JPH0761519B2 - Method for manufacturing spring with excellent fatigue resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method for manufacturing a high-strength spring used in an automobile engine and other fields in which fatigue resistance is highly required.

<Prior Art> Fatigue resistance of a metal material increases with its hardness,
It is said that if the hardness exceeds a certain level, it will decrease due to the influence of surface defects and non-metallic inclusions contained in the material.

Therefore, it is considered that a material having high strength and few defects is advantageous for fatigue resistance.

For example, a valve spring used in an automobile engine is highly required to have fatigue resistance and heat resistance due to its harsh usage environment. In addition to high strength, high-carbon steel wire and low-alloy steel wire have been put into practical use as materials for this valve spring in consideration of heat resistance, coiling formability, economy, and the like.

Recently, among the above, SiCr has excellent fatigue resistance and heat resistance.
The use of steel wire is widespread. Further, development using high alloy steel wire such as maraging steel wire has been promoted, and spring steel wire tends to have higher strength.

In recent years, in combination with this increase in strength, surface care techniques such as peeling, reduction of surface flaws by countermeasures against flaw generation sources and reduction of non-metallic inclusions by establishing clean steel melting technology have improved material defects. The fatigue resistance of the valve spring is further improved.

<Problems to be solved by the invention> However, looking at the recent development trend of automobile engines,
It is required to have a higher output and lighter weight than ever before.

Therefore, the valve spring is required to have a higher stress design and a longer life, and further improvement is desired.

BACKGROUND ART Conventionally, as a surface treatment method for improving the fatigue resistance of springs, shot peening treatment has been performed in which a large number of shots made of steel particles are hit at the surface of a wire at a high speed. This effect is due to the action of the compressive residual stress in the outer peripheral portion caused by the local plastic deformation that occurs when the shot collides with the strand, which extends over the entire surface of the strand and only the surface layer extends.

However, the indentation generated by shot peening causes stress concentration, which has a negative effect on fatigue resistance, and is not preferable.

<Means for Solving Problems> In view of the problems of the conventional method as described above, the present inventors have
As a result of investigation, they have found a new method for manufacturing a coil spring having excellent fatigue resistance.

That is, the present invention uses a carbon steel wire or an alloy steel wire to obtain a coil spring whose base material hardness is adjusted to Hv ≧ 550, in a manufacturing process, electrolytic polishing or chemical polishing after shot peening treatment performed after forming the coil spring. By removing the wire diameter to 10 to 100 μm by polishing, the surface roughness is 1 μm or more, 4 μ or more, according to JIS B0601 ten-point average roughness.
A method of manufacturing a spring having excellent fatigue resistance, which is characterized in that a cathode is arranged inside the coil spring when the polishing treatment is electrolytic polishing. By removing the negative influence on the fatigue resistance of the spring by applying it by electrolytic polishing or chemical polishing, a spring excellent in fatigue resistance that has never been obtained can be obtained.

<Operation> In the spring manufacturing process, shot peening is usually performed after coiling. As is well known, this is performed for the purpose of applying a compressive residual stress to the surface of the wire in order to improve fatigue resistance, but this treatment also causes flaws due to shot impressions on the entire surface of the wire.

On the other hand, in order to ensure fatigue resistance, it is necessary to adjust the hardness of the base material to at least Hv ≧ 550 to achieve high strength.

Therefore, the notch sensitivity becomes high, and when there are material defects in the outer peripheral portion where most of the load stress concentrates, conversely the fatigue resistance decreases.

Conventionally, such defects include non-metallic inclusions and surface flaws in the raw steel wire, but as mentioned above, these are now significantly improved, and flaws caused by shot peening remain as material defects. There is. Therefore, the fatigue resistance can be improved by polishing and removing the flaws caused by the shot peening.

Since the coil spring has a complicated shape, chemical polishing or electrolytic polishing can be considered for polishing this surface.

Further, in the case of electrolytic polishing, as shown in FIG. 1, the cathode can be provided inside the spring for efficient polishing.

The reason will be described below.

The high-strength spring is usually coiled after the strength is adjusted by wire drawing or heat treatment in the state of a single wire. Therefore, the influence of the tensile residual stress generated inside during coiling remains in the product as well, so that when the spring is compressed, higher stress is applied to the inside than to the outside of the coil.

Therefore, the purpose can be sufficiently achieved by performing the polishing process only on the inside of the coil. When electrolytic polishing is performed by providing a cathode inside the coil as shown in FIG. 1, only the inside of the coil is preferentially polished, so that the above-described polishing is possible.

The reason for limiting the amount of polishing of the wire surface to the wire diameter of 10 to 100 μm is as follows.

The thickness of the layer from which the spring wire has been removed when electrolytic polishing is applied to a spring subjected to shot peening, and JIS B-0601
The relationship between the maximum height (R _max ) and the ten-point average roughness (Rz) measured by is shown in FIG.

This shows almost the same tendency even by chemical polishing.
From this figure, it is recognized that it is necessary to polish at least the line surface layer to a thickness of 10 μm or more so that the ten-point average roughness according to JIS B-0601 is less than 4 μm.

On the other hand, the distribution of the compressive residual stress varies depending on the hardness of the steel wire, the hardness of the shot, the size, the speed, and the number of times of peening,
Since the maximum depth is usually 50 to 150 μm from the surface, it is not preferable that the thickness of the removed layer exceeds 50 μm and the ten-point average roughness according to JIS B-0601 is less than 1 μm.

From the above, in the case of uniformly polishing the wire surface, the range of the amount of polishing corresponds to a wire diameter of 20 to 100 μm.

However, the lower limit is set to 10 μm in consideration of the case where the inside of the coil is preferentially polished by electrolytic polishing.

The high-strength spring is usually subjected to strain relief annealing after shot peening and further set, but the polishing treatment can be performed at any stage after shot peening.

<Examples> Hereinafter, the present invention will be described in detail with reference to Examples.

As a bare steel wire, SWOSC-V (C 0.6%, Si 1.4%, Cr 0.7
%, Mn 0.7%, balance Fe and unavoidable impurities) steel wire stripped, wire-drawn, pickled, diameter 4.0m
After heating for 5 minutes at 850 ° C using a m-thick one, 50 in oil
Quenching was carried out at 400C, and tempering was continuously carried out at 400C for 15 minutes.

Then, after forming into springs with the specifications shown in Table 1 below, shot peening treatment was carried out to manufacture 30 springs as prototypes.

Of these, 15 springs were electrolytically polished by the electrolytic polishing apparatus shown in FIG. 1 under the conditions shown in Table 2.

In FIG. 1, 1 is a material to be treated, that is, a spring, and 2
Is an anode, 3 is a cathode, 4 is an anode support guide (insulator), 5
Is an electrolytic layer, and 6 is an electrolytic solution.

A spring fatigue test was carried out on 15 springs each subjected to the above-mentioned electrolytic polishing and 15 springs not subjected to polishing with an average tightening stress of 60 kgf / mm ² and a stress amplitude of 60 kgf / mm ² .

The wire diameter of the spring subjected to electrolytic polishing was calculated by measuring the mass before and after polishing and reducing the wire diameter by this weight difference.

The results of the fatigue test are shown in FIG.

From this figure, it was recognized that the fatigue resistance of the spring according to the present invention was extremely superior to that of the conventional method.

<Effects of the Invention> As described in detail above, the spring obtained by the present invention is
It has unprecedentedly high fatigue resistance, and when it is used as a valve spring for an automobile engine, in particular, its weight can be reduced, and improvement in engine performance is expected.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of an electrolytic polishing apparatus for electrolytic polishing carried out in carrying out the manufacturing method of the present invention, and FIG. 2 is a spring element in the case where electrolytic polishing is applied to a shot-peened spring. The thickness and maximum height of the stripped layer (R
_max ) and ten-point average roughness (Rz)
FIG. 3 is a distribution chart showing the fatigue test results of springs used for comparison with the springs obtained according to the present invention.

Claims (2)

[Claims] 1. In a manufacturing process for obtaining a coil spring having a base metal hardness adjusted to Hv ≧ 550 using a carbon steel wire or an alloy steel wire, electrolytic polishing or chemical polishing is performed after shot peening treatment performed after forming the coil spring. A spring with excellent fatigue resistance, characterized by finishing the surface roughness to 1 μm or more and less than 4 μm in terms of JIS B0601 ten-point average roughness by removing 10 to 100 μm of the wire diameter to the wire diameter by polishing. Manufacturing method. 2. The electrolytic polishing process, wherein a cathode is arranged inside a coil spring.
A method for manufacturing a spring having excellent fatigue resistance according to the item.