GB2099346A - Tube rolling mill - Google Patents

Abstract

In a rolling mill for making seamless tubes a solid square billet 16a is heated in a rotary hearth furnace and then passed through a press piercing mill 10a followed by two rotary elongator mills 11 and 12 and a four stage mandrel mill 13. If a rotary piercing mill is used in place of the press piercing mill, one rotary elongator only is necessary. The four-stage (instead of a normal eight-stage) mandrel mill allows the use of shorter mandrels. <IMAGE>

Description

SPECIFICATION Tube rolling mill This invention relates to seamless tubes and to their manufacture.

The most up to date methods of producing seamless tubes, particularly seamless steel tubes, involves the use of a multi-stand pipe mill which produces a high standard of accuracy of size of tube bore and surface finish and uniformity of wall thickness. This type of mill enables very long tubes to be produced of the order of 84ft or more in length.

The very long tubes are then sawn into the standard lengths (e.g. 42ft) with the advantage that the wastage at the end of the tube is minimised. There is normally wastage at the end of the tube because the shape at the ends of the tube is poor in any type of rolling operation.

Current practice is to use eight rolling stands in the multi-stand pipe mill. A tube is rolled on a mandrel bar which passes with the tube through the mandrel mill. Several mandrel bars have to be used and a large stock has to be carried to suit the wide range of tubes produced. The length of a mandrel bar is of the order of 16 metres. Bearing in mind that the mandrel bars must have a very high surface finish and hardness to give as long a life as possible under the arduous conditions of hot rolling, then it will be readily understood that the cost of these bars is a very significant part of tooling costs and a substantial part of the cost of running the mill.

The present invention is concerned with a new method of rolling wherein the length of the mandrel bars is significantly reduced and this in turn will significantly reduce the cost of mandrel bar tooling and thus reduce the cost per ton of tubers produced.

It is known to use a press piercing mill to produce a hollow shell from a solid billet, to follow this with a rotary elongator and then to pass the tube into an eight stand mandrel mill for further reduction to produce the finished length. The rolls of the mandrel mill are profiled to match the required outer diameter of the tube and many sets of rolls have to be held in stock; these rolls have to be changed every time a new size of tube is required. The internal mandrel bar used to obtain the correct inner diameter of the tube must be at least as long as the mill itself. In practice the mandrel bar is allowed to move forwards whilst the tube is being rolled so that the mandrel bar needs to be rather longer than the mill itself.In the case of an eight stand mill for rolling tubes up to, say, 103/W' diameter, the bar length would be of the order of 16 metres. 2/3(For bar travel 6 metres and mill length 9 metres). A wide range of bars would be held in stock in varying diameters to cover the size range of tube being produced.

Several bars are needed for each size of tube. After a bar has passed through the rolling mill it is stripped from the tube and then spray cooled and may be induction heated to achieve uniform temperature of approximately 250"F before being sprayed with a lubricant for further use of the mandrel bar in the rolling mill. Normallythere are 4 bars being recirculated continuously at least into and out of the mill and through the cooling, heating and lubrication stages in order to maintain a high throughput of tube through the mill. Extra bars must be held as spares in case of surface damage to any of the mandrel bars being used.Thus there are, in total, a very large number of mandrel bars to be provided in orderto maintain the throughput of the mill and to cover adequately the large range of tube sizes normally produced in such a mill. Cost of mandrel bar tooling is high and forms a very significant part of the running costs of the mill.

The object of this invention is to reduce these tooling costs by reducing the length of the mandrel bars.

In accordance with one aspect of this invention, a method of rolling a tube comprises the steps of: 1. Producing a hollow shell from a solid billet, 2. Rolling the hollow shell in a first elongator mill to achieve a further reduction in diameter and wall thickness.

3. Optionally rolling the reduced diameter shell in a second elongator mill to reduce the diameter and wall thickness further, and 4. Completing the production of the tube in a mandrel mill comprising only four mill stands, the first two stands effecting the final reduction in diameter and the third and fourth stands being light duty stands which merely "iron" the tube to produce uniform wall thickness and smooth surface finish.

If the tube is produced from round or substantially round solid billets using a rotary or cross-rolling piercing mill it will usually be possible to dispense with the second elongator mill because, in this instance the piercing mill itself produces a large elongation, but if a press piercing mill is used two elongator mills will be necessary.

Thus the first step may be carried out, for example, by producing a round hollow shell from a solid square or rectangular billet by means of a press piercing mill; or producing a round hollow shell from a solid round billet by means of a rotary or cross-rolling two orthree roll piercing mill.

From a second aspect of the invention a method of rolling a tube comprises: 1. Producing a round hollow shell from a solid substantially round billet using a rotary or crossrolling piercing mill, 2. Rolling the hollow shell in an elongator mill to achieve a further reduction in diameter and wall thickness; and 3. Completing the production of the tube in a mandrel mill comprising only four mill stands, the first two stands effecting the final reduction in diameter and the third and fourth stands being light duty stands which merely "iron" the tube to produce uniform wall thickness and smooth surface finish.

Preferably the second pair of stands of the mandrel mill produces a small reduction in tube wall thickness of the order of .01". Preferably the mandrel mill is operated in known manner with a restrained mandrel bar, the mandrel bar being controlled to move at an even speed through the mandrel mill during rolling. The mandrel bar may be controlled to, move in the direction of rolling or may be controlled to move in the opposite direction to rolling. Preferably the mandrel bar is permitted to move such a distance that part of it passes through at least two stands and maybe through all four stands of the mill during rolling of a length of tube.

From another aspect the invention comprises a tube rolling mill comprising in sequence for producing tube from a square billet, a press piercing mill, a first elongator mill, a second elongator mill and mandrel mill comprising only two pairs of stands, the second pair of stands of the mandrel mill being adapted to "iron" the tube i.e. to produce a uniform wall thickness and a smooth surface finish without effecting a substantial reduction in diameter.

Alternatively the invention comprises a tube rolling mill comprising in sequence for producing tube from round solid billets, a rotary piercing mill, an elongator mill, and a mandrel mill comprising only two pairs of stands the second pair of stands being adapted to "iron" the tube i.e. to produce a uniform wall thickness without effecting a substantial reduction in diameter.

The rotary or cross-rolling piercing mill produces large elongations so that it is not necessary to have in addition two elongator mills. When producing tube from a square billet a press piercing mill is used which produces very little elongation and in this case two elongator mills are required.

Preferably the elongators are: cross-rolling tube mills with two or three rolls with a skewed roll axis arranged about the pass centre line and an internal plug or alternatively a restrained mandrel bar is used to form the bore of the tube. The rolls are conical shaped and may have a humped working shoulder midway along the roll face. The tube is gripped and fed through the mill by the roll inlet surface and rolled down on the bar or plug. The wall thickness is established by the roll workshoul- der and the gap between the rolls and the internal mandrel bar or plug.

Elongator mills of this type are manufactured by the Aetna-Standard Engineering Company of U.S.A and under licence by the Head Wrightson Machine Company.

The cross-rolling mills or process may employ a fixed plug and the elongators may also employ fixed plugs.

If a restrained mandrel bar is employed in the cross-rolling elongator the working portion of the bar is preferably cylindrical and may be for example about one metre in length.

In the accompanying drawings: Figure 1 is a diagrammatic representation of a rolling mill including a rotary piercing mill and embodying the present invention; and Figure la is a diagrammatic representation of another rolling mill including a press piercing mill and embodying the present invention; Figure 2 illustrates diagrammatically the travel of the mandrel bar through the mandrel mill and the amount of float allowed; Figure 3 is a similar illustration to Figure 2 showing the mandrel bar in a different starting position and in this case the mandrel is to be pulied back through the mill during rolling in the opposite direction to movement of the tube; Figure 4 shows the way in which the mandrel bar is connected to a moving carriage by means of a connecting rod.The carriage is guided and the speed of the carriage is controlled in order to control the rate of movement and the direction of travel of the mandrel during rolling; Figure 5 shows diagrammatically the use of a restrained mandrel or floating bar in the elongator as well as in the mandrel mill; Figure 6 shows an alternative mill for dealing with round billets in which a two roll cross-roll piercer with a fixed plug and a two roll elongatorwith a fixed plug are employed, followed by a four stand mandrel mill; Figure 7shows another alternative mill for dealing with rectangular billets which includes a press piercing mill, a first two roll elongator with a fixed plug and a second two roll elongatorwith a fixed plug followed by a four stand mandrel mill with a restrained bar; and Figure 8shows a cross-rolling elongatorwith a restrained mandrel bar which may be used in place of the elongator shown in Figure 6, or two elongators of the type shown in Figure 8 may replace the two elongators shown in Figure 7.

The mill shown in Figure 1 comprises a rotary piercing mill 10, shown diagrammatically, a first elongator mill 11, and four stands of rolling mill 13 comprising a first pair of stands 14 and a second pair of stands 15. Solid round billets 17 are heated in a rotary hearth furnace 9 and supplied to the press piercing mill 10.

The rotary cross-rolling piercing mill 10 is a two roll piercing mill which produces a hollow shell 16 from a round solid billet 17 using a piercing plug 18.

The piercing mill is rotated in the direction shown by the arrows on the two rolls which causes rotation of the billet in the direction ofthe arrow 17a. Alternatively a press piercing mill 10a (Figure 1a) may be used in place of piercing mill 10 to produce a hollow round shell 18a from a square billet 16a. Both type of piercing mill are themselves well known in this art.

The rolls of the press piercing mill 10a have circular cross sectional profiles on the roll peripheries so that the incoming square billet is rolled and transformed to circular section.

The hollow shell then enters the elongator mill 11 where it is rolled to achieve further reduction in diameter and wall thickness. The diagrammatic end view of the elongator mill indicates by means of arrows the direction of rotation of the rolls. The second elongator 12 in Figure 1a is introduced to achieve a much greater reduction in tube diameter wall thickness before the tube enters the mandrel mill 13. The mandrel mill 13 itself consists of four stands only. The first two stands 14 effect the final tube reduction and the last two stands 15 of the mill have a very light duty only and achieve virtually no reduction at all - they are used to "iron" the tube to obtain a superior finish to the tube, together with uniform wall thickness and these finishing stands would be set so as to achieve a wall reduction which is very small, say at most .01" reduction in thickness.

The length of mandre! in the mandrel mill with this tube mill arrangement is now about 10.5 metres so that, in effect, the length and cost of mandrel bar tooling is drastically reduced.

The mandrel mill is operated in a conventional way with what is known as a "restrained" mandrel.

The mandrel is allowed to move slowly through the mill whilst rolling takes place and with a mill for producing tubes up to 103/4" diameter, a movement of the mandrel, or "float", of about 6 metres in length is provided for. The length of the mill itself would be about 4.5 metres from entry to exit stand, thus giving a total mandrel length required of 10.5 metres.

Figure 2 shows the mill comprising stands 14 and 15 set up with the mandrel bar 20 in the forward position for movement in the direction of rolling during the rolling operation.

Figure 3 shows the mandrel bar 20 in the back position whereby the mandrel can be pulled out of the mill in the opposite direction to rolling during the rolling operation.

The mandrel bar 20 is attached by means of a rod 21 to a moving carriage 22 in a guide 23 shown in Figure 4 so that the mandrel bar can be moved in direction of, or against, the travel of the tube during rolling.

The advantages are as follows: (a) reduction in mandrel bar length and consequent reduction in costs of mandrel bars, (b) reduction in size and cost of mandrel bar handling equipment; (c) when a second cross-rolling elongator mill is employed together with reduction in the amount of in-line rolling in the mandrel mill there will be an improvement in uniformity of wall thickness and concentricity in the tube.

The elongator mill is necessarily handling fairly long lengths of tube and if a stationary plug were used then it would tend to be overheated. To avoid this problem we propose to use a mandrel 20a (Figure 5) which would be restrained in the same way as the mandrel for the multistand pipe mill. The working length of the mandrel or restrained floating bar would probably be about one metre long and we would restrain it so that the mandrel bar travelled forwards a distance of about 3/4 metre during tube rolling. This would avoid local overheating and wear on the mandrel. It would also be possible to use a much longer mandrel bar with the two roll mill but this would be unnecessarily expensive.

In Figure 6 is shown a mill for producing tube from solid round billets 25 which are heated in a rotary furnace and fed to a conventional two-roll crossrolling piercer 27 with a fixed plug 28. The tube then passes through a conventional two-roll elongator 29 with a fixed plug 30. It will be noted that the elongator has no "hump" on the rolls. The elongator 29 is followed by a four stand mandrel mill 31 with a restrained mandrel bar 32. The input to this mill is either continuous cast round billets or round billets made by rolling billets to round billets in a conventional mill. This mill would normally be followed by a conventional sizing mill 33, reheatfurnace 34 and stretch reducing mill 35. From the stretch reducing mill 35 the tubes would proceed to the usual cooling beds, saws, etc.

In Figure 7 is shown a further alternative mill for producing tubes from rectangular billets 36 which are heated in a rotary hearth furnace 37 and then proceed through a press piercing mill 38 to a first two-roll elongator 39 with a fixed plug 40, followed by a second two-roll elongator 41 with a fixed plug 42. The press piercing mill 38 may, for example, be of the kind illustrated in U.S. Patent 4,006,618. From elongator 41 the tubes pass to a four-stand mandrel mill with a restrained mandrel bar. The remaining details may be as described with reference to Figure 6.

Incoming round billets would by-pass the press piercing mill 38 but incoming square billets would be pierced and also rounded in the press piercing mill 38.

In Figure 8 is shown a cross-rolling elongator 42 with only two rolls 43 and 44, and a restrained mandrel bar 45. The working portion of mandrel bar 45 is cylindrical and is preferably about one metre in length. The bar 45 is allowed to move in the direction of travel of the tube 46 while the tube is being rolled.

The elongator42 in Figure 8 may be used in place of the elongator 29 in Figure 6. Two elongators 42 may be used in place of elongators 39 and 41 in Figure 7.

In each of the processes described above the complete rolling process may include a billet heating furnace and further stages such as a sizing mill and/or a stretch reduction mill in known manner, as illustrated in Figure 6.

Claims (19)

1. A method of rolling a tube comprising the steps of: (A) Producing a hollow shell from a solid billet, (B) Rolling the hollow shell in a first elongator mill to achieve a reduction in diameter and wall thickness, (C) Optionally rolling the reduced diameter shell in a second elongator mill to reduce the diameter and wall thickness further, and (D) Completing the production of the tube in a mandrel mill comprising only four mill stands, the first two stands effecting the final reduction in diameter and the third and fourth stands being light duty stands which merely "iron" the tube to produce uniform wall thickness and smooth surface finish.

2. A method according to claim 1 in which step (A) is effected using round or substantially round solid billets and employing a rotary or cross-rolling two or three roll piercing mill and step (C) is omitted.

3. A method according to claim 1 in which step (A) is effected using rectangular or square section billets, a press piercing mill is used and step (C) is included.

4. A method of rolling a tube comprising: (A) Producing a round hollow shell from a solid substantially round billet using a rotary or crossrolling piercing mill, (B) Rolling the hollow shell in an elongator mill to achieve a further reduction in diameter and wall thickness; and (C) Completing the production of the tube in a mandrel mill comprising only four mill stands, the first two stands effecting the final reduction in diameter and the third and fourth stands being light duty stands which merely "iron" the tube to produce uniform wall thickness and smooth surface finish.

5. A method according to any preceding claim in which the second pair of stands of the mandrel mill produces a small reduction in tube wall thickness of the order of .01".

6. A method according to any preceding claim in which the mandrel mill is operated with a restrained mandrel bar, the mandrel bar being controlled to move at an even speed through the mandrel mill during rolling either in the direction of rolling or in the opposite direction to rolling.

7. A method according to claim 6 in which the mandrel bar is permitted to move such a distance that part of it passes through at least two stands of the mill during rolling of a length of tube.

8. A method according to claim 7 in which the mandrel bar is permitted to move so that part of it passes through all four stands of the mandrel mill during said rolling.

9. Atube rolling mill comprising in sequence for producing tube from a square billet, a press piercing mill, a first elongator mill, a second elongator mill and mandrel mill comprising only four stands, the third and fourth stands of the mandrel mill being adapted to "iron" the tube i.e. to produce a uniform wall thickness and a smooth surface finish without effecting a substantial reduction in diameter.

10. Atube rolling mill comprising in sequence for producing tube from round solid billets, a rotary piercing mill, an elongator mill, and a mandrel mill comprising only four stands the third or fourth stands being adapted to "iron" the tube i.e. to produce a uniform wall thickness without effecting a substantial reduction in diameter.

11. A mill according to claim 9 or claim 10 in which the elongators are cross-rolling tube mills with two orthree rolls with a skewed roll axis arranged about the pass centre line and an internal plug or a restrained mandrel bar used to form the bore of the tube.

12. A mill according to claim 11 in which the elongator rolls are conical shaped and have a humped working shoulder midway along the roll face.

13. A mill according to any of claims 9 to 12 in which the cross-rolling mills when used either as elongator or as piercing mills have fixed plugs.

14. A mill according to claim 11 in which said restrained mandrel bar is used in the or each elongator, the working portion of the bar being cylindrical.

15. A mill according to claim 16 in which the working portion of the bar is about one metre in length.

16. Tubes produced by the method of any of claims 1 to 8.

17. Tubes produced by a tube rolling mill according to any of claims 9 to 15.

18. A method of rolling a tube substantially as hereinbefore particularly described and as illustrated in the accompanying drawings.

19. Atube mill substantially as hereinbefore particularly described and as illustrated in the accompanying drawings.