Narrow gap gas shielded 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] Here, "narrow groove" 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 to weld steel plates, generally uses a consumable electrode, with CO2 gas alone or a mixture of Ar and CO2 gases 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 welding during the manufacturing process, especially in butt welding of steel plates, has increased, and the welding work has become more time-consuming, 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 weld a groove with a gap that is small compared to the plate thickness. Narrow-gap gas-shielded arc welding reduces the cross-sectional area of ​​the weld compared to regular gas-shielded arc welding, which 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 groove after the welding wire melts) used to fill the gap can be increased by the number of electrodes, making it possible to achieve higher welding efficiency than with single-electrode welding, and it is an effective means of increasing welding efficiency.

[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 method describes a first-layer welding process using two or more electrodes, with the first and second electrodes positioned along a predetermined parallel weld line, and the distance between the welding wire tips of the first and second electrodes controlled to a range of 5 mm to 16 mm. The method also describes controlling the angle of a line connecting the welding wire tips of the first and second electrodes with respect to a direction perpendicular to the weld line to a range of 45° or less. Furthermore, the method describes setting the fusion depth perpendicular to the weld line at the bottom of the steel plate to 1.5 mm or more. The method describes that these methods prevent welding defects and improve welding efficiency, even when groove preparation such as gas cutting or plasma cutting is performed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-223605 Summary of the Invention [Problem to be solved by the invention]

[0009] However, Patent Document 1 does not disclose a method for supplying a shielding gas. 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 result in 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 art and to provide a multi-electrode narrow-gap gas-shielded arc welding method that ensures good shielding properties, suppresses the entrainment 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 conducted extensive research into welding methods for improving shielding performance in multi-electrode narrow-gap gas-shielded arc welding, and have found that the shielding performance during welding can be improved by controlling the gas nozzle arrangement and the flow rate of the shielding gas within appropriate ranges.

[0012] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [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 shielding gas is disposed in front of the first electrode 3 in the welding direction within 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 characterized by the above. [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 characterized by the above. [3] The narrow gap gas-shielded arc welding method according to [1] or [2], wherein the cross-sectional shape of the gas discharge port 6a is rectangular. [4] A narrow-groove gas-shielded arc welding method according to any one of [1] to [3], characterized in that 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. [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 the line connecting the welding wire tips 3c and 4c of the first electrode 3 and the second electrode 4 with respect to the 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 characterized by the above. [Effects of the Invention]

[0013] According to the present invention, in narrow-gap gas-shielded arc welding, which has high welding efficiency, good shielding properties can be ensured and the amount of nitrogen in the weld metal can be reduced. Furthermore, the narrow-gap gas-shielded arc-welded joint obtained by this method can be manufactured at a significantly lower cost than conventional welded joints, making it extremely useful for general structures such as buildings, bridges, and ships. [Brief explanation of the drawings]

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

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

[0016] The present invention is a multi-pass narrow-gap gas-shielded arc welding method for steel plate with a 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" here includes thick steel material, thick steel plate material, 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, in a conventional square groove, increasing the groove angle and narrowing the groove gap may result in a smaller groove cross-sectional area than the groove targeted by the present invention, and the square groove may result in a more efficient welding with a smaller deposition amount. 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, 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 cases where the groove angle θ of the groove is 0°) with a gap at the bottom of the groove, and is a narrow groove. An example of this groove shape is shown in FIG. 3. Here, 1 is the steel plate (base material) and 2 is the groove face. The smaller the groove cross-sectional area of ​​the groove of the steel plate, the faster and more efficient welding becomes possible, but defects such as insufficient fusion are more likely to occur. Furthermore, when the groove angle θ of the groove exceeds 25°, it can be performed using conventional construction methods. For this reason, the present invention targets narrow grooves with a groove angle θ of 25° or less, which are difficult to perform using conventional construction methods and are expected to achieve even greater efficiency. In addition, when the groove angle θ of a V-shaped groove is 0°, it is called an I-shaped groove. Since this 0° angle is the most efficient in terms of deposition amount, the groove angle θ is set to 0° or more, including I-shaped grooves. However, since the groove may close during welding due to welding thermal 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 set to 10°. A more preferable groove angle θ is 5° to 10°.

[0021] [Bottom groove gap G: 7mm~18mm] The smaller the groove of a steel plate, the faster and more efficient welding becomes possible, but defects such as insufficient fusion are more likely to occur. Furthermore, welding with a bottom groove gap G, which is the gap between the bottoms of the grooves (groove width) before welding begins, exceeding 18 mm can be performed using conventional welding methods. Therefore, the present invention targets a bottom groove gap G of 18 mm or less, which is expected to achieve even higher efficiency compared to conventional welding 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-pass welded with three or more electrodes using the narrow groove described above. While it is preferable to perform welding with one pass per layer, it is also possible to perform multiple passes in which each layer is divided into separate welding passes.

[0023] [Shielding gas supply nozzle placement] The object of the present invention is to ensure good shielding in gas-shielded arc welding, 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 by positioning the shielding gas supply nozzle in front of the welding direction of the first electrode in the groove.

[0024] This will be explained in detail with reference to Figures 1 and 2. Figure 1 is a side cross-sectional view showing a schematic diagram of the relative positions of the shielding gas supply nozzle and the electrodes, and Figure 2 is a plan view showing the same relative positions as viewed 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 outlet 6a from which 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, supplying shielding gas while performing arc welding.

[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, resulting in insufficient shielding. 2 If the cross-sectional area A of the gas discharge port 6a is more than 100 mm 2 ~350mm 2 Preferably, 125 mm 2 ~300mm 2 is.

[0026] [Cross-sectional shape of gas outlet] 1 and 2 show only the tip portion of the shielding gas supply nozzle 6, but the cross-sectional shape of the gas discharge port 6a is not particularly limited. However, because 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 be rectangular.

[0027] [Shielding gas supply flow rate Q: 35L / min~70L / min] If the shielding gas supply flow rate Q is less than 35 L / min, the amount of gas supplied to the molten pool 7 will be insufficient, resulting in poor shielding performance. On the other hand, if the shielding gas supply flow rate Q exceeds 70 L / min, the shielding gas flow rate will be excessive, increasing the likelihood of welding defects. Therefore, the shielding gas supply flow rate Q is set to 35 L / min to 70 L / min, preferably 40 L / min to 65 L / min. The shielding gas flow rate changes depending on the combination of the shielding gas supply flow rate Q and the cross-sectional area A of the gas outlet port 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° to 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 will not reach the third electrode 5 sufficiently, resulting in insufficient shielding. On the other hand, if the angle X exceeds 80°, the gas will disperse 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 tip 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 of the gas discharge port 6a (the part closest to the electrode) and the welding wire tip 3c of the first electrode 3 exceeds 40 mm, the shielding gas will not reach the entire electrode sufficiently, and the gas will disperse 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 of the gas discharge port 6a (the part closest to the groove bottom 2b) and the groove bottom 2b exceeds 40 mm, the shielding gas will not be able to sufficiently cover the molten pool 7, resulting in insufficient shielding. 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 shield supply nozzle as described above, it is possible to suppress the incorporation of nitrogen from the atmosphere into 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, it may cause a decrease in the low-temperature toughness of the weld metal and the occurrence of welding defects such as blowholes. Therefore, 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, welding heat tends to concentrate at the center of the groove with a single electrode, resulting in insufficient melting of the steel sheet at the groove face, which can easily cause defects such as incomplete fusion (cold lap) and spatter and slag entrapment on the groove face. In particular, in the first layer welding, the steel sheet temperature is low and the melting depth is small, making defects due to incomplete fusion more 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 weld line 8. Note that 7 denotes the molten pool, and 9 denotes the weld metal.

[0033] [Distance between the tips of the welding wires of the first and second electrodes a: 5mm to 16mm] The distance a between the tips 3c, 4c of the welding wires 3b, 4b supplied from the 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 simply referred to 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 between the first and second electrodes a is less than 5 mm, the current (electrons) flowing between the electrodes reduces the heat generated by the arc itself, preventing sufficient melting of the groove surface 2. On the other hand, if the distance between the first and second electrodes a exceeds 16 mm, the outward electromagnetic force between the electrodes decreases inversely proportional to the distance. This prevents the arcs from repelling the inward electromagnetic force generated by the current flowing through the groove surface 2. As a result, the arcs are directed inward, causing heat to concentrate at the center of the groove. As a result, sufficient melting of the groove surface 2 is not achieved. Furthermore, in narrow gap welding, suppressing welding defects caused by spatter adhesion to the groove surface 2 is an issue. To address this issue, the first electrode 3 and the second electrode 4 are positioned along predetermined parallel welding lines 8, 8, and the distance between the first and second electrodes a is adjusted to a range of 5 mm to 16 mm. This allows the spatter to be absorbed by the molten metal, preventing spatter adhesion to the groove surface 2 and resulting in a sound weld. 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 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 negative wire (positive polarity) and the other as a positive wire (reverse polarity) to ensure melting of the groove surface 2 by utilizing the repulsion of the arc. However, if the angle α of the line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to the direction perpendicular to the welding line 8 exceeds 60°, a sufficient repulsive force of the arc cannot be obtained, and sufficient melting cannot be achieved at the groove surface 2. Therefore, the angle α of the line connecting the welding wire tips 3c, 4c of the first electrode 3 and the second electrode 4 with respect to the direction perpendicular to the welding line 8 (hereinafter simply referred to as the "first-second electrode arrangement angle"; see FIG. 2) is preferably 60° or less. More preferably, it is 50° or less. Note that the first-second electrode arrangement angle α may be 0°.

[0036] [Welding wire diameter f: 1.0mm~1.6mm] Welding wire for narrow-gap gas-shielded arc welding is generally manufactured with a diameter in the range of 0.6 mm to 2.0 mm. When welding with the same current, the thinner the wire diameter, the higher the deposition rate due to Joule heat. Therefore, to achieve highly efficient welding, 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 welding unstable. For this reason, 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 to 100mm] The distance b between the tips 4c, 5c of the welding wires 4b, 5b supplied from the power feed tips 4a, 5a at the tips of the welding torches of the second electrode 4 and the third electrode 5 (hereinafter simply referred to 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 this range improves hot cracking resistance. More preferably, it 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: A mixed gas containing 20% ​​or more CO2 by volume] Because the amount of oxygen in the weld metal is also significantly affected by the shielding gas composition, the shielding gas used in narrow-gap gas-shielded arc welding according to the present invention is preferably a mixed gas containing 20% ​​or more by volume of CO2 gas and the remainder an inert gas such as Ar. A single gas containing 100% by volume of CO2 gas is more preferable. 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, thereby steadily increasing the fusion 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] Furthermore, by satisfying the other welding conditions shown below, welding defects can be further suppressed and welding can be performed more efficiently.

[0041] (Welding wire feed angle φ to the bottom of the groove: 0° to 15° relative 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 this arc directionality 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 contact tip at the tip of the welding torch. The feed angle φ of the welding wire supplied from the power contact tip at the tip of the welding torch relative 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 supplied from the power feed tips 3a and 4a at the welding torch tips of the first and second electrodes 3 and 4 relative to the groove bottom 2b is less than 0° relative to the vertical, the current flows through a path with less resistance. As a result, the arc creeps up the wire electrode (arc creep), making it difficult to maintain the intended welding at the groove surface 2, particularly at the groove bottom 2b. On the other hand, when the feed angle φ exceeds 15°, the arc is directed too far toward the groove surface 2, resulting in a convex weld bead. This results in insufficient melting by the arc in the first and subsequent welding layers, making welding defects more likely to occur. For this reason, the feed angle φ is preferably in the range of 0° to 15° relative to the vertical. It is more preferably 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 positive 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 face: 0.1 mm to 3.0 mm) As shown in FIG. 4, the distance d between the side end portions of the tips 3c and 4c of the welding wires 3b and 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 this 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 the distance d exceeds 3.0 mm, the arc moves away 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 portion of the welding wire tip" refers to the side end portion of the steel plate bottom that is closer to the groove surface 2 to be melted by each electrode.

[0045] (The radius of curvature of the welding wire fed to the power supply tip: 150mm to 500mm) As described above, a bending power feed tip is used to control the feed angle φ of the welding wire 3b, 4b fed from the power feed tips 3a, 4a at the tip of the welding torch of the first electrode 3 and the second electrode 4. In this case, the welding wire passes through the bending power feed tip. To ensure smoother passage, it is preferable to pre-bend the welding wire using a so-called three-point roller or the like. If the radius of curvature of the welding wire is less than 150 mm, the wire feed resistance increases, making it difficult to feed the welding wire stably and maintaining an arc. On the other hand, if the radius of curvature of the welding wire exceeds 500 mm, the bending power feed tip is ineffective in reducing the wire feed resistance, making it difficult to feed the welding wire stably and maintaining an arc. For these reasons, the radius of curvature of the welding wire 3b, 4b fed to the power feed tips 3a, 4a of the first electrode 3 and the second electrode 4 is preferably 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 buildup 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, significantly reducing the risk of stacking defects in multi-layer welding. Note that the "groove center" 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 and second electrodes are wire-positive), the mutual electromagnetic force causes the arcs to be directed inward, concentrating heat at the center of the groove. This results in insufficient melting 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 generated by the welding currents generate strong outward electromagnetic forces, causing the arcs to repel each other. As a result, sufficient melting depth can be achieved at the groove surface 2. Therefore, it is more preferable to use wire-negative (positive polarity) and the other is 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 that are effective in reducing the size of inclusions in weld metal and improving toughness during welding. Furthermore, when welding is performed using a welding wire containing REM added to a positive electrode, droplet size can be reduced and transfer can be stabilized. This refinement of droplet transfer suppresses the generation of spatter and enables stable gas-shielded arc welding even with 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. Elements other than REM (rare earth elements) are generally contained in appropriate amounts depending on the grade (steel type) of 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 droplet refinement and stable transfer. On the other hand, if the REM content exceeds 0.060% by mass, cracks 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 in the range of 0.010% to 0.055% by mass. [Example]

[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 groove was 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 Nos. 1 to 14), and the nitrogen content (ppm) in the weld metal was measured using the inert gas fusion-thermal conductivity method. The nitrogen content in the weld metal was evaluated as follows: good (○) if the nitrogen content was 60 ppm or less, acceptable (△) if it was between 60 ppm and 100 ppm, and unacceptable (×) if it was more than 100 ppm.

[0053] In addition, the method for evaluating weld defects was to conduct a radiographic test on the welded joint, then cut the detected area and corrode the cross section of the weld with nital, and then check the cross section for the presence or absence of defects such as blowholes.

[0054] The results are also shown in Table 1. The welding conditions other than those listed in Table 1 are as follows: The steel types (grades) of the steel plates used were 490 MPa to 590 MPa grade steel. The steel type (grade) of the welding wire used was YGW18, and the diameter of the welding wire was 1.2 mm. The distance a between the first and second electrodes is 14 mm, the distance b between the second and third electrodes is 16 mm, and the first and second electrode arrangement angle α is 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 for the negative wire electrode is 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 face was 0.5 mm, and the radius of curvature 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-pass welding was performed using 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 Lines 9 Weld Metal 10 Backing material t Plate thickness θ Bevel angle G Bottom groove gap X: The angle between the cross section of the gas outlet and the bottom of the groove to be welded Y: Distance between the tip 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 welding wire tips of the first and second electrodes b Distance between the welding wire tips of the second and third electrodes α The angle of the straight line connecting the welding wire tips of the first and second electrodes relative to the perpendicular direction of the welding line d Distance between the side edge of the welding wire tip and the bottom of the groove face φ: The angle of the welding wire 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 within 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 characterized by the above.

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 section 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. 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 and 4c of the first electrode 3 and the second electrode 4 with respect to the 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. 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 and 4c of the first electrode 3 and the second electrode 4 with respect to the 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. 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 and 4c of the first electrode 3 and the second electrode 4 with respect to the 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. 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 and 4c of the first electrode 3 and the second electrode 4 with respect to the 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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