Monitoring device and method for manufacturing electric resistance welded steel pipe

Abstract

This method for manufacturing an electric resistance welded steel pipe comprises: a step in which both end parts (11L, 11R) of a steel plate (10) are formed into a tubular shape so as to face each other while conveying the steel plate (10); and a step in which both end parts (11L, 11R) are heated and melted, and then both end parts (11L, 11R) that approach each other and come into contact toward the downstream side in the conveyance direction are welded using a squeeze roll (32) including side rolls (321L, 321R). When the steel plate (10) melting point, carbon concentration, and conveyance speed are Tm, MCc, and Vf, respectively, a distance DV-SQC from a convergence point (V) of both end parts (11L, 11R) to a central axis (ASR) of the side rolls (321L, 321R) satisfies the following expression.　0 < D V-SQC ≤ {Tm-(1525.867-177 × MCc) } × Vf / 4200

Description

Electric resistance welded steel pipe manufacturing method and monitoring device

[0001] The present disclosure relates to a method for manufacturing an electric resistance welded steel pipe, more specifically, to a method for manufacturing an electric resistance welded steel pipe in which a steel plate is formed into a tubular shape while being conveyed, both ends of the steel plate are heated to melt, and then the both ends are upset and welded using a squeeze roll including a pair of side rolls.The present disclosure also relates to a monitoring device, more specifically, to a monitoring device for monitoring the manufacturing of an electric resistance welded steel pipe in which a steel plate is formed into a tubular shape while being conveyed, both ends of the steel plate are heated to melt, and then the both ends are upset and welded using a squeeze roll including a pair of side rolls.

[0002] Electric resistance welded steel pipes, also known as electric resistance welded steel pipes, are manufactured through a forming process, a welding process, a shaping process, and the like. In the forming process, a steel plate unwound from a coil is formed into an open pipe. In the welding process, both ends of the open pipe-shaped steel plate are heated and melted, and then upset and joined to form the electric resistance welded steel pipe. In the shaping process, the shape of the electric resistance welded steel pipe is adjusted.

[0003] In the manufacturing process of electric resistance welded steel pipes, a heating device and a squeeze roll are typically used in the welding process. Patent Document 1 discloses welding both ends of a steel plate using a work coil as a heating device and a squeeze roll including a pair of side rolls and a pair of top rolls. When a high-frequency current flows through the work coil, an induced current is generated at both ends of the steel plate, heating the both ends. The squeeze roll applies pressure to the open pipe-shaped steel plate from the outer periphery, pressing the both ends in a molten state. In Patent Document 1, a seam guide roll is disposed upstream of the work coil in the conveying direction of the steel plate. The seam guide roll adjusts the distance between both ends of the open pipe-shaped steel plate. The both ends of the steel plate approach each other as they move downstream from the seam guide roll and come into contact just before the squeeze roll.

[0004] JP 2009-241079 A

[0005] When welding both ends of a steel plate in the manufacturing process of electric resistance welded steel pipe, oxides are inevitably generated on the welded surfaces of these ends during the heating and melting process. If oxides remain between the welded surfaces, they can cause weld defects. Therefore, it is necessary to remove the oxides from between the welded surfaces and the inner and outer surfaces of the electric resistance welded steel pipe by upsetting using a squeeze roll. However, when upsetting is performed under low heat input conditions, the amount of molten steel is small, making it difficult to remove the molten steel from between the welded surfaces. Therefore, there is a possibility that oxides may remain between the welded surfaces. For this reason, in conventional electric resistance welded steel pipe manufacturing processes, it is necessary to take measures to suppress oxidation by purging the welded area, particularly for difficult-to-weld materials, or to install auxiliary equipment such as sealing, in order to obtain electric resistance welded steel pipe with good weld quality.

[0006] The present disclosure aims to provide a method for manufacturing electric-resistance welded steel pipes that can stably and easily ensure good welding quality by making it easier to expel oxides from between both ends of steel plates that are upset during the welding process.

[0007] The method for manufacturing an electric resistance welded steel pipe according to the present disclosure includes the steps of: forming a steel plate into a tubular shape while conveying the steel plate so that both ends face each other; and, after heating and melting both opposing ends, applying pressure to the steel plate using squeeze rolls including a pair of side rolls so that both ends, which approach each other and come into contact as they move downstream in the conveying direction of the steel plate, are pressed together to weld both ends. When the melting point of the steel plate is Tm [°C], the carbon concentration of the steel plate is MCc [mass %], and the conveying speed of the steel plate is Vf [mm / sec], the distance D in the conveying direction from the convergence point, where the both ends come into contact, to the central axis of the side roll is Ｖ－ＳＱＣ [mm] satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200

[0008] According to the method for manufacturing an electric resistance welded steel pipe according to the present disclosure, oxides are more easily expelled from between the two ends of the steel plate that is upset in the welding process, which makes it possible to stably and easily ensure good welding quality.

[0009] FIG. 1 is a schematic diagram showing the configuration of a manufacturing facility for electric resistance welded steel pipe according to a first embodiment. FIG. 2 is a front view of a squeeze roll included in the manufacturing facility shown in FIG. 1. FIG. 3 is a plan view of the manufacturing facility shown in FIG. 1, showing the squeeze roll of FIG. 2 and its vicinity. FIG. 4 is a cross-sectional view showing an example of a seam guide included in the manufacturing facility shown in FIG. 1. FIG. 5 is a schematic diagram showing a monitoring device for manufacturing electric resistance welded steel pipe according to a second embodiment. FIG. 6 is a diagram illustrating an example of the hardware configuration of the monitoring device shown in FIG. 5. FIG. 7 is a functional block diagram of the monitoring device shown in FIG. 5. FIG. 8A is a schematic diagram showing an example of an image used in the monitoring device shown in FIG. 5. FIG. 8B is a schematic diagram showing an example of an image used in the monitoring device shown in FIG. 5. FIG. 8C is a schematic diagram showing an example of an image used in the monitoring device shown in FIG. 5. FIG. 9 is a flowchart of a manufacturing method for electric resistance welded steel pipe according to the second embodiment. FIG. 10 is a schematic diagram showing changes in the welding state depending on the input power and the welding speed.

[0010] Generally, the welding condition (ERW welding) during the production of electric resistance welded steel pipes varies depending on the input power (heat input) and welding speed. Figure 10 is a schematic diagram (CPD diagram) showing the change in welding condition depending on the input power and welding speed. During high-frequency electric resistance welding, the ends of the steel plate gradually approach each other as they move downstream in the conveying direction. As the ends of the steel plate approach each other, they are strongly heated and melted due to the proximity effect. When molten steel accumulates at these ends, it is expelled by electromagnetic repulsion. Furthermore, melting at both ends progresses from the corners on the inner and outer surfaces of the open-pipe steel plate toward the center of the plate thickness. Line A in Figure 10 represents the condition under which the speed at which the ends of the steel plate approach each other coincides with the speed at which the molten steel produced by melting is expelled from between the ends. Line B in Figure 10 represents the condition under which the entire thickness of both ends of the steel plate is melted. The welding speed at the intersection of lines A and B is the critical welding speed Vm. The phenomenon of electric resistance welding changes significantly at the critical welding speed Vm.

[0011] In the CPD diagram of FIG. 10 , the region below line B corresponds to welding conditions known as Type 1. When welding is performed under Type 1 welding conditions, the entire thickness of both ends of the steel plate is not melted, resulting in a high probability of cold weld defects. The region above line A where the welding speed is equal to or greater than the critical welding speed Vm corresponds to welding conditions known as Type 2. Type 2 welding conditions are considered to enable welding with low defects. The region between lines A and B where the welding speed is equal to or greater than the critical welding speed Vm corresponds to welding conditions known as Type 1, where fusion welding is possible. When the welding speed is lower than the critical welding speed Vm, part of the region above line B corresponds to Type 2 welding conditions, but most of this region corresponds to so-called Type 3 welding conditions. Under Type 3 welding conditions, both ends of the steel plate experience excessive heat input, resulting in welding defects such as penetrators.

[0012] Typically, to ensure weld quality, it is considered preferable to perform welding at a welding speed above the critical welding speed Vm under type 2 welding conditions, or type 1 welding conditions that allow fusion welding. However, type 2 welding conditions create a region called a slit, where the approach speed of both ends of the steel sheet and the discharge speed of the molten steel are balanced. Arcing frequently occurs in the slit, which is known to cause spatter. If spatter adhering to the inner and outer surfaces of the steel sheet is pressed down, for example, by a squeeze roll, surface defects such as scratches may remain on the electric resistance welded steel pipe after welding.

[0013] For example, in small-diameter electric resistance welded steel pipes used in vehicles, surface defects caused by spatter may be unacceptable. In this case, electric resistance welding must be performed under Type 1 welding conditions that allow fusion welding. Under Type 1 welding conditions that allow fusion welding, the entire thickness of both ends of the steel plate melts during electric resistance welding, but no slits are formed, so no spatter occurs. However, because Type 1 welding conditions that allow fusion welding have a lower heat input than Type 2 welding conditions, the amount of molten steel generated at both ends of the steel plate is reduced when electric resistance welding is performed under Type 1 welding conditions that allow fusion welding. Ejection of molten steel due to electromagnetic repulsion occurs when the amount of molten steel at both ends of the steel plate overcomes surface tension. Under Type 1 welding conditions that allow fusion welding, the amount of molten steel is small, so ejection of molten steel due to electromagnetic repulsion is not possible. Instead, pressure from a squeeze roll must be used to eject the molten steel and oxides that are inevitably generated during electric resistance welding from between the ends of the steel plate.

[0014] If the distance from the position where both ends of the steel sheet come into contact to the squeeze rolls is large, the temperature of both ends of the steel sheet will drop before they reach the squeeze rolls, and the fluidity of the molten steel will decrease, making it difficult to discharge the molten steel containing oxides from between the ends of the steel sheet when it is pressed by the squeeze rolls. In this case, there is an increased possibility that oxides will remain between the ends of the steel sheet.

[0015] On the other hand, when the distance from the position where both ends of the steel sheet contact each other to the squeeze rolls is short, pressure can be applied by the squeeze rolls when both ends of the steel sheet are in a two-phase region of liquid and solid. In this case, the both ends of the steel sheet are pressed against each other while the fluidity of the molten steel is maintained at a high level, and therefore oxides can be discharged from between the ends of the steel sheet together with the molten steel.

[0016] When it is difficult to discharge molten steel from between the ends of steel sheets by applying pressure with squeeze rolls, conventional methods have minimized the oxides remaining between the ends of steel sheets by optimizing operating conditions under specific manufacturing conditions or by purging or sealing the weld. However, for example, in order to reliably manufacture electric resistance welded steel pipes using materials that require higher quality in the welds (high toughness, high strength, high workability, etc.), it is necessary to minimize oxides using unified welding conditions based on principles. Therefore, the inventors placed thermocouples on the weld surfaces at both ends of the steel sheet and observed the temperature transitions at both ends of the steel sheet when electric resistance welding was performed under type 1 welding conditions that allow fusion welding. As a result, it was found that under type 1 welding conditions that allow fusion welding, the heat loss in the circumferential direction at both ends of the open-pipe steel sheet was extremely large downstream of the position where the ends of the steel sheet come into contact and current flow ceases, i.e., the position where heating ceases. According to experiments conducted by the present inventors, the temperature of both ends of the steel plate dropped by approximately 42°C per 10 msec downstream of the position where the both ends contacted each other.

[0017] Based on the above considerations and experimental results, as well as on a carbon steel phase diagram (Fe-C equilibrium phase diagram), the inventors investigated the appropriate range of the distance from the position where both ends of the steel sheets come into contact (the convergence point) to the squeeze roll, and found standardized welding conditions for consistently obtaining electric resistance welded steel pipes with good weld quality.The inventors then completed a manufacturing method for electric resistance welded steel pipes according to an embodiment.

[0018] The manufacturing method of the electric resistance welded steel pipe according to the embodiment includes the steps of: forming the steel plate into a tubular shape while conveying the steel plate so that both ends face each other; and heating and melting both the opposed ends, and then using squeeze rolls including a pair of side rolls to press the steel plate so that both the ends, which approach each other and come into contact as they move downstream in the conveying direction of the steel plate, are pressed together to weld the both ends. When the melting point of the steel plate is Tm [°C], the carbon concentration of the steel plate is MCc [mass %], and the conveying speed of the steel plate is Vf [mm / sec], the distance D in the conveying direction from the convergence point, where the both ends come into contact, to the central axis of the side roll is Ｖ－ＳＱＣ [mm] satisfies the following formula (first configuration): 0<D Ｖ－ＳＱＣ≦{Tm-(1525.867-177×MCc)}×Vf / 4200

[0019] In the manufacturing method according to the first configuration, the distance D from the convergence points at both ends of the steel plate to the central axis of the squeeze roll Ｖ－ＳＱＣ is 0<D Ｖ－ＳＱＣ The formula is satisfied: ≦{Tm - (1525.867 - 177 × MCc)} × Vf / 4200. 1525.867 - 177 × MCc is a formula that shows the boundary between the two-phase region and the solid-phase region in the Fe-C equilibrium diagram (carbon concentration: 0.18 mass% or more, 2.14 mass% or less). Therefore, Tm - (1525.867 - 177 × MCc) is a formula that shows the allowable degree of temperature decrease depending on the melting point and carbon concentration of the steel sheet, that is, the degree of temperature decrease that does not cause both ends of the steel sheet to enter the solid-phase region. As described above, under the Type 1 welding conditions that allow fusion welding, both ends of the steel sheet are cooled at 42 ° C / 10 msec (4200 ° C / sec) after the convergence point, so the temperature decrease at both ends of the steel sheet from the convergence point to the central axis of the squeeze roll is D Ｖ－ＳＱＣ / Vf×4200. Therefore, D Ｖ－ＳＱＣ If / Vf × 4200 is equal to or less than Tm - (1525.867 - 177 × MCc), that is, if the above formula is satisfied, both ends of the steel sheet reach the squeeze roll in a two-phase region, even under Type 1 welding conditions that allow fusion welding. Therefore, pressure application by the squeeze roll can be performed while maintaining the fluidity of the molten steel, and oxides can be more easily expelled from between the ends of the steel sheet along with the molten steel. As a result, the occurrence of weld defects due to oxides can be easily suppressed, and good weld quality can be stably and easily ensured in the production of electric resistance welded steel pipes.

[0020] In the manufacturing method according to the first configuration, in the welding process, it is preferable that the angle formed by both ends of the steel plate that come closer to each other and come into contact as they move downstream in the conveying direction is 5.0° or more (second configuration).

[0021] In the second configuration, the angle formed by the ends of the metal plates that approach and come into contact with each other as they move downstream in the conveyance direction during the welding process (convergence angle) is ensured to be 5.0° or greater, thereby making it possible to more stably ensure good welding quality in the production of electric resistance welded steel pipes.

[0022] In the manufacturing method according to the first or second configuration, the carbon concentration MCc may be 0.18 mass % or more and 0.50 mass % or less (third configuration).

[0023] In the manufacturing method according to any one of the first to third configurations, the radius of curvature of the side surface of each side roll that is concavely curved toward the central axis may be 5 mm or more and 55 mm or less (fourth configuration).

[0024] In the manufacturing method according to any one of the first to fourth configurations, in the forming step, the distance between the opposing ends of the steel sheet may be adjusted by a seam guide. The seam guide is arranged upstream of the squeeze roll in the conveying direction. In this case, the diameter at the bottom of the side surface concavely curved toward the central axis of each of the side rolls included in the squeeze roll is defined as D ＳＲ [mm], the width of the seam guide is W ＳＧ [mm], the curvature radius of the side of the side roll is R ＳＲ When expressed in mm, it is preferable to satisfy the following formula (fifth configuration): W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ ≧0.25

[0025] Diameter D of the side roll included in the squeeze roll ＳＲ and seam guide width W ＳＧ is the distance D from the convergence points at both ends of the steel plate to the center axis of the squeeze roll Ｖ－ＳＱＣ The diameter of the side roll D ＳＲ and / or the width W of the seam guide ＳＧ By enlarging the diameter D of the side roll as in the fifth configuration, the convergence points at both ends of the steel sheet can be shifted to the squeeze roll side. ＳＲ and the width W of the seam guide ＳＧ W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ ≧0.25, the distance D from the convergence points at both ends of the steel plate to the center axis of the squeeze roll Ｖ－ＳＱＣ However, the above-mentioned 0<D Ｖ－ＳＱＣ≦{Tm−(1525.867−177×MCc)}×Vf / 4200 becomes easier to satisfy.

[0026] In another embodiment of a method for manufacturing an electric resistance welded steel pipe, a steel sheet is formed into a tubular shape while being conveyed, and both ends of the steel sheet are upset and welded using squeeze rolls including a pair of side rolls. This manufacturing method includes the steps of: capturing images of both ends of the steel sheet by an imaging device, and acquiring images including portions of the both ends that approach and come into contact as they move downstream in the conveying direction of the steel sheet; measuring the distance in the conveying direction from the convergence point, where the both ends come into contact, to the central axis of the side roll based on the images; and determining whether the distance satisfies the following formula (sixth configuration): 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200, where Tm is the melting point of the steel sheet [°C], MCc is the carbon concentration of the steel sheet [mass%], Vf is the conveying speed of the steel sheet [mm / sec], and D Ｖ－ＳＱＣ is the distance [mm].

[0027] The manufacturing method according to the sixth configuration can further include a step of adjusting the distance so that it satisfies the formula by increasing the width of a seam guide for adjusting the spacing between both ends before welding if it is determined in the determining step that the distance does not satisfy the formula (seventh configuration).

[0028] The manufacturing method according to the sixth or seventh configuration may further include a step of adjusting the distance so as to satisfy the formula by reducing the diameter of each of the side rolls if it is determined in the determining step that the distance does not satisfy the formula (eighth configuration).

[0029] A monitoring device according to an embodiment monitors the production of electric resistance welded steel pipes, in which a steel plate is formed into a tubular shape while being conveyed, and both ends of the steel plate are upset and welded using squeeze rolls including a pair of side rolls. The monitoring device comprises an image acquisition unit, a measurement unit, and a determination unit. The image acquisition unit acquires image data captured by an imaging device. The image data includes portions of both ends that approach and come into contact as they move downstream in the conveying direction of the steel plate. Based on the image data, the measurement unit measures the distance in the conveying direction from the convergence point, where the both ends come into contact, to the central axis of the side roll. The determination unit determines whether this distance satisfies the following formula (ninth configuration): 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200, where Tm is the melting point of the steel sheet [°C], MCc is the carbon concentration of the steel sheet [mass%], Vf is the conveying speed of the steel sheet [mm / sec], and D Ｖ－ＳＱＣ is the distance [mm].

[0030] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0031] <First embodiment> [Electrical resistance welded steel pipe manufacturing equipment] Fig. 1 is a schematic diagram showing the configuration of an electric resistance welded steel pipe manufacturing equipment 100. Referring to Fig. 1, the manufacturing equipment 100 manufactures an electric resistance welded steel pipe from a strip-shaped steel plate (steel strip) 10 while transporting the steel plate 10 along its longitudinal direction.

[0032] The manufacturing facility 100 includes a forming device 20, a welding device 30, and a shaping device 40. The forming device 20, the welding device 30, and the shaping device 40 are arranged in this order from upstream to downstream in the conveyance direction of the steel plate 10.

[0033] A steel sheet 10 unwound from a coil is continuously supplied to the forming device 20. Both widthwise ends 11L, 11R of the steel sheet 10 may be trimmed, for example, by an edge mirror 50, before being fed into the forming device 20. The forming device 20 includes, for example, a breakdown roll group 21, a fin pass roll group 22, and a seam guide 23. The breakdown roll group 21, the fin pass roll group 22, and the seam guide 23 are arranged in this order from upstream to downstream in the conveyance direction of the steel sheet 10.

[0034] The forming device 20 forms the transported steel sheet 10 into a tubular shape (open pipe shape). The breakdown roll group 21 and the fin pass roll group 22 each include a plurality of rolls. The breakdown roll group 21 performs bending processing on both end portions 11L, 11R of the steel sheet 10. The breakdown roll group 21 bends both end portions 11L, 11R of the steel sheet 10 upward on the plane of FIG. 1 , thereby curving the entire steel sheet 10 into an arc shape. The fin pass roll group 22 performs finish forming on the steel sheet 10 so that the steel sheet 10 has a substantially circular cross section. The seam guide 23 is arranged between both end portions 11L, 11R of the generally tubular steel sheet 10 and adjusts the distance between the opposing end portions 11L, 11R.

[0035] The welding device 30 welds both ends 11L, 11R of the steel sheet 10. The welding device 30 includes a heating device 31 and a squeeze roll 32. The heating device 31 is configured to heat and melt both ends 11L, 11R of the steel sheet 10. The heating device 31 is typically an electric resistance heating device. The heating device 31 may also be a high-frequency heating device. In this case, the heating device 31 supplies a high-frequency current to the steel sheet 10, melting both ends 11L, 11R of the steel sheet 10 by Joule heat. The heating device 31 uses electrodes such as a work coil or a contact tip, and can supply the high-frequency current to the tubular steel sheet 10 by induction or directly.

[0036] The squeeze roll 32 is disposed downstream of the heating device 31 in the conveying direction of the steel sheet 10. The squeeze roll 32 is configured to apply pressure to the steel sheet 10 and press both end portions 11L, 11R against each other.

[0037] The steel plate 10 becomes an electric-resistance welded steel pipe by welding both ends 11L, 11R by the welding device 30. The shaping device 40 adjusts the shape of the portion of the steel plate 10 that has become the electric-resistance welded steel pipe. A seam normalizer 60 may be disposed between the welding device 30 and the shaping device 40. The shaping device 40 includes, for example, a plurality of roll stands aligned in the conveying direction of the steel plate 10. Each of the roll stands may include a plurality of rolls that define the caliber.

[0038] [Method for manufacturing electric resistance welded steel pipe] Next, a method for manufacturing electric resistance welded steel pipe according to this embodiment will be described. The method for manufacturing electric resistance welded steel pipe according to this embodiment includes a forming step and a welding step. The manufacturing method can further include a shaping step.

[0039] (Forming Process) Continuing to refer to Fig. 1, in the forming process, the steel sheet 10 is formed into a tubular shape so that both end portions 11L, 11R face each other. More specifically, the steel sheet 10 is unwound from a coil, and then continuously supplied to a forming device 20, where it is formed into a tubular shape by the forming device 20. As the steel sheet 10 passes through the forming device 20, it is formed into an open pipe shape so that both end portions 11L, 11R face each other and are spaced apart from each other in the circumferential direction.

[0040] (Welding Process) In the welding process, both end portions 11L, 11R of the steel plate 10 are heated and melted, and then a squeeze roll 32 is used to press the steel plate 10 so that both end portions 11L, 11R are pressed against each other, thereby welding both end portions 11L, 11R. More specifically, the steel plate 10 formed into a tubular shape through the forming process is continuously fed to a welding device 30. The steel plate 10 is first fed to a heating device 31, and both end portions 11L, 11R are heated by the heating device 31. As a result, both end portions 11L, 11R of the steel plate 10 start to melt.

[0041] The steel sheet 10, with both ends 11L, 11R melted to produce molten steel, is supplied to the squeeze rolls 32. The both ends 11L, 11R of the steel sheet 10 approach each other and come into contact as they move downstream in the conveying direction. The squeeze rolls 32 press the steel sheet 10, with both ends 11L, 11R in contact, from the outer surface side. This causes the both ends 11L, 11R of the steel sheet 10 to be pressed against each other.

[0042] Fig. 2 is a view (front view) of the squeeze roll 32 as seen from the downstream side along the conveyance direction of the steel sheet 10. Fig. 3 is a plan view of the manufacturing equipment 100 when an electric resistance welded steel pipe is manufactured from the steel sheet 10, showing the squeeze roll 32 and its vicinity.

[0043] Referring to Fig. 2, the squeeze roll 32 includes a pair of side rolls 321L, 321R. The side rolls 321L, 321R are arranged on both sides of the tubular steel sheet 10. The side rolls 321L, 321R may be capable of moving toward and away from the steel sheet 10. One side roll 321L is arranged on the left side of the other side roll 321R on the paper surface of Fig. 2. Hereinafter, for convenience of explanation, the left and right on the paper surface of Fig. 2 will be simply referred to as left and right, and the top and bottom on the paper surface of Fig. 2 will be simply referred to as top and bottom.

[0044] Each of the side rolls 321L and 321R has a central axis A ＳＲ In the conveying direction of the steel sheet 10, the central axis A of the side roll 321L ＳＲ The position of the center axis A of the side roll 321R ＳＲ Each of the side rolls 321L and 321R is aligned with the central axis A. ＳＲ The shape is symmetrical (axially symmetrical) with respect to the central axis A ＳＲ Each of the side rolls 321L and 321R includes a side surface 321a. The side surface 321a is rotatable about the central axis A. ＳＲ In each of the side rolls 321L and 321R, the side surface 321a is an annular surface having a center axis A ＳＲ The side surface 321a has a concave curved shape. ＳＲ is, for example, 5 mm or more and 55 mm or less. ＳＲ is approximately half the outer diameter of the electric resistance welded steel pipe to be manufactured.

[0045] The side rolls 321L and 321R have a concavely curved side surface 321a, and therefore the diameter of the side rolls 321L and 321R decreases from the outside toward the center in the axial direction. The side rolls 321L and 321R each have a diameter D ＳＲThe diameter D of each of the side rolls 321L and 321R is ＳＲ is the radius of curvature R of the side surface 321a ＳＲ It is preferable that the ratio is 3.60 to 4.50 times.

[0046] The squeeze roll 32 may include other rolls in addition to the side rolls 321L, 321R. Although not shown, the squeeze roll 32 may include, for example, a pair of top rolls that come into contact with the steel sheet 10 near both end portions 11L, 11R. The squeeze roll 32 may also include a receiving roll that comes into contact with the steel sheet 10 from below.

[0047] 3, in the welding process, both end portions 11L, 11R of the steel sheet 10, which have been heated by the heating device 31 and are in a molten state, approach each other as they move downstream in the conveying direction and finally come into contact with each other. That is, both end portions 11L, 11R of the steel sheet 10 converge in a V-shape in a plan view of the manufacturing equipment 100. The position where both end portions 11L, 11R of the steel sheet 10 come into contact with each other is referred to as a convergence point V, and the central axes A of the side rolls 321L, 321R are referred to as a convergence point V. ＳＲ When the position of the squeeze roll center SQC is the distance D in the conveying direction of the steel plate 10 from the convergence point V to the squeeze roll center SQC, Ｖ－ＳＱＣ [mm] satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200

[0048] In the above formula, Tm is the melting point [°C] of the steel plate 10. MCc is the carbon concentration [mass%] of the steel plate 10. Vf is the conveying speed (pipe-making speed) of the steel plate 10 [mm / sec]. D Ｖ－ＳＱＣ >0 means that the convergence point V is located upstream of the squeeze roll center SQC in the conveying direction of the steel sheet 10.

[0049] The melting point Tm can be derived from, for example, an Fe-C equilibrium diagram based on the carbon concentration MCc. More specifically, the melting point Tm can be calculated using the following formula. The following formula is a melting point approximation formula that takes into account the overshoot (estimated value) due to high-speed heating in the Fe-C equilibrium diagram: Tm = -68 x MCc + 1550

[0050] The carbon concentration MCc of the steel plate 10 can be determined by a well-known analytical method. The carbon concentration MCc of the steel plate 10 may be measured from the steel plate 10 before it is introduced into the manufacturing facility 100, or may be measured from an electric resistance welded steel pipe manufactured from the steel plate 10.

[0051] The carbon concentration MCc of the steel sheet 10 is 0.18 mass% or more and 2.14 mass% or less. The carbon concentration MCc may be 0.18 mass% or more and 0.50 mass% or less. The conveying speed Vf of the steel sheet 10 is, for example, 667 mm / sec or more and 1333 mm / sec or less (40 mpm or more and 80 mpm or less).

[0052] When the convergence angle θ is the angle formed by the two ends 11L, 11R of the steel sheet 10 that approach and come into contact as they move downstream in the conveying direction, the convergence angle θ is 5.0° or more. The convergence angle θ may be 8.0° or less. The convergence angle θ can be controlled by a seam guide 23 ( FIG. 1 ) located upstream of the welding device 30. FIG. 4 is a cross-sectional view showing an example of the seam guide 23. Referring to FIG. 4 , the width W of the portion of the seam guide 23 that is located between the two ends 11L, 11R of the steel sheet 10 is ＳＧ is the radius of curvature R of the side surface 321a of the side rolls 321L and 321R. ＳＲ It is preferable that the width W of the seam guide 23 is 0.65 to 0.80 times the width W of the seam guide 23 (FIG. 2). ＳＧ [mm] is the diameter D of the side rolls 321L and 321R (FIG. 2) ＳＲ [mm] and the radius of curvature R of the side surface 321a ＳＲ It is preferable that the following formula is satisfied together with W [mm]. ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ ≧0.25

[0053] Returning to Fig. 1, both end portions 11L, 11R of the steel sheet 10 that have passed through the squeeze rolls 32 are solidified and joined by natural cooling as they are transported downstream. As a result, the steel sheet 10 becomes an electric resistance welded steel pipe having a weld extending in the longitudinal direction. The outer diameter of the electric resistance welded steel pipe is, for example, 10 mm or more and 110 mm or less.

[0054] Immediately after the welding process, weld beads are formed on the inner and outer surfaces of the electric resistance welded steel pipe. The weld beads can be cut and removed, for example, using a bead cutting machine (not shown). After welding, the welded portion may be heat treated using a seam normalizer 60.

[0055] (Shaping Process) In the shaping process, the shape of the electric resistance welded steel pipe formed through the welding process is adjusted. More specifically, the electric resistance welded steel pipe is continuously fed to a shaping device 40 and shaped into its final shape. For example, the electric resistance welded steel pipe may be subjected to a slight drawing process by passing through multiple roll stands included in the shaping device 40. This finishes the electric resistance welded steel pipe so that its cross section is perfectly circular and its outer diameter is within dimensional tolerances.

[0056] [Effect] In the method for manufacturing an electric resistance welded steel pipe according to this embodiment, the distance D from the convergence point V of both end portions 11L, 11R of the steel plate 10 to the squeeze roll center SQC is Ｖ－ＳＱＣ is 0 < D Ｖ－ＳＱＣ ≦{Tm - (1525.867 - 177 × MCc)} × Vf / 4200. In this case, even under type 1 welding conditions that allow fusion welding, the end portions 11L, 11R of the steel plate 10 contact each other and then reach the squeeze roll 32 in a two-phase interphase state. Therefore, the end portions 11L, 11R can be pressed together by applying pressure with the squeeze roll 32 while maintaining the fluidity of the molten steel between the end portions 11L, 11R. Because the fluidity of the molten steel is maintained, when the end portions 11L, 11R are pressed together, the molten steel and oxides generated during the welding process are easily expelled from between the end portions 11L, 11R. Therefore, the occurrence of weld defects due to oxides can be easily suppressed, and good weld quality can be stably and easily ensured in the production of electric resistance welded steel pipes. In other words, according to the method for producing an electric resistance welded steel pipe according to this embodiment, an electric resistance welded steel pipe with good weld quality can be easily obtained under uniform welding conditions without the need for special treatments to prevent oxide residue, as in the conventional method.

[0057] In this embodiment, D Ｖ－ＳＱＣBy appropriately setting the temperature, it is possible to make it difficult for weld defects caused by oxides to occur even under type 1 welding conditions that allow fusion welding. Therefore, the manufacturing method according to this embodiment can be suitably used for small-diameter electric resistance welded steel pipes that require electric resistance welding under type 1 welding conditions that allow fusion welding. According to the manufacturing method according to this embodiment, even small-diameter electric resistance welded steel pipes of 10 mm or more and 110 mm or less can be manufactured by applying type 1 welding conditions that allow fusion welding, without generating spatter due to the formation of slits, and welding defects caused by oxides that inevitably occur during welding can be easily suppressed.

[0058] In the method for producing an electric resistance welded steel pipe according to this embodiment, the convergence angle θ of both end portions 11L, 11R of the steel plate 10 in the welding step is preferably 5.0° or more, which makes it possible to more stably ensure good welding quality in the production of the electric resistance welded steel pipe.

[0059] In this embodiment, the seam guide 23 adjusts the distance between the two end portions 11L and 11R of the steel plate 10 immediately before the welding process. ＳＧ [mm], diameter D of side rolls 321L and 321R ＳＲ [mm], and the curvature radius R of the side surface 321a of the side rolls 321L and 321R ＳＲ [mm] is W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ It is preferable that the diameter D of the side rolls 321L and 321R satisfies ≧0.25. ＳＲ and / or the width W of the seam guide 23 ＳＧ By enlarging the diameter D of the side rolls 321L and 321R, the convergence point V of the both end portions 11L and 11R of the steel sheet 10 can be moved toward the squeeze roll 32. ＳＲ and the width W of the seam guide 23 ＳＧ W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ ≧0.25, the distance D from the convergence point V of both ends 11L, 11R of the steel plate 10 to the squeeze roll center SQCＶ－ＳＱＣ However, 0<D Ｖ－ＳＱＣ ≦{Tm−(1525.867−177×MCc)}×Vf / 4200 becomes easier to satisfy.

[0060] Width W of seam guide 23 ＳＧ [mm], diameter D of side rolls 321L and 321R ＳＲ [mm], and the curvature radius R of the side surface 321a of the side rolls 321L and 321R ＳＲ [mm] is W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ It is more preferable that W ≧ 0.35 is satisfied. ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ It is more preferable that W ≧ 0.55. ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ The larger the value of , the longer the distance D from the convergence point V of both ends 11L, 11R of the steel plate 10 to the squeeze roll center SQC. Ｖ－ＳＱＣ However, 0<D Ｖ－ＳＱＣ This is because it becomes easier to satisfy the condition of ≦{Tm-(1525.867-177×MCc)}×Vf / 4200. ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ There is no particular upper limit to the value of ＳＲ and the width W of the seam guide 23 ＳＧ Considering the upper limit of W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ It is preferable that the value satisfies ≦0.60.

[0061] In this embodiment, it is preferable that the side rolls 321L and 321R included in the squeeze roll 32 have a relatively small diameter. Specifically, the diameter D ＳＲ is the radius of curvature R of the concavely curved side surface 321a ＳＲBy making the diameter of the side rolls 321L, 321R small, the distance D from the convergence point V of the both end portions 11L, 11R of the steel sheet 10 to the squeeze roll center SQC is reduced. Ｖ－ＳＱＣ In addition, when the squeeze roll 32 includes a top roll, the diameter D of the side rolls 321L and 321R can be made smaller. ＳＲ Specifically, the diameter of the top roll is preferably set to a value smaller than the radius of curvature R of the side surface 321 a of the side rolls 321L and 321R. ＳＲ It is preferable that the value is 4.50 times or less.

[0062] In order to ensure the rigidity of the side rolls 321L, 321R and the bearings supporting the side rolls 321L, 321R, the diameter D ＳＲ is the radius of curvature R of the side surface 321a ＳＲ When the squeeze roll 32 includes a top roll, the diameter of the top roll is also preferably 3.60 times or more of the curvature radius R of the side surface 321 a of the side rolls 321L and 321R. ＳＲ It can be 3.60 times or more.

[0063] In this embodiment, the width W of the seam guide 23 ＳＧ It is preferable that the width W of the seam guide 23 is relatively large. ＳＧ is the radius of curvature R of the side surface 321a of each of the side rolls 321L and 321R. ＳＲ The width of the seam guide 23 is preferably 0.65 times or more, more preferably 0.70 times or more, and even more preferably 0.75 times or more. By making the width of the seam guide 23 wider, the convergence point V of the both end portions 11L, 11R of the steel sheet 10 can be moved toward the squeeze roll 32. Therefore, the distance D from the convergence point V of the both end portions 11L, 11R of the steel sheet 10 to the squeeze roll center SQC Ｖ－ＳＱＣ Furthermore, the width W of the seam guide 23 can be reduced. ＳＧBy increasing the width W of the seam guide 23, the convergence angle θ of both ends 11L, 11R of the steel plate 10 can be increased. When the convergence angle θ is increased, the critical welding speed Vm decreases, and the region of the first type welding conditions under which fusion welding is possible can be expanded. In other words, the process window in electric resistance welding can be expanded. However, in order to ensure good formability of the steel plate 10 in the production of electric resistance welded steel pipes, the width W of the seam guide 23 must be increased. ＳＧ is the radius of curvature R of the side surface 321a of the side rolls 321L and 321R. ＳＲ It is preferable that the value is 0.80 times or less.

[0064] Second Embodiment Fig. 5 is a schematic diagram showing a monitoring device 200 according to this embodiment. In this embodiment, a method for manufacturing an electric resistance welded steel pipe using the monitoring device 200 will be described.

[0065] As described in the first embodiment with reference to FIG. 5 , an electric resistance welded steel pipe is manufactured by forming a steel plate 10 into a tubular shape while conveying it, and then upsetting and welding both end portions 11L, 11R of the steel plate 10 using squeeze rolls 32. Hereinafter, upsetting and welding both end portions 11L, 11R of the steel plate 10, in other words, upsetting both end portions 11L, 11R of the steel plate 10 during the welding process, may be referred to as "upset welding." In this embodiment, an imaging device 70 is provided in an electric resistance welded steel pipe manufacturing facility 100 to monitor the production of the electric resistance welded steel pipe. The imaging device 70 is disposed above the steel plate 10 being conveyed. The imaging device 70 is positioned so as to be able to image the portions of both end portions 11L, 11R of the steel plate 10 that approach and come into contact as they move downstream in the conveyance direction, i.e., the V-shaped converging portions.

[0066] The imaging device 70 captures an image of the self-luminous pattern (radiation pattern) of a region including the V-shaped convergent portion of both end portions 11L, 11R of the steel plate 10. Although not particularly limited, the imaging device 70 may be, for example, a 3CCD color camera having 1920 x 512 pixels. The imaging device 70 captures an image of the region including the convergent portion under the following conditions: a field of view of 50 mm x 190 mm, a resolution of 100 μm / pixel, a frame rate of 500 fps, and an exposure time of 1 / 10,000 sec. The imaging device 70 may capture images of the steel plate 10 being transported continuously or intermittently at predetermined time intervals.

[0067] The monitoring device 200 is communicably connected to the imaging device 70. The monitoring device 200 is connected to the imaging device 70 via, for example, a network. The monitoring device 200 is, for example, a computer.

[0068] Fig. 6 is a diagram illustrating an example of the hardware configuration of the monitoring device 200. As shown in Fig. 6, the monitoring device 200 may include a central processing unit (CPU) 201, a main memory device 202, an auxiliary memory device 203, an input device 204, an output device 205, and the like.

[0069] The CPU 201 executes various programs loaded from the auxiliary storage device 203 to the main storage device 202 and performs information calculations. The CPU 201 executes the programs to realize various processes of the monitoring device 200. The main storage device 202 is, for example, a RAM, and is used as a work area for temporarily storing various programs executed by the CPU 201, information used by the CPU 201, calculation results by the CPU 201, and the like. The auxiliary storage device 203 is, for example, a HDD or ROM. The input device 204 is a device for an operator to perform input operations, and includes, for example, a pointing device such as a mouse or a touch panel, a keyboard, and the like. The output device 205 is a device for outputting processing results, etc., of the CPU 201. The output device 205 may include a display.

[0070] 7 is a functional block diagram of monitoring device 200. Monitoring device 200 includes image acquisition unit 206, measurement unit 207, and determination unit 208. The functions of image acquisition unit 206, measurement unit 207, and determination unit 208 are realized by CPU 201 (FIG. 6) executing predetermined programs.

[0071] The image acquisition unit 206 acquires data of images captured by the imaging device 70 (FIG. 5). That is, the CPU 201 (FIG. 6) of the monitoring device 200 acquires image data from the imaging device 70 via the communication interface and temporarily stores the image data in the main storage device 202. The image acquisition unit 206 may acquire the data of images captured by the imaging device 70 in real time, or may acquire the data collectively at a predetermined timing.

[0072] Fig. 8A is a diagram schematically illustrating an example of an image 80A captured by the imaging device 70 (Fig. 5). As shown in Fig. 8A, the image 80A includes a portion 81 of both end portions 11L, 11R of the steel sheet 10 that converges in a V-shape toward the downstream side in the conveying direction. The image 80A also includes high-heat regions 82L, 82R along the converging portion 81. The high-heat regions 82L, 82R are regions with a relatively high brightness level. In the image 80A, a region 83 exists downstream in the conveying direction of the steel sheet 10. In the region 83, a wavy pattern appears due to molten steel being discharged from both end portions 11L, 11R of the steel sheet 10.

[0073] Returning to FIG. 7, the measurement unit 207 includes, for example, a red component extraction unit 207a, a binarization unit 207b, a labeling unit 207c, a convergence point detection unit 207d, and a distance calculation unit 207e.

[0074] The red component extraction unit 207a extracts the red component (wavelength 590 nm to 680 nm) from each image data in order to clarify the contrast of the image data acquired by the image acquisition unit 206. That is, the CPU 201 (FIG. 6) of the monitoring device 200 reads the image data from the main storage device 202, extracts the red component, and temporarily stores the extracted image data of the red component in the main storage device 202.

[0075] The binarization unit 207b binarizes (inverts) the image data that has been processed by the red component extraction unit 207a. Specifically, the CPU 201 (FIG. 6) of the monitoring device 200 reads the red component image data from the main storage device 202, performs binarization processing on the read image data, and temporarily stores the binarized image data in the main storage device 202. For example, the binarization unit 207b assigns a pixel value of "0" to pixels included in the red component image data whose brightness level is equal to or greater than a threshold, and a pixel value of "1" to pixels whose brightness level is less than the threshold.

[0076] 8B is a diagram showing an example of a binarized image 80B. In FIG. 8B, the black portions of the binarized image 80B are indicated by hatching. In the example of FIG. 8B, the area corresponding to the V-shaped converging portion 81 is converted to white (pixel value "1"), and the areas corresponding to the high-heat areas 82L and 82R and the wavy area 83 are converted to black (pixel value "0").

[0077] Returning to FIG. 7 , the labeling unit 207c performs a labeling process on the image data processed by the binarization unit 207b, labeling each blob. A blob is an area formed by connecting multiple pixels. Specifically, if a pixel with a pixel value of "1" is adjacent to a pixel (four pixels adjacent to a pixel on the top, bottom, left, and right sides, and four pixels adjacent to a pixel diagonally), these eight pixels are connected. By performing this connecting process for each pixel, a blob can be obtained. The labeling process assigns a label number to each blob and derives the position, width, length, area, etc. of each blob within the image data. In this embodiment, the positions within the image data, the positions of each component of the electric resistance welded steel pipe manufacturing equipment 100 ( FIG. 5 ), and the position of the imaging device 70 ( FIG. 5 ) are expressed in common three-dimensional Cartesian coordinates (x, y, z coordinates) unless otherwise specified. In this embodiment, it is assumed that the longitudinal direction (conveying direction) of the steel plate 10 ( FIG. 5 ) is the x-direction, the width direction (left-right direction) of the steel plate 10 is the y-direction, and the direction perpendicular to the x-direction and y-direction (up-down direction) is the z-direction. The position of each blob in the image data derived by the labeling process is, for example, the maximum and minimum points of the x-coordinate and the maximum and minimum points of the y-coordinate. The value of the x-direction coordinate increases from the downstream side to the upstream side in the conveying direction of the steel plate 10. The CPU 201 ( FIG. 6 ) of the monitoring device 200 reads the binarized image data from the main memory device 202, performs the labeling process, and temporarily stores the result in the main memory device 202.

[0078] 8C is a diagram showing an example of a binarized image 80B after labeling. Fig. 8C shows an example in which three blobs 841, 842, and 843 are obtained from the binarized image 80B. These blobs 841, 842, and 843 are assigned label numbers 1, 2, and 3, respectively.

[0079] 7 , after the processing by the labeling unit 207c is completed, the convergence point detection unit 207d detects, in the binary image data, the convergence point V ( FIG. 8A ) of both end portions 11L, 11R of the steel plate 10. That is, the CPU 201 ( FIG. 6 ) of the monitoring device 200 detects the convergence point V from the binary image data and performs processing to derive its position (coordinates), and temporarily stores the result in the main memory device 202.

[0080] The convergence point detection unit 207d determines whether or not there is a blob that meets a predetermined condition among the blobs to which label numbers have been assigned by the labeling unit 207c. If the convergence point detection unit 207d determines that there is a blob that meets the predetermined condition, it extracts the blob as a blob that corresponds to the V-shaped convergence portion 81 ( FIG. 8A ). For example, if there is a blob that is in contact with the upstream end of the conveying direction in the binarized image data and meets a predetermined area condition, the convergence point detection unit 207d determines that the blob is a blob that corresponds to the convergence portion 81. The area condition may be, for example, if the area of ​​the blob is 15 mm ２ 150mm or more ２ The first condition is that the area of ​​the rectangular block circumscribing the blob is 25 mm ２ 320mm or more ２ In the case of the example of binarized image 80B shown in Fig. 8C, convergence point detection unit 207d extracts blob 842 assigned label number 2 as the blob corresponding to convergence portion 81 (Fig. 8A).

[0081] The convergence point detection unit 207d identifies the tip of the blob 842 corresponding to the converging portion 81 as the convergence point V ( FIG. 8A ), and acquires the position (x coordinate and y coordinate) of the tip as the position of the convergence point V. The convergence point detection unit 207d may acquire the position of the convergence point V for each of a plurality of temporally consecutive image data and use the average value of these as the position of the convergence point V, or may determine the position of the convergence point V based on a single image data.

[0082] After the processing by the convergence point detection unit 207d is completed, the distance calculation unit 207e calculates the distance D in the conveying direction (x direction) from the convergence point V of both end portions 11L, 11R of the steel plate 10 to the squeeze roll center SQC. Ｖ－ＳＱＣ As described above, the x-coordinate of the convergence point V is identified by the process performed by the convergence point detection unit 207d. The distance calculation unit 207e obtains the x-coordinate of the convergence point V and the x-coordinate of the squeeze roll center SQC, and calculates the distance D by subtracting the x-coordinate of the convergence point V from the x-coordinate of the squeeze roll center SQC. Ｖ－ＳＱＣ The position (coordinates) of the squeeze roll center SQC can be obtained by measuring the relative distance between the image reference position of the imaging device 70 and the position of the squeeze roll center SQC, for example, and is stored in advance in the auxiliary storage device 203 (FIG. 6) of the monitoring device 200 by an input operation by the operator. In this case, the position (coordinates) of the squeeze roll center SQC does not have to be included in the image captured by the imaging device 70. The CPU 201 (FIG. 6) of the monitoring device 200 reads out the x-coordinate of the convergence point V and the x-coordinate of the squeeze roll center SQC from the main storage device 202 or the auxiliary storage device 203, and calculates the distance D Ｖ－ＳＱＣ is calculated and temporarily stored in the main memory device 202.

[0083] The determination unit 208 determines the distance D in the conveying direction from the convergence point V of both end portions 11L, 11R of the steel sheet 10 to the squeeze roll center SQC. Ｖ－ＳＱＣ It is determined whether [mm] satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200

[0084] As explained in the first embodiment, Tm is the melting point [°C] of the steel sheet 10, MCc is the carbon concentration [mass %] of the steel sheet 10, and Vf is the conveying speed [mm / sec] of the steel sheet 10. The melting point Tm, the carbon concentration MCc, and the conveying speed Vf may be stored in advance in the auxiliary storage device 203 ( FIG. 6 ) of the monitoring device 200 by, for example, input operation by an operator. The CPU 201 ( FIG. 6 ) of the monitoring device 200 receives the distance D Ｖ－ＳＱＣ , melting point Tm, carbon concentration MCc, and conveying speed Vf.Ｖ－ＳＱＣ Determine whether the distance D is a positive value. Ｖ－ＳＱＣ If Tm is a positive value, the CPU 201 calculates the right side of the above equation using the melting point Tm, the carbon concentration MCc, and the transport speed Vf to obtain the distance D Ｖ－ＳＱＣ The CPU 201 obtains the upper limit value of the read distance D Ｖ－ＳＱＣ It is determined whether or not is equal to or less than the obtained upper limit value.

[0085] 9 is a flowchart of a method for manufacturing an electric-resistance welded steel pipe according to this embodiment. Referring to FIG. 9, the method for manufacturing an electric-resistance welded steel pipe includes an image acquisition step S1, a measurement step S2, and a determination step S3. In this embodiment, a method for manufacturing an electric-resistance welded steel pipe using the monitoring device 200 described above will be described. That is, steps S1 to S3 are steps executed by the CPU 201 ( FIG. 6 ) of the monitoring device 200. The image acquisition step S1, the measurement step S2, and the determination step S3 correspond to the processing of the image acquisition unit 206, the processing of the measurement unit 207, and the processing of the determination unit 208, respectively ( FIG. 7 ).

[0086] In the image acquisition step S1, the image acquisition unit 206 acquires an image 80A captured by the imaging device 70. The image includes the convergence point V of both end portions 11L, 11R of the steel plate 10.

[0087] After the image acquisition step S1, a measurement step S2 is executed. In the measurement step S2, the measurement unit 207 measures the distance D from the convergence point V of both end portions 11L and 11R of the steel plate 10 to the squeeze roll center SQC based on the image 80A. Ｖ－ＳＱＣ Measure.

[0088] The measurement step S2 includes a red component extraction step S2a, a binarization step S2b, a labeling step S2c, a convergence point detection step S2d, and a distance calculation step S2e. The red component extraction step S2a corresponds to the processing by the red component extraction unit 207a (FIG. 7). The binarization step S2b corresponds to the processing by the binarization unit 207b (FIG. 7). The labeling step S2c corresponds to the processing by the labeling unit 207c (FIG. 7). The convergence point detection step S2d corresponds to the processing by the convergence point detection unit 207d (FIG. 7). The distance calculation step S2e corresponds to the processing by the distance calculation unit 207e (FIG. 7). As already explained, the red component extraction unit 207a, the binarization unit 207b, the labeling unit 207c, the convergence point detection unit 207d, and the distance calculation unit 207e perform the processing in this order, thereby calculating the distance D Ｖ－ＳＱＣ is calculated.

[0089] In the determination step S3, the distance D calculated in the measurement step S2 is Ｖ－ＳＱＣ The determination unit 208 (FIG. 7) determines whether or not [mm] satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200

[0090] Distance D Ｖ－ＳＱＣ satisfies the above formula, even under the first type welding conditions that allow fusion welding, the steel plate 10 reaches the squeeze roll 32 in a two-phase region after the two end portions 11L, 11R come into contact. Therefore, the squeeze roll 32 can apply pressure to the end portions 11L, 11R while maintaining the fluidity of the molten steel between the end portions 11L, 11R, thereby pressing the end portions 11L, 11R against each other. Because the fluidity of the molten steel is maintained, when the end portions 11L, 11R are pressed against each other, the molten steel and oxides generated during the welding process are easily expelled from between the end portions 11L, 11R. Therefore, the occurrence of weld defects due to oxides can be easily suppressed, and good weld quality can be stably and easily ensured during the production of electric resistance welded steel pipes.

[0091] On the other hand, distance D Ｖ－ＳＱＣ If the above formula is not satisfied, there is a possibility that a welding defect occurs in the production of the electric resistance welded steel pipe. Ｖ－ＳＱＣIf the distance D is 0 or less, the upsetting by the squeeze rolls 32 will start before the both ends 11L, 11R of the steel sheet 10 come into contact with each other or at the same time as the both ends 11L, 11R of the steel sheet 10 come into contact with each other, and therefore the upsetting of the steel sheet 10 cannot be performed appropriately in the first place. Ｖ－ＳＱＣ is larger than {Tm-(1525.867-177×MCc)}×Vf / 4200, this means that upsetting by the squeeze roll 32 is performed in a state in which the temperature of the molten steel between the ends 11L, 11R has been lowered to the solid phase region, and therefore, the molten steel and oxides are less likely to be discharged from between the ends 11L, 11R.

[0092] Therefore, the manufacturing method of the electric resistance welded steel pipe according to this embodiment can further include an adjustment step S4 (FIG. 9). In the adjustment step S4, the distance D Ｖ－ＳＱＣ does not satisfy the above formula, the distance D Ｖ－ＳＱＣ The adjusting step S4 may be a step executed by the CPU 201 of the monitoring device 200, or may be a step including work by an operator.

[0093] For example, in the adjustment step S4, the distance D Ｖ－ＳＱＣ does not satisfy the above formula, the width W of the seam guide 23 described in the first embodiment ＳＧ By increasing the distance D Ｖ－ＳＱＣ For example, if the seam guide 23 is divided into a plurality of sections and the distance between the divided sections can be adjusted by gears or the like, the CPU 201 of the monitoring device 200 can issue a command to a device that controls the seam guide 23 to adjust the width W of the seam guide 23. ＳＧ Alternatively, the CPU 201 of the monitoring device 200 may output the distance D Ｖ－ＳＱＣ The operator may be notified that the above formula is not satisfied. Based on this notification, the operator may operate a gear that adjusts the distance between the divided portions of the seam guide 23, or may adjust the currently used seam guide 23 to a width W ＳＧ In the adjustment step S4, the width W of the seam guide 23 can be changed.ＳＧ By increasing the angle θ between the two end portions 11L, 11R of the steel plate 10 (convergence angle), the angle θ may be increased to, for example, 5.0° or more.

[0094] Also, for example, in the adjustment step S4, the distance D Ｖ－ＳＱＣ If it is determined that the above formula is not satisfied, the diameter D of each of the side rolls 321L and 321R is ＳＲ By reducing the distance D Ｖ－ＳＱＣ The CPU 201 of the monitoring device 200 outputs the distance D Ｖ－ＳＱＣ Based on this notification, the operator can adjust the diameter of the side rolls 321L and 321R to be smaller than the diameter D ＳＲ In the adjustment step S4, the distance D Ｖ－ＳＱＣ In order to satisfy the above formula, the diameter D of the side rolls 321L and 321R ＳＲ and the width W of the seam guide 23 is reduced. ＳＧ may be increased.

[0095] In the method for manufacturing an electric resistance welded steel pipe according to this embodiment, instead of adjusting step S4, the distance D Ｖ－ＳＱＣ If it is determined that the above formula is not satisfied, the distance D Ｖ－ＳＱＣ For example, the monitoring device 200 may include a step (not shown) for processing to exclude from the electric resistance welded steel pipe a portion that has been welded in a state where the distance D does not satisfy the above formula. Ｖ－ＳＱＣ does not satisfy the above formula, a mark is made on the position (coordinates) of the steel plate 10 where the determination was made. The marked portion of the welded steel plate 10 can be treated as a portion to be excluded from the electric resistance welded steel pipe as a product. The marked portion of the welded steel plate 10 may be cut and removed in a process subsequent to the welding process.

[0096] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0097] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to the following examples.

[0098] In order to confirm the effects of the present disclosure, a test was conducted to manufacture an electric resistance welded steel pipe by upset welding both ends of a steel plate while changing various conditions such as the melting point Tm [°C] of the steel plate, the carbon concentration MCc [mass %], the conveying speed Vf [mm / sec], the normalized SQ roll diameter, and the normalized SG width. In this test, the heat input condition (input power) was set to a condition that was 2% lower than the lower limit of the second type welding conditions (line A in Figure 10). The normalized SQ roll diameter is the smallest diameter D of each of the left and right side rolls. ＳＲ is the radius of curvature of the concave curved side surface R ＳＲ The normalized SG width is the width W of the seam guide. ＳＧ is the radius of curvature R of the concave curved side of the side roll. ＳＲ The value is obtained by dividing by . The conditions and results of each test example are shown in Table 1. Of the test examples shown in Table 1, electric resistance welded steel pipes with an outer diameter of 24.0 mm were manufactured in test examples 1 to 15, and electric resistance welded steel pipes with an outer diameter of 101.6 mm were manufactured in test examples 16 to 20.

[0099]

[0100] In Table 1, D Ｖ－ＳＱＣ The upper index limit is a value calculated by the formula: {Tm-(1525.867-177×MCc)}×Vf / 4200. Ｖ－ＳＱＣ The shortening index is expressed by the formula: W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ The value is calculated by: D Ｖ－ＳＱＣ The actual measured value is the distance D from the convergence point V at both ends of the steel plate to the squeeze roll center SQC measured in actual upset welding. Ｖ－ＳＱＣIn this test, for each test example, a Charpy impact test was conducted in warm conditions in accordance with JIS Z 2242:2018, using test specimens taken from the vicinity of the weld formed by upset welding. The fracture surfaces of the test specimens that fractured in the Charpy impact test were then observed under a microscope, and the defective fracture rate was calculated as an evaluation index for weld defects (oxide defects) caused by oxides. The defective fracture rate was calculated by observing the fracture surface of the test specimen under a microscope and dividing the area of ​​oxide defect regions that were visually distinguishable from other regions by the total area of ​​the observed surface. Test specimens with a defective fracture rate of 0.05% or less were judged to pass.

[0101] As shown in Table 1, in Test Examples 1 to 5, 7, 10, 13, 16, and 18, D Ｖ－ＳＱＣ The actual measured value is D Ｖ－ＳＱＣ The index exceeds the upper limit (D Ｖ－ＳＱＣ > {Tm - (1525.867 - 177 x MCc)} x Vf / 4200). In Test Examples 1 to 5, 7, 10, 13, 16, and 18, weld defects due to oxides occurred, and the test was judged to be unsuccessful. On the other hand, in Test Examples 6, 8, 9, 11, 12, 14, 15, and 17, D Ｖ－ＳＱＣ The actual measured value is D Ｖ－ＳＱＣ The index is below the upper limit (D Ｖ－ＳＱＣ ≦{Tm−(1525.867−177×MCc)}×Vf / 4200) In Test Examples 6, 8, 9, 11, 12, 14, 15, and 17, no welding defects caused by oxides occurred, and the test was judged to be pass.

[0102] Therefore, the distance D from the convergence point V at both ends of the steel plate to the squeeze roll center SQC Ｖ－ＳＱＣ It has been confirmed that when the following formula is satisfied, weld defects caused by oxides are prevented and an electric resistance welded steel pipe with good weld quality can be obtained. Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200

[0103] From Table 1, when comparing Test Examples 1 to 15, which have the same outer diameter condition for the electric resistance welded steel pipe, the larger the convergence angle θ, the smaller the D Ｖ－ＳＱＣ Therefore, by ensuring a large convergence angle θ, D Ｖ－ＳＱＣ The actual measured value is D Ｖ－ＳＱＣThe upper limit of the index ({Tm - (1525.867 - 177 × MCc)} × Vf / 4200) is less likely to be exceeded. When the convergence angle θ is 5.0° or more, the force pushing the convergence points V at both ends of the steel plate downstream becomes stronger, making it easier to maintain a steady state that satisfies the conditions of the above formula. Therefore, it is possible to more stably reduce the defective fracture rate, and it becomes easier to prevent weld defects caused by oxides.

[0104] From Table 1, D Ｖ－ＳＱＣ When the shortening index increases, D Ｖ－ＳＱＣ For example, when comparing Test Examples 2, 5, 8, 11, and 14 in which the melting point Tm, carbon concentration MCc, and conveying speed Vf of the steel plate are the same, D Ｖ－ＳＱＣ The shortening index is 0.25 or more (W ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ In Test Examples 5, 8, 11, and 14, where D Ｖ－ＳＱＣ Similarly, when comparing Test Examples 3, 6, 9, 12, and 15 in which the melting point Tm, carbon concentration MCc, and conveying speed Vf of the steel plate are the same, D Ｖ－ＳＱＣ In Test Examples 6, 9, 12, and 15, where the shortening index was 0.25 or more, D Ｖ－ＳＱＣ The actual measured value is significantly smaller. Ｖ－ＳＱＣ When the shortening index is 0.25 or more, D Ｖ－ＳＱＣ The actual measured value tends to be small, Ｖ－ＳＱＣ The actual measured value is D Ｖ－ＳＱＣ It can be said that the upper limit of the index ({Tm-(1525.867-177×MCc)}×Vf / 4200) is less likely to be exceeded. Ｖ－ＳＱＣ The larger the shortening index, the Ｖ－ＳＱＣ The actual measured value tends to be smaller, Ｖ－ＳＱＣ The actual measured value is D Ｖ－ＳＱＣ It becomes difficult to exceed the index limit.

[0105] In Test Examples 6, 8, 9, 11, 12, 14, 15, and 17, which were judged to pass this test, the normalized SQ roll diameter was 3.60 or more, but in Test Example 20, the normalized SQ roll diameter was less than 3.60. In Test Examples 6, 8, 9, 11, 12, 14, 15, and 17, good upset welding was performed, but poor SQ reduction was confirmed in Test Example 20. SQ reduction is a phenomenon in which the circumferential length of a tube is shortened when a steel plate formed into a tubular shape is pressed by a squeeze roll to promote the discharge of molten steel from the butt ends. Poor SQ reduction occurs when the diameter D of the side roll is ＳＲ In Test Example 20, it is believed that poor SQ reduction occurred due to insufficient rigidity of the side rolls.

[0106] Therefore, from the viewpoint of avoiding SQ reduction failure, the normalized SQ roll diameter (D ＳＲ / R ＳＲ On the other hand, the distance D from the convergence points V at both ends of the steel sheet to the squeeze roll center SQC is preferably 3.60 or more. Ｖ－ＳＱＣ In order to make the length shorter and make it easier to satisfy the above formula, the normalized SQ roll diameter is preferably 4.50 or less.

[0107] In Test Examples 6, 8, 9, 11, 12, 14, 15, and 17, which were judged to pass this test, the normalized SG width was 0.80 or less, but in Test Example 19, the normalized SG width was greater than 0.80. In Test Example 19, the electric resistance welded steel pipe manufactured from the steel plate did not have a circular shape, and defective forming was confirmed.

[0108] Therefore, from the viewpoint of avoiding molding defects, the normalized SG width (W ＳＧ / R ＳＲ On the other hand, the distance D from the convergence points V at both ends of the steel sheet to the squeeze roll center SQC is preferably 0.80 or less. Ｖ－ＳＱＣ In order to make the above formula easier to be satisfied by shortening the normalized SG width, it is preferable that the normalized SG width is 0.65 or more.

[0109] 10: Steel plate 11L, 11R: End portion 23: Seam guide 32: Squeeze roll 321L, 321R: Side roll 321a: Side surface 70: Imaging device 200: Monitoring device 206: Image acquisition section 207: Measuring section 208: Determining section

Claims

1. A method for manufacturing electric resistance welded steel pipe, comprising: a step of forming a steel plate into a tubular shape while conveying the steel plate so that both ends of the steel plate face each other; and a step of heating and melting the both opposing ends, and then using squeeze rolls including a pair of side rolls to press the steel plate so that the both ends, which approach and come into contact as they move downstream in the conveying direction of the steel plate, are pressed together to weld the both ends, wherein when the melting point of the steel plate is Tm [°C], the carbon concentration of the steel plate is MCc [mass %], and the conveying speed of the steel plate is Vf [mm / sec], the distance D in the conveying direction from the convergence point, which is the position where the both ends come into contact, to the central axis of the side roll is Ｖ－ＳＱＣ [mm] satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm-(1525.867-177×MCc)}×Vf / 4200 2. A manufacturing method according to claim 1, wherein in the welding step, the angle formed by the two ends that come closer to each other and come into contact as they move downstream in the conveying direction is 5.0° or more.

3. A manufacturing method according to claim 1, wherein the carbon concentration MCc is 0.18 mass % or more and 0.50 mass % or less.

4. A manufacturing method according to claim 1, wherein the radius of curvature of the side surface of each of the side rolls that curves concavely toward the central axis is 5 mm or more and 55 mm or less.

5. A manufacturing method according to claim 1, wherein in the forming step, the distance between the opposing ends is adjusted by a seam guide arranged upstream of the squeeze roll in the conveying direction, and the diameter at the bottom of the side surface of each of the side rolls that curves concavely toward the central axis is set to D. ＳＲ [mm], the width of the seam guide is W ＳＧ [mm], the radius of curvature of the side surface of the side roll is R ＳＲ When W is expressed as [mm], the following formula is satisfied: ＳＧ / R ＳＲ -0.0571 x D ＳＲ / R ＳＲ ≧0.25 6. A method for manufacturing an electric resistance welded steel pipe in which a steel plate is formed into a tubular shape while being conveyed, and both ends of the steel plate are upset and welded using squeeze rolls including a pair of side rolls, the method comprising the steps of: capturing images of both ends taken by an imaging device, including portions of the both ends that approach and come into contact as they move downstream in the conveying direction of the steel plate; measuring the distance in the conveying direction from a convergence point, which is the position where the both ends come into contact, to the central axis of the side roll based on the images; and judging whether the distance satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm−(1525.867−177×MCc)}×Vf / 4200, where Tm is the melting point of the steel sheet [°C], MCc is the carbon concentration of the steel sheet [mass%], Vf is the conveying speed of the steel sheet [mm / sec], and D Ｖ－ＳＱＣ is the distance [mm].

7. A manufacturing method as claimed in claim 6, further comprising the step of: if it is determined in the determining step that the distance does not satisfy the formula, adjusting the distance so that the formula is satisfied by increasing the width of a seam guide that adjusts the gap between the two end portions before welding.

8. A manufacturing method according to claim 6, further comprising the step of: if it is determined in the determining step that the distance does not satisfy the formula, adjusting the distance so that the formula is satisfied by reducing the diameter of each of the side rolls.

9. A monitoring device for monitoring the production of electric resistance welded steel pipes in which a steel plate is formed into a tubular shape while being conveyed and both ends of the steel plate are upset and welded using squeeze rolls including a pair of side rolls, the monitoring device comprising: an image acquisition unit that acquires image data captured by an imaging device, including portions of both ends that approach and come into contact as they move downstream in the conveying direction of the steel plate; a measurement unit that measures the distance in the conveying direction from a convergence point, which is the position where the both ends come into contact, to the central axis of the side roll based on the image data; and a determination unit that determines whether the distance satisfies the following formula: 0<D Ｖ－ＳＱＣ ≦{Tm−(1525.867−177×MCc)}×Vf / 4200, where Tm is the melting point of the steel sheet [°C], MCc is the carbon concentration of the steel sheet [mass%], Vf is the conveying speed of the steel sheet [mm / sec], and D Ｖ－ＳＱＣ is the distance [mm].

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