JPH04288978A - Welding method for thick electric resistance welded tube - Google Patents

Abstract

PURPOSE:To manufacture the thick electric resistance welded tube having an excellent quality by reducing a welding defect of the thick resistance welded tube. CONSTITUTION:At the time of welding an electric resistance welded tube, a resistance welding phenomenon of a thick resistance welded tube of >=8mm wall thickness can be stabilized by welding it by >=200kHz welding frequency, and also, >=1000Angstrom welding machine output, and generation of a welding defect can be prevented. This method is very effective for preventing the welding defect, and such excellent effects as the product yield is improved, and also, toughness of a weld zone is improved are obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing thick-walled electric resistance welded steel pipes.

0002

[Prior Art] As is well known, in the production of electric resistance welded steel pipes,
The edges of the pipe material (steel strip) to be welded are heated by electrical resistance or induction, and then pressure welded (welded) using squeeze rolls. Welded parts of electrical resistance welded steel pipes manufactured in this manner often have defects such as cold welding defects due to insufficient melting and welding defects due to residual oxides, which also deteriorate toughness and workability. In electric resistance welding, "Steel Research" No. 277 "Experiments on ERW steel pipe welding mechanism", it is effective to control welding phenomena to prevent welding defects, and there are appropriate welding conditions of high speed and large heat input, and under these conditions It has become clear that welding defects are less likely to occur even if slight variations in welding conditions occur. These are disclosed in Japanese Patent Publication No. 54-33784 and Japanese Patent Publication No. 54-40454 as "Method for monitoring and controlling high-frequency electric resistance welding phenomena" and "Monitoring and controlling device for high-frequency electric resistance welding phenomena." ing. In welding thick-walled ERW steel pipes, high welding power is required in order to satisfy the above-mentioned appropriate welding conditions of high speed and large heat input, so it is necessary to weld with a high-output welding machine. For example, when welding a thick-walled electric resistance welded steel pipe with a wall thickness of 8 mm, a welding speed of 20 m/min or higher is required to obtain appropriate welding conditions that are unlikely to cause welding defects; A heat input of 1000 kW or more is required. In other words, in order to prevent welding defects from occurring when welding thick-walled ERW steel pipes with a wall thickness of 8 mm, it is necessary to
It is necessary to weld with a welding machine having an output of 00 kW or more. On the other hand, due to its structure, high-frequency welding machines have the characteristic that when the output is increased, the welding frequency decreases.
The welding frequency of a welding machine with an output of 1000kW or more is generally 200kHz when considering the manufacturing cost of the welding machine.
less than z. However, this output of 1000kW or more,
A wall thickness of 8m was achieved using a welding machine with a welding frequency of less than 200kHz.
When welding thick-walled ERW steel pipes with a thickness of 20 m/m or more, the welding speed is 20 m/m, which is considered to be the appropriate welding condition in conventional technology.
Even under welding conditions where the welding heat input is 1000 kW or more and the welding heat input is 1000 kW or more, the welding phenomenon is not stable and weld defects occur frequently, resulting in the inconvenience that excellent weld quality cannot be obtained.

0003

[Problems to be Solved by the Invention] The present invention was invented to solve the above-mentioned problems.When welding thick-walled electric resistance welded steel pipes, welding frequency of 200kHz or more and welding machine output of 1000kW or more is required. By welding, welding defects in thick-walled ERW steel pipes with a wall thickness of 8 mm or more are significantly reduced,
At the same time, the aim is to improve the toughness and workability of the ERW welded part and to manufacture thick-walled ERW steel pipes of superior quality.

0004

[Means for Solving the Problems] The gist of the present invention is to weld thick-walled ERW steel pipes, which is characterized in that when welding ERW steel pipes, welding is performed at a welding frequency of 200 kHz or more and a welding machine output of 1000 kW or more. It's a method. The inventors,
Based on the prediction that the above-mentioned disadvantages of the conventional technology for thick-walled electric resistance welded steel pipes with a wall thickness of 8 mm or more are related to the welding frequency,
A prototype welding machine with a frequency of 00 kHz or more was manufactured, ignoring the manufacturing cost of the welding machine, and the following research was conducted. Wall thickness 8mm
Regarding the above thick-walled ERW steel pipes, welding frequency, welding speed,
Using a high-speed camera, we observed the series of heating → melting → pressurization phenomena that occur on both edges between the power supply position and the center of the squeeze roll when welding heat input was varied, and investigated the relationship between welding phenomena and welding frequency. As a result of my research, I discovered that there is a phenomenon that is completely different from conventional wisdom. The present invention has been made based on these findings, and details thereof will be described below.

[0005]

[Operation] In electric resistance welding, high frequency current is applied to both edges 1 and 2 as shown in Fig. 1 due to its skin effect and proximity effect.
The flow concentrates at the corner portions 3, 4, 5, and 6 of the electric resistance welded steel pipe, and as a result, a drum-shaped heat effect trace remains at the welded portion of the electric resistance welded steel pipe, as shown in FIG. This indicates that the central part of the thick wall is heated more slowly than the upper and lower surfaces. The heating delay for the upper and lower surfaces of the center of the wall thickness can be determined by the difference in heat influence width between the center of the wall thickness and the upper and lower surfaces. In FIG. 2, a is the heat affected width of the upper surface, and b is the heat affected width at the center of the wall thickness. Therefore, the heating delay for the upper and lower surfaces of the center of the wall thickness is
It can be expressed as a heating delay ratio (b/a). Heating delay ratio (b/
When the value of a) is 1.0, there is no heating delay, and the smaller the value of the heating delay ratio (b/a), the more severe the heating delay is. In this study, we found that this heating delay at the center of the wall thickness becomes more severe as the wall thickness increases and as the welding frequency decreases. This situation is shown in FIGS. 3 and 4. Furthermore, from FIGS. 3 and 4, it can be seen that this delay in heating at the center of the wall thickness becomes significant in the case of a thick wall of 8 mm or more and in the case of a low welding frequency of less than 200 kHz. Furthermore, as a result of observing the welding phenomenon using a high-speed camera, it was confirmed that this heating delay at the center of the wall thickness makes the welding phenomenon uneven in the thickness direction, causing the welding phenomenon to become unstable. As a result of the above research, it was found that in electric resistance welding of thick walls of 8 mm or more, a welding frequency of 200 kHz or more is required to stabilize the welding phenomenon. In electric resistance welding of thick walls of 8 mm or more, in order to stabilize the welding phenomenon and perform welding under appropriate welding conditions, it is necessary to weld at a welding frequency of 200 kHz or more and a welding machine output of 1000 kW or more. It is. Next, Table 1 shows examples of the present invention.

[0006]

【Example】

[Table 1]

[0007]

[Effects of the Invention] According to the present invention, it is very effective in preventing welding defects, and has the excellent effect of improving product yield as well as improving the toughness of the welded part, and the effects are extremely large. It is.

[Brief explanation of the drawing]

[Fig. 1] An explanatory diagram showing the heating status of the edge part of the electric resistance welding part immediately before joining,

[Fig. 2] An explanatory diagram showing the heat-affected state of the electric resistance welding part,

[Figure 3
] Diagram showing the relationship between wall thickness and heating delay ratio (b/a),

FIG. 4 is a diagram showing the relationship between welding frequency and heating delay ratio (b/a).

[Explanation of symbols]

1, 2 Edge 3, 4, 5, 6 Edge/corner part a Upper surface heat affected width b Thickness center heat affected width

Claims (1)

[Claims] 1. A method for welding thick-walled electric resistance welded steel pipes, which comprises welding the electric resistance welded steel pipes at a welding frequency of 200 kHz or more and a welding machine output of 1000 kW or more.