JPH07190173A - High strength gear - Google Patents

Abstract

PURPOSE:To provide a high strength gear improved in pitting resistance and wear resistance by a large margin and excellent in tooth face fatigue strength. CONSTITUTION:A high strength gear is formed of case-hardening steel for mechanical structure as material, and nitriding or carbonitriding is applied to a gear compact. The nitrogen content to the depth of at least 150/mum from the surface is 0.2% to 0.8%, and a hardening layer is formed of the mixed texture of martensite and 10-40% of residual austenite or the mixed texture of martensite, lower bainite and 10-40% of residual austenite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength gear having excellent tooth surface fatigue strength, which is used as a gear among mechanical elements of various mechanical structures such as automobiles, industrial machines, agricultural machines and the like.

[0002]

2. Description of the Related Art Gear hardened bodies are manufactured by plastic working or cutting after using carbon case-hardening steels for machine structures such as S15CK and S20CK and alloy case-hardening steels for machine structures such as SCr420 and SCM420. Carburizing or carbonitriding and quenching, and in some cases, shot peening are widely performed.

By performing such a treatment, a hardened layer harder than the core portion or a hardened layer having a compressive residual stress is formed on the tooth surface, whereby the tooth surface fatigue strength (mainly pitting strength). ) And abrasion resistance (for example, JP-A-62-33755).
No. 3-24257).

[0004]

On the other hand, due to recent demands for engine output improvement and weight reduction, gears having higher tooth surface fatigue strength, particularly superior pitting resistance, have been demanded. There is.

However, a large increase in the contact surface pressure on the tooth surface of the gear causes heat generation in the surface layer of the tooth surface, and in the conventional surface hardened layer, it softens with time and the compressive residual stress is released early. There were cases where it was accompanied by a phenomenon such as being lost.

For this reason, pitting and wear may occur and progress at an early stage, leading to problems such as noise generation. Therefore, it has been a problem to prevent such problems from occurring. .

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art. Even when the contact surface pressure on the tooth surface of the gear is significantly increased, the present invention has excellent pitting resistance and wear resistance. It is an object of the present invention to provide a high-strength gear having excellent tooth surface fatigue strength in which pitting and wear do not occur at an early stage because it is excellent, and problems such as noise generation do not occur easily.

[0008]

The high-strength gear having excellent tooth surface fatigue strength according to the present invention is made of case hardening steel for machine structure, and the gear compact is subjected to nitriding or carbonitriding treatment. A mixed structure of martensite and residual austenite of 10 to 40% with a nitrogen (N) content of 0.2% or more and 0.8% or less from the surface to a depth of at least 150 μm, or martensite, lower bainite and It is characterized by having a surface-hardened layer having a mixed structure of 10 to 40% of retained austenite.

Further, in the embodiment of the high-strength gear having excellent tooth surface fatigue strength according to the present invention, the case-hardening steel for machine structure contains 0.3 to 3.0% of Cr,
The V content may be 0.1 to 0.5%, and in the embodiment, shot peening may be performed.

In the high-strength gear having excellent tooth surface fatigue strength according to the present invention, its material is S09CK, S1.
5CK, S20CK, etc. for carbon hardened steel for machine structure, SN
Nickel / chromium based materials such as C415 and SNC815, SN
CM220, SNCM415, SNCM420, SNC
Nickel-chromium-molybdenum type such as M616, SNCM815, chromium type such as SCr415, SCr420,
SCM415, SCM418, SCM420, SCM4
21, SCM822 and other chromium-molybdenum-based, SMn
420, manganese such as SMnC420 and manganese
A case-hardening steel such as chrome-based alloy for machine structure is used.

In this case, in the case-hardening steel for mechanical structure containing chromium, it is more preferable to use one having a Cr content of 0.3 to 3.0%, and Cr is the case-hardening steel of this type. Since it has the effect of contributing to the improvement of the hardenability, it is desirable to set it to 0.3% or more in order to obtain such an effect, but if it is too much, the toughness will decrease, so 3.0 % Or less is desirable.

Further, in case-hardening steel for machine structure containing vanadium, it is more preferable to use one having a V content of 0.1 to 0.5%, and V is toughness in this kind of case-hardening steel. Since it has an effect of improving the toughness, it is desirable to set the content to 0.1% or more in order to obtain such an effect, but if the amount is too large, the effect is not improved and the toughness is rather lowered. Therefore, 0.5% or less is desirable.

In addition to the above, C, Si, Mn, P,
It goes without saying that S, Ni, Mo, etc. are also preferably in the range suitable for this type of case hardening steel.

In addition, A has a grain refining effect.
It may contain 1, Ti, Zr, Nb, N or the like, or may contain Pb, S, Ca, Se, Te, Bi or the like having a machinability improving effect.

Then, the case hardened steel for machine structure is used as a material to be processed into a gear compact by performing plastic working or cutting, and then subjected to nitriding or carbonitriding and quenching.

At this time, it is not necessary to employ a special method for nitriding or carbonitriding and quenching. By performing such heat treatment,
Nitrogen (N) content from the surface to a depth of at least 150 μm is 0.2% or more and 0.8% or less and a mixed structure of martensite and residual austenite of 10 to 40%, or martensite, lower bainite and 1
A surface hardened layer having a mixed structure of 0 to 40% retained austenite is obtained.

In this case, the nitrogen content up to a depth of at least 150 μm is 0.2% or more and 0.8% or less, whereby the pitting resistance and wear resistance can be greatly improved. is there.

The surface-hardened layer has a mixed structure of martensite and residual austenite of 10 to 40%, or martensite, lower bainite and 10 to 40%.
By using a mixed structure of retained austenite of
A dense and high-hardness martensite structure containing N is obtained, and the retained austenite, which is excellent in toughness and has an action of relaxing the stress at the fatigue crack tip and inhibiting its propagation, has a higher hardness than before. In addition, since it does not easily transform, it is possible to greatly improve the pitting resistance and wear resistance of the surface.

Furthermore, by performing such nitriding or carbonitriding and quenching, and then performing shot peening, it is possible to form a surface-hardened layer having a compressive residual stress on the surface, resulting in fatigue of the tooth surface. It is possible to further improve the strength.

[0020]

The high-strength gear according to the present invention has the above-mentioned constitution. By increasing and optimizing the surface nitrogen content, the softening resistance at high temperature (up to about 300 ° C.) is significantly improved. For example, even when the contact surface pressure of the tooth surface increases due to an increase in engine output or weight reduction, and heat is generated at the surface layer of the tooth surface, the surface hardened layer does not easily soften.

This is because the formation of fine carbonitrides such as Cr and V-based materials and the solid solution hardening of N are mainly obtained, so that the softening resistance at high temperature is greatly improved. Further, since the martensite containing nitrogen has a dense and high hardness structure and has a higher hardness than conventional even in retained austenite, and has a characteristic that it does not easily transform, As a result, pitting resistance and abrasion resistance are significantly improved.

[0022]

Example 1 SCr420, SCM420, and VCM-added steel of SCM420 having the chemical components shown in Table 1 were melted by vacuum melting, forged to a diameter of 32 mm, held at 920 ° C. for 60 minutes, and then air-cooled for normalization. After going, diameter 26mm x length 150
mm, and surface hardening heat treatment under the carbonitriding conditions shown in FIG. 1 (930 ° C. × 5 hr carburizing and diffusion →
Nitriding with NH ₃ gas at 840 ° C. × 2 hr (500 liters / hr) → immersion in 180 ° C. salt → 170 ° C. × 2
tempering of hr → air cooling).

Next, for some, arc height:
The present invention is performed by performing shot peening (S / P) under the conditions of 0.97 mA, coverage: 300% or more, shot grain size: 0.6 mm, shot hardness: Hv700, and then grinding (diameter 0.2 mm). Example roller pitting test pieces (invention products 1 to 6 shown in Table 2) were prepared.

Then, the surface hardness, surface nitrogen content, residual austenite content and high temperature softening resistance (tempering hardness of 300 ° C. × 3 hr) of the invention products 1 to 6 shown in Table 2 were examined. It was the result.

Comparative Example 1 SCr420, SCM420, and VCM-added steels of SCM420 having the chemical components shown in Table 1 were melted by vacuum melting, forged to a diameter of 32 mm, held at 920 ° C. for 60 minutes, and then air-cooled for normalization. After that, diameter 26 mm x length 150
mm, and surface hardening heat treatment under the carburizing conditions shown in FIG. 2 (930 ° C. × 5 hr carburizing and diffusion → 84
Hold at 0 ℃ x 2hrs → Immerse in 180 ℃ salt → 1
70 degreeC x 2 hr tempering-> air cooling) was performed.

Then, a portion of the roller pitting test piece of the comparative example (shown in Table 2) was shot peened (S / P) under the same conditions as in Example 1 and then ground (diameter 0.2 mm). Comparative products 1 to 3) were produced.

The surface hardness, surface nitrogen content, retained austenite content and high temperature softening resistance (tempering hardness of 300 ° C. × 3 hr) of Comparative Products 1 to 3 shown in Table 2 were also examined. It was the result.

Evaluation Test Example 1 Next, with respect to the roller pitting test pieces prepared in Example 1 (invention products 1 to 6) and Comparative example 1 (comparative products 1 to 3), roller pitting shown in FIG. A roller pitting test was performed according to the test method.

In FIG. 3, reference numeral 1 is a small roller (driving roller, test piece) having a contact portion with a diameter of 26 mm and a width of 28 mm, and 2 is a large roller (driven roller). Width of part is 8mm, diameter is 130mm
belongs to.

In this roller pitting test,
A carburized and hardened material made of SCr420 is used as the large roller (counterpart material) 2, and the test condition is a surface pressure: 3677.
MPa, slip ratio: -40%, lubricating oil: mission oil) were selected and implemented, and the pitting occurrence life was examined.
The results are also shown in Table 2.

[0031]

[Table 1]

[0032]

[Table 2]

As is clear from Tables 1 and 2, the surface nitrogen content is in the range of 0.2 to 0.8% and 10
Inventive products 1 to 6 capable of obtaining a surface-hardened layer having a mixed structure containing residual austenite of up to 40% are all high in tempering hardness at 300 ° C. × 3 hr and high in softening resistance at high temperature. And 4 × 10 ⁶
It is possible to obtain the number of occurrences of pitting occurrence times or more,
It was confirmed that the tooth surface fatigue strength and wear resistance were good.

On the other hand, in the comparative products 1 to 3 which are carburized and have a surface nitrogen content of 0%, they tend to be softened at a high temperature of 300 ° C. and have a low resistance to high temperature softening. The number of pitting lives was 4 × 10 ⁶ times or less.

Example 2 SCM420 and SCM420 having the chemical components shown in Table 1
After forming into a small gear compact using the V-added steel of No. 3 as a raw material, surface hardening heat treatment was performed under the carbonitriding conditions shown in FIG.

Next, shot peening (S / P) was performed under the same conditions as in Example 1 to produce small actual gears (invention products 7 and 8 shown in Table 3).

The surface hardness, surface nitrogen content, retained austenite content and high temperature softening resistance (tempering hardness of 300 ° C. × 3 hr) of the invention products 7 and 8 shown in Table 3 were examined. It was the result.

Comparative Example 2 SCM420 and SCM420 having the chemical components shown in Table 1.
After forming a small gear compact using the V-added steel as a raw material, a surface hardening treatment was performed under the carburizing conditions shown in FIG.

Next, shot peening (S / P) was performed under the same conditions as in Example 1 to fabricate small actual gears (Comparative products 4 and 5 shown in Table 3).

Then, the surface hardness, surface nitrogen content, residual austenite content and high temperature softening resistance (tempering hardness of 300 ° C. × 3 hr) of comparative products 4 and 5 shown in Table 3 were examined, and the results are also shown in Table 3. It was the result.

Evaluation Test Example 2 Next, the pitting occurrence life of the small actual gears produced in Example 2 (invention products 7 and 8) and Comparative Example 2 (comparison products 4 and 5) was examined by a power circulation type gear fatigue test. evaluated. The results are also shown in Table 3.

[0042]

[Table 3]

As is clear from Tables 1 and 3, the invention products 7 and 8 have a large high temperature softening resistance and a large number of tooth flapping occurrence times, and N = 1.5. The amount of wear on the tooth surface at x10 ⁶ times was small.

On the other hand, the comparative products 4 and 5 have a smaller high temperature softening resistance than the invention products 7 and 8, and the comparative product 5 has a tooth surface pitting occurrence life frequency comparable to that of the invention products. However, N = 1.5 × 1 for comparative products 4 and 5
Wear amount of the tooth surface at 0 ^six had become a more than inventions.

[0045]

The high-strength gear according to the present invention is made of case hardening steel for machine structure as a raw material, and the gear compact is subjected to nitriding or carbonitriding treatment.
A mixed structure of martensite and 10-40% retained austenite having a nitrogen content of 0.2% or more and 0.8% or less up to a depth of 0 μm, or martensite,
Since it has a surface-hardened layer consisting of a mixed structure of lower bainite and 10-40% retained austenite, it is possible to increase the surface nitrogen amount and optimize the temperature (30
Even when softening resistance (up to about 0 ° C) is significantly improved and the contact surface pressure of the tooth surface is increased to generate heat at the tooth surface layer,
The surface hardened layer is less likely to be softened, and further, the martensite containing nitrogen is dense and has high hardness, the retained austenite is also higher in hardness than before, and is not easily transformed. Therefore, the pitting resistance and wear resistance are greatly improved, and the tooth surface fatigue strength is excellent. Be brought.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing surface hardening heat treatment conditions (carbonitriding conditions) adopted in Examples 1 and 2 of the present invention.

FIG. 2 is an explanatory diagram showing surface hardening heat treatment conditions (carbonitriding conditions) adopted in Comparative Examples 1 and 2 of the present invention.

FIG. 3 is an explanatory diagram showing a roller pitting test method adopted in Example 1 and Comparative Example 1 of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Yoshiyoshi Matsushima Takeshi 1-3-3, Wakihama-cho, Chuo-ku, Kobe, Hyogo Prefecture Kobe Steel Works, Ltd. (72) Inventor, Morifumi Nakamura, Chuo-ku, Kobe, Hyogo Prefecture 1-38 Wakihamacho Kobe Steel Works, Ltd. (72) Inventor Yoichi Watanabe 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. (72) Shinji Asano Kanagawa, Yokohama, Kanagawa 2 Takara-cho, Tokyo Nissan Motor Co., Ltd. (72) Inventor, Makoto Ogura 2 Takara-cho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd.

Claims (4)

[Claims] 1. A case hardened steel for machine structure is used as a material, and a gear compact is subjected to nitriding or carbonitriding treatment,
Nitrogen content from the surface to at least 150 μm depth is 0.2% or more and 0.8% or less and mixed structure of martensite and 10-40% retained austenite, or martensite, lower bainite and 10-4
A high-strength gear having a surface-hardened layer having a mixed structure of 0% retained austenite. 2. The high-strength gear according to claim 1, wherein the case-hardening steel for machine structure contains 0.3 to 3.0% of Cr. 3. The high-strength gear according to claim 1, wherein the case-hardening steel for machine structure contains 0.1 to 0.5% of V. 4. The high-strength gear according to claim 1, which is shot peened.