JP4152364B2 - High strength coil spring and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength coil spring used for a damper spring of a clutch torsion of an automatic transmission or an engine valve spring in an automatic vehicle.

  At present, there are three types of JISG3561 SWO-V, SWOCV-V and SWOSC-V that are standardized as materials for valve springs. Among these, SWOSC-V, which is a silicon chrome steel oil tempered wire, is widely used since it has excellent fatigue strength and sag resistance. However, coil springs used for valve springs for automobile engines and damper springs for automatic transmissions have high strength and high fatigue resistance because there is a high need for high-speed rotation, compactness, and low cost. However, there is a constant demand for coil springs that are inexpensive.

The inventors have already proposed a high-strength coil spring that is inexpensive and has high impact resistance using a high-strength oil tempered wire as a raw material (see Patent Document 1). Recently, the coil spring has higher strength and higher fatigue resistance. Is now required.
JP 2003-193197 A

  An object of the present invention is to provide an inexpensive high-strength coil spring having higher strength and fatigue resistance than conventional high-strength coil springs, and a method for manufacturing the same.

The high-strength coil spring of the present invention is, in mass%, C: 0.63 to 0.68%, Si: 2.1 to 2.3%, Mn: 0.5 to 0.7%, Cr: 1. 1 to 1.3%, V: 0.1 to 0.3%, Co: 0.1 to 0.3% , the balance consists of Fe and inevitable impurities, P of the inevitable impurities, : 0.05% or less, S: 0.025% or less, Cu: 0.2% or less, maximum non-metallic inclusions of 15 μm, tensile strength of 2000 MPa or more, drawing of 38% or more of oil tempered wire The thickness of the white layer formed by nitriding is 4 μm or less, the hardness of the surface layer portion (around 0.02 mm from the surface) is Hv 700 to 900, and the internal hardness is 0.3 mm from the surface Hv 580 or more, the maximum surface roughness R _max is 5 μm or less , and the average stress is 750 MPa. And having an endurance of 1 × 10 ⁷ times or more when the amplitude stress is 640 MPa .

  A high-strength coil spring with high fatigue resistance can be obtained by manufacturing under a combination of the above conditions.

(Spring material)
The high-strength coil spring according to the present invention is in mass%, and in mass%, C: 0.63 to 0.68%, Si: 2.1 to 2.3%, Mn: 0.5 to 0.7% , Cr: 1.1 to 1.3%, V: 0.1 to 0.3%, Co: 0.1 to 0.3% , the balance being Fe and inevitable impurities, inevitable impurities Among them, P: 0.05% or less, S: 0.025% or less, Cu: 0.2% or less, and using a steel oil tempered wire having a maximum non-metallic inclusion of 15 μm as a material. It is a feature.

  Here, the reason for limiting the composition of the oil temper wire to the above range is as follows.

C: 0.55-0.75%
C is an essential element for increasing the strength of the steel wire. However, if it is less than 0.55%, sufficient strength cannot be obtained, and if it exceeds 0.75%, the toughness is lowered, and the steel wire is more susceptible to scratches. Increases and decreases reliability. Preferably, it is 0.63 to 0.68%.

Si: 1.80 to 2.70%
Si is an element effective for improving the strength of ferrite and improving sag resistance. If it is less than 1.80%, the sufficient effect is not obtained, and if it exceeds 2.70%, cold workability is lowered and decarburization by hot workability or heat treatment is promoted. That is, the moldability of the coil spring is reduced. Preferably, it is 2.1 to 2.3%.

Mn: 0.1 to 0.9%
Mn improves the hardenability of the steel and fixes S in the steel to prevent its damage. However, if it is less than 0.1%, there is no effect, and if it exceeds 0.9%, the toughness decreases. Preferably, it is 0.5 to 0.7%.

Cr: 0.70 to 1.50%
Cr, like Mn, improves the hardenability of steel and gives toughness by patenting after hot rolling, and after hardening, increases softening resistance during tempering and increases strength. It is an effective element. If it is less than 0.70%, the effect is small, and if it exceeds 1.50%, solid solution of carbides is suppressed, the strength is reduced, and the hardenability is excessively increased, resulting in a decrease in toughness. Preferably, it is 1.1 to 1.3%.

V: 0.05 to 0.50%
V is an element that forms carbides during tempering and increases the softening resistance. However, if it is less than 0.05%, the formation of carbides is extremely small, so the effect is small. On the other hand, if it exceeds 0.50%, the carbide becomes coarse during quenching heating, resulting in a decrease in toughness. Preferably, it is 0.1 to 0.3%.

Co: 0.05-0.40%
Co is an element effective for improving the toughness after quenching and tempering. In order to acquire the effect of a toughness improvement, it is preferable to contain 0.05% or more. However, even if the content is 0.40% or more, this effect is saturated and no further improvement in toughness is observed. More preferably, it is 0.1 to 0.3%.

Then, the maximum non-ferrous metal inclusions of the steel wire is Ru 15μm der. The non-metallic inclusions are often made of Al ₂ O ₃ or TiO, but these inclusions are hard, so if they are present near the surface of the steel wire, the fatigue strength is significantly reduced. If the maximum non-ferrous metal inclusion is smaller than 15 μm, it is not a problem.

Oil-tempered wire for use in the present invention has a tensile strength of at least 2000 MPa, Ru der diaphragm 38% or more. If the tensile strength is less than 2000 MPa, the fatigue strength of the spring is lowered, which is not desirable. Preferably it is 2100-2300 MPa. Further, if the aperture is less than 38%, the formation of the spring becomes a problem, so 38 to 55% is appropriate.

(Coil spring)
In the high-strength coil spring of the present invention having the above composition, the thickness of the white layer formed by nitriding is 4 μm or less.

  By nitriding the coil spring, a so-called white layer mainly composed of nitride or carbonitride is formed near the surface of the coil. Since this white layer has the property of being very hard and brittle, it becomes a factor of reducing the shock resistance of the coil spring. For this reason, the thickness of the white layer after nitriding is set to 4 μm or less. In particular, in a coil spring that receives a large impact force such as a damper spring for a clutch torsion of an automatic transmission, it is desirable that it be 2 μm or less.

  Moreover, the hardness of the surface layer part (around 0.02 mm from the surface) of the coil spring is Hv 700 to 900. If the surface layer hardness of the coil spring after nitriding is less than Hv700, a sufficient compressive residual stress cannot be formed by the two-step shot peening in the next step, and therefore a predetermined fatigue strength cannot be imparted. On the other hand, if it exceeds Hv900, the toughness decreases and the predetermined impact resistance cannot be obtained. More preferably, it is Hv750-850.

  Furthermore, the internal hardness of 0.3 mm from the surface of the coil spring is Hv580 or more. If the internal hardness is low, the spring may break from the inside, resulting in a decrease in fatigue resistance. The internal hardness is preferably Hv600 or more.

  For the thickness, surface layer hardness, and internal hardness of the coil spring as described above, the temperature and time of nitriding treatment or the temperature and time of low-temperature annealing treatment after coiling are appropriately selected. It can be adjusted by combining.

The surface of the coil spring after the shot peening, the maximum roughness R _max is Ru der less 5 [mu] m. If R _max exceeds 5 μm, it may be a starting point for fatigue failure, which is not appropriate. More preferably, it is 4 μm or less. In JISB0601 (revised January 2001), the description of the maximum height roughness was revised as Rz. However, in order to avoid misunderstanding with the previous ten-point average roughness Rz, in this specification the maximum roughness is defined as R. _{Expressed as max} .

(Manufacturing method of coil spring)
The high-strength coil spring of the present invention is a coiling process for forming a coil spring using the oil tempered wire having the above mechanical properties, a micro-shot process for removing surface scale, and a nitriding process for improving surface hardness. It can be obtained via a two-stage shot peening process that imparts residual stress.

  First, an oil temper wire having the above-described chemical composition is cold coiled to form a spring shape. Thereafter, it is preferable to perform low-temperature heat treatment to remove residual stress and residual distortion generated during coil forming. Generally, heat treatment is preferably performed at 350 to 500 ° C. for 3 to 60 minutes in an air atmosphere. Preferably, the heat treatment is performed at 400 to 480 ° C. for 5 to 20 minutes.

  Next, a descale process is performed prior to nitriding. The descaling process is a step of removing the oxide film on the surface of the coil-formed spring material, and enables uniform nitriding by removing the oxide film.

In the descaling process, it is preferable that the maximum roughness R _max of the surface of the spring material is 5 μm or less. When R _max exceeds 5 μm, the uniformity of nitriding becomes insufficient, and surface polishing of the obtained coil spring may be necessary. The descale treatment is performed by shot peening using a condition that causes relatively weak blasting, that is, micro-shot, so as not to increase the surface roughness of the spring material. Examples of micro shots include a method of using relatively soft glass beads and abrasive grains, using a fine cut wire having a diameter of 0.3 mm or less, or using a steel shot having a diameter of 0.3 mm or less. it can. By these methods, the maximum roughness R _max of the surface roughness of the spring material can be reduced to 5 μm or less. By performing descale by micro shot, the oxide film can be removed and nitriding in the next step can be facilitated.

  The nitriding treatment can be performed in an atmosphere such as nitrogen gas or ammonia gas. In order to suppress the formation of the white layer to 4 μm or less, soft nitriding treatment with ammonia gas is performed at 400 to 480 ° C. for 2 to 4 hours. Nitriding is desirable. When the nitriding temperature is less than 400 ° C., the surface layer of the coil spring becomes insufficiently cured, and when it exceeds 480 ° C., the internal hardness is lowered and a high-strength coil spring with high fatigue resistance cannot be obtained.

  Furthermore, if the nitriding time is less than 2 hours, the formation of a hardened layer accompanying the nitriding treatment becomes non-uniform, and if it exceeds 4 hours, a more effective nitrided layer suitable for long-time treatment cannot be obtained. Long-time treatment is not preferable in terms of cost.

  Next, two-stage shot peening is applied to the coil spring after nitriding. Shot peening is performed for the purpose of increasing the fatigue strength of the coil spring by applying residual stress to the coil spring that has been hardened by nitriding the surface portion. The residual stress is desirably high in the surface layer portion and deeply applied to the inside, and therefore, it is preferable to perform two-stage shot peening of the first and second different shot methods.

  In the first shot peening process, first, a shot having a large particle diameter is projected onto a coil spring at a high speed, for example, to apply a residual stress to a deep position inside the surface.

  In the second shot peening process, a shot having a particle diameter smaller than that used in the first shot peening process is used for projection, and a larger residual stress is applied to the surface portion. In this step, the effect can be further enhanced by using one having a higher hardness than the shot used in the first shot peening step or by projecting the shot at a high speed.

  By the above steps, since the residual stress on the surface is high and the residual stress can be applied to a deep position inside, the fatigue strength of the coil spring can be greatly improved.

  The shot used in the first shot peening step is preferably a shot having a diameter of 0.4 to 1.0 mm and a hardness in the range of Hv 500 to 800 in order to apply residual stress to a deep position inside.

In the second shot peening process, in order to increase the residual stress of the surface portion, a shot having a diameter of about 0.05 to 0.3 mm and a hardness of Hv 700 to 900 is used, which is smaller than that in the first shot peening process. It is preferable. In this case, it is desirable to project a shot with high-pressure air, and a remarkably high residual stress can be formed near the surface by this high-pressure projection. However, in order to set the maximum height roughness R _max of the surface to 5 μm or less, it is preferable to set the air pressure to 0.1 to 0.4 MPa and to perform shot peening for about 1 to 10 minutes.

  Subsequently, it is desirable to subject the coil spring after the two-stage shot peening to low temperature annealing. This low-temperature annealing is performed for the purpose of removing abnormally large internal strain caused by shot peening, and is generally a heat treatment at 200 to 300 ° C. for about 3 to 60 minutes.

  As described above, it is possible to obtain a high-strength coil spring having lower strength and higher strength and toughness than conventional ones. This high-strength coil spring can be suitably used as a damper spring for clutch torsion of an automatic transmission of an automatic vehicle or a valve spring of an engine.

  Hereinafter, specific examples will be described.

[Sample preparation]
( Reference Example 1)
As a material of the coil spring, C: 0.65% by mass (hereinafter referred to as mass% unless otherwise specified), Si: 2.23%, Mn: 0.54%, Cr: 1.18%, V: 0 .15%, Co: 0.20%, unavoidable impurities include P: 0.01%, S: 0.005%, Cu: 0.003%, with the balance being Fe, and tensile strength An alloy steel oil tempered wire having a length of 2246 MPa and a drawing of 50% was used. The maximum nonmetallic inclusion in the oil tempered wire was 15 μm.

  Coiled with the above oil tempered wire, wire diameter: 3.2 mm, coil center diameter: 20.2 mm, total number of turns: 5.9 turns, effective number of turns: 3.9 turns, free height: 47.0 mm, spring constant : A 30.0 N / mm coil spring was molded.

  Next, this coil spring was heat-treated at 435 ° C. for 10 minutes to remove residual stress and residual strain generated during coil forming. Thereafter, a micro-shot for 20 minutes was performed using a steel ball having a diameter of 0.2 mm to remove the oxide film on the surface.

  Next, nitriding treatment was performed at 440 ° C. for 3 hours in an ammonia gas atmosphere to form a nitride layer on the coil surface.

  In the first shot peening process, shot peening was performed for 10 minutes using a round cut wire having a diameter of 0.6 mm after nitriding.

In the second shot peening process, a round cut wire having a diameter of 0.25 mm was projected with air for 5 minutes to perform shot peening and to apply compressive residual stress to the coil surface. Next, low-temperature annealing at 230 ° C. for 10 minutes was performed to remove abnormally large internal strain, and a high-strength coil spring of Reference Example 1 was obtained.

(Example 2)
Exactly the same shape, manufacturing process and manufacturing conditions as in Reference Example 1 except that an oil tempered wire having the same composition as in Reference Example 1 and the same mechanical properties was used, and the nitriding conditions were 450 ° C. × 3 hours. Thus, a high-strength coil spring of Example 2 was obtained.

(Example 3)
Except for using an oil tempered wire having the same composition as in Reference Example 1 and the same mechanical properties, the shape, manufacturing process and manufacturing conditions were exactly the same as in Reference Example 1 except that the nitriding conditions were 475 ° C. × 3 hours. Thus, a high-strength coil spring of Example 3 was obtained.

(Comparative example)
As the material of the coil spring, C: 0.64%, Si: 2.06%, Mn: 0.78%, Cr: 0.70%, V: 0.07%, and unavoidable impurities are P: 0.008%, S: 0.014%, Cu: includes 0.04%, the balance being Fe, a tensile strength of 2103MPa, diaphragm with alloy steel oil-tempered wire 50.7% reference example 1 was formed, and then a comparative example coil spring was obtained in the same manner as in Reference Example 1.

[Evaluation]
(Evaluation methods)
About each coil spring (henceforth a sample) obtained above, the thickness of the white layer, surface layer part hardness, internal hardness, the maximum roughness of the surface, and the fatigue characteristic were measured . The white layer thickness, surface layer hardness, internal hardness, and maximum surface roughness were measured at several predetermined points on the inner peripheral side of the coil spring. The measuring method is as follows.

  (A) The thickness of the white layer is determined by polishing the cross section of each obtained sample, etching with nital to clarify the white layer, and measuring with a metallurgical microscope to obtain the maximum and minimum values in the field of view. It was.

(B) In the hardness test, the cross section of each obtained sample was polished, and the surface layer portion 0.02 mm from the surface of the spring to the inside and the inside containing 0.3 mm were obtained with micro Vickers (load 50 g). Two points were measured and each averaged. Incidentally, to measure the hardness distribution of the coil spring section for Comparative Examples and Best Embodiment 2 (FIG. 1).

(C) The maximum surface roughness R _max was measured according to JIS B0601 (1982 edition).

(D) Fatigue properties were tested with a star spring fatigue tester at an average stress of 750 MPa, a stress amplitude of 640 MPa, and a rotation speed of 1800 rpm, and at the same time up to 10 ⁷ times out of 8 samples subjected to the test. The number of samples that breaks was determined as the breakage probability.

  The measurement results are shown in Table 1 and FIG.

(Evaluation results)
Examples 2 and 3 were all manufactured under the same conditions except for the nitriding temperature. It can be seen that the hardness of the surface layer portion increases and the internal hardness tends to decrease as the nitriding temperature increases. Further, the surface roughness is reduced as the surface layer hardness increases. Although the white layer thickness is slightly recognized in Example 3, it is within the limited range of the present invention. Since Example 2, 3 having these properties, fracture probability found to have excellent fatigue properties and 0/8.

However, in the comparative example produced under the same conditions as in Reference Example 1 except for the material, both the surface layer hardness and the internal hardness were lower than in Reference Example 1. For this reason, in the fatigue test, 7 out of 8 pieces subjected to the test were broken at the same time.

Incidentally, the hardness distribution in the radial direction in the cross section of the spring shown for the comparative example and our Example 2 in Figure 1. It can be seen that Example 2 has a high hardness not only in the vicinity of the surface layer but also in the center as compared with the comparative example.

  As described above, according to the present invention, it has been found that a high-strength coil spring having much higher durability than that of the comparative example that is the prior art can be obtained without increasing the manufacturing cost.

  The high-strength coil spring of the present invention is suitable for use as a damper spring for a clutch torsion of an automatic transmission or an engine valve spring in an automatic vehicle.

It is a graph which shows an example of the hardness distribution of the radial direction in the cross section of a spring.

Claims (1)

In mass%, C: 0.63 to 0.68%, Si: 2.1 to 2.3%, Mn: 0.5 to 0.7%, Cr: 1.1 to 1.3%, V: 0.1 to 0.3%, Co: 0.1 to 0.3%, with the balance being Fe and unavoidable impurities, among the unavoidable impurities, P: 0.05% or less, S: 0.025% or less, Cu: 0.2% or less, the maximum non-metallic inclusions are 15 μm, the tensile strength is 2000 MPa or more, and the steel oil temper wire having a drawing of 38% or more is used as a raw material, and is formed by nitriding treatment. The thickness of the white layer is 4 μm or less, the hardness of the surface layer portion (around 0.02 mm from the surface) is Hv 700 to 900, the internal hardness of 0.3 mm from the surface is Hv 580 or more, and the maximum surface When the roughness R _max is 5 μm or less, the average stress is 750 MPa, and the amplitude stress is 640 MPa A high-strength coil spring characterized by having a durability of 1 × 10 ⁷ times or more .

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