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

Abstract

A coil spring according to an embodiment of the present invention is formed from a wire helically wound around a coil axis, and has an end-turn section and an effective section aligned in an axial direction parallel to the coil axis. According to one aspect of the embodiment, the surface of the wire has a first region and a second region that is more pliable than the first region, the first region and the second region are aligned in a circumferential direction centered on the axis of the wire, and in the end-turn section, the difference between the maximum value of tensile residual stress between the center of the second region in the circumferential direction and the axis and the tensile residual stress at a depth of 1 mm from the center or at a depth equal to 10% of the diameter of the wire is 500 MPa or less.

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 resistance to sag and corrosion fatigue.

[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] Known examples of studies on the sag resistance and corrosion fatigue resistance of coil springs are Patent Documents 1, 2, and 3. These documents attempt to improve sag resistance and corrosion fatigue resistance by softening part of the inside or surface of the wire (spring steel).

[0005] JP 2016-191445 A Japanese Patent No. 6053916 A JP 2010-133558 A

[0006] Even when Patent Documents 1, 2, and 3 are taken into consideration, there is still room for improvement in the sag resistance and corrosion fatigue resistance of coil springs. Furthermore, tensile residual stress can occur in the wire after coiling, particularly on the inner diameter side. This tensile residual stress is also one factor that affects the sag resistance and other characteristics of the coil spring.

[0007] An object of the present disclosure is to provide a coil spring with improved properties such as sag resistance and corrosion fatigue resistance, a suspension device including such a coil spring, and a method for manufacturing such a coil spring.

[0008] The coil spring according to the embodiment is formed of a wire wound in a spiral shape around a coil axis, and has an end turn portion and an active portion aligned in an axial direction parallel to the coil axis.

[0009] According to one aspect of the embodiment, the surface of the wire in the end turn portion has a first region and a second region softer than the first region, the first region and the second region are aligned in a circumferential direction centered on the axis of the wire, and in the end turn portion, the difference between the maximum value of the tensile residual stress between the center of the second region in the circumferential direction and the axis and the tensile residual stress at a position at a depth of 1 mm from the center or at a position at a depth of 10% of the diameter of the wire is 500 MPa or less.

[0010] The suspension device according to the 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.

[0011] A manufacturing method of a coil spring according to an embodiment includes forming a wire into a spiral shape around a coil axis, heating the wire so that a first region and a second region softer than the first region are formed on the surface of an end turn portion, and subjecting the wire after heating to shot peening, wherein the first region and the second region are aligned in a circumferential direction around the axis of the wire, and in the end turn portion after the shot peening, a difference between a maximum value of tensile residual stress between a center of the second region in the circumferential direction and the axis and a tensile residual stress at a position at a depth of 1 mm from the center or at a position at a depth of 10% of the diameter of the wire is 500 MPa or less.

[0012] According to the present disclosure, it is possible to provide, for example, a coil spring having improved properties such as sag resistance and corrosion fatigue resistance, a suspension device including the coil spring, and a method for manufacturing the coil spring.

[0013] 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 the wire in the depth direction of the first end winding portion. 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. FIG. 8 is a diagram showing an example of a local softening treatment according to the first embodiment. FIG. 9 is a schematic diagram for explaining the proximity effect. FIG. 10 is a schematic cross-sectional view of a first end winding portion heated by passing AC current without using a conductor. FIG. 11 is a schematic cross-sectional view of a first end winding portion heated by passing AC current using a conductor as shown in FIG. 8. FIG. 12 is a graph showing an example of the residual stress distribution of the wire during the manufacturing process of a coil spring. FIG. 13 is a graph showing another example of residual stress distribution of a wire during the manufacturing process of a coil spring. FIG. 14 is a schematic cross-sectional view showing a first modified example of a configuration applicable to a conductor of a heating device. FIG. 15 is a schematic cross-sectional view showing a second modified example of a configuration applicable to a conductor of a heating device. FIG. 16 is a schematic cross-sectional view showing a third modified example of a configuration applicable to a conductor of a heating device. FIG. 17 is a schematic cross-sectional view showing a fourth modified example of a configuration applicable to a conductor of a heating device. FIG. 18 is a schematic cross-sectional view showing a fifth modified example of a configuration applicable to a conductor of a heating device. FIG. 19 is a schematic cross-sectional view showing a sixth modified example of a configuration applicable to a conductor of a heating device. FIG. 20 is a diagram showing an example of local softening treatment according to the second embodiment. FIG. 21 is a graph showing an example of residual stress distribution of a wire when local softening treatment is performed by laser heating without preheating. FIG. 22 is a graph showing an example of residual stress distribution of a wire when local softening treatment is performed by laser heating after preheating.

[0014] Several embodiments will be described with reference to the drawings. Each embodiment illustrates a McPherson 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.

[0015] 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 vehicle suspension. The coil spring 1 includes a wire 2 wound in a spiral shape. The wire 2 is made of a metal material such as spring steel.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] For example, the first end winding portion 11 is in a range of 1.2 turns from the lower terminal 2a of the wire 2. The second end winding portion 12 is in a range of 1.2 turns from the upper terminal 2b of the wire 2.

[0025] 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 Figure 1. Hereinafter, as shown in Figure 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.

[0026] 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.

[0027] In this embodiment, the surface 20 of the wire 2 has a first region A1 and a second region A2 (the portion with a dotted pattern in FIG. 2 ) 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. The part of the surface 20 excluding the second region A2 is the first region A1.

[0028] 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 active portion 10, the distribution of hardness is substantially uniform over the entire circumference in the circumferential direction Dθ.

[0029] As shown in Figure 3, the wires 2 in the active section 10 are entirely formed of the first layer L1. The first area A1 corresponds to the surface of the first layer L1. The second end winding section 12 is also entirely formed of the first layer L1.

[0030] As shown in Figure 4, the first end winding portion 11 includes a first layer L1 and a second layer L2 that is softer than the first layer L1. The second region A2 corresponds to the surface of 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 greater than the proportion of the cross-sectional area of ​​the wire 2 occupied by the second layer L2. The axis X2 of the wire 2 passes through the first layer L1.

[0031] 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. The first position P1 is the position on the surface 20 that is farthest from the effective portion 10 in the axial direction DX. The second position P2 is the position on the surface 20 that is closest to the coil axis X1 in the radial direction DR. 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). 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.

[0032] From another perspective, the first position P1 is a position at which the coil spring 1 comes 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 perpendicular to the first center line CL1 and passes through the axis X2. The third position P3 is one of a pair of intersections between the first center line CL1 and the surface 20 that is opposite the first position P1 across the axis X2 (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 that 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.

[0033] 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 and lower side of the wire 2. A boundary B1 between the first region A1 and the second region A2 is located between the fourth position P4 and the first position P1 in the circumferential direction Dθ. Another boundary B2 between the first region A1 and the second region A2 is located between the second position P2 and the third position P3 in the circumferential direction Dθ. Furthermore, a 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.

[0034] The second region A2 is formed in a range from the boundary B1 to the boundary B2 in the circumferential direction Dθ and includes the first position P1 and the second position P2. The first region A1 is formed in a range from the boundary B2 to the boundary B1 in the circumferential direction Dθ and includes the third position P3 and the fourth position P4.

[0035] 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.

[0036] 1 (seat surface SF) at all times or when the coil spring 1 is compressed. That is, the second area A2 extends to at least a part of the portion of the first end winding portion 11 that can come into contact with the seat surface SF.

[0037] The second region A2 is preferably formed in at least a portion of a range of 1.2 turns from the terminal 2a in the direction in which the wire 2 extends helically (the direction along the axis X2). In the example of Fig. 2, the second region A2 is formed continuously from the terminal 2a over the entire range. As another example, the second region A2 may not be provided in a portion a certain distance from the terminal 2a, for example, in a portion that always contacts the first spring seat 4 regardless of the compression state of the coil spring 1.

[0038] In the example shown in Fig. 2, the second region A2 is not provided in the active portion 10 or the second end winding portion 12. As another example, the second region A2 may be provided in at least a portion of the active portion 10. This second region A2 may be continuous with the second region A2 of the first end winding portion 11 shown in Fig. 2. Alternatively, the second region A2 may be provided in at least a portion of the second end winding portion 12. In this case, the second region A2 of the second end winding portion 12 may be located in a portion of the surface 20 that contacts the second spring seat 5.

[0039] In the example of Fig. 4, the thickness t of the second layer L2 is greatest at the center C. The thickness t gradually decreases from the center C toward the boundaries B1 and B2. The maximum value of the thickness t is, for example, 0.6 mm or more, and preferably 1.0 mm or more. In relation to the diameter R of the wire 2, the maximum value of the thickness t is, for example, 2% to 8% of the diameter R.

[0040] The cross-sectional structure shown in Fig. 4 can be applied not only to the portion along line IV-IV in Fig. 2 but also to other portions of the first end winding portion 11. In one example, all cross sections of the wire 2 at the portion where the second region A2 is formed have the structure shown in Fig. 4.

[0041] 5 is a graph showing an example of the hardness distribution in the depth direction (direction from the surface 20 toward the axis X2) of the wire 2 in the first end winding portion 11. 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].

[0042] 5, the hardness of the first layer L1 is almost constant. In the second layer L2, the hardness is smallest at the surface 20 (center C), and the hardness gradually increases with increasing distance from the surface 20, reaching the hardness of the first layer L1 at the thickness t.

[0043] In this way, the second layer L2 is generally softer than the first layer L1. Furthermore, the hardness of the second layer L2 has a gradient according to the distance from the surface 20.

[0044] 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].

[0045] 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 boundary B1 and between the center C and the boundary B2, the hardness gradually increases with increasing distance from the center C, reaching the hardness of the first region A1 at the boundaries B1 and B2.

[0046] In this way, the second region A2 is generally softer than the first region A1. Furthermore, the hardness of the second region A2 has a gradient with the center C being the minimum value.

[0047] Next, a method for manufacturing the coil spring 1 will be described. Fig. 7 is a flowchart showing an example of a method for manufacturing the coil spring 1 according to this embodiment. First, the wire 2 is formed (coiled) into a spiral shape by a coiling machine, and the spirally formed portion is cut off by a cutter (step S1). At this point, the surface 20 of the wire 2 has the same hardness as the entire surface.

[0048] Next, the wire 2 is annealed (step S2). In this annealing, the wire 2 is heated to a temperature range of 400 to 500° C. for one minute or less by, for example, passing a direct current through the wire 2.

[0049] After the annealing, a local softening treatment is performed to form the second region A2 on the surface 20 (step S3). The details of the local softening treatment will be described later with reference to FIG.

[0050] After the local softening treatment, shot peening is performed on the wire 2 (step S4). This shot peening imparts compressive residual stress to the wire 2. After the shot peening, a coating film 21 is formed on the entire surface 20 of the wire 2 (step S5). Note that the local softening treatment may be performed before annealing.

[0051] 8 is a diagram showing an example of the local softening treatment in the first embodiment. In this embodiment, an AC current heating device 200 (hereinafter referred to as the heating device 200) for performing high-frequency current heating is used for the local softening treatment.

[0052] The heating device 200 includes a first terminal 210A, a second terminal 210B, a control device 220, and a conductor 230. In Fig. 8, the conductor 230 is indicated by a dotted pattern. Also, in Fig. 8, a part of the first end winding portion 11 of the wire 2 is shown, and the remaining part of the wire 2 is omitted.

[0053] 8 , each of the first terminal 210A and the second terminal 210B is divided into a first portion 211 and a second portion 212. The first terminal 210A and the second terminal 210B can be attached to the wire 2 by sandwiching a portion of the wire 2 between the first portion 211 and the second portion 212. In one example, the first portion 211 and the second portion 212 of each of the first terminal 210A and the second terminal 210B can be connected by at least one bolt 213. Note that the structure for attaching the first terminal 210A and the second terminal 210B to the wire 2 is not limited to this example.

[0054] 8, the first terminal 210A is connected to the first base 214A, and the second terminal 210B is connected to the second base 214B. The first base 214A and the second base 214B are, for example, plate-shaped conductors, and are stacked one on top of the other with an insulating material 216 interposed therebetween.

[0055] The first terminal 210A and the second terminal 210B are attached to both ends of the range in which the second region A2 is to be formed in the wire 2. In this embodiment, the second region A2 is formed in at least the first end winding portion 11. Therefore, at least one of the first terminal 210A and the second terminal 210B is attached to the first end winding portion 11.

[0056] 8, the first terminal 210A is attached near the terminal 2a, and the second terminal 210B is attached near the first turn from the terminal 2a. For example, if the second region A2 is formed in the range from the terminal 2a to 1.2 turns as in the example of FIG. 2, the second terminal 210B is attached near the 1.2th turn from the terminal 2a. In this case, the shapes of the first terminal 210A and the second terminal 210B may be appropriately changed from those shown in the figure so that they do not interfere with each other.

[0057] The control device 220 includes a power supply 221 that supplies alternating current. The first terminal 210A is connected to the power supply 221 via the first base 214A and wiring. Similarly, the second terminal 210B is connected to the power supply 221 via the second base 214B and wiring. The frequency of the alternating current supplied by the power supply 221 is not particularly limited, but as an example, a high frequency of 1 kHz or higher may be used.

[0058] The conductor 230 is disposed near the first end winding portion 11 of the wire 2 to which the first terminal 210A and the second terminal 210B are attached. The conductor 230 is in an electrically floating state and is insulated from the first terminal 210A and the second terminal 210B. The conductor 230 is also insulated from the wire 2 to which the first terminal 210A and the second terminal 210B are attached. The conductor 230 is supported by, for example, an insulating member (not shown).

[0059] The conductor 230 can be made of a metal material with excellent conductivity, such as copper or aluminum, or may have a laminated structure of a conductive layer made of a metal material and an insulating layer made of a resin or the like.

[0060] In the example of Figure 8, the conductor 230 has a bottom portion 231 located below the first end winding portion 11 and a side portion 232 located to the side of the first end winding portion 11. The bottom portion 231 and the side portion 232 are, for example, integrally formed. As another example, the bottom portion 231 and the side portion 232 may be separated from each other. Furthermore, at least one of the bottom portion 231 and the side portion 232 may include a plurality of separated portions.

[0061] The bottom portion 231 is inclined, for example, so that the distance D1 between the bottom portion 231 and the wires 2 in the axial direction DX is constant at each position on the wires 2. From the viewpoint of favorably generating eddy currents in the proximity effect described below, it is preferable that the bottom portion 231 be formed up to the vicinity of the intersection with the coil axis X1. The side portion 232 is curved, for example, so that the distance D2 between the bottom portion 231 and the wires 2 in the radial direction DR is constant at each position on the wires 2.

[0062] When performing the heating treatment using the heating device 200, the first terminal 210A and the second terminal 210B are attached to the wire 2 formed into a spiral shape. Furthermore, the conductor 230 is arranged so as to face at least the portion of the wire 2 between the first terminal 210A and the second terminal 210B (in this embodiment, the first end winding portion 11). When the first terminal 210A and the second terminal 210B are attached to the wire 2, a circuit is formed in which these elements are connected in series with the power source 221. Note that the order of the step of attaching the first terminal 210A and the second terminal 210B and the step of arranging the conductor 230 is not particularly limited.

[0063] In response to a switch operation by an operator or to reception of an external control signal, the control device 220 starts to energize the strands 2. At this time, the direction of the current flowing through the strands 2 is periodically switched in accordance with the frequency of the power source 221.

[0064] This current flow increases the temperature of the portion of the wire 2 located between the first terminal 210A and the second terminal 210B. When it is time to stop heating, the control device 220 stops the current supply from the power source 221.

[0065] Thereafter, the wires 2 are cooled. This cooling may be natural cooling, or, if rapid cooling is required, may be performed by spraying a fluid such as water or air onto the wires 2.

[0066] When an AC current flows through the wire 2, the skin effect increases the current density near the surface 20. Furthermore, in the wire 2 formed into a spiral shape, the current path passing through the inner diameter side region of the surface 20 is short, so the current density near the inner diameter side region is likely to increase.

[0067] Here, the role of the conductor 230 will be explained. When a current flows through a workpiece such as the wire 2, if an electrically floating conductor is placed nearby, a so-called proximity effect occurs, and the current flowing through the workpiece is biased toward the conductor. In the heating device 200 shown in Figure 8, this proximity effect is used to control the current density distribution (heating temperature distribution) of the wire 2. In other words, the conductor 230 is placed at a position where the proximity effect occurs in the wire 2.

[0068] 9 is a schematic diagram for explaining the proximity effect, showing a rod-shaped workpiece Ws and a conductor 230s arranged in the vicinity thereof. A current I from a power source flows into the workpiece Ws. Ａ When the magnetic field H flows, a magnetic field H ＩＡ occurs (Ampere's law).

[0069] In the conductor 230s, this magnetic field H ＩＡ The eddy current I caused by Ｅ１ is generated (Lenz's law). Furthermore, the eddy current I Ｅ１ The magnetic field H caused by ＩＥ is generated around the conductor 230s. This magnetic field H ＩＥ acts on the workpiece Ws, an eddy current I Ｅ２ occurs.

[0070] current I Ａ , eddy current I Ｅ１ and eddy current I Ｅ２ The direction of the current I flows is as shown by the arrow in the figure. That is, in the workpiece Ws, the current I flows in the vicinity of the side surface far from the conductor 230s. Ａ The direction of the flow and the eddy current I Ｅ２ On the other hand, in the vicinity of the side surface closer to the conductor 230s, the current I Ａ The direction of the flow and the eddy current I Ｅ２ As a result, the current density of the workpiece Ws becomes higher near the side surface closer to the conductor 230s.

[0071] By utilizing such a proximity effect, it is possible to control the current density distribution and heating temperature distribution of the workpiece Ws.

[0072] 10 is a schematic cross-sectional view of the first end winding portion 11 that is heated by passing an AC current without using a conductor 230. As described above, when an AC current flows through the spiral wire 2, the current density increases on the surface 20 of the wire 2 due to the skin effect. Furthermore, current flows mainly on the inner diameter side of the spiral wire 2. Therefore, the temperature of the surface 20 of the wire 2 increases preferentially mainly in the portion on the inner diameter side.

[0073] In one example, when an AC current flows, the inner diameter side portion of the wire 2 is heated to a temperature range below the austenitizing start temperature, thereby forming a heat affected zone (HAZ) 2H in the wire 2. In the example of Fig. 10, the heat affected zone 2H is formed in the inner diameter side portion centered on the second position P2.

[0074] 11 is a schematic cross-sectional view of the first end winding portion 11 that is heated by passing AC current through the conductor 230 as shown in FIG. The conductor 230 is positioned so that the bottom portion 231 faces the vicinity of the first position P1 and the side portion 232 faces the vicinity of the fourth position P4. The conductor 230 does not face the vicinity of the second position P2 or the vicinity of the third position P3.

[0075] In this case, a magnetic field caused by the current in the wire 2 causes a current to flow in the opposite direction to that of the wire 2 in the portion of the conductor 230 facing the wire 2. Therefore, due to the proximity effect described above, the current flowing in the wire 2 is attracted toward the conductor 230. As a result, the position of the heat-affected zone 2H also moves closer to the conductor 230 compared to the example in Figure 10. In the example in Figure 11, the heat-affected zone 2H is formed in a range including the first position P1 and the second position P2. When this heat-affected zone 2H is cooled, a second layer L2 is produced that has a lower hardness than the original wire 2.

[0076] If the temperature of the wire 2 during heating is too high, a portion of the wire 2 may be quench-hardened, increasing its hardness compared to before heating. The region of the heat-affected zone 2H (second layer L2) in the cross section of the wire 2 can be controlled by the frequency and current flow time of the AC current. If the AC current frequency is too low or the current flow time is too long, the entire surface 20 may soften, or the second layer L2 may extend to the vicinity of the axis X2 of the wire 2. Furthermore, if the AC current frequency is too low, the proximity effect described above may not be sufficiently obtained.

[0077] Considering these factors, the frequency of the AC current is preferably a high frequency of 1 kHz or more, and more preferably 100 kHz or more. The frequency of the AC current may be set to a higher range of 200 kHz or more. Furthermore, the duration of the AC current is preferably 5 seconds or less, and in one example, 0.5 seconds.

[0078] The region of the heat-affected zone 2H (second layer L2) in the cross section of the wire 2 can be controlled by the shape and position of the conductor 230. In the example of Fig. 11, the side portion 232 is curved in an arc shape toward the bottom portion 231. This prevents a sudden change in the distance between the conductor 230 and the wire 2, making it easier to guide the heat-affected zone 2H to the desired region.

[0079] 11, the distance D1 between the bottom portion 231 and the wire 2 (first position P1) is greater than the distance D2 between the side portion 232 and the wire 2 (fourth position P4). This example is not limiting, and the distance D1 may be the same as the distance D2, or the distance D1 may be smaller than the distance D2.

[0080] 12 and 13 are graphs showing an example of the residual stress distribution of the wire 2 in the manufacturing process of the coil spring 1. Specifically, Fig. 12 corresponds to the residual stress distribution in the portion between the first position P1 and the axis X2 in Fig. 4, and Fig. 13 corresponds to the residual stress distribution between the center C and the axis X2 in Fig. 4.

[0081] In each graph, the horizontal axis represents the distance [mm] from the surface 20, and the vertical axis represents the residual stress [MPa]. Residual stresses located in the positive (+) region on the vertical axis are tensile residual stresses. On the other hand, residual stresses located in the negative (-) region on the vertical axis are compressive residual stresses. Note that each graph shows the residual stress distribution from the surface 20 to a predetermined depth Dp, and the residual stress distribution from this position Dp to the axis X2 is omitted. The depth Dp is 1 mm from the surface 20 or 10% of the diameter R of the wire 2.

[0082] σ11 shown in Fig. 12 and σ21 shown in Fig. 13 are residual stress distributions in the wire 2 after annealing (step S2) and before local softening treatment (step S3). σ12 shown in Fig. 12 and σ22 shown in Fig. 13 are residual stress distributions in the wire 2 after local softening treatment (step S3) and before shot peening (step S4). σ13 shown in Fig. 12 and σ23 shown in Fig. 13 are residual stress distributions in the wire 2 after shot peening (step S4).

[0083] Immediately after coiling, tensile residual stress occurs particularly on the inner diameter side of the wire 2. After annealing, this tensile residual stress is reduced, but may remain near the surface layer as shown by the residual stress distributions σ11 and σ21.

[0084] After the local softening treatment, the tensile residual stress is reduced as shown by the residual stress distributions σ12 and σ22. After the shot peening, compressive residual stress is imparted to the surface layer of the wire 2 as shown by the residual stress distributions σ13 and σ23.

[0085] In the residual stress distribution σ13 in Fig. 12, the tensile residual stress has been entirely removed by shot peening. On the other hand, in the residual stress distribution σ23 in Fig. 13, although compressive residual stress has been imparted near the surface 20 by shot peening, tensile residual stress remains inside.

[0086] The tensile residual stress may have an unintended effect on the characteristics of the coil spring 1. Therefore, when manufacturing the coil spring 1, it is necessary to appropriately adjust the residual stress inside the wire 2.

[0087] Therefore, in this embodiment, the coil spring 1 is manufactured so as to satisfy one, preferably both, of the following conditions 1 and 2. The coil spring 1 satisfying these conditions 1 and 2 can be manufactured by adjusting the heating conditions in the local softening treatment, for example.

[0088] [Condition 1] In the first end turn portion 11, the maximum value σmax1 of the tensile residual stress between the first position P1 and the axis X2 is 500 MPa or less, preferably 200 MPa or less.

[0089] [Condition 2] In the first end turn portion 11, the difference between the maximum value σmax2 of the tensile residual stress between the center C of the second region A2 and the axis X2 and the tensile residual stress σp at the position of the depth Dp is 500 MPa or less, preferably 200 MPa or less.

[0090] In the graph of Fig. 12, the maximum value σmax1 of the residual stress distribution σ13 after shot peening is located between the first position P1 and the depth Dp. This maximum value σmax1 is 500 MPa or less, specifically 200 MPa or less. The residual stress distribution σ13 corresponds to the residual stress distribution of the coil spring 1 completed after the formation of the coating film 21 (step S5). Therefore, condition 1 is satisfied in the graph of Fig. 12.

[0091] In the graph of Figure 13, the maximum value σmax2 of the residual stress distribution σ23 after shot peening is located between the center C and the depth Dp. This maximum value σmax2 is equal to or less than the value obtained by adding 500 MPa to the tensile residual stress σp at the depth Dp (the dashed-dotted line in the graph), and more specifically, equal to or less than the value obtained by adding 200 MPa to the tensile residual stress σp. The residual stress distribution σ23 corresponds to the residual stress distribution of the coil spring 1 completed after the formation of the coating film 21 (step S5). Therefore, condition 2 is satisfied in the graph of Figure 13.

[0092] It is possible that the residual stress at the depth Dp is 0 or less, i.e., it is a compressive residual stress. For example, if the compressive residual stress at the depth Dp is 30 MPa and the maximum value σmax2 is 50 MPa, the "difference" in condition 2 is calculated as 50 MPa - (-30 MPa) = 80 MPa.

[0093] Conditions 1 and 2 are preferably satisfied in substantially all portions of the first end winding portion 11 in the direction along the axis X2, where the second region A2 is formed. However, conditions 1 and 2 may not be satisfied locally in the first end winding portion 11. Conditions 1 and 2 may also be satisfied by the average value of the residual stress distribution in each portion of the first end winding portion 11 in the direction along the axis X2, where the second region A2 is formed.

[0094] In the present embodiment described above, the first end turn portion 11 of the coil spring 1 has the second region A2 where the surface 20 of the wire 2 is soft. As a result, 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, by providing the second region A2 in the first end turn portion 11, the corrosion fatigue resistance of the first end turn portion 11 is improved.

[0095] If the second region A2 (second layer L2) were formed over the entire wire 2, the sag resistance of the coil spring 1 could be reduced. In contrast, in this embodiment, the second region A2 is not formed in the effective portion 10. Therefore, good sag resistance can be maintained in the effective portion 10. Because the first end winding portion 11 is a portion that is subjected to lower acting stress than the effective portion 10, even if the second region A2 is provided in the first end winding portion 11, the sag resistance of the entire coil spring 1 is unlikely to be affected.

[0096] The first end winding portion 11 is the portion that comes into contact with the first spring seat 4 (seat surface SF) located below it. Therefore, if sand or other foreign matter gets between the first end winding portion 11 and 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 that includes the first position P1 where the first end winding portion 11 comes into contact with the seat surface SF. Therefore, even if foreign matter gets between the first end winding portion 11 and the first spring seat 4 and causes a corrosion pit, cracks caused by the corrosion pit can be suppressed.

[0097] Furthermore, such foreign matter is likely to get under the portion of the first end winding portion 11 that comes into contact with and separates from the first spring seat 4. Therefore, forming the second region A2 to include such a portion can effectively improve corrosion fatigue resistance. As described above with reference to Figure 2, forming the second region A2 in the range from at least the terminal 2a to 1.2 turns can achieve good corrosion fatigue resistance while maintaining sag resistance.

[0098] 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 diameter side of the coil spring 1), and corrosion fatigue is also likely to occur in that portion. Therefore, as described above with reference to FIG. 4 , it is preferable to form the second region A2 below the wire 2 and closer to the inner diameter.

[0099] In the manufacturing method according to this embodiment, the second region A2 (second layer L2) is formed by electrical heating using a high-frequency alternating current. If electrical heating using a direct current or a low-frequency alternating current of about 50 Hz or 60 Hz were used, the heat-affected zone could extend up to the vicinity of the axis X2 of the wire 2. As a result, the second layer L2 would be formed over a wide area inside the wire 2, potentially reducing the sag resistance of the first end winding portion 11. In contrast, when electrical heating is performed using a high-frequency alternating current, the skin effect described above limits the area in which the heat-affected zone is formed to the vicinity of the surface 20. As a result, as shown in FIG. 4 , the first layer L1 remains hard near the axis X2, improving the sag resistance of the first end winding portion 11.

[0100] The coil spring 1 according to this embodiment satisfies at least one of the above conditions 1 and 2. This makes it possible to reduce the tensile residual stress, which is one of the causes of breakage.

[0101] It is important to properly manage tensile residual stress, particularly at positions that could be the starting point of breakage. The tensile residual stress generated by coiling is not uniform in the circumferential direction Dθ of the wire 2. That is, large tensile residual stress occurs near the second position P2 (the inner diameter side of the wire 2) shown in FIG. 4, but the tensile residual stress does not occur near the first position P1 as much as near the second position P2. To address this distribution of tensile residual stress, this embodiment specifies conditions 1 and 2 using maximum values ​​σmax1 and σmax2 of tensile residual stress at different positions in the circumferential direction Dθ. By satisfying at least one of these conditions 1 and 2, and preferably both, it is possible to provide a coil spring 1 that can suppress breakage and other problems caused by tensile residual stress.

[0102] The scope of the present invention is not limited to the configurations disclosed in the above embodiments. The present invention can be implemented by modifying the configurations disclosed in the embodiments in various ways. Some modifications of the embodiments are shown below.

[0103] 14 to 19 are schematic cross-sectional views showing first to sixth modified examples of configurations that can be applied to the conductor 230 of the heating device 200. In the first modified example shown in Fig. 14, the bottom 231 and the side 232 of the conductor 230 are connected vertically. In the second modified example shown in Fig. 15, the conductor 230 does not have a portion corresponding to the side 232. Furthermore, in the third modified example shown in Fig. 16, the conductor 230 does not have a portion corresponding to the bottom 231.

[0104] 17 , a conductor 230 has an upper portion 233 in addition to a bottom portion 231 and a side portion 232. One end of the upper portion 233 is connected to the upper end of the side portion 232. The upper portion 233 faces a third position P3 of the wire 2 in the axial direction DX.

[0105] 18, a conductor 230 also has a bottom portion 231, a side portion 232, and an upper portion 233. However, the side portion 232 has an arc-shaped cross section that faces the wire 2 with a certain gap therebetween.

[0106] 19, a conductor 230 has an arc-shaped cross section as a whole. The conductor 230 surrounds the wire 2 over a range of 180° or more, for example.

[0107] In addition to the first to sixth modified examples, various shapes may be applied to the conductor 230. The specific shape of the conductor 230 may be determined appropriately depending on the region where the second region A2 (second layer L2) is to be formed.

[0108] The heating device 200 may include a ferromagnetic material made of ferrite or the like in addition to or instead of the conductor 230. When such a ferromagnetic material is placed near the wire 2, the magnetic flux generated when a current is passed through the wire 2 is guided in a direction away from the ferromagnetic material. Therefore, the ferromagnetic material can also be used to control the region of the wire 2 where the temperature rises.

[0109] The conductor 230 does not necessarily have to be in an electrically floating state. For example, the conductor 230 may be connected to the power source 221 so that a current flows through the conductor 230 in the opposite direction to that of the strand 2. Even in such a case, the proximity effect described above occurs in the strand 2, so that the conductor 230 can control the current density distribution and heating temperature distribution of the strand 2.

[0110] Second Embodiment The method for heating the wire 2 in the local softening treatment (step S3) shown in Fig. 7 is not limited to AC current heating. In the second embodiment, an example is disclosed in which laser heating is used for the local softening treatment.

[0111] 20 is a diagram showing an example of the local softening process according to the second embodiment. In this embodiment, a laser device 300 is used for the local softening process. For example, the laser device 300 is a semiconductor laser, but is not limited to this example.

[0112] The laser device 300 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 the second region A2 is formed 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 may be tilted with respect to the axial direction DX. For example, the position where the irradiation axis XL intersects with the surface 20 of the wire 2 corresponds to the center C shown in Figure 4.

[0113] When irradiating the laser beam LZ, for example, the position of the laser device 300 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 300 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 300 may be moved.

[0114] The wire 2 is heated by irradiation with the laser beam LZ. As a result, a heat-affected zone similar to that in the case of using the heating device 200 in the first embodiment is formed in the wire 2. When the heat-affected zone is air-cooled, a second layer L2 having a reduced hardness compared to the original wire 2 is produced.

[0115] In one example, the portion of the wire 2 irradiated with the laser beam LZ is heated to a temperature range below the austenitizing start temperature. If the temperature of the wire 2 during heating is too high, a portion of the wire 2 may be quench-hardened, and may become harder than before irradiation with the laser beam LZ. The output [kW] and irradiation time [sec] of the laser beam LZ need to be adjusted so that such quench-hardening does not occur.

[0116] The wire 2 may be preheated prior to irradiating the area where the second region A2 is to be formed with the laser beam LZ. The heat generated during annealing may be used for this preheating. In one example, after heating for annealing, the wire 2 may be irradiated with the laser beam LZ in a state where the temperature of the surface 20 of the wire 2 has decreased to 300°C to 480°C.

[0117] When the wire 2 is irradiated with the laser beam LZ, the temperature of the region of the surface 20 irradiated with the laser beam LZ rises rapidly. Therefore, if preheating is not performed, a large difference in residual stress occurs between the region irradiated with the laser beam LZ and its surroundings. Specifically, tensile residual stress may occur in the region irradiated with the laser beam LZ. In this case, tensile residual stress may remain in the surface layer of the wire 2 even after shot peening.

[0118] On the other hand, if preheating is performed before irradiation with the laser beam LZ to raise the temperature of the wire 2 as a whole, the temperature change from the region irradiated with the laser beam LZ to its surroundings will have a gentle gradient, which will reduce the occurrence of tensile residual stress in the irradiated region.

[0119] Fig. 21 is a graph showing an example of the residual stress distribution in the wire 2 when the local softening treatment is performed by laser heating without preheating, and Fig. 22 is a graph showing an example of the residual stress distribution in the wire 2 when the local softening treatment is performed by laser heating after preheating.

[0120] These graphs correspond to the residual stress distribution in the portion between the center C and the axis X2 in Figure 4. In both graphs, the horizontal axis represents the distance [mm] from the surface 20, and the vertical axis represents the residual stress [MPa]. Residual stresses located in the positive (+) region on the vertical axis are tensile residual stresses. On the other hand, residual stresses located in the negative (-) region on the vertical axis are compressive residual stresses. In both graphs, the residual stress distribution from the surface 20 to a predetermined depth Dp is shown, and the residual stress distribution from this position Dp to the axis X2 is omitted. The depth Dp is 1 mm from the surface 20 or 10% of the diameter R of the wire 2. Note that the scale of the residual stress on the vertical axis shown in Figures 21 and 22 is the same.

[0121] σ31 shown in Fig. 21 and σ41 shown in Fig. 22 are residual stress distributions in the wire 2 after annealing (step S2) and before local softening treatment (step S3). σ32 shown in Fig. 21 and σ42 shown in Fig. 22 are residual stress distributions in the wire 2 after local softening treatment (step S3) and before shot peening (step S4). σ33 shown in Fig. 21 and σ43 shown in Fig. 22 are residual stress distributions in the wire 2 after shot peening (step S4).

[0122] When local softening treatment is performed by laser heating without preheating, tensile residual stress occurs in the surface layer, as shown by the residual stress distribution σ32 in Fig. 21. In this case, tensile residual stress may remain up to a depth Dp even after shot peening, as shown by the residual stress distribution σ33.

[0123] On the other hand, when the wire 2 is subjected to local softening treatment by laser heating in a preheated state, the tensile residual stress in the surface layer is reduced compared to the case without preheating, as shown by the residual stress distribution σ42 in Fig. 22. Therefore, the tensile residual stress after shot peening is also reduced compared to the case without preheating, as shown by the residual stress distribution σ43.

[0124] Even when performing local softening treatment using laser heating as in this embodiment, it is possible to manufacture the coil spring 1 so that one or both of the above-mentioned conditions 1 and 2 are satisfied. For example, in the example of FIG. 22 , the maximum value σmax2 of the residual stress distribution σ43 is equal to or less than the value obtained by adding 500 MPa to the tensile residual stress σp at the depth Dp (indicated by the dashed-dotted line in the figure), thereby satisfying condition 2. As with the residual stress distribution σ33 in FIG. 21 , condition 2 may be satisfied when preheating is not performed. Condition 1 may also be satisfied not only when preheating is performed, but also when preheating is not performed.

[0125] In the above-described embodiments, the wire 2 has the first layer L1 and the second layer L2. However, the wire 2 may have a multilayer structure including another layer having a hardness different from that of the first layer L1 and the second layer L2. 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.

[0126] In each of the above-described embodiments, 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.

[0127] 1...coil spring, 2...wire, 3...shock absorber, 4...first spring seat, 5...second spring seat, 10...active part, 11...first end winding part, 12...second end winding part, 20...surface of wire, 100...suspension device, A1...first region, A2...second region, L1...first layer, L2...second layer, 200...heating device, 210A...first terminal, 210B...second terminal, 220...control device, 221...power source, 230...conductor, 300...laser device.

Claims

1. A coil spring formed of a wire wound in a spiral shape around a coil axis, having a seat winding portion and an effective portion aligned in an axial direction parallel to the coil axis, wherein the surface of the wire in the seat winding portion has a first region and a second region softer than the first region, the first region and the second region are aligned in a circumferential direction around the axis of the wire, and in the seat winding portion, the difference between the maximum value of the tensile residual stress between the center of the second region in the circumferential direction and the axis and the tensile residual stress at a depth of 1 mm from the center or at a depth of 10% of the diameter of the wire is 500 MPa or less.

2. The coil spring according to claim 1, wherein in the end turn portion, the difference between the maximum value and the tensile residual stress at a position at a depth of 1 mm from the center or at a position at a depth of 10% of the diameter of the wire is 200 MPa or less.

3. The coil spring as described in claim 1, wherein the surface of the wire in the seat winding portion includes: a first position in contact with a seat surface that supports the coil spring; and a second position which is one of a pair of intersections where the surface intersects with a second center line that is perpendicular to a first center line that is a normal to the surface at the first position and passes through the axis of the wire, the second position being on the inner diameter side of the coil spring; and in the circumferential direction, the center of the second region is between the first position and the second position.

4. A suspension device comprising: a first spring seat; a second spring seat; and a coil spring according to any one of claims 1 to 3, disposed between the first spring seat and the second spring seat.

5. A method for manufacturing a coil spring, comprising: forming a wire into a spiral shape around a coil axis; heating the wire so that a first region and a second region softer than the first region are formed on the surface of an end turn portion; and subjecting the wire after heating to shot peening, wherein the first region and the second region are aligned in a circumferential direction around the axis of the wire, and in the end turn portion after the shot peening, the difference between the maximum value of the tensile residual stress between the center of the second region in the circumferential direction and the axis and the tensile residual stress at a depth of 1 mm from the center or at a depth of 10% of the diameter of the wire is 500 MPa or less.

6. The method for manufacturing a coil spring according to claim 5, wherein in the end turn portion after the shot peening, the difference between the maximum value and the tensile residual stress at a position at a depth of 1 mm from the center or at a position at a depth of 10% of the diameter of the wire is 200 MPa or less.

7. A method for manufacturing a coil spring as set forth in claim 5, wherein the surface of the wire in the seat winding portion includes: a first position in contact with a seat surface that supports the coil spring; and a second position which is one of a pair of intersections where the surface intersects with a second center line that is perpendicular to a first center line that is normal to the surface at the first position and passes through the axis of the wire, the second position being on the inner diameter side of the coil spring; and in the circumferential direction, the center of the second region is between the first position and the second position.

8. A method for manufacturing a coil spring as claimed in any one of claims 5 to 7, wherein the heating includes attaching a first terminal and a second terminal connected to a power source capable of supplying alternating current to the strand of wire such that at least one of the first terminal and the second terminal is located at the end turn portion of the strand of wire formed into a spiral shape, and supplying alternating current to the strand of wire through the first terminal and the second terminal, thereby raising the temperature of the portion of the strand of wire located between the first terminal and the second terminal.

9. The method for manufacturing a coil spring according to claim 8, wherein the heating further includes arranging a conductor so as to face at least the end turn portion, and the alternating current is supplied to the wire with the conductor arranged, so that currents flow in opposite directions through the wire and the conductor.

10. A method for manufacturing a coil spring as claimed in any one of claims 5 to 7, wherein the heating includes irradiating a range in which the second region is to be formed with laser light.

11. The method for manufacturing a coil spring according to claim 10, further comprising preheating the wire before irradiating it with the laser light.

Images