EP0423159B1 - Method and arrangement for reducing the outer diameter and wall thickness of an essentially cylindrical hollow rough-pierced tube blank by rolling - Google Patents

Abstract

In a process for reducing the outer diameter and wall thickness of an essentially cylindrical hollow rough-pierced tube blank by rolling, the front end of the tube blank is introduced into a rolling mill. To prevent piping defects during rolling and to obviate other disadvantages of conventional processes, the diameter and/or the wall thickness of the end section of the rough-pierced tube blank is reduced before the end section is introduced into the reduction zone of the rolling mill.

Description

The invention relates to a method for reducing the outer diameter and the wall thickness of a tube blank by rolling in a woodlouse mill, according to the preamble of patent claim 1.

When a steel tube is hot rolled, the diameter and wall thickness of a hollow tube blank are reduced at a temperature of approximately 1000 ° C. The reduction is carried out in a rolling mill with the aid of outer rolls which are arranged symmetrically around the pipe axis, an inner mandrel being introduced into the pipe blank. During rolling, the material mainly flows in the longitudinal direction of the tube and, due to the reducing forces and the rolling geometry, also has a tendency to expand in the transverse direction when it comes into the reduction zone. The expansion of the tube blank depends on the wall thickness of the tube. The thicker the wall thickness, the larger proportions expand in the transverse direction. When rolling the rear end part of thin-walled tubes, the expansion of the tube in the transverse direction and the associated enlargement of the tube diameter are so high that the rollers slide against the tube because you get a remaining funnel-shaped, triangular part of the tube and the tube gets stuck.

In addition to reducing the yield (the funnel must be cut off), the funnel creates many problems. It brings with it the fact that one cannot manufacture any thin-walled tube. The tubes stuck as a result of the funnel often have to be removed in a time-consuming and expensive disassembly. Subsequent pipes in the line must also be scrapped, because these then cool down too much and are not used in later work stages can be. The funnel continues to put great stress on the rollers and operating parts, with the result that even rollers can tear and clutches and gears can break. Another disadvantage is that the measurement of the tubes, which happens at the rear end part of the tubes, is made difficult.

From FR-A-10 96 090 a method for preparing the billet to be rolled out in a pilger rolling mill is known, in which the billet leaving a cross-rolling mill is pre-reduced at the front and rear ends. This reduction serves as preparation for pilgrimage and is intended to minimize the typical thickening that occurs with this method.

The invention has for its object to provide a method of the type mentioned in the preamble of claim 1, which avoids the formation of funnels during rolling in the finishing stretch and additionally ensures a reduction in temperature loss and high operating performance.

According to the invention, this is achieved in that the diameter and the wall thickness of the rear end part of the tube blank are pre-reduced during the rolling before the rear end part is introduced into the reduction zone of the woodlouse mill.

A more detailed description of the method according to the invention and the pipes produced with it and the arrangement of the devices is given with reference to the accompanying drawings.

Figur 1

eine Prinzipskizze für das Warmwalzen von Rohren in einem Assel-Walzwerk;

Figur 2

die sogenannte Schlingenbildung, die beim Walzen in einem Assel-Walzwerk entsteht;

Figur 3

schematisch eine Anordnung der Vorrichtung für die on-line Reduktion des hinteren Endteiles einer Rohrluppe und

Figur 4

einen durch das Verfahren gemäß der Erfindung erhaltenes hinteres Endteil einer Rohrluppe.

It shows:

Figure 1

a schematic diagram for the hot rolling of pipes in an Assel rolling mill;

Figure 2

the so-called loop formation that occurs when rolling in an Assel mill;

Figure 3

schematically shows an arrangement of the device for the on-line reduction of the rear end part of a tube blank and

Figure 4

a rear end part of a tube blank obtained by the method according to the invention.

Figure 1 shows how the hot rolling of a tube blank normally takes place in an Assel mill. In a hollow tube blank 1, which has been heated to about 1000 ° C, a elongated mandrel 5 introduced. The tube blank provided with the mandrel is then inserted into the pull-in part 3 between rollers 2a, 2b, 2c of the rolling mill and is pushed onto the shoulder 4, which is characteristic of an Assel rolling mill, where the reduction is carried out, after which the reduced tube is passed through the smoothing part 6 where the pipe gets its final shape. As a result of the friction between the rollers 2a, 2b, 2c and the tube 1, the tube rotates, whereby at the same time the feed is exerted in the rolling direction due to the inclined position of the rollers. Due to the relatively large width across the direction of rolling in tube blank 1 with a thin tube wall, a triangle-like so-called loop is created in the reduction zone, see Figure 2.

The loop formation is counteracted by the underlying, non-reduced material. In the rear end part of the tube blank, where the amount of material is low, particularly in the case of thin-walled tubes, this supporting effect is reduced and the loop can expand quite vigorously, as a result of which the tube 1 has an extended rear end part with a mainly triangular shape, which either entails that the pipe gets stuck or that slippage occurs between the rollers 2a, 2b, 2c and the pipe 1 during the feed.

In order to eliminate this funnel formation, which can bring about the aforementioned disadvantages, the diameter and the wall thickness of the rear end part of the tube are reduced before this part is introduced into the reduction zone. The pre-reduction can be carried out at any time during the ongoing rolling process, but before the end of the tube blank enters the Assel rolling mill, and with any suitable method, for example by plastic processing methods such as forging or rolling

A suitable arrangement to achieve the desired pre-reduction on-line is shown in Figure 3. Four pre-reduction rollers 7a, 7b, 7c, 7d are arranged self-centering around the tube blank 1 close to the Assel mill. These pre-reduction rollers are arranged in pairs on two arms 8a, 8b which can be rotated around an axis 9, the joint 9 being arranged under the tube blank 1 and the rollers. The arms can be pressed against one another by means of a force device, preferably a hydraulic cylinder, whereby the rollers 7a, 7b, 7c, 7d are also guided against each other and against the tube blank 1. In the normal position, the tube blank 1 goes freely through the pre-reduction rollers, but when the rear end part of the tube blank gets between the rollers, the power device is closed and the rollers are pressed against the tube blank, so that the tube thickness 1 and the wall thickness and diameter as the tube moves further be reduced, and the rear end part receives the appearance shown in Figure 4. The power device can be activated manually, but preferably the pre-reduction mill includes a scanning arrangement which, at a suitable distance, scans the passage of the rear end part and gives a signal, whereby the arms 8a, 8b and the pre-reduction rollers are pressed against the tube blank.

The hinge 9 is arranged on a pin 10, which can be placed in a corresponding hole in the frame of the Assel mill. This avoids the construction of a special and expensive frame for the reduction rollers.

Claims (8)

1. A method of reducing, by rolling in an Assel mill, the external diameter and the wall thickness of a tube blank which is introduced with its front part into the Assel mill, in which the tube receives its final shape, characterised in that the diameter and the wall thickness of the rear end part of the tube blank are pre-reduced during rolling before the rear end part is introduced into the reduction zone of the Assel mill.
2. A method according to Claim 1, characterised in that the rear end part is pre-reduced by means of plastic working.
3. A method according to Claim 2, characterised in that the rear end part is pre-reduced by means of rolling.
4. A method according to Claim 2, characterised in that the rear end part is pre-reduced by means of forging.
5. An apparatus for performing the method according to Claim 1, characterised by pre-reducing rolls (7a, 7b, 7c, 7d) arranged in the longitudinal direction of feed before the rolling mill, which rolls are adjustable against the tube blank (1) and can be moved away therefrom and that a scanning member is associated with the pre-reducing rolls (7a, 7b, 7c, 7d), by means of which the passage of the rear end part of the tube blank can be detected and which emits a signal for adjusting the pre-reducing rolls against the tube blank.
6. An apparatus according to Claim 5, characterised in that the pre-reducing rolls (7a, 7b, 7c, 7d) are arranged symmetrically, distributed around the tube blank (1), on two arms (8a, 8b) movable against the tube blank by means of a force device, which arms are mounted in an articulation.
7. An apparatus according to Claim 6, characterised in that the force device consists of a hydraulic cylinder with the aid of which the arms (8a, 8b) can be pressed against the tube blank.
8. An apparatus according to Claims 5 to 7, characterised in that the apparatus is connected securely to the roll stand.

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