Method for processing metal member, setting device for metal member, and metal member

Abstract

Provided is a metal member imparted with a compressive residual stress that is effective for improving resistance to permanent strain.　The surface portion of the metal member is rapidly heated. One example of the metal member is a spring member. A load is applied to the heated metal member so as to impose a larger plastic strain to the surface portion of the metal member than to the inside thereof. By removing the load, compressive residual stress is generated in an area from the surface of the metal member to a position at a first depth (D1) or deeper than the first depth (D1). The compressive residual stress increases in the direction from the inner side of the metal member toward the surface thereof and is substantially maximized at the surface. The compressive residual stress increases at a higher rate at portions close to the surface than on the inner side.

Description

METHOD FOR TREATING METAL COMPONENTS, SETTING DEVICE FOR METAL COMPONENTS, AND METAL COMPONENTS

[0001] The present invention relates to a method for treating a metal member capable of improving resistance to settling, a setting device for the metal member, and the metal member.

[0002] Setting is typically performed in the manufacturing process of spring members such as torsion bars, coil springs, and leaf springs. Spring members are an example of the metal members referred to in this invention. In particular, in the fields of vehicle suspension systems and engine valve trains, setting is important for improving the sag resistance of spring members. In general, setting involves applying a load to the spring member that exceeds the working stress, thereby causing plastic strain in the spring member. To enhance the setting effect, hot setting is also known, in which the spring member is heated to a warm temperature.

[0003] For example, Patent Documents 1 and 2 describe examples of hot setting. Patent Document 3 describes an example of a setting device and a setting method. In conventional hot setting, the entire spring member is heated in a furnace, and then a predetermined load is applied to the spring member removed from the furnace and held for a certain period of time (e.g., 70 minutes). This generates compressive residual stress in the surface of the spring member.

[0004] International Publication No. 2004 / 085685 Japanese Patent Application Laid-Open No. 2011-149036 Japanese Patent Application Laid-Open No. 4-224037

[0005] For example, in the field of vehicle suspension systems, it is sometimes desirable to increase the design stress of suspension spring members according to vehicle specifications. To increase the design stress of a spring member, it is desirable to impart residual stress deep into the spring member from its surface and to increase the absolute value of compressive residual stress near the surface. However, conventional hot setting has limitations in imparting compressive residual stress deep into the spring member.

[0006] An object of the present invention is to provide a processing method capable of imparting compressive residual stress, which is effective in improving settling resistance, to deep positions in a metal component, a setting device for the metal component, and the metal component.

[0007] A method for treating a metal component according to one embodiment includes heating at least a portion of a surface of the metal component and applying a load that generates plastic strain in the metal component while the surface is heated. After a predetermined time has elapsed, the load is removed, thereby generating a compressive residual stress portion from the surface of the metal component to a first depth or beyond the first depth. The compressive residual stress portion has an absolute value of compressive residual stress that increases from the interior of the metal component toward the surface. Furthermore, the compressive residual stress portion includes an internal residual stress portion and a surface residual stress portion. The internal residual stress portion extends from the first depth to a second depth that is shallower than the first depth. The surface residual stress portion extends from the second depth to the surface. The rate of increase in compressive residual stress in the surface residual stress portion is greater than the rate of increase in compressive residual stress in the internal residual stress portion.

[0008] In the method for treating a metal member, the surface of the metal member may be irradiated with laser light. By irradiating the surface portion of the metal member with laser light, the surface portion of the metal member is rapidly heated to a temperature range of 200°C to 400°C. Furthermore, shot peening may be performed on the surface of the metal member having the compressive residual stress portion. This shot peening may generate a peak portion in the surface portion where the absolute value of the compressive residual stress is maximum.

[0009] A setting device according to one embodiment includes a surface heating device that heats the surface of a metal member and a load application mechanism. The load application mechanism applies a load that causes plastic strain in the heated surface of the metal member, and removes the load after a predetermined time has elapsed. One example of the surface heating device may be a laser irradiation device that irradiates the surface of the metal member with laser light. Another example of the surface heating device may be an AC current application device that applies an AC voltage to the metal member.

[0010] A metal component according to one embodiment has a compressive residual stress portion extending from the surface to a first depth or beyond the first depth. The compressive residual stress portion has a compressive residual stress whose absolute value increases from the first depth toward the surface. The compressive residual stress has a maximum absolute value at approximately the surface. The compressive residual stress portion includes an internal residual stress portion and a surface residual stress portion. The internal residual stress portion extends from the first depth to a second depth shallower than the first depth. The surface residual stress portion extends from the second depth to the surface. The rate of increase in the compressive residual stress in the surface residual stress portion is greater than the rate of increase in the compressive residual stress in the internal residual stress portion. In this embodiment, the metal component may have a stable residual stress portion deeper than the first depth, where the magnitude of the compressive residual stress remains almost constant. The first depth may be a position where the compressive residual stress is zero. The metal component may be a torsion bar, a coil spring, or a leaf spring.

[0011] According to the method for treating a metal component and the setting device of one embodiment, compressive residual stress that is effective in improving sag resistance can be imparted to the metal component from its surface to a deep position. The metal component of one embodiment improves sag resistance.

[0012] 1. A front view showing a spring member as an example of a metal member. A front view schematically showing a setting device according to a first embodiment. A cross-sectional view schematically showing a surface heating portion of the metal member shown in FIG. 1. A diagram showing the relationship between the depth from the surface of a metal member that has been subjected to surface heating setting and the magnitude of residual stress. A diagram showing the relationship between the depth from the surface of a metal member that has been subjected to conventional hot setting and the magnitude of residual stress. A cross-sectional view schematically showing another example of a surface heating portion of a metal member. A cross-sectional view schematically showing yet another example of a surface heating portion of a metal member. A front view schematically showing a setting device according to a second embodiment. A diagram showing the relationship between the depth from the surface of a metal member that has been set by the setting device shown in FIG. 8 and the magnitude of residual stress. A front view showing another example of a metal member. A front view schematically showing a setting device according to a third embodiment. A front view of the setting device shown in FIG. 11 in a state where the load applied to the metal member has been removed. A front view schematically showing a setting device according to a fourth embodiment.

[0013] First Embodiment A method for treating a metal member, a setting device, and a metal member according to a first embodiment will be described below with reference to FIGS. 1 to 4. FIG.

[0014] FIG. 1 shows a torsion bar-type spring member 1A as an example of a metal member. The following description will use the spring member 1A as an example. The spring member 1A is made of spring steel and has a rod-shaped bar body 10, a first gripping portion 12, and a second gripping portion 14. The first gripping portion 12 is formed at a first end 11 of the bar body 10. The second gripping portion 14 is formed at a second end 13 of the bar body 10. When the first gripping portion 12 and the second gripping portion 14 are twisted relative to each other, torsional deformation occurs in the bar body 10, resulting in a spring action.

[0015] FIG. 2 schematically shows a setting device 20. The setting device 20 performs setting while the surface of the metal member is heated. An example of the metal member is a spring member 1A. The setting device 20 includes a torsion mechanism 21 and a laser irradiation device 30. The torsion mechanism 21 applies a torsional load to the spring member 1A. The laser irradiation device 30 irradiates the spring member 1A with laser light. The torsion mechanism 21 is an example of a load application mechanism for applying a torsional load to the spring member 1A. The laser irradiation device 30 is an example of a surface heating device for rapidly heating the surface of the spring member 1A.

[0016] One example of the torsion mechanism 21 has a base member 22, a first member 23, a second member 24, and a drive unit 25. The first member 23 holds the first gripping portion 12 of the spring member 1A. The second member 24 holds the second gripping portion 14 of the spring member 1A. The base member 22 is substantially rigid. The drive unit 25 can rotate the second member 24 in a direction that twists the spring member 1A.

[0017] One example of the laser irradiation device 30 includes a laser oscillator 31 which is a source of laser light, a laser irradiation unit 32, a control unit 33 which controls the laser oscillator 31, and an optical fiber 34. The laser oscillator 31 and the laser irradiation unit 32 are optically connected by the optical fiber 34. The control unit 33 includes an electrical configuration, software, etc. for controlling the laser oscillator 31.

[0018] The laser irradiation unit 32 can move in the length direction of the spring member 1A (indicated by arrow Y1 in FIG. 2 ) along the guide member 35. A torsional load is applied to the spring member 1A by the torsion mechanism 21. The spring member 1A to which the torsional load is applied is irradiated with laser light 36 emitted from the laser irradiation unit 32. The laser light 36 is irradiated toward the surface of the spring member 1A. The laser irradiation unit 32 then moves in the length direction of the spring member 1A. Therefore, the laser light 36 is irradiated onto the surface of the spring member 1A while moving in the length direction of the spring member 1A. The moving speed of the laser light 36 is, for example, 10 mm / sec.

[0019] The beam shape of the laser light 36 irradiated onto the surface of the spring member 1A is, for example, 3 mm wide and 18 mm long. The output of the laser irradiation device 30 is, for example, 0.4 kW, and the irradiation angle θ1 (shown in FIG. 2) of the laser light 36 is, for example, 15°. The irradiation angle θ1 here refers to the angle of the laser light 36 with respect to a line segment T1 perpendicular to the surface of the spring member 1A.

[0020] The laser irradiation device 30 irradiates the surface of the spring member 1A with laser light 36. By irradiating the laser light 36, a region (referred to as the surface portion) relatively shallow from the surface of the spring member 1A is rapidly heated to a temperature in the range of 200° C. to 400° C. The area schematically shown by hatching in FIG. 2 is the heated surface portion (referred to as the surface heated portion 38).

[0021] A torsional load (e.g., 1270 MPa) is applied to the spring member 1A by the torsion mechanism 21. With the torsional load applied, the laser beam 36 moves in the longitudinal direction of the spring member 1A. In this manner, a surface heated portion 38 having a length L1 is formed on the surface of the spring member 1A.

[0022] 3 is a cross-sectional view schematically illustrating a radial cross section of the spring member 1A heated by the laser irradiation device 30. The spring member 1A is an example of a metal member. In FIG. 3, the densely hatched areas are surface heating portions 38. As shown in FIG. 3, the surface heating portions 38 are formed in the range indicated by angle θ2 in the circumferential direction of the spring member 1A.

[0023] With the torsional load applied to the spring member 1A, the spring member 1A is irradiated with laser light 36. The surface of the spring member 1A is rapidly heated by the irradiation of laser light 36. The torsional load is maintained for a predetermined time (e.g., 2-3 minutes) by the torsion mechanism 21. This imparts greater plastic strain to the surface of the spring member 1A than to the interior of the spring member 1A. After the torsional load has been maintained for the predetermined time, the load is removed.

[0024] In this specification, the process of rapidly heating the surface of a metal member while applying a load to the metal member, maintaining the heat for a predetermined time, and then removing the load is referred to as "surface heating setting." If it is necessary to heat a wide area of ​​the surface of the metal member, the irradiation of the laser beam 36 may be repeated while changing the irradiation position.

[0025] Figure 4 shows the relationship between the depth from the surface of a metal component that has been subjected to surface-heat setting according to this embodiment and the magnitude of residual stress. Line segment L1 in Figure 4 shows the relationship between the depth from the surface of a metal component that has been subjected to surface-heat setting and the magnitude of residual stress. As shown by line segment L1, the metal component that has been subjected to surface-heat setting has compressive residual stress from the surface to a first depth D1 or to a position beyond the first depth D1. An example of the first depth D1 is a depth from the surface that exceeds 0.5 mm.

[0026] As shown by line segment L1 in Figure 4, a compressive residual stress portion S1 is generated from the surface of the metal component to a first depth D1 or beyond the first depth D1. The absolute value of the compressive residual stress in the compressive residual stress portion S1 increases from the first depth D1 toward the surface of the metal component, and the absolute value of the compressive residual stress reaches a maximum value P1 almost at the surface. "Almost at the surface" refers to the position of the compressive residual stress portion S1 closest to the surface.

[0027] The compressive residual stress portion S1 shown in Figure 4 includes an inner residual stress portion A1 extending from a first depth D1 to a second depth D2, and a surface residual stress portion A2 extending from the second depth D2 to the surface. The second depth D2 is shallower than the first depth D1. The rate of increase in compressive residual stress in the surface residual stress portion A2 is greater than the rate of increase in compressive residual stress in the inner residual stress portion A1. Furthermore, at a position deeper than the first depth D1, a residual stress-stable portion A3 is formed, where the magnitude of the compressive residual stress remains almost constant. The residual stress-stable portion A3 is an area that is not affected by setting or shot peening. The first depth D1 is a position where the compressive residual stress becomes zero. The first depth D1 may be the boundary between the compressive residual stress portion S1 and the residual stress-stable portion A3.

[0028] The heating depth of the surface portion heated by the irradiation of the laser light depends on various factors such as the thermal conductivity of the metal member and the intensity of the thermal energy applied to the metal member. The target depth of the surface portion to be heated may be, for example, near a first depth D1 from the surface, or near a second depth D2, or may be any other depth.

[0029] Line segment L2 in Figure 4 shows the relationship between the depth from the surface of a metal component that has been subjected to surface heating setting and then shot peening and the magnitude of residual stress. Shot peening is a process in which a large number of shots are fired at the surface of a metal component at high speed. Shot peening can create a peak portion P1 near the surface of the metal component, where the compressive residual stress is at its maximum.

[0030] As shown by line segment L2 in Figure 4, a metal component that has been subjected to surface heat setting and then shot peening has a peak P1 near the surface where the absolute value of the compressive residual stress is greatest. This compressive residual stress decreases toward the interior of the metal component, but compressive residual stress is maintained from the surface to a deep position. The compressive residual stress distribution obtained by shot peening is even more effective in improving the settling resistance of metal components.

[0031] Line segment L3 in Figure 5 shows the relationship between the depth from the surface of a metal component that has been subjected to conventional hot setting and the magnitude of residual stress. The metal component that has been subjected to hot setting shows a compressive residual stress portion S2. In the compressive residual stress portion S2, the compressive residual stress increases at a substantially constant rate from the first depth d1 toward the surface. A stable residual stress portion S3 is located deeper than the compressive residual stress portion S2. In the stable residual stress portion S3, the magnitude of the compressive residual stress remains almost constant. The absolute value of the compressive residual stress near the surface of the compressive residual stress portion S2 is significantly smaller than the absolute value of the compressive residual stress near the surface of the compressive residual stress portion S1 shown in Figure 4.

[0032] Line segment L4 in Figure 5 shows the relationship between the depth from the surface and the magnitude of residual stress in a metal component that has been subjected to conventional hot setting and then shot peening. As shown by line segment L4, the metal component that has been subjected to hot setting and then shot peening has a peak P3 near the surface, where the absolute value of the compressive residual stress is at its maximum. However, at a relatively shallower position, the compressive residual stress approaches zero. The depth of the compressive residual stress in the conventional shot peened material shown by line segment L4 is about half the depth of the compressive residual stress in the shot peened material of this embodiment shown by line segment L2 in Figure 4.

[0033] 6 is a cross-sectional view schematically illustrating another example of the surface heating portion 38 of the spring member 1A. In this example, a first surface heating portion 38a and a second surface heating portion 38b are formed symmetrically with respect to each other, within the ranges of θ3 and θ4, respectively. As in this example, surface heating portions may be formed at multiple locations around the circumference of the spring member 1A, and a load may be applied for a predetermined period of time, thereby generating more plastic strain in the surface portion of the spring member 1A than in the interior.

[0034] 7 is a cross-sectional view schematically illustrating another example of a surface heating portion of a spring member 1A. As shown in FIG. 7, a surface heating portion 38 is formed around the entire circumference of the spring member 1A. As in this example, by forming the surface heating portion 38 around the entire circumference of the spring member 1A and maintaining a load state for a predetermined time, a plastic strain greater than that in the interior may be generated around almost the entire circumference of the surface portion of the spring member 1A.

[0035] Second Embodiment Figure 8 is a front view schematically illustrating a setting device 40 according to a second embodiment. The setting device 40 of this embodiment includes a torsion mechanism 21 and an AC current application device 50. The torsion mechanism 21 applies a torsional load to a metal member (spring member 1A). The AC current application device 50 applies an AC voltage to the spring member 1A. The torsion mechanism 21 has a common configuration with the torsion mechanism 21 (shown in Figure 2) described in the first embodiment, and therefore common components to those of the first embodiment are designated by common reference numerals and will not be described again.

[0036] The AC current application device 50 is an example of a surface heating device for rapidly heating the surface portion of the spring member 1A. The AC current application device 50 includes a first electrode 51, a second electrode 52, and a power supply unit 53. The first electrode 51 is connected to the first end 11 of the spring member 1A. The second electrode 52 is connected to the second end 13 of the spring member 1A. The power supply unit 53 includes an AC power supply 54 and a control unit 55. The AC power supply 54 applies an AC voltage between the first electrode 51 and the second electrode 52. The control unit 55 controls the AC voltage.

[0037] A torsional load (e.g., 1270 MPa) is applied to the spring member 1A. While the torsional load is applied, an AC voltage is applied to the spring member 1A. For example, by applying an AC current for 0.5 seconds, the entire circumference of the surface portion is heated to, for example, 225°C. This AC current application forms a surface heated portion 38 (shown in FIG. 7) around the entire circumference of the bar body 10. With the surface heated portion 38 formed, a torsional load is applied to the spring member 1A by the torsion mechanism 21. This imparts a plastic strain greater than that in the interior of the spring member 1A to the surface portion of the spring member 1A. After this plastic strain is maintained for a certain period of time, the torsional load is removed. This surface heating setting imparts compressive residual stress to the surface portion of the spring member 1A.

[0038] Line segment L5 in Figure 9 shows the relationship between the depth from the surface of a metal component that has been set using the setting device 40 shown in Figure 8 and the magnitude of the residual stress. As shown by line segment L5, a compressive residual stress portion S2 is formed from the surface of the metal component to a first depth D3 or beyond the first depth D3. In the compressive residual stress portion S2, the absolute value of the compressive residual stress increases from the first depth D3 toward the surface, with the maximum absolute value of the compressive residual stress (peak portion) P4 occurring almost at the surface. Line segment L6 in Figure 9 shows the residual stress of a metal component that has been subjected to conventional hot setting.

[0039] As shown by line segment L5 in Figure 9, the compressive residual stress portion S2 includes an inner-side residual stress portion B1 and a surface-side residual stress portion B2. The inner-side residual stress portion B1 extends from a first depth D3 to a second depth D4. The surface-side residual stress portion B2 extends from the second depth D4 to the surface. The second depth D4 is shallower than the first depth D3. The rate of increase in compressive residual stress from the second depth D4 to the surface is greater than the rate of increase in compressive residual stress from the first depth D3 to the second depth D4. A stable residual stress portion B3, where the magnitude of the compressive residual stress remains almost constant, is formed at a position deeper than the first depth D3. The stable residual stress portion B3 is a region that is not affected by setting or shot peening. The first depth D3 is a position where the compressive residual stress becomes zero. The first depth D3 may be the boundary between the compressive residual stress portion S2 and the stable residual stress portion B3.

[0040] [Third Embodiment] Figure 10 is a front view showing a coil spring type spring member 1B as another example of a metal member. The spring member 1B has a wire 60 made of spring steel. The spring member 1B includes a first end 61 and a second end 62. In a free state where no compressive load is applied to the spring member 1B, the spring member 1B has a length H1. When a compressive load is applied to the spring member 1B, the length H1 decreases depending on the magnitude of the load.

[0041] 11 is a front view schematically showing a setting device 70 according to a third embodiment. The setting device 70 includes a base member 71, a compression mechanism 72, and a laser irradiation device 30. The base member 71 supports a spring member 1B. The compression mechanism 72 applies a compressive load to the spring member 1B. The laser irradiation device 30 irradiates the spring member 1B with laser light 36.

[0042] The compression mechanism 72 includes a pressure member 73 and a drive unit 74 that drives the pressure member 73. The compression mechanism 72 is an example of a load application mechanism. The load application mechanism applies a compressive load to the spring member 1B. The laser irradiation device 30 has a common configuration with the laser irradiation device 30 (shown in FIG. 2) described in the first embodiment, and therefore common components to the first embodiment are denoted by common reference numerals and will not be described again.

[0043] As shown in FIG. 11 , with the spring member 1B compressed to a length H2 by the compression mechanism 72, the surface of the spring member 1B is heated by irradiating it with laser light 36. The laser irradiation device 30 irradiates the surface of the spring member 1B with laser light 36. For example, the spring member 1B is rotated about the axis X1. With the spring member 1B rotated about the axis X1, the laser irradiation unit 32 is moved in a direction along the axis X1. This allows the laser light 36 to move along the length of the wire 60. With a compressive load applied to the spring member 1B in this manner, the surface of the spring member 1B is rapidly heated by the laser light, thereby imparting a large amount of residual strain to the surface.

[0044] 12 is a front view showing the state in which the compressive load applied to the spring member 1B has been removed. The length of the spring member 1B after the compressive load has been removed is H3. The length H3 after setting is smaller than the length H1 before setting, but compressive residual stress is imparted to the spring member 1B from the surface to a deep position. This spring member 1B has excellent resistance to settling.

[0045] [Fourth Embodiment] FIG. 13 is a front view schematically illustrating a setting device 80 according to a fourth embodiment. The setting device 80 includes a compression mechanism 72 and an AC current applying device 50. The compression mechanism 72 applies a compressive load to a spring member 1B, which is an example of a metal member. The AC current applying device 50 applies an AC voltage to the spring member 1B. The AC current applying device 50 has a common configuration with the AC current applying device 50 (shown in FIG. 8) described in the second embodiment, and therefore, common components to the second embodiment are denoted by common reference numerals and their descriptions are omitted. The compression mechanism 72 has a common configuration with the compression mechanism 72 (shown in FIG. 11) described in the third embodiment, and therefore, common components to the third embodiment are denoted by common reference numerals and their descriptions are omitted.

[0046] As shown in FIG. 13 , a first electrode 51 of an AC current applying device 50 is connected to a first end 61 of the spring member 1B. A second electrode 52 of the AC current applying device 50 is connected to a second end 62 of the spring member 1B. When the spring member 1B is in a compressed state, an AC voltage is applied to the spring member 1B by the AC current applying device 50. When an AC voltage is applied to the coil spring-shaped spring member 1B, the amount of heat generated mainly on the inside (the inner diameter side of the coil) of the spring member 1B becomes greater than the amount of heat generated on the outside (the outer diameter side of the coil). As a result, a large amount of plastic strain is imparted mainly to the inner surface portion of the compressed spring member 1B. After the compressed state is maintained for a predetermined time, the load is removed, and compressive residual stress is imparted mainly to the inner surface portion of the spring member 1B.

[0047] In carrying out the present invention, the metal member is not limited to a spring member such as a coil spring or a torsion bar, and can be applied to various metal members depending on the application. Furthermore, the laser irradiation device and AC current applying device as the surface heating device are not limited to the above-described embodiment, and can be embodied in various forms.

[0048] 1A, 1B...spring member, 20...setting device, 21...torsion mechanism (load application mechanism), 30...laser irradiation device (surface heating device), 38, 38a, 38b...surface heating section, 40...setting device, 50...alternating current application device (surface heating device), 70...setting device, 72...compression mechanism (load application mechanism), 80...setting device.

Claims

1. A method for treating a metal component, comprising: heating at least a part of a surface of the metal component; applying a load that generates plastic strain in the metal component while the surface is heated; and removing the load after a predetermined time has elapsed, thereby generating compressive residual stress portions (S1) (S2) in which the absolute value of compressive residual stress increases from the interior of the metal component toward the surface, from the surface to a first depth (D1) (D3) or a position beyond the first depth (D1) (D3); A method for treating a metal component, characterized in that the compressive residual stress portions (S1)(S2) include internal residual stress portions (A1)(B1) extending from the first depth (D1)(D3) to a second depth (D2)(D4) shallower than the first depth (D1)(D3), and surface residual stress portions (A2)(B2) extending from the second depth (D2)(D4) to the surface, and the rate of increase in compressive residual stress in the surface residual stress portions (A2)(B2) is greater than the rate of increase in compressive residual stress in the internal residual stress portions (A1)(B1).

2. A method for treating a metal component according to claim 1, wherein the surface of the metal component is heated to a temperature range of 200°C to 400°C by irradiating the surface with laser light.

3. A method for treating a metal component as described in claim 1, further comprising the step of performing shot peening on the surface of the metal component having the compressive residual stress portions (S1) (S2), thereby generating peak portions (P1) (P4) in the surface portion where the absolute value of the compressive residual stress is maximum.

4. A setting device for a metal member, comprising: a surface heating device that heats the surface of the metal member; and a load application mechanism that applies a load that causes plastic strain in the metal member whose surface has been heated, and removes the load after a predetermined time has elapsed.

5. A setting device according to claim 4, wherein the surface heating device is a laser irradiation device (30) that irradiates the surface of the metal member with laser light.

6. A setting device according to claim 4, wherein the surface heating device is an AC current applying device (50) that applies an AC voltage to the metal member.

7. A metal component having compressive residual stress portions (S1) (S2) extending from the surface of the metal component to a first depth (D1) (D3) or a position beyond the first depth (D1) (D3), wherein the absolute value of the compressive residual stress in the compressive residual stress portions (S1) (S2) increases from the first depth (D1) (D3) toward the surface, and the absolute value of the compressive residual stress is maximum approximately at the surface of the metal component; A metal component characterized in that the compressive residual stress portions (S1)(S2) include internal residual stress portions (A1)(B1) extending from the first depth (D1)(D3) to a second depth (D2)(D4) shallower than the first depth (D1)(D3), and surface residual stress portions (A2)(B2) extending from the second depth (D2)(D4) to the surface, and the rate of increase in compressive residual stress in the surface residual stress portions (A2)(B2) is greater than the rate of increase in compressive residual stress in the internal residual stress portions (A1)(B1).

8. A metal component according to claim 7, comprising residual stress stabilizing portions (A3) (B3) at positions deeper than the first depths (D1) (D3), where the magnitude of the compressive residual stress remains substantially constant.

9. A metal component according to claim 7, wherein the first depth (D1) (D3) is a position where the compressive residual stress becomes zero.

10. A metal member according to claim 7, wherein the metal member is a torsion bar type spring member (1A).

11. A metal member according to claim 7, wherein the metal member is a spring member (1B) of the coil spring type.