MXPA97000690A - Means of electromagnetic plugging for immersion coating crystal in calie - Google Patents

Abstract

A hot dip coating crucible having a strip inlet passage (3) and electromagnetic plugging means to prevent the spillage of bath liquid from the crucible through that passage, further characterized in that: the plugging means consist of: of two magnetic field generators (7) arranged one on each side of the passage, each generator projects an oscillatory magnetic field in the passage of at least two poles of opposite polarity that are adjacent to the passage and spaced apart in the transverse direction of the passage, the at least two mentioned poles of each generator are respectively in substantial alignment with the corresponding poles of the other in the transverse direction of the passage, the magnetic fields projected by the generators have flow patterns that are substantially mirror images in preference to a plane of reflection that coincide with a central plane of the passage, and both generators they exceed a frequency of more than seven kiloHertz, when there is no strip the fields combine and extend transversely of the passage, when a strip is present the fields not only plug the passage, but also provide restrictive forces to prevent the deviation of the pull your line of pa

Description

MEANS OF ELECTROMAGNETIC PLUGGING FOR HOT DIP COATING PAD TECHNICAL FIELD This invention relates to crucibles used to contain molten metal baths for use in the continuous hot dip coating of metal strip with liquid metal coatings. It was developed for use in continuous hot-dip galvanizing of steel strip, where the coating metal is essentially zinc. However, it will be apparent that it is applicable to any situation where the substrate strip is metal and the coating is a liquid metal, for example, hypereutectic alloys of zinc-aluminum and other alloys. More particularly, the invention is directed to electromagnetic plugging means to prevent the , spill of liquor of bapío from the crucible in those points where the crucible has an opening in this one that is below the level of surface of the liquid during the normal operation.

PREVIOUS TECHNIQUE In conventional continuous galvanizing procedures, the steel strip, after having been , Urnpiada and otherwise conditioned for the adherence acceptance of the coating, is fed from above in a bath of molten zinc or a zinc-based alloy. The strip passes around a so-called "collector roll" submerged in the bath, subsequently emerges from the bath and passes between coating thickness control devices, which return the excess coating liquid to the bath. The coating is allowed to solidify and the coated strip is finally wound up for storage, processing or sale. The collector roller, which is submerged in the bath, operates in a hostile environment and is thus a source of problems and deviations unless careful maintenance is given. Even when properly maintained, the inevitable wear and tear requires periodic replacement. Have you ever, ever crawled scum from the surface of the bathroom by the strip and The nut can be fixed to the collector roll and tend to form rough growths of alloys on the surface of the roll. That scum and those growths damage the strip, requiring the frequent quenching of the line to remove and replace the collector roller with a new or renewed roller. In this way, it would be desirable to eliminate the collector roller. With that desire in mind, it has been proposed to provide at least one opening in the crucible, positioned below the normal operating level of the Damage liquid so that a strip to be coated can enter the crucible, either deep or from below, and exit, either through a similar outlet opening or through the mouth of the crucible, without the need of a change of direction of the line of passage of the strip inside the bathroom. It is necessary, of course, to prevent the spillage of the bath liquid through the opening or openings and various means of electromagnetic plugging have been proposed for the purpose of "descriptive convenience", the surface of the liquid metal being supported. or otherwise contained by forces generated by the electromagnetic plugging means rather than by a solid component of the crucible, is hereinafter referred to as the "smooth" surface of the liquid metal.The previously proposed electromagnetic plugging means have fallen. normally within one of two categories, those that use either poly phase energizers or ulti pole electromagnets and switching devices, which provide moving magnetic fields that pass through the liquid or into the space in which the liquid could otherwise spill, and those that are analogous to electric motors that use either magne permanent coughs or BC or elect rornagnetos single phase in combination with an electric current transverse All these electromagnetic plugging devices are based on the interaction of electric currents and magnetic fields, either generated independently or induced by one another, and the currents are either DC or power frequency and the fields are equally either stable or oscillating at power frequencies. In both categories of clogging means, the magnetic field and / or the electric current passes through the bath liquid adjacent to the opening to generate containment forces therein. The previously proposed electromagnetic plugging means of the aforementioned type require assemblies of relatively complex components in close association with the liquid metal, and thus all operate in a hot and frequently cramped environment. This leads to design difficulties, limitations in the size and shape of the openings that can be plugged, and to high operating life expectations. Those previously proposed clogging means all suffer from functional deficiencies as well. For example, those that use DC currents that flow between electrodes in contact with the liquid metal, for example to generate a lifting force in the liquid to prevent it from falling through a strip entry opening in the crucible floor, they are inherently unstable. If an accidental projection located downwards is formed e / o the smooth surface of the liquid metal, then the current density in the projection becomes smaller than the average density of current in the skin of the smooth surface as a whole. In this way, the electromagnetic force directed upwards in the projection is reduced, and the hydrostatic pressure causes it to grow. The more it grows, the less the restoring force will be, until eventually the projection breaks off from the smooth surface like a droplet of bath liquid. After breaking, there is no current through the droplet, the restoring force on it drops to zero, and the droplet falls through the opening. In fact, in such plugging means any disturbance of the smooth surface usually leads to a continuous rain of droplets coming from it, in addition to the breaking of each droplet can cause a sufficient disturbance to initiate the formation of another. TarnbLen, all those previously known proposals that operate at power frequencies, produce liquid flows into the bath adjacent to the opening. In this way, the bath is turbulent in the position where it contacts the strip to be coated, and this turbulence seriously degrades the surface quality of the reverse in the finished product. Finally, it should be mentioned that the devices under description have or produce powerful magnetic poles adjacent to the opening, which interact with strips of ferrous substrate tending to attract the strip towards the poles. any accidental deviation from the central passage line towards one pole and outside another, causes the attraction of the strip towards one pole to increase and towards the other decreases, thus producing a deviation force that increases with the increase deviation. In this way, the situation is clearly inherently unstable. Eventually, successful operation with steel strip requires expensive guide rollers that are positioned closely adjacent the opening, and an objectionable high tension that is maintained in the strip to prevent deflection, a tension that is not easily obtained in the practice. In view of the foregoing and other deficiencies of what can be termed zero-plugging or power frequency devices, such devices have not been widely accepted or used within the metal coating industry. In another previous proposal it has been suggested - especially, the use of a high frequency electromagnetic field, oscillating but spatially stationary, positioned to exclude the bath liquid from the opening of the crucible. This proposal is based on the fact that such a field generates high frequency swirl currents inside the bath. Because of its high frequency, eddy currents flow uniquely into a thin surface layer of liquid (the so-called and well-known "skin effect"). The reaction between surface currents and the field is repulsion Hitua, and at sufficiently high frequencies, the field is effectively excluded from penetrating the liquid. In such circumstances the field behaves like an elastic cushion that can be deformed or compressed by the smooth liquid surface but resists the penetration of the liquid into the space occupied by the field. The strength of resistance is perpendicular to the direction of the flow lines of the field and the smooth surface of the liquid, and is proportional to the degree of deformation or compression of the field. In this way, unlike the lower frequency interactions, the situation is inherently stable since an accidental projection of the liquid surface into the field space produces a localized deformation of the field and an accompanying localized resistance increased for a subsequent intrusion. Moreover, in the absence of electric currents or electric fields within the body of the bath liquid there is no "liquid-induced rust caused by the clogging means being turned off. High frequency overcomes the main deficiencies of low or zero frequency plugging means, but they are subject to their own inherent limitations.In particular, a high density magnetic field is required if enough normal force is to be generated so that the smooth liquid surface resist hydrostatic pressure at the bottom of a liquid metal bath at a depth efficient to make it possible for a continuous and reliable strip coating operation to proceed. This in turn requires high energy generating coils and creates an interest in the use of crucible and crucible opening shapes and dimensions, which minimize the point of the smooth liquid area that will be supported or otherwise contained. This at least requires that the strip entry opening provide only small clearance for the strip to pass through. This, in turn, requires precautions to ensure that the strip does not deviate from the designed line of passage at any very wide point. A coating crucible provided with a simple conceptual form of high frequency electromagnetic plugging means is described in Japanese Patent No. 04-099160 (Mippon Steel). In this case the "crucible" is a rectangular and hollow silicon carbide prismatic cell approximately 100 m wide with a groove of / ^ approximately 20 mm wide on your floor. A sheet of steel moves up through the slot and through a galvanizing bath contained in the cell. The lower part of the cell is surrounded by a solenoid coil that is energized at 20 kHz and has a vertical central plane that coincides with that of the cell. The slot is placed next to that central piano. This is described as causing the lower part of the bath liquid to be pushed towards the center plane of the cell space of the slot but leaving the p ** rte superior of the unaffected bathroom. A strip of steel to be coated is shown traveling up through the slot and through the top of the bathroom. Taking into account the 20 kHz field would be effectively excluded from the bath and the strip, it is clear that the field of this device would be asymmetric with reference to the strip in a critical area immediately above the slot. In fact, due to the shielding effect of the strip, there would be very little, if any, field on the inside of the strip for pumping the liquid back from the inside of the strip immediately onto the slot. This earlier proposal would have three main deficiencies, (i) the aforementioned asymmetry would create substantial lateral forces off balance sheets in the strip that would require special arrangements and objectalemente a stress on the tension to reliably maintain the strip out of contact with the walls of the strip. the narrow slot, (n) the poor field adjacent to the interior - - < the strip would allow the liquid to fall through the slot in the interior of the strip, and (m) the extensive smooth surface area of liquid would require a high density and high volume field with consequent high power requirements for the generating coil One might think 'that (i) and di) could be overcome by centering the position of the groove .. In fact this could remove the force out of balance in the t produced by the plugging field and could prevent spillage when a strip is present in the slot, p ** ro would still keep the strip free of movement laterally from the pass line. More importantly, the naturally denser part of the field within the bore hole of the coil is substantially vertical and therefore not well oriented to provide a vertical restrictive force. Thus, in the absence of a strip, the restriction would depend on the divergent field of low density relatively at the upper end of the solenoid. This would require a very high power coil producing an unnecessarily dense field • completely if the spill is to be efficiently prevented.

DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a hot dip coating crucible with high frequency electromagnetic plugging means which at least overcome the aforementioned deficiency () of the electromagnetically high frequency plugged sols previously proposed, and in preferred reduces the deficiency (n) thereof. The invention achieves that object by providing high-frequency plugging means, which provide stable forces on the strip that tend to prevent it from deviating from its supposed line of passage through the plugged opening, and which are more effective to prevent spillage of the liquid metal in the absence of a strip, which J and clogging means of that type known above. The invention consists of a hot dip coating crucible having a strip inlet passage electromagnetic clogging means to prevent spillage of wash liquid from the crucible through that passage, further characterized in that: Clogging means of two magnetic field generators arranged one on each side of the passage; Each generator projects an oscillating magnetic field within the passage of at least two poles of opposite polarity that are adjacent to the passage and spaced apart in the transverse direction of the passage; The at least two mentioned poles of each generator are respectively in substantial alignment with the corresponding poles of the other in the transverse direction of the passage; • "" I The magnetic fields projected by the generated ones have flow patterns that are substantially mirror images with reference to a plane of reflection that coincides with a central piano of the passage; and Both generators operate at a frequency of more than three kiloHertz. As a result of their mirror image and mutual registration flow patterns, the protected fields provide identical repulsive forces on opposite sides of a strip. , centrally posed cloned, if one is present in the passage. If the strip deviates from the center of the passage, further compression of the field on one side and expansion of the field on the other increases and decreases the repulsive forces respectively to produce a storage-storing force tending to return the strip to the central position. To project a magnetic field, any magnetic field generator necessarily has at least two magnetic poles spaced apart. Poles are * necessarily of opposite polarity at any point and in the projected field they extend from one to the other along part of an endless flow path. These poles can be real (solid bodies from which the field emanates) or virtual (a spatial site from which the field emanates). Therefore, in order to project mirror image fields in the passage in accordance with the invention, each generator must have at least two poles of opposite polarity close to each other. -adjacent to the passage, spaced apart in the transverse direction of the passage, and in alignment with the corresponding poles of the other generator, as previously mentioned. However, in preferred embodiments of the invention there is another limitation, that is, that the polarities of the respectively aligned poles are such as to ensure that when a strip is not present in the passage, the projected fields combine, and the combined field extends. -t-tensely of the passage from each of the poles of one generator to the corresponding pole of the other generator. Using two high frequency generators as mentioned above, such a change in the field pattern becomes possible. This change provides the main benefits of the invention. In a pattern the combined field extends transversely of the passage and is ideally positioned to cap the passage when it is open in its full width due to the absence of the strip. In the other pattern, the fields extend along the passage on each side of the strip not only plugging respectively the narrow spaces of the passage on each side of the strip, but also reacting with the strip to keep it central to the entrance passage as a whole, in this way, making it possible for a narrow entry passage to be used together with a low voltage in ia t ra. The invention also extends to a continuous hot-dip galvanizing line or a similar metal coating apparatus, further characterized in that the coating crucible is a crucible according to the invention. In conventional coating lines, the crucible is necessarily long enough to accommodate the manifold roller and allow it to be partially or completely immersed in the liquid coating metal. An advantage of the present invention is that the crucible can be w-ucho smaller than what has been possible so far. In this way, the term "crucible" as used herein, includes small but elongated vessels a little different in shape and size from the normal concept of a conventional prior art crucible, although fulfilling the same function as the previous one.

BRIEF DESCRIPTION OF THE DRAWINGS By way of example, various embodiments of the invention described above are described in more detail herein with reference to the accompanying drawings. Figure 1 is a diagrammatic sectional view of a bottom portion of a coating crucible according to the invention showing a magnetic field pattern established in the absence of a strip to be coated. Figure 2 is a view similar to Figure 1, showing the matter of that figure and the magnetic field pattern established when a strip is present. Figure 3 is a perspective view of a pair of electromagnetic field generating coils, which are components of the crucible of Figure 1. Figure 4 is a view similar to Figure 3 of an alternative pair of coils. Figure 5 is a diagrammatic sectional view of bottom portion of a coating crucible according to another embodiment of the invention, showing a magnetic field pattern established in the absence of a strip to be coated. Figure 6 is a view similar to Figure 5, showing the material of that figure and the magnetic field pattern established when a strip is present. Figure 7 is a diagrammatic sectional view of a fork and a strip, which are components that appear in Figures 5 and 6 enlarged to a larger scale, showing the dimension signals as referred to in some part of the description. Figure 8 is a view similar to Figure 6 of another embodiment of the invention.

BEST WAY TO CARRY OUT THE INVENTION '^ - As shown in Figures 1 and 2, a hot dip coating crucible in a continuous line of strip coating contains a bath 1 of a molten metal coating material, for example zinc or an aluminum alloy and zinc. The crucible has a floor 2, with a downwardly directed duct 3 of generally transverse shape that defines a strip entry passage 4 that provides a space for the entrance in the crucible, of a metal beam 5 to be coated. .

The strip 5 is guided by rollers (not shown) to enter the crucible from below and travel up through the bath 1. Before reaching the passage 4, the strip 5 can be cleaned and conditioned in another way in a conventional manner to receive the reverse order. In this way, a strip of steel, for example, could normally be pre-conditioned and fed in a conventional manner from a heating furnace having a controlled reducing atmosphere, through a cover (not shown) of the same "shape containing a reducing atmosphere or at least inert 6, within the passage 4. Having emerged from the bath 1, the strip 5 would be treated, also in a completely conventional way, to become a final product.Therefore, apart from the provision of rollers to bring the strip towards the crucible from below and the shape of the aforementioned cover, the line equipment downstream and upstream of the coating crucible may be conventional in two ways. The crucible may be made of a ceramic or a refractory material, for example an alumina stabilized with titanium, silicon carbide or boron rutride. Two high-frequency magnetic field generators consisting of coils 7 are respectively arranged on opposite sides of the duct 3. Those coils can, for example, be optionally one or the other of the coils illustrated in FIGS. 3 and 4. These are shown in FIG. section in figures 1 . - 2, the section being taken on the line XX that appears in figures 3 and 4. In each instance, each coil 7 is of a single turn and consists of an upper coil side conductor 8, a lower coil side conductor 9 and a coil end conductor 10. The coils are connected to each other by means of interconnection conductors 11 and are advanced by supply conductors 12 extending to terminals 13. In Figure 3 the arrangement of the interconnection conductors 11 is such that the two coils are in series, while in figure 4 the arrangement of the coils is in parallel. In both cases, the coils preferably extend rigidly as self-supporting embedments from the terminals 13. For this purpose the coils can be manufactured from copper tube, preferably non-circular cross section tube so that each of the coils Side coil conductors 8 and 9 present a broad flat face towards the adjacent surfaces of the pot and conductor. For example, the coils can be manufactured from rectangular hollow section (HRS). The connections between conductors in the coils are preferably welded with bronze alloy. The terminals 13 can be lengths of copper pipe adapted to be clamped by pipe clamp formations in or on supply busbars extending to a power supply transformer of / "" 'ta frequency. The terminals may be internally threaded at their lower ends, as indicated in 14 in Figure 3 where a part of the terminal has been cut out, to receive cooling supply hoses (not shown) whereby cooler can be circulated to through coils 7 while in operation. From the foregoing it will be clear that the current instantaneous address in two conductors of the upper coil side 8 is always the same. Likewise, the vertical direction in the conductors of the lower coil side 9 is always the same and always in the opposite direction to that of the upper conductors. In this way, the coils are of the same polarity, and it can be considered that each coil 7 can have 3 virtual poles that are spaced apart in the longitudinal direction of step 4, namely, the region immediately adjacent to the side of the coil. upper coil 8, region 16 - ^ directly below the bottom coil side 9, and region 17 in the coil center. The fields starting from virtual poles 15 and 16 will always be in a common direction (that is, at a specific moment, towards step 4) and the field starting from virtual pole 17 will be in the opposite direction (at that moment, far from step 4), as indicated by arrows in the flow lines shown in figures 1 and 2. The virtual pole 17 can be considered as a single one because - ^ e twice the existence of each of the poles * 'irtuals 15 and 16 as two closely adjacent poles that are in the respective counterparts of poles 15 and 16. In the absence of a strip 5, the close proximity and mutual registration of the corresponding poles of the two generators, which with the coils identical perforated with simple air of the present embodiment are leveled with axial alignment of the coils, ensures that the electromagnetic fields generated by the coils join both coils. Also, for the same coil current, the transversal component of the field would have a value equal to twice that produced by any single acting coil. More importantly, in the absence of a strip, the registration of the poles 15 ensures that a portion of the field extends from one to the other of the passages transversely to provide a barrier to the descent of the liquid from the bath through the passage. This can be seen in Figure 1, where, although the relevant flow lines appear to be arched towards ~ \ t > When the garlic crosses the passage in response to liquid pressure, the force of repulsion on the surface of the liquid race is still mainly ascending. In general terms, since the repulsive forces between a high-frequency field and the liquid are effective in the field and liquid phase, the field of the poles 15 is effective, which is directly effective as an obturator. way is only those poles and the counterpart of pole 17 of each of them that are necessarily in register in the longitudinal direction of the step. Thus, in this and other embodiments having simple coils such as field generators, the coils are arranged so that at least their upper coil side conductors are in register with each other in the direction of the strip path. , that is, assuming the strip trajectories vertically through the conduit the upper coil sides rest on the same horizontal plane. Specifically, the lower coil sides could be more remote from the passage, for example they could rest in the same horizontal plane as the upper sides if desired. In other embodiments the arrangement can be further varied to maximize the field in the lower position of the poles 17 and to use the field from these poles for liquid lifting. This complementary polarity and the mutual registration of at least the sides of the upper coil are significant characteristics of the present embodiment of the invention, in which, in the absence of a strip, the horizontal component of the generated flow that intercepts the duct is maximized. immediately below the 2nd floor of the pot. This in turn increases to the maximum the lifting force acting on the surface race of the liquid metal 1. at the mouth of the conduit. The coils 7 are energized from an alternating voltage source preferably of magnitude inst in at a frequency of at least 3 kHz and preferably in excess of 7 kHz. If the strip 5 is present the field from each generating coil is substantially restricted to that part of the pitch and pulls on its side of the center plane of the strip. The generated fields then adopt mirror image patterns shown in FIG. 2, where the field of each pole 15 enters the passage more or less transversely and then retracts the passage lengthwise and pulls it for a distance, and then retracts again. more or less transversely of the passage towards the pole 17. The fields projected by the poles 15 and 17 then serve to obstruct the passage on each side of the strip, in this respect it must be observed that the field penetrates the strip, although at a depth very small, so that the strip and the field together provide complete repression for the liquid. The magnitude of the energy voltage needed to - * "* struir © 1 step in any case depends on the physical parameters of the installation (for example amplitude of the step, amplitude of the strip, number of turns in energizing the coil, etc.) and the pressure of the liquid The last mentioned depends on the density of the coating material and the depth of the bath The frequency of the energy source is chosen to produce an optimum balance between antagonistic effects. skin of the 'coil inductors', ie the thickness of the surface layer at which the current is mainly confined, is reduced and the resistance of the coil becomes higher. This leads to higher resistivity losses. On the other hand, the repulsive force in a conductive body, in this case the coating liquid, increases as the parasitic currents become more closely confined to its surface, which is how the frequency increases. The passage between one point and another between attraction and repulsion of the steel strip occurs at a frequency within the range of 3 to 7 kHz. In this way, frequencies in excess of up to about 100 kHz are preferable. In the present modality, all the fields of the respective poles, not only that of the poles 15 and 17 are projected in the passage and are mirror images of each other, thus, the fields projected in the passage through the poles 17 and 16 also it contributes to the restoration of forces that prevent the line or passage of the strip from deviating from the desired center position. The coils 7 may be effectively in air or other non-magnetic means as illustrated. Alternatively, the generating coils may be partially externally enclosed in C-sectioned magnetable covers. Such covers increase the magnetic flux for a given energizing current which is advantageous, but also increases the co-mductance of the coil, which requires a higher energizing voltage and is disadvantageous. In this way, they are The coated parts, a design balance has to be die-cut to optimize the efficiency of the means of oppression, Figures 5 to 7 illustrate another preferred embodiment of the invention, wherein each of the field generators is in nature of a romagnet elec comprising an energizing coil wound in a ferreornagnetic core, preferably in the form of a G. In this case, the pot may have a floor 20 that is thick enough to accommodate the high frequency magnetic field generators. 21 inside elongated recesses formed in the confronting faces of the floor 20 defining the entrance passage to the strip of the pot 22. Each of the generators 21 comprises an energizing coil 23 that surrounds the band of a core C- or G- ferrous-sectioned or ferri-magneti co 24 and, preferably, copper or other non-ferrous electrically-inductive coatings 25 and 26.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A hot dip coating crucible having a strip inlet passage and electromagnetic plugging means to prevent the spillage of bath liquid from the crucible through the passage, - / --- further characterized in that: the plugging means consist of two magnetic field generators arranged one on each side of the passage; each generator projects an oscillating magnetic field in the passage of at least two poles of opposite polarity that are adjacent to the passage and spaced apart in the transverse direction of the passage; the at least two mentioned poles of each generator are respectively in substantial alignment with the corresponding poles of the other in the transverse direction of the passage; fields • Gnéticos projected by the generators have flow patterns that are substantially mirror images with reference to a plane of reflection that coincides with a central plane of the passage; and both generators operate at a frequency of more than three kiloHertz.

2. A hot dip coating crucible according to claim 1, further characterized in that the crucible has a floor with a downwardly directed duct of rectangular cross section defining the p ^ s e to enter the strip.

3. A crucible of hot dip coating according to claim 2, further characterized in that the crucible is made of a refractory material.

4. A hot dip coating crucible according to claim 3, further characterized in that said refractory material is one of aluminum dioxide stabilized with titanium, silicon carbide and boron nitride.

5. A hot dip coating crucible according to claim 1, further characterized in that each generator consists of a coil of a conductor that extends rigidly as a self-supporting console from two ex-terminals.

6. A crucible of coating by immersion in lane according to claim 5, further characterized in that said L > ob? na is a one-turn coil.

7. A hot dip coating crucible according to claim 5, further characterized in that said conductor is tubular and said terminals are adapted for connection to a source of refrigerant to provide cooling flow through the interior of the conductor. ? .- A hot dip coating crucible according to claim 1, further characterized in that each of the field generators consist of '' an energizing coil wound on a magnetic center having end faces directed towards said passage, such end faces constituting said poles of the generator. 9. A hot dip coating crucible according to claim 8, further characterized in that said center is G-shaped, wherein one end face is smaller than the other. 10. A hot dip coating crucible according to claim 9, further characterized in that said generator is equipped with electrically conductive non-ferrous protections adapted to contain magnetic fields and positioned to increase the proportion of the total magnetic field generated by the coil energizer that emerges from the extreme faces of the center. 11. - A hot-dip coating crucible according to claim 8, further characterized in that each side of the passage is lined with an insulating, heat-resistant and non-metallic heat-insulating plate that provides a barrier between the molten metal from the bathroom and the upper parts of the generators. 12. A hot dip coating crucible according to claim 8, further characterized in that the center is made of a low loss material having high permeability and high saturation magnetization. 13.- A crucible of coating by immersion in "• encourages according to claim 12, further characterized in that said material is any of high density ferrite, magnetic metal crystal and iron powder. 14. A crucible hot dip coating according to claim 1, further characterized in that the distance between at least two poles in the transverse direction of the passage is within the range of from two to ten times the width of the space of air between a strip that is being reversed and one side of a passage. 15. A hot dip coating crucible according to claim 8, further characterized in that each energizing coil is a coil of a muiti filament conductor of no more than 10 turns. 16. A hot-dip coating crucible according to claim 10, further characterized in that said protectors are force-enfielded. 17. A crucible for coating by immersion in the lens according to claim 1, further characterized in that said frequency is within the range of from 7 kHz to 100 HHz. 18.- A crucible of hot dip coating according to any of the preceding claims, further characterized in that the polarities of the generated are such that when they are in operation in the absence of a strip to be coated, the projected fields are combined, and the combined field is spread transversally < • * the passage between the aligned poles of the respective generators. 19. A hot dip coating crucible as described herein with reference to any of Figures 1 to 4 or Figures 5 to 7 of the accompanying drawings. 20. An apparatus for the continuous hot dip coating of a metal strip with a metallic coating including a coating crucible according to claim 1.