Impedance sheet, mandrel for electric-resistance welded pipe manufacturing apparatus, and method for manufacturing electric-resistance welded pipe

Abstract

The present invention has been made with the objective of improving the efficiency of electric resistance welding easily and at low cost, without modifying or creating a new mandrel, impeder, or impeder case that constitutes a conventional inner current suppression device. The impedance sheet according to the present invention comprises a sheet that can be wrapped around the outer periphery of a mandrel of an electric-resistance-welded pipe manufacturing apparatus, and a plurality of ferromagnetic bodies that exhibit insulating properties and are arranged on the sheet.

Description

[Technical field]

[0001] The present invention relates to an impedance sheet, a mandrel for an electric resistance welded pipe manufacturing apparatus, and a method for manufacturing an electric resistance welded pipe. [Background technology]

[0002] Generally, methods for manufacturing metal pipes include, for example, a method for manufacturing electric resistance welded pipes or spiral pipes in which a metal strip is bent and welded to form a pipe shape, a method for manufacturing seamless pipes by directly drilling holes in a metal billet, and a method for manufacturing pipes by extrusion.

[0003] Electric-resistance welded pipes are mass-produced because they are particularly highly productive and can be manufactured inexpensively. In the manufacture of such electric-resistance welded pipes, a metal strip is continuously roll-formed by multiple forming rolls while traveling so that it gradually becomes cylindrical, and finally, butt welding is performed at the location where both ends of the metal strip in the width direction meet. In butt welding, a high-frequency current is generally passed through the metal strip to raise both ends of the metal strip in the width direction to their melting temperature, and then both end faces are pressure-welded by a squeeze roll to form a tubular shape.

[0004] Several methods for supplying electric current to both widthwise ends of the metal strips to be joined have already been disclosed. Patent Document 1 discloses a method of directly generating an induced current in a metal by winding an induction coil around the outside of the metal and passing a primary current through this induction coil, and a method of pressing metal electrodes against both ends of an open metal strip before it becomes a closed cross-section during forming, and passing a current directly through it from a power source.

[0005] When supplying current to both ends in the width direction of the metal strips to be joined, a high-frequency current of about 100 kHz to 400 kHz is generally used for the current passing through the induction coil or electrodes. Also, a ferromagnetic core such as soft ferrite is often placed on the inner surface of the tube as an impedance.

[0006] As will be described later, the current supplied to both widthwise ends tends to flow around the inner circumference of the metal strip (also called an "open tube") that has been bent into a cylindrical shape (hereinafter referred to as "inner circumference current"), which does not contribute to welding. Therefore, the above-mentioned impedance is used as a device for suppressing the inner circumference current of the metal strip that has been bent into a cylindrical shape (hereinafter referred to as "inner circumference current suppression device").

[0007] Hereinafter, a method for supplying a current to both widthwise ends of the metal band plates to be joined and a conventional inner current suppression device used therein will be described in detail with reference to Figs.

[0008] Fig. 13 is a plan view showing the main parts during production of an electric-resistance welded pipe using an electric-resistance welded pipe production apparatus 900 having an induction coil 2. Fig. 14 is a side view showing the state during production of an electric-resistance welded pipe using the electric-resistance welded pipe production apparatus 900 as shown in Fig. 13. As shown in Fig. 13 and Fig. 14, the electric resistance welded pipe manufacturing apparatus 900 is mainly composed of an induction coil 2, an impedancer as a conventional inner circumference current suppression device 6, a squeeze roll 901 that applies pressure from the side of the open pipe 1, and a head roll (not shown) that presses the joint from diagonally above the pipe. In Fig. 13 and Fig. 14, the reference numeral 1 denotes an open pipe, the reference numeral 11 denotes an open pipe opening, the reference numeral 111 denotes an open pipe opening end, the reference numeral 112 denotes an open pipe opening end, the reference numeral 12 denotes a joint, the reference numeral 3 denotes an induced current, the reference numeral 31 denotes a downstream induced current, the reference numeral 32 denotes a downstream induced current, the reference numeral 33 denotes an upstream induced current, and the reference numeral 34 denotes an upstream induced current. Here, the induced currents 31 to 34 are written as if they are flowing on the upper surface of the pipe for convenience, including the current flowing on the end surface of the open pipe 1 (not shown).

[0009] An induction coil 2 is wound around the outer periphery of an open tube 1, which is a metal strip formed into a cylindrical shape. By passing a primary current through the induction coil 2, an induced current 3 is generated in the open tube 1. An electric resistance welded pipe is manufactured by this induced current.

[0010] Here, the open pipe 1, which is a metal strip formed into a cylindrical shape, runs from the left to the right in the figure. In the following, the downstream side in the running direction is referred to as the "downstream" side, and the upstream side opposite to the running direction is referred to as the "upstream" side.

[0011] The electric resistance welded pipe is manufactured by the following steps. 1) A traveling open tube 1 that has been slit to a width that matches the diameter of the tube to be made is bent by multiple forming rolls into a cylindrical shape, and the open tube opening ends 111, 112 are arranged to face each other. 2) An induction current is generated in the open tube 1 formed into a cylindrical shape by the induction coil 2, which heats and melts the open tube opening ends 111, 112. 3) Downstream of the induction coil 2, the opposing open tube opening ends 111, 112 of the open tube 1 are butted together and pressed into tight contact with a squeeze roll 901 to join (weld) the open tube opening ends 111, 112. At this time, a head roll (not shown) is often used to press the joint 12 from diagonally above.

[0012] Most of the current flowing through the opposing open tube opening ends 111, 112 of the open tube 1 flows through the opposing ends. For the sake of simplicity, Fig. 13 illustrates the current flowing through the upper surface side of the opposing open tube opening ends 111, 112 of the open tube 1.

[0013] The open pipe 1, which is the material to be welded, is bent from a flat state by a plurality of forming rolls (not shown) while traveling, so that the open pipe opening ends 111, 112 face each other. Finally, the open pipe 1 that has been subjected to the bending process is pressed by a squeeze roll 901 so that the open pipe opening ends 111, 112 come into contact with each other at the joint 12.

[0014] An induction coil 2 is provided upstream of the squeeze roll 901 in order to melt and join the opposing open tube opening ends 111, 112. By passing a high-frequency current through this induction coil 2, an induced current 3 is generated on the surface of the open tube 1 bent into a cylindrical shape just below the induction coil 2.

[0015] The induced current 3 travels around the outer periphery of the open tube 1 along the induction coil 2 that travels around the open tube 1. Here, because the open tube opening ends 111, 112 of the open tube 1 are open as openings, the circulating induced current 3 cannot flow directly below the induction coil 2 in these open portions. Therefore, the induced current 3 is divided into currents 31, 32 flowing on the joint 12 side and currents 33, 34 flowing upstream of the induction coil 2, and can also be divided into an inner periphery current that tries to travel around the inner periphery of the open tube 1.

[0016] The inner circumferential current is a current that tries to flow on the inner circumferential side, opposite to the current generated on the outer circumferential side of the open tube 1 directly below the induction coil 2. If no measures are taken against the inner circumferential current, the amount of current flowing toward the joint 12 decreases and the amount of current that becomes the inner circumferential current increases, resulting in a significant decrease in welding efficiency.

[0017] A method for suppressing the above-mentioned inner circumferential current will be described with reference to Fig. 15. Fig. 15 is a schematic side cross-sectional view of the electric-resistance welded pipe manufacturing apparatus 900 taken from the side of the diagram shown in Fig. 13. As shown in the side cross-sectional view of Fig. 15, a conventional inner current suppression device 6 is disposed inside a cylindrically formed metal band plate 1. As will be described again below with reference to Fig. 16, this inner current suppression device 6 has an impedancer 61 made of a ferromagnetic material such as soft ferrite or electromagnetic steel housed in the internal space of an impedancer case 62.

[0018] By disposing the inner-circumferential current suppression device 6 inside the cylindrical open tube 1, when an inner-circumferential current tries to flow, a magnetic field that suppresses the flow of the inner-circumferential current is generated by electromagnetic induction. This increases the reactance inside the open tube 1, making it possible to suppress the inner-circumferential current.

[0019] However, this suppression of the inner current is possible only under the condition that the installed impeder 61 does not become magnetically saturated. When the impeder 61 becomes magnetically saturated, it loses its magnetism and no longer functions as an impeder. As a result, the inner current increases and the welding efficiency drops significantly. A possible cause of magnetic flux saturation is an increase in the strength of the magnetic field generated by the current flowing through the induction coil 2. When the current flowing through the induction coil 2 increases due to an increase in the line speed of the electric resistance welded pipe production line, an increase in the steel pipe diameter, or an increase in the plate thickness, the magnetic flux saturation of the impeder 61 becomes noticeable.

[0020] An example of a conventional inner current suppression device 6 will be described with reference to Fig. 16. Fig. 16 shows an example of a cross section obtained when a conventional inner current suppression device 6 having an impedancer 61 disposed around a mandrel 902 is cut along the AA cutting line in Fig. 14. 16 , the impeder 61 is rod-shaped and is often used in an electric resistance welded pipe manufacturing apparatus 900 having a mandrel 902 with a relatively large diameter. In this example, the rod-shaped impeder 61 is attached in the circumferential direction to an impeder case 62, and a lid is placed on the impeder case 62, allowing cooling water to flow inside the impeder case 62.

[0021] The conventional inner current suppression device 6 is used to suppress the induced current 3 generated by the induction coil 2 on the open tube 1 that has been bent into a cylindrical shape from flowing inside the tube that has been bent into a cylindrical shape, so that more current flows on the open tube opening ends 111, 112 of the open tube 1, thereby improving welding efficiency. In addition, these conventional inner current suppression devices 6 also have the effect of suppressing the formation of a closed circuit on the inner circumference of the tube due to the induced current flowing upstream of the induction coil 2, which is a high proportion of reactive current that does not contribute to welding, and increasing the induced current toward the joint 12 side.

[0022] The conventional inner current suppression device 6 described with reference to FIG. 16 generally has a structure in which cooling is performed through a cooling medium mainly made of water. In addition, the installation location of the conventional inner current suppression device 6 is usually such that it is attached to a mandrel 902 for grinding the bead of the pipe inner surface joint 12 as shown in Figure 15 and installed directly below the induction coil 2, or in front or behind it.

[0023] A plurality of mandrels 902 are usually prepared according to the inner diameter of the pipe to be manufactured. In accordance with the mandrels 902, a plurality of impedances 61 and impedance cases 62 constituting the conventional inner current suppression device 6 are also usually prepared.

[0024] Therefore, when it is desired to improve productivity by increasing heating efficiency or when it is desired to produce a metal pipe of a size that cannot be produced from a heating neck, etc., and the efficiency of suppressing the inner current is insufficient with the impeder 61 constituting the conventional inner current suppression device 6, it is necessary to increase the capacity of the impeder 61 by extending the length of the impeder 61 or increasing the cross-sectional area, etc., to increase the welding efficiency. In this case, the mandrel 902 and the impeder 61 and the impeder case 62 constituting the conventional inner current suppression device 6 must be modified, or a new impeder 61 and an impeder case 62 must be manufactured. However, in reality, the cost for this becomes very high, and there is a problem that it is often difficult to realize in terms of cost-effectiveness. [Prior art documents] [Patent documents]

[0025] [Patent Document 1] Japanese Patent Application Publication No. 53-44449 [Non-patent literature]

[0026] [Non-Patent Document 1] Kanjiro Takahashi (ed.), "Fundamentals and Applications of High Frequency - Key Technologies Supporting the Era", Tokyo Denki University Press, 1990, pp. 79, 80 Summary of the Invention [Problem to be solved by the invention]

[0027] The present invention has been made in consideration of the above problems, and has an object to provide an impedance sheet, a mandrel for an electric-resistance welded pipe manufacturing apparatus, and a method for manufacturing electric-resistance welded pipe, which make it possible to increase the efficiency of electric-resistance welding easily and at low cost, without modifying or creating a new mandrel, or the impedance and impedance case that constitute a conventional inner current suppression device. [Means for solving the problem]

[0028] In order to solve the above problems, the present inventors conducted extensive research into a method of configuring an inner current suppression device that improves heating efficiency during electric welded pipe manufacturing by taking into consideration the position, shape, ease of installation, and maintainability of the ferromagnetic material that constitutes the impedance of the inner current suppression device.

[0029] As a result, it was discovered that an impedance sheet having a ferromagnetic material arranged on it, which is durable, heat resistant, and can be freely wrapped around the outer periphery of the mandrel of an electric welded pipe manufacturing equipment, is extremely effective in constructing a simple, cost-effective inner current suppression device.

[0030] Furthermore, in the conventional inner current suppression device, it was thought that the impeller had to be housed in an impedance case and then cooled. However, the present inventors have found that, as described below, even in the conventional inner current suppression device, there are cases in which the impeller does not necessarily have to be housed in an impedance case and then cooled. The present invention has been made based on these findings, and the gist of the present invention is as follows.

[0031] (1) An impedance sheet comprising: a sheet that can be wrapped around the outer periphery of a mandrel of an electric-resistance-welded pipe manufacturing apparatus; and a plurality of ferromagnetic bodies exhibiting insulating properties that are arranged on the sheet. (2) The impedance sheet according to (1), wherein the sheet is made of a heat-resistant insulating material and does not cause dielectric loss due to high-frequency current. (3) An impedance sheet as described in (1), in which a bag-shaped holder capable of containing the ferromagnetic material in rod-like or powder form is provided on the surface of the sheet, and the ferromagnetic material is provided within the bag-shaped holder. (4) An impedance sheet described in any one of (1) to (3), wherein the plurality of ferromagnetic bodies are formed by stacking and integrating plate-shaped ferromagnetic bodies, and the ferromagnetic core is arranged on the sheet so that the stacking direction of the plate-shaped ferromagnetic bodies is perpendicular to the surface normal direction of the sheet. (5) The impedance sheet according to any one of (1) to (3), wherein the plurality of ferromagnetic bodies are made of a sintered magnetic material exhibiting ferromagnetism. (6) The impedance sheet according to any one of (1) to (5), wherein the sheet has connectors at ends thereof, allowing a plurality of the sheets to be connected to one another. (7) A mandrel of an electric-resistance welded pipe manufacturing apparatus, in which the impedance sheet according to any one of (1) to (6) is wrapped around the outer periphery of the mandrel of the electric-resistance welded pipe manufacturing apparatus so as to cover at least half of the circumference thereof. (8) A method for manufacturing an electric-resistance welded pipe, comprising electric-resistance welding a metal strip that has been bent into a cylindrical shape using an electric-resistance welded pipe manufacturing apparatus comprising an induction coil, a squeeze roll, and a mandrel, in which the impedance sheet according to any one of (1) to (6) is wrapped around the outer periphery of the mandrel so as to cover at least half of the circumference. (9) The method for manufacturing an electric-welded pipe according to (8), wherein the impedance sheet is wound around the mandrel so as to cover at least a portion of the mandrel that is located upstream of the induction coil in the traveling direction. Effect of the Invention

[0032] As described above, according to the present invention, it is possible to freely add an inner-circumference current suppression function to an existing mandrel later as necessary without modifying or newly manufacturing the mandrel, the impedancer, and the impedancer case that constitute the conventional inner-circumference current suppression device. Therefore, when it is desired to increase the inner-circumference current suppression capability from that at the time of the initial design due to a change in production conditions, it is possible to easily install an additional impedance with the length and thickness as required, which is a remarkable effect.

[0033] In addition, by enhancing the ability to suppress the inner circumferential current, significant effects can be achieved, such as reducing power consumption, increasing production speed, and enabling the production of electric-resistance-welded pipes of sizes that could not be produced due to power bottlenecks, etc.

[0034] Furthermore, the impedance sheet of the present invention does not use an impedance case as in the past. Therefore, it is easy to check and replace a damaged impedance without disassembling the impedance case, and there is also the advantage that maintenance can be minimized. This makes it possible to significantly reduce maintenance costs and time. Thus, the industrial effects of the present invention are extremely significant.

[0035] According to the present invention, the impedance capacity is improved, and therefore the welding efficiency is improved, so that the amount of power used can be reduced, and energy savings can be realized. Furthermore, when the same amount of power is input as in the past, the welding efficiency is improved by the present invention, so that the line speed can be increased, and productivity can be improved. Furthermore, it is now possible to manufacture electric-resistance-welded pipes of sizes and varieties that were previously difficult to manufacture due to limitations in power supply capacity, etc., and the industrial effects are extremely significant. [Brief description of the drawings]

[0036] [Figure 1] FIG. 1 is a diagram illustrating an impedance sheet according to a first embodiment of the present invention. [Figure 2A] 3A and 3B are diagrams illustrating an impedance sheet according to the embodiment. [Figure 2B] 3A and 3B are diagrams illustrating an impedance sheet according to the embodiment. [Diagram 3] 11A and 11B are diagrams illustrating an example in which a plurality of impedance sheets according to the embodiment are connected together. [Figure 4] FIG. 11 is a plan view illustrating an impedance sheet according to a second embodiment of the present invention. [Diagram 5] FIG. 5 is a side view illustrating the impedance sheet shown in FIG. [Figure 6] FIG. 11 is a plan view illustrating an impedance sheet according to a third embodiment of the present invention. [Figure 7] FIG. 7 is a side view illustrating the impedance sheet shown in FIG. [Figure 8] 1 is a diagram illustrating an example of a mandrel of an electric resistance welded pipe manufacturing apparatus in which an impedance sheet according to a first embodiment of the present invention is used as an inner current suppression device. [Figure 9] 1 is a diagram illustrating an example in which an inner loop current suppression device of the present invention is added upstream of an existing inner loop current suppression device. [Figure 10] 1 is a diagram illustrating an example of an inner circumferential current suppression device of the present invention in which inner circumferential current suppression devices of the present invention are stacked in the radial direction. [Figure 11]1 is a diagram illustrating an example of an inner circumferential current suppression device of the present invention in which inner circumferential current suppression devices of the present invention are stacked in the radial direction. [Figure 12A] FIG. 11 is an explanatory diagram illustrating a ferromagnetic core used in Example 4 of the present invention. [Figure 12B] FIG. 11 is an explanatory diagram illustrating a schematic view of a ferromagnetic core used in Comparative Example 2. [Figure 13] 1 is a plan view showing a state in which an electric-resistance welded pipe is manufactured using an electric-resistance welded pipe manufacturing apparatus having an induction coil. FIG. [Figure 14] 14 is a side view of an electric-resistance welded pipe during production using the electric-resistance welded pipe production apparatus shown in FIG. 13. [Figure 15] 14 is a side cross-sectional view of an electric-resistance welded pipe during production using the electric-resistance welded pipe production apparatus shown in FIG. 13. [Figure 16] FIG. 15 is a schematic cross-sectional view taken along line AA of FIG. 14, illustrating an example of a conventional inner current suppression device in which an impedance is disposed around a mandrel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals, and duplicated explanations will be omitted.

[0038] (First embodiment) The impedance sheet according to the first embodiment of the present invention is an impedance sheet having a sheet that can be wrapped around the outer periphery of a mandrel of an electric resistance welded pipe manufacturing apparatus, and a plurality of ferromagnetic bodies exhibiting insulating properties arranged on the sheet. Such an impedance sheet can be used in a manufacturing method of electric resistance welded pipe, for example, in which a metal strip of various steels, stainless steel, or the like is bent into a cylindrical shape while traveling and then welded to realize the tubular shape.

[0039] FIG. 1 is a plan view illustrating an impedance sheet 41 according to a first embodiment of the present invention. 1, reference numeral 41 denotes an impedance sheet, reference numeral 411 denotes a sheet, reference numeral 412 denotes a ferromagnetic body, and reference numeral 414 denotes a sheet connector. The impedance sheet 41 for manufacturing electric resistance welded pipes according to this embodiment is formed by directly laying and adhering the ferromagnetic body 412 to the sheet 411 at a predetermined density.

[0040] The sheet 411 according to this embodiment is used for manufacturing electric-resistance welded pipes by, for example, closely contacting and covering at least half or more of the outer periphery of the mandrel 902 of an electric-resistance welded pipe manufacturing apparatus, thereby constituting an inner current suppression device 4. For this reason, it goes without saying that the sheet 411 is flexible enough to be freely wrapped around the outer periphery of the mandrel 902, and is desirably durable and heat resistant.

[0041] More specifically, the sheet 411 according to this embodiment is generally made of a heat-resistant insulator and does not cause dielectric loss due to high-frequency current. If the sheet 411 is made of a material that does not have heat resistance, the sheet itself may be damaged by the heat generated during the manufacture of the electric resistance welded pipe. If the sheet 411 is not made of an insulator, an induced current may flow through the sheet itself due to the magnetic flux generated by passing a current through the induction coil 2, and the sheet may not be able to function as an impedancer. Even if the sheet 411 is an insulator, a material that causes dielectric loss due to high-frequency current may not only cause a decrease in efficiency, but may also be unable to maintain the shape of the sheet.

[0042] In this specification, "having heat resistance" means, for example, that the heat resistance temperature is 150°C or higher. There is no particular upper limit to the heat resistance temperature, and the higher the temperature, the better. The upper limit may be determined in consideration of the manufacturing cost, processability, life, etc. of the impedance sheet. In addition, in this specification, being an "insulator" means, for example, that the resistivity is 1×10 3 This refers to materials with a resistance of Ω·m or more.

[0043] Examples of materials for sheet 411 include rubbers such as fluororubber and silicone rubber, various epoxy resins such as bakelite and glass epoxy, silica sheets, welding sputter sheets, and materials coated with heat-resistant paint.

[0044] It should be noted that the state of sheet 411 according to this embodiment is not particularly limited as long as it is configured so as to be freely wrapped around the outer periphery of mandrel 902 .

[0045] For example, the sheet 411 according to this embodiment may be made of a freely curveable material (such as the various rubbers mentioned above) integrated into a sheet shape.

[0046] Furthermore, the sheet 411 according to the present embodiment may be configured to be freely curved without being broken, for example, by using a material that is relatively difficult to bend (for example, the above-mentioned Bakelite, glass epoxy, etc.). For example, the sheet 411 may be formed into a blind shape by cutting a material that is relatively difficult to bend into strips and connecting the strips together using a heat-resistant connecting member.

[0047] In the impedance sheet 41 according to this embodiment, a plurality of ferromagnetic bodies 412 exhibiting insulating properties are arranged on the surface of the sheet 411.

[0048] In this specification, the term "ferromagnetic material" refers to a material with an initial magnetic permeability (μi) of 1000 or more. In addition, when a ferromagnetic material "exhibits insulating properties," this means that the material or its mass has the characteristic that eddy currents are unlikely to occur when exposed to a high-frequency magnetic field, and the electrical conductivity is 1×10 6 This means that the value is greater than or equal to S / m.

[0049] In the impedance sheet 41 according to this embodiment, examples of materials that can be used as the ferromagnetic body 412 include soft ferrite, magnetic steel, and amorphous alloys that satisfy the above-mentioned initial permeability (μi).

[0050] In this embodiment, the overall shape of each ferromagnetic body 412 is not particularly restricted, and may be various shapes such as a rod shape, a plate shape, etc. The size of each ferromagnetic body 412 is also not particularly restricted. In this embodiment, each ferromagnetic body 412 may be a ferromagnetic core constituted by a laminate in which plate-shaped ferromagnetic bodies are laminated, or may be a sintered body of a magnetic material exhibiting ferromagnetism.

[0051] The number of ferromagnetic bodies 412 provided on the sheet 411 may be appropriately adjusted depending on the diameter of the electric resistance welded pipe to be manufactured or the diameter of the mandrel 902 so that the required cross-sectional area of ​​the ferromagnetic body can be secured.

[0052] These ferromagnetic bodies 412 etc. may be attached to the deformable sheet 411 as described above by, for example, adhering them with an adhesive or storing them in a resin storage space provided in advance, and thus attached closely to the sheet 411.

[0053] An example of a more preferable state of the ferromagnetic body 412 in this embodiment will be described below with reference to Figures 2A and 2B. Figure 2A is a schematic diagram showing an enlarged part of a cross section of the impedance sheet shown in Figure 1 taken along the AA cutting line in Figure 1.

[0054] In the impedance sheet 41 according to this embodiment, as shown in Fig. 2A, for example, a ferromagnetic core 413 formed by stacking a plurality of thin plate-like ferromagnetic bodies and integrating them may be used as the ferromagnetic body 412. Moreover, each of the ferromagnetic cores 413 is preferably disposed on the sheet 411 such that the stacking direction of the thin plate-like ferromagnetic bodies is perpendicular to the surface normal direction of the sheet 411, as shown in Fig. 2A.

[0055] In the ferromagnetic core 413 in which the thin ferromagnetic materials are stacked and integrated as described above and arranged as described above, even if magnetic flux enters the ferromagnetic core 413, the generation of eddy currents due to the magnetic flux can be prevented, and the impedance sheet 41 of this embodiment can continue to maintain its function as an impedance.

[0056] Here, the extent to which the thin plate-shaped ferromagnetic bodies should be laminated in each ferromagnetic core 413 is not particularly restricted since it depends on the operating environment in which the impedance sheet 41 according to this embodiment is used. For example, a thin plate-shaped ferromagnetic body having a thickness of about sub-mm is prepared using the above-mentioned soft ferrite, electromagnetic steel, amorphous alloy, etc., and this thin plate-shaped ferromagnetic body is laminated until the laminated thickness reaches about 3 to 30 mm to form the above-mentioned ferromagnetic core 413.

[0057] Here, since soft ferrite exhibits high resistance even when in lump or powder form, almost no eddy currents are generated even when the ferromagnetic core 413 made of laminated thin-plate soft ferrite is exposed to a high-frequency magnetic field.

[0058] In addition, although the surface of the electromagnetic steel is insulated by an insulating material, the cross section when the electromagnetic steel is cut is conductive. When electromagnetic steel is used as a thin plate-shaped ferromagnetic material, the electromagnetic steel is arranged and laminated so that the insulating materials located on the surface of the electromagnetic steel are adjacent to each other. In addition, the part where the cross section of the electromagnetic steel is exposed is insulated with an insulating material such as insulating varnish. This makes it possible to treat the ferromagnetic core 413 made of electromagnetic steel as a substantial insulator. However, since eddy currents are generated when magnetic flux enters the surface in the lamination direction, the ferromagnetic core 413 is arranged so that the lamination direction is perpendicular to the normal direction of the surface of the sheet 411, as described above.

[0059] FIG. 2B is a schematic diagram showing a state in which impedance sheet 41 using ferromagnetic core 413 as shown in FIG. 2A is mounted on mandrel 902 of an electric resistance welded pipe manufacturing apparatus. 2B, when the impedance sheet 41 using the ferromagnetic cores 413 shown in FIG. 2A is attached to the mandrel 902 of an electric-resistance-welded pipe manufacturing apparatus, it is preferable to arrange the impedance sheet 41 so that the lamination direction of the thin ferromagnetic material is along the circumferential direction of the mandrel 902 (so that the lamination direction of the thin ferromagnetic material is approximately parallel to the tangent line of the circumference of the mandrel 902 at the installation position of each ferromagnetic core 413). By attaching the impedance sheet 41 to the mandrel 902 in this manner, even if magnetic flux enters the ferromagnetic cores 413, it is possible to prevent the generation of eddy currents due to magnetic flux, and it is possible to continue to maintain the function of the impedance sheet 41 according to this embodiment as an impedance.

[0060] Furthermore, when a lump of sintered body using soft ferrite as a material is used as the ferromagnetic body 412, the thickness and diameter of the sintered body may be, for example, about 3 to 30 mm. Since soft ferrite exhibits high resistance even in a lump or powder form as described above, almost no eddy current is generated even when a lump of sintered body using soft ferrite as a material is exposed to a high-frequency magnetic field.

[0061] FIG. 3 is a plan view illustrating an example in which a plurality of impedance sheets 41 according to this embodiment are connected together. In FIG. 3, reference numeral 41 denotes an impedance sheet, reference numeral 411 denotes a sheet, reference numeral 412 denotes an impedance, and reference numeral 414 denotes a sheet connector. FIG. 3 shows an example in which four impedance sheets 41 (sheets 411) of FIG. 1 are connected with detachable sheet connectors 414 such as hook-and-loop fasteners.

[0062] In order to accommodate changes in the size of the mandrel 902, the impedance sheet 41 according to this embodiment is preferably structured so that the impedance sheets 41 having the structure as shown in FIG. 1 can be connected to each other, for example, as shown in FIG. 3, as shown in FIG. 1 and FIG. 3. This makes it possible to easily realize a sheet-like impedance having a desired size by using a plurality of impedance sheets 41. As a result, the impedance sheet 41 according to this embodiment can be applied to any size of existing mandrel 902.

[0063] As a method for connecting the impedance sheets 41 according to this embodiment, it is preferable to provide sheet connectors 414 on the four circumferential edges (i.e., four end sides) of the sheet 411 as illustrated in Fig. 1. However, as long as each impedance sheet 41 has one or more sheet connectors 414, the location of the connectors does not have to be on the four circumferential edges, and may be anywhere necessary.

[0064] The specific structure of the sheet connector 414 is not particularly restricted, and various known connecting structures and connecting members can be appropriately applied. In this case, the sheet connector 414, like the sheet 411, is preferably made of a heat-resistant insulating material that does not cause dielectric loss due to high-frequency current.

[0065] As the sheet connector 414 in this embodiment, for example, a method can be adopted in which a recess is formed at one end of each impedance sheet 411 while a protrusion is formed at the other end, and the protrusion of one impedance sheet 41 is fitted into the recess of the other impedance sheet 41.

[0066] Second Embodiment The impedance sheet according to the second embodiment of the present invention is an impedance sheet having a sheet that can be freely wrapped around the outer periphery of a mandrel of an electric resistance welded pipe manufacturing apparatus, and a bag-shaped holder that holds a rod-shaped ferromagnetic body that exhibits insulating properties, arranged on the sheet. This impedance sheet can be used in a manufacturing method of electric resistance welded pipe, for example, in which a metal strip of various steels, stainless steels, or the like is bent into a cylindrical shape while traveling and then welded to realize the tubular shape.

[0067] Fig. 4 is a plan view illustrating an impedance sheet 41 according to a second embodiment of the present invention, and Fig. 5 is a side view illustrating the impedance sheet 41 shown in Fig. 4.

[0068] 4 and 5, reference numeral 41 denotes an impedance sheet, reference numeral 411 denotes a sheet, reference numeral 420 denotes a holder, and reference numeral 421 denotes a ferromagnetic body. The impedance sheet 41 for manufacturing electric resistance welded pipe according to this embodiment is configured by providing holders 420 at a predetermined density on a sheet 411, and inserting rod-shaped ferromagnetic bodies 421 between the sheet 411 and the holder 420. In other words, the holder 420 is configured to be bag-shaped between the sheet 411 and the holder 420.

[0069] Since the sheet 411 in the impedance sheet 41 according to this embodiment is similar to the sheet 411 in the first embodiment, detailed description will be omitted below. Although not shown in Fig. 4, it is preferable that sheet connectors 414 are provided on the four peripheral edges of the sheet 411, similar to the impedance sheet 41 according to the first embodiment. This allows a plurality of impedance sheets 41 to be connected to each other via the sheet connectors 414.

[0070] Moreover, the ferromagnetic bodies 421 in the impedance sheet 41 according to this embodiment are similar to the ferromagnetic bodies 412 according to the first embodiment except that they are rod-shaped, so detailed description thereof will be omitted below.

[0071] In this embodiment, the number of holders 420 and ferromagnetic bodies 421 provided on the sheet 411 can be appropriately adjusted so as to ensure the required cross-sectional area of ​​the ferromagnetic body depending on the diameter of the electric resistance welded pipe to be manufactured or the diameter of the mandrel 902.

[0072] In the impedance sheet 41 according to this embodiment, the rod-shaped ferromagnetic material 421 may be a ferromagnetic material core 413 as shown in Fig. 2A. In this case, the ferromagnetic material core 413 is inserted into a holder 420 (described later) so that the lamination direction of the thin plate-shaped ferromagnetic material is perpendicular to the surface normal direction of the sheet 411.

[0073] Furthermore, when mounting the impedance sheet 41 using the ferromagnetic core 413 on the mandrel 902 of an electric welded pipe manufacturing apparatus, as in the first embodiment, it is preferable to arrange the impedance sheet 41 so that the stacking direction of the thin ferromagnetic material is along the circumferential direction of the mandrel 902 (so that at the installation position of each ferromagnetic core 413, the stacking direction of the thin ferromagnetic material is approximately parallel to the tangent to the circumference of the mandrel 902).

[0074] Moreover, the ferromagnetic body 421 may be a lump of sintered material made of soft ferrite.

[0075] Holder 420 is preferably a cloth-like material that has durability and heat resistance and does not cause dielectric loss due to high-frequency current, similar to sheet 411. As holder 420, for example, a sheet for preventing welding spatters is used.

[0076] Holder 420 may be provided, for example, by being sewn into sheet 411, or may be provided by being fixed to sheet 411 with a hook-and-loop fastener or the like. In this case, one end of holder 420 is provided so as to be open, and a rod-shaped ferromagnetic body 421 can be inserted from the opening (not shown) of holder 420.

[0077] 2A is used as the rod-shaped ferromagnetic body 421, it is preferable that the relationship between the lamination direction of the thin plate-shaped ferromagnetic body and the surface normal direction of the sheet 411 is maintained both when the ferromagnetic core 413 is inserted into the holder 420 and when the impedance sheet 41 is in use. For this reason, the cross-sectional shapes of the rod-shaped ferromagnetic body 421 and the holder 420 (shapes as shown in FIG. 5) may be designed to have anisotropy, for example, so that the user of the impedance sheet 41 can easily grasp the lamination direction.

[0078] As a method for fixing ferromagnetic body 421 to sheet 411, for example, ferromagnetic body 421 may be inserted into holder 420, and then the opening of holder 420 may be closed. The method for closing the opening of holder 420 is arbitrary, but for example, it may be closed with a hook-and-loop fastener, or may be closed with a cover with a resin button.

[0079] According to the impedance sheet 41 of this embodiment as described above, the holder 420 is fixed to the sheet 411, so that the ferromagnetic material 421 can be reliably held by the sheet 411 even in a high temperature environment, for example.

[0080] (Third embodiment) The impedance sheet according to the third embodiment of the present invention is an impedance sheet having a sheet that can be wrapped around the outer periphery of a mandrel of an electric resistance welded pipe manufacturing apparatus, and a bag-shaped holder that holds an insulating powder or granular impedance arranged on the sheet. This impedance sheet can be used in a manufacturing method of electric resistance welded pipe, for example, in which a metal strip of various steels or stainless steels is bent into a cylindrical shape while traveling and then welded to form a tubular shape.

[0081] Fig. 6 is a plan view illustrating an impedance sheet 41 according to a third embodiment of the present invention, and Fig. 7 is a side view illustrating the impedance sheet 41 shown in Fig. 6.

[0082] 6 and 7, reference numeral 41 denotes an impedance sheet, reference numeral 411 denotes a sheet, reference numeral 430 denotes a holder, and reference numeral 431 denotes a ferromagnetic material. The impedance sheet 41 for manufacturing electric resistance welded pipe according to this embodiment is configured by providing a holder 430 on the sheet 411, and inserting a powdered or granular ferromagnetic material 431 between the sheet 411 and the holder 430. In other words, the holder 430 is configured to be bag-shaped between the sheet 411 and the holder 430.

[0083] Since the sheet 411 in the impedance sheet 41 according to this embodiment is similar to the sheet 411 in the first embodiment, detailed description will be omitted below. Although not shown in Fig. 6, it is preferable that sheet connectors 414 are provided on the four peripheral edges of the sheet 411, similar to the impedance sheet 41 according to the first embodiment. This allows a plurality of impedance sheets 41 to be connected to one another via the sheet connectors 414.

[0084] In addition, the ferromagnetic material 421 in the impedance sheet 41 of this embodiment can be made of the same material as the ferromagnetic material 412 of the first embodiment, except that it has a powder or granular shape, so detailed explanation will be omitted below.

[0085] Holder 430 is preferably a cloth-like material that has durability and heat resistance and does not cause dielectric loss due to high-frequency current, similar to sheet 411. As holder 430, for example, a sheet for preventing welding spatters is used.

[0086] The holder 430 is provided, for example, by inserting powdered or granular ferromagnetic material 431 into the sheet 411 and sewing it into a quilt shape.

[0087] In this embodiment, the locations where the holder 430 and the ferromagnetic material 431 are provided on the sheet 411 can be appropriately adjusted so as to ensure the required cross-sectional area of ​​the ferromagnetic material depending on the diameter of the electric resistance welded pipe to be manufactured or the diameter of the mandrel 902.

[0088] As described above, the impedance sheet 41 according to this embodiment can provide the same effects as those of the second embodiment, and since the holder 430 is fixed to the sheet 411, the ferromagnetic material 431 can be securely held to the sheet 411 even in a high temperature environment, for example.

[0089] (Fourth embodiment) The fourth embodiment of the present invention relates to a mandrel of an electric resistance welded pipe manufacturing apparatus in which the impedance sheet according to the first embodiment is used as an inner current suppression device. Note that, in the following, an example will be described in which the impedance sheet according to the first embodiment is used as an inner current suppression device, but the impedance sheet according to the second or third embodiment may be used instead of the impedance sheet according to the first embodiment.

[0090] A mandrel 902 of an electric-resistance welded pipe manufacturing apparatus using the impedance sheet 41 according to the first embodiment as the inner current suppression device 4 will be described with reference to Fig. 8. Fig. 8 is a diagram illustrating an example of a mandrel 902 of an electric-resistance welded pipe manufacturing apparatus according to this embodiment when the impedance sheet 41 for electric-resistance welded pipe manufacturing according to the first embodiment is used as the inner current suppression device 4.

[0091] In FIG. 8, reference numeral 1 denotes an open pipe, reference numerals 111 and 112 denote the open pipe opening ends, reference numeral 4 denotes an inner current suppression device, reference numeral 41 denotes an impedance sheet, reference numeral 411 denotes a sheet, reference numeral 412 denotes a ferromagnetic body, and reference numeral 902 denotes a mandrel of an electric resistance welded pipe manufacturing apparatus. As shown in FIG. 8, by using one or more impedance sheets 41 according to the first embodiment, an inner current suppression device 4 can be constructed that covers one circumference of the outer periphery of the mandrel 902 of an electric welded pipe manufacturing apparatus.

[0092] The inner current suppression device 4 according to this embodiment can be attached to the outside of the mandrel 902 without being placed in an impedance case. Compared to conventionally used inner current suppression devices, the inner current suppression device 4 according to this embodiment can get closer to the inner circumference of the open tube 1 bent into a cylindrical shape, and therefore has a greater effect in suppressing the current.

[0093] Furthermore, by wrapping the impedance sheet 41 in layers so as to cover the outer periphery of the mandrel 902 in multiple layers, the effect of suppressing the inner periphery current can be further improved.

[0094] Furthermore, in the inner current suppression device 4 according to this embodiment, it is possible to obtain the above-mentioned inner current suppression effect with a simple configuration by simply attaching the impedance sheet 41 to the outside of the mandrel 902. Also, the attachment position of the inner current suppression device 4 using the impedance sheet 41 can be set at any position on the mandrel 902, but it should be set taking into consideration the magnetic field strength, dripping of molten metal, etc., and it is more preferable to attach it to a portion located upstream of the induction coil 2, for example.

[0095] In the example shown in Figure 8, the inner current suppression device 4 is illustrated as covering one entire circumference of the outer periphery of the mandrel 902, but in this embodiment, the inner current suppression device 4 using one or more impedance sheets 41 may be arranged to cover at least half the circumference of the mandrel 902 facing the open tube opening ends 111, 112.

[0096] Furthermore, when the impedance sheet 41 according to the second or third embodiment is used in the inner current suppression device 4 according to this embodiment, it is preferable to attach the flat surface side of the sheet 411 (i.e., the side on which the ferromagnetic bodies 421, 431 are not provided) to the outer circumferential surface side of the mandrel 902. This allows the impedance sheet 41 to be appropriately attached to the mandrel 902.

[0097] Next, the reason why no water-cooling structure is provided in the inner current suppression device 4 according to this embodiment will be described.

[0098] In the manufacture of electric-resistance welded pipes using electric-resistance welded pipe manufacturing apparatus 900 as shown in Fig. 14, the strength of the generated magnetic field becomes high directly below the induction coil 2. In the impeller 61 and impeller case 62 constituting the conventional inner current suppression device 6 as shown in Fig. 16, heat generation in the impeller 61 made of soft ferrite or electromagnetic steel due to the strength of the magnetic field as described above is unavoidable, so it was thought that the conventional impeller 61 would be cooled by passing cooling water through the impeller case 62.

[0099] However, when the inventors performed an electromagnetic field analysis of the magnetic field strength described above, they found that the magnetic field strength is strongest directly below induction coil 2 and in its vicinity, and in addition, depending on the input power, the magnetic field strength is weaker at a position about 50 mm to 100 mm upstream of induction coil 2, and therefore there is often no particular need to cool the impedancer.

[0100] Furthermore, even when there is concern about heat generation or scale or the like falling and accumulating from the opening of the open tube 1, it has been found that in the case of the inner current suppression device 4 of this embodiment, water can be simply injected into the opening of the open tube 1 from a nozzle or the like, and a cooling structure dedicated to the impedancer is not essential.

[0101] From the findings of the above electromagnetic field analysis, it is preferable that the impedance sheet 41 according to this embodiment is wound around the mandrel 902 so as to cover at least a portion located upstream of the induction coil 2 in the traveling direction.

[0102] Fifth embodiment The fifth embodiment of the present invention relates to a method for manufacturing an electric-resistance welded pipe using the inner current suppression device 4 according to the fourth embodiment. Note that, in the inner current suppression device 4 of this embodiment, the impedance sheet according to the second or third embodiment may be used instead of the impedance sheet according to the first embodiment.

[0103] The inner current suppression device 4 described in the fourth embodiment can be easily attached to any position of the mandrel 902 with any length and thickness without modifying or installing a new mandrel 902, conventional impedance 61, or impedance case 62. Therefore, by using such an inner current suppression device 4, the impedance capacity can be increased at low cost.

[0104] [Extension of existing inner current suppression device] FIG. 9 is a diagram for explaining an example in which an inner current suppression device 4 according to this embodiment is added to the upstream side of an existing inner current suppression device 6 of an existing mandrel 902. In FIG. In Figure 9, reference numeral 1 denotes an open tube, reference numerals 111 and 112 denote the open tube opening ends, reference numeral 12 denotes a joint, reference numeral 2 denotes an induction coil, reference numeral 4 denotes an inner circumferential current suppression device according to this embodiment, reference numeral 6 denotes a conventional inner circumferential current suppression device, and reference numeral 902 denotes a mandrel.

[0105] As illustrated in Figure 9, by adding a new inner current suppression device 4 of this embodiment upstream of an existing conventional inner current suppression device 6, it is possible to manufacture electric welded pipes while achieving the effect of essentially extending the existing conventional inner current suppression device 6 upstream.

[0106] In this case, the inner current suppression device 4 can suppress the inner current that tends to flow toward the upstream side of the induction coil 2 around the open tube opening ends 111 and 112 of the open tube 1 .

[0107] [Inner current suppression device with large cross-sectional area] 10 and 11 are diagrams illustrating an example in which the inner current suppression device 4 according to this embodiment is stacked in the radial direction of a mandrel 902 to form an impedance sheet 41 for manufacturing electric resistance welded pipes. 10 and 11, reference numeral 1 denotes an open tube, reference numerals 111 and 112 denote the open tube opening ends, reference numeral 4 denotes an inner current suppression device according to this embodiment, reference numeral 41 denotes an impedance sheet, reference numeral 411 denotes a sheet, reference numeral 412 denotes a ferroelectric, and reference numeral 902 denotes a mandrel.

[0108] In the example shown in FIG. 10, in the impedance sheet 41 provided in duplicate, the ferromagnetic bodies 412 are arranged so as to line up in a straight line along the radial direction. In the example shown in FIG. 11, in the impedance sheet 41 provided in duplicate, the ferromagnetic bodies 412 are arranged alternately in the radial direction.

[0109] 10 and 11, by providing two impedance sheets 41 according to this embodiment in the radial direction of the mandrel 902, it is possible to increase the cross-sectional area of ​​the portion corresponding to the inner current suppression device 4. In other words, by increasing the number of ferromagnetic bodies 412, the cross-sectional area of ​​the magnetic bodies in the inner current suppression device 4 can be increased, thereby decreasing the magnetic flux density of the ferromagnetic bodies 412. As a result, stable welding can be achieved by manufacturing an electric resistance welded pipe using such an inner current suppression device 4. EXAMPLES

[0110] In this example, the effect of the present invention was confirmed by a static heating experiment.

[0111] [Material to be heated] In this example, the heated material simulating an open pipe was a steel pipe with an outer diameter of 89 mm, a wall thickness of 3 mm, and a length of 1 m, which was laser processed to simulate the shape of the opening as shown in Fig. 13. In this laser processing, a part of the steel pipe was removed from the left end in Fig. 13 so that the parallel openings were spaced 15 mm apart and had a length of 400 mm, and a part of the steel pipe was removed over a length of 300 mm so that the angle between the apex, which is regarded as the joint 12, and both ends was 5.7 degrees, so that the openings were formed (the total length of the openings was 700 mm). The apex was 0.5R.

[0112] [ERW pipe manufacturing equipment] In the electric welded pipe manufacturing equipment used in the examples, an induction coil made of a water-cooled copper tube with a diameter of 10 mm (inner diameter 110 mm, 2 turns, width 50 mm) was used as the induction coil, with a distance of 10 mm between the copper tube and the steel tube, and the induction coil was positioned 150 mm away from the apex of the opening which was regarded as the joint. During heating, power with a frequency of 180 kHz to 20 kW was applied, and the time required for the maximum temperature to reach 800°C during static heating was measured. The heating temperature was measured by welding 50 μm K thermocouples to the end of the steel pipe at 20 mm intervals from the apex of the opening where the joint was assumed to be. In each of the following experiments, the temperature rise rates of the joints were compared. In each experiment, the squeeze roll was not provided.

[0113] [Example 1] <Impedance sheet> An impedance sheet was produced by adhesively attaching soft ferrite cores measuring 5 mm in thickness, 10 mm in width, and 130 mm in length at 5 mm pitch to a silicon rubber sheet measuring 170 mm in width, 140 mm in length, and 2 mm in thickness. <ERW pipe manufacturing> A 5 mm wide hook-and-loop fastener was attached to the end of the rubber sheet with an adhesive. This impedance sheet was wrapped around the outer periphery of a polycarbonate pipe, which acted as a mandrel, from a position 100 mm upstream of the induction coil and heated.

[0114] [Example 2] <Impedance sheet> The impedance sheet of Example 1 of the present invention was used. <ERW pipe manufacturing> As shown in Figure 9, first, a first inner-circumference current suppression device was constructed, and then a second inner-circumference current suppression device was installed in close contact with the end of the first inner-circumference current suppression device and extended upstream, and heating was then performed.

[0115] [Example 3] <Impedance sheet> The impedance sheet of Example 1 of the present invention was used. <ERW pipe manufacturing> As shown in FIG. 10, an inner current suppression device was constructed by using two layers of impedance sheets and heating was performed.

[0116] [Comparative Example 1] <ERW pipe manufacturing> As a comparative example, heating was carried out without using the impedance sheet according to the present invention.

[0117] The cylindrical soft ferrite core (outer diameter 35 mm, inner diameter 15 mm, length 250 mm) shown in Figure 16 was fixed to the center of the diameter of a polycarbonate pipe with an outer diameter of 50 mm and a thickness of 3 mm, and the inner and outer periphery of the soft ferrite core were designed to be water-coolable. The soft ferrite core was installed so that its tip was 50 mm upstream from the joint and its tail was 100 mm upstream from the induction coil. The polycarbonate pipe was placed in the center of the pipe, with the impeder inserted as a mandrel, over the entire length of the pipe.

[0118] [result] The results of the above-mentioned invention examples 1 to 3 and comparative example 1 are shown in Table 1 below. The heating rate ratio is the value obtained by dividing the heating rate of the comparative example by the heating rate at which the temperature of the apex of the opening, regarded as the joint, reaches 800°C. From Table 1, it can be seen that adding the impedance sheet or impedance of the present invention improves the heating rate by 6% (Example 1 of the present invention), and that doubling the length improves it by 13% (Example 2 of the present invention), and doubling the impedance thickness improves it by 27% (Example 3 of the present invention), confirming that the present invention has a significant effect. In addition, in the experiment, the impeder of the present invention was able to be used stably without generating any particular heat.

[0119] [Table 1]

[0120] [Invention Example 4, Comparative Example 2] In order to examine the difference in stability of a ferromagnetic core made of laminated thin ferromagnetic materials due to differences in the lamination direction, 30 μm thick electromagnetic steel sheets were laminated and integrated to form a ferromagnetic core having a length of 100 mm and a square cross section with a side length of 10 mm. Three such ferromagnetic cores were arranged adjacently in the longitudinal direction of the steel pipe to a length of 300 mm, and a comparison was made between a case in which the lamination direction was perpendicular to the surface normal direction of the sheets as shown in Fig. 12A (Example 4 of the present invention) and a case in which the lamination direction was parallel to the surface normal direction of the sheets as shown in Fig. 12B (Comparative Example 2).

[0121] In the experiment, a steel pipe with an outer diameter of 216.3 mm and a wall thickness of 4.5 mm was manufactured. The ferromagnetic core was wound around the mandrel 50 mm upstream of the induction coil, with the distance between the ferromagnetic core and the inner surface of the steel pipe being 13 mm, and then welding was performed. The welding frequency was 250 kHz. The results are shown in Table 2 below.

[0122] [Table 2]

[0123] [result] As shown in Table 2 above, Example 4 of the present invention was stable even after long-term use, only warming up a little. On the other hand, in Comparative Example 2, immediately after the induction coil was energized, heat and deformation occurred at the tip of the ferromagnetic core directly below the opening of the steel pipe near the induction coil, causing partial melting and leaving burn marks on the silicone rubber sheet material, so that the current to the induction coil had to be stopped immediately. The penetration depth of the electromagnetic steel sheet at the frequency of this experiment was about 25 μm. In the case of Example 4 of the present invention, eddy currents were unlikely to be generated and heat was unlikely to be generated, whereas in the case of Comparative Example 2, it is considered that eddy currents were easily generated and heat was generated because the magnetic flux entered the surface of the electromagnetic steel sheet.

[0124] Although the preferred embodiment of the present invention has been described in detail above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

[0125] The embodiments disclosed herein are illustrative and not restrictive in all respects. The above embodiments may be omitted, substituted, or modified in various forms 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 embodiments can be arbitrarily combined within a range that does not impair the effects of the components. Furthermore, the arbitrary combination naturally provides the actions and effects of each of the components in the combination, and also provides other actions and effects that are obvious to a person skilled in the art from the description of this specification.

[0126] In addition, the effects described in this specification are merely explanatory or exemplary, and are not limiting. In other words, the technology according to the present invention may achieve other effects that are apparent to a person skilled in the art from the description of this specification, in addition to or in place of the above effects. [Industrial Applicability]

[0127] The present invention can be used in the manufacture of electric resistance welded pipes. [Explanation of symbols]

[0128] 1 Open Tube 11 Open pipe opening 111 Open pipe opening end 112 Open pipe opening end 12 Joint 2 Induction coil 3 Induced current 31 Downstream induced current 32 Downstream induced current 33 Upstream induced current 34 Upstream induced current 4. Inner current suppression device 41 Impedance sheet 411 seats 412 Ferromagnetic material 413 Ferromagnetic Core 414 Seat connector 420 Holder 421 Ferromagnetic material 430 Holder 431 Ferromagnetic material 6. Conventional inner current suppression device 61 Impeder 62 Impeder Case 900 ERW pipe manufacturing equipment 901 Squeeze Roll 902 Mandrel

Claims

1. a sheet that can be wrapped around an outer periphery of a mandrel of an electric resistance welded pipe manufacturing apparatus; A plurality of ferromagnetic bodies exhibiting insulating properties, which are disposed on the sheet; An impedance sheet having

2. 2. The impedance sheet according to claim 1, wherein the sheet is made of a heat-resistant insulating material and does not cause dielectric loss due to high frequency current.

3. a bag-shaped holder capable of accommodating the rod-shaped or powder-shaped ferromagnetic material is provided on a surface of the sheet; The impedance sheet according to claim 1 , wherein the ferromagnetic body is provided inside the bag-shaped holder.

4. As the plurality of ferromagnetic bodies, a ferromagnetic core formed by stacking and integrating plate-shaped ferromagnetic bodies is used, The impedance sheet according to any one of claims 1 to 3, wherein the ferromagnetic core is arranged on the sheet so that the lamination direction of the plate-shaped ferromagnetic material is perpendicular to the surface normal direction of the sheet.

5. 4. The impedance sheet according to claim 1, wherein the plurality of ferromagnetic bodies are made of a sintered body of a magnetic material exhibiting ferromagnetism.

6. The impedance sheet according to any one of claims 1 to 3, wherein the sheet has connectors at ends thereof, enabling a plurality of the sheets to be connected to each other.

7. A mandrel of an electric-resistance welded pipe manufacturing apparatus, the mandrel being wound with the impedance sheet according to any one of claims 1 to 3 so as to cover at least half of the outer periphery of the mandrel of the electric-resistance welded pipe manufacturing apparatus.

8. A method for manufacturing electric-resistance welded pipes, comprising: using an electric-resistance welded pipe manufacturing apparatus including an induction coil, a squeeze roll, and a mandrel, electric-resistance welding is performed on a metal strip that has been bent into a cylindrical shape while being conveyed in a predetermined traveling direction; A method for manufacturing an electric-resistance welded pipe, comprising wrapping the impedance sheet according to any one of claims 1 to 3 around an outer periphery of the mandrel so as to cover at least half of the circumference.

9. The method for producing an electric-resistance welded pipe according to claim 8, wherein the impedance sheet is wound so as to cover at least a portion of the mandrel that is located upstream of the induction coil in the traveling direction.

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