GB2239414A

GB2239414A - Hot-rolling seamless pipes in a plug mill

Info

Publication number: GB2239414A
Application number: GB9023247A
Authority: GB
Inventors: Karl Heinz Hausler; Hermann Josef Frentzen
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1989-10-27
Filing date: 1990-10-25
Publication date: 1991-07-03
Anticipated expiration: 2010-10-25
Also published as: DE3936326A1; IT9021597A1; GB2239414B; FR2653689A1; GB9023247D0; IT9021597A0; IT1243979B

Abstract

In a process for hot-rolling seamless pipes in a plug mill, a pusher guides the hollow block into the roller nip of the plug stand, the working rolls of which roll out said block, drawing it over the plug of a stopper rod in a reduction pass to reduce the wall thickness, and form a pipe. After the reduction pass the pipe is conveyed back to the entry side of the plug stand in a return pass with the working rolls open by means of return rollers arranged in a stand on the exit side in order to perform at least a second reduction pass. The pipe is smoothed between the return rollers without or with only a slight reduction in the return pass.

Description

1 ;, 2 3 4E a 1- 4:

1 Pipe rolling process on a plug mill train The invention relates to a process for hotrolling seamless pipes in a plug mill. In such a process a pusher guides the hollow block into the roller nip of the plug stand, the working rolls of which roll out said block, drawing it over the plug of a stopper rod and reducing the wall thickness, to form a pipe. After the reduction pass the pipe is conveyed back to the entry side of the plug stand with the working rolls open by means of return rollers arranged in a stand on the exit side in order to perform at least one further pass in the same sequence.

There are two well known processes for the production of seamless pipes, namely the plug rolling process and the continuous rolling process. Both processes have the following drawbacks:

In the plug rolling process, a heated workpiece is rolled over a stationary plug supported on a rod. After rolling, the plug is removed and the tube blank is removed from the rod. This operation is repeated, normally two or three times until the tube blank is drawn to the predetermined wall thickness, with different plugs being used each time. The rolling process makes it necessary to feed the tube blank subsequently to smoothing rolls in the form of a skew rolling mill, so that the flank bulges remaining after rolling in the working rolls of the plug stand are removed. The rolling operation in the plug stand and in the subsequent smoothing rolls takes up so much time that the pipe is too cold for the final reduction rolling. It is therefore necessary to bring the pipe to rolling temperature again in an intermediate reheating furnace.

In the continuous rolling process, on the other hand, a workpiece is drawn out through roll stands arranged in series via a travelling rolling rod. The 1 2 number of stands is normally eight for a freely travelling rolling rod, and five to seven for a rod which travels in guided manner. With a freely travelling rod, reheating is likewise necessary, but not for a guided rod. What is disadvantageous in both processes is the relatively large number of costly roll stands.

The present invention is directed at a rolling process of the above general kind which can achieve at least equal quality without reheating and use a small number of roll stands. According to the invention, a method of hot-rolling seamless pipes comprises guiding a hollow workpiece into the roller nip of a plug stand, the working rolls of which roll out the workpiece, drawing it over the plug of a stopper rod and reducing the wall thickness in a reduction pass; conveying the workpiece back to the entry side of the plug stand in a return pass with the working rolls open and using return rollers arranged in a stand on the exit side, the workpiece being smoothed between the return rollers, and repeating at least one reduction pass to further reduce the wall thickness.

In a method according to the invention the pipe or workpiece is smoothed between the return rollers without or with only a slight reduction per pass when being conveyed back to the entry side of the plug stand. In thismanner. without using additional stands subsequent smoothing rolls become unnecessary due to the fact that the smoothing pass is performed in the rollers of the return rolling mill. This reduces not only the costs in terms of plant but also the processing time of the pipe, so that reheating may no longer be necessary.

A method of the invention is normally completed at the end of a return pass. However, after it has completely left the return rollers the pipe may be introduced into the return rollers in the opposite direction, rotated about its longitudinal axis, preferably through an angle substantially 90 degrees.

1 1 3 This rotation of the pipe means that the laterally freely widened bulges produced upon rolling in the working rolls of the plug stand come to lie in the base of the groove of the return rollers and are evened out there. This method also has the advantage that due to the rotation of the pipe on the exit side before the second pass renewed rotation of the pipe is no longer necessary, so that no time is lost.

According to the invention, apparatus for carrying out the above method apparatus comprises a plug mill with return rollers defining a return pass for a rolled workpiece, the apparatus including a guide on the exit side of the plug mill permitting the pipe to emerge freely, and which cooperates with a device for transporting the pipe into the return pass. Preferably, the stopper rod of the plug mill is guided in the direction of rolling pasing through the plug stand and the stand of the return rollers during rolling. This has advantages in terms of rolling and reduces the wear on the plug. The stopper rod may be held equally well on the entry side and on the exit side of the plug mill.

According to one variant of the above apparatus, the guide on the exit side consists of a non-driven. roller table, to which is allocated at least one driven drive roller which can be placed on the pipe. This drive roller is necessary because in the method proposed the pipe must leave the return rollers completely, so that arrangements must be made for transportation back and introduction into the return rollers upon the change of direction.

In another variant of the apparatus, provision is made for the rollers of the return stand to be arranged offset preferably by 90 degrees to the working rolls'of the plug stand. With this solution, rotation of the pipe at the exit side can be dispensed with.

In a typical process according to the invention the procedure is as follows:

4 In a first pass, a hollow workpiece introduced into the working rolls of the plug stand by the pusher is rolled out, with the workpiece being widened in known manner. The return rollers are opened, so that the rolled-out pipe can emerge unhindered, until it comes to lie in the exitside guide of a roller table. The pipe is rotated by 90 degrees using conventional means and is passed back to the return rollers and introduced therein with the aid of a driver in the form of a drive roller which can be lowered on to the pipe. The return rollers are calibrated in such a manner that in the closed state in addition to their transportation function for the pipe they act at the same time as smoothing rolls with which the flank bulge formed in the working rolls of the plug stand is smoothed. Optionally, a slight reduction in wall thickness may be carried out. Since the reduction in wall thickness is only very small, the light construction of the return roller stand can be retained.

A second and subsequent cycles can operate in the same manner, so that a pipe with a regular wall thickness and without a flank bulge is obtained which no longer has to pass through any subsequent smoothing rolls. Reheating of the pipe is likewise no longer necessary. Since the tube blank has already been rotated before the return rolling, it no longer needs to be rotated before the second pass and no time is lost.

The rod which is moved in its longitudinal direction during rolling is either held on the entry side or on the exit side. The head of the rod is rounded like a rolling plug. so that upon the first pass the hollow block is rolled in widening manner. The rod head lies with its cylindrical beginning directly under the working roll at the start of rolling.

1

Claims

1. A method of hot-rolling seamless pipe in a plug mill, comprising guiding a hollow workpiece into the roller nip of a plug stand, the working rolls of which roll out the workpiece, drawing it over the plug of a stopper rod and reducing the wall thickness in a reduction pass; conveying the workpiece back to the entry side of.the plug stand in a return pass with the working rolls open and using return rollers arranged in a stand on the exit side, the workpiece being smoothed between the return rollers, and repeating at least one reduction pass to further reduce the wall thickness.

2. A method according to Claim 1, wherein after it has completed a return pass the workpiece is introduced into the return rollers in the opposite direction, rotated about its longitudinal axis by substantially 90 degrees.

3. A method of hot-rolling seamless pipe according to Claim 1 is substantially as described herein.

4. Apparatus for hot-rolling seamless pipe comprising a plug mill with the return rollers defining a return pass for a rolled workpiece, the apparatus including a guide on the exit side of the plug mill permitting the pipe to emerge freely. and which cooperates with a device for transporting the pipe into the return pass.

5. Apparatus according to Claim 4 wherein the guide on the exit side is designed as a non-driven roller table, to which is allocated at least one driven drive roller which can be placed on the pipe.

6. Apparatus according to Claim 4 or Claim 5 wherein the return rollers are arranged offset relative to the working rolls of the plug stand.

7. Apparatus according to Claims 4 to 6, wherein the stopper rod of the plug mill is guided in the direction 6 of rolling passing through the plug stand and the stand of the return rollers.

8. Apparatus according to Claim 7 wherein the stopper rod is held on the entry side or exit side of the plug mill.

9. Apparatus for hot-rolling seamless pipe according to Claim 4 and substantially as herein described.

Published 1991 at 7he Patent Office. State House. 66171 High Holborn. London WC1R4TP. Further coples"y be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent.