GB2128121A - Producing seamless pipes by rolling - Google Patents

Abstract

The invention relates to the production of thin-walled seamless pipes from solid billets wherein, a billet is heated (1); pierced (2); longitudinally roughed (3,24); expanded (4); sized (5); and cooled (6). The longitudinal roughing device consists of a skew rolling mill (24) and/or a plug rolling mill (3), normally the latter. <IMAGE>

Description

SPECIFICATION A rolling installation for seamless pipes The invention relates to the production of seamless pipes fron solid billets. It relates particularly to the production of thin-walled such pipes from rods of limited length.

A rolling installation for the production of thinwalled seamless pipes is described in "Stahl und Eisen 1933" Pages 693 to 695. This installation comprises an heating device, a piercing device, a longitudinal roughing device, an expanding mill and a cooling bed, in which the billet is sequentially worked. The expanding mill is a so-called skewrolling mill, in which the axes of the two or more rolls are arranged inclined to the axis of the workpiece. The projection of each roll axis onto the plane in addition is arranged with an angle of twist of 2'to 35", preferably no more than 15 to the axis, and each roll is supported on both sides in the roll housing. All the rolls cooperate with a common, secured mandrel arranged between them and form with the latter a roll gap which narrows in the direction of rolling.

A pilger mill is proposed in this publication as a longitudinal roughing device, and in addition it is stated that the problem of producing thin-walled finished pipes in he skew-rolling mill remains unsolved. Installations according to this proposal with an expanding mill have not been developed in practice.

The demand forthin-walled pipes overapproxi- mately 350 mm diameter is constantly increasing, for example in the power industry. Also seamless pipes are required. The production of such pipes is not comparatively difficult owing to their thin wall thickness, but also is extremely costly in terms of equipment on the proposed pilger mill. The majority of such installations, for example a hot pilger rolling train of this dimensional range, are costly investments. Owing to the cooling of the rolled material and the station time of the installation dependent thereon, such rolling trains are only minimally adaptable to changing requirements. The present invention seeks to incorporate practically an expansion step in the manufacture of seamless pipes. This aim gives rise to a number of problems in that several of the objectives conflict.The primary objectives are as far as possible - a single heating step; equalizing the wall thickness in heat; progressive gradiation in wall thickness or roughing of the hollow body; and accuracy in finished dimensions.

We have found that these objectives can be at least in part fulfilled in a manufacturing process in which a billet is worked in a specific sequence of devices known per se. The process can be relatively sin ple, in terms of equipment but the resulting product can nevertheless be of technologically improved quality.

In a method according to the invention, a solid billet is sequentially heated; pierced; longitudinally roughed; expanded in a skew rolling mill; sized; and cooled, roughing being accomplished in at least one of a skew rolling mill and plug rolling mill. Preferably, the longitudinal roughing step comprises at least two passes in a plug rolling mill. In one preferred embodiment, the roughing step comprises skew rolling followed by plug rolling. The worked billet is normally fed directly from the expanding rolling mill to a sizing rolling mill.

A rolling installation according to the invention comprises heating; piercing; longitudinal roughing; expanding rolling; sizing; and cooling stages, wherein the expanding rolling stage is a skew rolling mill and the longitudinal roughing stage is at least one of a skew rolling mill and a plug rolling mill, the stages being arranged for sequential operation on a workpiece. An intermediate heating stage is not normally required.

The use of a plug rolling mill proves to be technologically advantageous in the proposed combination, although it is known that the pipes poduced therein do not have a regular wall thickness. However, the wall thickness which can be achieved in the usual manner in two passes with a plug rolling mill is sufficient for the application of the pipes into an expanding rolling mill. For longitudinal rolling, a plug rolling mill, compared for example with a pilger skew rolling mill, operates so quickly that no intermediate heating is required up to the sizing rolling mill, which has particular benefits having regard to high and rising heating costs.

A skew rolling mill can be used in the roughing stage, and is normally connected in series with a plug rolling mill, so that from the solid billet for example, hollow bodies are produced on two skew rolling mills arranged one behind the other, normally as the part-worked billet which is the preliminary material for a plug rolling mill, and are thereafter expanded.

The longitudinally roughed (stretched) workpieces are passed to the expanding rolling mill and are circumferentially enlarged to the required diameter.

The rolls of the expanding rolling mill are calibrated to the tapered mandrel such that a roll gap is produced, narrowing in the direction of rolling. The expanding rolling mill determines the finished pipe diameter. The finished pipe wall thickness is already prepared in the plug rolling mill for the respective expansion. Enlargements of, for example, 350 mm diameter to over 500 mm diameter are possible.

It is particularly advantageous to conststruct rolls and mandrel of the expanding rolling mill in a particular manner. Reference is directed to our copending Application No: in this respect.

According to that Application the rolls have the form of a hyperboloid of revolution and the mandrels are essentially truncated cones. Differing pipe diameters are thereby determined by mandrels of different lengths, whereby the installation and the necessary structural alterations to the installation when the dimension is changed are again substantially sim plfied.

In experimental examination of the present proposal we have found that the expansion, in particular in the application of a groove designing, as is proposed elsewhere, contributes to a high extent to the equalizing of the wall thickness, so that special skew-rolling (reeling) no longer needs to be provided after the plug rolling or after the expanding rolling mill, which reduces the cost of the installa tion.

The benefits of the invention are also apparent from the low heating costs, the need for an intermediate heating being obviated. In preferred embodiments of the invention, the pipe is passed immedi atelyfrom the expanding rolling mill to a sizing rolling mill in which it is rounded and edged by a few millimetres in diameter.

The invention will now be described by way of example and with reference to the accompanying diagrammatic drawings wherein: Figure 1 shows the technological arrangement of a rolling installation according to the invention; Figure 2 shows a longitudinal section in the plane of the pipe axis through the expanding rolling mill; and Figure 3 shows a longitudinal section in the plane of the roll axis through an expanding roll.

The rolling installation shown in Figure 1 has a rotary furnace 1, a piercing device 2, a plug rolling mill 3, an expanding rolling mill 4, a sizing rolling mill 5, and a cooling bed 6. A solid ingot is placed in the rotary furance 1 in the direction of arrow 7, and leaves it, heated to approximately 1250 , in the direction of arrow 8. The heated ingot 9 is descaied, centered and passed to he skew-rolling mill 2 where a piercing mandrel 10 is secured in a support. A hollow body is produced, which is still so hot that it can be subjected to further hot-forming procedures.

If the hollow body 11 is relatively thin-walled, it is passed immediately to the plug rolling mill 3.

The plug rolling mill 3 consists of a set of rolls 12, which cooperates with the mandrel 14 supported at 13. The hollow body 11 is thereby pushed onto the mandrel rod 15 and conveyed back by the pinch rolls 16 with the rolls 12 opened. Generally, the plug rolling mill stand 3 consists only of one set of rolls through which the billet is passed several times, twice as described (two passes); however, it can also consist of two such sets of rolls 12 arranged adjacent one another.

For the forming of the hollow body, between passes the roll setting is selected narrower and/or the mandrel diameter is selected larger.

After the final roughing pass, the hollow body 11 is withdrawn from the stand and expediently conveyed transversely to the expanding rolling mill 4. The expanding rolling mill 4 has inclined rolls 17 and an expanding mandrel 18, likewise held in a support (not shown). Figure 2 shows some details of the expanding rolling mill to an enlarged scale. The inclined position of the rolls can be seen with respect to the plane through the pipe axis 19, that is the plane of the paper. The upper roll dips to the right, the lower roll dips to the left through the plane of the paper. The pipe axis 19 and the projection shown of the roll axes 20, 21 onto the plane of the paper form the angle of twist of in each case 2 - 35 . The contour of the rolls is shown in Figure 3. It consists of a feed taper 22 and a hyperbolic part 17.As a result of the contour and the inclined positions a spiral line of the rolls is part of the hollow body taper 23, the axis 19 of which coincides with that of the taper 18 of the expanding mandrel.

The hollow body leaves the expanding rolling mill as a finished seamless pipe with a substantially uniform wall thickness and is passed at approximately 900"C to the sizing rolling mill 5 for diameter calibration and cools to room temperature on the cooling bed 6. After the cooling, the usual adjusting and testing procedures follow according to the relevant specifications.

An alternative installation is also shown in Figure 1, by the addition of a roughing skew rolling mill 24 between the piercing device 2 and the plug rolling mill 3. At this roughing rolling mill 24a hollow body is produced from the pierced billet which has sufficiently thin walls for the plug rolling. This is useful in installations where the pierced billet is not suitable directly for plug rolling.

Claims (11)

1. A rolling installation for the production of seamless pipes from solid billets conprising heating; piercing; longitudinal roughing; expanding rolling; sizing; and cooling stages, wherein the expanding rolling stage is a skew rolling mill and the longitudinal roughing stage is at last one of a skew rolling mill and a plug rolling mill, the stages being arranged for sequential operation on a workpiece.

2. An installation according to Claim 1 wherein the longitudinal roughing stage is a plug rolling mill and the installation is adapted to treat a workpiece in at least two passes in the plug rolling mill.

3. An installation according to Claim 1 or Claim 2 wherein the longitudinal roughing stage comprises a plug rolling mill and a skew rolling mill, the installation being adapted to pass a workpiece from the piercing stage to the skew rolling mill prior to the plug rolling mill in the roughing stage.

4. An installation according to any preceding Claim wherein the sizing stage is a sizing rolling mill and receives a workpiece directly from the expanding rolling mill.

5. A rolling installation substantially as descibed herein with reference to the accompanying drawings.

6. A method of producing a seamless pipe from a solid billet in which the billet is sequentially heated; pierced; longitudinally roughed; expanded in a skew rolling mill; sized; and cooled, roughing being accomplished in at least one of a skew rolling mill and a plug rolling mill.

7. A method according to Claim 6 wherein the longitudinal roughing step conprises at least two passes in a plug rolling mill.

8. A method according to Claim 6 or Claim 7 wherein the roughing step conprises skew rolling followed by plug rolling.

9. A method according to any of claims 6 to 8 wherein the worked billet is fed directly from the expanding rolling mill to a sizing rolling mill.

10. A method according to any of Claims 6 to 9 carried out with no intermediate heating of the billet.

11. A mehod of producing a seamless pipe substantially as described herein with reference to the accompanying drawings.