WO 95/15234 - 1 - PCT/FR94/01281 FEED DEVICE FOR THE DEPOSITION, SIMPLY BY GRAVITY, OF A GRANULAR MATERIAL ONTO THE SURFACE OF A CONTINUOUSLY CAST LIQUID METAL The present invention relates to the continuous 5 casting of metals, and in particular steel. More precisely, the invention relates to the feed and deposition of a layer of a granular material of a relatively fine particle size at the surface of the liquid metal in a continuous-casting mould. The functions 10 of this material are especially to thermally insulate the bath of molten metal, to pick up non-metallic inclusions which settle out from the liquid metal, to prevent re oxidation of the metal and to provide mould/product heat transfer and lubrication of the walls of the mould. This 15 material is normally called "covering powder" and, for convenience, it will be called hereafter "powder", it being understood that its particle size distribution and the shape of the particles may vary widely. This powder melts on contact with the liquid metal and infiltrates 20 between the metal and the walls of the mould, thus acting as a lubricant. This results in a continuous consumption of powder during casting, for which it is necessary to compensate by a fresh supply of powder. The magnitude of this supply varies depending on the characteristics of 25 the powder and on those of the casting plant and of the metal cast. In addition, the consumption of powder may vary in the course of casting depending on the various operating parameters, and especially on the temperature of the metal cast. 30 In order to provide a constant level of powder and therefore uniformity of the thickness of the layer of this powder, it is known to undertake gravity feed by means of a pipe coming from a hopper containing the said material and placed at a greater height than that of the 35 mould. This pipe emerges into the mould above the surface of the metal cast, at a distance from this surface corresponding to the desired thickness of powder, in the solid or molten state. Such a device is described, in -2 particular, in the document EP 473,521. This device makes it possible to provide an automatic powder feed because of the fact that, when the thickness of this powder tends to decrease, it flows in 5 the pipe under gravity until the upper level of the layer reaches the level of the discharge orifice of the pipe, interrupting the flow. In order to automate the powder feed, use is made of its ability to spread out substan tially uniformly over the entire cross-section of the 10 mould, while at the same time making use of the pro perties of particulate materials that they are deposited as a pile on exiting the feed pipe. The pipe, from the feed hopper to its lower end, the distance of which to the surface of the in-mould liquid metal determines the 15 thickness of the powder layer, is permanently filled with powder over its entire length and its entire cross section. The distributor, which is only a few tens of centimetres away from the continuous-casting mould, 20 overhangs the latter, as is the usual practice. It is therefore not possible to install the powder feed hopper above the mould and in the immediate vicinity of it. One consequence of this is that, over a length which may reach several tens of centimetres, the pipe must be 25 inclined with respect to the horizontal by an angle not exceeding a few tens of degrees. This low inclination, dictated by the local conditions of the space taken up by the casting zone, is an obstacle to correct sliding of the powder inside the pipe, which therefore runs the risk 30 of becoming blocked-up. This risk is especially present when a powder of fine particle size, less than 100pm is used. In fact, under these conditions, the rubbing of the particles against each other and against the wall of the pipe is exacerbated, compared to the case of powders of 35 larger particle size (0.3 to 0.8 mm, for example). This leads steelmakers to use the automatic powder distributing devices which have just been described only with powders of large particle size, and to add powder of fine particle size only manually. This manual addition -3 cannot guarantee satisfactory constancy of thickness of the powder layer, and this often leads operators to add more powder than would be necessary, either for safety or by misapprehension. Finally, this manual addition 5 requires the physical presence of the operators in the region of the mould, which it is desirable to avoid as much as possible, for reasons of health and safety (hot and dusty atmosphere, risk of liquid metal spilling over). However, it would be necessary to be able to add 10 powders, whether of small or large particle size, auto matically, since: - the powders most suited to the start-up phases of the casting are precisely those of small particle size; 15 - powders of large particle size are sometimes relatively rich in carbon, and it is not advisable to use them when casting steel having a very low carbon content, as they run the risk of contaminating; - powders of large particle size are approxi 20 mately twice as expensive as powders of small particle size. The object of the invention is to provide a plant for the automatic delivery of covering powder into a continuous mould, simply by gravity, which can operate 25 reliably both with powders of small particle size (less than 100pm) and with powders of large particle size. For this purpose, the subject of the invention is a feed device for the deposition, simply by gravity, of a layer of a granular material onto the surface of a 30 liquid metal contained in a continuous-casting mould, comprising a hopper containing the said granular material and connected to a feed pipe which includes a portion inclined at an angle, this angle being non-zero and not a right angle, with respect to the horizontal, and the 35 lower, discharge, end of which is permanently open and located above the said surface of the metal in the mould at a distance equal to or slightly greater than a set thickness of the layer of the said material, characterised in that, over at least part of the said -4 inclined portion, the said feed pipe is constituted by a straight tube and in that it comprises means for impar ting a rotational movement to the said straight tube about its axis, and means for connecting the ends of the 5 said straight tube to the rest of the said. pipe, allowing the said rotational movement. As will have been understood, the invention consists in imparting a rotation about its axis to at least one portion of that part of the feed pipe which is 10 inclined with respect to the horizontal. Agitation is thus created within the powder which minimises the risks of blocking-up, and enables powders of fine particle size to be used when the configuration of the continuous casting machine locally dictates a relatively small 15 inclination of the pipe. The invention will be better understood on reading the description which follows, given with refe rence to the following appended drawings: - Figure 1, which shows diagrammatically, seen in 20 longitudinal section, the upper part of the continuous casting mould equipped with a device for feeding with granular material according to the invention, this device also being seen in longitudinal section; - Figure 2, which shows diagrammatically a va 25 riant of the device for setting into rotation the straight tube included in the previous device. Figure 1 represents a continuous-casting mould 1 fed conventionally with liquid metal 2 by feed means, not shown, such as a distributor and an immersed nozzle. The 30 liquid metal 2 starts to solidify there by formation of a solid skin 3 against the walls 4 of the mould 1, these walls being strongly cooled by an internal circulation of water. On the surface of the liquid metal 2 there is a layer of covering powder 5 which melts on contact with 35 the metal 2, forming a liquid slag film 6 which flows progressively towards the walls 4 of the mould 1, infiltrates between these walls and the solid skin 3 and acts as a lubricant there. Thus, during the casting, there is a continuous consumption of powder 5 which has -5 to be permanently replaced in order to provide a substantially constant thickness of the layer of the powder 5. The powder 5 is fed from a hopper 7 to which a 5 feed pipe 8 is connected. Introduction of the powder 5 into this pipe 8 is controlled by a valve 32. Conven tionally, the terminal part of this pipe 8 is constituted by a sharply-bent tubular piece 9 made of a rigid material resistant to the relatively high temperatures 10 prevailing above the mould 1. Steel is completely suit able for this use. The lower end 10 of this sharply-bent tubular piece 9 is permanently open and is held (by means which will be described later) above the surface of the liquid metal 2, at a distance d from it. This distance d 15 is equal to or slightly greater than a set thickness of the layer of powder 5 and of slag 6. When the valve 9 [sic] is open, the pipe 8 is permanently supplied with powder 5 so as to be kept filled, and the powder 5 pours out,' simply by gravity, into the mould 1 until the layer 20 thus formed closes off the lower end 10 of the sharply bent tubular piece 9, thus interrupting the supply of powder. This phenomenon goes on continually throughout the casting, progressively with the consumption of the powder 5, the thickness of which, in the mould 1, is thus 25 kept constant. As has been stated, according to the invention, in order to permit the use of a powder 5 of fine particle size without running the risk of blocking up the pipe 8 in its slightly inclined portion, this pipe is made in 30 the following way. The sharply-bent tubular piece 9 which terminates it is connected, at its upstream end, to a straight tube 11. This tube 11 must be rigid and heat resistant and must exhibit, on its internal surface, a coefficient of friction with the powder 5 which is as low 35 as possible in order to permit easy descent of the powder 5. Here too, steel, in particular stainless steel, is a material particularly suited for making it. At its upstream end, the tube 11 is itself connected to the remaining part of the pipe 8. The internal diameter of -6 the tube 11 is of the same order of magnitude as that of the assembly comprising the pipe 8 and the sharply-bent tubular piece 9, namely of the order of 2 to 6 cm. According to the invention, the plant also includes means 5 for holding the longitudinal axis of the tube 11 in a position such that it makes an angle a, which is non-zero and is not a right angle, with the horizontal, and means for setting the tube 11 into rotation about this axis. In the example represented, these holding means include a 10 telescopic support arm 12 fixed to a base plate 13 fastened to the upper edge 14 of the mould 1. This support arm, in the (non-limiting) example represented, includes a vertical rod 15 rigidly fixed to the base plate 13, a tubular rod 16 fixed to the vertical rod 15 15 by an articulation 17 enabling the tubular rod 16 to be inclined. Inside the tubular rod 16 may slide a rod 18, the depth of penetration of which, into the tubular rod 16, may be adjusted and kept constant by a key 19 or by any other equivalent means. This rod 18 is, on the one 20 hand, rigidly connected via a lateral rod 20 to the sharply-bent tubular piece 9 constituting the terminal part of the pipe 8. On the other hand, the rod 18 carries, at its free end, a motor 21 (for example an electric or pneumatic motor) imparting to a spindle 22 a 25 rotational speed chosen by the operator. At its free end, this spindle 22 carries a roller 23, the outside surface of which is kept in contact with the external wall of the straight tube 11 and the shape (frustoconical in the example represented) of which is suitable for this 30 purpose. The roller 23 is made of a material exhibiting a high coefficient of friction with the material forming the straight tube 11, for example rubber if the straight tube 11 is made of steel. The result obtained by this configuration is that the rotation of the roller 23, 35 driven by the spindle 22 of the motor 21, causes, by friction, rotation of the straight tube 11 about its longitudinal axis. This rotation is carried out at an angular speed dependent on the rotational speed of the roller 23 and on the respective dimensions of the roller -7 23 and of the straight tube 11. These are chosen in such a way that the angular rotational speed of the straight tube 11 is of the order, for example, of 1 rpm. In order to ensure good friction between the roller 23 and the 5 straight tube 11, provision may also be made to roughen the external surface of the straight tube 11 in the zone where it is liable to be in contact with the roller 23. As has been stated, the effect of this rotation of the straight tube 11 is to induce, within the powder 5 10 contained therein, permanent agitation which prevents the risk of the straight tube 11 from becoming blocked-up. The angular a of inclination of the axis of the straight tube 11 and the distance d between the end 10 of the sharply-bent tubular piece 9 and the surface of the 15 liquid metal 2 in the mould 1 are adjusted in combination by the operator, who may vary, on the one hand, the depth of insertion of the rod 18 into the tubular rod 16 and, on the other hand, the orientation conferred on the tubular rod 16 by the articulation 17. In order to permit 20 this adjustment, it is, in addition, necessary that the pipe 8, in its part located upstream of the straight tube 11, possess articulation means or be made, over at least a portion of its length, of a flexible material. A plant is represented in Figure 1 which is such that, with the 25 chosen angle a and the chosen distance d, the sharply bent tubular piece 9 is perpendicular to the surface of the liquid steel 2 in the mould 1, although this feature is not at all obligatory. The connections between the straight tube 11 and 30 the pipe 8 on the one hand, and the sharply-bent tubular piece 9 on the other hand, must be designed to permit easy rotation of the straight tube 11. This assumes that the powder 5 cannot form an obstacle to this rotation. For this purpose, as represented in Figure 1, these 35 connections may be provided by joints 24, 25, each of which includes a rolling bearing fastened to the straight tube 11 or to the element to which it is connected. In addition, so that the powder 5 cannot jam up the ope ration of the joints 24, 25, it is preferable that: -8 - the downstream end 26 of the pipe 8 is inserted into the upstream end 27 of the straight tube 11 and emerges downstream of the joint 24; - and that [sic] the downstream end 28 of the 5 straight tube 11 is inserted into the upstream end 29 of the sharply-bent tubular piece 9 and emerges downstream of the joint 25. Figure 2 shows a possible variant of the mode of rotational driving of the straight tube 11. Instead of 10 the roller 23 of the previous example, it is a pinion 30 which the spindle 22 of the motor 21 carries, and this pinion meshes with a frustoconical crown gear 31 fastened to the external wall of the straight tube 11. The straight tube 11 is thus set into rotation more reliably 15 than if this were produced simply by friction of the roller 23 against the straight tube 11. The minimum value of the angle a of inclination of the straight tube 11 depends on the characteristics of the powder 5, but a value of 20 to 30* is sufficient to 20 ensure that the usual fine powders, the particles of which have a mean size of less than 100pm, fall uni formly. As regards the length of the straight tube 11, this depends on the geometry of the continuous-casting plant on which the device according to the invention is 25 fitted. However, it is quite clear that it is advan tageous for this rotary straight tube 11 to represent a significant fraction of the path effected by the powder 5 between the hopper 7 and the mould 1, at least from the non-vertical portion of this path, in which portion, 30 without it, problems of flow of the powder 5 could be encountered. Of course, although this device is principally designed to deliver powders 5 of fine particle size, it is perfectly compatible with the use of powders 5 of 35 coarser particle size. Likewise, it may be installed on plant for continuously casting other metals than steel.