JPH09168879A - Manufacture of laser beam welded steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an effective method for manufacturing a welded steel tube with few defective welded zone using laser irradiation. SOLUTION: In this manufacturing method, a hot rolled steel strip is formed into an open pipe with a forming roll, the amount of oxygen for welding opposing edges of the open pipe is specified to be the amount of oxygen in the welding zone shown with an area lower than a line A connecting (C)×(O)=0.006 and (O)=0.03. Next, the open pipe is pressed with a squeeze roll, both edges are butted, and then heated and molten with laser beam irradiation to join.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for manufacturing a welded steel pipe in which an open pipe is heated and melted by laser irradiation to be joined.

[0002]

2. Description of the Related Art In one of the methods for producing a steel pipe, first, a hot-rolled steel strip is continuously formed to prepare an open pipe, and then opposite edges of the open pipe are connected to each other. There is a method of heating along the longitudinal direction by high frequency induction heating or resistance heating and welding. The welding method used in this method is called an electric resistance welding method, and the steel pipe manufactured by this method is called an electric resistance welding pipe. ERW steel pipes are often used as steel pipes for machine structures, pipes, and the like. Steel pipes for machine structures include general carbon steel pipes and low alloy steel pipes.

According to the method for manufacturing a steel pipe described above, it is possible to manufacture a steel pipe with high efficiency as compared with other manufacturing methods for a welded steel pipe. However, when a steel pipe is manufactured by the above-mentioned electric resistance welding method, there is a problem that weldable minute inclusions are easily generated. The main reason for this is that, in the electric resistance welding method, unlike the normal fusion welding, a clear molten pool is not formed at the joint, so that the microscopic inclusions generated during welding are formed on the outside of the steel. It is considered that they are not discharged but remain at the bonding interface. Therefore, when manufacturing a steel pipe used for applications requiring high reliability, it was difficult to manufacture a steel pipe by the electric resistance welding method. Furthermore, it has been difficult to manufacture a steel pipe using a so-called high-grade material in which a large amount of alloy elements such as stainless steel is added by the electric resistance welding method.

As a method for solving the above-mentioned problems of the electric resistance welding method, a hot-rolled steel strip is formed to prepare an open pipe, and then the opposite edges of the open pipe are subjected to high frequency induction heating or. Japanese Patent Application Laid-Open No. 56-168981 discloses a method in which after preheating by resistance heating, the abutting point of the edge portion is melted by laser irradiation in the vicinity of the squeeze roll and pressure bonding is performed (hereinafter referred to as prior art).

Generally, heating by laser irradiation is an extremely excellent method for locally heating a steel strip to a high temperature in a short time. However, the energy that steel receives for the power it consumes is not necessarily large, and
High power laser generators are expensive. Heating by laser irradiation is a method for concentrating the applied energy within a narrow range, as described above. Therefore, when a laser is irradiated in manufacturing a steel pipe, a high temperature molten pool portion and a non-melted steel portion close to room temperature exist in a short distance. As a result, the molten pool formed by laser irradiation is rapidly deprived of heat by the surrounding low-temperature steel and rapidly cooled, so that the inclusions generated during welding are not discharged to the outside of the steel. Defects occur. Further, the strain near the weld is large and the residual stress is large. In order to solve the above-mentioned problems of laser welding, in the prior art, preheating is performed prior to laser welding. Preheating by high-frequency induction heating or resistance heating has a function of heating the vicinity of the edge portion of the open pipe in a wider range as compared with laser irradiation. Furthermore, this heating method has high heating efficiency per input energy.

The conventional electric resistance welding method as in the prior art,
According to the method combined with the laser heating method, the edge portion of the open pipe is melt-welded. Therefore, according to the prior art, a molten pool having a clear shape is formed at the joint portion, and therefore, it is expected to solve the problems caused by the minute inclusions generated during welding. Furthermore, since preheating is performed, insufficient penetration that often occurs when laser welding is performed at high speed is unlikely to occur.

However, the prior art has the following problems. That is, in order to reduce the occurrence of defects in the welded portion, the generated fine inclusions are discharged from the pool of steel by increasing the projected energy or slowing the pipe making speed, and the welded portion with less defects is generated. However, the manufacturing efficiency is significantly reduced. On the other hand, when trying to increase the manufacturing efficiency, defects occur in the welded portion.

In addition to inclusions such as oxides described above, welding defects include defects caused by gas and the like, and the cause thereof is similar to inclusions such as oxides. That is, bubbles and the like formed during melting float up in the molten pool and are discharged to the outside of the pool in the same manner as inclusions with the passage of time, but when the energy of laser irradiation is high, the molten portion is cooled. The speed is low, and the time required for bubbles to float is secured. On the other hand, when the energy by laser irradiation is low, the molten pool rapidly solidifies, and the molten portion solidifies before bubbles and the like are sufficiently discharged to the outside of the steel. As a result, a welded portion having a defect in which bubbles and the like existed in the steel is formed.

[0009]

Welded steel pipes are required to have not only excellent functions as steel pipes, but also to be manufactured efficiently and inexpensively. Therefore,
According to the prior art, a welded steel pipe cannot be efficiently obtained by simply performing preheating and joining by laser irradiation. Further, in the laser irradiation following the preheating of the open pipe, the following problems occur in the prior art. Here, it can be considered that the hot-rolled steel strip forming the open pipe is made of sufficiently deoxidized steel. Further, the passage of the laser beam near the welded portion is shielded with helium gas or the like. Therefore, it may be considered that it is not necessary to consider the effect of the amount of atmospheric components (especially oxygen) introduced into the steel pool during welding on the steel.

When the above-mentioned open pipe formed of the sufficiently deoxidized steel is preheated in the atmosphere, an oxide film is formed on the opposite edges of the open pipe. Next, the edge portion where the oxide film is formed is melted by laser irradiation. When steel is melted by laser irradiation, carbon in the steel reacts with oxygen in the oxide film to generate CO or CO ₂ gas, which causes bubbling, spatter, bubbles, and welding. Cause defects. Further, the decarburization of the steel described above reduces the strength of the steel. In order to prevent the production reaction of CO and CO ₂ gas described above, a method in which either O or C is not brought into the molten pool of steel can be considered, but a specific measure is not provided. Therefore, it is an object of the present invention to provide an efficient method for producing a welded steel pipe by laser irradiation capable of obtaining a welded steel pipe with few defects in the welded portion.

[0011]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. As a result, by controlling the relationship between the amount of oxygen contained in the weld and the amount of carbon contained in the steel by preheating the opposite edges of the open pipe within a predetermined range, a laser with few defects in the weld is obtained. It was found that a welded steel pipe can be efficiently obtained. That is, in order to prevent the production reaction of CO or CO ₂ gas, either carbon or oxygen should not be brought into the molten pool of steel. It is technically possible, if cost is neglected, to perform preheating almost completely isolated from the atmosphere. For example, it is possible to make an open pipe in vacuum, and of course, laser irradiation welding can be performed as it is. However, a significantly expensive equipment cost and operation cost are required.

Therefore, in reality, for example, at the time of preheating, the nitrogen gas is sent to the peripheral portion of the preheating portion for preheating, or
Alternatively, a method of performing preheating and laser welding premised on oxidation in the atmosphere can be considered. Oxidation in the atmosphere is affected by heating temperature and heating time. That is, as the heating temperature is higher and the heating time is longer, the oxidation progresses and a large amount of oxide is generated. As a result, the amount of oxygen brought into the molten pool also increases. On the other hand, for example, even if the amount of oxygen in the molten pool is large, if the carbon content is small, the above-described reaction of producing CO or CO ₂ gas does not occur. Thus, for example, when producing welded steel pipes by pre-heating followed by laser welding using steels that are almost free of carbon, the limits on pre-heating conditions are rather wide. Also, when producing welded steel pipes using steels with a high carbon content, the limits on preheating conditions are considerably narrower, the preheating temperature is considerably lower, and the preheating time must be shorter. Carbon is the most common and powerful alloying element in steel materials, and is added in an appropriate amount according to the purpose and application of steel. Therefore, if the oxygen content of the weld metal corresponding to the composition can be regulated, it has great industrial significance.

The present invention has been made on the basis of the above findings, in which a hot-rolled steel strip is prepared into an open pipe by a forming roll, and each of the opposite edge portions of the thus prepared open pipe is Preheat to the desired temperature,
Then, the open pipe is pressed by a squeeze roll to abut the edge portions, and the preheated and abutted edge portions are heated and melted by laser irradiation to be joined by welding. It is characterized by controlling the oxygen content of the weld metal forming the steel pipe within the region specified by the following with respect to the carbon content. (1) Carbon content 0.20 wt. % Of carbon steel, [C] × [O] ≦ 0.006 wt. %, And (2) carbon amount 0.20 wt. % Carbon steel,
[O] ≦ 0.03 wt. %. Further, the inside of the specified region is characterized by the following: (1) Carbon amount 0.20 wt. % Of carbon steel, [C] ×
[O] ≦ 0.004 wt. %, And (2) carbon content of 0.
20 wt. % Of carbon steel, [O] ≦ 0.02w
t. %.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for manufacturing a welded steel pipe. The outline of the method of the present invention is shown in FIG. In FIG. 2, 1 is a contact tip, 2 is a squeeze roll, 3 is a guide roll, 4 is laser irradiation, 5 is a steel pipe, 6
Indicates an open pipe. As shown in FIG. 2, the open pipe 6 is moved in the direction of the arrow by the guide rolls 3a and 3b and the squeeze rolls 2a and 2b. While preheating the opposite edge portions of the open pipe 6 by the high frequency induction heating of the contact tip 1, and then abutting the edge portions with the squeeze rolls 2a and 2b,
The abutting point of the edge portion is melted by laser irradiation 4 and joined to prepare a steel pipe 5.

The C content of the steel to which the method of the present invention is applied is 0.50 wt. %. 0.50w
t. When laser welding is performed on a steel containing C in excess of%, it is difficult to produce an excellent welded steel pipe due to poor ductility of the steel.

As a result of repeated diligent experiments, C <0.2: [C] on the graph shown in FIG. 1 in which the abscissa axis represents the C content of steel and the ordinate axis represents the oxygen content of the weld metal. × [O] =
It has been found that the incidence of welding defects is low in the region below the line A connecting 0.006 and C ≧ 0.2: [O] = 0.03. Furthermore, C <0.2: [C] ×
[O] = 0.004 and C ≧ 0.2: [O] = 0.02
It has been found that the occurrence rate of welding defects is even lower in the region below the line B connecting the and.

In the area above the line A, the occurrence rate of welding defects in the welded portion exceeds 2.0%, and in the area between the line A and the line B, the incidence rate of welding defects is higher. 1.0
~ 2.0%, and in the area below line B:
The occurrence rate of welding defects is 1.0% or less. It is considered that the above-mentioned reaction of generating CO and CO ₂ is influenced by the amount of the oxidizing elements (Al, Si, etc.) contained in the steel, but in the present invention, the oxidizing elements such as Si, Al, etc. Confirmed that it does not affect the occurrence of welding defects.

[0018]

EXAMPLES The method of the present invention will be described in detail with reference to examples. Steel No. 1 having the composition shown in Table 1. 1 to 10
Each was melted by converter melting and degassing, and then subjected to the processes of continuous casting, slab heating, and hot rolling to prepare a hot rolled steel strip. From the hot-rolled steel strip thus prepared, the open pipe 6 was continuously prepared by a multi-stage forming roll. Then, the opposite edges of each of the thus-prepared open pipes 6 are heated by the contact tip 1 to the preheating temperature shown in Tables 2 and 3, and then both edges are heated. , The steel of the welded portion is melted by laser irradiation 4 in the vicinity of the squeeze rolls 2a, 2b while pressing against each other by the squeeze rolls 2a, 2b, and then upset to perform the test piece No. of the welded steel pipe 5. 1 to 50 were prepared. The time for which the open pipe 6 was kept at the predetermined preheating temperature was about 5 seconds. Furthermore, the prescribed preheat temperature minus 5
The time required from the temperature of 0 ° C. to the predetermined preheating temperature was about 5 seconds.

The welding by laser irradiation described above is 800
Input 10k / w of electric power from the contact tip 1
min. The welding speed was set to 0 to 5 mm and the amount of upset was changed. Further, a laser with an output of 10 kW was used, the beam diameter at the focus position was adjusted to 0.5 mm, the focus was adjusted to the abutting point of the edge portion, and irradiation was performed from above the steel pipe 5 vertically.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

The welded steel pipes 5 each have an outer diameter of 60.
The thickness was 9.6φ and the wall thickness was 11.1 mm. Specimen No. of the prepared welded steel pipe 5 The occurrence of welding defects in each of the welds 1 to 50 was investigated. The investigation was carried out by microscopic observation of the weld. The observed cross section is a plane perpendicular to the welding line (longitudinal direction of the pipe). The observation results are also shown in Tables 2 and 3. FIG. 1 shows the results in relation to the oxygen content and carbon content of the weld metal. FIG.
Indicates that the weld defect occurrence rate is 2.0% or more,
Indicates that the weld defect occurrence rate is within the range of 1.0 to 2.0%, and ∘ indicates that the weld defect occurrence rate is 1.0% or less. The weld defect was measured in% without distinguishing between voids and inclusions.

As is clear from FIG. 1, C <0.2:
[C] × [O] = 0.006 and C ≧ 0.2: [O] =
When the oxygen content and the carbon content in the region below the line A connecting with 0.03 are excellent, the weld defect occurrence rate is 2.0% or less, which is excellent. Furthermore, C <0.2: [C]
× [O] = 0.004 and C ≧ 0.2: [O] = 0.0
When the oxygen content and the carbon content in the region below the line B connecting 2 with the weld defect rate are 1.0%.
The following is even better. Furthermore, from Table 1, Table 2, Table 3 and FIG. 1, even if the Si content of the steel changes in the range of 0.1 to 0.5% and the Al content in the range of 0.005 to 0.025%,
It can be seen that there is no effect on the occurrence of weld defects. This is because when these deoxidizing elements increase, the deoxidation is sufficiently performed and the reaction of generating CO and CO ₂ is suppressed, while the amount of inclusions increases. Therefore, when the Si and Al contents are within the above-mentioned ranges, it is considered that the occurrence of weld defects is not affected.

[0025]

According to the present invention, a laser welded steel pipe having few defects in the welded portion can be manufactured with high efficiency. The quality improvement effect, productivity, and economic value improvement effect are extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an oxygen content and a carbon content of a weld metal in a welded steel pipe and a weld defect occurrence rate.

FIG. 2 is a schematic diagram showing the method of the present invention.

[Explanation of symbols]

 1 Contact tip 2 Squeeze roll 3 Guide roll 4 Laser irradiation 5 Steel pipe 6 Open pipe

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Iwasaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Masanori Omura 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Co., Ltd. (72) Inventor Moriaki Ono 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A hot-rolled steel strip is prepared by a forming roll into an open pipe, each of the opposite edges of the thus prepared open pipe is preheated to a predetermined temperature, and then the open pipe is In the production of a welded steel pipe in which the edge portion is abutted by pressing with a squeeze roll, and the preheated and abutted edge portion is heated and melted by laser irradiation to join the welded steel pipe, the weld metal constituting the welded steel pipe The amount of oxygen in terms of carbon
A method for producing a laser-welded steel pipe characterized by controlling within a region specified by the following: (1) Carbon content 0.20
wt. % Of carbon steel, [C] × [O] ≦ 0.00
6 wt. %, And (2) carbon amount 0.20 wt. % Or more of carbon steel, [O] ≦ 0.03 wt. %. 2. The method for producing a laser-welded steel pipe according to claim 1, wherein the specified region is as follows: (1) Carbon content 0.20 wt. % Carbon steel,
[C] × [O] ≦ 0.004 wt. %, And (2) carbon amount 0.20 wt. % Of carbon steel, [O] ≦ 0.
02 wt. %.