Narrow gap gas shielded arc welding method

Abstract

A narrow gap gas shielded arc welding method capable of ensuring sufficiently high strength of the weld metal with high welding efficiency. 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 gap having a groove angle θ of 25° or less and a bottom groove gap G of 7 mm to 18 mm, wherein the multi-pass welding is performed by multi-electrode welding with three or more electrodes, the average value of the sum of the carbon equivalents (Ceq) of each welding wire defined by the following formula (1) divided by the number of each electrode is 0.400 mass% or more, the ratio of the maximum heat input to the minimum heat input of each electrode is 1.40 or less, and the sum of the heat inputs of each electrode is 10 kJ / cm to 60 kJ / cm. Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 ‥‥(1)

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 that can be applied to steel plates having a thickness of 22 mm or more and can ensure a tensile strength of the weld metal of 550 MPa or more. Here, "narrow groove" means that the groove angle is 25° or less and the width of the minimum groove gap between the steel plates to be welded (base metal) is 50% or less of the plate thickness of the steel plates. [Background technology]

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

[0003] In recent years, as steel structures have become larger and thicker, the amount of deposition in welding during the manufacturing process, particularly in butt welding of steel plates, has increased, and further welding work requires a lot of time, resulting in increased construction costs.

[0004] One possible method to improve this is the application of narrow-gap gas-shielded arc welding, which uses arc welding to multi-layer weld a gap that is small relative to the plate thickness. This narrow-gap gas-shielded arc welding reduces the amount of deposition compared to normal gas-shielded arc welding, making it possible to achieve high welding efficiency and energy savings, and is expected to reduce construction costs.

[0005] Recently, a technology has been proposed for performing this narrow-gap gas-shielded arc welding method using multiple electrodes. Multi-electrode welding can increase the amount of weld metal that fills the gap in one weld, making it possible to obtain higher welding efficiency than single-electrode welding, and is an effective means of increasing the efficiency of welding.

[0006] For example, Patent Document 1 discloses a narrow-gap gas-shielded arc welding method for joining steel plates by narrow-gap multi-layer welding. In Patent Document 1, the first layer welding is performed by multi-electrode welding with two or more electrodes. The first electrode and the second electrode are positioned along a predetermined parallel welding line, and the distance between the welding wire tips of the first electrode and the second electrode is controlled to a range of 5 mm to 16 mm. The angle of the straight line connecting the welding wire tips of the first electrode and the second electrode with respect to the perpendicular direction of the welding line is controlled to a range of 45° or less. It is described that these methods provide the effect of preventing the occurrence of welding defects and improving welding work efficiency even when groove processing such as gas cutting or plasma cutting is performed. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6137053 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the technique described in Patent Document 1 was unable to ensure sufficient tensile strength when high-strength steel was used as the base material, particularly when a high tensile strength of 550 MPa or more was required in the welded portion.

[0009] In view of the above-mentioned conventional techniques, an object of the present invention is to provide a narrow-gap gas-shielded arc welding method that can ensure sufficiently high strength of the weld metal with high welding efficiency. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have repeatedly studied a welding method for improving the strength of the weld metal. As a result, they have found that it is important to set the average value of the carbon equivalent of the welding wire divided by the number of wires to 0.400 mass% or more, the ratio of the maximum and minimum heat inputs of each electrode to 1.40 or less, and the total heat input to 10 kJ / cm to 60 kJ / cm.

[0011] The present invention was made based on this finding and further studies, and has the following gist and configuration. [1] A narrow-gap gas-shielded arc welding method for joining steel plates with a plate 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 multi-layer welding is performed by multi-electrode welding with three or more electrodes, the average value obtained by dividing the sum of the carbon equivalents (Ceq) of each welding wire defined by the following formula (1) by the number of each electrode is 0.400 mass% or more, the ratio of the maximum heat input to the minimum heat input of each electrode is 1.40 or less, and the sum of the heat inputs of each electrode is 10 kJ / cm to 60 kJ / cm. Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 ‥‥(1) Here, [element] indicates the element content (mass%) in the welding wire, and 0 is substituted if no element is added. [2] In the narrow gap gas-shielded arc welding method according to [1] above, one of the first electrode and the second electrode is a negative wire (positive polarity) and the other is a positive wire (reverse polarity), the distance a between the tips of the welding wires supplied from the power feed tips at the tip of the welding torch of each of the first electrode and the second electrode is 5 mm to 16 mm, the angle α of a straight line connecting the tips of the welding wires of the first electrode and the second electrode with respect to the groove width direction is 60° or less, the diameter of the welding wire used for each electrode is in the range of 1.0 mm to 1.6 mm, and the distance between adjacent electrodes after the second electrode is 10 mm to 100 mm. [3] The narrow gap gas-shielded arc welding method according to [2] above, in which a third electrode and subsequent electrodes are positioned in the center of the groove behind the first electrode and the second electrode. [4] The narrow gap gas-shielded arc welding method according to any one of the above [1] to [3], wherein a mixed gas containing 20 volume % or more of CO2 gas is used as the shielding gas. [5] In any one of the narrow gap gas-shielded arc welding methods [1] to [4] above, at least one of the welding wires used for the positive electrode contains 0.005% by mass to 0.060% by mass of REM (rare earth element). Effect of the Invention

[0012] According to the present invention, in narrow-gap multi-layer welding with high welding efficiency, even when the weld metal of the joint is required to have a tensile strength of more than 550 MPa, it is possible to fully cope with the demand. Furthermore, the narrow-gap gas-shielded arc-welded joint obtained by this process has a significantly reduced manufacturing cost compared to conventional welded joints, and is therefore extremely useful for application to general structures such as buildings, bridges, and ships. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is an explanatory diagram that shows a schematic diagram of a groove shape used in the present invention. [Diagram 2] FIG. 2 is an explanatory diagram showing an example of a procedure for performing welding using four electrodes in the present invention, where (a) is a front view and (b) is a top view. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present invention will be specifically described below.

[0015] The present invention is a narrow-gap gas-shielded arc welding method for joining steel plates with a plate 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 of 7 mm to 18 mm. Note that the "steel plate" referred to here includes thick steel materials, thick plate steel materials, etc.

[0016] [Steel plate thickness: 22mm or more] The thickness t (see FIG. 1) 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 by the present invention by increasing the groove angle and reducing the groove gap in a conventional R-shaped groove, and the R-shaped groove results in a highly efficient welding with a smaller amount of deposition. Preferably, the thickness t is 30 mm or more.

[0017] Furthermore, since the plate thickness of steel structures, including special structures, is often 200 mm or less, in the present invention, it is preferable that the upper limit of the plate thickness t of the steel plate is set to 200 mm.

[0018] The present invention can be applied to various types of thick steel plates, from mild steel plates to 780 MPa-class high-tensile steel plates. It is also possible to weld 590 MPa-class high-tensile thick steel plates without preheating.

[0019] [Bevel angle θ: 25° or less] The shape of the groove used in the present invention is a narrow groove shape of a V-shaped groove or an I-shaped groove (including θ=0°) having a gap at the bottom. An example of a V-shaped groove having a gap at the bottom is shown in FIG. 1. Here, 1 is a steel plate (base material), 2 is a groove surface, and the smaller the groove cross-sectional area of ​​the groove part of the steel plate 1, the more efficient welding is possible, but defects such as poor fusion are likely to occur. In addition, when the groove angle θ of the groove part exceeds 25°, it can be carried out even with a conventional construction method. For this reason, the present invention targets a case where the groove angle θ is 25° or less, which is difficult to carry out with the 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 deposition, this 0° case is the most efficient. 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. When the plate thickness t exceeds 100 mm, the upper limit of the suitable range is preferably 10°.The groove angle θ is more preferably 5° to 10°.

[0020] [Bottom Groove Gap G: 7mm~18mm] The smaller the groove of the steel plate, the faster and more efficient the welding can be performed, but defects such as incomplete fusion are more likely to occur. In addition, welding in which the bottom groove gap G (FIG. 1), which is the gap (groove width) between the bottoms of the groove before welding begins, exceeds 18 mm can be performed with the conventional construction method. For this reason, the present invention targets a bottom groove gap G of 18 mm or less, which is expected to be even more efficient than the conventional construction method. On the other hand, if the bottom groove gap G is less than 7 mm, welding with three or more electrodes may be difficult. Therefore, the bottom groove gap G is in the range of 7 mm to 18 mm. Preferably, it is in the range of 8 mm to 12 mm. In addition, it is preferable to use a backing material 7 during welding (see FIG. 2(a)).

[0021] [Multi-layer welding with three or more electrodes] In the present invention, the first layer and each layer after the second layer are joined by multi-pass welding using three or more electrodes using the narrow groove described above. Although it is permissible to form each layer by multiple passes of welding, it is preferable to form each layer by one pass of welding from the viewpoint of welding construction efficiency.

[0022] The reason why the number of electrodes is set to three or more is because it is difficult to improve the welding efficiency with two or less electrodes. The number of electrodes is preferably three to five. More than five electrodes is undesirable because it is easy for non-uniformity in the components of the weld metal to occur.

[0023] An example of the procedure for welding with four electrodes in the present invention is shown in Fig. 2. In Fig. 2, 3, 4, 5, and 6 are the first, second, third, and fourth electrodes, 3a, 4a, 5a, and 6a are the power feed tips of the electrodes 3, 4, 5, and 6, 3b, 4b, 5b, and 6b are the welding wires supplied to the electrodes 3, 4, 5, and 6, and 7 is a backing material. In addition, 8 is the welding line (the line along which the tip of the welding wire is placed), 9 is the weld bead of the first layer, 10 is the molten pool (second layer), and 11 is the weld bead (second layer) during welding. In the example of Fig. 2, the protruding parts of the welding wires 3b and 4b from the power feed tips 3a and 4a are curved, but the present invention is not limited to this, and the protruding parts may be straight.

[0024] [Average value obtained by dividing the sum of the carbon equivalents (Ceq) of each welding wire by the number of electrodes: 0.400 mass% or more] The carbon equivalent (Ceq) is defined by the following formula (1) and is an index that greatly affects the tensile strength of the weld metal. Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 ‥‥(1) Here, [element] indicates the element content (mass%) in the welding wire, and 0 is substituted if no element is added.

[0025] The average value obtained by dividing the sum of the carbon equivalents (Ceq) of the respective welding wires by the number of electrodes is hereinafter also referred to as the “average Ceq.” The average Ceq is defined by the following formula (2). Average Ceq = (Ceq #1 〕+〔Ceq #2 〕+〔Ceq #3 〕+…+〔Ceq #n 〕) / n ‥‥(2) Here, Ceq #i ] (i:1 to n) is the Ceq of the welding wire of the i-th electrode, and n is the total number of electrodes.

[0026] The inventors have found through experiments that in order to ensure a tensile strength of 550 MPa or more in the weld metal, the Ceq of the weld metal needs to be 0.320 mass% or more, and that in order to achieve this, the average Ceq of the welding wire used needs to be 0.400 mass% or more. Therefore, the average Ceq is set to 0.400 mass% or more, and preferably 0.420 mass% or more. Note that if the average Ceq is too high, there is a concern that it may have an adverse effect on the toughness of the weld metal, so it is preferable to set the upper limit to 0.520 mass%.

[0027] [Ratio of maximum and minimum heat input of each electrode: 1.40 or less] The amount of metal deposited from each welding wire is affected by the heat input of each electrode. Even if the average Ceq is 0.400 mass% or more, if there is a bias in the heat input between the electrodes, sufficient Ceq may not be imparted to the weld metal. In other words, if the ratio of the maximum heat input to the minimum heat input of each electrode (hereinafter also referred to as the "maximum / minimum heat input ratio") exceeds 1.40, the Ceq of the weld metal may not reach 0.320 mass% or more. Therefore, the maximum / minimum heat input ratio is set to 1.40 or less.

[0028] Here, the heat input is defined by the following equation (3). Heat input (kJ / cm) = {welding current (A) × welding voltage (V) / 1000} × 60 / {welding speed (cm / min)} ‥‥(3)

[0029] The heat inputs of each electrode calculated using this definition formula are compared to select the maximum and minimum heat inputs, and the ratio of the selected maximum and minimum heat inputs is the maximum / minimum heat input ratio. If the heat inputs of each electrode are equal, the maximum / minimum heat input ratio is 1.00.

[0030] By setting the welding current, welding voltage, and welding speed such that the maximum / minimum heat input ratio is 1.40 or less, Ceq is stably 0.320 mass% or more over the entire length of the weld metal, and a tensile strength of 550 MPa or more can be ensured. The maximum / minimum heat input ratio is preferably 1.20 or less.

[0031] [Total heat input of each electrode: 10kJ / cm~60kJ / cm] If the sum of the heat inputs of the electrodes is less than 10 kJ / cm, the strength of the weld metal becomes excessively high, and there is a concern of reduced toughness and low-temperature cracking. On the other hand, if the sum of the heat inputs of the electrodes exceeds 60 kJ / cm, the strength of the weld metal decreases, and it may not be possible to ensure a tensile strength of 550 MPa or more for the weld metal. Therefore, the sum of the heat inputs of the electrodes is set to 10 kJ / cm to 60 kJ / cm. Preferably, it is set to 15 kJ / cm to 45 kJ / cm.

[0032] In the present invention, the above-mentioned welding conditions related to the average Ceq of the welding wire, the maximum / minimum ratio of the heat input, and the sum of the heat inputs of the electrodes are essential. In addition, the following conditions can be mentioned as preferable welding conditions.

[0033] [Electrode polarity] If the first electrode and the second electrode have the same polarity (for example, both the first electrode and the second electrode are wire positive), the mutual arcs are directed inward by the electromagnetic force of attraction, and heat is concentrated in the center of the groove. For this reason, sufficient melting may not be obtained on 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 each electrode generate strong outward electromagnetic forces, and the arcs repel each other. As a result, it is possible to obtain a sufficient melting depth on the groove surface. For the above reasons, in the present invention, it is preferable that one of the first electrode and the second electrode is wire negative (positive polarity) and the other is wire positive (negative polarity). The polarity of the electrodes after the third electrode 5 is not particularly limited, and may be either wire negative (positive polarity) or wire positive (negative polarity).

[0034] [Arrangement of the first and second electrodes] In narrow gap multi-pass welding, when one pass is performed per layer, the welding heat tends to concentrate at the center of the groove with one electrode, which causes insufficient melting at the groove surface of the steel plate, and defects due to insufficient fusion (cold lap) and spatter and slag inclusion on the groove surface are likely to occur. In particular, in the first layer welding, the temperature of the steel plate is low and the melting depth is small, so defects due to insufficient fusion are likely to occur. Therefore, in the present invention, it is preferable to arrange the first electrode 3 and the second electrode 4 at positions along a predetermined parallel welding line 8 as shown in FIG. 2(b).

[0035] [Distance between the tips of the welding wires of the first and second electrodes a: 5mm~16mm] The distance a between the tips of the welding wires 3b, 4b supplied from the power feed tips 3a, 4a at the tip of the welding torch of the first electrode 3 and the second electrode 4 (hereinafter also referred to simply as the "distance between the first and second electrodes"; see FIG. 2(b)) is preferably adjusted to a range of 5 mm to 16 mm. Note that the "distance between the welding wire tips" referred to here refers to the distance between the centers of the welding wire tips of the respective electrodes.

[0036] If the distance a between the first and second electrodes is less than 5 mm, the heat of the arc itself decreases due to the current (electrons) flowing between the electrodes, and sufficient melting of the groove surface 2 may not be achieved. On the other hand, if the distance a between the first and second electrodes is more than 16 mm, the outward electromagnetic force between the electrodes decreases inversely proportional to the distance, and no arc repulsive force is obtained to overcome the inward electromagnetic force generated by the current flowing through the groove surface 2, so that the arcs face inward and heat is concentrated at the center of the groove. As a result, sufficient melting of the groove surface 2 may not be achieved.

[0037] In addition, in narrow gap welding, suppressing welding defects caused by spatter adhesion to the groove surface is an issue. However, the first electrode 3 and the second electrode 4 are arranged along the predetermined parallel welding lines 8, 8, one of which is wire negative (positive polarity) and the other is wire positive (reverse polarity), and the distance a between the first and second electrodes is adjusted to a range of 5 mm to 16 mm. This allows the spatter to be absorbed by the molten metals, suppressing the adhesion of spatter to the groove surface, and a sound weld can be obtained.

[0038] In view of the above, it is preferable to adjust the distance a between the first and second electrodes to within the range of 5 mm to 16 mm.

[0039] [Angle α of the straight line connecting the tips of the welding wires of the first and second electrodes: 60° or less] In the present invention, the repulsion of the arc is utilized to ensure melting of the groove surface. However, if the angle α of the straight line connecting the tips of the welding wires of the first electrode 3 and the second electrode 4 with respect to the groove width direction (perpendicular to the welding line 8) exceeds 60°, a sufficient repulsive force of the arc cannot be obtained, and sufficient melting cannot be obtained at the groove surface. Therefore, the angle α of the straight line connecting the tips of the welding wires of the first electrode and the second electrode (hereinafter simply referred to as the "first-second electrode arrangement angle", see FIG. 2(b)) is preferably 60° or less. More preferably, it is 50° or less. The first-second electrode arrangement angle α may be 0°.

[0040] [Welding wire diameter: 1.0mm~1.6mm] Welding wires for narrow gap gas shielded arc welding are generally manufactured with diameters in the range of 0.6 mm to 2.0 mm, but when welding with the same current, generally, the thinner the wire diameter, the higher the deposition rate due to Joule heat. Therefore, in order to realize highly efficient welding work, it is preferable to select a relatively thin wire diameter. On the other hand, if the wire diameter is too thin, the wire will soften due to Joule heat, making the welding unstable. Therefore, the diameter of the welding wire used in the present invention is preferably in the range of 1.0 mm to 1.6 mm.

[0041] [Distance between adjacent electrodes from the second electrode onwards: 10mm~100mm] The distance between adjacent electrodes after the second electrode 4 is, in the example of FIG. 2(b), the distance b between the tips of the welding wires 4b, 5b supplied from the power feed tips 4a, 5a at the tip of the welding torch of the second electrode 4 and the third electrode 5 (hereinafter, also simply referred to as the "second-third electrode distance"). Also, the distance c between the tips of the welding wires 5b, 6b supplied from the power feed tips 5a, 6a at the tip of the welding torch of the third electrode 5 and the fourth electrode 6 (hereinafter, also simply referred to as the "third-fourth electrode distance"). The distances b and c between adjacent electrodes after the second electrode 4 are preferably in the range of 10 mm to 100 mm. This is because hot cracking resistance is improved by setting them in this range. More preferably, they are in the range of 12 mm to 80 mm.

[0042] [Positioning of the third and subsequent electrodes] If the weld height exceeds the bottom groove gap G in the first layer welding, the risk of hot cracking increases. To avoid this, it is effective to position the third electrode 5 and subsequent electrodes in the center of the groove behind the first electrode 3 and second electrode 4. The reason for this is that the final solidification region changes when the third electrode 5 and subsequent electrodes apply heat to the center of the bead. This also makes it possible to further reduce the number of layers, greatly reducing the risk of stacking defects in multi-layer welding. The reason for this is that it is possible to prevent the weld metal from being biased in the groove. Note that the "center of the groove" here allows a range of ±10% of the bottom groove gap G from the center of the groove.

[0043] [Shielding gas: mixed gas containing 20% ​​or more CO2 gas by volume] Since the amount of oxygen in the weld metal is also greatly affected by the shielding gas composition, it is preferable to use a mixed gas containing 20 volume % or more of CO2 gas and the remainder of an inert gas such as Ar as the shielding gas used in narrow gap gas-shielded arc welding according to the present invention. Furthermore, a single gas of 100 volume % CO2 gas is more preferable. In the present invention, it is preferable to increase the oxygen concentration in the weld metal, which controls the flow of the weld metal, and direct the convection of the weld metal from the center outward to stably deepen the molten depth at the bottom of the steel plate in the groove.

[0044] [REM content of welding wire: 0.005% by mass to 0.060% by mass] REM (rare earth elements) are elements effective in refining inclusions in weld metal and improving toughness during welding. In addition, when welding is performed by supplying a welding wire containing REM to at least one electrode of positive polarity, it is possible to refine droplets and stabilize transfer. This refinement of droplet transfer suppresses the generation of spatter and enables stable gas shielded arc welding even in positive polarity. For these reasons, in the present invention, it is preferable to supply a steel wire containing REM (rare earth elements) to at least one electrode of positive polarity. Note that elements other than REM are included in the appropriate amounts normally contained according to the grade (steel type) of the welding wire, as specified in JIS Z 3312.

[0045] Here, if the REM content is less than 0.005% by mass, it is difficult to achieve the above-mentioned fine droplets and stable transfer. On the other hand, if the REM content exceeds 0.060% by mass, cracks will occur during the wire manufacturing process, making it difficult to manufacture the welding wire. For this reason, the REM content is preferably in the range of 0.005% to 0.060% by mass. More preferably, it is 0.010% to 0.055% by mass.

[0046] [Other welding conditions] In the present invention, the welding conditions other than those described above do not need to be particularly limited and may be in accordance with established standards, for example, a welding voltage of 32 V to 37 V, a welding speed of 30 cm / min to 90 cm / min, and a welding wire extension length of 15 mm to 30 mm.

[0047] In addition, the feed angle φ (hereinafter, also simply referred to as "feed angle φ") of the welding wires 3b and 4b of the first electrode 3 and the second electrode 4 to the bottom groove shown in FIG. 2(a) may be in the range of 0° to 15° with respect to the perpendicular line. The feed angle φ is defined as a positive angle in the direction toward the groove surface. The perpendicular line here refers to a perpendicular line to the bottom groove. The distance d (hereinafter, also simply referred to as "distance d") between the side end of the welding wire tip of the first electrode 3 and the second electrode 4 and the bottom of the groove surface 2 may be in the range of 0.1 mm to 3.0 mm. The radius of curvature of the welding wire supplied from the power feed tips 3a and 4a at the tip of the welding torch of the first electrode 3 and the second electrode 4 (hereinafter, also simply referred to as "wire curvature radius") may be in the range of 150 mm to 500 mm.

[0048] [Welding wire feed 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 the directivity of this 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 2(a).

[0049] When the feed angle φ of the welding wires 3b and 4b supplied from the power feed tips 3a and 4a 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 supplied from the power feed tip 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 after the first layer becomes insufficient, which makes it easy to cause welding defects. For this reason, the feed angle φ of the welding wires 3b and 4b of the first electrode 3 and the second electrode 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°.

[0050] The feed angle φ of the welding wire of first electrode 3 and second electrode 4 with respect to each bottom groove is the same as the inclination of the power feed tips 3a and 4a, particularly the inclination of the tips of the power feed tips. Therefore, the feed angle φ of the welding wire can be controlled by the inclination of the tips of the power feed tips.

[0051] [Distance d between the side end of the welding wire tip of the first electrode and the second electrode and the groove surface 2 of the steel plate: 0.1 mm to 3.0 mm] The distance d (FIG. 2(a)) between the side end of the welding wire tip of the first electrode 3 and the second electrode 4 and the bottom of the groove surface 2 of the steel plate is preferably 0.1 mm to 3.0 mm. If the distance d is less than 0.1 mm, an arc is generated between the upper part of the wire and the groove surface 2, and the groove surface 2 at the bottom of the steel plate may not be melted efficiently. On the other hand, if the distance d exceeds 3.0 mm, the arc may be separated from the groove surface 2, and the groove surface 2 may not be melted efficiently. For this reason, the distance d is preferably 0.1 mm to 3.0 mm. More preferably, it is 0.5 mm to 2.0 mm, and even more preferably, it is 0.5 mm to 1.0 mm. The "side end of the tip of the welding wire" referred to here refers to the side end on the side close to the groove surface 2 of the steel plate to be melted by each electrode.

[0052] [Curve radius of the welding wire fed to the power supply tip: 150mm~500mm] In the present invention, in order to control the feed angle φ of the welding wires 3b and 4b fed from the power feed tips 3a and 4a at the tips of the welding torches of the first electrode 3 and the second electrode 4, a power feed tip with a bent tip is used. At this time, the welding wire passes through the power feed tip with a bent tip, and in order to pass through it more smoothly, it is preferable to bend the welding wire in advance using a so-called three-point roller or the like. At this time, if the curvature radius of the welding wire is less than 150 mm, the wire feed resistance increases, the welding wire cannot be fed stably, and it becomes difficult to maintain the arc. On the other hand, if the curvature radius of the welding wire exceeds 500 mm, there is no effect in reducing the wire feed resistance when the power feed tip tip is bent, so the welding wire cannot be fed stably, and it becomes difficult to maintain the arc. From the above, it is preferable that the curvature radius of the welding wires 3b and 4b fed to the power feed tips 3a and 4a of the first electrode 3 and the second electrode 4 is 150 mm to 500 mm. More preferably, it is 175 mm to 475 mm. EXAMPLES

[0053] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0054] A base material (tensile strength 550 MPa or more) was prepared by gas cutting a steel plate with the groove shape shown in Table 1. Multi-layer narrow-gap gas-shielded arc welding was performed with one pass per layer under the welding conditions shown in Tables 1 to 3 to produce welded joints. In all cases, the third and subsequent electrodes were positioned in the center of the groove behind the first and second electrodes. The feed angle φ was 8°. The wire curvature radius was 300 mm.

[0055] The welded joints were inspected for the presence or absence of welding defects (hot cracks, incomplete fusion) after etching the welded cross sections with nital. No welding defects were found in any of the welded joints.

[0056] The weld metal of the welded joint was chemically analyzed, and Ceq was found to be in the range of 0.320 mass % or more in the inventive examples and less than 0.320 mass % in the comparative examples.

[0057] Round bar tensile test pieces were taken from the weld metal of the welded joint in accordance with the provisions of JIS Z 3111, and the tensile strength of the weld metal was evaluated. The evaluation results are shown in Table 4. As shown in Table 4, the tensile strength of the weld metal reached 550 MPa or more in all of the inventive examples. In contrast, the tensile strength of the weld metal fell below 550 MPa in Comparative Example No. 8, in which the average Ceq was less than 0.400 mass%, Comparative Example No. 9, in which the total heat input exceeded 60 kJ / cm, and Comparative Example No. 10, in which the maximum / minimum heat input ratio exceeded 1.40.

[0058] [Table 1]

[0059] [Table 2]

[0060] [Table 3]

[0061] [Table 4] [Explanation of symbols]

[0062] 1 steel plate 2 Bevel surface 3 1st electrode 4 Second electrode 5 Third electrode 6 4th electrode 3a, 4a, 5a, 6a Power supply chip 3b, 4b, 5b, 6b welding wire 7 Backing material 8 Welding seam 9 First layer weld bead 10 Molten pool (second layer) 11 Weld bead during welding (second layer)

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 of 7 mm to 18 mm, The multi-layer welding is performed by multi-electrode welding using three or more electrodes. The average value of the sum of the carbon equivalents (Ceq) of the respective welding wires defined by the following formula (1) divided by the number of the respective electrodes is set to 0.400 mass% or more, The ratio of the maximum heat input to the minimum heat input of each electrode is set to 1.40 or less, and the sum of the heat inputs of each electrode is set to 10 kJ / cm to 60 kJ / cm. Narrow gap gas shielded arc welding method. Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (1) Here, [element] indicates the content (mass%) of the element in the welding wire, and 0 is substituted when no element is added.

2. One of the first electrode and the second electrode is a wire negative (positive polarity), and the other is a wire positive (reverse polarity), a 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 electrode and the second electrode is set to 5 mm to 16 mm; The angle α of a straight line connecting the welding wire tips of the first electrode and the second electrode with respect to the groove width direction is set to 60° or less, The diameter of the welding wire used for each electrode is in the range of 1.0 mm to 1.6 mm; The distance between adjacent electrodes after the second electrode is set to 10 mm to 100 mm. The narrow gap gas-shielded arc welding method according to claim 1.

3. 3. The narrow gap gas-shielded arc welding method according to claim 2, wherein a third electrode and subsequent electrodes are disposed at the center of the groove behind the first electrode and the second electrode.

4. Shielding gas containing 20% ​​or more CO by volume ２ The narrow gap gas-shielded arc welding method according to any one of claims 1 to 3, wherein a mixed gas containing a gas is used.

5. The narrow gap gas-shielded arc welding method according to any one of claims 1 to 3, wherein at least one of the welding wires used for the positive electrode contains 0.005% by mass to 0.060% by mass of REM (rare earth element).

6. The narrow gap gas-shielded arc welding method according to claim 4, wherein at least one of the welding wires used for the positive electrode contains 0.005% by mass to 0.060% by mass of REM (rare earth element).

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