JP2003183808A - Mechanical structural part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cheap mechanical structural part excellent in face pressure resistance and bending fatigue strength. <P>SOLUTION: A microcarbide-dispersed carburized layer 2 with a thickness of 0.2 mm is formed by gas carburization and is strengthened by incorporating 25-45% remaining austenite structure therein formed by nitrogen penetration. A surface layer 1 on the microcarbide-dispersed carburized layer 2 is strengthened by microshot peening to ensure a remaining surface compression stress of 1,000 N/mm<SP>2</SP>or higher and a surface hardness of 900 HV or higher while suppressing the surface roughness to Ry3 μm or lower. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mechanical structural component such as a gear of a transmission or a camshaft of an engine drive system, which is required to have improved surface pressure resistance and bending fatigue strength.

[0002]

2. Description of the Related Art Conventionally, a carburized and hardened low alloy steel such as SCM420H has been used for mechanical structural parts such as gears and camshafts.

Recently, there has been an increasing demand for weight reduction and size reduction in transmissions and engines, and size reduction in mechanical structural parts such as gears and camshafts has also been required. In order to reduce the size of mechanical structural parts such as gears, it is necessary to further increase the surface pressure resistance and bending fatigue strength, but it is difficult to cope with this with ordinary carburizing and quenching such as SCM420H steel.

In order to eliminate such a point, for example, as a method for increasing the strength of gears, Japanese Unexamined Patent Application Publication No. 2000-18369 discloses a carbide that satisfies both the required strength characteristics of the tooth surface and the required strength characteristics of the tooth root. Dispersion strengthened gears and methods of making the same are disclosed.

As a method for improving the strength of gears,
Many shot peening methods using the air nozzle method have been reported. In these shot peening, the hardness is 700 to 800 HV and the particle size is φ0.3 mm.
A method of adding a deep compressive residual stress distribution of 0.1 mm or more by using the above shot grains is adopted.

[0006]

By the way, Japanese Unexamined Patent Application Publication No.
According to the strengthening method described in 0-18369,
There are the following problems.

(1) In order to suppress the decrease in bending fatigue strength of the root rounded portion due to the dispersed carbide, the dispersed amount of the carbide at the root is reduced (the tooth surface is reduced by 5 to 10% by volume and the root is reduced by 3% max). , The gear strength is increased by balancing the strength between the tooth root and the tooth surface.

However, in order to obtain such properties by carburization of fine carbide, it is necessary to carry out some kind of treatment during carburization to suppress the infiltration of C into the tooth root by a treatment such as application of a carburizing inhibitor. For this reason, the reheating nitrification treatment for the precipitation of carbide and the disappearance of the surface softening layer has to be performed in separate steps. Therefore, in the high strength method described in Japanese Patent Laid-Open No. 2000-18369, the heat treatment step involves two steps, which raises a problem of increasing the processing cost.

(2) Fine carbide dispersion is carried out by gas carburizing and nitrifying treatment. In such treatment, grain boundary oxidation (latent crack) occurs in the surface layer as shown in FIG. Since a certain amount of Cr is absorbed by the oxide, it is not possible to obtain carbide dispersion in the surface grain boundary oxidation forming layer. Therefore, dispersion strengthening by fine carbide cannot be expected in the surface layer, and such points and grain boundary oxidation (latent crack)
Due to the presence of, the strength cannot be sufficiently strengthened.
Therefore, in the gear used in the carburized state, improvement of the root bending strength and the tooth surface pitching strength cannot be expected.

Further, in Japanese Patent Laid-Open No. 2000-18369, as a strength strengthening method for solving the above-mentioned problems (1) and (2), improvement by removing erosion by shot peening and adding compressive residual stress. Is proposed. However, this improvement treatment does not consider improvement in surface roughness and surface compressive residual stress in view of the shot strength surface (arc height value: 0.3 to 0.8 mm). It is difficult to significantly improve the notch bending fatigue strength and the tooth surface pitching strength of the root rounded portion.

On the other hand, according to the shot peening technique used for increasing the strength of the conventional carburized gear, as shown in FIG. 12, it is a method of obtaining a deep compressive residual stress distribution of 0.1 mm or more. It is possible to obtain a great effect in improving the original strength.

However, there is a problem that the effect of improving the tooth surface pitching strength starting from the surface is impaired due to an increase in surface roughness and a decrease in surface layer compressive residual stress due to surface layer plastic deformation. As a countermeasure, two-stage peening is performed, such as adding fine particle peening to recover the surface roughness and surface compressive residual stress, and smoothing the unevenness of the surface. It becomes an issue.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an inexpensive mechanical structural component having both excellent surface pressure resistance and bending fatigue strength.

[0014]

The mechanical structural component of the present invention has a carburized layer of fine carbide dispersed by gas carburization of 0.15 m.
m or more, and contains 25 to 45% of residual austenite by nitrogen infiltration, and the surface roughness is obtained by micro shot peening for the upper layer part (surface layer) of the fine carbide dispersed carburized layer (see FIG. 1). The surface compression residual stress is 1000 N / mm ² or more, and the surface hardness is 900 HV or more.

In the mechanical structural component of the present invention, the Cr content is 0.85 to 1.25% and the Mo content is 0.5%.
The following steels are preferably used as raw materials.

Next, the present invention will be described in detail.

First, for example, the Cr content: 0.85-1.
25%, Mo content: 0.5% or less of steel as a material,
When fine carbide dispersion is performed by gas carburizing and nitriding heat treatment, a fine carbide-dispersed carburized layer is formed as shown in FIG. 11, but grain boundary oxidation (latent crack) and an unreinforced surface layer in which almost no carbide exists in the upper layer. (About 15 μm) is present.

The present invention intends to increase the surface hardness and the surface compressive residual stress by modifying such a defective surface layer. Specifically, the particle diameter is 0.1 m.
The surface layer is strengthened by performing micro shot peening using shot grains of m or less (see FIG. 1).
The surface layer reinforcement will be specifically described below.

As shown in FIG. 2, ordinary shot peening (conventional general treatment) using shot grains having a grain diameter of 0.3 mm or more can increase the compressive residual stress in the deep region. The compressive residual stress near the surface cannot be increased. On the other hand, in the case of micro shot peening using shot particles having a particle diameter of 0.1 mm or less,
Although the peening effect cannot be obtained in the deep region (30 μm or more), it is possible to increase the compressive residual stress near the surface. Therefore, the compressive residual stress can be applied to the surface layer by adopting the micro shot peening of φ0.1 mm or less. Further, the surface erosion effect by micro shot peening of φ0.1 mm or less can reduce the grain boundary oxidation depth of the surface layer (for example, 15 μm → 9 μm), and can eliminate the decrease in fatigue strength due to grain boundary oxidation. .

Furthermore, by adding micro shot peening of φ0.1 mm or less to the fine carbide dispersion by the gas carburizing and nitrogenizing heat treatment, the surface hardness can be improved without deteriorating the surface roughness.

As described above, according to the present invention, strengthening by fine carbide dispersion by gas carburizing and carbonitriding heat treatment, that is, formation of fine carbide-dispersed carburized layer (0.15 mm or more) and strength by containing 25 to 45% of retained austenite. In addition to the strengthening, the strength of the surface layer portion is strengthened by micro shot peening with a diameter of 0.1 mm or less, so that the surface pressure resistance and bending fatigue strength can be significantly increased.

Thus, for example, in a gear, the root notch bending fatigue strength and the tooth surface anti-pitting strength can be remarkably improved as compared with the conventional one, and the processing cost per transmission torque can be greatly reduced. The effect of becoming can be achieved.

Moreover, since it is not necessary to perform the gas carburizing and nitriding heat treatment and the two-step peening treatment in complicated steps, it is possible to provide a mechanical structural component excellent in surface pressure resistance and bending fatigue strength at low cost.

In the mechanical structural component of the present invention, a vulcanized layer of 3 to 5 μm may be laminated on the surface of the surface layer by room temperature electrolytic vulcanization treatment. If the vulcanized layer is formed on the surface of the surface in this way, the vulcanized layer remains in the surface recesses due to sliding, so that the smoothness of the surface of the surface can be improved. Is further reduced, so that the surface pressure resistance can be further improved.

[0025]

First, a method for manufacturing a mechanical structural component of the present invention will be described.

The composition range of the applicable material (steel) is such that the content of Cr, which is a carbide forming element, is 0.85% necessary for forming carbide.
In order to avoid deterioration of machinability due to bainite formation during normalizing, the upper limit of Cr content is 1.25%,
The upper limit of the Mo content, which is a toughness improving element, is 0.5%.

Materials of the above components (Mo content of 0.15
.About.0.30%) is subjected to the following gas carburizing and carbonitriding heat treatment and micro shot peening to obtain high strength mechanical structural parts.

<Gas Carburizing and Nitrogenizing Heat Treatment> The gas carburizing and nitrogenizing heat treatment is performed in the pattern shown in FIG. The process will be described in detail. First, a material (for example, SCM420H) is heated to a temperature above the transformation point Ar1 of 930 ° C. and kept at that temperature for 4.5 hours for carburizing (carbon potential C
p: 0.90 to 1.35%). Next, transformation point A
r1 or lower temperature: Reheated after cooling to 650 ° C., and fine carbide dispersion is performed by quenching and soaking at a temperature of 850 ° C. (0.5 hours, carbon potential Cp: 0.90%). In quenching and uniform heat treatment 1.
Nitrogen is infiltrated by adding 8% NH ₃ (0.5 hour) (nitriding treatment). Then, after oil cooling (90 ° C.), reheating is performed and tempering (200 ° C., 2 hours) is performed.

By the above gas carburizing and nitrogenizing heat treatment,
As shown in the schematic diagram of FIG. 1, a fine carbide-dispersed carburized layer 2 formed by gas carburizing can be formed to have a thickness of 0.15 mm or more,
Further, the residual austenite is reduced to 25 to 45 by nitrogen infiltration.
% Can be included.

<Micro shot peening> After the above carburizing and nitriding heat treatment, micro shot peening using shot grains with a grain size of 0.1 mm or less is performed.
The strength of the surface layer 1 on the fine carbide-dispersed carburized layer 2 is strengthened.

When such micro shot peening is carried out, the surface layer 1 on the fine carbide dispersed carburized layer 2 can have a maximum grain boundary oxidation depth of 9 μm due to its surface erosion action (5 to 6 μm). It is possible to eliminate the decrease in fatigue strength due to grain boundary oxidation. In addition, near the surface (~ 15μ
The residual stress of m) can be 1000 N / mm ² or more (FIGS. 3 and 4).

Furthermore, by using shot particles having a particle diameter of 0.1 mm or less, the surface hardness can be set to 900 HV or more with the surface roughness suppressed to Ry 3 μm or less (FIG. 5). The reason will be specifically described with reference to FIG.
As shown in, when micro shot peening is performed using the shot particles P of φ55 μm, the indentation D by the shot particles P is about 25 μm. In that case, the depth of the indentation D is geometrically about 3 μm, and therefore the surface roughness is Ry.
It can be suppressed to 3 μm or less.

Next, specific examples of the present invention will be described together with comparative examples.

<Example 1-1> SCM420H (Cr content: 0.85-1.25%, Mo content: 0.15-
(0.30%) as a raw material, fine carbide dispersion was performed by the above-mentioned gas carburizing and carbonitriding heat treatment (heat treatment condition: FIG. 6), and then micro shot peening was performed using shot grains with a grain diameter of 0.1 mm or less. A test piece was produced.

<Comparative Example 1-1> Using SCM420H as a material, only a fine carbide was dispersed by the above-mentioned gas carburizing and carbonitriding heat treatment (heat treatment condition: FIG. 6) to prepare a test piece.

<Comparative Example 1-2> Using SCM420H as a raw material, a gas carburizing and nitriding heat treatment (without fine carbide dispersion) having a pattern shown in FIG. 7 was performed to prepare a test piece.

With respect to the test pieces prepared in Example 1-1 and the test pieces prepared in Comparative Examples 1-1 and 1-2, a notch (shape factor α = 2.
The test piece with 4) was subjected to a 10 ⁷ times rotational bending fatigue test. The test results are shown in FIG.

As is clear from the test results of FIG. 8, Example 1-1 is 1.6 times as compared with Comparative Example 1-2 (carburizing and nitrifying).
It can be seen that double bending fatigue strength is obtained. Further, from the comparison between Comparative Example 1-1 and Comparative Example 1-2, it can be seen that the bending fatigue strength is lower than that of the gas carburized and nitrided material when fine carbide dispersion is performed by the gas carburized and nitrogenized heat treatment. This is considered to be due to the notch of the dispersed carbide.

Further, the test pieces prepared in Example 1-1 and the test pieces prepared in Comparative Examples 1-1 and 1-2 were evaluated for pitching resistance by a roller pitching test. The evaluation result is shown in FIG. Evaluation test is 6
1.6 L / m of 0 ° C engine lubricating oil (SAE # 20)
With the in-dripping state, the slip rate was added at -33%, and the rotation speed was 1350 rpm.

As is clear from the evaluation results of FIG. 9, Example 1-1 is 1.4 compared with Comparative Example 1-2 (carburizing and nitrifying).
It can be seen that double the anti-pitting strength is obtained. Also,
From the comparison between Comparative Example 1-1 and Comparative Example 1-2, it can be seen that the pitting resistance is not improved even if fine carbide dispersion is performed by the gas carburizing and carbonitriding heat treatment.

<Example 2-1> A helical gear (module 2.5) for power transmission of a marine transmission was manufactured by performing the same process as in Example 1-1.

<Comparative Example 2-1> A helical gear (module 2.5) for power transmission of a marine transmission was manufactured by performing the same treatment as that of Comparative Example 1-2.

The allowable transmission torques of the helical gears produced in the above Example 2-1 and the helical gears produced in Comparative Example 2-1 were evaluated with a power circulation type gear testing machine. The evaluation result is shown in FIG. The evaluation test uses engine lubricating oil (SAE # 30) at 80 ° C. and a rotation speed of 2600 rp.
m.

As is clear from the evaluation results of FIG. 10, Example 2-1 was compared with Comparative Example 2-1 (carburizing and nitrifying).
It can be seen that it has a transmission torque of 5 times.

[0045]

As described above, according to the present invention,
In addition to strengthening the strength of fine carbide dispersion by gas carburizing and nitriding heat treatment, the strength of the surface layer is strengthened by micro shot peening, providing inexpensive mechanical structural parts with excellent surface pressure resistance and bending fatigue strength. can do.

Therefore, by applying the present invention to a gear,
Notch root bending fatigue strength and tooth surface anti-pitting strength can be significantly improved compared to the past, and the processing cost per transmission torque can be significantly reduced, and the transmission can be made lightweight and compact. Cost reduction can be achieved.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a structure of a main part of a mechanical structural component of the present invention.

FIG. 2 is a graph showing the relationship between shot grain size and compressive residual stress distribution in shot peening.

FIG. 3 is a graph showing the relationship between the grain boundary oxidation depth and fatigue strength when micro shot peening is adopted.

FIG. 4 is a graph showing changes in surface residual stress when micro shot peening is added.

FIG. 5 is a graph showing changes in surface hardness when micro shot peening is added.

FIG. 6 is a diagram showing processing conditions for gas carburizing and nitrogenizing heat treatment set in an example of the present invention.

FIG. 7 is a diagram showing processing conditions for gas carburizing and nitrogenizing heat treatment set in a comparative example of the present invention.

FIG. 8 is a graph showing test results of bending fatigue tests of Examples and Comparative Examples of the present invention.

FIG. 9 is a graph showing the evaluation results of pitting resistance of Examples and Comparative Examples of the present invention.

FIG. 10 is a graph showing the evaluation results of the allowable transmission torque of the example of the present invention and the comparative example.

FIG. 11 is a view schematically showing a structure of a main part when a fine carbide dispersion is applied to a steel material by a gas carbonitriding heat treatment.

FIG. 12 is a graph showing a compressive residual stress distribution by general shot peening.

FIG. 13 is a diagram schematically showing the size of an indentation when performing micro shot peening using shot particles of φ55 μm.

[Explanation of symbols]

1 surface 2 Fine carbide dispersion carburized layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masahiro Nakamura             1-32 Chayamachi, Kita-ku, Osaka Yanmadi             Inside the company F-term (reference) 4K028 AA03 AB01 AC08

Claims (2)

[Claims] 1. A mechanical structural part made of steel, wherein a fine carbide-dispersed carburized layer formed by gas carburizing is formed to a thickness of 0.15 mm or more, and residual austenite is 25 to 4 due to nitrogen infiltration.
While containing 5%, by micro shot peening for the upper layer portion of the fine carbide-dispersed carburized layer, the surface roughness is kept at Ry 3 μm or less,
A mechanical structural component having a surface compressive residual stress of 1000 N / mm ² or more and a surface hardness of 900 HV or more. 2. A Cr content of 0.85 to 1.25%,
The mechanical structural component according to claim 1, wherein a steel having a Mo content of 0.5% or less is used as a raw material.