DESCRIPTION
TITLE OF INVENTION: PRODUCTION METHOD OF CARBURIZED STEEL
COMPONENT AND CARBURIZED STEEL COMPONENT
TECHNICAL FIELD
[OOOl]
The present invention relates to a production method of a steel component and
a steel component, and more specifically to a production method of a carburized steel
component by performing carburizing treatment, and a carburized steel component.
BACKGROUND ART
[0002]
Steel components as represented by gears and bearings are used in severe
environments and may be subjected to large loads during torque transmission and the
like. Therefore, high surface fatigue strength is required for such steel components.
[0003]
Steel components are generally produced in the following method. First, a
starting material is formed into a desired shape to produce an intermediate product.
The intermediate product is subjected to a case hardening treatment to obtain a steel
component. The casehardened steel component has high surface fatigue strength.
[0004]
A method for increasing surface fatigue strength has been proposed in
Japanese Patent Application Publication No. 2013-204645 (Patent Literature 1) in
which a surface unevenness is formed on the surface of a steel component by
pickling treatment. However, a pickling treatment is added in this method. Thus,
the number of processes increases compared with an ordinary production method of
a steel component. Increase in the number of processes will lead to increase in
production cost.
[0005]
Another method for improving surface fatigue strength is a method of
increasing Si content in a steel component. Si improves hardenability of a steel
component and further improves temper softening resistance in martensite. Thus,
Si increases strength of a core part of the steel component and also increases surface
fatigue strength.
[0006]
A further method for increasing surface fatigue strength is a method of
performing carburizing treatment as the casehardening treatment. Carburizing
treatment forms a carburized layer on the surface of a steel component, thereby
increasing surface fatigue strength of the steel component.
[0007]
Japanese Patent Application Publication No. 2008-280610 (Patent Literature
2) discloses a method for producing a steel component having an increased Si
content. In Patent Literature 2, a steel containing 0.5 to 3.0% of Si is subjected to a
vacuum carburizing treatment. However, performing continuous treatment is
difficult in such vacuum carburizing treatment. Moreover, tarring is likely to occur
in vacuum carburizing treatment. Further, the properties of a steel component is
difficult to control. Therefore, mass production of a steel component is difficult by
means of vacuum carburizing treatment, leading to low productivity.
[0008]
Another carburizing treatment different from the vacuum carburizing
treatment is gas carburizing treatment. Gas carburizing treatment does not have the
above described disadvantage of vacuum carburizing treatment. Therefore, gas
carburizing treatment is suitable for mass production of steel components.
[0009]
However, Si in steel deteriorates carburizing properties in gas carburizing
treatment. For example, a casehardening steel having a chemical composition
corresponding to SCr420 specified in JIS G4052 (hereafter, referred to as an ordinary
casehardening steel), and a case hardening steel having a higher Si content compared
to that of SCr420 (hereafter, referred to as a high-Si steel) are prepared. The
ordinary casehardening steel and the high-Si steel are subjected to a gas carburizing
treatment under the same condition. In this case, the depth of effective hardened
layer of the high-Si steel becomes smaller than that of the ordinary casehardening
steel.
[OO 1 01
It is reported in "IRON AND STEEL," 58th year (1 972), Vol. 7, (June 1, 1972,
published by The Iron and Steel Institute of Japan), P.926 (Non Patent Literature 1)
that increase in Si content results in decrease in gas carburized depth. Therefore,
there is a need for development of a production method which enables to achieve a
sufficient depth of effective hardened layer even when a high-Si steel is subjected to
gas carburizing treatment.
[OOll]
A gas carburizing method for increasing fatigue strength of a steel component
is disclosed in Japanese Patent Application Publication No. 02-1 56063 (Patent
Literature 3) and International Application Publication No. W0121077705 (Patent
Literature 4).
[OO 121
In Patent Literature 3, a steel material is subjected to preliminary
carburization at a carburizing temperature higher than A1 transformation point such
that the surface carbon concentration is not less than 1.0%. Next, the steel material
is gradually cooled to immediately above the A1 transformation point and is soaked.
Next, the steel material is reheated to a temperature less than the carburizing
temperature during preliminary carburization and is quenched.
[00 131
However, steel materials to be addressed in Patent Literature 3 are SCr steel,
SCM steel, SNCM steel, and casehardening steels specified in JIS Standard. The Si
contents of these steels are low. Therefore, when a steel having a high Si content is
subjected to the gas carburizing treatment of Patent Literature 3, sufficient surface
fatigue strength may not be achieved.
[00 141
Patent Literature 4 discloses the following items relating to a production
method including gas carburizing treatment of a high-Si steel. When a high-Si steel
is subjected to an ordinary gas carburizing treatment, oxide coating is formed on the
surface thereof in an early stage of the carburization. The oxide coating deteriorates
gas carburizing property. Accordingly, in Patent Literature 4, the following gas
carburizing treatment is performed. First, a steel material is subjected to primary
carburization under an atmosphere in which oxide coating is generated. Next, the
oxide coating formed on the steel material is removed by shot peening and chemical
polishing, etc. Next, the steel material whose oxide coating has been removed is
subjected to secondary carburization.
[00 1 51
However, in the method of Patent Literature 4, a process of removing oxide
coating is added compared to an ordinary carburizing treatment. Increase in the
number of processes will lead to deterioration in productivity and increase in
production cost.
CITATION LIST
PATENT LITERATURE
[00 1 61
Patent Literature 1: Japanese Patent Application Publication No. 2013-204645
Patent Literature 2: Japanese Patent Application Publication No. 2008-280610
Patent Literature 3: Japanese Patent Application Publication No. 02-156063
Patent Literature 4: International Application Publication No. W0121077705
NON PATENT LITERATURE
[00 1 71
Non Patent Literature 1: "IRON AND STEEL," 58th year (1972), Vol. 7,
(June 1, 1972, published by The Iron and Steel Institute of Japan), P.926.
SUMMARY OF INVENTION
[OO 1 81
It is an object of the present invention to provide a production method of a
carburized steel component, which can improve gas carburizing property for a steel
component having a high Si content, and suppress deterioration of productivity
thereof.
[00 191
The production method of a carburized steel component according to the
present embodiment includes a preliminary gas carburizing process, and a main gas
carburizing process. In the preliminary gas carburizing process, a steel component
having a chemical composition that consists of: by mass%, C: 0.1 to 0.4%, Si: 0.7 to
4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.15%, S: not more than 0.3%,
N: 0.003 to 0.03%, 0: not more than 0.0050%, P: not more than 0.025%, Nb: 0 to
0.396, Ti: 0 to 0.3%, V: 0 to 0.3%, Ni: 0 to 3.096, Cu: 0 to 3.096, Co: 0 to 3.0%, Mo:
0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0 to
0.05%, Te: 0 to 0.1%, and rare earth metals: 0 to 0.005%, with the balance being Fe
and impurities, and that satisfies Formula (I), is subjected to a gas carburizing
treatment at a carburizing temperature Tp("C) that satisfies Formula (A) for 10 to less
than 20 hours. The main gas carburizing process is performed following the
preliminary gas carburizing process. In the main gas carburizing process, a gas
carburizing treatment is performed at a carburizing temperature TI("C) that satisfies
Formula (B) for a carburizing time tr (minutes).
6.5 < 3.5[Si%]+[Mn%]+3[Cr%] < 18 (1)
800 I Tp < 163xln(CP+0.6) - 4lxln(3.5x[Si%]+[Mn%]+3x[Cr%]) + 950
(A)
4 < 1 3340/(Tr+273. 1 5)-ln(tr) < 7 (B)
Where, [Si%], [Mn%], and [Cr%] in the formulae are substituted by the Si
content, Mn content, and Cr content (in mass%) in the steel component. The term
In() represents natural logarithm. CP is substituted by a carbon potential during
carburization in the preliminary carburizing process.
[0020]
The production method of the present embodiment can improve gas
carburizing property for steel components having a high Si content, and also can
suppress deterioration of productivity thereof.
BRIEF DESCRIPTION OF DRAWING
to02 1 ]
[FIG. 11FIG. 1 is a cross sectional photograph of an outer layer of a carburized steel
component of the present embodiment.
DESCRIPTION OF EMBODIMENTS
[0022]
The present inventors have investigated and studied a method which can
suppress deterioration of gas carburizing property even when the Si content in a steel
component is increased.
[0023]
As described above, although increase in the Si content in the steel component
will lead to improvement in temper softening resistance, oxide coating is formed on
the surface of the steel component during gas carburization, thereby deteriorating gas
carburizing property. It is considered that the formation of oxide coating is related
to alloying elements which tend to form oxides, a carburizing temperature that
affects the diffusion coefficients of alloying elements and oxygen, and carbon
potential that affects oxygen partial pressure.
[0024]
As a result of subjecting a steel component consisting of in mass%, C: 0.1 to
0.4%, Si: 0.7 to 4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.15%, S: not
more than 0.3%, N: 0.003 to 0.03%, 0: not more than 0.0050%, P: not more than
0.025%, Nb: 0 to 0.3%, Ti: 0 to 0.3%, V: 0 to 0.3%, Ni: 0 to 3.0%, Cu: 0 to 3.0%,
Co: 0 to 3.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0
to 0.01%, Zr: 0 to 0.05%, Te: 0 to 0.1%, and rare earth metals: 0 to 0.005%, with the
balance being Fe and impurities, to an ordinary carburizing treatment, oxide coating
was formed on the surface of the steel component. As a result of performing
elemental analysis of the oxide coating by using characteristic X-rays, it is revealed
that major elements contained in the oxide coating are Si, Mn, Cr, and 0 (oxygen).
[0025]
Si, Mn, and Cr have strong affinity with oxygen, and are susceptible to
oxidation. Specifically, out of the above described chemical composition, elements
(for example, Ni, Cu, etc.) which have weaker affinity with oxygen than those of Si,
Mn, and Cr will not be oxidized, and therefore they have no effect on the formation
of oxide coating. On the other hand, since the content of elements (for example, Ti,
V, etc.) which have higher affinity with oxygen than that of Si, Mn, and Cr are
minute in quantity compared with the contents of Si, Mn, and Cr, they have
substantially no effect on the formation of oxide coating. Thus, elements that affect
the formation of oxide coating in the steel component having the above described
chemical composition are Si, Mn, and Cr. Hereafter, Si, Mn, and Cr are referred to
as "specific elements".
[0026]
Any of the specific elements improves the strength and hardenability of steel,
and also improves the temper softening resistance thereof. Therefore, when the
content of these specific elements is excessively low, the surface fatigue strength of
the carburized steel component decreases.
[0027]
F1 is defined as follows.
F1=3.5x[Si%]+[Mn%]+3x[Cr%]
Where, [Si%], [Mn%], and [Cr%] are substituted by the Si content, Mn
content, and Cr content in the steel component.
[0028]
When F1 is more than 6.5, it is possible to achieve strength and temper
softening resistance required of a carburized steel component such as a gear and a
bearing, and also to achieve excellent surface fatigue strength. Therefore, F1 needs
to be more than 6.5 in the carburized steel component in the present embodiment.
[0029]
On the other hand, as described above, each specific element forms oxide
coating, thereby deteriorating gas carburizing property. Accordingly, the present
inventors have further investigated the relationship between the content of specific
elements and the gas carburizing property in an ordinary gas carburizing treatment
by the following test method.
[0030)
Various steel materials containing: C: 0.1 to 0.4%, Al: 0.005 to 0.15%, S: not
more than 0.3%, N: 0.003 to 0.03%, 0: not more than 0.0050%, P: not more than
0.025%, and further Si: 0.1 to 4.0%, Mn: 0.1 to 3.0%, and Cr: 0.1 to 5.0% were
prepared. Each steel material was subjected to hot forging and a heat treatment.
Thereafter, the steel material was subjected to machining to fabricate a steel
component having a prismatic shape of 20 mm x 20 mm.
[003 11
Each steel component was subjected to an ordinary gas carburizing treatment
under the same gas carburizing condition (950°C - carbon potential of 0.8) to
fabricate a carburized steel component. The C content of the outer layer of the
carburized steel component was measured by EPMA. The condition of the content
of specific elements at which the C content of the outer layer to be observed becomes
not less than 0.5% was determined by multiple regression analysis.
[0032]
As a result of the test, it was revealed that in an ordinary gas carburizing
treatment, a carburized steel component in which the C content of the outer layer was
not less than 0.5% could not be obtained unless F1 was not more than 6.5. When
F1 was more than 6.5, oxide coating was formed on the surface of the steel
component, and therefore the carburizing property was low, and a carburized layer
was poorly formed.
[0033]
However, to achieve sufficient surface fatigue strength in a carburized steel
component, F1 must be more than 6.5. Accordingly, the present inventors have
studied a gas carburizing treatment method by which formation of oxide coating is
suppressed and sufficient gas carburizing property can be achieved even when F1 is
more than 6.5. As a result, the present inventors have obtained the following
findings.
[0034]
Decrease in carburizing temperature suppresses formation of oxide coating.
When the carburizing temperature is low, oxides become more likely to be formed
not on the surface of a steel component, but within the outer layer of the steel
component. That is, in this case, oxide coating is hard to be formed and, instead,
oxides are formed within the outer layer. Hereafter, oxides which are formed at a
grain boundary and in a grain within the outer layer of a steel component are referred
to as "internal oxides".
[0035]
FIG. 1 is a cross sectional photograph of an outer layer of a carburized steel
component according to the present embodiment. In FIG. 1, a large number of
oxides (black spots in FIG. 1) are formed within the outer layer of the steel
component. If such internal oxides are formed during gas carburizing treatment,
increase in the concentration of specific elements by diffusion is suppressed in the
outer layer of the steel component. For that reason, when a certain amount of
internal oxides is formed, oxide coating becomes less likely to be formed in the gas
carburizing treatment thereafter, and thus improving gas carburizing property.
[0036]
Accordingly, the following two-stage gas carburizing process is performed as
a method for suppressing formation of oxide coating even when F1 is more than 6.5.
The gas carburizing process of the present embodiment includes a preliminary gas
carburizing process and a main gas carburizing process which is to be performed
following the preliminary gas carburizing process.
[0037]
The preliminary gas carburizing process principally aims at formation of
internal oxides. In the preliminary gas carburizing process, carburizing temperature
is adjusted depending on the content of specific elements and carbon potential to
facilitate the generation of internal oxides.
[0038]
Specifically, in the preliminary gas carburizing process, gas carburizing
treatment is performed at a carburizing temperature Tp("C) that satisfies Formula (A)
by using a steel component having a chemical composition that satisfies the
following Formula (1).
6.5 < 3.5 [Si%]+[Mn%]+3[Cr%] I 18 (1)
800 ITp < 163xln(CP+0.6) - 41 xln(3.5x[Si%]+[Mn%]+3x[Cr%]) + 950
(A)
Where, [Si%l, [Mn%], and [Cr%] in the formulae are substituted by the Si
content, Mn content, and Cr content (in mass%) in the steel component. The term
ln () represents natural logarithm, and CP is substituted by a carbon potential during
carburization in the preliminary gas carburizing process.
[0039]
As shown in Formula (I), even if Fl is more than 6.5, when it is not more
than 18, it is possible to suppress formation of oxide coating on condition that a
preliminary gas carburizing treatment is performed at a carburizing temperature T
that satisfies Formula (A) for 10 minutes to less than 20 hours
[0040]
After the preliminary gas carburizing process, the main gas carburizing
process is successively performed. In the main gas carburizing process, a
carburized layer is formed on the surface of the base metal of the steel component.
[0041]
In the main gas carburizing process, to increase the surface fatigue strength of
the carburized steel component, gas carburizing treatment is performed at a
carburizing temperature Tr("C) that satisfies the following Formula (B) for a
carburizing time tr (minutes).
4 < 1 3340/(Tr+273. 1 5)-ln(tr) < 7 (B)
[0042]
When the carburizing temperature Tr("C) and the carburizing time tr (minutes)
satisfy Formula (B), the effective hardened layer of the carburized steel component
will have an appropriate depth, and the surface fatigue strength of the carburized
steel component will increase.
[0043]
Preferably, the carburizing temperature Tr("C) of the main gas carburizing
process is set to be higher than the carburizing temperature Tp("C) of the preliminary
gas carburizing process. In the present embodiment, internal oxides are generated
by the preliminary gas carburizing process that satisfies Formula (A). For that
reason, the concentration of specific elements is suppressed to be low in the outer
layer of the steel component during the main gas carburizing process. Therefore,
even when the carburizing temperature Tr("C) is set to be higher than the carburizing
temperature TP("C) in the main gas carburizing process, oxide coating is hardly
formed and thus gas carburizing property can be maintained provided that the main
gas carburizing process satisfies Formula (B). As a result, even for a steel
component having a high Si content, it is possible to form a carburized layer of a
sufficient thickness in a short period of time, and thus produce a carburized steel
component having excellent surface fatigue strength while suppressing deterioration
of productivity thereof.
[0044]
A production method of a carburized steel component according to the present
embodiment, which has been completed based on the above described findings,
includes a preliminary gas carburizing process and a main gas carburizing process.
In the preliminary gas carburizing process, a steel component having a chemical
composition that consists of: by mass%, C: 0.1 to 0.4%, Si: 0.7 to 4.0%, Mn: 0.2 to
3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.15%, S: not more than 0.3%, N: 0.003 to 0.03%,
0: not more than 0.0050%, P: not more than 0.025%, Nb: 0 to 0.3%, Ti: 0 to 0.3%,
V: 0 to 0.3%, Ni: 0 to 3.0%, Cu: 0 to 3.0%, Co: 0 to 3.0%, Mo: 0 to 1.0%, W: 0 to
l.O%,B: 0 to 0.005%, Ca: 0 toO.Ol%,Mg: 0 to 0.01%, Zr: 0 to 0.05%, Te: 0 to0.1%,
and rare earth metals: 0 to 0.005%, with the balance being Fe and impurities, and that
satisfies Formula (I), is subjected to a gas carburizing treatment at a carburizing
temperature TP("C) that satisfies Formula (A) for 10 to less than 20 hours. The
main gas carburizing process is performed following the preliminary gas carburizing
process. In the main gas carburizing process, a gas carburizing treatment is
performed at a carburizing temperature Tr("C) for a carburizing time tr (minutes),
which satisfy Formula (B).
6.5 < 3.5[Si%]+[Mn%]+3[Cr%] I 18 (1)
800 < Tp < 163xln(CP+0.6) - 41xln(3.5x[SiO/0]+[Mn%]+3x[Cr%]) + 950
(A)
4 < 1 3340/(Tr+273. 1 5)-ln(tr) < 7 (B)
Where, [Si%], [Mn%], and [Cr%] in the formulae are substituted by the Si
content, Mn content, and Cr content (in mass%) in the steel component. The term
In () represents natural logarithm. CP is substituted by a carbon potential during
carburization in the preliminary gas carburizing process.
[0045]
A carburized steel component according to the present embodiment includes:
a base metal having a chemical composition that consists of, in mass%, C: 0.1 to
0.4%, Si: 0.7 to 4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.15%, S: not
more than 0.3%, N: 0.003 to 0.03%, 0: not more than 0.0050%, P: not more than
0.025%, Nb: 0 to 0.3%, Ti: 0 to 0.3%, V: 0 to 0.3%, Ni: 0 to 3.0%, Cu: 0 to 3.0%,
Co: 0 to 3.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0
to 0.01%, Zr: 0 to 0.05%, Te: 0 to 0.1%, and rare earth metals: 0 to 0.005%, with the
balance being Fe and impurities, and that satisfies Formula (1); and a carburized
layer formed on the base metal. The C content of the outer layer of the carburized
layer is not less than 0.5%, and the Si content, Mn content, and Cr content of the
outer layer of the carburized layer satisfy Formula (2). The depth of effective
hardened layer is 0.3 to less than 1.5 mm, and an area fraction of oxide in a depth
range of 10 2 3 pm from the surface of the carburized layer is 7 to 50%.
6.5 < 3.5[Si%]+[Mn%]+3[Cr%] 5 18 (1)
3.5 [Sis%]+[Mns%]+3 [Crs%] I 9 (2)
Where, [Si%], [Mn%], and [Cr%] in Formula (1) are substituted by the Si
content, Mn content, and Cr content (in mass%) in the base metal, respectively, and
[Sis%], [Mns%], and [Crs%] in Formula (2) are substituted by the Si content, Mn
content, and Cr content (in mass%) of the outer layer of the carburized layer,
respectively.
[0046]
The above described chemical composition may contain one or more kinds
selected from the group consisting of Nb: 0.02 to 0.3%, Ti: 0.02 to 0.3%, and V: 0.02
to 0.3%.
[0047]
The above described chemical composition may contain one or more kinds
selected from the group consisting of Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%, Co: 0.2 to
3.0%, Mo: 0.05 to 1.0%, W: 0.05 to 1.0%, and B: 0.0006 to 0.005%.
100481
The above described chemical composition may contain one or more kinds
selected from the group consisting of Ca: 0.0005 to 0.01 %, Mg: 0.0005 to 0.01 %, Zr:
0.0005 to 0.05%, Te: 0.0005 to 0.1%, and rare earth metals: 0.0001 to 0.005%.
100491
Hereafter, a production method of a carburized steel component according to
the present embodiment will be described. The present production method includes
a preliminary gas carburizing process and a main gas carburizing process. In the
preliminary gas carburizing process, oxides (internal oxides) are formed within the
outer layer of a steel component having a high Si content, thereby suppressing
formation of oxide coating on the surface. In the main gas carburizing process, a
steel component in which formation of oxide coating is suppressed is subjected to a
gas carburizing treatment at a carburizing temperature higher than that in the
preliminary gas carburizing process, thereby improving productivity. Hereafter, the
preliminary gas carburizing process and the main gas carburizing process will be
described in detail.
[OOSO]
[Preliminary gas carburizing process]
In the preliminary gas carburizing process, a steel component having the
following chemical composition is prepared. The prepared steel component is
subjected to a preliminary gas carburization to generate internal oxides in steel and
suppress the concentration of specific elements in the outer layer.
[OOS 11
[Chemical composition of steel component]
The chemical composition of the steel component contains the following
elements. Hereafter, "%" regarding the elements represents mass%.
[0052]
C: 0.1 to 0.4%
Carbon (C) increases the strength of steel. More specifically, C increases the
strength of a core part of a steel component. When C content is excessively low,
the above described effect cannot be effectively achieved. C content further affects
the depth of effective hardened layer. On the other hand, when C content is
excessively high, the toughness of steel will decrease. Therefore, C content may be
0.1 to 0.4%. The lower limit of C content is preferably 0.16%, and more preferably
0.18%. The upper limit of C content is preferably 0.30%, and more preferably
0.28%.
[0053]
Si: 0.7 to 4.0%
Silicon (Si) deoxidizes steel. Si hrther increases the strength and
hardenability of steel, and also improves temper softening resistance. Therefore, Si
increases the strength of a core part of a steel component, thereby increasing surface
fatigue strength. Si fiu-ther forms internal oxides by satisfying the below described
production conditions. Internal oxides increase the surface fatigue strength of steel.
When Si content is excessively low, the above described effects cannot be effectively
achieved. On the other hand, when Si content is excessively high, steel becomes
susceptible to decarbonization during hot working such as hot forging. Therefore,
Si content may be 0.7 to 4.0%. The lower limit of Si content is preferably 0.8%,
and more preferably 1.0%. The upper limit of Si content is preferably 3.0%, and
more preferably 2.5%.
[0054]
Mn: 0.2 to 3.0%
Manganese (Mn) deoxidizes steel. Mn further increases the strength and
hardenability of steel, and also improves temper softening resistance. Thus, Mn
increases the strength of a core part of steel, as well as the surface fatigue strength
thereof. Mn further combines with S in steel to form MnS, thereby making S
harmless. Mn further forms internal oxides by satisfying the below described
production conditions. Internal oxides increase the surface fatigue strength of steel.
When Mn content is excessively low, the above described effects cannot be
effectively achieved. On the other hand, when Mn content is excessively high,
retained austenite remains in steel, thereby reducing strength, even when a sub-zero
treatment is performed. Therefore, Mn content may be 0.2 to 3.0%. The lower
limit of Mn content is preferably 0.4%, and more preferably 0.5%. The upper limit
of Mn content is preferably 2.0%, and more preferably 1.5%.
[0055]
Cr: 0.5 to 5.0%
Chromium (Cr) increases the strength and hardenability of steel, and also
improves temper softening resistance. Thus, Cr increases the strength of a core part
of a steel component, and also increases surface fatigue strength. Cr further forms
internal oxides by satisfying the below described production conditions. Internal
oxides increase the surface fatigue strength of steel. When Cr content is
excessively low, above described effects cannot be effectively achieved. On the
other hand, when Cr content is excessively high, the hardness of steel increases,
thereby deteriorating cold workability. Therefore, Cr content may be 0.5 to 5.0%.
The lower limit of Cr content is preferably 0.6%, and more preferably 0.8%. The
upper limit of Cr content is preferably 3.0%, and more preferably 2.5%.
[0056]
Al: 0.005 to 0.15%
Aluminum (Al) deoxidizes steel. A1 further combines with nitrogen to form
nitrides, thereby refining crystal grains. When A1 content is excessively low, the
above described effects cannot be effectively achieved. On the other hand, when A1
content is excessively high, nitrides become coarse, thereby embrittling steel.
Therefore, A1 content may be 0.005 to 0.15%. The lower limit of A1 content is
preferably 0.01%, and more preferably 0.02%. The upper limit of A1 content is
preferably 0.10%, and more preferably 0.05%. Note that the above described A1
content means a total A1 content.
[0057]
S: not more than 0.3%
Sulfur (S) is inevitably contained. Since S has an effect of increasing the
machinability of steel, S may be positively contained. When S content is
excessively high, the forgeability of steel deteriorates. Therefore, S content may be
not more than 0.3%. To achieve the effect of improving the machinability of steel,
the lower limit of S content is preferably 0.005%, and more preferably 0.01%. The
upper limit of S content is preferably 0.15'36, and more preferably 0.1 %.
[0058]
N: 0.003 to 0.03%
Nitrogen (N) combines with Al to form nitride, and refines crystal grains.
When N content is excessively low, this effect cannot be effectively achieved. On
the other hand, when N content is excessively high, forgeability of steel deteriorates.
Therefore, N content may be 0.003 to 0.03%. The lower limit of N content is
preferably 0.004%, and more preferably 0.005%. The upper limit of N content is
preferably 0.025%, and more preferably 0.02%.
[0059]
0 : not more than 0.0050%
Oxygen (0) is an impurity. Oxygen is present in steel as oxide-based
inclusions such as alumina and titania. When 0 content is excessively high, oxidebased
inclusions become coarse. A coarse oxide-based inclusion serves as a
starting point of a crack. For that reason, when the steel component is a power
transmitting part, crack may develop leading to breakage. Therefore, 0 content
may be not more than 0.0050%. 0 content is preferably as low as possible. 0
content is preferably not more than 0.0020%, and more preferably not more than
0.0015% when prolonging of service life is attempted.
[0060]
P: not more than 0.025%
Phosphorous (P) is an impurity. P segregates at grain boundaries, thereby
deteriorating the toughness of steel. Therefore, P content may be not more than
0.025%. P content is preferably as low as possible. P content is preferably not
more than 0.020%, and more preferably not more than 0.01 5% when prolonging of
service life of steel component is attempted.
[0061]
The balance of the chemical composition of the steel component according to
the present embodiment consists of Fe and impurities. Here, impurities refer to
elements which are mixed in from ores and scrap as the raw materials, or production
environments when steel is industrially produced, and which are tolerated within a
range not adversely affecting the steel component of the present embodiment.
100621
The chemical composition of the steel component according to the present
embodiment may further contain, in place of part of Fe, one or more kinds selected
from the group consisting of Nb, Ti, and V.
100631
Nb: 0 to 0.3%
Ti: 0 to 0.3%
V: 0 to 0.3%
Any of niobium (Nb), Titanium (Ti), and vanadium (V) is an optional element
and may not be contained. If contained, these elements combine with C and/or N to
form carbides, nitrides, and carbonitrides, thereby refining crystal grains. However,
when the contents of these elements are excessively high, the above described effect
will be saturated. Further, the hot workability and machinability of steel will
deteriorate. Therefore, Nb content may be 0 to 0.3%, Ti content 0 to 0.3%, and V
content 0 to 0.3%.
100641
To achieve the above described effect more effectively, the lower limit of Nb
content is preferably 0.02%, the lower limit of Ti content preferably 0.02%, and the
lower limit of V content preferably 0.02%. The upper limit of Nb content is
preferably 0.1 %, the upper limit of Ti content preferably 0.1 %, and the upper limit of
V content preferably 0.1 %.
[0065]
The chemical composition of the steel component according to the present
embodiment may further contain, in place of part of Fe, one or more kinds selected
from the group consisting of Ni, Cu, Co, Mo, W, and B.
[0066]
Ni: 0 to 3.0%
Cu: 0 to 3.0%
Co: 0 to 3.0%
Mo: 0 to 1 .O%
W: 0 to 1 .O%
B: 0 to 0.005%
Any of nickel (Ni), copper (Cu), cobalt (Co), molybdenum (Mo), tungsten
(W), and boron (B) is an optional element, and may not be contained. If contained,
any of these elements improves the hardenability of steel. However, when the
contents of these elements are excessively high, the above described effect will be
saturated, and production cost will increase. Therefore, Ni content is 0 to 3.0%, Cu
content is 0 to 3.09'6, Co content is 0 to 3.0%, Mo content is 0 to 1.0%, W content is
0 to 1.0%, and B content is 0 to 0.005%.
[0067]
To achieve the above described effect more effectively, the lower limit of Ni
content is preferably 0.2%, the lower limit of Cu content preferably 0.2%, and the
lower limit of Co content preferably 0.2%, the lower limit of Mo content preferably
0.05%, the lower limit of W content preferably 0.05%, and the lower limit of B
content preferably 0.0006%. The upper limit of Ni content is preferably 2.0%, the
upper limit of Cu content preferably 2.0%, and the upper limit of Co content
preferably 2.0%, the upper limit of Mo content preferably 0.3%, the upper limit of W
content preferably 0.3%, and the upper limit of B content preferably 0.001%.
[0068]
The chemical composition of the steel component according to the present
embodiment may further contain, in place of part of Fe, one or more kinds selected
from the group consisting of Ca, Mg, Zr, Te, and rare earth metals (REM).
[0069]
Ca: 0 to 0.01%
Mg: 0 to 0.01%
Zr: 0 to 0.05%
Te: 0 to 0.1 %
Rare earth metals (REM): 0 to 0.005%
Any of calcium (Ca), magnesium (Mg), zirconium (Zr), tellurium (Te), and
rare earth metals (REM) is an optional element, and may not be contained. If
contained, these elements improve the machinability of steel.
[0070]
Specifically, Ca decreases the melting point of oxides. In this case, oxides
are softened by heat generated in steel material during cutting work thereof, thereby
improving the machinability of steel. However, when Ca content is excessively
high, a large amount of hard CaS is generated, and the machinability of steel will be
rather deteriorated. Therefore, Ca content is 0 to 0.01%. To achieve the above
described effect more effectively, the lower limit of C content is preferably 0.0005%.
[007 1 ]
Mg, Zr, Te, and REM control the morphology of MnS, thereby improving the
machinability of steel. However, when Mg content is excessively high, MgS is
generated, thereby deteriorating the machinability of steel. Therefore, Mg content
is 0 to 0.01%. When Zr content is excessively high, the above described effect will
be saturated. Therefore, Zr content is 0 to 0.05%. When Te content is excessively
high, the above described effect will be saturated. Therefore, Te content is 0 to
0.1 %. When REM content is excessively high, coarse sulfides are generated,
thereby deteriorating the machinability of steel. Therefore, REM content is 0 to
0.005%.
[0072]
To achieve the above described effect more effectively, the lower limit of Mg
content is preferably 0.0005%, the lower limit of Zr content preferably 0.0005%, the
lower limit of Te content preferably 0.0005%, and the lower limit of REM content
preferably 0.0001 %.
100731
REM as used herein means a general term for 17 elements including yttrium
(Y) and scandium (Sc) in addition to the elements from lanthanum (La) of atomic
number 57 to lutetium (Lu) of atomic number 71 in the periodic table. The content
of REM means a total content of one or more kinds of these elements.
[0074]
[Formula (I)]
The chemical composition of the steel component of the present embodiment
further satisfies Formula (1).
6.5 < 3.5[Si%]+[Mn%]+3[Cr%] I 18 (1)
Where, [Si%], [Mn%], and [Cr%] in Formula (1) are substituted by the Si
content, Mn content, and Cr content (in mass%) in the steel component.
[0075]
As described above, Formula (1) is an indicator relating to the content of
specific elements (Si, Mn, and Cr). While the specific elements increase the surface
fatigue strength of steel, they are likely to form oxide coating in a gas carburizing
treatment.
LO0761
When F1 (=3.5[Si%]+[Mn%]+3[Cr%]) is excessively low, the specific
elements in the steel component become insufficient. For that reason, the temper
softening resistance of the carburized steel component deteriorates, thereby
decreasing surface fatigue strength. On the other hand, when F1 is excessively high,
even if gas carburizing treatment is performed under the below described production
conditions, oxide coating will be formed on the surface of the steel component, thus
deteriorating gas carburizing property. When F1 is more than 6.5 to 18, the surface
fatigue strength is sufficiently increased, and even if the below described gas
carburizing treatment is performed, oxide coating wil hardly be formed. Therefore,
the gas carburizing property can also be maintained.
[0077]
The above described steel component is produced, for example, by the
following method. Molten steel having the above described chemical composition
is produced. The molten steel is subjected to continuous casting to obtain a cast
piece. The molten steel may be subjected to an ingot-making process to obtain an
ingot (steel ingot). The cast piece of the ingot may be subjected to hot working to
obtain a billet (steel billet) or steel bar.
[0078]
The cast piece, ingot, billet, or steel bar is heated in a reheating furnace. The
heated cast piece, ingot, billet, or steel bar is subjected to hot working to produce a
steel component. The hot working is, for example, hot rolling or hot forging. Hot
working may be performed multiple times to produce a steel component. Hot
rolling and hot forging may be performed to produce a steel component.
[0079]
The intermediate product after hot forging may be subjected to cold working
as represented by cold forging to produce a steel component. The hot-worked
and/or cold worked intermediate product may be subjected to cutting work to
produce a steel component. When performing cold working to produce a steel
component, the intermediate product before cold working is preferably subjected to
spheroidizing annealing at 700 to 800°C. In this case, formability is improved.
[0080]
[Preliminary gas carburizing treatment]
The produced steel component is subjected to a preliminary gas carburizing
treatment. The preliminary gas carburizing treatment is performed by using a gas
carburizing furnace. After the steel component is charged into the gas carburizing
furnace, gas carburizing treatment is performed at the following conditions.
[0081]
[Preliminary gas carburizing temperature Tp]
The carburizing temperature Tp satisfies the following Formula (A).
800 5 Tp < 163xln(CP+0.6) - 41 xln(3.5x[Si%]+[Mn%]+3x[Cr%]) + 950
(A)
[0082]
It is defmed such that FA = 163xln(CP+0.6) -
41 xln(3.5x[Si%]+[Mn%]+3x[Cr%]) + 950. When the carburizing temperature Tp
is excessively higher than FA, oxygen partial pressure in the gas carburizing furnace
excessively increases. Further, diffusion coefficients of specific elements and
oxygen also increase. For that reason, even in a case of a steel component having a
chemical composition that satisfies Formula (I), oxide coating is formed on the
surface during a preliminary gas carburizing treatment. In this case, since the gas
carburizing property deteriorates, a sufficient carburized layer cannot be achieved
even after the next process, that is, a major gas carburizing process is performed.
Which will result in decrease in the surface fatigue strength of a carburized steel
component.
100831
On the other hand, when the carburizing temperature Tp is less than 800°C,
carburization efficiency in the preliminary gas carburizing treatment deteriorates.
In this case, the productivity decreases. Therefore, the lower limit of the
carburizing temperature T is 800°C.
100841
When the carburizing temperature Tp satisfies Formula (A), internal oxides
containing Si, Mn, and Cr are formed at grain boundaries and within grains within
the outer layer of the steel component in the preliminary gas carburizing treatment.
As a result, the concentration of specific elements within the outer layer will be
suppressed. For that reason, it is possible to suppress the formation of oxide
coating in the next process, that is, the main gas carburizing process.
[0085]
[Carbon potential CP]
A carbon potential CP in the preliminary gas carburizing treatment will not be
particularly limited provided that the carburizing temperature Tp satisfies Formula
(A). The lower limit of carbon potential is preferably 0.6, and the upper limit
thereof is preferably 1.2.
[0086}
[Preliminary gas carburizing time]
The carburizing time (preliminary gas carburizing time) at the above
described carburizing temperature T is 10 minutes to less than 20 hours. When the
carburizing temperature is less than 10 minutes, internal oxides will not be
sufficiently generated, and the concentration of specific elements within the outer
layer remains to be high. In this case, oxide coating becomes more likely to be
formed in the main gas carburizing treatment. On the other hand, when the
carburizing time is not less than 20 hours, the productivity decreases. Therefore,
the carburizing time is 10 minutes to less than 20 hours.
[0087]
[Main gas carburizing process]
After the above described preliminary gas carburizing process is performed,
successively, a main gas carburizing process is performed. The main gas
carburizing process is performed in the same gas carburizing hmace as in the
preliminary gas carburizing process. Specifically, the temperature of the gas
carburizing fimace is increased after the preliminary gas carburizing process. To
achieve high surface fatigue strength, it is necessary to appropriately manage the
depth of effective hardened layer which is obtained from the carburizing process.
For that end, the carburizing temperature Tr (OC) and the carburizing time tr
(minutes) satisfy the following Formula (B).
4 < 1 3340/(Tr+273. 1 5)-ln(tr) < 7 (B)
[0088]
It is defined such that FB = 13340/(Td273.1 5)-ln(tr). When FB is
excessively more than 7, the depth of effective hardened layer becomes excessively
small, and the surface fatigue strength of the carburized steel component decreases.
On the other hand, when FB is excessively less than 4, the depth of effective
hardened layer becomes excessively large, and the surface fatigue strength of the
carburized steel component decreases.
[0089]
Preferably, the carburizing temperature Tr of the main gas carburizing process
is set to be higher than the carburizing temperature Tp of the preliminary gas
carburizing process. In this case, the time for gas carburizing treatment can be
reduced, thereby improving the productivity. In the present embodiment, since the
preliminary gas carburizing process is performed at a condition that satisfies Formula
(A) to generate internal oxides, the concentration of specific elements within the
outer layer of the steel component is suppressed. Owing to performing such a
preliminary gas carburizing process, it is possible to achieve a sufficient depth of the
effective hardened layer, thereby achieving high surface fatigue strength, even when
the gas carburizing treatment is performed in a shorter period of time by raising the
carburizing temperature Tr during the main gas carburizing process that satisfies
Formula (B).
[0090]
The carbon potential in the main gas carburizing process will not be
particularly limited. The carburizing treatment may be performed in a well-known
range of carbon potential.
[0091]
The lower limit of the carburizing temperature Tr in the main gas carburizing
process is preferably 820°C, and more preferably 850°C. The upper limit of the
carburizing temperature Tr is preferably 1050°C. Further, the lower limit of the
carburizing time tr in the main gas carburizing process is preferably 20 minutes.
[0092]
[Processes after main gas carburizing process]
After the above described preliminary gas carburizing process and the main
gas carburizing process are performed, quenching and tempering are performed.
[0093]
After the main gas carburizing process is performed, quenching treatment is
performed by a well-known method. The quenching treatment is, for example,
water quenching or oil quenching. After the quenching treatment is performed,
tempering treatment is performed. Performing tempering treatment will increase
the toughness of a product member. The tempering treatment is performed at a
well-known condition.
lo0941
By the above described production processes, a carburized steel component is
produced. The produced carburized steel component has a sufficient depth of
effective hardened layer even when its Si content is high. Therefore, the present
carburized steel component has excellent surface fatigue strength. Hereafter, the
carburized steel component will be described.
[0095]
[Carburized steel component]
The carburized steel component produced by the above described production
method includes a base metal and a carburized layer.
[0096]
[Base metal]
The base metal has the chemical composition of the above described steel
component. That is, the chemical composition of the base metal contains the same
elements as those of the above described steel component, and satisfies Formula (1).
[0097]
[Carburized layer]
The carburized layer is formed on the surface of the base metal. The C
content of the outer layer of the carburized layer is not less than 0.5%. The C
content of the outer layer of the carburized layer is measured by the following
method. A sample having a cross section perpendicular to the surface of the
carburized steel component is taken. In a region from the surface to a depth of 30
pm of a cross section (hereafter referred to as "observation face") including the
surface of the carburized steel component, C concentration is measured at a pitch of
5 pm in the depth direction by using an EPMA (electron probe micro analyzer). An
average of the obtained C concentrations is defined as the C content of the outer
layer of the carburized steel component.
[0098]
When the C content of the outer layer is less than 0.5%, the hardness of the
outer layer decreases, and it is not possible to achieve excellent surface fatigue
strength. The lower limit of the C content of the outer layer is preferably 0.6%, and
the upper limit thereof is preferably 1.0%.
[0099]
Further, the depth of effective hardened layer of the carburized steel
component is 0.3 to less than 1.5 mm. The effective hardened layer is defined by a
depth (mm) from the surface at which a Vickers hardness of 550 Hv is obtained.
The depth of effective hardened layer is measured by the following method. In a
cross section of the carburized steel component, in a region from the surface to the
center, a hardness distribution is created by using a Vickers hardness meter based on
JIS 22244 (2009). In this occasion, the test force F is 1.96 N. In the obtained
hardness distribution, a depth at which the Vickers hardness is 550 Hv is determined,
and it is defined as an effective hardened depth (mm).
[O 1 001
When the depth of effective hardened layer is less than 0.3 mm, it is not
possible to achieve excellent surface fatigue strength. On the other hand, when the
depth of effective hardened layer is not less than 1.5 mm, compressive residual stress
decreases, and therefore the surface fatigue strength decreases. Therefore, the depth
of effective hardened layer is 0.3 to less than 1.5 mm.
[OlOl]
Further, Si content, Mn content, and Cr content of the outer layer of the
carburized layer satisfy Formula (2).
3.5 [Sis%]+[Mns%}+3[Crs%] 1 9 (2)
Where, [Sis%], [Mns%], and [Crs%] in Formula (2) are substituted by the Si
content, Mn content, and Cr content (in mass%) of the outer layer of the carburized
layer, respectively.
[O 1 021
The Si content, Mn content, and Cr content in the outer layer of the carburized
layer are defined in the same manner as the C content of the above described outer
layer. That is, in a region from the surface of the observation face of the sample to
a depth of 30 pm, Si concentration, Mn concentration and Cr concentration are
measured at a pitch of 5 pm in the depth direction by using an EPMA. An average
of the obtained concentrations of each element is defined as the Si content, Mn
content, and Cr content of the outer layer of the carburized layer, respectively.
[0 1031
It is defined such that F2 = 3.5[Sis%]+[Mns%]+3[Crs%]. Performing the
preliminary gas carburizing process at the above described conditions will result in
formation of internal oxides. In this case, specific elements which are dissolved in
the steel component are consumed. For that reason, it is considered that the content
of specific elements in the outer layer of the steel component at the start of the main
gas carburizing process decreases to a level at which F2 satisfies Formula (2).
Since the content of specific elements in the outer layer is suppressed, the gas
carburizing property in the main gas carburizing process is maintained so that a
carburized layer of a sufficient depth can be achieved. Performing the above
described production method will result in that F2 satisfies Formula (2) in the outer
layer of the carburized steel component (the outer layer of the gas carburized layer).
[0 1041
[Area fraction of internal oxide]
In the carburized steel component, the area fraction of oxide (internal oxide)
in a depth range of 10 + 3 pm from the surface of the carburized layer is 7 to 50%.
Hereafter, the area fraction of oxide in a depth range of 10 + 3 pm Erom the surface
of the carburized layer is referred to an "internal oxide fraction".
[0 1051
The internal oxide fraction is measured by the following method. An
element mapping of oxygen is obtained at an interval of 0.3 pm x 0.3 pm in the
observation face (400 pm x 400 pm) of the above described sample by using an
EPMA. From which, an 0 concentration profile at a depth of 200 pm from the
surface is extracted and binarized with a numerical value, which represents a
maximum oxygen concentration in metal iron excluding the second phase thereof
such as inclusions, as a threshold. Thereafter, a depth range of 10 + 3 pm from the
surface of the carburized layer is trimmed, and out of the trimmed range, the area
fraction of the region in which oxygen concentration is higher than the threshold is
determined. The determined area fraction is defined as an internal oxide fraction
(%).
[0 1 061
Performing the preliminary gas carburizing process and the main gas
carburizing process at the above described conditions will result in an internal oxide
fraction of 7 to 50%. In the preliminary gas carburizing process, when the
carburizing temperature T is more than FA, the area fraction of oxide will become
less than 7%. On the other hand, when the gas carburizing treatment of the present
embodiment (the preliminary gas carburizing process and the main gas carburizing
process) are performed, the internal oxide fraction will never be more than 50%.
[0 1 071
Note that when a steel component whose Si content is not less than 0.7% is
subjected to a conventional gas carburizing treatment, internal oxides will not be
formed within crystal grains, but will be formed at grain boundaries in a small
amount. Therefore, when a conventional gas carburizing treatment is performed,
the internal oxide fraction will be less than 7%.
EXAMPLES
[O 1 081
[Measurement of depth of effective hardened layer of carburized steel component
and measurement of internal oxide fraction]
Steel materials of Steel Nos. 1 to 34 having chemical compositions shown in
Table 1 were prepared. Each steel material was subjected to hot forging and heat
treatment to produce intermediate products. Each intermediate product was
subjected to cutting work (machining) to produce a steel component having a
prismatic shape of 20 mm x 20 mm.
[0 1 091
[Table 11

[OllO]
As shown in Table 2, the steel component of each Test No. was subjected to a
preliminary gas carburization and a main gas carburization under conditions shown
in Table 2.
[Olll]
[Table 21
Table 2
Test
No.
1
5
lo
l2
l3
l4
l5
l6
Steel
No.
lo
l2
l3
l4
l5
l6
Category
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Preliminary gas carburizing
FA
921
901
892
890
928
902
941
912
908
912
908
884
897
903
923
909
Temperature
Tp (OC)
870
8 90
860
870
890
850
860
8 90
850
880
860
850
860
880
890
830
FB
6.1
6.2
6.1
6.3
6.4
6.3
6.6
6.1
5.9
6.0
6.5
6.5
6.1
5.9
6.1
6.1
Main gas
Temperature
T, ("C)
95 0
900
930
95 0
920
910
880
900
930
920
910
890
95 0
980
93 0
900
conditions
Time
(min)
30
42
55
28
45
10
62
16
35
38
42
51
59
34
25
91
carburizing
Time tr
@in)
120
180
150
100
120
150
150
200
180
180
120
150
120
120
150
200
CP
0.8
0.9
0.8
0.7
0.9
0.6
1.2
0.8
0.8
0.8
0.9
0.7
0.8
0.8
0.9
1.0
conditions
CP
0.9
0.7
1.0
1.0
1.0
0.9
1.0
0.9
1.0
1.1
1.0
0.9
1.0
1.0
1.1
1.1
[0112]
For each of Test Nos. 1 to 30, and 33 to 36, the preliminary gas carburizing
process was performed at conditions (carburizing temperature, carburizing time, and
carbon potential CP) shown in Table 2. Further, following the preliminary gas
l7
18
l9
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
l7
18
l9
20
21
22
23
24
25
26
27
28
29
30
30
31
32
33
34
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Cornme
Example Cornme
Example
Cornpahve
Example Cornme
Example
Comparatrve
Example
Cornpatwe
Example
Cornparatrve
Example
h + v e
Example
Cornpatwe
Example
Example
880
890
850
870
860
850
8 80
900
870
860
870
890
870
880
890
9 1 0
850
880
55
32
29
40
35
52
26
31
35
21
31
42
25
18
8
25
31
45
0.8
0.9
0.7
0.8
0.9
0.8
0.8
0.9
0.8
0.9
0.8
0.7
0.8
0.8
0.9
0.8
0.8
0.8
920
920
898
911
933
918
918
924
914
931
925
915
885
929
922
907
920
917
95 0
93 0
930
1050
1050
920
94 0
920
930
930
950
93 0
930
890
950
950
920
930
850
1030
120
150
150
60
400
150
120
300
120
120
100
150
120
100
120
120
100
120
20
550
0.9
1.1
0.9
1.0
1.0
0.9
0.9
0.9
0.9
0.9
0.8
0.8
0.9
0.8
0.8
0.8
0.8
0.8
0.8
1.0
6.1
6.1
6.1
6.0
4.1
6.2
6.2
5.5
6.3
6.3
6.3
6.1
6.3
6.9
6.1
6.1
6.6
6.3
8.9
3.9
carburizing process, the main gas carburizing process was performed at conditions
(carburizing temperature, carburizing time, and CP) shown in Table 2. The steel
component after the main gas carburizing process was subjected to quenching in oil
at 130°C, and tempering at 150°C to produce a carburized steel component.
[0113]
For Test Nos. 3 1 and 32, the main gas carburizing process was performed at
conditions of Table 2 without performing the preliminary gas carburizing process.
After performing the main gas carburizing process, each steel component was
subjected to quenching in oil at 130°C and tempering at 150°C. By the above
described processes, carburized steel components (specimens) of Test Nos. 1 to 36
were produced.
[0114]
[Evaluation test]
[Measurement of C content and content of specific elements in outer layer of
carburized layer]
By the above described method, the C content, Si content, Mn content, and Cr
content in the outer layer of the carburized layer of the carburized steel component of
each Test No. were determined by using an EPMA. Based on the obtained Si
content, Mn content, and Cr content, F2 was determined by the above described
method. As the EPMA apparatus, one of a trade name JXA-8200 manufactured by
JEOL (Japan Electron Optics Laboratory) Ltd was used.
[0115]
[Measurement of depth of effective hardened layer and internal oxide fraction]
By the above described method, the depth (mm) of effective hardened layer of
each carburized steel component was determined. Further, by the above described
method, the area fraction of oxide (internal oxide fraction) in a depth range of 10 k 3
pm from the surface of the carburized layer of the carburized steel component was
determined.
[0116]
[Roller pitting fatigue test]
To evaluate surface fatigue strength of each produced carburized steel
component, a roller pitting fatigue test was conducted by using a large roller
specimen and a small roller specimen. Specifically, steel materials of Steel Nos. 1
to 34 of Table 1 were subjected to hot forging and heat treatment to produce
intermediate products. The intermediate products are subjected to machining to
fabricate small roller specimens and large roller specimens. The small roller
specimen had a diameter of 26 mm and a width of 28 mm. The large roller
specimen had a diameter of 130 mm and a width of 18 mm. The large roller
specimen further had a crowning of 150 mm in the outer circumference.
[0117]
In the Test Nos. 1 to 30, and 33 to 36, the fabricated small roller specimens
and large roller specimens were subjected to the preliminary gas carburizing process
and the main gas carburizing process at conditions shown in Table 2, and are hrther
subjected to oil quenching at 130°C and tempering at 150°C. In Test Nos. 3 1 and
32, the small roller specimens and the large roller specimens were not subjected to
the preliminary gas carburizing process, but subjected to the main gas carburizing
process at conditions shown in Table 2, and to oil quenching at 130°C and tempering
at 150°C.
[0118]
By using the small roller specimen and the large roller specimen after
tempering, the roller pitting test was performed as follows. The large roller
specimen was pressed against the small roller specimen. In this occasion, the
interfacial pressure was 3000 MPa in Hertzian stress. Each roller was rotated with
the circumferential velocity directions of both rollers being kept in the same direction
and a slip ratio therebetween being kept at -40% in a contact portion between the
small roller specimen and the large roller specimen. Specifically, the
circumferential velocity of the large roller specimen in the contact portion was made
larger by 40% than that of the small roller specimen. The number of rotational
cycles until pitting occurred in the small roller specimen was determined, and the
obtained number of rotational cycles was made an evaluation indicator of the surface
fatigue strength.
[0119]
During the roller pitting test, the temperature of gear oil to be supplied to the
contact portion was 80°C. The occurrence of pitting was detected by a vibration
meter installed. After detecting vibration, the rotation of both roller specimens was
stopped, and the occurrence of pitting and the number of rotational cycles were
confirmed. When no pitting occurred even after the number of rotational cycles
reached 10 million cycles, it was judged that the specimen had excellent surface
fatigue strength, and the test was stopped at 10 million cycles.
[O 1201
[Test results]
Test results are shown in Table 3.
[0121]
[Table 31
Table 3

[O 1 221
In Test Nos. 1 to 26, the chemical compositions of steel material were
appropriate, and F1 satisfied Formula (1). Further, the production conditions were
also appropriate, the carburizing temperature in the preliminary gas carburizing
process was less than FA, and FB satisfied Formula (2). For that reason, the C
content in the outer layer of the carburized layer of the carburized steel component
was not less than 0.5%, and F2 satisfied Formula (2). Further, the effective
hardened layer was 0.3 to less than 1.5 mm, and the internal oxide fraction was 7 to
50%. For that reason, in these Test Nos., each specimen endured 10 million cycles
in the roller pitting test, exhibiting excellent surface fatigue strength. Further, the
carburizing time in the gas carburizing process (the preliminary gas carburizing
process and the main gas carburizing process) was less than 50 hours, and it
compared favorably with an ordinary gas carburizing treatment.
[0 1231
On the other hand, in Test No. 27, the C content of steel material was
excessively low. For that reason, in the roller pitting fatigue test, damage of the
specimen occurred before the number of rotational cycles reached 10 million cycles,
exhibiting low surface fatigue strength. It was considered that since the C content
was excessively low, the strength of a core part, which was non-carburized layer, of
the carburized steel component was low.
[0 1241
In Test No. 28, Si content was excessively low. For that reason, in the roller
pitting fatigue test, damage of the specimen occurred before the number of rotational
cycles reached 10 million cycles, thus exhibiting low surface fatigue strength. It
was considered that since the Si content was excessively low, the temper softening
resistance was low, and consequently the surface fatigue strength decreased.
[0125]
13
l7
35
36
Comparative
Example
Comparative
Example
Endured 1.3
million cycles
Endured 7.4
million cycles
33
34
0.2
1.6
7.9
8.4
0.8
6.7
0.7
1 .O
In Test No. 29, although content of each element in steel material was
a~propriateF, 1 was more than the upper limit of Formula (1). For that reason, the
internal oxide fraction was less than 7%, the effective hardened layer was 0 mm, and
the C content of the outer layer was less than 5%. As a result, the surface fatigue
strength was low. It was considered that since F1 was more than the upper limit of
Formula (I), the content of specific elements was excessively large, and thus oxide
coating was formed on the surface of steel material in the main gas carburizing
treatment.
[0 1261
In Test No. 30, although the content of each element in steel material was
appropriate, F1 was less than the lower limit of Formula (1). For that reason, the
surface fatigue strength was low. It was also considered that since the temper
softening resistance was low, the surface fatigue strength decreased.
[0 1271
In Test No. 31, F1 was less than the lower limit of Formula (1). Further, the
preliminary gas carburizing process was not performed. For that reason, the surface
fatigue strength was low.
[0 1281
In Test No. 32, although the chemical composition was appropriate, and F1
satisfied Formula (I), the preliminary gas carburizing process was not performed.
For that reason, the depth of effective hardened layer was 0 mm, and the internal
oxide fraction was also low. As a result, the surface fatigue strength was low. It
was considered that oxide coating was formed during the main carburizing treatment,
and no carburization occurred.
[0 1291
In Test No. 33, although the chemical composition was appropriate, and F1
satisfied Formula (I), the carburizing time in the preliminary gas carburizing process
was excessively short. For that reason, F2 did not satisfy Formula (2), and the
effective hardened layer was 0 mm. As a result, the surface fatigue strength was
low.
[0 1303
In Test No. 34, although the chemical composition was appropriate and F1
satisfied Formula (I), the carburizing temperature Tp in the preliminary gas
carburizing treatment was not less than FA. For that reason, F2 did not satisfy
Formula (2), the effective hardened layer was 0 mm. As a result, the surface fatigue
strength was low.
[0131]
In Test No. 35, FB was more than the upper limit of Formula (B). For that
reason, the depth of effective hardened layer was excessively low, and the surface
fatigue strength decreased.
[0 1321
In Test No. 36, FB was less than the lower limit of Formula (B). For that
reason, the depth of effective hardened layer was more than 1.5 mm, and the surface
fatigue strength was low.
[0133]
So far, embodiments of the present invention have been described. However,
the above described embodiments are merely examples for practicing the present
invention. Therefore, the present invention can be practiced by appropriately
modifying the above described embodiments within a range not departing from the
spirit thereof, without being limited to the above described embodiments.
INDUSTRIAL APPLICABILITY
[0 1341
The production method of a carburized steel component according to the
present embodiment can be widely applied to the production of carburized steel
components. Particularly, a carburized steel component produced by the present
production method can enhance the power of automobiles, construction vehicles,
industrial machines, and the like, and improve the fuel economy thereof. For that
reason, the present production method is suitable for the production of carburized
steel members utilized in the above described field.
1. A production method of a carburized steel component comprising:
a preliminary gas carburizing process, wherein a steel component having a
chemical composition that consists of: by mass%, C: 0.1 to 0.4%, Si: 0.7 to 4.0%,
Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.15%, S: not more than 0.3%, N:
0.003 to 0.03%, 0: not more than 0.0050%, P: not more than 0.025%, Nb: 0 to 0.3%,
Ti: 0 to 0.3%, V: 0 to O.3%, Ni: 0 to 3.0%, Cu: 0 to 3.0%, Co: 0 to 3.0%, Mo: 0 to
1.0%, W: 0 to 1.0%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0 to 0.05%,
Te: 0 to 0.1%, and rare earth metals: 0 to 0.005%, with the balance being Fe and
impurities, and that satisfies Formula (I), is subjected to a gas carburizing treatment
at a carburizing temperature TpOC that satisfies Formula (A) for 10 minutes to less
than 20 hours; and
a main gas carburizing process, wherein following the preliminary gas
carburizing process, a gas carburizing treatment is performed at a carburizing
temperature T,"C that satisfies Formula (B) for a carburizing time t, minutes, wherein
the carburizing temperature T, is higher than the carburizing temperature T,.
6.5 < 3.5[Si%]+[Mn%]+3[Cr%] r 18 (1)
800 < Tp < 163xln(CP+0.6) - 41 xln(3.5x[Si%]+[Mn%]+3 x[Cr%]) + 950
(A)
4 < 1 3340/(Tr+273.1 5)-ln(t,) < 7 (B)
where, [Si%], [Mn%], and [Cr%] in the formulae are substituted by the Si
content, Mn content, and Cr content (in mass%) in the steel component, the term In()
represents natural logarithm, and CP is substituted by a carbon potential during
carburization in the preliminary gas carburizing process.
2. A carburized steel component, comprising:
a base metal having a chemical composition that consists of, by mass%,
C: 0.1 to 0.4%,
Si: 0.7 to 4.0%,
Mn: 0.2 to 3.0%,
Cr: 0.5 to 5.0%,
Al: 0.005 to 0.15%,
S: not more than 0.3%,
N: 0.003 to 0.03%,
0: not more than 0.0050%,
P: not more than 0.025%,
Nb: 0 to 0.396,
Ti: 0 to 0.3%,
V: 0 to 0.396,
Ni: 0 to 3.0%,
Cu: 0 to 3.0%,
Co: 0 to 3.0%,
Mo: 0 to 1.0%,
W: 0 to 1 .ox,
B: 0 to 0.005%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
Zr: 0 to 0.05%,
Te: 0 to 0.1%, and
rare earth metals: 0 to 0.005%, with the balance being Fe and impurities,
and that satisfies Formula (1); and
a carburized layer formed on the base metal, wherein
C content of an outer layer of the carburized layer is not less than 0.5%,
Si content, Mn content, and Cr content of the outer layer of the carburized
layer satisfy Formula (2),
a depth of effective hardened layer is 0.3 to less than 1.5 mm, and
an area fraction of oxide in a depth range of 10 + 3 pm from the surface of the
carburized layer is 7 to 50%:
6.5 < 3.5[Si%]+[Mn%]+3[Cr%] I 18 (1)
3.5[Sis%]+[Mns%]+3[Crs%] 2 9 (2)
where, [Six], [Mn%], and [Cr%] in Formula (1) are substituted by the Si
content, Mn content, and Cr content (in mass%) in the base metal, respectively, and
[Sis%Z [Mns%], and [Crs%] in Formula (2) are substituted by the Si content, Mn
content, and Cr content (in mass%) of the outer layer of the carburized layer,
respectively.
3. The carburized steel component according to claim 2, wherein
the chemical composition comprises one or more kinds selected from the
group consisting of:
Nb: 0.02 to 0.3%,
Ti: 0.02 to 0.3%, and
V: 0.02 to 0.3%.
4. The carburized steel component according to claim 2 or 3, wherein
the chemical composition comprises one or more kinds selected from the
group consisting of:
Ni: 0.2 to 3.0%,
Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%,
Mo: 0.05 to 1.0%,
W: 0.05 to 1.0%, and
B: 0.0006 to 0.005%.
5. The carburized steel component according to any one of claims 2 to 4,
wherein
chemical composition comprises one or more kinds selected from the group
consisting of:
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
Zr: 0.0005 to 0.05%~~
Te: 0.0005 to 0.1 %, and
rare earth metals: 0.0001 to 0.005%.