JPH06158158A - Production of coil spring - Google Patents

Abstract

PURPOSE:To provide the method for producing a coil spring by which high residual stress is applicable even to the surface part of a spring subjected to nitriding treatment and hardened and to furthermore improve the strength and fatigue resistance of the coil spring. CONSTITUTION:In the method for producing a coil spring in which steel wire rod is successively subjected to coiling, nitriding treatment and shot peening, and residual stress is applied thereto, the shot peening is executed by using shot grains having 0.8mm grain size and 1250Hv and constituted of zirconia having hardness higher than that of the surface part of the coil subjected to nitriding treatment. Thus, residual stress can be applied in such a manner that it is distributed over the relatively deep part from the topmost surface of the coil spring, and higher residual stress can be applied as it comes up to the surface part.

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 spring having high strength and fatigue resistance such as a valve spring used in an automobile engine.

[0002]

2. Description of the Related Art As a conventional method for producing a spring having high strength and high fatigue resistance, coiling molding, heat treatment, shot peening treatment is performed using a wire material having high tensile strength,
There is known a method of performing a polishing process and then performing each step of reducing the maximum surface roughness. Also, JP-A-63-76
Japanese Patent Publication No. 730 describes a method of manufacturing a coil spring in which a steel wire rod is coiled, nitrided, and then shot peened in several stages in which the shot grain size of the shot peening is successively reduced.

[0003]

Even by the conventional shot peening treatment, residual stress is generated on the surface of the coil spring, and the strength and fatigue resistance of the coil spring can be improved. It is an object of the present invention to provide a method for manufacturing a coil spring that can give a higher residual stress than that of the conventional processing method, and further improve the strength and fatigue resistance of the coil spring.

[0004]

The inventors of the present invention investigated the relationship between the hardness of the surface portion of the material to be treated and the hardness of shot grains used for shot peening. As a result, when the shot peening treatment is performed by using shot grains having a hardness higher than the hardness of the surface portion of the material to be treated after the nitriding treatment, it is possible to give a wide residual stress from the surface of the material to be processed deep inside, and The inventors have completed the present invention by finding that a large residual stress can be applied to the surface.

That is, in the method for manufacturing a coil spring of the present invention, in the method for manufacturing a coil spring in which coil wire forming, nitriding treatment and shot peening are sequentially performed on a steel wire material to give residual stress, the shot peening is performed by the nitriding. It is characterized in that it is carried out by using shot particles made of ceramics having a hardness higher than that of the surface of the coil which has been treated.

In the coil spring manufacturing method of the present invention, shot grains made of ceramics having a hardness higher than the hardness of the surface portion of the coil subjected to nitriding treatment after coiling forming of steel wire and nitriding treatment are used. By performing shot peening, high residual stress is applied to the surface of the coil spring. This significantly improves the strength and fatigue resistance of the coil spring.

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 and the hardness of the surface portion is increased. In particular, alloy steel oil tempered wires containing molybdenum and vanadium and hard drawn alloy steel wires which have been conventionally used for high strength springs are suitable. Further, in order to facilitate coiling molding, it is preferable that the wire as described above has an oxide film.

The alloy steel oil tempered wire for a high strength spring is cold coiled. Therefore,
After that, it is preferable to remove the residual stress and residual strain generated during coil forming by low temperature heat treatment, for example, low temperature annealing. Furthermore, when using a wire with an oxide film, before performing nitriding treatment after molding,
The surface of the spring material is preferably descaled. This descaling process is a process of removing the oxide film on the surface of the coiled spring material. By performing the descaling treatment to remove the oxide film from the surface of the spring material, the surface portion of the spring material can be nitrided more uniformly. In this descaling process, it is preferable to make the surface of the spring material as smooth as possible. If the surface of the spring material is not smooth, the uniformity of nitriding becomes insufficient, and the obtained coil spring needs to be surface-polished.

The above descaling treatment can be carried out, for example, by electrolytic polishing, pickling, shot blasting, shot peening and the like. When performing the descaling treatment by shot blasting, shot peening, etc., it is necessary to blast or shot the shot grains relatively weakly so as not to increase the surface roughness of the spring material. For example,
When performing descaling by shot peening, the surface of the spring material can be smoothed by using relatively soft glass beads as shot particles or by using steel shot with a diameter of 0.3 mm or less. . By performing descaling by shot blasting, shot peening, etc., the oxide film can be removed from the surface of the spring material, and the surface of the spring material is activated, facilitating the next step, nitriding. Will be able to be carried out.

The nitriding treatment takes about 0.2 from the surface of the spring material.
Nitriding to a depth of about mm, the hardness of the surface from 0.05 to 0.1 mm from the surface is 850 to 600 Hv.
It is a degree. This nitriding treatment can be performed in the same manner as the conventional one. For example, a predetermined nitride layer can be formed on the surface of the spring material by treating the spring material at 420 to 550 ° C. for 2 to 6 hours in an ammonia atmosphere.

Shot peening imparts a wide residual stress from the surface of the spring material hardened by nitriding the surface to a deep inside thereof, and also imparts a large residual stress to the surface of the spring material, thereby improving the resistance of the coil spring. It improves fatigue. Therefore, in the coil spring manufacturing method of the present invention, shot peening is performed using shot particles made of ceramics having a hardness higher than the hardness of the surface portion of the nitrided spring material. Zirconium oxide (Z
Zirconia grains obtained by stabilizing rO ₂ ) with yttrium oxide (Y ₂ O ₃ ), magnesium oxide (MgO) or calcium oxide (CaO), zirconium oxide (ZrO ₂ ).
And silicon oxide (SiO ₂₎ consisting of a zircon grain, corundum grains of aluminum oxide (Al ₂ O _3), aluminum oxide (Al ₂ O ₃₎ of silicon oxide (Si
Examples thereof include mullite grains composed of O ₂ ) and high-purity zirconia grains composed of 95% or more zirconium oxide (ZrO ₂ ).

This shot peening is performed by, for example, H
It can be carried out using shot particles made of ceramics with _v of 850 or more. If it is less than this hardness, the application of residual stress by shot peening is insufficient. Further, this shot peening has a grain size of, for example, 0.
It can be carried out using shot particles made of ceramics of 2 mm or more, preferably 0.2 to 1.5 mm. When the particle size of the shot particles is less than 0.2 mm,
Sufficient residual stress cannot be applied over a wide range from the surface of the spring material hardened by the nitriding treatment to deep inside, and the fatigue resistance of the coil spring cannot be sufficiently improved. If the grain size of the shot grains becomes too large, the residual stress is applied deep inside, but only a small residual stress can be applied to the surface. Therefore, the surface of the obtained coil spring is broken due to fatigue, and the fatigue resistance of the coil spring cannot be sufficiently improved.

After performing the above-mentioned shot peening, the spring material is subjected to a low temperature heat treatment to make the metallographic structure of the outermost surface of the spring material uniform. This low temperature heat treatment can be performed, for example, by performing low temperature annealing at 250 ° C. for about 15 minutes. After the shot peening is performed, a shot grain having a smaller shot energy is used, such as a shot grain having a diameter smaller than that of the shot grain made of the ceramic used for the shot peening. May be carried out. If such second-stage shot peening is performed, the residual stress is broadly applied from the surface to the deep inside by the first-stage shot peening,
Moreover, a larger residual stress can be applied to the surface of the spring material having a large residual stress applied to the surface thereof, and the fatigue resistance of the coil spring can be further improved. In addition, after the second stage shot peening is performed, the spring material is subjected to the low temperature heat treatment as described above to homogenize the metal structure of the outermost surface portion of the spring material.

[0014]

In the method of manufacturing the coil spring according to the present invention, the spring material whose surface is hardened by the nitriding treatment is made of a ceramic material having a hardness higher than the hardness of the surface of the nitriding-treated spring material. Shot peening is performed using the grains. As a result, it is possible to apply a wide residual stress to the spring material whose surface is hardened, from the surface to a deep inside, and to give a large residual stress to the surface. That is, the residual stress applied by the method for manufacturing a coil spring of the present invention is distributed from the outermost surface of the coil spring to a relatively deep portion, and becomes larger at a portion closer to the surface. Therefore, according to the coil spring manufacturing method of the present invention, it is possible to manufacture a coil spring having higher strength and fatigue resistance than ever before.

[0015]

EXAMPLES The method for manufacturing the coil spring of the present invention will be specifically described below with reference to examples. (Example) As a wire material for a coil spring, vanadium-added high-strength silicon-chromium alloy steel for valve springs, that is, 0.53% by weight of carbon (hereinafter,% means% by weight unless otherwise specified). , Silicon 0.23%, manganese 0.8%, phosphorus 0.011%, sulfur 0.008%, chromium 0.96%, vanadium 0.2%, copper 0.06%,
The alloy steel consisting of the balance iron was oil tempered to obtain an alloy steel oil tempered wire having a tensile strength σ _B = 2104 Mpa.

This wire is coiled to a wire diameter of 3.2.
mm, coil center diameter 20.0 mm, total number of turns 6.0, effective number of turns 4.0, free height 47.0 ± 0.2 mm, and spring constant 3.28 kgf / mm. The spring material is subjected to low temperature annealing at 450 ° C. for 15 minutes. After that, both ends of the spring material were ground in order to improve the seatability at the time of mounting.

Then, coiling forming, low temperature annealing treatment,
Grinded spring material with a diameter of 0.2 mm, Hv
Micro shot peening was carried out for 15 minutes using a steel ball of No. 500 to perform a surface activation treatment for removing an oxide film on the surface of the spring material. Next, the spring material subjected to such surface activation treatment is subjected to gas nitriding treatment at 450 ° C. for 6 hours in an ammonia gas atmosphere,
A nitride layer was formed on the surface of the spring material.

Finally, shot peening was carried out for 30 minutes by using shot grains made of zirconia having a Hv of 1250 and a grain size of 0.8 mm as the nitriding spring material. The shot particles made of zirconia were sprayed with an air shot machine at a projection pressure of 5 kgf /
Projected onto the spring material at cm ² . Furthermore, the spring material subjected to such shot peening was subjected to low temperature annealing at 250 ° C. for 15 minutes to homogenize the metal structure of the outermost surface portion of the spring material. The above-mentioned treatments were sequentially carried out to manufacture the coil spring of the example in which the compressive residual stress was applied to the surface portion. (Comparative example 1) Hv of 50 is applied to a spring material that has been subjected to nitriding
A coil spring of Comparative Example 1 was manufactured by the same manufacturing method as that of Example except that shot peening was performed for 30 minutes using a cut wire having a diameter of 0 and a diameter of 0.8 mm. (Comparative Example 2) Hv of 70 was applied to the spring material that was subjected to nitriding
A coil spring of Comparative Example 2 was manufactured by the same manufacturing method as that of the example except that shot peening was performed for 30 minutes using a cut wire having a diameter of 0 and a diameter of 0.8 mm. (First Evaluation Test) With respect to the compressive residual stress of the surface portion of the coil spring of the example, the coil spring of Comparative Example 1 and the coil spring of Comparative Example 2 manufactured as described above by the X-ray residual stress measuring method. The respective characteristic values were measured. The measurement results are shown in Table 1 below. The distribution of the compressive residual stress on the surface of these coil springs is shown in FIG. In addition, prior to the measurement of each characteristic value related to the compressive residual stress of the surface portion of each coil spring, a spring material only subjected to micro shot peening using a steel ball for surface activation treatment,
That is, each characteristic value relating to the compressive residual stress of the surface portion of the spring material before nitriding treatment and each characteristic value relating to the compressive residual stress of the surface portion of the spring material after nitriding treatment were measured in the same manner. . The measurement results are also shown in Table 1 below.

[0019]

[Table 1] As is clear from Table 1 and FIG. 1, on the outermost surface of the coil spring, the compressive residual stress applied to the coil springs of the examples is equal to the compressive residual stress applied to the coil springs of Comparative Example 1 and Comparative Example 2. And there is almost no difference. However, the maximum value of the compressive residual stress applied to the coil springs of the examples and the crossing points are significantly higher than those of the compressive residual stress applied to the coil springs of Comparative Examples 1 and 2. Here, the crossing point of the compressive residual stress means the depth at which the compressive residual stress applied by shot peening becomes zero, and is a measure of how deep the compressive residual stress is applied from the surface of the coil spring. .

That is, as shown by solid □ in FIG.
In the coil spring of the example, the compressive residual stress is distributed over a wide range from the surface to the inside deep, and a large residual stress is applied to the surface. Therefore, the coil spring manufactured by the manufacturing method of the embodiment has high strength and excellent fatigue resistance. Also,
The coil springs of Examples, Comparative Examples 1 and 2 are
The surface roughness R _Z was almost the same as that of the spring material after the nitriding treatment. (Second evaluation test) Using a star-type fatigue tester, fatigue resistance tests of the coil spring of the example, the coil spring of the comparative example 1 and the coil spring of the comparative example 2 were performed. In this fatigue resistance test, the fatigue limit τ _a (MPa) was measured when an average stress of 700 MPa was applied to each coil spring by repeating 1 × 10 ⁷ times. The measurement results are shown in Table 2 below.

[0021]

[Table 2] As is clear from Table 2, the fatigue limits of the coil springs of the examples are significantly higher than the fatigue limits of the coil springs of Comparative Example 1 and Comparative Example 2. Therefore, the coil spring manufactured by the coil spring manufacturing method of the embodiment has high strength and excellent fatigue resistance.

[0022]

As described above in detail, in the coil spring manufacturing method of the present invention, the hardness of the spring material whose surface portion is hardened by the nitriding treatment is higher than the hardness of the surface portion of the nitriding-treated spring material. Shot peening is performed using shot particles made of ceramics having hardness.
As a result, according to the method for manufacturing a coil spring of the present invention, residual stress can be distributed and applied from the outermost surface to a relatively deep portion of the coil spring efficiently and without increasing the number of steps required for manufacturing. In addition, a larger residual stress can be applied to the portion closer to the surface. Therefore, according to the coil spring manufacturing method of the present invention, it is possible to manufacture a coil spring having higher strength and fatigue resistance than conventional ones with high productivity and at low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a distribution of a compressive residual stress applied to a surface portion of a coil spring by a method for manufacturing a coil spring according to an example, a comparative example 1 and a comparative example 2.

Front page continuation (72) Inventor Toru Imura 2-58 Higashiyama Motomachi, Chikusa-ku, Nagoya-shi Higashiyama Corporus 102

Claims (1)

[Claims] 1. A method of manufacturing a coil spring in which a coiling process, a nitriding process, and a shot peening are sequentially performed on a steel wire rod to give residual stress, wherein the shot peening is a surface portion of the coil subjected to the nitriding process. A method for manufacturing a coil spring, which is carried out by using shot grains made of ceramics having a hardness higher than that of the above.