JPH05339763A - Coil spring manufacturing method - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for manufacturing a coil spring which does not require a polishing step, has a simplified manufacturing process, is inexpensive, and has high fatigue strength. A coil spring manufacturing method of the present invention comprises coiling an iron and steel wire rod having an oxide film, heat-treating it, descaling it to a maximum surface roughness Rmax of 5 μm or less, nitriding it, and residual stress by shot peening. The one that does the granting process. Surface roughness is kept low during the descaling process, and then nitriding is performed. Therefore, the increase in surface roughness due to shot peening in the residual stress application step is effectively suppressed, and the surface roughness is suppressed to a low level.
Therefore, the surface polishing step is not necessary after the residual stress imparting step.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high strength and high fatigue resistance spring such as a valve spring used in an automobile engine.

[0002]

2. Description of the Related Art As a method for producing a high-strength and fatigue-resistant spring, a wire having high tensile strength is used, coiled,
A method is known in which heat treatment is performed, residual stress is applied by shot peening, and then polishing is performed to perform each step of reducing the maximum surface roughness. In addition, Japanese Patent Laid-Open No. 2-1
In Japanese Patent No. 29422, a high-strength spring is manufactured by using a silicon chrome clean steel wire, coiling it, heat-treating it, applying residual stress by shot peening, and then polishing it so that the maximum surface roughness Rmax is 5 μm or less. How to do is described.

[0003]

The conventional method of applying residual stress by shot peening and then polishing it to obtain a maximum surface roughness of Rmax of 5 μm results in relatively large surface irregularities due to shot peening. Since it has to be thickly polished, there is a problem that the number of steps is increased.

The present invention provides a method for manufacturing a coil spring which can reduce the manufacturing cost and has higher fatigue resistance.

[0005]

DISCLOSURE OF THE INVENTION According to the present invention, a coiling molded article which has been subjected to a predetermined treatment as a means for increasing fatigue resistance and a means for suppressing an increase in surface roughness due to shot peening during residual stress imparting treatment is subjected to a nitriding treatment. Has been found to be effective, thus completing the present invention. That is,
The coil spring manufacturing method according to the present invention is carried out by coiling a steel wire rod having an oxide film, heat-treating the steel wire rod, and then the maximum surface roughness R
It is characterized in that descaling is performed so that the maximum is 5 μm or less, nitriding is performed, and residual stress imparting is performed by shot peening.

The coiling molded product is subjected to surface maximum roughness Rmax 5
By descaling to μm or less and then nitriding, the increase in roughness of the spring surface due to shot peening during residual stress application is suppressed, and polishing is not required. Since the cost is not applied, the manufacturing cost can be significantly reduced. Further, the surface of the spring is nitrided and hardened by the nitriding treatment, so that the surface is less likely to be scratched and the durability is improved accordingly.

The wire used in the method for manufacturing the coil spring of the present invention is a steel material in which the surface portion is nitrided by the nitriding treatment to increase the hardness of the surface portion. In particular, alloy steel oil tempered wire or alloy steel hard drawn wire which has been conventionally used for high strength springs is suitable. Such a wire needs to have an oxide film. The oxide film has a function of facilitating coiling molding in the subsequent steps.

The heat treatment is performed to remove residual stress and residual strain and to increase hardness. Specifically, the alloy steel oil tempered wire is subjected to low temperature annealing treatment at 420 ° C. for 30 minutes, for example. The residual stress and strain are removed. Further, the hardened alloy wire is subjected to quenching and tempering treatment to increase its hardness. The descaling process is a process of removing the oxide film on the surface of the spring material which has been coiled and heat treated. By removing the oxide film, more uniform nitriding treatment becomes possible.

In the descaling process, the maximum surface roughness of the spring material needs to be Rmax 5 μm or less.
If the maximum surface roughness exceeds Rmax of 5 μm, the uniformity of nitriding will be insufficient and surface polishing will be necessary. The descaling treatment can be performed by electrolytic polishing, pickling (eg, dipping in dilute hydrochloric acid of about 5% for several minutes), shot blasting, shot peening, or the like. In shot blasting and shot peening, it is necessary to select conditions for relatively weakly blasting so as not to increase the surface roughness of the spring material. For example, in shot peening,
By using relatively soft glass beads or abrasive grains, using a fine cut wire with a diameter of 0.3 mm or less, or using steel shot with a diameter of 0.3 mm or less, the maximum surface roughness of the spring material is Rmax 5 μm or less Can be

By performing descaling by shot blasting and shot peening, the oxide film can be removed and nitriding in the next step is facilitated. The nitriding treatment is performed by nitriding to a depth of about 0.2 mm from the surface, and nitriding to a depth of about 0.2 mm.
The hardness of the surface portion from 05 to 0.1 mm is about Hv 800 to 900. The nitriding process itself can be performed in the same manner as the conventional one. For example, a predetermined nitride layer can be formed by performing a treatment at 420 to 550 ° C. for 2 to 6 hours in an ammonia atmosphere.

The residual stress imparting process by shot peening is basically the same as the conventional one. It is preferable to apply the residual stress as deep as possible to the surface. According to the method of the present invention, a compressive stress that is highest at the surface portion and distributed deeper than the surface is applied as compared with the conventional method.

[0012]

【Example】

Example 1 As a wire material for a coil spring, 0.64% by weight of carbon (hereinafter,
% Means% by weight unless otherwise specified), silicon 1.43%, manganese 0.67%, phosphorus 0.015%,
Sulfur 0.006%, chromium 1.57%, molybdenum 0.
Oil tempered alloy steel consisting of 57%, vanadium 0.26% and balance iron, tensile strength σ _B = 209 kgf / mm
The alloy steel oil tempered wire of ² was used.

By coiling this wire rod, the wire diameter is 3.2 mm,
Coil center diameter 21.2 mm, total number of turns 6.5, effective number of turns 4.5, free height 50 mm, spring constant 2.445 kgf / mm
Molded into ² coil springs. Next, replace this coil spring with 50
It heat-processed at 0 degreeC for 30 minutes, and performed low temperature annealing. Then, using a steel ball having a diameter of 0.2 mm, micro shot peening was carried out for 10 minutes to remove the oxide film on the surface. The surface roughness of the coil in this state is Rmax
It was 2.5 μm.

Next, in an atmosphere of ammonia gas at 500 ° C.,
Gas nitriding was performed for 6 hours to form a nitride layer on the coil surface. After that, the end surface of the coil was cut and finished, and then a shot wire having a diameter of 0.8 mm was used for shot peening for 60 minutes to give a compressive residual stress to the coil surface. Finally, low-temperature annealing was performed at 250 ° C. for 30 minutes to remove an abnormally large internal strain and suppress a decrease in elastic limit. As a result, the coil spring of this example was obtained.

The surface roughness of this coil spring is Rmax 2.6.
was μm. The fatigue strength of this coil spring 5 × 10 ⁷ times was 60 ± 57 kgf / mm ² . For comparison, the same alloy steel oil tempered wire was used to make three types of coil springs of Comparative Example 1, Comparative Example 2 and Comparative Example 3 shown below. The coil spring of Comparative Example 1 omits the micro shot peening treatment and the nitriding treatment for removing the oxide film of Example 1. Comparative Example 1 is the same as Example 1 except for these oxide film removal and nitriding treatments.
Got a coil spring. The surface roughness of the coil spring of Comparative Example 1 was Rmax of 10 μm, and the fatigue strength of this coil spring at 5 × 10 ⁷ times was 60 ± 48 kgf / mm ² . The coil spring of Comparative Example 1 was not subjected to the oxide film removing treatment and the nitriding treatment, and was not subjected to the surface smoothing treatment after the shot peening for giving the residual stress. Therefore, the coil spring of Comparative Example 1 has the advantages that the manufacturing process is simple and the manufacturing cost is low, but on the other hand, the surface roughness of the coil spring of Comparative Example 1 is larger than that of the coil spring of Example 1. Moreover, the fatigue strength of the coil spring of Comparative Example 1 is far lower than that of the coil spring of Example 1.

In the coil spring of Comparative Example 2, the coil spring of Comparative Example 1 was finally electropolished to have a surface roughness of Rmax 3.
It is 5 μm. That is, the coil spring of Comparative Example 2 was manufactured in the same manner as in Example 1 except that the oxide film removal treatment and the nitriding treatment were not performed and the surface of the coil spring was finally smoothed by electrolytic polishing. The fatigue strength of this coil spring after 5 × 10 ⁷ cycles was 60 ± 51 kgf / mm ² . The coil spring of Comparative Example 2 has improved fatigue strength because the surface smoothing treatment is performed, but it is 5 × 10 ^7.
Tire fatigue strength is still low at 60 ± 51 kgf / mm ² . It is considered that this is because the nitriding treatment was not performed and a part of the residual compressive stress layer was polished and removed by the surface smoothing treatment.

In the coil spring of Comparative Example 3, instead of the micro-shot peening for removing the oxide film of Example 1, 30-minute shot peening was performed using a cut wire having a diameter of 0.8 mm, and other steps were performed. Is Example 1
The coil spring is obtained in exactly the same manner as. The surface roughness of this coil spring is Rmax 8 μm and is 5 × 1.
The fatigue strength at 0 ⁷ times was 60 ± 52 kgf / mm ² . The surface roughness of the coil spring after the oxide film removing step was Rmax of 9.0 μm. The coil spring of Comparative Example 3 has a rough surface in the oxide film removing step, and thus the surface roughness of the coil spring of the final product is also rough. Therefore, fatigue strength is low.

As can be understood from the comparison between the coil spring of Example 1 and the coil spring of Comparative Example 3, the surface roughness in the oxide film removing step is closely related to the surface roughness of the final product, and the surface in the oxide film removing step is By suppressing the roughness to be low and performing the nitriding treatment, the surface roughness of the coil spring of the final product can be reduced and the fatigue strength can be increased.

Example 2 An alloy steel having the same material as that of the wire of Example 1 was hard-drawn, and an alloy steel hard-drawn wire having a tensile strength σ _B = 135 kgf / mm ² was used as the wire. The same process as in Example 1 except that this wire was used, and instead of low-temperature annealing after coiling, heating was performed at 930 ° C. for 7 minutes, followed by quenching by air cooling and further tempering at 450 ° C. for 20 minutes. Was carried out to obtain a coil spring of this example.

The surface roughness of the coil spring of this embodiment is Rmax.
3.2 μm, the fatigue strength of 5 × 10 ⁷ times is 60 ± 5
It was 6 kgf / mm ² . The surface roughness of the coil spring after the oxide film removing step was Rmax 3.0 μm. Also in the case of this embodiment, since the surface roughness in the oxide film removing step is low, the surface roughness of the coil spring of the final product is Rma.
x could be as low as 3.2 μm, and fatigue strength could be increased.

[0021]

According to the method of manufacturing a coil spring of the present invention, the surface roughness Rmax after the oxide film removing step is reduced to 5 μm or less, and then the nitriding treatment is performed. Therefore, the surface roughness of the coil spring surface can be made relatively low even in the shot peening step of the surface residual stress application step, and a coil spring having high fatigue strength can be manufactured without requiring a surface polishing step for smoothing the coil spring surface. You can also Therefore, the manufacturing process can be shortened and the manufacturing cost can be significantly reduced.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location F16F 1/06 B 8917-3J // C21D 7/06 A 7412-4K 9/02 A (72) Inventor Mitsuaki Nakanishi Togo Manufacturing Co., Ltd. 1 Haruki, Toki-machi, Aichi-gun, Aichi Prefecture, Togo Manufacturing Co., Ltd. (72) Inventor Satoshi Kondo 1 Toku-cho, Tochi-machi, Aichi-gun, Aichi Prefecture, Tochi Manufacturing Co., Ltd. (72) Inventor Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Osamu Nakano 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Naoshi Uchida Otemachi, Chiyoda-ku, Tokyo 2-6-3 Shin Nippon Steel Co., Ltd. (72) Inventor Mitsuyoshi Onoda 7-5-1 Higashi Narashino, Narashino City, Chiba Suzuki Metal Industry Co., Ltd. The inner

Claims (4)

[Claims] 1. A steel wire rod having an oxide film is coiled, heat-treated, descaled so that the maximum surface roughness Rmax is 5 μm or less, nitrided, and subjected to residual stress imparting by shot peening. Coil spring manufacturing method. 2. The method for producing a coil spring according to claim 1, wherein the steel wire rod is an alloy steel oil temper wire, and the heat treatment is a low temperature annealing treatment. 3. The method for producing a coil spring according to claim 1, wherein the steel wire rod is an alloy steel hard drawn wire, and the heat treatment is quenching and tempering treatment. 4. The method for manufacturing a coil spring according to claim 1, wherein the descaling process is performed by shot peening.