Method for manufacturing a coil spring and coil spring

Abstract

To provide a method for manufacturing a coil spring with an excellent fatigue characteristic.SOLUTION: A method for manufacturing a coil spring, which is made of a steel material containing an inclusion whose difference from a linear expansion coefficient of a base at room temperature is 13.0×10-6K-1 or more, comprises a step of setting at room temperature or a temperature lower than the room temperature. The coil spring has a maximum residual stress of 300 MPa or less that acts perpendicularly on the maximum principal stress plane when a load is applied to the coil spring, in the base adjacent to the inclusion located at a depth of 0.50 to 1.50 mm from the surface.SELECTED DRAWING: Figure 15

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing a coil spring and to a coil spring itself. [Background technology]

[0002] In high-tensile steel machine parts subjected to high-cycle loads, it is necessary to reduce the risk of fracture due to internal fatigue failure initiated by internal defects such as inclusions. In internal fatigue failure, the size of the inclusion that initiates the failure and the probability of its presence in high-stress areas are thought to have a significant impact. Conventionally, methods for predicting the fatigue limit initiated by 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 in which compressive residual stress is imparted to the surface through shot peening or nitriding.

[0003] Japanese Patent Publication No. 3930715 discloses a high-strength spring made of steel with non-metallic inclusions of 15 μm or less in size, which is subjected to nitriding at 460°C or higher after coiling to achieve a surface hardness of Hv700 or higher, followed by at least two shot peenings and then setting at 210°C or higher.

[0004] Japanese Patent Publication No. 2010-117191 discloses a method for improving the fatigue limit of a surface-defect material by applying excessive stress to a member having surface defects and performing setting to form a tensile plastic deformation region at the tip of the defect, and then removing the excessive stress to form a compressive residual stress field at the tip of the defect, thereby improving the fatigue limit of the member. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 3930715 [Patent Document 2] Japanese Patent Publication No. 2010-117191 [Overview of the project] [Problems that the invention aims to solve]

[0006] While the method of imparting compressive residual stress by shot peening can improve the fatigue properties of steel around inclusions present up to a depth where compressive residual stress can be imparted by shot peening (crossing point, less than 0.5 mm), it cannot improve the fatigue properties of steel around inclusions in deeper regions. If the fatigue properties of steel around inclusions in deeper regions can be improved, it will be possible to achieve high fatigue strength even with materials with low cleanliness, which is expected to lead to reduced environmental impact through process reduction and lower manufacturing costs.

[0007] Japanese Patent Publication No. 2010-117191 describes a method for forming a compressive residual stress field at the tip of a defect in a member having a surface defect by applying excessive stress and performing setting. The publication details a method for calculating the magnitude of the excessive stress required to form the compressive residual stress field, but does not describe a specific method for applying the excessive stress. Furthermore, the publication assumes that the surface defect is a crack and does not relate to fatigue fracture due to inclusions.

[0008] The object of the present invention is to provide a method for manufacturing a coil spring with excellent fatigue properties, and to provide a coil spring with excellent fatigue properties. [Means for solving the problem]

[0009] A method for manufacturing a coil spring according to one embodiment of the present invention has a difference of 13.0 × 10⁻¹⁰ from the coefficient of linear expansion of the matrix at room temperature. -6 K -1A method for manufacturing a coil spring made of a steel material containing the inclusions as described above, including a step of setting at room temperature or a temperature lower than room temperature, wherein the maximum value of the residual stress acting perpendicular to the maximum principal stress plane when a load is applied to the coil spring at the base adjacent to the inclusions existing at a position 0.50 to 1.50 mm deep from the surface of the coil spring is 300 MPa or less.

[0010] The coil spring according to an embodiment of the present invention has a difference from the linear expansion coefficient of the base at room temperature of 13×10 -6 K -1 It is made of a steel material containing the inclusions as described above, and the maximum value of the residual stress acting perpendicular to the maximum principal stress plane when a load is applied to the coil spring at the base adjacent to the inclusions existing at a position 0.50 to 1.50 mm deep from the surface of the coil spring is 300 MPa or less.

Effect of the Invention

[0011] pAccording to the present invention, a coil spring excellent in fatigue characteristics can be obtained.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a diagram showing an example of the appearance of a coil spring. [Figure 2] FIG. 2 is an enlarged view of region A in FIG. 1, and is a diagram schematically showing the action of stress on the wire. [Figure 3] FIG. 3 is a diagram schematically showing the action of stress around the inclusions. [Figure 4] FIG. 4 is a diagram schematically showing the tensile stress generated around the inclusions when returning from the heat treatment temperature to room temperature. [Figure 5] FIG. 5 is a diagram schematically showing the mechanism of reducing tensile residual stress by setting at low temperature. [Figure 6] FIG. 6 is a flowchart of a method for manufacturing a coil spring according to an embodiment of the present invention. [Figure 7] FIG. 7 is a schematic diagram of an analysis model. [Figure 8]Figure 8 shows the stress-strain curve of the base. [Figure 9] Figure 9 shows the patterns of temperature history and load history added to the analysis model. [Figure 10] Figure 10 is a contour map showing the distribution of stress acting perpendicular to the y-plane at time A in Figure 9. [Figure 11] Figure 11 is a contour map showing the distribution of stress acting perpendicular to the y-plane at time point B in Figure 9. [Figure 12] Figure 12 is a contour plot showing the distribution of stress acting perpendicular to the y-plane at time point C in Figure 9. [Figure 13] Figure 13 is a contour map showing the distribution of stress acting perpendicular to the y-plane at time D in Figure 9. [Figure 14] Figure 14 is a contour plot showing the distribution of stress acting perpendicular to the y-plane at time point E in Figure 9. [Figure 15] Figure 15 is a graph showing the relationship between the temperature during setting and the maximum stress acting perpendicular to the y-plane in the matrix adjacent to the inclusion. [Modes for carrying out the invention]

[0013] In the process of investigating the relationship between the type of inclusions in steel and its fatigue properties, the inventors discovered a correlation between the coefficient of thermal expansion of the inclusions and the fatigue properties. Specifically, they found that the greater the difference between the coefficient of thermal expansion of the inclusions and the matrix, the lower the fatigue limit of the steel tends to be. This suggests that, in addition to the influence of external forces, thermal stress also plays a role in the decrease in fatigue strength around inclusions.

[0014] Based on this finding, the inventors modeled the minute elements around inclusions in steel and performed calculations using the finite element method to investigate the relationship between the stress distribution around the inclusions and the manufacturing process. As a result, they found that by performing setting at room temperature or a temperature lower than room temperature, which is conventionally done at a warm temperature, it is possible to reduce the tensile residual stress in the matrix around the inclusions or to impart compressive residual stress to the matrix around the inclusions.

[0015] Figure 1 shows an example of the appearance of a coil spring 1. Figure 2 is an enlarged view of region A in Figure 1, schematically illustrating the effect of stress on the wires. When a load is applied to the coil spring 1, a shear stress τ is generated on the surface of the wire element where the maximum principal stress occurs, as shown in Figure 2, among the wire elements where the principal stress is periodically distributed with each turn. This shear stress τ is larger near the surface of the wire element and decreases as it approaches the center of the wire element. This shear stress τ can be decomposed into a maximum principal stress σ1 in the direction 45° with respect to the wire axis of the wire element, and a minimum principal stress σ2 in the direction perpendicular to the direction of the maximum principal stress σ1.

[0016] Figure 3 schematically shows the stress action around the inclusion 11. When repeated load is applied to the coil spring, tensile stress is repeatedly applied around the inclusion 11 in the direction of the maximum principal stress described above.

[0017] Figure 4 schematically shows the tensile stress generated around the inclusion 11 when returning from the heat treatment temperature to room temperature. In the manufacturing process of coil springs, heat treatment is generally performed at around 300 to 600°C. When returning from the heat treatment temperature to room temperature, the coefficient of thermal expansion of the matrix is ​​greater than that of the inclusion, so the matrix cannot contract sufficiently around the inclusion, and the matrix around the inclusion is subjected to tensile stress in the circumferential direction of the inclusion. As a result, tensile residual stress is generated around the inclusion, and a decrease in fatigue strength occurs, especially around the inclusion where the difference in coefficient of thermal expansion between the matrix and the matrix is ​​large.

[0018] FIG. 5 is a diagram schematically showing the mechanism of reducing tensile residual stress by setting at a low temperature. By performing the setting at room temperature or a temperature lower than room temperature, the load due to an external force (torsion load) and the load due to thermal stress are superimposed, and an excessive load is applied around the inclusion. Due to this excessive load, the region where the excessive load is applied locally yields, and a plastic deformation region 10a occurs. When the external force is removed (unloaded) from this state, the region around the plastic deformation region elastically deforms, so the plastic deformation region 10a receives a compressive stress. As a result, the tensile residual stress of the base around the inclusion can be reduced, or a compressive residual stress can be imparted to the base around the inclusion. The lower the temperature at which the setting is performed, the greater the excessive load, and the more the tensile residual stress of the base around the inclusion can be reduced, or the greater the compressive residual stress can be imparted to the base around the inclusion.

[0019] The present invention has been completed based on the above findings. Hereinafter, a method for manufacturing a coil spring and a coil spring according to an embodiment of the present invention will be described.

[0020] [Method for Manufacturing Coil Spring] The coil spring targeted by the method for manufacturing a coil spring according to the present embodiment is such that the steel material constituting the coil spring has a difference in linear expansion coefficient from the base (matrix) of 13.0×10 -6 K -1 or more and includes inclusions. This is because the greater the difference in linear expansion coefficient between the base and the inclusion, the greater the decrease in fatigue strength around the inclusion, and the greater the effect of the setting process described later. The difference in linear expansion coefficient between the base and the inclusion is preferably 13.5×10 -6 K -1 or more, and more preferably 15.5×10 -6 K -1 or more. The upper limit of the difference in linear expansion coefficient between the base and the inclusion is not particularly limited, but for example, it is 23.0×10 -6 K -1 . Note that the "linear expansion coefficient" and the "difference in linear expansion coefficient" mean values at room temperature unless otherwise noted.

[0021] The coefficient of thermal expansion of the base depends on the type of steel, but for steel used in coil springs, it is approximately 23.0 × 10⁻⁶. -6 K -1 It is approximately 10.0 × 10⁻⁶. Therefore, the coefficient of linear expansion of the inclusion is 10.0 × 10⁻⁶. -6 K -1 The following is preferable. 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, the method for manufacturing a coil spring according to this embodiment may use one or more inclusions 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. The method for manufacturing a coil spring according to this embodiment is particularly suitable when the inclusion is VC.

[0022] The equivalent circular diameter of inclusions contained in the steel material constituting the coil spring is preferably 50.0 μm or less. In other words, it is preferable that the steel material constituting the coil spring does not contain inclusions with an equivalent circular diameter exceeding 50.0 μm. If inclusions exceeding 50.0 μm are present, it may be difficult to ensure sufficient fatigue strength. More preferably, the equivalent circular diameter of inclusions contained in the steel material constituting the coil spring is 45.0 μm or less.

[0023] Furthermore, the number density of inclusions with an equivalent circular diameter of 1.0 to 50.0 μm is as follows: 2 Preferably, there should be one or more inclusions per unit. The number density of inclusions should be 1 inclusion / 100mm 2 This is because, in cases where the number density is less than 1.0 μm, the influence of inclusions on the fatigue strength of the steel is considered to be inherently small. Furthermore, there are no particular limitations on the number density of inclusions with an equivalent circular diameter of less than 1.0 μm, as the influence of inclusions with an equivalent circular diameter of less than 1.0 μm on the fatigue strength of the steel is considered to be small.

[0024] Aside from the conditions regarding inclusions described above, there are no particular restrictions on the steel material that constitutes the coil spring. For example, high-tensile steel, which is commonly used as a steel material for coil springs, can be used as the steel material that constitutes the coil spring according to this embodiment. Preferably, the steel material that constitutes the coil spring according to this embodiment has a yield strength of 1600 MPa or more, more preferably 1700 MPa or more, and even more preferably 1800 MPa or more.

[0025] Figure 6 is a flow chart of a method for manufacturing a coil spring according to one embodiment of the present invention. This manufacturing method includes the steps of coiling (step S1), heat treatment (step S2), polishing (step S3), shot peening (step S4), secondary heat treatment (step S5), and setting (step S6).

[0026] The steel material described above is formed into a coil 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.

[0027] After heat treatment, the end faces are polished as needed (Step S3). Polishing may be omitted. After heat treatment or polishing, shot peening is performed to impart compressive residual stress to the surface of the steel (Step S4). Shot peening may be performed multiple times.

[0028] After shot peening, a secondary heat treatment is performed as needed (step S5). The secondary heat treatment may be omitted. The secondary heat treatment is performed to remove microscopic strain and improve fatigue resistance, and is generally performed at a lower temperature than the heat treatment after coiling (step S2). The temperature of the secondary heat treatment (step S5) is not particularly limited, but is, for example, 200-250°C.

[0029] After shot peening or heat treatment, setting is performed (step S6). Normally, setting is performed at a warm temperature, i.e., a temperature higher than room temperature, in order to improve the resistance of the coil spring to sagging. In contrast, in this embodiment, setting is performed at room temperature or a temperature lower than room temperature.

[0030] By performing setting at room temperature or a temperature lower than room temperature, the load due to external force (torsional load) and the load due to thermal stress are superimposed, resulting in an excessive load being applied around the inclusion. This excessive load causes the region under excessive load to locally yield, creating a plastic deformation region. When the external force is removed (unloaded) from this state, the region surrounding the plastic deformation region undergoes elastic deformation, and the plastic deformation region is subjected to compressive stress. This can either reduce the tensile residual stress in the matrix around the inclusion or impart compressive residual stress to the matrix around the inclusion.

[0031] Setting is preferably performed at a temperature lower than room temperature. The lower the temperature at which setting is performed, the greater the overload, which can either reduce the tensile residual stress in the matrix around the inclusion or impart a greater compressive residual stress to the matrix around the inclusion. The temperature during setting is more preferably 10°C or lower, even more preferably 0°C or lower, even more preferably -60°C or lower, even more preferably -90°C or lower, and even more preferably -190°C or lower.

[0032] Methods for setting at low temperatures include, but are not limited to, setting while spraying a refrigerant, setting while immersing in a refrigerant, and setting in a space cooled by heat exchange with a refrigerant. Examples of refrigerants, but are not limited to, include ice, ice-salt mixtures, dry ice, ethanol-dry ice mixtures, ethanol, liquid nitrogen, and fluorocarbons.

[0033] The coil spring according to this embodiment is manufactured through the above steps. The method for manufacturing the coil spring according to this embodiment only needs to include a setting step (step S6) at room temperature or a temperature lower than room temperature, and can be implemented by making various modifications to the manufacturing method shown in Figure 6. For example, the setting may be performed after the heat treatment (step S2) and before the secondary heat treatment (step S5). However, it is preferable not to perform heat treatment at a temperature of 300°C or higher after setting, as this will release residual stress.

[0034] [Coil spring] In one embodiment of the present invention, a coil spring has a maximum residual stress of 300 MPa or less acting perpendicular to the maximum principal stress plane when a load is applied to the coil spring, in a matrix adjacent to an inclusion located at a depth of 0.50 to 1.50 mm from the surface. In the following description, "residual stress acting perpendicular to the maximum principal stress plane when a load is applied to the coil spring" is referred to as "residual stress in the principal stress direction."

[0035] Here, "the plane of maximum principal stress when a load is applied to the coil spring" more specifically refers to the plane where the direction at 45° to the wire axis of the coil spring is the normal direction, as explained using Figure 2. "The base adjacent to the inclusion" more specifically refers to the base located in the region at a distance of 200 nm or less from the interface with the inclusion. "The maximum value of residual stress in the principal stress direction in the base adjacent to the inclusion" more specifically refers to the value of residual stress in the principal stress direction at the location where the residual stress in the principal stress direction is greatest among the bases adjacent to the inclusion of interest.

[0036] Residual stress around inclusions can be measured, for example, by the FIB-DIC ring core method. Specifically, in the FIB-DIC ring core method, a sample is cut from a coil spring, a fine marker is applied to the observed inclusion, and the residual stress is released by cutting away the matrix in the region close to the inclusion, and the released strain amount and residual stress value are measured.

[0037] The reason for focusing on the residual stress in the matrix surrounding inclusions located at a depth of 0.50 to 1.50 mm from the surface is as follows: First, in regions less than 0.50 mm from the surface, it is relatively easy to reduce residual stress by shot peening, etc., so inclusions in this region usually do not have a significant impact on the fatigue strength of the coil spring. Also, the stress generated when a load is applied to a coil spring decreases as you approach the center of the coil spring wires. Therefore, inclusions located deeper than 1.50 mm from the surface also do not have a significant impact on the fatigue strength of the coil spring.

[0038] In this embodiment, the maximum value of residual stress in the principal stress direction in the matrix adjacent to an inclusion located at a depth of 0.50 to 1.50 mm from the surface is set to 300 MPa or less. This makes it possible to suppress the reduction in fatigue strength due to the inclusion, even when an inclusion is present. The maximum value of residual stress in the principal stress direction is more preferably 100 MPa or less, even more preferably 50 MPa or less, and even more preferably 0 MPa or less (compressive residual stress).

[0039] The above describes a method for manufacturing a coil spring and a coil spring according to one embodiment of the present invention. According to this embodiment, a coil spring with excellent fatigue strength can be obtained. [Examples]

[0040] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0041] We modeled the minute elements surrounding inclusions in steel and performed calculations using the finite element method to investigate the relationship between the stress distribution around the inclusions and the manufacturing process. Figure 7 is a schematic diagram of the analysis model. For a 20 μm × 20 μm region centered on a circular inclusion with a radius of 2 μm, we considered symmetry and used a 1 / 4 region of 10 μm × 10 μm as the calculation target.

[0042] The material properties of the base are: Coefficient of thermal expansion: 23 × 10 -6 ×K -1 The Young's modulus was assumed to be 206 GPa, and the Poisson's ratio was assumed to be 0.3. The inclusions were assumed to be VC, and the coefficient of linear expansion was 6.88 × 10⁻⁶. -6 ×K -1 The Young's modulus was set to 262 GPa and the Poisson's ratio to 0.3. Figure 8 shows the stress-strain curve of the substrate. The yield strength of the substrate is approximately 1800 MPa.

[0043] Figure 9 shows the patterns of temperature history and load history applied to the analysis model. The upper panel shows the temperature history, and the lower panel shows the load history. First, residual stress relief annealing was simulated by heating to 420°C, and then the material was cooled to temperature T1 (400°C, 200°C, 20°C, or -196°C). A setting stress load of 800 MPa was applied at temperature T1. The load was removed while maintaining temperature T1, and then the temperature was reduced to room temperature (20°C).

[0044] As shown in Figure 7, the setting load was applied by applying a tensile stress of 800 MPa in the x-direction to one end of the analysis model in the y-direction, and a compressive stress of 800 MPa to the other end in the x-direction. The other end in the y-direction was constrained in the y-direction, and the other end in the x-direction was constrained in the x-direction. In Figure 7, the y-direction corresponds to the direction of the maximum principal stress, and the y-plane corresponds to the plane of the maximum principal stress. The calculation was performed assuming that there was an inclusion at a depth of 1.33 mm from the surface of a wire strand with a diameter of 4.0 mm.

[0045] Figures 10 to 14 show the stress acting perpendicular to the y-plane at time points A to E in Figure 9 (hereinafter referred to as "σ"). yy This is a contour map showing the distribution of ).

[0046] Figure 10 shows the σ at the point when heated to 420°C (point A in Figure 9). yy This is the distribution. Figure 10 shows that residual stress is removed by heating, and the stress of all elements becomes 0.

[0047] Figure 11 shows σ at the point when the temperature has cooled to T1 (400°C, 200°C, 20°C, or -196°C) (point B in Figure 9). yy This is the distribution. Because the coefficient of thermal expansion of the base is greater than that of the inclusion, the base cannot contract sufficiently around the inclusion, and the base around the inclusion experiences tensile stress in the circumferential direction of the inclusion. Therefore, near the x-edge of the inclusion, σ yy The value increases. Also, the lower the temperature T1, the greater the σ yy It gets bigger.

[0048] Figure 12 shows the σ at the point when a setting stress of 800 MPa is applied at temperature T1 (point C in Figure 7). yy This is the distribution. Figure 13 shows the σ at the point when the load was removed while maintaining the temperature T1 (time D in Figure 7). yy This is the distribution, and Figure 14 shows the σ at the point when the temperature was returned to room temperature (20°C) (time point E in Figure 7). yy This is the distribution.

[0049] As shown in Figure 14, in the analytical model with temperature T1 set to -196°C, the σ of the matrix adjacent to the inclusion yy The maximum value was observed at a position adjacent to the x-direction edge of the inclusion, and that value was -243 MPa. In other words, in the analytical model with temperature T1 set to -196°C, the σ of the matrix adjacent to the inclusion yy This was compressive stress across the entire circumference of the inclusion.

[0050] Similar analyses were performed for inclusion depths of 1.00 mm and 2.00 mm. Figure 15 shows the temperature T1 and the σ in the matrix adjacent to the inclusion. yy This graph shows the relationship with the maximum value. As shown in Figure 15, by lowering the temperature T1, the σ in the substrate adjacent to the inclusion yy It can be seen that the maximum value can be reduced.

[0051] The embodiments of the present invention have been described above. The embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and it is possible to carry out the present invention by appropriately modifying the embodiments described above without departing from the spirit of the invention.

Claims

1. With room temperature set at 20°C, the difference from the coefficient of linear expansion of the substrate at room temperature is 13.0 × 10 －６ K －１ A method for manufacturing a coil spring made of steel containing the above-mentioned inclusions, This includes a setting process at room temperature or a temperature lower than room temperature. A method for manufacturing a coil spring, wherein the coil spring has a maximum residual stress of 300 MPa or less acting perpendicular to the maximum principal stress plane when a load is applied to the coil spring, in the base adjacent to the inclusion located at a depth of 0.50 to 1.50 mm from the surface.

2. A method for manufacturing a coil spring according to claim 1, A method for manufacturing a coil spring, wherein the temperature during the setting process is lower than room temperature.

3. A method for manufacturing a coil spring according to claim 2, A method for manufacturing a coil spring, wherein the temperature during the setting process is 10°C or lower.

4. A method for manufacturing a coil spring according to claim 3, A method for manufacturing a coil spring, wherein the temperature during the setting process is 0°C or lower.

5. A method for manufacturing a coil spring according to any one of claims 1 to 4, The inclusion is TiB ２ , TiC, TiN, ZrB ２ , ZrC, ZrN, VB ２ , VC, VN, NbB ２ , NbC, TaB ２ , TaC, CrN, Mo ２ B ５ , Mo ２ C, W ２ B ５ , WC, B ４ C, SiC, SiB ６ , Si ３ N ４ , AlN, Al ２ O ３ , AlTi ４ , TiO ２ , and SiO ２ A method for manufacturing a coil spring, which is one or more selected from the group consisting of

6. A method for manufacturing a coil spring according to any one of claims 1 to 4, The equivalent circular diameter of the inclusion is 50.0 μm or less. The number density of the inclusions with an equivalent circular diameter of 1.0 to 50.0 μm is such that the cross-sectional area is 100 mm ２ A method for manufacturing coil springs, where each spring contains at least one spring.

7. A method for manufacturing a coil spring according to any one of claims 1 to 4, A method for manufacturing a coil spring, further comprising a step of heat treatment at 300 to 600°C before the setting step.

8. With room temperature set at 20°C, the difference from the coefficient of linear expansion of the substrate at room temperature is 13 × 10 －６ K －１ A coil spring made of steel containing the above-mentioned inclusions, The coil spring is such that the maximum value of residual stress acting perpendicular to the maximum principal stress plane when a load is applied to the coil spring in the base adjacent to the inclusion located at a depth of 0.50 to 1.50 mm from the surface is 300 MPa or less.

9. A coil spring according to claim 8, A coil spring in which the maximum value of the residual stress is 100 MPa or less.

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