EP0542021A1 - Electromagnetic stirring during continuous casting - Google Patents

Abstract

A method of electromagnetic stirring for a molten metal during continuous casting is provided. The windings of the inductors are supplied so as to create, in the molten metal (10), at least one primary rotary movement zone (14), which is offset with respect to the central flow axis. By cyclical commutation of the phases of the polyphase current, a secondary gyratory movement (18) around the central flow axis is imparted to this primary rotary movement zone (14). <IMAGE>

Description

The present invention relates to a method of stirring molten metal for continuous casting, according to which a mobile electromagnetic induction field is induced in a metal flow, defining a central casting axis by means of one or more inductors. arranged around the metal flow and supplied with polyphase current, said field generating in the molten metal at least one movement transverse to the metal flow.

It is well known to carry out electromagnetic stirring in continuous casting in order to obtain greater regularity of the cast metal, both with regard to its surface state and its internal properties, such as segregation and shrinkage.

It has thus been proposed to stir the metal either in the ingot mold itself, or at different locations below the ingot mold. The exact location of the various inductors carrying out the stirring is determined as a function of the casting speed, of the casting section, as well as of the quality of the metal to be treated.

• a) le brasseur rotatif qui entoure d'une manière aussi symétrique que possible le métal coulé ou à couler et agit perpendiculairement au flux du métal; un tel brasseur est par exemple décrit dans le fascicule de brevet FR-A-2 279 500;
• b) le brasseur linéaire directionnel qui agit le plus souvent parallèlement au flux du métal; un tel brasseur est par exemple décrit dans la revue allemande "Fachberichte Hüttenpraxis Metallverarbeitung", Vol.25, No. 7, 1987, p.676-681 "Electromagnetic Stirring using Voest-Alpine Pulsators on the Donawitz Continuous Bloom Caster". Ce dernier type de brasseur peut agir soit sur une seule surface du métal coulé, soit sur deux surfaces diamétralement opposées, soit même sur les quatre surfaces du métal. Il a aussi été proposé d'utiliser ce dernier type d'inducteur linéaire en lui imposant des pulsations destinées à engendrer un brassage supplémentaire du métal à traiter.

So far two main types of brewers are known:

• a) the rotary stirrer which surrounds the metal to be cast or to be cast as symmetrically as possible and acts perpendicular to the flow of the metal; such a brewer is for example described in the patent specification FR-A-2 279 500;
• b) the linear directional stirrer which most often acts parallel to the flow of the metal; such a brewer is for example described in the German review "Fachberichte Hüttenpraxis Metallverarbeitung", Vol.25, No. 7, 1987, p.676-681 "Electromagnetic Stirring using Voest-Alpine Pulsators on the Donawitz Continuous Bloom Caster". This last type of brewer can act either on a single surface of the cast metal, either on two diametrically opposite surfaces, or even on the four surfaces of the metal. It has also been proposed to use the latter type of linear inductor by imposing pulses thereon intended to generate additional mixing of the metal to be treated.

The patent specification FR-A-2 485 411 and the patent specification US-A-4,867,786 propose the use of linear inductors on two opposite faces of the metal. These inductors are then electrically divided into successive elementary inducing units producing juxtaposed rotary movements, rotating in opposite directions two by two. The process relates essentially to metallic products with an elongated quadrangular cross section.

It has also been proposed to generate in the metal to be treated by means of rotary stirrers a helical movement in order to cause upward inclusions contained in the liquid metal. Such a process has, for example, been described in the patent specification FR-A-2 426 516. This process has however not found application because it prevents correct lubrication of the mold.

In patent specification DE-A-3527387, the use of two rotating magnetic fields is proposed, working with different frequencies and amplitudes. The aim is to create two different coaxial rotary movements which are characterized in that the external movement has a lower speed of rotation than the internal movement. It will be noted that the axis of rotation of the two movements is coincident with the central axis of casting. Like the conventional rotary brewing, this method has, among other things, the disadvantage of creating a movement similar to a vortex in the center of the internal movement, when one wants to intensify the brewing movements.

The patent specification US-4,877,079 proposes to juxtapose in the flow of molten metal two rotary movements of opposite directions of rotation, so as to create, at the interface of the two movements, a flow of material transverse to the central axis of casting. The combined action of these movements results in a better movement of the bath in the center of the metal flow. The main drawback of this process is the orientation of the movement in a predominant direction, which is certainly not optimal from the point of view of homogeneity of the structure of the metal in a cross section.

It can therefore be seen that, while it is true that the various techniques used up to now have contributed to improving the internal and external structure of continuously cast products, the fact remains that the different types of brewing used entail significant new disadvantages. Thus it has for example been found that rotary and linear stirrers, used both in the mold itself, and below the mold, can at a certain time cause in the metal to be treated movements favoring the creation of defects, for example: the formation of inclusions and white lines, or the deterioration of the internal structure at the level of central segregation.

The aim of the present invention is to improve the quality of the internal and external structure of continuously cast metals by proposing a new particularly efficient brewing process.

To achieve this objective, the present invention provides a stirring process, as described in the preamble, according to which the inductor (s) are supplied so as to create in the molten metal at least one zone of primary rotary movement which is offset from the center to the central casting axis, and according to which a cyclic switching of the phases of the polyphase current of so as to impose on said primary rotary movement zone a secondary gyratory movement around the central casting axis.

It has been found that an excellent distribution of movements is obtained in this way in the flow of metal during solidification. Among other things, an exclusive movement around the casting axis is avoided, which produces a vortex movement causing inclusions or powders towards the center. In the same way, a unidirectional movement passing through the central casting axis is avoided, which makes it practically impossible to use submerged nozzles because these would be subject to excessive erosion. It will also be appreciated that the distribution of movements promotes an exchange of materials between the peripheral regions and the central region of the metal flow, without however favoring a particular direction. In this way one obtains, among other things, an excellent homogeneity of the structure in a cross section.

One can create a single primary circumscribed movement offset from the central axis of the metal casting and rotate it around this axis. In an advantageous variant embodiment, two primary movement zones with opposite directions of rotation are juxtaposed, each of these zones preferably extending from the edge of the metal flow to the central casting axis. At their interface the two primary movements are then superimposed to intensify the flow through the central regions. The secondary gyratory movement guarantees that this central flow has no predominant direction.

The polyphase current used is advantageously a three-phase current, respectively a two-phase current, supplying for example an inductor system with six coils, respectively with eight coils of design known per se.

In an advantageous embodiment, advantageously applicable to an ingot mold with a square section, eight coils are arranged symmetrically two by two along a four-phase current along the four sides of the ingot mold.

It will also be appreciated that it is possible to superimpose on said primary rotary movement and / or on said secondary gyratory movement a helical movement along the casting axis, without hampering correct lubrication of the mold.

The method is advantageously applicable to electromagnetic stirring in a cooled ingot mold, with circular, square, rectangular or other section, with or without a submerged central nozzle. However, it can also be applied to stirring in the various zones located below the mold for continuous casting. It should also be noted that the proposed method can be applied to the continuous casting of all known metals, such as steel, aluminum, copper, etc. Those skilled in the art will finally appreciate that the proposed process most often does not require modification of the inductors already installed. It suffices, in fact, to modify or replace the electrical supply, respectively to supplement it with an adequate switching installation.

• la Figure 1 représente schématiquement, dans une coupe transversale à travers le flux de métal en fusion, les mouvements dans le bain, avec un seul mouvement rotatif primaire ;
• la Figure 2 représente, dans une vue identique à la Figure 1, les mouvements dans le bain, avec deux mouvements rotatifs primaires de sens opposés ;
• la Figure 3 montre, pour un premier cycle, un schéma de répartition dans le temps des courants triphasés dans les différentes bobines de la Figure 2 ;
• la Figure 4 montre schématiquement une exécution d'une alimentation triphasée pour un brasseur à six bobines;
• les Figures 5 et 6 montrent schématiquement les mouvements dans le bain dans une lingotière de section carrée munie d'un brasseur à huit bobines.

Other features and advantages of the invention will emerge from the description of some advantageous embodiments, presented below with reference to the appended drawings, in which:

• Figure 1 shows schematically, in a cross section through the flow of molten metal, the movements in the bath, with a single primary rotary movement;
• Figure 2 shows, in a view identical to Figure 1, the movements in the bath, with two primary rotary movements of opposite directions;
• Figure 3 shows, for a first cycle, a distribution diagram over time of the three-phase currents in the different coils of Figure 2;
• Figure 4 schematically shows an execution of a three-phase supply for a six-coil stirrer;
• Figures 5 and 6 schematically show the movements in the bath in an ingot mold of square section provided with a stirrer with eight coils.

Reference 10 in the Figures schematically represents a horizontal section through either an ingot mold, a billet or a bloom below an ingot mold. The reference 10 therefore designates, in general, a metal flow perpendicular to the plane of the figures, the section of which can for example be square, rectangular or circular. Note that the molten metal is being solidified from its periphery.

The reference 12 designates an electromagnetic inductor known per se. This inductor can for example be constituted by an annular cage, in one or more parts, surrounding the metal flow 10. It comprises in Figure 1 for example six coils, numbered from 1 to 6, which can be selectively excited by three-phase current to induce an electromagnetic field in the molten metal. The coils, shown diagrammatically in black rectangles, can be part of the same annular inductor or be divided into several groups of coils belonging to several different inductors. The inductor can be with salient poles or with smooth poles.

Figure 1 schematically illustrates, for a circular section, the stirring movements obtained using a single primary movement zone, identified by the reference 14. This movement zone 14 is, in the case shown in Figure 1, generated by a sliding field produced by exciting coils 1-6-5 with the three phases of a three-phase current. It will be noted that this movement zone is off-center with respect to the central casting axis. According to the proposed method, the movement zone 14 of FIG. 1 is then imposed, as a whole, a gyratory movement around the central axis of the casting, which is symbolized by the arrow indicated by the reference 18.

To make the zone 14 of FIG. 1 gravitate around the central casting axis in the direction of the arrow 18, it suffices to carry out a phase switching so as to carry out the following sequence of cycles of excitation of the coils 1 to 6: (1-6-5), (2-1-6), (3-2-1), (4-3-2), (5-4-3), (6-5-4), (1-6-5), ....

Instead of rotating the movement zone 14 clockwise, it is of course also possible to rotate it in the opposite direction, by cyclically exciting the coils in a sequence opposite to that mentioned above. It can also be interesting to reverse the direction of movement 18 and / or movement 14 from time to time.

Figure 2 shows, for the device of Figure 1, the creation of a second primary rotary movement, identified by the reference. 16. This movement 16 is diametrically juxtaposed with the first primary rotary movement 14, but has a direction of rotation opposite to the latter. It will be noted that the second rotation zone 16 is, in the case shown in FIG. 2, generated by exciting the coils 2-3-4 in three-phase current, while the first rotation zone 14 is, still in the case shown in Figure 2, generated by exciting coils 1-6-5 in three-phase current. As in the case of FIG. 1, a primary gyratory movement is then imposed on the primary rotation zones 14 and 16 around the pouring axis, which is symbolized in FIG. 2 by the arrow indicated by the reference 18.

The graph in Figure 3 schematically represents the distribution of three-phase currents between the coils 1 to 6 during a first cycle. Note that during this first cycle, which generates the movements 14 and 16 shown in Figure 2, the coils 1 and 2 are connected to phase 1, the coils 3 and 6 to phase 2 and the coils 4 and 5 to phase 3. On the abscissa of the graph in Figure 3 we have plotted time. The black and white blocks are representative of the current in the phases over time. This diagram shows very well how it is necessary to feed the coils to create, in the case of Figure 2, the two sliding fields which move from 2 towards 3 towards 4, respectively from 1 towards 6 towards 5, by entraining the liquid metal and thereby generating the offset axes 14 and 16.

To create the gyratory movement 18 around the central axis of casting, one has only to change, in a second cycle, the connection of the phases to the different coils. Thus during the second cycle, the coils 3 and 2 will be connected to phase 1, the coils 4 and 1 to phase 2 and the coils 5 and 6 to phase 3. The zones of rotary movement 14 and 15 will thus be moved by a predetermined angle around the casting axis. This angle depends in particular on the number and configuration of the poles of the stirrer. In the above-mentioned case it is for example an angle of 60 °, so that a complete rotation around the central casting axis requires six cycles similar to that shown in FIG. 3.

The speed of rotation of this gyratory movement around the casting axis is notably a function of this angle and of the duration of the individual cycles. The latter is continuously variable, within the limits imposed by the electrical installation. In practice, the secondary rotation speed will for example be chosen as a function of the location of the inductor (s) in the installation of casting. Normally, the secondary rotation speed is between 5 and 200 revolutions per minute.

The frequency of the polyphase supply current will also be chosen according to the location of the inductor (s). Indeed, the inductor or inductors located at the level of the ingot mold will work mainly in the low frequency domain (2 to 15 Hz), likely to pass through the walls of the copper ingot mold, while the inductors located below the ingot mold may work at higher frequencies, for example between 15 and 70 Hz.

It will be noted that it is also possible to superimpose on the secondary gyratory movement and on the primary rotary movement (s), characteristics of the proposed process, a helical movement along the pouring axis. For this purpose, for example, in a manner known per se, the coil will be given an asymmetry inducing in the metal such a helical movement, or, in a manner known per se, one will shift in relation to the other, in the direction the various pole pieces of the inductor (s). It will be appreciated that the lubrication of the mold is significantly improved compared to the poor lubrication of the mold obtained during the application of a helical movement to conventional rotary stirring.

The coils are normally supplied with three-phase current, but it is not excluded to supply them with two-phase current. The use of polyphase currents with more than 3 phases is also possible.

There are many possibilities for supplying and controlling the different types of brewers and coils. FIG. 4 represents, by way of nonlimiting example, a three-phase supply, of the 6 coils of a brewer as shown in FIGS. 1 and 2. A three-phase supply network, represented schematically by the reference 22 , supplies a circuit of converters current 24, known per se. The latter makes it possible to create at its output a system of three-phase currents of adjustable frequency and amplitude. An electronic switch 26 makes it possible to switch the 3 phases of the three-phase current system thus formed, according to a predefined sequence, between the 6 coils of the inductor.

Figures 5 and 6 schematically illustrate, by way of example, a particular embodiment of the proposed electromagnetic stirring process, applied to an ingot mold 40 with square section. This mold is for example water-cooled and comprises a submerged nozzle 42. Along each of the four sides of the mold 40 are provided two coils belonging to one or more inductors. The supply is made with two-phase current, so as to obtain sliding magnetic fields generating zones of primary movement 44₁, 44₂ which are offset in the square section of the mold 40.

FIG. 5 represents the movements in a cross section in the direction of casting, during a first excitation cycle of the eight coils. It will be noted that the primary movement zones 44₁ and 44₂ are symmetrical with respect to a median plane. The first phase supplies the coils 1 and 3, respectively 8 and 6. The second phase supplies the coils 2 and 4, respectively 7 and 5.

FIG. 6 represents, in a similar view, the movements during the following cycle. It will be noted that the primary movement zones 44'₁ and 44'₂ are symmetrical with respect to a diagonal plane. The first phase supplies the coils 2 and 4, respectively 1 and 7. The second phase supplies the coils 3 and 5, respectively 8 and 6.

The arrows identified by the reference 46 in FIGS. 5 and 6 indicate the direction of the gyratory movement of the primary movement zones 44₁ and 44₂ around the submerged nozzle 42. Those skilled in the art will appreciate that an excellent result is thus obtained. stirring, without generating around the nozzle 42 a movement similar to a vortex, which risks entraining the slag cover, and without subjecting the submerged nozzle 42 to rapid erosion.

The invention has been described by way of illustration with reference to a vertical casting. It can however be applied with the same advantages, to an oblique or even horizontal casting.

It can be seen from the above that the proposed brewing process makes it possible to carry out, in a particularly simple and efficient manner, all the movements required by modern continuous casting, without presenting certain drawbacks of the brewing processes known from the prior art. .

Claims (8)

Process for stirring molten metal for continuous casting, according to which a mobile electromagnetic induction field is induced in a metal flow, defining a central casting axis, by means of a system of inductors arranged around the flow of metal and supplied with polyphase current, said field generating in the molten metal at least one movement transverse to the flow of metal, characterized in that the coils of the inductor system are supplied so as to create in the molten metal (10) at least one primary rotary movement zone (14) which is offset from the central casting axis and in that a cyclic switching of the phases of the polyphase current is carried out so as to impose this primary rotary movement zone ( 14) a secondary gyratory movement (18) around the central casting axis. Method according to claim 1, characterized in that two zones of rotary movement (14, 16) are created, which are juxtaposed and have opposite directions of rotation and each of which extends from the periphery of the metal flow up to to the central axis of casting. Method according to claim 1 or 2, characterized in that the polyphase current is a three-phase current. Method according to claim 3, characterized in that the inductor system comprises six coils arranged symmetrically around the metal flow. Method according to claim 1 or 2, characterized in that the polyphase current is a two-phase current. A method according to claim 5 characterized in that the inductor system comprises eight coils arranged symmetrically around the metal flow. Method according to claim 6 applied to a metal flow with a substantially square section, characterized in that the eight coils are arranged in pairs symmetrically along the four sides of the metal flow. A method according to claim 1, characterized in that one superimposes on said primary rotary movement (14, 16) and on said secondary rotary movement (18) a helical movement along the casting axis.

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