JP3028438B2 - Coil spring with excellent fatigue strength - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength coil spring having a base material hardness Hv of 700 or more, which is used in fields requiring a high degree of fatigue characteristics, such as a valve spring or a suspension spring in an automobile engine. Things.

[0002]

2. Description of the Related Art Generally, springs are JIS-G3502, 3
In many cases, steel materials specified in 506 or 4801 are simply quenched and tempered. However, recently, coil springs related to automobiles have been increasing in strength, and some of engine valve springs have been disclosed in Japanese Patent Publication No. 36-9406.
As described in the publication, there is even a carbonitrided material at a low temperature of around 500 ° C. On the other hand, when the strength of the spring increases, the fatigue characteristics of the spring are impaired by minute surface defects that seem inevitable from the current manufacturing process capability.

[0003] For that purpose, "Spring Technology Research Society Bulletin" N
o. 203 (AUG. 1988-P7) and JP-A-63-1988.
JP-A-52729, "Spring with excellent fatigue resistance and method of manufacturing the same", and JP-A-2-129421, "High-strength coil spring and method of manufacturing the same", and JP-A-2-12.
As described in Japanese Patent No. 9422, “High-strength coil spring and manufacturing method thereof”, it has been reported that by subjecting the spring to electrolytic polishing, the indentation caused by surface defects and shot peening is reduced to improve the fatigue life. I have.
This method of electropolishing the spring is considered to be one of the future methods for improving the fatigue strength of the spring. However, in the case of the coil spring, there is a problem in that the entire surface of the spring is uniformly finished, and the inside of the coil spring has a problem. Since the cathode needs to be arranged each time, the efficiency is extremely low.

[0004] Tumbling (putting a work in a barrel and rotating it to remove oil and scale on the work surface by collision of the work) and liquid honing are also methods of spring surface finishing. What is being considered,
"Spring Technology Research Association, Spring, Third Edition" (1982-P4)
37), all of which have been examined from the viewpoint of treatment similar to shot peening, that is, compressive residual stress, and have nothing to do with the following surface flaw countermeasures of the present invention. It is.

[0005]

For example, with the advancement of the performance of automobiles, springs having high fatigue strength are strongly desired.
In order to meet such a demand, it is necessary to increase the strength of the spring, particularly the surface hardness. Thus, there is a problem with minute surface defects that are considered impossible to completely eliminate at present. An object of the present invention is to provide a high-strength coil spring having no such surface defects and excellent in fatigue strength.

[0006]

The gist of the present invention is to provide a coil spring using a carbon steel wire or an alloy steel wire.
There are 90 distributed from the compression residual stress of ± 10 kgf / mm ² is the surface to the interior of 150μm depth, maximum roughness Rm
Base material hardness Hv having a surface roughness of 1.0 μm or less in ax
Is a coil spring having excellent fatigue strength of 700 or more.

[0007]

Hereinafter, the present invention will be described in detail. In recent years, techniques for inspecting flaws in coil spring materials have been increasing, and surface flaws having a depth of 60 μm or more can be almost detected. Base material hardness Hv
Is less than 500, there is little problem with such a surface flaw. However, in a coil spring using a carbon steel wire or an alloy steel wire having a base material hardness Hv of 500 or more and less than 700, the shear stress τ = 60 ± 50 in the fatigue test was used.
In the case where the number of repetitions is 5 × 10 ⁷ or more at kgf / mm ² , fatigue damage may occur from the surface flaw even if the depth is 10 to 30 μm as well as the surface flaw having a depth of 60 μm or more. . In particular, the surface hardness H
In the case of a coil spring having a v of 700 or more, the above-mentioned surface flaw guarantee always causes fatigue fracture starting from the surface flaw, so that reliable fatigue strength cannot be guaranteed.

In order to increase the fatigue strength of the coil spring, the surface hardness of the spring is increased, but the effect cannot be expected unless surface defects can be guaranteed. Therefore, the present inventors, in order to guarantee the surface flaw of the coil spring using a carbon steel wire or alloy steel wire having a base metal hardness Hv of 700 or more, immediately after shot peening in the current coil spring manufacturing process. It has been found that the above effects can be obtained by performing barrel polishing. Barrel polishing finish is 6-8
Workpiece (coil spring), abrasive grains such as alundum and carborundum of appropriate particle size, a small amount of water, and various polishing accelerators called compounds are placed in an appropriate amount in a cylindrical container having a square cross section. Put together and rotate or shake for several hours.

In the present invention, it is an important requirement to define surface roughness conditions by barrel polishing. That is, in a manufacturing process of obtaining a coil spring using a carbon steel wire or an alloy steel wire having a base material hardness Hv of 700 or more, barrel polishing is performed following a shot peening process performed after the coil spring is formed, so that the surface is polished. Finish the roughness to a maximum roughness Rmax of 1.0 μm or less. Maximum roughness Rmax is 1.0
If it exceeds μm, a spring having a high base material hardness, especially Hv of 70
If the value is 0 or more, the notch sensitivity increases, and the fatigue strength decreases. To make the base metal hardness Hv 700 or more, for example,
Stipulated in JIS-G3502, G3506, G4801
Quenching and tempering using steel wire or alloy wire
And then adjust the quenching or tempering temperature.
It is possible by doing.

The compressive residual stress is added to offset the tensile stress due to external force. However, even if the compressive residual stress is added beyond the upper limit, the shot peening treatment becomes inefficient. Since it is difficult to cancel the stress, the range is set to 90 ± 10 kgf / mm ² . The reason why the depth is set to 150 μm from the surface is that the maximum size in steel is 20 μm in a normal manufacturing method.
m of non-metallic inclusions, which are caused by external tensile stress
Therefore, if the tensile stress of the external force is offset to an internal depth of 150 μm , the occurrence of fatigue cracks due to the current maximum non-metallic inclusion size of 20 μm in steel can be suppressed. Although there is no problem in characteristics even if the distribution exceeds 150 μm, the range of the compressive residual stress is set to 150 μm from the surface layer in consideration of economic restrictions in the shot peening process. To apply residual stress,
Apply shot peening with wire
Can be. At that time, adjust the shot projection intensity and projection time
To adjust the magnitude and depth of residual stress
it can.

The barrel polishing is preferably performed immediately after shot peening in the current coil spring manufacturing process. That is, considering that the surface of the coil spring becomes uneven due to the shot peening, which is one of the causes of the decrease in the fatigue strength, it is preferable to immediately after the shot peening. However, since surface flaws may be hammered by shot peening, the effect is further improved by light barrel polishing, preferably before shot peening.

[0012] Since the coil spring needs to be set in order to improve the set characteristic, in the present invention also,
After performing shot peening and barrel polishing, setting is performed. As a coil spring material targeted by the present invention
Is, for example, JIS-G3502, G3506, G48
Piano wires and hard steels with components specified in 01 etc.
Use wire, spring steel, or other alloy wire
be able to. These spring materials are specified in these standards.
Hot-rolled after melting by the
Cold drawing, cutting, polishing or
It can be manufactured by performing a combination of cold working.
These spring steel materials are described in, for example, JIS-G4801.
Heat such as quenching and tempering at an appropriate temperature
By processing, the required base metal hardness can be obtained.
Wear. The coil spring obtained by these processes is described above.
Subject to shot peening, barrel polishing, etc.
Required residual stress and surface roughness
You.

[0013]

EXAMPLES The effects of the present invention will be more specifically described below with reference to examples. A spring average diameter of 2 was used using a φ3.0 mm silicon chrome steel oil-tempered wire for a valve spring in which the base metal hardness Hv was changed according to heat treatment conditions from 730 to 870.
A coil spring having 4.2 mm, a spring height of 52 mm, a total number of turns of 6.5, and an effective number of turns of 4.5 was formed. Then 0.8
Immediate shot peening with a cut wire of mm diameter to give compressive residual stress, immediately put 5 coil springs each in a hexagonal rotating barrel, mix appropriate amounts of abrasive grains, compound, water, etc. The surface roughness of the coil spring was changed by respectively changing the barrel polishing time of the final polishing, and thereafter, the setting was performed.

Here, in order to clarify the effects of the present invention,
The surface roughness and compressive residual stress of all these coil springs were measured. The residual stress was measured from the surface layer and from the surface layer to 150 μm at a pitch of 20 μm, the average value was shown as the value of the residual stress distribution, and ± 10 kgf / mm ^{2 of the} average value.
Table 1 shows the depths distributed in the range. In addition, the comparative material obtained the same hardness as above, after forming a coil spring, performed shot peening, then changed the surface roughness of the barrel-polished one, and set it as it was without the barrel-polishing. Was.

For the coil spring thus manufactured, the shear stress calculated from the spring shape (τm = 65 ± 5)
0 kgf / mm ² ) was repeatedly applied with a compressive load, and the fatigue life was determined up to 5 × 10 ⁷ times. Table 2 shows the results. As a result, the coil spring of the present invention, which was subjected to barrel polishing immediately after shot peening, exhibited a long service life, but had an optimum surface roughness, especially considering the base metal hardness, and a compressive residual stress distributed deep inside. The coil spring has a guaranteed fatigue life. On the other hand, in the case of the comparative example, most of the damage was caused by surface flaws.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

As described above, according to the present invention, the fatigue strength of a valve spring or a suspension spring of an automobile engine can be greatly improved, which is extremely useful in industry.

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-272524 (JP, A) JP-A-5-86418 (JP, A) JP-A-1-127634 (JP, U)

Claims (1)

(57) [Claims] 1. A coil using a carbon steel wire or an alloy steel wire.
A base material having a compressive residual stress of 90 ± 10 kgf / mm ² distributed from the surface layer to a depth of 150 μm and having a maximum roughness Rmax of not more than 1.0 μm has a base material hardness Hv of 700. A coil spring excellent in fatigue strength as described above.