Induction heating device and induction heating method

Abstract

An induction heating device according to one embodiment comprises a heating coil, a power supply device, and a conveyance device. The heating coil includes a first coil part, a second coil part that faces the first coil part, and a connection part that connects the first coil part and the second coil part. The first coil part, the second coil part, and the connection part are formed of one conductive material bent so as to form a plurality of loops. The power supply device supplies an alternating current to the heating coil. The conveyance device conveys a workpiece to be heated by the heating coil along a conveyance path passing between the first coil part and the second coil part. At least one of the first coil part and the second coil part includes a plurality of first wire portions arranged in an arrangement direction intersecting the conveyance path, and spacings between the plurality of first wire portions vary at least locally.

Description

Induction Heating Apparatus and Induction Heating Method

[0001] The present invention relates to an induction heating apparatus and an induction heating method for heating a workpiece by induction heating.

[0002] Conventionally, induction heating has been used as a method for heating a metal workpiece. In induction heating, a workpiece is placed near a heating coil to which an alternating current is supplied, and the workpiece is caused to self-heat by eddy currents generated in the workpiece.

[0003] Japanese Unexamined Patent Publication No. Hei 6-188068

[0004] When induction heating is performed, uneven heating temperature may occur in the workpiece depending on the arrangement of the conductive material forming the loop of the heating coil, the positional relationship between the conductive material and the workpiece, and the like.

[0005] One object of the present invention is to provide an induction heating apparatus and an induction heating method capable of improving the heating temperature distribution of a workpiece.

[0006] An induction heating apparatus according to one embodiment includes a heating coil, a power supply device, and a conveying device. The heating coil includes a first coil portion, a second coil portion facing the first coil portion, and a connecting portion connecting the first coil portion and the second coil portion, and is formed of a conductive material bent such that the first coil portion, the second coil portion, and the connecting portion form a plurality of loops. The power supply device supplies an alternating current to the heating coil. The conveying device conveys the workpiece to be heated by the heating coil along a conveying path passing between the first coil portion and the second coil portion. At least one of the first coil portion and the second coil portion includes a plurality of first wire portions arranged in an arrangement direction intersecting the conveying path. Further, intervals between the plurality of first wire portions differ at least locally.

[0007] Furthermore, an induction heating method according to one embodiment includes supplying an alternating current to a heating coil and transporting a workpiece to be heated along a transport path. The heating coil comprises a first coil section, a second coil section facing the first coil section, and a connecting section connecting the first coil section and the second coil section, and the first coil section, the second coil section and the connecting section are made of a conductive material bent to form a plurality of loops. The transport path passes between the first coil section and the second coil section. At least one of the first coil section and the second coil section is provided with a plurality of first line sections arranged in an array direction intersecting the transport path. Furthermore, the spacing between the plurality of first line sections differs at least locally.

[0008] According to the present invention, it is possible to provide an induction heating apparatus and induction heating method that can improve the heating temperature distribution of a workpiece.

[0009] Figure 1 is a schematic configuration diagram of an induction heating device according to the first embodiment. Figure 2 is a schematic perspective view of a heating coil according to the first embodiment. Figure 3 is a schematic cross-sectional view of the heating coil and workpiece along the line III-III in Figure 2. Figure 4 is a schematic plan view of the heating coil according to the first embodiment. Figure 5 is a schematic enlarged view of a portion of the multiple first line sections shown in Figure 4. Figure 6 is a diagram showing the verification conditions for uneven heating. Figure 7 is a graph showing the analysis results of the heating temperature distribution. Figure 8 is a schematic plan view of a heating coil according to the second embodiment. Figure 9 is a schematic plan view of a heating coil according to the third embodiment.

[0010] Several embodiments of induction heating devices and induction heating methods will be described with reference to the drawings.

[0011] [First Embodiment] Figure 1 is a schematic diagram of the induction heating device 1 according to the first embodiment. The induction heating device 1 comprises a heating coil 2, a power supply device 3, a cooling device 4, and a conveying device 5.

[0012] As will be described in more detail later, the heating coil 2 is made up of a conductive material 6 bent to form a loop. The conductive material 6 is, for example, a hollow metal member. A first terminal portion 61 is formed at one end of the conductive material 6, and a second terminal portion 62 is formed at the other end of the conductive material 6.

[0013] The power supply unit 3 supplies a high-frequency alternating current to the heating coil 2 via the first terminal section 61 and the second terminal section 62. The frequency of this alternating current is not particularly limited, but in one example it is 1 kHz or higher.

[0014] The cooling device 4 supplies coolant to the inside of the conductive material 6 via the first terminal portion 61 and collects the coolant that flows through the inside of the conductive material 6 and is discharged from the second terminal portion 62. This makes it possible to control the temperature of the conductive material 6 during heating.

[0015] The conveying device 5 conveys the workpiece W to be heated by the heating coil 2 along the conveying path F. In this embodiment, it is assumed that the conveying path F is generally straight. However, at least a part of the conveying path F may be curved. The workpiece W is, for example, a long, plate-shaped metal member having a first end E1 (tip) and a second end E2 (base). An example of such a workpiece W is a leaf spring. Note that the workpiece W does not necessarily have to be a plate-shaped member; it may be a member with a shape other than a plate, such as a rod with a circular or polygonal cross-section, or a hollow material.

[0016] In the following explanation, the direction along the transport path F is defined as the first direction X, the direction intersecting the transport path F (the longitudinal direction of the workpiece W) as the second direction Y, and the direction intersecting the first direction X and the second direction Y as the third direction Z. For example, the first direction X, the second direction Y, and the third direction Z are orthogonal to each other.

[0017] As shown by the dashed line in Figure 1, in this embodiment, it is assumed that the workpiece W is transported such that the portion near the first end E1 overlaps with the heating coil 2 in the third direction Z, and the portion near the second end E2 does not overlap with the heating coil 2 in the third direction Z.

[0018] The induction heating method according to this embodiment is carried out using an induction heating device 1. Specifically, when heating the workpiece W, the power supply device 3 supplies alternating current to the heating coil 2, and the cooling device 4 supplies coolant to the heating coil 2. Furthermore, the conveying device 5 conveys the workpiece W along the conveying path F.

[0019] Around the heating coil 2, to which an alternating current is supplied, a magnetic field is formed whose direction and intensity change periodically in accordance with the alternating current. When the workpiece W is transported to a position close to the heating coil 2, eddy currents corresponding to the magnetic field are generated in the workpiece W, causing the workpiece W to self-heat.

[0020] Induction heating using the induction heating device 1 can be applied to various heat treatments, such as partial quenching or tempering of the workpiece W. In other words, the induction heating device 1 constitutes part of the manufacturing apparatus for products including the heat-treated workpiece W. Furthermore, the induction heating method using the induction heating device 1 constitutes part of the manufacturing method for the said product.

[0021] Figure 2 is a schematic perspective view of a heating coil 2 according to the first embodiment. The heating coil 2 comprises a first coil section 10, a second coil section 20, and a connecting section 30. The first coil section 10, the second coil section 20, and the connecting section 30 are made of a single conductive material 6 bent to form multiple loops. The first terminal section 61 is connected to the outermost loop. The second terminal section 62 is connected to the innermost loop.

[0022] The first coil section 10 and the second coil section 20 face each other in the third direction Z (height direction). The connecting section 30 connects one end of the first coil section 10 and the second coil section 20 in the second direction Y. The other ends of the first coil section 10 and the second coil section 20 in the second direction Y are spaced apart. Also, the ends of the first coil section 10 and the second coil section 20 in the first direction X are spaced apart from each other.

[0023] The transport path F for the workpiece W passes between the first coil section 10 and the second coil section 20. Specifically, the workpiece W is transported such that its first end E1 passes through the space between the first coil section 10 and the second coil section 20, and its second end E2 passes outside that space.

[0024] In the example shown in Figure 2, the first coil section 10 comprises a plurality of first wire sections 11, a plurality of second wire sections 12, and a plurality of third wire sections 13. These first wire sections 11, second wire sections 12, and third wire sections 13 are all part of the conductive material 6. In this embodiment, the first wire sections 11, second wire sections 12, and third wire sections 13 are all linear, but the embodiment is not limited to this example.

[0025] Multiple first line sections 11 extend in a first direction X and are arranged in a second direction Y (arrangement direction). As illustrated by the outermost first line section 11, each first line section 11 has an upstream end Eu in the transport path F and a downstream end Ed in the transport path F.

[0026] Multiple second line sections 12 extend in the second direction Y and are aligned in the first direction X. Each second line section 12 is connected to the upstream end Eu of the first line section 11.

[0027] Multiple third line sections 13 extend in the second direction Y and are aligned in the first direction X. Each third line section 13 is connected to the downstream end Ed of the first line section 11.

[0028] The second coil section 20 can be configured in the same way as the first coil section 10. That is, the second coil section 20 comprises a plurality of first wire sections 11, a plurality of second wire sections 12, and a plurality of third wire sections 13. As another example, the shape of at least a part of the second coil section 20 may differ from that of the first coil section 10.

[0029] The connection section 30 comprises a plurality of first connecting pieces 31 arranged in a first direction X, and a plurality of second connecting pieces 32 arranged in a first direction X. Both the first connecting pieces 31 and the second connecting pieces 32 are part of the conductive material 6. Each first connecting piece 31 connects the second wire portion 12 of the first coil section 10 and the second coil section 20, respectively. Each second connecting piece 32 connects the third wire portion 13 of the first coil section 10 and the second coil section 20, respectively.

[0030] In the example shown in Figure 2, the first connecting piece 31 and the second connecting piece 32 are in a straight line extending in the third direction Z. In addition, a central region C is formed between the first connecting piece 31 and the second connecting piece 32, which are located furthest inward in the first direction X, where no conductive material 6 is placed.

[0031] For example, the width of the conductive material 6 is the same in the first wire section 11, the second wire section 12, the third wire section 13, the first connecting piece 31, and the second connecting piece 32. As another example, the width of the conductive material 6 may differ in at least some of these sections.

[0032] Figure 3 is a schematic cross-sectional view of the heating coil 2 and workpiece W along the line III-III in Figure 2. In the example in Figure 3, the first line portion 11 of the first coil portion 10 and the first line portion 11 of the second coil portion 20 face each other in the third direction Z.

[0033] As described above, the conductive material 6 has a hollow structure. The internal space of the conductive material 6 functions as a flow path S for the coolant supplied by the cooling device 4. This flow path S is continuously connected between the first terminal portion 61 and the second terminal portion 62.

[0034] As shown in Figure 3, a gap of height H is formed between the first coil section 10 and the second coil section 20. Specifically, height H corresponds to the distance in the third direction Z between the first wire section 11 of the first coil section 10 and the first wire section 11 of the second coil section 20. Although not shown in Figure 3, gaps of height H are also formed between the second wire section 12 of the first coil section 10 and the second wire section 12 of the second coil section 20, and between the third wire section 13 of the first coil section 10 and the third wire section 13 of the second coil section 20.

[0035] The workpiece W has a height h that is less than the height H in the third direction Z. For example, the workpiece W is transported such that the distance d1 between it and the first coil section 10 and the distance d2 between it and the second coil section 20 are equal. In this case, the distances d1 and d2 are each (H - h) / 2.

[0036] Figure 4 is a schematic plan view of the heating coil according to the first embodiment. In this embodiment, the spacing between the multiple first line portions 11 differs at least locally.

[0037] Specifically, in the example shown in Figure 4, of the multiple first line sections 11, the two closest to the connection section 30 in the second direction Y (first line sections 11a and 11b in the figure) are aligned at a first interval G1. Also, of the multiple first line sections 11, the two furthest from the connection section 30 in the second direction Y (first line sections 11c and 11d in the figure) are aligned at a second interval G2 which is smaller than the first interval G1 (G2 < G1). The spacing of the other first line sections 11 is, for example, the same as the first interval G1. However, at least one of these spacings may differ from the first interval G1 and the second interval G2.

[0038] Furthermore, the first line section 11a corresponds to a part of the innermost loop, and the first line section 11b corresponds to a part of the loop just outside of it. Also, the first line section 11c corresponds to a part of the outermost loop, and the first line section 11d corresponds to a part of the loop just inside of it.

[0039] Multiple second line sections 12 are aligned at a third interval G3. Multiple third line sections 13 are aligned at a fourth interval G4. In the example in Figure 4, the third interval G3 is smaller than the first interval G1 (G3 < G1). Also, the fourth interval G4 is smaller than the first interval G1 (G4 < G1). For example, the third interval G3 and the fourth interval G4 are the same as the second interval G2 (G2 = G3 = G4).

[0040] Figure 5 is a schematic enlarged view of the first line sections 11a and 11b shown in Figure 4. The upstream end Eu of the first line section 11b is connected to the first connecting piece 31 via the second line section 12. The downstream end Ed of the first line section 11b is connected to the second connecting piece 32 via the third line section 13.

[0041] On the other hand, in the example of Figure 5, the upstream end Eu of the first line portion 11a is connected to the first connecting piece 31 without going through the second line portion 12. Also, the downstream end Ed of the first line portion 11a is connected to the second connecting piece 32 without going through the third line portion 13. From another point of view, the distance D in the second direction Y between the first line portion 11a and the connecting portion 30 (first connecting piece 31 and second connecting piece 32) is zero.

[0042] In the induction heating method that conveys a workpiece along a conveyance path passing between a first coil section and a second coil section and heats a part of the workpiece as in the present embodiment, uneven heating may occur within the heating target region of the workpiece. With the configuration of the heating coil 2 according to the present embodiment, as can be understood from the verification results described below, such uneven heating can be suppressed.

[0043] FIG. 6 is a diagram showing verification conditions for uneven heating. A heating coil 2A shown in FIG. 6(a) corresponds to a comparative example with the present embodiment, and the distance between each first wire portion 11 is constant at a first distance G1.

[0044] On the other hand, a heating coil 2B shown in FIG. 6(b), similarly to the heating coil 2 shown in FIG. 4, differs from the heating coil 2A in that the distance between the first wire portion 11 of the outermost loop and the first wire portion 11 of the loop one inward therefrom is a second distance G2 that is smaller than the first distance G1. In this verification, the second distance G2 was set to one quarter of the first distance G1, and the heating temperature distribution of the workpiece W was analyzed when the workpiece W was heated using the heating coils 2A and 2B respectively.

[0045] FIG. 7 is a graph showing analysis results of the heating temperature distribution. The horizontal axis represents the position Px in the second direction Y starting from the first end E1 of the workpiece W, and the vertical axis represents temperature (in the range of T1 to T2° C.). The graph shows the temperature distribution on the surface of the workpiece W (the surface facing the first wire portions 11) for each of the heating coils 2A and 2B.

[0046] When the heating coil 2A is used, the surface temperature of the workpiece W decreases as the distance from the first end E1 increases. Even when the heating coil 2B is used, the surface temperature exhibits the same behavior in the region close to the first end E1 as when the heating coil 2A is used.

[0047] On the other hand, in the region distant from the first end E1, the surface temperature when the heating coil 2B is used is higher than that when the heating coil 2A is used. Accordingly, when the heating coil 2B is used, the overall temperature uniformity of the surface temperature is improved.

[0048] The above temperature rise in a region away from the first end E1 is caused by narrowing the interval between the first wire portion 11 of the outermost loop and the first wire portion 11 of the loop one inner side thereof in the heating coil 2B. That is, by adjusting the interval between the first wire portions 11 as in the heating coil 2 according to the present embodiment shown in FIG. 4, the heating temperature distribution of the workpiece W can be improved.

[0049] Further, the improved temperature uniformity of the workpiece W not only suppresses overheating of the portion to be heated, but also makes it possible to reduce the electric power to be supplied to the heating coil 2. That is, in order to impart predetermined properties to the portion to be heated by heat-treating the workpiece W, the entire portion to be heated needs to exceed the target temperature. Therefore, when there is large uneven heating in the workpiece W, it is necessary to increase the current supplied to the heating coil 2 or prolong the heating time so that portions whose temperature is less likely to rise can exceed the target temperature. In this case, portions where temperature easily rises, such as the vicinity of the first end E1, may be excessively heated. In contrast, when the temperature uniformity of the temperature distribution is high, it is possible to suppress local overheating and reduce power consumption. The reduction in power consumption leads to a reduction in carbon dioxide emissions required for manufacturing products including the workpiece W after heat treatment.

[0050] Further, by reducing the first gap G1, the total length of the conductive material 6 constituting the heating coil 2 can be shortened to lower the resistance, and the magnetic flux density can also be increased. From these viewpoints, it is also possible to reduce power consumption. For the same reason, as shown in FIG. 4, a configuration in which the plurality of second wire portions 12 and the plurality of third wire portions 13 are arranged at a third gap G3 and a fourth gap G4, respectively, which are smaller than the first gap G1, and a configuration in which the distance D is set to zero as shown in FIG. 5 also contribute to the reduction of power consumption.

[0051] As another method for improving the heating temperature distribution of the workpiece W, for example, a method of disposing a ferrite core around the heating coil to control the magnetic flux of the heating coil is also conceivable. However, in this case, the absorption of magnetic flux by the ferrite core may reduce the heating efficiency. In addition, it is necessary to design the device in consideration of the durability of the ferrite core, and the installation cost of the ferrite core and its cooling mechanism is also required.

[0052] In contrast, when the uniformity of heating is improved by adjusting the spacing of the first wire sections 11, as in the heating coil 2 according to this embodiment, the decrease in heating efficiency is suppressed compared to when a ferrite core is used. Furthermore, since there is no need to arrange components related to the ferrite core, the cost required to construct the induction heating device 1 can also be reduced.

[0053] It should be noted that, as an example, the effects are described in comparison to the case where a ferrite core is used, but this description does not necessarily negate the combination of the configuration of this embodiment with a ferrite core. In other words, in addition to the configuration of the heating coil 2 according to this embodiment, a ferrite core may be used to further improve the heating temperature distribution of the workpiece W.

[0054] [Second Embodiment] A second embodiment will now be described. Configurations not specifically mentioned can be those of the first embodiment.

[0055] Figure 8 is a schematic plan view of the heating coil 2 according to the second embodiment. The relationships between the first interval G1, second interval G2, third interval G3, and fourth interval G4 shown in this figure are the same as in the example in Figure 4.

[0056] In the example shown in Figure 8, the spacing between the multiple first line portions 11 decreases as they move away from the connection portion 30 in the second direction Y. That is, when the spacing between the first line portions 11 is formed in order from the connection portion 30 side as shown in the figure, the relationship G1 > Ga > Gb > Gc > Gd > Ge > G2 holds.

[0057] As is clear from the temperature distribution of the heating coil 2A shown in Figure 7, when the spacing of the first wire portions 11 is constant, a temperature distribution is formed in which the heating temperature decreases as you move away from the first end portion E1. Therefore, when the gap between the first wire portions 11 is gradually reduced as you move away from the connection portion 30, as in this embodiment, the gradient of temperature decrease is canceled out overall compared to when the spacing of the first wire portions 11 is constant, and the uniformity of the heating temperature distribution can be further improved.

[0058] [Third Embodiment] A third embodiment will now be described. Configurations not specifically mentioned can be those similar to those in the embodiments described above.

[0059] Figure 9 is a schematic plan view of the heating coil 2 according to the third embodiment. In this figure, as in Figure 5, a portion of the first wire sections 11a and 11b is shown in an enlarged view. In this embodiment, the first wire section 11a that forms the innermost loop also has a second wire section 12 connected to its upstream end Eu and a third wire section 13 connected to its downstream end Ed, just like the other first wire sections 11. That is, the distance D in the second direction Y between the first wire section 11a and the connection section 30 (first connecting piece 31 and second connecting piece 32) is not zero.

[0060] Here, we assume that the workpiece W is inserted to its maximum extent into the heating coil 2, that is, that the first end E1 is transported so that it overlaps with the first wire portion 11a in the third direction Z. In this case, if the first end E1 and the connecting portion 30 (first connecting piece 31 and second connecting piece 32) are too close together, sparks may occur between the workpiece W and the heating coil 2. Therefore, the distance D must be set to a predetermined distance or greater.

[0061] Similarly, the distances d1 and d2 shown in Figure 3 are set so that no sparks occur between the first wire portion 11 of the first coil portion 10 and the second coil portion 20 and the workpiece W. Therefore, distance D may be greater than or equal to distance d1 or distance d2. For example, when distances d1 and d2 are equal, these values ​​can be expressed as (H-h) / 2 using heights H and h as described above. Thus, in one example, distance D is greater than or equal to (H-h) / 2.

[0062] Furthermore, if the distance D is too large, the total length of the conductive material 6 constituting the heating coil 2 increases, which can lead to an increase in power consumption. Therefore, it is preferable that the distance D is 1.5 times or less of the distance d1 or distance d2. That is, if distances d1 and d2 are equal, it is preferable that the distance D is (H-h) / 2 × 1.5 or less.

[0063] The present invention is not limited to the configurations disclosed in the first to third embodiments described above. For example, various shapes other than those shown in each embodiment can be applied to the heating coil 2, taking into consideration the shape of the workpiece W and the temperature distribution required for the part to be heated.

[0064] In each embodiment, an example was given in which two terminal portions 61 and 62 are formed on the conductive material 6 of the heating coil 2. In other examples, three or more terminal portions may be formed on the conductive material 6. In this case, the portion of the heating coil 2 used for heating the workpiece W can be switched by selecting the terminal portion to which it is connected to the power supply unit 3. Alternatively, multiple power supply units 3 may be prepared, and each power supply unit 3 may be connected to a different terminal portion.

[0065] In each embodiment, the case where the transport path F is straight is illustrated. In other examples, the transport path F may include curved portions. In this case, at least one of the plurality of first line sections 11 may have a curved portion that curves along the transport path F. Furthermore, the plurality of second line sections 12 and the plurality of third line sections 13 do not necessarily have to be entirely straight, and may also include curved portions.

[0066] 1...Induction heating device, 2...Heating coil, 3...Power supply device, 4...Cooling device, 5...Conveying device, 6...Conductive material, 10...First coil section, 11...First wire section, 12...Second wire section, 13...Third wire section, 20...Second coil section, 30...Connection section, 31...First connecting piece, 32...Second connecting piece, 61...First terminal section, 62...Second terminal section, W...Workpiece.

Claims

1. An induction heating device comprising: a heating coil comprising a first coil section, a second coil section facing the first coil section, and a connecting section connecting the first coil section and the second coil section, wherein the first coil section, the second coil section and the connecting section are made of a conductive material bent to form a plurality of loops; a power supply device that supplies alternating current to the heating coil; and a transport device that transports a workpiece to be heated by the heating coil along a transport path passing between the first coil section and the second coil section, wherein at least one of the first coil section and the second coil section comprises a plurality of first line sections arranged in an array direction intersecting the transport path, and the spacing between the plurality of first line sections differs at least locally.

2. The induction heating apparatus according to claim 1, wherein the connecting portion connects one end of the first coil portion and the second coil portion in the direction of arrangement, and of the plurality of first wire portions, the two closest to the connecting portion in the direction of arrangement are arranged at a first interval, and the two furthest from the connecting portion in the direction of arrangement are arranged at a second interval smaller than the first interval.

3. The induction heating apparatus according to claim 2, wherein the spacing between the plurality of first line portions decreases as it moves away from the connection portion in the direction of arrangement.

4. The induction heating apparatus according to claim 2, wherein the workpiece has a first end and a second end in the arrangement direction, and the conveying device conveys the workpiece such that the first end passes through the space between the first coil portion and the second coil portion, and the second end passes outside the space.

5. The induction heating apparatus according to claim 2, wherein at least one of the first coil section and the second coil section further comprises a plurality of second wire sections connected to the upstream end of the plurality of first wire sections in the transport path, and the plurality of second wire sections are arranged at a third interval smaller than the first interval.

6. The induction heating apparatus according to claim 2, wherein at least one of the first coil section and the second coil section further comprises a plurality of third wire sections connected to the downstream end of the plurality of first wire sections in the transport path, and the plurality of third wire sections are arranged at a fourth interval smaller than the first interval.

7. An induction heating apparatus according to any one of claims 1 to 6, wherein a gap of height H is formed between the first coil portion and the second coil portion in the height direction intersecting the transport path and the arrangement direction, the workpiece has a height h in the height direction, and the distance D in the arrangement direction between the first line portion closest to the connection portion in the arrangement direction and the connection portion is (H - h) / 2 × 1.5 or less.

8. An induction heating method comprising supplying an alternating current to a heating coil comprising a first coil section, a second coil section facing the first coil section, and a connecting section connecting the first coil section and the second coil section, wherein the first coil section, the second coil section and the connecting section are bent to form a plurality of loops, and transporting a workpiece to be heated by the heating coil along a transport path passing between the first coil section and the second coil section, wherein at least one of the first coil section and the second coil section comprises a plurality of first line sections arranged in an array direction intersecting the transport path, and the spacing between the plurality of first line sections differs at least locally.

9. The induction heating method according to claim 8, wherein the connecting portion connects one end of the first coil portion and the second coil portion in the direction of arrangement, and of the plurality of first wire portions, the two closest to the connecting portion in the direction of arrangement are arranged at a first interval, and the two furthest from the connecting portion in the direction of arrangement are arranged at a second interval smaller than the first interval.

10. The induction heating method according to claim 9, wherein the workpiece has a first end and a second end in the arrangement direction, and is transported such that the first end passes through the space between the first coil portion and the second coil portion, and the second end passes outside the space.

11. The induction heating method according to claim 9, wherein the spacing between the plurality of first line portions decreases as it moves away from the connection portion in the direction of arrangement.

12. The induction heating method according to claim 9, wherein at least one of the first coil portion and the second coil portion further comprises a plurality of second wire portions connected to the upstream end of the plurality of first wire portions in the transport path, and the plurality of second wire portions are arranged at a third interval smaller than the first interval.

13. The induction heating method according to claim 9, wherein at least one of the first coil portion and the second coil portion further comprises a plurality of third wire portions connected to the downstream end of the plurality of first wire portions in the transport path, and the plurality of third wire portions are arranged at a fourth interval smaller than the first interval.

14. An induction heating method according to any one of claims 8 to 13, wherein a gap of height H is formed between the first coil portion and the second coil portion in the height direction intersecting the transport path and the arrangement direction, the workpiece has a height h in the height direction, and the distance D in the arrangement direction between the first line portion closest to the connection portion in the arrangement direction and the connection portion is (H - h) / 2 × 1.5 or less.

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