EP0875319A1 - Tundish provided with at least one plasma torch for reheating molten metal - Google Patents

Abstract

A refractory component (28), of annular form, to be implanted in a tundish (1) for the continuous casting of metals incorporating at least one plasma torch (18) for the reheating of the liquid metal and of which the internal wall (29) defines a space widening towards the bottom making up an upper opening (30) and a lower opening which allows the penetration of the lower end of the plasma torch (18) into this space. Also claimed is the tundish incorporating this refractory component.

Description

The invention relates to the field of continuous casting of metals, such as steel. It relates more precisely to continuous casting machines which include a torch plasma to heat the metal during its stay in the distributor.

During the continuous casting operation, the liquid steel contained in the casting, where its composition has been adjusted, does not flow directly into the bottomless ingot molds with cooled walls where it begins to solidify. It passes first in a container called "distributor", internally coated with refractories, the there are multiple functions. First, the bottom of the distributor is provided with one or generally several orifices called "nozzles" each overhanging an ingot mold, which allows to distribute the liquid metal in the different ingot molds while the pocket There is only one metal flow hole. On the other hand, the dispatcher constitutes a metal reserve which allows, when a pocket is emptied, to continue the pouring of the metal during the evacuation of the empty pocket and the positioning and opening a new pocket. We can thus run without interruption several successive pockets (operation known as "casting in sequence"). Finally, the dispatcher constitutes a privileged site for the settling of undesirable non-metallic inclusions present in the liquid steel, and this the more so as the average residence time of the metal is higher there.

• de diminuer l'amplitude des variations de la température de l'acier liquide sortant du répartiteur pendant la coulée: une poche met en général plusieurs dizaines de minutes à se vider, et pendant cette période l'acier liquide qu'elle contient peut perdre quelques dizaines de degrés; un apport d'énergie dans le répartiteur, notamment en fin de coulée, permet de compenser au moins en partie ces pertes thermiques, de manière à limiter les variations de la température du métal sortant du répartiteur dans une plage de quelques degrés pendant l'ensemble de la coulée;
• d'abaisser la température imposer au métal lors des étapes antérieures de son élaboration, d'où un gain de productivité de l'aciérie (on peut raccourcir les périodes de réchauffage du métal lors du traitement au convertisseur, au four électrique ou au four-poche) et des économies sur la consommation des matériaux réfractaires revêtant les divers récipients métallurgiques.

On certain continuous casting installations, the possibility is given to act on the temperature of the liquid steel by means of a heating device. This action can allow:

• to reduce the amplitude of the variations in the temperature of the liquid steel leaving the distributor during casting: a ladle generally takes several tens of minutes to empty, and during this period the liquid steel which it contains may lose a few tens of degrees; an energy supply in the distributor, especially at the end of casting, at least partially compensates for these heat losses, so as to limit the variations in the temperature of the metal leaving the distributor in a range of a few degrees during the assembly casting;
• lower the temperature imposed on the metal during the previous stages of its development, resulting in a gain in productivity of the steelworks (the heating periods of the metal can be shortened during treatment with a converter, an electric furnace or a furnace). pocket) and savings on the consumption of refractory materials coating the various metallurgical vessels.

In general, this increased control of the temperature makes it easier obtaining a temperature of the steel in a distributor relatively close to the temperature of liquidus from the casting shade. The difference between these two temperatures is called "overheating". From a metallurgical point of view, low overheating is favorable for obtaining a solidified product having in its section weak segregations in alloying elements such than carbon, manganese and sulfur, and therefore a good homogeneity of its properties mechanical. This advantage is particularly important when pouring shades of steel heavily loaded with alloying elements. On the other hand, low overheating allows to shorten the duration of solidification of the product: we can take advantage of this to pour the produced at a higher speed, resulting in a productivity gain in the steelworks, or for build a relatively compact continuous casting machine, saving on investments to put into play.

A first mode of supply of thermal energy to the metal passing through the distributor consists of scrolling at least part of said metal inside a channel surrounded by an inductor with appropriate characteristics, the currents induced in the metal causing it to heat up by the Joule effect. This solution is quite expensive, and the size of the inductor makes it difficult to apply to small installations size, or which were not originally designed to be equipped with it.

Another solution is to install a distributor over the metal, or even several plasma torches. The document WO 95/32069 in particular describes a distributor thus equipped. Remember that the operating principle of a torch plasma consists in blowing on the material to be heated a gas under pressure (gas plasmagen), such as nitrogen or argon, which is passed through an electric arc created between a cathode and an anode. The gas is thus partially ionized and is brought to very high temperature (4000 to 15000 K). It has a thermal conductivity and a power of very high radiation, which makes it suitable for achieving rapid heat transfers and intense with the material to be heated. By varying the gas pressure and the intensity of the current, it is easy to obtain the powers of several hundred kW necessary for the reheating of the steel in a distributor, while preserving the space required by the torch small enough to make it possible to install even on a size distributor scaled down. Two torch designs can be used for this application. In the plasma torches, the cathode and the anode are both integrated into the torch. In plasma torches "transferred", only the cathode is integrated into the torch, and the anode is consisting of the liquid metal to be heated. For this purpose, the sole of the distributor contains a electrically conductive element which is brought into contact with liquid metal during casting and connected to the positive terminal of the torch power supply. he is also possible to provide opposite polarities from those previously specified.

The area of the distributor in which the torch is installed must be covered by an internally coated refractory cover. This cover prevents the radiation from the arc does not blind the personnel working on the installation. Else hand, it is imperative that the torch acts on bare liquid metal, therefore not covered by the heat-insulating powder which it is usual to spread on its surface to protect it from atmospheric reoxidation and stop its radiation. The cover, under which you can inject a neutral gas such as argon in addition to the plasma gas (or in its place for periods when the torch is not used), allows to keep in the vicinity of the torch an almost oxygen-free atmosphere, therefore non-polluting for the metal liquid.

The refractories lining the distributor and its cover receive a share significant radiation from the arc emitted by the torch, and their surface is, therefore, carried at very high temperatures which can be higher than 1800 ° C when the torch is used at high power. At these temperatures, magnesia or alumina, which are the materials usually used, reaches its melting point, and the coatings deteriorate rapidly, which means that the restoration of the cover coating. In addition, the refractory which has become liquid tends to flow on the surface. of the metal bath, where it forms an insulating crust which hinders the heat transfers between the plasma and metal, and may even end up defusing the arc (in the case of a transferred plasma torch). This molten refractory can also flow from the cover on the metal tube surrounding the torch and degrade it. So we are forced to find a torch operating point which achieves a compromise between heating the sufficient metal and tolerable deterioration in refractories, to the detriment of the effectiveness of the heating that theoretically could offer the torch.

It is conceivable to produce the coverings of the distributor and the cover in one refractory material having an even higher melting temperature than conventional materials, for example silicon carbide or ceramic. But like the distributor coating must be completely renewed between each pour or between each sequence, this would considerably increase the cost of using the installation, and would negate much of the economic benefit of the torch.

The object of the invention is to propose an economical means for limiting the damage to the refractory lining of the distributor and the cover in the area of action plasma torch, without compromising the efficiency of the metal heating by this same torch.

To this end, the invention relates to a piece of refractory material, of shape ring, intended to be installed in a continuous metal flow distributor comprising at least one plasma torch for reheating the liquid metal, and the inner wall defines a downwardly flared space having an upper opening and a lower opening and allowing penetration of the lower end of said torch in said space.

The subject of the invention is also a distributor for continuous casting of metals. of the type comprising at least one plasma torch for reheating the liquid metal, and at least one cover crossed by said torch, characterized in that it comprises a part of refractory material of annular shape as defined above, said part being fixed to said cover, or to the refractory wall of said distributor and / or possibly to a or partitions delimiting a reheating compartment inside said distributor, and the flaring of its internal wall being turned towards the bottom of the distributor.

As will be understood, the invention consists in attaching to the distributor or to its cover an annular refractory piece whose inner wall surrounds the end of the plasma torch and deflects the radiation it receives towards the metal. This piece ring protects the coverings of the distributor and the cover, and can be the only part of the distributor to be made of a material having particularly resistance elevated to the radiation of the arc. It can be designed to be used during casting or a single sequence and therefore be changed with each repair of the coating of the distributor. It can also, especially if it is ceramic, be recoverable and usable during several flows or several sequences.

Another notable advantage of this annular part is that the return to the metal liquid from the arc radiation it receives improves the thermal efficiency of the plasma torch by increasing the share of radiation that actually reaches the metal.

• les figures 1a et 1b qui montrent respectivement vu de dessus et de profil en coupe transversale selon Ib-Ib un exemple de répartiteur de coulée continue de l'acier selon l'art antérieur;
• les figures 2a et 2b qui montrent vu de dessus et de profil en coupe transversale selon IIb-IIb le même répartiteur, modifié selon l'invention;
• la figure 3 qui montre vu de profil en coupe longitudinale un autre exemple de répartiteur selon l'invention.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

• Figures 1a and 1b which show respectively seen from above and in cross-sectional profile according to Ib-Ib an example of a distributor of continuous casting of steel according to the prior art;
• Figures 2a and 2b which show seen from above and in cross-sectional profile along IIb-IIb the same distributor, modified according to the invention;
• Figure 3 which shows a side view in longitudinal section another example of a distributor according to the invention.

Figures 1a and 1b show a distributor of continuous casting of steel 1 according to prior art. In the example shown, which is of course not limiting, it allows to feed a continuous casting machine (not shown) equipped with two ingot molds. he comprises an external metal carcass 2, internally coated with a refractory 3. The interior space of the distributor 1 has a flared shape upwards to allow after the casting easy removal of the refractory lining 1 by simply reversing the distributor 1. The liquid steel 4 (not shown in FIG. 1a) reaches distributor 1 coming from a pocket (not shown), and is introduced into it via a tube in refractory 5 connected to the outlet orifice of the pocket. This tube 5 protects the liquid steel 4 against atmospheric reoxidation. Draining of liquid steel 4 in the molds not shown is carried out by nozzles 6, 6 '. Refractory tubes 7 connected to nozzles 6, 6 'protect the liquid steel 4 against atmospheric reoxidation during its path between the distributor 1 and the mold which corresponds to each nozzle 6 6 '.

The example of distributor 1 shown is generally rectangular in shape and is internally divided into four compartments by refractory walls 8, 9, 10. Two walls 8, 9 are oriented perpendicular to the long sides of the distributor 1; the wall 10 is oriented parallel to the long sides of the distributor and connects the other two walls 8, 9. The walls 8, 9, 10 first define a first compartment 11 for the arrival of the metal liquid 4, into which opens the tube 5 connected to the bag. Liquid steel 4 crosses then the wall 10 which, for this purpose, is perforated by a pipe 12, and thus enters a second compartment 13 which, in the example shown, constitutes a projection side of the distributor 1 located opposite the liquid metal inlet tube 5 4. As shown will see, it is in this second compartment 13 that the liquid steel 4 is heated. He passes then in the third and fourth compartments 14 and 15, thanks to pipes 16, 17 which perforate the walls 8, 9, 10. It is in these compartments 14, 15 that are located the nozzles 6, 6 'overhanging the molds of the continuous casting machine.

The liquid steel reheating device 4 comprises a plasma torch 18 of a type known in itself. Schematically, it comprises a cathode 19 in one material such as thoriated tungsten, connected to the negative pole of the generator supplying the torch, and surrounded by a metallic envelope 20, for example made of copper, which can play the role of anode. In the case where the torch 19 is of the transferred plasma type as in the example shown, the metal casing 20 only behaves as an anode when striking the arc; but if the torch is of the blown plasma type, this metal casing 20 is constantly connected to the positive pole of the generator supplying the torch. Between the envelope 20 and the cathode 19 are injected with the plasma gas which may be argon, or possibly nitrogen if the cast steel grade can tolerate nitrogen content relatively high. In the sole 21 of the distributor 1, an anode 22 is installed, for example, by a steel bar cooled over at least part of its length, and connected to the positive pole of the generator supplying the torch. Between cathode 19 and metal liquid 4 which is in contact with anode 22 therefore creates an electric arc 23 through which passes plasma gas, so as to heat the liquid steel 4 present in the second compartment 13, which will be called "heating compartment".

It is necessary to cover the heating compartment 13 with a cover 24 (not shown in Figure la) through which the torch 18. This cover 24 is coated internally with a refractory layer 25, so that the electric arc 23 does not come blind personnel working near the casting machine. In addition, this cover 24 makes it possible to confine the atmosphere surrounding the heating compartment 13 by sheltering from the outside atmosphere and allowing to keep above the metal liquid 4 argon injected by the torch 18. This eliminates atmospheric reoxidation which would otherwise inevitably occur, especially since in this compartment of reheating 13, it is not possible to cover the surface of the liquid metal 4 with a insulating powder which would hinder the thermal and electrical transfers between the torch 18 and the metal 4. Such a powder 26 is present on the surface of the liquid metal 4 in the others compartments 11, 14, 15 of the distributor. At least during the periods when the torch 18 is not used, argon can also be injected under the cover 24 through a orifice 27.

As we said, with a distributor thus configured, the radiation of the arc 23 causes rapid wear of the refractory 3 covering the distributor 1 in the heating compartment 13, wall 10 and refractories 25 covering the cover 24. This wear can eventually go to their superficial melting, with all the previously mentioned problems it causes. All of the refractories exposed to arc 23 in a material having a very high resistance to sound radiation, with all the additional costs that this would entail.

The distributor according to the invention shown in Figures 2a and 2b is a improvement of the previous distributor (their common elements are designated by same references in FIGS. 1 and 2), in which the above problem is solved by economically. To this end, the heating compartment 13 was placed in the distributor 1 an annular piece 28 of a refractory material having a resistance high to the radiation of the electric arc 23. In the example shown, this part annular 28 bears on the refractory lining 3 of the casing of the distributor 1 and on the wall 10 which separates the heating compartment 13 from the compartment 11 for the arrival of the liquid steel 4 in the distributor 1. It could also be made integral with the coating 25 of the cover 24. The internal wall 29 of the annular part 28 has a frustoconical shape, and has its inclined slope turned towards the surface of the metal liquid 4. The placement and dimensions of this annular part 28 are such that the lower end of the plasma torch 18, when in use, is located below of the upper opening 30 of the annular part 28, and preferably substantially lower than said upper opening 30. In this way, the part of the radiation from the electric arc 23 which would normally strike the partition 10 and the refractories 3, 25 covering the heating compartment 13 and the cover 24 is very largely stopped by the internal wall 29 of the annular part 28, and is returned towards the metal liquid 4 present in the heating compartment 13. It is thus extended considerably the lifetime of the refractories 25 of the cover 24, and attenuates in the same way degradation during the casting of the refractories 3 coating the walls of the distributor and of the partition 10 in the heating compartment 13. It was thus possible to pass the duration of use of the coating 25 of the cover 24 from 20-30 hours to more than 100 hours. The annular part 28 was made of tabular alumina. Under the same conditions, it was found that equal torch usage power (about 300 kW), it could be increased by 14 ° C the temperature of the liquid steel 4 in the heating compartment 13, against 10 ° C. when the annular part 28 is not used. This improvement is due to the lesser degradation of refractories which reduces the formation of a crust on the surface of the liquid metal 4, but also to the fact that the annular part 28 as configured directly refers on that surface the part of the radiation from the arc which would normally strike the coating 25 of cover 24 and coating 3 of distributor 1, and would not reach the liquid metal 4 only after being attenuated by multiple reflections.

The annular part 28 according to the invention is made of a solid refractory capable of resist the radiation from the arc 23 for the entire duration of the use of the distributor 1 and of its coating 3, either a casting from a single pocket or a casting in sequence of several consecutive pockets. Materials such as tabular alumina, spinel alumina, silicon carbide are well suited for this use. The use of the annular part 28 avoids having to coat the entire heating compartment 13 of the distributor 1 and its cover 24 with such refractories, and thus decreases the overall cost of refractories the installation. In addition, if a material with radiation resistance is used particularly high, for example a ceramic with a melting temperature of around 2000 ° C, we can consider that the annular part could be reusable after have been separated from the coating of the used distributor. A ceramic would, moreover, the advantage of having an excellent reflection power of the radiation of the arc 23, which would further improve the thermal performance of the installation.

The interior and exterior shape of the annular part 28 shown in FIG. 2 is, of course, just one example. It is clear that its interior space can have, for example, the shape of a pyramid trunk and not a cone trunk. Its external shape is likewise to be adapted to the geometry of the heating compartment 13 of the distributor 1.

The distributor according to the invention shown in FIG. 3 constitutes an example adaptation of the invention to a distributor 31 of generally strictly rectangular shape, in which it is not possible, for reasons of space, to provide a single reheating compartment through which all the cast metal would pass, as in the example in FIGS. 1 and 2. It is provided, like the preceding distributor 1, two nozzles 32, 32 ', each extended by a refractory tube 33, 33' which plunges in an ingot mold not shown. The distributor 31 is supplied with liquid steel 34 by a refractory tube 35 connected by its upper end to a pocket, not shown. The liquid steel 34 leaving the tube 35 opens into a central compartment 36 materialized by a first pair of refractory partitions 37, 37 ′ blocking the distributor 31 over its entire width and located on either side of the tube 35. These first partitions 37, 37 ′ are provided perforations 38, 38 'which allow the passage of the liquid steel 34 in two heating compartments 39, 39 ′ contiguous to the central compartment 36. These heating compartments 39, 39 'are each delimited by one of the first partitions 37, 37 'and by another refractory partition forming part of a second pair 40, 40'. These second partitions 40, 40 'are provided with perforations 41, 41' allowing the passage of the liquid steel 34 in the pouring compartments 42, 42 ′ where the nozzles 32 are located, 32 '. The heating compartments 39, 39 'are each capped with a cover 43, 43' coated with refractory and traversed by a plasma torch 44, 44 'of a type similar to that previously described. In the case where, as shown, these torches 44, 44 ′ are of the plasma transferred, the floor 45 of the distributor 31 is crossed, in line with the compartments of reheating 39, 39 ', by anodes 46, 46' similar to those described above. We can thus establish in the heating compartments 39, 39 'between the torches 44, 44' and the liquid steel 34 of the electric arcs 47, 47 ′ which, in cooperation with the plasma gas blown into by the torches 44, 44 ', heat the liquid metal 34. The liquid metal 34 found in the distributor is covered with a layer of covering powder 48, except in the heating compartments 39, 39 'where it would interfere with the operation of the torches 44, 44 '. In this regard, the locations of the various perforations 38, 38 ', 41, 41' of the partitions 37, 37 ', 40, 40' are chosen so as to prevent the passage of the cover powder 48 in the reheating compartments 39, 39 '.

According to the invention, the refractory elements materializing the compartments of reheating 39, 39 'are completed by annular parts 49, 49' similar in their functions and their design in the annular part 28 previously described and represented on Figure 2. As for the previous one, their interior space is frustoconical, with a wall oriented towards the liquid metal 34 present in the compartment reheating 39, 39 'corresponding. In the example shown, the annular parts 49, 49 ' are mounted integral with partitions 37, 40, 37 ', 40' delimiting the compartments of reheating 39, 39 ', but they could also be fixed only to the refractory lining of the distributor 31, or to the covers 43, 43 '.

It goes without saying that the distributors which have been described and represented are only examples of implementation of the invention, which can easily be adapted to other types of distributors for continuous casting of steel or other metals. In particular, it is not to strictly speaking, it is essential that the distributor has one or more compartments space clearly delimited by one or more partitions: it is enough to stay in the spirit of the invention that the part of the arc radiation from the plasma torch which would come normally strike the cover crossed by the torch and the side walls of the distributor is stopped by the internal wall of the annular part and returned to the metal, therefore in direction of the dispatcher bottom. In the absence of such partitions, the annular part or parts may only be attached to the refractory wall of the distributor or to the cover.

Claims (7)

Piece of refractory material (28, 49, 49 '), of annular shape, intended to be installed in a continuous casting distributor for metals (1, 31) comprising at least a plasma torch (18, 44, 44 ') for reheating the liquid metal (4, 34), and the inner wall (29) defines a downwardly flared space having an upper opening (30) and a lower opening and allowing penetration of the lower end of said torch (18, 44, 44 ') in said space. Piece of refractory material (28, 49, 49 ') according to claim 1, characterized in that said downwardly flared space is frustoconical. Piece of refractory material (28, 49, 49 ') according to claim 1, characterized in that said downwardly flared space is in the form of a pyramid trunk. Piece of refractory material (28, 49, 49 ') according to one of claims 1 to 3, characterized in that it is based on alumina. Piece of refractory material (28, 49, 49 ') according to one of claims 1 to 3, characterized in that it is based on silicon carbide. Piece of refractory material (28, 49, 49 ') according to one of claims 1 to 3, characterized in that it is made of ceramic. Distributor for continuous casting of metals (1, 31) of the type comprising at least a plasma torch (18, 44, 44 ') for heating the liquid metal (4, 34), and at least a cover (24, 43, 43 ') crossed by said torch (18, 44, 44'), characterized in that it comprises a piece of refractory material (28, 49, 49 ') of annular shape according to one of the claims 1 to 6, said part (28, 49, 49 ') being fixed to said cover (24, 43, 43'), or the refractory wall (3) of said distributor (1, 31) and / or possibly to one or more partitions (10, 37, 40, 37 ', 40'), delimiting a heating compartment (13, 39, 39 ') inside of said distributor (1, 31), and the flaring of its internal wall (29) being turned towards the distributor base (1, 31).

Images