JPS60223676A - Production of welded pipe - Google Patents

Abstract

PURPOSE:To prevent formation of oxide which is a cause for self-cracking and to suppress generation of flush and the discharging amt. of the molten steel in a weld zone in a method for producing an electric welded steel pipe by maintaining the neighborhood of a heating part under a reducing atmosphere and maintaining welding temp. relatively low. CONSTITUTION:A steel strip ST is fed in a dropout arrow direction and is formed into U-shape, then to an approximately circular shape by various rolls, by which an open pipe OP is formed. The open pipe is fed to a high-frequency induction heating coil 1. A solvent of an adjusted amt. is oozed from the aperture in the bottom of a storage vessel 71 for the solvent such as alcohol and is coated on the end faces on the opposite sides of the pipe OP positioned between the coil 1 and the final fin pass roll 42. The side end faces of the pipe OP coated with the solvent on the side end faces thereof are heated by the coil 1 to which high-frequency current is supplied. The pipe is pressurized from both sides and is welded at a relatively low temp. of 1,150-1,350 deg.C at a melting point A, by which an electric welded steel pipe P is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of manufacturing a welded pipe such as an electric resistance welded steel pipe using a welding technique such as high frequency welding.

[Prior art]

In general, in the method of manufacturing ERW steel pipes, the steel strip is rolled through multiple breakdown rolls and side rolls.

It is continuously fed to a group of forming rolls consisting of fin pass rolls, etc., first formed into a U-shape, and then bent into a substantially circular open pipe.The open pipe is fed to a welding machine, and then A high-frequency current is passed through the opposite end face using an induction coil method or a therma tool method to heat it, and then it is pressed together with a squeeze roll, and the bead produced by this pressure welding is cut from the inside and outside with a bead cutting jig. As a result, ERW steel pipes are manufactured.

The great advantage of this method is that it is highly efficient compared to other pipe manufacturing methods, and it is possible to reduce the cost of oil country tubular goods, steel pipes for line pipes, steel pipes for boilers, steel pipes for NA machine structures, etc.

The welding temperature in this method of manufacturing ERW steel pipes generally refers to the temperature range from the opposite end of the open pipe, which is 30 mm upstream of the welding point where the open pipe is pressed with a squeeze roll and welded, to 50 mm downstream of the welding point. It refers to the temperature in the area up to the welding part, which is within the range of 1350 to 1550°C. This is because if the temperature is higher than this temperature range, welding defects called penetrators are likely to occur, whereas if the temperature is lower than this range, it becomes a cause of white cracking, which will be described later. However, in this temperature range, there is a large amount of molten steel at the opposite end of the open pipe, and therefore, when pressure welded with squeeze rolls, molten steel is inside the ERW steel pipe.
Scattered outward. This scattering is called a flash. When flash occurs, so-called flash clogging occurs between the inner and outer bead cutting jigs and the electric resistance welded steel pipe, which makes it easy for the bead cutting jig to be damaged or scratches to occur on the surface of the electric resistance welded steel pipe.

In addition, when molten steel is discharged during pressure welding, an umbrella-shaped bead force is created, but when there is a large amount of molten steel, the overall width becomes wider, and a large amount of cutting powder is generated, causing the beads on the inner and outer surfaces. Since so-called bead clogging is likely to occur between the cutting jig and the electric resistance welded steel pipe, the bead cutting jig may be damaged or the cutting operation may be difficult.

Although such bead shape and flash can be suppressed or prevented by lowering the welding temperature to 1350° C. or lower, white cracks are more likely to occur.

In Figure 1, the horizontal axis represents the aspect ratio (%), and the vertical axis represents the welding temperature (%).
C), compress the welded pipe from above and below with the welded part positioned horizontally, and when cracking occurs, the length of the minor diameter at the outer periphery is the numerator, and the outer diameter before compression is the denominator. This is a graph showing the relationship between the test results and welding temperature after conducting a flattening test to check the quality of the welded part using the flattening ratio of 0.12% C and 0.50% Mn. Results for ERW steel pipes: C: 0.22%, Mn: 0160%
The results are shown for ERW steel pipes. Note that the broken line in the figure indicates 1350°C. As can be understood from this figure, when the welding temperature is 1350° C. or lower, the workability expressed by the aspect ratio deteriorates. This is because defects called white cracks occur at the cracked parts of the weld.

This white cracking is caused by oxides that form in the weld at temperatures below about 1,350°C, and unlike chromium oxides that form at temperatures above 1,350°C, these oxides are formed on the opposite end of the open pipe. When the material is heated, the alloy components in the material are oxidized by oxygen in the atmosphere, and among the oxides produced, those with high melting points (oxides such as Mn, Si, 'Fe, etc.) are discharged to the bead part. It remains inside the welded area and causes minute welding defects on the entire surface of the welded area, and therefore significantly deteriorates the mechanical properties confirmed by tube expansion, flattening, and Charpy tests, and reduces the reliability of ERW steel pipes. This is one of the reasons for limiting its uses.

〔the purpose〕

The present invention has been made in view of the above circumstances, and its purpose is to prevent the generation of oxides that cause white cracks, and to suppress the occurrence of flash and the amount of molten steel discharged from the welded part. The present invention provides a method for manufacturing a welded pipe.

[Structure of the invention]

The method for manufacturing a welded pipe according to the present invention includes bending a metal strip into an open pipe and heating the opposite end of the pipe by passing an electric current through the same.
In a method of manufacturing a welded pipe by applying pressure from the side, the vicinity of the heating part is made into a reducing atmosphere, and 1150 to 13
It is characterized by welding at a welding temperature of 50°C.

〔Example〕

The method of the present invention will be explained in detail below based on the drawings showing its implementation state. FIG. 2 is a plan view schematically showing a main part of an electric resistance welded steel pipe manufacturing line using the method of the present invention, and FIG. 3 is a schematic vertical cross-sectional view taken along the line ■-■ in FIG. The steel strip ST, whose side edges have been finished with a surface suitable for welding using a mixing device (not shown), is fed in the direction of the white arrow to break down rolls 61a, 61b, 62a, 62b and side rolls 511, 51r, 52. N, 52r and 5
34! , 53r into a U-shape. Fin pass rolls 41 and 42 arranged vertically opposite each other (only the upper roll is shown in the figure) and the final side roll 541 between the fin pass rolls 41 and 42,
Open pipe OP formed into a substantially circular shape by 54r
The opposite edges of the fin plate of the upper fin pass roll are accurately guided and fed to the induction heating coil 1. An open pipe O located between the induction heating coil 1 and the last fin pass roll 42
Above P are the opposite end surfaces 8J and 8 of the open pipe OP.
Solvent 11 to be applied to r such as alcohol, ketone.

A container 71 is provided with an opening 71a at the bottom for storing ether etc. and discharging the solvent.

The conduit 73, one end of which is connected to the opening 71a, is equipped with a valve 72 at an appropriate position so that the amount of solvent oozing out from the inside of the conduit 73 can be adjusted. Further, the other end of the conduit 73 is divided into two openings 73.
N, 73r, with a brush 74j at the tip! ! .

74r is attached, and the solvent seeping out from the container 71 is transferred to the opposite end surface 84 of the oven pipe OP! , 8r. In this way, the oven pipe OP whose side end face is coated with solvent is heated from the high frequency power source 2 to 200~
A high frequency current of about 500 kHz is supplied to the induction heating coil 1, and an induced current is generated in the open pipe OP. This induced current causes the opposite end surface 811.
The oven pipe OP heated at 8r is pressurized from both sides by squeeze rolls 31.3r and welded at a relatively low welding temperature of 1150 to 1350°C at a welding point A to become an electric resistance welded steel pipe P.

When manufacturing an electric resistance welded steel pipe by such a method, when the opposing end surfaces 81t and 8r of the oven pipe OP are heated,
The opposite end surface 8j! , 8r has a very low boiling point, so it easily vaporizes, and its vapor pressure shields the area around the heat dissipation part from the atmosphere. In addition, the steam reacts with oxygen to perform a reducing action, thereby reducing the amount of oxygen gas around the heat dissipation section. Therefore, the oxidation of alloy components such as Mn, Si, and Fe in the material is prevented, and the generation of oxides can be effectively prevented.

In addition, since welding is performed at a relatively low welding temperature, the amount of molten steel can be appropriately reduced, and even if pressure is applied from both sides with squeeze rolls 51.5r, the amount of upcent can be adjusted appropriately, and therefore the amount of discharged molten steel can be reduced. The vehicle width at the bead portion can be reduced and flash can be prevented.

In the above explanation, the solvent is applied to the opposite end of the open pipe, but the present invention is not limited to solvents, but can also be applied to substances that have a low boiling point and exhibit reducibility in a gaseous state, such as Mg, CB, etc., or mixtures thereof. It goes without saying that the area to be coated is not limited to the opposing end of the open pipe, but may also be applied to the inner surface of the open pipe. It also has the effect of reducing the oxidized oxides.

Further, in the present invention, it goes without saying that the reducing atmosphere may be created by other methods than the coating method. For example, an open container for storing organic liquid is placed inside an oven pipe or ERW steel pipe, and the organic liquid is vaporized and sealed. Of course, it is also possible to use a method in which the periphery of the heat radiating portion is sealed with a reducing flame portion inside the combustion flame, or a method in which a reducing atmosphere is created using a seal box.

〔effect〕

Next, the effects of the present invention will be explained based on Examples. C:
0.30%, Si: 0.25%, Mn = 0.5
6%.

An electric resistance welded steel pipe was manufactured according to the present invention using a steel band containing Cr: 1.0% and Mo: 0.2%.

Figure 4 is a graph showing the defect occurrence rate (marked by .) in the above manufacturing case, with the defect occurrence rate (%) on the horizontal axis and the welding temperature (t') on the vertical axis. When only the welding temperature of the present invention is excluded from the conditions (O mark), and when the welding temperature is 13
Conventional welding method (X
The results for the cases marked with 〉 are also shown. As can be understood from this figure, in the case of the present invention, no defects occur, and when the welding temperature of the present invention is outside the condition of L7, 13
At high temperatures exceeding 50℃, the defect rate is zero, but
At low temperatures below 1150°C, welding failure occurred, that is, cold welding occurred. Furthermore, in the case of the conventional method, welding defects such as oxides occur regardless of the welding temperature because the area around the heat dissipation part is an oxidizing atmosphere.

Figures 5(a) and (b) show the shape of the bead of an ERW steel pipe manufactured according to the present invention at a welding temperature of 1300°C, and the bead part of an ERW steel pipe welded at 1450°C, which is outside the welding temperature range of the present invention. FIG. 3 is a schematic vertical cross-sectional view showing the shape or welding temperature of a bead portion according to a conventional method at a welding temperature of 1450° C., and the bead portion is shown by hunting. As can be understood from this figure, in the case of the present invention, the welding temperature is relatively low, so the amount of discharged molten steel is small, and the vehicle width is very small, making it possible to obtain a good width. When the welding temperature is outside the welding temperature range of the invention, the amount of discharged molten steel is large, resulting in a bead with a large overall width and poor shape as shown in FIG. The bead cutting efficiency was improved by 20 to 30% compared to cases where relatively high welding temperatures were used.

As detailed above, in the case of the present invention, since welding is performed at a relatively low welding temperature, the amount of molten steel is not excessive, and therefore flash during pressure welding can be prevented, and therefore the bead cutting jig can be welded. Corruption.

It is possible to prevent scratches on the electric resistance welded steel pipe and to make the cutting work smoother, and by optimizing the amount of molten steel, the umbrella shape of the repeat part can be improved, and therefore the efficiency of the cutting work can be improved. Furthermore, since the atmosphere is reducing, it is possible to prevent the generation of oxides that cause white cracks, and therefore, the present invention has excellent effects such as being able to use welded pipes without limiting the application.

[Brief Description of the Drawings] Figure 1 is a graph used to explain welding defects in the prior art, and Figure 2 is a plan view schematically showing the main parts of an ERW steel pipe manufacturing line using the method of the present invention. , Figure 3 is m- in Figure 2.
The schematic vertical cross-sectional view taken along the m-line, FIGS. 4 and 5 are graphs and diagrams showing the effects of the present invention. ST...Strip steel OP...Open pipe P...
ERW steel pipe 1...induction heating coil 71...container
Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Noboru Kono No. 5 020 40 Go 80 100

Claims (1)

[Claims] 1. A method of manufacturing a welded pipe by bending a metal strip into an open pipe, heating the opposite end of the pipe by passing an electric current through it, and applying pressure from the side. A method for manufacturing a welded pipe, characterized by welding at a welding temperature of °C.