Narrow gap gas shielded arc welding method

Abstract

Provided is a narrow-gap gas-shielded arc welding method that can produce a welded joint that realizes high welding efficiency and prevents defects such as poor penetration on the groove surface.The narrow-gap gas-shielded arc welding method joins steel plates with a plate thickness t of 22 mm or more by multi-layer welding with a narrow groove having a groove angle θ of 25° or less and a bottom groove gap G of 7 mm to 18 mm, characterized in that the distance d between the side end of the tip of the welding wire and the bottom groove surface of the steel plate is in the range of 3.0 mm or less, and the value of the variable K calculated from the following formula (1) is in the range of 24.0 to 66.0. K=(92.3×I / W+9.1)×H-0.5×d ··· (1) Here, H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feed speed (cm / min).

Description

[Technical field]

[0001] The present invention relates to a gas-shielded arc welding method, and in particular to a narrow-gap gas-shielded arc welding method applied to steel plates having a thickness of 22 mm or more (hereinafter also simply referred to as "thick steel plates"). Note that "narrow gap" 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 thickness of the steel plates. "x to y" representing a numerical range means from x to y, and includes the boundary value. [Background technology]

[0002] Gas-shielded arc welding, which is used for welding steel plates, is widely used in the manufacturing fields of automobiles, buildings, bridges, electrical equipment, etc. In recent years, as steel structures have become larger and thicker, the amount of deposition during welding in the manufacturing process, especially in butt welding of steel plates, has increased, and welding work requires a lot of time, leading to increased construction costs. Therefore, the application of narrow-gap gas-shielded arc welding, which uses gas-shielded arc welding to weld gaps that are small compared to the plate thickness, is being considered. The narrow-gap gas-shielded arc welding method reduces the amount of deposition compared to normal gas-shielded arc welding, achieving high welding efficiency and energy saving, and is expected to reduce construction costs.

[0003] In response to such demands, for example, Patent Document 1 discloses a double-sided multi-layer welding method for double-sided U-groove joints. In this welding method, lamination welding is performed by TIG welding using an inert gas, and the use of the inert gas suppresses the generation of slag and spatter and prevents lamination defects.

[0004] Moreover, Patent Document 2 discloses narrow gap welding in which a welding torch is weaved to suppress spatter and incomplete fusion.

[0005] Furthermore, Patent Document 3 discloses a narrow-gap gas-shielded arc welding method for joining thick steel materials by narrow-gap multi-layer welding. In the technology described in Patent Document 3, the first layer welding is performed by multi-electrode welding with two or more electrodes, one of the first and second electrodes is positive polarity and the other is reverse polarity, and the first and second electrodes are positioned along a predetermined parallel weld line. Furthermore, the distance between the welding wire tips of the first and second electrodes is 5 mm to 16 mm, the angle of the straight line connecting the welding wire tips of the first and second electrodes with respect to the perpendicular direction of the weld line is 45° or less, and the melting depth in the perpendicular direction of the weld line at the bottom of the thick steel material is 1.5 mm or more. As a result, even when groove processing such as gas cutting or plasma cutting is performed, there is no occurrence of defects, and the effect of improving welding construction efficiency is obtained. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2009-061483 A [Patent Document 2] JP 2010-115700 A [Patent Document 3] JP 2015-223605 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the TIG welding method, which is a non-consumable electrode type, described in Patent Document 1 is significantly inferior in welding construction efficiency compared to MAG welding and CO2 welding, which use steel wires as consumable electrodes. In addition, in the welding method described in Patent Document 2, the weaving direction of the welding torch is not in the groove depth direction but in the steel plate surface direction, so it is necessary to weave the welding torch before the molten metal drips. As a result, it becomes necessary to set the welding current to a low current of about 150A and suppress the deposition amount per pass. Therefore, when this welding method is applied to welding thick steel plates, it becomes a multi-pass stack welding with a small amount of welding, which increases stacking defects such as poor penetration and significantly reduces welding efficiency.

[0008] Furthermore, it is generally known that when gas-shielded arc welding is performed with positive polarity, the arc becomes unstable and a large amount of spatter is generated. In the technology described in Patent Document 3, a large amount of spatter is generated from the electrode using positive polarity, and the spatter adheres to the inside of the groove or the welding torch, which may cause welding defects. In addition, when multi-electrode welding with three or more electrodes is performed, high-temperature cracks may occur in the center of the weld metal.

[0009] As described above, the current situation is that a high-quality, highly efficient narrow-gap gas-shielded arc welding method that can be applied to welding thick steel plates has not yet been developed.

[0010] On the other hand, advances in automated welding technology, for example in the lightweight, high-performance, and high-precision welding robots, have made it possible to control the welding torch to suit the groove shape and welding position, which was previously difficult. Utilizing this technology, it is now possible to carry out welding that is suitable for the steel plate, groove shape, welding position, and welding material.

[0011] The present invention has been made in consideration of these problems and the current situation, and has an object to provide a narrow-gap gas-shielded arc welding method for steel plates that realizes high welding construction efficiency and can produce a welded joint that prevents the occurrence of defects such as poor penetration on the groove surface. [Means for solving the problem]

[0012] In order to achieve the above object, the inventors have thoroughly studied factors that affect the occurrence of defects such as poor penetration on the groove surface, and have found that in narrow-gap gas-shielded arc welding, in order to prevent the occurrence of instability or deflection of the arc and poor penetration, it is effective to appropriately control the welding heat input to the groove surface, the welding current, and the welding wire feed speed.

[0013] The present invention has been completed based on these findings and through further investigation. [1] A narrow-gap gas-shielded arc welding method for joining steel plates having a thickness t of 22 mm or more by multi-layer welding with a narrow gap having a groove angle θ of 25° or less and a bottom groove gap G in the range of 7 mm to 18 mm, wherein the distance d between the side end of the tip of the welding wire and the bottom groove surface of the steel plates is in the range of 3.0 mm or less, and the value of variable K calculated from the following formula (1) is in the range of 24.0 to 66.0. K=(92.3×I / W+9.1)×H-0.5×d ··· (1) Here, H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feed speed (cm / min). [2] The narrow gap gas-shielded arc welding method according to [1] above, wherein a distance h from a tip of a welding torch to a tip of the welding wire is in the range of 10 mm to 40 mm, and a feed angle φ of the welding wire to the bottom groove is in the range of 0° to 15° with respect to the perpendicular. [3] In the narrow gap gas-shielded arc welding method according to [1] or [2], the welding wire diameter f is in the range of 1.0 mm to 1.6 mm, the welding heat input H is in the range of 0.8 kJ / mm to 2.3 kJ / mm, the welding current I is in the range of 270 A to 370 A, and the welding wire feed speed W is in the range of 1000 cm / min to 1700 cm / min. [4] The narrow gap gas-shielded arc welding method according to any one of [1] to [3] above, wherein the penetration depth p at the groove face is 1.0 mm or more. [5] In any one of the above [1] to [4], a narrow gap gas-shielded arc welding method, in which a mixed gas containing 20 volume % or more of CO2 gas is used as a shielding gas in the gas-shielded arc welding. [6] In any one of the above [1] to [5], the welding wire fed to the power feed tip of the electrode is a welding wire curved with a radius of curvature ρ in the range of 150 mm to 300 mm. Effect of the Invention

[0014] According to the present invention, in a narrow gap gas shielded arc welding method, the welding heat input to the groove face, the welding current, and the welding wire feed speed can be appropriately controlled, the occurrence of defects such as poor penetration on the groove face can be prevented, and high welding efficiency can be realized. Therefore, compared with the conventional manufacturing of welded joints, a stable welded joint can be obtained, which is an industrially significant advantage. In addition, the present invention has the effect of being extremely useful for welding general structures such as buildings, bridges, and ships. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is an explanatory diagram illustrating a groove shape used in the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a relationship between a welding wire and a groove face during welding. [Diagram 3] FIG. 13 is a schematic diagram showing a feed angle of a welding wire with respect to a bottom groove. [Figure 4] FIG. 2 is a schematic diagram showing the penetration depth of molten metal in a groove face. [Diagram 5] FIG. 1 is a schematic diagram showing one embodiment of multi-layer welding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be specifically described with reference to FIG.

[0017] This embodiment is a narrow-gap gas-shielded arc welding method for joining steel plates having a thickness t of 22 mm or more by multi-layer welding with a narrow gap having a groove angle θ of 25° or less and a bottom groove gap G in the range of 7 mm to 18 mm. Note that the "steel plate" referred to here includes thick steel material, thick plate steel material, etc.

[0018] [Steel plate thickness: 22mm or more] The thickness t of the steel plate 1 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 in this embodiment by increasing the groove angle θ to reduce the groove gap in a conventional R-shaped groove, resulting in highly efficient welding. Preferably, the thickness t is 30 mm or more.

[0019] The upper limit of the plate thickness t is not specified. Since the plate thickness of steel structures, including special structures, is 200 mm or less, in this embodiment, the upper limit of the plate thickness t of the steel plate 1 is preferably set to 200 mm.

[0020] This embodiment can be applied to various grades of steel plates ranging from mild steel plates to high tensile steel plates of 780 MPa class. In particular, welding of high tensile steel plates of 590 MPa class is also possible without preheating.

[0021] [Bevel angle θ: 25° or less] The groove shape used in this embodiment is a V-shaped groove with a groove angle θ of 25° or less and a gap at the bottom. The groove shape in this embodiment includes an I-shaped groove with a groove angle θ of 0°. FIG. 1 shows an example of the groove shape. Here, 1 is a steel plate (base material) and 2 is a groove surface. The smaller the groove cross-sectional area of ​​the groove portion of the steel plate 1, the more efficient welding is possible, but defects such as poor penetration are likely to occur. In addition, when the groove angle θ of the groove portion exceeds 25°, it can be performed with a conventional construction method. For this reason, this embodiment 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 a V-shaped groove is 0°, it is called an I-shaped groove, and in terms of the amount of welding, it is most efficient when the groove angle θ is 0°. 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. Note that when the plate thickness t exceeds 100 mm, the upper limit of the suitable range is preferably set to 10°. More preferably, the groove angle θ is in the range of 5° to 10°.

[0022] [Bottom Groove Gap G: 7mm~18mm] The smaller the groove of the steel plate 1, the more efficient the welding, but the more likely defects such as poor penetration are 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 a conventional construction method. For this reason, this embodiment targets a bottom groove gap G of 18 mm or less, which is expected to be even more efficient than the conventional construction method. On the other hand, if the bottom groove gap G is less than 7 mm, it is difficult for the welding torch and welding wire to approach the bottom of the groove, which increases the variation in welding quality and may cause defects such as poor penetration. 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.

[0023] [Multi-layer welding] In this embodiment, the narrow groove is used to perform multi-layer welding with one or more electrodes for joining. An example of one form of multi-layer welding is shown in Fig. 5. The number of welding passes for forming the weld metal 7 of each layer may be one pass or two or more passes, and is not particularly limited. Here, 5 is a backing material.

[0024] [Distance d between the side end of the welding wire tip and the bottom groove surface: 3.0 mm or less] As shown in FIG. 2, the horizontal distance d between the side end of the tip 4a of the welding wire 4 of the first electrode and the bottom of the groove surface 2 (hereinafter, simply referred to as the "wire tip-groove surface distance") needs to be adjusted to a range of 3.0 mm or less. If the wire tip-groove surface 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 set to 3.0 mm or less. On the other hand, if the distance d is less than 0.1 mm, the arc may be generated between the upper part of the wire 4 and the groove surface 2, and the groove surface 2 at the bottom may not be melted efficiently, so the distance d is preferably set to 0.1 mm or more. More preferably, the distance d is in the range of 0.5 mm to 2.0 mm, and even more preferably, 0.5 mm to 1.0 mm. The "side end of the tip of the welding wire" refers to the side end on the side close to the groove surface 2 at the bottom of the steel plate to be melted.

[0025] [Control of variable K for adjusting distance d] This embodiment is a narrow gap gas shielded arc welding method in which the above-mentioned distance d is adjusted to stabilize the arc and suppress poor penetration at the groove face. To achieve this, the value of the variable K calculated from the following formula (1) is controlled to be in the range of 24.0 to 66.0. K=(92.3×I / W+9.1)×H-0.5×d ··· (1) Here, H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feed speed (cm / min).

[0026] This formula (1) was derived by examining the relationship between the welding heat input H, welding current I, welding wire feed speed W, and wire tip-groove face distance d and penetration characteristics from a large amount of experimental data.

[0027] It has been found that when the K value is in the range of 24.0 to 66.0, the arc becomes stable and there is no insufficient penetration, and the K value is preferably in the range of 30.0 to 60.0.

[0028] Furthermore, the welding heat input H is preferably in the range of 0.8 kJ / mm to 2.3 kJ / mm. If the heat input H is less than 0.8 kJ / mm, the thermal energy required to melt the bottom of the groove face 2 is insufficient, resulting in insufficient penetration. On the other hand, if the heat input H exceeds 2.3 kJ / mm, the arc becomes unstable, causing large variations in welding quality and possibly defects such as insufficient penetration. More preferably, the value of the variable K is in the range of 1.0 kJ / mm to 2.2 kJ / mm.

[0029] Also, the welding current I is preferably in the range of 270 A to 370 A. If the current I is less than 270 A, the directivity of the arc decreases and the heat input decreases, which may cause poor penetration, while if the current I exceeds 370 A, the droplet transfer becomes irregular and the welding target position may become unstable. More preferably, the welding current I is in the range of 290 A to 360 A.

[0030] The welding wire feed speed W is preferably in the range of 1000cm / min to 1700cm / min. If the feed speed W is less than 1000cm / min, the droplet transfer may become irregular and the welding target position may become unstable, while if the feed speed W exceeds 1700cm / min, the directivity of the arc may decrease and the heat input may become small, resulting in poor penetration. More preferably, the welding wire feed speed W is in the range of 1200cm / min to 1600cm / min.

[0031] [Other preferable requirements for welding method, welding conditions, etc.] (Distance from tip of power supply tip to tip of welding wire: 10mm~40mm) 2, the distance h from power feed tip 3 at the tip of the welding torch to tip 4a of welding wire 4 supplied from the power feed tip (hereinafter simply referred to as "tip-to-wire tip distance") is preferably adjusted to a range of 10 mm to 40 mm. Note that the distance referred to here refers to the distance along the direction in which power feed tip 3 faces.

[0032] If the tip-wire end distance h is less than 10 mm, spatters generated during welding will adhere to the power contact tip, making the arc discharge unstable. On the other hand, if the distance h exceeds 40 mm, droplet transfer will become irregular, and the welding target position may become unstable. Therefore, by adjusting the distance h to the range of 10 mm to 40 mm, the adhesion of spatters to the power contact tip can be suppressed and droplet transfer can be stabilized, resulting in a good welded joint. For these reasons, the tip-wire end distance h is preferably adjusted to the range of 10 mm to 40 mm. More preferably, it is in the range of 15 mm to 30 mm.

[0033] (Welding wire supply angle φ to the bottom 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 this directivity of the arc to melt the groove surface, it is advantageous to point the electrode tip toward the groove surface, 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 with respect to the bottom groove is shown in Figure 3.

[0034] When the feed angle φ of the welding wire 4 supplied from the power feed tip 3 at the tip of the welding torch to the bottom groove 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 wire electrode (arc creeping up), making it difficult to maintain the targeted fusion at the groove surface 2, especially at the bottom. On the other hand, when the feed angle φ of the welding wire 4 supplied from the power feed tip 3 to the bottom groove exceeds 15° with respect to the perpendicular line, the arc is too directed toward the groove surface 2, so the weld bead shape becomes convex, and the melting by the arc in the welding of the next layer and subsequent layers becomes insufficient, which makes it easy for welding defects to occur. For this reason, the feed angle φ of the welding wire 4 to the bottom groove is preferably in the range of 0° to 15° with respect to the perpendicular line. More preferably, it is 5° to 12°.

[0035] The feed angle φ of the welding wire 4 with respect to the bottom groove is the same as the inclination of the tip of the power feed tip 3, so the feed angle φ of the welding wire 4 can be controlled by the inclination of the tip of the power feed tip 3. The feed angle φ is defined as positive when it is facing the groove face. The perpendicular line here refers to the perpendicular line with respect to the bottom groove.

[0036] (The radius of curvature of the welding wire fed to the power feed tip ρ: 150mm~300mm) Furthermore, in order to control the feed angle φ of the welding wire 4 fed from the power feed tip 3 at the tip of the welding torch, 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. Therefore, in order to pass the power feed tip 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 ρ of the welding wire is less than 150 mm, the wire feed resistance becomes large, the welding wire cannot be fed stably, and it becomes difficult to maintain the arc. On the other hand, if the curvature radius ρ of the welding wire exceeds 300 mm, there is no effect in reducing the wire feed resistance when the power feed tip tip tip is bent, so the welding wire cannot be fed stably and it becomes difficult to maintain the arc. From the above, it is preferable that the curvature radius ρ of the welding wire 4 fed to the power feed tip 3 is 150 mm to 300 mm. More preferably, it is 175 mm to 275 mm.

[0037] (Welding wire diameter: 1.0mm~1.6mm) Welding wires for narrow gap gas shielded arc welding are generally manufactured with diameters ranging from 0.6 mm to 2.0 mm. When welding with the same current, generally, the thinner the wire diameter, the higher the deposition rate due to Joule heat. For this reason, 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. For this reason, the diameter of the welding wire used in this embodiment is preferably in the range of 1.0 mm to 1.6 mm.

[0038] (Penetration depth at groove face p: 1.0 mm or more) As shown in Fig. 4, the penetration depth p of the groove surface in narrow-gap gas-shielded arc welding is affected by the surface quality of the groove surface due to groove preparation such as gas cutting, particularly the recess depth and cleanliness. In general structures, the material is used for welding as is without any maintenance. Therefore, in order to effectively prevent welding defects such as high-temperature cracking and incomplete fusion without being affected by the variation in the surface quality of the groove surface due to groove preparation, it is necessary to deeply melt the groove surface during welding, especially the bottom groove surface, which has a low welding temperature and tends to have a small penetration depth.

[0039] For this reason, in this embodiment, the penetration depth p at the bottom groove face is preferably 1.0 mm or more. More preferably, it is 1.5 mm or more. However, if the penetration depth p exceeds 4.0 mm, the amount of molten metal at the groove face increases, and the molten metal tends to flow to the bottom of the groove under its own weight. Therefore, an undercut occurs at the top of the weld bead, which may cause a welding defect. Therefore, it is more preferable that the penetration depth is 4.0 mm or less.

[0040] (multi-electrode) As described above, the welding conditions in this embodiment, that is, the welding conditions for the first electrode in the case of multi-electrode welding with two or more electrodes, have been described. Below, suitable conditions for the second electrode, third electrode, etc. in the case of multi-electrode welding with two or more electrodes will be described.

[0041] Positional relationship between the first electrode and the second electrode In narrow gap multi-layer 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 poor fusion, i.e., cold lap, spatter attached to the groove surface, and slag inclusion, etc., tend to occur. In particular, in the first layer welding, the temperature of the steel plate is low and the penetration depth is small, so defects due to poor fusion tend to occur. Therefore, in this embodiment, it is preferable to arrange the first electrode and the second electrode at positions along a predetermined parallel welding line.

[0042] Distance between the tips of the welding wires of the first and second electrodes The distance a between the tips of the welding wires supplied from the power feed tips at the tips of the welding torches of the first and second electrodes (hereinafter, simply referred to as the "first-second electrode distance") is preferably adjusted to a range of 5 mm to 16 mm. The distance between the tips of the welding wires referred to here refers to the distance between the centers of the tips of the welding wires in each electrode. If the first-second electrode distance a 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 first-second electrode distance a exceeds 16 mm, the outward electromagnetic force between the electrodes decreases inversely proportional to the distance, and no arc repulsion force is obtained to overcome the inward electromagnetic force generated by the current flowing through the groove surface, so that the arcs are directed inward and heat is concentrated at the center of the groove. As a result, sufficient melting of the groove surface cannot be obtained. For these reasons, the first-second electrode distance a is preferably adjusted to a range of 5 mm to 16 mm. Furthermore, in order to obtain deeper and more stable melting of the groove face by stronger repulsion of the arc, it is more preferable that the distance a between the first and second electrodes is set in the range of 5 mm to 8 mm.

[0043] The angle of the line connecting the tips of the welding wires of the first and second electrodes If the angle α of the straight line connecting the welding wire tips of the first electrode and the second electrode with respect to the direction perpendicular to the weld line exceeds 60°, a sufficient repulsive force of the arc cannot be obtained, and sufficient melting cannot be obtained at the groove face. Therefore, the angle α of the straight line connecting the welding wire tips of the first electrode and the second electrode with respect to the direction perpendicular to the weld line (hereinafter simply referred to as the "first-second electrode arrangement angle") is preferably 60° or less. More preferably, it is 45° or less. This first-second electrode arrangement angle α may be 0°.

[0044] Electrode polarity The polarity of the first electrode and the second electrode is not particularly limited, and may be either wire negative (positive polarity) or wire positive (negative polarity). If the first electrode and the second electrode have the same polarity, for example, if both the first electrode and the second electrode are wire positive, the arcs of the two electrodes will be inward due to the electromagnetic force of attraction, and heat will be concentrated at the center of the groove. This makes it difficult to obtain sufficient melting at the groove surface. On the other hand, if one of the first electrode and the second electrode is wire negative (positive polarity) and the other is wire positive (negative polarity), and the arrangement of the first electrode and the second electrode is appropriately controlled, the magnetic fields due to the welding currents of the two electrodes will generate strong outward electromagnetic forces, and the arcs will repel each other. As a result, it is possible to obtain a sufficient penetration depth at the groove surface. For this reason, it is more preferable to use one of the first electrode and the second electrode as wire negative (positive polarity) and the other as wire positive (negative polarity).

[0045] Third and subsequent electrodes In multi-electrode welding with three or more electrodes, if the weld height exceeds the bottom groove gap in the first layer welding, the risk of hot cracking increases. To avoid this, it is effective to position the third and subsequent electrodes in the center of the groove behind the first and second electrodes. This also makes it possible to further reduce the number of layers, greatly reducing the risk of stacking defects in multi-layer welding. Note that the "center of the groove" is allowed to be within a range of ±10% of the groove gap G from the center of the groove.

[0046] From the viewpoint of hot cracking resistance, it is preferable that the third and subsequent electrodes are arranged such that the distance between adjacent leading electrodes is in the range of 10 mm to 100 mm. Here, the distance between adjacent leading electrodes means, in the case of four electrodes, the distance between the tip of the welding wire of the second electrode and the tip of the welding wire of the third electrode, or the distance between the tip of the welding wire of the third electrode and the tip of the welding wire of the fourth electrode. The polarity of the third and subsequent electrodes is not particularly limited, and may be either wire negative (positive polarity) or wire positive (reverse polarity).

[0047] (Shielding gas: mixed gas containing 20% ​​or more CO2 gas by volume) Since the heat source characteristics of the arc and the amount of oxygen in the weld metal are also greatly affected by the shielding gas composition, it is preferable to use a mixed gas containing 20% ​​or more CO2 gas by volume and the remainder inert gas such as Ar as the shielding gas used in the gas-shielded arc welding of this embodiment. More preferably, the gas contains 100% CO2 gas by volume. If the amount of CO2 gas is less than 20% by volume, the arc is relaxed and the heat input from the arc to the weld is dispersed, making it difficult to obtain a predetermined penetration depth. Therefore, it is preferable to make the amount of CO2 gas contained in the shielding gas 20% by volume or more. In addition, in this embodiment, it is preferable to increase the oxygen concentration in the weld metal that governs the flow of the weld metal and direct the convection on the surface of the molten pool from the periphery toward the center of the molten pool to stably and deeply penetrate the steel plate bottom in the groove.

[0048] (Other welding conditions) In this embodiment, the welding conditions other than those described above do not need to be particularly limited and may be in accordance with established standards, for example, welding voltage V: 28 V to 40 V, welding speed S: 300 mm / min to 1000 mm / min.

[0049] [Welding automation technology] As described above, in narrow-gap gas-shielded arc welding of steel plates, it is necessary to optimize the welding heat input to the groove surface, the welding current, the welding wire feed speed, and the like. However, in recent years, welding automation technology such as welding robots has become lighter, more functional, and more accurate, and welding control technology applicable to groove shapes and welding positions that were previously difficult has been developed. By utilizing this technology and incorporating the welding condition control method of the narrow-gap gas-shielded arc welding method according to this embodiment, welding suitable for the steel plate, groove shape, welding position, and welding material can be performed. Specifically, the distance d between the side end of the tip of the welding wire according to this embodiment and the steel plate groove surface is measured inline or online, and the distance d is automatically controlled based on the distance d, or other welding conditions are controlled. As a result, it is possible to suppress the occurrence of defects such as poor penetration on the groove surface and achieve high welding efficiency. EXAMPLES

[0050] Hereinafter, examples of the present invention will be described. 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] Using the steel type (grade) and welding wire shown in Table 1, multi-layer narrow-gap gas-shielded arc welding was performed with one electrode under the welding conditions shown in the same table, to obtain a narrow-gap gas-shielded arc-welded joint (weld length: 500 mm).

[0052] During narrow gap gas-shielded arc welding, the presence or absence of deviation, variation, or deflection of the arc from the target position was evaluated by visual inspection through a light-shielding surface and by analyzing videos taken with a high-speed camera. If the arc was stable, it was marked with an O, and if any of the deviation, variation, or deflection occurred, it was marked with an X.

[0053] The obtained welded joint was cut at five arbitrary cross sections in the longitudinal direction, and the penetration depth p at the groove face of the thick steel plate was determined for each cross section.

[0054] Regarding the penetration evaluation in Table 2, an evaluation of A was given when "the arc was stable and the penetration depth at the groove face of the thick steel plate was 1.0 mm or more." An evaluation of B was given when "the arc was stable and the penetration depth at the groove face of the thick steel plate was greater than 0 mm and less than 1.0 mm." An evaluation of F was given when the arc destabilized, causing deviation, variation, or deflection from the target position, or when poor penetration occurred at the groove face of the steel plate.

[0055] Table 2 summarizes these results.

[0056] [Table 1]

[0057] [Table 2]

[0058] As is clear from the results in Table 2, in Weld Nos. 1 to 9 shown as examples of the invention, the arc was stable and good welds without poor penetration were obtained. Of these examples of the invention, in Weld Nos. 1 to 7, the arc was stable and the penetration depth at the groove face of the steel plate was 1.0 mm or more. In contrast, in Weld No. 10, which is a comparative example, poor penetration occurred at the groove face of the steel plate mainly due to insufficient welding heat input H. In Weld Nos. 11 and 12, the arc did not satisfy the K value in formula (1), and thus the arc became unstable, causing deviation, variation, and deflection from the target position, and similarly, poor penetration occurred at the groove face of the steel plate. [Explanation of symbols]

[0059] 1 Steel plate (base material) 2 Bevel surface 3 Welding torch power tip 4 Welding Wire 4a welding wire tip 5 Backing material 6 Molten Metal 7 Welding metal t Plate thickness θ Bevel angle d Distance between the side end of the welding wire tip and the bottom groove surface h Distance between power tip and wire tip f welding wire diameter G Bottom Groove Gap φ Welding wire feed angle p Penetration depth of groove face

Claims

1. A narrow-gap gas-shielded arc welding method for joining steel plates having a plate thickness t of 22 mm or more by multi-layer welding with a narrow groove having a groove angle θ of 25° or less and a bottom groove gap G in the range of 7 mm to 18 mm, A narrow gap gas-shielded arc welding method, wherein a distance d between a side end of a tip of a welding wire and a bottom groove surface of the steel plate is in the range of 3.0 mm or less, and a value of a variable K calculated from the following formula (1) is in the range of 24.0 to 66.

0. K=(92.3×I / W+9.1)×H-0.5×d... (1) Here, H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feed speed (cm / min).

2. A distance h from a tip of a welding torch to a tip of the welding wire is set to a range of 10 mm to 40 mm, 2. The narrow gap gas-shielded arc welding method according to claim 1, wherein a feed angle φ of the welding wire to the bottom groove is in the range of 0° to 15° with respect to the perpendicular.

3. The diameter f of the welding wire is in the range of 1.0 mm to 1.6 mm, The welding heat input H is in the range of 0.8 kJ / mm to 2.3 kJ / mm, The welding current I is in the range of 270 A to 370 A, 3. The narrow gap gas shielded arc welding method according to claim 1, wherein the welding wire feed speed W is in the range of 1000 cm / min to 1700 cm / min.

4. 3. The narrow gap gas-shielded arc welding method according to claim 1, wherein a penetration depth p at the groove face is 1.0 mm or more.

5. The shielding gas in the gas-shielded arc welding contains 20% by volume or more of CO ２ 3. The narrow gap gas-shielded arc welding method according to claim 1, wherein a mixed gas containing a gas is used.

6. 3. The narrow gap gas-shielded arc welding method according to claim 1, wherein the welding wire fed to the power feed tip of the electrode is curved with a radius of curvature ρ in the range of 150 mm to 300 mm.

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