GB2179283A

GB2179283A - A method of hollow rolling

Info

Publication number: GB2179283A
Application number: GB08620464A
Authority: GB
Inventors: Karlhans Staat
Original assignee: Kocks Technik GmbH and Co KG
Current assignee: Kocks Technik GmbH and Co KG
Priority date: 1985-08-24
Filing date: 1986-08-22
Publication date: 1987-03-04
Also published as: IT1197101B; GB8620464D0; DE3530343C2; IT8621504A1; DE3530343A1; FR2586368A1; GB2179283B; IT8621504A0

Abstract

In a method of piercing a solid round billet 6 by rolling it over the plug or piercer 4 of a mandrel 2, the position of the piercer 4 of an adjustably supported mandrel 2 in the roll sizing gap is measured and is kept constant with the aid of regulating means by adjusting the mandrel 2 in dependence on changes in the rolling mill and mandrel geometry. On the assumption that the stress in the mandrel depends upon its strain and therefore upon changes in its geometry, the load on the mandrel 2 can be measured by measuring the pressure in a cylinder 10 acting on the rear end of the mandrel. <IMAGE>

Description

SPECIFICATION A method of hollow rolling The invention relates to hollow rolling in which work material is rolled over a piercerofa mandrel.

Pierce-rolling of solid round billets is conventional ly carried out in skew-rolling mills having two orthree skew rolls which are disposed at an angle to one another and to the axis ofthe round billetortothe mandrel and which are counter-rotated. The round billet is drawn in a screw-like manner into the conver gent roll gap formed between the mandrel plug or piercerand the rolls and is shaped into athick-walled bloom bythe piercing and transverse-rolling partof the rolls. The roll gap between the mandrel piercer and the outer surfaces of the rol Is determines the thickness of the wall orthetubular bloom.As, howev er, the work material and in particularthe mandrel are subject to changes in length during rolling as a result ofthe high rolling forces and the influence ofthework material temperature, changes in the roll gap auto matically occur during the usual stationary suspen- sion ofthe mandrel during the piercing process.

Whereas changes in the length ofthe rolling stand are generally negligible, the mandrel piercer is pushed out ofthe roll gap to a greater or lesserextent depending on the axial forces acting on it, and the mandrel rod is thus compressed owing to its fixed support. In addition,thefree length ofthe mandrel must be increased as the length ofthe tubular bloom is increased. As the diameter of the piercing rod is defined bytheinnerdiameterofthetubularbloom and as the piercing rod is often of a tubularform for reasons of cooling,the piercing rod is considerably compressed during rolling due to its relatively low buckling strength.When, in the final piercing phase, the axial stress of the mandrel suddenly sharply decreases as it penetrates the billet base, and the rolling orfeeding force acting on thetubular bloom issimultaneously considerably reduced because the billet or tubular bloom end upon which the piercing partofthe skew rolls acts becomes inreasingly smaller until the billet end leaves the roll gap, there is a sudden consid- erablereliefoftheload onthemandrel.Themandrel, which is subjected to maximum compression during the final piercing phase, is thus caused to elongate again in the opposite direction to the rolling direction.

This elongation causes the mandrel piercerto move into the sizing pass and hence the roll gap to become narrower. This can cause the end ofthe tubular bloom to become wedged inthe roll gap orcan lead to unwanted protuberances ortho a trumpet-shaped formation on the end ofthetubularbloom.

To combat this, it is known to retract the mandrel as soon as its tip penetrates the billet base. However, this automatically leads to an enlargementofthe roll gap andthisto a largerwall thickness and a corresponding waste of material, as the thicker end ofthe tubular bloom must be cut off before further processing in a skew-rolling mill. Furthermore, the position ofthe piercer in the sizing pass is also dependent on the resistance to deformation of the billet material which is a product of its composition, the thickness of the material and the temperature ofthe billet, which decreases during piercing.

In a further method known from German Patent Specification (Offenlegungsschrift) No.3328 269A, however, a recess is bored or melted into the billet base so that, in the final pierce-rolling phase, the mandrel initially enters this recess and, as a result, if the dimensioning of the recess is correct, trumpet shaped bloom ends can no longer be formed. Howev er, considerably complex apparatus is required in this method to produce the recess, and a not inconsider able wastage of material is involved.

Additionally, it is extremely difficult to dimension the recess correctly and changes in the material can occur if the recess is made, for example, by an electric arc.

Furthermore, the two above-mentioned known methods also have the disadvantage that it is not possibletocompensateforchangesinthedimensions ofthe mandrel and piercer rod, and hence changes in the roll gap, during rolling.

Similar problems to those found in pierce-rolling are also encountered in expanding mills and longitudinal rolling mills in which the position of the plug or piercerand hence the roll gap are also subjecttothe influence of changes in length and temperature as well as to the material composition.

It is an object ofthe invention to avoid the disadvantages of conventional hollow-rolling methods and in particularto provide a method which ensures a uniform wall thickness along the entire bloom ortube length together with low material wastage at the end ofthetubularbloom.

The invention resides in a method of hollow rolling in which work material is rolled over a plug of a roll mandrel and the position of the plug relative to the rolls is continuously or repeatedly measured and is adjusted accordingly.

Preferably, the position of the plug is keptsubstantially constant, with the aid of regulating means.

Accordingly, if the mandrel rod is shortened, the mandrel moves into the sizing pass. Conversely, the mandrel is automatically retracted when the mandrel tip penetrates the base ofthe billet and the axial stress ofthe mandrel is reduced orthe roll gap narrows during expanding or longitudinal rolling oftubes.

The position ofthe mandrel can be determined by measuring the longitudinal force acting on the mandrel rod, for example using wire strain gauges on the mandrel rod or mandrel support, and can be adjusted to produce a constant roll gap taking into accountthe empirically established spring characteristic ofthe rolling mill/roll mandrel system.

In a rolling mill according to the invention,the mandrel rod can be supported againstthe piston rod of an adjusting cylinder by a movable support. The cylinder moves the support along the mandrel axis according to the longitudinal force acting on the mandrel rod and takes into account the above-mentioned spring characteristic.

The invention is further described by way of example, with reference to the accompanying drawing which is a diagrammatic sectional view of a skew rolling mill for pierce-rolling solid round billets.

The skew-rolling mill comprises two skew rolls 1 with roll axes inclined at an angle to one another and a mandrel 2 having a mandrel rod 3 and a plug or piercer4. Thepiercer is disposed in the sizing gap or opening and forms, together with the three skew rolls 1,a roll gap Sthroughwhichthe round billet6 is drawn in a screw-like manner underthe influence of shear and compressive forces.

The mandrel rod 3 is held bya support8which is movable in the direction ofthe longitudinal axis 7 of the rod and upon whose rear end the piston rod 9 of a double-acting adjusting cylinder 10 acts. The piston 11 of the cylinder '0 determines the position ofthe piercer4relative to the rolls.

Each change in position ofthe piercer4 caused by changing mandrel stress or a change in the length of the mandrel is reflected in a change in pressure inside the adjusting cylinder 10. In order to maintain the position of the piercer relative to the inclined rolls 1 constant, this change in pressure merely has to be measured and transformed into an adjusting motion ofthe piston 11, taking into account the spring characteristicoftheskew-rolling mill/mandrel plug system, including the support. In this way, the roll gap 5 can be kept constant by continuous adjustment ofthe piercer4 in order to compensate for changes in posi tion dueto decreasing temperature ofthework mate- rial, increasing length ofthetubularbloom and changing rolling forces. On the other hand, it is also possible, by applying pressure to the piston 11 accordingly, to carry out rolling with an arbitrary change in the roll gap in order to produce atubular bloom whose wall thickness varies along its length.

The invention can be used in the same way in expanding rolling mills and when piercing round billets by longitudinal rolling. In all these cases, the position of the mandrel plug relative to the rolls determines the roll gap and hence the wall thickness ofthe expanded tube ortubular bloom.

Claims

1. A method of hollow rolling in which work material is rolled over a plug of a roll mandrel and the position of the plug relative to the rolls is continuously or repeatedly measured and is adjusted accordingly.

2. A method as claimed in claim 1, in which the longitudinal force in the mandrel rod is measured and the position ofthe plug is adjusted taking into account the spring characteristic of the rolling mill/mandrel system.

3. Carolling mill having a mandrel rod which is supported against the piston rod of an adjusting cylinder byway of a movable support.

4. A rolling mill as claimed in claim 3, which is a skew-rolling mill.

5. A method of hollow rolling substantially as herein described with reference to the accompanying drawing.

6. Askew-rolling mill constructed and adapted two operate substantially as herein described with referenceto and as illustrated in the accompanying drawing.