EP0097561B1 - Process and device for the electromagnetic stirring of continuously cast slabs, especially of steel - Google Patents

Abstract

In a continuous-casting method molten steel is continuously introduced into a continuous-casting mold to form therein a strand having a free surface in the mold, a pair of relatively wide faces, and a pair of relatively narrow faces. The mold and the steel therein are continuously cooled to externally solidify the molten-steel strand while leaving same internally molten and the externally solid and internally molten strand is continuously withdrawn from the lower end of the mold. The core of the strand solidifies increasingly as it moves from the mold and terminates downstream of the mold at a pool bottom. At each of a plurality of locations spaced apart about 1 m to 2 m longitudinally along the strand between the mold and the pool bottom a respective magnetic field is formed with the fields passing through the strand from between about 3 m to 7 m beneath the free surface to about 2 m to 6 m from the pool bottom. These fields are displaced transversely of and generally parallel to the side faces of the strand with each field moving opposite to the adjacent field or fields so as to magnetically transversely and oppositely displace respective portions of the molten core of the strand.

Description

The present invention relates to the electromagnetic stirring of metallic slabs, in particular of steel, continuously cast. It relates more precisely to the electromagnetic stirring operations of the molten metal in the secondary cooling zone of a continuous slab casting machine.

The electromagnetic stirring operations, which are discussed here, consist, as is known, of subjecting the cast product to one or more mobile magnetic fields, sliding in a determined direction, and whose action on the liquid metal is then manifested by a drive of the latter identical, in direction and direction, to the displacement of the magnetic field.

In the case of products of elongated section, such as slabs, continuously cast, it is known to entrain the liquid metal in the manner indicated above in a movement of horizontal translation, parallel to the large faces of the product.

The mobile magnetic field is generally created by a polyphase static inductor preferably placed in the immediate vicinity of the cast product and which can have different designs: for example a monobloc inductor, similar to a linear induction motor stator, placed either behind the holding rollers and guiding the slab during casting, either in substitution for one or more of these rollers (French patent n ° 2,068,803, German patent n ° 2,401,145), or even in spaces made available between two rollers consecutive (French patent n ° 2187 468). It has also been proposed an inductor of cylindrical structure introduced inside the same of one or more rollers, made tubular for this purpose (English patent n ° 1405312).

The advantage of controlled stirring of the liquid metal during casting, an interest which has now been known for a long time, lies in the systematic improvement of the internal quality of the stirred product compared to the non-stirred product. This improved quality, which is characterized in particular by a reduction in central porosity, as well as by a significant reduction in axial macro-segregation, results from the favorable influence of stirring on the solidification structure. The latter in fact reflects, in the case of brewed products, an early interruption of peripheral crystal growth of the “basaltic” type (dendritic growth) in favor of the formation and development of a central zone with non-oriented solidification structure, called "equiaxial" type, correspondingly more extensive.

However, although the cause and effect relationship between a large equiaxial zone and a weak axial segregation is now indisputable, the numerous metallographic observations made by the inventors show that axial segregation can still remain relatively large despite a well developed equiaxial zone.

The question which then arises, and to which the present invention aims to answer, consists in knowing whether there is an optimal type of stirring making it possible to obtain, together with a very wide central equiaxial zone, a level of the lowest possible macro-segregation, and in any case significantly more reduced than that obtained by the usual technique of electromagnetic stirring.

Another aim is to achieve the above-mentioned result with a minimum of mixing inducers.

With these objectives in view, the invention relates to a method of electromagnetic stirring of metallic slabs, in particular of steel, continuously cast, according to which, in the part of the liquid well located downstream of the mold in the direction of extraction. of the slab, the latter is subjected to at least one mobile magnetic field, sliding along the width of the slab and creating a driving movement of the liquid metal, process characterized in that, in order to stir the liquid metal on the portion of the product poured between 3 to 4 meters approximately below the free surface of the metal in the mold and approximately 2 to 3 meters from the bottom of the solidification well, a plurality of sliding magnetic fields produced by electromagnetic inductors are made to act. has a staggered arrangement on said portion, spacing them apart, in that the inductor closest to the ingot mold is located approximately 5 to 7 m below the free surface of the liquid metal and the inductor closest to the bottom is located of the solidification well about 4 to 6 m from this bottom, and in that the inductors are adjusted so that the magnetic field created by any inductor slides in a direction opposite to that of the magnetic fields created by the ind nearest neighbors.

In accordance with a particular implementation, using a minimum of electromagnetic inductors, these are arranged in a staggered configuration on either side of the two large faces of the slab.

According to a preferred variant, the electromagnetic inductor closest to the ingot mold is placed on the upper surface of the slab, that is to say facing the large face placed opposite the center of curvature of the machine. continuous casting.

As will no doubt have already been understood, the invention consists, in its fundamental features, in distributing the electromagnetic stirring energy transmitted to the cast metal over the major part of the metallurgical height so as to create convective movements which s '' establish in almost all of the solidification well.

• • d'une part, il est inutile de faire agir le champ magnétique au voisinage de l'extrémité de fermeture du puits de solidification, car en cet endroit le métal est déjà suffisamment pris en masse pour que l'on ne puisse y créer des mouvements de convection et ceci même avec des puissances électromagnétiques très importantes
  • d'un autre côté, il n'est pas souhaitable de brasser trop haut sur la hauteur métallurgique, c'est-à-dire au voisinage immédiat de la lingotière, car le jet d'alimentation du métal liquide en lingotière crée naturellement des mouvements de convection favorables qui s'étendent dans le puits liquide jusqu'à une distance égale à 2 ou 3 fois environ la hauteur de la lingotière et qu'il n'est pas opportun de perturber.

However, it is not necessary to stir over the entire height of the liquid well for the following reasons:

• • on the one hand, it is useless to make the magnetic field act in the vicinity of the closing end of the solidification well, because in this place the metal is already sufficiently solidified so that one cannot create movements therein even with very high electromagnetic powers
  • on the other hand, it is not desirable to stir too high over the metallurgical height, that is to say in the immediate vicinity of the ingot mold, because the jet of liquid metal feed into the ingot mold naturally creates movements favorable convection which extend into the liquid well up to a distance equal to 2 or 3 times approximately the height of the mold and which it is not advisable to disturb.

Under these conditions, it is understood that the portion of the metallurgical height to be subjected to electromagnetic stirring in accordance with the invention, lies between an upper limit, approximately 3 to 4 meters below the free surface of the metal in the mold, and a lower limit which can be situated approximately 2 or 3 meters from the closing point of the solidification well.

To now determine the location of the inductors ensuring such stirring, it must also be taken into account that the direct action of driving the magnetic field at any level of the metallurgical height induces recirculation movements of the liquid metal ( indirect drive) ensuring the looping of the current lines, and which flourish on either side of the direct drive area up to a distance of approximately 2 to 3 meters from the latter.

In view of these details, the action of the magnetic field closest to the ingot mold is located approximately 5 to 7 meters below the free surface of the liquid metal, and the action of the last magnetic field is located near the bottom of the well. solidification, at a distance of approximately 4 to 5 meters above said bottom.

Of course, the average distance separating a direct drive zone from a recirculation zone depends primarily on the electromagnetic force to which the liquid metal is subjected, that is to say essentially on the intensity of the magnetic field. acting on the metal, since the sliding speed of the field (ie frequency of the electric current in the inductor) is necessarily low, around 1 to 5 Hz approximately, to limit the weakening of the field between the active surface of the inductor and liquid metal.

It can be said, however, that, taking into account the technology currently available, electromagnetic inductors compatible with a continuous slab casting installation under normal operating conditions make it possible to deliver magnetic fields which are strong enough for the difference between ta- direct entrainment zone for liquid metal and the recirculation zone is around 2 meters or even beyond.

It may be useful to specify that it is easy to detect the levels on the metallurgical height where the recirculation zones are located. These appear in fact on metallographic observation, on a cross-section in a straight section of the solidified bar, in the form of crowns lighter than the rest of the metallic matrix (also called negative segregation zones or white zone ”), and whose the clarity is appreciably more blurred than that of the crowns of main negative segregation, which are formed, at the level of the direct entrainment of the sliding magnetic field. The depth at which these different negative segregation zones are located in the product relative to its surface depends on the local operating conditions of the casting machine and in particular on the initial overheating of the metal feeding the ingot mold, the extraction speed slab and speed of solidification of the product, that is to say the adjustment of the cooling system. Knowledge of these various parameters therefore makes it possible to easily link the depth of localization of negative segregation zones to the levels on the metallurgical height where the direct circulation and recirculation movements of liquid metal take place under the action of magnetic fields.

It should be emphasized that these same parameters make it possible to approach fairly finely, in each case, the upper and lower limit levels defining the portion of the metallurgical height subjected to stirring in accordance with the invention.

As an indication, it may be noted that with regard, for example, to the speed of extraction of the slabs, this can range from approximately 0.7 meters / minute to more than 3 meters / minute, that is to say - say vary in a ratio of 1 to 5 between different installations, or flowing different steel grades.

We will now describe, by way of illustration, a characteristic configuration of the method according to the invention using a minimum number of stirring inductors and suitable for the continuous casting of slabs at low extraction speed (0.7 meters / minute) and whose liquid well has a metallurgical height reduced to around 12 meters.

• la figure 1 montre une brame coulée en continu selon une vue en coupe longitudinale médiane parallèle aux grandes faces de la brame,
• la figure 2 est une vue analogue à celle de la figure 1 mais selon une coupe parallèle aux petites faces latérales de la brame,
• la figure 3 est une empreinte Baumann de la partie centrale d'une section droite de la brame solidifiée.

The description of this example will be made with reference to the plates of attached figures in which:

• FIG. 1 shows a slab continuously cast in a view in median longitudinal section parallel to the large faces of the slab,
• FIG. 2 is a view similar to that of FIG. 1 but in a section parallel to the small lateral faces of the slab,
• Figure 3 is a Baumann footprint of the central part of a cross section of the solidified slab.

There is shown diagrammatically in FIGS. 1 and 2, a mold 1, a nozzle 2 supplying the mold with liquid metal, the slab 3 during casting and having a solidified outer layer 4 and a core in the liquid state 5. The line plotted in 6 defines the closure of the solidification well by junction of increasing solidification fronts on the large faces of the product. The metallurgical height “H •, that is to say the distance separating the surface of the liquid metal 7 in the mold from the closure 6 of the solidification well, can be read directly in meters in FIG. 1 thanks to the marks placed on the small left side of the slab. In the figures, the regions of direct action of the sliding magnetic fields have been represented by the two hatched zones 9 and 10. These zones, as already said, define the regions for direct entrainment of the liquid metal whose lines of current have been represented by the loops 13 in thick lines in FIG. 1. The directions of sliding of the magnetic fields acting along the width of the slab are indicated by arrows on the left of the direct drive zones 9 and 10 of the Figure 1 and by the conventional symbols in fig. 2.

The implementation of the invention poses no particular problem and one can advantageously use, as shown very schematically in Figure 2, electromagnetic inductors with sliding field of cylindrical structure, placed inside the support rollers and guide the slab casting made tubular for this purpose. The assembly thus constituted by the roller and the internal inductor is a functional assembly, delivered ready for use, and usually designated in the technical field considered by the expression "roller-brewer, .. These rollers-brewers, do not not forming part of the proper object of the invention, will not be described here in detail. If desired, a detailed description of their design and technology can be found by referring to English Patent Application No. 1,405,312-IRSID.

In order not to overload the figures unnecessarily, these brewing rollers have not been shown in FIG. 1. In FIG. 2, only the brewing rollers 11, 11 'and 12, 12' have been illustrated, excluding all the other ordinary rollers which normally mark out in tight rows the large faces of the slab.

The minimum configuration representative of the distribution of the action of the magnetic field over the metallurgical height, is characterized here, as can be seen, by the installation of a first pair of brewer rollers 11, 11 'on the upper surface of the slab, downstream of the ingot mold, at an average distance of 6 meters from the free surface 7 of the cast metal, and a second pair of rollers 12, 12 'offset downward relative to the pair 11, 11' d 'an average distance of 1.50 meters. Furthermore, the direction of sliding of the magnetic field created by the pair 11, 11 'is opposite to that created by the pair 12, 12'.

Under these conditions, the electromagnetic stirring caused by the sliding fields acting on the two levels 9 and 10 offset in height creates within the liquid metal a movement of forced convection in the form of triple "0" or, if one prefers, " butterfly wings •, which develops over the major portion of the metallurgical height, that is to say over the portion between the upper limit level at about 3.5 meters and the lower limit level close to the dimension 10 meters. More precisely, this movement in “butterfly wings” comprises, as can be seen, a central body 13 with relatively intense circulation because it is generated by the combined effect of the two direct drive zones 9 and 10 and, on both sides. other of this central body 13, recirculation regions 14, 15, which flourish respectively upwards and downwards up to the levels, on the metallurgical height, of 3.5 meters and 10 meters approximately.

The metallographic analyzes carried out show that the slabs poured and stirred continuously in the manner which has just been described have a very large equiaxed solidification zone which is already initiated at a skin depth corresponding to the level, over the metallurgical height, about 3.5 meters. In addition, these analyzes also show that the core of the slabs is practically free of macrosegregation phenomena. These results can be seen directly in FIG. 3 where the axis of the slab is shown at 16 (which moreover merges with the casting axis) and where the width has been designated at 17 equiaxed solidification zone bordered on both sides by a fringe 18 of oriented basaltic solidification, which is rather difficult to discern on this reproduction of a photographic print. However, we can clearly see on the latter, within the equiaxial phase 17 the two concentric light rings 19 and 20 at close distance from each other and characterizing the zones of negative segregation formed by the mixing action in the regions direct drive 9 and 10. We also distinguish, around and at a distance from these dark circles, another crown (21) of negative segregation, of much more attenuated contrast, and reflecting at this location the presence of the upper recirculation region referenced 14 in FIG. 1. It should be noted that the negative segregation ring corresponding to the low recirculation region 15 is not observable on the metallographic section, since we are located at this level in a region where the proportion of solid in the liquid is very important and therefore forms a rigid skeleton and where, consequently, the sweeping of the solidification front by the forced convection movements of the liquid metal, responsible for the formation negative segregation zone, no longer operates in this respect significantly.

It goes without saying that the invention cannot be limited to the example described and extends to numerous variants and equivalents insofar as the characteristics set out in the appended claims are respected.

This is particularly the case with the number of sliding magnetic fields, that is to say the number of direct drive zones which are stepped over the metallurgical height, provided however that the direction of sliding of the fields is reversed between two consecutive direct training zones.

Likewise the staggered arrangement of mixing inductors defining the direct drive zones is justified if the number of available inductors is limited. If not, it is entirely possible to couple the inductors one next to the other at the level of the direct drive zones, the important thing in this case being of course that the magnetic fields created by the inductors paired at the level of a specific direct training area slides in the same direction.

Likewise, the fact that the direct drive zones 9 and 10 are each created by a pairing of two electromagnetic inductors 11, 11 ′ and 12, 12 ′, cannot in any way constitute a limitation of the present invention. These provisions can only be explained by the desire to work during tests with electromagnetic powers of the order of 250 KVA for each direct drive zone, while the nominal power of the available inductors was 125 KVA.

Under conditions, it is considered, for the understanding of the invention, that the inductor units paired on the same face of the slab such as 11, 11 ′ to 12, 12 ′ and / or paired at the same level on the two faces opposite, constitute one and the same inductor, because they are intended to produce one and the same direct entrainment zone of the liquid metal. In particular, the direction of sliding of the magnetic field is uniform for these inducing units.

Claims (4)

1. A process for electromagnetically stirring continuously cast metal slabs, in particular of steel, according to which, in the part of the solidification pool located downstream of the ingot mould, in the direction of extraction of the slab, the latter is subjected to at least one mobile magnetic field, sliding according to the width of the slab and creating a driving movement of the liquid metal, characterized in that, in order to stir the liquid metal on the portion of the cast product located between approximately 3 to 4 m below the free surface (7) of the metal in the ingot mould and approximately 2 to 3 m from the bottom (6) of the solidification pool, a plurality of sliding magnetic fields are activated, these fields being produced by electromagnetic inductors (11, 11', 12, 12') which are disposed in stepped fashion spaced apart on said portion, in that the inductor closest to the ingot mould is located approximately 5 to 7 m below the free surface (7) of the liquid metal, and the inductor closest to the bottom (6) of the solidification pool is located approximately 4 to 6 m from said bottom, and in that the inductors are regulated in such a way that the magnetic field created by any inductor slides in an opposite direction to that one of the magnetic fields created by the closest adjacent inductors.

2. A process according to claim 1, characterized in that the inductors are disposed according to a staggered configuration on each side of the two large surfaces of the slab.

3. A process according to claim 2, characterized in that the inductor closest to the ingot mould is disposed on the extrados of the slab.

4. A process according to any of the above claims, characterized in that polyphase, cylindrical electromagnetic inductors placed longitudinally within support and guide rolls for the slab, made tubular for this purpose, are used, as are known for stirring liquid metal.

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