Method and apparatus for manufacturing metal product

Abstract

A method for manufacturing a metal product according to one embodiment includes: preparing a metal workpiece which has a curved part; attaching a pair of terminals connected to a power supply to the workpiece; disposing a jig which has a conductive part having conductivity and an insulating part having insulating properties in such a manner that the insulating part is positioned between the conductive part and the workpiece and that the conductive part faces the curved part; and heating the workpiece by supplying an AC current to the workpiece through the pair of terminals.

Description

Method and apparatus for manufacturing metal products

[0001] This invention relates to a method for manufacturing metal products and an apparatus for manufacturing them.

[0002] Conventionally, metal products have been subjected to various heat treatments such as quenching and tempering (see, for example, Patent Documents 1 to 4). In addition, known heat treatment methods include heating using a heating furnace, induction heating using a high-frequency coil, or electrostatic heating, which generates heat by passing an electric current through the workpiece itself for processing into a metal product.

[0003] JP2002-331326A JP10-297242A JP11-189022A Patent No. 6258243

[0004] For example, when applying the above-mentioned electric heating to a curved workpiece, such as a stabilizer used in a vehicle suspension system, it is difficult to control the temperature distribution in the curved portion. That is, the current tends to flow more easily through the inner circumference of the curved portion, causing the temperature in that inner circumference to rise easily. Therefore, even when the heat treatment should be applied to the workpiece at a uniform temperature throughout, temperature unevenness may occur in the curved portion, and the properties of the metal product after heat treatment may become non-uniform at different points in the curved portion.

[0005] Therefore, one of the objectives of the present invention is to provide a method and apparatus for manufacturing metal products that can achieve a desired heating temperature distribution in the curved portion of the workpiece during heat treatment.

[0006] A method for manufacturing a metal product according to one embodiment includes preparing a metal workpiece having a curved portion, attaching a pair of terminals connected to a power source to the workpiece, arranging a jig having a conductive portion having an insulating portion having an insulating portion such that the insulating portion is located between the conductive portion and the workpiece and the conductive portion faces the curved portion, and heating the workpiece by passing an alternating current through the pair of terminals.

[0007] Furthermore, a metal product manufacturing apparatus according to one embodiment includes a pair of terminals connected to a metal workpiece having a curved portion, a jig having a conductive portion and an insulating portion, a moving device that moves the jig so that the insulating portion is located between the conductive portion and the workpiece and the conductive portion faces the curved portion, and a heating device that heats the workpiece by passing an alternating current through the pair of terminals to the workpiece.

[0008] According to the present invention, it is possible to provide a method and apparatus for manufacturing metal products that can achieve a desired heating temperature distribution in the curved portion of the workpiece during heat treatment.

[0009] Figure 1 is a schematic perspective view showing a part of a vehicle equipped with a stabilizer, which is an example of a metal product. Figure 2 is a schematic plan view of a stabilizer according to one embodiment. Figure 3 is a flowchart showing a method for manufacturing a stabilizer according to one embodiment. Figure 4 is a schematic diagram showing a part of a manufacturing apparatus for a stabilizer according to one embodiment. Figure 5 is a schematic cross-sectional view showing a configuration applicable to a heating jig according to one embodiment. Figure 6 is a schematic cross-sectional view of a heating jig along the line VI-VI in Figure 5. Figure 7 is a flowchart showing the flow of the heat treatment using the manufacturing apparatus according to one embodiment. Figure 8 is a schematic diagram showing the terminal attachment process in the heat treatment. Figure 9 is a schematic diagram showing the placement process of the heating jig in the heat treatment. Figure 10 is a schematic cross-sectional view of the workpiece and heating jig after the placement process. Figure 11 is a schematic cross-sectional view of the workpiece and heating jig along the line XI-XI in Figure 10. Figure 12 is a schematic diagram for explaining the operation of the moving device in the placement process. Figure 13 is a schematic diagram for explaining the proximity effect. Figure 14 shows an example of the heating temperature distribution of a curved section in two cases: (a) when AC current heating is performed without a conductive part, and (b) when AC current heating is performed with a conductive part. Figure 15 is a schematic cross-sectional view of a conductive part and a workpiece without an insulating part. Figure 16 is a graph showing the relationship between the amount of displacement between the workpiece and the conductive part in a certain direction and the heating temperature. Figure 17 is a graph showing the relationship between the amount of displacement between the workpiece and the conductive part and the heating temperature in a direction different from that shown in Figure 16.

[0010] One embodiment will be described with reference to the drawings. In this embodiment, a vehicle stabilizer is disclosed as an example of a metal product subjected to heat treatment. However, the manufacturing method and manufacturing apparatus disclosed in this embodiment can be applied to various other metal product manufacturing methods and apparatus.

[0011] Figure 1 is a schematic perspective view showing a part of a vehicle 100 equipped with a stabilizer 1 according to this embodiment. The stabilizer 1 is connected to the suspension mechanism of the vehicle body 101 via connecting members 110A and 110B. In the example of Figure 1, support parts 120A and 120B are provided on the stabilizer 1 for supporting the stabilizer 1 on the vehicle body 101. For example, rubber bushings can be used as support parts 120A and 120B.

[0012] In this embodiment, it is assumed that the stabilizer 1 is made of hollow steel. However, the stabilizer 1 may be solid. For example, a resin-based coating is formed on the surface of the stabilizer 1.

[0013] Figure 2 is a schematic plan view of the stabilizer 1 according to this embodiment. As shown in the figure, the X, Y, and Z axes are defined to describe the shape of the stabilizer 1. In the following description, the directions parallel to these X, Y, and Z axes will be referred to as the X direction, Y direction, and Z direction. The X, Y, and Z directions are orthogonal to each other.

[0014] The stabilizer 1 has a torsion section 2, a pair of arm sections 3A and 3B, and a pair of curved sections 4A and 4B. The torsion section 2 extends linearly in the X direction. Arm section 3A is connected to one end of the torsion section 2 via curved section 4A. Arm section 3B is connected to the other end of the torsion section 2 via curved section 4B.

[0015] The end 30A of arm portion 3A and the end 30B of arm portion 3B are connected to connecting members 110A and 110B shown in Figure 1, respectively. The support portions 120A and 120B shown in Figure 1 are attached to the torsion portion 2, for example.

[0016] In the example shown in Figure 2, the arm portions 3A and 3B extend in the Y direction. That is, in the curved portions 4A and 4B, the stabilizer 1 is bent at approximately a right angle. The example is not limited to this, and the arm portions 3A and 3B may be inclined with respect to the Y direction. Also, the arm portions 3A and 3B may have one or more curved portions.

[0017] When the vehicle 100 travels around a curve, opposing loads are applied to the arm sections 3A and 3B. At this time, opposing bending forces are applied to the arm sections 3A and 3B, as well as bending and torsional forces to the curved sections 4A and 4B, and the torsion section 2 is twisted. At this time, a rebound load is generated to suppress the rolling behavior of the vehicle body 101.

[0018] Next, the manufacturing method and manufacturing apparatus for the stabilizer 1 will be described. Figure 3 is a flowchart showing the manufacturing method for the stabilizer 1 according to this embodiment. In manufacturing the stabilizer 1, first a rod-shaped workpiece is bent (step P1). This forms the curved portions 4A and 4B shown in Figure 2 on the workpiece. In other words, step P1 prepares a workpiece having the curved portions 4A and 4B. In this embodiment, the workpiece is a hollow steel pipe.

[0019] Next, the workpiece is subjected to quenching (step P2). As will be described in detail later, in this embodiment, AC current heating is used for quenching. After quenching, the workpiece is subjected to tempering (step P3). The method of tempering is not particularly limited, but in one example, DC current heating can be used.

[0020] After tempering, the workpiece is shot peened (step P4). Subsequently, a coating is formed on the surface and the support parts 120A and 120B are assembled (step P5), completing the stabilizer 1.

[0021] Figure 4 is a schematic diagram showing a part of the manufacturing apparatus 200 for stabilizer 1, illustrating an example of a configuration for performing a hardening process P2 on a workpiece. The manufacturing apparatus 200 includes a heating device 50, a pair of terminals 51A and 51B, a pair of heating jigs 6A and 6B, and a pair of moving devices 7A and 7B.

[0022] The heating device 50 is equipped with a power supply 52 that supplies alternating current. The frequency of the alternating current supplied by the power supply 52 is not particularly limited, but in one example, a high frequency of 1 kHz or higher may be used. Terminals 51A and 51B are connected to the power supply 52.

[0023] The heating fixtures 6A and 6B are held by moving devices 7A and 7B, respectively. The moving devices 7A and 7B move the heating fixtures 6A and 6B. In this embodiment, it is assumed that the moving devices 7A and 7B are 6-axis robot arms. That is, the moving devices 7A and 7B can move the heating fixtures 6A and 6B linearly along three axes that define three-dimensional space, and can also rotate them around these three axes. As a result, the moving devices 7A and 7B can control the position and orientation of the heating fixtures 6A and 6B with 6 degrees of freedom.

[0024] Figures 5 and 6 are schematic cross-sectional views showing an example of a configuration applicable to the heating jig 6A. As shown, the U-axis, V-axis, and W-axis are defined to describe the shape of the heating jig 6A. In the following description, the directions parallel to these U-axis, V-axis, and W-axis are referred to as the U-direction, V-direction, and W-direction. The U-direction, V-direction, and W-direction are orthogonal to each other.

[0025] Figure 5 shows a cross-section of the heating jig 6A along the U-V plane, which is parallel to the U and V directions. Figure 6 shows a cross-section of the heating jig 6A along the VI-VI line in Figure 5 (a cross-section along the V-W plane, which is parallel to the V and W directions). The same configuration as the heating jig 6A shown in Figures 5 and 6 can be applied to the heating jig 6B.

[0026] The heating jig 6A is equipped with a conductive part 8 (induction plate) that has electrical conductivity. As the material of the conductive part 8, a metal material with excellent electrical conductivity such as copper or aluminum can be used. The conductive part 8 is, for example, in the form of a thin plate and is insulated from the moving device 7A.

[0027] As shown in Figure 6, the conductive portion 8 has a first portion 81, a second portion 82, and a third portion 83. In Figure 6, dashed lines are drawn at the boundaries between the first portion 81 and the second portion 82, and between the first portion 81 and the third portion 83.

[0028] The first portion 81 has a curved cross-sectional shape. The second portion 82 and the third portion 83 are flat plates parallel to the U-V plane and extend from the first portion 81. The second portion 82 and the third portion 83 face each other in the W direction.

[0029] In the cross-section shown in Figure 5, the first portion 81 has a straight region 81a parallel to the U direction, a straight region 81b parallel to the V direction, and a curved region 81c. In each of these regions 81a, 81b, and 81c, the first portion 81 has the cross-sectional shape shown in Figure 6. The straight region 81a may be inclined with respect to the U direction. Similarly, the straight region 81b may be inclined with respect to the V direction.

[0030] The heating jig 6A further comprises first insulating parts 91a, 91b (first spacers), second insulating parts 92a, 92b (second spacers), and third insulating parts 93a, 93b (third spacers). These insulating parts 91a to 93a and 91b to 93b are all located inside the conductive part 8. The method of attaching the insulating parts 91a to 93a and 91b to 93b to the conductive part 8 is not particularly limited, but in one example, screw fastening or adhesive bonding can be applied.

[0031] As will be described in more detail later, these insulating parts 91a to 93a and 91b to 93b may come into contact with the workpiece to be heated. Therefore, it is preferable that the insulating parts 91a to 93a and 91b to 93b be made of a material that has excellent insulating properties, toughness, heat insulation properties, impact resistance, and wear resistance. Examples of materials that satisfy these properties include ceramics such as silicon nitride, alumina, and zirconia, and ceramic fibers made of alumina fibers and silica fibers.

[0032] The first insulating portion 91a is attached to the inner surface of the first portion 81 in the linear region 81a. The first insulating portion 91b is attached to the inner surface of the first portion 81 in the linear region 81b. In the example in Figure 6, the surface F1 of the first insulating portion 91a is flat. The same shape as the first insulating portion 91a can be applied to the first insulating portion 91b.

[0033] If the first insulating parts 91a and 91b are provided in the curved region 81c, the shape of the first insulating parts 91a and 91b may become complex. In contrast, if the first insulating parts 91a and 91b are provided in the straight region 81a and 81b, the shape of these first insulating parts 91a and 91b can be made simpler, making manufacturing easier. However, an insulating part may be placed in the curved region 81c instead of, or together with, the first insulating parts 91a and 91b.

[0034] As shown in Figure 5, the second insulating parts 92a and 92b are attached to the inner surface of the second part 82. The second insulating part 92a is aligned with the first insulating part 91a in the V direction. The second insulating part 92b is aligned with the first insulating part 91b in the U direction.

[0035] As shown in Figure 6, the third insulating parts 93a and 93b are attached to the inner surface of the third portion 83. The third insulating parts 93a and 93b are opposite the second insulating parts 92a and 92b in the W direction, respectively.

[0036] At least one of the second insulating portion 92a and the third insulating portion 93a may have a tapered portion TP whose thickness decreases as it moves away from the first portion 81. In the example in Figure 6, both the second insulating portion 92a and the third insulating portion 93a have tapered portions TP. These tapered portions TP form inclined surfaces on the surface F2 of the second insulating portion 92a and the surface F3 of the third insulating portion 93a, where the distance between surfaces F2 and F3 increases as you move towards the entrance side to the interior of the conductive portion 8 (downward in Figure 6). The second insulating portion 92b and the third insulating portion 93b also have tapered portions TP similar to those of the second insulating portion 92a and the third insulating portion 93a.

[0037] FIG. 7 is a flowchart showing the flow of the heat treatment using the manufacturing apparatus 200. The heat treatment includes an attachment step P11 of attaching terminals 51A and 51B to the workpiece, an arrangement step P12 of arranging the heating fixtures 6A and 6B at predetermined positions of the workpiece to control the position and orientation of the workpiece, and a power supply step P13 of supplying power to the workpiece through the terminals 51A and 51B. Hereinafter, the details of these steps will be described.

[0038] FIG. 8 is a schematic diagram showing the attachment step P11. In the attachment step P11, the terminals 51A and 51B are attached to the workpiece 1a (stabilizer semi-finished product) in which the bent portions 4A and 4B are formed by bending (step P1 in FIG. 4).

[0039] In the example of FIG. 8, the terminal 51A is attached to the end portion 30A of the arm portion 3A, and the terminal 51B is attached to the end portion 30B of the arm portion 3B. The terminals 51A and 51B also serve to fix (grip, restrain) the end portions 30A and 30B so that the workpiece 1a is in a posture suitable for the heat treatment. In one example, the terminals 51A and 51B are composed of a plurality of divided members, and the end portions 30A and 30B are respectively sandwiched by these members. However, the structure for attaching the terminals 51A and 51B (the structure for fixing the end portions 30A and 30B of the workpiece 1a) is not limited to this example.

[0040] FIG. 9 is a schematic diagram showing the arrangement step P12 of the heating fixtures 6A and 6B. In the arrangement step P12, the moving device 7A moves the heating fixture 6A to a position covering the bent portion 4A. Further, the moving device 7B moves the heating fixture 6B to a position covering the bent portion 4B.

[0041] FIG. 10 is a schematic cross-sectional view of the workpiece 1a and the heating fixture 6A after passing through the arrangement step P12. FIG. 11 is a schematic cross-sectional view of the workpiece 1a and the heating fixture 6A along the line XI-XI in FIG. 10. For example, the heating fixture 6A is arranged such that the U direction, the V direction, and the W direction (see FIGS. 5 and 6) coincide with the X direction, the Y direction, and the Z direction (see FIG. 2) of the workpiece 1a, respectively.

[0042] Specifically, the insulating portions 91a-93a and 91b-93b are positioned on the workpiece 1a side. In the examples shown in Figures 10 and 11, the torsion portion 2 of the workpiece 1a is in contact with the surface F1 of the first insulating portion 91a. Furthermore, the arm portion 3A of the workpiece 1a is in contact with the surface F1 of the first insulating portion 91b. This ensures that the positional relationship between the workpiece 1a and the conductive portion 8 in the X direction (U direction) and the Y direction (V direction) is stably determined. In addition, the movement of the curved portion 4A in the X direction (U direction) and the Y direction (V direction) is restricted.

[0043] In the example shown in Figure 11, the torsion portion 2 is located between the second insulating portion 92a and the third insulating portion 93a, and the arm portion 3A is located between the second insulating portion 92b and the third insulating portion 93b. The curved portion 4A is located between the second portion 82 and the third portion 83.

[0044] As shown in Figure 11, the distance D between the second insulating portion 92a and the third insulating portion 93a in the portion excluding the tapered portion TP is greater than the diameter R of the workpiece 1a (D > R). That is, in the Z direction (W direction), there is play between the insulating portions 92a, 93a and the workpiece 1a. A similar distance D is formed between the second insulating portion 92b and the third insulating portion 93b, which also creates play between the insulating portions 92b, 93b and the workpiece 1a. This play can absorb variations in the shape of the workpiece 1a or thermal deformation of the workpiece 1a during heating. Furthermore, the movement of the curved portion 4A in the Z direction (W direction) is restricted to within the range of the above-mentioned play.

[0045] The workpiece 1a may be in contact with the second insulating portion 92a, 92b, or the third insulating portion 93a, 93b. This ensures that the positional relationship between the workpiece 1a and the conductive portion 8 in the Z direction (W direction) is stably determined. In the example shown in Figure 11, the workpiece 1a is in contact with the third insulating portion 93a, 93b.

[0046] By providing tapered portions TP in the second insulating portions 92a, 92b and the third insulating portions 93a, 93b, respectively, it becomes easier to position the heating jig 6A relative to the workpiece 1a. The tapered portions TP have a shape that increases in distance from the workpiece 1a to be inserted into the heating jig 6A as they move away from the first portion 81. That is, at the entrance side of the heating jig 6A, the distance between the second insulating portion 92a and the third insulating portion 93a, and the distance between the second insulating portion 92b and the third insulating portion 93b increases, thereby suppressing interference between the insulating portions 92a, 92b, 93a, 93b and the workpiece 1a when inserting the workpiece 1a.

[0047] In the example shown in Figure 10, the first portion 81 of the conductive portion 8 faces the outer circumferential surface of the curved portion 4A (the surface located on the outer circumferential side of the bend of the curved portion 4A) through a gap. The second portion 82 and the third portion 83 of the conductive portion 8 also face the curved portion 4A through a gap. On the other hand, the conductive portion 8 does not face the inner circumferential surface of the curved portion 4A (the surface located on the inner circumferential side of the bend of the curved portion 4A).

[0048] In other areas as well, the conductive part 8 is not in contact with the workpiece 1a. In other words, the conductive part 8 remains electrically floating even after the placement process P12.

[0049] Figure 12 is a schematic diagram illustrating the operation of the moving device 7A in the placement process P12. In the illustrated example, the moving device 7A comprises a base 70 and links 71, 72, 73, and 74. By configuring the connection points of the base 70 and links 71, 72, 73, and 74 to be rotatable as appropriate, six-axis movement can be achieved. Note that the configuration of the moving device 7A for achieving six-axis movement is not limited to that shown in Figure 12, and various configurations can be adopted.

[0050] The moving device 7A is equipped with multiple sensors S. These sensors S detect the loads in the U, V, and W directions applied to the heating jig 6A when the heating jig 6A comes into contact with the workpiece 1a.

[0051] In the placement step P12, the moving device 7A moves the heating jig 6A to a position that covers the curved portion 4A. For example, the movement path and orientation of the heating jig 6A are set in advance. As another example, the workpiece 1a may be photographed with a camera, and the moving device 7A may move the heating jig 6A to a position that covers the curved portion 4A based on the image data.

[0052] As the heating jig 6A moves, the workpiece 1a is inserted into the heating jig 6A. When any of the insulating parts 91a-93a or 91b-93b come into contact with the workpiece 1a, a load is detected by each sensor S. The moving device 7A preferably has a load control function that realizes an appropriate positional relationship between the workpiece 1a and the heating jig 6A based on this load. For example, the moving device 7A may control the position and orientation of the heating jig 6A so that the loads it receives from the workpiece 1a in the U, V, and W directions are within predetermined ranges. Alternatively, at least one of the heating jig 6A and the moving device 7A may be given elasticity, and a mechanism may be provided to change the orientation of the heating jig 6A to conform to the position of the workpiece 1a when the workpiece 1a and the heating jig 6A come into contact. As for the elasticity, for example, the deflection of the arm of the moving device 7A can be used. As another example, the above elasticity may be provided by adding a spring mechanism to the heating jig 6A (insulating parts 91a to 93a, 91b to 93b).

[0053] By using this load control function, even if there are variations in the shape of each workpiece 1a, as shown by the dashed and solid lines in Figure 12, the heating jig 6A can be positioned to follow the shape. Furthermore, the position and orientation of the heating jig 6A can be controlled in accordance with the thermal deformation of the workpiece 1a.

[0054] The workpiece 1a may undergo thermal deformation due to heating during the energizing process P13. During the energizing process P13, the position of the heating jig 6A can be controlled based on the load control function, making it possible to make the position and orientation of the heating jig 6A follow the thermal deformation.

[0055] Furthermore, the relationship between the heating jig 6B and the workpiece 1a after the placement process P12 can be the same as the relationship between the heating jig 6A and the workpiece 1a shown in Figures 10 and 11. Also, the configuration and control method of the moving device 7B can be the same as the configuration and control method of the moving device 7A explained using Figure 12.

[0056] When the placement of the heating jigs 6A and 6B is complete, the moving devices 7A and 7B notify the heating device 50 that placement is complete. Upon receiving this notification, the heating device 50 supplies alternating current from the power supply 52 to the workpiece 1a via terminals 51A and 51B. As a result, the workpiece 1a heats up and hardens. The frequency, amplitude, and energizing time of the alternating current can be appropriately determined according to the target heating temperature, etc.

[0057] After heating, the moving devices 7A and 7B move the heating jigs 6A and 6B away from the position covering the curved sections 4A and 4B. Furthermore, the workpiece 1a is cooled. This cooling may be done by natural cooling, or if rapid cooling is required, the workpiece 1a may be exposed to a liquid such as water, or a gas such as air or a liquid such as water may be blown onto the workpiece 1a.

[0058] When alternating current flows through the workpiece 1a, the current density increases near its surface due to the skin effect. Furthermore, in the curved sections 4A and 4B, the current path passing near the inner surface of the bend is short, which tends to increase the current density near the inner surface. As a result, the heating temperature near the inner surface becomes higher than that near the outer surface. This temperature difference can be particularly pronounced when the radius of curvature of the curved sections 4A and 4B is small, the bending angle is large, or the diameter of the workpiece 1a is large.

[0059] Here, we will explain the role of the conductive part 8 of the heating fixtures 6A and 6B. When an electric current flows through a workpiece such as workpiece 1a, if an electrically floating conductive part is placed nearby, a so-called proximity effect occurs. In the heating process, this proximity effect is used to control the current density distribution (heating temperature distribution) of workpiece 1a. In other words, the conductive part 8 is positioned at a location where this proximity effect occurs on workpiece 1a.

[0060] FIG. 13 is a schematic diagram for explaining the proximity effect, showing a rod-shaped workpiece Ws and a conductive portion 8s disposed in the vicinity thereof. When a current I from a power source flows through the workpiece Ws Ａ a magnetic field H ＩＡ is generated around the workpiece Ws (Ampere's law).

[0061] In the conductive portion 8s, an eddy current I ＩＡ is generated due to this magnetic field H Ｅ１ (Lenz's law). Further, a magnetic field H Ｅ１ is generated around the conductive portion 8s due to the eddy current I ＩＥ . When this magnetic field H ＩＥ acts on the workpiece Ws, an eddy current I Ｅ２ is generated in the workpiece Ws.

[0062] The directions in which the currents I Ａ , eddy current I Ｅ１ and eddy current I Ｅ２ flow are as indicated by the arrows in the figure. That is, in the workpiece Ws, near the side surface on the side far from the conductive portion 8s, the direction in which the current I Ａ flows and the direction in which the eddy current I Ｅ２ flows are opposite. On the other hand, near the side surface on the side close to the conductive portion 8s, the direction in which the current I Ａ flows and the direction in which the eddy current I Ｅ２ flows coincide. As a result, the current density of the workpiece Ws becomes high near the side surface close to the conductive portion 8s. By utilizing such a proximity effect, it is possible to control the current density distribution and the heating temperature distribution of the workpiece Ws.

[0063] FIG. 14 is a diagram showing an example of the heating temperature distribution of the curved portion 4A in each of the cases of (a) performing AC energization heating without disposing the conductive portion 8 and (b) performing AC energization heating with the conductive portion 8 disposed. The darker the hatching in the figure, the higher the temperature.

[0064] When the conductive portion 8 is not disposed, as described above, the current density near the inner peripheral surface of the curved portion 4A increases. Therefore, as shown in FIG. 14(a), the vicinity of the inner peripheral surface is heated at a higher temperature than the vicinity of the outer peripheral surface.

[0065] In contrast, when the conductive part 8 is placed, the current is attracted to the outer surface due to the proximity effect described above. Therefore, as shown in Figure 14(b), the heating temperature can be made uniform.

[0066] In Figure 14(a), uneven heating temperatures occur not only in the curved portion 4A but also in the surrounding torsion portion 2 and arm portion 3A. In such cases, if the conductive portion 8 faces a part of the torsion portion 2 and arm portion 3A, as shown in Figure 10, it is possible to equalize the heating temperature in the torsion portion 2 and arm portion 3A. Furthermore, the same effect as near the curved portion 4A can be obtained near the curved portion 4B by the conductive portion 8 of the heating jig 6B.

[0067] In this example, we assumed a case where the heating temperature of workpiece 1a is made uniform, but this is not the only example. That is, it is also possible to control the heating temperature distribution using the conductive part 8 so that a desired temperature gradient is formed.

[0068] In this embodiment, the heating jigs 6A and 6B are equipped with insulating parts 91a to 93a and 91b to 93b, thereby improving the accuracy of controlling the heating temperature distribution. The details of this effect will be explained below.

[0069] Figure 15 is a schematic cross-sectional view of the conductive portion 8 and workpiece 1a, where the insulating portions 91a to 93a and 91b to 93b are not provided. As shown in this figure, the outer peripheral position Q1, inner peripheral position Q2, first lateral position Q3, and second lateral position Q4 on the surface of workpiece 1a are defined. The outer peripheral position Q1 is included in the outer peripheral surface described above. The inner peripheral position Q2 is included in the inner peripheral surface described above.

[0070] The center line CL1 shown in Figure 15 is a straight line passing through the axis AX of the workpiece 1a and parallel to the Y direction. The center line CL2 is a straight line passing through the axis AX of the workpiece 1a and parallel to the Z direction. The outer circumference position Q1 corresponds to one of the two intersection points of the center line CL1 and the surface of the workpiece 1a that is closer to the first portion 81 of the conductive part 8. The inner circumference position Q2 corresponds to the other of these two intersection points. The first lateral position Q3 corresponds to one of the two intersection points of the center line CL2 and the surface of the workpiece 1a that is closer to the second portion 82 of the conductive part 8. The second lateral position Q4 corresponds to the other of these two intersection points and faces the third portion 83 of the conductive part 8.

[0071] The heating temperature distribution when alternating current flows through the workpiece 1a varies depending on the distance between the workpiece 1a and the conductive part 8. To explain this variation, we define the amount of displacement d1 in the Z direction and the amount of displacement d2 in the Y direction of the conductive part 8 relative to the workpiece 1a.

[0072] Figure 16 is a graph showing the relationship between the displacement amount d1 [mm] and the temperature difference ΔT [°C] between the heating temperature at positions Q1 to Q4 and the reference temperature. Here, it is assumed that the displacement amount d1 = 0 occurs when the distance between the first lateral position Q3 and the second part 82 is equal to the distance between the second lateral position Q4 and the third part 83. The reference temperature mentioned above is the average value of the heating temperatures at positions Q1 to Q4 when the displacement amounts d1 and d2 = 0.

[0073] As is clear from this graph, the more the displacement d1 increases, that is, the shorter the distance between the second part 82 and the first lateral position Q3, the higher the temperature difference ΔT at the first lateral position Q3. On the other hand, the more the displacement d1 increases, that is, the longer the distance between the third part 83 and the second lateral position Q4, the lower the temperature difference ΔT at the second lateral position Q4. Note that the effect of fluctuations in the displacement d1 on the temperature difference ΔT between the outer circumference position Q1 and the inner circumference position Q2 is negligible.

[0074] Figure 17 is a graph showing the relationship between the displacement amount d2 [mm] and the temperature difference ΔT [°C] between positions Q1 to Q3. When the displacement amount d2 = 0, the temperature difference ΔT between positions Q1 to Q3 is approximately 0. Note that the temperature difference ΔT at the second lateral position Q4 is equivalent to, for example, the temperature difference ΔT at the first lateral position Q3.

[0075] As is clear from this graph, the greater the displacement d2, that is, the greater the distance between the first part 81 and the outer peripheral position Q1, the lower the temperature difference ΔT at the outer peripheral position Q1. On the other hand, the greater the displacement d2, the higher the temperature difference ΔT at the inner peripheral position Q2. Note that the effect of fluctuations in the displacement d2 on the temperature difference ΔT at the first lateral position Q3 is negligible.

[0076] As is clear from the above, when controlling the heating temperature distribution of AC current heating using the proximity effect, it is important to accurately determine the distance between the conductive part 8 and the workpiece 1a. However, as in the example in Figure 15, if insulating parts 91a to 93a and 91b to 93b are not placed between the conductive part 8 and the workpiece 1a, it is difficult to appropriately adjust the gap between the conductive part 8 and the workpiece 1a.

[0077] In contrast, when insulating portions 91a to 93a and 91b to 93b are arranged as in this embodiment, the distance between the conductive portion 8 and the workpiece 1a can be determined with high precision. That is, as shown in Figures 10 and 11, by pressing the workpiece 1a against the first insulating portions 91a and 91b, the distance between the workpiece 1a and the first portion 81 can be set to a distance corresponding to the thickness of the first insulating portions 91a and 91b. Furthermore, the second insulating portions 92a and 92b and the third insulating portions 93a and 93b can provide appropriate distances between the workpiece 1a and the second portion 82, and between the workpiece 1a and the third portion 83, respectively.

[0078] From another perspective, by appropriately determining the thickness of the insulating portions 91a to 93a and 91b to 93b, the desired distances can be accurately established between the workpiece 1a and the first portion 81, between the workpiece 1a and the second portion 82, and between the workpiece 1a and the third portion 83. This makes it possible to achieve the desired heating temperature distribution in and near the curved portions 4A and 4B of the workpiece 1a.

[0079] The configuration disclosed in this embodiment can be modified in various ways. For example, in this embodiment, it is assumed that the heat treatment using the manufacturing apparatus 200 is applied to the quenching process P2. However, the heat treatment using the manufacturing apparatus 200 may also be applied to heat treatments other than the quenching process P2, such as the tempering process P3.

[0080] The stabilizer 1 may further include curved sections other than the curved sections 4A and 4B. In this case, the manufacturing apparatus 200 may further include a heating jig for controlling the heating temperature distribution of the curved sections and a moving device for moving the heating jig. The same configuration as that of the heating jigs 6A and 6B and the moving devices 7A and 7B disclosed in this embodiment can be applied to these heating jigs and moving devices.

[0081] Furthermore, the heat treatment performed by the manufacturing apparatus 200 according to this embodiment is not limited to stabilizers, but can be applied to the manufacturing of various metal products having curved portions. For example, the metal product to be heat-treated may be a coil spring in which a metal wire is wound in a spiral shape. In this case, each part of the spirally wound wire corresponds to a curved portion.

[0082] The moving devices 7A and 7B do not necessarily have to be 6-axis robot arms. For example, the moving devices 7A and 7B may be 5-axis or less robot arms, or they may be sliders or cylinders that move the heating jigs 6A and 6B on a single axis.

[0083] The shape of the conductive part 8 and the arrangement of the insulating part in the heating jigs 6A and 6B are not limited to those shown in Figures 5 and 6. For example, the conductive part 8 does not have to have at least one of the second part 82 and the third part 83. Also, the insulating part does not have to be divided into multiple parts such as insulating parts 91a to 93a and 91b to 93b, but may be a single, integrated shape. Furthermore, the insulating part may be divided in a manner different from insulating parts 91a to 93a and 91b to 93b. In addition, the shape and arrangement of the conductive part 8 and the insulating part can be appropriately modified according to the shape of the metal product to be heated.

[0084] 1...Stabilizer, 1a...Workpiece, 2...Torsion section, 3A, 3B...Arm section, 4A, 4B...Bending section, 6A, 6B...Heating jig, 7A, 7B...Moving device, 8...Conductive section, 50...Heating device, 51A, 51B...Terminals, 52...Power supply, 81...First section, 82...Second section, 83...Third section, 91a, 91b...First insulating section, 92a, 92b...Second insulating section, 93a, 93b...Third insulating section, 200...Manufacturing device.

Claims

1. A method for manufacturing a metal product, comprising: preparing a metal workpiece having a curved portion; attaching a pair of terminals connected to a power source to the workpiece; arranging a jig having a conductive portion and an insulating portion such that the insulating portion is located between the conductive portion and the workpiece and the conductive portion faces the curved portion; and heating the workpiece by passing an alternating current through the pair of terminals.

2. The manufacturing method according to claim 1, wherein the conductive portion comprises a first portion, a second portion extending from the first portion, and a third portion extending from the first portion and facing the second portion, and the jig is arranged such that the curved portion is located between the second portion and the third portion, and the first portion faces the outer peripheral surface of the curved portion.

3. The manufacturing method according to claim 2, wherein the insulating portion includes a first insulating portion provided on the first portion, and the jig is arranged such that the first insulating portion is in contact with the workpiece.

4. The manufacturing method according to claim 3, wherein the insulating portion further includes a second insulating portion provided in the second portion and a third insulating portion provided in the third portion, and the jig is arranged such that the workpiece is located between the second insulating portion and the third insulating portion.

5. The manufacturing method according to claim 4, wherein at least one of the second insulating portion and the third insulating portion has a tapered portion that increases in distance from the workpiece as it moves away from the first portion.

6. The manufacturing method according to claim 1, wherein the metal product is a stabilizer comprising: a torsion portion that generates an elastic restoring force; an arm portion that extends in a direction different from the torsion portion; and a curved portion that connects the torsion portion and the arm portion.

7. The manufacturing method according to any one of claims 1 to 6, further comprising controlling the position or orientation of the jig or constraining the position of the workpiece in accordance with the load the jig receives from the workpiece.

8. A metal product manufacturing apparatus comprising: a pair of terminals connected to a metal workpiece having a curved portion; a jig having a conductive portion and an insulating portion; a moving device for moving the jig so that the insulating portion is located between the conductive portion and the workpiece and the conductive portion faces the curved portion; and a heating device for heating the workpiece by passing an alternating current through the pair of terminals to the workpiece.

9. The manufacturing apparatus according to claim 8, wherein the conductive portion comprises a first portion, a second portion extending from the first portion, and a third portion extending from the first portion and facing the second portion, and the moving device moves the jig such that the curved portion is located between the second portion and the third portion, and the first portion faces the outer peripheral surface of the curved portion.

10. The manufacturing apparatus according to claim 9, wherein the insulating portion includes a first insulating portion provided in the first portion, and the moving device moves the jig so that the first insulating portion contacts the workpiece.

11. The manufacturing apparatus according to claim 10, wherein the insulating portion further includes a second insulating portion provided in the second portion and a third insulating portion provided in the third portion, and the moving device moves the jig so that the workpiece is positioned between the second insulating portion and the third insulating portion.

12. The manufacturing apparatus according to claim 11, wherein at least one of the second insulating portion and the third insulating portion has a tapered portion that increases in distance from the workpiece as it moves away from the first portion.

13. The manufacturing apparatus according to claim 8, wherein the metal product is a stabilizer comprising: a torsion portion that generates an elastic restoring force; an arm portion that extends in a direction different from the torsion portion; and a curved portion that connects the torsion portion and the arm portion.

14. The manufacturing apparatus according to any one of claims 8 to 13, wherein the moving device controls the position or orientation of the jig or restrains the position of the workpiece in accordance with the load the jig receives from the workpiece.

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