Narrow groove gas shield arc welding method

Abstract

To provide a narrow groove gas shield arc welding method with multiple electrodes, capable of securing preferable shield property, suppressing entrainment of nitrogen in atmosphere, and reducing reduction of low temperature toughness of weld metal and generation of weld flaw.SOLUTION: There is provided a narrow groove gas shield arc welding method for joining a steel plate whose plate thickness t is 22 mm or more, by multi layer welding in which an angle of bevel θ is 25° or lower, and a bottom bevel gap G is 7 mm to 18 mm. The multi layer weld is performed using multiple electrodes of three or more electrodes, in a front side of a weld direction of a first electrode 3 in the bevel, a shield gas supply nozzle 6 is arranged, a cross section A of a gas discharge hole 6a of the nozzle is 100 mm2 to 350 mm2, and a supply flow rate Q of the shield gas is 35 L / min to 70 L / min. In addition, an angle X formed by a cross section of the gas discharge hole and the bevel bottom part 2b is 40° to 80°, and a distance Y between an outermost tip part 6c of the gas discharge hole and a weld wire tip end part 3c of the first electrode, is preferably 40 mm or lower.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a gas-shielded arc welding method, and more particularly to a narrow-gap gas-shielded arc welding method applicable to steel plates with a plate thickness of 22 mm or more.

[0002] The term "narrow groove" used here means that the groove angle is 25° or less, and the width of the minimum groove gap between the steel plates to be welded (base metal) is 50% or less of the plate thickness of the steel plates. [Background technology]

[0003] Gas-shielded arc welding, which is used in welding steel plates, generally uses a consumable electrode system in which CO2 gas alone or a mixture of Ar and CO2 gas is used to shield the molten area, and is widely used in the manufacturing fields of automobiles, construction, bridges, electrical equipment, etc.

[0004] In recent years, as steel structures have become larger and thicker, the amount of deposition in welding during the manufacturing process, especially in butt welding of steel plates, has increased, and the welding work requires more time, resulting in increased construction costs.

[0005] One possible way to improve this is to apply narrow-gap gas-shielded arc welding, which uses arc welding to multi-layer welds to weld gaps that are small relative to the plate thickness. This narrow-gap gas-shielded arc welding has a smaller cross-sectional area of ​​the weld than regular gas-shielded arc welding, so it is expected to achieve high welding efficiency and energy savings, and ultimately reduce construction costs.

[0006] Furthermore, recently, a technology has been proposed for performing this narrow-gap gas-shielded arc welding method using multiple electrodes. With multi-electrode welding, the amount of deposited metal (metal that adheres to the inside of the gap after the welding wire melts) for filling the gap can be increased by the number of electrodes, making it possible to obtain higher welding efficiency than with single-electrode welding, and it is an effective means for increasing the efficiency of welding.

[0007] For example, Patent Document 1 discloses a narrow-gap gas-shielded arc welding method for joining steel plates by narrow-gap multi-layer welding. The first layer welding is performed by multi-electrode welding with two or more electrodes, with the first and second electrodes positioned along a predetermined parallel weld line, and the distance between the tips of the welding wires of the first and second electrodes is controlled to a range of 5 mm to 16 mm. The angle of the straight line connecting the tips of the welding wires of the first and second electrodes with respect to the perpendicular direction of the weld line is controlled to a range of 45° or less. Furthermore, the fusion depth perpendicular to the weld line at the bottom of the steel plate is set to 1.5 mm or more. It is described that these methods provide the effect of preventing the occurrence of welding defects and improving welding work efficiency even when groove processing such as gas cutting or plasma cutting is performed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2015-223605 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, a method for supplying a shielding gas is not described in Patent Document 1. If the supply of the shielding gas is unstable and gas shielding during welding is insufficient, a large amount of nitrogen from the atmosphere will be entrained in the weld metal, which may cause a decrease in the low-temperature toughness of the weld metal and the occurrence of welding defects such as blowholes.

[0010] The present invention aims to solve the problems of the conventional technology and to provide a multi-electrode narrow-gap gas-shielded arc welding method that ensures good shielding properties, suppresses the inclusion of nitrogen from the atmosphere, and reduces the deterioration of the low-temperature toughness of the weld metal and the occurrence of weld defects. [Means for solving the problem]

[0011] In order to achieve the above object, the present inventors have repeatedly studied a welding method for improving the shielding performance in narrow-gap gas-shielded arc welding using multiple electrodes, and have found that the shielding performance during welding can be improved by controlling the arrangement of the gas nozzle and the flow rate of the shielding gas within an appropriate range.

[0012] The present invention has been completed based on these findings and through further investigation. [1] A method for multi-layer narrow-gap gas-shielded arc welding in which a steel plate 1 having a plate thickness t of 22 mm or more has a groove angle θ of 25° or less and a bottom groove gap G of 7 mm to 18 mm, The multi-layer welding is performed using three or more electrodes, A nozzle 6 for supplying a shielding gas is disposed in front of the first electrode 3 in the welding direction in the groove, The cross-sectional area A of the gas discharge port 6a of the nozzle 6 is 100 mm 2 ~350mm 2 in, The supply flow rate Q of the shielding gas is 35 L / min to 70 L / min. A narrow gap gas shielded arc welding method. [2] In the above [1], the angle X between the cross section of the gas discharge port 6a and the groove bottom 2b to be welded is 40° to 80°, The distance Y between the most distal end 6c of the gas discharge port 6a and the welding wire tip 3c of the first electrode 3 is 40 mm or less. A narrow gap gas shielded arc welding method. [3] The narrow gap gas-shielded arc welding method according to [1] or [2], characterized in that the cross-sectional shape of the gas discharge port 6a is rectangular. [4] A narrow gap gas-shielded arc welding method according to any one of [1] to [3], characterized in that the distance Z between the lowermost end 6b of the gas discharge port 6a and the groove bottom 2b to be welded is 40 mm or less. [5] In any one of [1] to [4], the distance a between the welding wire tips 3c, 4c supplied from the power feed tips 3a, 4a at the tip of each welding torch of the first electrode 3 and the second electrode 4 of the multi-electrode is 5 mm to 16 mm, The angle α of a straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to a direction perpendicular to the welding line 8 is 60° or less, The diameter f of the welding wire used in all the electrodes of the multi-electrode is 1.0 mm to 1.6 mm, The distance b between the tips of the welding wires after the second electrode 4 is 10 mm to 100 mm. A narrow gap gas shielded arc welding method. Effect of the Invention

[0013] According to the present invention, in narrow-gap gas-shielded arc welding with high welding efficiency, good shielding properties can be ensured and the amount of nitrogen in the weld metal can be suppressed. Furthermore, the narrow-gap gas-shielded arc-welded joint obtained by this process has a significantly reduced manufacturing cost compared to conventional welded joints, and is therefore extremely useful for application to general structures such as buildings, bridges, and ships. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a side cross-sectional view that illustrates a schematic relationship between a gas supply nozzle and an electrode according to the present invention. [Diagram 2] FIG. 2 is a plan view showing a schematic relationship between a gas supply nozzle and an electrode according to the present invention. [Diagram 3] FIG. 2 is a front cross-sectional view showing a typical groove shape according to the present invention. [Figure 4] FIG. 2 is a front cross-sectional view showing a schematic relationship between a welding wire and a groove surface according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will be specifically described below.

[0016] The present invention is a narrow-gap gas-shielded arc welding method for multi-layer welding of steel plates with a plate thickness t of 22 mm or more, a groove angle θ of 25° or less, and a bottom groove gap G of 7 mm to 18 mm. Note that the term "steel plate" as used herein includes thick steel materials, thick plate steel materials, etc.

[0017] [Steel plate thickness: 22mm or more] The thickness t of the steel plate is 22 mm or more. If the thickness t is less than 22 mm, the groove cross-sectional area may be smaller than that of the groove targeted by the present invention by increasing the groove angle and reducing the groove gap in a conventional R-shaped groove, and the R-shaped groove may result in a highly efficient welding with a smaller amount of deposition. Preferably, the thickness t is 30 mm or more.

[0018] Furthermore, since the plate thickness of steel structures, including special structures, is 200 mm or less, in the present invention, it is preferable that the upper limit of the plate thickness t of the steel plate is set to 200 mm.

[0019] The present invention can be applied to various steel plates ranging from mild steel plates to high-tensile steel plates of 780 MPa class. It is also possible to weld high-tensile steel plates of 590 MPa class without preheating.

[0020] [Bevel angle θ: 25° or less] The groove shape used in the present invention is a V-shaped groove (including the case where the groove angle θ of the groove is 0°) having a gap at the bottom of the groove, and is a narrow groove. An example of the groove shape is shown in FIG. 3. Here, 1 is a steel plate (base material) and 2 is a groove surface. The smaller the groove cross-sectional area of ​​the groove of the steel plate, the faster and more efficient welding is possible, but defects such as poor fusion are more likely to occur. In addition, when the groove angle θ of the groove exceeds 25°, it can be performed with a conventional construction method. For this reason, the present invention targets a narrow groove with a groove angle θ of 25° or less, which is difficult to perform with a conventional construction method and is expected to be even more efficient. In addition, when the groove angle θ of the V-shaped groove is 0°, it is called an I-shaped groove, and from the viewpoint of the amount of deposition, the groove angle θ of 0° is the most efficient, so the groove angle θ is 0° or more, including an I-shaped groove. However, since the groove may close during welding due to welding heat distortion, it is preferable to set the groove angle θ according to the plate thickness t in anticipation of this. Furthermore, when the plate thickness t exceeds 100 mm, the upper limit of the suitable range is preferably 10°. More preferably, the groove angle θ is 5° to 10°.

[0021] [Bottom Groove Gap G: 7mm~18mm] The smaller the groove of the steel plate, the faster and more efficient the welding can be performed, but defects such as incomplete fusion are more likely to occur. In addition, welding in which the bottom groove gap G, which is the gap (groove width) between the bottoms of the grooves before welding begins, exceeds 18 mm can be performed with conventional construction methods. For this reason, the present invention targets a bottom groove gap G of 18 mm or less, which is expected to be even more efficient than conventional construction methods. On the other hand, if the bottom groove gap G is less than 7 mm, it may be difficult to perform multi-electrode welding with three or more electrodes. Therefore, the bottom groove gap G is set to a range of 7 mm to 18 mm. Preferably, it is set to a range of 8 mm to 12 mm.

[0022] [Multi-layer welding] The present invention is a welding method in which each weld metal layer is multi-layer welded with three or more electrodes using the narrow groove described above. Although it is preferable to weld one layer with one pass, it is also possible to weld one layer with multiple passes.

[0023] [Arrangement of shielding gas supply nozzles] In order to ensure good shielding in gas-shielded arc welding, which is an object of the present invention, and to suppress the inclusion of nitrogen from the atmosphere and reduce the deterioration of the low-temperature toughness of the weld metal and the occurrence of welding defects, a shielding gas supply nozzle is positioned in front of the welding direction of the first electrode in the groove.

[0024] A detailed description will be given based on Fig. 1 and Fig. 2. Fig. 1 is a side cross-sectional view showing a schematic arrangement of the shielding gas supply nozzle and the electrodes, and Fig. 2 is a plan view showing the same arrangement from above. These figures show an example of three-electrode welding in which a first electrode 3, a second electrode 4, and a third electrode 5 are arranged. A shielding gas supply nozzle 6 is arranged in front of the first electrode 3 in the welding direction, and the tip of the shielding gas supply nozzle 6 is a gas discharge port 6a from which the shielding gas is discharged, and is directed toward the first electrode 3. During welding, the shielding gas supply nozzle 6 moves in conjunction with the three electrodes along the welding direction, and arc welding is performed while supplying the shielding gas.

[0025] [Gas outlet cross-sectional area A: 100 mm 2 ~350mm 2 ] The cross-sectional area A of the gas outlet 6a is 100 mm 2 If the cross-sectional area A of the gas discharge port 6a is less than 350 mm, the area onto which the gas is blown becomes small, and the shielding performance becomes insufficient. 2 If the cross-sectional area exceeds 100 mm, the gas will disperse to areas other than the groove bottom 2b, resulting in insufficient shielding. 2 ~350mm 2 Preferably, 125 mm 2 ~300mm 2 It is.

[0026] [Cross-sectional shape of gas outlet] 1 and 2, only the tip portion of the shielding gas supply nozzle 6 is shown, but the cross-sectional shape of the gas discharge port 6a is not particularly limited. However, since a nozzle with a larger cross-sectional area can be inserted into the groove when the cross-sectional shape of the gas discharge port 6a is rectangular rather than circular, it is preferable that the cross-sectional shape of the gas discharge port 6a is rectangular.

[0027] [Shielding gas supply flow rate Q: 35L / min~70L / min] If the supply flow rate Q of the shielding gas is less than 35 L / min, the amount of gas supplied to the molten pool 7 becomes insufficient, and the shielding performance deteriorates. On the other hand, if the supply flow rate Q of the shielding gas exceeds 70 L / min, the flow rate of the shielding gas becomes excessive, and the possibility of occurrence of welding defects increases. Therefore, the supply flow rate Q of the shielding gas is set to 35 L / min to 70 L / min, and preferably to 40 L / min to 65 L / min. Note that the flow rate of the shielding gas changes depending on the combination of the supply flow rate Q of the shielding gas and the cross-sectional area A of the gas outlet 6a, but it is preferable that the flow rate be 2 m / s to 7 m / s.

[0028] [Angle X between the cross section of the gas outlet and the bottom of the groove to be welded: 40°~80°] If the angle X between the cross section of the gas discharge port 6a and the groove bottom 2b to be welded is less than 40°, the shielding gas does not reach the third electrode 5 sufficiently, resulting in insufficient shielding. On the other hand, if the angle X exceeds 80°, the gas is dispersed to areas other than the groove bottom 2b, resulting in insufficient shielding. Therefore, the angle X between the cross section of the gas discharge port 6a and the groove bottom 2b to be welded is preferably 40° to 80°. More preferably, it is 45° to 75°.

[0029] [Distance Y between the most distal end of the gas outlet and the tip of the welding wire of the first electrode: 40 mm or less] Furthermore, the distance between the gas discharge port 6a and the electrode is also important. That is, if the distance Y between the most distal end 6c (the part closest to the electrode) of the gas discharge port 6a and the welding wire tip 3c of the first electrode 3 exceeds 40 mm, the shielding gas does not reach the entire electrode sufficiently, and the gas disperses to areas other than the groove bottom 2b, resulting in insufficient shielding. Therefore, the distance Y is preferably 40 mm or less. More preferably, it is 5 mm to 35 mm. Note that the "distance" here refers to the distance in the welding direction.

[0030] [Distance Z between the bottom of the gas outlet and the bottom of the groove to be welded: 40 mm or less] The distance between the gas discharge port 6a and the groove bottom 2b is also important. That is, if the distance Z between the lowest end 6b (the part closest to the groove bottom 2b) of the gas discharge port 6a and the groove bottom 2b exceeds 40 mm, the shielding gas cannot sufficiently cover the molten pool 7, and the shielding performance becomes insufficient. Therefore, the distance Z is preferably 40 mm or less, and more preferably 35 mm or less. Note that the "distance" here refers to the distance in the thickness direction of the steel plate 1.

[0031] [Nitrogen content in weld metal] By adjusting the gas shielding conditions and the arrangement and shape of the gas shielding supply nozzle described above, it is possible to suppress the inclusion of nitrogen from the atmosphere in the weld metal, and to control the nitrogen content (ppm) in the weld metal. If the nitrogen content in the weld metal exceeds 100 ppm by mass, this may cause a decrease in the low-temperature toughness of the weld metal and may lead to the occurrence of welding defects such as blowholes, so the nitrogen content in the weld metal is preferably 100 ppm by mass or less, and more preferably 60 ppm or less.

[0032] [Arrangement of the first and second electrodes] In narrow gap multi-pass welding, when one pass is performed per layer, the welding heat tends to concentrate at the center of the groove with one electrode, which causes insufficient melting at the groove surface of the steel plate, and defects due to incomplete fusion (cold lap) and spatter and slag inclusion on the groove surface are likely to occur. In particular, in the first layer welding, the temperature of the steel plate is low and the melting depth is small, so defects due to incomplete fusion are likely to occur. Therefore, in the present invention, as shown in Figure 2, it is preferable to position the first electrode 3 and the second electrode 4 of the three electrodes along a predetermined parallel welding line 8. Note that 7 is a molten pool and 9 is a weld metal.

[0033] [Distance between the tips of the welding wires of the first and second electrodes a: 5mm~16mm] The distance a between tips 3c, 4c of welding wires 3b, 4b supplied from power feed tips 3a, 4a at the tip of the welding torch of each of the first electrode 3 and the second electrode 4 (hereinafter also referred to simply as the "distance between the first and second electrodes"; see FIG. 2) is preferably adjusted to a range of 5 mm to 16 mm. Note that the "distance between the welding wire tips" here refers to the distance between the centers of the welding wire tips of each electrode.

[0034] If the distance a between the first and second electrodes is less than 5 mm, the heat of the arc itself decreases due to the current (electrons) flowing between the electrodes, and sufficient melting of the groove surface 2 cannot be obtained. On the other hand, if the distance a between the first and second electrodes exceeds 16 mm, the outward electromagnetic force between the electrodes decreases inversely proportional to the distance, and the arc repulsion force to overcome the inward electromagnetic force generated by the current flowing through the groove surface 2 is not obtained, and the arcs turn inward and heat is concentrated at the center of the groove. As a result, sufficient melting of the groove surface 2 cannot be obtained. In addition, in narrow gap welding, suppression of welding defects caused by adhesion of spatter to the groove surface 2 is an issue. To address this, the first electrode 3 and the second electrode 4 are arranged along predetermined parallel welding lines 8, 8, and the distance a between the first and second electrodes is adjusted to a range of 5 mm to 16 mm. As a result, the spatter is absorbed by the respective molten metals, and adhesion of spatter to the groove surface 2 is suppressed, so that a sound weld can be obtained. In view of the above, it is preferable to adjust the distance a between the first and second electrodes to within the range of 5 mm to 16 mm.

[0035] [Angle α of the straight line connecting the tips of the welding wires of the first and second electrodes: 60° or less] In the present invention, it is preferable to use one of the first electrode 3 and the second electrode 4 as a wire negative (positive polarity) and the other as a wire positive (negative polarity) to ensure melting of the groove surface 2 by utilizing the repulsion of the arc. However, if the angle α of the straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to the perpendicular direction of the welding line 8 exceeds 60°, a sufficient repulsive force of the arc cannot be obtained, and sufficient melting cannot be obtained at the groove surface 2. Therefore, it is preferable that the angle α of the straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to the perpendicular direction of the welding line 8 (hereinafter also simply referred to as the "first-second electrode arrangement angle", see FIG. 2) is 60° or less. More preferably, it is 50° or less. The first-second electrode arrangement angle α may be 0°.

[0036] [Welding wire diameter f: 1.0mm~1.6mm] Welding wires for narrow gap gas shielded arc welding are generally manufactured with diameters in the range of 0.6 mm to 2.0 mm, but when welding with the same current, generally the thinner the wire diameter, the higher the deposition rate due to Joule heat. Therefore, in order to realize highly efficient welding work, it is preferable to select a relatively thin wire diameter. On the other hand, if the wire diameter is too thin, the wire will soften due to Joule heat, making the welding unstable. Therefore, the diameter f of the welding wire used in the present invention is preferably in the range of 1.0 mm to 1.6 mm.

[0037] [Distance between the tips of the welding wires after the second electrode b: 10mm~100mm] The distance b between tips 4c, 5c of welding wires 4b, 5b supplied from power feed tips 4a, 5a at the tips of the welding torches of the second electrode 4 and the third electrode 5 (hereinafter also referred to simply as the "second-third electrode distance"; see Figs. 1 and 2) is preferably in the range of 10 mm to 100 mm. This is because hot cracking resistance is improved by setting the distance in this range. More preferably, the distance is 12 mm to 80 mm. Similarly, the distance between the tips of the welding wires of the third electrode and the fourth electrode (not shown) is preferably in the range of 10 mm to 100 mm.

[0038] [Shielding gas: mixed gas containing 20% ​​or more CO2 gas by volume] Since the amount of oxygen in the weld metal is also greatly affected by the shielding gas composition, it is preferable to use a mixed gas containing 20 volume % or more of CO2 gas and the remainder of an inert gas such as Ar as the shielding gas used in narrow gap gas-shielded arc welding according to the present invention. More preferably, it is a single gas of 100 volume % CO2 gas. In the present invention, it is preferable to increase the oxygen concentration in the weld metal, which controls the flow of the weld metal, and direct the convection of the weld metal from the center outward to stably deepen the molten depth at the bottom of the thick steel plate in the groove.

[0039] [Other welding conditions] In the present invention, the welding conditions other than those described above do not need to be particularly limited and may be in accordance with established methods, such as a welding voltage of 32 V to 37 V, a welding speed of 30 cm / min to 90 cm / min, a welding wire extension length of 15 mm to 30 mm, and a welding heat input per pass of 10 kJ / cm to 50 kJ / cm.

[0040] In addition, by satisfying the other welding conditions shown below, welding defects can be further suppressed and welding can be performed more efficiently.

[0041] (Welding wire supply angle φ to the bottom of the groove: 0° to 15° to the perpendicular) Arcs have directionality and tend to point in the direction that the tip of the electrode (welding wire) is pointing. To effectively utilize the directionality of this arc to melt the groove surface 2, it is advantageous to point the electrode tip toward the groove surface 2, and the direction that the electrode tip points varies greatly depending on the feed angle φ of the welding wire supplied from the power feed tip at the tip of the welding torch. The feed angle φ of the welding wire supplied from the power feed tip at the tip of the welding torch with respect to the bottom of the groove is shown in Figure 4. Here, 10 is the backing material.

[0042] When the feed angle φ of the welding wires 3b and 4b fed from the power feed tips 3a and 4a at the tips of the welding torches of the first electrode 3 and the second electrode 4 to the groove bottom 2b is less than 0° with respect to the perpendicular line, the current flows through a path with less resistance. As a result, the arc creeps up the wires (arc creeps up), which is the electrode, making it difficult to maintain the melting at the target groove surface 2, especially the groove bottom 2b. On the other hand, when the feed angle φ exceeds 15°, the arc is too directed toward the groove surface 2, so the weld bead shape becomes convex, and melting by the arc in the welding after the first layer becomes insufficient, which makes it easy to cause welding defects. For this reason, the feed angle φ is preferably in the range of 0° to 15° with respect to the perpendicular line. More preferably, it is 5° to 12°.

[0043] Since the feed angle φ is the same as the inclination of the power feed tips 3a and 4a, particularly the tip of the power feed tip, the feed angle φ of the welding wire can be controlled by the inclination of the tip of the power feed tip. Note that the feed angle φ is defined as + in the direction toward the groove face 2. The perpendicular line here refers to the perpendicular line to the groove bottom 2b.

[0044] (Distance d between the side end of the welding wire tip and the bottom of the groove surface: 0.1mm~3.0mm) As shown in FIG. 4, the distance d between the side end of the tip 3c, 4c of the welding wire 3b, 4b of the first electrode 3 and the second electrode 4 and the bottom of the groove surface 2 is preferably in the range of 0.1 mm to 3.0 mm. If the distance d is less than 0.1 mm, an arc is generated between the upper part of the wire and the groove surface 2, and the groove surface 2 at the bottom cannot be melted efficiently. On the other hand, if it exceeds 3.0 mm, the arc is separated from the groove surface 2, and the groove surface 2 cannot be melted efficiently. For this reason, the distance d is preferably 0.1 mm to 3.0 mm. More preferably, it is 0.5 mm to 2.0 mm, and even more preferably, it is 0.5 mm to 1.0 mm. Note that the "side end of the tip of the welding wire" refers to the side end close to the groove surface 2 at the bottom of the steel plate to be melted by each electrode.

[0045] (The radius of curvature of the welding wire fed to the power feed tip: 150mm to 500mm) As described above, in order to control the feed angle φ of the welding wires 3b and 4b fed from the power feed tips 3a and 4a at the tips of the welding torches of the first electrode 3 and the second electrode 4, a power feed tip with a bent tip is used. At this time, the welding wire passes through the power feed tip with a bent tip, and in order to pass through it more smoothly, it is preferable to bend the welding wire in advance using a so-called three-point roller or the like. At this time, if the curvature radius of the welding wire is less than 150 mm, the wire feed resistance increases, the welding wire cannot be fed stably, and it becomes difficult to maintain an arc. On the other hand, if the curvature radius of the welding wire exceeds 500 mm, there is no effect in reducing the wire feed resistance when the power feed tip tip is bent, so the welding wire cannot be fed stably, and it becomes difficult to maintain an arc. From the above, it is preferable that the curvature radius of the welding wires 3b and 4b fed to the power feed tips 3a and 4a of the first electrode 3 and the second electrode 4 is 150 mm to 500 mm. More preferably, it is 175 mm to 475 mm.

[0046] (Positioning of the third and subsequent electrodes) If the weld build-up height exceeds the bottom groove gap G in the first layer welding, the risk of hot cracking increases. To avoid this, it is effective to position the third electrode 5 and subsequent electrodes in the center of the groove behind the first electrode 3 and second electrode 4. This also makes it possible to reduce the number of layers, greatly reducing the risk of stacking defects in multi-layer welding. Note that the "center of the groove" here allows a range of ±10% of the bottom groove gap G from the center of the groove.

[0047] (Electrode polarity) If the first electrode 3 and the second electrode 4 have the same polarity (for example, both the first electrode and the second electrode are wire positive), the arcs of each electrode are inward due to the electromagnetic force of attraction, and heat is concentrated in the center of the groove. As a result, sufficient melting cannot be obtained at the groove surface 2. On the other hand, if one of the first electrode 3 and the second electrode 4 is wire negative (positive polarity) and the other is wire positive (reverse polarity), and the arrangement of the first electrode 3 and the second electrode 4 is appropriately controlled, the magnetic fields due to the welding currents of each electrode generate strong outward electromagnetic forces, and the arcs repel each other. As a result, it is possible to obtain a sufficient melting depth at the groove surface 2. Therefore, it is more preferable to use one of the first electrode 3 and the second electrode 4 as wire negative (positive polarity) and the other as wire positive (reverse polarity). The polarity of the third electrode 5 and subsequent electrodes is not particularly limited, and may be either wire negative (positive polarity) or wire positive (reverse polarity).

[0048] (REM content of welding wire: 0.005% by mass to 0.060% by mass) REM (rare earth elements) are elements effective in refining inclusions in weld metal and improving toughness during welding. In addition, when welding is performed by supplying a welding wire containing REM to a positive electrode, it is possible to refining droplets and stabilizing transfer. This refining of droplet transfer suppresses the generation of spatter and enables stable gas shielded arc welding even in positive polarity. For these reasons, in the present invention, it is preferable to supply a steel wire containing 0.005% to 0.060% by mass of REM (rare earth elements) to a positive electrode. Note that elements other than REM (rare earth elements) are included in the appropriate amounts normally contained according to the grade (steel type) of the welding wire, as specified in JIS Z 3312.

[0049] If the REM (rare earth element) content is less than 0.005% by mass, it is difficult to achieve the above-mentioned fine droplets and stable transfer. On the other hand, if the REM (rare earth element) content exceeds 0.060% by mass, cracks will occur during the wire manufacturing process, making it difficult to manufacture the welding wire. For this reason, the REM (rare earth element) content is preferably in the range of 0.005% to 0.060% by mass. More preferably, it is 0.010% to 0.055% by mass. EXAMPLES

[0050] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0051] The grooves were prepared by gas cutting, and narrow-gap gas-shielded arc welding was performed with one pass per layer under the groove shape and gas shielding conditions shown in Table 1. The total number of layers in the multi-layer welding was also calculated.

[0052] Test pieces were taken from the weld metal of the prepared welded joints (Test No. 1-14), and the nitrogen content (ppm) in the weld metal was measured by the inert gas fusion-thermal conductivity method. The nitrogen content in the weld metal was evaluated as follows: 60 ppm or less is good (○), 60 ppm to 100 ppm is acceptable (△), and over 100 ppm is unacceptable (×).

[0053] In addition, the method of evaluating welding defects was as follows: a radiographic test was performed on the welded joint, the detected area was cut, and the cross section of the weld was corroded with nital, and then the presence or absence of defects such as blowholes on the cross section was confirmed.

[0054] The results are also shown in Table 1. The welding conditions other than those shown in Table 1 were as follows. The steel types (grades) of the steel plates used are 490 MPa class steel to 590 MPa class steel. The steel type (grade) of the welding wire used was YGW18, and the diameter of the welding wire was 1.2 mm. The 1st-2nd electrode distance a was 14 mm, the 2nd-3rd electrode distance b was 16 mm, and the 1st-2nd electrode arrangement angle α was 55°. The welding speed is 60 cm / min, and the welding heat input is 28 kJ / cm to 41 kJ / cm. The REM content of the welding wire used in the negative wire electrode was 0.025% by mass to 0.030% by mass. The feed angle φ of the welding wire was 8°, the distance d between the tip end of the welding wire and the bottom of the groove surface was 0.5 mm, and the curvature radius of the welding wire was 300 mm.

[0055] [Table 1]

[0056] In all of the examples of the present invention, even when narrow-gap multi-layer welding was performed with three-electrode welding, the nitrogen content in the weld metal was reduced to 100 ppm or less, and no welding defects occurred. On the other hand, in the comparative examples outside the range of the present invention, the nitrogen content in the weld metal increased or welding defects occurred, and a sound narrow-gap gas-shielded arc-welded joint was not obtained. [Explanation of symbols]

[0057] 1 Steel plate (base material) 2 Bevel surface 2b Bevel bottom 3 1st electrode 4 Second electrode 5 Third electrode 3a, 4a, 5a power supply chip 3b, 4b, 5b welding wire 3c, 4c, 5c Welding wire tip 6 Shielding gas supply nozzle 6a Gas outlet 6b Lowermost end of gas outlet 6c The most advanced part of the gas outlet 7 Molten Pool 8 Welding seam 9 Welding metal 10 Backing material t Plate thickness θ Bevel angle G Bottom Groove Gap X is the angle between the cross section of the gas outlet and the bottom of the groove to be welded Y: Distance between the most distal end of the gas outlet and the tip of the welding wire of the first electrode Z Distance between the bottom of the gas outlet and the bottom of the groove to be welded a Distance between the tips of the welding wires of the first and second electrodes b Distance between the tips of the welding wires of the second and third electrodes α Angle of the straight line connecting the tips of the welding wires of the first and second electrodes with respect to the perpendicular direction of the welding line d Distance between the side end of the welding wire tip and the bottom of the groove surface φ The angle at which the welding wire is fed to the bottom of the groove

Claims

1. A narrow-gap gas-shielded arc welding method for multi-layer welding of a steel plate 1 having a plate thickness t of 22 mm or more, a groove angle θ of 25° or less, and a bottom groove gap G of 7 mm to 18 mm, The multi-layer welding is performed using three or more electrodes, A nozzle 6 for supplying a shielding gas is disposed in front of the first electrode 3 in the welding direction in the groove, The cross-sectional area A of the gas discharge port 6a of the nozzle 6 is 100 mm 2 ~350mm 2 in, The supply flow rate Q of the shielding gas is 35 L / min to 70 L / min. A narrow gap gas shielded arc welding method.

2. The angle X between the cross section of the gas discharge port 6a and the groove bottom 2b to be welded is 40° to 80°, The distance Y between the most distal end 6c of the gas discharge port 6a and the welding wire tip 3c of the first electrode 3 is 40 mm or less.

2. The narrow gap gas-shielded arc welding method according to claim 1 .

3. The cross-sectional shape of the gas discharge port 6a is rectangular.

3. The narrow gap gas shielded arc welding method according to claim 1 or 2.

4. The distance Z between the lowest end 6b of the gas discharge port 6a and the groove bottom 2b to be welded is 40 mm or less.

3. The narrow gap gas shielded arc welding method according to claim 1 or 2.

5. The distance Z between the lowest end 6b of the gas discharge port 6a and the groove bottom 2b to be welded is 40 mm or less.

4. The narrow gap gas shielded arc welding method according to claim 3.

6. In the multi-electrode, the distance a between the welding wire tips 3c, 4c supplied from the power feed tips 3a, 4a at the tip of each welding torch of the first electrode 3 and the second electrode 4 is 5 mm to 16 mm, The angle α of a straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to a direction perpendicular to the welding line 8 is 60° or less, The diameter f of the welding wire used in all the electrodes of the multi-electrode is 1.0 mm to 1.6 mm, The distance b between the tips of the welding wires after the second electrode 4 is 10 mm to 100 mm.

3. The narrow gap gas shielded arc welding method according to claim 1 or 2.

7. In the multi-electrode, the distance a between the welding wire tips 3c, 4c supplied from the power feed tips 3a, 4a at the tip of each welding torch of the first electrode 3 and the second electrode 4 is 5 mm to 16 mm, The angle α of a straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to a direction perpendicular to the welding line 8 is 60° or less, The diameter f of the welding wire used in all the electrodes of the multi-electrode is 1.0 mm to 1.6 mm, The distance b between the tips of the welding wires after the second electrode 4 is 10 mm to 100 mm.

4. The narrow gap gas shielded arc welding method according to claim 3.

8. In the multi-electrode, the distance a between the welding wire tips 3c, 4c supplied from the power feed tips 3a, 4a at the tip of each welding torch of the first electrode 3 and the second electrode 4 is 5 mm to 16 mm, The angle α of a straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to a direction perpendicular to the welding line 8 is 60° or less, The diameter f of the welding wire used in all the electrodes of the multi-electrode is 1.0 mm to 1.6 mm, The distance b between the tips of the welding wires after the second electrode 4 is 10 mm to 100 mm.

5. The narrow gap gas-shielded arc welding method according to claim 4.

9. In the multi-electrode, the distance a between the welding wire tips 3c, 4c supplied from the power feed tips 3a, 4a at the tip of each welding torch of the first electrode 3 and the second electrode 4 is 5 mm to 16 mm, The angle α of a straight line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to a direction perpendicular to the welding line 8 is 60° or less, The diameter f of the welding wire used in all the electrodes of the multi-electrode is 1.0 mm to 1.6 mm, The distance b between the tips of the welding wires after the second electrode 4 is 10 mm to 100 mm.

6. The narrow gap gas shielded arc welding method according to claim 5.

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