BACKGROUND OF THE INVENTION
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This invention relates to improvements in a case hardening
steel and a carburized part using the case hardening steel, and
more particularly to the case hardening steel and carburized part
belonging to ferrous material to be used structural parts and to be
used for parts required to be high in hardness at their surface
layer upon being subjected at their surface layer to a case
hardening treatment such as carburizing, carbonitriding and the
like (including gas carburizing, solid carburizing, liquid
carburizing, salt bath carburizing, plasma carburizing, vacuum
carburizing and the like), the parts including engine parts (such as
piston pin), gears, shafts and the like sued in engines,
transmissions, differentials and the like of an automotive vehicle.
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Hitherto, case hardening steels have been known and
identified as SCr420H, SCM420H and SNCM420H according to
JIS (Japanese Industrial Standard). However, recently it has been
eagerly required to improve impact strength of parts for power
transmission to meet an increase in power output and
weight-lightening made in transportation machines such as
automotive vehicles or the like. Accordingly, the above case
hardening steels according to JIS seem to be insufficient in impact
strength.
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In order to meet such a requirement, a method of producing
a bevel gear high in impact strength by improving forging and heat
treatment manners has been proposed as disclosed in Japanese
Patent Provisional Publication No. 9-201644. However, this
method has encountered difficulties in which material cost and
processing cost are high. Additionally, the impact strength of the
bevel gear cannot be largely improved.
SUMMARY OF THE INVENTION
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It is an object of the present invention to provide an
improved case hardening steel which can overcome drawbacks
encountered in conventional case hardening steels.
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Another object of the present invention is to provide an
improved case hardening steel which is high in impact strength
without causing a large increase in material cost and processing
cost as compared with conventional case hardening steels, and a
carburized part using the improved case hardening steel.
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As a result of eager studies of the present inventors, it has
been found to overcome the above problems encountered in the
conventional case hardening steels by controlling amounts of
elements of C, Mn, Mo, P and S inherently contained in case
hardening steel and of B and the like within specified content
ranges thereby establishing a suitable balance between crystal
grain size and a carburized case (hardened layer).
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A first aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3 % by weight, silicon in an amount of from more than
0.3 to 1.0 % by weight, manganese in an amount of from 0.3 to
1.7 % by weight, phosphorus in an amount of not more than 0.03 %
by weight, sulfur in an amount of not more than 0.03 % by weight,
molybdenum in an amount of not more than 1.0 % by weight,
aluminum in an amount of not more than 0.04 % by weight,
nitrogen in an amount of not more than 0.03 % by weight, and
balance being iron and inevitable impurities. The case hardening
steel meets the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + [Mo %] + 1.8) / 8
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A second aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3 % by weight, silicon in an amount of from more than
0.3 to 1.0 % by weight, manganese in an amount of from 0.3 to
1.7 % by weight, phosphorus in an amount of not more than 0.03 %
by weight, sulfur in an amount of not more than 0.03 % by weight,
aluminum in an amount of not more than 0.04 % by weight,
nitrogen in an amount of not more than 0.03 % by weight,
chromium in an amount of from more than 0 to 1.6 % by weight,
and balance being iron (Fe) and inevitable impurities. The case
hardening steel meets the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + 1.8) / 8
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A third aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3 % by weight, silicon in an amount of not more than
0.3 % by weight, manganese in an amount of from 0.3 to 1.7 % by
weight, phosphorus in an amount of not more than 0.03 % by
weight, sulfur in an amount of not more than 0.03 % by weight,
molybdenum in an amount of not more than 1.0 % by weight,
aluminum in an amount of not more than 0.04 % by weight,
nitrogen in an amount of not more than 0.03 % by weight, and
balance being iron and inevitable impurities. The case hardening
steel meets the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + [Mo %] + 1.8) / 8
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A fourth aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3 % by weight, silicon in an amount of not more than
0.3 % by weight, manganese in an amount of from 0.3 to 1.7 % by
weight, phosphorus in an amount of not more than 0.03 % by
weight, sulfur in an amount of not more than 0.03 % by weight,
aluminum in an amount of not more than 0.04 % by weight,
nitrogen in an amount of not more than 0.03 % by weight, and
balance being iron and inevitable impurities. The case hardening
steel meets the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + 1.8) / 8
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A fifth aspect of the present invention resides in a
carburized part formed of a case hardening steel which consists
essentially of carbon in an amount of from 0.1 to 0.3 % by weight,
silicon in an amount of from more than 0.3 to 1.0 % by weight,
manganese in an amount of from 0.3 to 1.7 % by weight,
phosphorus in an amount of not more than 0.03 % by weight,
sulfur in an amount of not more than 0.03 % by weight,
molybdenum in an amount of not more than 1.0 % by weight,
aluminum in an amount of not more than 0.04 % by weight,
nitrogen in an amount of not more than 0.03 % by weight, and
balance being iron and inevitable impurities. The case hardening
steel meets the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + [Mo %] + 1.8) / 8
Additionally, the carburized part has a hardened layer of
carburized case including fine austenite whose austenite grain size
number according to JIS G 0551 is not smaller than 7.
BRIEF DESCRIPTION OF THE DRAWINGS
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- Fig. 1 is a cross-sectional view of a gear specimen used in
Experiment 1 for evaluating performance of case hardening steels
according to the present invention;
- Fig. 2 is a graphic representation showing a heating
pattern for carburizing hardening and tempering for obtaining the
gear specimen of Fig. 1;
- Fig. 3 is a plan view illustrating an impact test by using a
drop impact tester, in Experiment 1;
- Fig. 4 is a graph showing the relationship between the
impact torque (Nm) and the frequency (times) of application of
impact load, in connection with the impact test in Experiment 1;
- Fig. 5A is a cross-sectional view of an example of a gear
specimen used in the impact test in Experiment 2;
- Fig. 5B is a cross-sectional view similar to Fig. 5A but
showing another example of the gear specimen;
- Fig. 6 is a graph showing the relationship between the
(cold) forging load ratio and the hardness upon undergoing the
spheroidizing annealing, for the steels of Examples and
Comparative Examples in connection with Experiment 2; and
- Fig. 7 is a graph showing the relationship between the (100
times) impact strength ratio and the value of [(left side) - (right
side) of Eq. (1)], for the steels of Examples and Comparative
Examples in connection with Experiment 2.
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DETAILED DESCRIPTION OF THE INVENTION
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The present invention is based on the present inventors'
findings that, in order to improve impact strength of a case
hardening steel, it is effective to control the amounts of elements
of C, Mn, Mo, P and S inherently contained in the case hardening
steel and of B and the like within specified content ranges thereby
establishing a suitable balance between crystal grain size and a
carburized case (hardened layer) corresponding to an effective case
depth. In other words, the present invention depends on the
present inventors' knowledge that the impact strength of case
hardening steel can be improved upon strong contribution of
decreasing the amount of P and S as impurity elements in place of
addition of a large amount of Mo and the like which are high in
cost, addition of Mn in place of Mo, addition of B, and refining
crystal grain.
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A first embodiment of a case hardening steel according to
the present invention consists essentially of carbon (C) in an
amount of from 0.1 to 0.3 % by weight, silicon (Si) in an amount of
from more than 0.3 to 1.0 % by weight, manganese (Mn) in an
amount of from 0.3 to 1.7 % by weight, phosphorus (P) in an
amount of not more than 0.03 % by weight, sulfur (S) in an amount
of not more than 0.03 % by weight, molybdenum (Mo) in an amount
of not more than 1.0 % by weight, aluminum (Al) in an amount of
not more than 0.04 % by weight, nitrogen (N) in an amount of not
more than 0.03 % by weight, and balance being iron (Fe) and
inevitable impurities. Additionally, the case hardening steel is
prepared to meet the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + [Mo %] + 1.8) / 8
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Here, C of component elements of the case hardening steel
functions to increase the hardness of the case hardening steel
obtained after carburizing hardening and to increase the strength
of a carburized (component) part. The content of C is within a
range of from 0.1 to 0.3 % by weight. If the content of C is smaller
than 0.1 % by weight, the effect of addition of C is insufficient. If
the content of C exceeds 0.3 % by weight, the resultant case
hardening steel is lowered in toughness and impact strength.
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Si promotes intergranular oxidation after carburizing and
may lower the strength of the resultant case hardening steel, and
therefore it is preferable that the case hardening steel contains not
more than 0.3 % by weight of Si. However, the content of Si may
not be limited to not more than 0.3 % in a heat treatment such as
vacuum carburizing or a plasma carburizing which can suppress
the intergranular oxidation. It is to be noted that an excessive
content of Si largely degrades machinability and cold forgeability
of the case hardening steel, and therefore the upper limit of the Si
content is set at 1.0 % by weight.
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Mn is an element which is effective for improving
hardenability of the case hardening steel. The lower limit of the
Mn content is set at 0.3 % by weight for the reason why austenite
in a suitable amount is necessary to be retained after carburizing
in order to improve the toughness of the case hardening steel. An
excessive Mn content over 1.7 % by weight lowers the cold
forgeability of the case hardening steel and promotes the
intergranular oxidation after the carburizing.
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P is an element which functions to lower the toughness of a
carburized case (hardened layer) of the case hardening steel.
Particularly in case that the content of P exceeds 0.03 % by weight,
lowering in impact strength of the case hardening steel becomes
conspicuous. Additionally, P is an impurity element, and therefore
it is preferable that the P content approaches 0 % by weight as
much as possible.
-
S is an elelment which also functions to lower the
toughness of the carburized case (hardened layer). Similarly to P,
if the content of S exceeds, lowering in impact strength of the case
hardening steel becomes conspicuous. Additionally, S is also an
impurity element, and therefore it is preferable that the S content
approaches O % by weight as much as possible.
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Mo is an element which is effective for improving the
hardenability of the case hardening steel and effective for
improving the toughness of the carburized case (hardened layer).
The Mo content is excessive so as to exceed 1.0 % by weight, such
effects become saturated.
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Al reacts with N in the case hardening steel to form AlN
thereby being effective to prevent coarsening of austenite grain
size during carburizing. If the Al content exceeds 0.04 % by weight,
the effect of preventing the grain size coarsening becomes
saturated. Additionally, for the similar reason, the content of N
exceeds 0.03 % by weight, the effect of preventing the grain size
coarsening becomes saturated.
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It will be understood that Fe occupies an almost whole part
of the case hardening steel of the present invention, other than the
above-discussed components (elements). The case hardening steel
of the present invention further contains Cu, O and the like as
inevitable impurities.
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The above equation Eq. (1) is a formula for optimizing the
contents of C, P, S, Mn and Mo in the case hardening steel in order
to suppress formation and propagation of crack at crystal
boundary which crack serves as a starting point of breaking. In
other words, the toughness of the carburized case (hardened layer)
obtained after carburizing can be increased by addition of suitable
amounts of Mn and Mo, while the toughness of the crystal
boundary can be increased by reducing the contents of P and S as
the impurities, so that the case hardening steel can be improved in
impact strength.
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A second embodiment of a case hardening steel according to
the present invention consists essentially of carbon (C) in an
amount of from 0.1 to 0.3 % by weight, silicon (Si) in an amount of
from more than 0.3 to 1.0 % by weight, manganese (Mn) in an
amount of from 0.3 to 1.7 % by weight, phosphorus (P) in an
amount of not more than 0.03 % by weight, sulfur (S) in an amount
of not more than 0.03 % by weight, aluminum (Al) in an amount of
not more than 0.04 % by weight, nitrogen (N) in an amount of not
more than 0.03 % by weight, optionally chromium in an amount of
from more than 0 to 1.6 % by weight, and balance being iron (Fe)
and inevitable impurities. Additionally, the case hardening steel is
prepared to meet the following equation:
[C %] + 5([P %] + [S %]) ≤ ([Mn %] + 1.8) / 8
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The case hardening steel of this embodiment is similar in
composition and in effects to be produced, to that of the first
embodiment with the exception that Mo is not contained. In this
embodiment, Mn is added in place of Mo thereby omitting use of
Mo which is high in cost. Additionally, the case hardening steel of
this embodiment may contain not more than 1.6 % by weight of Cr.
Cr is an element which is effective for improving the hardenability
of the case hardening steel. However, addition of an excessive
amount of Cr may invite embrittlement of crystal grain boundary,
and therefore the Cr content is preferably not more than 1.6 % by
weight. The lower limit of the Cr content is decided in accordance
with a required hardenability and therefore is not particularly set.
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Next, a third embodiment of the case hardening steel
according to the present invention will be discussed. This case
hardening steel consists essentially of carbon (C) in an amount of
from 0.1 to 0.3 % by weight, silicon (Si) in an amount of not more
than 0.3 % by weight, manganese (Mn) in an amount of from 0.3 to
1.7 % by weight, phosphorus (P) in an amount of not more than
0.03 % by weight, sulfur (S) in an amount of not more than 0.03 %
by weight, molybdenum (Mo) in an amount of not more than 1.0 %
by weight, aluminum (Al) in an amount of not more than 0.04 % by
weight, nitrogen (N) in an amount of not more than 0.03 % by
weight, and balance being iron (Fe) and inevitable impurities.
Additionally, the case hardening steel is prepared to meet the
above equation Eq. (1).
-
Thus, the case hardening steel of the third embodiment is
similar to the case hardening steel of the first embodiment with
the exception that the Si content is not more than 0.3 % by weight.
Si is an element which is effective for improving the hardenability
of the case hardening steel; however, addition of an excessive
amount of Si promotes intergranular oxidation after carburizing
thereby inviting lowering in strength of the case hardening steel.
Accordingly, the Si content is limited to not more than 0.3 % by
weight.
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Next, a fourth embodiment of the case hardening steel
according to the present invention will be discussed. This case
hardening steel consists essentially of carbon (C) in an amount of
from 0.1 to 0.3 % by weight, silicon (Si) in an amount of not more
than 0.3 % by weight, manganese (Mn) in an amount of from 0.3 to
1.7 % by weight, phosphorus (P) in an amount of not more than
0.03 % by weight, sulfur (S) in an amount of not more than 0.03 %
by weight, aluminum (Al) in an amount of not more than 0.04 % by
weight, nitrogen (N) in an amount of not more than 0.03 % by
weight, and balance being iron (Fe) and inevitable impurities.
Additionally, the case hardening steel is prepared to meet the
above equation of Eq. (2). Thus, the case hardening steel of this
embodiment is the same as that of the second embodiment with
the exception that the Si content is not more than 0.3 % by weight
and no Cr is contained.
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The above case hardening steels of the third and fourth
embodiments may contain not more than 1.6 % by weight of Cr. Cr
is an element which is effective for improving the hardenability of
the case hardening steel. However, addition of an excessive
amount of Cr may invite embrittlement of crystal grain boundary,
and therefore the Cr content is preferably not more than 1.6 % by
weight. The lower limit of the Cr content is decided in accordance
with a required hardenability and therefore is not particularly set.
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Preferably, the above case hardening steels of the third and
fourth embodiments have an elemental composition to meet the
following equation:
80 [Si %] + 24 [Mn %] + 33 [Mo %] + 13 ≤ 40
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With this elemental composition, the hardness of the
material (case hardening steel) before cold forging can be lowered
thereby making it possible to lower deformation resistance and
improve deformability of the material and additionally to lower a
pressing load (or cold forging load) during cold forging. In other
words, cold forgeability of the case hardening steel can be
improved by preparing the case hardening steel to meet the above
equation Eq. (4).
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The case hardening steels of the first to fourth
embodiments may contain boron (B) in an amount of from 0.001 to
0.005 % by weight, niobium (Nb) in an amount of from 0.01 to
0.10 % by weight and/or titanium (Ti) in an amount of from 0.01 to
0.10 % by weight. The above-mentioned B is an element which is
effective for improving the hardenability of the case hardening
steel, and also effective for strengthening grain boundary of the
carburized case (hardened layer) upon its segregation at the grain
boundary of the carburized case (hardened layer). In order to
obtain such effects, addition of not less than 0.001 % by weight of B
is preferable. However, addition of B in an amount exceeding
0.005 % by weight is not preferable because not only the effect of
improving hardenability becomes saturated but also hot or cold
machinability is degraded.
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It will be understood that at least one of the
above-mentioned Nb and Ti may be contained in the case
hardening steel. In case that the case hardening steel contains
both Nb and Ti, each of Nb and Ti is preferably contained in an
amount of from 0.01 to 0.10 % by weight. Nb and Ti react with C
and N to form carbide and nitride thereby preventing coarsening of
austenite crystal grain. If the content of Nb or Ti is less than
0.01 % by weight, it is difficult to obtain a sufficient effect of
preventing the crystal grain coarsening. If the content of Nb or Ti
exceeds 0.10 % by weight, the effect of preventing the crystal grain
coarsening becomes saturated.
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It is preferable that the case hardening steels of the first to
fourth embodiments have an elemental composition to meet the
following equation:
[C %] + 5.2 ([P %] + [S %]) ≤ ([Mn %] + [Mo %] + 3.8) / 22 +
96 [B %] + [austenite grain size number according to JIS G 0551] / 111
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With this elemental composition, impurities at crystal
grain boundary can be removed under the effect of addition of B,
thereby achieving strengthening the grain boundary. Additionally,
crystal grain size becomes small, thereby suppressing breaking at
crystal grain boundary.
-
Particularly in case of meeting the above equations Eq. (1)
and Eq. (3), achievement can be made on optimizing the contents
of C, P, S, Mn and Mo for suppressing the formation and
propagation of crack at crystal grain boundary which crack serves
as the breaking starting point, and on reinforcement of crystal
grain boundary under the effects of addition of B and crystal grain
refining. In other words, addition of a suitable amount of Mn and
Mo is preferable to improve the toughness of the carburized case
(hardened layer) after carburizing, and decreasing the contents of
P and S as impurities is preferable to improve the toughness of
crystal grain boundary. These concepts lead to the limitation of the
equation Eq. (1). It is preferable that impurities at crystal grain
boundary are removed under the effect of addition of B thereby to
achieve strengthening of grain boundary. Additionally, it is also
preferable that crystal grain size is lowered thereby preventing
breaking at grain boundary. These concepts lead to the limitation
of the equation Eq. (3).
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Furthermore, the case hardening steels of the embodiments
1 to 4 may contain lead (Pb) in an amount of not more than 0.3 %
by weight, bismuth (Bi) in an amount of not more than 0.15 % by
weight and/or calcium (Ca) in an amount of not more than 0.1 % by
weight. Pb, Bi and Ca may be contained in any combinations.
These elements are effective for improving machinability of the
case hardening steel; however, not only the machinability
improving effect may become saturated but also the toughness
may lower if the Pb content exceeds 0.3 % by weight, the Bi
content exceeds 0.15 % by weight or the Ca content exceeds 0.1 %
by weight.
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A carburized (component) part according to the present
invention will be discussed. The carburized part is formed of the
above-mentioned case hardening steel and has a carburized case
(hardened layer) includes fine austenite whose austenite grain size
number according to JIS (Japanese Industrial Standard) G 0551 is
not smaller than 7. Such refining crystal grain size is
accomplished during carburizing and effective for improving
resistance to the crack propagation upon input of impact. If
austenite in the carburized case is not so refined that the austenite
grain size number is smaller than 7, the carburized part cannot
obtain an excellent impact strength characteristics.
-
Thus, the case hardening steel of the present invention is
used as parts whose surface layer requires a high hardness, such
as gears, shafts and the like in a transmission, a differential and
the like of an automotive vehicle.
EXAMPLES
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The present invention will be more readily understood with
reference to the following Examples in comparison with
Comparative Examples; however, these Examples are intended to
illustrate the invention and are not to be construed to limit the
scope of the invention. Additionally, although the case hardening
steels of Examples and Comparative Examples are directed to
gears, it will be understood that the principle of the present
invention are not limited to gears and therefore may be applied to
all machine structural parts which particularly regard impact
strength characteristics as important.
EXPERIMENT 1
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Steels A to I, K to M and R of Examples (according to the
present invention) and steels N to Q of Comparative Examples (not
according to the present invention) in an amount of 150 Kg were
produced in a usual manner under vacuum melting. The steels A to
R and the steels N to Q had chemical compositions shown in Table
1. The steel N of Comparative Example corresponded to a
conventional case hardening steel identified as SCr420H according
to JIS. Subsequently, each of these steels was subjected to rolling
and normalizing in a usual manner, and thereafter was machined
to a gear shape having a module of 1.5 as shown in Fig. 1. The gear
shaped steel had an outer diameter (corresponding to addendum
circle) of 64.5 mm and a width (axial dimension) of 26 mm as
illustrated in Fig. 1. Thereafter, each gear shaped steel was
subjected to carburizing hardening and tempering in a heating
pattern as shown in Fig. 2, followed by finish machining, thereby
obtaining gear specimen 10 shown in Fig. 1.
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An impact test was conducted on each gear specimen 10 by
using a drop impact tester as shown in Fig. 3. With the impact
tester, gear specimen 10 was fixedly mounted on a first shaft and
engaged with opposed gear 12 fixedly mounted on a second shaft
supported by a supporting base 14. A torque arm 16 has a base end
section fixedly mounted on the first shaft. A free end section of the
torque arm 16 has a position 18 to which impact load was
repeatedly applied so as to apply impact (load) torque (Nm) to gear
specimen 10. In this impact test, the frequency or number (times)
of application of the impact load to torque arm 16 at a time when
breaking of the gear specimen 10 had occurred was measured. This
measurement of the frequency of impact load application was
made plural times by changing the impact torque to be applied to
the gear specimen, thereby obtaining an upper group of data for
each gear specimen of Example and a lower group of data for each
gear specimen of Comparative Example. In the upper group of data,
each black dot indicates the measured frequency of impact load
application at a value of the impact torque. In the lower linear
data, each light triangle indicated the measured frequency of
impact load application at a value of the impact torque.
-
From each of the upper group of data and the lower group
of data in Fig. 4, an impact (load) torque (Nm) applied to the gear
specimen in case that the (measured) frequency of impact load
application was 100 times (at which the gear specimen was
broken) was determined from a relational expression between the
impact torque and the frequency of impact load application, i.e. in
a manner using a dotted arrow as illustrated in Fig. 4. The dotted
arrow is drawn from a straight line representing the upper or
lower group of data. The thus determined impact torque is referred
to as "100 times impact strength (Nm)". The 100 times impact
strength for each of the gear specimens of Examples and
Comparative Examples is shown in Table 2.
-
Additionally, austenite grain size of the gear specimens of
Examples and Comparative Examples were determined by a
judgment method using crossover line segments, according to JIS
G 0551. The thus determined austenite grain size of the gear
specimens are shown in Table 2.
-
As apparent from the experimental results shown in Table
2, the gear specimens formed of the steels A to I, K to M and R of
Examples meet either one of the above equations Eq. (1) and Eq.
(2) and the equation Eq. (3) by optimizing balance between the
impurity elements and the added elements, and therefore are high
in impact strength as compared with those formed of the steels of
Comparative Example. In contrast, the gear specimen formed of
the steel N of Comparative Example cannot meet the above
equations Eq. (2) and Eq. (3) and therefore are low in impact
strength. The gear specimen formed of the steel O of Comparative
Example contains much Cr and cannot meet the above equations
Eq. (2) and Eq. (3), and therefore is low in impact strength. The
gear specimen formed of the steel P of Comparative Example is
lower than 7 in the grain size number and cannot meet the above
equations Eq. (1) and Eq. (3), and therefore low in impact strength.
The gear specimen formed of the steel Q is lower than 7 in the
grain size number and cannot meet the above equations Eq. (2)
and Eq. (3), and therefore is low in impact strength.
EXPERIMENT 2
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Steels 1 to 3 of Examples (according to the present
invention) and steels 4 to 9 of Comparative Examples (not
according to the present invention) in an amount of 150 Kg were
produced in a usual manner under vacuum melting. The steels A to
R and the steels N to Q had chemical compositions shown in Table
3. The steels 1 to 3 met all the equations Eq. (1), Eq. (3) and Eq. (4),
whereas the steels 4 to 9 cannot meet at least one of the equations
Eq. (1), Eq. (3) and Eq. (4). The steel 8 of Comparative Example
corresponded to a conventional case hardening steel identified as
SCM418H according to JIS.
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Subsequently, each of the steels of Examples and
Comparative Examples was subjected to rolling in a usual manner
and formed into a bar material, and thereafter underwent cutting,
spheroidizing annealing, shot blasting, a treatment for forming
lubricating coating, and cold teeth forging. Thereafter, the bar
material was subjected to cutting such as turning or the like so as
to be formed into a gear of the final shape as shown in Fig. 5A. The
bar material might be formed into the final shape of a gear as
shown in 5B. The gear of the final shape was then subjected to
carburizing hardening and tempering, followed by finish grinding,
thereby obtaining a gear specimen of each of the steels 1 to 3 of
Examples and the steels 4 to 9 of Comparative Examples, as shown
in Fig. 5A.
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The gear of the final shape shown in Fig. 5A or 5B may be
produced by another production method in which the steel is
formed into a certain blank shape under cold forging and
thereafter subjected to turning and gear cutting. Concerning the
steels 1 to 3 of Examples, forming of the gear may be sufficiently
accomplished even if the spheroidizing annealing as a softening
heat treatment made before the cold forging is omitted.
-
The impact test was conducted on each of the gear
specimens of the steels 1 to 3 of Examples and steels 4 to 9 of
Comparative Examples by using the drop impact tester in the
same manner as that for the gear specimens in Experiment 1, in
which the 100 times impact strength was measured for each gear
specimen. Then, calculation was made for each gear specimen to
determine a ratio of the 100 times impact strength of each gear
specimen to the 100 times impact strength of the gear specimen of
the steel 8 of Comparative Example (corresponding to the case
hardening steel SCM418H according to JIS) on the assumption
that the 100 times impact strength of the steel 8 was 100. This
ratio was referred to as "100 times impact strength ratio" and
shown in Table 4.
-
Additionally, in order to determine cold forgeability of each
of the gear specimens of the steels 1 to 3 of Examples and the gear
specimens of the steels 4 to 9 of Comparative Examples, a press
load (or cold forging load) applied to the bar material for each gear
specimen was measured during the above cold teeth forging by
using a load cell equipped with a press work machine. It will be
understood that the cold forgeability is excellent as the press load
is low. Then, calculation was made for each gear specimen to
determine a ratio of the press load of each gear specimen to the
press load of the gear specimen of the steel 8 of Comparative
Example (corresponding to the case hardening steel SCM418H
according to JIS) on the assumption that press load of the steel 8
was 100. This ratio was referred to as "cold forging load ratio" and
shown in Table 4. Furthermore, each of the gear specimens of the
steels 1 to 3 of Examples and the steels 4 to 9 of Comparative
Examples was subjected to measurement of Rockwell hardness
(B-scale). The measured Rockwell hardness (HRB) of the gear
specimens were shown in Table 4.
-
As apparent from the experimental results shown in Table
4, it is confirmed that the steels 1 to 3 of Examples meet the
equations Eq. (1), Eq. (3) and Eq. (4) and therefore are excellent
both in cold forgeability and impact strength. In contrast, it is
confirmed that the steels 4, 5 and 9 of Comparative Examples meet
the equations Eq. (1) and Eq. (3) and therefore excellent in impact
strength; however, they cannot meet the equation Eq. (4) and
therefore are inferior in cold forgeability. The steels 7 and 8 of
Comparative Examples meet the equations Eq. (1) and Eq. (4) and
cannot meet the equation Eq. (3), and therefore are inferior in
impact strength.
-
While the present invention has been discussed
particularly on examples of gears, it will be appreciated that the
principle of the present invention may be applied to all machinery
structural parts in which impact strength is particularly regarded
as important.
-
As appreciated from the above, according to the present
invention, the amounts of elements of C, Mn, Mo, P and S
inherently contained in case hardening steel and of B and the like
are controlled within specified content ranges thereby establishing
a suitable balance between crystal grain size and a carburized case
(hardened layer). This can provide the case hardening steel high in
impact strength without large increase in material cost and
processing cost, and the carburized part using the thus improved
case hardening steel.
-
The entire contents of Japanese Patent Applications
P2001-216990 (filed July 17, 2001) and P2002-075624 (filed March
19, 2002) are incorporated herein by reference.
-
Although the invention has been described above by
reference to certain embodiments and examples of the invention,
the invention is not limited to the embodiments and examples
described above. Modifications and variations of the embodiments
and examples described above will occur to those skilled in the art,
in light of the above teachings. The scope of the invention is
defined with reference to the appended claims.
| | | Austenite
grain size | (left side)-(right side)
of Eq. (1) | (left side)-(right side)
of Eq. (2) | 100 times impact strength
(Nm) |
Alloy of
Example | A | 8.0 | (1)-0.160 | -0.049 | 12663 |
| B | 7.5 | (2)-0.063 | -0.016 | 12518 |
| C | 8.1 | (1)-0.160 | -0.040 | 12653 |
| D | 8.3 | (2)-0.039 | -0.002 | 12397 |
| E | 9.1 | (2)-0.033 | -0.019 | 12454 |
| F | 8.9 | (2)-0.013 | -0.170 | 13123 |
| G | 8.8 | (1)-0.036 | -0.147 | 12813 |
| H | 11.0 | (1)-0.031 | -0.172 | 13024 |
| I | 9.4 | (1)-0.100 | -0.144 | 13008 |
| K | 8.3 | (1)-0.061 | -0.153 | 13001 |
| L | 9.0 | (1)-0.058 | -0.155 | 13017 |
| M | 9.5 | (1)-0.039 | -0.133 | 12939 |
| R | 9.0 | (2)-0.063 | -0.017 | 12530 |
Alloy of
Compr.
Example | N | 8.4 | (2)-0.080 | 0.125 | 11917 |
| O | 8.0 | (2)-0.051 | 0.065 | 12134 |
| P | 5.1 | (1)-0.018 | 0.126 | 11914 |
| Q | 4.3 | (2)-0.070 | 0.000 | 12289 |
| Eq. (1) : [C%] + 5[P%+S%] ≦ ([Mn%] + [Mo%] + 1.8) / 8 |
Eq. (2) : [C%] + 5.2[P%+S%] ≦ ([Mn%] + [Mo%] + 3.8) / 22
+ 96[B%] + [JIS austenite grain size number] /111 |
| | C | Si | Mn | P | S | Cr | Mo | B | Nb |
Alloy of
Example | 1 | 0.19 | 0.07 | 0.41 | 0.006 | 0.017 | 0.97 | 0.15 | 0.0014 | 0.05 |
| 2 | 0.18 | 0.05 | 0.74 | 0.007 | 0.012 | 1.09 | 0.01 | 0.0017 | 0.05 |
| 3 | 0.18 | 0.10 | 0.50 | 0.010 | 0.018 | 1.00 | 0.20 | 0.0015 | 0.05 |
Alloy of
Compr.
Example | 4 | 0.17 | 0.07 | 0.59 | 0.010 | 0.017 | 0.92 | 0.41 | 0.0013 | 0.05 |
| 5 | 0.18 | 0.07 | 0.40 | 0.008 | 0.015 | 0.95 | 0.40 | 0.0015 | 0.05 |
| 6 | 0.19 | 0.07 | 1.44 | 0.009 | 0.015 | 0.97 | 0.00 | - | 0.00 |
| 7 | 0.19 | 0.06 | 0.82 | 0.008 | 0.014 | 1.07 | 0.41 | - | 0.00 |
| 8 | 0.19 | 0.19 | 0.79 | 0.013 | 0.017 | 0.97 | 0.16 | - | 0.00 |
| 9 | 0.21 | 0.15 | 0.60 | 0.015 | 0.020 | 1.20 | 0.25 | 0.0030 | 0.07 |
