WO2000037199A1 - Side wall for closing the casting space of an installation for continuous casting of thin metal strips - Google Patents

Abstract

The invention concerns a side wall (3) for closing the casting space in an installation for continuous casting of thin metal strips between two unwinding cooled casting surfaces, comprising an active part (5) designed to be in contact with the liquid metal and to rub against the sides of the casting surfaces, a casing (7) enclosing said active part, a frame (8), means for pressing (9, 10) the side wall (3) against the sides of the casting surfaces integral with the frame (8) and the casing (7), an inductor arranged inside the casing (7), whereof the coil (11) is formed by the winding of a tube in internally cooled conductive material extending rearwards of the side wall (3) passing through the casing (7) in the form of two rigid branches (14, 15) then of flexible conductors (16, 17) connected to a supply of electric power and cooling fluid (18). The invention is characterised in that said inductor comprises shielding means including a magnetic shield placed behind the coil (11), such as a ferrite stack (19), an electric shield such as a cooled copper plate (23) placed behind said magnetic shield and supporting it, and at least a sheath (27, 29) encloses said rigid branches (14, 15) and said flexible conductors (16, 17) over at least part of their length.

Description

 SIDE SIDE FOR SEALING THE CASTING SPACE OF A CONTINUOUS CASTING INSTALLATION OF METAL THIN STRIP

The invention relates to the continuous casting of metals. It relates more particularly to installations for the continuous casting of thin metal strips between two cooling moving surfaces, in particular those of the type called "casting between cylinders", and very precisely the lateral faces which close off their casting space.

The casting of thin metal strips a few mm thick directly from liquid metal (steel or cui \ τe, for example) can take place on an installation called "casting between cylinders". supplied with liquid metal from a distributor by means of a pouring nozzle. The machine includes an ingot mold, the casting space of which is delimited on its long sides by a pair of internally cooled cylinders with parallel horizontal axes and rotating around these axes in opposite directions, and on its short sides by closure plates (called side faces) in refractory matéπau, applied against the flat ends of the cylinders. The solidification of the liquid metal must occur only on the cylindrical surfaces of the rolls cooled by the formation of solidified shells that ^'is caused to join at the neck (the area where the gap between the roll surfaces is minimal) to form the strip, which is continuously extracted from the casting space. The cylinders can be replaced by two cooled scrolling bands which are given curved shapes.

In practice, it is however difficult to permanently avoid the appearance of solidifications called “parasitic solidifications”, that is to say layers of solidified metal which arise on certain zones of the lateral faces, often in the immediate vicinity of the cylinders. . They are due to the fact that the liquid metal surrounding these zones may be at a temperature substantially lower than its nominal temperature and therefore be capable of solidifying locally on contact with the lateral faces. This low temperature may be due to the influence of the cooled cylinders, or to unfavorable hydrodynamic conditions which do not allow sufficient renewal of the liquid metal in these zones, or to an insufficient temperature of the lateral faces due to poor preheating before casting. . Of course, several of these factors can combine. The parasitic solidifications which occur in the vicinity of the contact zones between the cylinders and the lateral faces cause degradations of the lateral faces which compromise the sealing of the casting space. Metal can then infiltrate between the side faces and the cylinders and solidify there, which further increases the deterioration of the side faces. In the most severe cases, the metal can even escape in a liquid state out of the machine, with all the risks that this entails for the installation and the personnel. In addition, when the solid metal resulting from these parasitic solidifications is entrained in the bottom of the casting space, it must pass between the cylinders, creating a thickness of metal which is added to the normal thickness of the product. It follows that the cylinders must temporarily absorb an additional effort which forces them to temporarily move away from each other to avoid their deterioration. The quality of the tape is adversely affected. We can also witness a decline in the lateral face, with the risks of losing the tightness of the casting space associated with it. Usually, we try to limit the appearance of parasitic solidifications by adopting particular nozzle configurations. They aim to impose on the liquid metal located in the casting space flow conditions which are supposed to ensure a permanent renewal of the metal in front of the zones where the parasitic solidifications are most likely to occur, for example by bringing out liquid metal of the nozzle directly near these areas. This can however lead to a deficit of liquid metal supply in other parts of the casting space.

Another method consists in providing a possibility of permanent reheating of the lateral faces during casting, or even also an induction reheating of the metal in their vicinity. Document JP-A-62-077155 shows a casting installation the lateral faces of which are heated by means of plasma torches pointed on their rear faces. The document EP-B-0 593 383 shows an installation where conductive elements (called "susceptors") are embedded in the side faces in refractory and are heated under the action of an inductor placed behind the side faces. The heat of the susceptor is thus transmitted to the lateral faces by conduction. The documents JP-A-62-077156 and EP-B-0 491 641 describe installations where the liquid metal surrounding the lateral faces is heated by induction, by means of inductors placed behind the lateral faces, the latter being sufficiently thin to not not prevent the magnetic field from entering the liquid metal. The first of these documents teaches to use for this purpose thin side faces of refractory or metallic material, while the second document recommends only the use of metallic side faces, possibly coated with a refractory protective film on their face in contact with liquid metal. The refractory constituting the lateral face can be electrically conductive, which has the advantage of providing the possibility of obtaining a reheating of the small face itself by the Joule effect. Document JP-A-2- 155543 (which relates to the lateral faces of a casting installation between two moving bands) teaches to integrate the inductor within the lateral face itself, which has the advantage of minimize the total size of the heating installation and not hinder the installation and use of the means for applying the lateral face against the sides of the casting surfaces (cylinders or strips).

However, the use of induction heating has the drawback of causing the Joule effect to heat the metal parts of the support members of the lateral face which are located in the vicinity of the inductor, at the rear thereof. . Likewise, the electromagnetic fields emitted by the inductor, risk disturbing the operation of the multiple sensors and actuators which manage the operation of the organs controlling the application of the side faces against the cylinders or the strips.

The object of the invention is to provide a constitution of an induction heating device, intended to prevent the appearance of parasitic solidifications on the lateral faces of a thin metal strip casting installation, which does not influence the behavior of the side face applicators and sensors that manage their operation.

To this end, the subject of the invention is a lateral face for closing off the casting space of an installation for casting thin metal strips between two casting surfaces cooled in movement, comprising an active part intended to be in contact liquid metal and to rub against the sides of the casting surfaces, a box enclosing said active part, a frame, means for applying the lateral face against the sides of the casting surfaces integral with the frame and the box, an inductor arranged inside the box, the coil of which is formed by the winding of a tube of internally cooled conductive material which extends towards the rear of the side face passing through the box in the form of two rigid branches then flexible conductors connected to a supply of electricity and cooling fluid, characterized in that said inductor comprises shielding means comprising a magnified screen tick placed behind said coil, such as a stack of ferrites, an electric screen such as a cooled copper plate placed behind said magnetic screen and supporting it, and in that at least one sheath surrounds said rigid branches and said conductors flexible over at least part of their length.

In a particular embodiment, the lateral face also includes a susceptor interposed between the insulating layer and the active part or its support plate, intended to be heated by the flux coming from the coil and to transmit its heat to said active part. , possibly via the support plate.

As will be understood, the invention consists in providing in the environment of the inductor coil effective shielding means, both from the magnetic point of view and from the electrical point of view. This shielding is obtained by installing behind the coil successively a magnetic screen, such as a stack of ferrite blocks, and an electric screen such as a cooled copper plate. Likewise, the conductors which bring the electric current to the coil and which extend behind the lateral face must also be electrically isolated from their environment. The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

- Figure 1 which shows, seen in profile and in section along II, a first example of a side face according to the invention; - Figure 2 which partially shows this same first example seen from the front along II-II;

- Figure 3 which shows, seen in profile and in section along I-I, a second example of a side face according to the invention. In Figure 1, there is shown in dotted lines the outer contour of one of the casting rolls 1, against the lateral end 2 of which the lateral face 3 according to the invention is applied. The horizontal level 4 of the neck is also located in broken lines, that is to say of the zone where the cylindrical lateral surfaces of the cylinders 1 are separated by a minimum distance, equal to the thickness targeted for the cast strip. In accordance with the prior art, this lateral face 3 comprises the following elements:

- An active part 5 intended to rub against the cylinders 1 and to be in direct contact with the liquid metal; it can be in one piece or be formed by the juxtaposition of several elements, made of different materials: a core which provides most of the contact with liquid metal and for which the insulating properties are favored, and lateral parts which enclose the core by ensuring contact with the cylinders 1 and for which the properties of resistance to friction and to corrosion by liquid metal are favored in order to obtain the most impermeable side face-cylinder contacts possible;

- A support plate 6 of refractory material, on which the active part 5 is fixed; - A box 7 which encloses the active part 5 and its support plate 6, and which consists essentially of a metal part cooled by internal circulation of water in channels not shown;

- A frame 8 which can be at will approached or distant from the cylinders 1 and be held in position; - Means for applying the side face 3 against the cylinders 1, which are integral with both the frame 8 and the box 7, and which can be constituted by actuated cylinders 9, 10 or by spring devices; these means of application are often three in number as shown; two of them (like the jack 9 in FIG. 1) act on the upper part of the lateral face and are arranged on either side of the longitudinal plane of symmetry of the casting installation (the jack 9 is therefore behind the cutting plane of Figure 1); the third application means acts on the lower part of the lateral face 3 and in said plane of symmetry, for example in an area located substantially at level 4 of the neck of the casting installation (which is the case of the jack 10 in Figure 1). As these bodies are part of the prior art and do not form part of the invention, they have only been shown very schematically in FIG. 1.

According to a first embodiment of the invention, there is an inductor inside the box 7 and behind the support plate 6. This inductor comprises a coil 1 1 formed by a tube of conductive material, such as copper, impregnated insulation, to inside which a cooling fluid such as water is circulated. In the example shown, the conductor is wound so as to form the coil 11 of a bipolar plane inductor which ensures a return of the flux from one pole to the other. But it would remain in the spirit of the invention to provide a unipolar inductor. Likewise, the winding forms only one layer, but could form several to increase the flux of the electromagnetic field. As is apparent in Figure 2, the outer contour of the coil 1 1 has an approximately triangular shape, so as to match the perimeter of the casting space defined by the cylindrical outer surfaces of the cylinders 1, and thus avoid the action of the flow of the coil 11 does not heat the peripheries of the cylinders 1.

This inductor has the function of heating, thanks to the currents induced by the flux produced by the coil 11, a susceptor 12 made of a highly conductive material such as graphite. This susceptor 12 is fixed to the rear of the support plate 6, and transmits its thermal energy to it. The support plate 6 in turn transmits this thermal energy to the active part 5 of the lateral face 3, so as to heat it to reduce the risk of parasitic solidification formation of liquid metal on its face facing the casting space. An intermediate layer of insulator 13, such as a silica-based fibrous material, separates the coil 11 and the susceptor 12, and thus prevents the susceptor 12 from being in contact with the coil 11 which, as has been said, is cooled by internal circulation of water. As shown, the susceptor 12 does not necessarily have the same dimensions as the support plate 6 and the active part 5. It suffices that it is present at the level of the zones of the active part 5 on which the parasitic solidifications are liable to produce, in order to achieve a preferred heating of said zones.

The copper tube constituting the coil 1 1 extends towards the rear of the side face 3 in the form of two rigid horizontal branches 14, 15. Hollow flexible conductors 16, 17 are fixed to said rigid branches 14, 15 and are connected to an installation 18 which supplies the electric current which supplies the inductor and the water which cools it.

In order to prevent the posterior parts of the lateral face 3 and of its holding device (of which the cylinders 9, 10 and the frame 8 are the main elements) from also being heated by the flux coming from the coil 1 1 of l 'inductor, effective shielding means must be provided. According to the invention, magnetic shielding means and electrical shielding means are provided. Immediately behind the coil 11, there is a stack of ferrites 19 which provide the magnetic shielding by stopping the magnetic flux which is emitted towards the rear of the coil 11 and by returning it in the direction of the susceptor 5. This thus increases the efficiency of the installation. This stack of ferrites 19 has a recess 20 in its central zone, so as to allow passage to the portions 21, 22 of the copper tube which connect the coil 11 to the rigid horizontal branches 14, 15. The ferrites 19 are themselves fixed on a copper plate 23 which provides, in addition to the support function of the ferrites 19 and the coil 1 1, the electrical shielding of the installation. This copper plate 23 is cooled on its rear face by circulation of water in channels 24. A recess 25 formed in the copper plate 23 allows passage to the portions 21, 22 of the copper tube. Preferably, as shown, the edges of the copper plate 23 are folded so as to provide a frame for the assembly of the inductor and the insulator 13. Of course, the ferrites can be replaced by any other suitable material to fulfill their magnetic screen function.

The rigid horizontal branches 14, 15 of the copper tube forming the coil 11 must also pass through the box 7. For this, a recess 26 is made in said box 7. But to avoid heating the surrounding metal parts and disturbing the measurements made by the various sensors present on the installation, it is also necessary to ensure effective shielding of these rigid horizontal branches 14, 15. For this purpose, there is around them and inside the recess 26 of the box 7 a first sheath 27 in copper, cooled externally by a coil 28 with water circulation. A second sheath 29 cooled by a coil 30 surrounds the flexible conductors 16, 17 over their entire portion where, in the absence of shielding, they would likewise risk disturbing the operation of the side face 3. The connection between the two sleeves 27, 29 is provided by a copper ring 31 which extends the first sleeve 27 and is in contact with the second sleeve 29 by means of an articulated connection comprising a ball joint 32 surrounding the second sleeve, and allowing lateral movements from one scabbard to the other. The articulated link must be able to ensure the electrical continuity of the shielding. This constitution provides the shielding of the flexible conductors 16, 17 with a freedom of movement which makes it capable of following the movements imposed on the lateral face 3 during casting. In order to further increase this freedom of movement, provision may be made for the flexible conductors 16, 17 to be surrounded not by a single sheath 29 but by a succession of such sheaths connected to each other by articulated connections.

If the susceptor 12 is made of a combustible material such as graphite, it is advisable to provide a blowing of a neutral gas, such as argon, at the interface between the susceptor 12 and the insulator 13. To this end, a pipe 33 is housed inside the box 7, connected to an argon tank and opening out towards said interface. Grooves 34 can be made on the surface of the insulator 13 which is in contact with the susceptor 12 in order to promote the circulation of argon on the interface.

For a good control of the operation of the installation, it is preferable to give the possibility of measuring the temperature of the susceptor 12. This can be achieved by pyrometric sighting at one or more points. In the example shown, perforations 35, 36 are made for this purpose in the box 7, the copper plate 23 and the insulator 13, so that they can be passed through, for example, by optical fibers (not shown) connected pyrometers, these perforations 35, 36 giving access to two zones of the face rear of the susceptor 12. This measurement can be replaced by pyrometry with thermocouples brought into contact with the susceptor 12, or jointly use these two measurement systems, the advantages and disadvantages of which are complementary. Thermocouples are more fragile, while the functioning of pyrometers is likely to be disturbed by the electromagnetic fields of the environment.

The variant of the invention shown in Figure 3 (its common elements with the variant of Figures 1 and 2 are designated by the same references) differs from the previous version in that, this time, we no longer seek to heat the active part 5 of the lateral face 3 via the susceptor 12, but an attempt is made to reheat the liquid metal present in the vicinity of said active part 5. For this purpose, compared with the previous version shown in FIGS. 1 and 2 , the susceptor 12 has been eliminated, as well as the perforations 35, 36 allowing pyrometric sighting for controlling its temperature and the means 33, 34 for injecting argon at the susceptor 12-insulator interface 13. So that the magnetic field reaches the liquid metal, it is necessary to choose the dimensions of the various parts of the installation and the operating parameters of the inductor. To this end, compared to the previous version, it is desirable to decrease the frequency of the electric current flowing through the inductor to increase the thickness of penetration of the magnetic field, and to decrease the thickness of the insulator 13 in order to bring the as much as possible the coil 11 of the liquid metal without compromising the mechanical resistance of the refractories of the lateral face 3, the thickness of which can be preserved. The materials used to compose the support plate 6 and the active part 5 must be as transparent as possible to the magnetic flux. In general, it is necessary to avoid using refractories having a relatively high electrical conductivity, such as highly graphite refractories or carbides, such as silicon carbide.

As an indication, provision can be made to supply the coil 11 of the inductor with an alternating current of frequency 1 to 100 kHz (typically around 30 kHz if the active part 5 of the lateral face 3 is reheated by the intermediate of a susceptor 12), at a voltage of 100 to 2000 V and an intensity of 200 to 2000 A. In the case where a susceptor 12 is used, it must be possible to bring it to a temperature of the order of 1350 to 1500 ° C.

Without departing from the spirit of the invention, modifications can be made to the installations described which do not affect the ability of the inductor and its environment to heat either the active part 5 of the lateral face 3 at least in its parts most likely to be affected by the parasitic solidifications, ie the liquid metal itself in the vicinity of the lateral face 3, and this without disturbing the operation of the other members of the device for maintaining and managing the lateral face 3. Thus, one can consider removing the support plate 6 to put the active part 5 of the side face 3 directly in contact with the susceptor 12 or the insulator 13. Similarly, one can reduce the minimum the length of the rigid branches 14, 15 and connect the flexible conductors 16, 17 almost directly to the portions 21, 22 of the copper tube which extend the coil 11 of the inductor towards the rear. It is thus possible to merge the two shielding sleeves 27, 29 into a single one. The invention can also be applied to an installation for casting thin strips between two strips cooled in movement.

Claims

1) Side face (3) for closing the casting space of a device for casting thin metal strips between two casting surfaces cooled in movement such as the lateral surfaces of two cylinders (1) with horizontal axes internally cooled and rotating in opposite directions or two curved cooled strips, of the type comprising an active part (5) intended to be in contact with the liquid metal contained in the casting space and to rub against the sides of the casting surfaces, and optionally fixed to a support plate (6), a box (7) enclosing said active part (5) and the possible support plate (6), a frame (8) can be at will approached or removed from the casting surfaces, means for application (9, 10) of the lateral face (3) against the sides of the casting surfaces integral with the frame (8) and the box (7), an inductor arranged inside the box (7), the coil ( 11) is formed by the winding a tube of internally cooled conductive material which extends towards the rear of the lateral face (3) passing through the box (7) in the form of two rigid branches (14, 15) then of flexible conductors (16, 17) connected to a supply of electricity and coolant (18), said coil (11) being separated from the active part (5) or from its support plate (6) by an insulating layer (13), characterized in that that said inductor comprises shielding means comprising a magnetic screen placed behind said coil (11), such as a stack of ferrites (19), an electric screen such as a cooled copper plate (23) placed behind said magnetic screen and supporting it, and in that at least one sheath (27, 29) surrounds said rigid branches (14, 15) and said flexible conductors (16, 17) over at least part of their length. 2) side face according to claim 1, characterized in that it also comprises a susceptor (12) interposed between the insulating layer (13) and the active part (5) or its support plate (6), intended to be heated by the flow from the coil (11) and to transmit its heat to said active part (5), possibly via the support plate (6). 3) side face according to claim 2, characterized in that it comprises means (33, 34) for injecting a neutral gas at the interface between the insulating layer (13) and the susceptor (12) . 4) side face according to claim 2 or 3, characterized in that it comprises recesses (35, 36) formed in the box (7), the electrical screen, the magnetic screen and the insulating layer (13 ) allowing the measurement of the temperature of the susceptor (12) by pyrometric sighting. 5) Side face according to one of claims 1 to 4, characterized in that the inductor is a bipolar inductor.