JP7593427B2 - Narrow gap gas shielded arc welding method for thick steel plate - Google Patents

Description

The present invention relates to a gas-shielded arc welding method, and in particular to a narrow-gap gas-shielded arc welding method for thick steel plates. Here, "narrow gap" means that the groove angle is 25° or less, and the minimum groove width between the thick steel plates to be welded is 50% or less of the plate thickness of the steel.

Gas-shielded arc welding, which is used to weld steel materials, 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 materials, has increased, and welding work requires a lot of time, leading to increased construction costs. Therefore, the application of narrow-gap gas-shielded arc welding, which uses gas-shielded arc welding to multi-layer welds in gaps that are small compared to the plate thickness, has been considered. Narrow-gap gas-shielded arc welding reduces the amount of deposition compared to normal gas-shielded arc welding, making welding more efficient and energy-saving, and is expected to reduce construction costs.

More recently, a technology has been proposed for performing this narrow-gap gas-shielded arc welding method using multiple electrodes. With multi-electrode welding, the amount of deposited metal (metal that adheres to the inside of the gap when the welding wire melts) used to fill the gap can be increased by the number of electrodes, making it possible to obtain higher welding efficiency than with single-electrode welding, and is an effective means of increasing the efficiency of welding.

For example, Patent Document 1 describes a narrow-gap gas-shielded arc welding method for joining thick steel materials by multi-layer welding of narrow gaps. In the technology described in Patent Document 1, the first layer welding is performed by multi-electrode welding with two or more electrodes, one of the first and second electrodes is positive polarity and the other is reverse polarity, and the first and second electrodes are positioned along a predetermined parallel weld line. Furthermore, the distance between the tips of the welding wires of the first and second electrodes is set to a range of 5 mm to 16 mm, the angle of the straight line connecting the tips of the welding wires of the first and second electrodes with respect to the perpendicular direction of the weld line is set to a range of 45° or less, and the melting depth at the bottom of the thick steel material in the perpendicular direction to the weld line is set to 1.5 mm or more. As a result, even when groove processing such as gas cutting or plasma cutting is performed, there is no occurrence of defects and the effect of improving welding construction efficiency can be obtained.

Patent No. 6137053

However, 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. With the technology described in Patent Document 1, a large amount of spatter is generated from the electrode using positive polarity, and the spatter may adhere to the inside of the groove or the welding torch, causing welding defects. Furthermore, it has been found that when multi-electrode welding with three or more electrodes is performed with the technology described in Patent Document 1, high-temperature cracks may occur in the center of the weld metal.

In view of the above-mentioned conventional techniques, the present invention aims to provide a narrow gap gas-shielded arc welding method for thick steel plates that can prevent the occurrence of welding defects and achieve high welding efficiency.

In order to achieve the above-mentioned object, the inventors have intensively studied factors that affect the arc stability of electrodes that adopt positive polarity. As a result, the multi-layer welding is performed as a multi-electrode welding with two or more electrodes, the first and second electrodes are arranged along a predetermined parallel welding line, and one of the first and second electrodes is positive polarity and the other is reverse polarity. Furthermore, the relative positions of the first and second electrodes during welding are appropriately adjusted, and the welding wire supplied to the electrode that adopts positive polarity is a steel wire containing 0.015 to 0.100 mass% REM (rare earth element). As a result, the arc is stabilized, spatter generated during welding can be significantly reduced, and it has been found that multi-layer welding of thick steel plates is possible with high welding efficiency while ensuring a melting depth at the bottom of the groove (thick steel plate) of 1.5 mm or more. Furthermore, the causes of hot cracking in multi-electrode welding with three or more electrodes have been intensively studied. As a result, they found that if the electrode distance from the second electrode onwards is outside the appropriate range, a welding defect called hot cracking occurs in the centre of the weld metal. They also found that if the electrode distance from the second electrode onwards is adjusted to within the range of 10 to 100 mm, a welded joint with a sound bead (weld metal) can be obtained.

The present invention has been completed based on these findings and through further investigation.
[1] A method for narrow-gap gas-shielded arc welding of thick steel plates in which a thick steel plate having a plate thickness t of 22 mm or more is joined 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 to 18 mm,
The multi-layer welding is performed by multi-electrode welding using two or more electrodes,
Among the multiple electrodes, a first electrode and a second electrode are disposed at positions along a predetermined parallel welding line, and one of the first electrode and the second electrode is a wire negative (positive polarity) and the other is a wire positive (reverse polarity);
Furthermore, the distance a between the tips of the welding wires supplied from the power feed tips at the tips of the welding torches of the first electrode and the second electrode is set in the range of 5 mm to 16 mm, and 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 a direction perpendicular to the welding line is set in the range of 60° or less,
The welding wire to be supplied to the electrode having a positive polarity is a steel wire containing 0.015 to 0.100 mass% of REM (rare earth element),
A method for narrow gap gas-shielded arc welding of thick steel plates, characterized in that the fusion depth p in the direction perpendicular to the weld line at the bottom of the thick steel plate is 1.5 mm or more.
[2] The narrow gap gas-shielded arc welding method for thick steel plate described in [1], characterized in that the feed angle φ of the welding wire of the first electrode and the second electrode to the bottom groove is 0° or more and 15° or less with respect to the perpendicular line.
[3] A method for narrow gap gas-shielded arc welding of thick steel plate according to [1] or [2], characterized in that the distance d between the side end of the welding wire tip of the first electrode and the second electrode and the groove surface of the thick steel plate is 0.5 mm or more and 3.0 mm or less, respectively.
[4] A narrow gap gas-shielded arc welding method for thick steel plate according to any one of [1] to [3], characterized in that the welding wire fed to the power feed tips of the first electrode and the second electrode is a welding wire curved with a radius of curvature in the range of 150 mm or more and 300 mm or less.
[5] A narrow gap gas-shielded arc welding method for thick steel plates as described in any one of [1] to [4], characterized in that a third electrode and subsequent electrodes are positioned in the center of the groove behind the first electrode and the second electrode, and the inter-electrode distance from the second electrode and subsequent electrodes is in the range of 10 mm to 100 mm.
[6] A method for narrow gap gas-shielded arc welding of thick steel plates according to any one of [1] to [5], characterized in that a mixed gas containing 60 volume % or more of _CO2 gas is used as the shielding gas in the gas-shielded arc welding.

According to the present invention, when multi-layer welding thick steel plates with narrow-gap gas-shielded arc welding is performed, defects such as incomplete fusion can be prevented without the need to prepare the groove surface, even when low-cost groove preparation such as gas cutting is performed. Furthermore, the amount of spatter generated during welding can be reduced, the amount of welding defects can be reduced, the welding efficiency is improved, and the welding cost can be significantly reduced compared to the manufacturing of conventional welded joints, providing a significant industrial effect. In addition, the present invention is also extremely useful for welding general structures such as buildings, bridges, and ships.

FIG. 2 is an explanatory diagram illustrating a groove shape used in the present invention. 1A and 1B are schematic diagrams showing a procedure for welding using four electrodes, in which (a) is a front view and (b) is a top view. FIG. 2 is an explanatory diagram showing an example of a cross section of a welded joint after multi-layer welding with four electrodes.

The present invention will be described in detail below.

This invention is a narrow-gap gas-shielded arc welding method for joining thick steel plates with a plate thickness t of 22 mm or more by multi-layer welding with a narrow gap, a groove angle θ of 25° or less, and a bottom groove gap G of 7 to 18 mm. Note that "thick steel plate" here includes not only thick steel plate, but also thick material, thick plate steel material, etc.

The groove used in this invention is a V-groove (including an I-groove (θ: 0°)), and the groove shape is shown in Figure 1. 1 is the thick steel plate, and 2 is the groove surface.

[Thickness of steel plate 1: 22mm or more]
The thickness t of the thick steel plate 1 is 22 mm or more, and preferably 200 mm or less. When the thickness t of the thick steel plate 1 is less than 22 mm, in a conventional R-shaped groove, by increasing the groove angle and reducing the groove gap, the groove cross-sectional area becomes smaller than that of the groove targeted by the present invention, and the R-shaped groove may result in highly efficient welding with a smaller amount of deposition.

In addition, since the plate thickness of steel structures, including special structures, is 200 mm or less, in the present invention, it is preferable to set the upper limit of the plate thickness of the thick steel plate to 200 mm. Furthermore, the present invention can be applied to various types of thick steel plate, from mild steel plate to 780 MPa class high tensile steel plate. Welding of 590 MPa class high tensile thick steel plate is also possible without preheating.

[Bevel angle θ: 25° or less]
The groove used in the present invention is a V-groove including an I-groove.

The smaller the groove (groove angle θ), the more efficient the welding. However, if the groove angle θ of the groove is small, defects such as insufficient fusion are likely to occur. For a V-groove, a groove angle θ of 0°, i.e., an I-groove, is the most efficient in terms of deposition volume, but the groove closes during welding due to welding heat distortion. For this reason, it is preferable to set the groove angle θ according to the plate thickness t, taking into account welding heat distortion.

Specifically, the groove angle θ is preferably in the range of (0.5×t/20) to (2.0×t/20)°, and more preferably in the range of (0.8×t/20) to (1.2×t/20)°. Note that when the plate thickness t exceeds 100 mm, the upper limit of the preferred range is 10°.

On the other hand, welding with a groove angle θ exceeding 25° can be performed using conventional construction methods, so in this invention, the upper limit of the groove angle θ is set to 25°.

[Bottom groove gap G: 7 to 18 mm]
If the bottom groove gap G is less than 7 mm, it is difficult to perform welding with multiple electrodes (two or more electrodes). On the other hand, welding with a bottom groove gap G of more than 18 mm can be performed using conventional welding methods. For this reason, in the present invention, the range of the bottom groove gap G is set to 7 mm or more and 18 mm or less, which is a range in which welding is difficult using conventional welding methods and in which further high efficiency is expected. The range is preferably 8 mm or more and 12 mm or less.

In the present invention, the narrow groove described above is used to weld and join the first layer and each layer thereafter by multi-layer welding using two or more electrodes.

In the present invention, the groove preparation of thick steel plates is performed by gas cutting, plasma cutting, laser cutting, etc. However, mechanical processing is not excluded. The fusion depth required for the groove surface in narrow-gap gas-shielded arc welding is mainly determined by the surface properties of the groove surface (particularly the recess depth and cleanliness). In the most common groove preparation by gas cutting, except for special steels and stainless steels, the finish of the cut surface varies greatly depending on the gas flow rate and the nozzle selected during gas cutting. For example, when the gas flow rate and the nozzle are well adjusted, the recess depth on the groove surface is about 0.2 mm or less, but in special cases, such as when the flame flow speed is lower than usual due to wear of the nozzle, a recess depth of more than 1 mm may occur. However, even if such a recess occurs, it will be used for welding as is without any maintenance in general structures, etc.

Therefore, to effectively prevent defects due to hot cracks and insufficient fusion, it is necessary to melt the groove surface more deeply during welding, especially the bottom of the groove (thick steel plate), where the temperature during welding is low and the melting depth tends to be small. In addition, since the cut surface is covered with a thick oxide film generated by the processing heat, it is also necessary to melt the groove surface more deeply during welding.

For this reason, in the present invention, the fusion depth at the bottom of the groove (thick steel plate) is limited to 1.5 mm or more. It is preferably 2.0 mm or more. However, if the fusion depth exceeds 4 mm, an undercut will occur at the top of the weld bead on the groove face, which can cause welding defects, so it is preferable that the fusion depth be 4 mm or less.

In the present invention, the narrow groove described above is used to perform multi-layer welding using two or more electrodes for the first layer and each layer after the second layer, and a welded joint is obtained by welding and joining with high welding efficiency while preventing defects such as poor fusion and welding defects. In addition, in the present invention, the narrow groove described above is used and the welding conditions are appropriately adjusted so that the fusion depth p at the bottom of the groove (thick steel plate) is 1.5 mm or more.

[Multi-layer welding is performed by multi-electrode welding using two or more electrodes, and the first electrode and the second electrode are arranged at positions along a predetermined parallel weld line]
In the present invention, from the viewpoint of improving the efficiency of welding, multi-layer welding is performed using two or more electrodes, and preferably four or less electrodes from the viewpoint of hot cracking resistance.

In narrow gap multi-pass welding, when one pass is performed per layer, with one electrode, the welding heat tends to concentrate in the center of the groove, resulting in insufficient melting at the groove surface 2 of the thick steel plate 1, and defects due to poor fusion (cold lap) and spatter and slag entrapment on the groove surface are likely to occur. In particular, in the first layer welding, the temperature of the thick steel plate is low and the melting depth is small, so defects due to poor fusion are likely to occur. For this reason, in the present invention, among multiple electrodes with two or more electrodes, the first and second electrodes are positioned along a predetermined parallel welding line and welding is performed. Figure 2 shows the construction procedure for multi-pass welding with four electrodes. Note that 7 is a backing material.

[One of the first electrode and the second electrode is wire negative (positive polarity), and the other is wire positive (reverse polarity)]
If the first and second electrodes have the same polarity (for example, both the first and second electrodes are wire positive), the electromagnetic force of attraction will cause the arcs to be inward, and heat will be concentrated in the center of the groove. As a result, sufficient melting will not be obtained on the groove surface. 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 due to the welding currents will generate strong outward electromagnetic forces, and the arcs will repel each other. As a result, it is possible to obtain a sufficient melting depth on the groove surface.

For this reason, in the present invention, one of the first electrode and the second electrode is the wire negative (positive polarity) and the other is the wire positive (reverse polarity).

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

If the distance a between the first and second electrodes is less than 5 mm, the heat of the arc itself is reduced due to the current (electrons) flowing between the electrodes, and sufficient melting of the groove surface 2 is not achieved. On the other hand, if the distance a between the first and second electrodes exceeds 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 in the center of the groove. As a result, sufficient melting of the groove surface 2 is not achieved.

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 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, making it possible to obtain a sound weld.

For this reason, the distance a between the tips of the welding wire supplied from the power feed tips at the tip of each welding torch of the first electrode 3 and the second electrode 4 should be adjusted to a range of 5 mm to 16 mm. Note that in order to obtain deeper and more stable melting of the groove surface through stronger arc repulsion, it is preferable to set the distance a between the first and second electrodes to a range of 5 mm to 8 mm.

[Angle α of the straight line connecting the tips of the welding wires of the first electrode and the second electrode with respect to the perpendicular direction of the welding line: 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 direction perpendicular to the welding line exceeds 60°, 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 with respect to the direction perpendicular to the welding line (hereinafter, also simply referred to as the "first-second electrode arrangement angle", see FIG. 2(b)) is limited to 60° or less. It is preferably 45° or less. The first-second electrode arrangement angle α may be 0°.

[Welding wire supplied to the electrode with positive polarity: steel wire containing 0.015 to 0.100 mass% REM (rare earth element)]
REM (rare earth elements) are effective elements for refining inclusions during steelmaking and casting, and for improving the toughness of weld metal during welding. In addition, when welding is performed by supplying a welding wire containing REM to a positive electrode, 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 with positive polarity. For this reason, in the present invention, a steel wire containing 0.015 to 0.100 mass% of REM (rare earth elements) is supplied to the positive electrode. Note that elements other than REM (rare earth elements) are included in the appropriate amounts normally contained depending on the grade (steel type) of the welding wire, as specified in JIS Z 3312.

If the REM (rare earth element) content is less than 0.015% by mass, the droplets will not be finer and the transfer will not be stabilized. On the other hand, if the content exceeds 0.100%, cracks will occur during the wire manufacturing process, making it difficult to manufacture the welding wire. For this reason, the REM (rare earth element) content is limited to the range of 0.015 to 0.100% by mass. The preferred range is 0.025 to 0.050% by mass.

Welding wire for gas-shielded arc welding is generally manufactured with a diameter ranging from 0.6mm to 2.0mm, but when welding with the same current, the thinner the wire diameter, the higher the deposition rate due to Joule heat. For this reason, it is preferable to select a relatively thin wire diameter to achieve highly efficient welding work. 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, it is preferable to use a diameter of welding wire in the range of 1.0mm to 1.6mm.

The above welding conditions are the basic welding conditions in this invention, and by performing welding under these conditions, a welded joint can be produced that has a weld metal part that can ensure a fusion depth p of 1.5 mm or more from the groove face perpendicular to the weld line at the bottom of the groove.

In addition to the basic welding conditions described above, the present invention also satisfies the welding conditions shown below, making it possible to obtain a deeper and more stable fusion depth p from the groove face at the bottom of the thick steel plate in the narrow groove.

[Feed angle φ of the welding wire of the first electrode and the second electrode to the bottom groove: 0° to 15° with respect to the perpendicular]
The arc has a directivity and tends to point in the direction in which the tip of the electrode (welding wire) points. In order to effectively utilize the directivity of the arc in melting the groove surface, it is advantageous to point the direction of the electrode tip toward the groove surface, and the direction in which 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 to the bottom groove is shown in FIG. 2(a). If 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, which is the electrode (crawling up of the arc), making it difficult to maintain the targeted melting at the groove surface 2, especially at the bottom. On the other hand, if 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 faces the groove surface 2 too much, so the shape of the weld bead becomes convex, and 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 each bottom groove is preferably in the range of 0° to 15° with respect to the perpendicular line. More preferably, it is 5° to 12°. The feed angle φ of the welding wires of the first electrode 3 and the second electrode 4 to each bottom groove is the same as the inclination of the power feed tips 3a and 4a, particularly the tip of the power feed tip, so that the feed angle φ of the welding wire can be controlled by the inclination of the tip of the power feed tip. The feed angle φ is defined as + in the direction facing the groove surface. The perpendicular line here refers to the perpendicular line to the bottom groove.

[Distance d between the side end of the tip of the welding wire of the first electrode and the second electrode and the groove surface of the thick steel plate: 0.5 mm or more and 3.0 mm or less]
The distance d between the side end of the tip of the welding wire of the first electrode 3 and the second electrode 4 and the groove surface 2 of the thick steel plate is shown in FIG. 2(a). If the distance d between the side end of the tip of the welding wire 3b and 4b at the bottom groove and the groove surface 2 of the thick steel plate is less than 0.5 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 thick steel plate cannot be efficiently melted. On the other hand, if it exceeds 3.0 mm, the arc is separated from the groove surface 2 and the groove surface 2 cannot be efficiently melted. For this reason, the distance d between the side end of the tip of the welding wire and the groove surface 2 of the thick steel plate is preferably 0.5 mm or more and 3.0 mm or less. More preferably, it is 0.5 to 2.0 mm, and even more preferably, it is 0.5 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 thick steel plate to be melted by each electrode.

[Curvature radius of welding wire fed to power feed tips of the first electrode and the second electrode: 150 mm or more and 300 mm or less]
In the present invention, a power feed tip with a bent tip is used 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. At this time, the welding wire passes through the power feed tip with the 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.

If the radius of curvature 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 radius of curvature of the welding wire exceeds 300 mm, there is no effect in reducing the wire feed resistance when the tip of the power feed tip is bent, so the welding wire cannot be fed stably and it becomes difficult to maintain the arc. For this reason, it is preferable that the radius of curvature 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 or more and 300 mm or less. More preferably, it is 175 mm or more and 275 mm or less.

[Third electrode and subsequent electrodes: arranged in the center of the groove behind the first and second electrodes, and distances b and c from adjacent preceding electrodes: 10 mm or more and 100 mm or less]
If the weld build-up height H 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 (the third electrode 5 and the fourth electrode 6 in the example of FIG. 2) in the center of the groove behind the first electrode 3 and the second electrode 4. This also makes it possible to further reduce the number of layers, greatly reducing the risk of stacking defects in multi-layer welding. Note that the "groove center" is allowed to be within a range of ±10% of the groove gap G from the center of the groove.

From the viewpoint of hot crack resistance, it is preferable to arrange the third electrode 5 and subsequent electrodes at a distance of 10 mm to 100 mm from the adjacent preceding electrodes. Here, the distance from the adjacent preceding electrodes refers to the distance b between the tip of the welding wire 4b of the second electrode 4 and the tip of the welding wire 5b of the third electrode 5, and the distance c between the tip of the welding wire 5b of the third electrode 5 and the tip of the welding wire 6b of the fourth electrode 6, as shown in FIG. 2(b). The polarity of the third electrode 5 and subsequent electrodes is not particularly limited, and may be either wire negative (positive polarity) or wire positive (reverse polarity).

[Shielding gas: mixed gas containing 60% 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 60 volume % or more of _CO2 gas and the remainder of an inert gas such as Ar as the shielding gas used in the gas-shielded arc welding of the present invention. More preferably, it is 100 volume % _CO2 gas. In the present invention, it is preferable to increase the oxygen concentration in the weld metal that governs 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 thick steel plate in the groove.

The other conditions do not need to be particularly limited and may be in accordance with established practice. For example, welding current: 280-360A, welding voltage: 32-37V (increases with current), welding speed: 30-90cm/min, wire extension length: 15-30mm, welding heat input per pass: 10-50kJ/cm.

The present invention will be further explained below based on examples.

For various steel plates of the steel types (grades) shown in Table 1, grooves were processed by gas cutting to obtain the groove shapes shown in Figure 1 and Table 2. Note that no grinding or other preparation was performed on the groove surface 2. For the processed groove surface 2, a laser displacement meter was used to measure the surface properties of the groove surface 2, and the maximum recess depth was determined, which is shown in Table 2.

For thick steel plate (length: 500 mm) with the groove shape shown in Table 2, narrow-gap gas-shielded arc welding was performed as the first layer welding under the welding conditions shown in Tables 3 and 4 using welding wire (steel wire) of the steel type (grade) shown in Table 1. Multi-layer welding was then performed from the second layer onwards to obtain a narrow-gap gas-shielded arc-welded joint (welding length: 500 mm). The welding conditions for the second layer onwards were the same as for the first layer. The electrode with positive polarity was supplied with REM-containing welding wire as shown in Table 3. The third and fourth electrodes were both placed in the center of the groove behind the first and second electrodes, and were set so that the distance between them and the adjacent leading electrodes was in the range of 10 to 100 mm.

The obtained welded joint was cut into five cross sections in the longitudinal direction, and the bottom fusion width wi (w1 to w5) was measured for each cross section (cross section 1 to cross section 5) as shown in Figure 3. For each cross section, the length of the bottom groove gap G was subtracted from the fusion width wi obtained for each cross section, and the obtained value was divided by 2 to determine the fusion depth pi (p1 to p5) for that cross section. The average value of the five cross sections was calculated, and this average value was used as the bottom fusion depth p of the welded joint. The results are shown in Table 4.

In addition, ultrasonic inspection was performed on the obtained welded joints to evaluate the presence or absence of weld defects. Cases where no defects were detected were marked with ◎, cases where only passable defects with a length of 3 mm or less were detected were marked with 〇, and cases where defects with a length of more than 3 mm were detected were marked with ×. The results obtained are shown in Table 4.

All of the examples of the present invention showed a bottom fusion length of 1.5 mm or more, even when narrow-gap multi-layer welding was performed using multi-pole welding with two or more electrodes. In addition, even without groove surface preparation, no welding defects were detected, resulting in a sound narrow-gap gas-shielded arc-welded joint, and it can be seen that the welding construction efficiency has been improved. On the other hand, in the comparative examples outside the scope of the present invention, the bottom fusion length was less than 1.5 mm or welding defects were detected, and a sound narrow-gap gas-shielded arc-welded joint was not obtained.

Reference Signs List 1 Thick steel plate 2 Groove surface 3 First electrode 4 Second electrode 5 Third electrode 6 Fourth electrode 3a, 4a, 5a, 6a Power feed tip 3b, 4b, 5b, 6b Welding wire 7 Backing material 8 Weld line 9 First layer weld bead 10 Weld pool (second layer)
11 Weld bead during welding (second layer)

Claims (9)

A narrow-gap gas-shielded arc welding method for thick steel plates, in which a thick steel plate having a plate thickness t of 22 mm or more is joined 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 to 18 mm,
The multi-layer welding is performed by multi-electrode welding using two or more electrodes,
Among the multiple electrodes, a first electrode and a second electrode are disposed at positions along a predetermined parallel welding line, and one of the first electrode and the second electrode is a wire negative (positive polarity) and the other is a wire positive (reverse polarity);
Furthermore, the distance a between the tips of the welding wires supplied from the power feed tips at the tips of the welding torches of the first electrode and the second electrode is set in the range of 5 mm to 16 mm, and 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 a direction perpendicular to the welding line is set in the range of 60° or less,
The welding wire to be supplied to the electrode having a positive polarity is a steel wire containing 0.015 to 0.100 mass% of REM (rare earth element),
The fusion depth p in the direction perpendicular to the weld line at the bottom of the thick steel plate is 1.5 mm or more,
The third electrode and subsequent electrodes are disposed in the center of the groove behind the first electrode and the second electrode, and the inter-electrode distance from the second electrode and subsequent electrodes is set in the range of 10 mm to 100 mm.
A narrow gap gas shielded arc welding method for thick steel plates. The narrow gap gas shielded arc welding method for thick steel plates according to claim 1, characterized in that the feed angle φ of the welding wire of the first electrode and the second electrode to the bottom groove is set to be 0° or more and 15° or less with respect to the perpendicular line. The method for narrow-gap gas-shielded arc welding of thick steel plates according to claim 1 or 2, characterized in that the distance d between the side end of the welding wire tip of the first electrode and the second electrode and the groove surface of the thick steel plate is 0.5 mm or more and 3.0 mm or less, respectively. The narrow gap gas shielded arc welding method for thick steel plates according to claim 1 or 2, characterized in that the welding wire fed to the power feed tips of the first electrode and the second electrode is curved with a radius of curvature in the range of 150 mm to 300 mm. The narrow gap gas shielded arc welding method for thick steel plates according to claim 3, characterized in that the welding wire fed to the power feed tips of the first electrode and the second electrode is curved with a radius of curvature in the range of 150 mm to 300 mm. 3. A method for narrow gap gas-shielded arc welding of thick steel plates according to claim 1, characterized in that a mixed gas containing 60 volume % or more of _CO2 gas is used as the shielding gas in the gas-shielded arc welding. 4. A method for narrow gap gas-shielded arc welding of thick steel plates according to claim 3, characterized in that a mixed gas containing 60 volume % or more of _CO2 gas is used as the shielding gas in the gas-shielded arc welding. A method for narrow gap gas-shielded arc welding of thick steel plates according to claim 4, characterized in that a mixed gas containing 60 volume % or more of _CO2 gas is used as the shielding gas in the gas-shielded arc welding. A method for narrow gap gas-shielded arc welding of thick steel plates according to claim 5, characterized in that a mixed gas containing 60 volume % or more of _CO2 gas is used as the shielding gas in the gas-shielded arc welding.

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