Forming method

Abstract

To locally heat only a punched end while a current value of a heating electrode is stabilized during heating.SOLUTION: A forming and processing method includes a punching step of punching a steel plate, and a heating step of disposing a heating coil so as to face an end face of a punched end punched in the punching step in a non-contact manner along the end face of the punched end and applying a current to the heating coil to generate an induced electromotive force in the steel plate, thereby heating the end face.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for forming a steel sheet. [Background technology]

[0002] One of the problems with press forming of high tensile steel is stretch flange cracking. This stretch flange cracking occurs due to residual strain on the shear end surface, such as the punched end, caused by the punching process. A heating method is already known as a method for reducing this residual strain. In addition, as a method for heating the residual strain portion, a forming method is known in which a heating electrode is brought into contact with the punched end, and the punched end is heated by passing an electric current through it to remove the residual strain (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-116635 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, according to one embodiment of the above-mentioned forming method, the heating electrode is in direct contact with the punched end. Therefore, when the heating electrode repeatedly comes into contact with the punched end due to mass production or the like, the heating electrode is worn. This wear of the heating electrode changes the contact area between the heating electrode and the punched end, and the current value of the heating electrode during heating is not stable. In contrast, according to another embodiment of the above-mentioned forming method, the punched end is sandwiched between a pair of heating electrodes without contact, and electrical heating is performed. In this case, the heating electrode is not worn, but on the other hand, a problem occurs in that the heating range extends beyond the vicinity of the punched end.

[0005] The present invention has been made to solve such problems, and its main object is to provide a molding method that can locally heat only the punched end while stabilizing the current value of the heating electrode during heating. [Means for solving the problem]

[0006] In order to achieve the above object, one aspect of the present invention is to A punching process for punching out steel plates; a heating step of disposing a heating coil along and facing an end surface of the punched end produced by the punching step in a non-contact state, and heating the end surface by passing a current through the heating coil to generate an induced electromotive force in the steel sheet; A molding method comprising the steps of: It is. In this aspect, the heating coil may have a diameter larger than a thickness of the steel plate. In this one aspect, in the punching step, a hole may be formed in the steel plate by punching out the steel plate, and in the heating step, the heating coil may be inserted in the hole and heat an end surface of the hole. In this one aspect, the punching step may be performed by punching the steel plate to form an open punched end having a portion of an end face of the steel plate, and the heating step may be performed by positioning the heating coil along and facing an end face of the open punched end to heat the end face. In this one aspect, in the heating process, the steel plate punched in the punching process is placed in a heating jig and then heated by the heating coil, and the heating jig may have a positioning guide that positions the steel plate punched in the punching process at a predetermined position, and the heating coil that heats an end face of the punched end of the steel plate positioned by the positioning guide. In this embodiment, the heating step may be performed during any step in the press. In one aspect of this method, the method further includes a stretch flange forming process for forming a stretch flange on the punched end, and the heating process may be performed in an idling process for adjusting a conveying pitch when conveying the steel plate from the punching process to the stretch flange forming process. In this one aspect, while the steel plate formed product is continuously transported by a gripping portion within the press machine, at least the punching process and the idling process are continuously performed at a predetermined transport pitch, and the heating jig is positioned at a position corresponding to the idling process, so that the idling process can be replaced with the heating process. In this embodiment, the heating coil may have three turns. In this embodiment, the central axis of the heating coil may be inclined at a predetermined angle with respect to the central axis of the end face of the punched end of the steel plate, and the predetermined angle θ may be set within the range of -15°≦θ≦15°. In this embodiment, the heating coil may be configured as a single pipe-shaped coil wire, and a cooling liquid may flow inside the coil wire. In this one aspect, the heating coil may be formed in a loop shape along an end surface of the hole formed in the steel plate, and ends of the loop may overlap each other. Effect of the Invention

[0007] According to the present invention, it is possible to provide a forming method capable of locally heating only the punched end while stabilizing the current value of the heating electrode during heating. [Brief description of the drawings]

[0008] [Figure 1] 1A to 1C are schematic diagrams illustrating steps of a molding method according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram illustrating a processing step. [Diagram 3] FIG. 2 is a perspective view of a heating coil according to the embodiment. [Figure 4] FIG. 4 is a top view of the heating coil shown in FIG. 3 as viewed from above. [Diagram 5]FIG. 2 is a side view of the heating coil and the steel plate. [Figure 6] FIG. 13 is a diagram showing an example of the configuration of a heating coil having three turns. [Figure 7] FIG. 13 is a diagram showing an example of the configuration of a heating coil having one turn. [Figure 8] FIG. 2 shows a punched end with an open end face. [Figure 9] FIG. 13 is a diagram showing an example of the configuration of a heating coil having two turns. [Figure 10] FIG. 1 is a diagram showing perforations in a steel plate punched by press working. [Figure 11] FIG. 1 is a diagram showing test results of a Vickers hardness test. [Figure 12] FIG. 1 is a diagram showing test results of a Vickers hardness test. [Figure 13] FIG. 2 is a diagram showing a molded product including a stretch flange forming portion. [Figure 14] FIG. 1 is a diagram showing a specific example of a molded product of a FR lower arm. [Figure 15] FIG. 1 is a diagram showing a specific example of a molded part for an A-pillar lower. [Figure 16] FIG. 1 is a diagram showing a specific example of a molded product of a B-pillar outer lower. [Figure 17] FIG. 1 is a diagram showing an example of a heating jig. [Figure 18] 1 is a diagram comparing steps of a molding method according to the present embodiment with steps of a conventional molding method. [Figure 19] FIG. 2 is a diagram showing an example of a process in a press machine. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] EMBODIMENT 1 The present invention will be described below through embodiments of the invention, but the invention according to the claims is not limited to the following embodiments. Furthermore, not all of the configurations described in the embodiments are necessarily essential as means for solving the problems.

[0010] 1 is a schematic diagram illustrating the steps of a forming method according to the present embodiment. The forming method described below involves providing a hole 101 in a steel plate 100 as a workpiece, and deforming the periphery of the hole 101 to form a flange 102.

[0011] The forming method according to this embodiment includes, for example, a punching process of punching out a steel plate 100, a heating process of heating the punched end 103, a cooling process of cooling the heat generated by the heating process, and a stretch flange forming process of forming a flange 102 around the punched end 103, as shown in FIG.

[0012] The punching process is a process in which a steel plate 100 fixed to a punching die (not shown) is punched out by a punch 200. The heating process, which will be described in detail later, is a process in which a punched end 103 formed in the punching process is heated.

[0013] In the heating step, as shown in Fig. 2, the punched steel sheet 100 is placed in a heating jig. Then, high-frequency induction heating is performed on the punched end 103 of the steel sheet 100. After the heating, the steel sheet 100 is removed from the heating jig.

[0014] 1, the cooling process is a process for cooling the punched end 103 heated in the heating process. Specifically, the steel sheet 100 is left in a room temperature environment for a certain period of time. The stretch flange forming process (burring process) is a process for inserting a flange die 300 into the perforation 101 and plastically deforming the peripheral portion of the perforation 101 to form a flange 102.

[0015] Residual strain generated in the peripheral portion of the perforation 101 by the punching process is removed in the heating process. After cooling, the steel sheet 100 is introduced into the stretch flange forming process, so that damage to the flange die 300 can be reduced compared to performing the process on the steel sheet 100 in a heated state. In particular, in this embodiment, as described later, the punched end 103 can be locally heated, so that the residual strain can be effectively removed while the heating efficiency is also improved.

[0016] According to one embodiment of the conventional forming method, the heating electrode directly contacts the punched end. Therefore, when the heating electrode repeatedly contacts the punched end due to mass production, the heating electrode wears out. This wear of the heating electrode changes the contact area between the heating electrode and the punched end, and the current value of the heating electrode during heating is not stable. In contrast, according to another embodiment of the forming method, the punched end is sandwiched between a pair of heating electrodes without contact, and electrical heating is performed. In this case, the heating electrode does not wear out, but on the other hand, a problem occurs in that the heating range extends beyond the vicinity of the punched end.

[0017] In contrast, the forming method according to this embodiment includes a heating step in which, for example, as shown in FIG. 3, a heating coil 1 is disposed in a non-contact state with an end face 104 of a punched end 103 produced by the punching step, facing the end face 104, and a current is passed through the heating coil 1 to generate an induced electromotive force in the steel plate 100, thereby heating the end face 104.

[0018] Fig. 3 is a perspective view of the heating coil according to this embodiment. Fig. 4 is a top view of the heating coil shown in Fig. 3. The heating coil 1 is arranged in a non-contact state with the end surface 104 of the punched end 103 of the steel plate 100 to heat the end surface 104, so that the above-mentioned wear of the heating coil 1 does not occur, and the current value of the heating coil 1 during heating can be stabilized.

[0019] Furthermore, it is no longer necessary to heat the steel plate from above and below while holding it as in the conventional method, and the heating coil 1 can be placed against the end face 104 of the punched end 103 of the steel plate 100 and heated.

[0020] Furthermore, the heating coils 1 are disposed along and facing the end face 104 of the punched end 103, generating an induced electromotive force in the steel sheet 100. This allows an induced current to be generated along the end face 104 of the punched end 103, and only the end face 104 can be locally heated. And, since only the end face 104 of the punched end 103 that forms the stretch flange can be locally heated, softening of the entire steel sheet 100 can be suppressed.

[0021] According to the heating process of this embodiment, the heating range does not expand from the vicinity of the punched end 103, but is limited to a limited range extending outward from the punched end 103 by, for example, about 1 to 2 mm, and only the vicinity of the punched end 103 can be heated locally.

[0022] The punched end 103 of the steel plate 100 is formed into a circular hole shape, for example, as shown in Fig. 4. When the punched end 103 is hole-shaped like this, the induced current flowing through the end surface 104 is connected to form a loop, so that heating can be performed more efficiently.

[0023] When the punched end 103 of the steel plate 100 is formed into a circular hole shape, the heating coil 1 is formed into a substantially circular shape along the hole shape, and is inserted and arranged within the hole shape, as shown in Fig. 4. In this state, the heating coil 1 heats the end surface 104 of the hole. The hole shape of the punched end 103 is not limited to a circular shape, and may be, for example, an elliptical shape, a rectangular shape, a triangular shape, or the like.

[0024] 5 is a side view of the heating coil and the steel plate. The central axis L1 of the heating coil 1 may be inclined at a predetermined angle θ with respect to the central axis L2 of the end face 104 of the punched end 103 of the steel plate 100. The predetermined angle θ is preferably set within the range of -15°≦θ≦15°, and it is more preferable that the absolute value of θ is smaller. In other words, it is most preferable that the central axis L1 of the heating coil 1 is parallel (θ=0) to the central axis L2 of the end face 104 of the punched end 103 of the steel plate 100.

[0025] The number of turns of the heating coil 1 is 2, as shown in Fig. 5. However, the number of turns of the heating coil 1 is most preferably 3, as shown in Fig. 6. This not only enables the entire end face 104 of the punched end 103 to be heated in a short time with optimal inductance, but also effectively suppresses heat conduction in the longitudinal direction of the steel sheet 100, and enables more appropriate local heating of only the vicinity of the end face 104 of the punched end 103.

[0026] The number of turns of the heating coil 1 may be 1 or 4 or more. By increasing the number of turns of the heating coil 1, the inductance of the heating coil 1 can be increased and the magnetic field can be strengthened, thereby shortening the heating time. When the number of turns of the heating coil 1 is 1 as shown in Fig. 7, it is preferable that the diameter of the heating coil 1 is larger than the plate thickness of the steel plate 100. This allows the entire end surface 104 of the punched end 103 to be effectively heated.

[0027] As shown in Fig. 5, it is more preferable that the center C of the heating coil 1 as viewed from the side coincides with the center C of the perforation 101 in the steel sheet 100, but the heating coil 1 may be slightly shifted upward or downward. It is more preferable that the heating coil 1 is shifted upward than downward.

[0028] The heating coil 1 is configured as a single coil wire, but for example, a single coil wire may be formed by bundling a number of thin wires. In consideration of cooling efficiency, it is more preferable to configure the heating coil 1 as, for example, a single pipe-shaped coil wire and run a cooling liquid inside it.

[0029] 4, the heating coil 1 preferably penetrates the entire circumference of the punched end 103 of the steel sheet 100 by overlapping the circular end 11. This allows sufficient induced electromotive force to be generated around the entire circumference of the punched end 103 of the steel sheet 100. Note that the heating coil 1 preferably penetrates at least 3 / 4 or more of the wire length of the punched end 103 of the steel sheet 100.

[0030] The distance d between the heating coil 1 and the end face 104 of the punched end 103 of the steel sheet 100 is preferably equal to or less than twice the diameter of the heating coil 1 (FIG. 4). The distance d between the heating coil 1 and the end face 104 of the punched end 103 of the steel sheet 100 is more preferably as small as possible within a range that does not cause sparks (for example, equal to or greater than the thickness of the steel sheet 100). The heating coil 1 may be covered with an insulator. In this case, the heating coil 1 covered with an insulator and the end face 104 of the punched end 103 of the steel sheet 100 may be in contact with each other.

[0031] The punched end 103 of the steel plate 100 may be an open punched end 103 in which a part of an end face 104 is open, as shown in Fig. 8. The heating coil 1 is formed in a substantially elliptical shape along the shape of the end face 104 of the open punched end 103, and is disposed so as to face the end face 104 of the open punched end 103 along the end face 104 of the open punched end 103.

[0032] Furthermore, even in the configuration of the open punched end 103, the heating coil 1 may have a plurality of turns in order to increase the inductance of the heating coil 1. Fig. 9 is a diagram showing an example of the configuration of a heating coil having two turns.

[0033] The heating temperature by the heating coil 1 is adjusted so that the punched end 103 is, for example, 200°C or higher and lower than the Ac1 point. Heating within this temperature range allows for proper removal of residual strain. In particular, if the steel sheet 100 is heated to the Ac1 point or higher, austenitic transformation occurs in the steel sheet 100, which softens and reduces strength when air-cooled, and increases hardness when rapidly cooled by running water, etc., reducing formability in the stretch flange forming process, so it is preferable to keep the temperature below the Ac1 point.

[0034] Next, the effects of the molding method according to this embodiment will be described in detail. In this embodiment, specifically, tests were carried out under the following conditions.

[0035] As shown in Fig. 10, a 30 mm diameter hole 101 is formed in the center of a 1.2 mm thick steel plate JAC1180 (a zinc-plated steel plate with a tensile strength of about 1180 MPa) by punching using a press. A 20 mm diameter circle is formed in a loop shape using a 5 mm diameter pipe to form a heating coil 1. The heating coil 1 thus constructed is inserted into the punched hole 101. The heating coil 1 is disposed approximately parallel to the steel plate 100.

[0036] A high frequency current of 150 to 400 kHz is applied to the heating coil 1 for one second by feedback control so that the end surface 104 of the perforation 101 (punched end 103), which is the heated portion, is heated to 600°C.

[0037] A Vickers hardness test was performed on the end surface 104 of the perforation 101 heated under the above-mentioned conditions. In this test, the measurements were made as follows.

[0038] Measurements are taken at a position 0.1 mm from the surface in the sheet surface direction. A total of 10 points are measured at 0.2 mm intervals from the edge 104 to 2 mm. A total of 16 points are measured at 0.5 mm intervals from 2 to 10 mm from the edge 104. A load of 300 g is applied.

[0039] Fig. 11 and Fig. 12 are diagrams showing the test results of the above-mentioned Vickers hardness test. Fig. 11 shows the hardness near the edge of the perforation 101 on cross section A, and Fig. 12 shows the hardness near the edge of the perforation 101 on cross section B. In Fig. 11 and Fig. 12, the vertical axis shows Vickers hardness [HV], the horizontal axis shows the distance [mm] from the end face 104, and shows the Vickers hardness [HV] of the back and front surfaces of the steel plate 100.

[0040] 11 and 12, in cross sections A and B, the Vickers hardness gradually increases from about 320 HV as the distance from the end face 104 increases from 0 to 2 mm. When the distance from the end face 104 exceeds 2 mm, the Vickers hardness is about 380 HV.

[0041] In addition, the change in Vickers hardness was almost the same in cross section A and cross section B, and there was no difference between the cross sections. In other words, it can be considered that only the area 2 mm from the end face 104 along the circumferential direction (the area near the hole edge) was uniformly heated and softened.

[0042] It can be seen that the forming method according to this embodiment maintains the base material strength in the area 2 mm or more away from the end face 104, and locally softens only the area near the hole edge within 2 mm away from the end face 104. This makes it possible to ensure product performance without reducing the base material strength while improving stretch flangeability.

[0043] Next, an example of a molded product produced by the molding method according to the present embodiment will be described. The molded product produced by the molding method according to the present embodiment includes a stretch flange forming part, such as a hole expanding part for expanding a hole or a flange forming part for erecting a flange with a curvature, as shown in Fig. 13, and includes a forming part in which the line length of the end face is significantly expanded before and after processing.

[0044] A specific molded product is assumed to be a front engine rear lower arm used in a vehicle suspension, as shown in Fig. 14. In Fig. 14, a bush press-in portion surrounded by a dotted line, a hole enlarging portion such as a work hole, and a flange forming portion such as a crotch portion may be molded by the molding method according to this embodiment.

[0045] In the above-mentioned forming method, the case where the perforations 101 are formed in the steel sheet 100 has been described as an example, but the punching step of punching out the steel sheet 100 is not limited to the case where the perforations 101 are formed, and it may also be the case where unnecessary portions are cut off. In the stretch flange forming step, a flange die 300 is pressed against the punched end 103 from which the unnecessary portions have been cut off to form a stretch flange. As will be described below, the flange forming portions of the A-pillar lower and the B-pillar outer lower are formed in this manner.

[0046] As the molded product, an A-pillar lower used for a window pillar of a vehicle is assumed as shown in Fig. 15. In Fig. 15, a flange molding portion such as a corner portion surrounded by a dotted line may be molded by the molding method according to this embodiment.

[0047] As an example of the other molded product, a B-pillar outer lower of a vehicle is assumed, as shown in Fig. 16. In Fig. 16, a flange molding portion such as a corner portion surrounded by a dotted line may be molded by the molding method according to this embodiment.

[0048] As described above, the forming method according to this embodiment includes a punching step of punching out the steel sheet 100, and a heating step of disposing the heating coil 1 along and in a non-contact state with the end surface 104 of the punched end 103 produced by the punching step, and heating the end surface 104 by passing a current through the heating coil 1 to generate an induced electromotive force in the steel sheet 100. This makes it possible to locally heat only the punched end 103 while stabilizing the current value of the heating coil 1 during heating.

[0049] EMBODIMENT 2 In the present embodiment 2, the steel plate molded product punched out in the punching process is placed in a heating jig in a heating process, and heated. Fig. 17 is a diagram showing an example of a heating jig. The left side of Fig. 17 shows a state before the steel plate molded product X is placed in the heating jig 400, and the right side shows a state after the steel plate molded product X is placed in the heating jig 400.

[0050] 17, the heating jig 400 has a heating coil 1 for heating the punched end 103 of the steel plate product X, a positioning guide 2 for positioning the steel plate product X at a predetermined position, and a base part 3. An AC power source 4 for supplying power to each heating coil 1 is connected to each heating coil 1.

[0051] The shape and arrangement of the positioning guide 2 are set so that the position of the steel plate molded product X is automatically set by simply placing the steel plate molded product X on the positioning guide 2. The heating coil 1 and the positioning guide 2 are arranged on the base part 3 in accordance with the shape of the steel plate molded product X and the position of the heating part.

[0052] The position, number, and shape of the heating coil 1 are not limited to the example shown in Fig. 17, and can be set arbitrarily. Similarly, the position, number, and shape of the positioning guide 2 are not limited to the example shown in Fig. 17, and can be set arbitrarily.

[0053] By using the heating jig 400 according to this embodiment, it is no longer necessary to heat the steel plate molded product X from above and below while holding it, as in the conventional method, and the steel plate molded product X can be simply placed on the heating jig 400 and heated.

[0054] The heating step according to this embodiment may be performed in an idling step between the punching step and the stretch flange forming step, as shown in Fig. 18. The idling step is a step for adjusting the conveying pitch when conveying the steel sheet product X from the punching step to the stretch flange forming step, and is a waiting step in which no processing is performed.

[0055] This makes it possible to efficiently heat the steel sheet formed product X by utilizing the idling process in which the punched steel sheet formed product X was kept waiting after the punching process and before the stretch flange forming process in the past. Therefore, there is no need to introduce an additional heating process, and productivity is improved.

[0056] Furthermore, according to this embodiment, by simply arranging the heating jig 400 in an idling process in a normal press machine, the idling process can be converted into a heating process, so that the number of processes does not increase. Fig. 19 is a schematic diagram showing an example of the processes in a press machine.

[0057] In the press machine 500, for example, the steel sheet formed product X is continuously transported by fingers (gripping parts) 501 shown in the lower part of Fig. 19, while the forming process, punching process, idling process, idling process, and punching process shown in the upper part of Fig. 19 are continuously performed at a predetermined transport pitch. Therefore, the idling process in the press machine 500 can be easily replaced with the heating process by simply placing the heating jig 400 at a position corresponding to the idling process.

[0058] The heating step may be performed during any step in the press machine 500, such as during a molding step or punching step, as long as the heating jig 400 can be placed there.

[0059] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0060] 1 heating coil, 2 positioning guide, 3 base portion, 4 AC power source, 100 steel plate, 101 perforation, 102 stretch flange, 103 punching end, 104 end face, 200 punching punch, 300 flange die, 400 heating jig, 500 press machine, 501 finger

Claims

1. A punching process for punching out steel plates; a heating step of arranging a coil wire of a heating coil in a non-contact manner with an end surface of the punched end produced by the punching step so as to face the end surface along the end surface, and causing a current to flow through the heating coil to generate an induced electromotive force in the steel sheet, thereby heating the end surface; Including, The distance between the coil wire of the heating coil and the end face of the punched end of the steel plate is not more than twice the diameter of the coil wire of the heating coil. Molding processing method.

2. The molding method according to claim 1, A forming method, wherein the diameter of the coil wire of the heating coil is larger than the plate thickness of the steel plate.

3. The molding method according to claim 1 or 2, In the punching step, the steel plate is punched to form a hole in the steel plate, In the heating step, the heating coil is inserted into the hole and heats an end surface of the hole.

4. The molding method according to claim 1 or 2, In the punching step, the steel plate is punched to form an open punched end in which a part of an end surface of the steel plate is opened, In the heating step, the heating coils are disposed along and facing an end surface of the open punched end to heat the end surface.

5. The molding method according to any one of claims 1 to 4, In the heating step, the steel plate punched in the punching step is placed in a heating jig and then heated by the heating coil, The heating jig has a positioning guide that positions the steel plate punched in the punching process at a predetermined position, and the heating coil that heats the end face of the punched end of the steel plate positioned by the positioning guide, in a forming processing method.

6. The molding method according to claim 1, The molding method, wherein the heating step is carried out during a molding step or a punching step in a press.

7. The molding method according to claim 5, The method further includes a stretch flange forming step of forming a stretch flange on the punched end, The forming method, wherein the heating step is performed in an idling step that adjusts a conveying pitch when the steel sheet is conveyed from the punching step to the stretch flange forming step.

8. The molding method according to claim 7, In the press machine, the steel plate formed product is continuously transported by a gripping unit, and at least the punching step and the idling step are continuously performed at a predetermined transport pitch, The molding method includes arranging the heating jig at a position corresponding to the idling step, thereby replacing the idling step with the heating step.

9. The molding method according to any one of claims 1 to 8, A molding method, wherein the heating coil has three turns.

10. The molding method according to any one of claims 1 to 9, The central axis of the heating coil is inclined at a predetermined angle with respect to the central axis of the end surface of the punched end of the steel plate, and the predetermined angle θ is set within the range of −15°≦θ≦15°. Molding processing method.

11. The molding method according to any one of claims 1 to 10, The heating coil is configured as a single pipe-shaped coil wire, and a cooling liquid flows inside the heating coil. Molding processing method.

12. The molding method according to claim 3 or 4, The heating coil is formed in a loop shape along the end surface formed on the steel plate, The looped ends overlap.

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