is HV 50 or
less, is regarded as having a softening resistance, it
has been found that, when the Si content is at least
0.45 wt.~, there is a region where a softening
resistance is exhibited even at a repeating
temperature as high as 300°C
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On the other hand, as mentioned hereinbefore,
there is problems that the addition of Si elevates the
Y ~ a phase transformation initiating temperature of
the steel, and that a ferrite phase is generated at
the time of quenching subsequent to carburization. As
means for coping with these problems, the positive
effect of the addition of an austenite stabilizing
element on the lowering of the phase transformation
initiating temperature of the steel was utilized in
the present invention. In particular, with respect to
Ni as an alloying element, it has been noted that not
only inhibition of ferrite formation but also
improvement of toughness being a property important
for a gear steel can be expected therefrom, and thus
application of Ni has been attempted. First, the
above test pieces prepared from the test materials
indicated in Tables 1 and 2 were carburized, quenched
and tempered under the conditions indicated in FIG. 4.
With respect to each of the resultant carburized test
pieces, the microstructure after quenching at a depth
of 3 mm from the surface thereof was observed under an
optical microscope to examine the formation of any
ferrite. At the examined depth, the carbon
concentration was satisfactorily low. Exemplary results
obtained by these microscopic observations are shown in
FIGS. 10-15. It is apparent therefrom that when the Ni
content is as low as about 0.10 wt.%, an increase of the Si
content to about 1.00 wt.% causes ferrite formation in the
carburized microstructure (compare steel type No. d shown in
FIGS. 10 and 13 with steel type No. f shown in FIGS. 11 and
14). The degree of the formation is more marked at a lower
21~~~~~
-l0a-
quenching temperature of 820~'C, as shown in FIGS. 10, 11 and
12. On the other hand, even if the Si content is as high as
about 1.00 wt.%, it is apparent that ferrite formation does
not occur when the Ni content is increased to about 1.00
wt.% (compare steel type No. f shown in FIGS. 11 and 14 with
steel type No. g shown in FIGS. 12 and 15).
~1~7'~4~
P
Next, for confirming the effect of Ni on the
inhibition of ferrite formation in greater detail,
experiments were conducted in which the contents of Si
and Ni were varied. While the chemical components of
the test materials and the procedure of machining the
test pieces were as described above, the heat
treatment of the test pieces was carried out under the
conditions as shown in FIG. 5. With respect to each
of the test pieces after the heat treatment, the
microstructure thereof was observed under an optical
microscope to examine the formation of any ferrite.
The results are shown in Table 3. Therein, the mark
"o" indicates that no ferrite formation was observed,
the mark " ~," that the formation of a small amount of
ferrite was observed, and mark "x" that the formation
of a large amount of ferrite was observed. The table
shows that each of the steels in which only the Si
content has been increased without regulating the Ni
content, such as comparative steels a and b, a and f
and h to 1, exhibits a hardness decrease after
reheating up to 300°C of not greater than IiV 50, thus
having a softening resistance, but suffers from
ferrite formation at quenching at 820 to 840°C. By
contrast, it has been found that each of comparative
steel g and steels of the present invention m to w in
which the Si content has been increased wile
regulating the Ni content not only has a softening
resistance but also suffers from no ferrite formation
at any of the quenching temperatures: Further, the
Table shows that comparative steels c and d and
currently used steels x to z each having a low Si
content do not suffer from ferrite formation at any of
''" ~i~77~~
-12-
i
the quenching temperatures, though each exhibits a
hardness decrease after reheating at 300°C of greater
than HV 50, thus having no softening resistance. From
. the above results, it has been found that there is a
compositional range in which improvement of the
softening resistance by Si without the formation of
any ferrite even at a quenching temperature of 820°C
or higher can be attained by regulating the Ni content
of the steel.
213'~v44
-13-
Table 3
Hard ness rease Obs ervationof fer rite
No. after dec ng format ion each rdening
reheati(HV) at ha
te mp.
220C 260C 300C 820C 840C 860C 880C
a -16 -19 -32 x x x x
b -1~ -23 -27 x x x x
c -45 -85 -113 0 0 0 0
d -17 -56 -77 0 0 0 0
a -1 -2 -29 x x x x
f 2 -6 -9 x x x x
g -10 -15 -28 0 0 0 0
h -20 -40 -41 x x is x
i -32 -30 -34 x x x x
-23 -30 -33 x x x a
k -22 -36 -40 x x x a
1 2 -5 -32 x a o 0
m -25 -15 -10 0 0 0 0
n 33 21 -10 0 o O o
o -5 -5 -22 0 0 0 0
p 7 16 -7 0 o O o
q -43 -37 -45 0 0 0 0
r -23 -13 -25 0 0 0 0
s 27 22 -21 0 0 0 0
t 6 31 5 0 0 0 0
a 15 -5 -25 0 o 0 0
13 -7 -28 0 0 0 0
w 12 -10 -32 0 0 0 0
x -24 -57 -94 0 0 0 0
y -9 -40 -94 0 0 0 0
-34 -55 -132 0 0 0 0
o: no ferrite formation observed.
e:'ferrite formation slightly observed.
x: marked ferrite formation observed.
Nos. m - 1: Comparative Steels
Nos. m - w! Invention Steels
Nos. x - z: Current Steels
''" 2237744
-1 4-
t
Finally, the occurrence of intergranular
oxidation by the addition of Si has been studied.
Although Si is believed to promote intergranular
oxidation as mentioned hereinbefore, the behavior
thereof has been investigated in the ranges broader
than the conventional. As a result, a compositional
range has been found in which the intergranular
oxidation can be suppressed. Table 4 shows the
chemical composition (by weight ~) of the test pieces
having been investigated. The procedure of machining
the test pieces was as described above, and the
prepared test pieces were carburized and quenched
under the conditions indicated in FIG. 1. With
respect to each of the carburized test pieces, the
structure of the carburized surface thereof was
observed under an optical microscope to thereby
measure the intergranular oxidation depth.
''~ 2137744
-15-
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21~7'~4~
-1 6-
FIG. 6 shows the relationship between the above
intergranular oxidation depth and the Si content of
the steel. Therefrom, it is apparent that, as pointed
out in the art, the intergranular oxidation depth
proportionally increases up to an Si content of 0.25
wt.%, and that, however, the depth contrarily
decreases when the Si content exceeds the above value
and is limited to approximately 10 p.m when the Si
content is 0.45 wt.% or greater. Accordingly, it has
been found that, in a region where the Si content is
0.45 wt.% or greater to thereby have a softening
resistance, the intergranular oxidation depth does not
pose any problem.
On the basis of the above fundamental studies,
particular means has been found for improving the
softening resistance to thereby improve the fatigue
resistance or endurance life while solving the
problems of ferrite formation and increased occurrence
of intergranular oxidation attributed to Si.
Therefore, the present invention provides a
steel for carburized gear having softening resistance,
consisting essentially of, in weight percentages, 0.18
to 0.25% C, 0.45 to 1.00% Si, 0.40 to 0.70% Mn, 0.30
to 0.70% Ni, 1.00 to 1.50% Cr, 0.30 to 0.70% Mo, up to
0.50 Cu, 0.015 to 0.030% A1, 0.03 to 0.30% V, 0.010 to
0.030% Nb, up to 0.0015% O, 0.0100 to 0.0200$ N and
the balance consisting of Fe and inevitable impurity
elements, wherein quenching at 820°C or higher after
carburization does not cause any ferrite to be formed
in a hardened structure of the core part of the
carburized steel, and wherein, while tempering is
generally performed at 160 to 180°C after the
quenching, reheating at any of temperatures inclusive
of the tempering temperature and up to 300°C does not
''~ 2~37~44
~...,
-17-
I
cause the hardness of a carburized case of the
carburized steel to decrease by HV 50 or more from the
one after the carburization, quenching and tempering.
- Moreover, according to necessity, the carburized steel
for gear is characterized by further including, in its
y
material, at least one member selected from among
0.005 to 0.020 wt.% S, 0.03 to 0.09 wt.% Pb and 0.003
to 0.030 wt.% Te, as an element capable of improving
the machinability of the steel.
With respect to the above composition according
to the present invention, the reasons for the
numerical limitations will be described below.
C: 0.18 to 0.25%
The addition of C in an amount of at least 0.18%
is required for obtaining a core part hardness of HRC
35 to 45 to be possessed by gears. When the amount of
C is too small, the Y -~ a phase transformation
initiating temperature is excessively high, so that
the control thereof by the addition of an austenite
stabilizing element becomes difficult. On the other
hand, the addition of excess C causes the hardness of
the core part to increase so excessively that not only
is satisfactory introduction of a residual surface
compression stress unfeasible after quenching but also
the toughness of the core part is deteriorated. For
w . ~ avoiding this, the upper limit must be restricted to
0.25%.
Therefore, the amount of C to be added ranges
from 0.18% to 0.25%.
Si: 0.45 to 1.00%
Si is the most important of the elements to be
incorporated in the steel of the present invention.
That is, Si is an element capable of most effectively
increasing the softening resistance at a temperature
~r
2137744
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I
ranging from 200 to 300°C which is believed to be
reached during the rolling of gears, etc. For
effectively exhibiting the above capability, it is
requisite that at least 0.45% Si be added. However,
since Si is a ferrite stabilizing element as generally
recognized, the addition of excess Si raises the Ac3
transforming point, so that the ferrite formation at
the core part at which the carbon content is low
becomes marked in the conventional quenching at
temperatures ranging from 820 to 860°C, thereby
inviting a strength deterioration. Further, the
excess Si would diminish the carburizability of the
steel and cause the steel prior to carburization to
become too hard, thereby deteriorating the cold
forgeability and machinability of the steel. For
avoiding these, the upper limit must be restricted to
1.00%.
Therefore, the amount of Si to be added ranges
from 0.45% to 1.00%.
Mn: 0.40 to 0.70%
Mn must be added in an amount of at least 0.40%
in order to ensure the hardenability of the steel.
However, Mn is likely to cause an intergranular
oxidation. For reducing this likelihood, the upper
limit of the amount of Mn must be restricted to 0.70%.
w ~ Therefore, the amount of Mn to be added ranges
from 0.40% to 0.70%.
Ni: 0.30 to 0.70%
In the steel of the present invention, Ni is an
element as important as Si. That is, since Ni is an
austenite stabilizing element in contrast with Si, Ni
lowers the Y ~ a phase transformation initiating
temperature elevated by the addition of Si. Further,
simultaneously, Ni is an element which improves not
213'744
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I
only the hardenability of the steel but also the
toughnesses of the carburized case and the core part.
For exercising these effects, Ni must be added in an
- amount of at least 0.30%. However, since Ni is an
expensive element, the addition of excess Ni is not
desirable from the economic point of view. Moreover,
it rather intensifies the formation of residual
austenite to thereby invite lowering of the hardness
of the surface of the steel. For avoiding these, the
uPPer limit of the amount of Ni must be restricted to
0.70%.
Therefore, the amount of Ni to be added ranges
from 0.30% to 0.70%.
Cr: 1.00 to 1.50%
Cr is an element required for ensuring the
hardenability of the steel. Also, it is an element
from which precipitation of a fine carbide can be
expected. For attaining these desired effects, Cr
must be added in an amount of at least 1.00%. However
Cr is an element which is likely to cause an
intergranular oxidation, like Mn, so that the addition
. of excess Cr renders the core part too hard, thereby
deteriorating the toughness thereof. For avoiding
this, the upper limit of the amount of Cr must be
restricted to 1.50%.
w ~ Therefore, the amount of Cr to be added ranges
from 1.00% to 1.50%.
Mo: 0.30 to 0.70%
Mo is an element which improves not only the
hardenability of the steel but also the toughnesses of
the carburized case and the core part like Ni. For
exercising these effects, Mo must be added in an
amount of at least 0.30%. However, the addition of
excess Mo not only renders the softening treatment of
213?44
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the steel prior to carburization difficult to thereby
deteriorate the machinability of the steel, but also
renders the core part so excessively hard as to
deteriorate the toughness thereof. For avoiding
these, the upper limit of the amount of Mo must be
restricted to 0.70%.
Therefore, the amount of Mo to be added ranges
from 0.30% to 0.70.
Cu: up to 0.50
Cu is an element from which precipitation
s, hardening can be expected at a relatively high
temperature ranging from 400 to 600C. Therefore, Cu
is preferably added to the steel for use under severe
conditions, such as gear tooth and rolling contact
surfaces where an extreme temperature elevation is
caused, is presumed, or when it is used in a high
temperature environment, e.g., in aircraft materials
disposed in the vicinity of jet propulsion machinery
or a turbine. However, the addition of excess Cu
intensifies the hot brittleness of the steel and
deteriorates the carburizability of the steel. For
avoiding these, the upper limit of the amount of Cu
must be restricted to 0.50.
Therefore, the amount of Cu to be added is
limited to 0.50 or less.
A1: 0.015 to 0.030
A1 is an element which is bonded to N to from
A1N, thereby acting to refine the grain size of
austenite crystal: Through the refining activity, it
contributes to improvement of the toughnesses of the
carburized case and the core part. For this purpose,
it is necessary to add A1 in an amount of at least
0.015. However, the addition of excess A1 increases
the formation of A1203 as an inclusion hazardous for
''' 21377~~
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the fatigue strength of the steel. For avoiding this,
the upper limit of the amount of A1 must be restricted
to 0.030%. Therefore, the amount of A1 to be added
ranges from 0.015% to 0.030%.
V: 0.03 to 0.30%
Even at relatively low temperatures close to the
carburizing temperature, V forms a carbide, from which
a hardness improvement can be expected. For attaining
the hardness improvement, it is necessary to add V in
an amount of at least 0.03%. However, the addition of
excess V deteriorates the toughness of the carburized
case of the steel. For avoiding this, the upper limit
of the amount of V must be restricted to 0.30%.
Therefore, the amount of V to be added ranges
from 0.03% to 0.30%.
Nb: 0.010 to 0.030%
Nb is an element which is bonded to the C and N
in the steel to form a carbonitride, thereby acting to
refine the grain size of austenite crystal, like A1N.
Through the refining activity, it contributes to
improvement of the toughnesses of the carburized case
and the core part. Accordingly, the amount of Nb to
be added is determined depending on the quantitative
balance between coexistent Al and N. When the amount
is too small, no desired effect can be exercised.
Thus, it is requisite that Nb be added in an amount of
at least 0.010%. However, the addition of excess Nb
causes grain coarsening of carbonitride precipitated,
thereby deteriorating the toughness of the carburized
case of the steel. For avoiding this, the upper limit
of the amount of Nb must be restricted to 0.030%.
Therefore, the amount of Nb to be added ranges
front 0.010% to 0.030%.
O: up to 0.0015%
2~~77~~
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I
O is an element which is present in the steel as
an oxide inclusion, causing the fatigue strength of
the steel to be deteriorated.
Therefore, the upper limit of the amount of 0 is
set at 0.0015%.
N: 0.0100 to 0.0200%
N is an element which is bonded to A1 and Nb to
form A1N and NbCN, thereby acting to refine the grain
size of austenite crystal. Through the refining
activity, it contributes to improvement of the
toughnesses of the carburized case and the core part.
Accordingly, the amount of N to be added is determined
depending on the quantitative balance between
coexistent A1 and Nb. When the amount is too small,
no desired effect can be exercised. Thus, it is
requisite that N be added in an amount of an least
0.0100%. However, the addition of excess N invites
not only the occurrence of pores in the surface part
of a steel ingot at the time of solidification but
also deterioration of the forgeability of the steel.
For avoiding these, the upper limit of the amount of N
must be restricted to 0.0200%.
Therefore, the amount of N to be added ranges
from 0.0100% to 0.0200%.
S: 0.005 to 0.020%
S is an element which is mostly present in the
form of a sulfide inclusion in the steel, thus being
effective in the improvement of machinability of the
steel. The machinability is important for gears and
other parts shaped by cutting work. For ensuring the
above effect, it is necessary to add S in an amount of
at least 0.005%. However, the addition of excess S
invites deterioration of the fatigue strength of the
steel. For avoiding these, the upper limit of the
''' 2~37~44
-23-
amount of S must be restricted to 0.020%.
Therefore, the amount of S to be added ranges
from 0.005% to 0.020%.
Pb: 0.03 to 0.09%
Pb is an element which is effective in the
improvement of machinability of the steel like, the
machinability being important for gears and other
parts shaped by cutting work. For ensuring the above
effect, it is necessary to add Pb in an amount of at
least 0.03%. However, the addition of excess Pb
invites deterioration of the fatigue strength of the
steel. Further, when the amount is 0.10% or more, the
use of Pb falls under legal regulations regarding air
pollution. For avoiding these, the upper limit of the
amount of Pb must be restricted to 0.09%.
Therefore, the amount of Pb to be added ranges
from 0.03% to 0.09%.
Te: 0.003 to 0.030%
Te is an element which improves the
machinability of the steel. For attaining this
effect, it is necessary to add Te in an amount of at
least 0.003%. However, the addition of excess Te
causes the steel to have a hot brittleness. For
avoiding this, the upper limit of the amount of Te
must be restricted to 0.030%.
w Therefore, the amount of Te to be added ranges
from 0.003% to 0.030%.
The present invention will now be described in
greater detail with reference to the following
Example.
Example
In order to confirm that the improvement of the
pitting fatigue strength, which is the primary object
of the present invention, can be attained on the basis
1.
-24-
of the above results, a test steel ingot comprising
the chemical composition (by weight %) shown in Table
was produced as a steel according to the present
invention by the use of a high-frequency induction
5 vacuum melting furnace, and the pitting fatigue life
thereof was evaluated by the roller pitting fatigue
test.
Table 5
Balance: Fe
C Si Mn P S Ni Cr Mo
0.22 0.77 0.42 0.012 0.012 0.50 1.24 0.34
Cu A1 Nb Pb V Te ~0) [N]
0.09 0.027 0.020 0.00 0.16 0.000 0 0009 0 0154
FIG. 7 (a) shows an outline of a roller pitting
fatigue tester. Therein, numeral 1 denotes a test
piece, numeral 2 a load roller, numerals 3, 4 gearing
gears, numeral 5 a bearing, numeral 6 a coupling,
numeral 7 a transmission belt, and numeral 8 a motor.
FIG. 7(b) shows the configuration of a test piece.
20. FIG. 7(c) shows the configuration of a load roller.
The dimensions indicated in FIGS. 7(b) and (c) are all
in millimeters. The test was conducted under the
conditions such that the maximum Hertz's contact
surface pressure was 3430 MPa, and that the slip ratio
was 40%. The test steel ingot was hot-forged,
normalized and machined into a test piece. The test
piece was carburized, quenched and tempered under the
conditions indicated in FIG. 1. A part was cut off
the test piece, and, with respect to the part, the
hardness distribution of the carburized case was
determined and the microstructure thereof was
observed. The results are shown in FIG. Is and Table
213774
-25-
6.
Table 6
Carburization Characteristics
Surface Effective Hardness Depth of
hardness hardened of core intergranular
case death part oxide layer
HV 756 0.90 mm HV 468 8.5 um
As a result, first, it has been confirmed that,
in the steel of the present invention, there is no
ferrite formation in its core part, and that the depth
of intergranular oxidation therein is as small as 8.5
um. FIG. 8 shows the results of the roller pitting
fatigue test. Therein, the pitting fatigue life of
the steel of the present invention, together with
those of the conventional steels, is shown in terms of
cumulative fracture probability. It is apparent from
the results thereof that the pitting fatigue life of
the steel of the present invention is prolonged beyond
the range of those of the conventional steels. FIG. 9
20I shows the results obtained by interrupting the fatigue
test at each given repeat count and measuring the
surface hardness for grasping the decrease with time
of the hardness during rolling in the fatigue test'.
w ~ The results of the steel of the present invention are
shown together with those of the conventional steels.
It is apparent therefrom that the surface hardness
decrease during rolling of the steel of the present
invention is less than the range of those of the
conventional steels. Therefore, in accordance with
the alloying design concept, it can be interpreted
that, as the effects of the increase in Si content,
the softening resistance is improved; the surface
~rrr
21~'~741
-26-
I
hardness decrease under the influence of frictional
heat during rolling at a high contact surface pressure
including slip, which surface hardness is the most
r important factor for the pitting fatigue strength, is
suppressed; there is no ferrite formation at the core
part; and the intergranular oxidation depth is small,
so that the fatigue life is prolonged. As apparent
from the above, the steel of the present invention
exhibits a prolonged pitting fatigue life and has
advantageous properties as compared with those of the
current steels.
As demonstrated by the above results, the steel
of the present invention is strikingly excellent in
the pitting fatigue strength now being the most
important requirement for gears as compared with the
conventional steel. Therefore, the employment of the
steel of the present invention makes it possible not
only to effect miniaturization and weight reduction of
the steel gear while utilizing the conventional
carburization and quenching conditions and design
items as they are, but also to realize higher output
even with the same configuration and size.
Therefore, the effects of the present invention
permit wide contributions to cost reduction and
reliability improvement in industries where gears are
utilized under severe conditions.
