Vertical narrow gap gas shielded arc welding method

Abstract

The present invention provides a high-quality and highly efficient vertical narrow gap gas shielded arc welding method whereby it is possible to achieve stabilization of a weld bead shape including the suppression of sagging molten metal which represents a problem in vertical welding and prevent weld defects from being generated while obtaining a welded joint that has high toughness and particularly excellent low-temperature weld metal toughness. The present invention is a vertical narrow gap gas shielded arc welding method in which two pieces of thick steel material are joined by single-layer welding or multi-layer welding using weaving under a predetermined gap condition in which the pieces of steel material each have a plate thickness of 10 mm or more, wherein a welding wire containing a prescribed amount of REM is used, weaving of an initial layer weld is performed using a welding torch comprising a bent section and a tip section defined by the bent section, and when weaving with respect to the gap surface of the thick steel material on this occasion, the angle θ1 of the tip section of the welding torch with respect to the horizontal direction at a reference position is set to an angle exceeding 45°, and the tip section of the welding torch is made to oscillate toward the gap surface of the thick steel material under predetermined conditions.

Description

Vertical narrow gap gas shielded arc welding method

[0001] The present invention relates to a narrow-groove gas-shielded arc welding method, particularly to a vertical narrow-groove gas-shielded arc welding method applicable to butt welding of two thick steel sheets. Here, "narrow groove" means a groove angle of 20° or less and a groove gap of 20 mm or less.

[0002] Gas-shielded arc welding, which is used for welding steel, is generally a consumable electrode type that uses CO2 alone or a mixture of Ar and CO2 to shield the molten part. This type of gas-shielded arc welding is widely used in the manufacturing fields of automobiles, construction, bridges, electrical equipment, etc.

[0003] In recent years, as steel structures have become larger and thicker, the amount of welding during the manufacturing process, particularly in butt welding of steel materials, has increased, and the welding work has become more time-consuming, resulting in increased construction costs.

[0004] One possible way to improve the above-mentioned problems 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 relative to the plate thickness. Narrow-gap gas-shielded arc welding reduces the amount of deposition compared to regular gas-shielded arc welding, which is expected to achieve high welding efficiency and energy savings, and ultimately reduce construction costs.

[0005] On the other hand, electroslag welding is usually used for high-efficiency vertical welding, but it is based on single-pass, large-heat-input welding, and there are concerns that excessive heat input will reduce the toughness of the weld metal and the weld heat-affected zone when welding plate thicknesses exceed 60 mm.In addition, there is a limit to the plate thickness that can be achieved with single-pass welding, and the technology has not yet been established, particularly for plate thicknesses exceeding 65 mm.

[0006] Therefore, it is desired to develop a high-quality and highly efficient welding method that applies narrow-gap gas-shielded arc welding to vertical welding.

[0007] Patent No. 6439882 Patent No. 6119940 Patent No. 5884209 Patent No. 6119948

[0008] As a welding method applying such narrow-gap gas-shielded arc welding to vertical welding, for example, the methods described in Patent Documents 1 and 2 are characterized by pointing the tip of the welding torch toward the groove wall to promote melting of the groove wall and weaving in a U-shape along the groove wall. However, when the angle of the wire tip with respect to the horizontal is between 10° and 45°, the welding current fluctuates significantly during the reciprocating weaving motion, resulting in a decrease in the low-temperature toughness of the weld metal. Furthermore, the methods described in Patent Documents 3 and 4 have the problem that, because the welding torch does not have a bent portion, when the torch angle increases, the height of the torch base increases, making it more likely to interfere with the welded parts, and the problem of insufficient melting of the left and right grooves.

[0009] As mentioned above, a high-quality, high-efficiency vertical narrow-gap gas-shielded arc welding method that can be applied to welding thick steel materials has not yet been fully developed. On the other hand, advances in welding automation technology (welding robots) have made it possible to weave a welding torch that is suitable for the groove shape and welding position, which was previously difficult. By utilizing this, it has become possible to perform welding (setting of conditions) that are suitable for the steel material, groove shape, welding position, and welding material (wire).

[0010] An object of the present invention is to provide a vertical narrow-groove gas-shielded arc welding method that utilizes highly functional and highly accurate welding automation technology to perform precise weaving of the welding torch in accordance with the groove shape, welding position, etc., thereby enabling high-quality and highly efficient welding of thick steel materials.

[0011] The inventors conducted further studies to solve the above problems and found that by using a welding wire containing 0.015 to 0.100 mass % REM (Rare Earth Metal), and by controlling the welding conditions in the first layer more precisely, i.e., by performing weaving with a welding torch having a bent portion and a tip defined by the bent portion, and by holding the tip of the welding torch at an appropriate angle relative to the horizontal plane when weaving the groove face of the thick steel material and oscillating the tip of the welding torch toward the groove face of the thick steel material under appropriate conditions, it is possible to ensure sufficient joint depth while preventing the occurrence of welding defects, even when the groove angle is small, and to achieve stable bead shape including suppression of molten metal dripping, which is a problem in high-current vertical welding, and to achieve high toughness of the welded joint, particularly improved low-temperature toughness of the weld metal. The present invention was completed based on the above findings and further studies.

[0012] That is, the gist of the present invention is as follows.

[0013] 1. A vertical narrow gap gas-shielded arc welding method for joining two thick steel sheets having a plate thickness of 10 mm or more by single-layer welding or multi-layer welding using weaving, with a groove angle of 20° or less and a groove gap of 20 mm or less, wherein a welding wire containing 0.015 to 0.100 mass% REM is used, and weaving of the first layer weld is performed with a welding torch having a bent portion and a tip portion defined by the bent portion, and during weaving of the groove face of the thick steel material, the tip portion of the welding torch is swung toward the groove face of the thick steel material, and a position where the tip portion of the welding torch is aligned with the weld line direction as viewed from the plate thickness direction of the thick steel material is set as a reference position, and the angle θ1 of the tip portion of the welding torch with respect to the horizontal direction at the reference position is set to an angle exceeding 45°, and the swing angle θ2 of the tip portion of the welding torch from the reference position is set to be 5° or more and 60° or less, and the joint depth of the first layer weld is set to be 10 mm or more. Vertical narrow gap gas shielded arc welding method.

[0014] 2. The vertical narrow gap gas-shielded arc welding method according to 1 above, wherein the joining is performed by single-layer welding and the groove gap is set to 25% or less of the plate thickness of the thick steel material.

[0015] 3. The vertical narrow gap gas-shielded arc welding method according to 1 above, wherein the joining is multi-layer welding and the joining depth in the first layer welding is 10 mm or more and 70 mm or less.

[0016] 4. The vertical narrow groove gas-shielded arc welding method according to any one of 1 to 3, wherein in the weaving of the first layer weld, the weaving pattern of the welding torch is U-shaped as viewed from the weld line direction.

[0017] According to the present invention, even when welding thick steel plates with a thickness of 10 mm or more under narrow groove conditions, high-quality, highly efficient narrow-gap gas-shielded arc welding can be performed while stabilizing the bead shape and preventing the occurrence of weld defects, including suppressing molten metal dripping, which is a problem in vertical welding. This allows for the production of welded joints with high toughness, particularly excellent low-temperature toughness of the weld metal. Furthermore, the welding method of the present invention reduces the deposition rate compared to conventional gas-shielded arc welding and achieves energy savings through high welding efficiency, thereby significantly reducing welding costs. Furthermore, the welding method of the present invention does not require a water-cooled copper backing pressing mechanism to prevent molten metal dripping, as is required in electrogas arc welding equipment, thereby avoiding the need for complex equipment. Furthermore, by performing multiple passes with a predetermined groove shape, the welding heat input per pass can be reduced, making it easier to ensure the desired mechanical properties of the weld metal and the heat-affected zone of the steel.

[0018] 1 shows examples of various groove shapes. 2 shows a procedure for performing first layer welding in a V-shaped groove shape by the welding method of the present invention. 3 is a schematic diagram showing the swinging state of a welding torch when weaving the groove surface of a thick steel material. 4 shows an example of a groove cross section after first layer welding in a V-shaped groove shape. 5 shows the weaving pattern of a welding torch viewed from the weld line direction in weaving of first layer welding.

[0019] The present invention will be described in detail below. Figures 1(a) to 1(c) show examples of various groove shapes. In the figures, reference numeral 1 denotes a thick steel material, 2 denotes the groove face of the thick steel material, and 3 denotes the groove at the lower stage of the steel material (in a Y-shaped groove). Symbols θ denote the groove angle, G denotes the groove gap, t denotes the plate thickness, and h denotes the groove height at the lower stage of the steel material (in a Y-shaped groove). As shown in the figures, the groove shapes considered here can be either V-shaped grooves (including I-shaped grooves and V-shaped grooves) or Y-shaped grooves. A multi-stage Y-shaped groove is also possible, as shown in Figure 1(c). Note that, as shown in Figures 1(b) and 1(c), the groove angle and groove gap in the case of a Y-shaped groove are the groove angle and groove gap at the groove at the lower stage of the steel material. Here, the groove on the lower part of the steel material refers to the area from the surface of the steel material that will become the back side during welding (the surface on the welding device (welding torch) side is the front side, and the opposite side is the back side) to approximately 20 to 40% of the plate thickness.

[0020] FIG. 2 also shows the procedure for performing a first-layer weld using a welding method according to one embodiment of the present invention in a V-shaped groove. In the figure, reference numeral 4 denotes a welding torch, 5 denotes a welding wire, and 6 denotes a backing material. The weld line, molten pool, and weld bead are not shown. As shown in FIG. 2 , this welding method is gas-shielded arc welding in which two thick steel sheets of a predetermined thickness are butted together and joined by vertical welding using weaving. The welding method is basically upward welding, with the welding direction being upward. Then, when weaving the groove surface of the thick steel sheets, the tip of the welding torch is swung toward the groove surface of the thick steel sheets. While a V-shaped groove shape is shown here as an example, the same applies to other groove shapes.

[0021] Furthermore, Figure 3 is a schematic diagram showing the swinging state of the welding torch when weaving the groove face of a thick steel material. Figures 3(a) and (b) respectively show the welding torch in the reference position and the welding torch swung at an angle θ2 as viewed from the plate thickness direction (the back surface of the thick steel material (the side with the backing material) in Figure 2), and Figure 3(c) is a view as viewed from the X arrow in Figure 3(a). Note that the reference position is the position where the tip of the welding torch (the center line, i.e., the direction in which the welding wire protrudes) is aligned with the welding line direction as viewed from the plate thickness direction, as shown in Figure 3(a). Also, in Figures 3(a) and (b), the groove face of the thick steel material to be melted (not shown) is on the left side of the paper. In the figures, reference numeral 7 denotes the main body of the welding torch, 8 denotes the power supply tip, 9 denotes the bent portion, and 10 denotes the tip. Here, the tip 10 is closer to the welding wire (not shown) than the bent portion 9. Although bent portion 9 may be provided on either main body 7 or power feed tip 8 of the welding torch, it is preferable to provide it on power feed tip 8 from the viewpoint of ease of installation. θ1 is the angle of the tip of the welding torch relative to the horizontal at the reference position, θ2 is the swing angle of the tip of the welding torch from the reference position, θ3 is the bending angle at the bent portion of the welding torch, and 1 is the length of the tip of the welding torch, all of which are based on the center line of each part of the welding torch.

[0022] 4 shows an example of a groove cross section after first layer welding in a V-shaped groove shape. In the figure, reference numeral 11 denotes a weld bead, symbol D denotes the joint depth in the first layer welding, and symbol W denotes the weld bead width in the first layer welding (the gap between the grooves after the first layer welding). Note that the joint depth D in the first layer welding is the minimum value of the weld bead height in the first layer welding when the steel surface that becomes the back surface during welding is used as the starting point (the first layer weld bead height closest (lowest) to the steel surface at the starting point). Here, a V-shaped groove shape is shown as an example, but D and W are similar for other groove shapes.

[0023] Next, the reasons for limiting the groove angle, groove gap, and steel plate thickness to the above ranges in the welding method of the present invention will be explained.

[0024] Groove angle θ: 20° or less. The smaller the groove of the steel material, the faster and more efficient the welding, but the more likely defects such as incomplete fusion are. Furthermore, welding with a groove angle greater than 20° can be performed using conventional construction methods. Therefore, this welding method targets groove angles of 20° or less, which are difficult to perform using conventional construction methods and offer the promise of even greater efficiency. In a V-groove, a groove angle of 0° is called an I-groove, and this 0° angle is the most efficient in terms of deposition rate. While a groove angle of 0° (I-groove) is acceptable, the groove will close during welding due to welding thermal strain. Therefore, it is preferable to set the groove angle according to the plate thickness t (however, in the case of a Y-groove, the groove height h of the lower steel portion). Specifically, the groove angle is preferably (0.5 × t / 20)° or more, and more preferably (0.8 × t / 20)° or more. Furthermore, the groove angle is preferably (2.0 × t / 20)° or less, and more preferably (1.2 × t / 20)° or less. For example, when the plate thickness t is 100 mm, the groove angle is preferably 2.5° or more, and more preferably 4° or more. Furthermore, the groove angle is preferably 10° or less, and more preferably 6° or less. However, when the plate thickness t exceeds 100 mm, the upper limit of the preferred range of the groove angle exceeds 10°, and in this case, the upper limit of the preferred range is 10°.

[0025] Groove gap G: 20 mm or less The smaller the groove of the steel material, the faster and more efficient the welding. Furthermore, welding with a groove gap greater than 20 mm is difficult because the molten metal tends to drip. To address this issue, it is necessary to keep the welding current low, but this makes welding defects such as slag entrapment more likely to occur. Therefore, this method is intended for groove gaps of 20 mm or less. The groove gap is preferably 4 mm or more, and more preferably 12 mm or less. Furthermore, particularly when joining using a single-layer weld consisting of only the first layer weld, the groove gap is more preferably 25% or less of the plate thickness of the steel material to be welded, and even more preferably 20% or less.

[0026] Plate thickness t: 10 mm or more The plate thickness of the steel material is 10 mm or more. If the plate thickness of the steel material is less than 10 mm, a sound joint may be obtained while suppressing the welding heat input even when using a conventional welding method, such as semi-automatic CO2 arc welding using a flux cored wire. The plate thickness is preferably 15 mm or more, and more preferably 20 mm or more. Note that when general rolled steel material is used, the plate thickness generally has an upper limit of 100 mm. Therefore, it is preferable that the upper limit of the plate thickness of the steel material used here be 100 mm or less.

[0027] Furthermore, high-tensile steels (e.g., extra-thick YP460MPa-class steel for shipbuilding (tensile strength 570MPa-class steel) and TMCP SA440 steel for construction (tensile strength 590MPa-class steel)) are particularly suitable as the steel to be welded. High-tensile steels have strict restrictions on welding heat input, are prone to cracking in the weld metal, and do not achieve the required joint strength and toughness due to the effects of welding heat. In contrast, this welding method allows for efficient welding with reduced heat input, and is also capable of welding 590MPa-class high-tensile steel plates and high-alloy 590MPa-class corrosion-resistant steel. This welding method can also be applied to mild steel without any problems.

[0028] The reasons for limiting the groove angle, groove gap, and steel thickness in this welding method have been explained above. In this welding method, it is preferable to use welding wire with a chemical composition that matches that of the steel to be welded. Furthermore, it is important to use welding wire that contains REM. The chemical composition of the welding wire used in this welding method will be explained below.

[0029] REM: 0.015-0.100% by mass. REM is an effective element for reducing the size of inclusions during steelmaking and casting and improving the toughness of weld metal. Furthermore, REM also contributes to the refinement of droplets, stabilization of droplet transfer, and the effective suppression of arc generation at the groove face, particularly when using positive-polarity (negative wire) welding wire or high welding currents. This refinement of droplets and stabilization of droplet transfer suppress spatter and enable stable gas-shielded arc welding. Additionally, the formation of REM oxides on the molten metal surface also reduces dripping of the weld metal, even when the welding torch tip is angled to the horizontal. Here, REM content less than 0.015% by mass does not achieve the effects of reducing droplet size and stabilizing droplet transfer. On the other hand, REM content greater than 0.100% by mass can lead to cracking during the welding wire manufacturing process and reduced toughness of the weld metal. Therefore, the REM content of the welding wire is set to a range of 0.015 to 0.100 mass %, and preferably the REM content of the welding wire is 0.025 mass % or more and 0.050 mass % or less.

[0030] The REM components are not particularly limited and may be selected appropriately depending on the type of steel to be welded, etc. For example, when welding high-tensile steel plates as described above, the composition may contain, in addition to the REM components, 0.10 to 0.20 mass% C, 0.05 to 2.5 mass% Si, 0.25 to 3.5 mass% Mn, 0.05 mass% or less P, 0.02 mass% or less S, 0.005 to 3.00 mass% Al, 0.008 mass% or less O, and 0.008 mass% or less N, with the balance being Fe and unavoidable impurities.

[0031] In addition, the polarity of the welding wire used is preferably negative (positive polarity) from the viewpoint of fully obtaining the effects of fine droplets and stabilization of droplet transfer due to the inclusion of REM.

[0032] In this welding method, it is important to use a welding wire containing the above-mentioned REM, and to perform efficient welding with a heat input appropriate for the groove shape while properly controlling the first-pass welding conditions to obtain the desired joint depth. These welding conditions and joint depth are explained below.

[0033] The angle θ1 of the welding torch tip relative to the horizontal at the reference position: an angle exceeding 45° relative to the horizontal. As shown in Figure 3, a welding torch with a bent portion and a tip defined by the bent portion is used to perform weaving while oscillating the tip of the welding torch toward the groove face of the thick steel material. This allows the wire tip to approach the groove face while avoiding contact between the power supply tip and the groove face of the thick steel material. Furthermore, since the wire tip also faces the groove face, direct melting of the groove face by the arc is possible. Therefore, even when the welding heat input per pass is reduced, the groove face can be sufficiently melted to prevent welding defects. Furthermore, the expansion of the arc heat input range by weaving the welding torch can suppress molten metal dripping and stabilize the bead shape. However, if θ1 is less than 45°, the welding current fluctuates significantly during the reciprocating weaving motion, resulting in unstable welding, poor shielding, and reduced low-temperature toughness of the weld metal. Therefore, θ1 is set to an angle greater than 45°. On the other hand, if θ1 exceeds 90°, the weld metal tends to flow forward of the groove and the angle of the bent portion becomes large, so the preferred range of the angle θ1 of the tip of the welding torch relative to the horizontal direction is preferably 90° or less.

[0034] Oscillation angle θ2 of the welding torch tip from the reference position: 5° to 60°. As described above, by using a welding torch with a bent portion and a tip defined by the bent portion, weaving can be performed while oscillating the welding torch tip toward the groove face of the thick steel material. This allows the wire tip to approach the groove face while avoiding contact between the power feed tip and the groove face of the thick steel material. Furthermore, since the wire tip also faces the groove face, direct melting of the groove face by the arc is possible. Therefore, even when the welding heat input per pass is reduced, the groove face can be sufficiently melted to prevent welding defects. Furthermore, the expansion of the arc heat input range by weaving the welding torch can also prevent molten metal dripping and stabilize the bead shape. However, when θ2 is less than 5°, the above effect is not fully achieved, resulting in welding defects and molten metal dripping. On the other hand, when θ2 exceeds 60°, the groove face melts excessively, resulting in welding defects due to undercutting of the groove face. For this reason, the swing angle θ2 of the tip of the welding torch from the reference position is set to 5° or more and 60° or less, preferably 10° or more and 45° or less.

[0035] The bending angle θ3 of the bent portion of the welding torch and the length l of the tip of the welding torch are not particularly limited, but from the viewpoint of controlling θ1 and θ2 within the above ranges, it is preferable that θ3 be in the range of 10 to 45° and l be in the range of 10 to 50 mm.

[0036] First-pass welding joint depth D: 10 mm or more. To weld thick steel materials, especially thick steel materials with a plate thickness of 40 mm or more, into a predetermined groove shape, the first-pass welding joint depth must be 10 mm or more. Furthermore, if the first-pass welding joint depth is less than 10 mm, welding heat will concentrate, causing molten metal dripping. Therefore, the first-pass welding joint depth must be 10 mm or more. Preferably, it is 25 mm or more. The upper limit of the first-pass welding joint depth is the same as the upper limit of the steel plate thickness, i.e., approximately 100 mm. However, when performing multi-pass welding, especially when the steel plate thickness is 80 mm or more, if the first-pass welding joint depth exceeds 70 mm, excessive welding heat input is likely to occur, and welding defects such as hot cracking, poor fusion at the groove face due to heat dispersion during welding, and slag entrapment may occur. Therefore, when performing multi-layer welding, the joint depth in the first layer welding is preferably 10 mm or more, more preferably 20 mm or more, and even more preferably 25 mm or more. Also, the joint depth in the first layer welding is preferably 70 mm or less, more preferably 60 mm or less, and even more preferably 55 mm or less. In the case of single-layer welding, the joint depth D is more preferably 15 mm or more and 65 mm or less.

[0037] The basic conditions have been explained above, but in the welding method of the present invention, it is preferable that the following conditions be further satisfied.

[0038] Weaving depth L in the plate thickness direction during welding torch weaving: 10 mm to 70 mm. This welding method involves weaving a welding torch. It is also important to properly control the weaving depth L in the plate thickness direction and the maximum weaving width M in the plate thickness direction and the direction perpendicular to the weld line, as described below. The weaving depth L in the plate thickness direction and the maximum weaving width M in the plate thickness direction and the direction perpendicular to the weld line for various weaving patterns are shown in Figures 5(a) to 5(d). The weaving depth L and the maximum weaving width M in the plate thickness direction and the direction perpendicular to the weld line, as described below, refer to the weaving depth and maximum weaving width of the welding wire tip, determined without taking into account the oscillation of the welding torch tip, assuming that the welding torch tip is always in the reference position. The weaving pattern referred to here refers to the trajectory of the welding wire tip, assuming that the welding torch tip is always in the reference position, without taking into account the oscillation of the welding torch tip.

[0039] In vertical upward welding, which is the basis of this welding method, the joint depth and the weaving depth in the plate thickness direction are approximately the same. Therefore, if the weaving depth in the plate thickness direction is less than 10 mm, it is difficult to obtain the desired joint depth. On the other hand, if the weaving depth in the plate thickness direction exceeds 70 mm, not only is it difficult to obtain the desired joint depth, but the welding heat input becomes excessive, making it difficult to obtain the desired mechanical properties in the weld metal and the heat-affected zone of the steel. Furthermore, welding defects such as hot cracking, incomplete fusion of the groove face due to heat dispersion during welding, and slag inclusion are likely to occur. Therefore, the weaving depth in the plate thickness direction is preferably 10 mm or more, more preferably 15 mm or more, and preferably 70 mm or less, and more preferably 65 mm or less. In single-layer welding, the weaving depth in the plate thickness direction is more preferably 20 mm or more, and even more preferably 60 mm or less. In multi-layer welding, the weaving depth in the plate thickness direction is more preferably 25 mm or more, and even more preferably 55 mm or less.

[0040] The maximum weaving width M of the welding torch in the plate thickness direction and perpendicular to the weld line: (W-6) mm or more and W mm or less (W: weld bead width in first-layer welding). To prevent unmelted groove surfaces, the maximum weaving width in the plate thickness direction and perpendicular to the weld line is preferably (W-6) mm or more. On the other hand, if the maximum weaving width in the plate thickness direction and perpendicular to the weld line exceeds W mm, molten metal may drip and the weld may not be successful. Therefore, the maximum weaving width in the plate thickness direction and perpendicular to the weld line is preferably in the range of (W-6) mm or more and W mm or less. More preferably, it is (W-4) mm or more, and more preferably, it is (W-1) mm or less. In the case of single-layer welding, W is the groove width on the surface of the steel material that will become the surface (the surface facing the welding device (welding torch)) during welding.

[0041] The weaving pattern of the welding torch is not particularly limited, and can be U-shaped, V-shaped, trapezoidal, triangular, or the like, as seen from the weld line direction (which is the same as the welding direction, usually vertical), as shown in Figures 5(a) to 5(d). For example, when the weaving pattern is U-shaped or trapezoidal, weaving from point A to point B and from point C to point D, as shown in Figures 5(a) and 5(b), corresponds to weaving on the groove face of the thick steel material. In this case, when weaving from point A to point B, the tip of the welding torch is swung toward the groove face of the thick steel material on the left side of the paper, while when weaving from point C to point D, the tip of the welding torch is swung toward the groove face of the thick steel material on the right side of the paper. Note that when weaving from point B to point C (or from point D to point A in the case of a trapezoid), the tip of the welding torch does not need to be swung. In addition, in Figures 5(a) to (d), the trajectory of the welding torch at each point where the direction of the welding torch changes (points B and C in Figure 5(a)) may be angular or rounded.

[0042] However, in vertical upward welding, weaving near the weld surface side is likely to cause molten metal to drip. Furthermore, if the welding torch movement deviates from the groove surface, uniform melting of the groove surface is not achieved, and welding defects such as insufficient fusion are likely to occur. In particular, while typical trapezoidal and triangular weaving patterns that do not require a reversal movement have a small load on the equipment, they are prone to cause molten metal to drip due to welding torch movement near the weld surface side (point D → point A in the trapezoidal weaving pattern in FIG. 5( b ) and point C → point A in the triangular weaving pattern in FIG. 5( d )). Therefore, from the perspective of suppressing molten metal dripping, a U-shaped or V-shaped weaving pattern without torch movement on the weld surface side is preferred. Furthermore, with V-shaped or triangular weaving patterns, if the groove gap is large (e.g., 6 mm or more), the welding torch movement deviates from the groove surface (for example, in the movement from point A to point B in Figure 5(c) , the trajectory of the welding torch tip becomes non-parallel to the groove surface (the side closer to the welding torch)), preventing uniform melting of the groove surface and making welding defects such as insufficient fusion more likely to occur. Therefore, in such cases, a U-shaped weaving pattern is optimal, which makes it easy to move the welding torch parallel to the groove surface.

[0043] The distance a from the back surface of the steel material to the deepest point of the tip of the welding wire during weaving in the plate thickness direction (for example, points B and C in Figures 5(a) and 5(b) and point B in Figures 5(c) and 5(d)) is usually about 2 to 5 mm. When U-shaped weaving or trapezoidal weaving is applied to the groove shape described above, M1, M2, and M3 in Figures 5(a) and 5(b) are about 2 to 18 mm, 0 to 10 mm, and 0 to 10 mm, respectively. Furthermore, the frequency and stop time during weaving (stop time at each point such as point A in Figure 5) are not particularly limited; for example, the frequency may be about 0.25 to 0.5 Hz (preferably 0.4 to 0.5 Hz), and the stop time may be about 0 to 0.5 seconds (preferably 0.2 to 0.3 seconds).

[0044] Other conditions are not particularly specified. However, an average welding current below 270 A results in a small weld pool, and the surface side experiences repeated melting and solidification with each torch weave, similar to multi-layer welding, resulting in poor fusion and slag entrapment. On the other hand, an average welding current above 360 ​​A increases the likelihood of molten (welded) metal dripping, and welding fumes and spatter make it difficult to identify the arc point, making adjustments during welding difficult. For this reason, an average welding current of 270 to 360 A is preferred. Furthermore, an average welding current of 270 to 360 A minimizes welding fumes and spatter while achieving stable penetration, further enhancing the practicality of this welding method. Other conditions can be determined by standard methods, such as a welding voltage of 28 to 37 V (increasing with current), a welding speed (upward): 1 to 15 cm / min (preferably 4 to 9 cm / min), a wire extension length of 20 to 45 mm, and a wire diameter of approximately 1.2 to 1.6 mm. Furthermore, the composition of the shielding gas is not particularly limited, and CO2 gas alone or a mixed gas of Ar and CO2 may be used.

[0045] In addition, when multi-layer welding is performed, the number of layers to be stacked until welding is complete is preferably about 2 to 4 layers from the viewpoint of preventing stacking faults. The welding conditions for each layer other than the first layer are not particularly limited and may be in accordance with established methods, for example, the same as the welding conditions for the first layer described above. Note that the welding method of the present invention is based on one pass of welding per layer.

[0046] Two steel sheets with the groove shape shown in Table 1 were subjected to narrow-gap vertical upward gas-shielded arc welding under the welding conditions shown in Table 2 using a welding torch (θ3: 15°, l: 20 mm) with a bent power feed tip as shown in Figure 3. The steel sheets used contained 0.04-0.06 mass% C, 0.1-0.2 mass% Si, 1.8-2.0 mass% Mn, 0.01 mass% or less P, 0.005 mass% or less S, 0.02-0.06 mass% Al, 0.003 mass% or less O, and 0.005 mass% or less N, with the remainder being Fe and unavoidable impurities. Gas cutting was used to prepare the grooves, and no grinding or other preparation was performed on the groove faces. The welding wire used was a 1.2 mm diameter solid wire for high-strength steel or one grade higher. The chemical composition of all welding wires other than the REM listed in Table 2 was 0.10-0.20% by mass of C, 0.6-0.8% by mass of Si, 1.8-2.0% by mass of Mn, 0.01% by mass or less of P, 0.005% by mass or less of S, 0.005-0.03% by mass of Al, 0.003% by mass or less of O, and 0.005% by mass or less of N, with the remainder being Fe and unavoidable impurities. Furthermore, the welding current was 260-330 A, the welding voltage was 28-46 V (increasing with current), the average welding speed was 0.7-42.7 cm / min (adjusted during welding), the average wire extension was 30 mm, and the weld length was 400 mm. In all cases, CO2 was used as the shielding gas, and welding was performed using a gas shielding system separate from the conventional arc welding nozzle.

[0047] Note that Nos. 8 to 15 and 17 were multi-layer welded, and for the welding of each layer except the first layer, the welding current was set to 270 to 360 A and the welding voltage was set to 28 to 37 V, and gas shielded arc welding with weaving was used to finish the welded joints. Additionally, Nos. 1 to 7 and 16 were welded in one layer to finish the welded joints.

[0048] After the first layer welding, the bead width and joint depth were measured at five randomly selected points. The maximum value of the bead width measurement was taken as the bead width W in the first layer welding, and the minimum value of the joint depth measurement was taken as the joint depth D in the first layer welding.

[0049] In addition, dripping of molten metal during the first layer welding was visually evaluated as follows: ⊚: no dripping of molten metal; ◯: dripping of molten metal in less than three places; ×: dripping of molten metal in three places or more, or welding interrupted

[0050] Furthermore, ultrasonic flaw detection was carried out on the finally obtained welded joints, and the results were evaluated as follows: ◎: No defects were detected ○: Only acceptable defects with a defect length of 3 mm or less were detected ×: Defects with a defect length of more than 3 mm were detected

[0051] In addition, the finally obtained welded joints were subjected to a Charpy impact test in accordance with JIS Z 2242 (test temperature: -40°C) with the notch located at the center of the weld metal, and the absorbed energy vE-40 (J) at the test temperature was measured and the low-temperature toughness of the weld metal was evaluated according to the following criteria: ◎: vE-40 (J) is 47J or more ○: vE-40 (J) is less than 47J and 27J or more ×: vE-40 (J) is less than 27JThe results are also shown in Table 2.

[0052]

[0053]

[0054] As shown in Table 2, in Examples 1 and 4 to 12, there was no dripping of molten metal in the first layer welding, or if there was, it was in two or fewer locations. Furthermore, ultrasonic testing did not detect any defects. Furthermore, all of these Examples achieved excellent low-temperature toughness in the weld metal. On the other hand, in Comparative Examples 13 to 17, in which the groove gap was greater than 20 mm, dripping of molten metal was observed in three or more locations in the first layer welding, or welding could not be continued due to excessive dripping. Furthermore, in Comparative Examples 2 and 3, in which the angle θ1 did not exceed 45°, defects longer than 3 mm were detected in ultrasonic testing, and the molten metal did not achieve sufficient low-temperature toughness.

[0055] 1: Thick steel material 2: Groove surface of thick steel material 3: Groove of lower steel material 4: Welding torch 5: Welding wire 6: Backing material 7: Main body 8: Power supply tip 9: Bent portion 10: Tip 11: Weld bead

Claims

1. A vertical narrow gap gas-shielded arc welding method for joining two thick steel sheets of 10 mm or more in thickness by single-layer or multi-layer welding using weaving, with a groove angle of 20° or less and a groove gap of 20 mm or less, wherein a welding wire containing 0.015 to 0.100 mass% REM is used, and weaving of the first layer is performed with a welding torch having a bent portion and a tip portion defined by the bent portion, and during weaving of the groove face of the thick steel, the tip of the welding torch is swung toward the groove face of the thick steel, and the position where the tip of the welding torch is aligned with the weld line direction as viewed from the thickness direction of the thick steel is set as a reference position, and the angle θ1 of the tip of the welding torch with respect to the horizontal at the reference position is set to an angle exceeding 45°, and the swing angle θ2 of the tip of the welding torch from the reference position is set to be 5° or more and 60° or less, A vertical narrow gap gas-shielded arc welding method, in which the joining depth in the first layer welding is 10 mm or more.

2. A vertical narrow gap gas-shielded arc welding method according to claim 1, wherein the joining is a single-layer weld and the groove gap is 25% or less of the plate thickness of the thick steel material.

3. A vertical narrow gap gas-shielded arc welding method according to claim 1, wherein the joining is multi-layer welding and the joining depth in the first layer welding is 10 mm or more and 70 mm or less.

4. A vertical narrow gap gas-shielded arc welding method according to any one of claims 1 to 3, wherein the weaving pattern of the welding torch when viewed from the direction of the weld line in the weaving of the first layer weld is U-shaped.

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