JP3375221B2 - Steel for carburized gear - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

[0001] The present invention relates to a steel for carburized gears for realizing high fatigue strength and durable life by heat treatment by a normal gas carburizing quenching-tempering process, and its industrial application field is gears. It is widely used in the automobile, construction vehicle and industrial machinery industries used.

[0002]

2. Description of the Related Art Conventionally, a technique for improving fatigue strength and durability life of gears carburized and tempered by gas carburization causes fatigue cracks as disclosed in, for example, Japanese Unexamined Patent Publication No. 4-83848. For the purpose of reducing the grain boundary oxidation and the incompletely hardened layer, the elements such as Si, Mn, and Cr that are more easily oxidized than Fe are reduced, and the hardenability is reduced by the elements such as Ni and Mo that are less easily oxidized than Fe. Techniques for adjusting mechanical properties, or by increasing the carbon potential during carburization, such as high-concentration carburization, plasma carburization or excess carburization, to deposit fine spherical carbides on the surface and improve surface hardness There are techniques and techniques for imparting surface compressive residual stress by shot peening and delaying the development of fatigue cracks.

However, these improved techniques are all related to the characteristics before the gear is used, and do not consider the state in which the gear is actually used and meshed. In particular, when the driving surface and the driven surface of the gear are in contact with each other at a high surface pressure, a surface fatigue phenomenon which cannot be dealt with only by the characteristics before use occurs. Particularly in the recent damage modes of gears, surface fatigue becomes more dominant in combination with the demand for high engine output and downsizing of gears. That is, it is estimated that when gears are used and meshed with each other, the temperature of the tooth contact surface rises to 200 to 300 ° C. due to friction due to contact pressure including slippage. It has been observed that when exposed to such high temperatures, the hardness of the carburized layer is lower than before use. Maintaining the hardness of the carburized layer is the most important factor for surface fatigue, but even if the hardness of the carburized layer before use is improved by the above-mentioned improvement technology, carburization caused by friction heat during use There is a problem that surface fatigue occurs due to the decrease in layer hardness.

[0004]

SUMMARY OF THE INVENTION In order to solve the above problems inexpensively and easily, the present invention adjusts the chemical composition of the steel of the carbonized material so that a normal gas can be used regardless of a special heat treatment process. The purpose is to develop steel for carburized gears that can give temper softening resistance to gear materials in the carburizing and quenching-tempering process. The main content is to utilize Si, which is an element effective for temper softening resistance. It is considered that Si has a function of delaying the diffusion of carbon due to a chemical repulsive force relationship with C, and prevents aggregation / formation of carbide which is one of the causes of softening. However, Si is a strong ferrite stabilizing element, and by increasing the phase transformation start temperature of steel,
After carburizing, there is a problem that ferrite is generated in the core structure having a low carbon content at a normal quenching temperature. The generation of ferrite makes the microstructure non-uniform in strength and promotes crack development preferentially, which is extremely undesirable. Also, Si
Has a problem in that it is an element that easily causes grain boundary oxidation during carburization. The present invention intends to solve the above problems associated with Si and to provide steel that remarkably exerts the effect of contributing to its softening resistance.

[0005]

The constitution of the present invention for solving the above-mentioned problems is C = 0.18 to 0.25 in% by weight.
%, Si = 0.45 to 1.00%, Mn = 0.40
0.70%, Ni = 0.30 to 0.70%, Cr = 1.
00 to 1.50%, Mo = 0.30 to 0.70%, Cu
= 0.50% or less, Al = 0.015 to 0.030%,
V = 0.03 to 0.30%, Nb = 0.10 to 0.0
30%, O = 0.0015% or less, N = 0.0100-
It contains 0.0200% and consists of balance Fe and unavoidable impurity elements, and does not generate ferrite in the quenched structure of the core even after quenching at a temperature of 820 ° C. or higher after carburization, and Generally, after the quenching, tempering is performed at a temperature of 160 to 180 ° C., or even when reheated at any temperature up to 300 ° C. including this temperature, the hardness of the carburized layer is carburized and quenched. It is a steel for carburized gears having a softening resistance, which is characterized by not lowering by HV50 or more as compared with the hardness after returning.

Furthermore, an element improving the machinability in the material in addition to the above components, and in weight% fatigue properties as significantly inhibit not elements, S = 0.00 6 ~0.020%, Pb =
0.03-0.09%, Te = 0.003-0.030
% Of the steel for carburized gears. In the first place, the starting point of the present invention is the development of a technology for improving the fatigue strength of steel for carburized gears, and one of the achievements was the above-mentioned Japanese Patent Laid-Open No. 4-83848. However, in recent years, the surface pressure applied to gears has increased, and damage due to surface fatigue has become noticeable. Therefore, in addition to the same invention,
In particular, for the purpose of improving surface fatigue strength, research has been conducted on the effect of alloying elements on the resistance to the decrease in the hardness of the carburized layer, that is, the softening resistance, even if there is a heat generation phenomenon due to the contact of gears.

As test materials, test steel ingots having the chemical compositions shown in Tables 1 and 2 were manufactured in a high frequency melting furnace, hot forged to 30 mmφ, and normalized at 920 ° C. for 1 hour. These steel materials,
A test piece was prepared by machining to 25 mmφ, and was carburized and tempered under the conditions shown in FIG. For these post-carburizing test pieces, a reheating experiment was carried out under the conditions shown in FIG. 2, and the hardness of the carburized layer having a depth of 50 μm from the surface was measured. This hardness is called hardness after reheating. Table 3 shows the tempering hardness and 22 at 180 ° C, which is the temperature generally tempered after carburizing and quenching.
The difference in hardness after reheating at 0 to 300 ° C, that is, the degree of softening is shown as the decrease in hardness after reheating. The softening resistance was evaluated based on the degree of this softening, and the smaller the decrease in hardness after reheating, the larger the softening resistance. FIG. 3 shows the relationship between the decrease in hardness after reheating and the Si content. It can be seen that in the region where the Si content is low, the hardness decreases more as the reheating temperature increases. That is, when the reheating temperature is 220 ° C., the decrease in hardness is Hv50 at the maximum, and there is almost no correlation with the Si content.
At 60 ° C., Hv50 is exceeded in the region where the Si content is 0.25 wt or less, and at 300 ° C., this decrease becomes more remarkable. Here, assuming that the decrease in hardness after reheating is HV50 or less to have softening resistance, it is found that there is a region having softening resistance even if the tempering temperature is 300 ° C. when the Si content is 0.45 wt% or more. It was

[0008]

[Table 1]

[0009]

[Table 2] On the other hand, as described above, the addition of Si raises the phase transformation start temperature of the steel material, and there is a problem that a ferrite phase is generated during quenching after carburization. As a method for coping with this, in the present invention, the fact that the phase transformation start temperature is lowered by the addition of the austenite stabilizing element was used. In particular, with regard to Ni, which is an alloying element, attention has been paid to the fact that ferrite generation can be suppressed and toughness, which is important as a steel for gears, can be expected, and its application was tried. First, the test pieces prepared from the test materials shown in Tables 1 and 2 were carburized and tempered under the conditions shown in FIG. For these post-carburizing test pieces, the microstructure after quenching at a position of 3 mm from the surface where the carbon concentration is considered to be sufficiently lowered was observed with an optical microscope to determine the occurrence of ferrite. An example of those results is shown in FIGS. From this, the Ni content is about 0.10.
When it is as low as wt%, it is found that when the Si content is increased to about 1.00 wt%, ferrite is generated in the carburized microstructure (comparison between steel types No.d and No.f). And, the degree is more remarkable at 820 ° C. where the quenching temperature is low. On the other hand, even if the Si content is about 1.00 wt%, when the Ni content is increased to about 1.00 wt%, the generation of ferrite is not recognized (steel type No.f and steel type No.g comparison). More in detail N
In order to confirm the ferrite suppressing effect of i, an experiment was performed in which the Si content and the Ni content were varied. The chemical composition of the test material and the processing procedure of the test piece are as described above,
The heat treatment conditions of the test piece are as shown in FIG. The microstructure of these test pieces after heat treatment was observed with an optical microscope to determine the occurrence of ferrite. The results are shown in Table 3. Here, ◯ indicates that no ferrite was observed at all, Δ indicates that ferrite was slightly observed, and x indicates that ferrite was remarkably observed. From this, the comparative steels ab, ef and h
Steels whose Ni content is not adjusted as in the case of ˜1 and whose Si content is simply increased certainly have a softening resistance at a hardness decrease of HV50 or less after reheating up to 300 ° C.
It was found that ferrite was generated under the condition of the quenching temperature of ˜840 ° C. However, the steels whose Ni content is increased and Si content is increased like Comparative Steel g and Steels m to t of the present invention are:
It was found that it has softening resistance and that ferrite is not generated at any quenching temperature. On the other hand, comparative steel c
Steels with low Si content, such as ~ d and current steels x ~ z, do not generate ferrite at any quenching temperature.
The hardness decrease after reheating at 00 ° C exceeds HV50,
It was found that there was no softening resistance. From these results, N
It has been found that by adjusting the i content, there is a component range in which the softening resistance of Si can be improved without generating ferrite even at a quenching temperature of 820 ° C. or higher.

[0010]

[Table 3]

[0011]

[Table 4] Finally, the generation of grain boundary oxidation due to the addition of Si was examined.
As described above, Si is said to enhance the grain boundary oxidation. Here, as a result of investigating its behavior in a wider range than conventionally investigated, a range of components in which the generation thereof is suppressed was found. Table 4 shows the chemical components of the test pieces used for the investigation. The processing procedure of these test pieces is as described above,
The produced test piece was processed under the carburizing and quenching conditions shown in FIG. For these carburized test pieces, the carburized structure on the surface was observed with an optical microscope to measure the depth of the grain boundary oxide layer.

[0012]

[Table 5] FIG. 6 shows the relationship between the grain boundary oxide layer depth and the Si content. As a result, as has been pointed out in the past, up to a Si content of 0.25 wt%, the depth of the grain boundary oxide layer becomes uniformly deep, but when the Si content is higher than that, it becomes shallower. It can be seen that when it is 0.45 wt% or more, it can be suppressed to about 10 μm. Therefore, it was found that the grain boundary oxide layer is not a problem in the region where the Si content is 0.45 wt% or more where the softening resistance is exhibited.

From the above basic researches, a specific method has been invented for improving the softening resistance and, in turn, improving the fatigue life and the durable life while solving the problems such as the generation of ferrite by Si and the increase in grain boundary oxidation. That is, the present invention is, by weight%, C = 0.18 to 0.25%, Si = 0.45.
1.00%, Mn = 0.40 to 0.70%, Ni = 0.
30 to 0.70%, Cr = 1.00 to 1.50%, Mo
= 0.30 to 0.70%, Cu = 0.50% or less, Al
= 0.015 to 0.030%, V = 0.03 to 0.30
%, Nb = 0.10 to 0.030%, O = 0.001
5% or less, N = 0.0100 to 0.0200% is contained, and the balance is Fe and inevitable impurity elements. Even after quenching at a temperature of 820 ° C. or more after carburizing, the core To generate a ferrite phase in the quenched structure of
There is no further, after the tempering was Tsu line at 160~180 ℃, used
During use it is exposed to temperatures above this temperature and below 300 ° C.
H be the hardness of the carburized layer in comparison to the hardness of the carburized quenching and Modonochi
A steel for carburized gears having a softening resistance that does not decrease by V50 or more.
= 0.006 to 0.020%, Pb = 0.03 to 0.0
9%, and is characterized in that it contains one or more from among Te = 0.003 to .030%.

Next, the reasons for limiting the composition of the present invention will be described. C: 0.18 to 0.25% C needs to be added at least 0.18% or more in order to obtain the core hardness HRC35 to 45 required for the gear. Further, when C is low, the phase transformation start temperature rises too much, and it becomes difficult to control it by adding an austenite stabilizing element. However, the excessive addition thereof causes the hardness of the core to be excessively increased, the residual compressive stress on the surface cannot be sufficiently introduced after quenching, and the toughness of the core is deteriorated. In order to avoid this, it is necessary to limit the upper limit to 0.25%. Therefore, the addition amount of C is set to the range of 0.18 to 0.25%. Si: 0.45-1.00%

Si is the most important element in the steel of the present invention. That is, Si is an element that most enhances the softening resistance in the temperature range of 200 to 300 ° C., which is considered to reach during the rolling of gears, etc., and at least 0.45% or more must be added to exert the result. Is. But S
As is well known, i is a ferrite stabilizing element, and its excessive addition raises the Ac3 transformation point, and ferrite appears in the core with a low carbon content in the normal quenching temperature range (820 to 860 ° C). Becomes remarkable, resulting in a decrease in strength. Further, the carburizing property is hindered and the steel material before carburizing becomes too hard, which deteriorates the cold forgeability and the machinability. In order to avoid this, it is necessary to limit the upper limit to 1.00%.

Therefore, the addition amount of Si is 0.45 to 1.0.
The range was 0%. Mn: 0.40 to 0.70% Mn needs to be added at least 0.40% or more in order to secure hardenability. However, Mn easily forms an abnormal carburizing layer. To reduce this, the upper limit is 0.70
Must be limited to%. Therefore, the addition amount of Mn is 0.
The range was 40 to 0.70%. Ni: 0.30 to 0.70% Ni is an important element together with Si in the steel of the present invention. That is, since Ni is an austenite stabilizing element contrary to Si, it lowers the phase transformation start temperature increased by adding Si. At the same time, Ni is an element that improves the hardenability and also the toughness of the carburized layer and the core. In order to exert these effects, it is necessary to add at least 0.30%. However, since Ni is an expensive element, excessive addition of Ni is not desirable from an economical point of view, but rather promotes the formation of retained austenite, resulting in a decrease in surface hardness. In order to avoid this, it is necessary to limit the upper limit to 0.70%.

Therefore, the addition amount of Ni is 0.30 to 0.7.
The range was 0%. Cr: 1.00 to 1.50% Cr is an element necessary for ensuring hardenability. It is also an element that can be expected to precipitate fine carbides. For that purpose, it is necessary to add at least 1.00% or more.
However, like Mn, Cr is an element that tends to form an abnormal carburizing layer, and excessive addition of Cr causes the core to become too hard, which deteriorates toughness. In order to avoid this, it is necessary to limit the upper limit to 1.50%. Therefore, Cr
Was added in the range of 1.00 to 1.50%. Mo: 0.30 to 0.70% Mo is an element that improves hardenability like Ni and improves toughness of the carburized layer and the core. For that purpose, it is necessary to add at least 0.30%. However, the excessive addition thereof makes it difficult to soften the steel material before carburizing, so that the machinability is impaired, and the core becomes too hard and the toughness deteriorates. In order to avoid this, it is necessary to limit the upper limit to 0.70%.

Therefore, the addition amount of Mo is 0.30 to 0.7.
The range was 0%. Cu: 0.50% or less Cu is an element for which precipitation hardening can be expected in a relatively high temperature range of 400 to 600 ° C. Therefore, it is desirable to add it when a severe usage situation in which the temperature of the tooth surface or rolling surface rises significantly, or when it is used in a high temperature environment near a jet propulsion machine or turbine like aircraft materials. . However, its excessive addition increases hot brittleness and impairs carburization. In order to avoid this, it is necessary to limit the upper limit to 0.50%. Therefore,
The amount of Cu added was 0.50% or less. Al: 0.015 to 0.030%

Al is an element having a function of forming AlN by combining with N and refining the austenite crystal grain size, and contributes to the improvement of the toughness of the carburized layer and the core through the grain refinement. For that purpose, it is necessary to add at least 0.015% or more. However, its excessive addition promotes the formation of Al ₂ O ₃ inclusions which are harmful to fatigue strength. In order to avoid this, it is necessary to limit the upper limit to 0.030%. Therefore, the added amount of Al is 0.015-0.030%
And the range. V: 0.03 to 0.30% V forms a carbide even at a relatively low temperature near the carburizing temperature, and improvement in hardness due to them can be expected. For that purpose, it is necessary to add at least 0.03% or more.
However, its excessive addition deteriorates the toughness of the carburized layer.
In order to avoid this, it is necessary to limit the upper limit to 0.30%.

Therefore, the addition amount of V is 0.03 to 0.30.
The range is%. Nb: 0.010 to 0.030% Nb combines with C and N in steel to form carbonitride, and Al
Similar to N, it is an element effective in reducing the grain size of austenite, and contributes to the improvement of the toughness of the carburized layer and the core through the grain refinement. Therefore, the addition amounts are Al and N which coexist.
However, if the amount is small, the effect cannot be exhibited. Therefore, it is necessary to add at least 0.010%. However, its excessive addition forms and precipitates coarse carbonitrides, impairing the toughness of the carburized layer. In order to avoid this, it is necessary to limit the upper limit to 0.030%. Therefore, the amount of Nb added is in the range of 0.010 to 0.030%. O: 0.0015% or less O is an element existing in the steel as an oxide inclusion and impairing fatigue strength.

Therefore, the upper limit of O is defined as 0.0015% or less. N: 0.0100 to 0.0200% N combines with Al and Nb to form AlN and NbCN,
It is an element effective in reducing the grain size of austenite, and contributes to the improvement of the toughness of the carburized layer and the core through the reduction of grain size. Therefore, the addition amount is determined by the quantitative balance of coexisting Al and Nb, but if the addition amount is small, the effect cannot be exhibited, so at least 0.0100% or more is required to be added. However, its excessive addition causes the generation of bubbles on the surface of the steel ingot during solidification and the deterioration of the forgeability of the steel material. In order to avoid this, it is necessary to limit the upper limit to 0.0200%.
Therefore, the amount of N added is in the range of 0.0100 to 0.0200%. S: 0.00 6 ~0.020%

Most of S is present in steel as sulfide inclusions, and is an element effective for improving machinability in parts such as gears formed by cutting. Prior to the
Even if the technology is avoided, it is necessary to add at least 0.006 % or more. However, its excessive addition causes a decrease in fatigue strength. To avoid this, the upper limit is 0.0
It needs to be limited to 20%. Therefore, the addition amount of S is set to the range of 0.006 to 0.020%. Pb: 0.03 to 0.09% Similar to S, Pb is an element effective for improving machinability in a component formed by cutting like a gear. For that purpose, it is necessary to add at least 0.03% or more.
However, its excessive addition is an element that causes a decrease in fatigue strength. Further, if it is 0.10% or more, the handling of Pb is subject to legal restrictions such as dust collectors and methods. In order to avoid this, it is necessary to limit the upper limit to 0.09%.

Therefore, the amount of Pb added is 0.03 to 0.0.
The range was 9%. Te: 0.003 to 0.030 Te is an element that increases the interface energy between the sulfide-based oxide and Fe that is the parent phase, and makes the shape spindle-shaped to improve machinability. In order to achieve that effect, it is necessary to add at least 0.003%. However, its excessive addition causes hot brittleness. In order to avoid this, it is necessary to limit the upper limit to 0.030%. Therefore, T
The amount of e added was 0.003 to 0.030. Hereinafter, the present invention will be described in more detail with reference to examples.

[0024]

EXAMPLES Next, based on these results, in order to confirm that the original purpose of improving the pitching fatigue strength was achieved, test steel ingots having the chemical composition shown in Table 5 as the steel of the present invention were subjected to high-frequency vacuum. It was manufactured in a melting furnace, and its pitching fatigue life was evaluated by a roller pitching fatigue test.

[0025]

[Table 6] FIG. 7A shows an outline of the roller / pitting fatigue testing machine. Here, 1 is a test piece, 2 is a load roller, 3 and 4
Is a meshing gear, 5 is a bearing, 6 is a coupling, 7 is a transmission belt, and 8 is a motor. 7B shows the shape of the test piece, and FIG. 7C shows the shape of the load roller. The test conditions are a maximum Hertzian surface pressure of 3430 MPa and a slip rate of 4
It is 0%. The test ingot was hot forged and machined into a test piece after normalizing, and carburized and tempered under the conditions shown in FIG. 1. A part of these test pieces was cut and the hardness distribution and microstructure of the carburized layer were observed. The results are shown in FIG. 13 and Table 6.
Shown in.

[0026]

[Table 7] From this, it was first confirmed that in the steel of the present invention, ferrite did not occur in the core, and the grain boundary oxidation depth was as shallow as 8.5 μm. FIG. 8 shows the results of the roller pitching fatigue test. This figure shows the pitting fatigue life of the steel of the present invention in terms of cumulative failure probability together with those of the current steels that have been used up to now. From this, it can be seen that the pitting fatigue life of the steel of the present invention is longer than those of the current steel. FIG. 9 shows the results of measuring the surface hardness by interrupting the fatigue test at a certain number of repetitions in order to grasp the decrease in hardness during rolling in the fatigue test with time. Similarly, although it is shown together with the range of the current steel, it can be seen that the steel of the present invention has a smaller decrease in surface hardness during rolling than the current steel. Therefore, according to the intended design concept, the effect of increasing the Si content improves the softening resistance, and the surface hardness that is most important for the pitching fatigue strength against friction heat during rolling under high surface pressure including slip. It can be construed that the fatigue life was extended because the decrease in the grain size was suppressed, ferrite was not generated in the core, and the oxidation depth at the grain boundaries was shallow. As described above, the invented steel has a longer pitting fatigue life than the currently used steel and has excellent properties.

[0027]

As can be seen from the above results, the steel of the present invention is far superior to the current steel in the pitching fatigue strength, which is the most important requirement for steel for gears at present. Therefore,
By using the steel of the present invention, it is possible to reduce the size and weight of the carburized gear under the current carburizing and quenching conditions and design specifications.
Further, even if the shape and size are the same, higher output can be achieved.
For these reasons, the effect of the present invention is that it can widely contribute to cost reduction and reliability improvement in the industrial world where gears are used under severe conditions.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing carburizing and quenching-tempering conditions,

FIG. 2 is an explanatory view showing heat treatment conditions of a reheating experiment,

FIG. 3 is a graph showing the relationship between hardness and Si content after reheating,

FIG. 4 is a heat treatment condition of an experiment simulating carburizing and quenching of the core,

FIG. 5 Carburizing and quenching conditions for test pieces,

FIG. 6 is a graph showing the relationship between grain boundary oxide layer depth and Si content,

7A is a schematic view of a roller / pitting fatigue tester, FIG. 7B is a schematic view of a test piece of a roller / pitting fatigue test, and FIG. 7C is a schematic view of a load roller of a roller / pitting fatigue test.

FIG. 8 is a graph showing the pitting fatigue life of the invention steel and the current steel,

FIG. 9 is a graph showing changes with time in surface hardness of the invention steel and the current steel during rolling,

10 is a micrograph showing the microstructure of a metal test piece carburized under the conditions shown in FIG.

FIG. 11 is a micrograph showing a microstructure of a metal test piece carburized under the conditions shown in FIG.

12 is a micrograph showing a microstructure of a metal test piece carburized under the conditions shown in FIG. 4,

FIG. 13 is a photomicrograph showing the microstructure of a metal test piece carburized and tempered under the conditions shown in FIG.

[Explanation of symbols]

1 test piece 2 load roller 3,4 meshing gear 5 bearings 6 coupling 7 transmission belt 8 motor

Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/60 C22C 38/60 (56) Reference JP-A-5-140696 (JP, A) JP-A-5-125437 (JP, A) JP-A-63-60257 (JP, A) JP-A-62-1843 (JP, A) "The 3rd edition Iron and Steel Handbook ▲ IV ▼" Page 129, right column, Japan Iron and Steel Institute, January 20, 1984 Published by Maruzen Co., Ltd. (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 6/00 C21D 9/32

Claims (2)

(57) [Claims] 1. In weight%, C = 0.18 to 0.25%,
Si = 0.45-1.00%, Mn = 0.40-0.7
0%, Ni = 0.30 to 0.70%, Cr = 1.00
1.50%, Mo = 0.30 to 0.70%, Cu = 0.
50% or less, Al = 0.015 to 0.030%, V =
0.03-0.30%, Nb = 0.10-0.030
%, O = 0.0015% or less, N = 0.0100-0.
Containing 0200% and the balance Fe and unavoidable impurity elements, even after the hardening was performed at 820 ° C. or higher temperature after carburizing, without causing the ferrite phase hardenability tissue heart portion, further after the tempering was Tsu line at 160~180 ℃,
Exposed to temperatures above 300 ° C during use
Also the hardness of the carburized layer is steel carburized gear having a softening resistance, characterized in that not reduced carburization quenching and the hardness of Modonochi compared to HV50 or more. 2. An element that improves machinability in the material and does not significantly impair fatigue properties, and is S = wt%.
0.006-0.020%, Pb = 0.03-0.09
%, Te = 0.003~0.030% of one or steel for carburized gear is according to claim 1, characterized in that it contains two or more from among.