Coil spring, suspension device, and method for manufacturing coil spring

Abstract

A coil spring according to one embodiment comprises a spirally wound element wire, and has a seat winding part and an effective part. Furthermore, the seat winding part comprises a first layer and a second layer that is softer than the first layer and contains ferrite.

Description

Coil spring, suspension device, and method for manufacturing coil spring

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

[0002] For example, coil springs are used in suspension systems for vehicles such as automobiles, and these types of coil springs are required to have good sag resistance and corrosion fatigue resistance.

[0003] Increasing the overall hardness of the wire that forms the coil spring is expected to improve sag resistance. However, in this case, if corrosion pits form on the surface of the wire, for example, due to partial peeling of the coating applied to the wire, cracks may occur in the wire starting from these corrosion pits. If the hardness of the wire is high, the cracks will progress quickly, which may lead to early breakage of the coil spring. On the other hand, if the hardness of the wire is low overall, sag resistance will decrease.

[0004] Patent Document 1 is known as an example of a study into the corrosion fatigue resistance of a coil spring. In the coil spring described in this document, a portion of the wire is softened in the end turn portion. This makes it possible to slow the progression of cracks even if the above-mentioned corrosion pits occur in the end turn portion, resulting in improved corrosion fatigue resistance of the coil spring.

[0005] Patent No. 7203910

[0006] Even in light of Patent Document 1, there is still room for improvement in the corrosion fatigue resistance of coil springs. For example, if the softened portion as described above is formed of a crystalline structure that is highly susceptible to hydrogen embrittlement, there is a possibility that this portion will be damaged by hydrogen embrittlement.

[0007] Therefore, one of the objects of the present disclosure is to improve the corrosion fatigue resistance of a coil spring.

[0008] A coil spring according to one embodiment includes a wire wound in a spiral shape, an end turn portion, and an active portion. The end turn portion further includes a first layer and a second layer that is softer than the first layer and includes ferrite.

[0009] A suspension device according to one embodiment includes a first spring seat, a second spring seat, and the coil spring disposed between the first spring seat and the second spring seat.

[0010] A method for manufacturing a coil spring according to one embodiment includes forming a coil spring having an end turn portion and an active portion by spirally winding a wire, heating a portion of the end turn portion including a first layer, and cooling the heated end turn portion to form a second layer in the end turn portion, the second layer being softer than the first layer and including ferrite.

[0011] According to the present disclosure, for example, it is possible to improve the corrosion fatigue resistance of a coil spring.

[0012] FIG. 1 is a schematic cross-sectional view of a suspension device according to the first embodiment. FIG. 2 is a schematic perspective view of a coil spring according to the first embodiment. FIG. 3 is a schematic cross-sectional view of the coil spring taken along line III-III in FIG. 2. FIG. 4 is a schematic cross-sectional view of the coil spring taken along line IV-IV in FIG. 2. FIG. 5 is a graph showing an example of the hardness distribution of a first end winding portion in the depth direction. FIG. 6 is a graph showing an example of the hardness distribution of the surface of the first end winding portion in the circumferential direction. FIG. 7 is a flowchart showing an example of a method for manufacturing a coil spring according to the first embodiment. FIG. 8 is a diagram showing an example of a heating step in a local softening treatment. FIG. 9 is a schematic perspective view of a coil spring according to a second embodiment.

[0013] Several embodiments will be described with reference to the drawings. Each embodiment illustrates a MacPherson strut type suspension system, a coil spring used in the suspension system, and a method for manufacturing the coil spring. The coil spring disclosed in each embodiment can be used in other types of suspension systems and can also be used for purposes other than suspension systems.

[0014] 1 is a schematic cross-sectional view of a suspension system 100 according to a first embodiment. The suspension system 100 includes a coil spring 1 for suspending a vehicle. The coil spring 1 includes a wire 2 wound in a spiral shape. The wire 2 is made of, for example, spring steel.

[0015] The suspension device 100 further includes 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 attached to the suspension device 100 in a compressed state between the first spring seat 4 and the second spring seat 5.

[0016] 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 generating mechanism provided inside the cylinder 30, and a cover member 32 that covers the sliding portion of the rod 31. The rod 31 can extend and retract relative to the cylinder 30 in parallel to the axis X0 of the shock absorber 3. The damping force generating mechanism provides resistance to the movement of the rod 31.

[0017] The upper end of the shock absorber 3 is attached to the vehicle body 7 via a mount insulator 6. The mount insulator 6 includes a vibration-isolating rubber 60 and a support member 61 fixed to the vehicle body 7. The lower end 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.

[0018] The coil spring 1 is mounted in a compressed state between the first spring seat 4 and the second spring seat 5, elastically supports the load applied from above, and expands and contracts within a predetermined range of deflection (between full rebound and full bump) depending on the magnitude of the load.

[0019] Fig. 2 is a schematic perspective view of the coil spring 1 according to this 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. The cross sections in Figs. 3 and 4 both intersect perpendicularly with the axis X2 of the wire 2.

[0020] As shown in Figure 2, the coil spring 1 has an active portion 10, a first end winding portion 11, and a second end winding portion 12. The first end winding portion 11 is a portion that can come into contact with the first spring seat 4. The second end winding portion 12 is a portion that can come into contact with the second spring seat 5. The active portion 10 is a portion located between the first end winding portion 11 and the second end winding portion 12.

[0021] In this embodiment, the first seat winding portion 11 may include not only a portion that is always in contact with the first spring seat 4, but also a portion that moves away from the first spring seat 4 when the compressive load applied to the coil spring 1 is less than a predetermined value, and that comes into contact with the first spring seat 4 when the compressive load applied to the coil spring 1 exceeds the predetermined value.

[0022] Similarly, the second seat winding portion 12 may include not only a portion that is always in contact with the second spring seat 5, but also a portion that moves away from the second spring seat 5 when the compressive load applied to the coil spring 1 is less than a predetermined value, and that comes into contact with the second spring seat 5 when the compressive load applied to the coil spring 1 exceeds the predetermined value.

[0023] For example, the first end winding portion 11 is in a range of 1.2 turns from the lower end 2a of the wire 2. The second end winding portion 12 is in a range of 1.2 turns from the upper end 2b of the wire 2.

[0024] In the effective portion 10, the wire 2 is 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 and the axis X0 of the shock absorber 3 shown in FIG. 1. Hereinafter, as shown in FIG. 2, an axial direction DX parallel to the coil axis X1 and a radial direction DR centered on the coil axis X1 are defined. The first end winding portion 11 and the second end winding portion 12 are each aligned with the effective portion 10 in the axial direction DX.

[0025] 3, the surface 20 of the wire 2 may be entirely covered with a coating film 21. As an example, the diameter R of the wire 2 is 8 to 18 mm, and the thickness of the coating film 21 is 40 μm or more.

[0026] In this embodiment, the surface 20 of the wire 2 has a first region A1 and a second region A2 that is softer than the first region A1. In the example of Fig. 2, the second region A2 is provided in a part of the first end winding portion 11. For example, the part of the surface 20 excluding the second region A2 is the first region A1.

[0027] That is, in at least a portion of the surface 20 of the first end winding portion 11, the distribution of hardness in the circumferential direction Dθ about the axis X2 of the wire 2 shown in Figure 4 is not uniform. On the other hand, in the surfaces 20 of the second end winding portion 12 and the effective portion 10, the distribution of hardness is uniform over the entire circumference in the circumferential direction Dθ.

[0028] In the example shown in Figure 3, the wires 2 in the active portion 10 are entirely formed of the first layer L1. The first region A1 corresponds to the surface of the first layer L1. For example, the second end winding portion 12 is also entirely formed of the first layer L1, but this is not limiting.

[0029] 4, the first end winding portion 11 includes a first layer L1, a second layer L2, and a third layer L3. The second layer L2 and the third layer L3 are both softer than the first layer L1.

[0030] The second layer L2 is located on the surface of the first end winding portion 11. The second region A2 corresponds to the surface of the second layer L2. The third layer L3 is located between the first layer L1 and the second layer L2. In the example of Figure 4, the proportion of the cross-sectional area of ​​the wire 2 occupied by the first layer L1 is larger than the proportions of the cross-sectional area occupied by the second layer L2 and the third layer L3. The axis X2 of the wire 2 passes through the first layer L1.

[0031] The first layer L1 is mainly formed of, for example, tempered martensite. In one example, the content of tempered martensite in the first layer L1 is approximately 99%. The first layer L1 may also contain carbides and the like.

[0032] The second layer L2 is mainly formed of ferrite and retained austenite. Preferably, the ferrite content in the second layer L2 is 60% or more. The retained austenite content in the second layer L2 is 20% or less. The second layer L2 may contain carbides and the like in addition to ferrite and retained austenite. However, the second layer L2 may not contain retained austenite and may be composed of only ferrite or ferrite and carbides and the like.

[0033] The third layer L3 has, for example, the same crystalline structure as the first layer L1, i.e., the third layer L3 may contain, for example, 99% tempered martensite and carbides.

[0034] The "content" in this specification can be measured by SEM / EBSD (Electron Backscatter Diffraction) method. Specifically, a phase map (phase distribution diagram) of the α phase and the γ phase is created using, for example, a JSM-7900F (TSL Solutions OIM-Analysis Ver. 7.3.1) manufactured by JEOL Ltd. For example, the observation range of the phase map is 8 μm × 24 μm, and the observation magnification is 10,000 times. The proportion of the area occupied by the α phase in the phase map corresponds to the content of ferrite. Furthermore, the proportion of the area occupied by the γ phase in the phase map corresponds to the content of retained austenite.

[0035] Here, as shown in FIG. 4 , a first position P1, a second position P2, a third position P3, and a fourth position P4 are defined on the surface 20 of the wire 2. For example, the first position P1 is the position on the surface 20 that is farthest 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 closest to 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 farthest from the coil axis X1 in the radial direction DR (the position opposite the second position P2 across the axis X2). The first position P1, the second position P2, the third position P3, and the fourth position P4 are arranged in order at 90-degree intervals in the circumferential direction Dθ.

[0036] From another perspective, the first position P1 is a position that can come into contact with the seat surface SF that supports the coil spring 1. The seat surface SF is, for example, a part of the first spring seat 4 shown in FIG. 1 . The second position P2 is one of a pair of intersections between the surface 20 and a second center line CL2 that is perpendicular to the first center line CL1, which is a normal to the surface 20 at the first position P1 (a line perpendicular to the seat surface SF), and passes through the axis X2. The second position P2 is one of a pair of intersections between the surface 20 and the second center line CL2, which is on the inner diameter side of the coil spring 1 (the side closer to the coil axis X1). The third position P3 is one of a pair of intersections between the first center line CL1 and the surface 20, which is on the opposite side of the axis X2 from the first position P1 (the side closer to the active portion 10). The fourth position P4 is one of a pair of intersections between the second center line CL2 and the surface 20, which is on the outer diameter side of the coil spring 1 (the side farther from the coil axis X1). In the example of FIG. 4, the first center line CL1 is parallel to the coil axis X1, but this is not limiting.

[0037] In the first end 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 closer to the inner diameter of the wire 2 and closer to the lower side.

[0038] In the example of Figure 4, the end E1 of the second region A2 is located between the fourth position P4 and the first position P1 in the circumferential direction Dθ. The other end E2 of the second region A2 coincides with the second position P2. Furthermore, the center C of the second region A2 in the circumferential direction Dθ is located between the first position P1 and the second position P2 in the circumferential direction Dθ. In this configuration, the second region A2 includes the first position P1. The first region A1 includes the third position P3 and the fourth position P4.

[0039] For example, the center C is located equidistant from the ends E1 and E2 in the circumferential direction Dθ. The center C is preferably located between the fourth position P4 and the second position P2 in the circumferential direction Dθ, and more preferably between the first position P1 and the second position P2.

[0040] 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 in a narrower range in the circumferential direction Dθ than the first region A1. However, this is not limiting, and the second region A2 may be formed in a wider range in the circumferential direction Dθ than the first region A1.

[0041] The first position P1 and the area nearby can be in contact with the seating surface SF of the first spring seat 4 shown in Fig. 1 at all times or when the coil spring 1 is compressed. In other words, the second area A2 extends to at least a part of the portion of the first end winding portion 11 that can be in contact with the seating surface SF.

[0042] The range of the second region A2 and the position of the center C are not limited to the example shown in Fig. 4. For example, the end E2 does not need to coincide with the second position P2, and may be located between the first position P1 and the second position P2 in the circumferential direction Dθ. Furthermore, the end E2 may be located between the second position P2 and the third position P3 in the circumferential direction Dθ.

[0043] The first end winding portion 11 does not need to have the cross-sectional structure shown in Fig. 4 at all positions. In the example of Fig. 2, the second region A2 is not provided in a portion 11a that is a certain distance from the terminal 2a. This portion 11a is a portion that always contacts the first spring seat 4 regardless of the compression state of the coil spring 1 when the coil spring 1 is installed in the suspension device 100, for example. From another perspective, in the example of Fig. 2, the second region A2 is provided in a portion that comes into contact with or separates from the first spring seat 4 depending on the load applied to the coil spring 1.

[0044] The second region A2 is preferably formed in at least a portion of the range of 1.2 turns from the terminal 2a. In one example, the second region A2 is formed in a range of at least 0.4 turns and 0.9 turns from the terminal 2a. However, the second region A2 may be formed beyond this range or smaller than this range. Furthermore, the second region A2 may be formed continuously within a certain range from the terminal 2a.

[0045] The third layer L3 is formed, for example, along the second layer L2. In the example of FIG. 4 , both end portions of the third layer L3 in the circumferential direction Dθ coincide with the end portions E1 and E2 of the second layer L2 in the circumferential direction Dθ. However, both end portions of the third layer L3 may be offset from the end portions E1 and E2 of the second layer L2. In other words, the third layer L3 may be formed over a narrower or wider area than the second layer L2 in the circumferential direction Dθ.

[0046] In the example of Fig. 4, the thickness t2 of the second layer L2 and the thickness t3 of the third layer L3 are both greatest at the center C. The thickness t2 gradually decreases from the center C toward the ends E1 and E2 of the second layer L2. The thickness t2 at the center C, i.e., the maximum value of the thickness t2, is, for example, 0.1 mm or more, and preferably 0.6 mm or more.

[0047] Similarly, the thickness t3 of the third layer L3 gradually decreases from the center C toward both end portions of the third layer L3. In the example of Fig. 4, the thickness t3 is greater than the thickness t2 at any position in the circumferential direction Dθ. However, the thickness t3 may be equal to or less than the thickness t2 at least in a part of the circumferential direction Dθ.

[0048] The total thickness ts (= t2 + t3) of the second layer L2 and the third layer L3 is, for example, 0.6 mm or more, and preferably 1.0 mm or more, at the center C. In terms of its relationship with the diameter R of the wire 2, the thickness ts at the center C is, for example, 2% or more and 8% or less of the diameter R.

[0049] 5 is a graph showing an example of the hardness distribution of the first end winding portion 11 in the depth direction (direction from the surface 20 toward the axis X2). This hardness distribution corresponds to that along a line segment passing through the center C and the axis X2. In this graph, the horizontal axis represents the distance [mm] from the surface 20 (center C) of the wire 2, and the vertical axis represents Rockwell hardness [HRC].

[0050] In the example of Figure 5, the hardness of the first layer L1 is almost constant. In the second layer L2 and the third layer L3, the hardness is lowest at the surface 20 (second region A2), and the hardness gradually increases with increasing distance from the surface 20, reaching the hardness of the first layer L1 at a thickness ts. In this way, the second layer L2 and the third layer L3 are generally softer than the first layer L1. The hardness of the second layer L2 and the third layer L3 has a gradient that depends on the distance from the surface 20.

[0051] In the example of Fig. 5, the second layer L2 and the third layer L3 have the same hardness gradient. As another example, the second layer L2 and the third layer L3 may have different hardness gradients. Furthermore, the third layer L3 may include a portion having a lower hardness than the second layer L2.

[0052] 6 is a graph showing an example of the hardness distribution in the circumferential direction Dθ on the surface 20 of the first end winding portion 11. In this graph, the horizontal axis represents the position [mm] in the circumferential direction Dθ on the surface 20, and the vertical axis represents Vickers hardness [HV].

[0053] In the example of Figure 6, the hardness of the first region A1 is almost constant. In the second region A2, the hardness is lowest near the center C. Between the center C and the end E1, and between the center C and the end E2, the hardness gradually increases with increasing distance from the center C, reaching the hardness of the first region A1 at the ends E1 and E2. In this way, the second region A2 is softer overall than the first region A1. The hardness of the second region A2 has a gradient with the hardness at the center C being the minimum value.

[0054] 7 is a flowchart showing an example of a method for manufacturing the coil spring 1. First, the wire 2 is formed into a spiral shape by a coiling machine, and this spirally formed portion is cut off by a cutter (step S1). For example, at this point, the wire 2 is entirely formed of a first layer L1 mainly composed of tempered martensite.

[0055] Next, the wire 2 is subjected to electric annealing (step S2). In this electric annealing, the wire 2 is heated, for example, at a temperature range of 400 to 500°C for a predetermined time of one minute or less. After the electric annealing, the wire 2 is subjected to a local softening treatment to form the second layer L2 and the third layer L3 (step S3). The local softening treatment in this embodiment includes a preheating step S3a, a heating step S3b, and a cooling step S3c. Here, the electric annealing step S2 may also serve as the preheating step S3a.

[0056] In the preheating step S3a, the wire 2 is heated to a first temperature that is lower than the austenitizing start temperature. For example, the austenitizing 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.

[0057] In the preheating step S3a, the heating time is determined so that at least the first end winding portion 11, preferably the entire wire 2, is heated to the first temperature. There are no particular limitations on the method for heating the wire 2 in the preheating step S3a, but one example is electrical heating.

[0058] In the heating step S3b, the wires 2 are further heated in a state in which the temperature of the wires 2 has been increased in the preheating step S3a. In the cooling step S3c, the wires 2 that have been subjected to the heating step S3b are cooled. Details of the heating step S3b and the cooling step S3c will be described later with reference to FIG.

[0059] After the local softening treatment, the wire 2 is subjected to hot setting, in which an overload is applied to the wire 2 while the wire 2 is heated (step S4).

[0060] Next, the wire 2 is subjected to shot peening (step S5). In this shot peening, compressive residual stress is imparted to the wire 2. Thereafter, the wire 2 is presetting (step S6). Furthermore, a coating film 21 is formed over the entire surface 20 of the wire 2 (step S7). Note that the local softening treatment may be performed, for example, before the shot peening, or may be performed after the hot setting.

[0061] 8 is a diagram showing 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.

[0062] The laser device 200 irradiates a laser beam LZ onto an area of ​​the first end winding portion 11 where the second region A2 is to be formed. When forming the second region A2 in an area closer to the inner diameter and lower side of the wire 2 as shown in Figure 4, the irradiation axis XL of the laser beam LZ is tilted with respect to the axial direction DX. For example, the position where the irradiation axis XL intersects with the surface 20 corresponds to the center C shown in Figure 4.

[0063] When irradiating the laser beam LZ, for example, the position of the laser device 200 may be fixed, and the wire 2 may be rotated around the coil axis X1. In this case, the wire 2 may be moved in the axial direction DX as the wire 2 rotates, so that the distance between the portion of the wire 2 irradiated with the laser beam LZ and the laser device 200 remains constant. This makes it possible to prevent the focus of the laser beam LZ from shifting. As another example, the wire 2 may be fixed, and the laser device 200 may be moved.

[0064] The wire 2 is heated by being irradiated with the laser beam LZ. This heating forms a heat affected zone (HAZ) in the wire 2. When the heat affected zone is cooled in the cooling step S3c, a second layer L2 and a third layer L3 having reduced hardness compared to the original wire 2 are formed.

[0065] In the heating step S3b, at least the portion where the second layer L2 is to be formed is heated to a second temperature equal to or higher than the austenitization start temperature. In the cooling step S3c, the portion is naturally cooled. 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 manner, ferrite and retained austenite are generated in the portion.

[0066] The preheating step S3a contributes to realizing slow cooling in the cooling step S3c. In other words, if the wire 2 were heated by the method shown in Fig. 8 without performing the preheating step S3a, a large temperature difference would occur between the portion heated in the heating step S3b and 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 generated.

[0067] In contrast, if the preheating step S3a is performed, the temperature difference between the portion heated in the heating step S3b and the other portions is reduced, and therefore, in the cooling step S3c, the temperature of the portion heated in the heating step S3b decreases more slowly, thereby transforming the crystalline structure of the portion into ferrite and retained austenite.

[0068] The heating method in the heating step S3b is not limited to the method using the laser device 200. As another example, the heating method in the heating step S3b may be AC ​​current heating of the wire 2. Specifically, terminals are connected to both ends of the range in the winding direction of the wire 2 where the second layer L2 is to be formed, and 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.

[0069] The slow cooling in the cooling step S3c can also be achieved by a method other than preheating. As another example, slow cooling can be achieved even when the cooling step S3c is performed in a furnace capable of heating the wires 2. In this case, the preheating step S3a may be omitted.

[0070] In the above-described embodiment, the wire 2 has a first region A1 and a second region A2 in the first end turn portion 11 of the coil spring 1. The second layer L2 constituting the second region A2 and the third layer L3 located inside thereof are softer than the first layer L1 constituting the first region A1. Therefore, even if a corrosion pit occurs in the second region A2, the corrosion pit is less likely to develop into a crack in the wire 2. Furthermore, even if a crack does occur, its progression can be slowed. In other words, the configuration of this embodiment improves the corrosion fatigue resistance of the first end turn portion 11.

[0071] If the entire wire 2 were to be softened, the sag resistance of the coil spring 1 would be reduced. In contrast, in this embodiment, the effective portion 10 is entirely formed of the first layer L1. Therefore, the effective portion 10 can maintain good sag resistance. The first end winding portion 11 is a portion that is subjected to lower acting stress than the effective portion 10. Therefore, even if the first end winding portion 11 includes the second layer L2 and the third layer L3, the sag resistance of the entire coil spring 1 is unlikely to be affected.

[0072] The first end winding portion 11 is the portion that comes into contact with the first spring seat 4 located below it. Therefore, if sand or other foreign matter gets between the first end winding portion 11 and the seating surface SF of the first spring seat 4, the coating film 21 is damaged as the suspension 100 is used, and corrosion pits are likely to occur in the first end winding portion 11. In contrast, in this embodiment, the second region A2 is provided in a range of the first end winding portion 11 that can come into contact with the seating surface SF. Therefore, even if foreign matter gets between the seating surface SF and the first end winding portion 11 and causes corrosion pits, cracks caused by these corrosion pits can be suppressed.

[0073] Furthermore, such foreign matter is likely to penetrate below the portion of the first end turn portion 11 that comes into contact with and separates from the seat surface SF. Therefore, by locally forming the second region A2 in such a portion, it is possible to effectively improve the corrosion fatigue resistance of the coil spring 1 while maintaining the sag resistance. In this regard, the range of 0.4 turns or more and 0.9 turns or less from the terminal 2a is a region where corrosion pits due to foreign matter are likely to occur. Furthermore, this range is more susceptible to stress when the coil spring 1 is compressed than the range from the terminal 2a to 0.4 turns or less. Therefore, as described above with reference to FIG. 2 , it is preferable to form the second region A2 at least in the range of 0.4 turns or more and 0.9 turns or less from the terminal 2a.

[0074] Furthermore, stress is likely to be applied to the portion of the surface 20 of the wire 2 on the coil axis X1 side (the inner side of the coil spring 1), and corrosion fatigue is also likely to occur. Therefore, as described above with reference to Figure 4, it is preferable to form the second region A2 closer to the inner side of the coil spring 1.

[0075] Generally, tempered martensite is highly susceptible to hydrogen embrittlement, and therefore, if the portion softened by the local softening treatment (step S3) is mainly formed of tempered martensite, hydrogen embrittlement may occur in that portion, potentially leading to breakage of the first end winding portion 11.

[0076] In contrast, in this embodiment, a second layer L2 containing ferrite and retained austenite is formed on the surface of the portion softened by the local softening treatment. This second layer L2 has a lower susceptibility to hydrogen embrittlement than the portion formed of tempered martensite. This reduces hydrogen embrittlement in the softened portion, and as a result, it is possible to further suppress damage such as breakage of the first end winding portion 11.

[0077] Furthermore, because ferrite is a soft structure, shot peening (step S5) can impart compressive residual stress deep into the wires 2. Therefore, by providing the second layer L2, it is expected that the stress characteristics of the first end turn portion 11 will also be improved.

[0078] When a laser beam is used in the heating step S3b, it may be difficult to sufficiently raise the temperature inside the wire 2. In contrast, if the preheating step S3a is performed before the heating step S3b, the temperature can be sufficiently raised to a deep position in the wire 2. As a result, the second layer L2 and the third layer L3 can be formed thick. In addition to the above, various other advantageous effects can be obtained from this embodiment.

[0079] Second Embodiment The second embodiment illustrates another configuration that can be applied to the coil spring 1. The configuration of the coil spring 1 and the configuration of the suspension device 100 that are not specifically mentioned are the same as those in the first embodiment.

[0080] Fig. 9 is a schematic perspective view of a coil spring 1 according to the second embodiment. As shown in Fig. 9, in this embodiment, a second region A2 (hereinafter referred to as second region A2a) is also formed in the surface 20 of the wire 2 in the effective portion 10, in an area that may come into contact with the second end winding portion 12 during compression.

[0081] In the example of Fig. 9, the second region A2a is not formed in the portion 12a that is a certain distance from the terminal 2b. Specifically, in the example of Fig. 9, the second region A2a is formed in a range of one to two turns from the terminal 2b. As another example, the second region A2a may extend to the second end winding portion 12. Furthermore, the second region A2a may extend to the terminal 2b.

[0082] For example, the cross-sectional structure of the portion including the second region A2a is the same as the cross-sectional structure shown in Figure 4, which is inverted upside down. That is, this portion includes the first layer L1, the second layer L2, and the third layer L3. However, the range in which the second region A2a is formed in the circumferential direction Dθ can be changed as appropriate.

[0083] In the example of Figure 9, a second region A2a is formed on the surface 20 of the active portion 10 in a portion facing the second end winding portion 12. This portion is a location that may come into contact with the second end winding portion 12 when the coil spring 1 is compressed. Even if a corrosion pit due to this contact occurs in the wire 2, the second region A2a prevents the pit from developing into a crack. Furthermore, the second layer L2 containing ferrite and retained austenite reduces hydrogen embrittlement in this portion.

[0084] The scope of the present invention is not limited to the configurations disclosed in the first and second embodiments described above. The present invention can be implemented by modifying the configurations disclosed in the respective embodiments in various ways.

[0085] For example, in the first and second embodiments, at least one of the second layer L2 and the third layer L3 may be formed on the surface 20 of the first end winding portion 11 in a portion facing the active portion 10. Also, at least one of the second layer L2 and the third layer L3 may be formed on a portion of the second end winding portion 12 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 at least one of the second layer L2 and the third layer L3 is formed on the second end winding portion 12, the first end winding portion 11 does not necessarily have to have the second layer L2 and the third layer L3.

[0086] In the first and second embodiments, the wire 2 does not necessarily have to include the third layer L3. The wire 2 may further include another layer having a hardness different from that of the first layer L1, the second layer L2, and the third layer L3. For example, if the other layer extends to the surface 20 of the wire 2, another region having a hardness different from that of the first region A1 and the second region A2 may be additionally formed on the surface 20.

[0087] In each embodiment, the coil spring 1 is disclosed in which the wire 2 is wound in a cylindrical shape. However, the coil spring 1 may have another shape, such as a barrel shape in which the diameter decreases toward the first end winding portion 11 and the second end winding portion 12.

[0088] 1...coil spring, 2...wire, 3...shock absorber, 4...first spring seat, 5...second spring seat, 10...active part, 11...first end turn part, 12...second end turn part, 20...surface of wire, 100...suspension device, A1...first region, A2...second region, L1...first layer, L2...second layer, L3...third layer.

Claims

1. A coil spring comprising a wire wound in a spiral shape, having an end turn portion and an active portion, the end turn portion comprising: a first layer; and a second layer that is softer than the first layer and contains ferrite.

2. The coil spring according to claim 1, further comprising a third layer positioned between the first layer and the second layer, the third layer being softer than the first layer and containing tempered martensite.

3. The coil spring according to claim 1, wherein the ferrite content in the second layer is 60% or more.

4. The coil spring according to claim 1, wherein the second layer further contains retained austenite.

5. The coil spring according to claim 4, wherein the content of the retained austenite in the second layer is 20% or less.

6. The coil spring according to claim 1, wherein the maximum thickness of the second layer is 0.1 mm or more.

7. A suspension device comprising: a first spring seat; a second spring seat; and a coil spring according to any one of claims 1 to 6, disposed between the first spring seat and the second spring seat.

8. A method for manufacturing a coil spring, comprising: forming a coil spring having an end turn portion and an active portion by spirally winding a wire; heating a portion of the end turn portion including a first layer; and cooling the heated end turn portion to form a second layer in the end turn portion, the second layer being softer than the first layer and containing ferrite.

9. The method for manufacturing a coil spring according to claim 8, wherein the heating includes irradiating the surface of the end turn portion with laser light.

10. The method for manufacturing a coil spring according to claim 8, wherein, in the heating step, the temperature of the portion where the second layer is to be formed is raised to a temperature equal to or higher than the austenitization start temperature, and in the cooling step, the portion is slowly cooled, thereby generating the ferrite in the portion.

11. The method for manufacturing a coil spring according to claim 10, wherein the cooling is carried out so that the temperature of the portion is maintained at 500°C or higher for 5 seconds or more.

12. The method for manufacturing a coil spring according to claim 10, further comprising preheating the coil spring to a temperature below the austenitization start temperature before the heating, and by carrying out the heating in a state in which the temperature of the coil spring has been raised by the preheating, the portion that has been raised to a temperature equal to or higher than the austenitization start temperature during the cooling is slowly cooled.

13. The method for manufacturing a coil spring according to any one of claims 8 to 12, wherein the heating further forms a third layer that is located between the first layer and the second layer, is softer than the first layer, and contains tempered martensite.

14. A method for manufacturing a coil spring according to any one of claims 8 to 12, wherein the ferrite content in the second layer is 60% or more.

15. A method for manufacturing a coil spring according to any one of claims 8 to 12, wherein the second layer further contains retained austenite.

16. A method for manufacturing a coil spring according to claim 15, wherein the content of the retained austenite in the second layer is 20% or less.

17. A method for manufacturing a coil spring according to any one of claims 8 to 12, wherein the maximum thickness of the second layer is 0.1 mm or more.

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