EP0249565A1 - Device for regulating the level of the contact line of the free metal surface with the continuous casting mould in a vertical casting - Google Patents

Abstract

The invention relates to an apparatus for regulating the level of the line of contact of the free surface of the metal with the mould in vertical casting. It comprises a mould formed by a material having a level of resistivity of higher than 5 mu omega .cm, surrounded by an annular coil in which at least one alternating electrical current flows. It finds application in the casting of metallurgical semi-manufactured products, in particular of aluminium and alloys thereof, such as for example lithium-bearing alloys, and in which there is a wish to have both a cortical zone of virtually zero thickness, a fine grain without the previous addition of refining agents, and an absence of pitting.

Description

The invention relates to a device for adjusting the level of the line of contact of the free surface of the metal with the 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, essentially due 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 ingot mold. In addition, other heterogeneities, 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. liquid metal 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, the skilled person has recommended the use of electromagnetic casting, a technique in which, thanks to the confinement of the metal using forces of electromagnetic origin, the ingot mold can be eliminated and thus the appearance of the cortical layer is avoided since there is no has more 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) de grande intensité (de l'ordre de 5000 A) 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 non-industrial frequency currents (500 to 4000 Hz) of high intensity (of the order of 5000 A) to create a suitable containment 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,
• - personnel safety can be jeopardized when casting aluminum and its alloys because in the event of an electrical failure, the liquid metal, which is no longer confined, spreads outside the ingot mold and can come into contact with the 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 the 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 ingot mold or less change the height of the band. What confers a lack of flexibility to a solution bringing in the end only a partial elimination 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 traditionnelle,
• - 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 lingotiêre,
• - 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 traditionnelle.

This is why the Applicant, aware of the problems posed by these techniques, has, 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 dots. , sought and developed a method which has, compared to those of the prior art, the following advantages:

• - use of less complicated electrical installations than those imposed by traditional 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,
• - risks of explosion by leakage of liquid metal smaller than with traditional 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 filtre en tissu de verre qui régularise 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 when the level of metal in the mold is high. Indeed, with a low level, the glass cloth filter which regulates the metal supply to the ingot mold approaches the solidification front and risks, for semi-finished products of small dimensions, to be blocked by an untimely solidification. 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 flow with a height of metal in the ingot mold as low as possible, thereby limiting the height of metal contact with the wall of the ingot mold and therefore reduces the thickness of the cortical area which, as we saw above, is mainly 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 sufficient height of metal so as not to interfere with the operation of the filter, to be able to limit the contact height as much as possible metal with the surface of the mold which basically amounted to finding a way to level the contact line of the free surface of the metal liquid with: the wall of the mold.

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

In fact, it has been found that by placing around the mold at least one 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 is possible to achieve 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 mold, all the more so since the variations in the supply voltage and correlatively in the intensity of the field created, were great.

Thus, by increasing the intensity of the field, one could lower the level and consequently decrease the height of the metal-ingot 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 refinement of the pouring grain.

The coil which creates the magnetic field preferably has a shape close to that of the ingot mold, so that it creates a direction field substantially parallel to the axis of the ingot mold. It is arranged along this axis so that the zone where the field exerts a maximum action is located between the top of the liquid meniscus and the point of contact with the mold.

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 this, a field of low intensity and possibly zero is used so as to minimize 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 then to renew it for all the castings 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 makes it possible to modify the intensity of the field to adapt it to variations in speed and to determine as before the maximum value of the admissible intensity for each of these speeds.

The process can be better understood with the help of FIG. 1 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 process described above. .

There is a supply nozzle (1) made of liquid metal, a stopper rod (2) for level regulation, a lippotiere (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 lowering of the level of the contact line of the surface of the metal with the mold of a point (10) in the casting of the prior art at the point (11) along the leiprocédén, point which is located at the intersection (12) of the solidification front (13) resulting from indirect cooling and from 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 ₁ to an extremely small height h ₂ which can be assimilated to point (11).

This process can be illustrated using 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 stopper regulated the level of metal at mid-height of the mold and the coolant came into contact with the skin of the cast billet approximately 1 cm below the base of the 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 the 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 leads to a progressive reduction in the thickness of the cortical layer as the electric voltage across the coil is increased in proportions such that this thickness becomes zero for a voltage of 180 volts.

Simultaneously, 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.

However, during its tests, the applicant noted that the conventional devices used, whether for casting plates or for casting billets, were not the best suited for this adjustment process. Thus she found that it was possible, while maintaining the results obtained on the cortical layers and the grain size, to decrease the voltage necessary for the adjustment, that is to say the electric power consumed if the 'We increased the electrical resistance of devices located near the coil and especially that of the mold itself.

This increase can be obtained by modifying the composition of the material constituting the devices to give it greater resistivity, or the structure of the devices to increase their resistance.

According to the invention, the two paths have been followed, one characterized in that an ingot mold consisting of a solid material having a resistivity greater than 5 μΩ.cm is used, the other characterized in that that an ingot mold shared in the height direction in at least two sectors separated from each other by an electrical insulator is used. According to the first way, if the increase in resistivity, for water boxes for example, is easy to obtain using materials such as stainless steels or fiber-reinforced resins, on the other hand, for ingot molds, the solution is less practical due to the traditional use of aluminum or copper, metals with very low resistivity (<3µΩ.cm)

During the tests, the Applicant has found that it is possible to reduce the electrical voltage necessary for adjustment, while maintaining suitable casting conditions, by using materials belonging to the group consisting of ceramics, metals such as for example non-magnetic stainless steels, titanium. However, the best solution is to use aluminum alloys with elements such as manganese, chromium, titanium, vanadium, which allow not too high concentrations to obtain relatively high solid solution contents and as a result of the higher resistivities.

Thus, one can cite among the alloys that which contains by weight about 1.8% Mn- 0.25% Cr- 0.2% Ti and 0.1 Z V and whose resistivity is equal to 9.3 µΩ.cm. This resistivity can however be improved by adding Mg up to 5%, in which case values of 11 to 12 µΩ.cm can be obtained. The addition of Li up to 1% or Zr up to 0.15% is also favorable.

Other solutions consist in using ingot molds constituted by composite products such as for example a stainless steel coated internally with a thin layer of aluminum.

According to the second path, to reduce the electrical voltage necessary for adjustment, an ingot mold is used such that it allows the electrical resistance to be increased. This structure can be obtained by dividing the mold into sectors which are separated from each other by an electrical insulator and assembled by means known to those skilled in the art.

Figure 2 shows in perspective such an ingot mold of diameter 320mm and 120 mm high, on which there are 4 sectors separated over - their entire height by a mica blade (16). These parts are kept assembled together by means of stainless steel pins (17) passing through the edge of the mold and dowels (18) of insulating material, all of these means being placed in the mass of the mold. To obtain a suitable result, it has been found that it is however necessary to have sectors whose width is not too large. Thus widths between 10 and 30 cm have given the best results.

The ingot molds designed according to the invention have the result, when they are subjected to the action of a magnetic field created by an industrial current of frequency 50 to 60 Hz, obtaining cast products having both a cortical layer of very little or no thickness and a fine grain.

However, the applicant has found that their operation can be optimized by subjecting them to the action of several magnetic fields of different frequency, capable of separately exacerbating each of the two desired results. It should be known that a high frequency field exerts its influence near the surface of the cast product, while a low frequency field can extend its influence to the middle of the cast product. Consequently, provided that it crosses the ingot mold, a high frequency field N _{1 will be} suitable for obtaining a zero cortical thickness. On the other hand, to obtain a refining of the grain, which therefore concerns the entire section of the product, a lower frequency N _{2 will be required} and adapted to said section.

Preferably, N ₁ is chosen from the frequencies between 50 Hz and 1 kHz depending on the nature and geometry of the ingot mold and N ₂ is at least of the order of Hz for cast products of very thick (from about 60 cm thick).

The simultaneous application of these frequencies N1 and N2 therefore makes it possible to act at the same time on the two desired results. It should be noted that if the electrical assembly allowing the two frequency currents NI and N ₂ to flow in a single turn seems too complicated, we can use two concentric turns, the turn traversed by the current of frequency N ₂ surrounding the turn traversed by the current of frequency Nl.

It will be noted that the application of these intense fields can lead to levitation of the molds made of aluminum alloys. This phenomenon can be avoided by securely fixing the mold to the casting loom or by replacing its upper part with a piece of stainless steel. We can obviously combine the two solutions.

The invention can be illustrated using the following example of application:

Molds with dimensions 1100 x 300 mm and height 120 mm were used with different resistivities. These ingot molds were surrounded by a turn constituted, for reasons of electrical safety, of a single winding and traversed by a current of frequency 50 Hz. We measured the voltage necessary at the terminals of the turn to obtain in all cases the same height adjustment of the liquid metal - ingot mold contact line, leading to zero cortical layer thickness. The results are as follows:

It can be seen that the electrical voltage at the terminals of the turn is considerably reduced, and consequently the power consumed by using molds with higher resistivity. However, for very large resistivities the gain obtained is relatively low.

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 fine grain without prior addition of refining agents such as AT5B and the absence of pins.

Claims (11)

1. Device for adjusting the level of the line of contact of the free surface of the metal with the ingot mold (3) in a vertical casting by means of at least one annular coil (8) traversed by a periodic electric current and surrounding said ingot mold , characterized in that, in order to reduce the electrical voltage necessary for adjustment, an ingot mold made of a solid material having a resistivity greater than 5 µΩ.cm is used. 2. Device for adjusting the level of the line of contact of the free surface of the metal with the ingot mold, in a vertical casting by means of an annular coil traversed by a periodic electric current and surrounding said ingot mold, characterized in that, in the purpose of reducing the electric voltage necessary for the adjustment, an ingot mold is used which is divided in the height direction in at least two sectors separated from each other by an electrical insulator. 3. Device according to claim 1, characterized in that the material belongs to the group consisting of ceramics, metals and alloys with high resistivity. 4. Device according to claim 1, characterized in that the material is an aluminum alloy of high resistivity. 5. Device according to claim 4 characterized in that the material is an aluminum alloy of composition by weight: 1.8% Mn- 0.25% Cr-0.2% Ti- 0.1 XV and 5 X Mg . 6. Device according to claim 1 characterized in that the material is a composite product. 7. Device according to claim 6 characterized in that the material is a non-magnetic stainless steel coated internally with a thin layer of aluminum. 8. Device according to claim 2 characterized in that the sectors have a width between 10 and 30 cm. 9. Device according to claims 1 and 2, characterized in that the annular coil is connected simultaneously to two sources of electric current of different frequency N1 and N2. 10. Device according to claim 9 characterized in that one of the sources has an industrial frequency NI and the other a lower frequency N2. 11. Device according to claims 1 and 2 characterized in that it comprises two annular coils, one supplied by an industrial frequency source N1, the other by a lower frequency source N2.

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