JPH07214216A - Manufacture of high-strength spring - Google Patents

Abstract

PURPOSE:To improve fatigue resistance and durability by successively applying coiling formation low temp. annealing, bearing surface grinding, gas nitriding, shot peening and low temp. annealing to a steel wire rod. CONSTITUTION:The steel wire rod is cold drawn and, after that, an oil tempered steel wire is formed by executing quenching and tempering treatment. A coiled spring is formed by cold coiling it after electrical polishing. After annealing the coiled spring at a lower temp., the bearing surfaces are ground. After that, gas nitriding is applied. Then, shot peening is executed. Next, low temp. annealing is executed. These stages are successively executed. The shot peening is constituted of 1st shot peening and 2nd shot peening in which shots having diameters smaller than them used in the 1st shot peening are used. In this way, compressive residual stress is imparted to a deeper 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 high-strength, high-fatigue resistant spring such as a valve spring used in an automobile engine or a clutch spring.

[0002]

2. Description of the Related Art As a method for producing a high-strength spring, an oil tempered wire having high tensile strength is used for coiling, heat treatment, grinding, residual stress imparting treatment by shot peening, and then polishing treatment for maximum surface treatment. A method of performing each step of reducing roughness is known. Further, in JP-A-3-310439, in a method of manufacturing a coil spring in which a steel wire rod is sequentially subjected to coiling forming, heat treatment, nitriding treatment and shot peening, the shot peening is performed by a first shot peening step and a subsequent low temperature firing. Second step performed using a shot having a diameter smaller than the shot used in the annealing step and the first shot peening step
A method of manufacturing a spring is described which comprises a shot peening step. Further, in Japanese Patent Application Laid-Open No. 4-376346, a steel wire obtained by quenching and tempering after drawing a steel wire for spring is subjected to electrolytic polishing or chemical polishing to determine the surface roughness.
A spring having excellent fatigue resistance is described, which is finished to have a ten-point average roughness (Rz) of 5 μm or less according to ISB-0601 and is subjected to shot peening treatment before or after forming into a spring.

[0003]

In order to increase the output of automobile engines and reduce the weight of vehicle bodies, coil springs, which are engine parts, are required to have high strength and high fatigue resistance. For this coil spring, an oil tempered wire is used in which a steel wire rod for spring is drawn and then quenched and tempered.

In the conventional coil spring manufacturing method, coiling molding, heat treatment, grinding, nitriding, and shot peening are sequentially performed using this oil tempered wire. However, since the oxide film generated in the manufacturing process of the oil tempered wire remains on the surface of the spring until nitriding, it is necessary to remove the oxide film before nitriding. However, the conventional method has a problem that the gas nitriding cannot be effectively performed because the oxide film cannot be sufficiently removed.

Further, the oil tempered wire has minute flaws and decarburized layers generated at the manufacturing stage thereof. In order to solve these problems, the surface roughness is measured by electrolytic polishing or chemical polishing to obtain the ten-point average roughness (Rz) of JISB-0601.
To 5 μm or less, and shot peening is performed before or after forming into a spring. However, the conventional method has a problem that the surface roughness increases because the nitriding treatment is not performed before the shot peening.

[0006]

Means for Solving the Problems As a result of thorough study of means for increasing fatigue resistance and method for increasing strength, the present inventors have conducted coiling forming using a steel wire rod that is electrolytically polished on an oil temper wire, It was found that a high strength and high fatigue resistance spring can be formed by sequentially performing low temperature annealing, bearing surface grinding, gas nitriding, then shot peening, and low temperature annealing. Furthermore, after coiling forming using the above steel wire,
It has been found that a spring having high strength and high fatigue resistance can be formed by sequentially performing degreasing, gas nitriding, bearing surface grinding, shot peening, and low temperature annealing. It has also been found that this shot peening is more effective by combining the first shot peening with the second shot peening performed by using a shot having a diameter smaller than the shot used in the first shot peening. The present invention has been completed based on such findings.

That is, in the method for manufacturing a high-strength spring according to the first aspect of the present invention, a steel wire rod which has been subjected to quenching and tempering after drawing and then electrolytic polishing is subjected to coiling forming, low temperature annealing, bearing surface grinding, gas nitriding, and then shot. The feature is that peening and low temperature annealing are sequentially performed. Further, the manufacturing method of the high-strength spring of the second invention is, after wire drawing quenching, tempering, then coiling forming on steel wire rod subjected to electrolytic polishing, degreasing, gas nitriding, seat surface grinding, then shot peening, low temperature annealing. It is characterized by performing sequentially.

The first and second inventions will be described together as the present invention. The wire rod used in the method for manufacturing the high-strength coil spring of the present invention is preferably an alloy steel containing vanadium or silicon, and this wire rod is a steel wire rod whose surface portion is nitrided by nitriding so that the hardness of the surface portion becomes high. This steel wire rod is drawn by cold drawing, then subjected to quenching and tempering for increasing hardness, and subjected to electrolytic polishing treatment to obtain an electrolytically polished oil tempered wire. Since the surface of this electrolytically-polished oil tempered wire is polished, there are almost no minute flaws.
Next, this electropolished oil tempered wire is subjected to coiling for forming into a spring shape, the residual stress and residual strain generated in the spring are removed by the low temperature annealing treatment, and then the bearing surface is ground.

The electropolishing oil tempered wire that has undergone these steps is nitrided. This nitriding is the same as the conventional one, and a predetermined nitriding layer can be formed by, for example, treating in an ammonia atmosphere at 420 to 550 ° C. for 2 to 6 hours. The obtained nitriding layer can form a harder surface than the conventional oil-tempered wire which is not electrolytically polished.
A surface that is harder than conventional ISOS standard SWOSC-V steel can be formed.

Further, due to nitriding, a compressive residual stress is formed not only on the surface but also inside, and the fatigue resistance is increased. The electrolytically-polished oil-tempered wire of the present invention has a deeper and stronger compressive residual stress than the conventional oil-tempered wire which is not subjected to electrolytic polishing. Further, the alloy steel used this time has a deeper and stronger compressive residual stress formed than the conventional SWOSC-V steel.

In the shot peening step, a shot is struck on the surface of the coil spring which is hardened by nitriding the surface to give a compressive residual stress from the surface to the inner deep part. In this shot peening process, the normal diameter is 0.6 to
A hardness of 1.0 mm and a hardness of Hv in the range of 600 to 800 are used. It is necessary to perform strong shot peening to give compressive residual stress to the inner deep part. However, strong shot peening tends to give insufficient compressive residual stress near the surface. Therefore, it is preferable that the shot peening step is performed in two stages including the first shot peening step and the second shot peening step. The shot used in the first shot peening step is usually one having a diameter of 0.6 to 1.0 mm and a hardness of Hv in the range of 600 to 800, and compressive residual stress is applied to deep inside positions. Preferably, it is formed. In the second shot peening step, the shot used is weaker than the first shot peening step, and the shot used has a diameter of about 0.05 to 0.2 mm and a hardness of Hv in the range of 700 to 900. Is good. This weak shot effectively forms the compressive residual stress on the surface portion.

After that, low temperature annealing is performed to relax abnormal stress and stabilize the compressive residual stress to manufacture a spring having high fatigue resistance and high strength. In the second invention, a degreasing process is performed instead of the low temperature annealing process of the first invention. Also in this case, the electropolished oil-tempered wire of the present invention can form a harder surface than the oil-tempered wire which has not been subjected to conventional electropolishing. In addition, a nitriding step is performed after the degreasing step, and then seat surface grinding is performed. The other steps are the same as the steps of the first invention.

[0013]

In the method for manufacturing a high-strength spring according to the first aspect of the present invention, the steel wire rod subjected to quenching and tempering after wire drawing and then electrolytic polishing is subjected to coiling forming, low temperature annealing, bearing surface grinding, gas nitriding, and then shot peening and low temperature. The feature is that annealing is performed sequentially. Further, in the method for manufacturing a high-strength spring according to the second aspect of the present invention, quenching after wire drawing, tempering, and coiling forming, degreasing, gas nitriding, bearing surface grinding, and then shot peening, low temperature annealing are performed on steel wire rods that have been electrolytically polished. It is characterized by performing sequentially.

In any of the methods, since a thick oxide film such as black skin does not remain in the pre-nitriding step, nitriding effectively enters the steel wire rod. Therefore, the hardened layer becomes deeper, and the subsequent application of compressive residual stress by shot peening forms a compressive residual stress from the outermost surface portion to a relatively deep portion, and a larger compressive residual stress is imparted to the portion closer to the surface. Therefore, a spring having high strength and high fatigue resistance can be manufactured.

[0015]

EXAMPLES The present invention will be described below with reference to examples. (Example 1) As the wire material of the spring used in this example, 0.65% by weight of carbon (hereinafter,% means% by weight unless otherwise specified), silicon 1.36%, manganese 0. An alloy steel composed of 67%, 0.007% phosphorus, 0.01% sulfur, 0.71% chromium, 0.16% vanadium, and the balance iron is cold-drawn, and then quenched and tempered, An alloy steel oil tempered wire having a tensile strength σ в = 2,130 MPa was used.

After that, this alloy steel oil tempered wire was electrolytically polished to remove a thick black skin oxide film and surface scratches to obtain an electrolytically polished wire. The electrolytically polished wire was cold coiled to obtain a wire diameter of 3.2 mm, a coil center diameter of 21.2 mm,
Total number of turns 6.5, effective number of turns 4.5, free length 52mm,
A coil spring having a spring constant of 23.54 N / mm was formed.

Thereafter, this coil spring is heated at 450 ° C. for 1
After low temperature annealing for 5 minutes, the bearing surface was ground. The coil spring whose ground surface was ground was subjected to gas nitriding treatment at 450 ° C. for 5 hours in an ammonia gas atmosphere. This formed a nitride layer on the coil surface. Then, shot peening processing was implemented. Shot peening was performed in two stages. The first shot peening is 0.8mm in diameter,
Shot peening was carried out for 30 minutes under the condition of 70 m / s using a Hv700 cut wire. After that, second shot peening was performed. In the second shot peening, a steel ball having a diameter of 0.15 mm and Hv800 was used, and shot peening was carried out for 30 minutes under the condition of a projection pressure of 5 kgf / cm ² .

Next, low temperature annealing was carried out at 225 ° C. for 5 minutes to remove an abnormally large internal strain, and compressive residual stress was applied to the coil surface to obtain a coil spring of this example. To see the characteristics of the coil spring obtained by the method of this embodiment,
The endurance limit of the coil spring was examined with a star fatigue tester. In the test, average stress σm = 637 MPa, number of tests N = 5 × 1
The test piece was censored 0 ⁷ times, and the number of breakages for each stress amplitude was examined, and the endurance limit was measured at the maximum stress amplitude that 8 of 8 pieces did not break. The durability limit of the coil spring manufactured by the method of this example was ± 560 MPa (average stress σm = 637 MPa).

For comparison, the same alloy steel is used, and electrolytic polishing is not performed but electrolytic polishing is replaced with oxide film removal treatment shot peening (φ0.2SB) before the nitriding step.
Hv500 × 70 m / s × 15 minutes), and a coil spring was manufactured by the same process as in the other examples. The durability of the coil spring obtained by using the non-electrolytic polishing wire for this comparison was ± 510 MPa (average stress σm = 637 MPa).

As described above, the coil spring using the electrolytic polishing wire of this embodiment has a durability limit of +50 MPa (average stress σm = 637) as compared with the coil spring using the non-electrolytic polishing wire.
(MPa) was also large. In order to find out the reason why the coil spring obtained by the method of the present example has a high durability limit, the nitriding in the method of the present example using the electrolytic polishing wire and the method of the comparative example using the non-electrolytic polishing wire The hardness of the nitrided layer after the treatment and the residual stress of the manufactured coil spring were measured.

Regarding the hardness of this nitride layer, the Vickers hardness was measured from the surface to the deep portion on the cross section by cutting the coil spring after nitriding treatment and before shot peening.
The measurement results are shown in FIG. The relationship between the Vickers hardness (Hv) of the coil spring using the electrolytically-polished wire of this example and the depth from the surface is shown by a black circle (● in the figure) and a broken line. Similarly, the relationship between the Vickers hardness (Hv) of the coil spring using the non-electrolytic polishing wire of the comparative example and the depth from the surface is indicated by an open circle (○ in the figure).
Is indicated by a solid line.

As is apparent from FIG. 1, the hardness of the outermost surface of the coil spring using the electrolytic polishing wire of this embodiment has a hardness of about Hv76.
The hardness was extremely high as 0, and the hardness of the outermost surface of the coil spring using the non-electrolytic polishing wire was about Hv 720, and was 40 different at Hv. The hardness of the surface portion of the coil spring using the electrolytic polishing wire of this example is 0.1 mm deeper than the surface, and is higher than the hardness of the surface portion of the coil spring using the non-electrolytic polishing wire. Regarding the hardness at a depth exceeding 0.1 mm, there was no great difference between the coil spring using the electrolytic polishing wire of the present example and the coil spring using the non-electrolytic polishing wire.

It is clear that the hardness of the surface layer portion of the nitrided layer shown in FIG. 1 is caused by the presence or absence of electrolytic polishing. When electrolytic polishing is performed, the hardness of the surface portion is further improved by nitriding. Example 1
FIG. 2 shows the result of examining the distribution of the compressive residual stress of the coil spring using the electrolytic polishing wire of No. 1. The relationship between the residual stress (MPa) and the depth from the surface of the coil spring using the electrolytically-polished wire of this example is shown by a black triangle (▲ in the figure). Similarly, the residual stress of the coil spring using the non-electrolytic polishing wire of the comparative example (MP
The relationship between a) and the depth from the surface is indicated by a white triangle circle (Δ in the figure). The residual stress of the coil spring using the electrolytic polishing wire of the example is about -930 MPa on the surface, and the residual stress of the coil spring using the non-electrolytic polishing wire of the comparative example is about -850 MPa on the surface, which is about -80 MPa. There was a difference.

It is considered that this difference in hardness and residual stress may be the difference in durability of the manufactured coil spring. (Example 2) In this example, instead of the alloy steel used in Example 1, JIS standard SWOSC-V steel material (carbon 0.57) was used.
% By weight, 1.42% silicon, 0.67% manganese, 0.1% phosphorus.
012%, sulfur 0.005%, chromium 0.72%, balance iron) cold drawn, then quenched and tempered,
An alloy steel oil tempered wire having a tensile strength σв = 1,961 Mpa was used. Then, the same steps as in Example 1 were carried out to manufacture a coil spring.

The durability of the coil spring manufactured by the manufacturing method of Example 2 is ± 530 MPa (average stress σm = 63).
7 MPa). For comparison, using the same alloy steel as that used in this example, instead of electrolytic polishing without electrolytic polishing, an oxide film removal treatment was performed before the nitriding step, and the other examples were used. A coil spring was manufactured by the same process as above.

The durability of the coil spring obtained by using the non-electrolytic polishing wire for comparison is ± 470 MPa (average stress σm
= 637 MPa). As described above, the coil spring using the electrolytic polishing wire of the present example had a large durability limit of +60 MPa (average stress σm = 637 MPa) as compared with the coil spring using the non-electrolytic polishing wire.

(Example 3) In this example, the same alloy steel as that used in Example 1 was cold-drawn and then quenched,
Tempered, same as in Example 1, tensile strength σв =
An alloy steel oil tempered wire of 1,961 Mpa was used. This alloy steel oil tempered wire was electrolytically polished in the same manner as in Example 1 to remove a thick black skin oxide film and surface scratches to obtain an electrolytically polished wire. After that, the electrolytic polishing wire was cold coiled in the same manner as in Example 1.

After that, the coil spring was degreased in place of low temperature annealing and seat surface grinding in Example 1. This degreasing step was performed with an organic solvent (eg, CFC substitute, ketones, alcohols, etc.). Then, nitriding was performed in exactly the same manner as in Example 1. Then, the bearing surface of the nitriding coil spring was ground. Then, two steps of shot peening and low temperature annealing were performed in exactly the same manner as in Example 1 to manufacture a coil spring.

In order to examine the characteristics of the coil spring obtained by the method of this embodiment, the durability limit of the coil spring was examined by the same method as in the first embodiment. The durability of the coil spring manufactured by the manufacturing method of this embodiment is ± 565 MPa (average stress σm = 637 M).
It was Pa). For comparison, the same alloy steel used in this example was used, and electrolytic film removal was performed without electrolytic polishing, and instead of electrolytic polishing, oxide film removal treatment (φ0.
2SBHv500 × 70 m / s × 15 minutes), and a coil spring was manufactured by the same process as in the other examples.

The durability limit of the coil spring obtained by using the non-electrolytic polishing wire for this comparison is ± 510 MPa (average stress σm
= 637 MPa). As described above, the coil spring using the electrolytic polishing wire of the present example had a large durability limit of +55 MPa (average stress σm = 637 MPa) as compared with the coil spring using the non-electrolytic polishing wire. (Example 4) In this example, the same JIS standard SW as that used in Example 2 instead of the alloy steel used in Example 3 was used.
OSC-V steel (carbon 0.57% by weight, silicon 1.42)
%, Manganese 0.67%, phosphorus 0.012%, sulfur 0.0
(05%, chromium 0.72%, balance iron) is cold-drawn, then quenched and tempered to obtain tensile strength σв = 1,
A 961 MPa alloy steel oil tempered wire was used. Then, the same steps as in Example 3 were carried out to manufacture a coil spring.

The durability of the coil spring manufactured by the manufacturing method of Example 4 is ± 525 MPa (average stress σm = 63).
7 MPa). For comparison, using the same steel material as that used in this example, instead of electrolytic polishing without electrolytic polishing, an oxide film removal treatment was performed before the nitriding step, and the other examples. A coil spring was manufactured in exactly the same process.

The durability of the coil spring obtained by using the non-electrolytic polishing wire for comparison is ± 465 MPa (average stress σm
= 637 MPa). As described above, the coil spring using the electrolytic polishing wire of the present example had a larger durability limit of +60 MPa (average stress σm = 637 MPa) than the coil spring using the non-electrolytic polishing wire.

[0033]

INDUSTRIAL APPLICABILITY In the method for manufacturing a high strength spring of the present invention, nitriding is more effective because the oil-tempered wire that has been conventionally used is electrolytically polished to remove the thick oxide film on the black skin before nitriding. Done deeper. Therefore, in the next step, the compressive residual stress due to shot peening is applied to the outermost surface portion to be larger and deeper. Therefore, fatigue resistance and durability are improved.

Further, in the method for manufacturing a high strength spring of the present invention,
In order to perform shot peening after increasing the hardness by nitriding without performing surface area activation shot before nitriding, by performing electrolytic polishing on the oil-tempered wire that has been conventionally used to remove minute flaws by polishing the surface, The surface roughness of the final product is reduced, and the fatigue resistance and strength are further improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a hardness distribution from the surface to the inside due to nitriding of a coil spring using an electrolytic polishing wire of Example 1 and a coil spring using a conventional non-electrolytic polishing wire.

FIG. 2 is a diagram showing the distribution of the compressive residual stress from the surface to the inside of the coil spring using the electrolytic polishing wire of Example 1 and the coil spring using the conventional non-electrolytic polishing wire.

Claims (4)

[Claims] 1. A steel wire rod, which has been subjected to quenching and tempering after drawing and then electrolytic polishing, is subjected to coiling forming, low temperature annealing, bearing surface grinding, gas nitriding, and then shot peening,
A method for manufacturing a high-strength spring, which comprises sequentially performing low-temperature annealing. 2. The high strength spring according to claim 1, wherein the shot peening comprises a first shot peening and a second shot peening performed by using a shot having a diameter smaller than the shot used in the first shot peening. Production method. 3. A high-strength material, which is obtained by sequentially performing quenching, degreasing, gas nitriding, bearing surface grinding, shot peening and low temperature annealing on an iron and steel wire rod that has been hardened after wire drawing, tempered, and then electrolytically polished. Spring manufacturing method. 4. The high strength spring according to claim 3, wherein the shot peening comprises a first shot peening and a second shot peening performed by using a shot having a diameter smaller than the shot used in the first shot peening. Production method.