EP0195793B1 - Method for regulating the level of the contact line of the free metal surface with the ingot mould in a continuous vertical casting - Google Patents

Abstract

In a continuous vertical casting operation carried out in a mould 3, a coil 7 applies a periodic magnetic field of variable strength whose direction is substantially parallel to the axis of the mould 3 to the liquid 5 as it solidifies, in order to regulate the level of the line of contact of the free surface of the metal 5 with the mould 3. The height of the contact of the metal with the mould can be reduced from h1 to h2 by applying the magnetic field, and the strength of the field is adjusted in dependence on the desired level. The procedure is used in casting semifinished metallurgical products, in particular in aluminium and its alloys, in which there is a wish to obtain a cortical region of zero thickness, a fine grain without the preliminary addition of refining agents, and an absence of pitting.

Description

The invention relates to a method for adjusting the level of the contact line of the free surface of the metal with an ingot mold in a vertical casting.

During the manufacture of metallurgical semi-products by casting ferrous or light metals such as aluminum and its alloys, those skilled in the art seek to obtain ingots, billets, plates, etc. having the best physical and chemical homogeneity possible, this in order to avoid the appearance of certain defects during the subsequent transformation of these products into sheets, threads, etc.

However, most of the casting methods currently used in industry give rise, when the metal passes from the liquid state to the solid state, to the formation of more or less significant homogeneity defects, due essentially to different cooling conditions from one point to another of the cast products. Thus in the casting in a vertical passage mold where the metal is successively cooled indirectly through the mold, then directly by a water slide, we find the presence on the semi-finished products of a layer external called "primary cortical layer". This layer, the structure and composition of which differs from that of the internal part of the semi-finished product, results from the indirect cooling of the metal in contact with the mold. In addition, other heterogeneities which are much less pronounced, but all equally troublesome, may appear, such as "spikes" or small pitting due in particular to a dispersion in the metallic mass of the oxide layer which forms on the surface of the metal. liquid in contact with the atmosphere.

Admittedly, the person skilled in the art did not remain inactive before these problems and he brought a certain number of more or less satisfactory solutions aiming at eliminating or at least reducing the importance of these heterogeneities.

Thus in French patent 1 509 962, a person skilled in the art recommended the use of electromagnetic casting, a technique in which thanks to the confinement of the metal using forces of electromagnetic origin, we can remove the ingot mold and thus avoid the appearance of the cortical layer since there is no longer indirect cooling.

We thus manage to improve the homogeneity of semi-finished products.

• - il faut équiper le métier de coulée avec une installation électrique relativement complexe et chère à cause de la nécessité de disposer de courants de fréquence non industrielle (500 à 4000 Hz) pour créer un champ de confinement convenable,
• - le risque d'hétérogénéité par picots est accru en raison, d'une part de l'absence de lingotière et donc de l'augmentation de la surface de métal liquide susceptible d'être oxydé, d'autre part du phénomène de brassage de la masse liquide causé par le champ de confinement qui contribue fortement à la dislocation du film d'oxyde et à sa dispersion dans le métal,
• - il est souvent difficile de créer un confinement convenable lors du démarrage de la coulée électromagnétique,
• - la sécurité du personnel peut être mise en cause quand on coule l'aluminium et ses alliages car en cas de défaillance électrique, le métal liquide n'étant plus confiné se répand à

• l'extérieur de la lingotière et peut entrer en contact avec le fluide de refroidissement direct en provoquant une explosion.

However, this technique has the following drawbacks:

• - it is necessary to equip the casting profession with a relatively complex and expensive electrical installation because of the need to have currents of non-industrial frequency (500 to 4000 Hz) to create a suitable confinement field,
• the risk of heterogeneity by pins is increased due, on the one hand, to the absence of an ingot mold and therefore to the increase in the surface area of liquid metal capable of being oxidized, on the other hand to the phenomenon of mixing the liquid mass caused by the confinement field which greatly contributes to the dislocation of the oxide film and to its dispersion in the metal,
• - it is often difficult to create suitable confinement when starting electromagnetic casting,
• - the safety of personnel can be called into question when casting aluminum and its alloys because in the event of an electrical failure, the liquid metal being no longer confined spreads to

• the outside of the mold and can come into contact with direct coolant causing an explosion.

Other simpler solutions have also been proposed to reduce the thickness of the cortical layer. For example, French patent 1,398,526 teaches the use of a strip of fiberfrax bonded to the ingot mold so as to reduce the height of metal in contact with the ingot mold, and therefore to reduce the effects due to indirect cooling. However, this reduction in height cannot be fixed once and for all, since it depends in particular on the speed of casting. Also, when this parameter varies, you must either change the mold, or at least modify the height of the strip. What confers a lack of flexibility to a solution bringing in the end only a partial suppression of heterogeneities.

In French patent 1,496,241, the drawbacks of indirect cooling are eliminated by using an uncooled graphite ingot mold, but we then come up against problems of maintenance and frequent change of the ingot mold due to the brittleness of this material.

Another solution consists in using molds with a striated interior surface by means of which the thickness of the cortical layer is reduced by more than 30% when aluminum 1050 is poured for example. However, in addition to the machining of these molds which significantly increases the price, there are drawbacks due to the adaptation of the mold, and here streaks, at each casting speed.

We still know the loaded casting with enhancement, called "HOT TOP", but it also has the disadvantage of both leading to a periodic solidification of the meniscus, cause of small folds on the surface of semi-finished products and s' accompany difficulties during start-up.

Finally, more recently, French patent 2,417,357 has claimed a process in which the axial length of the part of the ingot mold in contact with the liquid metal is varied by using a cuff sliding on the inner wall of the ingot mold. Such a system has the disadvantage during untimely solidification of the metal that it leads to adhesion between the ingot mold and the cuff, thus causing the parts present to be torn off when the sliding movement is carried out.

• - utilisation d'installations électriques moins compliquées que celles imposées par la coulée électromagnétique,
• - passage aisé de la phase de démarrage de la coulée au régime de croisière,
• - adaptation facile à des variations de paramètres tels que la vitesse de coulée puisque le procédé ne nécessite aucune modification du matériel tel que le changement de lingotière,
• - application à n'importe quel type de lingotière classique,
• - absence de tout dispositif dans lequel on met des pièces en mouvement,
• - risques d'explosion par fuite de métal liquide moins grands qu'avec la coulée électromagnétique.

Therefore, the conscious plaintiff problems posed by these techniques a, in order to obtain homogeneous semi-products in which the thickness of the cortical layer is practically zero, the grain is refined and the skin is free of spikes, sought and developed a process which has the following advantages over those of the prior art:

• - use of less complicated electrical installations than those imposed by electromagnetic casting,
• - easy transition from the start of casting phase to cruising speed,
• - easy adaptation to variations in parameters such as the casting speed since the process does not require any modification of the material such as the change of ingot mold,
• - application to any type of conventional ingot mold,
• - absence of any device in which moving parts are put,
• - lower risk of explosion by leakage of liquid metal than with electromagnetic casting.

• - d'une part, le démarrage de la coulée est d'autant plus facile que le niveau de métal dans la lingotière est haut. En effet, avec un niveau bas, le flotteur qui règle le niveau et l'alimentation en métal de la lingotière se rapproche du front de solidification et risque, pour des semi-produits de petites dimensions, d'être bloqué par une solidification intempestive du métal et de ne plus pouvoir assurer sa fonction. De même, le phénomène de cambrure qui se manifeste avec les semi-produits de grande largeur interdit également un démarrage en niveau bas.
• - d'autre part, en régime de croisière, il est préférable de couler avec une hauteur de métal dans la lingotière la plus faible possible car on limite ainsi la hauteur de contact de métal avec la paroi de la lingotière et de ce fait on réduit l'épaisseur de la zone corticale qui, comme on l'a vu plus haut, est essentiellement due au refroidissement du métal par l'intermédiaire de la lingotière.

To achieve this result, the plaintiff started with the following observations:

• - On the one hand, the start of the casting is all the easier the higher the level of metal in the mold. Indeed, with a low level, the float which regulates the level and the metal supply of the ingot mold approaches the solidification front and risks, for semi-finished products of small dimensions, to be blocked by an inadvertent solidification of the metal and no longer able to perform its function. Likewise, the camber phenomenon which manifests itself with large semi-finished products also prohibits starting at a low level.
• - on the other hand, in cruising mode, it is preferable to sink with a metal height in the ingot mold as low as possible, thereby limiting the height of metal contact with the wall of the ingot mold and thereby reducing the thickness of the cortical zone which, as we saw above, is essentially due to the cooling of the metal through the ingot mold.

It was therefore necessary, starting from a conventional ingot mold with its contingencies, that is to say while keeping in the ingot mold a constant height of metal since fixed by the position of the float and sufficient to not interfere with the operation of the float, being able to limit as much as possible the height of contact of the metal with the surface of the ingot mold which amounted, in short, to finding a means of adjusting the level of the line of contact of the free surface of the liquid metal with the wall of the ingot mold.

This means consists, according to the invention, in applying to the liquid metal in the course of solidification a periodic electromagnetic field of variable intensity and direction substantially parallel to the axis of the ingot mold and in adapting its intensity as a function of the desired level.

In fact, it has been observed that by placing around the ingot mold a circular coil constituted by an electrical circuit formed by one or more windings, and by supplying it with an alternating current of sufficient industrial voltage, it was possible to modify the profile of the metal meniscus and in particular to vary the level of this line of contact of the metal with the ingot mold and this all the more so as the variations in the supply voltage and correlatively in the intensity of the field created, were large.

Thus, by increasing the intensity of the field, one could lower the level and consequently decrease the height of the metallingotiere contact zone or on the contrary by making it decrease, one could raise this level and consequently increase this height.

The advantage of such a method is therefore to allow the metal-ingot contact height and consequently the thickness of the cortical layer to be reduced at will simply with a coil supplied by a current of industrial frequency 50 or 60 Hz knowing that any electrical failure will only have the effect of varying the height of metal in the mold, that is to say that any risk of leakage of liquid metal will be eliminated which is not the case in electromagnetic casting .

In addition, the presence of an ingot mold, while limiting the possibility of oxidation of the liquid metal at the meniscus, prevents, by the contact it has with the metal, any displacement of the oxide film towards the side wall. and therefore any risk of tingling on the surface of the semi-finished product.

In addition, the field applied to the metal also has the effect of creating forces inside the liquid which homogenize the cooling and tend to cause a refinement of the pouring grain.

The coil which creates the magnetic field preferably has a shape symmetrical to that of the mold, so that it creates a direction field substantially parallel to the axis of the mold. It is arranged along this axis so that the zone where the field exerts a maximum action is located at a level of the mold between half and a third of its height from the base.

Such a method makes it possible, during a casting operation, to carry out a normal start-up under the best possible conditions, that is to say with a high height of metal in the ingot mold. For that, one decreases the intensity of the field until possibly canceling it so as to remove any modification of the normal level of the metal. Subsequently, to switch to cruising mode, the intensity of the field is increased until reaching a minimum height leading to a minimum thickness of the cortical layer. The maximum value of the admissible field is easily detected by the appearance, when it is exceeded, of deformation of the surface of the cast product.

It is therefore sufficient to determine this value during the start of a test casting and the then repeat for all flows of the same type.

This value generally corresponds to the moment when the level reached by the contact line corresponds to the level at which the line of intersection between the solidification front due to indirect cooling and the solidification front due to direct cooling in conventional casting takes place. The contact height is then practically reduced to a circular line and the cortical layer nonexistent.

Depending on the type of alloy cast, we know that we will have to sink at different speeds. The method according to the invention makes it possible to modify the intensity of the field to adapt it to variations in speed and to determine as previously the maximum value of the admissible intensity for each of these speeds.

The invention will be better understood using the attached figure which shows in vertical section two half-molds, the one on the left is used according to the prior art and the one on the right according to the method of the invention.

There is a liquid metal supply nozzle (1), a level control float (2), an ingot mold (3) cooled directly by a fluid (4) which then cools the metal (5) directly at point (6 ). The right half ingot mold is equipped with a coil (7) which is supplied with an alternating voltage (8) in order to create the magnetic field of direction (9) and to cause the level of the contact line to drop. of the surface of the metal with the ingot mold of a point (10) in the casting of the prior art at point (11) according to the invention, point which is located at the intersection (12) of the front solidification (13) resulting from indirect cooling and the front (14) resulting from direct cooling. It can thus be seen that the contact height of the metal with the ingot mold has been reduced from a height h to an extremely small height h which can be assimilated to point (11).

The invention can be illustrated with the aid of the following application examples:

In an aluminum ingot mold of diameter 320 mm, height 100 mm, an aluminum alloy of the type 2214 was poured according to the standards of the Aluminum Association, at the speed of 60 mm / minute. The float adjusted the metal level halfway up the ingot mold and the coolant came into contact with the skin of the cast billet approximately 1 cm below the base of the ingot mold.

In a first test, the casting was carried out under the conditions of the prior art and a micrographic examination of different sections of the billet showed that the cortical layer had an average thickness of 18 mm.

A series of tests was then carried out during which the ingot mold was surrounded by an annular coil of internal diameter 372 mm, external 465 mm, height 48 mm, formed by 120 turns of enameled copper wire of diameter 3, 35 mm and powered by an alternating current of 50 Hz.

Each of the tests was conducted under a different electrical voltage and the corresponding mean cortical thicknesses and grain size were measured by the intersection method.

The results are shown in the table below:

It can therefore be seen that the application of the method according to the invention leads to a progressive reduction in the thickness of the cortical layer as the electric voltage at the terminals of the coil is increased in proportions such that this thickness becomes zero for a voltage of 180 volts.

At the same time, the grain size decreases so that, from a metal which has grains of 500 μm in conventional casting, grains of 180 μm on average are obtained according to the invention.

In addition, we do not note the presence of any pin.

The present invention finds its application in the casting of metallurgical semi-products, in particular aluminum and its alloys such as, for example, lithium alloys and in which it is desired to obtain both a cortical zone of practically zero thickness, a grain end without prior addition of refining agents such as ATSB and the absence of pins.

Claims (7)

1. A process for regulating the level of the line of contact of the free surface of the metal with a mould in a continuous vertical casting operation characterised by applying to the liquid metal in the course of solidification a periodic electronagnetic field of variable strength and whose direction is substantially parallel to the axis of the mould and adjusting the strength of the field in dependence on the desired level.

2. A process according to claim 1 characterised in that the field is at an industrial frequency.

3. A process according to claim 1 characterised in that the region in which the field exerts its maximum action is at a level of the mould which is between half and a third of its height measured from its base.

4. A process according to claim 1 characterised by increasing the strength of the field to lower the level.

5. A process according to claim,l characterised by reducing the strength of the field to raise the level.

6. A process according to claim 1 characterised in that, in a casting operation, the strength of the field is reduced at the time of starting up and it is then progressively increased to a maximum value above which the surface of the cast product begins to suffer deformation.

7. A process according to claim 1 characterised by modifying the strength of the field when the casting speed varies.

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