JP2810799B2 - Manufacturing method of coil spring - Google Patents

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 for an automobile engine.

[0002]

2. Description of the Related Art As a method of manufacturing a high-strength, high-fatigue-resistant spring, a wire having a high tensile strength is used, coiled and formed.
A method is known in which heat treatment is performed, a residual stress imparting process is performed by shot peening, and then each process is performed to reduce the maximum surface roughness by performing a polishing process. Also, Japanese Patent Application Laid-Open No.
No. 29422 discloses that a high strength spring is manufactured by using a silicon chromium clean steel wire, coiling, heat-treating, applying a residual stress by shot peening, and then polishing to reduce the maximum surface roughness Rmax to 5 μm or less. A method is described.

[0003]

The conventional method of applying a residual stress by shot peening and then polishing the surface to obtain a maximum surface roughness of Rmax 5 μm requires a relatively large surface unevenness due to shot peening. There is a problem that the polishing has to be thick and the man-hour is increased.

[0004] The present invention is to provide a method of manufacturing a coil spring which can reduce the manufacturing cost and has higher fatigue resistance.

[0005]

SUMMARY OF THE INVENTION The present invention provides a nitriding treatment for a coiled molded product 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 a residual stress applying treatment. Are found to be effective, and the present invention has been completed. That is,
In the method for manufacturing a coil spring according to the present invention, a steel wire having an oxide film is coiled, heat-treated, and subjected to a maximum surface roughness R.
It is characterized in that it is subjected to a descaling treatment, a nitriding treatment, and a residual stress imparting treatment by shot peening so as to have a maximum of 5 μm or less.

[0006] The coiled molded product is subjected to the maximum surface roughness Rmax5.
By performing nitriding after de-scaling to less than μm, the increase in roughness of the spring surface due to shot peening during the treatment of residual stress is suppressed, eliminating the need for polishing, and the cost required for polishing. , So that the manufacturing cost can be greatly reduced. In addition, since the spring surface is nitrided and hardened by the nitriding treatment, the surface is less likely to be scratched, and the durability is improved accordingly.

The wire used in the method of manufacturing a coil spring according to the present invention is a steel material whose surface is nitrided by nitriding to increase the hardness of the surface. In particular, alloy steel oil-tempered wire or alloy steel hard drawn wire conventionally used for high-strength springs is suitable. Such a wire must have an oxide film. The oxide film has the function of facilitating the subsequent coil forming.

[0008] The heat treatment is performed to remove residual stress and residual strain and to increase the hardness. Specifically, the alloy steel oil-tempered wire is subjected to, for example, low-temperature annealing at 420 ° C for 30 minutes. And remove residual stress and distortion. The hardened wire of the alloy is quenched and tempered 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 subjected to the coil forming and heat treatment. Removing the oxide film enables a more uniform nitriding treatment.

In the descaling process, the maximum surface roughness of the spring material must be less than Rmax 5 μm.
When the maximum surface roughness exceeds Rmax of 5 μm, the uniformity of nitriding becomes insufficient and the surface needs to be polished. The descaling treatment can be performed by electrolytic polishing, pickling (for example, immersion 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 a condition for relatively weak 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 fine cut wires with a diameter of 0.3 mm or less, or using steel shots with a diameter of 0.3 mm or less, the maximum surface roughness of the spring material can be reduced to Rmax 5 μm or less. Can be

By performing the descaling by shot blasting and shot peening, nitriding in the next step becomes easy in addition to removing the oxide film. In the nitriding treatment, nitriding is performed to a depth of about 0.2 mm from the surface.
The hardness of the surface portion from 05 to 0.1 mm is about Hv 800 to 900. The nitriding treatment itself can be carried out in the same manner as in the prior art. For example, a predetermined nitrided layer can be formed by treating in an ammonia atmosphere at 420 to 550 ° C. for 2 to 6 hours.

The processing for applying residual stress by shot peening is basically the same as in the prior art. Preferably, the residual stress is applied as deep as possible to the surface. According to the method of the present invention, a compressive stress which is highest at the surface and is distributed deeper than the surface compared with the conventional method is applied.

[0012]

【Example】

Example 1 As a wire rod of a coil spring, 0.64% by weight of carbon (hereinafter, referred to as carbon)
% Means wt% unless otherwise specified), 1.43% silicon, 0.67% manganese, 0.015% phosphorus,
0.006% sulfur, 1.57% chromium, 0.1% molybdenum.
An alloy steel consisting of 57%, 0.26% vanadium and the balance iron is oil-tempered and has a tensile strength σ _B = 209 kgf / mm.
The alloy steel oil tempered wire of No. ² was used.

[0013] This wire rod is coiled to have a wire diameter of 3.2 mm.
Coil center diameter 21.2mm, total number of turns 6.5, effective number of turns 4.5, free height 50mm, spring constant 2.445kgf / mm
Formed into a coil spring of No. ² . Next, this coil spring is
Heat treatment was performed at 0 ° C. for 30 minutes to perform low-temperature annealing. Thereafter, using a steel ball having a diameter of 0.2 mm, microshot peening was performed for 10 minutes to remove an oxide film on the surface. The surface roughness of the coil in this state is Rmax
It was 2.5 μm.

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

The surface roughness of this coil spring is Rmax 2.6.
μm. The fatigue strength of the coil spring at 5 × 10 ⁷ times was 60 ± 57 kgf / mm ² . For comparison, three types of coil springs of Comparative Example 1, Comparative Example 2, and Comparative Example 3 shown below were made using the same alloy steel oil-tempered wire. The coil spring of Comparative Example 1 is obtained by omitting the micro shot peening process and the nitriding process for removing the oxide film of Example 1. Comparative Example 1 was the same as Example 1 except for the removal of the oxide film and the nitriding treatment.
A coil spring was obtained. The coil spring of Comparative Example 1 had a surface roughness Rmax of 10 μm, and the coil spring had a fatigue strength of 5 × 10 ⁷ times of 60 ± 48 kgf / mm ² . The coil spring of Comparative Example 1 did not perform the oxide film removal treatment and the nitriding treatment, and did not perform the treatment for surface smoothing after the shot peening for imparting the residual stress. Therefore, the coil spring of Comparative Example 1 has an advantage that the manufacturing process is simple and the manufacturing cost is low. On the other hand, the surface roughness of the coil spring of Comparative Example 1 is larger than the surface roughness of the coil spring of Example 1. Further, the fatigue strength of the coil spring of Comparative Example 1 is much lower than the fatigue strength of the coil spring of Example 1.

The coil spring of Comparative Example 2 was electrolytically polished from the coil spring of Comparative Example 1 to reduce its surface roughness to Rmax 3.
5 μm. That is, the coil spring of Comparative Example 2 was produced 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 at 5 × 10 ⁷ times was 60 ± 51 kgf / mm ² . The coil spring of Comparative Example 2 has improved fatigue strength because of the surface smoothing treatment, but 5 × 10 ^7.
The fatigue strength per cycle is still as low as 60 ± 51 kgf / mm ² . This is considered to be because 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, shot peening was performed for 30 minutes using a cut wire having a diameter of 0.8 mm in place of the micro shot peening for removing the oxide film of Example 1, and other steps were performed. Is Example 1
A coil spring was obtained in exactly the same way as in the first embodiment. The surface roughness of this coil spring is Rmax 8 μm and 5 × 1
0 ⁷ times of fatigue strength was 60 ± 52kgf / mm ^2. The surface roughness of this coil spring after the oxide film removing step was Rmax 9.0 μm. Since the surface roughness of the coil spring of Comparative Example 3 is rough in the oxide film removing step, the surface roughness of the final product coil spring is also rough. Therefore, the 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. By performing the nitriding treatment while keeping the roughness low, the surface roughness of the final product coil spring can be reduced and the fatigue strength can be increased.

Example 2 An alloy steel of the same material as that of the wire of Example 1 was hard drawn to obtain an alloy steel hard drawn wire having a tensile strength σ _B = 135 kgf / mm ² , which was used as the wire. Exactly 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, 5 × 10 ⁷ fatigue strength 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 this embodiment, since the surface roughness in the oxide film removing step is reduced, the surface roughness of the coil spring of the final product is reduced to Rma.
x could be as low as 3.2 μm and the fatigue strength could be increased.

[0021]

According to the method of manufacturing the coil spring of the present invention, the surface roughness Rmax after the oxide film removing step is reduced to 5 μm or less, and a nitriding treatment is performed thereafter. 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 applying step, and a coil spring having high fatigue strength can be manufactured without the need for a surface polishing step for smoothing the coil spring surface. Can also. Therefore, the manufacturing process can be shortened and the manufacturing cost can be significantly reduced.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F16F 1/06 F16F 1/06 B // C21D 7/06 C21D 7/06 A 9/02 9/02 A (72) Inventor Mitsuaki Nakanishi 1 Hiruki, Haruki, Togo-cho, Aichi-gun, Aichi Prefecture Inside Togo Seisakusho Co., Ltd. (72) Inventor Satoru Kondo 1st Hiruki, Haruki, Oaza-cho, Togo-cho, Aichi Gun, Aichi Prefecture Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor Osamu Nakano 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Takashi Uchida Otemachi, Chiyoda-ku, Tokyo 2-6-3 Nippon Steel Corporation (72) Inventor Mitsuyoshi Onoda 7-5-1, Higashi Narashino, Narashino City, Chiba Prefecture Suzuki Metal Industry Co., Ltd. (56) Reference JP-A-2-129422 (JP, A) JP-A-64-31927 (JP, A) JP-A-63-227791 (JP, A) JP-A-63-176430 (JP, A) JP-A-2-125813 (JP, A) JP-A-52-21551 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23F 17/00 B21F 35/00 C23C 8/26 F01L 3/10 F16F 1 / 02 F16F 1/06 C21D 7/06 C21D 9/02 C21D 9/52 B24C 1/10

Claims (4)

(57) [Claims] 1. A steel wire having an oxide film is coil-formed, heat-treated, descaled so as to have a maximum surface roughness Rmax of 5 μm or less, nitrided, and subjected to residual stress applying treatment by shot peening. Manufacturing method of coil spring. 2. The method for manufacturing a coil spring according to claim 1, wherein the steel wire is an alloy steel oil-tempered wire, and the heat treatment is a low-temperature annealing treatment. 3. The method for manufacturing a coil spring according to claim 1, wherein the steel wire is an alloy steel hard drawn wire, and the heat treatment is quenching and tempering. 4. The method according to claim 1, wherein the descaling is performed by shot peening.