Submerged arc welding method and submerged arc welding system

Abstract

The present invention provides a submerged arc welding method and a submerged arc welding system that eliminate the slag removal step when performing multi-layer welding. [Solution] A submerged arc welding method for multilayer welding, wherein the second voltage command value in the second welding pass is greater than the first voltage command value in the first welding pass, the second welding speed command value in the second welding pass is less than the first welding speed command value in the first welding pass, and the second welding pass is performed without removing the slag from the first welding pass.

Description

【Technical Field】 【0001】 The present invention relates to a submerged arc welding method and a submerged arc welding system. 【Background Art】 【0002】 Conventionally, submerged arc welding is known. Submerged arc welding involves spraying granular flux onto the workpiece, feeding a welding wire into the flux, and generating an arc between the tip of the welding wire and the workpiece to perform welding. In submerged arc welding, by passing a large current through a thick welding wire, thick plates can be welded with high efficiency. 【0003】 The flux shields the periphery of the arc to stabilize the arc. Also, a part of the flux melts by the arc heat to become slag, protecting the molten metal from the atmosphere and shaping the bead when the molten metal solidifies. The unmolten flux is recovered and reused. 【0004】 When performing multi-layer welding in submerged arc welding, the slag is removed for each welding pass before starting the next welding pass. Generally, the flux used in submerged arc welding is adjusted in composition so that the slag has good peelability, and slag removal after welding is often easy. Slag removal is generally performed manually using a chisel and a hammer. Also, in some cases, it may be automatically removed by a slag removal device disclosed in Patent Document 1. 【Prior Art Documents】 【Patent Documents】 【0005】 【Patent Document 1】 Japanese Patent Application Laid-Open No. 11-267833 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0006】 However, the detachability of slag is influenced by various factors other than its chemical composition, such as the width of the slag and the shape of the weld metal. The greater the solidification shrinkage of the slag, the better its detachability. Therefore, the wider the slag, the easier it is to detach. Also, if the surface of the weld metal is convex, when the slag solidifies, it shrinks and adheres to the weld metal from both sides in the width direction, worsening the detachability of the slag. On the other hand, if the surface of the weld metal is flat or concave, adhesion to the weld metal does not occur during the solidification shrinkage of the slag, so the detachability of the slag is good. Thus, depending on the welding conditions, the detachability of the slag may worsen, making slag removal difficult. This is particularly noticeable in first-layer welding within a groove. In first-layer welding within a groove, the groove width is small, resulting in a small slag width. Also, in first-layer welding, low-voltage welding conditions are often used to achieve deep penetration. In this case, the surface of the weld metal tends to be convex. Therefore, in the initial layer welding within a groove, the detachability of the slag tends to deteriorate, making slag removal difficult. When slag removal is difficult, it is necessary to use electric or compressed air-driven tools. When the deterioration of slag detachability is particularly pronounced, it is necessary to use large and high-powered tools, in which case the welding quality may deteriorate due to deformation of the surface of the previous bead or the groove. In addition, handling large tools in a very noisy environment places a heavy burden on the worker. Furthermore, even when using the slag removal equipment mentioned above, slag with deteriorated detachability may remain without being removed. 【0007】 The present invention was conceived under the circumstances described above, and aims to provide a submerged arc welding method and a submerged arc welding system that can omit the slag removal step when performing multilayer welding. [Means for solving the problem] 【0008】 A submerged arc welding method provided by a first aspect of the present invention is a submerged arc welding method for multilayer welding, wherein a second voltage command value in the second welding pass is greater than a first voltage command value in the first welding pass, and the slag from the first welding pass is melted in the second welding pass. 【0009】 In a preferred embodiment of the present invention, the second welding speed command value in the second welding pass is smaller than the first welding speed command value in the first welding pass. 【0010】 In a preferred embodiment of the present invention, the second voltage command value is 40V or more, and the second welding speed command value is 3 / 4 or less of the first welding speed command value. 【0011】 In a preferred embodiment of the present invention, the second current command value in the second welding pass is greater than the first current command value in the first welding pass. 【0012】 In a preferred embodiment of the present invention, welding is performed at an advance angle in the second welding pass. 【0013】 In a preferred embodiment of the present invention, a submerged arc welding system having a leading electrode and a trailing electrode with an advance angle is used, and in the first welding pass, welding is performed using only the leading electrode, and in the second welding pass, welding is performed using only the trailing electrode. 【0014】 A submerged arc welding system provided by a second aspect of the present invention is a submerged arc welding system for multilayer welding, comprising: a first setting unit for setting a first voltage command value for the first layer welding pass; and a second setting unit for setting a second voltage command value for the second layer welding pass, wherein the second setting unit sets a second voltage command value greater than the first voltage command value based on the first voltage command value, and melts the slag from the first layer welding pass in the second layer welding pass. [Effects of the Invention] 【0015】 According to the present invention, in the second welding pass, a second voltage command value greater than the first voltage command value of the first pass is set. Since the melting of the slag by the first welding pass can be promoted in the second welding pass, it is possible to perform the second welding pass without removing the slag from the first welding pass. As a result, the submerged arc welding method according to the present invention can omit the slag removal step when performing multi-layer welding. [Brief explanation of the drawing] 【0016】 [Figure 1] This diagram illustrates a welding system according to the first embodiment, where (a) is a block diagram showing the overall configuration of the welding system, and (b) is a block diagram showing the internal configuration of the welding power supply and control device. [Figure 2] This is a diagram illustrating a modified example of the first embodiment, and is a block diagram showing the internal configuration of the control device according to the modified example. [Figure 3] This diagram illustrates a welding system according to a second embodiment, where (a) is a block diagram showing the carriage of the welding system and the equipment mounted on the carriage, and (b) is a block diagram showing the internal configuration of the control device. [Figure 4] This diagram illustrates a welding system according to a third embodiment, where (a) is a block diagram showing the overall configuration of the welding system, and (b) is a block diagram showing the internal configuration of the welding power supply and control device. [Modes for carrying out the invention] 【0017】 Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 【0018】 [First Embodiment] Figure 1 is a diagram illustrating a welding system A1 according to the first embodiment. Figure 1(a) is a block diagram showing the overall configuration of the welding system A1. Figure 1(b) is a block diagram showing the internal configuration of the welding power supply unit 2 and the control device 1. 【0019】 The welding system A1 is a welding system for performing submerged arc welding. As shown in Fig. 1(a), the welding system A1 includes a control device 1, a welding power supply device 2, a welding torch 3, a carriage 4, a wire feeding device 5, a wire reel 6, a spraying device 71, a recovery device 72, and an electrode 8. The welding system A1 moves the carriage 4 along the weld line of the workpiece W, while the spraying device 71 sprays granular flux 79, and the wire feeding device 5 feeds the welding wire into the flux 79. The welding wire is supplied from the wire reel 6. The welding power supply device 2 converts the AC power supplied from the commercial power supply P into power suitable for welding and outputs it, and generates an arc between the electrode 8, which is the tip portion of the welding wire, and the workpiece W inside the flux 79. Welding is performed by the heat of the arc. Thereby, welding is performed along the weld line of the workpiece W. A part of the sprayed flux 79 melts by the arc heat to form slag, protecting the molten metal from the atmosphere. The unmolten flux 79 is recovered by the recovery device 72 and reused. Instead of using the carriage 4, the workpiece W may be moved or rotated. 【0020】 The control device 1 performs various controls on the welding system A1. The control device 1 may be a general-purpose computer installed with a program for performing various controls on the welding system A1, or may be a dedicated device for controlling the welding system A1. The control device 1 instructs the carriage 4 to start, stop, and the moving speed. The control device 1 controls the welding speed by controlling the moving speed of the carriage 4. The moving speed is set according to the material and thickness of the workpiece W, etc. In addition, an operator may instruct the start, stop, and moving speed of the carriage 4. 【0021】 The control device 1 instructs the welding power supply device 2 to start and stop the power output. In addition, the control device 1 instructs the set values of the welding voltage and welding current. 【0022】 The welding torch 3 guides the welding wire fed by the wire feeder 5 to the welding location. The tip of the welding wire becomes the electrode 8 that protrudes from the tip of the welding torch 3. The welding torch 3 includes a contact tip (not shown) disposed at the tip portion and electrically connected to the welding power supply device 2. The welding power supply device 2 supplies a welding current to the welding wire that contacts the contact tip. The welding torch 3 moves as the carriage 4 moves. 【0023】 The control device 1 instructs the wire feeder 5 to start, stop, and control the feeding speed of the welding wire. The feeding speed is set according to the set welding current and the like. Further, the control device 1 instructs the spraying device 71 to start and end the spraying of the flux 79. Note that the operator may instruct the start and end of the spraying of the flux 79. Also, the operator may manually spray the flux 79 before the start of welding. In this case, the welding system A1 may not include the spraying device 71. The recovery device 72 may be instructed by the control device 1 to start and end the recovery of the flux 79, or may start and end the recovery in accordance with the start and end of the spraying of the spraying device 71. Also, the operator may manually recover the flux 79 after the end of welding. In this case, the welding system A1 may not include the recovery device 72. 【0024】 In the present embodiment, the control device 1 switches the welding conditions depending on whether it is the first layer welding pass or the second layer welding pass. Specifically, the control device 1 switches the welding voltage, welding current, and welding speed (the moving speed of the carriage 4). Note that the welding current may not be switched. Details of the switching of the welding conditions between the first layer welding pass and the second layer welding pass by the control device 1 will be described later. 【0025】 The welding power supply device 2 converts the AC power supplied from the commercial power supply P into AC power of a desired frequency and outputs it. As shown in FIG. 1(b), the welding power supply device 2 includes a rectifying and smoothing circuit 21, an inverter circuit 22, a transformer 23, a rectifying and smoothing circuit 24, an inverter circuit 25, a current sensor 26, a voltage sensor 27, and a control circuit 28. 【0026】 The rectifier and smoothing circuit 21 converts the AC power input from the commercial power supply P into DC power and outputs it. The inverter circuit 22 converts the DC power input from the rectifier and smoothing circuit 21 into high-frequency power and outputs it by switching a switching element according to the output control drive signal input from the control circuit 28. The transformer 23 transforms the high-frequency voltage output by the inverter circuit 22 and outputs it to the rectifier and smoothing circuit 24. 【0027】 The rectifier and smoothing circuit 24 converts the high-frequency power input from the transformer 23 into DC power and outputs it. The inverter circuit 25 converts the DC power input from the rectifier and smoothing circuit 24 into AC power and outputs it by switching a switching element in response to a switching drive signal input from the control circuit 28. The inverter circuit 25 switches between positive polarity, where the potential of output terminal a (connected to the workpiece W) is higher than the potential of output terminal b (connected to the welding wire), and reverse polarity, where the potential of output terminal a is lower than the potential of output terminal b. 【0028】 The current sensor 26 detects the output current of the welding power supply unit 2, and in this embodiment, it is located on the connecting wire that connects one output terminal of the inverter circuit 25 to output terminal a. The output current of the welding power supply unit 2 detected by the current sensor 26 is approximately equal to the current flowing through the electrode 8. The current sensor 26 outputs a current value signal corresponding to the detected instantaneous current value to the control circuit 28 and the control device 1. The voltage sensor 27 detects the output voltage of the welding power supply unit 2, and in this embodiment, it detects the terminal voltage between output terminal a and output terminal b. This voltage is approximately equal to the voltage applied between the workpiece W and the tip of the electrode 8. The voltage sensor 27 outputs a voltage value signal corresponding to the detected instantaneous voltage value to the control circuit 28 and the control device 1. The voltage sensor 27 may also detect the voltage between the lead wire attached to the welding torch 3 and the lead wire attached to the workpiece W. 【0029】 The control circuit 28 is a circuit for controlling the welding power supply unit 2 and is implemented by, for example, a microcomputer. The control circuit 28 receives a current value signal from the current sensor 26, a voltage value signal from the voltage sensor 27, and various command signals and various setting values ​​from the control device 1. The various setting values ​​include a voltage command value, which is the setting value for the welding voltage, and a current command value, which is the setting value for the welding current. The control circuit 28 then outputs drive signals to the inverter circuit 22 and the inverter circuit 25, respectively. 【0030】 When the control circuit 28 receives a command signal from the control device 1 instructing it to start power output, it starts outputting drive signals to the inverter circuit 22 and inverter circuit 25, respectively, thereby starting power output. Conversely, when the control circuit 28 receives a command signal from the control device 1 instructing it to stop power output, it stops outputting drive signals, thereby stopping power output. 【0031】 Furthermore, the control circuit 28 calculates the effective current value from the current value signal input from the current sensor 26 and the effective voltage value from the voltage value signal input from the voltage sensor 27. The control circuit 28 then generates an output control drive signal so that the effective current value matches the current command value and the effective voltage value matches the voltage command value, and outputs it to the inverter circuit 22. 【0032】 Furthermore, the control circuit 28 generates a switching drive signal to control the switching elements of the inverter circuit 25 based on the current value signal input from the current sensor 26 and a waveform command signal generated internally, and outputs it to the inverter circuit 25. In other words, the control circuit 28 performs feedback control so that the waveform of the output current matches the waveform commanded by the waveform command signal. In this embodiment, the waveform command signal is a square wave (trapezoidal wave) signal. The waveform command signal may be other waveform signals such as a sine wave signal. By generating a switching drive signal based on the waveform command signal and outputting it to the inverter circuit 25, the inverter circuit 25 outputs a sinusoidal AC current corresponding to the waveform command signal. The control circuit 28 may also generate the switching drive signal based only on the waveform command signal without using the instantaneous value of the output current. 【0033】 Furthermore, when the control circuit 28 receives a DC output command signal from the control device 1, it outputs a switching drive signal to the inverter circuit 25 that fixes a predetermined switching element in the ON state and the other switching elements in the OFF state. For example, when the state of each switching element is fixed such that the positive output terminal of the rectifier and smoothing circuit 24 is connected to output terminal a, and the negative output terminal of the rectifier and smoothing circuit 24 remains connected to output terminal b, the welding power supply unit 2 outputs DC power with output terminal a as the positive terminal and output terminal b as the negative terminal. In other words, welding system A1 is a dual AC / DC welding system that can output not only AC power but also DC power. Note that the configuration of the welding power supply unit 2 is not limited. 【0034】 Next, we will explain in detail how the control device 1 switches welding conditions between the first welding pass and the second welding pass. 【0035】 Normally, if the second layer of welding is performed without removing the slag from the first layer of welding, the slag ahead in the welding direction cannot be sufficiently melted. As a result, the welding wire comes into contact with the slag of the first layer, making it impossible to proceed with welding. This can also lead to the trolley 4 tipping over. In particular, the ends of the slag in the welding direction of the first layer (the welding start position and the welding end position) cannot be sufficiently melted, which can lead to slag entrapment. Therefore, in normal welding procedures, it is not possible to perform the second layer of welding without removing the slag of the first layer. In this embodiment, in order to sufficiently melt the slag of the first layer, the welding conditions for the second layer are set to be higher voltage, higher current, and lower speed than the welding conditions for the first layer. Specifically, in the second layer, the voltage command value and current command value are set higher than in the first layer, and the welding speed command value, which is the set value for the welding speed, is set lower. 【0036】 The control device 1 includes a first setting unit 11, a second setting unit 12, and a switching unit 13, as shown in Figure 1(b), for switching welding conditions between the first and second welding passes. The control device 1 also includes other components, but these are not shown or described. 【0037】 The first setting unit 11 is configured for setting each welding condition in the first welding pass. The first setting unit 11 sets values ​​entered by the operator through the operation of an operation unit (not shown). The first setting unit 11 sets various setting values ​​such as welding current, welding voltage, welding speed (movement speed of the trolley 4), overhang length, torch angle, joint shape, groove angle, wire feeding speed, wire material, wire diameter, material of the workpiece W, thickness of the workpiece W, and flux type indicating the type of flux 79. The first setting unit 11 may also set values ​​calculated from the setting values ​​of the welding conditions as other setting values. The first setting unit 11 includes a current setting unit 111, a voltage setting unit 112, and a welding speed setting unit 113. 【0038】 The current setting unit 111 sets the current command value for the first welding pass. The voltage setting unit 112 sets the voltage command value for the first welding pass. The welding speed setting unit 113 sets the welding speed command value for the first welding pass. 【0039】 The first setting unit 11 outputs the setting values ​​for each welding condition, including the current command value set by the current setting unit 111, the voltage command value set by the voltage setting unit 112, and the welding speed command value set by the welding speed setting unit 113, to the switching unit 13. 【0040】 The second setting unit 12 is configured for setting each welding condition in the second welding pass. The second setting unit 12 sets the values ​​entered by the operator through the operation of the control unit. The second setting unit 12 sets the setting values ​​for each welding condition in the same way as the first setting unit 11. In addition, for welding conditions where the same setting values ​​as the first setting unit 11 are set, the second setting unit 12 may reuse the setting values ​​set in the first setting unit 11. Furthermore, the second setting unit 12 may set values ​​calculated from the setting values ​​of the welding conditions as other setting values. The second setting unit 12 includes a current setting unit 121, a voltage setting unit 122, and a welding speed setting unit 123. 【0041】 The current setting unit 121 sets the current command value for the second welding pass. The current command value for the second welding pass is set to a value greater than the current command value for the first welding pass. A larger welding current results in greater heat input from the arc to the first layer of slag, thus promoting the melting of the slag. However, the current command value for the second welding pass may be the same as the current command value for the first welding pass. 【0042】 The voltage setting unit 112 sets the voltage command value for the second welding pass. The voltage command value for the second welding pass is set to a value higher than the voltage command value for the first welding pass. The higher the welding voltage, the greater the heat input from the arc to the first layer of slag, thus promoting the melting of the slag. Also, since welding voltage and arc length are proportional, the higher the welding voltage, the longer the arc length can be made, increasing the distance between the tip of the welding wire and the workpiece W. This suppresses contact between the tip of the welding wire and the first layer of slag. 【0043】 When the weld area is observed using X-rays during welding, the position of the welding wire tip can be observed. If the welding voltage is less than 40V, the welding wire tip penetrates below the surface of the molten metal, resulting in a so-called buried arc. In this case, it is difficult to prevent the welding wire tip from contacting the first layer of slag. Therefore, in this embodiment, the voltage command value for the second welding pass is set to 40V or higher. In submerged arc welding, the voltage command value is usually set to less than 40V from the viewpoint of ensuring penetration depth and preventing excessive bead width. Therefore, under normal welding conditions, the first layer of slag cannot be sufficiently melted. Furthermore, a voltage command value of 45V or higher is more desirable for the second welding pass. The distance between the welding wire tip and the workpiece W with respect to the welding voltage varies somewhat depending on the welding material, etc. Therefore, if the voltage command value is less than 45V, a buried arc may occur depending on the welding material. However, if the voltage command value is 45V or higher, it is possible to create an open arc, rather than a buried arc, regardless of the welding material, thus preventing the tip of the welding wire from coming into contact with the initial layer of slag. 【0044】 The welding speed setting unit 113 sets the welding speed command value for the second welding pass. The welding speed command value for the second welding pass is set to a value smaller than the welding speed command value for the first welding pass. The smaller (slower) the welding speed, the more the melting of the slag in front of the welding direction due to heat conduction can be promoted. Also, by reducing the welding speed and increasing the heat input, the decrease in penetration depth due to the increase in welding voltage can be mitigated. In this embodiment, the welding speed command value for the second welding pass is set to 3 / 4 or less of the welding speed command value for the first welding pass. This makes it easier to melt the slag in the first layer. Note that the welding speed command value for the second welding pass may be the same as the welding speed command value for the first welding pass. 【0045】 The second setting unit 12 outputs the setting values ​​for each welding condition, including the current command value set by the current setting unit 121, the voltage command value set by the voltage setting unit 122, and the welding speed command value set by the welding speed setting unit 123, to the switching unit 13. 【0046】 The switching unit 13 switches between and outputs the welding conditions input from the first setting unit 11 and the welding conditions input from the second setting unit 12. When performing the first layer welding pass, the switching unit 13 outputs the welding conditions input from the first setting unit 11, and when performing the second layer welding pass, it outputs the welding conditions input from the second setting unit 12. Whether it is the first layer welding pass or the second layer welding pass may be entered by the operator or may be switched automatically. 【0047】 The switching unit 13 outputs current command values ​​and voltage command values ​​to the control circuit 28 of the welding power supply unit 2. In addition, the control device 1 instructs the trolley 4 to move at a speed according to the welding speed command value output by the switching unit 13. 【0048】 In welding system A1, when performing multi-layer welding, first, the control device 1 has the switching unit 13 set the welding conditions input from the first setting unit 11, and the first layer welding pass is performed. Next, without removing the slag formed in the first layer welding pass, the control device 1 has the switching unit 13 set the welding conditions input from the second setting unit 12, and the second layer welding pass is performed. In the second layer welding pass, welding conditions are set that can promote the melting of the slag, so it is possible to perform welding while melting the slag of the first layer. 【0049】 In the second welding pass, it is desirable to start the arc using steel wool rolled to a larger diameter than in the first welding pass, generate an arc, and then wait until the slag of the first layer directly beneath the tip of the welding wire is melted. This ensures that the slag at the start of the weld is reliably melted. Also, compared to the conventional method (removing the slag before performing the second welding pass), the chemical composition of the weld metal in the second layer may change, so it is desirable to perform a welding test beforehand to confirm the mechanical performance of the joint. 【0050】 Since the slag from the second layer onwards is much easier to remove than the slag from the first layer, the slag may be removed manually or by other means, as in the conventional method, before welding the next layer. Alternatively, for welding passes from the third layer onwards, welding may be performed without removing the slag from the previous layer, by setting the welding conditions input from the second setting unit 12 via the switching unit 13 in the control device 1. In this case, the entire slag removal process can be omitted. 【0051】 Multilayer welding was performed under the following conditions. First, using a 4.8mm diameter welding wire, the first layer of a butt weld on a 25mm thick plate with a 50° groove was welded under the following conditions: current command value 800A, voltage command value 24V, and welding speed command value 60cm / min. Then, without removing the slag, the second layer was welded under the following conditions: current command value 800A, voltage command value 45V, and welding speed command value 40cm / min. In the second welding pass, the slag from the first layer melted, and welding was performed without any problems. Furthermore, the slag from the second layer could be easily removed. 【0052】 Next, the operation and effects of the submerged arc welding method and submerged arc welding system according to this embodiment will be described. 【0053】 According to this embodiment, the voltage command value for the second welding pass is set to a value greater than the voltage command value for the first welding pass. Since the melting of the slag by the first welding pass can be promoted in the second welding pass, it is possible to perform the second welding pass without removing the slag from the first welding pass. As a result, the submerged arc welding method according to this embodiment can omit the slag removal step when performing multi-layer welding. 【0054】 Furthermore, according to this embodiment, the welding speed command value for the second welding pass is set to a smaller value than the welding speed command value for the first welding pass. This further promotes the melting of the slag by the first welding pass. In addition, by increasing the heat input, the decrease in penetration depth due to the increased welding voltage can be mitigated. 【0055】 Furthermore, according to this embodiment, the current command value in the second welding pass is set to a value greater than the current command value in the first welding pass. This further promotes the melting of the slag by the first welding pass. 【0056】 Furthermore, according to this embodiment, the voltage command value in the second welding pass is 40V or higher. Therefore, the occurrence of a buried arc can be suppressed. This prevents the tip of the welding wire from coming into contact with the slag of the first layer. Also, if the voltage command value in the second welding pass is 45V or higher, an open arc can be achieved regardless of the welding material. Therefore, it is possible to prevent the tip of the welding wire from coming into contact with the slag of the first layer. In addition, according to this embodiment, the welding speed command value in the second welding pass is 3 / 4 or less of the welding speed command value in the first welding pass. Therefore, it is easier to melt the slag of the first layer. 【0057】 In this embodiment, the case in which the switching unit 13 sets the corresponding current command value, voltage command value, and welding speed command value depending on whether it is the first layer welding pass or the second layer welding pass has been described, but it is not limited to this. For example, the operator may set the corresponding current command value, voltage command value, and welding speed command value depending on whether it is the first layer welding pass or the second layer welding pass in the control device 1. 【0058】 Furthermore, although this embodiment describes a case where the control device 1 is equipped with the first setting unit 11, the second setting unit 12, and the switching unit 13, it is not limited to this. The welding power supply device 2 may be equipped with the first setting unit 11, the second setting unit 12, and the switching unit 13, or a part thereof. 【0059】 Furthermore, although this embodiment describes a case in which the second setting unit 12 sets values ​​entered by the operator through the operation of the control unit, it is not limited to this. The second setting unit 12 may automatically set each welding condition for the second welding pass based on each welding condition set in the first setting unit 11. Figure 2 is a diagram illustrating this modified example and is a block diagram showing the internal configuration of the control device 1 according to the modified example. 【0060】 The modified second setting unit 12 includes a current changing unit 125, a voltage changing unit 126, and a welding speed changing unit 127 instead of the current setting unit 121, the voltage setting unit 122, and the welding speed setting unit 123. The current changing unit 125 sets a value obtained by applying a predetermined correction to the current command value set in the current setting unit 111 and increasing it, as the current command value for the second welding pass. The voltage changing unit 126 sets a value obtained by applying a predetermined correction to the voltage command value set in the voltage setting unit 112 and increasing it to 40V or more, as the voltage command value for the second welding pass. The welding speed changing unit 127 sets a value obtained by applying a predetermined correction to the welding speed command value set in the welding speed setting unit 113 and reducing it to 3 / 4 or less, as the welding speed command value for the second welding pass. Note that each correction may be performed using a predetermined calculation formula, or it may be read based on a correspondence table stored in a storage unit (not shown). According to this modified example, the welding conditions in the second setting unit 12 are set automatically, thus reducing the effort required from the operator. 【0061】 [Second Embodiment] Figure 3 is a diagram illustrating the welding system A2 according to the second embodiment. Figure 3(a) is a block diagram showing the trolley 4 and the devices mounted on the trolley 4 of the welding system A2. Figure 3(b) is a block diagram showing the internal configuration of the control device 1. In Figure 3, elements that are the same as or similar to those in the first embodiment are denoted by the same reference numerals as in the first embodiment. Note that the other configurations of the welding system A2 are the same as in the first embodiment, so their description and explanation are omitted. The welding system A2 according to this embodiment differs from the welding system A1 according to the first embodiment in that the torch angle of the welding torch 3 can be changed. 【0062】 The welding system A2 according to this embodiment further includes a drive unit 9. The drive unit 9 has a fixed welding torch 3 and, for example, includes a servo motor, which changes the torch angle of the welding torch 3 in response to a command from the control device 1. The specific structure of the drive unit 9 is not limited. 【0063】 The first setting unit 11 according to this embodiment further includes a torch angle setting unit 114. The torch angle setting unit 114 sets the torch angle for the first layer welding pass. For example, the torch angle for the first layer welding pass is set to "0°" (perpendicular to the surface). The second setting unit 12 according to this embodiment further includes a torch angle setting unit 124. The torch angle setting unit 124 sets the torch angle for the second layer welding pass. For the second layer welding pass, a positive value (advancing angle) is set to the torch angle. The control device 1 according to this embodiment outputs a command to the drive unit 9 according to the torch angle output by the switching unit 13. As a result, welding is performed perpendicular to the surface in the first layer welding pass (see welding torch 3 shown by a solid line in Figure 3(a)), and welding is performed at an advancing angle in the second layer welding pass (see welding torch 3 shown by a dashed line in Figure 3(a)). In the case of an advance angle, the arc is formed ahead of the tip of the welding wire in the direction of welding, making it easier to melt the slag before the welding wire comes into contact with it. 【0064】 In this embodiment as well, the voltage command value for the second welding pass is set to a value greater than the voltage command value for the first welding pass. Since the melting of the slag by the first welding pass can be promoted in the second welding pass, it is possible to perform the second welding pass without removing the slag from the first welding pass. As a result, the submerged arc welding method according to this embodiment can omit the slag removal step when performing multi-layer welding. Furthermore, the submerged arc welding method according to this embodiment has a common configuration with the submerged arc welding method according to the first embodiment and thus achieves the same effects as the submerged arc welding method according to the first embodiment. Moreover, according to this embodiment, welding is performed at an advance angle in the second welding pass, making it easier to melt the slag before the welding wire and slag come into contact. 【0065】 In this embodiment, the case in which the drive unit 9 changes the angle of the welding torch 3 in response to a command from the control device 1 has been described, but this is not the only case. The welding system A2 may not have a drive unit 9, and the operator may manually change the torch angle of the welding torch 3. 【0066】 [Third Embodiment] Figure 4 is a diagram illustrating the welding system A3 according to the third embodiment. Figure 4(a) is a block diagram showing the overall configuration of the welding system A3. Figure 4(b) is a block diagram showing the internal configuration of the welding power supply unit 2 and the control device 1. In Figure 4, elements that are the same as or similar to those in the first embodiment are denoted by the same reference numerals as those in the first embodiment. The welding system A3 according to this embodiment differs from the welding system A1 according to the first embodiment in that it is a system for performing so-called tandem welding. 【0067】 The welding system A3 according to this embodiment includes two welding power supply units 2, two wire feeders 5, two wire reels 6, two welding torches 3, and two electrodes 8. The welding system A3 performs so-called tandem welding, where the two electrodes 8 pass over the same welding point with a time difference, and each of the two electrodes 8 generates an arc, enabling highly efficient and high-speed welding. The two electrodes 8 are arranged side by side in the direction of travel of the trolley 4. Therefore, as the trolley 4 travels along the welding line of the workpiece W, after one electrode 8 passes over a certain welding point, the other electrode 8 passes over the same welding point with a delay. Each electrode 8 melts the workpiece W, forming a single weld bead. 【0068】 In the following, the leading electrode 8 that passes through the welding area first will be referred to as electrode 8a (leading electrode), and the trailing electrode 8 that passes through the welding area later than electrode 8a will be referred to as electrode 8b (trailing electrode). Also, the welding power supply device 2 that supplies power to electrode 8a will be referred to as welding power supply device 2a, and the welding power supply device 2 that supplies power to electrode 8b will be referred to as welding power supply device 2b. Furthermore, the welding torch 3 that causes the welding wire to protrude as electrode 8a will be referred to as welding torch 3a, and the welding torch 3 that causes the welding wire to protrude as electrode 8b will be referred to as welding torch 3b. In this embodiment, the torch angle of welding torch 3a is set to, for example, "0°" (perpendicular to the surface) or a negative value (backward angle), and the torch angle of welding torch 3b is set to a positive value (forward angle). 【0069】 In this embodiment, when performing the first layer welding pass, the switching unit 13 outputs current command values ​​and voltage command values ​​only to the welding power supply unit 2a to instruct it to output power, and instructs the wire feeder 5, which feeds the welding wire to the welding torch 3a, to feed the welding wire. In other words, in the first layer welding pass, welding is not performed with electrode 8b, and welding is performed using only electrode 8a. On the other hand, when performing the second layer welding pass, the switching unit 13 outputs current command values ​​and voltage command values ​​only to the welding power supply unit 2b to instruct it to output power, and instructs the wire feeder 5, which feeds the welding wire to the welding torch 3b, to feed the welding wire. In other words, in the second layer welding pass, welding is not performed with electrode 8a, and welding is performed using only electrode 8b. 【0070】 In this embodiment as well, the voltage command value for the second welding pass is set to a value greater than the voltage command value for the first welding pass. Since the melting of the slag by the first welding pass can be promoted in the second welding pass, it is possible to perform the second welding pass without removing the slag from the first welding pass. As a result, the submerged arc welding method according to this embodiment can omit the slag removal step when performing multi-layer welding. Furthermore, the submerged arc welding method according to this embodiment has a common configuration with the submerged arc welding method according to the first embodiment and thus achieves the same effects as the submerged arc welding method according to the first embodiment. Moreover, according to this embodiment, welding is performed at an advance angle in the second welding pass, making it easier to melt the slag before the welding wire and slag come into contact. 【0071】 In this embodiment, the control device 1 has been described as switching between welding using only electrode 8a and welding using only electrode 8b by issuing instructions to the welding power supply device 2 and the wire feed device 5, but this is not the only case. The welding system A3 may also be configured so that the operator manually switches the settings of the welding power supply device 2 and the wire feed device 5 to switch between welding using only electrode 8a and welding using only electrode 8b. 【0072】 The submerged arc welding method and submerged arc welding system according to the present invention are not limited to the embodiments described above. The specific configuration of each part of the submerged arc welding method and submerged arc welding system according to the present invention can be modified in various ways. [Explanation of symbols] 【0073】 A1-A3: Welding system, 11: First setting unit, 12: Second setting unit, 13: Switching unit

Claims

[Claim 1] A submerged arc welding method for multilayer welding, The second voltage command value in the second welding pass is greater than the first voltage command value in the first welding pass. In the second welding pass, the slag from the first welding pass is melted. Submerged arc welding method. [Claim 2] The second welding speed command value in the second welding pass is smaller than the first welding speed command value in the first welding pass. The submerged arc welding method according to claim 1. [Claim 3] The aforementioned second voltage command value is 40V or higher. The second welding speed command value is 3 / 4 or less of the first welding speed command value. The submerged arc welding method according to claim 2. [Claim 4] The second current command value in the second welding pass is greater than the first current command value in the first welding pass. The submerged arc welding method according to any one of claims 1 to 3. [Claim 5] In the second welding pass, welding is performed at an advance angle. The submerged arc welding method according to any one of claims 1 to 3. [Claim 6] Using a submerged arc welding system having a leading electrode and a trailing electrode with an advance angle, In the first welding pass, welding is performed using only the leading electrode. In the second welding pass, welding is performed using only the trailing electrode. The submerged arc welding method according to claim 5. [Claim 7] A submerged arc welding system for multi-layer welding, A first setting unit sets the first voltage command value for the first welding pass, A second setting unit for setting the second voltage command value in the second welding pass, Equipped with, The second setting unit sets a second voltage command value that is greater than the first voltage command value based on the first voltage command value. In the second welding pass, the slag from the first welding pass is melted. Submerged arc welding system.

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