GB2036622A - Manufacture of seamless metal tubes - Google Patents

Abstract

Seamless steel tubes are made from solid blanks by, firstly, piercing the blank (at 2), while elongating the blank by a factor of, typically, 1.8; subjecting the blank to a circumferential rolling process in a device (3) which may be generally similar to the piercing device, and which produces an elongation roughly equal to that in the piercing step; and passing the blank through a continuous rolling mill (4) to produce a seamless tube, with the elongation produced by the rolling mill being by approximately the same factor as the combined elongation produced in the piercing and circumferential rolling steps, typically about 4. The number of sets of rolls in the rolling mill may be fewer than is conventional, for example six, giving less chance for the blank to cool before rolling is completed. The method can be used for large diameter tubes, for example about 250 mm in diameter, with relatively thin walls. <IMAGE>

Description

SPECIFICATION Manufacture of seamless metal tubes This invention relates to methods of manufacturing seamless metal tubes. It is particularly, although not exclusively, applicable to the manufacture of large-diameter steel tubes with relatively thin walls.

Various methods have been proposed for the production of seamless metal tubes. In general, the first step is to pierce a solid blank, to produce a short, hollow cylindrical blank with thick walls, sometimes known as a bottle. This piercing step is generally carried out with the blank at a high temperature, and is generally accompanied by a certain amount of elongation of the blank. (Numerically, the elongation is the ratio of the length of the blank after the processing step to its length before the processing step, or alternatively, the ratio of the cross-sectional area of the blank before the processing step to the cross-sectional area of the blank after the step. In this specification, the elongation will be designated by A.) The pierced blank or bottle has then to be elongated further, in order to produce a finished tube.One method of producing this further elongaton is to use a continuous rolling mill, having a number of sets of rolls which progressively reduce the wall thickness of the pierced blank to produce the finished tube. The bore of the tube is supported by a long plain mandrel, which travels through the rolling mill with the blank. In most known processes of this type, eight sets of rolls are used, in order to produce a sufficient elongation of the pierced blank; of course, each set of rolls has to be driven at a different speed, to match the increasing speed of the blank as it is elongated by the successive rolling steps. To help achieve uniformity of wall thickness around the tube, successive sets of rolls are arranged with their axes at right angles to one another; for example, one set of rolls might have its axes horizontal; the next set of rolls would then have its axes vertical.

Continuous rolling processes of this type are rapid, and produce tubes having good uniformity of wall thickness. However, these processes are generally limited to the production of tubes with outside diameters not greater than 140 mm, although a diameter of 168 mm can be reached if special care is taken. In fact, the criterion which mainly determines whether a particular size of tube can be successfully rolled is the ratio Dle between its diameter D and its wall thickness e. The maximum value of D/e in known processes is about 38 (D = 152.5 mm, e = 4 mm). Above this value difficulties arise in maintaining the uniformity of the finished tubes; the usual tolerance on thickness which has to be satisfied is + 12.5%. There may also be excessive wear on the rolls and the mandrel.At this point, it may be mentioned that the mandrels represent a considerable part, possibly half, of the investment required in a continuous tube rolling mill, so that it is important to keep their wear to an acceptable level.

The applicant has discovered that the difficulties in producing a tube with a high value of D/e by a continuous rolling process arise from the fact that the tube cools rapidly as it passes through the continuous rolling mill. This makes rolling more difficult in the later sets of rolls; a greater lateral flow of metal begins to occur, instead of a longitudinal flow, resulting in non-uniformities of thickness, and excessive wear of the rolls and the mandrel.

According to one aspect of the present invention, a method of making a seamless metal tube from a solid blank comprises: piercing the blank by means of a piercing device having rollers which act to roll the blank circumferentially while also moving the blank axially over a piercing head, the piercing operation producing an elongation of the blank; producing an intermediate elongation of the blank by circumferential rolling; and longitudinally rolling the blank in a continuous rolling mill to produce from the blank a tube of the required diameter and wall thickness; the continuous rolling mill producing an elongation of the blank approximately equal to the overall elongation resulting from the piercing and intermediate elongation operations.

By inserting an extra stage of elongation before the continuous rolling mill the elongation which has to be produced by the rolling mill can be less than in the known processes, and therefore the rolling mill can have fewer sets of rolls than previously (that is, seven or fewer sets of rolis), and be shorter in length, so that the tubes does not cool excessively before the rolling process is completed.In this way, it may be possible to produce seamless metal tubes of large diameter (more than about 170 mm), without departing from the normal conditions for satisfactory production. (In this specification, the term 'normal conditions' will be used to indicate that the tolerance of -1- ~ 12.5% on wall thickness is not exceeded, that the surfaces of the tube are free from defects, and that there is no abnormal wear of the rolls and mandrels used in producing the tube).

As a particular example, it may be possible to produce seamless metal tubes having a D/e ratio of 45, for example, with D = 255 mm and e = 5.56 mm.

In a preferred embodiment, the elongation produced by the piercing device is approximately equal to the intermediate elongation. More specifically, the elongation A produced by the piercing device may be between 1.3 and 2, the intermediate elongation A may be between 1.4 and 2, and the elongation A produced by the continuous rolling mill may be not more than 4.5.

By distributing the required elongation in this ways, it may be possible to use as blanks, underthe normal conditions mentioned above, round bars produced by continuous casting; the blanks do not have to be especially high-grade. Also, the blank, after piercing by the piercing device, will have a low D/e ratio, and this will in general result in good concentricity and a low rate of wear on the piercing device. Quite long blanks can be used, giving a high throughput.

In the preferred embodiment, the intermediate elongation of the blank involves moving the blank longitudinally over a head carried by an elongate mandrel, whereby, after the intermediate elongation, the blank encircles the mandrel, and the blank and the mandrel are then transferred together to the continuous rolling mill, and travel together through the continuous rolling mill, with the mandrel supporting the interior surface of the blank, and the mandrel is then released from the resulting tube for re-use in the intermediate elongation and longitudinal rolling of a further blank. The mandrel may be allowed to rotate freely as the circumferential rolling takes place to produce the intermediate elongation, but be held stationary in the axial sense relative to the rolls performing the circumferential rolling.During the subsequent passage of the blank and the mandrel through the continuous rolling mill, it may be advantageous that the speed of travel of the mandrel should be positively controlled. In particular, the wall thickness of the resulting tube may be more uniform as a result.

The invention also extends, in a second aspect, to apparatus for making seamless metal tubes from solid blanks, comprising: a piercing device for piercing a solid blank, while also producing an elongation of the blank; an intermediate elongating device for receiving the pierced blank from the piercing device, and having rolls which circumferentially roll the blank to produce an intermediate elongation of the blank; and a continuous rolling mill for receiving the blank from the intermediate elongating device, and longitudinally rolling the blank to produce a further elongation of the blank and thereby produce a tube of required diameter and wall thickness; the piercing device and the intermediate elongating device being arranged to produce elongations which, together, are approximately equal to the further elongation produced by the continuous rolling mill.The intermediate elongating device may comprise two rotatable rolls whose axes are somewhat skewed realtive relative to the axis of the blank, and between which rolls the blank is squeezed.

The piercing device may be of similar construction, comprising two rotatable rolls whose axes are somewhat skewed relative to the axis of the blank, and between which rolls the blank is squeezed, and two rotatable guide discs bearing against the surface of the blank to maintain the blank in position between the two rolls.

The intermediate elongating device may also include rotatable guide discs of this type.

As indicated above, the continuous rolling mill preferably has no more than seven sets of rolls, and possibly fewer, for example five, and the grooves of these rolls are preferably 'closed' grooves, that is to say, grooves which embrace the tube around a considerable angle. In the preferred embodiment, each roll embraces the tube around about 1000 of the tube circumference.

The invention may be carried into practice in various ways, but one specific embodiment will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic illustration of a plant embodying the invention, for manufacturing seamless tubes; Figure 2 is a view of a piercing device forming part of the plant of Figure 1, and also shows a blank for the production of a tube being pierced, the blank being shown in section on a plane containing its axis; Figure2A is a view, partly in section on the line A-A of Figure 2; Figure 3 is a view, similar to Figure 2, but showing a device also forming part of the plant of Figure 1, for circumferentially rolling an already pierced blank, also shown, in section, in Figure 3; and Figures 4A to 4H are views in cross-section, illustrating how the blank is subjected to longitudinal rolling after being processed by the devices of Figures 2 and 3.

The plant of Figure 1 uses as its starting material ingots of steel, which may be produced either elsewhere or on site, possibly by continuous casting. The ingots are cut into blanks, preferably of circular cross-section, although other shapes can be used. The blanks are raised to a temperature of the order of 1300"C in a heating furnace 1. After heating, scale is removed from the blanks. If the blanks have an initial cross-section departing considerably from a circle, for example a square, the blanks can then be subjected to a first rolling operation which brings their cross-section to a shape approximating a circle, such as an octagon. A starting hole is then made at one end of each blank in a "centring press" machine.The apparatus used for these preliminary operations is not shown in the drawings, but operates at very high speed, so that each blank is still at a temperature of the order of 1200"C after these operations.

Next, each blank is supplied to a piercing device 2, whose preferred form is illustrated in more detail in Figure 2. The piercing device 2 comprises two rolls 21 and 22 whose axes lie roughly parallel to the axis (shown at 29) of the blank (shown at 20). The two rolls 21 and 22 are rotated, both in the same direction by motor and reduction gear assemblies 24 and 25 (Figure 1), so that the blank 20, which is squeezed between the rolls, is rotated in the opposite direction.In addition, the rotation of the rolls 21 and 22 drives the blank 20 longitudinally over a piercing head 23 which is mounted on the end of a bar 26; to produce this longitudinal movement, the axes of the rolls 21 and 22 are both somewhat skewed relative to the axis 29 of the blank 20, so that their circumferential movement at their point of engagement with the blank 20 has a component in the direction axial of the blank. The circumferential speed of the rolls 21 and 22 can be adjusted, for example in the range 0 to 7 m/s; also, their angle of skewing relative to the axis 29 can be adjusted, for example over the range from 0 to 17 . The bar 26 is supported by a mounting 17 (Figure 1), which holds the bar 26 axially stationary during piercing, but allows the bar 26 to rotate freely. The blank 20 can also rotate freely, so that both the piercing head 23 and the blank 20 rotate at speeds controlled by the rotation of the rolls 21 and 22.

The piercing device 2 also has two guides 27 and 28 (Figure 2A), which preventthe blank 20 from moving laterally out of the nip between the rolls 21 and 22. These guides could take various forms, such as freely rotatable rollers with their axes parallel to the axis 29, or stationary guides having a concave surface to match the exterior of the blank 20, but in their preferred form, as shown in Figure 2A, they comprise discs 27 and 28 whose peripheries bear against the blank 20, the discs 27 and 28 being rotatable about axes lying at right angles to and laterally displaced from the axis 29. The periphery of each disc is made concave in profile, to match the exterior of the blank 20.Piercing devices incorporating the preferred form of guides 27 and 28 can be used in the manufacture of large diameter tubes, and the resulting tubes should be virtually free from surface defects caused by the guides. Also, there should be virtually no interruptions in production resulting from a need to change the guides 27 and 28.

In Figure 2, the heavily shaded part of the blank 20 indicates the region in which the metal is deformed, to give a pierced blank having an outside diameter d1 and a wail thickness e1. In this preferred embodiment, the elongation A produced by the piercing device is between 1.3 and 2.

After piercing, the piercing head 23 and its bar 26 are withdrawn by the mounting 27 and the pierced blank is automatically transferred to a circumferential rolling device 3 (Figure 1). Since the piercing head 23 suffers negligible wear, it is returned more or less directly to the piercing device 2 for re-use, without any special preparation for a new piercing operation being required.

The circumferential rolling device 3 may have two or three rolls; in the preferred example illustrated, two rolls are used. The rolling device 3 is arranged to produce an elongation A of between 1.4 and 2, accompanied by a reduction in the wall thickness of the blank; it may also, if required, produce a reduction of 5% to 10% in the outer diameter of the blank.

Figure 3 illustrates the circumferential rolling device 3 in some detail; as can be seen, the device 3 is quite similar to the piercing device 2. The two rolls are shown at 31 and 32, and are driven by motor and reduction gear units 34 and 35 (Figure 1), with a circumferential speed which can be varied between 0 and 7 m/s. The axes of the rolls 31 and 32 are skewed relative to the axis of the pierced blank, at an angle which can be adjusted between 0 and 17 . In operation, the rolls 31 and 32 force the already-pierced blank over a mandrel having a head 33 and a shank 36, the mandrel being supported from a mounting 37 (Figure 1) by means of its shank 36. The mounting 37, like the mounting 17, keeps the mandrel axially stationary, but allows it to rotate freely.

As the blank undergoes the circumferential rolling process, it is delivered on to an output table which has means (not shown) for maintaining the axes of the shank 36 and the blank in the correct position. After the circumferential rolling, the blank has an outside diameter of d2 (Figure 3).

The blank is then transferred, with the mandrel still outside, to a longitudinal rolling mill 4 (Figure 1). In this miil, the blank is rolled between six sets of rolls 41 to 46, each of which reduces the wall thickness of the blank, and produces a corresponding elongation of the blank. Obviously, other numbers of sets of rolls could be used. During this rolling process, the shank 36 acts as a mandrel, supporting the hollow blank from inside, and largely determining the bore of the resulting tube. The longitudinal speed of the mandrel 33, 36 is controlled during the longitudinal rolling operation by a holding device 40, in the manner described in French Patent specification No. 7231 888.

Figure 1 also shows, for each set of rolls, a driving motor and a reduction gear.

Because the mandrel 33, 36 does not have to be withdrawn from the blank and replaced by a different mandrel in preparation for the longitudinal rolling operation, the blank can be rapidly transferred from the rolling device 3 to the rolling mill 4, without much drop in temperature. Also, there need be only a small clearance between the shank 36 and the bore of the blank after the circumferential rolling operation.

Figures 4A to 4H illustrate the progress of the longitudinal rolling operation. Figure 4A shows the cross-section of the blank emerging from the circumferential rolling device 3, with the mandrel 36 inside.

Figure 4B shows how, in the first set of rolls 41, the blank is rolled between generally semi-circular grooves in two rolls whose axes are horizontal. Figure 4C shows how the second set of rolls 42 is generally similar to the first set, except that the axes of the rolls are vertical. Similarly, the following sets of rolls have alternatively horizontal and vertical axes, as shown by Figures 4D to 4G. The total elongation A produced by the whole rolling mill 4 will not usually be more than 4.5; the elongation produced by any one set of rolls will not usually exceed 1.4, in the most heavily loaded sets of rolls (which are not necessarily those producing the greatest elongation).

Friction on the mandrel is reduced and its wear is kept at a normal level. Moreover, with the longitudinal speed of the mandrel 36 being controlled, the profile of the grooves in the rolls can be such as to embrace the tube around a considerable part of the tube circumference (closed grooves), improving the longitudinal flow of metal, and the uniformity of the wall thickness of the resulting tube, and facilitating the release of the finished tube from the mandrel 36. Figure 4G shows the cross-section of the tube after the last rolling operation; it will be seen that the tube is now fully circular, so that there is a slight clearance all around the mandrel 36, which is therefore released. Figure 4H shows the tube after withdrawal of the mandrel.

The main parameters of the tube manufacturing process illustrated in the accompanying drawings are as follows: Starting blank: solid bar, 290 mm diameter, 2 m long Blankafterpiercing: Diameter = 305 mm, e = 43.5 mm, 3.6 m long A 1.85 Blankafter circumferentialrolling: Diameter = 288 mm, e = 21.56 mm, 7.13 m long A 1.98 Continuous rolling mill with six sets ofrolls: Roll set No.1 A=1.55 Roll set No.2 A = 1.36 Roll set No.3 A = 1.35 Roll set No.4 A = 1.25 Roll set No. A=1.12 Roll set No.6 A = 1.06

A = 4.21 Rolled tube: Diameter = 255 mm, e = 5.56 mm, 30 m long.

As Figure 1 shows, when the tubes leave the rolling mill 4, they are transferred to a bench 5 where the tubes are separated from the mandrels, the tubes preferably being drawn off the mandrels by means of a further pair of rolls which also act as sizing rolls. The tubes are conveyed to a lateral conveyor 6, while the mandrels are returned along a path 50 to the circumferential rolling device 3 for re-use, after having been inspected, cooled and lubricated if required.

Next, the tubes can be sized again, sent through a reheating furnace, and be supplied to a reducing and drawing device, if the final required diameter of tube is less than the diameter of the tube produced by the rolling mill 4. The tubes then undergo finishing treatment.

Of course, modifications may be made to the described embodiment. For example, in order to manufacture thick tubes, a by-pass route can be provided at arrows 39 in Figure 1, thus transferring the blank directly from the furnace 1 to the circumferential rolling device 3, which will then operate as a piercing device.

Claims (18)

1. A method of making a seamless metal tube from a solid blank, which method comprises: piercing the blank by means of a piercing device having rollers which act to roll the blank circumferentially while also moving the blank axially over a piercing head, the piercing operation producing an elongation of the blank; producing an intermediate elongation of the blank by circumferential rolling; and longitudinally rolling the blank in a continuous rolling mill to produce from the blank a tube of the required diameter and wall thickness; the continuous rolling mill producing an elongation of the blank approximately equal to the overall elongation resulting from the piercing and intermediate elongation operations.

2. A method as claimed in Claim 1, in which the continuous rolling mill comprises a number of sets of rolls not exceeding seven, the blank being rolled once by each set of rolls.

3. A method as claimed in claim 1 or Claim 2, in which the elongation produced by the piercing operation is approximately equal to the said intermediate elongation.

4. A method as claimed in Claim 1 or Claim 2 or Claim 3, in which the elongation produced by the piercing device is between 1.3 and 2, the said intermediate elongation is between 1.4 and 2, and the further elongation produced by the continuous rolling mill is not greater than 4.5.

5. A method as claimed in any of the preceding claims, in which the intermediate elongation of the blank involves moving the blank longitudinally over a head carried by an elongate mandrel, whereby, after the intermediate elongation operation, the blank encircles the mandrel, and the blank and the mandrel are then transferred together to the continuous rolling mill, and travel together through the continuous rolling mill, with the mandrel supporting the interior surface of the blank, and the mandrel is then released from the resulting tube for re-use in the intermediate elongation and longitudinal rolling of a further blank.

6. A method as claimed in Claim 5, in which the speed of travel of the mandrel through the continuous rolling mill is positively controlled.

7. A method as claimed in any of the preceding claims, in which the continuous rolling mill comprises sets of rolls having closed grooves, between which the blank is rolled.

8. Apparatus for making seamless metal tubes from solid blanks, comprising: a piercing device for piercing a solid blank, while also producing an elongation of the blank; an intermediate elongating device for receiving the pierced blank from the piercing device, and having rolls which circumferentially roll the blank to produce an intermediate elongation of the blank; and a continuous rolling mill for receiving the blank from the intermediate elongating device, and longitudinally rolling the blank to produce a further elongation of the blank and thereby produce a tube of the required diameter and wall thickness; the piercing device and the intermediate elongating device being arranged to produce elongations which, together, are approximately equal to the further elongation produced by the continuous rolling mill.

9. Apparatus as claimed in Claim 8, in which the intermediate elongating device comprises two rotatable rolls whose axes are somewhat skewed relative to the axis of the blank, and between which rolls the blank is squeezed.

10. Apparatus as claimed in Claim 9, in which the intermediate elongating device also includes two rotatable guide discs having their axes at right angles to but spaced from the axis of the blank, the peripheries of the guide discs bearing aginst the surface of the blank to maintain the blank in position between the two rolls of the intermediate elongating device.

11. Apparatus as claimed in Claim 8 or Claim 9 or Claim 10, which also includes: at least one mandrel having a head, over which head the blank is moved axially during the said intermediate elongation; a transfer device for conveying the mandrel, encircled by the blank, to the continuous rolling mill, for longitudinal rolling of the blank; an extractor device for separating the mandrel from the tube produced by longitudinal rolling; and a further transfer device for returning the mandrel to the intermediate elongating device.

12. Apparatus as claimed in Claims 9 and 11 or Claims 10 and 11, which includes supporting means for supporting the mandrel to maintain the head thereof in the region between the two rolls during the intermediate elongation of the blank, while permitting free rotation of the head of the mandrel.

13. Apparatus as claimed in Claim 11 or Claim 12, which further includes means for controlling the speed of travel of the mandrel through the continuous rolling mill during longitudinal rolling.

14. Apparatus as claimed in any of Claims 8 to 13, in which the continuous rolling mill comprises a number of sets of rolls not exceeding seven, the blank being rolled once by each of the sets of rolls.

15. Apparatus as claimed in any of Claims 8 to 14, in which the continuous rolling mill has sets of rolls which have closed grooves for rolling the blank.

16. Apparatus as claimed in any of Claims 8 to 15, in which the piercing device comprises two rotatable rolls whose axes are somewhat skewed relative to the axes of the blank, and between which rolls the blank is squeezed, and two rotatable guide discs having their axes at right angles to but spaced from the axis of the blank, the peripheries of the guide discs bearing against the surface of the blank to maintain the blank in position between the two rolls.

17. A method of manufacturing a seamless metal tube, the method being substantially as herein described, with reference to the accompanying drawings.

18. Apparatus for manufacturing a seamless metal tube, the apparatus being substantially as herein described, with reference to the accompanying drawings.