JP2007260684A - Multi-electrode submerged arc welding method for thick steel plate - Google Patents

Abstract

Provided is a thick steel plate submerged arc welding method capable of effectively reducing welding heat input and obtaining a high-quality welded portion with few welding defects while ensuring a penetration depth as before.
In multi-electrode submerged arc welding with three or more electrodes, a wire having a wire diameter of 3.2 mm or less is used as the first electrode and the second electrode, and a flux cored wire is applied to at least the first electrode. The first electrode is fed using a DC constant voltage power supply with a current of 700 A or more and the current density is 130 A / mm ² or more, while the second electrode is fed using an AC power supply with a current density of 95 Weld under conditions of A / mm ² or more.
[Selection] Figure 1

Description

  The present invention relates to a multi-electrode submerged arc welding method for thick steel plates suitable for applications such as seam welding of large-diameter steel pipes. In particular, when welding thick steel plates, an attempt is made to obtain high-quality welds under high efficiency. To do.

Submerged arc welding is characterized by excellent weld quality, beautiful bead appearance, and slag shielding of the molten pool, enabling welding with high heat input. is doing.
For this reason, submerged arc welding with two or more electrodes is generally applied to the welding of thick steel plates with long straight welding lines, and high-quality and high-efficiency welding is performed.

In order to increase the welding efficiency of the thick steel plate, it is necessary to increase the penetration depth (distance from the steel plate surface to the lower end of the weld metal) and the welding speed.
Submerged arc welding can apply a larger current than gas shielded arc welding, so that deep penetration can be obtained and it is suitable for increasing the welding efficiency of thick steel plates. However, due to the advantage of high current and high heat input welding, too much emphasis is placed on welding efficiency and defect suppression (such as prevention of slag entrainment), resulting in excessive welding heat input, especially in the heat affected zone (HAZ zone). There is a problem that the toughness of the steel deteriorates.

Without this problem, if the penetration depth and welding speed of submerged arc welding can be further increased, problems such as deterioration of joint toughness due to excessive heat input and reduced joint strength due to HAZ softening will occur. Can be overcome.
Note that when the welding heat input is lowered, the welding amount inevitably decreases, so it is necessary to reduce the groove cross-sectional area in accordance with the decrease in the welding amount. For this reason, unless further deep penetration welding is performed, penetration will be insufficient.
Therefore, in order to solve the above problem, it is necessary to satisfy both conflicting problems of reduction of input heat input and increase of penetration depth.

For example, Patent Document 1 discloses a small input consisting of a diameter of at least one preceding electrode wire preceding the last electrode smaller than that of the last electrode and using a DC power source for at least one pole of the preceding electrode. Thermal multi-electrode submerged arc welding methods have been proposed.
However, this method has a problem that a satisfactory penetration depth and welding speed cannot be obtained as expected in the present invention, and defects such as slag entrainment increase. This is because the current density is insufficient, so not only is the effect of increasing the welding amount with the increase of the penetration depth, but also the strong flow to the rear of the weld pool caused by the application of small diameter wire is controlled. Because you can't.

Patent Document 2 proposes a submerged arc welding method in which the penetration depth is increased by increasing the current density according to the electrode diameter.
However, in this method, since current and current density are insufficient, it is difficult to achieve both a significant reduction in heat input and an increase in penetration depth.

JP-A-52-82652 JP-A-10-109171

  In the welding of large diameter steel pipes, submerged arc welding that can perform seam welding with large current and high heat input is frequently used in order to obtain a high-efficiency and sound-free welded part. However, submerged arc welding with high heat input melts the base material not only in the plate thickness direction but also in the plate width direction. In other words, since a large amount of heat energy is consumed for melting the base metal in the plate width direction, which does not originally require the input of heat energy, the welding heat input increases, leading to deterioration of the toughness of the weld metal and weld heat affected zone. It was as described above.

  However, excessive welding heat input can be omitted if the necessary penetration depth can be ensured and the melting of the base metal in the plate width direction can be suppressed by supplying arc energy only in the plate thickness direction as much as possible. The heat reduction effect makes it possible to improve the toughness of the welded HAZ (heat affected zone).

  The present invention advantageously responds to the above requirements, and by effectively reducing the welding heat input while ensuring the penetration depth as before, high quality welds with few welding defects are obtained, and toughness is achieved. It aims at proposing the multi-electrode submerged arc welding method of a thick steel plate which can suppress effectively deterioration of welded part characteristics, such as joint strength.

That is, the gist configuration of the present invention is as follows.
In multi-electrode submerged arc welding of 1.3 electrodes or more, wires having a wire diameter of 3.2 mm or less are used as the first electrode and the second electrode, and a flux cored wire is applied to at least the first electrode. The power supply to the electrode is a DC constant voltage power supply with a current of 700 A or more and the current density is 130 A / mm ² or more, while the power supply to the second electrode is an AC power supply with a current density of 95 A / mm A multi-electrode submerged arc welding method for thick steel plates, characterized by welding under conditions of mm ² or more.

2. 2. The multi-electrode submerged arc welding method according to 1 above, wherein the distance between the first electrode and the second electrode measured at the center of the welding wire is 18 mm or less.

3. 1 or 2 above, wherein the inclination angle of the first electrode is −15 ° to + 15 ° with respect to the welding direction, and the inclination angle of the subsequent electrode is 0 to 30 ° with respect to the immediately preceding electrode. The multi-electrode submerged arc welding method according to 2.

4). Groove processing is applied to both the front and back sides of the steel sheet, and the groove cross-sectional area S is given by
S ≦ 3.15t-14 --- (1)
Where S: groove cross-sectional area (mm ² )
t: Thickness (mm)
The multi-electrode submerged arc welding method according to any one of the above items 1 to 3, wherein welding is performed under a condition that satisfies the following conditions.

The effects of the present invention are as follows.
a) Welding heat input can be significantly reduced while maintaining an appropriate penetration depth, and as a result, excellent low temperature toughness can be obtained in the weld metal and the weld heat affected zone.
b) Since the softening of the weld heat affected zone, which is a problem with high strength steel pipes, can be suppressed, a stable joint strength can be obtained.
c) As in the present invention, welding defects such as slag entrainment that are likely to occur when welding with a large penetration depth ratio to bead width is significantly suppressed by applying a flux cored wire. As a result, a high-quality weld metal can be obtained.
d) From the manufacturer's side, it is possible to apply a low-cost steel material in which the toughness is not sufficient, for example, the amount of C is high and the number of alloy elements such as Cu and Ni is small, and the manufacturing cost can be reduced. .

The present invention will be specifically described below.
It is the first electrode and the second electrode that greatly contribute to the penetration depth in multi-electrode welding.
Therefore, the inventors have repeatedly studied the optimum form of the first electrode and the second electrode.
As a result, the following knowledge was obtained.
a) For at least the first electrode, it is effective to use a thin flux-cored wire (also referred to as a flux cored wire).
b) In addition, the power supply method for the first electrode and the second electrode is also important. The power supply to the first electrode is a DC constant voltage power supply with a current of 700 A or more, while the power supply to the second electrode is an AC power supply. It is advantageous to use.
c) Further, regarding the current density of the first electrode and the second electrode, it is advantageous that the first electrode is 130 A / mm ² or more, while the second electrode is 95 A / mm ² or more.

Hereinafter, the above points a to c will be described in detail.
First, the flux cored wire will be described. Since the flux cored wire includes a powder filler in the wire, the metal portion on the cross-sectional area is smaller than that of the solid wire having the same wire diameter. Therefore, a substantial current density is increased. When the current density is increased, the arc is concentrated and the arc energy is input in the thickness direction, so that the penetration depth and the welding speed are larger than those of the solid wire having the same diameter. In other words, by using a small-diameter flux cored wire, a deep penetration depth can be obtained, and further, by reducing the diameter, the wire melting rate can be improved and the amount of welding per welding heat input can be increased. Both high welding speed and deep penetration depth can be achieved with low heat input welding.
In this regard, in the case of a solid wire, even if the diameter is the same, the cross-sectional area of the metal portion is large, and a reduction in current density is inevitable.

  Here, the wire diameter of the first electrode and the second electrode needs to be 3.2 mm or less. This is because when the wire diameter exceeds 3.2 mm, it is not possible to expect a sufficient improvement in current density, and hence an increase in penetration depth and welding speed. The lower limit of the wire diameter is preferably 1.8 mm. This is because if the wire diameter is smaller than this, not only the wire feeding speed becomes too fast and the welding machine can not cope, but the resistance heat of the wire becomes too large and the wire is too melted, so the arc length is constant. This is because the welding conditions become unstable.

In addition, as a merit of applying the flux cored wire, it is possible to effectively suppress the occurrence of slag entrainment. The reason for this is not clear, but it is thought that the droplets become finer when transferred and the arc is stabilized.
In order to obtain this effect, the metal oxide component contained in the powder filled in the flux cored wire is desirably 1 mass% or more of the powder weight. Moreover, in order to obtain a high welding speed, it is desirable that the metal component contained in the powder is 80 mass% or more of the powder weight.

In the present invention, the flux cored wire as described above is preferably applied to both the first electrode and the second electrode, but only the first electrode may be used.
Moreover, there is no restriction | limiting in particular about the electrode after the 3rd electrode, A normal solid wire can be used. Moreover, there is no problem even if the flux cored wire of the present invention is used.
Moreover, there is no restriction | limiting in particular also about a wire diameter, The wire of the diameter of about 4.0-6.4 mm used conventionally can be used.

FIG. 1 shows the penetration behavior of submerged arc welding according to the present invention in comparison with the penetration behavior of conventional submerged arc welding.
As shown in Fig. 3 (a), since the conventional approach is directed to large current and large heat input welding, the groove cross-section is large, and high-efficiency welding is performed under such large groove cross-section. Therefore, the base material is melted not only in the plate thickness direction but also in the plate width direction. As a result, the heat energy is not consumed unnecessarily, and the toughness of the weld heat affected zone is forced to deteriorate.
On the other hand, in the present invention, as shown in FIG. 5 (b), the diameter of the wire can be reduced, the arc can be reduced, and deep penetration can be obtained. The amount can be reduced. As a result, welding with low heat input becomes possible, and toughness deterioration of the weld heat affected zone can also be prevented.

Next, FIG. 2 shows the results of examining the relationship between the welding current and the welding amount when the wire diameters are variously changed under the same welding conditions.
As shown in the figure, the thinner the wire diameter, the larger the amount of welding at the same welding current value, and this tendency becomes remarkable when the welding current is 700 A or more.
In addition, when an appropriate range of current density (current value / wire cross-sectional area) at this time was examined, it was found that it was necessary to set the current density to 130 A / mm ² or more. In addition, the wire cross-sectional area was made into the area enclosed by the wire outer diameter for convenience.

In other words, by increasing the current to 700A or higher, the strength of the arc digging the base metal becomes stronger, and by increasing the current density to 130A / mm ² or higher, the arc concentrates, so the arc energy is effectively reduced. Even if it is a case where the groove cross-sectional area is reduced and the total amount of welding is reduced, deep penetration can be obtained and welding with low heat input can be performed. It became possible.

If the welding current becomes too large, an increase in heat input is unavoidable and adversely affects the toughness of the HAZ part. Therefore, the upper limit of the welding current is preferably about 1400A.
In addition, if the current density becomes too high, the stability of the arc deteriorates, so the upper limit of the current density is preferably about 300 A / mm ² . Preferably, it is 200 A / mm ² or more.

  In the present invention, the first electrode is a thin wire, and therefore needs to be fed at high speed. For such high speed feeding, a power source with a DC constant voltage characteristic is used. It is advantageous to use. This DC constant voltage characteristic power supply has an effect of stabilizing the arc and suppressing the occurrence of defects.

On the other hand, the second electrode usually does not have a strong function of obtaining penetration, but a small-diameter wire (preferably a flux cored wire) is applied to provide an appropriate distance between the electrode and an appropriate current. By applying an AC power supply under the density, it is possible to further deepen the deep penetration obtained in the first electrode.
In other words, the first electrode and the second electrode are regarded as one electrode, and the second electrode can also exert a strong gouging function to obtain deep penetration. For this purpose, the distance between the first electrode and the second electrode measured at the center of the welding wire is preferably 18 mm or less.

In order to obtain a deep penetration, it is necessary to bring the first electrode and the second electrode closer as described above. However, if the distance between the electrodes is too narrow, there is a disadvantage in that both interfere with each other when a DC power source is used as a power feeding method. .
Therefore, in the present invention, in order to avoid such interference, an AC power supply that does not interfere with the first electrode is used as the power supply for the second electrode.

When an AC power supply is used, the arc voltage is fed back to the wire feed speed and the welding conditions (arc length) are controlled. However, in order to perform welding at a high current density with a thin wire, the wire feed speed is increased. There is a problem that the feedback control becomes difficult. To avoid these problems, it is desirable to use a low inertia servo motor of 1.0 × 10 ⁻⁴ kg · m or less for the wire feed motor. Generally, motors used in submerged arc welders have a large inertia of approximately 6.0 × 10 ⁻⁴ kg · m, and the feedback response tends to be delayed during high-speed wire feeding, and arc breaks due to short circuits are likely to occur. It is. By the above operation, the arcs of the first and second electrodes are stabilized, and a high-quality welded part having deep penetration and no defects can be obtained.

Here, the current density of the second electrode needs to be 95 A / mm ² or more. This is because if the current density is less than 95 A / mm ² , it is difficult to further deepen the deep penetration obtained in the first electrode. In addition, if the current density of the second electrode becomes too large, it is inevitable that the bead becomes convex and the shape of the bead deteriorates. Therefore, the upper limit of the current density is preferably about 250 A / mm ^2. . Preferably, it is 130 A / mm ² or more.
As described above, by using thin wires for the first electrode and the second electrode and performing welding at a high current density, the amount of heat generated by energizing the wire increases, and the welding speed also increases. Therefore, it is possible to further reduce welding heat input.

Note that, due to the violent gouging of the first and second electrodes described above, the molten metal flows violently behind the second electrode. Therefore, in order to decelerate the hot water flow and adjust the bead shape, a third electrode having the function is required.
For this purpose, the distance between each electrode after the second electrode and the third electrode is preferably 30 mm or less (preferably 13 mm or more) in terms of the distance between the wire centers at the steel plate surface position.
Regarding the inclination angle of the electrodes, the line perpendicular to the steel plate to be treated with respect to the welding progress direction is set to 0 °, the inclination angle of the first electrode is set to −15 ° to + 15 °, and the inclination of the electrodes after the second electrode is set. The angle is preferably 0 to 30 ° with respect to the immediately preceding electrode. Regarding the tilt angle, the-side means the receding angle side, and the + side means the advancing angle side.

As described above, the first electrode and the second electrode are welded with extremely high energy density, so the arc pressure is high, the molten metal behind the first electrode and the second electrode flows violently, and a convex bead is formed. It is easy to be done. However, by optimizing the arrangement of the third and subsequent electrodes, it is possible to keep the bead shape appropriate and obtain a high-quality bead having no defects.
That is, by setting the distance between the electrodes to 30 mm or less, the oscillation of the molten pool is mitigated, the convex bead is suppressed, and the electrode disposed at the rear is 0 to 30 ° forward angle side with respect to the immediately preceding electrode. By inclining, the flow of the molten metal from the first electrode can be relaxed, the movement of the molten pool can be stabilized, and a high-quality bead without defects can be obtained.

Next, in FIG. 3, when the groove processing is performed on both the front and back surfaces of the steel sheet and the submerged arc welding is performed under the same conditions, the plate thickness and the groove cross-sectional area that can perform appropriate welding are shown. The results of investigating the relationship are organized and shown.
As shown in the figure, when groove processing is performed on both the front and back surfaces of the steel sheet, the groove cross-sectional area S is expressed by the following equation (1)
S ≦ 3.15t-14 --- (1)
Where S: groove cross-sectional area (mm ² )
t: Thickness (mm)
When the above relationship was satisfied, good submerged arc welding could be performed.
Thus, in the present invention, a desired effect can be obtained by keeping the groove cross-sectional area small.

The thick steel plate to be used in the present invention refers to a steel plate having a thickness of 10 mm or more.
Moreover, in this invention, it is not necessary to weld both front and back by this invention method, and the effect of this invention can be acquired also as a method of performing deep penetration welding only from one side.

Example 1
After subjecting the steel sheet having the composition shown in Table 1 to groove processing as shown in FIG. 4 under the processing conditions shown in Table 2, welding conditions are set for each plate thickness, and welding shown in Tables 3 and 4 is performed. After performing inner surface side welding under the conditions, outer surface side welding was performed under the welding conditions shown in Tables 5 and 6.
C1, C3, and C5 in Table 3 and D1, D4, and D7 in Table 5 were flux cored wires only for the first electrode, and the others were the first electrode and the second electrode, respectively. The weight ratio between the outer shell made of mild steel and the powder to be filled was 3: 1, the flux component in the powder was 1.5 mass%, and the metal component was 98 mass%. As the flux, a melt type flux mainly composed of SiO ₂ —CaO—CaF ₂ was used. Moreover, as a welding wire other than the flux cored wire, a solid wire containing C: 0.07 mass%, Si: 0.5 mass%, and Mo: 0.5 mass% was used.

Tables 4 and 6 show the welding heat input in the multi-electrode submerged arc welding described above in comparison with the welding heat input when the conventional method is followed.
Tables 4 and 6 also show the results of examining the penetration depth and the welded portion appearance.
Further, Tables 4 and 6 also show the results of collecting Charpy test pieces from the bond portion of the weld and measuring the Charpy absorbed energy at −50 ° C. in order to evaluate HAZ toughness.
Here, the penetration depth refers to the distance from the bottom of the groove to the penetration tip, and does not include the bead height even if there is a temporary bead in the groove.

The inner surface side welding shown in Tables 3 and 4 are all invention examples. As is apparent from Table 4, when the present invention is followed, a deep penetration depth can be obtained with low heat input, and HAZ. The toughness was also good.
Among the outer surface side weldings shown in Tables 5 and 6, symbols D1, D3, D4, D7, D8, and D10 are invention examples. In this case, good welding results were obtained. That is, deep penetration depth was obtained with low heat input, and HAZ toughness was also good.
On the other hand, in the symbol D2, since the electrode angle of the third electrode was too large, defects occurred and the bead shape was disturbed.
D5 caused a slag entrainment defect because the distance between the second electrode and the third electrode was too large, and also caused a lack of penetration because the current density of the second electrode was insufficient.
The symbol D6 was insufficiently melted because the current of the first electrode was insufficient.
The symbol D9 was insufficiently melted because the current density of the first electrode was insufficient.

It is the figure which showed the penetration behavior (b) of submerged arc welding according to this invention compared with the penetration behavior (a) in the conventional submerged arc welding. It is the figure which showed the relationship between welding current and the amount of welding in the case of varying a wire diameter variously on the same welding conditions. It is the figure which showed the relationship between plate | board thickness t and groove | channel cross-sectional area S which can perform appropriate welding, when groove processing was given to the front and back both surfaces of a steel plate, and submerged arc welding was performed on the same conditions. . It is the figure which showed the groove shape in an Example.

Claims (4)

In multi-electrode submerged arc welding of three or more electrodes, wires having a wire diameter of 3.2 mm or less are used as the first electrode and the second electrode, and a flux cored wire is applied to at least the first electrode. The DC power supply uses a DC constant voltage power supply with a current of 700A or more and a current density of 130 A / mm ² or more, while the power supply to the second electrode uses an AC power supply with a current density of 95 A / mm ^2. A multi-electrode submerged arc welding method for thick steel plates characterized by welding under the above conditions.   The multi-electrode submerged arc welding method according to claim 1, wherein the distance between the first electrode and the second electrode measured at the center of the welding wire is 18 mm or less.   The inclination angle of the first electrode is -15 ° to + 15 ° with respect to the welding progress direction, and the inclination angle of the subsequent electrode is 0 to 30 ° with respect to the immediately preceding electrode. Or the multi-electrode submerged arc welding method according to 2; Groove processing is applied to both the front and back sides of the steel sheet, and the groove cross-sectional area S is given by
S ≦ 3.15t-14 --- (1)
Where S: groove cross-sectional area (mm ² )
t: Thickness (mm)
The multi-electrode submerged arc welding method according to any one of claims 1 to 3, wherein the welding is performed under a condition that satisfies the following conditions.

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