JP2001082518A - Coil spring and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To give higher strength to a coil spring by enhancing fatigue resistance of the coil spring, by specifying a compressed residual stress from the surface of the coil spring till a specific depth and the surface roughness of the coil, as for the coil spring formed by coiling a steel wire for a spring. SOLUTION: A coil spring suitable for a valve spring for an engine, a damper spring for a clutch disk and the like is manufactured by coiling a steel wire for a spring, and is so prepared that a compressed residual stress from the surface of the coil spring till the internal depth of 280 μm is not less than 200 MPa and the surface roughness of the coil is not more than Rma×20 μm. It is also so prepared that the crossing point of the compressed residual stress is at a point having a depth of one tenth of the wire diameter or more. A composite containing 0.62-0.66 wt.% of carbon, 1.20-1.60 wt.% of silicon, 0.60-0.80 wt.% of manganese, 1.35-1.65 wt.% of chromium and 0.40-0.60 wt.% of molybdenum is used for the steel wire for the spring.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, high-fatigue-resistant coil spring which is used for a valve spring for an engine, a damper spring for a clutch desk which is a drive system component, a coil spring for suspension of an automobile, and the like. It relates to the manufacturing method.

[0002]

2. Description of the Related Art Coil springs are widely used in various kinds of machines such as automobiles, but the demands for coil springs are becoming stricter year by year. For example, in the case of a valve spring for an automobile engine, the stress acting on the coil spring is increasing with the increase in output and weight, and the coil spring must have higher strength and higher fatigue resistance. Is required. In various fields, lightweight, compact, light, thin and small devices are also demanded, and a coil spring, which is one of these components, is also required to be lightweight and compact. Of course, the desired spring characteristics (strength,
On the premise of having fatigue resistance, spring constant, etc., it is required to have a compact shape (coil outer diameter, maximum height, close contact height, etc.) and light weight.

[0003]

However, when the wire diameter is reduced by reducing the number of windings to reduce the weight, for example, the torsional shear stress τ applied to the wire material of the coil spring is 1 / d ³ (wire diameter: d). , The torsional shear stress τ increases sharply. For this reason, in order to reduce the weight, higher strength is also required. In order to increase the strength of the coil spring, it is usual to make the coil spring harder. For example, in the case of a coil spring made of steel, a high hardness can be achieved by increasing the carbon content (C weight%) and performing heat treatment under optimal conditions. In general, increasing the hardness increases the fatigue limit, so that the fatigue resistance is also improved, which is favorable.

[0004] However, there is a limit in increasing the hardness, and when the hardness exceeds a certain hardness (for example, Rockwell hardness of 50 to 60 HRC), the fatigue limit tends to decrease. When the strength (tensile strength σb) is increased, the notch sensitivity becomes more sensitive. Therefore, it is not easy to achieve both strength and fatigue resistance. Especially when there are minute scratches or non-metallic inclusions near the surface of the coil spring and stress concentrates there, even if it can be solved in terms of strength, but it becomes a problem from the point of fatigue resistance There is also. For this reason, it is very important to increase the strength of the coil spring together with the improvement of the fatigue resistance. As described above, there is a limit to the improvement in fatigue resistance due to the increase in strength. Therefore, it is desired to improve the fatigue resistance by a method different from the increase in strength.

[0005] Fatigue fracture is generally caused by the propagation of a crack from a starting point near the material surface after a repetitive stress exceeding a fatigue limit acts a certain number of times or more. Therefore, to improve fatigue resistance, surface modification, that is, surface hardening and application of compressive residual stress are effective at present, and the former is heat treatment such as nitriding or low-temperature carbonitriding, and the latter is mechanical treatment by shot peening. It is performed by processing. In particular, shot peening is often performed because the application of a deeper and larger compressive residual stress is effective in improving the fatigue resistance of a coil spring.

[0006] In shot peening, a shot material having a particle size of about several tens of μm to several mm is projected on a coil spring by a pneumatic or centrifugal shot peening machine, and the surface of the coil spring is cooled (heated). (Interim) processing. The surface layer is plastically worked into a satin shape, and a compressive residual stress is given to the vicinity thereof by the extension of the surface layer. In reality, work hardening is further added, and the fatigue resistance of the coil spring is further improved. Here, as a result of research conducted by the present inventors, even when shot peening is used, the conventional method has a limit in applying compressive residual stress to a coil spring, and a deeper and larger compressive residual stress is applied. Was difficult.
Also, no such proposal has been made, and there is no record of using such a coil spring.

FIG. 5 shows an example of a conventional high strength and high fatigue resistance coil spring.
μm, and the peak value of the compressive residual stress is 1 at the outermost layer.
Although it is as high as 900MPa, the compressive residual stress is only 1250MP in the inside (point of 50μm from the surface)
It can be seen that it has rapidly decreased to a. Of course, if only a large compressive residual stress is applied, a large-size (large-mass) shot can be projected at a high speed to cause a large composition deformation in the coil spring. However, this significantly increases the surface roughness of the coil spring, and conversely reduces the fatigue resistance. Further, in the case of a coil spring having a wire diameter of about several millimeters, the occurrence of large plastic deformation may impair the predetermined dimensions and characteristics of the coil spring.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and has been developed to provide a coil spring and a method of manufacturing the coil spring, which can remarkably increase fatigue resistance in order to cope with higher strength of the coil spring. The purpose is to provide.

[0009]

Accordingly, the present inventors have conducted intensive studies and conducted various systematic experiments from a completely different viewpoint, and as a result, have accomplished the present invention.

(Coil Spring) In other words, the coil spring of the present invention is a coil spring obtained by coiling a steel wire for a spring, wherein the compression residual stress from the surface of the coil spring to an inner depth of 280 μm is 200 MPa or more, Is characterized by having a surface roughness of Rmax 20 μm or less.

Unlike the prior art, a coil spring having a compressive residual stress of 200 MPa or more from the surface to 280 μm can be obtained, so that the fatigue resistance of the coil spring can be further improved. Thereby, the strength of the coil spring can be further increased. In addition, since the surface roughness at this time is not more than Rmax 20 μm, the fatigue resistance does not decrease. Rmax means the maximum height, and the unit is μm. Here, the compressive residual stress is 2
If the pressure is less than 00 MPa, it is not always possible to sufficiently meet severe demands for high stress and fatigue resistance. The surface roughness is 20μ.
If it exceeds m, the requirement for severe fatigue resistance cannot always be sufficiently satisfied due to an increase in the notch effect. In order to further improve the fatigue resistance, the compressive residual stress must be 4%.
More preferably, the surface roughness is not less than 00 MPa or the surface roughness is not more than Rmax 15 μm.

It is preferable that the crossing point of the residual compressive stress is located at a point deeper than 1/10 of the wire diameter of the spring steel wire. Thereby, a larger compressive residual stress can be applied as a whole, and the fatigue resistance can be further improved. Specifically, the crossing
When the point becomes 1/10 or more of the wire diameter, 36
% Or more is given compressive residual stress. In addition, since the existence region is the outer peripheral portion where the shear stress and the bending stress are increased, it is understood that the present invention is more effective in improving the fatigue resistance. Here, if the crossing point is shallower than 1/10 of the wire diameter, it is not preferable to further increase the fatigue resistance. The crossing point is
This is the depth at which the compressive residual stress becomes zero, and serves as an indication of how much the compressive residual stress is applied from the surface of the coil spring.

It is preferable that the compressive residual stress from the surface of the coil spring to the inner depth of 50 μm is 1400 MPa or more. The shear stress and the bending stress acting on the coil spring become maximum near the outer peripheral portion. The compressive residual stress from the surface corresponding to that portion to the internal depth of 50 μm is 14
Since it is as large as 00 MPa or more, it is more preferable to improve fatigue resistance. In order to further improve the fatigue resistance, it is more preferable that the compressive residual stress up to an internal depth of 200 μm is 900 MPa or more.

The spring steel wire contains 0.62 to 0.66% of carbon and 1.20 to 1.60 of silicon in weight%.
%, Manganese 0.60 to 0.80%, chromium 1.3
5 to 1.65%, 0.40 to 0.60% of molybdenum,
It is preferable that vanadium is contained in an amount of 0.15 to 0.25% and the balance substantially consists of iron.

This is one of the most suitable compositions for obtaining the coil spring of the present invention, and by using a wire having this composition, it is possible to achieve high fatigue strength and high strength. In particular, this spring steel wire is suitable for oil-tempering, and a coil spring having both high strength and high fatigue resistance can be obtained by obtaining a high-strength and high-toughness oil-tempered wire.

Carbon (C) is an element that affects the strength (hardness) of a quenched and tempered steel wire. If it is less than 0.62%, the strength required as a coil spring cannot be obtained, and it exceeds 0.66%. Even if there is no further advantage in strength, the upper limit is set to 0.
66%. Silicon (Si) is an element for forming a solid solution in ferrite ground to increase strength and improve fatigue resistance.
If it is less than 0%, the effect is low, and if it exceeds 1.60%, the toughness is reduced and decarburization during production becomes remarkable, which is not preferable. Manganese (Mn) is an element for enhancing hardenability and securing strength and toughness after heat treatment. For that reason, 0.60% or more is required, but if added in excess of 0.80%, toughness is impaired. It is not preferable.

Chromium (Cr) is an element that enhances hardenability and temper softening resistance and imparts strength and toughness to a spring by precipitating fine carbides. For this purpose, 1.35% or more is added. Is required, but 1.65%
If it exceeds, "slack" tends to occur, which is not preferable. Molybdenum (Mo) is also an element that enhances hardenability and temper softening resistance and imparts strength and toughness to a spring by precipitating fine carbides. However, if less than 0.40%, the effect is not recognized. If the content exceeds 0.60%, the effect is saturated, which is not preferable. Vanadium (V)
Is an element that improves strength and sag resistance due to crystal grain refinement and precipitation hardening. If the content is less than 0.15%, there is no effect, and if the content exceeds 0.25%, the effect is saturated. Not preferred. In addition, when phosphorus (P) and sulfur (S) are contained as impurities, both are preferably set to 0.025% or less from the viewpoint of fatigue resistance.

In the vicinity of the surface of the coil spring (for example, 100 to 3)
Non-metallic inclusions (particle size: about 20 μm)
The use of the coil spring according to the present invention is particularly effective in the case where causes fatigue fracture and hinders the increase in strength.

(Method of Manufacturing a Coil Spring) Next, a method of manufacturing a coil spring according to the present invention includes a coiling step of forming a steel wire for a spring into a coil, and projecting a shot having a sphericity of 50 μm or less onto the coil. And a shot peening step of imparting compressive residual stress by applying a pressure.

By performing a shot peening step of projecting a shot having a sphericity of 50 μm or less, a deeper and larger compressive residual stress can be applied to the entire worked layer of the coil formed in the coiling step. It is.
Further, since the shot has a good sphericity, the surface roughness of the processed surface after the shot peening step is also good.
Since the surface roughness has a large effect on the fatigue resistance, the fatigue resistance can be improved from this viewpoint as well.

The details of the mechanism by which the compressive residual stress becomes deeper and larger are not clear, but are considered as follows. Conventional shots (cut wire shots, cast steel shots, etc.) have different shapes, one by one. In particular, cut wire shots have rounded corners, but in reality, such as rugby balls and cocoon-shaped shots It had a shape.
When a shot having such a shape collides with the surface of a coil spring, which is a workpiece, a large force easily acts on only a part thereof. For this reason, when viewed microscopically, large plastic deformation occurs only in a very small portion, and the portion with low compressive residual stress increases,
It is considered that the overall compressive residual stress decreases.

On the other hand, when the shot of the present invention is used, the plastic deformation on the machined surface of the coil spring becomes almost uniform irrespective of the direction of the shot due to its good sphericity.
The distribution of compressive residual stress is also uniform. Therefore, it is considered that as a result, the overall compressive residual stress also increased.

Here, the sphericity refers to a value represented by (maximum diameter-minimum diameter) / 2 for one shot. If the sphericity exceeds 50 μm, the plastic deformation becomes non-uniform, and the distribution of compressive residual stress decreases. In order to further improve the effect of shot peening, the sphericity is more preferably set to 20 to 30 μm.

It is preferable that the shot has a particle diameter of 1/4 or more of the wire diameter of the coil spring and a Vickers hardness of 700 to 1200 HV. By setting the particle diameter of the shot to 1/4 or more of the wire diameter of the coil spring, the plastic deformation becomes more uniform on the surface and the distribution of the compressive residual stress can be expanded to a deeper level. In addition, by setting the hardness of the shot to 700 to 1200 HV, the energy absorbed by the shot at the time of projection is reduced, so that a large projection energy can be given to the surface of the coil, and a large compression residue can be obtained even inside. Stressed. Because of the good sphericity of the shot, even if the particle size of the shot was increased, the surface roughness did not deteriorate, and no harmful plastic deformation occurred.

Here, the particle diameter means an average value of diameters when one shot is measured from at least two orthogonal directions. If the particle diameter is less than 1/4 of the wire diameter, a large projection energy cannot be obtained, which is not preferable for giving a compressive residual stress in a short time from the viewpoint of productivity. The reason for setting the Vickers hardness to 700 to 1200 HV is that if the Vickers hardness is less than 700, the amount of shot energy absorbed by shots increases, the compressive residual stress applied to the coil decreases, and the fatigue resistance is undesirably increased. When the Vickers hardness exceeds 1200 HV, although there is no problem in improving the fatigue resistance of the coil, the life of the shot is shortened, which is not preferable.

In general, the higher the strength of a coil spring, the higher the hardness. Therefore, shot peening is preferably performed using a shot having a Vickers hardness of 700 to 1200 HV.
In order to further improve the fatigue resistance of the workpiece, the Vickers hardness of this shot is more preferably set to 800 to 900 HV, particularly preferably equal to or higher than the hardness of the workpiece.

It is more preferable that a gas nitriding process is performed between the coiling process and the shot peening process. This nitriding step can increase the hardness of the surface layer of the coil spring, and is effective in improving fatigue resistance by applying compressive residual stress. In particular, it is advantageous to perform a shot peening step after that, since the compressive residual stress in the surface layer portion is further increased and the surface roughness can be kept better. This nitriding step is, for example,
By performing the treatment in an ammonia atmosphere at 420 to 550 ° C. for 1 to 48 hours, a predetermined nitrided layer can be formed, and the Vickers hardness near the surface can be set to 600 to 1100 HV. The reason for performing the nitriding process after the coiling process is that
This is because the coiling resistance increases before the coiling step.

After the shot peening step, Vickers hardness is 1200 to 1600 HV, specific gravity is 12 to 16,
It is preferable to perform a second shot peening step of projecting a shot having a smaller particle size than the shot. By performing the second shot peening step further after the shot peening step, the peak value of the compressive residual stress can be brought near the surface of the coil spring, and the compressive residual stress can be applied further inside.

Since the shot used in the second shot peening step has a smaller grain size than the shot in the first shot peening step, the surface roughness of the coil after the second shot peening step is improved. it can. Moreover, the Vickers hardness of the shot is 1200 to 160
Since the hardness is as high as 0 HV and the specific gravity is as large as 12 to 16, further compressive residual stress is applied even after the first shot peening step.

For example, in the first shot peening step, impeller projection is performed at a speed of 70 to 100 m / s to form a compressive residual stress to a deep position inside,
In the second shot peening step, the pressure is 0.2 to 0.7.
It is also possible to increase the compressive residual stress on the surface by blasting the air with MPa. In addition, stress peening or warm peening may be performed.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The coil spring of the present invention has high strength and high fatigue resistance, such as a valve spring for an engine, a damper spring for a clutch disk or a lock-up type torque converter, or a suspension coil spring for an automobile or the like. Suitable for the required coil spring.

Taking the valve spring as an example, the valve spring is a compression coil spring that biases the valve so that the valve closes the supply / exhaust port of the cylinder head. Usually, an automobile engine rotates several thousand revolutions per minute, and a motorcycle engine rotates 10,000 to 20,000 revolutions per minute. Moreover, both the shear stress and the torsional shear stress caused by the load itself act inside the coil, and the repetitive stress is large. Therefore, a large fatigue strength is required. In particular, in the case of a valve spring used in a harsh environment, since weight reduction, high fatigue resistance and high strength are required in high dimensions, using the coil spring of the present invention in such a valve spring. Is more suitable.

The valve spring which is a compression coil spring has been described as an example, but the shape of the coil spring of the present invention is not limited. For example, it may be a linear coil spring, a non-uniform pitch coil spring, or a non-linear coil spring such as a drum-shaped or barrel-shaped coil spring. Further, it may have a circular cross section or a rectangular cross section.

Next, an example of a related process of the method for manufacturing a coil spring according to the present invention will be described. For coil spring steel, for example, flaw removal, hot rolling, peeling, annealing,
By performing the steps of cold drawing in order, a cold wire for manufacturing a coil spring can be formed. The cold wire is subjected to an oil tempering process to obtain an oil-tempered wire, and cold coiling is performed as an example of the coiling step of the present invention. Thereafter, low-temperature annealing (or bluing) is preferable because processing strain is removed, harmful residual stress (tensile residual stress) is released, and fatigue resistance of the coil spring increases. Although depending on the degree of processing, the heating time is preferably shorter for making the crystal structure finer, and the heating temperature is preferably lower than the recrystallization temperature (usually 150 to 550 ° C.).

Thereafter, gas nitriding, which is the nitriding step of the present invention, is performed to increase the hardness of the surface layer of the coil spring and apply compressive residual stress. Thereby, the fatigue resistance is further improved. In addition, when combined with shot peening performed later, there is an effect that the surface roughness can be kept good, and the compressive residual stress in the surface layer portion is further increased, which is advantageous. Thereafter, the end face is ground to make the coil spring a predetermined size. Edge grinding may be performed before nitriding.

Next, shot peening, which is the shot peening step of the present invention, is performed to apply a compressive residual stress to the surface layer of the coil spring. Thereby, the fatigue resistance is particularly improved. In particular, it is preferable that the second shot peening step of the present invention is performed in a plurality of stages. Thereby, the peak value of the compressive residual stress can be brought near the surface of the coil spring, and a deeper and large compressive residual stress can be applied to the inside.

For a coil spring having a constant load direction, setting is preferably performed after shot peening. This is because the elastic limit (proportional limit) is significantly improved by giving a plastic strain. Further, it is preferable to perform low-temperature annealing after this, because the elastic limit is further increased due to strain aging and the sag resistance is improved. Usually 150 to 30
Perform low temperature annealing at 0 ° C.

Further, by performing warm setting at 200 to 400 ° C., the sag resistance can be particularly improved, and especially for a suspension coil spring of an automobile or a valve spring for an engine having a large acting stress. preferable.

[0039]

The present invention will be described below in more detail with reference to examples. (Manufacture of Coil Spring) (1) First Example In this example, 0.64% by weight of carbon (hereinafter,% means% by weight unless otherwise specified) and 1.4 of silicon.
3%, manganese 0.67%, phosphorus 0.015%, sulfur 0.
006%, chromium 1.57%, molybdenum 0.57%,
0.24% vanadium, the remainder uses alloy steel consisting essentially of iron for springs, and after each processing of flaw removal, hot rolling, scalping, annealing, cold drawing, An oil tempering treatment was performed to obtain a spring steel wire (oil-tempered wire (SWOCX-VS)) as referred to in the present invention.

Next, cold coiling, which corresponds to the coiling step referred to in the present invention, was carried out, and the wire diameter was 3.2 mm, the coil center diameter was 20.0 mm (the coil outer diameter was 23.2 m).
m), a coil spring having a total of 6.0 turns and an effective number of 4.0 turns was obtained. At the time of this coiling, the surface of the oil-tempered wire was covered with the oxide scale and had lubricity, and showed good coiling properties.

Thereafter, low-temperature tempering was performed at 500 ° C. for 15 minutes to remove harmful residual stress. Next, both seat surfaces were ground to form a coil spring having a free length of 47.0 mm. At this time, the spring constant was 32.0 N / mm 2. Next, gas nitriding treatment corresponding to the nitriding step according to the present invention was performed. When treated at 500 ° C. for 4 hours in an ammonia atmosphere, the Vickers hardness became 800 to 1000 HV at a depth of 0.02 mm of the nitrided layer near the surface.

Next, the shot peening step of the present invention is performed. Shot peening is performed using a particle size φ0.
915 mm, sphericity 4-5 μm, Vickers hardness 900
Using a shot of ~ 1000 HV and an air type shot peening machine, a projection pressure of 0.5 MPa x a projection time of 2
Performed for 5 minutes. The projection time is a total time in which shot peening of 60 seconds is repeated 25 times. The distance from the nozzle of the pneumatic shot peening machine to each test piece (coil spring) was within about 500 mm. In order to keep the shot sphericity constant, shots that were deformed or damaged were appropriately removed. The arc height at this time is
It was 0.9 mmA or more and the coverage was 99% or more.

The specifications of the almen strip and the measuring method using an almen gauge were in accordance with the Japan Spring Association Standard JSMA. In addition, the coverage was measured by using a magnifying glass with a magnification of 10 to compare with the sample. After the shot peening step, the mixture was heated by an electric furnace and subjected to low-temperature annealing at 225 ° C. for 15 minutes.

(2) Second Embodiment In this embodiment, the wire diameter of the spring steel wire is φ3.2 mm,
Using a shot having a particle diameter of 0.915 mm, a sphericity of 4 to 5 μm, and a Vickers hardness of 900 to 1000 HV, using a pneumatic shot peening machine, a projection pressure of 0.5 MPa ×
The shot peening process was performed for a projection time of 25 minutes. The projection time is a total time in which shot peening of 60 seconds is repeated 25 times.

Thereafter, a second shot peening step is performed using a carbide shot, and the other points are the same as in the first embodiment. The cemented carbide shot is a substantially spherical particle made of tungsten carbide (WC), and has a particle diameter smaller than that of the first shot, about φ0.1 mm, a specific gravity of 14, and a Vickers hardness of 1,400 HV. The Vickers hardness is a value on the shot surface, and this Vickers hardness is defined as “Vickers hardness” in the present invention.

In the second shot peening step, shot peening was performed with a pneumatic shot peening machine at a projection pressure of 0.2 MPa and a projection time of 5 minutes. The projection time is a total time in which shot peening of 60 seconds is repeated five times. At this time, the arc height is 0.26 mmN
As described above, the shots having coverage of 99% or more and deformed or damaged were appropriately removed.

(3) Third Embodiment In this embodiment, the diameter of the spring steel wire rod is φ3.2 mm,
RC with particle size φ1.05mm and Vickers hardness 800HV
The shot peening step is performed for 20 minutes using W (round cut wire), and the other points are the same as in the first embodiment. The projection time is a total time in which shot peening of 60 seconds is repeated 25 times.

In this RCW shot, a hard steel wire is drawn, formed into a wire with a diameter of 1.05 mm, cut into the same diameter (1.05 mm), and then projected onto a hard wall. Thus, the edge is rounded. In this case, the arc height is 0.9 mmA or more, and the coverage is 99%.
As described above, shots deformed or damaged were appropriately removed.

(4) Fourth Embodiment In this embodiment, the wire diameter of the spring steel wire rod is φ3.2 mm,
RC with particle size φ1.05mm and Vickers hardness 800HV
The shot peening is performed for 25 minutes using W, and then the second shot peening step is performed using the above-described carbide shot, and the rest is the same as the first embodiment.

In the second shot peening step, shot peening was performed with a pneumatic shot peening machine at a projection pressure of 0.2 MPa and a projection time of 5 minutes. The projection time is a total time in which shot peening of 60 seconds is repeated five times. At this time, the arc height is 0.26 mmN
As described above, the shots having coverage of 99% or more and deformed or damaged were appropriately removed.

(5) Conventional example This conventional example has the same wire rod (wire diameter: φ
3.2 mm), shot peening is performed for 80 minutes using RCW having a particle size of 0.8 mm and a Vickers hardness of 700 HV, and then shot peening again using the carbide shot used in the second embodiment. It was done.

For the second shot peening, shot peening was performed with a pneumatic shot peening machine at a projection pressure of 0.2 MPa and a projection time of 5 minutes. The projection time is a total time in which shot peening of 60 seconds is repeated five times. At this time, the arc height is 0.26 mmN
As described above, the shots having coverage of 99% or more and deformed or damaged were appropriately removed.

(Shot of φ 0.915 mm) (1) Manufacturing JISG4805SUJ bearing steel is drawn and processed to φ1.0
mm, and the drawn wire has the same length (1.0 mm) as its diameter.
mm) and then projected onto a rigid wall to round the edges. This was ground (polished) with a target value of a particle diameter of 0.915 mm. The grinding was performed by inserting a shot whose rounded edge was only between a fixed upper plate and a rotating lower plate having a grindstone surface on the upper surface, and rotating the lower plate. Therefore, the shot is ground to a predetermined sphericity while rolling on the grindstone surface of the lower plate while being in contact with the upper plate.

Thereafter, quenching and tempering were performed to obtain a predetermined Vickers hardness. Quenching is done in a vacuum quenching furnace.
After heating at 50 ° C. for 120 minutes, it was quenched in quenched oil. Tempering was performed in an electric furnace at 170 ° C. for 180 minutes and then air-cooled. As a result, a shot having a very good sphericity and hardness of 5 μm or less and Vickers hardness of 800 HV or more (near the surface of the shot) was obtained. The particle size of the shot is φ0.915 mm
Therefore, it is 1/4 or more of the coil diameter φ3.2 mm.

(2) Shot Measurement Sphericity The sphericity was measured as follows. One of a large number of shot materials was randomly extracted from the V-groove measurement block.
Place it in the groove, securely fix the dial gauge to the arm tip of the magnet base, contact the measuring part (tip part) of the dial gauge from above the blast material, read the needle runout of the dial gauge after reset It is. By rotating the projectile so that the entire projectile can be measured on average,
Similar measurements were made at 0 points. The minimum value and the maximum value were selected, and 差 of the difference was defined as the sphericity, and the measurement was performed for four lots.
m, 4.0 μm, 4.5 μm, and 3.2 μm. It was confirmed that the sphericity was very good regardless of the lot.

Vickers hardness Vickers hardness is measured according to the Vickers hardness test method specified in JISZ2244. The test load was 500 g and the load time was 5 seconds. Based on this, Table 1 shows the Vickers hardness measured for one shot.

[Table 1] From this table, it can be seen that this shot has a substantially constant hardness up to the inside.

Particle Size The particle size of the shot was determined by measuring the diameter of the shot in the example in two directions perpendicular to each other using an optical microscope, and the average of the two was taken as the diameter in that direction. Change the measurement direction of the shot,
This was repeated 10 times. And the average of these 10 data was made into particle diameter. For example, when a target value of φ0.915 mm was measured for several lots,
φ0.9149mm, φ0.9151mm, φ0.94
It was 95 mm and had a uniform particle size.

(Evaluation) (1) First Example The distribution of compressive residual stress in this example was measured, and the results are shown in FIG. As a measuring method, a general X-ray residual stress measuring method using X-ray diffraction was used as a non-destructive method.

From this, the coil spring of the present embodiment has a large compressive residual stress up to the inside, and a compressive residual stress of 400 MPa or more exists at 280 μm, which is the crossing point of the conventional coil spring. . In addition, it can be seen that the crossing point is as deep as 350 μm or more, which is at least 1/10 of the wire diameter (φ3.2 mm) of the coil spring. Also from the surface 2
Since a large compressive residual stress of 900 MPa or more exists up to 00 μm, fatigue resistance is significantly improved.
When the surface roughness of this coil spring was measured by a surface roughness meter, it was good as Rmax 11.3 μm.

(2) Second Example The distribution of compressive residual stress in this example was measured, and the results are shown in FIG. The measuring method is the same as in the first embodiment. Also in the coil spring of this embodiment, a large compressive residual stress is generated inside, and a compressive residual stress of 400 MPa or more exists at a point of 280 μm which is a crossing point of the conventional coil spring. In addition, the crossing point is 350 μm or more (wire diameter: 1/10 of φ3.2 mm)
Above) and deep. In addition, 18 to 70 μm from the surface
A large compressive residual stress of 00 MPa or more exists, and a compressive residual stress of 900 MPa or more exists from the surface to 200 μm. Therefore, fatigue resistance is significantly improved. The surface roughness of this coil spring is also Rmax1
It was as good as 0.1 μm.

(3) Third Example The distribution of compressive residual stress in this example was measured, and the results are shown in FIG. The measuring method is the same as in the first embodiment. Also in the coil spring of this embodiment, a large compressive residual stress is generated inside, and a compressive residual stress of 400 MPa or more exists at a point of 280 μm which is a crossing point of the conventional coil spring. In addition, the crossing point is as deep as 420 μm or more (wire diameter: １ ／ of φ3.2 mm or more). In addition, a large compressive residual stress of 1400 MPa or more exists from the surface to 100 μm, and a compressive residual stress of 900 MPa or more exists from the surface to 200 μm. Therefore, fatigue resistance is significantly improved. The surface roughness of this coil spring is also Rma
x11.9 μm, which was good.

(4) Fourth Example The distribution of compressive residual stress in this example was measured, and the results are shown in FIG. The measuring method is the same as in the first embodiment. Also in the coil spring of this embodiment, a large compressive residual stress occurs inside, and at the point of 280 μm which is the crossing point of the conventional coil spring, the compressive residual stress is 600
Mpa or more. In addition, the crossing point is as deep as 430 μm or more (wire diameter: 以上 of φ3.2 mm or more). Furthermore, a very large peak value of compressive residual stress of 1900 MPa or more exists on the surface.
In addition, a large compressive residual stress of 1500 MPa or more exists from the surface to 100 μm,
A compressive residual stress of 1000 MPa or more exists up to 0 μm. Therefore, the fatigue resistance is particularly improved. The surface roughness of this coil spring is also Rmax 11.8 μm.
m and good.

(5) Conventional Example The distribution of compressive residual stress of this conventional example was measured, and the results are shown in FIG. The measuring method is the same as in the first embodiment.
This conventional coil spring has a crossing point of 280 μm. The compressive residual stress is 190
Although it is as large as 0 MPa, it decreases sharply toward the inside, and drops to 1250 MPa at a point 50 μm from the surface. The surface roughness of this coil spring is Rm
ax 5.5 μm.

[0064]

According to the coil spring of the present invention, since the residual compressive stress is large and the surface roughness is good, the fatigue resistance can be remarkably increased, and the coil spring can be further strengthened. Further, the method for manufacturing a coil spring according to the present invention is suitable for manufacturing a coil spring in which higher strength and higher fatigue resistance are desired.

[Brief description of the drawings]

FIG. 1 is a graph showing a distribution of compressive residual stress according to a first example of the present invention.

FIG. 2 is a graph showing a distribution of compressive residual stress according to a second embodiment of the present invention.

FIG. 3 is a graph showing a distribution of compressive residual stress according to a third embodiment of the present invention.

FIG. 4 is a graph showing a distribution of compressive residual stress according to a fourth embodiment of the present invention.

FIG. 5 is a graph showing a distribution of compressive residual stress of a conventional example.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/28 C22C 38/28 38/34 38/34 (72) Inventor Yuji Ishikawa Togo-cho, Aichi-gun, Aichi Prefecture No. 1 Haruki, Haruki, Togo Mfg. Co., Ltd. (72) Inventor Yoshihiro Watanabe, 481, Oda, Nitta-ji, Omagachi, Kamo-gun, Aichi Prefecture F-term in Toyo Seiko Co., Ltd. 3J059 AB11 AD04 BA01 BC02 EA08 EA20 GA02 GA08 GA14 4K042 AA02 BA03 BA04 BA09 CA06 CA08 CA13 DA02 DA06 DC02 DC03

Claims (8)

[Claims] 1. A coil spring obtained by coiling a steel wire for a spring, wherein a compressive residual stress from a surface of the coil spring to an inner depth of 280 μm is 200 MPa or more, and a surface roughness of the coil is Rmax 20 μm or less. Characteristic coil spring. 2. The coil spring according to claim 1, wherein a crossing point of the residual compressive stress is a point deeper than 1/10 of a wire diameter of the coil spring. 3. An internal depth of 50 μm from the surface of the coil spring.
The coil spring according to claim 1, wherein a residual compressive stress up to m is 1400 MPa or more. 4. The spring steel wire has a carbon content of 0.1% by weight.
62-0.66%, silicon 1.20-1.60%, manganese 0.60-0.80%, chromium 1.35-1.
The coil spring according to any one of claims 1 to 3, comprising 65%, 0.40 to 0.60% of molybdenum, 0.15 to 0.25% of vanadium, and the balance substantially consisting of iron. 5. A coiling step of forming a steel wire for a spring in a coil, and a shot peening step of projecting a shot having a sphericity of 50 μm or less to the coil to impart a compressive residual stress. Manufacturing method of coil spring. 6. The shot has a particle diameter of at least 1/4 of a wire diameter of the coil spring, and has a Vickers hardness of 7 or more.
The method for producing a coil spring according to claim 5, wherein the pressure is from 00 to 1200 HV. 7. The method for manufacturing a coil spring according to claim 5, further comprising a nitriding step of performing a gas nitriding treatment between said coiling step and said shot peening step. 8. After the shot peening step, Vickers hardness is 1200 to 1600 HV, specific gravity is 12 to 16,
The method of manufacturing a coil spring according to claim 5, wherein a second shot peening step of projecting a shot having a smaller particle diameter than the shot is performed.