JPH10259470A - Gear excellent in pitting resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a gear capable of producing at a low cost and excellent in pitting resistance and to provide a method for producing the same. SOLUTION: This gear is obtd. by forming a case hardened steel into a gear shape and thereafter executing carburizing treatment. The case hardened steel has a compsn. contg., by weight, 0.10 to 0.30% C, 0.50 to 1.50% Si, 0.30 to 1.00% Mn, 0.50 to 200% Cr and <=0.50% Mo and satisfying 1.5<=3×Si(%)-Mn(%)+Cr(%)/4+Mo(%). The gear 1 after the carburizing treatment has a carburizing layer 12 in which the concn. of C is regulated to be >=0.65%, and the amt. of retained austenite is regulated to be <=25%, and moreover, a carburizing abnormal layer 11 as the external layer thereof. The maximum depth D of the carburizing abnormal layer 11 is regulated to be 5 to 40 μm, and the area occupied by the carburizing abnormal layer 11 in the cross-section from the position of the maximum depth to the surface 15 is regulated to be >=70%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gear widely used in automobiles, construction vehicles, construction equipment, and the like, particularly to a gear excellent in pitching resistance, and a method of manufacturing the same.

[0002]

2. Description of the Related Art In the environment surrounding automobiles, construction vehicles and construction equipment, there is a social demand for energy saving and further improvement in performance, and efforts are being made to reduce body weight and increase engine output. I have. For this reason, the operating environment of gears used in automobiles and construction vehicles and equipment, especially gears used in drive train transmission parts, has become more severe, and superior tooth root fatigue strength and pitting resistance have been required. Gears are required.

Conventional gears use case hardening steel such as chrome steel Jls-SCr420 steel or chromium molybdenum steel Jls-SCM420 steel as gear steel for producing the same. Then, after forming these case hardened steels into gears, carburizing, quenching and tempering (hereinafter, referred to as
Carburizing) to form a so-called carburized gear.

[0004]

However, the above conventional gear has the following problems. In other words, in recent years, the demand for lighter bodies and higher power of engines, which are required for automobiles, construction vehicles, and construction equipment, have been increasing. for that reason,
With carburized gears that are merely carburized from conventional steel, they are no longer able to meet these requirements.

[0005] On the other hand, many proposals have been made for carburized gear steel with improved strength, and there have been many proposals regarding manufacturing methods such as shot peening. And these proposals have shown a dramatic improvement, especially in the root strength.

However, the improvement in the tooth surface strength is not so large as compared with the improvement in the tooth root strength. As a result, the gear failure mode changed from tooth root fatigue to tooth surface fatigue, that is, pitting failure. In particular, gears used at high contact pressure and high speed rotation cannot cope with the environment, causing a problem that the pitting resistance is insufficient.

[0007] To cope with such lack of pitting resistance, control of inclusion morphology by reducing the amount of oxygen in steel, addition of trace elements, and abnormal carburization by reducing the elements forming abnormal carburized layers. The method of suppressing the formation of layers or the carburizing steel for gears with tempering softening resistance
Various proposals have been made.

For example, Japanese Patent Application Laid-Open No. 1-52467 discloses an attempt to define the shape of nonmetallic inclusions. However, such proposals require advanced treatment in the steelmaking process, which increases production costs and ultimately increases the cost of steel products, which meets the needs of users who insist on cost reduction. It's gone.

Japanese Patent Application Laid-Open No. 2-85343 discloses that
It has been proposed to reduce the amount of Si added to suppress the occurrence of an abnormal carburized layer and improve the tooth surface strength (pitting resistance). There are many other similar proposals. However, according to the detailed investigation and research conducted by the developers, the method of suppressing the abnormal carburized layer not only does not provide excellent pitting resistance but also promotes the variation in pitting life of individual gears. I found out.

The present invention has been made in view of such a conventional problem, and an object of the present invention is to provide a gear which can be manufactured at low cost and has excellent pitting resistance and a method of manufacturing the same.

[0011]

According to a first aspect of the present invention, there is provided a gear obtained by forming a case hardened steel into a gear shape and then carburizing the case hardened steel. .30%, S
i: 0.50 to 1.50%, Mn: 0.30 to 1.00
%, Cr: 0.50 to 2.00%, Mo: 0.50% or less, and 1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4
+ Mo (%), the balance being composed of Fe and unavoidable impurities, and the carburized gear having a C concentration of 0.1%.
It has a carburized layer of 65% or more and the amount of retained austenite is 25% or less, and has an abnormal carburized layer composed of an incompletely hardened structure as an outer layer of the carburized layer, and The maximum depth of the abnormal layer is 5 to 40 μm,
In addition, the gear having excellent pitting resistance is characterized in that the area occupied by the abnormal carburized layer in the cross section from the maximum depth position to the surface is 70% or more.

What should be noted in the present invention is that case hardening steel having the above specific composition is used, the C concentration and the amount of retained austenite in the carburized layer are within the above specific ranges, and furthermore, the maximum depth of the abnormal carburized layer And the area occupied by it is within the above specific range. The most important point is that the abnormal carburized layer composed of the incompletely hardened structure was positively provided only at the specific depth.

The abnormal carburized layer is a layer having an incompletely hardened structure as described above. What is an incompletely quenched structure?
It is a structure composed of troostite or bainite generated during quenching in a series of carburizing treatments. If the carburized abnormal layer is corroded with a corrosive liquid such as nital after the cross section of the treated product is mirror-finished, it becomes black and corroded, so that its form can be easily observed. This abnormal carburized layer is generated as follows.

That is, for example, in the case of a gas carburizing process, a certain amount of oxygen is contained in the carburizing atmosphere. When this oxygen enters from the surface of the steel, the Si, Cr, Mn, Ni, and Mo contained (solid solution) contained in the matrix near the grain boundaries.
Among them, Si, Cr, and Mn combine with oxygen diffused through crystal grain boundaries to form oxides. For this reason,
The hardenability decreases near where the oxide is formed. Therefore, martensite is not generated during quenching, and troostite or bainite is generated. This layer of incompletely quenched structure composed of troostite or bainite is an abnormal carburized layer.

This abnormal carburized layer is provided on the outermost surface of the gear, and its maximum depth is 5 to 40 μm as described above.
Further, the area occupied by the abnormal carburized layer in the cross section from the maximum depth position to the surface (hereinafter, referred to as occupied area ratio) is set to 70% or more. This abnormal carburized layer is usually formed with a variation in depth, as shown in FIG. Therefore, in the present invention, the thickness of the abnormal carburized layer is defined by the maximum depth, and the degree of unevenness of the depth is defined by the occupied area ratio of the abnormal carburized layer.

When the maximum depth of the abnormal carburized layer is less than 5 μm, there is a problem that the effect of the initial conformability described later cannot be sufficiently exhibited. On the other hand, if it exceeds 40 μm, the amount of wear due to the wear of the abnormal carburized layer will be too large, and the gear contact will deteriorate, noise during operation will increase, and the stress distribution acting on the tooth surface will worsen. There's a problem.

If the area occupied by the abnormal carburized layer from the maximum depth position to the surface is less than 70%, the variation in the depth of the abnormal carburized layer becomes extremely large. Therefore, even after the abrasion of the abnormal carburized layer described later, a large number of soft incompletely quenched structures remain on the surface of the carburized layer in a wedge shape. Therefore, there is a problem that a crack originating from this is easily generated.

On the other hand, the upper limit of the occupied area ratio of the abnormal carburized layer is ideally preferably 100%. That is,
The carburized abnormal layer has a uniform depth as the occupied area ratio is higher, the unevenness of the depth is reduced. for that reason,
After the wear of the abnormal carburized layer, the carburized layer of high hardness is exposed on the gear surface, and the subsequent pitting resistance can be improved.

Next, as described above, the carburized layer located below the abnormal carburized layer has a C (carbon) concentration of 0.65% or more and a residual austenite amount of 25% or less. If the C concentration is less than 0.65%, there is a problem that the hardness of the martensite structure obtained by carburizing and quenching does not become too high. However, the upper limit of C concentration is
Cementite is formed at the grain boundary, which may reduce fatigue strength and pitting resistance. Therefore, the content is preferably set to 1.2%. Further, when the amount of retained austenite in the carburized layer exceeds 25%, there is a problem that the hardness of the carburized layer cannot be sufficiently increased.

Next, as the material of the gear of the present invention, case hardened steel having the above specific composition is used. The reasons for limiting the range of each chemical component will be described below.

C: 0.10 to 0.30%, in order to sufficiently satisfy the strength required for the carburized and quenched and tempered gear parts, that is, to set the internal hardness Hv 200 to 500 of the carburized gear parts. 0.10 to get
% Of C must be contained. However, 0.30%
If the content exceeds the upper limit, the toughness of the inside deteriorates, the strength of the gear decreases, the machinability decreases, and the cold forgeability deteriorates.

Si: 0.50 to 1.50%, during carburizing, Si in the carburized layer reacts with oxygen in the carburizing atmosphere to form an oxide. Therefore, the hardenability decreases near the surface layer of the article to be treated, and a so-called abnormal carburized layer is formed. That is, Si is an element that has an important effect on the formation of the abnormal carburized layer and is an element that enhances the tempering softening resistance of the martensitic structure. In the present invention, in order to obtain a desired form of the abnormal carburized layer and to increase the temper softening resistance, it is necessary to contain Si in an amount exceeding 0.50%. However, if the content exceeds 1.50%, not only the above-mentioned abnormal carburized layer is not obtained, but also the cold forgeability, machinability and toughness are reduced, so the upper limit is set to 1.50%.
%.

Mn: 0.30 to 1.00%, Mn is an element having a remarkable effect on improving hardenability.
Hardness (Hv2
In order to guarantee (M.00 to 500), it is necessary to contain 0.30% or more of Mn. Further, since Mn is also an element that forms an abnormal carburized layer, the amount of Mn affects the form of the abnormal carburized layer. For this reason, it must not exceed 1.00% in order to obtain the carburized abnormal layer required for the gear.

Cr: 0.50 to 2.00%, Cr is an element for improving hardenability, and must be contained at least 0.30% after carburizing and quenching to obtain the above internal hardness. is there. On the other hand, if the content exceeds 2.00%, the cold forgeability and machinability are remarkably deteriorated, so the upper limit is set to 2.00%.

Mo: 0.50% or less Mo has an effect of improving hardenability and toughness, and also has an effect of refining crystal grains after carburizing. Further, it has the effect of suppressing the abnormal carburized layer, and can be used together with the effect of forming the abnormal carburized layer of Si to obtain a desired form of the abnormal carburized layer. When this element is added in a large amount, not only a desired carburized abnormal layer cannot be obtained, but also the cost is increased, and further, the cold forgeability and machinability are deteriorated. Therefore, the upper limit is 0.50%. Note that the lower limit is preferably set to 0.10% in order to exert the above effect.

Next, the following relational expression for controlling the chemical components in the case hardening steel will be described. Relational expression that regulates chemical components, 3 x Si (%)-Mn (%) + Cr
(%) / 4 + Mo (%) is a parameter that regulates the tempering resistance of martensite. That is, the tooth surface of the gear is exposed to an environment of 200 to 500 ° C. due to heat generated by friction, and the surface is tempered. As a result, when the decrease in the hardness of the tooth surface is large, that is, when the tempering resistance is low, it causes pitting failure.

In the above relational expression, when the value is 1.5 or more, the tempering resistance of martensite is improved,
During use of the gear, a decrease in the hardness of martensite on the tooth surface can be suppressed to Hv 100 or less, and pitting resistance can be improved in terms of tooth surface hardness. In addition,
The upper limit of the value of the above relational expression is preferably 3.0 for reasons of deterioration in workability (machinability) due to an increase in material hardness and cost increase due to an increase in the amount of alloying elements.

Next, the operation of the present invention will be described. As described above, the gear of the present invention is provided with the specific amount of abnormal carburized layer on the outer layer of the carburized layer having the specific C concentration and the retained austenite. Therefore, the gear of the present invention exhibits excellent initial conformability at the stage of actual use and exhibits excellent pitting resistance.

That is, in the case where the carburized gear is not polished after the carburizing process, it is inevitable that the shape of the tooth surface includes a certain error (strain). Needless to say, each gear has a slightly different shape from tooth to tooth within a single gear. This error (strain) greatly affects the distribution of contact pressure applied to the tooth surface. As a result, the maximum contact pressure generated on the tooth surface during the conventional meshing driving of the gears is extremely high, and often reaches the limit value of the load capacity. These greatly influence the pitching life, of which the influence of the surface pressure as well as the gear strength is a dominant factor.

In this regard, the gear of the present invention has the abnormal carburized layer having the specific thickness as the outer layer of the hardened carburized layer. Therefore, the adverse effect of the error existing in the tooth surface can be greatly reduced by the running-in operation of the gear incorporated in the device. That is, the abnormal carburized layer is a soft structure composed of an incompletely quenched structure. Therefore, it has a characteristic that it is easily worn. This property greatly improves the initial adaptability of the gear. Specifically, by actually driving the gears into engagement, the abnormal carburized layer is worn to alleviate the uneven stress distribution occurring on the tooth surface, and the shape of the tooth surface is self-corrected.

The thickness of the abnormal carburized layer in the present invention is 5 to 40 μm. Therefore, this abnormal carburized layer is sufficiently removed by the above-mentioned initial adaptation. On the tooth surface from which the abnormal carburized layer has been removed, the underlying carburized layer is exposed on the surface. This carburized layer has a martensitic structure having a C concentration of 0.65% or more as described above, and further has an extremely high hardness with a residual austenite content of 25% or less.

[0032] Therefore, in the gear that has been subjected to the initial run-in, the tooth surface has a shape capable of obtaining a uniform contact state and a uniform high hardness. Therefore, the gear of the present invention exhibits very good pitting resistance after running-in operation.

On the other hand, the gears have 20
It is tempered by being exposed to an environment of 0 to 500 ° C. If the hardness of the tooth surface, that is, the hardness of the carburized layer is reduced by the tempering, the above excellent pitting resistance is impaired. In this respect, the gear of the present invention uses the case hardened steel having the above specific component range as a material. Therefore, it has excellent resistance to softening due to tempering. Therefore, the tooth surface strength can be maintained high even during operation exposed to high temperatures, and the excellent pitting resistance can be exhibited.

The case hardened steel according to the present invention has not been subjected to compositional changes that would increase costs compared to conventional materials. Further, the cost of forming into a gear shape and carburizing can be the same as the conventional cost. Therefore, in the present invention, the gear having the excellent pitching resistance can be obtained at low cost.

The abnormal carburized layer itself is, for example, Jls-
It is possible to form even when a conventional gear steel such as SCr420 steel or SCM420 steel is used.
However, these carburized abnormal layers of conventional steel have a small occupied area ratio (less than 70%). That is, microscopically, a layer in which an incompletely quenched structure and a martensite structure are mixed is present on the tooth surface.

Therefore, in a gear using conventional steel,
Even if an abnormal carburized layer is provided, not only a good initial break-in state cannot be obtained, but also a pitting failure often starts from the residual incomplete quenched structure. Also,
The martensite structure of the steel is also poor in softening resistance.

Next, as in the second aspect of the present invention, the case hardening steel has an Al content of 0.020 to 3.0 in addition to the above composition.
It is preferable to contain 0.060% and N: 0.0080 to 0.0200%. Thereby, the strength of the gear can be further improved.

Al: 0.020 to 0.060%, Al combines with N in steel and has the effect of improving the toughness by refining the crystal grains after carburizing and quenching as AlN. In order to obtain this effect, it is necessary to contain 0.020% or more of Al. However, if the content exceeds 0.060%, excessive Al ₂ O ₃ is generated in the steel,
Since the strength is reduced, the upper limit is set to 0.060%.

N: 0.0080 to 0.0200%, as described above, N combines with Al and refines crystal grains as AlN. To obtain such an effect, 0.0
It is necessary to contain 080% or more of N. On the other hand, 0.0
Even if the content exceeds 200%, the above effect is saturated, and N may be gasified during steel making, which may make steel production difficult. Therefore, the upper limit is set to 0.0200%.

Further, as in the third aspect of the present invention, the case hardening steel further includes, in addition to the above composition, Ti: 0.20% or less, Nb: 0.20% or less, V: 0, 30% or less. Among them, it is preferable that one or more kinds are contained. Thereby, the strength of the gear can be further increased.

Ti: 0.20% or less, V: 0.30% or less, Nb: 0.20% or less. These elements improve toughness, for example, by making crystal grains after carburization finer, and increase fatigue. Improve strength. However, even if it is added in a large amount, these effects not only saturate, but also form coarse precipitates and lower the strength.
The upper limit was set to 0.20% for Ti, 0.30% for V, and 0.20% for Nb. In order to sufficiently exhibit the above effects, the lower limits are 0.01% for Ti, 0.03% for V, and N for N.
b is preferably set to 0.01%.

Next, C: 0.10 to 0.30%, Si: 0.50 to 1.
50%, Mn: 0.30-1.00%, Cr: 0.50
~ 2.00%, Mo: 0.50% or less, and
1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) /
4 + Mo (%) was satisfied, and the balance was made of case hardened steel composed of Fe and unavoidable impurities. The case hardened steel was formed into a gear shape, and then carburized to obtain a C concentration of 0.6.
A carburized layer of 5% or more and a residual austenite amount of 25% or less is formed, and a carburized abnormal layer made of an incompletely hardened structure is formed on the outer layer of the carburized layer. A method for manufacturing a gear excellent in pitting resistance, characterized in that the depth is 5 to 40 μm and the area occupied by the abnormal carburized layer in the cross section from the maximum depth position to the surface is 70% or more. .

The above carburizing process refers to a series of processes including carburizing, quenching, and tempering. Further, for carburizing, various treatment methods similar to those in the related art can be used. Then, for example, in the case of performing gas carburization, the C concentration of the carburized layer can be easily adjusted by adjusting the carbon potential, temperature, treatment time, and the like of the gas to the optimal values, as is well known.

After the C concentration is adjusted by carburizing, quenching is performed immediately after cooling or after cooling. Quenching may be performed by combining primary quenching and secondary quenching,
Direct quenching may be performed. Further, in order to adjust the amount of retained austenite, a sub-zero treatment can be performed after quenching. In addition, tempering after quenching is performed at about 130 to 180 ° C., as is usually performed.

Such a series of carburizing treatments are performed on the case hardened steel in the above-described specific component range formed into a gear shape. Thereby, a gear having the specific carburized layer and the specific abnormal carburized layer can be easily manufactured. The reason for limiting each numerical value is the same as above.

According to a fifth aspect of the present invention, in addition to the above composition, the case hardened steel further comprises Al: 0.020 to 0.020.
It is preferable to contain 0.060% and N: 0.0080 to 0.0200%. Further, as in the sixth aspect of the present invention, the case hardening steel further includes Ti:
0.20% or less, Nb: 0.20% or less, V: 0, 30
% Or less, it is preferable that one or more kinds are contained. In these cases, the same effects as above can be obtained. The reason for limiting the numerical values is the same as above.

[0047]

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment A gear having excellent pitting resistance and a method of manufacturing the gear according to an embodiment of the present invention will be described with reference to FIGS. The gear 1 of this embodiment is an example of a general spur gear as shown in FIG. The spur gear is an example, and is applicable to a helical gear, a bevel gear, a bevel gear, a screw gear, a worm gear, and various other gears. In manufacturing the gear 1, first, steel A shown in Table 2 described later was prepared as a case hardened steel.

As shown in Table 2, the steel A was made of C, Si, M
This steel is a steel in which the contents of n, Cr, and Mo are restricted within the range of the present invention, and 0.030% of Al and 0.0120% of N are added, and 1.5 ≦ 3 × Si (%)-Mn. (%) + C
The steel satisfies r (%) / 4 + Mo (%).

In manufacturing the gear 1 using steel A, the gear 1 is first formed into a gear 1 having the shape shown in FIG. 2 by cutting. Next, the formed gear 1 is carburized. The carburizing treatment in this example was performed by using a gas carburizing method, specifically, as shown in FIG.

That is, first, the carbon potential (C.
P) is maintained at 0.8-1.2% and the temperature is 950 ° C.
After carburizing for 75 minutes in a gas atmosphere kept at a temperature of 950.degree. Carbon is diffused into steel in a gas atmosphere with reduced P. Then, C.I. P
While maintaining the temperature, the temperature was lowered to 850 ° C., soaked for 30 minutes, and then quenched into oil at 130 ° C. After that, temperature 1
Tempering treatment was performed at 60 ° C. for 1 hour.

The processing conditions can be changed depending on the type of steel or the like. For example, the initial carburizing and diffusion temperature can be changed between 930 to 980 ° C., the time for 0.5 to 5 hours, the soaking temperature for 840 to 870 ° C., and the time for about 0.5 to 2 hours. .

As shown in FIG. 1, the gear 1 obtained by the carburizing treatment has a surface state in which a carburized layer 12 is formed on a matrix 10 and a carburized abnormal layer 11 is further formed thereon. As a result of analysis by EPMA, the carburized layer 12 in this example had a C concentration of 0.70%, and an X-ray diffraction analysis revealed that the amount of retained austenite was 22%. In addition, the hardness of the carburized layer 12 was very high at Hv810.

As shown in FIG. 1, the abnormal carburized layer 11 has a maximum depth D of 12 μm and an area occupied by the abnormal carburized layer 11 in the section A from the maximum depth position to the surface is 80 μm. %Met.

Next, the operation of the gear 1 of this embodiment will be described. As described above, the gear 1 of the present embodiment has a carburized layer 1 of high hardness.
The outer layer 2 is further provided with the specified amount of abnormal carburized layer 12. Therefore, as shown in FIG. 1, by driving the two gears 1 in mesh with each other, excellent initial adaptability can be exhibited.

That is, in the gear 1, the abnormal carburized layer 11 is worn to alleviate the uneven stress distribution generated on the tooth surface 15, and the shape of the tooth surface 15 is self-corrected. Then, on the tooth surface 15 from which the abnormal carburized layer 11 has been removed by abrasion,
The lower carburized layer 12 is exposed on the surface.

Therefore, in the gear 1 of this embodiment, after a certain initial running-in operation, a uniform contact state of the tooth surface 15 is obtained, the stress distribution becomes uniform, and the contact surface has a high hardness carburized layer. Is constituted by As a result, the stress state at the time of contact can be improved, and the pitting resistance can be improved, as compared with a conventional gear in which a countermeasure for reducing the abnormal carburized layer is minimized.

Further, the gear 1 of this embodiment uses the steel A having the above specific composition as a raw material. Therefore, a decrease in hardness due to frictional heat during operation of the gear is small. Therefore, the pitching resistance of the gear 1 of the present embodiment does not deteriorate so much even when used for a long time, and the gear 1 has a long life.

Embodiment 2 In this embodiment, the steel A shown in Table 1 was used instead of steel A of Embodiment 1.
Gears were prepared by preparing zero types of test steels, and the softening resistance of the carburized layer was quantitatively evaluated. The gears were manufactured using the test steels according to the same manufacturing method as in the first embodiment. However, in order to alleviate the influence of the difference in hardenability due to the difference in components, all the gears were subjected to a subzero treatment at a temperature of -40 to -70 ° C for 1 hour after tempering in the carburizing treatment.

Further, in order to evaluate the softening resistance, after the above-mentioned sub-zero treatment, a re-tempering treatment was further performed at a temperature of 250 ° C. for 4 hours. Then, the cross-sectional hardness of the carburized layer before and after the re-tempering treatment was measured. Table 1 and FIG. 4 show the measurement results.
Shown in In addition, the following relational expression for the chemical composition of each steel, 3
× Si (%)-Mn (%) + Cr (%) / 4 + Mo
Table 1 also shows the values of (%). In FIG. 4, the values of the relational expression are plotted on the horizontal axis and the hardness (Hv) is plotted on the vertical axis, and the hardness before re-tempering (（) and the hardness after re-tempering (●) are plotted.

As can be seen from Table 1 and FIG. 4, the hardness of the carburized layer before re-tempering was 8
It showed a very high hardness of around 00 Hv. In contrast,
After re-tempering, the smaller the value of the above relational expression, the lower the hardness tended to be. In particular, as shown in the figure, when the value of the relational expression was less than 1.5, the hardness was greatly reduced to a low hardness below 700 Hv.

FIG. 5 shows the relationship between the decrease in hardness and the value of the above relational expression in order to clarify the relationship. In FIG. 5, the horizontal axis represents the value of the relational expression, and the vertical axis represents the hardness reduction value (Hv). As can be seen from the figure, there is a correlation between the value of the relational expression and the value of the decrease in hardness due to re-tempering.
And, when the value of the relational expression exceeds 1.5, it can be seen that the decrease in hardness due to re-tempering is 100 Hv or less.

From the above results, the value of the above relational expression was set to 1.5
It has been clarified that the gear of the present invention using the steel regulated as described above can suppress softening due to frictional heat when the gear is used, and can maintain the pitting resistance of the gear for a long time.

[0063]

[Table 1]

Embodiment 3 In this embodiment, as shown in Table 2, in addition to the A steel used in Embodiment 1, eight types (a total of nine types) of test steels were prepared.
We quantitatively evaluated how the composition of the case hardening steel and the depth of the abnormal carburized layer affect the pitting strength and the like. First, Table 2 shows a list of the chemical components of the prepared test steels.

[0065]

[Table 2]

Next, each of the test steels was melted, forged, forged, and normalized, then processed into test specimens for each test described later, and carburized. As shown in Table 3 below, the carburizing treatment was performed so as to change the maximum depth and the occupied area ratio of the abnormal carburized layer while maintaining the C concentration of the carburized layer at 0.65% or more. Specifically, for the case where normal gas carburizing was performed, the carburizing method of Embodiment 1 (FIG. 3) was basically used.
Temperature, time, C.I. This was performed by finely adjusting each condition of P. For those that did not produce any abnormal carburized layers, vacuum carburizing was performed.

Next, as an alternative test for evaluating the tooth surface strength of the gear, a roller pitching test was performed as shown in FIGS. As shown in FIGS. 6 and 7, the roller pitching test is performed using a small roller 41 having a test portion 411 at the center and a large roller 42 in a disk shape. The small roller 41 and the large roller 42 are manufactured by performing the same manufacturing process and carburizing using the same steel type.

As shown in FIG. 7A, the small roller 41 has a width W of 28 m at a central portion having an overall length L of 130 mm.
m, an outer diameter D ₁ is provided with a testing portion 411 of 26 mm. As shown in FIG. 7B, the large roller 42
The thickness T is 18 mm, the outer diameter D ₂ is a disc of 130 mm, the end portion 421 as shown in FIG. 7 (c), are the curved surface of the crowning radius 300 mm.

As shown in FIG. 6, with the end portion 421 of the large roller 42 set on the shaft 429 and the test portion 411 of the small roller 41 in contact with each other, they are rotated at different peripheral speeds. Let it. Specifically, the rotation speed of the small roller 41 is set to 2000 rpm, and the slip ratio (peripheral speed difference) is set to -4.
0%. Lubrication was performed by ATF (Automatic Transmission Fluid) at an oil temperature of 120 ° C.

Then, the small roller 41 is rotated with a constant surface pressure applied between the small roller 41 and the large roller 42 to determine the total number of rotations of the small roller 41 until the occurrence of pitching. This is repeated by changing the surface pressure. Note that a roller pitching tester manufactured by Komatsu Engineering Co., Ltd. was used.

[0071] Then, in this example, was used for evaluation as the surface pressure (10 ⁷ times endurance surface pressure) pitching strength pitching even when the small roller 41 is rotated more than 10 ⁷ times is not generated (MPa). Table 3 shows the test results.

As can be seen from Table 3, the case where the maximum depth of the abnormal carburized layer is 0 and the case where the maximum depth exceeds 40 μm (sample N
o. 1, 6, 12) all had low pitching strengths of 1800 MPa or less. Even if the maximum depth of the abnormal carburized layer is in the range of 5 to 40 μm, when the occupied area ratio of the abnormal carburized layer is less than 70% (Sample Nos. 3 and 14), the same is also 1800 MPa or less It was a low value. Conversely, the occupied area ratio of the abnormal carburized layer is 7
Even in the case of 0% or more, when the maximum depth of the abnormal carburized layer exceeds 40 μm as described above (Sample No. 6), the value was also as low as 1800 MPa or less.

Further, even if the maximum depth and the occupied area ratio of the abnormal carburized layer are good, the composition range of the steel is out of the range of the present invention (Sample No. 13). Even when the occupied area ratio is good and the component range of the steel is within the range of the present invention, when the value of the above relational expression is less than 1.5 (Sample No. 10), the pitting strength is low. The result was.

Next, an Ono-type rotary bending fatigue test was performed as an alternative test for estimating the tooth root strength of the gear. The Ono-type rotating bending fatigue test was performed according to JIS-Z2274 "Rotating bending fatigue test method of metal material" using a test piece 5 having a shape as shown in FIG. The rotation speed is 360
It was set to 0 rpm. In this example, a bending stress (10 ⁷ times endurance stress) that does not break even when rotated 10 ⁷ times or more was used for evaluation.

Table 3 shows the test results. As known from Table 3, sample no. 52 kgf / mm other than 6
^{It was 2} or more, and good results were obtained. On the other hand, for sample no. In No. 6, Ono-type rotary bending strength was the lowest value. This is considered to be because the maximum depth of the abnormal carburized layer was too deep.

[0076]

[Table 3]

From the above test results, it can be seen that the pitting strength can be surely improved by the range of steel components, the provision of the relational expressions, and the appropriate formation of the abnormal carburized layer.
Also, within the range where the pitting strength can be improved, the Ono-type rotary bending strength is also good, and it can be seen that the tooth root strength of the gear can be maintained well.

[0078]

As described above, according to the present invention, it is possible to provide a gear which can be manufactured at low cost and has excellent pitting resistance, and a method for manufacturing the same.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a formation state of an abnormal carburizing layer in a first embodiment.

FIG. 2 is an explanatory view showing a gear according to the first embodiment.

FIG. 3 is an explanatory view showing carburizing conditions in the first embodiment.

FIG. 4 is an explanatory diagram showing the hardness of a carburized layer before and after re-tempering in a second embodiment.

FIG. 5 is an explanatory view showing a state in which hardness is reduced by re-tempering in a second embodiment.

FIG. 6 is an explanatory view showing a roller pitching test method in a third embodiment.

FIG. 7A illustrates a small roller according to the third embodiment.
FIG. 3B is an explanatory view showing the shape of a large roller, and FIG.

FIG. 8 is an explanatory view showing the shape of a test piece in an Ono-type rotating bending fatigue test according to the third embodiment.

[Explanation of symbols]

 1. . . Gears, 10 . . Mother phase, 11. . . 11. abnormal carburized layer; . . Carburized layer,

────────────────────────────────────────────────── ─── Continuing on the front page (51) Int.Cl. ⁶ Identification symbol FI C22C 38/28 C22C 38/28 (72) Inventor Tsuyoshi Matsuda 1 Wanowari, Arao-cho, Tokai-shi, Aichi Prefecture Inside Aichi Steel Co., Ltd. 72) Inventor Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Masaaki Kondo 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation

Claims (6)

[Claims] 1. A gear obtained by forming a case-hardened steel into a gear shape and then carburizing the same, wherein the case-hardened steel has a weight ratio of:
C: 0.10 to 0.30%, Si: 0.50 to 1.50
%, Mn: 0.30 to 1.00%, Cr: 0.50 to
2.00%, Mo: 0.50% or less, and 1.5 ≦ 3 × Si (%)-Mn (%) + Cr (%) / 4
+ Mo (%), the balance being composed of Fe and unavoidable impurities, and the carburized gear having a C concentration of 0.1%.
It has a carburized layer of 65% or more and the amount of retained austenite is 25% or less, and has an abnormal carburized layer composed of an incompletely hardened structure as an outer layer of the carburized layer, and The maximum depth of the abnormal layer is 5 to 40 μm,
A gear excellent in pitting resistance, wherein an area occupied by the abnormal carburized layer in a cross section from the maximum depth position to the surface is 70% or more. 2. The case hardening steel according to claim 1, wherein the case hardening steel further comprises Al: 0.020 to 0.060 in addition to the composition.
%, N: 0.0080 to 0.0200%. 3. The case hardening steel according to claim 1, wherein the case hardening steel further contains Ti: 0.20% or less in addition to the above composition.
A gear excellent in pitting resistance characterized by containing one or more of Nb: 0.20% or less and V: 0, 30% or less. 4. C: 0.10 to 0.30 by weight ratio
%, Si: 0.50 to 1.50%, Mn: 0.30 to
1.00%, Cr: 0.50 to 2.00%, Mo: 0.
50% or less, and 1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4
+ Mo (%), the balance being Fe and unavoidable impurities, using a case hardened steel, forming the case hardened steel into a gear shape, and then carburizing to reduce the C concentration to 0.65. % And a carburized layer having a residual austenite amount of 25% or less, and an outer layer of the carburized layer is formed with an abnormally carburized abnormal layer made of incompletely quenched structure. And the area occupied by the abnormal carburized layer in the cross section from the maximum depth position to the surface is 7 μm.
A method for producing a gear excellent in pitting resistance, characterized by being 0% or more. 5. The steel according to claim 4, wherein the case hardening steel further comprises Al: 0.020 to 0.060 in addition to the composition.
%, N: 0.0080 to 0.0200%. A method for manufacturing a gear having excellent pitting resistance. 6. The case hardening steel according to claim 4, wherein the case hardening steel further contains Ti: 0.20% or less and N
b: 0.20% or less, V: 0, 30% or less,
A method for producing a gear excellent in pitting resistance, characterized by containing one or more kinds.