Coil spring, suspension system, and method for manufacturing a coil spring

Abstract

To improve the corrosion resistance of coil springs. [Solution] A coil spring according to one embodiment is a coil spring comprising spirally wound wires, wherein a portion of the cross-section of the wires has a compressive residual stress of 200 MPa or more at a depth of 0.6 mm or more from the surface of the wires.

Description

Technical Field

[0001] The present invention relates to a coil spring, a suspension device, and a method for manufacturing a coil spring.

Background Art

[0002] For example, coil springs are used in suspension devices for vehicles such as automobiles. This type of coil spring is required to have excellent sag resistance, corrosion durability, improved fatigue strength, and the like.

[0003] If the overall hardness of the wire forming the coil spring is increased, an improvement in sag resistance can be expected. However, in this case, for example, if part of the coating film applied to the wire peels off and corrosion pits form on the surface of the wire, cracks originating from these corrosion pits may occur in the wire. The higher the hardness of the wire, the faster the crack propagates, which may lead to early breakage of the coil spring. On the other hand, if the overall hardness of the wire is low, the sag resistance decreases.

[0004] Patent Document 1 is known as an example of studies concerning the sag resistance and corrosion durability of coil springs. In this document, attempts are made to improve sag resistance and corrosion durability by softening the interior or a portion of the surface of the wire (spring steel). Further, Patent Document 2 discloses a method for manufacturing a spring that can obtain desired mechanical properties.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of Invention

Problems to be Solved by the Invention

[0006] Even with reference to Patent Documents 1 and 2, there is still room for improvement in enhancing the corrosion resistance of coil springs. Therefore, one of the objectives of this disclosure is to improve the corrosion resistance of coil springs. [Means for solving the problem]

[0007] A coil spring according to one embodiment is a coil spring comprising spirally wound wires, wherein a portion of the cross-section of the wires has a compressive residual stress of 200 MPa or more at a depth of 0.6 mm or more from the surface of the wires.

[0008] A suspension device according to one embodiment comprises a first spring seat, a second spring seat, and the coil spring disposed between the first spring seat and the second spring seat.

[0009] A method for manufacturing a coil spring according to one embodiment includes forming a coil spring by winding a wire in a spiral shape, heating a portion of the wire, and cooling the heated portion of the wire to form a softened layer containing ferrite, thereby applying a compressive residual stress of 200 MPa or more to the portion of the wire to a depth of 0.6 mm or more from the surface of the wire. [Effects of the Invention]

[0010] According to this disclosure, it is possible to improve the corrosion resistance of coil springs. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic cross-sectional view of a suspension system according to one embodiment. [Figure 2] Figure 2 is a schematic perspective view of a coil spring according to one embodiment. [Figure 3] Figure 3 is a schematic cross-sectional view of the coil spring along the line III-III in Figure 2. [Figure 4] Figure 4 is a schematic cross-sectional view of the coil spring along the IV-IV line in Figure 2. [Figure 5]Figure 5 is a graph showing an example of the hardness distribution of the first winding section in the depth direction. [Figure 6] Figure 6 is a flowchart showing an example of a method for manufacturing a coil spring according to one embodiment. [Figure 7] Figure 7 shows an example of the heating process in local softening treatment. [Figure 8] Figure 8 is a graph showing an example of the residual stress distribution in the depth direction of a coil spring. [Figure 9] Figure 9 is a diagram illustrating crack propagation in a corrosion pit. [Modes for carrying out the invention]

[0012] One embodiment will be described with reference to the drawings. In this embodiment, a MacPherson strut type suspension system, a coil spring used in the suspension system, and a method for manufacturing the coil spring will be illustrated. The coil spring disclosed in this embodiment can be used in other types of suspension systems, or in applications other than suspension systems.

[0013] Figure 1 is a schematic cross-sectional view of the suspension device 100 according to this embodiment. This suspension device 100 includes a coil spring 1 for vehicle suspension. The coil spring 1 comprises a spirally wound wire 2. The wire 2 is made of, for example, spring steel.

[0014] The suspension system 100 further comprises a shock absorber 3, a first spring seat 4, and a second spring seat 5. The second spring seat 5 is located above the first spring seat 4 in the vertical direction Z. The coil spring 1 is mounted to the suspension system 100 in a compressed state between the first spring seat 4 and the second spring seat 5.

[0015] The shock absorber 3 includes a cylinder 30 containing a fluid such as oil, a rod 31 inserted into the cylinder 30, a damping force generation mechanism provided inside the cylinder 30, and a cover member 32 covering the sliding portion of the rod 31. The rod 31 can extend and retract relative to the cylinder 30 in parallel with the axis X0 of the shock absorber 3. The damping force generation mechanism provides resistance to the movement of the rod 31.

[0016] The upper end portion of the shock absorber 3 is attached to the vehicle body 7 via a mount insulator 6. The mount insulator 6 includes a vibration-proof rubber 60 and a support member 61 fixed to the vehicle body 7. The lower end portion of the shock absorber 3 is attached to a knuckle member 8 that supports the axle via a bracket 9. In the example of FIG. 1, the axis X0 of the shock absorber 3 is inclined at an acute angle θ0 with respect to the vertical direction Z.

[0017] The coil spring 1 is mounted in a compressed state between the first spring seat 4 and the second spring seat 5, elastically supports a load applied from above, and expands and contracts within a predetermined deflection range (between full rebound and full bump) according to the magnitude of the load.

[0018] FIG. 2 is a schematic perspective view of the coil spring 1 according to the present embodiment. FIG. 3 is a schematic cross-sectional view of the coil spring 1 taken along line III-III in FIG. 2. FIG. 4 is a schematic cross-sectional view of the coil spring 1 taken along line IV-IV in FIG. 2. Both the cross sections of FIG. 3 and FIG. 4 intersect perpendicularly with the axis X2 of the wire 2.

[0019] As shown in FIG. 2, the coil spring 1 has an effective portion 10, a first end coil portion 11 (end coil portion), and a second end coil portion 12. In the present embodiment, a part of the wire 2 is the first end coil portion 11, but a part of the wire 2 may be the effective portion 10 or the second end coil portion 12. The first end coil portion 11 is a portion that can come into contact with the first spring seat 4. The second end coil portion 12 is a portion that can come into contact with the second spring seat 5. The effective portion 10 is a portion located between the first end coil portion 11 and the second end coil portion 12.

[0020] In this embodiment, the first spring seat portion 11 may include not only a portion that is always in contact with the first spring seat 4, but also a portion that separates from the first spring seat 4 when the compressive load applied to the coil spring 1 is less than a predetermined value, and contacts the first spring seat 4 when the compressive load applied to the coil spring 1 exceeds the predetermined value.

[0021] Similarly, the second spring seat portion 12 may include not only a portion that is always in contact with the second spring seat 5, but also a portion that separates from the second spring seat 5 when the compressive load applied to the coil spring 1 is less than a predetermined value, and contacts the second spring seat 5 when the compressive load applied to the coil spring 1 exceeds the predetermined value.

[0022] For example, the first winding section 11 extends 1.2 turns from the lower end 2a of the wire strand 2. The second winding section 12 extends 1.2 turns from the upper end 2b of the wire strand 2.

[0023] In the effective section 10, the strands 2 are wound multiple times around the coil axis X1. For example, the coil axis X1 is inclined at an acute angle with respect to the vertical direction Z shown in Figure 1 and the axis X0 of the shock absorber 3. Hereinafter, as shown in Figure 2, we define the axial direction DX parallel to the coil axis X1 and the radial direction DR centered on the coil axis X1. The first and second winding sections 11 and 12 are aligned with the effective section 10 in the axial direction DX, respectively.

[0024] As shown in Figure 3, the entire surface 20 of the wire strand 2 may be covered with a coating 21. For example, the diameter R of the wire strand 2 is 8 to 18 mm, and the thickness of the coating 21 is 40 μm or more.

[0025] As shown in Figure 4, the first winding section 11 comprises a first layer L1, a second layer L2, and a third layer L3. In this embodiment, the first layer L1 is an example of a hardened layer, and the second layer L2 is an example of a softened layer. The first layer L1 and the third layer L3 are located on the surface of the first winding section 11. In other words, the first layer L1 and the third layer L3 include the surface 20 of the wire strand 2. The axis X2 of the wire strand 2 passes through the third layer L3.

[0026] The second layer L2 is located between the first layer L1 and the third layer L3. In the example in Figure 4, the proportion of the third layer L3 to the cross-sectional area of ​​wire 2 is greater than the proportion of the first layer L1 and the second layer L2 to that cross-sectional area.

[0027] In the example shown in Figure 3, the strands 2 in the effective portion 10 are entirely formed by the third layer L3. For example, the second winding portion 12 is also entirely formed by the third layer L3, but this is not the only example.

[0028] The first layer L1 is formed, for example, along the second layer L2. In the example in Figure 4, the ends E1 and E2 of the first layer L1 in the circumferential direction Dθ coincide with the ends of the second layer L2 in the circumferential direction Dθ, respectively. However, the ends E1 and E2 of the first layer L1 may be offset from the ends of the second layer L2. That is, the first layer L1 may be formed over a narrower or wider area than the second layer L2 in the circumferential direction Dθ.

[0029] In this embodiment, the surface 20 of the wire strand 2 has a first region A1 and a second region A2. The first region A1 corresponds to the surface layer of the third layer L3. The second region A2 corresponds to the surface layer of the first layer L1. In the example in Figure 2, the second region A2 is provided in a part of the first winding portion 11. For example, the portion of the surface 20 excluding the second region A2 is the first region A1.

[0030] Here, as shown in Figure 4, we define the first position P1, second position P2, third position P3, and fourth position P4 on the surface 20 of the wire 2. For example, the first position P1 is the position on the surface 20 that is furthest from the effective portion 10 in the axial direction DX. For example, the second position P2 is the position on the surface 20 that is furthest from the coil axis X1 in the radial direction DR. For example, the third position P3 is the position on the surface 20 that is closest to the effective portion 10 in the axial direction DX (the position opposite the first position P1 across the axis X2). For example, the fourth position P4 is the position on the surface 20 that is closest to the coil axis X1 in the radial direction DR (the position opposite the second position P2 across the axis X2). The first position P1, second position P2, third position P3, and fourth position P4 are arranged sequentially at 90-degree intervals in the circumferential direction Dθ.

[0031] In other terms, the first position P1 is a position that can come into contact with the seating surface SF that supports the coil spring 1. The seating surface SF is, for example, a part of the first spring seat 4 shown in Figure 1. The second position P2 is one of a pair of intersection points where the surface 20 intersects with the second centerline CL2, which is perpendicular to the first centerline CL1 (a line perpendicular to the seating surface SF) of the surface 20 at the first position P1 and passes through axis X2, and is on the outer diameter side of the coil spring 1 (the side farther from the coil axis X1). The third position P3 is one of a pair of intersection points where the first centerline CL1 intersects with the surface 20, on the opposite side of axis X2 from the first position P1 (the side closer to the effective portion 10). The fourth position P4 is one of a pair of intersection points where the second centerline CL2 intersects with the surface 20, on the inner diameter side of the coil spring 1 (the side closer to the coil axis X1). In the example shown in Figure 4, the first center line CL1 is parallel to the coil axis X1, but this is not the only example.

[0032] In the first seat winding portion 11, the first region A1 and the second region A2 are aligned in the circumferential direction Dθ. In this embodiment, the second region A2 is formed downwards. Specifically, the second region A2 includes the first position P1, and the first region A1 includes the second position P2, the third position P3, and the fourth position P4.

[0033] In the example in Figure 4, the center of the second region A2 in the circumferential direction Dθ coincides with the first position P1. The first center line CL1 passes through the center of the second region A2 in the circumferential direction Dθ. Also, as shown in Figure 4, in the direction from the first position P1 toward axis X2, the first layer L1, the second layer L2, and the third layer L3 are arranged in this order. That is, in this direction, the second layer L2 is located closer to the center than the first layer L1, and the third layer L3 is located closer to the center than the second layer L2. Here, "closer to the center" refers to the position closer to axis X2 than the surface 20 of the wire 2 in a line segment passing through the surface 20 of the wire 2 and axis X2. Specifically, in a line segment passing through the surface 20 of the wire 2 and axis X2, the second layer L2 is located closer to axis X2 than the first layer L1, and the third layer L3 is located closer to axis X2 than the second layer L2. The first centerline CL1 passes through the centers of the first layer L1 and the second layer L2 in the circumferential direction Dθ.

[0034] In this embodiment, a portion of the cross-section of the wire 2 corresponds to a range including the center (e.g., first position P1) and axis X2 in the circumferential direction Dθ of the second region A2, as shown in the example of Figure 4. In this embodiment, a portion of the cross-section is located in the first winding portion 11. In other words, the first winding portion 11 includes a portion of the cross-section. In the example of Figure 4, a portion of the cross-section of the wire 2 includes the first layer L1, the second layer L2, and the third layer L3.

[0035] In the example in Figure 4, the end E1 of the second region A2 lies between the fourth position P4 and the first position P1 in the circumferential direction Dθ, and the other end E2 of the second region A2 lies between the second position P2 and the first position P1 in the circumferential direction Dθ. For example, the center of the second region A2 in the circumferential direction Dθ is equidistant from the ends E1 and E2 in the circumferential direction Dθ.

[0036] In the example shown in Figure 4, the length of the second region A2 in the circumferential direction Dθ is shorter than the length of the first region A1 in the circumferential direction Dθ. That is, the second region A2 is formed over a narrower area in the circumferential direction Dθ than the first region A1. This example is not limited to this case; the second region A2 may be formed over a wider area in the circumferential direction Dθ than the first region A1.

[0037] Furthermore, the first position P1 and the surrounding area may come into contact with the seating surface SF of the first spring seat 4 shown in Figure 1, either under normal circumstances or when the coil spring 1 is compressed. In other words, the second region A2 extends to at least a portion of the first seat winding portion 11 that may come into contact with the seating surface SF.

[0038] The range and center position of the second region A2 are not limited to the example in Figure 4. For example, the center of the second region A2 in the circumferential direction Dθ may be located between the first position P1 and the fourth position P4, or between the first position P1 and the second position P2, in the circumferential direction Dθ.

[0039] As shown in the enlarged view below Figure 4, for example, the thickness t1 of the first layer L1 and the thickness t2 of the second layer L2 are both greatest at the first center line CL1. In other words, the thickness t1 of the first layer L1 and the thickness t2 of the second layer L2 are both greatest at the center in the circumferential direction Dθ of the second region A2.

[0040] The thickness t1 gradually decreases from the first position P1 toward the ends E1 and E2 of the first layer L1. The thickness t1 at the first center line CL1, i.e., the maximum value of the thickness t1, is, for example, 0.2 mm or less, preferably 0.15 mm or less, and more preferably 0.08 mm or less. In the depth direction (direction from the surface 20 toward axis X2), in one example, the first layer L1 is formed to a depth of 0.2 mm or less from the surface 20 of the wire 2. In other examples, the first layer L1 is formed to a depth of 0.15 mm or less from the surface 20 of the wire 2. In yet another example, the first layer L1 is formed to a depth of 0.08 mm or less from the surface 20 of the wire 2.

[0041] Similarly, the thickness t2 of the second layer L2 gradually decreases from the first position P1 toward both ends of the second layer L2. In the depth direction, the second layer L2 is formed at a deeper position than the first layer L1 (towards axis X2), extending to a depth of 0.7 mm or less from the surface 20 of the wire 2. The maximum value of the combined thickness ts(t1+t2) of the first layer L1 and the second layer L2 is, for example, 0.7 mm or less.

[0042] Furthermore, the third layer L3 is formed at a deeper position (towards axis X2) than the second layer L2. For example, if the second layer L2 is formed to a depth of 0.7 mm from the surface 20, the third layer L3 will be formed at a position deeper than 0.7 mm from the surface 20.

[0043] In the example shown in Figure 4, the thickness t2 is greater than the thickness t1 at any point along the circumferential direction Dθ. However, in at least a portion of the circumferential direction Dθ, the thickness t2 may be less than or equal to the thickness t1.

[0044] The first spring seat portion 11 does not need to have the cross-sectional structure shown in Figure 4 at all positions. In the example in Figure 2, the second region A2 is not provided in the portion 11a at a certain distance from the end 2a. This portion 11a is the part that is always in contact with the first spring seat 4 regardless of the compression state of the coil spring 1 when the coil spring 1 is incorporated into the suspension device 100. From another point of view, in the example in Figure 2, the second region A2 is provided in the part that comes into contact with or separates from the first spring seat 4 depending on the load applied to the coil spring 1.

[0045] The second region A2 is preferably formed in at least a portion of the range from terminal 2a to 1.2 turns. In one example, the second region A2 is formed in a range of at least 0.4 turns and 0.9 turns from terminal 2a. However, the second region A2 may be formed beyond this range or smaller than this range. Also, the second region A2 may be formed continuously over a certain range from terminal 2a.

[0046] Here, we will explain the first layer L1, the second layer L2, and the third layer L3.

[0047] The first layer L1 has a body-centered cubic structure (BCC) proportion of 90% or more. The body-centered cubic structure (BCC) contains at least one of tempered martensite (α), work-induced martensite (α), and ferrite (α'). Furthermore, the content of work-induced martensite in the first layer L1 is 5% or more and 15% or less. The first layer L1 may also contain retained austenite and carbides.

[0048] The second layer L2 has a body-centered cubic structure (BCC) of 80% or more, and a face-centered cubic structure (FCC) of 10% to 20% or less. The face-centered cubic structure (FCC) contains retained austenite (γ). The second layer L2 may also contain carbides. The third layer L3 has a body-centered cubic structure (BCC) of 90% or more. The third layer L3 may also contain carbides.

[0049] The first layer L1 is mainly formed of ferrite, and the content of work-induced martensite in the first layer L1 is 5% or more and 15% or less. The second layer L2 is mainly formed of ferrite, and the content of retained austenite in the second layer L2 is 10% or more and 20% or less. The ferrite content in the second layer L2 is 80% or more. The third layer L3 is mainly formed of tempered martensite, and the retained austenite content in the third layer L3 is 5% or less. The tempered martensite content in the third layer L3 is 90% or more.

[0050] Furthermore, the proportion of body-centered cubic structures (BCCs) in the first layer L1 and the third layer L3 is greater than the proportion of body-centered cubic structures (BCCs) in the second layer L2. The proportion of face-centered cubic structures (FCCs) in the second layer L2 is greater than the proportion of face-centered cubic structures (FCCs) in the first layer L1 and the third layer L3.

[0051] Furthermore, the ferrite content in the first layer L1 and the second layer L2 is greater than the ferrite content in the third layer L3. The retained austenite content in the second layer L2 is greater than the retained austenite content in the first layer L1 and the third layer L3. In addition, the first layer L1 contains work-induced martensite, but the second layer L2 and the third layer L3 do not. Furthermore, the first layer L1, the second layer L2, and the third layer L3 may contain tempered martensite. From the above, it can be said that the content of tempered martensite, work-induced martensitic, and ferrite in the first layer L1 is 90% or more, the content of tempered martensite and ferrite in the second layer L2 is 80% or more, and the content of tempered martensite and ferrite in the third layer L3 is 90% or more.

[0052] In this specification, "proportion" and "content" can be measured by the SEM / EBSD (Electron Back Scatter Diffraction) method. Specifically, for example, a phase map (phase distribution diagram) of the α phase and γ phase can be created using a JSM-7900F (TSL Solutions OIM-Analysis Ver.7.3.1) manufactured by JEOL Ltd. For example, the observation area of ​​the phase map is 8 μm × 24 μm, and the observation magnification is 10,000x. In the phase map, the proportion of the area occupied by the α phase corresponds to the proportion of body-centered cubic structure (BCC). Also, the proportion of the area occupied by the α phase in the phase map corresponds to the content of ferrite and martensite. In the phase map, the proportion of the area occupied by the γ phase corresponds to face-centered cubic structure (FCC). Also, the proportion of the area occupied by the γ phase in the phase map corresponds to the content of retained austenite. Furthermore, in the phase map, the content of processing-induced martensite in the first layer L1 and the third layer L3 can be calculated from the change in the ratio of the area of ​​the α phase and the γ phase before and after processing by shot peening.

[0053] Figure 5 is a graph showing an example of the hardness distribution of the first coiled section 11 in the depth direction. This hardness distribution corresponds to the line segment passing through the first position P1 (the center in the circumferential direction Dθ of the second region A2) and axis X2. In this graph, the horizontal axis is the distance [mm] from the surface 20 of the wire 2, and the vertical axis is the Vickers hardness [Hv]. The diameter of the wire 2 in Figure 5 is 12.2 mm. The hardness in Figure 5 was measured using a Vickers hardness tester. In Figure 5 and Figure 6, the first layer L1 is formed to a depth of 0.2 mm from the surface 20, and the second layer L2 is formed to a depth of 0.7 mm from the surface 20.

[0054] In the example shown in Figure 5, the hardness is highest at surface 20 (second region A2) of the first layer L1, and gradually decreases as you move away from surface 20, with the hardness being lowest in the second layer L2. In other words, the second layer L2 is softer than the first layer L1. In the example shown in Figure 5, the maximum hardness is approximately 650 Hv, and the minimum hardness is approximately 360 Hv.

[0055] The hardness gradually increases from the point of lowest hardness in the second layer L2 towards the third layer L3, and further increases as the distance from the surface 20 increases. In the third layer L3, the hardness also gradually increases as the distance from the surface increases. In the example in Figure 5, the hardness reaches 500 Hv at a distance of approximately 2.3 mm from the surface 20.

[0056] Thus, the hardness of the first layer L1 has a gradient depending on the distance from the surface 20. The first layer L1 is generally harder than the second layer L2. In other words, the second layer L2 is softer than the first layer L1. Also, a portion of the first layer L1, including the surface 20, is harder than the third layer L3. Within the third layer L3, the proportion of the portion that is harder than the second layer L2 is greater than the proportion of the portion that is softer than the second layer L2.

[0057] In one example, the first layer L1 has a hardness of 500 Hv or more, while the second layer L2 has a hardness of less than 500 Hv. Furthermore, the first layer L1 includes portions with a hardness of 550 Hv or more and portions with a hardness of 600 Hv or more. In the example in Figure 5, the hardness of the third layer L3 at the position of axis X2 is approximately 550 Hv. Therefore, the third layer L3 at the position of axis X2 is harder than the second layer L2.

[0058] Focusing on the surface 20 of the wire strand 2, the second region A2 is harder than the first region A1. In at least a portion of the surface 20 of the first winding portion 11, the hardness distribution in the circumferential direction Dθ around the axis X2 of the wire strand 2 is not uniform. In contrast, in the surface 20 of the second winding portion 12 and the effective portion 10, the hardness distribution is uniform over the entire circumference of the circumferential direction Dθ.

[0059] Figure 6 is a flowchart showing an example of a method for manufacturing a coil spring 1. First, the wire strands 2 are formed into a spiral shape by a coiling machine, and this spirally formed portion is cut by a cutter (step S1). For example, at this point, the wire strands 2 are formed entirely of a third layer L3 mainly composed of tempered martensite.

[0060] Next, the wire strands 2 are subjected to electroannealing (step S2). In this electroannealing, the wire strands 2 are heated for a predetermined time of 1 minute or less at a temperature range of, for example, 400 to 500°C. After electroannealing, the wire strands 2 are subjected to a local softening treatment to form a second layer L2 (softened layer) (step S3). In this embodiment, the local softening treatment includes a preheating step S3a, a heating step S3b, and a cooling step S3c. Here, the electroannealing step S2 may also serve as the preheating step S3a.

[0061] In the preheating step S3a, the wire 2 is heated to a first temperature, for example, below the austenitization start temperature. This austenitization start temperature is set to a temperature above the A3 transformation point. For example, the austenitization start temperature of the wire 2 is 727°C or higher. In this case, the first temperature is preferably in the range of 300 to 450°C.

[0062] In the preheating step S3a, the heating time is determined so that at least the first winding portion 11, preferably the entire wire 2, rises to the first temperature inside. The method of heating the wire 2 in the preheating step S3a is not particularly limited, but one example is that electrical heating can be used.

[0063] In the heating step S3b, the wire 2 is further heated while its temperature has risen due to the preheating step S3a. In the cooling step S3c, the wire 2 that has gone through the heating step S3b is cooled. Details of the heating step S3b and the cooling step S3c will be described later with reference to Figure 7.

[0064] After local softening treatment, the wire strand 2 is heated and then subjected to hot setting, which involves applying an overload to the wire strand 2 (step S4).

[0065] Next, the wire strands 2 are subjected to shot peening (step S5). In this shot peening process, compressive residual stress is applied to the wire strands 2.

[0066] Subsequently, presetting is performed on the wire strands 2 (step S6). Furthermore, a coating film 21 is formed on the entire surface 20 of the wire strands 2 (step S7). Note that the local softening treatment may be performed, for example, before shot peening or after hot setting.

[0067] Figure 7 shows an example of the heating step S3b in the local softening treatment. The heating step S3b can be performed using a laser device 200. For example, the laser device 200 is a semiconductor laser, but is not limited to this example.

[0068] The laser device 200 irradiates the area in the first winding portion 11 where the second region A2 is to be formed with laser light LZ. For example, the position where the irradiation axis XL intersects the surface 20 corresponds to the center of the circumferential direction Dθ of the second region A2.

[0069] When irradiating with laser light LZ, for example, the position of the laser device 200 may be fixed, and the strand 2 may be rotated around the coil axis X1. In this case, the strand 2 may be moved axially DX as it rotates so that the distance between the part of the strand 2 irradiated by the laser light LZ and the laser device 200 remains constant. This suppresses the shift in the focal point of the laser light LZ. As another example, the strand 2 may be fixed, and the laser device 200 may be moved.

[0070] When the laser beam LZ is irradiated, the wire 2 is heated. This heating causes a heat-affected zone (HAZ) to form on the wire 2. When the heat-affected zone is cooled by the cooling process S3c, a second layer L2 is formed that is less hard than the original wire 2.

[0071] In the heating step S3b, at least the portion where the second layer L2 is to be formed is heated to a second temperature above the austenitization start temperature. In the cooling step S3c, the portion is allowed to cool naturally. This natural cooling slowly cools the portion heated in the heating step S3b. For example, in the cooling step S3c, it is preferable that the portion where the second layer L2 is to be formed is maintained at a temperature of 500°C to 700°C for 5 to 10,000 seconds. By slowly cooling the portion in this way, ferrite and retained austenite are generated in that portion. As a result, the second layer L2 containing ferrite and retained austenite is formed.

[0072] After step S3, the second layer L2 is formed up to the surface 20 of the wire 2. In other words, in the first winding portion 11, the surface layer of the second layer L2 is located on the surface 20 of the wire 2. As described above, the second layer L2 contains retained austenite. Therefore, when shot peening is applied to the surface layer of the second layer L2 (step S5 in Figure 6), some of the retained austenite is transformed into work-induced martensite. As a result, a first layer L1 (hardened layer), which is harder than the second layer L2, is formed on the surface 20 of the wire 2.

[0073] The preheating step S3a contributes to achieving slow cooling in the cooling step S3c. In other words, if the wire 2 were heated using the method shown in Figure 7 without performing the preheating step S3a, a large temperature difference would occur between the heated portion in the heating step S3b and the other portions. As a result, the heated portion would be rapidly cooled in the cooling step S3c, and there is a possibility that ferrite and retained austenite would not be sufficiently formed.

[0074] In contrast, if the preheating step S3a is performed, the temperature difference between the part heated in heating step S3b and the other parts becomes smaller. Therefore, in the cooling step S3c, the temperature decrease of the part heated in heating step S3b becomes slower. As a result, the crystal structure of that part transforms into ferrite and retained austenite.

[0075] The heating in heating step S3b is not limited to the method using the laser device 200. As another example, the heating in heating step S3b may be AC ​​current heating of the wire 2. Specifically, terminals are connected to both ends of the area in the winding direction of the wire 2 where the second layer L2 is to be formed, and an AC current is passed between these terminals. The heating temperature distribution (current density distribution) in AC current heating can be controlled, for example, by utilizing the proximity effect that occurs when a conductor is placed near the wire 2, or by placing a ferromagnetic material around the wire 2.

[0076] Furthermore, slow cooling in the cooling process S3c can also be achieved by methods other than preheating. As another example, slow cooling can be achieved even when the cooling process S3c is carried out in a furnace capable of heating the wire strands 2. In this case, the preheating process S3a may be omitted.

[0077] Here, we show an example of the distribution of residual stress values ​​in the depth direction of a coil spring. Figure 8 is a graph showing an example of the distribution of residual stress values ​​in the depth direction of a coil spring. In this graph, the horizontal axis is the distance [mm] from the surface 20 of the wire 2, and the vertical axis is the residual stress value [MPa]. If the residual stress value is less than 0, it is compressive residual stress, and if it is greater than 0, it is tensile residual stress. Compressive residual stress is the internal stress in the compressive direction of the surface layer of the wire when no external force is acting on it. The compressive residual stress in Figure 8 was measured using an X-ray diffraction type residual stress measuring device.

[0078] In Figure 8, the residual stress values ​​of the coil spring 1 according to this embodiment are shown by a dashed line. This stiffness distribution corresponds to the line segment passing through the first position P1 (the center in the circumferential direction Dθ of the second region A2) and axis X2 in the cross-section of the first seat winding portion 11.

[0079] Furthermore, in Figure 8, the residual stress value of the coil spring according to the comparative example is shown by a solid line. The coil spring according to the comparative example has not undergone local softening treatment (step S3 in Figure 6). Therefore, the coil spring according to the comparative example differs from the coil spring 1 according to this embodiment in that it does not have layers corresponding to the first layer L1 and the second layer L2 in this embodiment.

[0080] The compressive residual stress of coil spring 1 is maximum at a depth of approximately 0.07 mm from the surface 20 of wire 2. The maximum compressive residual stress of coil spring 1 is approximately 1000 MPa. In the example in Figure 8, the compressive residual stress at a depth of 0.3 mm is approximately 600 MPa. The compressive residual stress at a depth of 0.4 mm is approximately 500 MPa.

[0081] In this embodiment, the coil spring 1 has a compressive residual stress in the cross-section of the first seat winding portion 11 that is at least half of the maximum compressive residual stress applied to a depth of 0.3 mm or more from the surface 20. In the example in Figure 8, the coil spring 1 has a compressive residual stress at a depth of 0.4 mm from the surface 20 that is at least half of the maximum compressive residual stress applied.

[0082] Coil spring 1 is subjected to a compressive residual stress of 300 MPa or more at a depth of 0.5 mm from the surface 20. Furthermore, coil spring 1 is subjected to a compressive residual stress of 200 MPa or more at a depth of 0.6 mm from the surface 20. In the example shown in Figure 8, the compressive residual stress at a depth of 0.6 mm is approximately 250 MPa, and the compressive residual stress at a depth of 0.7 mm is approximately 160 MPa.

[0083] Furthermore, the coil spring 1 is subjected to compressive residual stress at a depth of 0.8 mm from the surface 20. In the example in Figure 8, the compressive residual stress at a depth of 0.8 mm is approximately 10 MPa. In the coil spring 1, a portion of the cross-section of the first seat winding portion 11 has a point where the compressive residual stress is zero at a depth of 0.8 mm or more from the surface 20.

[0084] This point corresponds to the depth from the surface where the compressive residual stress inside the wire is zero under no load. In the example in Figure 8, the compressive residual stress is zero at a depth of approximately 0.81 mm from the surface 20. This point is sometimes called the crossing point.

[0085] Focusing on the relationship between the first layer L1, the second layer L2, and the third layer L3 in Figure 4, the coil spring 1 has the maximum compressive residual stress in, for example, the first layer L1. In the coil spring 1, the depth from the surface 20 to 0.6 mm corresponds to the second layer L2. Also, the point where the compressive residual stress is zero is located in the third layer L3. Since ferrite has a soft structure, by applying shot peening to the second layer L2 containing ferrite, as in this embodiment (step S5 in Figure 6), compressive residual stress can be applied to the depth of the wire 2 in the region overlapping with the second layer L2 in the depth direction. Specifically, by applying shot peening to the second layer L2, a compressive residual stress of 200 MPa or more can be applied to the wire 2 at a depth of 0.6 mm or more from the surface 20.

[0086] In contrast, the compressive residual stress of the coil spring in the comparative example is maximum at a depth of approximately 0.05 mm from the surface 20 of wire 2. Furthermore, the compressive residual stress at a depth of 0.3 mm is approximately 200 MPa. In other words, the coil spring in the comparative example has a compressive residual stress at a depth of 0.3 mm from the surface 20 that is less than half of the maximum value of the applied compressive residual stress.

[0087] Furthermore, the point at which the compressive residual stress of the coil spring in the comparative example becomes zero is located at a depth of 0.4 mm or less from the surface 20. In other words, compared to the coil spring in the comparative example, the coil spring 1 according to this embodiment has a deeper point at which the compressive residual stress becomes zero, meaning that compressive residual stress is applied to a deeper position from the surface 20 of the wire 2. In the comparative example, tensile residual stress is applied at a depth of 0.6 mm from the surface.

[0088] In this embodiment as well, the area near the third position P3 (shown in Figure 4) on the surface 20 is not laser heated. Therefore, near the third position P3, compressive residual stress is not applied to a deep position, similar to the coil spring in the comparative example. The coil spring 1 has different depths to which compressive residual stress is applied on either side of the axis X2. In other words, the coil spring 1 has different depths at which the compressive residual stress becomes zero in the circumferential direction Dθ.

[0089] In this embodiment, the coil spring 1 has a compressive residual stress of 200 MPa or more at a depth of 0.6 mm or more from the surface 20 of the wire 2. Figure 9 is a diagram illustrating the propagation of crack C1 in a corrosion pit CP. As shown in Figure 9, a corrosion pit CP may be formed on the surface 20.

[0090] A corrosion pit CP can serve as the starting point for a crack C1. The coil spring 1 is subjected to compressive residual stress to a deep position. This suppresses the tensile stress around the surface 20 of the wire strand 2, reducing stress concentration around the corrosion pit CP. As a result, the growth rate of the crack C1 slows down even when subjected to repeated loading, and the progression of corrosion fatigue is suppressed.

[0091] For example, it was confirmed that the lifespan of coil spring 1 until fracture in repeated load tests under corrosive conditions was improved by approximately 20% compared to the coil spring in the comparative example (shown in Figure 8). In other words, as in this embodiment, applying a compressive residual stress of 200 MPa or more to a depth of 0.6 mm or more from the surface 20 improves the corrosion resistance of coil spring 1.

[0092] Furthermore, as in this embodiment, by forming a second layer L2 that is softer than the first layer L1, even if a corrosion pit CP occurs in the second region A2, this corrosion pit CP is less likely to develop into a crack C1 in the wire 2. Also, even if a crack C1 does occur, its progression can be slowed because compressive residual stress is applied to a deep position. Specifically, by having a point in a part of the cross-section of the wire 2 where the compressive residual stress is zero at a depth of 0.8 mm or more from the surface 20, the progression of the crack C1 can be slowed, further improving the corrosion resistance of the coil spring 1. In addition, from the viewpoint of improving corrosion resistance, it is preferable that the compressive residual stress at a depth of 0.3 mm or more from the surface 20 is at least half of the maximum value of the compressive residual stress applied to the wire 2.

[0093] Furthermore, if the entire strand 2 softens, the sag resistance of the coil spring 1 may decrease. In contrast, in this embodiment, the effective portion 10 is entirely formed of the third layer L3. Therefore, good sag resistance can be maintained in the effective portion 10. The first seat winding portion 11 is a portion with lower applied stress compared to the effective portion 10. Therefore, even if the second layer L2 is provided in the first seat winding portion 11, it does not significantly affect the overall sag resistance of the coil spring 1.

[0094] The first seat coil portion 11 is the part that contacts the first spring seat 4 located below it. Therefore, if foreign matter such as sand gets between the seat surface SF of the first spring seat 4 and the first seat coil portion 11, the coating 21 will be damaged during use of the suspension device 100, and corrosion pits are likely to form in the first seat coil portion 11. In contrast, in this embodiment, a second region A2 is provided in the area of ​​the first seat coil portion 11 that can come into contact with the seat surface SF. Therefore, even if foreign matter gets between the seat surface SF and the first seat coil portion 11 and corrosion pits form, cracks caused by these corrosion pits can be suppressed.

[0095] Furthermore, foreign matter as described above tends to get trapped beneath the portion of the first seat coil 11 that comes into contact with or separates from the seat surface SF. Therefore, by locally forming a second region A2 in such a portion, it is possible to effectively improve corrosion resistance while maintaining the sag resistance of the coil spring 1. In this regard, the range from terminal 2a to 0.4 turns or more and 0.9 turns or less is a region where corrosion pits due to foreign matter are likely to occur. Also, this range is more susceptible to stress when the coil spring 1 is compressed compared to the range from terminal 2a to 0.4 turns. Therefore, it is preferable to form the second region A2 in at least the range from terminal 2a to 0.4 turns or less and 0.9 turns or less.

[0096] When laser light is used in the heating step S3b, it may be difficult to sufficiently raise the temperature inside the wire 2. In contrast, if preheating step S3a is performed before heating step S3b, the temperature can be sufficiently raised to a deep location within the wire 2. As a result, it becomes possible to form a thicker second layer L2.

[0097] As described above, this embodiment can improve the corrosion resistance of coil springs. In addition to the above, various other desirable effects can be obtained from this embodiment.

[0098] The above embodiments do not limit the scope of the present invention to the configurations disclosed in these embodiments. The present invention can be implemented by modifying the configurations disclosed in the embodiments in various ways.

[0099] In this embodiment, an example is disclosed in which the first layer L1 and the second layer L2 are formed on the first seat winding portion 11. However, the first layer L1 and the second layer L2 may be formed on at least one of the effective portion 10 and the second seat winding portion 12. By appropriately changing the position in which the second layer L2 is formed, compressive residual stress can be applied to a desired region in the circumferential direction Dθ to a deep position.

[0100] For example, in this embodiment, the first layer L1 and the second layer L2 may be formed on the surface 20 of the first spring seat portion 11 in the portion facing the effective portion 10. Also, in the second spring seat portion 12, the first layer L1 and the second layer L2 may be formed on the portion that comes into contact with or separates from the second spring seat 5 depending on the load applied to the coil spring 1. Furthermore, when the first layer L1 and the second layer L2 are formed on the second spring seat portion 12, the first layer L1 and the second layer L2 do not need to be formed on the first spring seat portion 11.

[0101] Furthermore, the position where the second region A2 is formed can be any position in the circumferential direction Dθ, and the laser device 200 irradiates the area where the second region A2 is to be formed with laser light LZ.

[0102] Furthermore, the wire strand 2 may further include other layers with different hardness from the first layer L1, the second layer L2, and the third layer L3, respectively. For example, if the other layers extend to the surface 20 of the wire strand 2, additional regions with different hardness from the first region A1 and the second region A2 may be formed on the surface 20.

[0103] In the embodiment, a coil spring 1 in which strands 2 are wound in a cylindrical shape is disclosed. However, the coil spring 1 may have other shapes, such as a barrel shape in which the diameter decreases towards the first seat winding portion 11 and the second seat winding portion 12. [Explanation of Symbols]

[0104] 2...wire strand, 10...effective portion, 11...first winding portion, 12...second winding portion, 20...surface, 21...coating film, 100...suspension device, 200...laser device, A1...first region, A2...second region, L1...first layer, L2...second layer, L3...third layer.

Claims

1. A coil spring having spirally wound wires, A portion of the cross-section of the aforementioned wire has a compressive residual stress of 200 MPa or more at a depth of 0.6 mm or more from the surface of the wire. Coil spring.

2. The aforementioned portion of the cross-section has a point where the compressive residual stress is zero at a depth of 0.8 mm or more from the surface. The coil spring according to claim 1.

3. The portion of the cross-section has a compressive residual stress of at least half the maximum value of the compressive residual stress applied to the portion of the cross-section at a depth of 0.3 mm or more from the surface. The coil spring according to claim 1.

4. The portion of the cross-section comprises a first layer including the surface of the wire, and a second layer that is softer than the first layer and located closer to the center than the first layer. A depth of 0.6 mm from the surface of the aforementioned wire is included in the second layer. The coil spring according to claim 1.

5. The unit further comprises a seat coil portion including the part of the cross-section, The coil spring according to claim 1.

6. The aforementioned portion of the cross-section contains ferrite at a depth of 0.6 mm from the surface of the wire. The coil spring according to claim 1.

7. The first spring constellation and The second spring constellation, A coil spring according to any one of claims 1 to 6, disposed between the first spring seat and the second spring seat, A suspension system equipped with [a specific feature].

8. A coil spring is formed by winding the wires in a spiral shape. A portion of the aforementioned wire is heated, By cooling the heated portion of the wire, a softened layer containing ferrite is formed. This includes applying a compressive residual stress of 200 MPa or more to a portion of the aforementioned wire at a depth of 0.6 mm or more from the surface of the aforementioned wire. A method for manufacturing coil springs.

9. The heating includes irradiating the surface of the wire with laser light. The method for manufacturing a coil spring according to claim 8.

10. By applying shot peening to the softened layer, a compressive residual stress of 200 MPa or more is imparted at the aforementioned depth. A method for manufacturing a coil spring according to claim 8 or 9.

11. In the heating described above, the portion to which the softened layer is to be formed is raised to a temperature above the austenitization initiation temperature. In the cooling process, by slowly cooling the portion, the softened layer containing the ferrite is formed in that portion. A method for manufacturing a coil spring according to claim 10.

12. By applying shot peening to the softened layer, a hardened layer harder than the softened layer is formed on the surface of the wire. A method for manufacturing a coil spring according to claim 10.

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