EP0723487B1 - Method of adjusting the composition of a molten metal such as steel, and plant for implementation thereof - Google Patents

Abstract

PCT No. PCT/FR94/01161 Sec. 371 Date May 20, 1996 Sec. 102(e) Date May 20, 1996 PCT Filed Oct. 5, 1994 PCT Pub. No. WO95/10377 PCT Pub. Date Apr. 20, 1995A process for adjusting the composition of a liquid metal, such as steel, is provided wherein liquid metal is sucked up from a start receptacle into a reactor which is at a reduced pressure with respect to the start receptacle. First, the liquid metal is made to flow inside the reactor under conditions close to plug flow where it undergoes a metallurgical treatment. Next, after the liquid metal has been conveyed through the reactor it is completely discharged into a finish receptacle which is also at a reduced pressure with respect to the reactor. To insure thorough mixing, the reactor is divided into a plurality of cells, each of which has its own independent gas injector for agitating the liquid metal therein.

Description

The invention relates to the production of liquid steel. It is particularly applicable to the production of high purity steels, with extremely low carbon contents, and even also nitrogen and oxygen.

• d'une cuve de grande hauteur et de forme grossièrement cylindrique, revêtue intérieurement de réfractaires, et dont la partie supérieure est connectée à une installation d'aspiration des gaz capable de maintenir dans cette cuve une pression réduite qui peut descendre jusqu'à moins de 1 torr lorsque le réacteur est en fonctionnement (on rappelle que 1 torr ≈ 133 Pa);
• de deux tubes ou plongeurs, de section cylindrique ou ovale, revêtus intérieurement et extérieurement de réfractaires, dont l'une des extrémités débouche dans le fond de la cuve ; l'un de ces plongeurs est muni d'un dispositif permettant d'y insuffler un gaz, habituellement de l'argon.

The use during the production of liquid steel of vacuum reactors of the type called "RH" is common today. Remember that these reactors are made up of:

• a very tall, roughly cylindrical tank, lined with refractories, the upper part of which is connected to a gas extraction system capable of maintaining in this tank a reduced pressure which can drop to less than 1 torr when the reactor is in operation (remember that 1 torr ≈ 133 Pa);
• two tubes or plungers, of cylindrical or oval section, coated internally and externally with refractories, one end of which opens into the bottom of the tank; one of these plungers is fitted with a device for blowing in a gas, usually argon.

• une décarburation partielle, par combinaison du carbone avec de l'oxygène déjà dissous dans le métal ou y étant insufflé à cet effet par une lance ou des tuyères insérées dans la paroi de la cuve ;
• une addition d'éléments d'alliages qui est ainsi effectuée à l'abri de l'air et du laitier de poche, donc avec un rendement optimal ;
• un réchauffage du métal par aluminothermie : on lui ajoute de l'aluminium, puis on y insuffle de l'oxygène, et l'oxydation de l'aluminium qui en résulte provoque ce réchauffage.

These facilities are used as follows. The pocket containing the liquid metal to be treated is brought under the RH, and the lower ends of the plungers are immersed there. After which, the tank is put under vacuum, which causes the suction of a certain amount of metal inside. Finally, the gas blowing into the diver equipped for this purpose begins. The function of this insufflation is to carry towards the tank the metal which is in this plunger, called for this reason "ascending plunger". The metal passing through the tank then descends into the pocket by borrowing the other plunger, called "descending plunger". A continuous circulation of metal is thus obtained between the pocket and the tank. During the entire duration of the treatment (generally around ten to thirty minutes), the same metal droplet therefore makes several stays inside the tank. Their duration average is a function of the metal circulation rate in the plungers and the ratio between the respective capacities of the bag and the tank. The passage of the liquid metal in the tank maintained under vacuum makes it possible mainly to decrease its contents in hydrogen and, to a lesser extent, in dissolved nitrogen. The other metallurgical operations likely to occur in the tank are:

• partial decarburization, by combining carbon with oxygen already dissolved in the metal or being blown into it for this purpose by a lance or nozzles inserted into the wall of the tank;
• an addition of alloying elements which is thus carried out in the absence of air and pocket slag, therefore with optimum yield;
• reheating of the metal by aluminothermy: aluminum is added thereto, then oxygen is injected into it, and the resulting oxidation of the aluminum causes this reheating.

At the same time, the circulation of the metal between the pocket and the tank causes gentle agitation of the metal in the pocket, favorable to good decantation of the non-metallic inclusions.

The last few years have seen an increase in demand from industries consuming steel for steel products with an extremely low carbon content (less than 50 ppm), in particular for cold rolled sheets with high ductility and tensile strength, for steels for. deep drawing, for ferritic chromium-molybdenum stainless steels, etc. RH quickly emerged as the pocket metallurgy reactor best suited to obtaining such steels under industrial conditions. Indeed, the kinetics of decarburization is favorably influenced there by the massive insufflation of gas which is carried out in the ascending plunger, even also inside the tank. Thus, for a pocket containing 300 tonnes of liquid steel, an RH tank containing 15 tonnes, and a circulation rate of 240 tonnes / min, a processing time of 10 minutes may be enough to lower the carbon content in the steel from 300 ppm to 20 ppm.

The increased demand for steels of increasingly high purity will probably make it necessary, in the very near future, to be able to routinely obtain even lower carbon contents (5 to 10 ppm) with at least equivalent productivity. to that of current installations (around 10 rpm in large integrated factories). However, in conventional RH, there is a marked slowdown in the decarburization kinetics when the average carbon content of the liquid steel becomes less than 30 ppm. A significant acceleration of these kinetics would make it possible to obtain the desired metallurgical performance in a time always compatible with optimal operation of the other workshops of the steelworks. But it would only be conceivable by considerably increasing the speed of circulation of the metal and the quantity of gas injected. This would result in excessively accelerated wear of the refractories, therefore more frequent shutdowns and less reliable operation of the installation. Ultimately, obtaining industrial contents of carbon contents significantly lower than 10 ppm under satisfactory technical and economic conditions does not seem to be within the reach of an HR of traditional design.

Obtaining as low a carbon content as possible in the liquid steel is all the more important since, in the course of the production and casting operations, the steel will have multiple opportunities to re-carburize, by example, in contact with refractories and covering powders of the distributor and the mold.

The object of the invention is to propose a new type of metallurgical reactor, which gives access to the carbon contents in the liquid steel of the order of 10 ppm and less under satisfactory productivity conditions. This reactor should also be able to be used to produce less deeply decarburized steels but with very low content of oxidized inclusions.

The subject of the invention is a method for adjusting the composition of a liquid metal, according to which, from a starting container, said liquid metal is sucked inside a reactor which is placed under reduced pressure with respect to said first container, it is subjected to a metallurgical treatment, and said metal is discharged into an inlet container with respect to which said reactor is also placed under reduced pressure, characterized in that said metal is run off inside said reactor according to a regime close to a piston flow.

Preferably, said starting container is supplied with metal continuously, and said metal is made to flow from said arrival container also continuously.

In an exemplary implementation of the method, the liquid metal is steel, the metallurgical treatment comprises decarburization, denitriding or deoxidation by carbon under vacuum, and the arrival container is a continuous casting distributor.

The invention also relates to an installation for adjusting the composition of a liquid metal, such as steel, which comprises a starting container containing said liquid metal, a reactor, and an arrival container, said reactor. comprising a tank provided with means allowing its interior to be maintained at a reduced pressure compared to those prevailing in said starting container and said arrival container, of a first plunger called ascending plunger, one end of which dips in the liquid metal contained in said starting container and the other end of which is connected to the bottom of said tank, of a second plunger, said descending plunger, one end of which dips in the liquid metal contained in said arrival container and the other end of which is connected at the bottom of said tank, and means for ensuring a continuous circulation of said liquid metal between said departure container and said arrival container e through said plungers and said reactor vessel, characterized in that said vessel is shaped so as to impose on said metal liquid a flow close to a piston type flow on its path between said plungers.

• un engin d'élaboration primaire de l'acier liquide ;
• des moyens pour déverser de manière continue ledit acier liquide issu de l'engin d'élaboration primaire dans un premier récipient pourvu de moyens d'introduction de désoxydants dans le métal liquide et de moyens de brassage ;
• un réacteur de décarburation grossière du métal liquide comportant une cuve pourvue de moyens pour sa mise sous pression réduite, d'un premier plongeur trempant dans le métal liquide contenu dans ledit premier récipient et d'un deuxième plongeur trempant dans un récipient intermédiaire, ladite cuve comportant également des moyens d'insufflation d'oxygène dans ledit métal liquide ;
• des moyens pour assurer une circulation continue dudit métal liquide entre ledit premier récipient et ledit récipient intermédiaire ;
• au moins une installation de réglage de la composition de l'acier liquide du type que l'on vient de décrire pour laquelle ledit récipient intermédiaire constitue ledit récipient de départ.

This installation can, according to the invention, be inserted in an installation for adjusting the composition of the liquid steel comprising:

• a machine for the primary production of liquid steel;
• means for continuously pouring said liquid steel from the primary production machine into a first container provided with means for introducing deoxidizers into the liquid metal and with stirring means;
• a coarse decarburization reactor for liquid metal comprising a tank provided with means for putting it under reduced pressure, a first plunger dipping in the liquid metal contained in said first container and a second plunger dipping in an intermediate container, said tank also comprising means for blowing oxygen into said liquid metal;
• means for ensuring continuous circulation of said liquid metal between said first container and said intermediate container;
• at least one installation for adjusting the composition of the liquid steel of the type just described for which said intermediate container constitutes said starting container.

As will be understood, the new proposed reactor has points in common with RH, namely the presence of a tank placed under vacuum and two plungers, through which the metal enters the tank and leaves. But the principle of continuous circulation of the liquid metal between the ladle and the reactor is abandoned: the metal leaving the reactor here pours into another container than its starting container and will no longer return to this same reactor. In addition, while the RH behaved like a perfectly stirred reactor, the liquid steel in the reactor according to the invention must have a flow close to a piston flow. If necessary, as will be seen, this pseudo-piston flow is obtained by fragmenting the reactor into a multiplicity of perfectly mixed cells between which the exchange of matter is reduced to a minimum.

Finally, this reactor can advantageously be inserted in a continuous or semi-continuous production line for liquid steel.

• la figure 1 qui schématise, vu en coupe longitudinale, un exemple de réacteur métallurgique selon l'invention ;
• la figure 2 qui schématise l'ensemble d'un exemple de chaîne d'élaboration et de coulée d'un acier à ultra-basse teneur en carbone, dans laquelle est inséré un réacteur selon l'invention.

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

• Figure 1 which shows schematically, seen in longitudinal section, an example of a metallurgical reactor according to the invention;
• FIG. 2 which shows diagrammatically the whole of an example of a production and casting line for an ultra-low carbon steel, into which a reactor according to the invention is inserted.

It has appeared to the inventors that one of the reasons why very extensive decarburization was not accessible quickly enough in an RH is that, in fact, a given elementary portion of liquid metal is only exposed to it during a vacuum. relatively short average time. If we take the example given above, of an RH with a capacity of 15 t of liquid metal coupled to a bag of 300 t with a circulation of 240 t / min, the average residence time of a portion metal in the RH tank is only 30 seconds for 10 minutes of treatment. During these 30 seconds, most of it is consumed by the first stages of decarburization, which make it possible to pass relatively easily from a content of 200-400 ppm to 30 ppm. There is then no longer enough time available to complete the decarburization, up to levels of 10 ppm and less, in a manner compatible with the productivity of the other steelworks workshops (converter or electric furnace, and casting keep on going).

dans l'expression du coefficient cinétique de la réaction. Pour obtenir une décarburation rapide, le rapport

doit être aussi proche que possible de 1. Il est de l'ordre de 0,6 en moyenne dans les RH classiques, et varie en fonction du stade d'avancement de la décarburation et du débit de recirculation du métal.On the other hand, the principle of the continuous recirculation of the metal between the ladle and the tank means that the tank is constantly supplied with less decarburized metal than is the metal being decarburized which is already there. As the tank behaves like a perfectly stirred reactor, we can define at any time an average carbon content C _V of the metal in the tank, as well as an average carbon content C _L of the metal in the ladle. As C _L is permanently greater than C _V , it is understood that the decrease in C _V is slowed down by the addition of less decarburized metal, of carbon content C _L. This is well reflected in the mathematical models of decarburization which involve the relationship

in the expression of the kinetic coefficient of the reaction. To obtain rapid decarburization, the report

must be as close as possible to 1. It is of the order of 0.6 on average in conventional RH, and varies according to the stage of advancement of decarburization and the rate of recirculation of the metal.

est idéalement réalisée si on met simultanément sous vide l'ensemble du métal à traiter ; cela correspond au cas d'une installation de vide en poche classique. Mais si on utilise ce type d'installation, on se prive d'un des avantages fondamentaux du RH, à savoir la possibilité d'accélérer la cinétique de décarburation en faisant passer dans le métal des quantités considérables de gaz rapportées à la masse de métal en jeu. Faire passer ces quantités de gaz dans une simple poche mise sous vide provoquerait des projections de métal excessives qui détérioreraient très rapidement l'installation. Un autre avantage du RH est d'exposer au vide une grande surface de métal par rapport au volume que renferme la cuve, et cet avantage n'existe plus si on utilise une poche sous vide. De fait, industriellement, les poches simples ou les fours-poches sous vide ne sont que rarement utilisés pour l'obtention de teneurs en carbone inférieures à 40 ppm, car globalement la décarburation y est insuffisamment rapide.This condition $$\frac{{\text{VS}}_{\text{V}}}{{\text{VS}}_{\text{L}}}\text{= 1}$$

is ideally carried out if all the metal to be treated is placed under vacuum simultaneously; this corresponds to the case of a conventional bagged vacuum installation. But if we use this type of installation, we deprive ourselves of one of the fundamental advantages of RH, namely the possibility of accelerating the kinetics of decarburization by passing through the metal considerable quantities of gases relative to the mass of metal. Passing these quantities of gas through a simple vacuum bag would cause excessive metal splashes which would very quickly deteriorate the installation. Another advantage of RH is to expose a large surface of metal to the vacuum relative to the volume contained in the tank, and this advantage no longer exists if a vacuum bag is used. In fact, industrially, simple bags or vacuum bag ovens are only rarely used for obtaining carbon contents of less than 40 ppm, because overall decarburization is insufficiently rapid there.

serait d'augmenter le débit de recirculation du métal entre la poche et la cuve, en jouant sur la géométrie des plongeurs et le débit de gaz insufflé dans le plongeur ascendant. Mais l'usure des réfractaires s'en trouverait considérablement accélérée.Another way to approach the ideal condition $$\frac{{\text{VS}}_{\text{V}}}{{\text{VS}}_{\text{L}}}\text{= 1}$$

would be to increase the rate of metal recirculation between the pocket and the tank, by playing on the geometry of the plungers and the flow of gas blown into the plunger ascending. But the wear of the refractories would be considerably accelerated.

• une cuve 6 destinée à renfermer à un instant donné une certaine quantité d'acier liquide 2 en circulation ; comme les cuves des RH, cette cuve 6 doit être suffisamment haute pour que sa partie supérieure ne risque pas d'être trop endommagée par des projections de métal liquide ;
• une installation 7 d'aspiration des gaz destinée à établir une pression réduite dans la cuve 6 ;
• un plongeur ascendant 8 traversant le couvercle 3 et trempant dans le récipient intermédiaire 4 ; il peut être équipé de moyens 9 d'injection d'un gaz tel que de l'argon, destinés à provoquer ou assisster la circulation du métal liquide 2 dans la cuve 6 ;
• un plongeur descendant 10 par lequel le métal liquide sort de la cuve 6.

FIG. 1 schematically represents the principle of an elementary reactor according to the invention, and an example of its insertion in an installation for the continuous production of steel. This installation comprises a channel 1 for supplying the liquid steel 2 which flows continuously with a controllable flow rate from a metallurgical container not shown, such as a steelworks pocket or a primary production machine. metal similar to a converter or an electric furnace. This primary production machine also works continuously. This channel 1 is connected to a cover 3 which covers an intermediate container 4 exposed to atmospheric pressure, in which the liquid steel flows 2. Optimally, injections of a neutral gas, such as argon, take place in the channel 1 and under the cover 3 to protect the metal 2 from the ambient air. It is this liquid steel 2 that one wishes to rid as much as possible of the dissolved gases (hydrogen, nitrogen) which it contains, and also of its carbon. For this purpose, the metallurgical reactor 5 according to the invention is used. Like conventional reactors of the RH type, this reactor 5, internally coated with refractory, comprises:

• a tank 6 intended to contain at a given instant a certain quantity of liquid steel 2 in circulation; like the RH tanks, this tank 6 must be high enough so that its upper part is not likely to be too damaged by projections of liquid metal;
• a gas suction installation 7 intended to establish a reduced pressure in the tank 6;
• an ascending plunger 8 passing through the cover 3 and dipping into the intermediate container 4; it can be equipped with means 9 for injecting a gas such as argon, intended to cause or assist the circulation of the liquid metal 2 in the tank 6;
• a descending plunger 10 by which the liquid metal leaves the tank 6.

Unlike RH, this descending plunger 10 does not open into the same container as the ascending plunger 8, but into an inlet container 11 which, in the example shown, is a continuous casting distributor. As it should be, this distributor 11 is equipped with at least one outlet nozzle 12 thanks to which the liquid metal 2 flows continuously, with a flow controllable by a stopper rod or a drawer not shown, in at least one ingot mold 13 bottomless, with walls energetically cooled by an internal circulation of water. It is in this ingot mold 13 that the solidification of a crust 14 of steel begins which gives rise to a steel product 15, slab, bloom or billet according to the format of the ingot mold 13.

où ρ est la masse volumique de l'acier liquide (environ 7000 kg/m³) et g l'accélération de la pesanteur (9,8 m/s²), soit : $${\text{Δh (en m) ≈ 1,46.10}}^{\text{-5}}\text{ΔP (en Pa)}$$

The RH tanks have a substantially cylindrical shape, with an inside diameter of a few meters at most. This configuration gives RH the properties of a perfectly stirred reactor. On the contrary, according to the invention, the tank 6 must confer on the reactor 5 properties as close as possible to those of a piston flow reactor, where the steel which has undergone degassing and / or decarburization to given levels cannot then be mixed with less purified steel. To this end, one solution consists in giving the inside of the tank 6 the shape of a corridor, that is to say a channel of rectangular or approximately rectangular section, long and narrow, at the ends of which are connected the divers 8, 10. The ratio between the distance separating the divers 8, 10 and the width of the channel is at least equal to 6. It is possible to give the tank 6, for example, a width of 1 m to 1.50 m and a length separating the divers 8, 10 from 8 to 10 m. The efficiency of decarburization and degassing is largely a function of the ratio between the surface of liquid metal 2 offered under vacuum and the volume of this same liquid metal 2 present in the tank 5. This ratio must be as high as possible, which implies that, for a given volume of metal, the depth "e" of the metal present in the tank 5 must not be too great (0.40 to 0.80 m for example). This depth is governed by the geometry of the set of the installation, (in particular the differences in level between the tank 6, and the intermediate container 4 and the distributor 11) and also by the pressure difference ΔP between the inside of the tank 6 and the atmosphere to which the surfaces are exposed 16 and 17 of the liquid steel 2, respectively in the intermediate container 4 and in the distributor 11. If Δh is called the average difference in level between the surface of the liquid steel 2 in the tank 6 of the reactor and said surfaces 16 , 17 exposed to the atmosphere, we have the relation $$\text{Δh =}\frac{\text{ΔP}}{\text{ρ.g}}$$

where ρ is the density of the liquid steel (approximately 7000 kg / m ³ ) and g the acceleration of gravity (9.8 m / s ² ), that is: $${\text{Δh (in m) ≈ 1.46.10}}^{\text{-5}}\text{ΔP (in Pa)}$$

Ainsi, si P est de 50 torrs, Δh ≈ 1,40 m
et si P est de 1 torr, Δh ≈ 1,50 m.If we call P the absolute pressure in the tank 6, and if we assume that the intermediate container 4 and the distributor 11 are exposed to a pressure of 1 atm (i.e. 101,325 Pa), we therefore have: $${\text{Δh ≈ 1.46-10}}^{\text{-5}}\text{(101325 - P)}$$

So, if P is 50 torr, Δh ≈ 1.40 m
and if P is 1 torr, Δh ≈ 1.50 m.

The depth e of the liquid metal 2 in the tank 5 is therefore relatively little dependent, in the usual pressure ranges, on the level of vacuum obtained in the tank 5.

As has been said, the circulation of metal between the containers 4, 11 through the reactor 5 is governed in part by the injection of gas into the ascending plunger 8 if it is practiced there. But in all cases, the existence of this circulation and the flow of metal which it brings into play depend on the difference in level between the surface 16 of the liquid metal 2 in the intermediate container 4 and the surface 17 of the liquid metal 2 in the distributor 11. This difference in level is linked in particular to the difference between the supply flow rate of the intermediate container 4 and the flow rate of metal 2 leaving the distribution valve 11.

The corridor shape for the tank 6 of the reactor 5 is the most suitable for establishing a piston-type flow for the liquid metal 2. However, the arrival of the metal in coming from the ascending plunger 8 and the gaseous releases due to the argon possibly injected into this plunger 8 and the decarburization of the metal (at least in the upstream zone of the tank 6 where this decarburization is most intense) cause strong agitation which can significantly degrade and in an uncontrollable manner the conditions of this flow. This is why it is advisable to carry out a flow approaching a piston flow by dividing the tank 6 of the reactor 5 into a series of perfectly mixed cells and between which the exchanges of liquid metal 2 are as limited as possible. For this purpose, it is possible to install in the bottom 18 of the tank 6 devices 19-23 for injecting a fluid such as argon, for example permeable elements or nozzles, similar to those used on the converters. , electric ovens and steelworks pockets. These injection devices 19-23 ensure in their respective implantation zones an intense mixing of the liquid metal 2. If the injection devices 19-23 are far enough from each other, it can be considered that these zones of influence are sufficiently distinct so that communications between them take place without backscattering of liquid metal 2 from one to the other. We would thus achieve inside each cell obtaining a homogeneous and constant carbon concentration over time (insofar as the other operating conditions would be constant), which corresponds to the ideal conditions for decarburization provided by the mathematical models. In addition, these injections of argon considerably accelerate the kinetics of decarburization.

Optimally, it is possible, as shown in FIG. 1, to provide a physical separation of the different cells by installing dams 24-28 in the tank 6. These dams 24-28 are arranged transversely to the general orientation of the tank 6 and delimit cells 29-34, each equipped with a fluid injection device 19-23 (except possibly the first cell 29, if an injection of fluid is carried out in the ascending plunger 8; it is then this injection which causes the agitation of metal in this first cell 29). The first dam 24, that is to say that which separates the first cell 29 from the second cell 30, preferably, starts from the bottom 18 of the tank 6 and has a height such that its upper part provides a threshold 35 that the liquid metal 2 can pass during its progression in the tank 6. This configuration contributes to good control of the flow rate of the metal 2 in circulation. Openings 36-39 are provided in the other dams 25-28 to allow minimal communication of the cells 30-34 between them ensuring the progression of the liquid metal 2 in the tank 6. These openings 36-39 are preferably placed in the parts lower dams 25-28 to allow complete emptying of the tank 6. In this way, the respective zones of influence of the fluid injection devices 19-23 are well delimited, and a large number of such devices can be provided without increasing the risk of excessive metal exchanges between two neighboring cells. This brings us closer to the conditions of an ideal piston flow than in the absence of such dams.

It is also conceivable that means for stirring the metal 2 by suitably oriented mobile electromagnetic fields replace or, possibly, add to the gas injection devices. This thus constitutes another possibility of controlling the flows of metal 2 which could even make it possible to dispense with the dams 24-28 to achieve a satisfactory delimitation of the cells 29-34.

Furthermore, the reactor 5 is provided with all the equipment (not shown) that can usually be encountered on RH, namely: one or more television cameras allowing operators to observe the surface of the liquid metal 2 in the tank, one or more devices for taking metal samples (it is advantageous to provide several staggered along the tank 5 to follow the evolution of the composition of the metal during vacuum treatment), one or more devices introduction of alloying elements, one or more oxygen blowing devices, one or more graphite resistors providing a preheating of the refractories of the tank 6. It is advantageous to install an oxygen insufflation device (lance or nozzle) at least in the first cell 29. It is thus possible, when necessary, to increase the initial content of the liquid metal 2 in dissolved oxygen and eliminate any deoxidizing elements (Al, Si) present in the metal which would prevent decarburization. It may also be advantageous to install it in the last cell 34, if this cell is also equipped with means for introducing aluminum into the liquid metal 2: it is then possible to carry out a final reheating of the metal by aluminothermy. However, this implies that one can be satisfied with the carbon content obtained just before the entry of the liquid metal 2 into the last cell 34. Indeed, the introduction of aluminum implies the capture in the form of alumina dissolved oxygen, and therefore the definitive cessation of the decarburization reaction. This also implies that we give ourselves the possibility, in the course of the preparation, of removing the alumina thus formed within the metal 2.

As a variant, the fluid injected into the bottom 18 of the tank 6 by at least some of the devices 19-23 may be not argon, but a gas capable, initially, of partially dissolving in the liquid steel , and whose departure under the effect of the vacuum tends to favor decarburization. This gas can be nitrogen and, above all, hydrogen. In doing so, of course, it is accepted that the content of the metal in this gas in most of the tank is higher than with the usual practice of argon insufflation. But since hydrogen is a relatively easy gas to remove from liquid steel, it is sufficient to inject argon in its place in the very last cell (s) to find the levels of hydrogen in metal 2 usually encountered at the outlet of a vacuum reactor. With regard to nitrogen, its departure from the liquid metal is slower and risks not being total: it is better not to use it as a stirring gas if very low nitrogen contents are sought in conjunction with very low contents carbon.

This reactor 5 can also be used not as an advanced decarburization reactor, but as a vacuum deoxidation reactor. For this purpose, once the carbon content has reached a level deemed sufficiently low (for example 80 ppm), carbon is added to the bath in solid or gaseous form (for example in the form of CH ₄ ) in one or more places of tank 6, so that it combines with dissolved oxygen and decreases its concentration. The advantage of such a mode of deoxidation is that it saves a large part of the aluminum usually used to deoxidize the bath, and, thereby, avoid the massive formation of inclusions of alumina. which should then be removed before the metal is poured. One can thus cast a steel with high cleanliness inclusiveness, which results in a very low oxygen content on the final product. This method can also be applied to the production of stainless steels for which carbon deoxidation can be a prerequisite for the massive addition of chromium.

Another advantage of the installation according to the invention is that the metal flow rate which passes through it is very moderate compared to that which circulates in an RH (10 rpm against 240 rpm). The refractories therefore wear considerably less quickly, in particular at the level of the plungers.

Instead of inserting this reactor 5 into a continuous production line for liquid metal 2, it can also be used simply so as to decant said metal from a container initially containing a certain quantity of liquid steel (and no longer receiving it later) in another container, pocket or distributor, initially empty. For this purpose, it is however necessary to arrange so that the surface of the metal in the arrival container is permanently at an altitude lower than that of the surface of the metal in the departure container. This may require moving these devices relative to each other during operation in a rather complex way, and requiring the departure and arrival containers to be shallow, so that their movements do not have a too large amplitude.

If the initial carbon content of the liquid steel is not very high at the inlet of reactor 5 (for example of the order of 80 ppm), a single reactor of reasonable dimensions makes it possible to reduce this content to levels of the order of 5 ppm, as will be seen in the example described below. If the initial carbon content is a few hundred ppm, an additional reactor can be added to reactor 5 located upstream of it in the continuous production line, which would have the function of bringing the carbon content to the level required at the inlet of reactor 5 (80 ppm in our example). As the transition, during a vacuum treatment, from a carbon content of 200-800 ppm to a content of 80 ppm is very rapid provided that the decarburization is assisted by a strong injection of gas, a reactor not comprising that a single cell perfectly brewed (and therefore much shorter than the reactor 5) would suffice for this use. A reactor that would have the configuration of a conventional RH but whose divers would each soak in a different container would be quite suitable.

We will now describe an example of a production line and continuous casting of an ultra-low carbon steel, or even of other elements such as sulfur, nitrogen and oxygen. This example is shown in Figure 2.

This production line firstly comprises a primary production machine 40. Its function is to produce, continuously or discontinuously, a liquid steel whose composition must be adjusted in the course of operations. This machine 40 can, as shown in Figure 2, be a conventional LD type converter, that is to say in which the liquid iron which is introduced therein is transformed into liquid steel by decarburization. This decarburization is obtained by insufflation of oxygen by means of an emerged lance 42. But of course, any other known primary production machine may be suitable, for example a converter of the LWS type with oxygen blowing from the bottom, a blown converter, or an electric furnace producing steel liquid from scrap. Periodically, this primary production machine 40 feeds liquid steel 2 to a large steelworks pocket 43 which acts as a buffer container. The contents of this pocket 43 pours continuously into a first container 44. The feed rate of this first container 44 is controlled by a drawer closure (not shown) of a type known per se (or its functional equivalent) ) located on the pocket 43. The liquid steel 2 is introduced into the first container 44 by a protective tube 45 in refractory designed to limit the absorption of atmospheric oxygen and nitrogen by the jet of liquid metal. In steady state, the flow of steel leaving the pocket 43 is, for example, around 10 rpm, corresponding to the average productivity of the production machine 40.

In the first container 44 can take place various operations for adjusting the composition of the liquid steel 2, in particular the addition of alloying elements, and above all a desulfurization. It is indeed advisable to desulfurize the steel 2, when necessary, before the vacuum treatment. One reason for this is that this operation requires the addition of materials such as lime which may have a high moisture content. They are therefore capable of supplying hydrogen to the liquid steel 2. In addition, the desulfurization requires intense mixing of the liquid steel 2 which thus risks absorbing atmospheric nitrogen. It is therefore necessary for the degassing of the liquid steel 2 to follow the desulfurization in order to compensate for the negative effects on the dissolved gas content. In addition, the departure of nitrogen during this degassing is all the easier the lower the sulfur content of the liquid steel 2.

• une très faible teneur en oxygène dissous de l'acier liquide 2 ; elle est obtenue par une addition périodique ou continue d'aluminium qui se combine avec l'oxygène dissous pour former de l'alumine ;
• la présence à la surface de l'acier liquide 2 d'une couche d'un laitier 50 à forte teneur en chaux et d'une grande fluidité, de manière à faire réagir la chaux avec le soufre du métal qui est alors piégé dans le laitier sous forme de CaS ; il faut maintenir en permanence une composition satisfaisante pour ce laitier, alors qu'il se sature progressivement en CaS au fur et à mesure de l'opération, et qu'il s'enrichit progressivement en alumine provenant de la désoxydation ; il est donc nécessaire d'éliminer périodiquement une fraction de ce laitier, et d'ajouter simultanément de la chaux (et éventuellement d'autres constituants assurant une bonne fluidité au laitier) pour compenser ce prélèvement.

Advantageously, the first container 44 is divided into two compartments 46, 47 by a transverse partition 48, the lower part of which is provided with one or more openings 49 providing communication between the two compartments. The first compartment 47 is that into which the liquid steel 2 coming from the bag 43 is introduced, and where desulfurization takes place. To this end, conventional means for introducing solid materials (not shown) make it possible to permanently maintain in this first compartment 47 metallurgical conditions favorable to the desulfurization of the metal, namely:

• a very low dissolved oxygen content of the liquid steel 2; it is obtained by a periodic or continuous addition of aluminum which combines with dissolved oxygen to form alumina;
• the presence on the surface of the liquid steel 2 of a layer of a slag 50 with a high lime content and a high fluidity, so as to react the lime with the sulfur of the metal which is then trapped in the dairy in the form of CaS; it is necessary to permanently maintain a satisfactory composition for this slag, while it gradually becomes saturated with CaS as the operation progresses, and it is gradually enriched with alumina originating from deoxidation; it is therefore necessary to periodically remove a fraction of this slag, and simultaneously add lime (and possibly other constituents ensuring good fluidity to the slag) to compensate for this removal.

The first compartment 46 is also equipped with means for stirring the liquid steel 2, such as means 51 for blowing argon, making it possible to ensure the intense stirring between the metal 2 and the slag 50 necessary for the performing desulfurization, and removing much of the alumina inclusions formed during the introduction of aluminum.

The sulfur content of the liquid steel 2 obtained as a result of this treatment also depends on the sulfur content of the raw materials from which it was produced, the quantity of slag 50 surmounting the liquid steel 2, and the average residence time of the liquid steel 2 in this first compartment 46. The capacity and the geometry of this first compartment 46 must therefore be calculated to guarantee the liquid steel 2 an average residence time high enough for the sulfur content to reach the desired level.

In the second compartment 47 of the first container 44 therefore practically only penetrates liquid steel 2 desulfurized and freed from a large part of the alumina inclusions formed in the first compartment 46. It is in this second compartment 47 that quenching the lower end of the ascending plunger 52 of a coarse decarburization reactor 53. This reactor 53 has, in the example shown, a configuration quite similar to that of a conventional RH. It comprises a cylindrical tank 54 provided with two plungers, namely said ascending plunger 52 which can be equipped with an argon injection device 55, and a descending plunger 56, the lower end of which dips in an intermediate container 57 separate from the first container 44. The tank 54 is equipped with a gas suction device 58 making it possible to maintain a reduced pressure in the reactor 53, for example of the order of 50 torr or less, under the effect of which liquid metal 2 is sucked inside the tank 54. The gas possibly blown into the ascending plunger 55 contributes to ensuring the circulation of the metal 2 between the first container 44 and the intermediate container 57, with a flow rate which, in operation permanent, is substantially equal to the feed rate of the first container 44. This maintains stable operating conditions throughout the installation if we manage to maintain a drop constant designation between the surfaces of the metal in the first container 44 and the intermediate container 57. If no argon is injected into the ascending plunger 52, it is nevertheless necessary to inject it into the tank 54 itself to accelerate the decarburization of the metal 2. The vessel 54 of the reactor 53 is equipped with a lance 59 (or an equivalent device) making it possible to inject oxygen into the liquid metal, preferably in an area situated at the level of the ascending plunger 52. The oxygen thus introduced on the one hand consumes the aluminum present in the bath, and on the other hand, dissolves in steel 2 where it can thus combine with carbon to effect the coarse decarburization required. This should bring the carbon content of metal 2 from 200-800 ppm to, for example, about 80 ppm. Since decarburization is very rapid in this range of contents, this perfectly stirred reactor 53 is sufficient to obtain this result.

At the outlet of the reactor 53, the liquid steel 2 therefore has a carbon content already considerably reduced compared to its initial content, and contains sufficient dissolved oxygen so that decarburization can continue until an ultra-low content if it is treated in a reactor of the type shown in FIG. 1. Such treatment constitutes the next step in the production chain.

For this purpose, the liquid steel 2 is withdrawn from the intermediate container 57 by an advanced decarburization reactor 5, identical to that previously described and shown in FIG. 1. For this reason, it is not useful to describe it here in more detail. The intermediate container 57 here fulfills the functions of the intermediate container 4 in FIG. 1. The other elements common to the two installations are designated in FIG. 2 by the same reference signs as in FIG. 1. Means 60 have been added to it. the continuous addition to the liquid steel 2 of alloying elements such as aluminum, silicon, manganese in the last cell or cells of the advanced decarburization reactor 5. At this stage, decarburization is considered as completed, and the metal can be definitively nuanced after the capture by the aluminum of the residual dissolved oxygen.

As in the installation previously described and shown in FIG. 1, the decarburized and nuanced liquid steel 2 enters a continuous casting distributor 11, in which the descending plunger 10 of the advanced decarburization reactor 5 is immersed. Then it begins to solidify in the ingot mold (s) 13 to form in each of them a slab, a bloom or a billet 15. This distributor 11 is preferably equipped with all the known improvements which make it possible to pour high-quality metallurgical products, in particular from the point of view of inclusion cleanliness: obstacles increasing the residence time of the metal, devices for mixing by gas blowing or by induction, cover covering the distributor and under which neutral gas is blown. These devices are particularly useful here insofar as the last deoxidation of the metal 2 is carried out here just before it enters the distributor, and it is therefore necessary to quickly eliminate the inclusions resulting from this deoxidation. On the other hand, the presence in the distributor 11 of a metal heating device by induction or plasma torch, as is now well known, is recommended. Indeed, the multiple transfers undergone by the liquid metal 2 during its treatment are costly in calories, especially when the installation has not yet reached its full thermal equilibrium. The possibility of reheating the metal 2 at various stages of its treatment therefore avoids pouring it from the production machine 40 at too high a temperature, which would shorten the duration of use of its refractory lining. Such reheating is also one of the functions of burning aluminum by injecting oxygen into the coarse decarburization reactor 53.

Advantageously, the metallurgical vessels 43, 44, 57 are provided with covers and means for blowing a neutral gas under these covers (not shown) to limit the contact of the liquid metal with the ambient air.

• élaboration dans le convertisseur 40 de capacité 300 t, au rythme de une coulée toutes les 30 minutes (soit une productivité moyenne de 10 t/min), d'un acier de teneur en carbone de 400 à 800 ppm environ ; coulée du métal 2 dans la poche 43 ;
• transvasement en continu avec un débit de 10 t/min du métal liquide 2 dans le premier compartiment 46 du premier récipient 44 où il est désoxydé et désulfuré ;
• aspiration du métal liquide 2 désoxydé et désulfuré à partir du deuxième compartiment 47 du premier récipient 44 dans le réacteur de décarburation grossière 53, avec un débit de 10 t/min ; dans ce réacteur 53, oxydation de la totalité de l'aluminium et d'une partie du carbone présents dans le métal 2 ; caractéristiques du réacteur 53 : capacité de la cuve : 15 t de métal liquide, déterminant un temps de séjour moyen du métal 2 de 90 sec ; pression : 50 torrs ; débit d'argon dans le plongeur ascendant 52 ou la cuve 54 : 60 à 65 l/sec ; ces conditions conduisent à l'obtention à la sortie du réacteur de décarburation grossière 53 d'un métal 2 contenant environ 80 ppm de carbone et 160 ppm d'oxygène dissous, qui passe dans le récipient intermédiaire 57 ;
• aspiration du métal liquide 2 à partir du récipient intermédiaire 57 dans le réacteur de décarburation poussée 5, avec un débit de 10 t/min ; ce réacteur 5 a les caractéristiques suivantes : longueur : 9 m ; largeur : 1 m ; profondeur moyenne "e" du métal 2 : 40-45 cm ; pression de travail : 50 torrs, voire moins : nombre de cellules : 9, y compris la première cellule 29 dans laquelle débouche le plongeur ascendant 8 ; la dernière de ces cellules est consacré à la mise à nuance et ne participe pas à la décarburation si on y ajoute des éléments désoxydants (aluminium, silicium, etc.) et d'autres éléments d'alliage ; longueur de la première cellule 29 : 1,8 m ; longueur des autres cellules : 0,9 m ; débit d'argon dans chaque cellule : 15 l/sec environ ; temps de séjour moyen du métal dans chaque cellule : 15 sec, sauf dans la première cellule 29 où il est de 30 sec, d'où un temps de séjour total de 150 sec dans le réacteur ; teneur en carbone dans la première cellule : 30 ppm ; teneur en carbone dans les 8e et 9e cellules : 5 ppm ;
• introduction du métal liquide 2 décarburé, désoxydé et mis à nuance dans le répartiteur 11 ;
• coulée du métal 2 dans les lingotières 13 pour former des produits sidérurgiques 15.

By way of example, one can imagine the following production scheme, intended to produce a steel with a carbon content of the order of 5 ppm:

• elaboration in the converter 40 of capacity 300 t, at the rate of a casting every 30 minutes (that is to say an average productivity of 10 t / min), of a steel with carbon content of 400 to 800 ppm approximately; pouring the metal 2 into the pocket 43;
• continuous transfer with a flow rate of 10 rpm of the liquid metal 2 into the first compartment 46 of the first container 44 where it is deoxidized and desulfurized;
• aspiration of the deoxidized and desulphurized liquid metal 2 from the second compartment 47 of the first container 44 into the coarse decarburization reactor 53, with a flow rate of 10 rpm; in this reactor 53, oxidation of all of the aluminum and part of the carbon present in the metal 2; characteristics of reactor 53: tank capacity: 15 t of liquid metal, determining an average residence time of metal 2 of 90 sec; pressure: 50 torr; argon flow in the ascending plunger 52 or the tank 54: 60 to 65 l / sec; these conditions lead to the production at the outlet of the coarse decarburization reactor 53 of a metal 2 containing approximately 80 ppm of carbon and 160 ppm of dissolved oxygen, which passes into the intermediate container 57;
• aspiration of the liquid metal 2 from the intermediate container 57 in the advanced decarburization reactor 5, with a flow rate of 10 rpm; this reactor 5 has the following characteristics: length: 9 m; width: 1 m; average depth "e" of metal 2: 40-45 cm; working pressure: 50 torr, or even less: number of cells: 9, including the first cell 29 into which the ascending plunger 8 opens; the last of these cells is devoted to nuancing and does not participate in decarburization if deoxidizing elements (aluminum, silicon, etc.) and other alloying elements are added thereto; length of the first cell 29: 1.8 m; length of other cells: 0.9 m; argon flow in each cell: approximately 15 l / sec; average residence time of the metal in each cell: 15 sec, except in the first cell 29 where it is 30 sec, hence a total residence time of 150 sec in the reactor; carbon content in the first cell: 30 ppm; carbon content in the 8th and 9th cells: 5 ppm;
• introduction of the decarburized, deoxidized and nuanced liquid metal 2 into the distributor 11;
• pouring of metal 2 into the molds 13 to form steel products 15.

Compared to a conventional HR of the same productivity, we can therefore, in this example, multiply by 8 the time during which a portion of metal is exposed to vacuum and can decarburize, if the residence times of the metal 2 are added in the coarse decarburization reactor 53 and in the reactor 5 according to the invention.

The ultra-low carbon contents obtained often go hand in hand with very low nitrogen contents (30 ppm and less), provided that the nitrogen content at the outlet of the production machine 40 is not too high, and that the following stages (in particular the transfers from one container to another and the stirring of the metal) do not bring about significant nitrogen recovery. In the event that the greatest possible reduction in the nitrogen content is sought, the preceding procedure would not be optimal. In fact, the denitriding in the last cells would be very low since the deeply decarburized metal would have a high content of dissolved oxygen. The CO sweep produced by decarburization would therefore be too reduced to be able to offset the negative effect of dissolved oxygen on the denitriding kinetics.

We can then increase the flow of argon in the first cells of the advanced decarburization reactor 5, and / or replace the argon with hydrogen to accelerate the decarburization kinetics, and obtain the targeted carbon content sooner than in previous practice. The addition of deoxidizing aluminum is then carried out not in the very last cell, but more upstream. The last cells can thus be devoted to denitriding, by injecting a large volume of gas (argon or hydrogen) into the deoxidized metal.

On the other hand, as mentioned above, the reactor 5 can no longer be used as an advanced decarburization reactor, but as a reactor for deoxidation of the metal by carbon under vacuum. The advantage is to obtain a final metal with a very low content of inclusive oxygen, since a minimum quantity of aluminum is necessary for the final deoxidation of the liquid metal. To this end, in the previous production scheme, we can be satisfied with the carbon content of 80 ppm (for example) obtained thanks to the coarse decarburization reactor 53. In the reactor 5 according to the invention, the various injections of argon are replaced by injections of a liquid or gaseous hydrocarbon such as methane CH ₄ , which decomposes by cracking into carbon and hydrogen. Carbon combines with the oxygen dissolved in metal 2 to form CO, the formation kinetics of which is accelerated by hydrogen. The quantity of hydrocarbon injected can optionally be modulated in each cell so that the carbon content of the bath remains constant as the deoxidation takes place. It is thus possible to obtain a dissolved oxygen content of approximately 10 ppm while retaining the initial 80 ppm of carbon.

Finally, it is possible to envisage simultaneously obtaining an ultra-low carbon content (5 ppm) and an ultra-low dissolved oxygen content (10 ppm). For this, it is necessary to lower the pressure in the reactor to 10 torr or less, in order to decrease the oxygen content in thermodynamic equilibrium with a given carbon content.

As a variant of the production line which has just been described, it is possible to envisage replacing the primary production unit 40 and the bag 43 with a primary production unit which would operate continuously. It would permanently pour liquid metal directly into the first container 44, with a flow rate substantially equal to the flow rate of circulation of the liquid metal in the installation.

Of course, the invention is not limited to the examples which have been described and shown, and modifications can be made to the constitution of the metallurgical reactor according to the invention and to the process for the preparation of the liquid steel which produces it. use. The main thing is to keep the general principle governing the design of the reactor, namely, a flow of liquid metal within it as close as possible to the ideal case of a piston flow. One can, for example, use a cascade of reactors according to the invention. Likewise, it is clear that the reactor which has been described can without problems also be used for the production of steels whose carbon content is not particularly low, and for which no research that a high inclusive cleanliness and a low dissolved gas content. Finally, the invention can find applications in the preparation of metals other than steel.

Claims (23)

1. Process for adjusting the composition of a liquid metal, in which the said liquid metal is sucked up from a start receptacle into a reactor which is at a reduced pressure with respect to the said first receptacle, the said metal is made to undergo a metallurgical treatment and is discharged into a finish receptacle with respect to which the said reactor is also at a reduced pressure, characterized in that the said metal is made to flow inside the said reactor under conditions close to plug flow.
2. Process according to Claim 1, characterized in that the said start receptacle is fed continuously with liquid metal and in that the said metal is made to flow, also continuously, from the said finish receptacle.
3. Process according to Claim 2, characterized in that the said finish receptacle is a continuous casting tundish.
4. Process according to one of Claims 1 to 3, characterized in that the said reactor is divided into a plurality of cells, each of which forms a thoroughly mixed zone.
5. Process according to one of Claims 1 to 4, characterized in that the said liquid metal is steel and in that the said metallurgical treatment includes decarburization.
6. Process according to one of Claims 1 to 5, characterized in that the said liquid metal is steel and in that the said metallurgical treatment includes denitriding.
7. Process according to one of Claims 1 to 6, characterized in that the said liquid metal is steel and in that the said metallurgical treatment includes a vacuum deoxidation using carbon.
8. Plant for adjusting the composition of a liquid metal (2), such as steel, which includes a start receptacle (4) containing the said liquid metal, a reactor (5) and a finish receptacle (11), the said reactor (5) including a vessel (6) provided with means (7) enabling its interior to be maintained at a reduced pressure with respect to the pressures existing in the said start receptacle (4) and the said finish receptacle (11), with a first dip tube (8) called ascending dip tube, one end of which dips into the liquid metal (2) contained in the said start receptacle (4) and the other end of which is connected to the bottom (18) of the said vessel (6), with a second dip tube (10) called descending dip tube, one end of which dips into the liquid metal (2) contained in the said finish receptacle (11) and the other end of which is connected to the bottom (18) of the said vessel, and with means for ensuring continuous flow of the said liquid metal between the said start receptacle (4) and the said finish receptacle (11) through the said dip tubes (8, 10) and the said vessel (6) of the reactor (5), characterized in that the said vessel (6) is shaped so as to impose on the said liquid metal a flow close to plug-type flow over its path between the said dip tubes (8, 10).
9. Plant according to Claim 8, characterized in that the vessel (6) has an approximately rectangular cross-section and in that the ratio between the distance separating the dip tubes (8, 10) and the width of the vessel (6) is at least equal to 6.
10. Plant according to Claim 8 or 9, characterized in that the said vessel (6) includes a plurality of means for agitating the liquid metal (2), dividing the said vessel (6) into a plurality of cells (29-34), each of which is thoroughly mixed.
11. Plant according to Claim 10, characterized in that the said agitating means include means (19-23) for injecting a fluid into the said liquid metal.
12. Plant according to Claim 11, characterized in that, for at least part of the said agitating means, the said fluid is argon.
13. Plant according to Claim 11, characterized in that, for at least part of the said agitating means, the said fluid is hydrogen.
14. Plant according to Claim 11, characterized in that, for at least part of the said agitating means, the said fluid is a hydrocarbon.
15. Plant according to one of Claims 10 to 14, characterized in that the said agitating means include means for applying moving electromagnetic fields to the said liquid metal (2).
16. Plant according to one of Claims 10 to 15, characterized in that the said cells (29-34) are separated by dams (24-28), each of which allows communication between the cells separated by it.
17. Plant according to one of Claims 8 to 16, characterized in that the said means for ensuring continuous flow of the said liquid metal between the said start receptacle (4) and the said finish receptacle (11) include means (9) for injecting a gas into the liquid metal (2) contained in the said ascending dip tube (8).
18. Plant according to one of Claims 8 to 17, characterized in that it includes means (1) for continuously feeding the said start receptacle (4) with liquid metal (2) and means (12) for continuously extracting the liquid metal (2) from the said finish receptacle (11).
19. Plant according to Claim 18, characterized in that the said finish receptacle (11) is a continuous casting tundish.
20. Plant for adjusting the composition of the liquid steel, characterized in that it includes:

    - an apparatus (40) for the primary smelting of the liquid steel (2);

    - means (43) for continuously pouring the said liquid steel (2) from the primary smelting apparatus (40) into a first receptacle (44) provided with means for introducing deoxidizing agents into the liquid metal (2) and with mixing means (51);

    - a reactor (53) for the rough decarburization of the liquid metal (2), including a vessel (54) provided with means (58) for putting it at a reduced pressure, with a first dip tube (52) dipping into the liquid metal (2) contained in the said first receptacle (44) and with a second dip tube (56) dipping into an intermediate receptacle (57), the said vessel (53) also including means (59) for blowing oxygen into the said liquid metal;

    - means for ensuring continuous flow of the said liquid metal (2) between the said first receptacle (44) and the said intermediate receptacle (57); and

    - at least one plant for adjusting the composition of the liquid steel according to one of Claims 8 to 19, in which plant the said intermediate receptacle (57) forms the said start receptacle (4).
21. Plant according to Claim 20, characterized in that the said primary smelting apparatus (40) itself forms the said means (43) for continuously pouring the said liquid steel (2) into the said first receptacle (44).
22. Plant according to Claim 20 or 21, characterized in that the said first receptacle (44) includes a partition (48) separating it into two communicating compartments (46, 47), in that one (46) of these compartments receives just the said liquid metal (2) coming from the said means (43), which pour it into the said first receptacle (44), and the said deoxidizing agents and in that the said first dip tube (52) of the said rough decarburization reactor (53) dips into the other compartment (47).
23. Plant according to one of Claims 20 to 22, characterized in that the said means for ensuring continuous flow of the said liquid metal (2) between the said first receptacle (44) and the said intermediate receptacle (7) include means (55) for blowing a gas into the liquid metal contained in the said first dip tube (52).

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