JPH10109184A - Manufacture of resistance welded tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a resistance welded tube with a stable seam quality from a comparatively thick open tube without causing any flash by preheating both edge parts of the open tube to an unmelting temperature area of a specific temperature or over, and next, abutting and joining both edge parts while heating regularly to a melting temperature area with a frequency of a specific frequency or below. SOLUTION: Before abutting and joining opposing both edge parts of the open tube 1 by squeeze rolls 5, first of all, both edge parts are preheated to the unmelting temperature area of Curie point (about 770 deg.C), for instance, by a work coil 3 for preheating. Next, the resistance welded tube is manufactured by heating regularly to the melting temperature area with a frequency of 200kHz or below by a work coil 4 for regular heating. A cooling (air-cooling) process exists between the preheating and the regular heating. However, it is preferable that this air-cooling time is regulated to 0.05 seconds or below. By heating the edge parts by this order, a melting width is uniformalized in the whole area of the edge parts of the comparatively thick open tube, and the seam quality of the resistance welded tube after joining is stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an electric resistance welded tube.

[0002]

2. Description of the Related Art Normally, an electric resistance welded tube is an open tube having a substantially circular cross section in which both sides are opposed to each other by passing a skeleton through a forming roll. It is manufactured by a method of joining by heating and passing through a squeeze roll and applying upset.

[0003] When induction heating or resistance heating is performed using a high-frequency current, the current density at the ridges on both side edges increases due to the proximity effect and the skin effect. Therefore, when the entire end face of the edge is melted, At the end in the plate thickness direction, that is, at the inner and outer surface sides of the open pipe, compared to the central part of the end face in the plate thickness direction, the spread of the melted portion in the pipe circumferential direction (referred to as a melt width) is higher and the temperature is higher.

[0004] Therefore, at the joining point of the skelp, since molten steel droplets called flashes generated by the flow and scattering of molten steel adhere to the pipe wall, there is a problem that a pressing flaw is generated on the pipe surface when pressed with a squeeze roll. was there. As a countermeasure for preventing the indentation flaw, a method has conventionally been adopted in which the heating temperature is reduced as much as possible by, for example, adjusting a high-frequency current to suppress the occurrence of flash. But,
In this method, there is a problem that the heating temperature becomes too low, and an unmelted portion appears at the center of the end face in the thickness direction, and the seam quality becomes unstable.

To solve this problem, for example, Japanese Patent Laid-Open
No. 2-299782 or Japanese Patent Application Laid-Open No. 2-299783, discloses a method of heating the edge portion of an open tube in two stages and raising the temperature above the Curie point in the first stage heating and the frequency for the first stage heating from 45 kHz to 25.
A method of setting the frequency to 0 kHz has been proposed. However, these methods are effective for relatively thin ERW pipes having a wall thickness of 4.5 mm or less, but for relatively thick ERW pipes having a wall thickness of more than 4.5 mm, flashing is not possible. It is still insufficient to obtain stable seam quality under welding conditions that can suppress the occurrence.

[0006]

SUMMARY OF THE INVENTION In view of the above-mentioned problems in the prior art, the present invention provides an electric resistance welded pipe that can produce a relatively thick open pipe into an electric resistance welded pipe having a stable seam quality without generating flash. It is an object to provide a method for manufacturing a pipe.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a method for manufacturing an electric resistance welded tube in which both opposing edges of an open pipe are heated and melted by high frequency heat and joined by means of a squeeze roll. The gist of the present invention is to preheat to an unmelting temperature range above a point and then to perform full-heating to a melting temperature range at a frequency of 200 kHz or less to perform abutment joining.

In the present invention, it is preferable that the air cooling time between the preheating and the main heating is 0.05 seconds or more. In the present invention, it is preferable to perform the preheating at a high frequency of 300 kHz or more.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of an embodiment of the present invention in which a temperature history of an end face of an open pipe edge is shown in a main part of an electric resistance welded pipe manufacturing line. Sewing tube, 3 is a pre-heating work coil, 4 is a main heating work coil, 5 is a squeeze roll, 6 is an extra bank, and 7 is an extra cutting blade.

[0010] According to the present invention, the electric resistance welded pipe 2 is first joined, for example, by the preheating work coil 3 to the Curie point ( Preheating to the unmelting temperature range of about 770 ° C) or higher, and then 200 kHz by the main heating work coil 4.
It is manufactured by main heating to the melting temperature range at the following frequency. A cooling (air cooling) process is interposed between the preheating and the main heating, and the air cooling time is preferably regulated to 0.05 seconds or less as described later.

[0011] By heating the edge portion in this order, the melting width becomes uniform over the entire edge portion of the relatively thick open tube, and the seam quality of the joined electric resistance welded tube is stabilized. The operation and effect of this configuration will be described below. Since the steel undergoes magnetic transformation from a ferromagnetic material to a paramagnetic material in a temperature range above the Curie point, the relative magnetic permeability (vs. vacuum ratio) becomes a value close to 1 as shown in FIG. On the other hand, the penetration depth of the high-frequency current is
Given by (1).

S = α {ρ / (μ _rf )} ^1/2 (1) where, S: penetration depth (m), ρ: resistivity (Ω · m),
μ _r: relative permeability, f: Frequency (kHz), α: a constant. FIG. 3 is a characteristic diagram showing an example of the temperature and frequency dependence of the penetration depth according to the equation (1). Therefore relative permeability mu _r is decreased by heating the edge portion above the Curie point, the temperature of the edge portion end face is penetration depth given is increased by the formula (1) is directed toward the homogenization. However, the end portion in the thickness direction of the end portion of the edge portion (referred to as a corner portion) at a stretch is a melting temperature range where flash does not occur, preferably
When heated to 1600 to 1650 ° C, especially when the wall thickness is relatively large, unmelted portions remain in the center in the thickness direction (called the center), and the joining tends to be incomplete, and the seam quality is stable. do not do.

On the other hand, the corner portion is once preheated to an unmelting temperature range above the Curie point, preferably below 1000 ° C.,
From there, by performing main heating to the melting temperature range by high-frequency heating at a frequency of 200 kHz or less, the temperature within the end face at the joining point becomes more uniform and the isothermal surface inside the edge part below the end face approaches the state parallel to the end face, The difference in the temperature and the melting width between the corner and the central portion is reduced, and a joining point where no unmelted portion remains can be obtained with high probability under the condition where no flash occurs, and the seam quality is stabilized. If the main heating is performed using a frequency exceeding 200 kHz, this effect cannot be obtained.

As a heating method for preheating, a high-frequency heating method is preferable as in the main heating, but a local heating method using a laser beam, an electron beam, a plasma beam or the like is also applicable. In the present invention, the air cooling time between the preheating and the main heating is preferably set to 0.05 seconds or more. As a result, the amount of heat stored at the corners at the end of preheating is diffused to the center and the temperature within the end face becomes more uniform, so the temperature distribution at the junction is further improved and the seam quality is further stabilized I do.

In the present invention, when preheating is performed by high-frequency heating, it is preferable to use a frequency of 300 kHz or more from the viewpoint of energy saving because a required current value can be reduced. Although the present invention can be applied to the production of a thin-walled electric resistance welded tube, the effect is particularly remarkable in the production of a thick-walled electric resistance welded tube.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The electric resistance welded pipe production line shown in FIG.
While passing through an open pipe made of 19% carbon steel at 30 m / min, preheat it under the conditions shown in Table 1, perform main heating, and abut it to form a 6.0 mm thick sheet.
The pipes were made into thick electric resistance welded pipes A to I having an outer diameter of 60.5 mm. The air cooling time between preheating and main heating is 0.02 seconds for tube H,
0.1 seconds. For these ERW pipes, a flattening test was performed using the rules shown in FIG. 4, and the flattening ratio (h / D) when the seam cracked was measured. ,
Tables 1 and 2 show those exceeding 2 t / D and 4 t / D or less, and those exceeding 4 t / D as x.

[0017]

[Table 1]

Table 1 shows the difference in the melting width between the corner portion and the center portion at the end face at the end of the main heating (simulated calculation value), the required current value for preheating (a less than 2000 A is a suitable level, and Was given as a non-preferred level) and the overall evaluation based on the results of the flattening test and the current required for preheating. From Table 1, it can be seen from the results of the flattening test that the preheating was stopped at 700 ° C below the Curie point and the main heating was performed at a frequency of over 400 kHz to 400 kHz, and the difference in the melting width between the corner and the center was large. X. The preheating was stopped at 700 ° C. below the Curie point, and the tubes E and F which were fully heated at a frequency of 200 kHz improved the difference in the melting width between the corner and the center compared to the tubes A and B. The flattening test result was x as in the tubes A and B.

On the other hand, although the preheating was performed up to the temperature range not lower than the Curie point, the flat test results of the tubes C and D which were fully heated at a frequency of more than 200 kHz and 400 kHz were Δ, In the tubes G, H, and I satisfying the requirements of the present invention under both heating conditions, the difference in the melting width between the corner portion and the central portion was remarkably reduced, and the flattening test result became 初 め て for the first time. And among these, the tube G preheated at a frequency (300 kHz or more) of 400 kHz in a further preferable range and the air cooling time between preheating and main heating is set to 0.1 second in a further preferable range (0.05 seconds or more) The difference in the melting width between the corner portion and the central portion was smaller than that of the tube H in which the air cooling time was 0.02 seconds, and the required current value was significantly reduced as compared with the tube I preheated at a frequency of 10 kHz.

[0020]

According to the present invention, there is an excellent effect that a relatively thick open pipe can be formed into an electric resistance welded pipe having a stable seam quality without generating flash.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an embodiment of the present invention in which a temperature history of an end face of an open pipe edge portion is shown together with a main part of an electric resistance welded pipe production line.

FIG. 2 is a graph showing temperature dependence of relative magnetic permeability of steel.

FIG. 3 is a graph showing the temperature and frequency dependence of the current penetration depth for steel.

FIG. 4 is an explanatory diagram of a flat test outline.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Open pipe 2 ERW pipe 3 Work coil for preheating 4 Work coil for heating 5 Squeeze roll 6 Excessive 7 Excessive cutting blade

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Motoaki Itaya 1-1-1, Kawasaki-cho, Handa-city, Aichi Prefecture Inside the Chita Works, Kawasaki Steel (72) Inventor Akira Ito 1-1-1, Kawasaki-cho, Handa-city, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Toshio Onishi 1-1-1, Kawasakicho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Yuji Hashimoto 1-1-1, Kawasakicho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works

Claims (3)

[Claims] 1. A method of manufacturing an electric resistance welded pipe in which opposite edges of an open pipe are heated and melted by high-frequency welding and joined by squeeze rolls, first, both edges are brought to an unmelted temperature range equal to or higher than the Curie point. Preheat, then 200kHz
A method for producing an electric resistance welded tube, comprising subjecting a main heating to a melting temperature range at the following frequency to perform abutment joining. 2. An air cooling time between preheating and main heating is set to 0.05.
The method for producing an electric resistance welded tube according to claim 1, which takes at least one second. 3. The method according to claim 1, wherein the preheating is performed at a high frequency of 300 kHz or more.