Mandrel equipped with impeder device, electric resistance welded tube manufacturing device, and manufacturing method of electric resistance welded tube

Abstract

The purpose of the present invention is to appropriately suppress deterioration of and damage to an electric resistance welded tube manufacturing impeder device caused by induction heating, decreases of welding efficiency caused thereby, and obstacles to mandrel breakage prevention and production. A mandrel and an impeder device according to the present invention are an electric resistance welded tube manufacturing mandrel and impeder device provided inside an open tube in which ends melted by an induction current are welded to each other. The mandrel and the impeder device include an impeder core comprising a magnetic material; and an electric resistance welded tube manufacturing mandrel which is positioned inside the impeder core, serves as a support member for the impeder core, and is extended in a predetermined direction. A recess region is formed, in a portion of the impeder core, having a size within a predetermined range along the extension direction of the mandrel, and the size of the recess region in a direction orthogonal to the traveling direction is 20 mm or more and 1 / 3 or less of the length of the outer dimension of the mandrel.

Description

Mandrel equipped with an impeder device, electric resistance welded pipe manufacturing apparatus, and electric resistance welded pipe manufacturing method

[0001] The present invention relates to a mandrel equipped with an impeder device used in the manufacture of electric resistance welded pipe, an electric resistance welded pipe manufacturing apparatus, and a method for manufacturing electric resistance welded pipe.

[0002] Generally, methods for manufacturing metal pipes include a method in which a metal strip is bent and welded to form a tubular shape (electric resistance welded pipes, spiral pipes, etc.), a method in which holes are drilled into a metal billet (seamless pipes), an extrusion method, etc. Of these, electric resistance welded pipes are mass-produced because they are particularly highly productive and can be manufactured inexpensively.

[0003] Electrical resistance welded pipes are manufactured using the following manufacturing method. This manufacturing method will be briefly described with reference to Fig. 18. Fig. 18 is an explanatory diagram that schematically shows the manufacturing method of electric resistance welded pipes. Note that in Fig. 18, the forming rolls are not shown in order to simplify the illustration.

[0004] As shown in FIG. 18 , a metal strip made of steel, stainless steel, or the like is first formed into a cylindrical shape while traveling to form an open pipe (see cross sections A-A to C-C in FIG. 18 ). At this time, a mandrel is inserted into the open pipe through the opening, as shown in the figure. Next, a high-frequency current is passed through the opposing end faces of the open pipe (hereinafter simply referred to as the "ends of the open pipe") across the opening to heat them to their melting temperature. In this state, the end faces of the open pipe are pressure-welded together using a squeeze roll (not shown) to form a tubular shape. As a result of the welding, a weld bead is formed at the welded area, as shown in cross section D-D in FIG. 18 . This weld bead is removed using a cutting tool attached to the tip of the mandrel, and finally, an electric resistance welded pipe having a shape as shown in cross section E-E in FIG. 18 is produced.

[0005] One method of supplying high-frequency current to the end of an open pipe is to provide an induction coil (solenoid coil) along the outer periphery of the open pipe and pass a primary current through this induction coil to directly generate an induced current in the open pipe, as shown in Figure 18. In this case, a high-frequency current of approximately 100 kHz to 400 kHz is typically used as the current passing through the induction coil. Here, to prevent the induced current induced by this high-frequency current from circulating around the inner periphery of the open pipe and not involved in the welding, a ferromagnetic material such as ferrite or electromagnetic steel called an impeder is often placed inside the open pipe (typically around the outer periphery of the mandrel) (the impeder is not shown in Figure 18).

[0006] When an induction current is generated on the surface of an open pipe using an induction coil as described above, the generated magnetic flux penetrates into the open pipe, creating a strong magnetic field at the opening of the open pipe located directly below the induction coil. When a strong magnetic field occurs inside the open pipe, the generated magnetic flux can cause the impedance to saturate or burn out. This can lead to an uncontrollable inner current, which can heat the mandrel connecting the inner bead cutting tool due to induction heating, resulting in its breakage. If this mandrel breakage occurs, stable operation over long periods of time becomes impossible.

[0007] Therefore, as a structure for preventing damage to the mandrel due to heating of the mandrel itself caused by induced current resulting from magnetic flux saturation or burnout of the impeder, Patent Document 1 discloses a structure to which a protector can be applied to prevent burnout of the outer tube provided in the impeder device. According to Patent Document 1, a semi-cylindrical ceramic protector is provided on the outer periphery of at least the upper half of the outer tube, thereby insulating the outer tube and suppressing the occurrence of burnout accidents of the outer tube.

[0008] Furthermore, Patent Document 2 discloses a structure in which the outer periphery of a strength support member provided inside an impedance core is coated with a material with high electrical conductivity to prevent the strength support member from being induction heated. According to Patent Document 2, deterioration of the strength support member due to induction heating is prevented without reducing the cross-sectional area of ​​the impedance core.

[0009] Japanese Utility Model Publication No. 59-8869

[0010] However, if the impeller is damaged by the strong magnetic field inside the open pipe as described above (specifically, if the impeller is exposed to a magnetic field exceeding magnetic saturation), the impeller will lose its magnetism and lose its ability to suppress the inner circumferential current, which may make it unable to block the induced current flowing inside the pipe. As a result, welding efficiency will be significantly reduced, the mandrel will break, and welding will become impossible.

[0011] As described above, Patent Documents 1 and 2 each disclose structures for preventing damage to the outer tube provided on the outside of the impeder and the strength support provided inside the impeder. However, they do not disclose any method for protecting the impeder by focusing on the structure of the impeder core itself. In other words, conventional electric resistance welded pipe manufacturing apparatuses do not address the cause of the problem.

[0012] The present invention has been made in consideration of the above circumstances, and aims to appropriately suppress deterioration and damage to an impeder device for manufacturing electric resistance welded pipes due to induction heating, as well as the resulting decrease in welding efficiency and damage to the mandrel, thereby preventing disruptions to production.

[0013] The gist of the present invention, which was completed in light of the above circumstances, is as follows: (1) A mandrel equipped with an impeder device for manufacturing electric resistance welded pipes, comprising: an impeder device having an impeder core made of a magnetic material; and a mandrel positioned inside the impeder core and elongated in a predetermined direction to serve as a support member for the impeder core, wherein a recessed region is formed in a portion of the impeder core along the elongation direction of the mandrel and has a size within a predetermined range, and the size of the recessed region in a direction perpendicular to the elongation direction is 10 mm or more and one-third or less of the outer circumferential length of the mandrel. (2) The mandrel described in (1), wherein the length of the recessed region in the elongation direction is more than 100 mm and not more than the entire length of the impeder core. (3) The mandrel according to (1) or (2), wherein the impedance device further includes a hollow impedance case disposed outside the impeder core such that a gap exists between the impedance device and the hollow portion of the impeder case, and cooling water is passed through the hollow portion of the impeder case. (4) The mandrel according to any one of (1) to (3), wherein the recessed region is formed by bending downward at least an upper portion of the impeder device. (5) The mandrel according to any one of (1) to (3), wherein the recessed region is a cutout portion formed by removing at least a portion of the impeder core. (6) The mandrel according to (5), wherein the surface of the mandrel is exposed at the bottom of the recessed region. (7) The mandrel according to (6), wherein an electromagnetic shielding material is provided on the exposed surface of the mandrel. (8) The mandrel according to (7), wherein the electromagnetic shielding material is copper mesh. (9) The mandrel according to any one of (6) to (8), wherein a slit extending in the running direction is formed in the exposed portion of the mandrel at the bottom of the recessed region. (10) The mandrel according to (9), wherein the width of the metal portion forming the slit is smaller than the penetration depth of the induced current. (11) The mandrel according to (9) or (10), wherein cooling water flows inside the mandrel and a sealing member is provided in the portion of the mandrel where the slit is formed.(12) The mandrel according to any one of (1) to (11), wherein a through hole penetrating vertically is formed in the mandrel immediately below at least a part or all of the recessed region. (13) The mandrel according to any one of (1) to (12), wherein the impedance core is provided only on the outer periphery of an upper half of the mandrel. (14) An electric-resistance welded pipe manufacturing apparatus having a group of forming rolls for forming a desired electric-resistance welded pipe shape, an induction coil for generating an induced current, and a mandrel for manufacturing electric-resistance welded pipe that is elongated in a predetermined direction, wherein an impeder device having an impeder core made of a magnetic material is provided in a part of the mandrel, and a recessed region is formed in a part of the impeder core with a size within a predetermined range along the elongation direction of the mandrel, the size of the recessed region in the elongation direction is at least (the width of the induction coil + 100 mm) and is not more than the total length of the impeder core, and the size of the recessed region in a direction perpendicular to the elongation direction is at least 10 mm and is not more than 1 / 3 of the outer circumferential length of the mandrel. (15) A method for manufacturing an electric-resistance welded pipe, in which ends of an open pipe that has been bent into a cylindrical shape while being transported in a predetermined running direction are melted by induced current and then the ends are butted together and electric-resistance welded, comprising: disposing the mandrel according to any one of (1) to (13) inside the open pipe so that the recessed region faces the end of the open pipe and so that at least a portion of an arrangement position of an induction coil for generating the induced current is encompassed by the recessed region in the extension direction of the mandrel. (16) A method for manufacturing an electric-resistance welded pipe according to (15), in which, in a cross section of the open pipe, the impeder device, and the mandrel taken in a radial direction of the open pipe, a distance between an upper end of the impeder device in the recessed region and a lower end of the end of the open pipe is greater than a distance between an upper end of the impeder device and a lower end of the end of the open pipe upstream of the recessed region in the running direction.(17) A method for manufacturing an electric-welded pipe described in (15) or (16), wherein the length of the recessed region in the extension direction is a length that can encompass a range from a position 50 mm upstream of the induction coil to a position 50 mm downstream of the induction coil, based on a position directly below the induction coil that generates the induced current.

[0014] As described above, according to the present invention, deterioration and damage to an impeder device for manufacturing electric resistance welded pipes due to induction heating can be appropriately suppressed.

[0015] 6 is a schematic diagram showing an outline of the configuration of a mandrel equipped with a conventional impeder device used in the production of electric-resistance welded pipe.

[0033] FIG. 6 is a schematic diagram showing an outline of the A-A cross section of a mandrel equipped with a conventional impeder device used in the production of electric-resistance welded pipe.

[0034] FIG. 6 is a side view showing an outline of the configuration of a conventional electric-resistance welded pipe manufacturing apparatus.

[0035] FIG. 6 is a cross-sectional view showing an outline of the A-A cross section of the configuration of a conventional electric-resistance welded pipe manufacturing apparatus.

[0036] FIG. 6 is a side view showing an outline of the configuration of a mandrel equipped with an impeder device according to a first embodiment of the present invention.

[0037] FIG. 6 is a cross-sectional view showing an outline of the A-A cross section of the configuration of a mandrel equipped with an impeder device according to the same embodiment.

[0038] FIG. 6 is a perspective view showing an outline of the configuration of an electric-resistance welded pipe manufacturing apparatus according to the same embodiment.

[0039] FIG. 6 is a side view showing an outline of the configuration of an electric-resistance welded pipe manufacturing apparatus using a mandrel equipped with an impeder device according to the same embodiment.

[0039] FIG. 6 is a cross-sectional view showing an outline of the A-A cross section of the configuration of an electric-resistance welded pipe manufacturing apparatus using a mandrel equipped with an impeder device according to the same embodiment.

[0039] FIG. 6 is a side view showing a modified example of the impeder device equipped in the electric-resistance welded pipe manufacturing apparatus of FIG. 16 is a cross-sectional view of the A-A section illustrating the circumferential width of the recessed region in the impeder device according to the embodiment. FIG. 17 is a cross-sectional view showing a modified example of the impeder device according to the embodiment. FIG. 18 is a side view showing a modified example of the impeder device according to the embodiment. FIG. 19 is a top view showing a modified example of the impeder device according to the embodiment. FIG. 19 is a cross-sectional view of the A-A section of the impeder device shown in FIG. 13. FIG. 19 is a partially enlarged view showing a modified example of the mandrel and impeder device according to the embodiment. FIG. 19 is a partially enlarged view showing a modified example of the mandrel and impeder device according to the embodiment. FIG. 19 is a side view showing a modified example of the mandrel and impeder device according to the embodiment. FIG. 19 is a side view showing a schematic outline of the configuration of an electric-resistance welded pipe manufacturing apparatus according to a second embodiment of the present invention. FIG. 19 is a side view showing a modified example of the mandrel and impeder device provided in the electric-resistance welded pipe manufacturing apparatus of FIG. 16. FIG. 19 is an explanatory view showing a schematic view of how an electric-resistance welded pipe is manufactured.

[0016] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0017] In addition, in the description of this specification, the terms "upstream" and "downstream" refer to "upstream" and "downstream", respectively, in the running direction of a metal strip plate or an open pipe, which will be described later.

[0018] (Conventional Electric-Resistance Welded Pipe Manufacturing Apparatus) First, a conventional electric-resistance welded pipe manufacturing apparatus will be described. Figures 1A and 1B are schematic diagrams showing the outline of the configuration of a mandrel having a conventional impeder device used in manufacturing electric-resistance welded pipe, and Figures 2 and 3 are a side view and a cross-sectional view, respectively, showing the outline of the configuration of a conventional electric-resistance welded pipe manufacturing apparatus 1.

[0019] In the production of electric resistance welded pipes as described above, a mandrel 3 having an impeder device 1 as shown in Figures 1A and 1B has been used. Here, the impeder device 1 is a device that is installed on the mandrel 3 as needed. As shown in Figures 1A and 1B, the impeder device 1 has a magnetic core called an impeder core 2.

[0020] The mandrel 3 is a member extending in the z-axis direction in FIG. 1A and is cantilevered, for example, upstream of the welding point V. The mandrel 3 is an annular member made of a strong, non-magnetic material such as stainless steel SUS304. The annular mandrel 3 may have a solid structure or a hollow structure. Furthermore, a core 3a may be provided inside the hollow mandrel 3 as shown in FIG. 1B, or cooling water for cooling the impedancer device 1 may flow through it.

[0021] Furthermore, a magnetic core called an impeder (i.e., impeder core 2) is provided in a predetermined area around the outer periphery of the mandrel 3 to prevent the flow of induced current generated by the induction coil 8 that is not involved in welding and that tends to circulate around the inner periphery of the open tube 9. The impeder core 2 is made of a ferromagnetic material such as soft ferrite, electromagnetic steel, or amorphous alloy.

[0022] The impeder core 2 is housed in an impeder case 6 made of, for example, a resin material, and is protected from damage such as breakage. Cooling water, for example, is supplied to the inside of the impeder case 6, and the cooling water cools the impeder core 2. In the following description, the rollers 4 and the impeder case 6 are omitted from the illustrations for the sake of simplicity.

[0023] The impeder device 1 is inserted into the open pipe 9. The tip of the mandrel 3 is provided with rollers 4 to facilitate insertion of the impeder device 1 into the open pipe 9, and cutting tools 5 to cut the weld bead extending along the running direction of the open pipe. The rollers 4 press against the cutting tools 5, stabilizing the cutting of the weld bead by the cutting tools 5. The cutting tools 5 are attached to the tip of the mandrel 3 downstream of the welding point V, and cut off the weld bead that forms on the inner surface of the pipe during welding. There is also a cutting tool (not shown) on the outer surface of the pipe that cuts off the weld bead that forms on the outer surface of the pipe.

[0024] 2 and 3, in the electric resistance welded pipe manufacturing apparatus 7, a metal strip to be welded is gradually bent from a flat state by forming rolls (not shown in FIGS. 2 and 3) while traveling, and roll-formed into a cylindrical shape into a cylindrical open pipe 9. Opposing ends 9b of the open pipe 9 are pressed together through an opening 9a by a squeeze roll (not shown in FIGS. 2 and 3), which is one of the components constituting the electric resistance welded pipe manufacturing apparatus 1. As a result, the open pipe 9 is passed through so that it comes into contact at the welding point V, and typically, a head roll (not shown) presses down on this weld from diagonally above.

[0025] An induction coil 8 is provided on the outer periphery of the open pipe 9, upstream of the welding point V in the traveling direction R, to generate an induction current for melting the end 9b of the open pipe 9. The induction coil 8 is provided so as to surround the outer periphery of the open pipe 9. The end 9b of the open pipe 9 is melted by the induced current and then pressure-welded by the squeeze roll, thereby being formed into a tubular shape. Note that the induction coil 8 is not shown in FIG. 3 .

[0026] As described above, the induction coil 8 in the conventional electric resistance welded pipe manufacturing apparatus 7 is wound around the entire circumference of the open pipe 9. In other words, the induction coil 8 is disposed so as to straddle the opening 9a of the open pipe 9. In such a case, the high-density magnetic flux F generated by the induction coil 8 directly enters the impedance device 1, which may cause magnetic flux saturation in the impedance core 2 and result in burnout.

[0027] According to the research conducted by the present inventors, it was found that the burning of the impedance core 2 is caused by the following two points in particular: (1) As shown in Fig. 3, magnetic flux F that enters the inside of the open tube 9 from the opening 9a enters the impedance core 2 directly, causing magnetic flux saturation. (2) As shown in Fig. 3, because the impedance core 2 is located near the lower end of the end 9b, the magnetic flux F generated by the induced current flowing through the end 9b increases the magnetic flux density, causing magnetic flux saturation.

[0028] When the magnetic flux is saturated, the impeder device 1 cannot suppress the inner circumferential current, and the induced current flowing around the inner circumferential surface of the tube accelerates the generation of heat, causing the temperature to exceed the Curie point at which the impeder core 2 loses its magnetic properties. If the material of the impeder core 2 is soft ferrite, the heat generation will cause cracks due to thermal stress, rendering the impeder core unusable. If the material of the impeder core 2 is electromagnetic steel, the heat generation will cause melting, rendering the impeder core unusable.

[0029] Here, ferromagnetic materials such as soft ferrite, which are the material of the impedance core 2, have the characteristic of being able to block the flow of induced current when placed near the induced current. On the other hand, these ferromagnetic materials have an initial permeability more than 1,000 times that of air (magnetic resistance is 1 / 1,000 or less), so they also have the property of collecting magnetic flux. As a result, magnetic flux selectively passes through the ferromagnetic material (impedance core 2) with low magnetic resistance, which tends to result in a particularly high magnetic flux density.

[0030] Furthermore, as described above, the mandrel 3 is cantilevered on the upstream side in the traveling direction R. Therefore, in order to bring the cutting tool 5 into contact with the inner bead, the position of the impedance core 2 may be forced to be eccentric upward inside the open tube 9. As a result, the impedance core 2 may be brought closer to the induction coil 8. As such, the distance between the impedance core 2 and the induction coil 8, which is the magnetic flux generating source, and the distance between the impedance core 2 and the lower end of the end 9b, tend to be small, which further increases the magnetic flux density.

[0031] Therefore, the inventors have investigated a method for reducing the magnetic flux F entering the impedance core 2 to below the saturation magnetic flux density and maintaining this state below the saturation magnetic flux density. Specifically, since the region of high magnetic flux density in the impedance core 2 is often concentrated in a specific region near the lower end of the end 9b (more specifically, the region on the upper side of the impedance core 2), the inventors have investigated a method for reducing the magnetic flux density in this region.

[0032] As a result, the inventors have noticed that by utilizing the fact that magnetic flux density is inversely proportional to the square of the distance from the magnetic flux source, it is possible to use the impedance core 2 without magnetic flux saturation. Specifically, they have discovered that the magnetic flux density of the impedance core 2 can be reduced by increasing the distance between the impedance core 2 and the induction coil 8 and the lower end of the end 9b only in the region of the impedance core 2 close to the induction coil 8 where the magnetic flux density is high, compared to the rest of the region. Below, a preferred embodiment of the present invention, which was completed based on this discovery, is described.

[0033] (First embodiment) <Configuration of a mandrel having an impedance device> First, the configuration of a mandrel having an impedance device according to a first embodiment of the present invention will be described with reference to Figures 4A and 4B. Figure 4A is a side view showing an outline of the configuration of a mandrel having an impedance device according to this embodiment, and Figure 4B is a cross-sectional view taken along line A-A of the mandrel having the impedance device shown in Figure 4A.

[0034] 4A and 4B, the impeder device 110 according to this embodiment includes an impeder core 111 and an impeder case 6. The mandrel 3 is provided with a roller 4 and a cutting tool 5.

[0035] Here, the mandrel 3, roller 4, cutting bit 5, and impedance case 6 in the impedance device 110 of this embodiment are similar to the mandrel 3, roller 4, cutting bit 5, and impedance case 6 in the conventional impedance device 1 shown in Figure 1, so description thereof will be omitted below.

[0036] The impedance core 111 in this embodiment is provided within a predetermined range on the outer periphery of the mandrel 3, and is provided to prevent the induced current generated by the induction coil from flowing around the inner surface of the open pipe, which is not involved in welding.

[0037] An insulating ferromagnetic material is used for the impedance core 111. In this specification, the term "ferromagnetic material" refers to a material having an initial magnetic permeability of 1000 or more. Furthermore, a ferromagnetic material having insulating properties refers to a material having an electrical conductivity of 1×10 －３ This means that the resistance is S / m or less.

[0038] Materials that can be used for the impedance core 111 according to this embodiment include, for example, soft ferrite, electromagnetic steel, amorphous alloy, and the like, which satisfy the above-mentioned initial permeability.

[0039] In addition, as shown in Figure 4A, a recessed region 113 is formed in a part of the impedance core 111 according to this embodiment, with a size within a predetermined range, along the extension direction of the mandrel 3 (the z-axis direction in Figure 4A).

[0040] The impedance device 110 according to this embodiment is used together with the induction coil 8. The range in which the recessed region 113 is formed (more specifically, the range in the z-axis direction shown in FIG. 4A ) is set in consideration of the range affected by the magnetic flux generated by the induction coil 8, as will be described again below.

[0041] For example, in the impedance core 111 according to this embodiment, it is preferable that the size of the range in the z-axis direction in which the recessed region 113 is provided (length L1 in FIG. 4A) is greater than 100 mm and less than the total length of the impedance core 111.

[0042] More specifically, the size of the range in the z-axis direction in which the recessed region 113 is provided depends on the size of the induction coil 8 used, as described above, but it is preferable to design it to be, for example, the length of the induction coil 8 + 100 mm or more and the length of the induction coil 8 + 300 mm or less.

[0043] On the other hand, the size of the recessed region 113 in the direction (x-axis direction in FIG. 4A) perpendicular to the elongation direction (z-axis direction in FIG. 4A) of the mandrel 3 (more specifically, the length in the x-axis direction at the bottom surface of the recessed region 113, length L2 in FIG. 4B) is set to 10 mm or more and 1 / 3 or less of the outer circumferential length of the mandrel 3. This size roughly corresponds to the width of the opening 9a in the open pipe 9 used in the production of electric resistance welded pipes.

[0044] 4A and 4B , the recessed region 113 is formed by removing the impedance core 111 in the thickness direction of the mandrel 3, for example, from a portion of the upper part of the impedance core 111. In other words, the recessed region 113 according to this embodiment can be said to be a cutout portion formed by cutting out at least a portion of the impedance core 111.

[0045] The depth of the recessed region 113 formed in the impedance core 111 is not particularly limited, and may be formed by removing a portion of the impedance core 111 to reduce the thickness of the impedance core 111, or by removing the relevant portion of the impedance core 111 to a depth that is approximately equal to the thickness (i.e., removing the entire impedance core 111 in that portion). Also, it is possible to avoid providing the impedance core 111 in that portion from the beginning.

[0046] 4A and 4B show a case where the recessed region 113 is removed from the impedance core 111 to a depth that is approximately equal to the thickness of the impedance core 111. In such a case, the surface of the mandrel 3 is exposed at the bottom of the recessed region 113.

[0047] Forming such a recessed region 113 in the upper portion (portion on the positive y-axis side) of the impedance core 111 makes it possible to increase the distance between the impedance core 111 and the lower end of the induction coil 8 and the end 9b of the open tube 9 in the recessed region 113. As will be explained again below, the range where the recessed region 113 is provided is a portion where high-density magnetic flux concentrates, and therefore, forming such a recessed region 113 makes it possible to reduce the magnetic flux density of the impedance core 111. This makes it possible to appropriately suppress deterioration and damage to the impedance core 111 due to induction heating.

[0048] As shown in FIGS. 4A and 4B, the impedance core 111 according to this embodiment is housed in an impedance case 6 made of, for example, a resin material, and is protected from damage due to breakage or the like.

[0049] 4B, the impedance case 6 is preferably provided outside the impedance core 111 so that there is a gap between the impedance case 6 and the impedance core 111. In this case, it is preferable that cooling water be passed through the hollow portion of the impedance case 6 (the gap between the impedance core 111).

[0050] As explained above, the recessed region 113 according to this embodiment is formed by removing at least a portion of the impedance core 111 to reduce the thickness of the impedance core 111 in the recessed region 113 (or, in some cases, removing the entire portion). By passing cooling water through the hollow portion of the impedance case 6, more cooling water flows through the portion where the recessed region 113 is formed than through portions other than the recessed region 113, making it possible to cool the portion where the recessed region 113 is formed more efficiently. This makes it possible to more effectively prevent heat generation in the recessed region 113 and to more effectively prevent deterioration and damage to the impedance core 111.

[0051] <Configuration of Electric Welded Pipe Manufacturing Apparatus 100> Next, the configuration of an electric welded pipe manufacturing apparatus 100 using an impeder device 110 according to this embodiment will be described with reference to Figures 5 to 8. Figures 5, 6, and 8 are side views showing an outline of the configuration of the electric welded pipe manufacturing apparatus 100. Figure 7 is a cross-sectional view taken along the line A-A of the electric welded pipe manufacturing apparatus 100 shown in Figure 6. In the following description, for the sake of simplicity, the rollers 4 provided on the mandrel 3 and the impeder case 6 of the impeder device 110 are not shown. In addition, in the configuration of the electric welded pipe manufacturing apparatus 100, elements having substantially the same functional configuration as those of the electric welded pipe manufacturing apparatus 7 shown in Figures 1A to 3 are designated by the same reference numerals, and redundant description will be omitted.

[0052] As shown in FIG. 5 , the electric-resistance welded pipe manufacturing apparatus 100 according to this embodiment is an apparatus used to manufacture electric-resistance welded pipes. The apparatus melts the ends of an open pipe 9 that has been bent into a cylindrical shape while being transported in a predetermined running direction using an induced current, and then butts and welds the ends together. In addition to the impeder device 110 (not shown in FIG. 5 ) described above, the electric-resistance welded pipe manufacturing apparatus 100 also includes a forming roll group 101 for forming the desired electric-resistance welded pipe shape and an induction coil 8. Also, as shown in FIG. 5 , a pair of squeeze rolls 103 are provided downstream of the induction coil 8. The open pipe 9 is made of, for example, steel or stainless steel. The following description of the electric-resistance welded pipe manufacturing apparatus 100, as shown in FIG. 5 , focuses particularly on the induction coil 8 and the impeder device 110.

[0053] As shown in Figures 6 and 7, the electric resistance welded pipe manufacturing apparatus 100 according to this embodiment has an induction coil 8 arranged along the outer periphery of the open pipe 9, and an impedancer device 110 attached to the mandrel 3 arranged inside the open pipe 9.

[0054] As described above, the recessed region 113 is formed in a portion of the impedance core 111 according to this embodiment. The region in which this recessed region 113 is formed will be explained again below, focusing on its relationship with the induction coil 8 that constitutes the electric resistance welded pipe manufacturing apparatus 100.

[0055] That is, the region where the recessed region 113 is formed in the impedance core 111 according to this embodiment preferably corresponds to the region where the magnetic flux density is high, as described above. Therefore, the impedance device 110 according to this embodiment is disposed inside the open tube 9 so that the recessed region 113 faces the end 9 b of the open tube 9 and at least a portion of the position where the induction coil 8 is disposed is encompassed by the recessed region 113 in the extension direction of the mandrel 3, as shown in FIG.

[0056] Specifically, the recessed region 113 is preferably formed to include at least a part of a range X2 directly below the induction coil 8 that is particularly strongly affected by the magnetic flux, within the range from approximately 50 mm upstream of the induction coil 8 to the welding point V in the running direction R of the open pipe 9 (hereinafter referred to as the "magnetic flux affected range X1"). For example, the recessed region 113 is preferably formed to have a length that can include the range from a position 50 mm upstream of the induction coil 8 to a position 50 mm downstream of the induction coil 8, with the position directly below the induction coil 8 as the reference position.

[0057] In this case, the recessed region 113 may be formed over the entire magnetic flux affected range X1, as shown in FIG. 6, or may be formed only in the range X2 directly below and its upstream side (the negative z-axis side in FIG. 8), as shown in FIG.

[0058] That is, in this embodiment, as shown in Figures 6 and 7, the impedance device 110 is configured so that the separation distance H1 between the upper end of the impedance device 110 in the portion where the recessed region 113 is formed and the lower end of the end 9b of the open tube 9 is greater than the separation distance H2 between the upper end of the impedance device 110 in the portion where the recessed region 113 is not formed (in this embodiment, for example, upstream of the recessed region 113) and the lower end of the end 9b of the open tube 9.

[0059] In addition, the upper end of the impedance device 110, in the case shown in Figure 6 as an example, refers to, for example, the upper end of the exposed mandrel 3 in the portion where the recessed region 113 is formed, and refers to the upper end of the impedance core 111 in the portion where the recessed region 113 is not formed (upstream of the recessed region 113).

[0060] According to this embodiment, the recessed region 113 is formed in the impeder core 111 so as to include at least the directly below range X2, thereby increasing the distance between the impeder core 111 and the lower end of the end 9b of the induction coil 8 or the open tube 9, which is a magnetic flux generating source. This prevents magnetic flux F from entering the impeder core 111 and attenuates the magnetic flux F entering the impeder core 111, thereby preventing magnetic flux saturation in the impeder core 111. As a result, the impeder device 110 according to this embodiment suppresses heat generation in the impeder core 111 and damage to the impeder core 111 and the impeder device 110, allowing stable production of electric resistance welded pipes to continue for a long period of time. Furthermore, this reduces the number of downtimes of the production line due to replacement of the impeder device 110, thereby reducing the time and cost required for maintenance of the impeder device 110.

[0061] 9, the circumferential width Y1 of the recessed region 113 in the impedance core 111 is preferably at least the same as the opening width Y2 of the opening 9a of the open tube 9 located directly above it, and more preferably is larger than the opening width Y2. By forming the width Y1 of the recessed region 111 to be the same as or more preferably larger than the opening width Y2 of the opening 2a, it is possible to more appropriately prevent the magnetic flux F from entering the impedance core 111.

[0062] According to this embodiment, the recessed region 113 is preferably formed by removing the impedance core 111 by a width Y1 that is at least equal to or greater than the opening width Y2 of the opening 9 a above the impedance core 111. According to this embodiment, even if the impedance core 111 is not provided along the entire outer circumferential surface of the mandrel 3, the induced current that sneaks around from the end 9 b can be appropriately suppressed.

[0063] This point will be explained with reference to FIG. 10 . FIG. 10 is a cross-sectional view showing a modified example of the impedance device 110 according to this embodiment attached to the mandrel 3. After extensive study, the inventors have found that the induced current that flows around from the end 9 b to the inner periphery of the open tube 9 can be suppressed if the impedance core 111 is provided on a portion of the outer periphery of the mandrel 3 (more specifically, at least the upper half of the mandrel 3). That is, the impedance core 111 may be provided only on the outer periphery of the upper half of the impedance device 110 (i.e., only in a region that extends in an arc shape by half the circumference), as shown in FIG. 10 . This makes it possible to appropriately suppress the induced current that flows around from the end 9 b to the inner periphery of the open tube 9, and also reduces the mass and cross-sectional area of ​​the impedance device 110.

[0064] The modified example shown in FIG. 10 is particularly useful when the impeder device 110 according to this embodiment is applied to an electric-resistance welded pipe manufacturing apparatus for manufacturing electric-resistance welded pipes with small diameters. That is, when the diameter of the electric-resistance welded pipe to be manufactured is small, the diameter of the mandrel 3 provided in the open pipe 9 also becomes small. Therefore, unless the mass of the impeder core 111 attached to the mandrel 3 is reduced, problems such as bending of the mandrel 3, preventing effective bead cutting, and reducing the impeder's effectiveness may occur. Therefore, by arranging the impeder core 111 only in a portion where the impeder functions effectively (in the example of FIG. 10 , only the outer periphery of the upper half of the mandrel 3), the weight of the impeder core 111 can be reduced, thereby preventing the mandrel 3 from bending. This allows for effective bead cutting and prevents the impeder's effectiveness from being reduced, even when manufacturing electric-resistance welded pipes with small diameters.

[0065] As shown in FIG. 11 , an electromagnetic shielding material 120 for protecting the mandrel 3 may be provided on the surface of the mandrel 3 exposed by removing the impedance core 111 (in other words, the portion of the impedance core 111 where the recessed region 113 is formed). Like the mandrel 3, the electromagnetic shielding material 120 is preferably a non-magnetic material. Examples of materials that can be used for the electromagnetic shielding material 120 include an insulated copper plate or copper mesh, carbon, and carbon fiber. The electromagnetic shielding material 120 preferably has a structure and shape that makes it less likely to generate heat due to a magnetic field, such as a braided wire mesh with an insulating surface. Instead of the electromagnetic shielding material 120, an additional impeder may be provided on the exposed surface of the mandrel 3 (the portion of the impedance core 111 where the recessed region 113 is formed) so as to cover the recessed region 113.

[0066] 12, a through-hole 3b may be formed vertically through at least a portion or the entire length of the mandrel 3 exposed by forming the recessed region 113. By forming the through-hole 3b in this manner, the magnetic flux F is less likely to pass through a space with high magnetic resistance. As a result, more magnetic flux enters the end 9b of the open tube 9, and heat generation in the mandrel 3 can be further suppressed.

[0067] 12 illustrates an example in which the through hole 3b is formed when the impedance core 111 is installed in the upper half portion along the outer circumferential surface of the mandrel 3. However, even when the impedance core 111 is installed around the entire circumference of the mandrel 3, as shown in FIG. 7, for example, the through hole 3b can be formed by removing the impedance core 111 from the upper and lower portions of the impedance device 110 where the through hole 3b is to be formed. In such a case, by extending the impedance core 111 to the lower portion of the mandrel 3, damage to the impedance core 111 and the mandrel 3 can be prevented, and the induced current that flows around the inner periphery of the open tube 9 can be further suppressed.

[0068] 13 and 14, the mandrel 3 exposed by removing the impedance core 111 may be cut out in part to form a plurality of slits 3d spaced apart from one another and sandwiching a lattice 3c which is the metal portion of the remaining mandrel 3. Therefore, in the portion of the mandrel 3 where the slits 3d are formed, the lattice 3c where the mandrel 3 remains and the slits 3d where the mandrel 3 is cut out are arranged alternately along the circumferential direction of the mandrel 3.

[0069] The slit 3d is preferably formed so as to cover at least the range X2 directly below the open tube 9 in the running direction R, where the influence of the magnetic flux is strong, and it is more preferable that the slit 3d is formed so as to extend within a range of approximately 50 mm before and after the range X2 directly below the open tube 9 in the running direction R.

[0070] Furthermore, the width Y3 and depth H3 (see FIG. 14 ) of each lattice 3 c, which is the metal portion formed between the slits 3 d, need only be large enough so that the induced current from the openings 9 a and the ends 9 b does not form a closed circuit. In other words, the width Y3 and depth H3 of each lattice 3 c are not particularly limited as long as they are equal to or less than twice the penetration depth of the induced current, which is determined by the current frequency and the conductivity and initial permeability of the mandrel 3. The penetration depth of the induced current can be calculated based on the following formula (1):

[0071] δ=5.03×(ρ / μf) ０．５ ...Equation (1) In the above equation (1), δ: penetration depth (unit: cm), ρ: specific resistivity of the mandrel 3 (i.e., the reciprocal of the conductivity) (unit: μΩ·cm), μ: initial permeability of the mandrel 3, and f: frequency of the current (unit: Hz).

[0072] If the mandrel 3 is made of steel, the resistivity of the mandrel 3 is approximately 20 μΩ·cm at room temperature and approximately 128 μΩ·cm at 1000°C. Since stainless steel, a non-magnetic material, is often used as the material for the mandrel 3, the initial permeability of the mandrel 3 can be treated as 1. If the mandrel 3 is made of a magnetic material, the initial permeability changes depending on the strength of the magnetic field. For example, if the mandrel 3 is made of steel, it will exhibit a value of approximately 20 to 1000 upon heating.

[0073] 14, the formation width Y3 of the grid 3c is the remaining width in the circumferential direction of the mandrel 3 in a cross-sectional view, and can also be rephrased as the formation interval of the slits 3d. Therefore, in this embodiment, the formation interval Y3 of the slits 3d is not particularly required to be uniform as long as it is equal to or less than twice the penetration depth of the induced current and does not form a closed circuit.

[0074] In the mandrel 3 having the slits 3d formed therein, the induced currents that enter each slit 3d from the openings 9a and ends 9b and flow along the surface of the lattices 3c are offset and attenuated by being in opposite phase to each other on the surfaces of adjacent lattices 3c. As a result, in the mandrel 3 having the slits 3d formed therein, damage to the impedance core 111 and the mandrel 3 due to the induced current can be more effectively prevented, while the induced current that flows around to the inner periphery of the open tube 9 can be more effectively suppressed.

[0075] Note that when slits 3d are formed in mandrel 3 in this manner and cooling water is passed through mandrel 3 instead of core material 3a, the formation of slits 3d may cause the cooling water to leak out of mandrel 3. Therefore, if it is desired to prevent the leakage of cooling water when cooling water is passed through mandrel 3, slits 3d may be sealed with a sealing material 115 that does not cause dielectric loss, such as an adhesive or resin, as shown as an example in Figure 15A. In this case, sealing material 115 does not need to completely fill the entire length of depth H3 of slit 3d. As shown in Figures 15B and 15C, it is sufficient that sealing material 115 is present in at least a portion of depth H3 of slit 3d, as long as it is sufficient to prevent the leakage of cooling water from the inside.

[0076] Although not shown in the figures, instead of filling the slit 3d with the sealing material 115 in this manner, an insulating film / layer or the electromagnetic shielding material 120 shown in Figure 11 may be installed along at least one of the outer and inner surfaces of the mandrel 3, in other words, so as to block at least one of the upper and lower parts of the slit 3d.

[0077] Therefore, at least one of a sealing material 115 as a sealing member to prevent cooling water from leaking out from inside the mandrel 3, an insulating film / layer, or an electromagnetic shielding material 120 may be provided in the portion of the mandrel 3 where the slit 3d is formed.

[0078] Second Embodiment In the first embodiment described above, the recessed region 113 is formed by removing a portion of the impedance core 111 so as to reduce the thickness of the impedance core, or by completely removing the impedance core 111. However, the method for forming the recessed region 113 is not limited to such examples. A second embodiment of the present invention will now be described.

[0079] The configuration of an electric-resistance welded pipe manufacturing apparatus 200 having an impeder device 210 according to a second embodiment of the present invention attached to a mandrel 3 will be described below with reference to Figures 16 and 17. Figures 16 and 17 are side views showing an outline of the configuration of an electric-resistance welded pipe manufacturing apparatus 200 having an impeder device 210 according to this embodiment attached to a mandrel 3. Note that in the configuration of the electric-resistance welded pipe manufacturing apparatus 200, elements having substantially the same functional configuration as the electric-resistance welded pipe manufacturing apparatus 1 or the electric-resistance welded pipe manufacturing apparatus 100 are designated by the same reference numerals, and redundant description will be omitted.

[0080] 16, an electric-resistance welded pipe manufacturing apparatus 200 according to a second embodiment of the present invention has, similar to the electric-resistance welded pipe manufacturing apparatus 100 shown in Fig. 5, a group of forming rolls (not shown in Fig. 16), an induction coil 8 provided along the outer periphery of an open pipe 9, and an impedancer device 210 provided inside the open pipe 9. The impedancer device 210 also has an impedancer core 211.

[0081] Similar to the recessed region 113 according to the first embodiment, a recessed region 213 is formed in the impedance core 211 in the region of high magnetic flux density described above. Specifically, the recessed region 213 is formed in the running direction R of the open tube 9 so as to include at least a part of the magnetic flux affected range X1, that is, the range X2 directly below the induction coil 8, which is particularly strongly affected by the magnetic flux.

[0082] 16 , the recessed region 213 is formed by, for example, bending the mandrel 3 and the impedance core 211 downward within an arbitrary range in the magnetic flux affected range X1. In other words, the recessed region 213 formed in the impedance core 211 can be rephrased as a curved region of the impedance core 211.

[0083] The depth of the recessed region 213, i.e., the curvature depth of the impedance core 211, is set to be approximately equal to the thickness of the impedance core 211, as in the recessed region 113 according to the first embodiment, for example.

[0084] According to this embodiment, the impedance core 211 is curved to form the recessed region 213 within a range including at least a portion of the range X2 directly below the induction coil 8. This increases the distance from the induction coil 8, which serves as a magnetic flux generating source, or the lower end of the end 9b, compared to the upstream side, thereby attenuating the magnetic flux entering the impedance core 211. This also prevents the impedance core 211 from becoming saturated with magnetic flux, thereby suppressing heat generation in the impedance core 211 and damage to the impedance core 211 and the impeder device 210, allowing stable production of electric resistance welded pipes to continue for a long period of time. This also reduces the number of production line downtimes due to replacement of the impeder device 210, thereby reducing the time and cost required for maintenance of the impeder device 210.

[0085] Furthermore, according to this embodiment, unlike the first embodiment, the recessed region 213 can be formed without removing the impedance core 211. Therefore, according to this embodiment, it is possible to obtain an impedance effect equivalent to that of a conventional impeder device, and furthermore, it is possible to suppress damage to the impeder device 210, particularly the impeder core 211.

[0086] As explained in the first embodiment, the impedance core 211 may be provided only in the upper half of the outer peripheral surface of the impedance device 210, in an area extending on an arc for half the circumference.

[0087] Furthermore, if the strength of the impedance device 210 after bending can be ensured, the recessed region 213 may be formed by bending only the mandrel 3 and the upper surface of the impedance core 211, as shown in FIG. 17, for example.

[0088] Here, the configuration of the impedancer device 210 according to the second embodiment of the present invention as described above can be applied as appropriate to the impedancer device 110 according to the first embodiment of the present invention.

[0089] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0090] In the following examples, we assumed that an open pipe made of ordinary steel with a diameter of 216.3 mm and a wall thickness of 6 mm was heated by an induction coil with a width of 150 mm, installed 150 mm upstream from the welding point in the longitudinal direction. The difference in magnetic flux density due to differences in the configuration of the impedance device was determined by electromagnetic field analysis. The induction coil was placed with a gap of 5 mm from the open pipe, and a high-frequency current of 4000 A at a frequency of 300 kHz was passed through it.

[0091] The impedance core was assumed to be made of a soft ferrite having a thickness of 10 mm, a circumferential width of 15 mm, and a longitudinal length of 50 mm (relative permeability 1500, saturation magnetic flux density 0.45 T), with multiple soft ferrite pieces attached adjacent to each other with an adhesive to the outer periphery of a SUS304 mandrel having a diameter of 140 mm. The installation conditions for the impedance core were as follows:

[0092] (Example 1) Example 1 of the present invention is an electromagnetic field analysis of the situation shown in Figure 8 in the first embodiment. In Example 1 of the present invention, a recessed region with a width of 16 mm in the x-axis direction in Figure 8 was formed in a range from 50 mm upstream of the induction coil to 250 mm toward the welding point, and the mandrel was exposed. An impeller was installed in this recessed region, with a distance from the upper end of the impeller to the open tube of 23 mm and a distance from the impeller device surface (i.e., from the impeller core) to the open tube of other locations of 13 mm. This assumption corresponds to the case where the impeller is installed in the region closer to point V within the range of length X1 but outside the range of length X2 in Figure 8.

[0093] (Example 2 of the Invention) Example 2 of the invention is an electromagnetic field analysis of the situation shown in Fig. 6 in the first embodiment. In Example 2 of the invention, a recessed region with a width of 16 mm in the x-axis direction in Fig. 6 was formed in the range from 50 mm upstream of the induction coil to the downstream end of the impedance core (i.e., the entire range affected by magnetic flux), and the mandrel was assumed to be exposed. The separation distance from the mandrel to the open tube was 33 mm, and the separation distance from the impedance device surface (i.e., from the impedance core) to the open tube in other locations was 13 mm.

[0094] (Example 3 of the Invention) Example 3 of the invention is based on the assumption that a 5 mm thick copper mesh is provided as an electromagnetic shielding material on the exposed mandrel in Example 2 of the invention, as shown in Figure 10 of the first embodiment.

[0095] (Comparative Example) The comparative example is a conventional electric resistance welded pipe manufacturing apparatus in which a recessed region is not formed as shown in Figures 2 and 3, i.e., the distance from the impedance device to the open pipe is fixed at 13 mm.

[0096] (Electromagnetic field analysis results) Table 1 below summarizes the maximum magnetic flux density and mandrel heat generation ratio as results of electromagnetic field analysis for each of Invention Examples 1 to 3 and the Comparative Example. Note that the mandrel heat generation ratio in the table is the ratio of the heat generation amount of each Invention Example when the heat generation amount of the Comparative Example is set to 1.0.

[0097]

[0098] As shown in Table 1, in the comparative example, the maximum magnetic flux density of the impedance core was 0.62 T, a value that greatly exceeded the saturation magnetic flux density of 0.45 T. In contrast, in invention example 1, in which a recessed region was formed in the range from 50 mm upstream of the induction coil, which has a high magnetic flux density, to 250 mm toward the welding point, and the impedancer was installed in this recessed region, the maximum magnetic flux density of the impedance core was 0.43 T, which was below the saturation magnetic flux density of the impedance core.

[0099] Furthermore, in Example 2 of the present invention, in which the recessed region was formed all the way to the downstream end of the impeder core, the maximum magnetic flux density was 0.42 T, which was below the saturation magnetic flux density of the impeder core. Meanwhile, when the mandrel was covered with an impeder as in Example 1 of the present invention, only a small amount of magnetic flux entered the mandrel. However, in Example 2 of the present invention, the absence of an impeder allowed the magnetic flux to enter the mandrel directly, causing the mandrel to heat up, resulting in a 12% increase in the total heat generation ratio. However, although the amount of heat generation increased, it was not to a level that would cause damage, and the magnetic flux density of the impeder was lowered, enabling stable production.

[0100] In contrast, in the case of Example 3 of the present invention, in which a copper mesh electromagnetic shielding material was provided on the exposed mandrel, the maximum magnetic flux density of the impedance core was 0.42 T, which was equivalent to the result of Example 2 of the present invention, and the total heat generation ratio was also suppressed to the same level as Example 1 of the present invention.

[0101] As can be seen from the above results, by providing a recessed region in the impedance core, i.e., by increasing the distance between the induction coil as a magnetic flux generating source and the lower end of the opening, it is possible to attenuate the magnetic flux entering the impedance core and reduce the magnetic flux density.

[0102] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0103] The embodiments disclosed herein are illustrative in all respects and are not limiting. The above-described embodiments may be omitted, substituted, or modified in various ways without departing from the scope of the appended claims, the technical scope of the present invention as described below, and the spirit thereof. For example, the components of the above-described embodiments may be arbitrarily combined within the scope that does not impair the effects of the components. Furthermore, such an arbitrary combination naturally provides the functions and effects of each of the components involved in the combination, and also provides other functions and effects that are apparent to those skilled in the art from the description of this specification.

[0104] Furthermore, the effects described in this specification are merely descriptive or exemplary and are not limiting. In other words, the technology according to the present invention may achieve other effects that will be apparent to those skilled in the art from the description of this specification, in addition to or in place of the above-described effects.

[0105] The present invention is useful for an electric-resistance welded pipe welding apparatus that bends a traveling metal band into a cylindrical shape, inductively heats the metal band, and welds both end faces of the metal band using the current induced in the metal band. The electric-resistance welded pipes manufactured in this manner are used for pipes that require lightweight construction, such as oil country tubular goods and pipes for motorcycles and automobiles.

[0106] REFERENCE SIGNS LIST 1 Impeder device 2 Impeder core 3 Mandrel 3a Core material 3b Through hole 3c Lattice 3d Slit 4 Roller 5 Cutting tool 6 Impeder case 7 Electric-resistance welded pipe manufacturing device 8 Induction coil 9 Open pipe 9a Opening 9b End 100 Electric-resistance welded pipe manufacturing device 101 Forming roll group 103 Squeeze roll 110 Impeder device 111 Impeder core 113 Recessed region 115 Sealing material 120 Electromagnetic shielding material 200 Electric-resistance welded pipe manufacturing device 210 Impeder device 211 Impeder core 213 Recessed region V Welding point X1 Magnetic flux affected area X2 Directly below area Y1 Width Y2 Opening width

Claims

1. A mandrel equipped with an impedance device for manufacturing an electric welding tube, comprising: an impedance device having an impedance core made of a magnetic material; a mandrel extending in a predetermined direction, located inside the impedance core and serving as a support member for the impedance core; and a concave region is formed in a part of the impedance core with a size within a predetermined range along the extending direction of the mandrel; the size of the concave region in the direction perpendicular to the extending direction is 10 mm or more and not more than 1 / 3 of the outer peripheral length of the mandrel.

2. The mandrel according to claim 1, wherein the length of the concave region in the extending direction is greater than 100 mm and not more than the total length of the impedance core.

3. The impedance device further comprises: a hollow impedance case provided outside the impedance core with a gap existing between the impedance core and the impedance case; cooling water is passed through the hollow portion of the impedance case. The mandrel according to claim 1 or 2.

4. The mandrel according to claim 1 or 2, wherein the concave region is formed by bending at least the upper part of the impedance device downward.

5. The mandrel according to claim 1 or 2, wherein the concave region is a notch formed by removing at least a part of the impedance core.

6. The mandrel according to claim 5, wherein the surface of the mandrel is exposed at the bottom of the concave region.

7. An electromagnetic shielding material is provided on the exposed surface of the mandrel. The mandrel according to claim 6.

8. The electromagnetic shielding material is a copper mesh. The mandrel according to claim 7.

9. Slits extending in the traveling direction are formed in the exposed portion of the mandrel at the bottom of the concave region. The mandrel according to claim 6.

10. The width of the metal part forming the slit is smaller than the penetration depth of the induced current. The mandrel according to claim 9.

11. Cooling water is passed through the inside of the mandrel, and a sealing member is provided at the portion where the slit is formed in the mandrel. The mandrel according to claim 9.

12. The mandrel directly below at least a part or all of the concave region has a through-hole penetrating in the vertical direction, the mandrel according to claim 1 or 2.

13. The impedance core is provided only on the outer periphery of the upper half of the mandrel, the mandrel according to claim 1 or 2.

14. An electric resistance welded pipe manufacturing apparatus having a forming roll group for forming a desired shape of an electric resistance welded pipe, an induction coil for generating an induction current, and a mandrel for an electric resistance welded pipe extended in a predetermined direction, An impedance device having an impedance core made of a magnetic material is provided on a part of the mandrel, A concave region is formed in a part of the impedance core with a size within a predetermined range along the extending direction of the mandrel, The size of the concave region in the extending direction is not less than (the width of the induction coil + 100 mm) and not more than the total length of the impedance core, The size of the concave region in a direction orthogonal to the extending direction is not less than 10 mm and not more than 1 / 3 of the outer peripheral length of the mandrel, the electric resistance welded pipe manufacturing apparatus.

15. In a method of manufacturing an electric resistance welded pipe in which an open pipe bent into a cylindrical shape while being conveyed in a predetermined traveling direction has its end melted by an induction current and then the ends are butted against each other for electric resistance welding, The mandrel according to claim 1 or 2 is disposed inside the open pipe such that the concave region faces the end of the open pipe and at least a part of the arrangement position of the induction coil for generating the induction current is included in the concave region in the extending direction of the mandrel, the method of manufacturing an electric resistance welded pipe.

16. In a cross section when the open pipe, the impedance device, and the mandrel are cut in the radial direction of the open pipe, The separation distance between the upper end of the impedance device in the concave region and the lower end of the end of the open pipe is larger than the separation distance between the upper end of the impedance device and the lower end of the end of the open pipe on the upstream side of the concave region in the traveling direction, the method of manufacturing an electric resistance welded pipe according to claim 15.

17. The method for manufacturing an electric welded pipe according to claim 15, wherein the length of the recessed region in the elongation direction is a length capable of encompassing a range from a position 50 mm upstream of the induction coil to a position 50 mm downstream of the induction coil, with reference to a position directly below the induction coil that generates the induced current.