Electric resistance welded pipe manufacturing method, induction coil for electric resistance welded pipe manufacturing device, and electric resistance welded pipe manufacturing device

Abstract

[Problem] To improve heating efficiency in manufacturing an electric resistance welded pipe while preventing damage to an impeder and overheating of the surface of an open pipe. [Solution] In an electric resistance welded pipe manufacturing method according to the present invention, an induction coil includes: a pair of inclined conductor parts provided so as to be inclined at a predetermined angle with respect to a vertical direction in a side view; a connection conductor part that is provided at an end section of the pair of inclined conductor parts on the downstream side in a traveling direction so as to connect the pair of inclined conductor parts to each other, and that is arranged so as to extend across an opening in a plan view; and power source connection conductor parts that are provided at end sections of the pair of inclined conductor parts on the upstream side in the traveling direction and that are connected to a power source, wherein the pair of inclined conductor parts are arranged so as to gradually separate from the opening toward the upstream side in the traveling direction in a plan view.

Description

Electric-resistance welded pipe manufacturing method, induction coil for electric-resistance welded pipe manufacturing device, and electric-resistance welded pipe manufacturing device

[0001] The present invention relates to a method for manufacturing electric-resistance welded pipe in which a traveling metal band is bent into a cylindrical shape and induction-heated, and both end faces of the metal band are welded together using a current induced in the metal band; an induction coil for use in an electric-resistance welded pipe manufacturing apparatus; and an electric-resistance welded pipe manufacturing apparatus.

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

[0003] Electric-resistance welded pipes are particularly highly productive and inexpensive to manufacture, and are therefore mass-produced. In the manufacture of such electric-resistance welded pipes, a metal strip is first formed into a cylindrical shape by running it through a moving metal plate. Next, the opposing end faces of the open pipe (hereinafter simply referred to as "open pipe ends") across the opening are pressed against the outside of the open pipe with squeeze rolls to form a contact point, and a high-frequency current is applied upstream of the contact point to raise the pipe end temperature to its melting point. This pressure-welds the end faces of the previously formed contact points to form a tubular shape.

[0004] There are two methods for supplying current to the end of the open pipe. The first method involves, for example, providing a non-contact induction coil (solenoid coil) in the open pipe and passing a primary current through this induction coil to directly generate an induced current in the open pipe (see, for example, Figures 2 and 5 of Patent Document 1). The second method involves pressing a metal electrode against the end of the open pipe and directly passing current from a power source (see, for example, Figure 13 of Patent Document 2). In this case, a high-frequency current of approximately 100 to 400 kHz is typically used as the current flowing through the induction coil or electrode. To block the induced current induced by this high-frequency current that does not contribute to welding and tends to flow around the inner circumference of the open pipe, a ferromagnetic material called an impedancer is often placed on the inner surface of the pipe.

[0005] In the first method using an induction coil described above, the induction coil shown in Fig. 2 of Patent Document 1 is arranged so as to surround the outer periphery of the open tube, whereas the induction coil shown in Fig. 5 of Patent Document 1 is an air-core coil that is arranged above the opening without surrounding the outer periphery of the open tube so as to form a primary current circuit across the opening.

[0006] Furthermore, in the second method using a metal electrode described above, FIGS. 1 to 12 of Patent Document 2 describe an apparatus in which, in addition to a contact (contact tip) connected to a welding power source having a frequency of about 100 to 400 kHz, a preheating coil connected to another preheating power source having a frequency of about 1 to 20 kHz is provided upstream of the contact.

[0007] Japanese Patent Publication No. 2015-134379 Japanese Patent Publication No. 62-176085 U.S. Patent Publication No. 2,931,885 U.S. Patent Publication No. 3,127,674

[0008] However, when an induction coil is installed so that a portion of the induction coil overlaps the opening of the open pipe in a plan view, as in the case of the induction coil described in Patent Document 1, a strong magnetic field is generated even inside the open pipe during welding to manufacture electric resistance welded pipes. When a strong magnetic field is generated inside the open pipe, magnetic flux concentrates on the impeller, which has low magnetic resistance, causing the impeller to heat up. For this reason, impellers are typically housed in a resin case and cooled with cooling water. However, if the magnetic field is stronger than the saturation magnetic flux density, the impeller loses its magnetism and loses its ability to block current around the inner circumference, or even becomes damaged. In addition, metal rods connecting the cutting tool for inner bead cutting can also break due to heat generation. Thus, a magnetic field stronger than the saturation magnetic flux density can sometimes prevent stable operation for long periods of time.

[0009] Furthermore, such problems of impeller damage and rod breakage are common in small-diameter pipes (e.g., pipes with an inner diameter of approximately 100 mm or less), and are particularly pronounced when manufacturing thick-walled pipes (e.g., pipes with a wall thickness of more than 6 mm) by electric resistance welding. The method described in Patent Document 2 uses a contact tip, but in such cases, it is often used for large-diameter pipes. Furthermore, in the case of electric resistance welding of such large-diameter pipes, the inner circumference is overwhelmingly longer than the round-trip distance from the induction coil or contact to the weld (also called the joint). As a result, the resistance of the path around the inner circumference of the pipe is greater, resulting in an increase in current flow toward the weld, which has lower resistance. For this reason, the effect of an impeller is small, and impellers are often not installed. Thus, problems such as impeller damage are unlikely to occur in the case of large-diameter electric resistance welding of pipes.

[0010] Furthermore, as explained above, it is preferable to position the induction coil closer to the weld zone because a shorter round-trip distance from the induction coil to the weld zone compared to the inner periphery increases the current flowing toward the weld zone and improves heating efficiency. However, various equipment is disposed above the open tube near the squeeze roll, such as leads from a power source and a roll that presses the weld zone from diagonally above. Therefore, when the induction coil circumferentially surrounds the open tube, as in the case of the induction coil shown in FIG. 2 of Patent Document 1, there is a limit to how close the induction coil itself can be to the squeeze roll. As a result, the position where the induced current is generated in the open tube is far from the joint, which poses a problem of difficulty in improving heating efficiency.

[0011] Furthermore, when manufacturing medium-diameter pipes (e.g., pipes with an outer diameter of approximately 300 to 600 mm) by electric resistance welding, as the roll diameter of the squeeze roll increases, the induction coil must be moved away from the weld. Therefore, the above-mentioned problem is even more pronounced for medium-diameter pipes. The preheating coil described in Patent Document 2 is connected to a preheating power source with a frequency of approximately 1 to 20 kHz, and a contact (contact tip) connected to a welding power source with a frequency of approximately 100 to 400 kHz, separate from the preheating power source, is provided between the preheating coil and the joint. Therefore, Patent Document 2 does not propose any solution to the above-mentioned problem of the positional relationship between the induction coil and the squeeze roll.

[0012] Furthermore, in Patent Documents 3 and 4, attempts are made to move a portion of the induction coil closer to the weld by providing a portion of the induction coil with a shape that conforms to the opening (see, for example, Figure 4 of Patent Document 3 and Figure 2 of Patent Document 4). However, as a result of investigations by the present inventor, it became clear that in the shapes described in Patent Documents 3 and 4, the spacing between the coils facing each other across the opening is too narrow, resulting in excessive cancellation of the magnetic field, and the induced current density is concentrated in the surface portion on the outer circumferential surface of the open pipe. As a result, the surface of the open pipe may generate heat, causing overheating. Therefore, a method capable of improving heating efficiency while preventing overheating of the surface of the open pipe is desired.

[0013] The present invention has been made in consideration of the above points, and aims to provide an electric-resistance welded pipe manufacturing method, an induction coil for an electric-resistance welded pipe manufacturing apparatus, and an electric-resistance welded pipe manufacturing apparatus that can improve heating efficiency when manufacturing electric-resistance welded pipe while preventing damage to the impeder and overheating of the surface of the open pipe.

[0014] In order to achieve the above object, according to one aspect of the present invention, there is provided a method for manufacturing an electric resistance welded pipe, in which both end surfaces of a pipe material of an open pipe having an opening extending in a running direction are melted by an induced current generated by an induction coil, and the end surfaces are melted by pressing the end surfaces together against the outer surface of the pipe with a squeeze roll while gradually narrowing the distance between the opening, wherein the induction coil comprises a pair of inclined conductors arranged to be inclined at a predetermined angle with respect to the vertical in a side view, a connecting conductor arranged at downstream ends of the pair of inclined conductors in the running direction so as to connect each of the pair of inclined conductors to each other and arranged so as to straddle the opening in a plan view, and a power supply connecting conductor arranged at upstream ends of the pair of inclined conductors in the running direction and connected to a power supply, and the pair of inclined conductors are arranged so as to be gradually separated from the opening as they move upstream in the running direction in a plan view.

[0015] In the present invention, the size and shape of the opening of the open pipe are predetermined according to the diameter of the electric resistance welded pipe and the welding conditions before the electric resistance welded pipe is manufactured. Therefore, in the method for manufacturing an electric resistance welded pipe according to the present invention, the detailed size and shape of the induction coil are determined based on the opening and the shapes of the squeeze roll portion and the upper pressure roll.

[0016] According to the method for manufacturing an electric-resistance welded pipe of the present invention, the induction coil has a pair of inclined conductors and a connecting conductor that straddles the opening of the open pipe and connects the pair of inclined conductors to each other. Furthermore, by connecting a power supply connection conductor to the inclined conductors, a closed circuit can be formed. Furthermore, the pair of inclined conductors are arranged so that, in a plan view, they gradually become farther away from the opening toward the upstream side in the running direction. This prevents magnetic flux from directly entering the impeller from the induction coil, thereby reducing the magnetic flux density of the impeller. Furthermore, because the inclined conductors are arranged so that they gradually become farther away from the opening, current density does not concentrate on the outer peripheral surface of the open pipe in the longitudinal direction. As a result, the method for manufacturing an electric-resistance welded pipe of the present invention maintains the function of the impeller and prevents damage, preventing overheating of the outer peripheral surface of the open pipe and improving the heating efficiency of the open pipe.

[0017] Such an effect is particularly useful when producing small diameter pipes (for example, pipes with an inner diameter of about 100 mm or less), particularly thick-walled pipes (for example, pipes with a wall thickness of more than 6 mm).

[0018] Although details will be described in the embodiments, the connecting conductor portion in the present invention crosses the opening above the opening whose width gradually narrows toward the squeeze roll portion, at a position where the opening width is such that the influence of the magnetic flux generated in the connecting conductor portion directly entering the impedance does not become a problem.

[0019] Furthermore, by properly arranging the inclined conductor portion and the connecting conductor portion, the induction coil of the present invention can secure a large, obstruction-free space above the open pipe. As a result, the induction coil can be placed closer to the weld than before. This allows for the installation of auxiliary equipment such as measuring devices in the space above the open pipe.

[0020] Furthermore, by bringing the induction coil closer to the weld, the induced current flows more easily along the end face of the opening, and is more likely to be concentrated at the weld.

[0021] More specifically, the induction coil according to the present invention has a shape in which a portion of the conductor is closer to the welded portion, so that the induced current induced in the open tube exhibits directionality such that it is concentrated toward the welded portion. As a result, the induction coil according to the present invention generates more heat toward the welded portion than conventional coils. As a result, the end surface can reach a temperature sufficient for welding in a shorter distance than conventional coils. The induced current that was generated upstream of the induction coil is drawn toward the welded portion, making it easier for the current to concentrate near the welded portion and increasing the amount of heat generated. This improves welding efficiency and reduces thermal diffusion by shortening the heating time. As a result, the effects of reducing the heat-affected area of ​​the metal structure in the welded portion and suppressing the generation of oxides due to heating, as well as improving welding quality due to the reduction in the welded area and the suppression of oxide generation, are achieved.

[0022] The above-mentioned effects are also useful when manufacturing medium-diameter pipes (for example, pipes with an outer diameter of about 300 to 600 mm).

[0023] In the method for manufacturing an electric-resistance-welded pipe, the pair of inclined conductor portions are preferably provided so as to be inclined at an angle of 20 to 70 degrees with respect to the vertical direction in a side view.

[0024] In the method for manufacturing an electric resistance welded pipe, when the induction coil is viewed from above, the distance from the end of the connecting conductor on the welding point side to the welding point is defined as L1 (mm), the distance from the end of the connecting conductor on the welding point side to the position where the induction coil reaches the bottom of the open pipe is defined as L2 (mm), the distance between the end face of the opening and the inclined conductor at the position where the induction coil reaches the bottom of the open pipe is defined as d (mm), and the ape angle of the opening is defined as α, the spacing D (mm) between the pair of inclined conductors at the position where the induction coil reaches the bottom of the open pipe preferably satisfies the relationships expressed by the following formulas (1) and (2): D = 2d + (L1 + L2) × tan(α / 2) ... formula (1) 10 (mm) ≦ D ≦ (diameter of open pipe + 10) (mm) ... formula (2)

[0025] In the method for manufacturing an electric resistance welded pipe, it is preferable that, when the induction coil is viewed in a plan view, an angle φ formed by a center line of each of the inclined conductor portions is larger than an aperture angle α of the opening.

[0026] In order to achieve the above object, according to another aspect of the present invention, there is provided an induction coil for use in an electric-resistance welded pipe manufacturing apparatus for manufacturing electric-resistance welded pipe, the induction coil having a pair of inclined conductor portions arranged so as to be inclined at a predetermined angle with respect to the vertical direction in a side view, a connecting conductor portion arranged integrally with the pair of inclined conductor portions at one end of the pair of inclined conductor portions so as to connect each of the pair of inclined conductor portions to each other, and a power supply connection conductor portion arranged integrally with the pair of inclined conductor portions at the other end of the pair of inclined conductor portions and connected to a power supply, wherein the pair of inclined conductor portions are arranged so as to be inclined at an angle of 20 to 70 degrees with respect to the vertical direction in a side view.

[0027] In the induction coil for an electric resistance welded pipe manufacturing apparatus, the induction coil may have a substantially elliptical shape when viewed from above.

[0028] In the induction coil for an electric-resistance-welded pipe manufacturing apparatus, the inclined conductor portion may have a linear shape when viewed from the side.

[0029] In the induction coil for an electric-resistance-welded pipe manufacturing apparatus, at least a portion of the inclined conductor may be curved when viewed from the side.

[0030] In order to achieve the above object, according to yet another aspect of the present invention, there is provided an electric-resistance welded pipe manufacturing apparatus having a group of forming rolls for forming a desired electric-resistance welded pipe shape, and an induction coil for generating an induced current, wherein the induction coil has a pair of inclined conductors arranged so as to be inclined at a predetermined angle with respect to the vertical direction in a side view, a connecting conductor arranged integrally with the pair of inclined conductors at one end of the pair of inclined conductors so as to connect each of the pair of inclined conductors to each other, and a power supply connection conductor arranged integrally with the pair of inclined conductors at the other end of the pair of inclined conductors and connected to a power supply, wherein the pair of inclined conductors are arranged so as to be inclined at an angle of 20 to 70 degrees with respect to the vertical direction in a side view.

[0031] In the electric-resistance-welded pipe manufacturing apparatus, the induction coil may have a substantially elliptical shape when viewed from above.

[0032] In the induction coil for an electric-resistance-welded pipe manufacturing apparatus, the inclined conductor portion may have a linear shape when viewed from the side.

[0033] In the induction coil for an electric-resistance-welded pipe manufacturing apparatus, at least a portion of the inclined conductor may be curved when viewed from the side.

[0034] As described above, according to the present invention, when manufacturing electric resistance welded pipes, damage to the impeller can be prevented by reducing the magnetic flux density of the impeller while maintaining a heating condition equivalent to that of conventional welding methods. As a result, stable operation for long periods of time is possible without interrupting the operation. Furthermore, in the present invention, the induction coil has an inclined conductor portion, which allows the power supply connection conductor connected to the power source to be spaced away from the weld, preventing current density from concentrating on the outer peripheral surface of the open pipe in the longitudinal direction. Furthermore, because the connecting conductor portion does not encircle the open pipe, the induction coil can be positioned closer to the weld than in the past. As a result, heating efficiency is improved, and overheating of the outer peripheral surface of the open pipe is prevented, thereby further improving welding quality.

[0035] FIG. 1 is a perspective view schematically showing the outline of the configuration of a conventional electric-resistance welded pipe manufacturing apparatus. FIG. 2 is a schematic plan view showing an example of an electric-resistance welded pipe manufacturing apparatus according to the prior art, which uses an induction coil in which a closed circuit is formed so as to surround the outer circumferential surface of a metal band plate bent into a cylindrical shape. FIG. 3 is a schematic side view of the electric-resistance welded pipe manufacturing apparatus shown in FIG. 2. FIG. 4 is a schematic longitudinal sectional view of the electric-resistance welded pipe manufacturing apparatus shown in FIGS. 2 and 3. FIG. 5 is a perspective view schematically showing the outline of the configuration of an electric-resistance welded pipe manufacturing apparatus according to an embodiment of the present invention. FIG. 6 is a schematic plan view of an electric-resistance welded pipe manufacturing apparatus according to an embodiment of the present invention. FIG. 7 is a schematic side view of the electric-resistance welded pipe manufacturing apparatus shown in FIG. 6. FIG. 8 is a schematic side view of the electric-resistance welded pipe manufacturing apparatus shown in FIG. 6. FIG. 9 is a schematic plan view of an electric-resistance welded pipe manufacturing apparatus according to an embodiment of the present invention. Fig. 1 is a plan view schematically showing an electric-resistance welded pipe manufacturing apparatus according to a modified example of an embodiment of the present invention; Fig. 2 is a side view schematically showing an electric-resistance welded pipe manufacturing apparatus according to a modified example of an embodiment of the present invention; Fig. 3 is a side view schematically showing an electric-resistance welded pipe manufacturing apparatus according to a modified example of an embodiment of the present invention; Fig. 4 is a plan view schematically showing an electric-resistance welded pipe manufacturing apparatus according to a modified example of an embodiment of the present invention; Fig. 5 is a side view schematically showing an electric-resistance welded pipe manufacturing apparatus according to a modified example of an embodiment of the present invention.

[0036] 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.

[0037] (Conventional Electric-Resistance Welded Pipe Manufacturing Apparatus) First, a conventional electric-resistance welded pipe manufacturing apparatus as described in Patent Document 1 will be described with reference to FIGS.

[0038] Fig. 1 is a perspective view showing a schematic outline of the configuration of a conventional electric resistance welded pipe manufacturing apparatus. Fig. 2 is a schematic plan view illustrating an electric resistance welded pipe manufacturing apparatus in which an induction coil is wound around an open pipe and a primary current is passed through the induction coil to generate an induced current in the open pipe, thereby manufacturing the electric resistance welded pipe. Fig. 3 is a schematic side view of Fig. 2. Fig. 4 is a schematic longitudinal sectional view of the apparatus shown in Figs. 2 and 3.

[0039] As shown in Fig. 1, a conventional electric-resistance welded pipe manufacturing apparatus 900 is an apparatus used for manufacturing electric-resistance welded pipes by melting the ends of an open pipe 1 that has been bent into a cylindrical shape while being transported in a predetermined running direction R using an induced current and then butting the ends together to perform electric-resistance welding. This electric-resistance welded pipe manufacturing apparatus 900 has a group of forming rolls 901 for forming the desired electric-resistance welded pipe shape, and an induction coil 3. As shown in Fig. 1, a pair of squeeze rolls 6 is provided further downstream of the induction coil 3.

[0040] Here, most of the current flowing through the end of the open tube 1 flows through the opposing end faces, as shown schematically in Figure 2. For ease of explanation, Figure 2 depicts the current flowing through the upper surface (outer peripheral surface) of the end face of the open tube 1 (the same applies below). In this case, the primary current flowing through the induction coil 3 and the induced current generated in the open tube 1 are alternating currents (AC) flowing in opposite directions. However, for convenience, the drawings depict the current as a vector with the magnitude and direction at a given moment (the same applies below).

[0041] The impeder 7 is usually housed in a resin case and is cooled by cooling water that flows inside the case. This impeder case also prevents the impeder 7 from being damaged if it is hit during installation. In the following explanation, the impeder case is not shown in the drawings to avoid making the illustrations more understandable.

[0042] In general, in the manufacture of electric resistance welded pipes, as shown schematically in FIG. 1 , a traveling metal strip, slit to a width corresponding to the diameter of the pipe to be made, is first bent by a group of forming rolls 901, with both widthwise end faces of the strip facing each other to form a cylindrical open pipe 1. An induction current is then passed through the open pipe 1 by an induction coil 3, heating and melting the end faces of the open pipe 1 (the end faces facing the opening). Then, downstream of the process, squeeze rolls 6 press the opposing end faces of the open pipe 1 together, removing the molten and softened portions from the front and back surfaces along with oxides that are prone to defects, thereby completing the welding. The ejected and solidified bead is then removed by cutting, resulting in an electric resistance welded pipe with a defect-free, sound weld.

[0043] Here, "downstream" as described in this specification means downstream in the running direction R of the metal strip or open pipe 1, and hereinafter, when we refer to "upstream" and "downstream", they will refer to "upstream" and "downstream", respectively, in the running direction R of the metal strip or open pipe 1.

[0044] As shown in Figures 1 to 4, a metal strip such as steel or stainless steel, which is the material to be welded, is bent from a flat state by a group of forming rolls 901 while traveling, and is formed into a cylindrical open pipe 1 with both end face portions 2a, 2b facing each other. Next, both end face portions 2a, 2b of the open pipe 1 are pressed against each other by squeeze rolls 6, and the open pipe 1 is passed through while forming a joint (squeeze roll portion, welded portion) 5. An induction coil (solenoid coil) 3 such as that shown in Figures 1 to 4 is provided upstream of the squeeze rolls 6 in order to melt the opposing both end face portions 2a, 2b and join them at the joint portion 5.

[0045] By passing a high-frequency current (usually on the order of 100 to 400 kHz) through this induction coil 3, induced currents 4c and 4d are generated in the surface layer on the outer peripheral surface of the cylindrical open tube 1 directly below the induction coil 3. This induced current circulates around the outer peripheral surface of the open tube 1 along the induction coil 3 that encircles the open tube 1. At this time, the opening 2 of the open tube 1 is present midway along the induced current's circulation, and therefore the induced current cannot flow directly below the induction coil 3 at this opening 2. As a result, the induced current tries to branch and flow in roughly three directions, including the circulation route directly below the induction coil 3.

[0046] 2, the primary current of the induction coil 3 and the induced current of the open pipe 1 are shown in their respective directions at a given moment. The current flowing in the first direction is currents 4a and 4b that pass through the joint 5 along the end surfaces 2a and 2b of the open pipe 1. The current flowing in the second direction is currents 4c and 4d that flow from the opening 2 of the open pipe 1 around the outer periphery. The current flowing in the third direction is minor currents 4e and 4f that flow upstream of the induction coil 3. Due to the proximity effect specific to high-frequency currents, the currents 4a and 4b that pass through the joint 5 are concentrated on the surfaces of the end surfaces 2a and 2b of the open pipe 1 that face the opening 2, heating and melting these areas. Finally, the joint 5 is pressed together by a squeeze roll 6, completing the welding.

[0047] 2 and 4, a ferromagnetic core made of ferrite or the like, called an impedance 7, is usually placed inside the open pipe 1 to increase the impedance of the inner circumferential surface of the open pipe 1, thereby preventing current from flowing along the inner circumferential surface. Furthermore, if the diameter of the electric resistance welded pipe to be manufactured is large compared to the round trip length to the joint 5 and the inner circumferential surface of the open pipe 1 is sufficiently long, the impedance of the inner circumferential surface may be sufficiently large even without placing the impedance 7, and the current circulating along the inner circumferential surface may be suppressed. For the above reasons, the current attempting to circulate along the inner circumferential surface of the open pipe 1 is not shown in FIG. 2.

[0048] A tool (not shown) is disposed inside the electric resistance welded pipe downstream of the joint 5 to cut the inner surface bead after welding (cutting the inner surface bead is also called inner surface bead cutting). This tool is supported by a rod 8 (see FIG. 4; when manufacturing electric resistance welded pipes with large diameters, a highly rigid mandrel may be used instead of the rod 8) disposed approximately in the center of the open pipe 1.

[0049] In this example, an impeder 7 is disposed midway along the rod 8. The impeder 7 has, for example, a thick, generally cylindrical shape. The rod 8 is disposed approximately at the center of the impeder 7 so as to pass through the rod 8 with a cooling water passage 9 interposed between the rod 8 and the inner diameter side of the impeder 7. By passing cooling water through the cooling water passage 9, both the impeder 7 and the rod 8 are cooled.

[0050] (Background to the Invention) As described above, the induction coil 3 in a conventional electric-resistance welded pipe manufacturing apparatus is arranged so that its conductor portion extends across the opening 2 of the open pipe 1. In this case, the magnetic flux generated by the induction coil 3 enters the impeller 7 directly through the opening 2. In particular, the impeller 7 is made of a ferromagnetic material such as ferrite or electromagnetic steel, which has high magnetic permeability and allows the magnetic flux to selectively enter the impeller 7. When the magnetic flux enters the impeller 7, the impeller 7 begins to generate heat and loses its magnetism when its temperature exceeds the Curie temperature. As a result, the impeller 7 no longer functions as an impeller, and the induced current begins to circulate around the inner circumference of the pipe. This can lead to problems such as a decrease in the current contributing to the weld, making welding impossible, damage to the impeller 7, or fracture of the rod 8. When this occurs, stable operation over long periods of time becomes difficult. In particular, in a small diameter pipe where the impeder 7 and the induction coil 3 are likely to be close to each other, the above problem is likely to occur.

[0051] According to the inventor's investigation, it was found that the causes of damage to the impeller 7 are: (1) high-density magnetic flux generated by the induction coil 3 arranged across the opening 2 of the open pipe 1 directly enters the impeller 7, which has high magnetic permeability; and (2) magnetic flux generated by the induced current (welding current) flowing through the end surfaces 2a, 2b of the opening 2 of the open pipe 1 enters the impeller 7, causing magnetic flux saturation and damage to the impeller 7. This situation becomes more pronounced when the electric resistance welded pipe being manufactured is a small-diameter pipe (for example, a pipe with an inner diameter of about 100 mm or less), and particularly when it is a thick-walled pipe (for example, a pipe with a wall thickness of more than 6 mm).

[0052] That is, when manufacturing a small-diameter, thick-walled electric resistance welded pipe, the space between the induction coil 3 and the impeder 7 becomes narrow, exposing the impeder 7 to a strong magnetic field. Furthermore, because the cross-sectional area of ​​the impeder 7 that receives such magnetic flux cannot be ensured sufficiently, the magnetic flux density of the impeder 7 itself tends to be high. In addition, the distance between the end faces 2a, 2b of the opening 2 of the open pipe 1 and the impeder 7 is short, and a large current flows near the impeder 7, which increases the magnetic flux density of the impeder 7. As a result, the impeder 7 heats up, loses its magnetism, and is severely damaged.

[0053] Furthermore, if the impedance 7 becomes magnetically saturated or damaged, it loses its effect of increasing the impedance of the inner surface of the pipe, and the induced current generated on the outer periphery of the pipe will circulate along the inner surface of the open pipe 1 instead of flowing through the end surfaces 2a and 2b of the opening 2. As a result, the current required to weld the end surfaces 2a and 2b cannot be secured, the amount of heat generated at the end surfaces 2a and 2b decreases, and welding may become impossible. Furthermore, the induced current circulating along the inner surface of the open pipe 1 may also heat the rod 8, resulting in its breakage.

[0054] Furthermore, if the cooling water flowing through the resin impeller case (not shown) that houses the impeller 7 to cool the impeller 7 becomes too hot due to the heat generated by the impeller 7, the resin impeller case (not shown) can easily deform due to radiant heat from the heated end faces 2a and 2b at the top of the impeller case. This can cause holes in the impeller case, allowing the cooling water to leak out, making it impossible to cool the impeller 7 and preventing operation.

[0055] Therefore, the present inventors have studied a method for reducing the magnetic flux density of the impeder 7 by avoiding the magnetic flux that directly enters the impeder 7 from the induction coil.

[0056] As a result, the inventor discovered that by positioning the induction coil so that the magnetic flux generated from the induction coil does not enter the high-permeability impeder 7 directly from the opening 2 of the open tube 1, and by having the induction coil form a closed circuit, the magnetic flux density of the impeder 7 can be reduced.

[0057] The present inventors also investigated the use of induction coils shaped to fit the opening, as described in Patent Documents 3 and 4. As a result, it became clear that in the portions facing each other across the opening, the magnetic fields are canceled out because the gap between the coils is too narrow, and the induced current density is concentrated on the outer circumferential surface side in the longitudinal direction of the open tube 1. If the current density is concentrated on the outer circumferential surface side in the longitudinal direction of the open tube 1, the surface of the open tube may generate heat, resulting in overheating of the surface.

[0058] As a result of the inventor's investigation into this point, he discovered that by improving the shape of the portion along the opening, it is possible to prevent the current density from concentrating on the outer circumferential surface of the open tube 1 in the longitudinal direction, while concentrating the current density more on both end face portions 2a, 2b facing the opening 2.

[0059] Specifically, in the embodiment of the present invention described below, when the induction coil is viewed from above, a pair of inclined conductors are provided so that they gradually move away from the opening toward the upstream side in the running direction of the open pipe 1, and a connecting conductor is provided so as to connect the pair of inclined conductors to each other across the opening 2 of the open pipe 1. In addition, a conductor (power supply connecting conductor) connected to a high-frequency power supply is disposed at the other end of the inclined conductor, thereby forming the closed circuit mentioned above.

[0060] Here, the connecting conductor portion of the induction coil crosses the opening 2 at a position where the opening width is such that the magnetic flux generated in the connecting conductor portion has little effect on directly entering the impedancer. By arranging the induction coil near the end surfaces 2a and 2b of the opening 2 of the open tube 1 in this manner, the induced current generated at the outer periphery of the induction coil is drawn toward the induction coil. Furthermore, as described in detail below, by controlling the degree of inclination of the inclined conductor portion and its position from the opening 2, the magnetic flux generated at the opposing end surface can be effectively canceled out to the extent that excessive current density is not concentrated on the outer periphery of the open tube 1 in the longitudinal direction. Therefore, the induction coil according to the present invention reduces the effect of the magnetic flux entering the impedancer 7. As a result, the magnetic flux to the impedancer 7 can be controlled.

[0061] Furthermore, by setting the inclination of the inclined conductor portion of the induction coil within a predetermined range and by having the connecting conductor portion straddle the opening 2 at a position where the magnetic flux does not affect the impedance 7, the induced current flowing through the end surfaces 2a and 2b and the induced current flowing directly below the coil merge together. This allows the induced current to concentrate at the welded portion (the portion where welding is performed) and its vicinity, within a range that does not cause overheating of the outer peripheral surface in the longitudinal direction of the open pipe 1. As a result, the magnetic flux generated by the induction coil that directly enters the impedance 7 is kept low, and heating efficiency is improved.

[0062] Furthermore, the induction coil according to the embodiment of the present invention has a shape in which a portion of the conductor (more specifically, the connecting conductor) is closer to the welded portion. This narrows the opening, resulting in a low impedance on the welded portion side. As a result, the current is concentrated toward the welded portion, and the amount of heat generated toward the welded portion is increased compared to conventional methods. This allows the end face portion to reach a temperature sufficient for welding in a shorter distance than conventional methods. As a result, it is possible to obtain the following effects: improved welding efficiency due to the increased amount of heat generated near the welded portion; a reduced welded area and suppressed oxide generation at the welded portion; and improved welding quality due to the reduced welded area and suppressed oxide generation.

[0063] A preferred embodiment of the present invention, developed based on the above findings, is described below. Note that various equipment, such as leads from a power source and a roll that presses the weld zone from above at an angle, is disposed above the open tube 1 near the squeeze roll 6. Therefore, when the induction coil 3 of the prior art is wound around the open tube 1, there is a natural limit to how close the induction coil 3 itself can be to the weld zone. In such a case, the position at which an induced current is generated in the open tube 1 must be located some distance from the joint 5, resulting in poor heating efficiency. In contrast, by using the induction coil described above as in the present invention, a larger space can be secured above the open tube 1 than in the prior art. As a result, heating efficiency can be improved by positioning the induction coil closer to the weld zone depending on the secured space. Furthermore, as disclosed in International Publication No. 2013 / 69748, auxiliary equipment, such as measuring devices and devices for observing the welding status, can be installed in the space above the open tube 1, enabling operation while controlling welding quality.

[0064] <Regarding an Electric-Resistance Welded Pipe Manufacturing Apparatus According to an Embodiment of the Present Invention> The electric-resistance welded pipe manufacturing apparatus according to an embodiment of the present invention, which was developed based on the findings described above, is an electric-resistance welded pipe manufacturing apparatus for manufacturing an electric-resistance welded pipe by melting both end surfaces of a pipe blank facing the opening extending in the running direction of the pipe using an induction current generated by an induction coil, and then bringing the end surfaces into contact with each other in a squeeze roll section while gradually narrowing the gap between the openings to weld them. Here, the pipe blank for the open pipe is made of, for example, steel or stainless steel.

[0065] An electric-welded pipe manufacturing apparatus according to an embodiment of the present invention has, as the induction coil, a pair of inclined conductor sections that are inclined at a predetermined angle relative to the vertical direction in a side view, a connecting conductor section that is provided at the downstream ends of the pair of inclined conductor sections in the running direction so as to connect each of the pair of inclined conductor sections to each other and that is arranged so as to straddle the opening in a plan view, and a power supply connection conductor section that is provided at the upstream ends of the pair of inclined conductor sections in the running direction and is connected to a power supply, and the pair of inclined conductor sections are equipped with an induction coil that is arranged so as to gradually move away from the opening as they move upstream in the running direction in a plan view.

[0066] The inclined conductor is spaced apart from the outer peripheral surface of the open pipe, and the connecting conductor is connected to an end of the inclined conductor that is closer to the weld, and the power supply connecting conductor is connected to an end of the inclined conductor that is farther from the weld.

[0067] The configuration of an electric resistance welded pipe manufacturing apparatus 10 and an induction coil 100 according to an embodiment of the present invention will be described in detail below with reference to Figures 5 to 19. Note that although the impeller is depicted in the figures, in reality the impeller is housed in an impeller case. However, because depicting the impeller case in a small space would make the drawings more complicated, the impeller case will not be shown in the following drawings either.

[0068] FIG. 5 is a perspective view that schematically shows the outline of the configuration of an electric-resistance-welded pipe manufacturing apparatus according to this embodiment, and FIG. 6 is a plan view that schematically shows the electric-resistance-welded pipe manufacturing apparatus according to this embodiment.

[0069] As shown in Fig. 5, an electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment is an apparatus used to manufacture electric-resistance welded pipe by melting the ends of an open pipe 1 that has been bent into a cylindrical shape while being transported in a predetermined running direction R using an induced current and then butting the ends together to perform electric-resistance welding. This electric-resistance welded pipe manufacturing apparatus 10 has a group of forming rolls 11 for forming the desired electric-resistance welded pipe shape, and an induction coil 100. As shown in Fig. 5, a pair of squeeze rolls 6 are provided further downstream of the induction coil 100.

[0070] As shown in Figures 5 and 6, an electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment bends a metal band traveling in a traveling direction R by a group of forming rolls 11 into a cylindrical shape so that both end face portions (end face portions) 2a, 2b in the width direction of the metal band face each other with a gap therebetween to form an open pipe 1. Then, a high-frequency current is passed through an induction coil 100 disposed near an opening 2 of the open pipe 1, and the generated induced current melts both end face portions 2a, 2b. That is, the electric-resistance welded pipe manufacturing apparatus 10 uses the induction coil 100 to induce a high-frequency current near the opening 2 of the open pipe 1. Typically, the induced current is generated and flows directly below the induction coil 100. When high-frequency currents of opposite polarities flow nearby, a proximity effect occurs, causing the high-frequency currents to move closer to each other, reducing inductance (i.e., narrowing the space surrounded by these currents).

[0071] In this embodiment, since both end face portions 2a, 2b of the open pipe 1 are positioned close to each other, the space surrounded by both end face portions 2a, 2b is a space surrounded by induced currents of opposite polarity. In addition, the induced current generated outside the opening 2 is diverted and flows through both end face portions 2a, 2b. This current heats and melts both end face portions 2a, 2b.

[0072] The opening 2 is gradually narrowed by pressing both sides of the open pipe 1 with squeeze rolls 6, so that both end face portions 2 a, 2 b come into contact and are welded. More specifically, the electric resistance welded pipe manufacturing apparatus 10 according to this embodiment is an apparatus for manufacturing electric resistance welded pipes in which both end face portions 2 a, 2 b of a pipe material facing the opening 2 from both sides (in other words, the end face portions facing each other across the opening 2) of the open pipe 1 having the opening 2 extending in the running direction R are melted by an induced current generated by an induction coil 100, and the end face portions 2 a, 2 b are brought into contact with each other at a joint portion 5 while the gap between the opening 2 is gradually narrowed, thereby welding the end face portions 2 a, 2 b.

[0073] The inclined conductor portion according to this embodiment is not provided to extend in the running direction along each of the end surfaces on both sides of the opening 2. Specifically, as will be described below, the inclined conductor portion according to this embodiment is arranged so as to be inclined at a predetermined angle with respect to the vertical direction in a side view.

[0074] The electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment will be described in more detail below with reference to Figures 6 to 17. Figure 6 is a plan view schematically showing the electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment. Figure 7 is a side view of the electric-resistance welded pipe manufacturing apparatus 10 shown in Figure 6, where (a) is a right side view as seen from the right side in the direction of travel, and (b) is a left side view as seen from the left side in the direction of travel.

[0075] As previously explained, the electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment has an induction coil 100 having a shape as shown in FIGS. 5 to 7, instead of the induction coil 3 in the conventional electric-resistance welded pipe manufacturing apparatus 900 shown in FIG. 1.

[0076] As shown in Figures 6 and 7, the induction coil 100 includes a pair of inclined conductor portions 110A, 110B (hereinafter, A refers to the side of one end surface portion 2a, and B refers to the side of the other end surface portion 2b. The pair of inclined conductor portions may be collectively referred to as the "inclined conductor portions 110"), a connecting conductor portion 120, a pair of partial circumferential conductor portions 130A, 130B (hereinafter, the pair of partial circumferential conductor portions may be collectively referred to as the "partial circumferential conductor portions 130"), and a pair of power supply connecting conductor portions 140A, 140B (hereinafter, the pair of power supply connecting conductor portions may be collectively referred to as the "power supply connecting conductor portions 140").

[0077] The material of the induction coil 100 used in this embodiment is not particularly limited, but it may be formed from a pipe, wire, plate, or the like made of a good conductor with low resistance such as copper.

[0078] The inclined conductor portion 110 is provided at a distance from the outer peripheral surface of the open pipe 1, and is arranged so as to be inclined at a predetermined angle θ with respect to the vertical direction in a side view, as shown in Fig. 7. In this inclined conductor portion 110, the connecting conductor portion 120 is located at an end of the inclined conductor portion 110 closer to the squeeze roll 6, and the partial surrounding conductor portion 130 and the power supply connecting conductor portion 140 are located at ends of the inclined conductor portion 110 farther from the squeeze roll 6.

[0079] <Regarding the inclined conductor portion 110> As shown in Figure 6, when the electric-welded pipe manufacturing apparatus 10 is viewed in a plan view from above in the vertical direction, the inclined conductor portions 110A, 110B are positioned at a distance from the outer peripheral surface of the open pipe 1 so that they gradually move away from the opening 2 as they move upstream in the traveling direction R.

[0080] This arrangement prevents magnetic flux generated by the inclined conductors 110A and 110B from directly entering the impedance 7, while directly inducing an induced current on the outer surface of the open pipe 1 near the end faces 2a and 2b. Furthermore, the induced current moves to the end faces 2a and 2b, which have low impedance, due to the proximity effect before dissipating. This allows the end faces 2a and 2b to be heated efficiently, improving heating and welding efficiency and welding quality. Furthermore, in a plan view as shown in FIG. 6 , the inclined conductors 110A and 110B are positioned farther away from the end faces 2a and 2b as they move upstream in the running direction R away from the joint 5. This prevents excessive concentration of the induced current on the outer peripheral surface of the open pipe 1 and prevents overheating of the outer surface of the open pipe 1.

[0081] 7 is in the range of 20 to 70°. When the inclination angle θ is 20° or more, even if an upward pressing roll is provided above the open pipe 1, the tip of the connecting conductor 120 can be brought close to the welded portion 5. The inclination angle θ of the inclined conductor 110 is preferably 30° or more.

[0082] Furthermore, by setting the inclination angle θ to 70° or less, even if an upward pressing roll is provided above the open pipe 1, it is possible to bring the tip of the connecting conductor 120 close to the welding portion 5 while preventing the upward pressing roll from being heated. The inclination angle θ of the inclined conductor 110 is preferably 60° or less.

[0083] 6 and 7 show the case where the inclined conductor 110 is inclined at a certain constant angle θ, but the inclination angle θ of the inclined conductor 110 may not be constant. １ In another part, the inclination angle is θ ２ (≠θ １ ) the inclination angle θ may be changed stepwise or continuously.

[0084] 6 and 7, the inclined conductors 110A and 110B are linear, but the shapes of the inclined conductors 110A and 110B are not limited to these examples. The inclined conductors 110A and 110B may have bent or curved portions as long as they have a shape such that the farther they are from the joint 5 toward the upstream side in the running direction R, the farther they are from the end surface portions 2a and 2b.

[0085] <Regarding the connecting conductor portion 120> As shown in Figures 6 and 7 , the connecting conductor portion 120 is provided at the downstream end of the pair of inclined conductor portions 110A, 110B in the running direction so as to connect each of the pair of inclined conductor portions 110A, 110B to each other, and is positioned at a distance from the outer peripheral surface so as to straddle the opening 2 of the open pipe 1 in a plan view.

[0086] As described above, the spacing between the openings 2 narrows as the openings 2 approach the joint 5. The connecting conductor 120 straddles the openings 2 at a position where the spacing (opening width) between the openings 2 narrows to a degree that prevents the magnetic flux generated by the connecting conductor 120 from penetrating into the open pipe 1.

[0087] With this configuration, even if the connecting conductor part 120 crosses the opening 2, the magnetic flux will not directly enter the impedance 7. Note that the specific opening width that prevents the magnetic flux from entering the inside of the open tube 1 will be described later.

[0088] Here, Figure 6 illustrates an example in which the connecting conductor portion 120 has a linear shape, but the shape of the connecting conductor portion 120 is not limited to the example shown in Figure 6, and it may be an approximately V-shape with a bent portion, or an approximately U-shape with a curved portion.

[0089] A linear connecting conductor 120 is easy to manufacture. A generally U-shaped connecting conductor 120 having a curved portion allows current to flow easily. On the other hand, a generally V-shaped connecting conductor 120 having a bent portion is prone to power concentration at its vertex, whereas linear and generally U-shaped connecting conductors 120 are less prone to such power concentration. Furthermore, the generally V-shaped connecting conductor 120 is smaller in shape than the generally U-shaped connecting conductor 120. Therefore, the generally V-shaped connecting conductor 120 is particularly effective when manufacturing small-diameter electric resistance welded pipes, where the space in which the connecting conductor 120 can be placed tends to be narrow. Thus, when using a generally U-shaped or generally V-shaped connecting conductor 120, it is preferable to determine which connecting conductor 120 to use depending on the application of the connecting conductor 120.

[0090] <Regarding the partial circumferential conductor portion 130> As shown in Figures 6 and 7 , the pair of partial circumferential conductor portions 130A, 130B are integrally provided at the end portions of the inclined conductor portions 110A, 110B that are farther from the longitudinal joint portion 5, and are arranged at a distance from the outer peripheral surface of the open pipe 1 in a position that does not overlap with the opening 2 in a planar view (more specifically, so as to circle around the portion of the outer peripheral surface of the open pipe 1 excluding the opening 2).

[0091] <Regarding the power supply connection conductor portion 140> As shown in Figures 6 and 7, one end of the pair of power supply connection conductor portions 140A, 140B is integrally provided at a position facing any part of the partial surrounding conductor portions 130A, 130B in the outer circumferential direction of the open tube 1, and the other end is connected to a high-frequency power supply (not shown).

[0092] 6 and 7 show an example of the connection position between the partial surrounding conductor portion 130 and the power supply connecting conductor portion in the outer circumferential direction of the open tube 1, which is the bottom portion farthest from the opening 2 of the open tube 1. However, the connection position is not limited to the example shown in the drawings, and may be, for example, a horizontal position approximately 90° around the opening 2.

[0093] 6 and 7 show an example in which the power supply connection conductors 140A, 140B extend vertically downward, but the power supply connection conductors 140A, 140B may be disposed so as to extend horizontally on the side of the pipe.

[0094] Furthermore, when horizontally extending power supply connection conductors 140A, 140B are provided, the power supply connection conductors 140A, 140B may be connected to the inclined conductor portion 110 without the partial surrounding conductor portion 130. Since the induction coil 100 according to this embodiment only needs to be disposed above the open tube 1 without surrounding it, the configuration including the horizontally extending power supply connection conductor 140 as described above can be applied to both small and medium diameter electric resistance welded pipes to be manufactured. In other words, according to this embodiment, there is no need to prepare and replace induction coils 100 each having a surrounding portion that matches the diameter of the electric resistance welded pipe to be manufactured, thereby reducing the cost and labor associated with manufacturing and replacing induction coils.

[0095] 8A and 8B are side views of the electric-resistance welded pipe manufacturing apparatus 10 shown in FIG. 6, where FIG. 8A is a right side view as seen from the right side in the running direction, and FIG. 8B is a left side view as seen from the left side in the running direction. As shown in FIG. 8, in the electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment, a primary current C Ｐ is passed through (primary current C shown in FIG. 8) Ｐ The direction of the alternating current is merely a convenient indication of the direction of the alternating current at a given moment, and naturally includes the case where the current alternates and flows in the opposite direction.) At this time, a high-frequency primary current C Ｐ 9 and 10 (alternating induced currents cut at a certain moment) in the open pipe 1. Of the induced current distributions shown in Fig. 10, Fig. 9 shows the induced current distribution on the side surface on the right side in the running direction of the open pipe 1, and the induced current distribution on the side surface on the left side in the running direction is not shown.

[0096] As shown in FIG. 8(a), when a certain moment of the alternating high frequency current is taken, the primary current C flowing through the induction coil 100 is Ｐ flows from bottom to top in FIG. 8(a) through the power supply connection conductor 140B connected to a high frequency power supply (not shown), and then flows in the same direction as the running direction (from left to right in FIG. 8(a)) through the partial surrounding conductor 130B and the inclined conductor 110B. Next, the primary current C Ｐ After flowing through the connecting conductor portion 120, as shown in FIG. 8(b), which is a snapshot of the same moment as FIG. 8(a), the primary current C flows through the inclined conductor portion 110A in the direction opposite to the direction of travel (from left to right in FIG. 8(b)). Furthermore, the primary current C Ｐ After flowing through the partial surrounding conductor portion 130A, it flows from the top to the bottom in FIG. 8B through the power supply connecting conductor portion 140A connected to the high frequency power supply, and returns to the high frequency power supply.

[0097] The primary current C Ｐ 9 and 10, which are cut out from the same instant as FIG. 8, a primary current C Ｐ As a result, near the inclined conductor portions 110A and 110B, induced currents 40b and 40d flowing near the end surfaces 2a and 2b, respectively, and induced currents 40a and 40c flowing on the outer surface of the open tube 1 are generated in a branched manner. Also, near the connecting conductor portion 120, induced currents 40e and 40f flowing through the end surfaces 2a and 2b are generated so as to be connected via the joint portion 5. Furthermore, near the partial surrounding conductor portions 130A and 130B, induced currents 40g and 40h flow with their origin and end points near the upstream ends of the inclined conductor portions 110A and 110B, respectively, so as to continuously connect these induced currents. In this way, the primary current C flowing through the induction coil 100 ＰAs a result, main current loops (closed circuits) 40g, 40a+40b, 40e, 40f, 40d+40c, and 40h are formed near the opening 2. In addition, induced currents 50a and 50b branching upstream from the main current loops and current loops circulating around the outer periphery of the tube are further formed, with the upstream ends of the inclined conductors 110A and 110B as their starting and ending points.

[0098] Among the induced currents, induced currents 40b and 40d flowing near the end faces 2a and 2b and induced currents 40e and 40f flowing through both end faces 2a and 2b on the joint 5 side heat up and melt the end faces 2a and 2b, thereby welding the joint 5. At this time, the distance between the openings 2 narrows as the distance approaches the joint 5, lowering the impedance. Furthermore, the proximity effect also contributes to the synergistic effect of current concentration and increased temperature. Furthermore, the inclined conductor 110 is positioned so that it gradually approaches both end faces 2a and 2b downstream in the running direction. Therefore, the induced current does not excessively concentrate on the outer surface of the open pipe 1, but tends to flow near the inclined conductor 110 in the region where the inclined conductor 110 is located near both end faces 2a and 2b. This, combined with the proximity effect of both end faces 2a and 2b, allows the induced current to flow effectively to both end faces 2a and 2b without excessively concentrating on the outer surface. This allows the joint 5 to be heated effectively without causing overheating of the outer surface of the open pipe 1.

[0099] In the induction coil 100 according to this embodiment, the inclined conductor portion 110 and the partial surrounding conductor portion 130 can form a closed circuit by induced current without crossing the opening 2 of the open tube 1. Furthermore, although the connecting conductor portion 120 crosses the opening 2, it crosses the opening 2 at a position where the width of the opening 2 narrows enough to prevent magnetic flux from penetrating the interior of the open tube 1. As a result, magnetic flux from the induction coil 100 can be prevented from directly entering the impeder 7. Furthermore, since the magnetic flux density of the impeder 7 can be reduced, the magnetic flux entering the impeder 7 can also be reduced, preventing damage to the impeder. This effect is particularly useful when manufacturing small-diameter pipes (e.g., pipes with an inner diameter of approximately 100 mm or less), especially thick-walled pipes (e.g., pipes with a wall thickness of more than 6 mm).

[0100] Furthermore, the induction coil 100 according to this embodiment, having the inclined conductor portion 110 as described above, can position the connecting conductor portion 120 as close as possible to the joint 5 while positioning the partial surrounding conductor portion 130 farther from the joint 5 than the conventional induction coil described in Patent Document 1. This allows space to be secured above the open pipe 1 near the upstream side of the joint 5. Even if an upper pressing roll (not shown) is installed above the open pipe 1, the induction coil 100 can be positioned close to the joint 5 without interfering with the upper pressing roll. This space can also be used to install auxiliary equipment such as a measuring device or a weld observation device. Furthermore, because the connecting conductor portion 120 is provided at the end of the inclined conductor portion 110 on the joint 5 side without encircling the open pipe 1, the encircling portion of the induction coil 100 does not interfere with the squeeze roll 6 as in the conventional induction coil. This allows the induction coil 100 to be positioned closer to the joint 5. This improves heating efficiency when manufacturing electric resistance welded pipes. Furthermore, this improved heating efficiency also contributes to improved welding quality. Such an effect is also useful when manufacturing pipes with an outer diameter of, for example, about 100 to 700 mm.

[0101] Furthermore, in the induction coil 100 according to this embodiment, heating efficiency is improved by arranging the induction coil 100 close to the weld. Here, as shown in Fig. 7, the shortest distance G between the tip of the connecting conductor 120 and the squeeze roll 6 is preferably 20 mm or more (G ≥ 20 mm). This makes it possible to concentrate the induced current on the end face portion up to the weld 5 while suppressing induction heating of the squeeze roll 6, thereby enabling more effective welding.

[0102] In the example of Figure 7, the direction of the shortest distance G between the connecting conductor part 120 and the squeeze roll 6 is shown in an oblique direction when viewed from the side, but this direction is determined depending on the relationship between the distance between the connecting conductor part 120 and the open pipe 1 and the curvature of the curved part when viewed from the side of the squeeze roll 6 and the roll (not shown) that presses the weld part from obliquely above.

[0103] Furthermore, the conventional preheating coil described in Patent Document 2 is connected to a preheating power supply with a frequency of approximately 1 to 20 kHz, and furthermore, a contact (contact tip) connected to a welding power supply with a frequency of approximately 100 to 400 kHz, separate from the preheating power supply, is provided between the preheating coil and the joint. Therefore, this preheating coil cannot be brought close to the squeeze roll. In contrast, the induction coil 100 of this embodiment is connected to a high-frequency power supply via a power supply connection conductor 140, and no other power supply is provided between the induction coil 100 and the joint 5. Therefore, the induction coil 100 can be brought close to the joint 5.

[0104] Furthermore, the device described in Patent Document 2 has two power supplies: one connected to the preheating coil and one connected to the contacts. In such cases, the two power supplies may be inductively coupled to each other. This inductive coupling may cause the load to become unstable, resulting in a loss of matching and the resulting oscillation, which may result in a lack of current flow. As a result, it may be impossible to heat the open pipe, and the electric resistance welded pipe may not be properly manufactured. In contrast, the electric resistance welded pipe manufacturing device 10 of this embodiment is equipped with only one high-frequency power supply for the induction coil 100, thereby stabilizing the load.

[0105] Furthermore, unlike the induction coils disclosed in Patent Documents 3 and 4, the induction coil 100 according to this embodiment has the inclined conductor portion 110 arranged so that, when the electric resistance welded pipe manufacturing apparatus 10 is viewed vertically from above in a plan view, it becomes increasingly farther away from the opening 2 as it moves upstream in the traveling direction R. Therefore, the induced current does not concentrate excessively on the outer surface of the open pipe 1, and therefore the surface of the open pipe 1 is not overheated, making it possible to manufacture electric resistance welded pipe with excellent surface quality.

[0106] The electric resistance welded pipe manufacturing apparatus 10 of this embodiment is not limited to small diameter pipes, but can also be applied to medium diameter pipes, which have an outer diameter of, for example, about 300 to 600 mm.

[0107] In the case of electric resistance welding of medium-diameter pipes, the amount of power input for welding is generally greater than in the case of small-diameter pipes, and the opening 2 of the open pipe 1 is also wider, so there is a concern that the strong magnetic field generated by the induction coil 3 as shown in Fig. 1 may directly affect the impedance 7. However, by using the induction coil 100 having the configuration of this embodiment, this effect can be minimized, enabling stable production.

[0108] That is, in typical electric resistance welded pipe welding, the opening 2 of the open pipe 1 is unobstructed. Therefore, through this opening 2, magnetic flux from the strong magnetic field generated by the induction coil 3 selectively penetrates the impeder 7, which has high magnetic permeability. Therefore, if the impeder 7 does not have a cross-sectional area sufficient to prevent magnetic flux saturation, the impeder 7 will undergo magnetic flux saturation and generate heat, which will not prevent damage to the impeder 7 and will also eliminate the effect of suppressing current flow around the inner surface of the open pipe 1. In contrast, in the induction coil 100 configured according to this embodiment, the inclined conductor portion 110 and the partial surrounding conductor portion 130 do not overlap the opening 2 in a plan view. Furthermore, even if the connecting conductor portion 120 straddles the opening 2, the width of the opening 2 at the straddling portion is configured to be sufficiently small. Therefore, the magnetic flux generated by the induction coil 100 does not enter the interior of the open pipe 1 through the opening 2. As a result, damage to the impeder 7 can be prevented even when power is increased, enabling stable production.

[0109] Furthermore, in the case of electric resistance welding of medium-diameter pipes, the electric power required for welding is generally greater than that required for small-diameter pipes, so the applied current value must also be increased, resulting in poorer heating efficiency than for small-diameter pipes. In contrast, in this embodiment, the induction coil 100 can be placed closer to the squeeze roll 6, thereby improving heating efficiency. Therefore, this effect is also useful when manufacturing medium-diameter pipes.

[0110] The configuration of the induction coil 100 according to this embodiment is not limited to the above example, and may include at least the inclined conductor portion 110, the linking conductor portion 120, and the power supply connecting conductor portion 140, as shown in Fig. 11 to Fig. 13. Fig. 11 shows an example of a linear inclined conductor portion 110, Fig. 12 shows an example of an inclined conductor portion 110 having a bent portion, and Fig. 13 shows an example of an inclined conductor portion 110 having a curved portion.

[0111] <Preferred Arrangement of Induction Coil 100> Next, a preferred arrangement of the induction coil 100 in this embodiment will be described. From the viewpoint of efficiency, it is preferable that the distance between the inner peripheral surface of the induction coil 100 and the outer peripheral surface of the open tube 1 is as close as possible. However, in order to avoid contact between the induction coil 100 and the open tube 1, it is preferable that the induction coil 100 is spaced 5 mm or more from the open tube 1 and arranged at as short a distance as possible while avoiding contact with the open tube 1. Furthermore, it is preferable that the distance between the inner peripheral surface of the induction coil 100 and the outer peripheral surface of the open tube 1 is constant over the entire circumference of the open tube 1.

[0112] In this embodiment, as described above, the inclined conductors 110A and 110B are arranged at positions that do not overlap the openings 2 in plan view. However, as shown in Fig. 14 , the distance between the end portions of the inclined conductors 110A and 110B that are closer to the openings 2 in plan view and the end face portions 2a and 2b of the openings 2 is preferably determined in accordance with the spacing between the openings 2.

[0113] In other words, it is important to determine the positions of the inclined conductors 110A and 110B through electromagnetic field analysis or experiments, taking into consideration the viewpoints of efficiently heating the end face portions 2a and 2b and the viewpoint that if the inclined conductors 110A and 110B are positioned as close as possible to the end face portions 2a and 2b, the magnetic fluxes generated in the inclined conductors 110A and 110B will interfere with each other, making it difficult for induced currents to be generated in the end face portions 2a and 2b.

[0114] 14 , when the induction coil 100 is viewed from above, the distance from the end of the connecting conductor 120 on the welding point 5 side to the welding point 5 is defined as L1 (mm). The distance from the end of the connecting conductor 120 on the welding point 5 side to the position where the induction coil 3 reaches the bottom of the open tube 1 is defined as L2 (mm), and the distance between the end face of the opening 2 and the inclined conductor 110 at the position where the induction coil 3 reaches the bottom of the open tube 1 is defined as d (mm). Furthermore, the Apex angle of the opening 2 is defined as α. At this time, the distance D (mm) between the pair of inclined conductors 110 at the position where the induction coil 3 reaches the bottom of the open tube 1 preferably satisfies the relationship expressed by the following formulas (101) and (102).

[0115] D = 2d + (L1 + L2) × tan (α / 2) ...Equation (101) 10 (mm) ≦ D ≦ (open pipe diameter + 10) (mm) ...Equation (102)

[0116] The relationships expressed by the above formulas (101) and (102) were obtained from the results of electromagnetic field analysis conducted by the inventors on steel pipes with outer diameters of approximately 30 to 500 mm. Within the range in which the above relationships are satisfied, the maximum magnetic flux density of the impedance 7 is significantly reduced without any problems. It has also been confirmed that the amount of heat generated at the end face portions 2a and 2b is also significantly reduced due to the magnetic flux interference between the induction coils 100.

[0117] Furthermore, when the induction coil 100 according to this embodiment is viewed vertically from above in a plan view, as shown in FIG. 14, it is preferable that the angle φ between the center lines of the inclined conductor portions 110 is larger than the ape angle α of the opening (i.e., the relationship φ > α holds). Here, when manufacturing steel pipes with an outer diameter of approximately 30 to 500 mm, the ape angle α is often set to approximately 3°. The fact that the relationship φ > α holds means that the shape of the inclined conductor portions 110 gradually moves away from the opening 2 as it moves upstream in the running direction.

[0118] Furthermore, in this embodiment, the spacing of the openings 2 at the positions where the connecting conductor parts 120 are provided is set to a value that does not affect the impedance 7 even if magnetic flux penetrates into the open tube 1, as described above, and does not affect equipment such as the squeeze roll 6.

[0119] Specifically, although it depends on the pipe diameter of the open pipe 1, the opening angle of the openings 2 (i.e., the ape angle α), and the molten state of both end faces, the spacing of the openings 2 at the positions where the connecting conductors 120 are provided (the spacing of the openings 2 at a distance L1 from the joint 5) is preferably 5 mm or less, at which point the influence of the magnetic field of the induction coil is substantially small. Furthermore, in order to efficiently flow the end face current through the weld, it is preferable to make the distance L1 from the joint 5 as short as possible, taking into account the arrangement of the squeeze roll 6.

[0120] Generally, in the opening 2 within a range of about 30 mm upstream from the joint 5, the end face portions 2 a, 2 b that have melted near the welded portion are in contact with each other. If the connecting conductor portion 120 is provided above the opening 2 within this fusion range, the opening 2 is essentially blocked by the fused material, and it is thought that this makes it possible to prevent magnetic flux from penetrating into the open pipe 1.

[0121] The induction coil 100 according to this embodiment has been described in detail above.

[0122] <Modification of induction coil 100> In Figure 6 and other figures, a case is illustrated in which one induction coil 100 according to this embodiment is used, but the electric-resistance welded pipe manufacturing apparatus 10 according to this embodiment may use multiple induction coils 100 as described above.

[0123] In addition, in the examples shown in Figures 6 and 7, the induction coil 100 according to this embodiment has a shape that does not surround the open tube 1, but it is also possible to realize a shape of the induction coil 100 provided with the inclined conductor portion 110 that surrounds the open tube 1, as shown in Figures 15 to 17.

[0124] Fig. 15 is a plan view schematically showing a modified example of the electric-resistance welded pipe manufacturing apparatus 10 according to the present embodiment, Fig. 16 is a side view schematically showing the modified example of the electric-resistance welded pipe manufacturing apparatus 10 according to the present embodiment, Fig. 17 is a side view schematically showing another modified example of the electric-resistance welded pipe manufacturing apparatus 10 according to the present embodiment, Fig. 18 is a plan view schematically showing yet another modified example of the electric-resistance welded pipe manufacturing apparatus 10 according to the present embodiment, and Fig. 19 is a side view schematically showing yet another modified example of the electric-resistance welded pipe manufacturing apparatus 10 according to the present embodiment.

[0125] As shown in Figures 15 and 16 , in a modified example of the induction coil 100 according to this embodiment, the induction coil 100 may have a generally elliptical shape. In this case, one end of the elliptical shape in the major axis direction, located above the open tube 1, functions as the connecting conductor 120, and a power supply connection conductor 140 is provided at the other end of the elliptical shape in the major axis direction, located below the open tube 1. In the generally elliptical induction coil 100, the portion other than the portion that functions as the connecting conductor 120 functions as an inclined conductor 111 utilizing part of the elliptical shape. Even with the above-described elliptical induction coil 100, it is possible to realize the inclined conductor 111, as shown in Figure 16 .

[0126] 17 , in the inclined conductor 111 utilizing a portion of an elliptical shape as described above, a partial surrounding conductor 130 may be provided at the end opposite to the side functioning as the linking conductor 120. In this case, a power supply connecting conductor 140 is provided at a portion of the partial surrounding conductor 130. The induction coil 100 having the shape shown in FIG. 17 can be said to be a combination of the induction coil 100 shown in FIGS. 6 and 7 with the inclined conductor 111 utilizing a portion of an elliptical shape as shown in FIGS. 15 and 16 .

[0127] Furthermore, as shown in Figures 18 and 19, the induction coil 100 of this modified example may have a connecting conductor portion 120 that is approximately V-shaped, an inclined conductor portion 111 that utilizes part of an elliptical shape, and a power supply connecting conductor portion 140 that is provided at the end of the inclined conductor portion 111 opposite to the side to which the connecting conductor portion 120 is connected.

[0128] Although Figures 15 to 19 illustrate an induction coil with one turn and an elliptical shape as the induction coil 100 according to this modification, the induction coil 100 according to this modification may be an induction coil with two turns and an elliptical shape, or an induction coil with three or more turns and a multi-turn structure.

[0129] The electric resistance welded pipe manufacturing apparatus, induction coil, and electric resistance welded pipe manufacturing method according to the embodiment of the present invention have been described in detail above with reference to FIGS.

[0130] In order to confirm the effects of the induction coil and electric resistance welded pipe manufacturing apparatus according to the above-described embodiment of the present invention, an analysis of the effects was carried out in examples. Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0131] A magnetic field analysis was performed on an electric resistance welded pipe made of ordinary steel with a diameter of 89.1 mm and a wall thickness of 6 mm, using an electric resistance welded pipe manufacturing apparatus having the induction coils of Examples 1 to 3 and Comparative Example 1 shown below, in which a primary current of 3000 A was passed through the pipe. In this magnetic field analysis, the specific magnetic field state of a three-dimensional model of the electric resistance welded pipe manufacturing apparatus having the induction coils of Examples 1 to 3 and Comparative Example 1 was determined using the finite element method (FEM) with a numerical calculation application, and Maxwell's equations were used to determine the current density generating eddy currents, as well as the Joule heating density and magnetic flux density based on this current density. An impeder with a diameter of 16 mm and a length of 300 mm was positioned inside the open pipe, extending from a position 30 mm upstream from the weld toward the upstream side. The magnetic field analysis was performed using the ratio of the total heat generation at the open pipe end face within a range of 700 mm upstream from the joint and the maximum magnetic flux density of the impedancer installed inside the open pipe. The ratio of the total heat generation was calculated by normalizing the sum of the heat generation at the end face of each example with the sum of the heat generation in the comparative example. The ratio of the maximum magnetic flux density was calculated by normalizing the maximum magnetic flux density of each example with the maximum magnetic flux density in the comparative example.

[0132] Example 1: The electric resistance welded pipe manufacturing apparatus of Example 1, equipped with the induction coil shown in Figures 15 and 16, was used for the analysis. Specifically, an inclined conductor was positioned connecting a position 50 mm upstream from the weld to a position 140 mm upstream from the weld. A connecting conductor was positioned at the end of the inclined conductor on the weld side, spanning the open pipe. At the other end of the inclined conductor, a power supply connection conductor extended below the open pipe was connected to a high-frequency power supply. The inclination angle of the induction coil relative to the vertical direction when viewed from the side was 60°. Then, assuming a situation in which a high-frequency current of 3000 A was flowing, the total heat generation ratio at the open end of the open pipe and the maximum magnetic flux density of the impedance were calculated.

[0133] Example 2: The electric resistance welded pipe manufacturing apparatus of Example 2 was analyzed using the electric resistance welded pipe manufacturing apparatus equipped with the induction coil shown in FIG. 12 . Specifically, an inclined conductor was positioned connecting a position 50 mm upstream from the weld and a position 140 mm upstream from the upstream position. A generally V-shaped connecting conductor was positioned at the end of the inclined conductor on the weld side, straddling the open pipe. At the other end of the inclined conductor, a power supply connection conductor extending below the open pipe was connected to a high-frequency power supply. The inclination angle of the induction coil relative to the vertical direction when viewed from the side was 60°. Then, assuming a situation in which a high-frequency current of 3000 A was flowing, the total heat generation ratio at the open end of the open pipe and the maximum magnetic flux density of the impedance were calculated.

[0134] Example 3: The electric resistance welded pipe manufacturing apparatus of Example 3, equipped with an induction coil, was used for the analysis. Specifically, an inclined conductor was positioned connecting a position 50 mm upstream from the weld and a position 140 mm upstream from the weld. A generally V-shaped connecting conductor was positioned at the end of the inclined conductor on the weld side, spanning the open pipe. A partial circumferential conductor and a power supply connection conductor were positioned at the other end of the inclined conductor. The inclination angle of the induction coil relative to the vertical direction when viewed from the side was 60°. Then, assuming a situation in which a high-frequency current of 3000 A was flowing, the total heat generation ratio at the open end of the open pipe and the maximum magnetic flux density of the impedance were calculated.

[0135] Comparative Example 1: The electric resistance welded pipe manufacturing apparatus of Comparative Example 1 was installed 100 mm from the weld, and had a circular induction coil encircling a normal open pipe 50 mm wide upstream in the longitudinal direction, spaced 10 mm radially outward from the outer surface of the open pipe. The inclination angle of the induction coil relative to the vertical when viewed from the side was 0°.

[0136] Table 1 shows the measurement and calculation results of the total heat generation ratio and the maximum magnetic flux density of the impeder for the electric resistance welded pipe manufacturing apparatus according to Examples 1 to 3 and Comparative Example 1 configured as described above.

[0137] In Table 1, the total heat generation amount is shown as the total heat generation amount ratio. This total heat generation amount ratio indicates the ratio of the total heat generation amount at the end face of the steel pipe in the example and comparative example 1 to the total heat generation amount at the end face of the opening when a normal induction coil that goes around across the opening of an open pipe, arranged with the same width and the same distance from the weld, is used to generate heat with the same current (in other words, the ratio of the total heat generation amount at each example when the total heat generation amount in comparative example 1 is set to 1).

[0138]

[0139] As shown in Table 1, when electric resistance welded pipe welding was performed using an electric resistance welded pipe manufacturing apparatus equipped with an induction coil having an inclined conductor and a connecting conductor according to Examples 1 to 3, it was found that the total amount of heat generated at the end surface of the opening could be increased by 20% compared to the amount of heat generated in Comparative Example 1. In particular, it was found that in Examples 1 and 3, the total amount of heat generated at the end surface of the opening could be increased by about 20%, while the magnetic flux entering the impeller (i.e., the maximum magnetic flux density of the impeller) could be reduced by about 10%. In other words, it was found that by using an electric resistance welded pipe manufacturing apparatus equipped with an induction coil having an inclined conductor and a connecting conductor as in the present invention, it is possible to improve the heating efficiency when manufacturing electric resistance welded pipe while preventing damage to the impeller.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] The present invention is useful for an electric resistance welded pipe manufacturing 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 by the current induced in the metal band.

[0144] REFERENCE SIGNS LIST 1 open pipe 2 opening 2a, 2b end surface 5 joint (squeeze roll portion, welded portion) 6 squeeze roll 7 impeder 8 rod 9 cooling water channel 10 electric resistance welded pipe manufacturing device 100 induction coil 110, 111 inclined conductor portion 120 connecting conductor portion 130 partial surrounding conductor portion 140 power supply connection conductor portion C Ｐ Primary current R Running direction 40a, 40b, 40c, 40d, 40e, 40f, 40g, 40h Induced current 50a, 50b Induced current

Claims

1. A method for manufacturing an electric-resistance welded pipe, in which both end surface portions of a pipe material of an open pipe having an opening extending in a running direction, which mutually face the opening from either side, are melted by an induced current generated by an induction coil, and the end surface portions are melted by pressing and contacting the outer surface of the pipe with a squeeze roll while gradually narrowing the distance between the opening, wherein the induction coil has: a pair of inclined conductor portions arranged to be inclined at a predetermined angle with respect to the vertical in a side view; a connecting conductor portion arranged on the downstream ends of the pair of inclined conductor portions in the running direction so as to connect each of the pair of inclined conductor portions to each other, and arranged so as to straddle the opening in a plan view; and a power supply connection conductor portion arranged on the upstream ends of the pair of inclined conductor portions in the running direction and connected to a power supply, wherein the pair of inclined conductor portions are arranged so as to be gradually separated from the opening as they move toward the upstream side in the running direction in a plan view.

2. The method for manufacturing an electric-resistance welded pipe as set forth in claim 1, wherein the pair of inclined conductor portions are inclined at an angle of 20 to 70 degrees with respect to the vertical direction when viewed from the side.

3. The method for manufacturing an electric resistance welded pipe according to claim 1 or 2, wherein, in a plan view of the induction coil, a distance from an end of the connecting conductor on the welding point side to the welding point is L1 (mm), a distance from the end of the connecting conductor on the welding point side to a position where the induction coil reaches the bottom of the open pipe is L2 (mm), a distance between the end face of the opening and the inclined conductor at the position where the induction coil reaches the bottom of the open pipe is d (mm), and an ape angle of the opening is α, a distance D (mm) between the pair of inclined conductors at the position where the induction coil reaches the bottom of the open pipe satisfies the relationships expressed by the following formulas (1) and (2): D = 2d + (L1 + L2) × tan (α / 2) ... formula (1) 10 (mm) ≦ D ≦ (diameter of open pipe + 10) (mm) ... formula (2) 4. A method for manufacturing an electric welded pipe as described in any one of claims 1 to 3, wherein when the induction coil is viewed in a plane, the angle φ between the center lines of each of the inclined conductor portions is larger than the ape angle α of the opening.

5. An induction coil for electric welded pipe manufacturing equipment used in an electric welded pipe manufacturing apparatus for manufacturing electric welded pipe, comprising: a pair of inclined conductor sections arranged to be inclined at a predetermined angle with respect to the vertical direction in side view; a connecting conductor section arranged integrally with the pair of inclined conductor sections at one end of the pair of inclined conductor sections so as to connect each of the pair of inclined conductor sections to each other; and a power supply connection conductor section arranged integrally with the pair of inclined conductor sections at the other end of the pair of inclined conductor sections and connected to a power supply, wherein the pair of inclined conductor sections are arranged to be inclined at 20 to 70 degrees with respect to the vertical direction in side view.

6. The induction coil for use in an electric-resistance-welded pipe manufacturing apparatus according to claim 5, wherein the induction coil has a substantially elliptical shape when viewed from above.

7. An induction coil for an electric-resistance-welded pipe manufacturing apparatus as set forth in claim 5 or 6, wherein the inclined conductor portion has a linear shape when viewed from the side.

8. An induction coil for an electric-resistance-welded pipe manufacturing apparatus as set forth in claim 5 or 6, wherein at least a portion of said inclined conductor portion is curved when viewed from the side.

9. An electric welded pipe manufacturing apparatus having a group of forming rolls for forming a desired electric welded pipe shape, and an induction coil for generating an induced current, wherein the induction coil has: a pair of inclined conductor sections arranged to be inclined at a predetermined angle with respect to the vertical direction in side view; a connecting conductor section arranged integrally with the pair of inclined conductor sections at one end of the pair of inclined conductor sections so as to connect each of the pair of inclined conductor sections to each other; and a power supply connection conductor section arranged integrally with the pair of inclined conductor sections at the other end of the pair of inclined conductor sections and connected to a power supply, wherein the pair of inclined conductor sections are arranged to be inclined at 20 to 70 degrees with respect to the vertical direction in side view.

10. An electric-resistance-welded pipe manufacturing apparatus as set forth in claim 9, wherein the induction coil has a substantially elliptical shape when viewed in a plan view.

11. An electric-resistance-welded pipe manufacturing apparatus according to claim 9 or 10, wherein the inclined conductor portion has a linear shape when viewed from the side.

12. An electric-resistance-welded pipe manufacturing apparatus as set forth in claim 9 or 10, wherein at least a portion of the inclined conductor portion is curved when viewed from the side.

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