FR2856321A1 - CONTINUOUS CASTING INSTALLATION FOR ELECTRO-MAGNETIC ROTATION OF LIQUID METAL IN TRANSIT IN THE CASTING BUSH - Google Patents

Abstract

The continuous casting plant for metals, in particular steel, in which the immersed nozzle (8) is surrounded by an annular electromagnetic inductor (1) with a magnetic field rotating around the casting axis intended to drive in rotation axial with it the molten metal, is characterized in that this inductor is polyphase type through magnetic field, provided with a pair of poles (3) per phase, each pole having at its end opposite the nozzle a shrinkage lateral (12) which increases the distance separating the polar faces (4) between them. Thus, it is possible to realize a highly compact inductor delivering a high magnetic field of several thousand gauss to the central part of the nozzle and with a high frequency primary current so as to obtain efficient rotation of the liquid metal in transit through the nozzle.The invention is advantageously applied to the casting Continuous slabs using a submerged nozzle with gills lateral outlets.

Description

i

  Continuous casting apparatus for electromagnetic rotation of the liquid metal in transit through the casting nozzle.

  The present invention relates to the continuous casting of metals, especially steel, employing a submerged casting nozzle which is immersed in an ingot mold placed underneath. More specifically, the invention relates to the axial rotation of the liquid metal in transit in such a nozzle between the tundish and the mold.

  It is known that the axial rotation of the metal already within the casting nozzle is a recognized means for controlling the flows in the mold by modifying the distribution of the gas bubbles and inclusions present in the liquid metal before it arrives in the mold. . It is thus possible to: reduce or even eliminate the inclusion deposits along the inner wall of the nozzle and, in the case of a nozzle with lateral outlet openings for slab casting, at its gills and its bottom bowl; - greatly reduce the penetration depth of the gas bubbles and inclusions in the liquid well of the product being cast, thus also the risk of their entrapment on the intrados face of the products cast on a bent machine - reduce the speed of circulation of the liquid metal under the meniscus as well as the level fluctuations thereof; - Limit the flow instabilities, like jets swings in the mold by generating a "gyroscopic" effect on the flows in the nozzle.

  The rotation of the flows in the pouring nozzle thus appears as an effective means of combating the appearance of surface appearance defects, such as blistering and exfoliations, on cold-rolled sheets of steel grades. automotive application and steels for packaging. This technique therefore makes it possible to reduce the writing operations on continuously cast slabs (reduction or even elimination of surface defects on exfoliation-type sheets), the elimination of downgrades and disputes for blistered defects, as well as the increase in productivity of the machines by the lengthening of the sequences and the increase of the speeds of casting.

  The rotating of the liquid metal in the pouring nozzle has already been proposed using different types of actuators. Two types of actuators can be schematically distinguished: "passive" actuators and "active" ones.

  The "passive" actuators are, among other things, the design modifications of the inner wall of the nozzle (for example: spirals), the members such as the helix, the internal helical nozzle, etc. which are implanted in the body of the nozzle itself. or the modifications of the upper part of the nozzle at the junction with the distributor (for example: acceleration cone) or else the modifications of the actual body for regulating the metal flow rate in the nozzle. The major drawbacks of this type of actuators are to generate a rotation speed directly dependent on the metal flow passing through the nozzle and to constitute preferred sites of deposits of inclusions in the nozzle, hence a potential increase in the risks of clogging. .

  The "active" actuators are essentially of electromagnetic nature: a static annular electromagnetic inductor surrounds the nozzle at a small distance over part of its length and generates a magnetic field rotating around the casting axis intended to drive in axial rotation with it the liquid metal present in the nozzle.

  However, all the electromagnetic devices heretofore proposed are based on tangential rotating field linear stator technology operating at low or very low frequency (<10 Hz). These devices have the disadvantages of: generating rotational speeds that are often too low, given the current frequencies used, to obtain the desired effects (for example, at 4 Hz three-phase 15 usable for an internal nozzle diameter of 80 mm) , the maximum theoretical rotation speed is 80 rpm), - generating in the liquid metal a highly concentrated force field near the inner wall of the nozzle, which has the effect of creating a zone of strong depression in the central portion of the nozzle where the metal is then accelerated in the downward vertical direction; - must operate with high current (> 300-500 A), which leads to large devices in order to ensure their cooling, so not easy to implement on a continuous casting machine and more require the use of very expensive electric generator.

  The object of the present invention is to propose a solution for an electromagnetic rotation of the liquid metal within a casting nozzle which does not have the drawbacks of known solutions.

  To this end, the subject of the invention is a continuous casting plant for metals, in particular steel, in which the immersed nozzle through which the molten metal to be cast arrives in the mold from a casting distributor located in the above is surrounded by a static annular magnetic electromagnetic field magnet rotating around the casting axis for driving in axial rotation with it the molten metal, characterized in that said inductor is a polyphase inductor having a magnetic field through provided a pair of poles per phase.

  According to a preferred embodiment, each pole of the inductor is formed by an electric winding wound around an inwardly projecting pole tooth ending in a polar face disposed facing the nozzle, the polar teeth being interconnected. by an outer magnetic peripheral yoke closing the magnetic flux, each tooth having a lateral narrowing (a bevel for example) at the end of its protruding portion, which increases the distance separating the polar faces therebetween.

  According to an advantageous variant, the annular inductor is formed in two pivoting articulated half-cups that can close around the nozzle.

  As will probably be understood, the idea underlying the invention is the implementation of a magnetic field said "through", ie in this case passing through the axis of the nozzle without noticeable weakening of its intensity between the edge and the center of it.

  Thanks to the inductor technology adopted by the invention, namely that to a pair of poles per phase of the power supply supplying a polyphase annular inductor with wound salient poles distributed around the nozzle, the rotating magnetic field produced is of the type "crossing" sought. In other words, at each moment, the casting axis is at the center of the air gap of the inductor and the field produces prosperous in this gap through the casting axis for, from a given magnetic pole, 15 join the counterpart opposite magnetic pole located opposite and not beside it as would be the case with an inductor with distributed poles or with several pairs of poles per phase.

  As we know, this type of technology is not new in itself. It is even quite widely used for rotating the poured liquid metal, not in a nozzle, but in the mold itself, so in the case of induced rotation (the liquid metal column). ) of much larger apparent diameter than that of the metal jet in the nozzle and with a correlatively lower rotational angular speed requirement. However, contrary to popular belief, it turns out that the transfer of this technology from the mold to the casting nozzle can be accompanied by a reduction in size of the inductor such that it becomes compatible with the assembly the inductor around and as close to an installed nozzle provided that one preserves the character "through", in any case essentially "through" the product field, and this without affecting then its necessary cooling.

  And it is not the least merit of the authors of the invention to have discovered that one could, without penalizing the performance of the inductor, in terms of available power in particular, preserve this character "crossing" of the field despite the compactness of the inductor by agreeing to a minimal magnetic mass loss located at selected locations of the salient poles, namely the edges of the active faces, to counter-carrage the natural tendency of the field to propagate in the gap according to 35 the least reluctant paths by looping between neighboring poles close to each other.

  Tests performed on steel have confirmed the ability of such an inductor to rotate the metal flowing in a submerged nozzle under casting conditions much more severe than those encountered in industrial machines 40 blooms or slabs . These tests were carried out with a nozzle of the straight type (single axial opening opening in the bottom) in which the metal flowed at an average speed of the order of 3.5 to 4.2 m / s, knowing that in a slab casting nozzle, the average flow rates are rather between 1.5 and 2.0 m / s.

  The invention will in any case be well understood and other aspects and advantages will become apparent from the following description given by way of an exemplary embodiment and with reference to the attached drawing plates in which: FIG. diagram showing, seen in cross section, the inductor in two butted half-shells provided with its internal heat shield bordering the gap; FIG. 2 is a diagram similar to the previous one but intended to show the propagation of the lines of force of the magnetic field passing through the air gap as frozen at any given moment in the operation of the inductor; FIG. 3 is a block diagram showing the articulation of the two half-shells constituting the inductor; FIG. 4 shows the map of the velocities of the liquid metal rotating within the casting nozzle 15 under the effect of the magnetic field in a cross sectional plane of the nozzle; FIG. 5 shows the evolution of the intensity B of the magnetic field in the air gap along a diameter D of the nozzle taken in a plane situated at half height of the inductor; FIG. 6 shows, in correspondence with the representation of FIG. 5, the correlative evolution of the field of magnetic forces FB along a diameter D of the nozzle according to a radial profile R and according to a orthoradial profile OR.

  In the figures, the same elements are designated by identical references.

  As can be seen with reference to FIGS. 1 to 3 together, the inductor 25 1 is a linear motor stator closed on itself, constituted for this purpose by two equal independent hemi-tubular parts 2a and 2b, (the half -Shells). Each half-shell comprises three coiled protruding poles 3, the polar face 4 of which faces inwards, these magnetic poles, made of assembled stacked soft iron plates, being conventionally interconnected by an outer peripheral hemi-tubular cylinder head 5a, 30 5b. The assembly is sized so that the two paired heads come to abut in the junction plane J when the inductor is in the closed working position shown in Figures 1 and 2.

  A cap 7a, 7b, also of corresponding hemi-tubular shape internally capes the polar faces of each half-shell and forms, once the inductor 35 in the closed position, a thermal protection screen 7 which surrounds at a short distance the nozzle of casting. This thermal protection is desirable for the electric windings 3 of the inductor with respect to the radiation emitted by the pouring nozzle 8 shown in FIG. 3 and channeling the flow of molten metal towards the mold. Details on the possible constitution of this screen will be given later.

  The electrical winding 6 of each coiled pole 3 is connected to a phase of a three-phase power supply (not shown) intended to supply the primary current of the inductor. With the inductor in the closed position, any salient pole of one of the half-shells 2a is diametrically opposite a salient pole of the other half-shell 2b. These two poles form a "pair of poles" in the sense that they are both connected to the same phase of the power supply, but in opposition (for example via a different winding direction) so that, at every moment, their active faces are of opposite signs. This condition is necessary for the magnetic field produced to be of the through type.

  The poles 3 and the magnetic flux return yoke 5a, Sb are laminated into grain oriented Fe-Si plates having an initial thickness of 0.3 mm so as to minimize the hysteresis losses. Their operating height (height of the active face 4) is between 50 (minimum value) and 500 mm, depending on the space available between the distributor and the top of the mold between which the inductor will take place. Their internal diameter (diameter of the air gap) is of the order of the outer diameter of the pouring nozzle increased by about ten mm to preserve a separation but in order to ensure the best possible inductive coupling.

  The primary windings 6 consist of a large number (several hundred) of very small diameter copper wire turns supporting high current densities (> 10 A / mm 2). They are provided within them water-cooled copper heat extractors (not shown).

  These coils are supplied with three-phase currents at medium frequency ranging from 50 Hz to 600 Hz. In the proposed technology, it will be noted that operating at a high frequency, greater than 50 or 60 Hz, makes it possible, at constant current intensity, to increase the driving torque that the electromagnetic forces exert on the metal flowing in the nozzle. However, this option requires the use of a frequency converter, unlike the operation at the mains frequency (50 or 60 Hz).

  As shown in the diagram of FIG. 5, this static motor constituting the inductor 1 can generate in its gap occupied by the nozzle a transverse electromagnetic field (said through) of high intensity (between 1000 and 1500 gauss) for values weak inductive currents (a few tens of amperes).

  This field, as seen in the diagram, is almost uniform in the central part of the gap. This essential characteristic of the invention makes it possible to generate in the liquid metal a uniformly decreasing field of forces from the wall 35 to the center, as shown in the diagram of FIG. 6. This allows, as is also clearly shown by the FIG. speed of Figure 4, to rotate the liquid metal with a speed that remains high even in the axial portion of the nozzle. This specificity is necessary to avoid a too strong depression in the central part of the nozzle where the metal would then tend to "leak" and undergo a strong downward vertical acceleration, thus canceling part of the beneficial effect of rotating .

  As clearly shown in FIG. 2, it is thanks to the shrunken shape of the radial magnetic teeth 3 at their free end 4 (the polar faces) that, at any given moment, the lines of force of the magnetic field in the gap essentially connect two diametrically opposed poles and that only a residual part of the field loops between neighboring poles. This result, essential to the implementation of the invention, is obtained, despite the necessary compactness of the inductor, thanks to this shrunken shape of the end of the poles, which makes that despite their mutual rapprochement as As we move towards the center, the distance separating their free ends two to two remains sufficient to avoid significant bridging of the field lines between them. This is what, in the case of a small compact inductor, is a guarantee of the high relative intensity of the magnetic field in the axis (see Fig. 5), otherwise the imperatively "traversing" nature of this field without which the invention does not produce the desired effects. As can be seen in FIG. 1 and more clearly in FIG. 2, this shape shrinkage of the radial teeth 3 is obtained thanks to a bevel precut 12 of the ends of the sheets to be stacked to form them.

  The angle of the bevel is to be adjusted according to the outer diameter of the nozzle to be surrounded.

  It should be noted, however, that the polar face 4 should not be less than half of the cross-section of the tooth 3 at the surface and that the beginning of the constriction bevel 12 on the body of the tooth can be initiated. two-thirds of the length. It is not necessary to start before and it is even desirable to do it as late as possible to maximize the magnetic mass of the nducer.

  By supplying the inductor with a resonant circuit, the intensity of the primary currents can be greatly increased. The proposed technique makes it possible, in a wide range of intensity of the primary currents, to increase very strongly the intensity of the electromagnetic field in the gap, by increasing the intensity of these currents to values well beyond the Threshold intensity corresponding to the magnetic saturation of the cylinder head 5. This enables the magnetic field lines to be channeled and to increase, in the air gap of the motor, the intensity of this magnetic field until last reaches its saturation value in the cylinder head. Beyond this threshold value, it is the magnetic field generated by the inductor directly in the air which contributes to the increase of the intensity of the field in the air gap of the motor.

  In operation, the inductor is very close (at about 5 mm distance) from the casting nozzle 8, the external temperature of which is of the order of 1100 to 1200 C. Its thermal protection, vis-à-vis the radiation emitted by the nozzle, is then provided by the segmented copper screen 7, thin, cooled by water circulation and transparent to the electromagnetic field through this segmentation.

  The constitution of the inductor 1 into two independent hemi-tubular parts 5a and 5b makes it easy to place it around the nozzle and withdraw it at any time without any modification of the standard casting process. Referring again to FIG. 3, it can be seen that, in order to be put in place around the casting nozzle 8, the inductor is advantageously held by a support consisting of two arms 9 hinged about a pivot axis 10 The arms are driven by jacks 11 which ensure their closing-opening and make it possible to exert a sufficient contact force (greater than 200 kgf) between the yokes 5a and 5b of the two hemi-tubular parts 2a and 2b once those As shown in FIG. 1, on the one hand, a close contact between the yokes 5a and 5b is necessary for a good looping of the magnetic field lines between the two component parts of the inductor and therefore for a good one. electromagnetic efficiency. On the other hand, a large force of closing two hemi-tubes is necessary to prevent the vibrations that would inevitably be generated by the oscillating electromagnetic forces.

  It goes without saying that the invention can not be limited to the embodiment described but that it extends to multiple variants and equivalents to the extent that is respected its definition given by the appended claims.

Claims (7)

  1) Continuous casting plant for metals, in particular steel, in which the submerged nozzle (8) through which the molten metal to be poured arrives in the mold from a distributor located above is surrounded by an annular electromagnetic inductor magnetic field rotating around the casting axis for driving in axial rotation with it the molten metal, characterized in that said inductor (1) is a polyphase inductor with a magnetic field 10 provided with a pair of poles (3) by phase.   2) installation for continuous casting of metals according to claim 1 characterized in that each pole of the inductor is formed by a wound electrical winding (6) around a polar tooth (3) protruding inwards ending in a Polar face (4) arranged facing the nozzle (8), the pole teeth being connected to each other by an outer magnetic peripheral yoke (5a, 5b) closing the magnetic flux, each tooth having, at the end of its protruding part, a lateral narrowing (12) which increases the distance separating the polar faces between them.   3) A continuous casting installation according to claim 1 characterized in that the submerged nozzle (8) is a nozzle with side outlet openings.   4) A continuous casting installation according to claim 2 characterized in that the inductor comprises at its inner periphery a thermal protection screen (7) surrounding the remote nozzle.   5) A continuous casting installation according to claim 2 characterized in that the annular inductor is formed in two pivoting articulated half-shells (2a, 2b). 30 6) A continuous casting installation according to claim 1 characterized in that it further comprises a resonant electrical circuit in which the inductor is connected in series with an adjustable capacity.   7) A continuous casting installation according to claim 5, characterized in that the inductor is mounted at the end of support arm 9 holding in position, the support arm being retractable and provided with controlled means (11) actuating each half-hole (2a, 2b) pivoting.