POWDER-FILLED WELDED STEEL TUBE AND METHOD FOR THE CONTINUOUS MANUFACTURE THEREOF

Abstract: A powder-filled welded steel tube has a tube wall (1) with a welded longitudinal seam (6) and is wound into a coil for despatch. To reduce the risk that the seam opens when the tube is twisted, e.g. on uncoiling, the tube wall is provided with a continuous longitudinal groove (5). Preferably the groove (5) is symmetrical and directed radially inwards. In the continuous manufacture of such tube, a metal strip is formed into an open channel, powder is introduced into the channel, the channel is closed and the edges welded to form a closed tube. The diameter is then reduced and the finished product is coiled. The longitudinal groove (5) is rolled in continuously during the diameter reduction of the powder-filled welded tube.

Description: The invention relates to a powder-filled steel tube in coiled form, having a tube wall with a welded longitudinal seam and which is wound into a coil and to a method of continuous manufacture of such a tube. EP-A-0003370 illustrates a tube of the kind described above and a method of its manufacture. This product can be used in steel making for alloying liquid steel or for adding oxidizing agents into a steel ladle. It is therefore known as "ladle wire". This ladle wire, with a typical diameter of 5 to 18 mm, is injected vertically into the liquid steel bath at a speed of between 1 to 10 metres per second. At present this ladle wire is supplied in parallel wound coils. "Parallel wound" means a coil in which the different windings of wire are wound layer-wise next to each other and against each other. In order to uncoil the wire without twisting it, it is necessary to set up such a coil so that it revolves around its axis whereby the wire is pulled off from the coil by the injection device. However, some users find it troublesome to set up such a coil rotatably and they prefer to place the coil on the floor and draw off the wire end starting from the innermost layer. Clearly this means that the wire becomes twisted. In practice, this twisting can impose so much stress on the seam of the welded steel tube that the ladle wire ruptures at the seam and part of the powder spills out. The object of the invention is to improve a powder-filled welded steel tube so as to prevent or minimize the danger of untimely rupture of the seam resulting from twisting. The coiled tube according to the invention is characterized in that the tube wall is provided with a continuous longitudinal groove of depth greater than the wall thickness. This groove thus has an open mouth at the tube circumference and is directed inwardly therefrom. The groove preferably has a depth less than the radius of the tube. This longitudinal groove provides the advantage that it absorbs torsional stresses, so that the seam is protected. It is remarked here that FR-A-1 471 221 discloses a hollow tube, having at least one rib projecting inwardly. The rib acts as a support for a tool when screwing the tube into another body. FR-A-1 471 221 is not concerned with preventing a powder-filled welded steel tube from rupturing untimely. Preferably, as seen in transverse cross section of the tube, the longitudinal groove is symmetrical and directed radially inwards. The invention also provides a method for the continuous manufacture of a powder-filled welded steel tube, in which (i) a metal strip is formed into an open channel, (ii) powder is introduced into the channel, (iii) the channel is closed to form a tube with a longitudinal joint by bringing together the edges of the strip, (iv) the longitudinal joint is welded into a seam to form a closed tube, (v) the diameter of the tube is reduced in at least one reduction stage, and (vi) the tube is wound, e.g. on a reel, into a coil suitable for despatch, characterized in that a longitudinal groove is rolled continuously in the tube wall during the diameter reduction of the powder-filled welded tube, said groove having a depth greater than the final wall thickness. In this method at the time of first rolling of the groove in the tube wall, preferably the tube has an outer diameter of about 16 mm and a wall thickness of about 0.4 mm and the groove created has a width of about 3 mm and a depth of about 3 mm. After further reduction, the tube can have an outer diameter of about 13 mm, preferably between 13 and 13.5 mm and the groove can have a width of about 1 mm and a depth of about 5 mm. A preferred embodiment of the invention is described below by way of non-limitative example with reference to the accompanying drawing, in which Fig. 1 is an enlarged cross-sectional view of the unfinished powder filled tube of the invention, and Fig. 2 shows, also enlarged, the final cross-section of the finished powder-filled tube. In Fig. 1 there is shown a powder-filled welded steel tube at an intermediate stage when the tube wall 1 has a thickness of about 0.4 mm. The outer diameter of the powder-filled welded steel tube is 16.02 mm at this stage. Of course the numerical dimensions given here are not limitative. During the formation of this tube, after welding of the seam, by rolling-in, a continuous longitudinal groove 3, symmetrical in cross-section and directed radially inwards, is produced in the tube wall 1. This longitudinal groove 3 has a base of radius of curvature 1.5 mm and so has an internal width of 2 × 1.5 mm = 3.0 mm and by means of curved surfaces 2 and 4 smoothly passes into the cylindrical tube wall 1 with an external curvature radius a and a₁ of 1.8 mm. The depth of the groove is therefore 1.8 + 1.5 = 3.3 mm. This tube is formed from a metal strip which is shaped into a channel. Powder is introduced into the channel and the strip edges brought together and sealingly welded to produce a longitudinal seam. Diameter reduction, using reducing rolls, follows. The groove 3 is formed during this diameter reduction. When the tube shown in Fig. 1 is further reduced in diameter (by rolling or drawing), the groove 3 which has been rolled in becomes narrower and deeper, and finally reaches the shape 5 shown in Fig. 2. This groove 5 is about 5 mm deep and about 1 mm wide. The tube diameter has become about 13 mm. The tube is then coiled, e.g. on a reel. Preferably the groove 5 is produced at an angular distance of 90° from the welded seam 6, shown in Fig. 2. This 90° angle occurs as a result of a convenient arrangement of the rolls during reducing, assuming that a symmetrical rolled section is to be maintained. However, it is possible that, if the quality of the welded seam 6 is excellent, the longitudinal groove can equally well coincide with this welded seam. When, especially when manufacturing thin-walled tube, this tube is filled with powder for obtaining so-called filled tube (such as is intended as wire or filled ladle wire), it is important that the welded seam produced should not rupture on further processing such as diameter reduction or coiling. If the tube according to Figure 1 is filled with a powder of a density ρ which is still to be compacted, then, on reducing the diameter from d₁ to d₂, because of folding in the rolled-in groove, the final density ρ₂ of this powder will increase more than in inverse ratio to the diameter, in the idal case of no reduction in circumference at all roughly according to the relationship: This means that the increase in length of the tube at this reduction of the diameter can, in principle, be nil, and that in order to achieve a particular powder density, it is only necessary to reduce the diameter less. The torsional stiffness of the wire-shaped tube according to Fig. 2 is significantly less compared to a wire of corresponding outer diameter, which means that less stress in the welded seam occurs on twisting, so that the chance of rupture of this seam 6 on later use is markedly less.

Claims: Powder-filled welded steel tube, the tube wall (1) of which has a welded longitudinal seam (6) and which is wound into a coil characterised in that the tube wall is provided with a continuous longitudinal groove (5) of depth greater than the wall thickness. Powder-filled welded steel tube according to claim 1 wherein, as seen in transverse cross-section of the tube, the longitudinal groove (5) is symmetrical and directed radially inwards. Method for the continuous manufacture of a powder-filled welded steel tube, in which (i) a metal strip is formed into an open channel, (ii) powder is introduced into the channel, (iii) the channel is closed to form a tube with a longitudinal joint by bringing together the edges of the strip, (iv) the longitudinal joint is welded into a seam to form a closed tube, (v) the diameter of the tube is reduced in at least one reduction stage, and (vi) the tube is wound into a coil characterised in that a longitudinal groove (5) is rolled in continuously in the tube wall (1) during the diameter reduction of the powder-filled welded tube, said groove having a depth greater than the final wall thickness. Method according to claim 3, wherein at the time of the first rolling of the groove, the tube has an outer diameter of about 16 mm and a wall thickness of about 0.4 mm, and the groove (3) created has a width of about 3 mm and a depth of about 3 mm. Method according to claim 4, wherein after further diameter reduction, the tube has an outer diameter of about 13 mm and the groove (5) has a width of about 1 mm and a depth of about 5 mm. Method according to any one of claims 3 to 5 wherein said groove (5) is formed extending radially inwardly with respect to the tube wall (1). Method according to any one of claims 3 to 6 wherein, by virtue of the formation of the groove, the diameter reduction of the tube is performed with substantially no length increase of the tube.

Inventors: DACKUS, ARNOLD JOZEF GERARDUS, IR.
Applicants: RIJNSTAAL B.V.
Application year: 1987
Main classification: B23K35
IPC codes: B23K35, C21C7