Production method of coil spring and processing device of coil spring

Abstract

To provide a production method of a coil spring which is excellent in fatigue characteristics.SOLUTION: A production method of a coil spring includes a step in which an entire peripheral surface compression load is applied to an element wire to which bending for molding it in a coil shape is performed.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The present invention relates to a manufacturing method of a coil spring and a processing apparatus for 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 No. 4328047 discloses a swaging machine that reduces the cross-sectional area of ​​a workpiece such as a solid wire or a circular pipe by repeatedly striking the workpiece in the radial direction with a plurality of dies.

[0004] WO 2014 / 077326 discloses a strut-type suspension device equipped with a compression coil spring in which the wire diameter differs between the inside and outside of a shock absorber. The document describes that a wire with a variable diameter is formed by swaging or the like, and then formed into a spiral shape by a bending process to produce a compression coil spring. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4328047 [Patent Document 2] International Publication No. 2014 / 077326 [Non-patent literature]

[0006] [Non-Patent Document 1] RW Landgraf and RC Francis, “Material and processing effects on fatigue performance of leaf springs,” SAE Transactions, pp. 1485-1494, 1979 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] An object of the present invention is to provide a method for manufacturing a coil spring having excellent fatigue characteristics. Another object of the present invention is to provide a coil spring processing device capable of manufacturing a coil spring having excellent fatigue characteristics. [Means for solving the problem]

[0009] A method for manufacturing a coil spring according to an embodiment of the present invention includes a step of applying a compressive load all around the circumference to a wire that has been bent to be formed into a coil shape.

[0010] A coil spring processing apparatus according to one embodiment of the present invention is a coil spring processing apparatus that applies a full-circumferential compressive load to a wire that has been bent to form it into a coil shape, and is equipped with a guide member having a hole, at least four dies arranged inside the hole, on both radial sides and both height sides of the coil spring of the wire that passes through the hole, and a striking member that presses at least one of the four dies into one radial side of the coil spring, and the hole has a tapered shape in which the height dimension of the coil spring becomes smaller toward the one side. 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 plan view showing a schematic configuration of an example of an apparatus for applying a compressive load to an entire circumferential surface of a bent wire. [Diagram 3] FIG. 3 is a front view of the coil spring processing device of FIG. [Figure 4] FIG. 4 is a plan view showing a schematic configuration of another example of an apparatus for applying an all-circumferential compressive load to a bent wire. [Figure 5A] FIG. 5A is a nominal stress-nominal strain diagram (σ-ε diagram) obtained by a tensile test. [Figure 5B] FIG. 5B is a true stress-plastic strain curve (στ-εp curve) obtained by a tensile test. [Figure 6] FIG. 6 shows a monotonic tensile, cyclic stress-strain curve assumed as input data for the numerical calculation. [Figure 7] FIG. 7 is a schematic diagram of the finite element model used in the numerical calculations. [Figure 8A]FIG. 8A shows the distribution of residual equivalent stress (Mises stress (σmises)) in the cross section of the central part of the model after the deformation is released. [Figure 8B] Figure 8B shows the distribution of residual normal stress (σx) in the cross section of the central part of the model after the deformation is released. [Figure 8C] Figure 8C shows the distribution of residual shear stress (τxy) in the cross section of the central part of the model after the deformation is released. [Figure 9] FIG. 9 shows the elastic and elastoplastic stress distributions at the elastic limit and the full cross-sectional yield moment. [Figure 10] FIG. 10 is a diagram showing the relationship between the maximum value of the residual equivalent stress σmises and the compressive stress σcyl in the central cross section of the model. [Figure 11] FIG. 11 is a diagram showing the relationship between the wire diameter and the compressive stress σcyl after the entire circumferential surface is subjected to a compressive load. [Figure 12A] FIG. 12A shows the distribution of residual equivalent stress (Mises stress (σmises)) when a compressive load of σcyl=2000 MPa was applied to the entire circumference and then removed. [Figure 12B] FIG. 12B shows the distribution of residual normal stress (σx) when a circumferential compressive load of σcyl=2000 MPa is applied and then removed. [Figure 12C] FIG. 12C shows the distribution of residual shear stress (τxy) when a compressive load of σcyl=2000 MPa was applied to the entire circumference and then released. [Figure 13A] FIG. 13A shows the distribution of residual equivalent stress (Mises stress (σmises)) when a circumferential compressive load of σcyl=2200 MPa is applied and then removed. [Figure 13B] FIG. 13B shows the distribution of residual normal stress (σx) when a circumferential compressive load of σcyl=2200 MPa is applied and then removed. [Figure 13C] FIG. 13C shows the distribution of residual shear stress (τxy) when a circumferential compressive load of σcyl=2200 MPa is applied and then removed. [Figure 14]FIG. 14 shows the distribution of residual equivalent stress (Mises stress (σmises)) when a surface compressive load of 2200 MPa is applied from both sides in the Y direction and then removed. [Figure 15] FIG. 15 shows the distribution of residual equivalent stress (Mises stress (σmises)) when a surface compressive load of 2200 MPa is applied from both sides in the Z direction and then unloaded. 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, after the wire is formed into a coil (coiling), a heat treatment (residual stress relief annealing) is generally performed at about 300 to 600°C to remove residual stress caused by coiling. 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 fatigue strength is reduced especially around the inclusions, which have a large difference in linear expansion coefficient from the matrix.

[0015] The inventors have investigated a method for removing the residual stress generated during coiling without using heat treatment. Specifically, they used the finite element method to calculate the residual stress generated when a deformation equivalent to the bending process during coiling is applied, and investigated a processing method that can remove this residual stress. As a result, they confirmed that the residual stress can be significantly reduced by applying a full-circumferential compressive load that is equal to or greater than the tensile strength.

[0016] For this reason, it is believed that by applying a compressive load to the entire circumference while maintaining the coil spring shape after coiling, it is possible to omit the heat treatment after coiling. If the heat treatment after coiling can be omitted, the residual stress around the inclusions caused by the difference in linear expansion coefficient between the matrix and the inclusions can also be reduced. This makes it possible to prevent a decrease in fatigue strength even if inclusions, which were previously considered harmful, are present. In addition, as a secondary effect, it is expected that the omission of heat treatment will improve productivity and the hardness of the coil spring.

[0017] The present invention has been completed based on the above findings. Hereinafter, a coil spring manufacturing method and a coil spring processing apparatus according to one embodiment of the present invention will be described.

[0018] [Manufacturing method of coil springs] The coil spring to be manufactured by the coil spring manufacturing method according to the present embodiment has a steel material (wire) constituting the coil spring, and the difference in linear expansion coefficient between the base material (matrix) is 3.0×10 -6 K -1 It is preferable that the alloy contains inclusions having a linear expansion coefficient of 10.0×10 or more. This is because the greater the difference in linear expansion coefficient between the matrix and the inclusions, the greater the decrease in fatigue strength that occurs around the inclusions when heat treated, and the greater the effect of omitting heat treatment. The difference in linear expansion coefficient between the matrix and the inclusions is more preferably 10.0×10 -6 K -1 More preferably, it is 13.0×10 -6 K -1 The upper limit of the difference in linear expansion coefficient between the matrix and the inclusions is not particularly limited, but is, for example, 23.0×10 -6 K -1 Note that the "linear expansion coefficient" and the "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 23.0 x 10 -6 K -1 Therefore, the linear expansion coefficient of the inclusion is about 20.0×10 -6 K -1It is preferable that the inclusions are equal to or less than the above. Examples of such inclusions include TiB2, TiC, TiN, ZrB2, ZrC, ZrN, VB2, VC, VN, NbB2, NbC, TaB2, TaC, CrN, Mo2B5, Mo2C, W2B5, WC, B4C, SiC, SiB6, Si3N4, AlN, Al2O3, AlTi4, TiO2, and SiO2. 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 TiB2, TiC, TiN, ZrB2, ZrC, ZrN, VB2, VC, VN, NbB2, NbC, TaB2, TaC, CrN, Mo2B5, Mo2C, W2B5, WC, B4C, SiC, SiB6, Si3N4, AlN, Al2O3, AlTi4, TiO2, and SiO2.

[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 circular 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] The steel material constituting the coil spring according to this embodiment may be, for example, a high-tensile steel material generally used as a steel material for coil springs. The steel material (strand) constituting the coil spring according to this embodiment preferably has a tensile strength of 1800 MPa or more (tensile strength in the longitudinal direction of the strand; the same applies below), and more preferably has a tensile strength of 2000 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. This manufacturing method includes the steps of coiling (step S1) and full circumferential surface compression loading (step S2).

[0024] First, the above-mentioned steel material is bent and formed into a coil shape (coiling (step S1)).

[0025] Next, a compressive load is applied to the entire circumferential surface of the wire that has been bent to form it into a coil (step S2). Specifically, a uniform compressive load is applied to the entire circumferential surface of the bent wire in the radial direction. The process of applying the compressive load to the entire circumferential surface of the wire (step S2) may be performed simultaneously with the coiling. Specifically, a coil spring processing device described later may be incorporated into the coiling device, and the entire circumferential compressive load may be applied to the wire immediately after it has been bent.

[0026] The magnitude of the compressive stress applied in the process of applying a full circumferential compressive load (step S2) is preferably equal to or greater than the magnitude of the stress corresponding to the tensile strength of the wire. Even if the magnitude of the compressive stress is small, the effect of reducing the residual stress can be obtained to a certain extent, but by making the magnitude of the compressive stress equal to or greater than the magnitude of the stress corresponding to the tensile strength of the wire, the residual stress can be significantly reduced. The lower limit of the magnitude of the compressive stress is more preferably 105% of the tensile strength, and even more preferably 110% of the tensile strength. On the other hand, even if the magnitude of the compressive stress is excessively large, it is believed that the effect will saturate. The upper limit of the magnitude of the compressive stress is preferably 150% of the tensile strength, more preferably 140% of the tensile strength, and even more preferably 130% of the tensile strength.

[0027] Alternatively, the process of applying a compressive load to the entire circumference (step S2) may be such that the diameter reduction ratio {(diameter of the wire before processing-diameter of the wire after processing) / diameter of the wire before processing} is 0.005 (0.5%) or more. The lower limit of the diameter reduction ratio is more preferably 0.01, and even more preferably 0.02. The upper limit of the diameter reduction ratio may be as large as possible as long as the wire (wire) is not damaged, and is preferably 0.16, and more preferably 0.08.

[0028] Alternatively, the process of applying a compressive load all around (step S2) may be such that the cross-sectional area reduction rate {(cross-sectional area of ​​the wire before processing-cross-sectional area of ​​the wire after processing) / cross-sectional area of ​​the wire before processing} is 0.01 (1.0%) or more. The lower limit of the cross-sectional area reduction rate is more preferably 0.02, and even more preferably 0.04. The upper limit of the cross-sectional area reduction rate may be as large as possible as long as the wire (wire) is not damaged, and is preferably 0.30, and more preferably 0.15.

[0029] The manufacturing method of the coil spring according to the present embodiment may further include a step of performing shot peening as necessary after applying the full circumferential compressive load.

[0030] In the method for producing a coil spring according to this embodiment, it is preferable not to carry out a heat treatment in which the coil spring is heated to a temperature of 300°C or higher after coiling. If the coil spring is heated to a temperature of 300°C or higher, tensile residual stress may occur around the inclusions. In the method for producing a coil spring according to this embodiment, it is more preferable not to carry out a heat treatment in which the coil spring is heated to a temperature of 200°C or higher after coiling, and it is even more preferable not to carry out a heat treatment after coiling.

[0031] The coil spring is manufactured by the above steps. According to this embodiment, a coil spring having excellent fatigue properties is obtained.

[0032] [Coil spring processing equipment] There is no particular limitation on the method for applying a compressive load to the wire all around, and any method can be used. For example, the method can be carried out using the device described below.

[0033] [Example of processing equipment 1] Fig. 2 is a plan view showing a schematic configuration of a coil spring processing apparatus 10, which is an example of an apparatus for applying an all-circumferential compressive load to a bent wire. Fig. 3 is a front view of the coil spring processing apparatus 10. The coil spring processing apparatus 10 includes a guide member 11, at least four dies 121-124, a striking member 13, and a support member 14 for fixing the guide member 11.

[0034] The guide member 11 has a hole 11a that is open on at least one surface, and the dies 121-124 are disposed inside this hole 11a. The guide member 11 further has a hole 11b (FIG. 3) for passing a wire w of a coil spring therethrough. The hole 11b passes through the guide member 11. The hole 11b is also continuous with the hole 11a inside the guide member 11.

[0035] The wire w is fed into the hole 11b of the guide member 11 by a feeding device (not shown). The holes 11a and 11b are configured so that the wire w, which has entered the inside of the guide member 11 from one opening of the hole 11b, can exit the outside of the guide member 11 from the other opening of the hole 11b after passing through the hole 11a.

[0036] The guide member 11 has a shape that does not interfere with other parts of the coil spring when the wire w is passed through the hole 11b. For example, the guide member 11 can be shaped such that the radial dimension of the coil spring is smaller than the inner diameter of the coil spring, and the height dimension of the coil spring (direction perpendicular to the radial direction) is smaller than twice the pitch of the coil spring. Alternatively, the guide member 11 may have a shape that has a recess or slit at the portion through which the wire w passes. The support member 14 is also positioned so as not to interfere with the coil spring. This makes it possible to maintain the shape of the coil spring being machined.

[0037] The dies 121 to 124 are arranged inside the hole 11a so as to surround the wire w passing through the hole 11a from four directions. More specifically, the dies 121 to 124 are arranged on both sides of the wire w passing through the hole 11a in the radial direction of the coil spring and on both sides in the height direction of the coil spring. To explain in more detail, the die 121 is arranged on a side farther from the center of the coil spring than the wire w passing through the hole 11a in the radial direction of the coil spring. The die 122 is arranged on a side closer to the center of the coil spring than the wire w passing through the hole 11a in the radial direction of the coil spring. The die 123 is arranged on one end side of the wire w passing through the hole 11a in the height direction of the coil spring. The die 124 is arranged on the other end side of the wire w passing through the hole 11a in the height direction of the coil spring. It is preferable that each of the dies 121 to 124 has a surface in contact with the wire w that is curved to follow the shape of the wire w passing through the hole 11a.

[0038] Hole 11a has a tapered shape with a diameter decreasing toward the center in the radial direction of the coil spring. More specifically, hole 11a has a tapered shape with a dimension in the height direction (y direction) of the coil spring decreasing toward the center in the radial direction of the coil spring. Furthermore, each of dies 121-124 has a surface that contacts the inner peripheral surface of hole 11a with a shape that conforms to the inner peripheral surface of hole 11a, and is configured to be able to slide along the inner peripheral surface of hole 11a.

[0039] The striking member 13 is connected to an actuator (not shown) and vibrates in the radial direction of the coil spring to apply an impact to the die 121. The striking member 13 also applies an impact to the other dies 122-124 indirectly via the die 121. Specifically, each of the dies 121-124 receives an impact in a direction moving toward the center of the coil spring. At this time, since the hole 11a has a tapered shape with a diameter decreasing toward the center of the coil spring, each of the dies 123 and 124 moves toward the center of the coil spring and is restricted by the inner peripheral surface of the hole 11a, moving in a direction approaching the wire w in the height direction of the coil spring. The die 122 also receives a reaction force from the inner peripheral surface of the hole 11a in a direction moving away from the center of the coil spring.

[0040] With this configuration, compressive stress is applied to the wire w from four directions as shown by the white arrows in Figure 2. By vibrating the striking member 13 while feeding the wire w, a compressive load can be applied to the entire wire w.

[0041] The impact energy applied by the impact member 13 is, for example, 10 to 80 J. The energy E (J) is proportional to the volume V (mm 3 ) can be converted to hydrostatic pressure p (MPa) using the following formula: p=1000E / V For example, when an impact energy of 20 J is applied to a cylinder with a wire length of 0.758 mm and a diameter of 4 mm, the generated hydrostatic pressure p is 2100 MPa. By adjusting the impact energy and the dimensions of the dies 121 to 124, the magnitude of the compressive stress applied to the wire can be adjusted.

[0042] The coil spring processing device 10 may be incorporated into a coiling device. For example, the coil spring processing device 10 may be disposed after the coiling pin in the feed direction of the coiling device, so that a full circumferential compressive load is applied to the wire w immediately after it has been bent by the coiling pin. In this case, the coil spring processing device 10 and the coiling device may share a feed device.

[0043] 2 and 3, a case has been described in which the striking member 13 is disposed farther from the center of the coil spring than the wire w to be processed in the radial direction of the coil spring, and the striking member 13 is pressed toward the center of the coil spring to apply a compressive load to the wire w over the entire circumference. However, conversely, the striking member 13 may be disposed closer to the center of the coil spring than the wire w to be processed in the radial direction of the coil spring, and the striking member 13 may be pressed toward the side farther from the center of the coil spring to apply a compressive load over the entire circumference of the wire w. The direction in which the striking member 13 is pressed may also be inclined from the radial direction of the coil spring.

[0044] [Example of processing equipment 2] 4 is a plan view showing a schematic configuration of a coil spring processing apparatus 20, which is another example of an apparatus for applying an all-circumferential compressive load to a bent wire. The coil spring processing apparatus 20 includes an annular member 21, a first member 22, and a second member 23. The first member 22 and the second member 23 are connected by a hinge 24.

[0045] The wire w is fed by a feeder (not shown) so as to pass through the inside of the annular member 21. At least one of the first member 22 and the second member 23 is connected to an actuator (not shown) and configured to rotate around a hinge 24. The annular member 21 is disposed between the first member 22 and the second member 23 and configured to be sandwiched between the first member 22 and the second member 23 as at least one of the first member 22 and the second member 23 rotates. As the annular member 21 is sandwiched between the first member 22 and the second member 23, the diameter of the annular member 21 is reduced, and the wire w passing through the inside of the annular member 21 is subjected to a compressive load on the entire circumferential surface.

[0046] By periodically driving the first member 22 and the second member 23 while feeding the wire w, a compressive load can be applied to the entire wire w on its entire circumferential surface.

[0047] The above describes examples of devices for applying a circumferential compressive load to a bent wire, but these are merely examples, and the method of applying a circumferential compressive load to a bent wire is not limited to using these devices. EXAMPLES

[0048] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0049] [Material model and test materials] The material model used was the Oak Ridge National Laboratory model (ORNL), which takes into account the kinematic hardening law. This material model is a model in which the stress-strain response gradually changes due to repeated loading, and hardens or softens every 10 cycles. It is believed that by using this material model, it is possible to simulate the release of residual stress due to the repeated processing softening characteristics unique to spring steel.

[0050] In order to obtain the material properties necessary for numerical calculations, tests were conducted using actual materials (test materials). The test material used was a wire made of SAE9254 spring standard steel with vanadium added. The mechanical properties are shown in Table 1. This test material was made through the same processing and heat treatment processes as an actual coil spring, and is suitable for use in evaluating the plastic deformation characteristics of coil springs.

[0051] [Table 1]

[0052] Small test pieces were prepared from the test materials and tensile tests were performed. The nominal stress-nominal strain curve (σ-ε curve) and true stress-plastic strain curve (σ τ -ε p The line diagram is shown in Figures 5A and 5B.

[0053] It is recognized that the test material undergoes repeated softening. Therefore, the nominal stress-nominal strain curve of the tensile test was used as the standard, and the monotonic tensile and cyclic stress-strain curves shown in Figure 6 were assumed to have the same ratio as the tensile and cyclic stress-strain characteristics in the previous study (RW Landgraf and RC Francis, “Material and processing effects on fatigue performance of leaf springs,” SAE Transactions, pp. 1485-1494, 1979), and were used as input data for the numerical calculation. This input data represents a 33% decrease in yield stress. The input material properties are shown in Table 2. Parameters were set in the Johnson Cook model below for static properties, and the relationship between direct stress and plastic strain was input for cyclic properties. σ=A+B(ε p ) n

[0054] [Table 2]

[0055] [Finite element model, loads, and constraints] A simple round bar model shown in Figure 7 was used for the finite element model. More specifically, a cylindrical shape with a diameter of 4 mm and a total length of 8 mm was created using 8-node solid elements of the first order. The number of elements in the model is 5120. This model also has 21 nodes on the diameter. In order to simplify the modeling, a model was used in which one cross section was symmetrically constrained and the other cross section was multipoint constrained (MPC constrained) to apply a bending moment around the Z axis, targeting the area where a uniform bending load is applied. Since the model is not fixed by symmetric constraint alone, the symmetric constraint surface was calculated by constraining the Y and Z directions and the three-axis rotation. These symmetric constraints and MPC constraints caused high strain and stress at the edge parts, so the evaluation was performed on the central cross section of the model (X = 4 mm surface) which is considered to be uniformly deformed.

[0056] For the above model, a moment of -23,600 mm·N was applied to the end face, which is the MPC restraint part, as monotonic bending. The amount of deformation when a moment of -23,600 mm·N is applied is equivalent to the amount of deformation when the end face, which is the MPC restraint part, is rotated 30°, which is equivalent to the amount of deformation when a coil spring with an average diameter of 30 mm is coiled.

[0057] The residual stress distribution in the cross section of the central part of the model after the deformation is released is shown in Fig. 8A to Fig. 8C. Fig. 8A shows the equivalent stress (Mises stress (σ mises ) and Fig. 8B shows the distribution of normal stress (σ x ) and Fig. 8C shows the distribution of shear stress (τ xy ) distribution.

[0058] As shown in Fig. 8A, σ mises The maximum value of occurs at the neutral axis, and τ in Fig. 8C xy Generally, in elastic bending of a beam, the maximum stress occurs at the top and bottom surfaces of the beam (Fig. 9(a)), and the maximum shear stress occurs at the neutral plane (Fig. 9(e)). Furthermore, in the case of plastic bending considering a perfect elastic-plastic model, the stress is constant in the region from the top to the elastic limit (Fig. 9(c)), and the total cross-sectional yield moment M p When a load is applied, extreme shear plastic stress occurs at the neutral axis (Fig. 9(h)). Plastic deformation caused by this extreme shear plastic stress occurs locally at the neutral axis, and large residual stress remains when the load is removed. Looking at the results of this analysis, σ mises and τ xy It is clear that the main cause of residual stress is shear deformation due to full cross-sectional yielding.

[0059] [Reduction of residual stress due to compressive load on all sides] After monotonic bending, a calculation was performed to apply a surface compressive load (all-around compressive load) equivalent to the drawing load. Specifically, a compressive stress σ of 600 to 2400 MPa was applied to the curved surface of the model. cyl A load was applied and the residual stress after removal was evaluated.

[0060] Residual equivalent stress σ at the central cross section of the modelmises and compressive stress σ cyl The relationship between the wire diameter and compressive stress σ after the entire circumference is compressed is shown in Fig. 10. cyl The relationship between the compressive stress and the cyl With the increase of σ mises In particular, when a compressive load greater than the tensile strength of the wire (2076 MPa) is applied to the entire circumference, σ mises In addition, as shown in Fig. 11, when a compressive load was applied to the entire circumference that was greater than the tensile strength of the wire (2076 MPa), the wire diameter also decreased rapidly. cyl = 2000MPa and σ cyl = 2200 MPa, the wire diameters were 3.992 mm (diameter reduction rate: 0.0020 (0.2%), area reduction rate: 0.0040 (0.4%)) and 3.897 mm (diameter reduction rate: 0.0258 (2.58%), area reduction rate: 0.0508 (5.08%)), respectively.

[0061] Suddenly σ mises Before and after the decrease in σ cyl The residual stress distribution at σ = 2000 MPa is shown in Fig. 12A to Fig. 12C. cyl The residual stress distribution at σ = 2200 MPa is shown in Figs. 13A to 13C. As in Figs. 8A to 8C, Figs. 12A and 13A show the residual stress distribution at σ = 2200 MPa. mises ) and Fig. 12B and Fig. 13B show the distribution of normal stress (σ x ) distribution, and Figs. 12C and 13C show the distribution of shear stress (τ xy ) distribution. From Fig. 13A and Fig. 13B, σ mises The point where is maximum is σ x is the region where σ mises Even if the stress is high, compressive residual stress occurs in the high value region, and it is considered that the effect on fatigue strength is minor or that fatigue strength is improved.

[0062] Figure 14 shows the residual stress distribution when, instead of applying a compressive load all around, a compressive load of 2400 MPa is applied to a region of 45° on both sides in the Y direction (direction perpendicular to the axial direction (Z direction) of the bending moment). Figure 15 shows the residual stress distribution when, instead of applying a compressive load all around, a compressive load of 2400 MPa is applied to a region of 45° on both sides in the Z direction (direction parallel to the axial direction (Z direction) of the bending moment). In both cases, residual stress remained, and the improvement was smaller than when a compressive load was applied all around.

[0063] 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. [Explanation of symbols]

[0064] 10,20 Coil spring processing equipment 11 Guide member 121~124 Dice 13 Striking member 14 Support member 21 Annular member 22 First member 23 Second member 24 Hinge w Bare wire

Claims

1. A method for manufacturing a coil spring, comprising a step of applying a compressive load to an entire surface of a wire that has been bent to form it into a coil shape.

2. A method for manufacturing a coil spring according to claim 1, comprising the steps of: a compressive stress applied in the step of applying an entire circumferential compressive load is equal to or greater than a stress corresponding to a tensile strength of the wire.

3. A method for manufacturing a coil spring according to claim 1, comprising the steps of: a diameter reduction rate of the wire in the step of applying an entire circumferential compressive load is 0.00125 or more.

4. A method for manufacturing a coil spring according to claim 1, a cross-sectional reduction rate of the wire in the step of applying an entire circumferential compressive load is 0.00250 or more.

5. A method for manufacturing a coil spring according to any one of claims 1 to 4, A manufacturing method for a coil spring in which the spring is not subjected to heat treatment at a temperature of 300°C or higher after being formed into a coil shape.

6. A method for manufacturing a coil spring according to any one of claims 1 to 4, The difference between the linear expansion coefficient of the wire and that of the matrix at room temperature is 3.0 × 10 －６ K －１ A method for manufacturing a coil spring including the above inclusion.

7. A method for manufacturing a coil spring according to any one of claims 1 to 4, The method for manufacturing a coil spring, wherein the wire has a tensile strength of 2000 MPa or more.

8. A coil spring processing device that applies a circumferential compressive load to a wire that has been bent to form it into a coil shape, comprising: A guide member having a hole; At least four dies are arranged inside the hole on both sides of the wire passing through the hole in a radial direction of the coil spring and on both sides of the wire in a height direction of the coil spring; a striking member for pressing at least one of the four dies toward one radial side of the coil spring, The hole has a tapered shape in which the height dimension of the coil spring decreases toward the one side.

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