JP2009270150A - Method for manufacturing coil spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a coil spring which can stably show high fatigue characteristics. <P>SOLUTION: The method for manufacturing the coil spring includes: a forming step of forming the coil spring by employing an oil-tempered alloy-steel wire as a raw material; a nitriding treatment step of enhancing the surface hardness of the coil spring; and a shot peening step of imparting residual stress to the coil spring. The method further includes electrolytically polishing the coil spring after the shot peening step to remove the surface layer of the coil spring, control the thickness of a white layer formed in the nitriding treatment step to 5 μm or less, simultaneously control the surface roughness to 4 μm or less by Rc and also control the compressive residual stress of the outermost surface to 1,800 MPa or higher. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a coil spring.

  High energy shot peening as a method of manufacturing high strength and high fatigue resistance springs by applying high tensile strength wire to coiling into a predetermined shape and applying high compressive residual stress after surface hardening by nitriding treatment The method of applying is known (for example, Patent Document 1).

However, when higher strength and higher fatigue properties are required, high compressive residual stress that matches the required fatigue properties can be applied to the spring surface unless shot peening treatment with higher energy than before is applied. I can't. When such strong shot peening is performed after nitriding, the spring surface becomes extremely rough and fine surface defects are generated on the entire surface. For this reason, the spring has a problem that the sensitivity to scratches is increased, resulting in a decrease in fatigue characteristics.
Japanese Patent Laid-Open No. 10-118930

  The present invention has been made to solve the above-mentioned problems. The surface of the coil spring is subjected to nitriding treatment to increase the hardness, and the shot peening is applied with higher strength than before to give high compressive residual stress. Even so, it is an object to provide a method of manufacturing a coil spring that can stably exhibit high fatigue characteristics.

  The method for manufacturing a coil spring according to the present invention includes a forming step of forming a coil spring using an alloy steel oil temper wire as a raw material, a nitriding treatment step for improving the surface hardness of the coil spring, and applying residual stress to the coil spring. In a method for manufacturing a coil spring having a shot peening process, the surface layer of the coil spring is removed by performing electropolishing after the shot peening process, so that the thickness of the white layer generated by the nitriding treatment is 5 μm or less. In addition, the surface roughness is 4 μm or less in terms of Rc, and the compressive residual stress on the outermost surface is 1800 MPa or more.

  In the coil spring manufacturing method of the present invention, the surface hardness of the coil spring after the nitriding treatment is desirably 850 to 1400 HV, and the surface layer of the coil spring is preferably removed by 3 to 20 μm by electrolytic polishing.

  Moreover, in the manufacturing method of the coil spring of this invention, the said alloy steel oil temper wire is C: 0.55-0.75% in mass%, Si: 1.00-2.70%, Mn: 0.10. -1.50%, Cr: 0.4-2.50%, V: 0.05-0.50%, preferably consisting of the balance Fe and unavoidable impurities, Mo: 0.05- Any one of 2.0% and Co: 0.05 to 0.40% can be included.

  In the present invention, by performing electropolishing after shot peening, the surface layer portion of the coil spring is removed so that the thickness of the harmful white layer formed on the surface of the coil spring in the nitriding process is 5 μm or less. Therefore, according to the present invention, the impact resistance and fatigue characteristics of the coil spring can be improved.

  Further, in the present invention, fine surface defects due to shot indentation are removed by performing electropolishing so that the surface roughness of the spring surface is set to 4 μm or less in terms of Rc. The working stress of the spring is maximized on the surface. Therefore, according to the present invention, the notch or wrinkle of the coil spring can be reduced to improve the impact resistance and fatigue characteristics.

  In general, the compressive residual stress applied by shot peening is maximized somewhat inside from the surface of the spring. In the present invention, since the surface layer portion of the coil spring is removed by electrolytic polishing, the compressive residual stress on the outermost surface of the coil spring can be brought close to the maximum value. In addition, since the surface of the working stress of the spring is maximized, according to the present invention, the applied compressive residual stress can be most effectively used to improve the fatigue characteristics of the coil spring.

  Hereinafter, a preferred embodiment of the present invention will be described in detail. In the present embodiment, C: 0.55 to 0.75%, Si: 1.00 to 2.70%, Mn: 0.10 to 1.50%, Cr: 0.4 to 2% by mass%. An alloy steel oil tempered wire containing 50%, V: 0.05 to 0.50%, and the balance Fe and unavoidable impurities may be used as a material. As an oil tempered wire having such a composition, those disclosed in JP-A-2-107746 are known.

  Further, an alloy steel oil tempered wire containing 0.05 to 2.0% of Mo in the composition (disclosed in Japanese Patent Laid-Open No. 2003-3241) and an alloy containing 0.05 to 0.40% of Co. A steel oil tempered wire (disclosed in JP-A No. 2004-190116) can also be suitably used. As these alloy steel oil tempered wires, those having a tensile strength of 1900 to 2300 MPa and a drawing of 35% or more may be used.

  First, an alloy steel oil temper wire having a desired chemical composition and tensile strength is cold-coiled and formed into a desired spring shape. Thereafter, a low-temperature heat treatment is performed at 350 to 500 ° C. for 3 to 60 minutes in an air atmosphere to remove residual stress and residual strain generated during coil forming.

  Next, a descale process is performed prior to the nitriding process. The descaling process is a process of removing the oxide film on the surface of the spring material that has been coil-formed, and enables uniform nitriding by removing the oxide film. In this descale process, the maximum height Rz of the surface of the spring material is preferably 5 μm or less. If the maximum height Rz of the spring material surface exceeds 5 μm, nitriding uniformity may not be obtained, which is not preferable. As the descaling process, conditions such that the spring material is relatively weakly blasted, for example, shot peening using micro shots, are applied so as not to increase the surface roughness of the spring material. Examples of the micro shot include relatively soft glass beads and abrasive grains, a cut wire having a diameter of 0.3 mm or less, or a steel shot having a diameter of 0.3 mm or less. By these micro shots, the maximum height Rz of the spring material surface can be reduced to 5 μm or less.

  The nitriding treatment can be performed in an atmosphere such as nitrogen gas or ammonia gas, but gas nitriding treatment with ammonia gas at 400 to 500 ° C. for 2 to 48 hours is desirable. When the nitriding temperature is less than 400 ° C., the surface layer of the coil spring becomes insufficiently cured, while when it exceeds 500 ° C., the internal hardness is lowered and a high-strength coil spring having excellent fatigue characteristics 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 even if it exceeds 48 hours, the hardening is saturated and a more effective nitrided layer suitable for long-time processing can be obtained. It is uneconomical because it cannot be done.

  By performing the nitriding treatment as described above, the hardness (from the surface to the vicinity of 0.02 mm) of the coil spring after the nitriding treatment is set to 850 to 1400 HV. If the surface hardness after the nitriding treatment is less than 850 HV, a sufficient compressive residual stress cannot be formed by shot peening in the next step, and thus a desired fatigue strength cannot be imparted. On the other hand, if it exceeds 1400 HV, the toughness decreases and the desired impact resistance cannot be obtained. More preferably, it is 850-1200HV.

  Next, shot peening is applied to the coil spring after nitriding. In this embodiment, two or more stages of multi-step shot peening with different shot methods are applied in order to impart a high compressive residual stress at the surface layer and deep inside.

  For example, in the first-stage shot peening process, first, a shot having a large particle size is projected onto a coil spring at a high speed to apply a residual stress from the surface to a deep position inside. In the second-stage shot peening process, a shot having a particle size smaller than that used in the first-stage shot peening process is used for projection, and a larger residual stress is applied to the extreme surface layer 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.

  As a shot used in the first shot peening step, in order to give a residual stress to a deep position inside, a shot having a diameter of 0.4 to 1.0 mm and a hardness of 500 to 800 HV, for example, Conditioned cut wires are preferred.

  In the second shot peening process, it is preferable to use a shot having a smaller diameter and a higher hardness than the shot used in the first shot peening process in order to increase the residual stress on the surface portion. Specifically, it is preferable to use a shot having a diameter of about 0.05 to 0.3 mm and a hardness of 700 to 1500 HV, such as a conditioned cut wire or a carbide shot. 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. Further, third-stage shot peening may be performed as necessary. By performing such multi-stage shot peening, a high compressive residual stress can be applied to the surface and the fatigue characteristics of the coil spring can be improved.

  Subsequently, the coil spring after shot peening is annealed at a low temperature. 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.

  Next, the coil spring after the low-temperature annealing is subjected to electrolytic polishing. In the present embodiment, as described above, the coil spring after descaling is subjected to nitriding treatment, and a hardened layer by nitriding is formed on the surface layer portion, so that the strength of the coil spring is further increased. However, when nitriding is performed, a white layer mainly composed of nitride or carbonitride is formed on the surface of the coil spring. The thickness of the white layer to be formed is about 1 to 10 μm, although it varies depending on the nitriding conditions and the site. Since this white layer has the property of being very hard and brittle, it causes a reduction in the shock resistance and fatigue characteristics of the coil spring. Therefore, the surface layer portion of the coil spring is removed by electropolishing so that the thickness of the harmful white layer is 5 μm or less (preferably 3 μm or less). By making the thickness of the white layer 5 μm or less, the impact resistance and fatigue characteristics of the coil spring can be improved.

  Further, the surface of the coil spring after peening is extremely roughened with a maximum surface height Rz of 7 μm or more because of high-intensity shot peening. At the same time, many fine surface defects are generated due to shot indentations. For this reason, this surface defect is removed by electropolishing, and the surface roughness is set to 4 μm or less (preferably 3 μm or less) in terms of Rc to reduce the scratch sensitivity of the coil spring. As parameters indicating the surface roughness, JIS B 0601 defines an average roughness Ra, a maximum height Rz, an average height Rc, and the like. The reason why the average height Rc is evaluated here is that Rc is a parameter calculated by averaging the heights, and is considered optimal for expressing the overall surface roughness.

  The compressive residual stress imparted by shot peening varies depending on the hardness, size, speed, number of peenings, etc. of the shot, but is generally maximum within 2 to 10 μm from the spring surface. Therefore, the surface layer of the coil spring is preferably removed by electrolytic polishing so that the compressive residual stress is closer to the maximum value on the outermost surface.

  As described above, the surface layer of the coil spring is subjected to electropolishing after the shot peening process in order to realize removal of harmful white layers, removal of minute surface defects due to shot indentations, and removal of low compressive residual stress portions. Is preferably removed by 3 to 20 μm. If the removal amount of the surface layer is less than 3 μm, a white layer of 5 μm or more may remain, which is not preferable. Further, if it is removed by 20 μm or more, the compressive residual stress on the outermost surface may be lowered, which is not preferable. More preferably, 3 to 10 μm is removed. In addition, as described above, the coil spring is subjected to low-temperature annealing and further setting after the shot peening process, but this electrolytic polishing treatment process should be performed at any stage after the shot peening process. Can do.

Example 1
As a material of the coil spring, C: 0.65 mass% (hereinafter the same), Si: 2.23%, Mn: 0.54%, Cr: 1.18%, V: 0.15%, Co: 0 An alloy steel oil tempered wire having a balance of Fe and inevitable impurities at 20%, tensile strength of 2252 MPa, and drawing of 50% was used.

  The oil temper wire is coiled, and the wire diameter is 3.2 mm, the coil outer diameter is 23.2 mm, the total number of windings is 5.9, the effective number of windings is 3.9, the free height is 47 mm, and the spring constant is 32 N / A coil spring of mm was formed.

  Next, these coil springs were soaked at 450 ° C. for 7 minutes to remove residual stress and residual strain generated during coil forming. Thereafter, a micro-shot for 20 minutes was performed using a 0.2 mm steel ball to remove the oxide film on the surface.

  Next, nitriding treatment was performed at 500 ° C. for 4 hours in an ammonia gas atmosphere to form a nitrided layer on the coil surface. The surface hardness after nitriding was 850 to 950 HV.

  The first shot peening was performed for 60 minutes using a conditioned cut wire having a diameter of 0.8 mm after the nitriding treatment. Subsequently, a conditional cut wire having a diameter of 0.25 mm was projected with air for 5 minutes to give a second shot peening, thereby applying a compressive residual stress to the coil surface.

  Subsequently, abnormally large internal strain was removed by low-temperature annealing at 225 ° C. × 30 minutes. After the coil spring forming process, the same treatment was applied to all the formed coil springs until the low-temperature annealing process, but half of them were collected as samples before electropolishing.

  The remaining half was further electropolished under the conditions shown in Table 1 to remove the surface layer portion by 5 μm, and a sample after electropolishing was obtained. In addition, the removal thickness of the surface layer part by electropolishing was calculated | required by calculation from the weight change of the coil spring before and behind electropolishing.

[Evaluation]
(Evaluation methods)
About the sample before and behind electropolishing obtained above, the thickness of the white layer, surface roughness (Ra, Rz, Rc), distribution of compressive residual stress, and fatigue characteristics were measured. In addition, the thickness and surface roughness of the white layer were measured at several points on the inner peripheral side of the coil spring. The measuring method is as follows.
(A) The thickness of the white layer was measured with a metal microscope after the cross section of each obtained sample was polished and etched with nital to clarify the white layer.
(B) The surface roughness (Ra, Rz, Rc) was measured according to JIS B 0601 (2001 edition).
(C) The compressive residual stress was measured under the conditions of a Cr tube, 45 kV, and 40 mA using an X-ray stress measuring device (PSPC type manufactured by Rigaku Corporation).
(D) Fatigue characteristics were tested using a star spring fatigue tester at an average stress of 750 MPa, a stress amplitude of 700 MPa, and a rotation speed of 1800 rpm, and 1 × 10 ⁷ out of 8 samples subjected to the test at the same time. The number of samples that broke by the time was evaluated as the breakage probability.

  The measurement results are shown in Table 2.

(Evaluation results)
The white layer thickness was observed as a considerably thick layer of 2 to 10 μm before electropolishing. However, by electropolishing, the thickness was less than 5 μm (0 to 3 μm), and it was made harmless to the impact resistance and fatigue characteristics of the coil spring. Further, it can be seen that the surface roughness Rc was reduced from 4.5 μm to 2.9 μm by electrolytic polishing, and the coil surface was smoothed.

  FIG. 1 is a graph showing the residual stress distribution in the depth direction from the surface before and after electropolishing. A (●) represents the residual stress distribution before electropolishing and B (◯) represents the residual stress distribution after electropolishing. As shown in FIG. 1, the compressive residual stress before electropolishing is about 1500 MPa at the surface, but is maximum at about 1950 MPa at a position 5 to 10 μm deep from the surface. However, since 5 μm of the surface layer portion was removed by electropolishing, the coil surface became the maximum compressive residual stress site at about 2000 MPa after electropolishing. Therefore, the compressive residual stress on the outermost surface was improved by about 500 MPa by applying electropolishing.

The fatigue test was carried out under severe test conditions of average stress: 750 MPa, stress amplitude: 700 MPa, 1 × 10 ⁷ times, and the breakage probability of the coil spring was 2/8 before electropolishing. However, the breakage probability of the electropolished coil spring was 0/8, and it was confirmed that the coil spring had extremely excellent fatigue characteristics.

  INDUSTRIAL APPLICABILITY The present invention is useful when applied to the manufacture of a damper spring for an automatic transmission clutch and a valve spring for an engine in an automatic vehicle.

It is a graph which shows the residual stress distribution of the depth direction from the surface before and behind electropolishing.

Claims (5)

Production of a coil spring comprising a forming step of forming a coil spring using an alloy steel oil tempered wire, a nitriding treatment step for improving the surface hardness of the coil spring, and a shot peening step for imparting residual stress to the coil spring In the method
The surface layer of the coil spring is removed by electropolishing after the shot peening process, the thickness of the white layer generated by the nitriding treatment is 5 μm or less, and the surface roughness is 4 μm or less with Rc, And the compression residual stress of the outermost surface shall be 1800 Mpa or more, The manufacturing method of the coil spring characterized by the above-mentioned.   The method of manufacturing a coil spring according to claim 1, wherein the surface hardness of the coil spring after nitriding is 850 to 1400 HV.   The method for manufacturing a coil spring according to claim 1, wherein a surface layer of the coil spring is removed by 3 to 20 μm by the electrolytic polishing.   The alloy steel oil tempered wire is C: 0.55-0.75%, Si: 1.00-2.70%, Mn: 0.10-1.50%, Cr: 0.4-% by mass. The manufacturing method of the coil spring in any one of Claims 1-3 which contains 2.50% and V: 0.05-0.50%, and consists of remainder Fe and an unavoidable impurity.   Furthermore, the manufacturing method of the coil spring of Claim 4 containing any 1 type of Mo: 0.05-2.0%, Co: 0.05-0.40%.

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