Narrow-groove gas-shielded arc welding method

Abstract

Provided is a narrow-groove gas-shielded arc welding method capable of achieving high welding execution efficiency and capable of manufacturing a welded joint that prevents the occurrence of defects such as poor penetration in the groove surface. This narrow-groove gas-shielded arc welding method joins steel plates having a plate thickness t of 22 mm or greater by narrow-groove multilayer welding in which the groove angle θ is 25° or lower and the bottom groove gap G is 7-18 mm, and is characterized in that the distance d between a side end of a welding wire tip and the bottom groove surface of the steel plate is set to a range of 3.0 mm or less, and the value of a variable K obtained from equation (1) below is set to a range of 24.0 to 66.0.　(1): K=(92.3×I / W+9.1)×H-0.5×d where H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feeding speed (cm / min).

Description

Narrow gap gas shielded arc welding method

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

[0002] Gas-shielded arc welding, 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 the welding process has become much more time-consuming, resulting in increased construction costs. Therefore, the application of narrow-gap gas-shielded arc welding, which uses gas-shielded arc welding to create multi-layer welds in grooves with gaps that are small compared to the plate thickness, has been considered. Narrow-gap gas-shielded arc welding reduces deposition compared to conventional gas-shielded arc welding methods, enabling more efficient and energy-efficient welding, 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, layer 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 stacking faults.

[0004] Furthermore, Patent Document 2 discloses narrow gap welding in which the 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 using multi-electrode welding with two or more electrodes, with one of the leading first and second electrodes being positive polarity and the other being opposite polarity, and the first and second electrodes being 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 line connecting the welding wire tips of the first and second electrodes relative to the direction perpendicular to the weld line is 45° or less, and the fusion depth perpendicular to the weld line at the bottom of the thick steel material is 1.5 mm or more. This is said to result in no defects and improved welding efficiency even when groove preparation such as gas cutting or plasma cutting is performed.

[0006] JP 2009-061483 A JP 2010-115700 A JP 2015-223605 A

[0007] However, the TIG welding method described in Patent Document 1, which uses a non-consumable electrode, is different from MAG welding, which uses a steel wire as a consumable electrode, and CO ２ Compared to conventional welding, the welding efficiency is significantly lower. Furthermore, 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 the welding torch needs to be weaved before the molten metal drips. As a result, the welding current needs to be low, about 150 A, and the deposition amount per pass needs to be reduced. Therefore, when this welding method is applied to welding thick steel plates, it results in low-volume, multi-pass stack 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 may adhere to the groove or the welding torch, resulting in welding defects. Furthermore, when multi-electrode welding with three or more electrodes is performed, hot cracks may occur in the center of the weld metal.

[0009] As described above, 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 welding automation technology, such as 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 perform 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 aims to provide a narrow-gap gas-shielded arc welding method for steel plates that achieves high welding efficiency and can produce a welded joint that prevents defects such as poor penetration on the groove face.

[0012] In order to achieve the above object, the inventors have conducted extensive research into factors that affect the occurrence of defects such as poor penetration on groove faces, and have found that in narrow-gap gas-shielded arc welding, appropriate control of the welding heat input to the groove face, the welding current, and the welding wire feed speed is effective in preventing arc instability or deflection and the occurrence of poor penetration.

[0013] The present invention was completed based on these findings and further studies. Specifically, the gist of the present invention is as follows: [1] A narrow-gap gas-shielded arc welding method for joining steel plates with a thickness t of 22 mm or more by multi-pass 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, 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 set to 3.0 mm or less, and the value of variable K calculated from the following formula (1) is set to 24.0 to 66.0: K = (92.3 × I / W + 9.1) × H - 0.5 × d (1) where H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feed rate (cm / min). [2] The narrow groove gas-shielded arc welding method of [1] above, wherein the distance h from the tip of the power feed tip of the welding torch to the tip of the welding wire is in the range of 10 mm to 40 mm, and the feed angle φ of the welding wire with respect to the bottom groove is in the range of 0° to 15° with respect to the perpendicular. [3] The narrow groove gas-shielded arc welding method of [1] or [2] above, wherein 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, and the welding wire feed rate W is in the range of 1000 cm / min to 1700 cm / min. [4] The narrow groove gas-shielded arc welding method of 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], the shielding gas in the gas-shielded arc welding contains 20% by volume or more of CO ２ [6] The narrow-gap gas-shielded arc welding method according to any one of [1] to [5], 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.

[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, thereby preventing defects such as poor penetration at the groove face and achieving high welding efficiency. Therefore, compared with conventional welded joint manufacturing, a more stable welded joint can be obtained, which is an industrially significant advantage. Furthermore, the present invention has the effect of being extremely useful in welding general structures, particularly in buildings, bridges, and ships.

[0015] Fig. 1 is an explanatory diagram schematically showing a groove shape used in the present invention; Fig. 2 is a schematic diagram showing the relationship between a welding wire and a groove surface during welding; Fig. 3 is a schematic diagram showing the feed angle of a welding wire with respect to a bottom groove; Fig. 4 is a schematic diagram showing the penetration depth of molten metal at a groove surface; Fig. 5 is a schematic diagram showing one form of multi-layer welding;

[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 narrow-gap multi-layer welding with 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 term "steel plate" here includes thick steel material, thick plate steel material, etc.

[0018] [Thickness t of steel plate: 22 mm or more] The thickness t of the steel plate 1 is 22 mm or more. If the thickness t is less than 22 mm, in a conventional V-groove, increasing the groove angle θ to reduce the groove gap may result in a smaller groove cross-sectional area than the groove targeted in this embodiment, 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, it is preferable that the upper limit of the plate thickness t of the steel plate 1 is 200 mm.

[0020] This embodiment can be applied to various grades of steel plates, 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 possible without preheating.

[0021] [Groove 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 of this embodiment includes an I-shaped groove with a groove angle θ of 0°. An example of this groove shape is shown in FIG. 1. Here, 1 is the steel plate (base material) and 2 is the groove face. The smaller the groove cross-sectional area of ​​the groove portion of the steel plate 1, the more efficient welding becomes, but defects such as poor penetration are more likely to occur. Furthermore, when the groove angle θ of the groove portion exceeds 25°, it can be performed using conventional construction methods. Therefore, this embodiment 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 a V-shaped groove, when the groove angle θ is 0°, it is called an I-shaped groove, and in terms of the amount of welding, a groove angle θ of 0° is the most efficient. 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. Note that when the plate thickness t exceeds 100 mm, the upper limit of the suitable range is preferably 10°. A more preferable groove angle θ is in the range of 5° to 10°.

[0022] [Bottom Groove Gap G: 7 mm to 18 mm] 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. Furthermore, welding with a bottom groove gap G (groove width), which is the gap between the bottoms of the grooves before welding begins, exceeding 18 mm can be performed using conventional construction methods. Therefore, this embodiment 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 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 result in 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 described above is used to perform multi-layer welding with one or more electrodes for joining. An example of multi-layer welding is shown in Fig. 5. The number of welding passes for forming the weld metal 7 in each layer is not particularly limited and may be one pass or two or more passes. 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 4 a of the welding wire 4 of the first electrode and the bottom of the groove surface 2 (hereinafter also referred to simply as the “wire tip-groove surface distance”) needs to be adjusted to a range of 3.0 mm or less. If this wire tip-groove surface distance d exceeds 3.0 mm, the arc moves away from the groove surface 2, making it impossible to efficiently melt the groove surface 2. For this reason, this distance d is set to 3.0 mm or less. On the other hand, if the distance d is less than 0.1 mm, an arc may be generated between the top of the wire 4 and the groove surface 2, making it impossible to efficiently melt the bottom groove surface 2. Therefore, 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 that is closer to the groove face 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 that adjusts the distance d to stabilize the arc and prevent poor penetration at the groove face. To achieve this, the value of variable K calculated from the following equation (1) is controlled to fall within the range of 24.0 to 66.0: K = (92.3 × I / W + 9.1) × H - 0.5 × d (1) where H is the welding heat input (kJ / mm), I is the welding current (A), and W is the welding wire feed rate (cm / min).

[0026] This formula (1) is a relational formula derived from a study of 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, etc.

[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. 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. This is because 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 will be insufficient, resulting in insufficient penetration. On the other hand, if the heat input H exceeds 2.3 kJ / mm, the arc will become unstable, resulting in greater variation in welding quality and the possibility of defects such as poor penetration. More preferably, the value of the variable K is in the range of 1.0 kJ / mm to 2.2 kJ / mm.

[0029] 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 result in 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] Furthermore, the welding wire feed speed W is preferably in the range of 1000 cm / min to 1700 cm / min. If the feed speed W is less than 1000 cm / min, droplet transfer may become irregular and the welding target position may become unstable, while if the feed speed W exceeds 1700 cm / min, the directivity of the arc may decrease and the heat input may become small, which may result in poor penetration. More preferably, the welding wire feed speed W is in the range of 1200 cm / min to 1600 cm / min.

[0031] [Other Preferred Requirements, Welding Conditions, etc. Related to the Welding Method] (Distance h from the Power Feed Tip to the Tip of the Welding Wire: 10 mm to 40 mm) In this embodiment, as shown in Fig. 2, the distance h from the power feed tip 3 at the tip of the welding torch to the tip 4a of the welding wire 4 supplied from the power feed tip (hereinafter simply referred to as the "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 the power feed tip 3 is facing.

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

[0033] (The feed angle φ of the welding wire relative to the bottom groove: 0° to 15° relative to the perpendicular) The arc has directionality and tends to point in the direction that the tip of the electrode (welding wire) points. To effectively utilize this arc direction in melting 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 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 relative 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 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 desired fusion at the groove face 2, particularly 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 vertical, the arc is directed too far toward the groove face 2, resulting in a convex weld bead shape. This results in insufficient fusion by the arc in subsequent welding layers, making welding defects more likely 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 vertical. 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 pointing toward the groove face. The perpendicular line here refers to the perpendicular line to the bottom groove.

[0036] (Radius of curvature ρ of welding wire fed to power feed tip: 150 mm to 300 mm) Furthermore, a power feed tip with a bent tip is used 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. At this time, the welding wire passes through the power feed tip with a bent tip. Therefore, to allow the welding wire to pass more smoothly through the power feed tip, it is preferable to pre-bend the welding wire using a so-called three-point roller or the like. In this case, if the radius of curvature ρ 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 ρ of the welding wire exceeds 300 mm, the bending of the 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 ρ of the welding wire 4 fed to the power feed tip 3 is preferably 150 mm to 300 mm. More preferably, it is 175 mm to 275 mm.

[0037] (Welding wire diameter f: 1.0 mm to 1.6 mm) 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, generally, the thinner the wire diameter, the higher the deposition rate due to Joule heat. Therefore, in order 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 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 p at groove surface: 1.0 mm or more) As shown in Figure 4, the penetration depth p at 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 depth of the recess and cleanliness. In general structures, the material is used for welding as is without any preparation. Therefore, in order to effectively prevent welding defects such as hot cracking and insufficient fusion without being affected by variations in the surface quality of the groove surface due to groove preparation, it is necessary to deeply melt the groove surface during welding, particularly the bottom groove surface, which tends to have a low temperature during welding and a small penetration depth.

[0039] For these reasons, 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. As a result, undercuts occur at the top of the weld bead, which can cause welding defects. Therefore, it is more preferable that the penetration depth be 4.0 mm or less.

[0040] (Multiple electrodes) As described above, the welding conditions for this embodiment, i.e., 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-pass welding, when one pass is performed per layer, the welding heat tends to concentrate at the center of the groove with one electrode, resulting in insufficient melting at the groove surface of the steel sheet, and defects due to incomplete fusion, i.e., cold lap, spatter adhering to the groove surface, and slag inclusion, are likely to occur. In particular, in the first layer welding, the temperature of the steel sheet is low and the penetration depth is small, making defects due to incomplete fusion more likely to occur. Therefore, in this embodiment, it is preferable to position the first electrode and the second electrode along a predetermined parallel weld line.

[0042] Distance between the welding wire tips of the first and second electrodes: The distance a between the welding wire tips supplied from the power feed tips at the welding torch tips 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 welding wire tips here refers to the distance between the centers of the welding wire tips of each electrode. If the first-second electrode distance a is less than 5 mm, the current (electrons) flowing between the electrodes reduces the heat of the arc itself, preventing sufficient melting of the groove surface 2. 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. This prevents the arc from generating a repulsive force sufficient to overcome the inward electromagnetic force generated by the current flowing through the groove surface. Consequently, the arcs are directed inward, causing heat to concentrate at the center of the groove. As a result, sufficient melting of the groove surface is not achieved. 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 in the range of 5 mm to 8 mm.

[0043] Angle of the Line Connecting the Tips of the Welding Wires of the First and Second Electrodes If the angle α of the line connecting the tips of the welding wires of the first and second electrodes 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 achieved at the groove face. Therefore, the angle α of the line connecting the tips of the welding wires of the first and second electrodes 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. Note that this first-second electrode arrangement angle α may be 0°.

[0044] Electrode Polarity: The polarities of the first and second electrodes are not particularly limited and may be either wire-negative (positive polarity) or wire-positive (reverse polarity). If the first and second electrodes have the same polarity, for example, if both the first and second electrodes are wire-positive, the mutual electromagnetic force will cause the arcs to be directed inward, concentrating heat at the center of the groove. This makes it difficult to achieve sufficient melting at the groove face. On the other hand, if one of the first and second electrodes is wire-negative (positive polarity) and the other is wire-positive (reverse polarity), and the arrangement of the first and second electrodes is appropriately controlled, the magnetic fields generated by the welding currents will generate strong outward electromagnetic forces, causing the arcs to repel each other. As a result, sufficient penetration depth can be achieved at the groove face. For these reasons, it is more preferable to use one of the first and second electrodes as wire-negative (positive polarity) and the other as wire-positive (reverse polarity).

[0045] - Third and subsequent electrodes In multi-electrode welding with three or more electrodes, if the weld buildup height exceeds the bottom groove gap during 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, significantly reducing the risk of stacking defects in multi-layer welding. Note that the "groove center" allows a range of ±10% of the groove gap G from the groove center.

[0046] From the viewpoint of hot cracking resistance, it is preferable that the third and subsequent electrodes be 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 a four-electrode welding, 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: 20% or more by volume of CO ２ 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. Therefore, the shielding gas used in gas-shielded arc welding in this embodiment is a mixed gas containing CO ２ It is preferable to use a mixed gas containing 20% ​​by volume or more of CO gas and the remainder being an inert gas such as Ar. ２ The gas is 100% by volume. ２ If the gas is less than 20% by volume, the arc is less constricted and the heat input from the arc to the weld is dispersed, making it difficult to obtain the desired penetration depth. ２ In this embodiment, it is preferable to increase the oxygen concentration in the weld metal, which controls the flow of the weld metal, and direct the convection on the molten pool surface from the periphery toward the center of the molten pool, thereby stably increasing the penetration depth at the bottom of the steel sheets 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, such as a welding voltage V of 28 V to 40 V and a welding speed S of 300 mm / min to 1000 mm / min.

[0049] [Welding Automation Technology] As mentioned above, narrow-gap gas-shielded arc welding of steel plates requires optimization of the welding heat input to the groove surface, welding current, welding wire feed rate, and the like. However, in recent years, welding automation technologies such as welding robots have become lighter, more functional, and more accurate, leading to the development of welding control technologies that can be applied to groove shapes and welding positions that were previously difficult to achieve. By utilizing this technology and incorporating the welding condition control method for the narrow-gap gas-shielded arc welding method according to the present 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 and the groove surface of the steel plate according to the present embodiment is measured inline or online, and the distance d is automatically controlled based on the measurement, and other welding conditions are controlled. This suppresses defects such as poor penetration at the groove surface and achieves high welding efficiency.

[0050] The following examples of the present invention will be described. However, the following examples are merely for the purpose of illustrating the present invention in more detail and are not intended to limit the scope of the present invention.

[0051] Using the steel types (grades) and welding wires 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 narrow-gap gas-shielded arc-welded joints (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 video footage taken with a high-speed camera. If the arc was stable, it was marked with a ○, and if any deviation, variation, or deflection occurred, it was marked with an ×.

[0053] The obtained welded joint was cut at any five 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, when "the arc is stable and the penetration depth at the groove face of the thick steel plate is 1.0 mm or more," the evaluation was A. When "the arc is stable and the penetration depth at the groove face of the thick steel plate is greater than 0 mm and less than 1.0 mm," the evaluation was B. 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, the evaluation was F.

[0055] Table 2 summarizes these results.

[0056]

[0057]

[0058] As is clear from the results in Table 2, in Welding 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, Welding Nos. 1 to 7 had stable arcs and penetration depths of 1.0 mm or more at the groove face of the steel plate. In contrast, in Welding No. 10, a comparative example, poor penetration occurred at the groove face of the steel plate, mainly due to insufficient welding heat input H. In Welding Nos. 11 and 12, the K value of formula (1) was not satisfied, and therefore, deviation, variation, and deflection of the arc from the target position occurred due to arc instability, and similarly, poor penetration occurred at the groove face of the steel plate.

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

Claims

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 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 set to 3.0 mm or less, and the value of the variable K calculated from the following formula (1) is set to 24.0 to 66.0: K = (92.3 x I / W + 9.1) x H - 0.5 x d ... (1) where 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 narrow gap gas-shielded arc welding method as set forth in claim 1, wherein the distance h from the tip of the welding torch power tip to the tip of the welding wire is in the range of 10 mm to 40 mm, and the 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. A narrow gap gas-shielded arc welding method according to claim 1 or 2, wherein 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. A narrow gap gas-shielded arc welding method according to any one of claims 1 to 3, wherein the 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% or more by volume of CO. ２ The narrow gap gas-shielded arc welding method according to any one of claims 1 to 4, wherein a mixed gas containing a gas is used.

6. A narrow gap gas-shielded arc welding method according to any one of claims 1 to 5, 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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