JPH0871779A - Laser welding equipment for tube and its method - Google Patents

Abstract

PURPOSE: To eliminate possibility of deforming a molten pool and to enable laser welding with a high shielding performance by exhausting a lubricant decomposed gas by means of a strong cylindrical gas flow and shielding water vapor and the like by means of a weak columnar gas flow. CONSTITUTION: When a welding part W2 of a metallic belt W is irradiated with a laser beam 18 from outside a tube, the shield gas inside the tube of a welding part W2 is constituted of a weak columnar gas flow 21 and a strong cylindrical gas flow 22 enclosing the weak one, and the inside diameter of the cylindrical gas flow 22 is made larger than that of a molten pool W3 . The weak columnar gas flow 21 prevents the air or water vapor of which the shield gas from a second gas nozzle 16 overflows from a thorough-hole 13 and heads for the molten pool W3 ; the strong cylindrical gas flow 22 is formed from a first gas nozzle 15, preventing the air or water vapor heading for the lower face of the molten pool W3 and exhausting the heat decomposed gas of a lubricant. Thus, a laser welding with a high shielding performance is made possible without deforming the molten pool W3 .

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to improvements in laser welding technology for pipes.

[0002]

2. Description of the Related Art In laser welding, the energy density of the heat source is higher than other welding methods, deep penetration is possible and high-speed welding is possible, and since the total heat input can be reduced, the performance of the welded portion is also good. Therefore, it has attracted attention as a welding method for steel pipes and is being applied. In addition, a technique has been proposed (for example, Japanese Patent Laid-Open No. 56-168981) in which high-frequency preheating is added to the laser welding method to further increase the speed.

By the way, since the laser torch is generally provided with a shield gas discharge hole, there is no fear that the welded portion will be oxidized in the atmosphere by surrounding the welded portion with the shield gas. However, when the laser torch is placed outside the tube, the outer surface of the tube can be gas-sealed, but the inner surface of the tube cannot be sealed. Then, the welded portion in the pipe is oxidized.

The applicant of the present invention has previously filed Japanese Unexamined Patent Publication No. 6-142959.
In the publication "Pipe-making method by laser welding method and its apparatus", an invention has been proposed in which an inert gas is blown to the inner surface of a pipe below a welded portion and a water flow is formed in the pipe while cooling the welding spatter. The point is to prevent the oxidation of the welded part with an inert gas and render the spatter harmless with a water flow. Specifically, a damage prevention member having a shield gas injection port and a water discharge port is provided with a through hole for avoiding the laser beam, and the through hole is formed into a downward conical shape, and a horizontal flow of the shield gas is formed in this through hole. To do. The narrow through hole on the top hinders the upward flow of water vapor, and also shields the water vapor reliably with a shield gas.

[0005]

However, since the through-hole has a conical shape that spreads downward, the flow of water vapor can be blocked well, but the gas seal at the lower part of the weld becomes weak. On the other hand, when bending a flat metal strip into a tubular shape, a lubricant is indispensable, and a part of the lubricant remains in the welded part, and this residual lubricant is decomposed by welding heat and gasified. . Weak gas seals have poor ability to block decomposed gases and can lead to welding defects.

[0006]

Therefore, the inventors of the present invention may find a more preferable shield mechanism and shield method by reexamining the shape of the through hole and reassessing the distribution of the shield gas. From this point of view, research has been repeated and a satisfactory shield mechanism and shield method have been established. Specifically, a laser torch is placed outside the tube of a metal band bent into a tubular shape, a shield mechanism is placed inside the tube, and a metal beam is welded to the metal band by a laser beam emitted from the laser torch toward the shield mechanism. In a laser welding device for laser welding parts, a shield mechanism is opened in order to let a laser beam pass through the mandrel inserted into the pipe, the box attached to the tip of this mandrel, and the center of this box. A through hole, a gas chamber formed inside the box body, and a first gas nozzle opened in the box body so as to form an upward gas flow in a ring shape having a larger diameter than the molten pool of the welded portion, , A second gas nozzle opened in the box body to form horizontally oriented gas in the through hole, and a gas supply path for supplying a shield gas to the gas chamber.

Preferably, the gas chamber is divided into a first gas chamber communicating with the first gas nozzle and a second gas chamber communicating with the second gas nozzle, and a first gas supply path is connected to the first gas chamber.
The second gas supply passage is connected to the second gas chamber, and the gas ejection amount of the first gas nozzle and the gas ejection amount of the second gas nozzle can be controlled independently of each other.

It is preferable that mandrel rotating means is attached to the mandrel so that the box body can be swung according to the movement of the welded portion.

In the laser welding method for a pipe, a laser beam is irradiated from the outside of the welded portion of a metal band bent into a tubular shape, and the inside of the welded portion is surrounded by a shield gas to prevent oxidation. The shield gas inside the pipe is composed of a weak cylindrical gas flow and a strong cylindrical gas flow surrounding this cylindrical gas flow, and the inner diameter of this cylindrical gas flow is at least larger than the weld pool of the welded part, The cylindrical gas flow blocks air and steam toward the molten pool, and the cylindrical gas flow blocks air and steam toward the molten pool and scavenges the pyrolysis gas of the lubricant.

Argon gas is used as the shield gas inside the tube, and helium gas is blown from the outside of the tube together with the laser beam.

[0011]

Embodiments of the present invention will be described below with reference to the accompanying drawings. The drawings should be viewed in the direction of reference numerals. FIG. 1 is a schematic view of a roll-type pipe forming equipment equipped with a laser welding apparatus according to the present invention, and the roll-type pipe forming equipment 1 is equipment for gradually bending a flat metal strip W into a tubular shape.
A lubricant is used for high speed molding. The high frequency heating means 2, the steam blowing means 3 and the laser welding device 4 are arranged in this order on the exit side of the roll-type pipe forming equipment 1.

FIG. 2 is a plan view of the laser welding apparatus according to the present invention, in which the laser torch 5 of the laser welding apparatus is arranged between a pair of squeeze rolls 6 and 6, and below the laser torch 5, a tubular metal strip W. A shield mechanism 10 is arranged inside. Then, the gas purge nozzles 3a, 3a of the vapor blowing means 3 are arranged on the upstream side of the laser torch 5, and the contact tips 2a, 2a of the high frequency heating means 2 are arranged on the upstream side of the gas purge nozzles 3a, 3a. 2b is a high frequency oscillator.
The contact tips 2a, 2a fed from b are contacts for energizing the ends W1, W1 of the metal strip W before laser welding. The contact tip 2a is an example, and may be, for example, an annular coil for induction heating.

FIG. 3 is a longitudinal sectional view of the laser welding apparatus according to the present invention. Since the metal strip W is squeezed by the drum-shaped squeeze roll 6, the metal strip W is close to the upstream side of the laser torch 5 (for convenience, a close contact portion is shown). ///, the weld bead part is XXX
Hatched. ), The gas purge nozzle 3a and the shield mechanism 10 are arranged so as to blow the upstream portion from the close contact portion. The gas ejected from the gas purge nozzle 3a is preferably compressed air, nitrogen gas, argon gas, or helium gas. However, the compressed gas used is one that has been cleaned and dried. on the other hand,
The gas ejected from the shield mechanism 10 is argon gas. The reason will be described later.

FIG. 4 is a first embodiment of the shield mechanism according to the present invention. The shield mechanism 10 includes a mandrel 11 inserted into a pipe, a box body 12 attached to the tip of the mandrel 11, and this box. A through hole 13 formed in the center of the body 12 for passing a laser beam, and the box body 1
2 and a first gas nozzle 15 (...) formed in the box body 12 so as to form a ring-shaped upward gas flow having a diameter larger than that of the molten pool of the welded portion. The same shall apply hereinafter), a second gas nozzle 16 opened in the box body to form a horizontally oriented gas in the through hole 13, and a gas supply path 17 for supplying a shield gas to the gas chamber 14. Consists of.

FIG. 5 is a plan view of FIG. 4, in which a box 12 has a through hole 13 formed substantially in the center thereof, and two rows of first gas nozzles 15 are formed so as to surround the through hole 13.

The operation of the laser welding apparatus for pipes and the operation of the shield mechanism (first embodiment) described above will be described below.
In FIG. 1, a flat tube-shaped metal strip W is gradually bent into a tubular shape by a roll-type tube forming facility 1. Use a lubricant for high speed molding. The metal strip W is preheated by the high frequency heating means 2 to vaporize the lubricant. In FIG. 2, of the generated steam, the portion outside the pipe is blown by the steam blowing means 3, and the inside portion of the pipe is blown by the shield mechanism 10, and the welded portion is welded by the laser torch 5.

FIG. 6 is an operation diagram of the shield mechanism (first embodiment) of the present invention. When the welded portion W2 of the metal strip W is irradiated with the laser beam 18 from the outside of the pipe, the welded portion W2 of the welded portion W2 is exposed. The shield gas inside the pipe is composed of a weak cylindrical gas flow 21 and a strong cylindrical gas flow 22 surrounding the cylindrical gas flow 21, and the inner diameter of this cylindrical gas flow 22 is formed at least in the welded portion W2. The diameter is larger than that of the molten pool W3. The columnar gas flow 21 refers to a flow of the shield gas filled in the through hole 13 through the second gas nozzles 16 ... From the upper and lower openings of the through hole 13.
It is much weaker than the cylindrical gas flow 22 formed directly by the first gas nozzles 15 ... A cylindrical gas flow 21 blocks air and steam from the bottom to the top of the drawing through the through holes 13 toward the molten pool W3, and a cylindrical gas flow 22 blocks the molten pool W.
(3) Blocks air and steam from the outside that is directed horizontally to the lower surface and scavenges the pyrolysis gas of the lubricant.

The above operation was confirmed by experiments, and the results will be described.

[0019]

[Table 1]

Table 1 shows the experimental conditions for the shield mechanism (first embodiment). That is, the test material is carbon steel,
The product size is a steel pipe having an outer diameter of 101.6 mm and a wall thickness of 6.0 mm, and is preheated to about 1100 ° C. by the high frequency heating means 2 of FIG. 1 (that is, FIGS. 2 and 3) to evaporate the residual lubricant, and vapor The outside of the tube is blown beforehand by the blowing means 3, and the inside of the tube is shown in FIG.
Blow by the shield mechanism 10 of. Shield mechanism 10
The through hole 13 of is a cylinder of 15 mm, and the first gas nozzle 1
5 ... Can be opened with a diameter of 1 mm × 16 pieces × 2 rows, and the second gas nozzles 16 can be opened with a diameter of 1 mm × 8 pieces × 4 rows (4 steps). A 25 kW carbon dioxide laser welder is used as the welding device, the welding speed is 12 m / min, and 100% pure helium gas is injected as a shield gas from the laser torch so as to surround the laser beam.

[0021]

[Table 2]

Table 2 is a welding quality evaluation table for the shield mechanism (first embodiment). The welding quality was evaluated by taking 30 samples with a welding length of 200 mm, subjecting them to a transmission X-ray inspection, and determining the welding quality as "defective" if even one of them failed. The evaluation was rated as "x".
This determination is common thereafter. In the shield mechanism 10 of the first embodiment, the flow rate distribution of the first and second gas nozzles depends only on the nozzle area ratio, so the shield gas flow rate (liter / minute) is shown as the sum of the two. Comparative Example 1: The shield gas flow rate was set to 0. As a matter of course, bubbles were mixed in the welded portion, and the welding quality was poor. Comparative Example 2: Therefore, the flow rate of the shield gas (argon gas) was set to 40 liters / minute, but the amount of gas was still insufficient and the welding quality remained poor.

Examples 1 and 2, shield gas flow rate is 80,
When it was set to 120 liters / minute, there was no risk of bubbles remaining in the welded part, and the welding quality was good. Comparative Example 3: However, the shield gas flow rate is 160 liters /
However, since the columnar gas flow pushed up the molten pool significantly, the bead shape became defective and undercuts appeared, resulting in poor welding quality. Therefore, it was found that the gas flow directly hitting the molten pool needs to be weakened.

FIG. 7 is a second embodiment of the shield mechanism according to the present invention, and its plan view is the same as that of FIG. The shield mechanism 30 includes a first gas chamber 31 that communicates the gas chamber with the first gas nozzles 15 ...
6 and the second gas chamber 32 leading to the first gas chamber 3
1, the first gas supply path 33 is connected, the second gas chamber 32 is connected to the second gas supply path 34, and the gas ejection amounts of the first gas nozzles 15 ... And the gas ejection amounts of the second gas nozzles 16 ... It is characterized in that it can be controlled independently.

[0025]

[Table 3]

Table 3 shows the experimental conditions for the shield mechanism (second embodiment). That is, the test material is carbon steel,
The product size is a steel pipe having an outer diameter of 101.6 mm and a wall thickness of 6.0 mm. The high-frequency heating means 2 in FIG. 1 preheats it to about 1100 ° C. to evaporate the residual lubricant, and the steam blow means 3 outside the tube. In advance, and the inside of the pipe is shielded by the shield mechanism 3 of FIG.
Blow at 0. The through hole of the shield mechanism 30 is 15 m
m is a cylinder, and the first gas nozzle 15 ... Has a diameter of 1 mm × 16
Opening with the specification of 2 pieces x 2 rows, the second gas nozzle 16 ... 1m in diameter
It can be opened with the specifications of m x 8 pieces x 1 row (1 step). Welding equipment is 2
Using a 5kW carbon dioxide laser welder, welding speed is 1
At a rate of 2 m / min, 100% pure helium gas is injected as a shield gas from the laser torch so as to surround the laser beam. That is, the second embodiment is characterized in that the shield mechanism 30 can arbitrarily change the flow rates of the first and second gas nozzles and the number of the second gas nozzles is extremely small.

[0027]

[Table 4]

Table 4 is a welding quality evaluation table relating to the shield mechanism (second embodiment). Comparative Example 4: The total amount of shield gas was 30 liters / minute (2
0 + 10), the sealing effect could not be expected and the welding quality was poor. Examples 3 to 6: The flow rate of the second gas nozzle was 10 liters /
Fixed to the minute, the flow rate of the first gas nozzle is 40, 60, 12
The welding quality was good at 0,160 liters / minute. Comparative Example 5: The flow rate of the first gas nozzle is 160 liters /
As a result, when the flow rate of the second gas nozzle was set to 0, bubbles were generated in the welded portion and the welding quality was poor. This is because the second gas nozzle did not function and air reached the welded portion through the through hole. Therefore, the flow rate of the second gas nozzle is necessary. Comparative Example 6: When the flow rate of the first gas nozzle was set to 0 and the flow rate of the second gas nozzle was set to 10 l / min, the welded part was defective. In the above, the shield gas was argon gas, but this was replaced with helium gas for an experiment.

Comparative Examples 7 and 8: The flow rate of the first gas nozzle was set to 4
0,160 l / min, the flow rate of the second gas nozzle is 10
The welding quality was poor, even though it was set to 1 / min. It can be inferred that helium gas has a low density, and the blocking ability of the cylindrical gas flow formed by the first gas nozzle is significantly smaller than that of argon gas. That is, since the blocking ability depends on the dynamic pressure of the gas flow, and the dynamic pressure is linearly proportional to the density, it is not appropriate to use helium gas having a density of 1/10 of that of argon gas.

On the other hand, in laser welding of 10 kW or more, it is common knowledge that argon gas cannot be used because gas plasma self-grows, and helium gas is used.
The present inventors have found a method of sealing the outside of a tube having high laser beam energy with helium gas, and sealing the inside of a tube whose energy is weakened by penetrating a metal band with argon gas, and the operation and effect thereof are carried out as described above. This is confirmed in the examples and comparative examples.

Examples 7 and 8: These are examples in which a shielding gas in which 5% oxygen was mixed with argon gas was adopted, and the welding quality was good. Since argon gas is a sufficiently heavy gas and has a large ability to block outside air, it can be said that it does not matter if it contains some oxygen or air. On the contrary, since a small amount of oxygen has an effect of improving the welding quality, it is convenient that a small amount of oxygen or the like can be mixed in the shield gas in this way. Further, although high-purity argon gas is used in the experiment, it is also possible to use inexpensive and slightly low-purity argon gas.

FIGS. 8A and 8B are views showing a third embodiment of the shield mechanism according to the present invention. In the shield mechanism 40, a mandrel moving means 41 is attached to the base of the mandrel 11.
The rotation is controlled by the laser beam position control mechanism described below. Since the flat metal strip W is formed into a tubular shape by bending and the welded portion is located right above, ideally the welded portion should be on the center of the line. However, in a large-diameter pipe, the actual welded portion may deviate by several mm to several tens of mm in a plan view, and it is necessary to move the laser torch 5 by monitoring the amount of this deviation.

A laser beam position control mechanism 43 for that purpose
Is composed of an end face position measuring device 44 (comprising a light projector 45 and a light receiving device 46), a surface thermometer 47 (for example, a two-color thermometer), and a calculator 48, as shown in FIG. The end face position measuring device 44 optically measures the position of the end face of the metal strip. The surface thermometer 47 measures the temperature distribution of the welded part and its vicinity.
That is, the abutting point can be estimated from the fact that the end of the metal strip is hot, the other portions of the metal strip are cold, and the gap is naturally cold. The calculator 48 calculates the position of the end face of the metal strip by the end face position calculation unit 49, calculates the collision point by the collision point position calculation unit 51, and calculates the deviation amount by the control amount calculation unit 52 based on both information. .

The content of the control amount calculation unit 52 is shown in FIG. 5A, which shows that the abutting point 53 and the deviation amount δ can be calculated geometrically. The control unit 54 in (b) moves the laser torch 5 in a direction to make the shift amount δ to 0, and moves the box body 12 at the tip of the mandrel via the mandrel moving means 41. That is, the box 12 is moved to match the laser torch 5. With the above configuration, the laser torch 5 and the box body 12 can be exposed to the actual welded portion.

[0035]

[Table 5]

Table 5 shows the experimental conditions for the shield mechanism (third embodiment). This shield mechanism 40 is for preheating the outside of the tube by the high-frequency heating means 2, blowing the outside of the tube with a steam blowing means (not shown), and blowing the inside of the tube with a vertically divided shield mechanism similar to that shown in FIG. 100% for gas nozzle
Purity (correctly, the purity is 99.995% or more, but is expressed as 100% purity for convenience. The same applies hereinafter), 160 liters / min of argon gas is flowed, and 10 is supplied to the second gas nozzle.
The experiment is conducted while flowing 0% pure argon gas at 10 l / min. However, for the convenience of the experiment, the laser torch 5 and the mandrel 11 (box 12) are attached to the welded part by 2 to 2.
It is moved by 10 mm artificially, and the shield device of FIG.
It was decided to confirm the necessity of (Example).

[0037]

[Table 6]

Table 6 is a welding quality evaluation table relating to the shield mechanism (third embodiment). Examples 9-11: Laser torch and mandrel 2,
After moving 4 and 6 mm, the welding quality was good. Comparative Example 9: When the laser torch and the mandrel were moved by 8 mm, the welding quality became poor due to the inclusion of bubbles. Comparative Example 10: When the laser torch and the mandrel were moved by 10 mm, the welding quality became poor due to the inclusion of bubbles and the occurrence of undercut. Therefore, if there is a deviation amount of a certain amount (for example, 8 mm) or more, welding failure occurs, so that it is necessary to move not only the laser torch 5 but also the mandrel 11 (box 12) particularly in a large diameter pipe.

The first gas nozzles 15 of the present embodiment can be arbitrarily changed in the number, the number of rows and the blowing angle as long as they form a strong cylindrical gas flow. In addition, the second gas nozzle 1
6 ..., the number of nozzles is arbitrary as long as it forms a weak columnar gas flow.

[0040]

The present invention has the following effects due to the above configuration. The laser welding device according to claim 1 has a shield mechanism arranged in the pipe, a mandrel, a box attached to the tip of the mandrel, and a through hole formed in the center of the box for passing a laser beam. And a gas chamber formed inside the box, and a first opening opened in the box to form a ring-shaped upward gas flow having a diameter larger than the molten pool of the welded part.
It is composed of a gas nozzle, a second gas nozzle opened in the box body to form horizontally oriented gas in the through hole, and a gas supply path for supplying a shield gas to the gas chamber, and a strong cylindrical gas flow is generated by the first gas nozzle. It eliminates the lubricant decomposition gas by forming it, and forms a weak columnar gas flow by the second gas nozzle to block water vapor, etc., so that there is no concern that the molten pool will be deformed due to the weak columnar gas flow. Since the gas flow is strong, the barrier performance is high and good laser welding can be performed.

In the laser welding apparatus of the second aspect, the gas chamber is
It is divided into a first gas chamber communicating with the first gas nozzle and a second gas chamber communicating with the second gas nozzle, the first gas supply passage is connected to the first gas chamber, and the second gas supply passage is connected to the second gas chamber. Then
Since the amount of gas ejected from the first gas nozzle and the amount of gas ejected from the second gas nozzle can be controlled independently of each other, the strength of the cylindrical gas flow / columnar gas flow can be freely changed, and the appropriate shield gas can be obtained. Easy distribution.

In the laser welding apparatus according to the third aspect of the present invention, the mandrel moving means is attached to the mandrel so that the box body can be moved according to the movement of the welded portion, so that the welded portion is located outside the shield gas. It does not cause any inconvenience such as being oxidized.

In the laser welding method of the fourth aspect, the shield gas inside the pipe is constituted by a weak cylindrical gas flow and a strong cylindrical gas flow surrounding the cylindrical gas flow, and the inner diameter of the cylindrical gas flow is set to The diameter of the molten pool is at least larger than that of the welded part, and the cylindrical gas flow blocks air and steam toward the molten pool, and the cylindrical gas flow blocks air and steam toward the molten pool and heats the lubricant. Since the decomposed gas is scavenged, there is no fear of deforming the molten pool due to the weak columnar gas flow, and the high cylindrical gas flow provides high barrier performance and good laser welding.

The laser welding method of claim 5 is a method of sealing the outside of the tube having a high laser beam energy with helium gas and sealing the inside of the tube whose energy is weakened by penetrating the metal band with argon gas. By sufficiently improving the gas shield property on the inner side of the pipe with argon gas, it is possible to improve the welding quality and achieve economical efficiency by using an inexpensive gas.

[Brief description of drawings]

FIG. 1 is a schematic view of roll-type pipe forming equipment equipped with a laser welding apparatus according to the present invention.

FIG. 2 is a plan view of a laser welding device according to the present invention.

FIG. 3 is a longitudinal sectional view of a laser welding apparatus according to the present invention.

FIG. 4 is a diagram of a first embodiment of a shield mechanism according to the present invention.

5 is a plan view of FIG.

FIG. 6 is an operation diagram of the shield mechanism (first embodiment) of the present invention.

FIG. 7 is a diagram of a second embodiment of the shield mechanism according to the present invention.

FIG. 8 is a diagram of a third embodiment of the shield mechanism according to the present invention.

[Explanation of symbols]

4 ... Laser welding device, 5 ... Laser torch, 6 ... Squeeze roll, 10, 30, 40 ... Shield mechanism, 11 ... Mandrel, 12 ... Box body, 13 ... Through hole, 14 ... Gas chamber, 1
5 ... 1st gas nozzle, 16 ... 2nd gas nozzle, 17 ... Gas supply path, 21 ... Weak cylindrical gas flow, 22 ... Strong cylindrical gas flow, 31 ... 1st gas chamber, 32 ... 2nd gas chamber, 3
3 ... 1st gas supply path, 34 ... 2nd gas supply path, 41 ... Mandrel moving means, 43 ... Laser beam position control mechanism,
W ... metal strip, W1 ... end portion, W2 ... welded portion, W3 ... molten pool.

Claims (5)

[Claims] 1. A laser torch is arranged outside the tube of a metal band bent into a tubular shape, and a shield mechanism is arranged inside the tube,
In a laser welding apparatus for a pipe for laser welding a welded portion of a metal band with a laser beam emitted from the laser torch toward a shield mechanism, the shield mechanism is attached to a mandrel inserted into the pipe and a tip of the mandrel. Box, a through hole opened in the center of the box for passing a laser beam, a gas chamber formed inside the box, and a ring shape with a diameter larger than the weld pool of the welded part. A first gas nozzle opened in the box body to form an upward gas flow, a second gas nozzle opened in the box body to form a horizontally oriented gas in the through hole, and a shield gas to the gas chamber. A laser welding device for a pipe, characterized by comprising a gas supply path for supplying the gas. 2. The gas chamber is divided into a first gas chamber communicating with the first gas nozzle and a second gas chamber communicating with the second gas nozzle, and a first gas supply passage is connected to the first gas chamber, The second gas supply path is connected to the gas chamber, and the gas ejection amount of the first gas nozzle and the gas ejection amount of the second gas nozzle can be controlled independently of each other. Laser welding equipment. 3. The laser welding apparatus for a pipe according to claim 1, wherein a mandrel moving means is attached to the mandrel so that the box body can be moved according to the movement of the welded portion. . 4. A laser welding method for a pipe, comprising: irradiating a laser beam from the outside of a welded portion of a metal band formed into a tubular shape and surrounding the inside of the welded portion with a shield gas to prevent oxidation. The shield gas inside the pipe is composed of a weak cylindrical gas flow and a strong cylindrical gas flow surrounding the cylindrical gas flow, and the inner diameter of this cylindrical gas flow is at least larger than that of the molten pool of the welded part. The columnar gas flow blocks air and steam toward the molten pool, the cylindrical gas flow blocks air and steam toward the molten pool, and sweeps the pyrolysis gas of the lubricant. Laser welding method for pipes. 5. The laser welding method for a pipe according to claim 4, wherein the shield gas inside the pipe is argon gas, and helium gas gas is blown together with the laser beam from outside the pipe.