Method for manufacturing coil spring

Abstract

To provide a method for manufacturing a coil spring having excellent fatigue characteristics.SOLUTION: A method for manufacturing a coil spring is a method for manufacturing a coil spring made of a steel material containing inclusions whose linear expansion coefficient at room temperature differs by 2.0×10-6K-1 or more from that of a base. The method includes a step of applying a double swing load to an intermediate product of a coil spring molded by forming a wire into a coil shape so that equivalent stress generated in a surface layer of the intermediate product of the coil spring becomes larger than a shear yield strength of the wire.SELECTED DRAWING: Figure 12

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a coil spring. [Background technology]

[0002] For high-tensile steel machine parts that are subjected to high-cycle loads, it is necessary to reduce the risk of breakage of parts due to internal fatigue fracture originating from internal defects such as inclusions. It is believed that the size of the inclusion that is the origin of internal fatigue fracture and the probability of inclusions being present in high stress areas have a large effect on internal fatigue fracture. Conventionally, methods for predicting the fatigue limit originating from inclusions have been formulated, and under uniform stress, the fatigue limit can be predicted using such estimation formulas. Based on this idea, coil springs have been developed that have been subjected to shot peening or nitriding to impart compressive residual stress to the surface.

[0003] Japanese Patent Application Publication No. 2010-117191 discloses a method for improving the fatigue limit of a material with a surface defect, in which excessive stress is applied to a component having a surface defect and setting is performed to form a tensile plastic deformation region at the tip of the defect, and the excessive stress is then removed to form a compressive residual stress field at the tip of the defect, thereby improving the fatigue limit of the component.

[0004] WO 2014 / 042066 discloses a method for manufacturing a highly durable compression coil spring, specifically describing a method for coiling a coil spring wire in a normal state after heating it to the austenite region, and a method for carburizing the coil spring to form a C-enriched layer on the surface. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2010-117191 A [Patent Document 2] International Publication No. 2014 / 042066 [Non-patent literature]

[0006] [Non-Patent Document 1] Motoaki Hayama, Masakazu Kikuchi, Jun Komodori: Evaluation of compressive residual stress release behavior on steel surface by in-situ X-ray stress measurement, Tetsu-to-Haganen, Vol. 107, No. 2, 2021, p. 137-145. [Non-Patent Document 2] Akihiko Kato, Masayoshi Kurihara, Masatomo Kawahara: Journal of the Society of Naval Architects of Japan, Vol. 1983, No. 153, pp. 336-343, 1983. Summary of the Invention [Problem to be solved by the invention]

[0007] The method of imparting compressive residual stress by shot peening can improve the fatigue properties of steel around inclusions that exist up to the depth at which compressive residual stress can be imparted by shot peening (crossing point, less than 0.5 mm), but cannot improve the fatigue properties of steel around inclusions that exist in deeper regions. If it were possible to improve the fatigue properties of steel around inclusions that exist in deeper regions, it would be possible to achieve high fatigue strength even in materials with low cleanliness, which is expected to lead to reduced environmental impact and reduced manufacturing costs by reducing processes.

[0008] JP 2010-117191 A describes a method of forming a compressive residual stress field at the tip of a defect by applying an excessive stress to a member having a surface defect and performing setting. The publication describes in detail a method of calculating the magnitude of the excessive stress required to form a compressive residual stress field, but does not describe a specific method of applying the excessive stress. In addition, the publication assumes that the surface defect is a crack, and does not relate to fatigue fracture caused by inclusions.

[0009] An object of the present invention is to provide a method for manufacturing a coil spring having excellent fatigue properties. [Means for solving the problem]

[0010] In the method for producing a coil spring according to an embodiment of the present invention, the difference between the linear expansion coefficient of the matrix at room temperature is 2.0×10 -6 K -1 The method for manufacturing a coil spring made of steel material containing inclusions as described above includes a step of applying a reciprocating load to an intermediate coil spring product formed by forming a wire into a coil shape, so that an equivalent stress generated in a surface layer of the intermediate coil spring becomes larger than the shear yield strength of the wire. Effect of the Invention

[0011] According to the present invention, a coil spring having excellent fatigue properties can be obtained. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a flow diagram of a method for manufacturing a coil spring according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing a schematic diagram of a loaded state of a coil spring. [Diagram 3] FIG. 3 is a diagram showing a schematic diagram of stress generated in a wire element when a load is applied to a coil spring. [Figure 4] FIG. 4 is a schematic diagram showing the mechanism by which tensile residual stress around an inclusion is reduced by applying a cyclic load. [Diagram 5] FIG. 5 is a diagram showing the shape of a torsion fatigue test piece. [Figure 6] FIG. 6 shows the results of the torsional fatigue test. [Figure 7] FIG. 7 is a micrograph of the fracture surface of a test piece to which a preload was applied after a torsional fatigue test. [Figure 8] FIG. 8 is a micrograph of the fracture surface of a test piece to which a preload was applied after a torsional fatigue test. [Figure 9] FIG. 9 is a micrograph of the fracture surface of the test piece. [Figure 10] FIG. 10 is an enlarged photograph of area A in FIG. [Figure 11]FIG. 11 is a diagram showing the distribution of shear stress. [Figure 12] FIG. 12 is a graph showing the results of the torsional fatigue test in FIG. 6, organized by the effective stress intensity factor range ΔKeff. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In the course of investigating the relationship between the type of inclusions in steel and fatigue properties, the inventors found that there is a correlation between the linear expansion coefficient of the inclusions and fatigue properties. Specifically, they found that the fatigue limit of the steel tends to decrease as the difference between the linear expansion coefficient of the inclusions and the linear expansion coefficient of the matrix increases. This suggests that the decrease in fatigue strength around the inclusions is due to the influence of thermal stress in addition to the influence of external forces.

[0014] In the manufacturing process of coil springs, heat treatment is generally performed at about 300 to 600°C. When returning to room temperature from the heat treatment temperature, the matrix is ​​unable to shrink sufficiently around the inclusions because the linear expansion coefficient of the matrix is ​​greater than that of the inclusions, and the matrix around the inclusions is subjected to tensile stress in the circumferential direction of the inclusions. This generates tensile residual stress around the inclusions, and it is believed that a decrease in fatigue strength occurs especially around the inclusions, which have a large difference in linear expansion coefficient from the matrix.

[0015] On the other hand, there are reports that residual stress in steel materials is released at a relatively early stage by applying repeated loads. For example, Non-Patent Document 1 (Hayama, Motoaki, Kikuchi, and Komodori, "Evaluation of the release behavior of compressive residual stress on steel surfaces by in-situ X-ray stress measurement," Iron and Steel, Vol. 107, No. 2, 2021, p. 137-145.) shows that residual stress is released after about 1 to 10 cycles in tension-compression fatigue tests on S45C carbon steel for mechanical structures.

[0016] The inventors came up with the idea that it might be possible to reduce the tensile residual stress around inclusions by applying a reversible cyclic load to a coil spring before use. They then conducted torsional fatigue tests on spring steel containing inclusions, both with and without a cyclic load, and confirmed that applying a cyclic load improved the fatigue properties.

[0017] The present invention has been completed based on the above findings. A method for manufacturing a coil spring according to one embodiment of the present invention will now be described.

[0018] The coil spring to be manufactured by the coil spring manufacturing method according to the present embodiment is a coil spring having a steel material that has a linear expansion coefficient difference of 2.0×10 -6 K -1 The greater the difference in linear expansion coefficient between the matrix and the inclusions, the greater the decrease in fatigue strength around the inclusions, and the greater the effect of preloading, which will be described later. The difference in linear expansion coefficient between the matrix and the inclusions is preferably 3.0×10 -6 K -1 The "linear expansion coefficient" and "difference in linear expansion coefficient" refer to values ​​at room temperature (25°C).

[0019] The linear expansion coefficient of the matrix depends on the type of steel, but for steels used for coil springs, it is approximately 11.0 x 10 -6 K -1 Therefore, the linear expansion coefficient of the inclusion is 9.0 to 13.0 × 10 -6 K -1It is preferable that the inclusions are Al2O3, AlN, TiO2, TiC, MnO, MnS, MgO, CaO, CaS, SiO2, Si3N4, SiC, ZrO2, ZrN, ZrC, Nb2O5, NbC, CrO2, VN, VC, TaC, and WC. That is, in the manufacturing method of the coil spring according to the present embodiment, the inclusions may be one or more selected from the group consisting of Al2O3, AlN, TiO2, TiC, MnO, MnS, MgO, CaO, CaS, SiO2, Si3N4, SiC, ZrO2, ZrN, ZrC, Nb2O5, NbC, CrO2, VN, VC, TaC, and WC. The manufacturing method of the coil spring according to the present embodiment is particularly suitable when the inclusions are VC.

[0020] The circle equivalent diameter of the inclusions contained in the steel material constituting the coil spring is preferably 50.0 μm or less. In other words, the steel material constituting the coil spring preferably does not contain inclusions with a circle equivalent diameter exceeding 50.0 μm. If inclusions exceeding 50.0 μm are present, it may be difficult to ensure sufficient fatigue strength. The circle equivalent diameter of the inclusions contained in the steel material constituting the coil spring is more preferably 45.0 μm or less.

[0021] In addition, the number density of inclusions with a circle equivalent diameter of 1.0 to 50.0 μm is 2 It is preferable that the number density of inclusions is 1 / 100 mm or more. 2 When the number density of inclusions with an equivalent circle diameter of less than 1.0 μm is less than 1.0 μm, the effect of the inclusions on the fatigue strength of the steel material is considered to be small to begin with. Note that the number density of inclusions with an equivalent circle diameter of less than 1.0 μm is not particularly limited. This is because the effect of inclusions with an equivalent circle diameter of less than 1.0 μm on the fatigue strength of the steel material is considered to be small.

[0022] Other than the above-mentioned conditions regarding the inclusions, there are no particular limitations on the steel material constituting the coil spring. As the steel material constituting the coil spring according to this embodiment, for example, a high tensile steel material generally used as a steel material for coil springs can be used. The steel material constituting the coil spring according to this embodiment preferably has an ultimate shear strength of 900 MPa or more, more preferably has an ultimate shear strength of 1000 MPa or more, and further preferably has an ultimate shear strength of 1100 MPa or more.

[0023] 1 is a flow diagram of a method for manufacturing a coil spring according to an embodiment of the present invention, which includes the steps of coiling (step S1), heat treatment (step S2), and preloading (step S3).

[0024] The above-mentioned steel material is formed into a coil shape to manufacture an intermediate coil spring (coiling (step S1)). After coiling, a heat treatment (low-temperature annealing) is performed to remove residual stress caused by coiling (step S2). The temperature of this heat treatment is not particularly limited, but is, for example, 300 to 600°C. The lower limit of the heat treatment temperature is preferably 360°C, and more preferably 400°C. The upper limit of the heat treatment temperature is preferably 480°C, and more preferably 440°C.

[0025] A reciprocating load is applied to the intermediate coil spring that has been subjected to coiling and heat treatment (preload application step (step S3)). In other words, the intermediate coil spring is expanded and contracted (see FIG. 2). At this time, the equivalent stress generated in the surface layer of the intermediate coil spring is set to be greater than the shear yield strength of the wire.

[0026] Figure 3 is a schematic diagram showing the stress generated in the wire element when a load is applied to the coil spring. When a load is applied to the coil spring, shear stress is generated on the surface of the wire element. This shear stress τ can be decomposed into a maximum principal stress in a direction at 45° to the linear axis of the wire element, and a minimum principal stress in a direction perpendicular to the direction of the maximum principal stress.

[0027] Figure 4 is a schematic diagram showing the mechanism by which tensile residual stress around inclusions is reduced by applying cyclic loads. As mentioned above, tensile residual stress occurs in the area around the inclusions in the wire matrix during the process of returning to room temperature from the heat treatment temperature. This tensile residual stress can be released by applying cyclic loads. At this time, applying bidirectional cyclic loads can release the tensile residual stress not only in one direction, but also in the direction perpendicular to the one direction.

[0028] If the load applied is too small, the effect of reducing the tensile residual stress is not sufficiently obtained. Therefore, the magnitude of the applied load is set so that the equivalent stress generated in the surface layer of the intermediate coil spring is greater than the shear yield strength of the wire. It is preferable that the load applied is such that the intermediate coil spring is slightly plastically deformed. It is preferable that the load applied to the intermediate coil spring is set so that the equivalent stress generated in the surface layer of the intermediate coil spring is 103% or more of the shear yield strength of the wire, and more preferably 110% or more.

[0029] On the other hand, if the load applied is too large, the intermediate part of the coil spring may be significantly plastically deformed, which may result in a decrease in fatigue strength. The load applied to the intermediate part of the coil spring is preferably set to a value that causes the equivalent stress generated in the surface layer of the intermediate part of the coil spring to be 140% or less of the shear yield strength of the wire, and more preferably to be 130% or less.

[0030] The number of repetitions is preferably 1 to 10. If the number of repetitions is excessively large, the effect of reducing the residual stress around the inclusions becomes saturated, and there is a possibility that the matrix may be damaged. The number of repetitions is more preferably 1 to 5.

[0031] If necessary, shot peening may be further performed on the intermediate coil spring. When shot peening is performed on the intermediate coil spring, it is preferable to perform the shot peening after the preload application step (step S3). If the preload application step is performed after the shot peening, the compressive residual stress introduced by the shot peening may also be released.

[0032] In addition, after the preload application step (step S3), it is preferable not to perform a heat treatment in which the intermediate coil spring is heated to a temperature of 300° C. or higher. If the intermediate coil spring is heated to a temperature of 300° C. or higher, tensile residual stress may occur again around the inclusions.

[0033] The coil spring is manufactured by the above steps. According to this embodiment, a coil spring having excellent fatigue properties is obtained. EXAMPLES

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0035] A torsional fatigue test specimen with the shape shown in Figure 5 was made from a wire rod having the mechanical properties shown in Table 1. This wire rod was manufactured by rolling and drawing an ingot of SAE9254 spring steel with vanadium (V) added, and then by subjecting it to heat treatment by quenching and tempering. The strength in the shear direction (tensile strength and yield strength) was 1 / 3 of the strength in the perpendicular direction. 1 / 2 It was calculated by multiplying.

[0036] [Table 1]

[0037] A preload was applied to this test piece by repeatedly applying a reversible torsional load, separate from the torsional fatigue test described later. The load was set so that the equivalent stress generated in the surface layer of the test piece was 1400 MPa (approximately 120% of the yield stress of the wire (1167 MPa)), with 3 repetitions and a stress ratio of -1 (reversible). After the preload was applied, shot peening was performed to impart compressive residual stress to the surface.

[0038] The test piece was preloaded and shot peened, and the stress ratio was 0.1 (pulse), the frequency was 10Hz, and the number of cutoffs was 1×10 7 A torsional fatigue test was conducted under cycle conditions. For comparison, a torsional fatigue test was also conducted under the same conditions on a test piece that had been shot peened without a preload. The results of the torsional fatigue test are shown in Figure 6.

[0039] Figures 7 and 8 are micrographs of the fracture surfaces of the test pieces that were preloaded after the torsional fatigue test. All of the test pieces that were preloaded showed fatigue fracture originating from the inclusion (VC). As shown in Figures 9 and 10 (enlarged photographs of area A in Figure 9), the maximum principal stress σ pri The crack propagated perpendicular to the specimen (approximately 45° to the axial direction of the specimen). σ pri = τ a (1) τ a is the shear stress amplitude during the torsional fatigue test.

[0040] As shown in Fig. 11, the shear stress is maximum near the surface of the test piece and decreases toward the center. Taking this stress gradient into consideration, the principal stress amplitude σ pri,eff was calculated. σ pri,eff = τ a (1-D / R) (2) D is the starting depth from the surface of the test piece, and R is the radius of the test piece.

[0041] From equation (1) and equation (2), Δσ pri,eff was calculated as follows: τa =(τ a,max -τ a,min ) / 2 From equation (1), σ pri,max = τ a,max σ pri,min = τ a,min From equation (2), Δσ pri,eff = τ a (1-D / R) Here, τ a,max is the maximum shear stress, and τ a,min is the minimum shear stress.

[0042] Calculated Δσ pri,eff was substituted into the following equation (3) to calculate the stress intensity factor range ΔK. ΔK=0.5Δσ pri,eff ·{π(area) 1 / 2} 1 / 2 (3) The area is the area of ​​the inclusion projected in the direction of the maximum principal stress.

[0043] Effective stress intensity factor range ΔK from aperture ratio U and ΔK eff was calculated. ΔK eff =U ΔK (4) The opening ratio U was taken from the value given in Akihiko Kato, Masayoshi Kurihara, and Masato Kawahara, Journal of the Society of Naval Architects of Japan, Vol. 1983, No. 153, pp. 336-343, 1983.

[0044] Figure 12 shows the results of the torsional fatigue test in Figure 6 in terms of the effective stress intensity factor range ΔK eff It can be seen from Fig. 12 that the fatigue properties are significantly improved by applying a preload.

[0045] The above describes the embodiments of the present invention. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out within the scope of the invention.

Claims

1. The difference with the linear expansion coefficient of the matrix at room temperature is 2.0 x 10 －６ K －１ A method for manufacturing a coil spring made of a steel material containing the above inclusions, comprising the steps of: A method for manufacturing a coil spring, comprising: applying a reciprocating load to an intermediate coil spring, formed by forming a wire into a coil shape, so that an equivalent stress generated in a surface layer of the intermediate coil spring becomes greater than the shear yield strength of the wire.

2. A method for manufacturing a coil spring according to claim 1, comprising the steps of: an equivalent stress generated in a surface layer portion of the intermediate coil spring is 103% or more and 140% or less of a shear yield strength of the wire.

3. A method for manufacturing a coil spring according to claim 1 or 2, The method for manufacturing a coil spring, wherein the wire has an ultimate shear strength of 1000 MPa or more.

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