U.S. patent application number 13/064686 was filed with the patent office on 2012-10-04 for carburized steel and its process of manufacture.
This patent application is currently assigned to NIPPON STEEL CORPORATION. Invention is credited to Yuji Adachi, Susumu Kato, Shuji Kozawa, Manabu Kubota, Koji Obayashi, Hirokazu Sato, Keita Taguchi.
Application Number | 20120247619 13/064686 |
Document ID | / |
Family ID | 44712345 |
Filed Date | 2012-10-04 |
United States Patent
Application |
20120247619 |
Kind Code |
A1 |
Obayashi; Koji ; et
al. |
October 4, 2012 |
Carburized steel and its process of manufacture
Abstract
A carburized steel member is manufactured by specific
carburizing, cooling, and quenching steps. The steel member
contains: C: 0.1% to 0.4%, Si: 0.35% to 3.0%, Mn: 0.1% to 3.0%, P:
0.03% or less, S: 0.15% or less, Al: 0.05% or less, and N: 0.03% or
less, and a content of Cr is less than 0.2%, a content of Mo is
0.1% or less, and remainder is constituted of Fe and unavoidable
impurities. A surface layer thereof includes: a first layer having
a carbon concentration of 0.60 mass % to 0.85 mass % and including
a martensitic structure in which no grain boundary oxide layer
caused by Si exists; a second layer having a carbon concentration
of 0.1 mass % to 0.4 mass % and including a martensitic structure;
and a third layer having a carbon concentration of 0.1 mass % to
0.4 mass % and including no martensitic structure.
Inventors: |
Obayashi; Koji; (Toyoake,
JP) ; Taguchi; Keita; (Agui, JP) ; Kato;
Susumu; (Anjyo, JP) ; Kozawa; Shuji; (Muroran,
JP) ; Kubota; Manabu; (Muroran, JP) ; Adachi;
Yuji; (Tokai, JP) ; Sato; Hirokazu; (Tokai,
JP) |
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
AISIN AW CO., LTD.
Anjo-Shi
JP
|
Family ID: |
44712345 |
Appl. No.: |
13/064686 |
Filed: |
April 8, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2011/057935 |
Mar 30, 2011 |
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13064686 |
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Current U.S.
Class: |
148/233 ;
148/319; 420/104; 420/119; 420/120; 420/121; 420/123; 420/127;
420/128; 420/87 |
Current CPC
Class: |
C22C 38/14 20130101;
C21D 9/30 20130101; Y02P 10/25 20151101; Y02P 10/253 20151101; F16H
55/06 20130101; C21D 1/18 20130101; C21D 9/32 20130101; C22C 38/02
20130101; C21D 1/06 20130101; C21D 9/28 20130101; C22C 38/001
20130101; C23C 8/80 20130101; C21D 1/10 20130101; C22C 38/18
20130101; C23C 8/22 20130101; C22C 38/06 20130101; C22C 38/04
20130101 |
Class at
Publication: |
148/233 ;
148/319; 420/87; 420/104; 420/120; 420/121; 420/128; 420/123;
420/119; 420/127 |
International
Class: |
C23C 8/00 20060101
C23C008/00; C22C 38/18 20060101 C22C038/18; C22C 38/08 20060101
C22C038/08; C22C 38/00 20060101 C22C038/00; C22C 38/12 20060101
C22C038/12; C22C 38/60 20060101 C22C038/60; C22C 38/04 20060101
C22C038/04 |
Claims
1. A carburized steel member that is manufactured by: a carburizing
step of forming a carburized layer on a surface layer by heating a
steel member to an austenitizing temperature or higher in a
carburizing atmosphere where oxygen concentration is lower than
that of the atmosphere; a cooling step of, subsequent to the
carburizing step, cooling the steel member at a cooling rate slower
than a cooling rate at which martensitic transformation occurs, and
cooling the steel member to not higher than a temperature at which
structural transformation caused by cooling is completed; and a
quenching step of heating a desired portion of the steel member
with high-density energy to an austenitic region, and thereafter
cooling the steel member at a cooling rate of not slower than the
cooling rate at which martensitic transformation occurs, the
carburized steel member wherein as a basic chemical composition,
the steel member contains, by mass %, C: 0.1% to 0.4%, Si: 0.35% to
3.0%, Mn: 0.1% to 3.0%, P: 0.03% or less, S: 0.15% or less, Al:
0.05% or less, and N: 0.03% or less, and a content of Cr is less
than 0.2%, a content of Mo is 0.1% or less, and remainder is
constituted of Fe and unavoidable impurities, and the surface layer
of the desired portion on which the quenching step is performed
includes: a first layer having a carbon concentration higher than
the range of the basic chemical composition and including a
martensitic structure in which no grain boundary oxide layer caused
by Si exists; a second layer located inside the first layer, having
a carbon concentration within the range of the basic chemical
composition, and including a martensitic structure; and a third
layer located inside the second layer, having a carbon
concentration within the range of the basic chemical composition,
and including no martensitic structure.
2. The carburized steel member according to claim 1, wherein the
carbon concentration of a surface portion of the first layer is
within a range of 0.6 mass % to 0.85 mass % and is gradually
lowered to approach the range of the basic chemical composition
toward a boundary with the second layer.
3. The carburized steel member according to claim 2, wherein the
second layer has a larger thickness than that of the first
layer.
4. The carburized steel member according to claim 1, wherein the
basic chemical composition further includes Ti: 0.005% to 0.2% and
B: 0.0006% to 0.005%.
5. The carburized steel member according to claim 1, wherein Mo
content in the basic chemical composition is less than 0.01%.
6. The carburized steel member according to claim 1, wherein the
basic chemical composition further includes either or both of Nb:
0.01% to 0.3% and V: 0.01% to 0.2%.
7. The carburized steel member according to claim 1, characterized
in that the basic chemical composition further includes Ni: 0.1% to
3.0%.
8. A method of manufacturing a carburized steel member, comprising:
a carburizing step of forming a carburized layer on a surface layer
by heating a steel member to an austenitizing temperature or higher
in a carburizing atmosphere where oxygen concentration is lower
than that of the atmosphere; a cooling step of, subsequent to the
carburizing step, cooling the steel member at a cooling rate slower
than a cooling rate at which martensitic transformation occurs, and
cooling the steel member to not higher than a temperature at which
structural transformation caused by cooling is completed; and a
quenching step of heating a desired portion of the steel member
with high-density energy to an austenitic region, and thereafter
cooling the steel member at a cooling rate of not slower than the
cooling rate at which martensitic transformation occurs, wherein as
a basic chemical composition, the steel member contains, by mass %,
C: 0.1% to 0.4%, Si: 0.35% to 3.0%, Mn: 0.1% to 3.0%, P: 0.03% or
less, S: 0.15% or less, Al: 0.05% or less, and N: 0.03% or less,
and a content of Cr is less than 0.2%, a content of Mo is 0.1% or
less, and remainder is constituted of Fe and unavoidable
impurities.
9. The method of manufacturing a carburized steel member according
to claim 8, wherein the cooling step is started immediately after
the carburizing step so as not to precipitate solid solution of
carbon in austenite created in the carburizing step as graphite
grain.
10. The method of manufacturing a carburized steel member according
to claim 8, wherein the carburizing step is performed under a
condition that a surface carburizing concentration of the steel
member after diffusion is 0.8% or less.
11. The method of manufacturing a carburized steel member according
to claim 10, wherein the steel member on which the carburizing step
is performed includes a first portion and a second portion whose
diffusion rate of carbon entering when the carburizing treatment is
performed is different from that of the first portion because of
shapes thereof, the entering carbon in the second portion has a
lower diffusion rate than that in the first portion, and the
carburizing step is performed under a condition that a surface
carburizing concentration of the first portion is within a range of
0.65.+-.0.1 mass %.
12. A case hardening steel for the carburized steel member
according to claim 1, wherein the case hardening steel has a basic
chemical composition including, by mass %, C: 0.1% to 0.4%, Si:
0.35% to 3.0%, Mn: 0.1% to 3.0%, P: 0.03% or less, S: 0.15% or
less, Al: 0.05% or less, and N: 0.03% or less, and a content of Cr
is less than 0.2%, a content of Mo is 0.1% or less, and remainder
is constituted of Fe and unavoidable impurities.
13. The case hardening steel according to claim 12, wherein the
basic chemical composition further includes Ti: 0.005% to 0.2% and
B: 0.0006% to 0.005%.
14. The case hardening steel according to claim 12, wherein the Mo
content in the basic chemical composition is less than 0.01%.
15. The case hardening steel according to claim 12, wherein the
basic chemical composition further includes either or both of Nb:
0.01% to 0.3% and V: 0.01% to 0.2%.
16. The case hardening steel according to claim 12, wherein the
basic chemical composition further includes Ni: 0.1% to 3.0%.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2010-079437 filed on Mar. 30, 2010 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a carburized steel member,
for example, such as a gear manufactured by performing carburizing
treatment, and a manufacturing method thereof.
DESCRIPTION OF THE RELATED ART
[0003] For example, a steel member such as a gear is, in many
cases, subjected to carburizing treatment and quenching treatment
as treatment for increasing surface hardness while keeping
toughness (hereinafter, a steel member on which the carburizing
treatment is performed will be referred to as a carburized steel
member as appropriate). The carburizing treatment is treatment to
increase carbon concentration of a surface in a state where a steel
member is heated to an austenitizing temperature or higher, and
quenching treatment is generally performed right after the
carburizing treatment is performed. Toughness of the core of the
steel member can be ensured and surface hardness of the steel
member can be increased by performing the carburizing treatment and
the quenching treatment.
[0004] A material to which Cr and Mo are added has been employed as
a conventional carburized steel member in order to improve
quenching properties and a resistance to temper softening.
Furthermore, as a method of manufacturing a steel member by
performing the quenching treatment after the carburizing treatment
is performed, a new method has been proposed that is advantageous
for improving the quality, in place of a conventional combination
in which the quenching treatment and the carburizing treatment are
simply combined in a continuous manner (see WO2006/118242).
[0005] In the manufacturing method described in WO2006/118242,
"carburizing, slow cooling, and high-frequency quenching treatment"
is employed in which a steel member is once slowly cooled after the
carburizing treatment, and then rapidly cooled right after a
desired portion is high-frequency heated for quenching. The
employment of the "carburizing, slow cooling, and high-frequency
quenching treatment" allows to restrain occurrence of deformation
associated with the quenching treatment while ensuring high
hardness of a surface layer portion of a specific part.
[0006] Hereinafter, in the specification, heat treatment including
the "carburizing treatment" and the "quenching treatment" will be
expressed as "carburizing and quenching treatment" in a general
meaning. Unless otherwise clearly noted, the "carburizing and
quenching treatment" is used as a term including the conventional
method in which the quenching treatment is performed right after
the carburizing treatment, and the carburizing, slow cooling, and
high-frequency quenching treatment. When a distinction therebetween
is necessary, a description will be added as such necessity
arises.
SUMMARY OF THE INVENTION
[0007] It is ideal that at least an inner structure near a surface
has a uniform martensitic structure in a carburized steel member
after carburizing and quenching treatment is performed. However,
especially when heating in a quenching step after the carburizing
treatment is performed by heat treatment for a very short time,
such as heating using high-density energy, it has been found that
structural unevenness, which may be caused by alloy carbide, etc.
remaining in martensite, readily occurs in an inner structure after
the quenching treatment is performed. Such structural unevenness is
not preferable because the unevenness influences durability, etc.
of the carburized steel member.
[0008] For example, in a case where the carburizing, slow cooling,
and high-frequency quenching treatment is employed using a steel
material whose content rate of Cr, which has been conventionally
used for obtaining a steel member having high hardness and high
accuracy, is high (for example, Cr: 0.2 mass % or more), Cr carbide
is produced in a carburized layer when the steel material is slowly
cooled after the carburizing treatment. When high-frequency heating
subsequent to the carburizing treatment and the slow cooling
treatment is performed at a relatively high temperature (for
example, 950.degree. C. or higher), the Cr carbide is dissolved in
a matrix to form uniform austenite transformation, and a uniform
quenched structure can be obtained and the hardness also becomes
uniform. On the other hand, when the steel material is heated at a
relatively low temperature (for example, lower than 950.degree.
C.), the Cr carbide is not readily dissolved in a matrix, uniform
austenite transformation is not readily obtained, and hardness
unevenness may occur. Also when the content rate of Mo is high (for
example, 0.1 wt % or more), Mo carbide is produced, which
contributes to occurrence of hardness unevenness similar to the
case of Cr although not so remarkable as with the case of Cr.
[0009] The carburized layer of a part to be treated is heated with
high-density energy such as high-frequency heating. During such
heating to quench the part, how much the temperature is raised
changes depending on the shape of the part, required properties,
etc. For example, in case of a gear for which high accuracy is
required and whose surface layer portion is required to have high
hardness and inner portion is required to have low hardness
providing high toughness, it is required to perform the quenching
after only the surface layer portion of the gear is heated in
outline when high-frequency quenching is performed. In order to
achieve the processing, it is required to execute the
high-frequency heating with a high power and for a short time. The
heating for a short time prevents heat conduction to the inner
portion to allow heating in outline. At this heating, because
heating is performed with a high output, the surface layer portion
becomes a relatively high temperature totally, and in particular, a
tooth tip portion may be heated to a temperature near the melting
point. In the heating method, as described above, the Cr carbide
and Mo carbide are readily dissolved in a matrix, so that quenched
structure having uniform hardness can be obtained at the surface
layer portion.
[0010] On the other hand, for example, in a high torque
transmission gear such as a differential ring gear, a stress is
applied not only to a surface layer but also to an inner portion of
a tooth portion when the gear is used, so that it is necessary to
increase the hardness of the inner portion. Therefore, when the
high torque transmission gear is high-frequency heated, it is
necessary that not only the surface layer but also the inner
portion of the gear is heated to the austenite transmission
temperature or higher. In this case, because heat conduction to the
inner portion is to be aggressively used so as to heat the inner
portion, heating with a low power and for a relatively long time is
employed in contradiction to the former heating. This also
restrains rising of temperature of the tooth tip and allows heating
of a relatively good temperature distribution. However, because of
austenitic transformation at a relatively low temperature, when Cr
carbide and Mo carbide exist in a large amount especially in a
tooth root portion, etc. whose temperature is not readily
increased, the Cr carbide and Mo carbide cannot be completely
dissolved in the austenite and remains therein. This may cause
lowered hardness and unevenness.
[0011] To address such a problem, it is important to optimize each
processing method in the carburizing and quenching treatment.
However, it has been also strongly required to optimize the
chemical composition itself of the carburized steel member so as to
restrain structural unevenness and hardness unevenness caused by
such structural unevenness even when treatment conditions are
different.
[0012] The present invention is made in light of such a problem,
and provides a method of manufacturing a carburized steel member in
which a chemical composition that can restrain hardness unevenness
regardless of carburizing and quenching treatment conditions is
aggressively employed, and further provides a method of
manufacturing a carburized steel member including further
preferable manufacturing conditions that take advantage of the
chemical composition.
[0013] As described above, the present invention is specialized in
a carburized steel member manufactured through a carburizing step
performed under a low oxygen concentration atmosphere, a cooling
step in which slow-cooling is performed under the above-mentioned
specific conditions, and a quenching step employing heating with
the high-density energy, and a method of manufacturing the same.
The chemical composition within the above ranges is employed as the
optimum chemical composition when the carburized steel member is
manufactured by the manufacturing method.
[0014] Notable points in the chemical composition are that Cr and
Mo are contained as impurities or the contents thereof are limited
to less than 0.2% and 0.1% or less respectively even when added,
and that the content of Si is increased to the range of 0.35% to
3.0%.
[0015] Cr is an element useful for improving quenching properties
and increasing a resistance to temper softening. However, when Cr
is added by 0.2% or more, Cr carbide is readily produced when the
slow cooling is performed after the carburizing treatment. Once Cr
carbide is produced, it may be difficult to dissolve the Cr carbide
in a matrix when heating is subsequently performed by using
high-density energy.
[0016] Furthermore, Mo is also an element useful for improving
quenching properties and increasing a resistance to temper
softening. However, when Mo is added to 0.1% or more, Mo carbide is
readily produced when the slow cooling is performed after the
carburizing treatment. Once Mo carbide is produced, it may be
difficult to dissolve the Mo carbide in a matrix when heating is
subsequently performed by using high-density energy.
[0017] Remaining of Cr carbide and Mo carbide in the structure
causes structural unevenness, and eventually causes hardness
unevenness.
[0018] Therefore, as described above, Cr is aggressively regarded
as an optional component, and the content thereof is limited to
less than 0.2% even when Cr is added. In the same manner, Mo is
regarded as an optional component, and the content thereof is
limited to 0.1% or less even when Mo is added. The improvement
effects of the quenching properties and the resistance to temper
softening when Cr and Mo are added are achieved by increasing the
content of Si that may exert functions thereof to the range of
0.35% to 3.0%.
[0019] Therefore, formation of Cr carbide and Mo carbide is
restrained when the slow cooling is performed after the carburizing
treatment while ensuring quenching properties and a resistance to
temper softening. Accordingly, even when a heating unit used in the
carburizing step is limited to heating using high-density energy,
and even when the heating is performed at a relatively low
temperature, structural unevenness due to the presence of Cr
carbide and Mo carbide can be reduced because formation itself of
Cr carbide and Mo carbide is originally restrained, and a desired
hardness can be obtained. Furthermore, material cost can be also
reduced by reducing the additive amount of expensive Cr and Mo.
[0020] Furthermore, conventionally, an increase in the content of
Si may cause grain boundary oxidization due to Si when the
carburizing treatment is performed. On the contrary, as described
above, occurrence of grain boundary oxidization can be restrained
and reduction in structural properties associated with an increase
in the content of Si can be prevented by restricting processing
conditions of the carburizing step so that the carburizing step is
executed under a carburizing atmosphere where oxygen concentration
is lower than that of the atmosphere.
[0021] The resultant carburized steel member thus obtained has: a
first layer having a carbon concentration higher than the range of
the basic chemical composition and including a martensitic
structure in which no grain boundary oxide layer caused by Si
exists; a second layer located inside the first layer, having a
carbon concentration within the range of the basic chemical
composition, and including a martensitic structure; and a third
layer located inside the second layer, having a carbon
concentration within the range of the basic chemical composition,
and including no martensitic structure, on the surface layer of the
desired portion on which the quenching step is performed.
[0022] The thickness of the second layer (size in a depth direction
from the surface) may be larger than the thickness of the first
layer. Accordingly, hardness of the carburized layer on the inner
side can also be highly hardened in a relatively wide range. To
increase the thickness of the second layer, that is, a layer in
which the carburized structure is obtained while maintaining the
carbon concentration of the base material, which is disposed inside
the carburized layer, it is necessary for heating in the
carburizing step to be performed at a relatively low temperature
for a long time to deeply advance austenitizing. Also in this case,
as described above, production of Cr carbide and Mo carbide can be
restrained, so that structural unevenness can be reduced.
[0023] Accordingly, in the thus obtained carburized steel member,
hardness unevenness is reduced as structural unevenness is reduced,
thereby achieving excellent cost performance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is an explanatory diagram showing a configuration of
heat treatment equipment used in an embodiment;
[0025] FIG. 2 is an explanatory diagram showing a heat pattern in a
carburizing step and a cooling step in the embodiment;
[0026] FIG. 3 is an explanatory diagram showing a heat pattern in a
quenching step in the embodiment;
[0027] FIG. 4 is an explanatory diagram showing a carburized steel
member (differential ring gear) in the embodiment;
[0028] FIG. 5 is an explanatory diagram showing a detail of a tooth
mold part of the differential ring gear in the embodiment;
[0029] FIG. 6 is a photograph showing a metal structure state near
the tooth mold part of Test No. 3 in the embodiment; and
[0030] FIG. 7 is an explanatory diagram illustrating a structure of
the metal structure of FIG. 6 in the embodiment.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0031] First, a reason for restricting the basic chemical
composition will be described. Hereinafter, mass % in the
composition will be simply referred to as %. Herein, the expression
of "basic chemical composition" denotes a basal chemical
composition of a base material before carbon concentration of the
surface layer is increased by carburizing treatment, that is, of
steel itself.
[0032] C: 0.1% to 0.4%
[0033] C is an element to be added for ensuring strength of steel,
in particular, strength of the central portion. Therefore, the
strength of an interior portion is ensured by setting the lower
limit of the amount of C to 0.1%. However, the upper limit is set
to 0.4% because the toughness is lowered and cutting properties of
the material are deteriorated due to the fact that the hardness is
increased when the content thereof exceeds 0.4%. The most
preferable additive amount is 0.15% to 0.3%.
[0034] Si: 0.35% to 3.0%
[0035] Si is an element useful for deoxidation of the steel, and as
described above, is an element useful for providing strength and
quenching properties necessary for the steel to improve a
resistance to temper softening. The present developed steel has a
characteristic that expensive Cr and Mo can be substantially
reduced. However, there is a case in that the reduction of Cr and
Mo becomes disadvantageous for a part for which a resistance to
temper softening, such as a pitching strength of a gear, is
required as compared with CrMo steel, which is conventional steel.
Accordingly, Si is to be contained by 0.35% or more in order to
obtain a necessary resistance to temper softening even when Cr and
Mo are reduced. However, when the content of Si exceeds 3.0%, the
effect is not only saturated but also causes an increase in
hardness to deteriorate cutting properties of the material, so that
it is necessary to set the content within the range of 0.35% to
3.0%. The most preferable range is more than 0.45% to 1.5%.
[0036] Furthermore, when a material to which a large amount of Si
is added is used, in gas carburizing treatment using an oxidized
gas, grain boundary oxidation occurs and a carburized abnormal
layer is produced on the surface layer by Si in a carburizing step,
because Si readily forms oxidation products. Therefore, the
strength of the carburized layer is lowered to cause lowering of
flexural fatigue strength and rolling contact fatigue strength.
Herein, in the method of manufacturing the carburized steel member,
as described above, a step is employed in which carburizing
treatment is performed in a carburizing atmosphere where the oxygen
concentration is lowered. This makes it possible to positively
increase the Si content because a problem of grain boundary
oxidation can be eliminated.
[0037] Mn: 0.1% to 3.0%
[0038] While Mn is an element useful for deoxidation of the steel
and is an element useful for improving quenching properties, but
the effect is insufficient when the content is less than 0.1%.
However, when the content exceeds 3.0%, the effect is not only
saturated but also causes an increase in hardness to deteriorate
cutting properties of the material, so that it is necessary to set
the content within the range of 0.1% to 3.0%. The most preferable
range is 0.5% to 1.5%.
[0039] P: 0.03% or Less
[0040] It is necessary that P is reduced as much as possible
because P is segregated in a grain boundary to lower toughness, so
that it is necessary to restrict the content thereof to 0.03% or
less although it is difficult to reduce the content to 0.
[0041] S: 0.15% or Less
[0042] S forms MnS in the steel and is an element to be added in
order to improve cutting properties of the material because of the
formation. However, the effect obtained by the content of S is
saturated when the content thereof exceeds 0.15%, and rather causes
grain boundary segregation, resulting in grain boundary
embrittlement. Accordingly, it is necessary to reduce the content
of S to 0.15% or less. When the S amount is less than 0.001%, the
improvement effect of cutting properties is not remarkable.
Accordingly, the content of S should be 0.001% or more when the
lower limit is defined. A more preferable range is within the range
of 0.005% to 0.06%.
[0043] Al: 0.05% or Less
[0044] Al is precipitated and dispersed in the steel as a nitride.
For this reason, Al is regarded as an element useful for preventing
austenitic structure from being coarsened when the carburizing
treatment is performed. However, when the content thereof exceeds
0.05%, precipitate is coarsened to cause embrittlement of steel.
Accordingly, the upper limit is set to 0.05%. It is noted that a
portion on which high-density energy quenching treatment is
performed is recrystallized, and coarsened austenitic structure is
fined. Accordingly, when only the portion on which the high-density
energy quenching treatment is performed needs a high strength, and
a region that is not heated with high-density energy does not need
a high strength, it is not necessary to contain Al. However, when
the region that is not heated with the high-density energy also
needs a high strength, it is necessary to contain Al. In this case,
when the amount of Al is less than 0.005%, the effect of preventing
the austenitic structure from being coarsened is not remarkable.
Accordingly, when the lower limit is defined, the amount is set to
0.005% or more. A more preferable range is 0.02% to 0.04%.
[0045] N: 0.03% or Less
[0046] N forms various types of nitrides, and has an effect to
prevent the austenitic structure from being coarsened when the
carburizing treatment is performed. However, when the content
thereof exceeds 0.03%, forgeability is remarkably deteriorated.
Accordingly, the upper limit is set to 0.03%. It is noted that a
portion on which the high-density energy quenching treatment is
performed is recrystallized, and coarsened austenitic structure is
fined. Accordingly, when only the portion on which high-density
energy quenching treatment is performed needs a high strength, and
a region that is not heated with high-density energy does not need
a high strength, it is not necessary to contain N. However, when
the region that is not heated with the high-density energy also
needs a high strength, it is necessary to contain N. In this case,
when the amount of N is less than 0.003%, the effect of preventing
the austenitic structure from being coarsened is not remarkable.
Accordingly, when the lower limit is defined, it is necessary that
the amount is set to 0.003% or more. A more preferable range is
0.005% to 0.02%.
[0047] Cr: Less than 0.2%
[0048] As described above, Cr is an element useful for improving
quenching properties and a resistance to temper softening. However,
when added by 0.2% or more, Cr carbide is readily produced during
carburizing and slow cooling.
[0049] Specifically, when a heating temperature in a quenching step
after the carburizing and the slow cooling is relatively low, solid
solution of the Cr carbide once produced in a matrix becomes
difficult, so that structural unevenness and hardness unevenness
due to the structural unevenness readily occur.
[0050] The formation itself of the Cr carbide can be restrained by
setting the Cr amount at the requested minimum, so that it is
possible to prevent an adverse effect to the structure caused by
the Cr carbide, even when subsequent heat conditions of the
high-density energy are variously changed.
[0051] Even when the content of Cr is 0.2% or more, formation of
the Cr carbide can be considerably restrained by employing a
chemical composition satisfying a condition of [Si %]+[Ni %]+[Cu
%]-[Cr %]>0.5. However, the hardness unevenness is not
completely restrained. The most preferable range of the content of
Cr is 0.1% or less.
[0052] Mo: 0.1% or Less
[0053] Although it is one of objects of the present application to
reduce Mo, which is a rare metal and is relatively expensive among
rare metals, Mo may be added by a very small amount. Mo is an
element useful for improving quenching properties and a resistance
to temper softening. However, containing of a large amount of Mo
not only increases the cost but also may cause the problem of
hardness unevenness as described above, so that it is necessary to
set the upper limit to 0.1%. It is preferable to add no Mo when
necessary quenching properties can be ensured by other elements. In
this case, it is preferable that the content thereof be less than
0.01%, which is a condition that no Mo is aggressively added.
[0054] Furthermore, it is preferable that the basic chemical
composition further contain Ti: 0.005% to 0.2%, B: 0.0006% to
0.005%.
[0055] Ti: 0.005% to 0.2%
[0056] While Ti is an element that is readily combined with N, so
that Ti provides an effect to prevent B from becoming BN to lose an
improved effect of quenching properties by B, and is an element
useful for preventing the austenitic structure from being coarsened
when the carburizing treatment and the high-frequency quenching
treatment are performed by being precipitated and dispersed in the
steel as a carbonitride, the effect is insufficient when the
content thereof is less than 0.005%. However, when the content
exceeds 0.2%, precipitate is coarsened to make the steel brittle,
so that the upper limit is set to 0.2%. The most preferable range
of the addition is 0.01% to 0.03%.
[0057] B: 0.0006% to 0.005%
[0058] While B is an element useful for providing quenching
properties and strengthening grain boundary strength by being
added, the effect is insufficient when the content thereof is less
than 0.0006%. However, when the content exceeds 0.005%, the effect
is saturated, so that it is necessary to set the content within the
range of 0.0006% to 0.005%. The most preferable range is 0.001% to
0.003%.
[0059] Furthermore, it is preferable that the basic chemical
composition further include either or both of Nb: 0.01% to 0.3% and
V: 0.01% to 0.2%.
[0060] Nb: 0.01% to 0.3%, V: 0.01% to 0.2%
[0061] While Nb and V are elements useful for preventing the
austenitic structure from being coarsened when the carburizing
treatment and the high-frequency quenching treatment are performed
by being precipitated and dispersed in the steel as a carbonitride,
the effect is insufficient when the content of each of Nb and V is
less than 0.01%. However, when the additive amount thereof is too
large, precipitate is coarsened to make the steel brittle, so that
the upper limits of Nb and V are set to 0.3% and 0.2%,
respectively. The most preferable ranges of the addition are 0.01%
to 0.02% for Nb, and 0.01% to 0.05% for V.
[0062] Furthermore, the basic chemical composition may further
include Ni: 0.1 to 3.0%.
[0063] Ni: 0.1% to 3.0%
[0064] While Ni is an element useful for improving toughness, the
effect is insufficient when the content thereof is less than 0.1%.
However, when the content exceeds 3.0%, the effect is not only
saturated but also causes an increase in hardness to deteriorate
cutting properties of the material, so that it is necessary to set
the range within 0.1% to 3.0%. The most preferable range is 0.5% to
1.5%.
[0065] Next, it is preferable to perform each step in the
manufacturing method as described below.
[0066] Specifically, first, the carburizing step is performed in a
carburizing atmosphere where the oxygen concentration is lower than
that in the atmosphere. Specific examples include a method that is
performed in a reduced pressure carburizing gas where the pressure
is lowered than the atmospheric pressure. In other words, it is
effective to employ a reduced pressure carburizing step. In the
reduced pressure carburizing step, the carburizing treatment can be
performed more efficiently than the conventional method because the
carburizing treatment can be executed by a relatively small amount
of carburizing gas while the high temperature inner portion of a
carburizing oven is kept in a reduced pressure state. Furthermore,
treatment time can be reduced, energy consumption can be reduced,
and carburizing-quenching equipment itself can be downsized,
because conventional long-time heating treatment using a
large-sized heat treating oven is no longer required.
[0067] Then, when the reduced pressure carburizing step is applied,
Cr is aggressively regarded as an optional ingredient, and an
effect obtained by restricting the upper limit is especially
effectively exerted. In other words, setting of conditions can be
made by preliminarily calculating carbon potential, because
carburizing is executed by an equilibrated reaction in the case of
general gas carburizing. However, the setting of conditions by the
calculation above is difficult because the reduced pressure
carburizing treatment is a non-equilibrated reaction. Furthermore,
when a steel member having irregularity portions such as a gear is
subjected to the reduced pressure carburizing treatment, for
example, a difference occurs in diffusion rate of entering carbon
depending on the portions, the carburizing concentration of the
surface to be obtained varies depending on the portions, and an
intended modifying effect may not be obtained at a portion whose
surface should be modified by the carburizing treatment. When the
variation of the carburizing level becomes larger than that in the
case of the general gas carburizing, content of a large amount of
Cr may remarkably advance the formation of Cr carbide. Accordingly,
restriction of the content of Cr is especially effective when the
reduced pressure carburizing step is employed.
[0068] Furthermore, employment of the reduced pressure carburizing
allows to restrain the amount of oxygen in the atmosphere to a low
level by reducing pressure of the carburizing atmosphere with
respect to the atmospheric pressure in the carburizing step. This
makes it possible to prevent grain boundary oxidation from
occurring in the carburized layer.
[0069] Furthermore, a carburizing method performed in a carburizing
atmosphere where the oxygen concentration is lower than that in the
atmosphere is not limited to the reduced pressure carburizing
method. For example, a method can be employed in which grain
boundary oxidation of the carburized layer is prevented by
restraining the amount of oxygen in the atmosphere to a low level
by filling nitrogen gas or inactive gas without reducing the
pressure of the atmosphere.
[0070] The reduced pressure carburizing is also called as vacuum
carburizing, and is carburizing treatment that is performed by
reducing the pressure of the atmosphere in an oven and directly
injecting gas of hydrocarbon system (for example, methane, propane,
ethylene, and acetylene) as carburizing gas in the oven. The
reduced pressure carburizing treatment generally includes a
carburizing stage and a diffusion stage. In the carburizing stage,
active carbon that is generated by resolution when the carburizing
gas contacts a surface of the steel becomes carbide on the surface
of the steel to be accumulated in the steel. In the diffusion
stage, the carbide is resolved, and the accumulated carbon is
dissolved in a matrix to be diffused toward an interior portion. It
is noted that the supply route of the carbon is not limited to a
route via the carbide, and it is alleged that there exists a route
through which the carbon is directly dissolved in a matrix.
[0071] Furthermore, it is preferable that the carburizing step be
performed under a reduced pressure condition of 1 to 100 hPa. When
the reduced pressure during the carburizing in the reduced pressure
carburizing step is lower than 1 hPa, there may be a possibility
that a problem occurs such that expensive equipment is required for
maintaining a degree of vacuum. On the other hand, when the reduced
pressure exceeds 100 hPa, a problem may occur that soot is
generated during the carburizing to cause carburizing concentration
unevenness.
[0072] Furthermore, hydrocarbon series gas, such as acetylene,
propane, butane, methane, ethylene, and ethane, may be applied as
the carburizing gas.
[0073] Furthermore, it is preferable that the carburizing step be
performed under a condition that the surface carburizing
concentration of the steel member after diffusion is 0.8% or less.
In this case, the carbon amount in the carburized layer is to be
set to not more than that of eutectoid steel, austenitic
transformation can be provided by heating during the quenching, and
thus martensitic structure can be obtained without causing
cementite precipitation by the subsequent rapid cooling.
[0074] Furthermore, it is preferable that the steel member on which
the carburizing step is performed include a first portion and a
second portion whose diffusion rate of carbon entering when the
carburizing treatment is performed is different from that of the
first portion because of the shapes, that the diffusion rate of the
carbon entering the second portion be slower than that in the first
portion, and that the carburizing step be performed under a
condition that the surface carburizing concentration of the first
portion is within a range of 0.65.+-.0.1 mass %.
[0075] As described above, because the reduced pressure carburizing
treatment is a non-equilibrated reaction, it has been found that
when a steel member having irregularity portions such as a gear is
subjected to a reduced pressure carburizing treatment, for example,
the diffusion rate of the entering carbon differs depending on the
portions, the carburizing concentration of the surface obtained
varies depending on the portions, and an intended modifying effect
may not be obtained at a portion whose surface should be modified
by the carburizing treatment.
[0076] The carburizing concentration of the surface obtained
changes between the time right after the carburizing stage in which
the carburizing is advanced, and the time after the diffusion stage
in which diffusion of carbon entering the interior portion
thereafter is advanced because the reduced pressure carburizing
treatment is a non-equilibrated reaction. The carburizing
concentration is expressed as a value after the diffusion stage has
passed. Accordingly, every carburizing concentration in the present
specification is not a value obtained at the time right after the
carburizing stage, but is a value obtained after the diffusion
stage has passed (hereinafter, expressed in the same manner).
[0077] In light of such a circumstance, in order to optimize the
condition when the reduced pressure carburizing step is performed
on the steel member having the first portion (carbon readily
diffused portion) and the second portion (carbon hardly diffused
portion) whose diffusion rate of the carbon entering when the
carburizing treatment is performed is different from that of the
first portion because of the shapes, it has been found that it is
effective to perform the reduced pressure carburizing step under a
condition that the surface carburizing concentration of the first
portion is within the range of 0.65.+-.0.1 mass %.
[0078] It has been found by many experimental results for the first
time that the surface carburizing concentration of the second
portion of the steel member to be obtained, that is, the portion
where the diffusion rate is slower than that of the first portion
and the carburizing concentration of the surface after the
carburizing treatment is higher than that of the first portion can
be restrained within the range of 0.85 mass % or less by performing
the reduced pressure carburizing treatment under the above
conditions. Accordingly, the surface carburizing concentration of
almost the whole surface of the portion whose surface is to be
modified by the carburizing treatment of the steel member can be
set within the range of 0.55 mass % to 0.85 mass %. With the
surface carburizing concentration set within the range, quenching
effect can be sufficiently obtained also at a portion whose surface
carburizing concentration is near the lower limit (first portion),
a failure due to excess carbon can be restrained at a portion whose
surface carburizing concentration is near the upper limit (second
portion), and an excellent modified surface can be obtained after
the quenching, by subjecting the steel member to a special
quenching step in which the steel member is subsequently heated
using the high-frequency energy, and then rapidly cooled.
[0079] For the carburizing condition, it is necessary to perform a
plurality times of preliminarily experiments in which temperature,
type of carburizing gas, pressure, treatment time, etc. in the
reduced pressure carburizing step are changed to find conditions by
which the surface carburizing concentration of the first portion
becomes within the above-mentioned specific range. It is noted that
when the steel members that are members to be treated have a same
shape, the number of times of the preliminarily experiments can be
optionally reduced by accumulation of data. Furthermore,
determination of the first portion and the second portion of the
steel member may be made by measuring the carburizing concentration
in a plurality of portions practically in the preliminarily
experiments. Alternatively, the determination may be made by
observing the shape because the determination can be relatively
easily made based on the shape.
[0080] Furthermore, the cooling step is performed under at least a
slow cooling condition in which the cooling rate is slower than the
cooling rate at which martensitic transformation occurs in the
steel member during cooling. Accordingly, generation of deformation
due to the martensitic transformation can be restrained, and the
carburizing treatment can be ended in an excellent state in the
shape accuracy.
[0081] To be more specific, it is preferable that the cooling step
be performed under a slow cooling condition by which the cooling
rate during the time when at least the temperature of the steel
member is Al transformation point temperature or more is
0.1.degree. C./sec to 3.0.degree. C./sec. When the cooling rate of
the cooling step exceeds 3.0.degree. C./sec during the time when
the temperature of the steel member is not less than the Al
transformation point temperature, an effect to restrain occurrence
of deformation during the cooling may not possibly be sufficiently
obtained. On the other hand, when the cooling rate in the cooling
step is set to less than 0.1.degree. C./sec during the time when
the temperature is not less than the Al transformation point
temperature of the steel member, it requires a long time before the
temperature reaches the Al transformation point temperature, and
diffusion of the carburized carbon is advanced in the steel member
during the time. The diffusion during the cooling may cause
unevenness of diffusion speed due to the temperature difference
depending on the portions. As a result, the carbon amount may
vary.
[0082] Furthermore, it is preferable that the cooling step be
performed in the cooling gas under a state where the pressure of
the cooling gas is lowered than the atmospheric pressure, that is,
be a reduced pressure cooling step. This makes it possible to
further restrain the occurrence of deformation during the
cooling.
[0083] In other words, when the cooling gas is agitated during the
cooling, reducing of the pressure of the cooling gas enables to
reduce a difference in the cooling rate of circulating cooling gas
between an upwind side and a downwind side compared with the
atmospheric pressure state. In other words, when the cooling is
performed in the atmospheric pressure, thermal exchange is advanced
when a member to be cooled is only made into contact with the
cooling gas in the atmospheric pressure, and cooling of the member
to be cooled is started. In this case, a gas convective flow forms
an upwind side and a downwind side due to active gas agitation or
heat, thereby causing a cooling rate difference. The cooling rate
difference causes a temperature difference in the member to be
cooled, whereby heat treatment deformation occurs. On the other
hand, thermal exchange rate is originally slow on either of the
upwind side and the downwind side by reducing the pressure of the
cooling gas, so that a difference in cooling rate is difficult to
occur. Accordingly, when the reduced pressure cooling in which the
pressure of the cooling gas is reduced is employed, occurrence of
heat treatment deformation is rare because the cooling is
relatively uniformly advanced. Furthermore, even when no agitating
is performed at all, in the case of a reduced pressure state, a
difference of the cooling rate due to the stagnating flow of the
cooling gas at different temperatures can be reduced compared with
the case of the atmospheric pressure.
[0084] By using the effect obtained by reducing the pressure of the
cooling gas, occurrence of deformation of the steel member on which
the reduced pressure cooling step is performed can be restrained,
and the steel member can be advanced to the quenching step while
keeping the size accuracy to a high accuracy. The effect becomes
most remarkable when slow cooling in which the cooling rate is
lowered is employed as a reduced pressure slow cooling step.
Accordingly, the steel member can be less deformed and highly
accurately formed even after the quenching is performed, by making
use of the advantages obtained by the above quenching step using
the high-density energy.
[0085] Furthermore, by employing the reduced pressure carburizing
step as the carburizing step and the reduced pressure cooling step
as the cooling step, and performing these steps continuously, a
reduced pressure carburizing chamber and a slow cooling chamber can
be directly joined, and it is not necessary to provide an auxiliary
chamber, etc. for adjusting a reduced pressure level between the
chambers in practical equipment. In other words, a pressure
difference between the chambers can be reduced because both of the
reduced pressure carburizing step and the slow cooling step are
performed in a reduced pressure state. Accordingly, the reduced
pressure slow cooling treatment can be performed without exposing a
product subjected to the reduced pressure carburizing treatment to
a normal pressures state, which makes it possible to execute
efficient treatment in which occurrence of deformation is
restrained.
[0086] Furthermore, it is preferable that the reduced pressure
condition of the cooling gas in the cooling step be, more
specifically, 100 hPa to 650 hPa. In the reduced pressure cooling
treatment, when the pressure is higher than the range of 100 hPa to
650 hPa, the effect of the reduced pressure may not be fully
obtained, and on the other hand, it may be difficult to lower the
pressure than the range because of the equipment structure.
[0087] Therefore, more preferably, a reduced pressure state of the
cooling gas in the slow cooling step is set in the range of 100 hPa
to 300 hPa.
[0088] Furthermore, it is important that the quenching step is
performed by heating the desired portion of the steel member to
austenitizing temperature or more by using the high-density energy,
and then rapidly cooling the steel member at not slower than a
rapid cooling critical cooling rate at which the martensitic
transformation occurs at least in the carburized layer. The
high-density energy includes, for example, a high-density energy
beam such as an electron beam or a laser beam, and high-density
energy such as high-frequency heating, which is not a beam. By
using the high-density energy, it becomes possible to reduce the
heat time, and sufficient hardness effects can be surely obtained
because the composition is adjusted so as to be readily
austenitized by the reduction of the amount of Cr.
[0089] Furthermore, it is preferable that the heating in the
quenching step be performed by high-frequency heating. In this
case, by employing the high-frequency heating, non-contact heating
can be accurately performed by induction heating, and high
effectiveness can thus be achieved. Furthermore, a known method can
be applied as the high-frequency heating.
[0090] Furthermore, it is preferable that the rapid cooling be
performed by water cooling. In other words, partial heating of the
parts, instead of overall heating, can be precisely performed when
the high-density energy is used, so that even when water quenching
is performed thereafter by using water having very high cooling
effect, occurrence of quenching deformation can be substantially
restrained compared with the case of a conventional gas heating.
Further, quenching properties can be improved, and strength of the
quenched portion can be further increased, by the excellent rapid
cooling effect achieved by the water cooling. Furthermore, by using
this increased strength, simplification of the carburizing
treatment (shortening of the treatment time) may be achieved, that
is, a requested strength may be fulfilled even when the thickness
of the carburized layer is reduced. In this case, the time of the
entire heat treatment step can be further reduced.
[0091] Furthermore, it is preferable that the heating with the
high-frequency heating be performed on the steel members each
flowing one by one, and that the steel member be cooled by spraying
cooling water toward the steel member from the periphery while
rotating the steel member during cooling after the heating. In this
case, the steel member can be cooled evenly when the cooling is
performed, and occurrence of deformation can be further
restrained.
[0092] Furthermore, it is preferable that the heating in the
quenching step be performed under a condition that the whole range
of not less than 1 mm in depth from the surface is austenitized
without the surface being melt. The heating with the high-frequency
energy allows only the uppermost surface of the steel member to be
heated by applying very high-density energy within a very short
time of, for example, 10 sec or less. Accordingly, only a region of
less than 1 mm in depth from the surface can be heated to a desired
high temperature. However, by selecting a relatively lower range of
temperature (for example, 750.degree. C. to 950.degree. C.) as the
austenitizing temperature of steel and by making input energy of
the high-density energy relatively low so as to lengthen the heat
time to a relatively long time (for example, about more than 10
sec), the heating can be performed on a portion of not less than 1
mm in depth from the surface by heat transmission. It has been
found that, by executing the low-temperature and long-time heating
with the high-density energy, quenching effect can be provided not
only to the region of the carburized layer, but also to a depth
region of not less than 1 mm in depth, and that the effect of
restraining of deformation in the quenching step is increased as
the austenitizing temperature becomes lower.
[0093] Considering that structure unevenness may occur if the
content of Cr is not limited to the above-mentioned specific range,
the effect obtained by the reduction of Cr is significant also in
this case.
[0094] Furthermore, when the carburized steel member is a gear part
including a number of protruded tooth portions, it is preferable
that the heating be performed under a condition by which the
surface and whole interior portions of the tooth portion are
austenitized. It is desired for the gear member to increase surface
hardness of the tooth portion, increase the toughness of the
interior portion, and have excellent shape accuracy of the tooth
portion. Accordingly, as described above, by selecting a relatively
lower range of temperature (for example, 750.degree. C. to
950.degree. C.) as the austenitizing temperature of the steel, and
by making input energy of the high-density energy relatively low so
as to lengthen the heat time to a relatively long time, the
quenching effect can be provided not only to a region of the
carburized layer but also to the deep portion, and the effect of
restraining of deformation is increased, which is very
preferable.
[0095] Furthermore, the carburized steel members include drive
system parts of vehicles. There are severe requirements to the
drive system parts of the vehicles with regard to strength and size
shape accuracy in addition to cost reduction. The above-mentioned
manufacturing method is very useful to satisfy the
requirements.
[0096] The drive system parts of the vehicles include, for example,
a gear for an automatic transmission, a ring member, and other
parts.
[0097] Furthermore, as described above, the heating treatment
including the carburizing step, the cooling step, and the quenching
step is a treatment method capable of restraining occurrence of
deformation. In other words, in the cooling step after the
carburizing, cooling is performed at a rate slower than the rate at
which the martensitic transformation occurs. Accordingly,
martensitic structure is not formed in the carburized layer nor in
a core portion, and any one of ferrite, pearlite, and bainite, or a
mixed structure thereof is formed. Accordingly, deterioration of
product accuracy due to heat treatment deformation can be prevented
because the martensitic transformation does not occur and
deformation due to the martensitic transformation is not caused by
the cooling in the cooling step.
[0098] The quenching using the high-density energy, such as
high-frequency quenching, is performed after the cooling step. The
heating using the high-density energy, such as high-frequency
quenching, enables to heat only a region located in a predetermined
depth from the surface and only a portion, so that a part of the
volume of the product can be heated. Accordingly, a region that is
hardened by the martensitic transformation caused by the subsequent
rapid cooling can be reduced, occurrence of deformation due to the
martensitic transformation can be reduced, and deterioration of the
product accuracy can be prevented.
EXAMPLES
[0099] Hereinafter, examples of the method of manufacturing the
carburized steel member will be specifically described. It is noted
that the examples are designed to illustrate the present invention,
and not intended to restrict the scope of the invention.
[0100] In these examples, hot rolling materials having respective
basic chemical compositions (Test Nos. 1 to 32) shown in Table 1
were prepared, and after being subjected to annealing treatment
after hot forging, a steel member (differential ring gear) 8 was
manufactured by mechanical processing as shown in FIG. 4. The
mechanical processing was used as a testing (cutting testing) for
determining cutting properties of the steel member.
[0101] As is understood from Table 1, Test Nos. 1 to 23 are
examples, and Test Nos. 24 to 32 are comparative examples of the
basic chemical compositions of the materials. Among the comparative
examples, Test Nos. 31 and 32 are examples in which SCM 420, which
is conventional steel, is used as a material.
[0102] Next, after the mechanical processing, the steel member 8 is
subjected to a carburizing step of heating the steel member 8 in a
carburizing atmosphere to form a carburized layer on a surface.
Then, after the carburizing step, the steel member 8 is at least
subjected to a cooling step of cooling the steel member 8 to not
more than the temperature at which structure transformation caused
by cooling is completed, and a quenching step of rapidly cooling
the steel member 8 after a desired portion is heated to an
austenitic region with high-density energy. A material examination
and fatigue testing are performed on the obtained steel member
(carburized steel member) 8.
TABLE-US-00001 TABLE 1 Test Basic chemical composition (mass %) No.
Classification C Si Mn Cr P S Al Ti B N Nb V Ni Mo 1 Inventive
example 0.25 0.65 1.25 0.11 0.013 0.019 0.032 -- -- 0.0054 -- -- --
-- 2 Inventive example 0.36 0.51 1.59 0.09 0.017 0.016 0.031 -- --
0.0125 -- -- -- -- 3 Inventive example 0.21 0.81 0.81 0.12 0.015
0.012 0.032 0.015 0.0019 0.0038 -- -- -- -- 4 Inventive example
0.15 0.75 1.00 0.12 0.016 0.013 0.029 0.031 0.0031 0.0038 -- -- --
-- 5 Inventive example 0.10 0.81 0.82 0.11 0.015 0.001 0.033 -- --
0.0038 -- -- -- -- 6 Inventive example 0.40 0.81 0.83 0.03 0.014
0.015 0.033 0.030 0.0019 0.0040 -- -- -- -- 7 Inventive example
0.35 0.36 0.60 0.10 0.015 0.055 0.029 -- -- 0.0040 -- -- -- -- 8
Inventive example 0.20 2.98 0.91 0.10 0.014 0.005 0.031 -- --
0.0041 -- -- -- -- 9 Inventive example 0.21 0.81 0.10 0.05 0.015
0.015 0.031 -- -- 0.0040 -- -- -- -- 10 Inventive example 0.20 0.80
2.99 0.10 0.015 0.149 0.030 -- -- 0.0038 -- -- -- -- 11 Inventive
example 0.38 1.49 0.83 -- 0.015 0.015 0.030 -- -- 0.0037 -- -- --
-- 12 Inventive example 0.20 0.51 1.10 0.19 0.015 0.011 0.033 -- --
0.0040 -- -- -- -- 13 Inventive example 0.20 0.80 0.82 0.11 0.030
0.015 0.033 -- -- 0.0041 -- -- -- -- 14 Inventive example 0.19 0.50
1.49 0.10 0.015 0.014 0.005 -- -- 0.0041 -- -- -- -- 15 Inventive
example 0.30 0.80 0.82 0.10 0.015 0.014 0.049 -- -- 0.0040 -- -- --
-- 16 Inventive example 0.20 1.30 1.20 0.10 0.014 0.014 0.030 0.005
0.0006 0.0038 -- -- -- -- 17 Inventive example 0.19 0.45 0.82 0.07
0.015 0.008 0.032 0.190 0.0049 0.0042 -- -- -- -- 18 Inventive
example 0.20 0.80 0.81 0.10 0.015 0.009 0.029 -- -- 0.0031 -- -- --
-- 19 Inventive example 0.37 0.80 0.83 0.10 0.014 0.015 0.034 -- --
0.0296 -- -- -- -- 20 Inventive example 0.19 0.91 0.81 0.11 0.015
0.014 0.030 -- -- 0.0040 0.015 -- -- -- 21 Inventive example 0.20
0.81 1.21 0.10 0.016 0.015 0.031 -- -- 0.0041 -- 0.04 -- -- 22
Inventive example 0.20 0.37 0.83 0.10 0.015 0.014 0.030 -- --
0.0040 -- -- 1.01 -- 23 Inventive example 0.20 0.80 0.71 0.10 0.014
0.015 0.033 -- -- 0.0037 -- -- -- 0.06 24 Comparative example 0.21
0.30 0.79 0.10 0.015 0.014 0.030 0.015 0.0021 0.0056 -- -- -- -- 25
Comparative example 0.41 0.79 0.83 0.10 0.015 0.060 0.033 -- --
0.0039 -- -- -- -- 26 Comparative example 0.20 0.50 0.83 0.30 0.015
0.015 0.030 -- -- 0.0041 -- -- -- -- 27 Comparative example 0.21
0.91 0.66 0.23 0.016 0.019 0.030 -- -- 0.0120 -- -- -- -- 28
Comparative example 0.30 0.80 0.82 1.05 0.015 0.015 0.029 0.015
0.0015 0.0043 -- -- -- -- 29 Comparative example 0.20 0.79 1.10
0.10 0.032 0.015 0.032 -- -- 0.0039 -- -- -- -- 30 Comparative
example 0.32 1.29 0.60 0.08 0.013 0.161 0.033 -- -- 0.0045 -- -- --
-- 31 Comparative example 0.20 0.25 0.81 1.10 0.015 0.015 0.033 --
-- 0.0081 -- -- -- 0.15 32 Comparative example 0.21 0.34 0.79 1.10
0.015 0.014 0.031 -- -- 0.0078 -- -- -- 0.15
[0103] First, heat treatment equipment 5 for performing the
carburizing step to the quenching step will be simply
described.
[0104] As shown in FIG. 1, the heat treatment equipment 5 is
equipped with a prewashing tank 51 for washing the steel member
before the carburizing and quenching treatment, a reduced pressure
carburizing slow cooling apparatus 52 equipped with a heating
chamber 521, a reduced pressure carburizing chamber 522, and a
reduced pressure slow cooling chamber 523, a high-frequency
quenching machine 53, and a magnetic flaw detector 54 for
inspecting defects.
[0105] Next, the carburizing step and the cooling step performed by
using the heat treatment equipment 5 will be described.
[0106] The carburizing step of this example is a reduced pressure
carburizing step to be performed in carburizing gas under a reduced
pressure lower than the atmospheric pressure. A heat pattern A in
the step is shown in FIG. 2. In the drawing, the horizontal axis
denotes time and the vertical axis denotes temperature.
[0107] As is understood from the drawing, in the heat pattern A in
the carburizing step, temperature was raised to a carburizing
temperature in a temperature rise region a, and then the
temperature was kept to be constant in keeping regions b1 and b2.
The temperature was constantly kept at 950.degree. C., which is a
temperature not less than the austenitizing temperature. The
introductory region b1 of the keeping region is a region for the
carburizing stage in the carburizing treatment, and the subsequent
region b2 is a region for the diffusion stage in the carburizing
treatment.
[0108] In the example, a reduced pressure condition of the reduced
pressure carburizing treatment was 1 hPa to 3.5 hPa, and acetylene
was used as the carburizing gas in the region b1 of the carburizing
stage. Furthermore, conditions that were preliminarily confirmed by
preliminary experiments were employed for the carburizing
conditions, and performed as described below.
[0109] Specifically, the steel member 8 of the example is a
differential ring gear and has a first portion (a carbon readily
diffused portion) and a second portion (a carbon hardly diffused
portion) whose diffusion rate of carbon entering during the
carburizing treatment is different from that of the first portion
because of the shapes. As shown in FIG. 5, the first portion is a
tooth bottom 815 and a tooth surface 811, and the second portion in
which the diffusion rate of the entering carbon is slower than that
of the first portion is a tooth tip corner portion 813 (a corner
portion between the tooth surface 811 and a tooth tip 812). In the
example, the condition by which the surface carburizing
concentration of the tooth bottom 815, which is the first portion,
becomes within the range of 0.65.+-.0.05 mass % was employed.
[0110] Next, after the diffusion stage of the reduced pressure
carburizing treatment was ended, the treatment for a cooling region
c was performed as the cooling step. In the example, a reduced
pressure slow cooling step was employed, and the reduced pressure
condition was 600 hPa. Furthermore, the cooling atmosphere gas was
nitrogen (N.sub.2). Furthermore, the cooling rate of the reduced
pressure slow cooling step was set within the range of 0.1.degree.
C./sec to 3.0.degree. C./sec until the temperature was lowered to
150.degree. C., which is lower than the Al transformation point,
from the austenitizing temperature or more right after the
carburizing treatment.
[0111] The heat pattern A and the other conditions shown herein are
only an example, and can be modified to the optimum conditions for
the steel member to be treated by preliminary experiments, etc.
[0112] Next, the quenching step performed by using the heat
treatment equipment 5 will be described.
[0113] The quenching step of the example employed high frequency
heating as a heating unit, and employed water cooling as a rapid
cooling unit. A heat pattern B thereof is shown in FIG. 3. In the
drawing, the horizontal axis denotes time, and the vertical axis
denotes temperature.
[0114] As shown in the drawing, the quenching step of the example
includes a temperature rise region d1 in which a teeth profile
portion 81, which is an outer periphery portion of the steel member
8, is heated to the austenitizing temperature or more with the
high-frequency heating, and a rapid cooling region d2 in which
water is sprayed for water quenching so that a cooling rate of not
less than the rapid cooling critical cooling rate at which
martensitic transformation occurs in the carburized layer can be
readily obtained.
[0115] In the temperature rise region d1, the energy amount to be
input was set lower than that in a condition under which normal
high-frequency heating is performed, and the treatment was
performed for 26.8 sec, which is relatively a long time, so that
the whole interior portion of the teeth profile portion 81 became a
temperature within the range of 750.degree. C. to 960.degree.
C.
[0116] The treatment for the rapid cooling region d2 was performed
for about 13 sec, and the cooling rate was 50.degree. C./sec to
65.degree. C./sec. A method that can restrain occurrence of
deformation most was employed for the heating with the
high-frequency heating. In the method, the steel members 8 were
carried (conveyed) one by one and subjected to heating treatment
one by one, and during cooling after the heating, cooling water was
sprayed toward the steel member 8 from the periphery while rotating
the steel member 8.
[0117] The heat pattern B and the other conditions shown herein are
only an example, and can be modified to the optimum conditions for
the steel member to be treated by preliminary experiments, etc.
[0118] Hereinafter, conditions of each testing performed on the
steel members will be described.
[0119] <Cutting Property Testing>
[0120] The cutting property testing was performed when the steel
member 8 was cut into a desired shape by the mechanical processing
as described above.
[0121] In the cutting property testing, by using a carbide tool,
lathe turning was performed under conditions that rim speed was 250
m/min, feed was 0.3 mm/rev, insection was 1.5 mm under a dry
environment. Furthermore, ten steel members 8 were manufactured for
conducting evaluation in the testing, and each of the ten steel
members 8 was evaluated as a good one when there was no crack in
the carbide tool, and was evaluated as a defective one when there
was a crack in the carbide tool.
[0122] <Material Examination>
[0123] A material examination was performed by collecting a sample
from the tooth surface 811 of the steel member 8 on which no
fatigue testing was performed.
[0124] Vickers hardness was measured with a load of 2.9 N for every
0.05 mm from a surface of the sample, and an effective curing depth
prescribed in JIS G 0557 was measured.
[0125] Crystal grain was evaluated by obtaining an austenite
crystal grain size number prescribed in JIS G 0551 at a depth
portion of 0.4 mm from the surface of the sample.
[0126] In determination of the evaluation, the one whose effective
curing depth was ensured by 0.8 mm or more, which had a difference
in Vickers hardness at each of adjacent measuring points of less
than HV 50 and no hardness unevenness, and whose austenite crystal
grain size number was 6 or more, which means fine grains, was
determined as a good one.
[0127] However, as for the one having a low strength in the fatigue
testing, SEM observation was performed on a fracture surface and
whether grain boundary was a brittle fracture surface was
confirmed.
[0128] <Fatigue Testing>
[0129] The fatigue testing was performed by a power circulating
gearwheel testing machine. Module, pressure angle, torsion angle
that are the specifications of a gear under test are 2.03,
18.degree., and 27.degree., respectively.
[0130] Dedendum bending fatigue strength (Nm) was evaluated by
input torque that allowed durability of ten million times under a
condition in which the temperature of a lubricant was 80.degree. C.
and the rotating speed was 2000 rpm. ATF was used as the
lubricant.
[0131] Pitching (Nm) strength was evaluated by input torque by
which pitching aerial proportion became 3% or less of the area per
all teeth after 50 million times test under a condition in which
the lubricant temperature was 120.degree. C. and the rotating speed
was 4000 rpm. ATF was used as the lubricant.
[0132] Results of the above-mentioned testing are shown in Table
2.
TABLE-US-00002 TABLE 2 Fatigue testing result Input Test Cutting
property torque (Nm) No. Classification testing result Material
testing result Dedendum Pitching 1 inventive example good good 406
270 2 inventive example good good 400 253 3 inventive example good
good 418 282 4 inventive example good good 406 276 5 inventive
example good good 424 288 6 inventive example good good 419 290 7
inventive example good good 406 206 8 inventive example good good
365 300 9 inventive example good good 383 276 10 inventive example
good good 412 288 11 inventive example good good 365 306 12
inventive example good good 394 222 13 inventive example good good
377 282 14 inventive example good good 406 259 15 inventive example
good good 412 282 16 inventive example good good 371 306 17
inventive example good good 377 247 18 inventive example good good
418 282 19 inventive example good good 406 270 20 inventive example
good good 371 282 21 inventive example good good 388 300 22
inventive example good good 377 211 23 inventive example good good
418 276 24 comparative example good good 412 194 25 comparative
example cutting defect (testing piece manufacturing impossible) 26
comparative example good hardness unevenness 330 194 27 comparative
example good hardness unevenness 325 188 28 comparative example
good hardness unevenness 324 188 29 comparative example good grain
boundary brittle fracture surface 294 164 30 comparative example
good grain boundary brittle fracture surface 303 165 31 comparative
example good hardness unevenness 342 200 32 comparative example
good hardness unevenness 340 205
[0133] As shown in Table 2, it becomes apparent that the ring gears
of Test Nos. 1 to 23 in the example have good cutting properties of
the material and have excellent material properties and fatigue
strength. Specifically, compared with the conventional steel SCM
420 (Test Nos. 31 and 32), in every example, the dedendum fatigue
strength was fairly larger, and the pitching strength was also
equal or larger.
[0134] On the other hand, the fatigue strength of Test No. 24 of
the comparative example was low. This is because that quenching
properties were low and improvement of a resistance to temper
softening could not be fully obtained because of the steel material
whose content of Si is less than 0.35%.
[0135] The cutting properties of the material were poor in Test No.
25 of the comparative example. This is because that the material
was hardened too much because of the steal material whose content
of C exceeded 0.4%.
[0136] In Test Nos. 26 to 28 of the comparative examples, each of
the fatigue strength was low. This is because that production of a
Cr carbide was recognized in the material examination because of
the steel material whose content of Cr exceeded 0.2%, and hardness
unevenness occurred.
[0137] In Test No. 29 of the comparative example, the fatigue
strength was low. The reason of the low fatigue strength is that a
grain boundary brittle fracture surface occurred because of the
steel material whose content of P exceeded 0.03%.
[0138] In Test No. 30 of the comparative example, the fatigue
strength was low. The reason of the low fatigue strength is that a
grain boundary brittle fracture surface occurred because of the
steel material whose content of S exceeded 0.15%.
[0139] Test Nos. 31 and 32 of the comparative examples are examples
in which similar treatment was performed on the SCM 420 of JIS,
which is conventional steel. Hardness unevenness occurred in the
comparative examples because both of Cr and Mo were included and
the compositions were not optimized. Furthermore, it becomes
apparent, from comparison between the examples and the comparative
examples, that the carburized steel members of the examples have
excellent dedendum fatigue strength and other properties equivalent
to or superior than those of conventional Cr--Mo alloy steel even
when the contents of Cr and Mo are reduced to reduce the material
cost.
[0140] <Observation of First to Third Layers>
[0141] The thicknesses of first to third layers were measured by
observing metal structure of a cross section of the steel member of
Test No. 3. The steel member was corroded with alcohol solution
having nitric acid of 3% to 5%, and the metal structure was
observed with an optical microscope. A photograph of the metal
structure is shown in FIG. 6, and an explanatory diagram made to
simply show the structure of each layer is shown in FIG. 7. In the
drawing, reference numeral S1 denotes the first layer, reference
numeral S2 denotes the second layer, and reference numeral S3
denotes the third layer, respectively.
[0142] The first layer S1 is a portion of the outermost layer on
which the quenching step is performed, is a portion whose carbon
concentration of the surface is 0.60 mass % to 0.85 mass % and in
which the carbon concentration is gradually lowered to the carbon
concentration of the base material (within the range of the basic
chemical composition) toward inside, and is a layer including
martensitic structure in which no grain boundary oxide layer caused
by Si exists. It was observed that the first layer S1 existed, and
it was found that the thickness of the layer was within the range
of 0.7 mm to 1.3 mm.
[0143] The second layer S2 is located inside the first layer S1,
and the carbon concentration is 0.1 mass % to 0.4 mass %, that is,
the carbon concentration of the base material (within the range of
the basic chemical composition), and is a layer including
martensitic structure. It was observed that the second layer S2
existed, and it was found that the thickness of the layer was
within the range of 2 mm to 5 mm.
[0144] The third layer S3 is located inside the second layer S2,
and the carbon concentration is 0.1 mass % to 0.4 mass %, that is,
the carbon concentration of the base material (within the range of
the basic chemical composition), and is a layer including no
martensitic structure. It was confirmed that the third layer S3
existed.
[0145] As a result of the structural observation, it became
apparent that the thickness of the second layer S2 was larger than
the thickness of the first layer S1 in the steel member of Test No.
3.
[0146] Graphite grain was not produced in any of the first to third
layers. It has been know that graphite grain is produced by keeping
the temperature of the steel member for a certain amount of time
within the temperature range of, for example, 650.degree. C. to
720.degree. C. after the carburizing treatment. However, as
described above, when at least slow cooling is performed right
after the carburizing treatment, production of graphite grain can
be prevented.
* * * * *