EP0178477A1 - Method for producing seamless steel tubes with a great diameter - Google Patents

Abstract

A method of forming seamless metal tubing and pipes of large diameter in excess of 500 mm diameter. A thickwalled hollow body is made by continuous casting. Rolling the continuously cast hollow body over a mandrel in an oblique rolling mill to the desired weight per unit length. Radially expanding the rolled hollow body while maintaining its weight per unit length.

Description

The invention relates to a method of making seamless large diameter steel tubes, i.e. with an outer diameter larger than approx. 500 mm.

Steel pipes of large diameter, especially for pipelines, are almost exclusively made from welded pipes. The reason lies essentially in the economical production method of welded steel pipes compared to seamless steel pipes of large dimensions. However, it would be desirable to use seamless tubes for most of the applications of large diameter steel tubes to avoid the risk of uncertainty of a weld both during installation and operation. As is known, weld seams are always a problem of strength, which means that even in the manufacture of such pipes, a high outlay is required both for the inspection of weld seams and for the subsequent tempering.

Methods are known in which pipes are produced by rolling out large hollow blocks. The technical and economic difficulties lie on the one hand in the production of the large blocks, on the other hand in the use of special, complex rolling mills.

A method has also already been proposed, starting from hollow blocks which are produced by continuous casting, by producing the final dimension of the tube by rolling the hollow block in a multi-roll cross-rolling mill. However, this manufacturing process is only suitable for pipes up to max. medium size, since the shaping work, even if it is carried out in several passes, involves technical risks if you want to go to diameters over 500 mm and lengths over 10 m. Costly rolling mills and costly tests are then necessary to avoid cracking or dimensional deviations.

• a) Herstellung von Hohlblöcken durch an sich bekanntes Stranggießen,
• b) Einsatz der Hohlblöcke in ein an sich bekanntes Schrägwalzwerk mit auf Zug beanspruchtem Dorn zum Auswalzen dieser Blöcke auf das vorgesehene Metergewicht,
• c) Aufweiten unter ungefährer Beibehaltung des Metergewichts ohne I Streckung in axialer Richtung auf die gewünschte Endabmessung in an sich bekannten Aufweitewalzwerken mit einer auf Zug beanspruchten Dornstange.

The disadvantages mentioned are avoided in a method for producing seamless steel tubes of large diameter by using the method steps according to the invention:

• a) production of hollow blocks by continuous casting known per se,
• b) use of the hollow blocks in a known cross-rolling mill with a mandrel subjected to tensile stress to roll these blocks out to the intended weight per meter,
• c) Widening while roughly maintaining the meter weight without I stretching in the axial direction to the desired final dimension in known widening rolling mills with a mandrel bar subjected to tension.

The inventive combination of these process steps makes it possible to economically produce seamless steel pipes of large diameter and thus to avoid the known risks of the welded large pipes due to the uncertainty of the weld seam. This is particularly important for pipes that are made from difficult to weld steels or that are under a very high internal pressure. This is also important for pipes that are to be used in difficult climatic conditions or whose handling does not require special care.

In the drawing, an embodiment of the process sequence according to the invention is shown schematically in comparison to the conventional welded manufacture of large pipes.

Fig. 1 shows the process for producing a welded large pipe. The first production unit is the steel mill with a slab caster. Both are shown symbolically in box 1. The starting product is the slab 2, which is rectangular in cross section in the next processing step after intermediate heating, fed to a sheet metal rolling mill, symbolically represented by box 3.

After the finishing line, the starting material for tube production, the heavy plate, is shown at 4. As the starting material for the final pipe production in a large pipe mill, this first comes into a so-called U-press, in which a U-shaped body is first produced in the longitudinal direction of the sheet. This U-body is completely bent in the so-called 0-press into a tube with a round cross-section and then welded in the longitudinal direction at the joints. After the subsequent heat treatment and adjustment, the manufacturing process for the finished tube is finished. This manufacturing process is symbolized by box 5. The end result is the welded large pipe 6.

The process for producing a seamless large pipe is shown schematically in the same way in FIG. 2. In the first production stage, symbolically designated by box 7, there is a steel mill with a hollow continuous casting system known per se. The result of this manufacturing stage is a round, thick-walled, hollow steel body, the so-called hollow block, as shown in cross section at 8. In principle, this hollow block can be of any length.

After a possibly necessary reheating, this hollow block is used in the next process stage 9 in a cross-rolling mill, the main task of which is to produce the intended final length of the finished tube. This cross rolling mill is primarily represented by a known cross rolling mill, the mandrel bar of which is subjected to tension.

Then the stretched hollow block is fed to one or more series-connected expansion rolling mills with mandrel bars that are subjected to tension, which only have the task of bringing the diameter of the hollow block to the desired dimension with the desired wall thickness and thus to produce an extension only in the tangential direction below approximate maintenance of the meter weight or the axial dimension. The exact final dimension is in a sizing mill or expander generated. Once this has been achieved, there is a seamless tube 10 after the finishing, the dimensions of which, depending on the deformation units used and on the hollow block coming from the hollow continuous casting installation, are quite comparable to those of the welded large tubes. Furthermore, it is also conceivable to achieve dimensions without great difficulty, which are superior to the welded large pipes of conventional production both in the length of the large pipe and in its diameter and wall thickness. Finally, it is also possible to use this type of large pipe production to process high-quality steel more safely and economically because of the lack of welding risk.

The advantages of this manufacturing process are, in particular, a lower expenditure on plants and thus investments, as well as a lower expenditure on energy compared to welded large pipes. This makes it possible to produce large pipes in an economically justifiable manner that do not have the risk of a weld seam, are more economical to manufacture and can be kept variable in their dimensions, in particular with regard to the pipe length, combined with the advantage of the possibility of processing high-quality steels.

Claims (1)

Process for producing seamless steel pipes of large diameter, characterized in that the following process steps are used: a) production of hollow blocks by continuous casting known per se, b) use of the hollow blocks in a known cross-rolling mill with a mandrel subjected to tensile stress to roll these blocks out to the intended weight per meter, c) widening while roughly maintaining the weight per meter without stretching in the axial direction to the desired final dimension in expanding rollers known per se.

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