EP2167697A2

EP2167697A2 - Method and device for controlling the thickness of coating of a flat metal product

Info

Publication number: EP2167697A2
Application number: EP08762808A
Authority: EP
Inventors: Andrea Codutti; Fabio Guastini; Milorad Pavlicevic; Alfredo Poloni; Anatoly Kolesnichenko
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2007-06-08
Filing date: 2008-06-09
Publication date: 2010-03-31
Anticipated expiration: 2028-06-09
Also published as: WO2008149218A2; ITMI20071166A1; EP2167697B1; WO2008149218A3; WO2008149218A8

Abstract

The weight and distribution of the coating material applied to a moving steel strip, extracted from a bath of molten zinc, are advantageously controlled by combined action of magnetic forces that are induced in a layer of coating by means of a non- continuous magnetic field and a jet of gas. The synergistic effect of the electromagnetic forces and the jet of gas enables removal of the coating material in excess, prevention of a premature solidification of the coating layer in the area of impact of the gas, and prevention at the same time of any oscillation of the strip. Thanks to the jointed action of the magnetic forces and of the gas jets a uniform coating is obtained.

Description

METHOD AND DEVICE FOR CONTROLLING THE THICKNESS OF COATING OF A FLAT METAL PRODUCT

Field of the invention

The present invention relates to a method and a device for controlling the thickness of a coating on a flat metal product, such as a steel strip, during the continuous galvanizing process of the strip by hot immersion, also referred to briefly "hot dip" by the English term.

Prior Art

In the galvanizing process by immersion in a hot bath a metal strip, suitably thermally pre-treated in a non-oxidising /reducing atmosphere, is dipped in a bath of melted Zn (440°C-470°C) and is guided out in a vertical direction by rollers immersed in the bath.

The amount of liquid Zn extracted by the strip during the passage through the meited÷-hath-is determined by the balance between the force of gravity and the viscous- forces, and the thickness of the layer of liquid Zn which is deposited on both surfaces of the strip, results as proportional to the speed of the strip and the physical properties of the melted Zn, such as kinematic viscosity and surface tension.

In order -to reduce the thickness of the Zn layer deposited on the strip-to those values required by final application specifications of the strips, jets or blades of air, known in English as "Air Knives", or of some other gas, usually steam or N2, are commonly used.

The devices employed generally comprise two nozzles having a rectangular section or a section having some other form, positioned at the sides of the strip at a predetermined distance from both the strip and the free surface of the Zn bath, from which a gas jet exits advantageously at room temperature. These gas jets act to reduce the thickness of the zinc layer that covers the surface of the strip, forcing part of the liquid metal to return towards the bath.

Since the final thickness of the coating is proportional to the speed of the strip, in order to obtain the same thickness at increasing speed, the pressure exercised by the air knives must be increased. This effect is obtained by an increase in the gas flow rate or the reduction of the opening of the air knife nozzles.

There exists, however, a speed limit for the strip feeding over which the surface of the coating layer is subject to instability and wave formation to the point of releasing liquid and solid drops or particles in the environment in proximity of the air knives. This phenomenon, referred to as "splashing", is generally amplified by the vibrations and oscillations that always occur on the strip. "Splashing" provokes large problems both for product quality and for environmental safety because of the dust- released, and this represents one of the main causes that limits the strip speed and therefore the productivity.in actual galvanizing plants. Another problem is due to the different fluid-dynamic and thermal situation present on the centre of the strip with respect to the strip edges. In fact, this situation leads to the fact that the thickness is not uniform but is greater at the edges. In fact, the edges of the strip cool more rapidly than the centre of the strip creating variations in the physical properties of the liquid Zn, in particular in the kinematic viscosity, that generate surface forces (Marangoni effect) provoking an accumulation of coating near the edges.. The problem is resolved only partially using- knives or masks to deflect the gas jet at the edges of the strip, or using butterfly nozzles that increase the gas flow rate on the edges.

The accumulation of the coating near the edges, in addition to create problems with winding, and successively problems of flatness of the galvanized strip, causes- also problems .of uniformity in the coating properties when the strip is subjected to successive treatments, for example a heating and a holding for an appropriate time at a temperature close to the melting point of the zinc, a treatment referred to as "galvannealing" in English. Furthermore, this accumulation does not permit to reduce to a minimum the amount of Zn necessary to obtain a given coating, with the consequential economical disadvantages.

A limit of air-knife technology is represented also by the fact that the airflow produces a coating oxidation that increases in intensity in proportion to the increase in speed and gas flow rate. This generates defects in the final product and contributes towards releasing dust into the environment. The realization of cutting systems using inert gas, such as N2, used to prevent this drawback, are only able to resolve the problem partially and in any case at a higher cost when compared to common air knife systems. Another limit of this technology is that of provoking a strong cooling and therefore the premature solidification of the Zn under the action of the air knife, especially when the supply pressure is increased with the purpose of obtaining increasingly thinner coatings. This signifies diminishing the efficacy of Zn thickness reduction. A further limit regards the pressure of the air, or gas, which must be maintained within certain limits in order to prevent reaching supersonic air speeds with the consequential problems of vibration, beating and instability in the strip position, and excessive noise levels in the plant.

As- a consequence of this, in the case where the-firral thickness of the coating is fixed at a relatively reduced value, since it is not possible to increase the air pressure too much, the strip speed must be reduced, and therefore also the production line productivity, and this is in contrast with current needs in sales competitiveness, which require speeds over 200 metres/min. In order to solve at least some of the problems described above, various solutions have -been proposed that use^' magnetic fields to reduce the thickness of the coating.

If implemented alone, these magnetic solutions, albeit solving in many cases some of the problems described above, are unable, however, to increase the current maximum productivity of galvanizing lines, since the volume force produced by said devices on the la_yer of zinc, which has an intensity such as not to cause problems of overheating of the zinc and of the strip, is at the most equal to the one generated by current pneumatic systems used on galvanizing lines. As an alternative, solutions have been proposed that provide the combined use of magnetic fields and air knives. Also these combined solutions, however, present disadvantages. An example of these combined solutions is described in the document JP61227158, in which there are provided the direct injection of currents on the moving strip, in the direction of its width, and the action of an induced magnetic field acting in a direction perpendicular to the surface of the strip. Said solution is of difficult practical application because it involves various problems of contact between the moving part, i.e., the strip, and the stationary part, represented by the electrodes. There is then the possibility of damage to the strip, which must be able to move at high speed. Furthermore, superposition of the jet of gas on the area of action of the magnetic forces can be obtained only by inclining the jet of the gas towards said area of action and, hence, reducing the force of pressure on the layer of zinc and increasing the shear stress, with the consequent increase in the risk of inducing surface instability and hence the undesirable phenomenon of "splashing". There is hence felt the need to provide a method and a related device for controlling the thickness of a coating-of metal products, at output from a hot bath, which is able is to overcome the aforesaid drawbacks. Summary of the invention A purpose of the present invention is to provide a method and a related device for carrying out an operation of controlled removal of the coating in excess in the final stage of continuous galvanization by hot dipping of a flat metal product, such as for example a steel strip, by means of jointed use of electromagnetic fields and jets of gas in such a way as to increase the maximum productivity of current galvanization lines. Another purpose of the invention regards the possibility of effective control of the weight of the coating and the uniformity of distribution thereof. A further purpose of the present invention is to reduce and possibly eliminate the problem of "splashing". A final purpose-of the present invention is to control_and reduce to a minimum the oscillations of the strip induced by the operation of removal of the coating in excess.

In order to achieve the purposes mentioned, according to a first aspect of the present invention, a method is provided for controlling the thickness of coating of a flat metal product, which defines a feeding direction when it exits from a bath of molten coating material in continuous hot-dip galvanization processes, in which there are provided two inductors, each supplied with single-phase non-continuos current, having magnetic cores, substantially C-shaped, and windings, wound on said cores, arranged on each side of said flat metal product at its surfaces of major extension, suitable for producing electromagnetic forces induced on said flat metal product and cooperating with nozzles suitable for producing at least one jet of gas directed on at least one of the surfaces of said flat metal product, said method, in accordance with claim 1 , comprising the stages of: a) blowing jets of gas through the nozzles on an area of impact of the surfaces of the flat metal product coated by the molten coating material after exit from a dip in said bath; b) activating at least one of said inductors and producing said electromagnetic forces so that they act on said area of impact to make more efficient the action of removal of the material by said jets of gas and to control oscillation of the flat metal product.

A second aspect of the invention provides a device for controlling the thickp.es.s-.of coating of a flat metal product, suitable for defining a feeding direction wherrit exits from a bath of coating material in continuous hot-dip galvanization processes, which, in accordance with claim 9, comprises

- two inductors, which can be supplied with single-phase non-continuos current, arranged respectively at the surfaces of major extension of the flat metal product, each inductor having a substantially C-shaped magnetic core and at least one winding, wound around said core, suitable for producing electromagnetic forces acting on at least one surface of the flat metal product;

- nozzles, cooperating respectively with said inductors, suitable for producing at least one jet of gas acting on at least one surface of the flat metal product, said device being characterized in that there are provided, within each inductor, means for supplying said nozzles so that the action of said at least-one jet of gas and of said electromagnetic forces is concentrated on one and the same area of impact of the surface of the flat metal product in order to make more efficient the action of removal of the coating material in excess and to control oscillation of the strip itself. In the device of the invention the inductors are suitable for producing three magnetic field loops, among which two loops are generated by each inductor respectively and a third loop is generated in common by the two inductors. The means for supplying the nozzles, comprising a gas manifold, are at least partially made of magnetic material with high electrical resistivity. The method of the invention provides using a non-continuous magnetic field, either alternating or pulsed, which impinges upon both of the layers of molten material of the coating and upon the strip. In particular, the spatial components of the electromagnetic force produced by the non-continuous magnetic field that are oriented downwards, i.e. tangentially along the surface of the strip, together with the transverse ones, i.e. directed orthogonal to said surface, are used advantageously for removing the coating material in excess from the steel strip that moves upwards as it exits from the bath of molten material. Furthermore, the transverse components of the electromagnetic force are used for controlling oscillation of the strip and for keeping the latter aligned at the centre of the working gap. Advantageously, oscillation or deformation of the-strip during feeding thereof is thus avoided.

In this way, the combination of the non-continuous- magnetic field, generating the electromagnetic forces, and of the jets of gas produces the force necessary for effective removal of the coating in excess, together with control of the oscillations of the strip in the area of removal for favouring uniformity of the coating thickness. Advantageously, the more gradual distribution of said electromagnetic forces with respect to the pneumatic ones reduces the problem of splashing up to the point where it is completely solved. Furthermore, thanks to the non-continuous magnetic field, there is advantageously generated an induction heating of the strip and of the coating material directly in the area of action of the jets of gas, thus preventing intensive cooling of the coating material by the gas and the risk of a premature solidification thereof. Induction heating, in addition to increasing the surface temperature of the coating material, advantageously reduces the surface tension and viscosity thereof. Thus, thanks to induction heating and to the jointed action of the jets of gas and of the electromagnetic forces, a coating is obtained of much smaller and more uniform thickness than in current plants, as well as higher production rates. According to the present invention it is possible to coat, for example, steel strips with zinc, zinc-iron alloys and zinc-aluminium alloys, aluminium, and aluminium-tin alloys.

Brief description of the figures

Further characteristics and advantages will emerge more clearly from the detailed description of preferred, but non-exclusive, embodiments of the method and of the device of the invention, with the aid of the annexed drawings, in which:

Figure 1 illustrates a cross section of the entire device in accordance with the present invention;

Figure 2 illustrates the distributions of the magnetic field in the area of operation for two extreme values of the phase shift angle between the magnetic fluxes in the left-hand and right-hand inductors (as viewed in Figure 1);

Figure 3 illustrates a cross section of a variant of the entire device in accordance with the present invention; Figure 4 illustrates the trend of electromagnetic forces that are generated for removing the coating material in excess;

Figure 5 illustrates the trend of the thickness of the coating for different phase shift angles in the windings of the left-hand and right-hand inductors and in the case where activation of said inductors is not provided; Figure 6 illustrates a distribution of the fields, of the induced currents, and of the electromagnetic forces on the strip and on the layers of coating, suitable both for removing the excess of coating material from the strip and for stabilizing the strip in the gap between the inductors;

Figure 7 illustrates a graph regarding the means producing a change of direction of the- electromagnetic forces that hold the strip at the centre of the magnetic gap;

Figure 8 illustrates a graph with the trend of the maximum induction heating of the coating material and of the strip in the active area;

Figure 9 illustrates a cross section of another variant of the invention with electromagnetic shields inside and outside of the inductors; Figure 10 illustrates the effect of the internal electromagnetic shield on the temperature of the jets of gas;

Figure 11 illustrates a distribution of the positioning of the sensors for detecting the position of the strip.

Detailed description of preferred embodiments of the invention With reference to Figure 1, the device according to the present invention comprises means for generating non-continuous electromagnetic fields for removal of the coating material in excess by means of the electromagnetic forces induced on the coating layers, said means being advantageously combined with means for generating jets of gas, for example air, for removal of the coating material in excess also by means of fluid-dynamic forces.

In particular, the means for generating electromagnetic fields comprise two inductors, each constituted for example by two windings or coils 5 wound around a core 4, substantially C-shaped, whilst the means for generating jets of gas comprise, for each inductor, support and/or supply means for supporting and/or supplying nozzles 2, comprising a gas supply manifold 1 and the nozzles themselves, placed in proximity of each surface of major extension of the steel strip at output from the molten bath of the coating material. The pressure of supply of the nozzles is preferably comprised between 0,1 barand 1 bar.

The cores 4, substantially C-shaped, are of the laminated type, or compact, made of ferromagnetic or magneto-dielectric or ferritic material, whilst the coils 5 are arranged facing one another on each side of the steel strip 3 and- are- water- cooled. There is advantageously provided the control of the frequency of the alternating magnetic field according to the type and quality of the coating to be removed.

In accordance with the present invention, the ensemble of the device constituted by the inductors together with the gas manifold 1 and the gas nozzles 2 can be inclined accordingJo different angles and displaced in the direction of the strip-by appropriate movement means. The variation of the orientation of the inductors and the nozzles, which can take place in a fixed way or else in an uncoupled way, enables to modify the conditions of removal of the coating in excess. Advantageously, since the support and/or supply means, which comprise the gas- supply manifold 1 and the nozzles 2, are arranged within the ferromagnetic cores 4, the superposition of the gas jets on the area of action of the magnetic forces is always guaranteed without this implying any reduction of the force of pneumatic pressure on the layer of the zinc coating or any increase in the shear stress that would cause the undesirable phenomenon of "splashing". The nozzles 2, arranged in proximity of the magnetic yoke poles 14', 14" of each ferromagnetic core 4, can be located inside or outside the inductors.

Advantageously, the combined effect of the induced electromagnetic forces and of the fluid-dynamic forces of the gas jets enables an increase in the efficiency of reduction of the coating thickness, as compared to gas knives alone, and enables a more uniform and thinner layer of coating material 11 to be obtained. In fact, by means of the inductors of the device of the invention it is possible to:

• prevent cooling and premature solidification of the coating layers 11 thanks to the heating, by the Joule effect, of the strip 3 and of the coating layers 11 generated by the induced parasitic currents; • reduce the viscosity and the surface tension of the layer of coating, which is still liquid, once again by the Joule effect, facilitating the task of removal of the material in excess by the gas knives.

The gas knives, in turn, advantageously perform the function of control of the temperature, preventing an excessive induction heating both of thexoating layers 11 and of the steel strip 3. In this way_^then, the induced currents never overheat the coating layers 11 and the steel strip 3, thus preventing any undesirable saturation and loss of the ferromagnetic properties of the strip. Since the ferromagnetic properties are preserved thanks to the cooling produced by the gas, the steel strip 3 concentrates the magnetic flux on its own surface, more precisely on the interface between the coating layer and the strip, and in this way, the electromagnetic forces are increased several times making more efficient the effect of removal of the coating material in excess. In accordance with the present invention, it is advantageous to supply- the coils 5 with a single-phase alternate current having a medium-frequency of a value comprised in the range 50 to 1000 Hz, preferably between 100 and 500 Hz. With such a frequency range it is possible to maintain the ferromagnetic properties of the steel strip unalterated, because said strip is not overheated; it is possible to obtain an electromagnetic force sufficiently intense- to remove the coating material in excess and to keep the strip aligned in the central position in the magnetic gap 13. Optimal results have been obtained, in particular, with a frequency range of 100÷480 Hz.

In accordance with a variant, the invention provides the possibility of using just the means for generating electromagnetic fields individually. When the inductors are preferably used together with the gas knives, in order to concentrate the electromagnetic power in the area of impact of the gas on the strip, the distance between the magnetic yoke poles or polar expansions 14', 14", top and bottom respectively, of the ferromagnetic cores 4, i.e. the distance between the common branches of the magnetic flux, is as small as is allowed by the nozzles 2 that generate the gas knife, which are arranged advantageously inside or outside the inductors in proximity of said poles 14', 14". Said distance between the poles is preferably comprised between 15 and 50 mm, in order to concentrate the electromagnetic force along a strip stretch longitudinally extending for 5÷30 mm that coincides with the stretch on which the pneumatic force acts. With respect to a device exploiting a "travelling electromagnetic field", the device of the invention allows to obtain higher electromagnetic forces, having a maximum intensity higher of about 20% with respect to that obtained by aforesaid travelling field devices, and to better exploit the concentrated cooling action of the air knives.

In accordance with another variant illustrated in Figure 3, the magnetic yoke itself performs also the function of air knife. This is possible in so far as the polar expansions or poles 14' and 14" are shaped appropriately in order to define the nozzles 2 adapted to generate the gas jets. In said variant, partitions 30, or slots, are advantageously provided at the inlet section of said nozzles 2, said slots having the purpose of equalizing the flow rate within the nozzles themselves. The nozzles 2, in this case, are therefore defined by the configuration of the polar expansion 14', 14" and have a passage orefice, which, in cross section (Figure 3), has a tapered shape in the feeding direction of the strip. In the -embodiment of Figure 3, in particular, said passage orefice comprises two successive tapered stretches defining mutually incident directions. In this case, the distance between the magnetic yoke poles 14', 14", top and bottom respectively, is comprised between 0,5 and 5 mm. In this- variant, the means for generating electromagnetic fields comprise two inductors, each constituted, for example, by a winding or coil 5 wound around the core 4, as illustrated in Figure 3, which is substantially C-shaped. The windings 5 are supplied with alternate or pulsed alternate current. Inside each core 4 there are provided the supply means for supplying the nozzles, comprising a manifold not illustrated.

Figure 2 shows, with reference to the variant of Figure 1 , the lines of magnetic flux 15 generated by the coils 5 in the ferromagnetic core 4 and outside the core (dispersed flux). Each inductor creates its own loop of magnetic flux 152, 153, which closes between pairs of poles 14', 14" of the ferromagnetic core 4, as illustrated in the right-hand part of Figure 2, and a common loop 151 of magnetic flux embracing both of the ferromagnetic cores 4, as illustrated in the left-hand part of Figure 2. Thus, the magnetic flux 152, 153 of each inductor passes along both of the surfaces of the strip 3 in a substantially vertical direction (right-hand part of Figure

2), i.e. tangentially to the surfaces, and simultaneously the loop of common magnetic flux 151 , which flows between the two inductors, passes twice through the steel strip 3 in directions substantially perpendicular to the surfaces 11 of the strip 3 and opposite each other according to the arrows 18', 18", visible in Figure

6, respectively in the area of the top magnetic poles 14' and in the area of the

-bottom magnetic poles 14".

—Figure 6- shows that the component of the magnetic flux oriented in a direction perpendicular to the strip 3 induces in the strip two loops of induced current 17', 17", which surround, respectively, the magnetic flux indicated by the arrows 18', 18". These two current loops 17' and 17" join in the impact area 12 of the jets of gas, for example air, up to the point of possibly being superposed. Thanks to the interaction of these induced currents 17', 17" with the magnetic flux 18', 18", longitudinal electromagnetic forces- (Lorentz forces) are produced, oriented upwards 20' and downwards 20", respectively. The electromagnetic forces oriented downwards 20" produce a shear effect and hence a effect of removal of the coating material in excess, and they are advantageously concentrated along a strip stretch longitudinally extending for 5÷30 mm, preferably 10÷25 mm, thanks to aforesaid configurations of the magnetic poles. Since the electromagnetic forces 20" are generated by the interaction of the current loop 17' with the magnetic flux 18", in order to maximize the intensity thereof the shape of the two magnetic yoke poles 14', 14" is, in both of the variants, tapered and optimized for increasing to a maximum the intensity of current in the loop 17' and for concentrating the magnetic flux 18" on the strip 3 and on the coating layers 11. In this way, the undesirable electromagnetic forces directed upwards 20', produced by the interaction of the current loop 17" with the magnetic fluxes 18' and 18", are reduced considerably until they are almost eliminated. At the same time, the electromagnetic forces directed downwards 20" have a distribution such as to produce a more gradual reduction of the layer of zinc, with respect to the reduction that would be produced by the pneumatic action only, so as to overcome the problem of splashing. The interaction of the current loops 17', 17" with the magnetic flux 19 of each ferromagnetic core 4 (associated to the loops of magnetic flux 152, 153 visible in Figure 2) produces the electromagnetic forces 21 , which have an orientation perpendicular to the surfaces of the strip, as illustrated in Figure 6. If the overall thickness of the strip 3 and of the coating layers 11 is comparable to the depth of penetration of the magnetic flux in the materials, the magnetic flux 19 of each inductor induces a current loop 22 that surrounds the strip 3 and the coating layers 11. The interaction between the currents 22 and the electromagnetic-flux 19- creates transverse electromagnetic forces 23', 23", which are substantially perpendicular both to the strip 3 and to the coating layers 11. The gradient of the electromagnetic forces 23', 23" also produces a shear effect for removing the coating material in excess from the strip 3. From the difference between the forces 23' and 23" there can be generated a resultant force perpendicular to the strip 3. Advantageously, the tapered shape of the two magnetic yoke poles 14', 14" is such as to maximize also the electromagnetic forces that act in a direction perpendicular to the strip 3.

When the inductors are used in combination with the air knives, it is possible to obtain better results in terms of reduction of the coating layers 11 by means of superposition of the electromagnetic forces 20" directed downwards and of the maximum gradient of the electromagnetic forces 23', 23" with the area of impact 12 of the gas jets.

Represented in Figure 4 is a graph of the longitudinal electromagnetic forces 20', 20" and transverse electromagnetic forces 23', 23" that appear in the coating layers 11 as a result of application of an alternating or pulsed magnetic field when the two inductors generate the common magnetic flux. Represented on the axis of the ordinates is the density of these electromagnetic forces or Lorentz forces in N/m³; represented, instead, on the axis of the abscissae is the spatial coordinate along the vertical feeding direction of the strip. The relation between the longitudinal magnetic flux 19 in each inductor and the common transverse magnetic fluxes 18', 18" in the two inductors changes as a function of the phase shift between the currents in the windings of each inductor. When the currents in the windings of the two left-hand and right-hand inductors are in phase, i.e., the phase shift of the currents is Δφ = 0 (as illustrated in Figure 2 on the left), the magnetic fluxes 18', 18" join in a common loop 151 of magnetic flux, and the longitudinal electromagnetic forces 20', 20" reach their maximum, as likewise does induction heating of the strip.

When the currents in the windings of the left-hand and right-hand inductors are in phase opposition, i.e., the phase shift of the currents is Δφ = 180° (as illustrated in -Eigure 2 on the right), the loop of magnetic flux 151 vanishes and there remains only the longitudinal magnetic- flux 19, generated by the loops 152, 153 of magnetic flux. In this case, the-indtrctiorrheating of the strip is minimum. By varying the phase shift angle in the range ±180°, longitudinal 19 and transverse 18', 18" magnetic fluxes having an intermediate intensity comprised between the minimum and maximum values are generated. Since the steel strip 3 is ferromagnetic, it is strongly attracted by the magnetic yoke poles 14' and 14". Consequently, to counter said attraction, the electromagnetic force or Lorentz force generated by the difference between the. values of phase shift of the currents as a function of the displacement of the strip

3 from the centre of the magnetic gap 13 is advantageously exploited. As already mentioned above, when the currents in the windings have a phase shift comprised in the range ±180°, in the area of action of the electromagnetic forces there exist two magnetic fluxes 19 and 18', 18", and there also arise transverse integral forces 21 and the difference between the transverse electromagnetic forces 23' and 23" that tends to move the strip 3 in a horizontal direction. The direction of the resultant of the forces varies as a function of the orientation of the longitudinal magnetic flux 19, and the orientation of the total Lorentz force, which results from the sum of the forces 21 , 23' and 23", changes with the variation of the phase shift between the currents in the windings arranged on one side and in those arranged on the opposite side of the strip 3. By reversing the sign of the phase angle of the current and keeping the same values of the modulus of the current, the force changes sign. This phenomenon can be used for countering the ferromagnetic attraction of the strip and for suppressing the oscillations of position of the strip 3.

For this reason, said total Lorentz force is defined as "force of repulsion".

The position of the strip in the magnetic gap 13 between the two inductors is measured with sensors of an optical, or capacitive, or inductive type 14, as illustrated in Figure 11 , which are suitable for sending the signal necessary to the power source of the inductors in order to change the sign of the phase angle and the amplitude of the electrical parameters of the inductors when the strip 3 deviates from the position centred in the gap 13. The variation of electrical parameters of the supply system of the inductors, such as the value and/or phase of the voltage and the amplitude and/or phase of the current, can be used also for measuring the_- pjosition of the strip and for generating the signal necessary for the power source to-cnange the phase shift. Figure 5 shows the trend of the thickness of the coating as a function of the different phase shift in the inductors on both sides of the strip. The curve 200 represents the trend of the thickness of the coating in the case where there is not provided removal of the coating material in excess also by means of the electromagnetic forces. The curves 201 and 202 represent, respectively, the trend of the thickness ofthe coating on the left-hand side and on the right-hand side of the strip in the case where these electromagnetic forces of removal are provided. The thickness of the coating is reduced from approximately 15 μm to approximately 5,5 μm when the action of removal of the coating in excess by the electromagnetic field is added to the normal jet of gas, in the case where the variation of phase between the currents in the inductors is equal to zero. It may be noted that the thickness of the coating remains almost constant, with a value lower than 6 μm of thickness, until the phase shift reaches a value of Δφ = 100°. In the graph of Figure 7 it may be noted that the force of repulsion 24 (Lorentz force), which results from the sum of the forces 21, 23' and 23" in Figure 6 for countering the ferromagnetic attraction exerted by the poles of the inductor, is maximum on the strip 3 when the phase shift is approximately 90°.

In the same Figure 7, the curves 240 and 241 represent the trend as the phase shift Δφ, respectively, of the Maxwell force and of the sum of the Maxwell force and of the force of repulsion 24 vary. The graph of Figure 8 illustrates the dependence of the maximum temperatures that can be generated on the steel strip 3 (curve 100) and on the two coating layers 11 (curve 101) by induction heating, without the use of gas knives, when the phase shift between variations of currents in the inductors varies in the range ±180°. It is possible to obtain a minimum local heating for Δφ = ±30°. In this way, the optimal phase shift between the supply currents in the inductors of both sides of the strip can be determined in the range Δφ = ±90°. In this range it is possible to obtain the necessary repulsive forces, without losing the possibility of obtaining a small thickness of the coating. In order to reduce the induction heating of the support and supply means for supporting and supplying the-gas knives, said means being arranged inside each ferromagnetic core 4 and comprising the manifold 1 and possibly the nozzles 2, at least one-high-conductivity electrical shield 16 can be provided, arranged between said means and the core 4 (as illustrated in Figure 9), which fulfils two functions: - preventing induction overheating of the air knife; and

- concentrating the magnetic flux directly in the area 12 where the gas jet acts. Figure 10 shows that, using the high-conductivity shield, with a phase shift of Δφ = ±30° the shield enables a reduction of the temperature of the gas knives to a regular level. Supplementary high-conductivity electrical shields 160', 160" can be provided, arranged outside each ferromagnetic core 4 and in proximity of the poles of the magnetic yokes 14', 14", in order to reduce the induction heating on the strip 3 and on the coating layer 11 , when the temperatures become excessive for the process. By means of this appropriate positioning of the shields 160', 160', it is possible to limit the reduction of the magnetic flux in the area 12^"where the gas jet acts to maintain the effectiveness of the system of removal of the coating material in excess.

When the inductors are used in combination with gas knives, in order to increase the magnetic flux in the area 12 where the gas acts and to increase the electromagnetic forces, it is possible to make all the support and supply means of the gas knives, or alternatively only the nozzles 2, by using a magnetic material having a high electrical resistance, e.g., iron or laminated steel, ferrite or magneto- dielectric material. According to a further variant, the aforesaid electromagnetic shields, internal or external to the magnetic cores 4, can be shaped in such a way as to constitute themselves the nozzles for the gas jets. In this case, then, the nozzles 2 will be defined by the configuration of the electromagnetic shields. In a particular embodiment of the invention, a concentration of the horizontal electromagnetic forces, acting in a direction substantially orthogonal to the strip, is obtained at the edges of the strip for removing the material in excess on the edges.

In an advantageous variant of the invention, the process of removal of the coating material in excess provides the use of the device getting only the inductors to work, without setting in operation the air kniv.es... It is moreover possible to get the device to act only on one of the faces of the strip, leaving the coating unaltered on the second face, or else it is possible to get the inductors and the air-knives to- work in various combinations on one or both sides of the strip.

Claims

1. A method for controlling the thickness of coating of a flat metal product (3), the product defining a feeding direction when it exits from a bath of molten coating material in continuous hot-dip galvanization processes, wherein there are provided two inductors, each-supplied with single-phase non-continuos current, having magnetic cores (4), substantially C-shaped, and windings_(5), wound on said cores, arranged on each side of said flat metal product at its surfaces (11) of major extension, suitable for producing-electromagnetic forces induced on said flat metal product and cooperating with nozzles (2) suitable for producing at least one jet of gas directed on at least one of the surfaces (11) of said flat metal product, said method comprising the stages of: a) blowing jets of gas through the nozzles (2) on an area of impact (12) of the surfaces (11) of the flat metal product (3) coated by the molten coating material after exit from a dip in said bath; b) activating at least one of said inductors and producing said electromagnetic forces so that they act on said area of impact (12) to make more efficient the action of removal of the material by said jets of gas and to control oscillation of the flat metal product (3).

2. The method according to claim 1, wherein in stage b) there is provided -the activation of both the inductors.

3. The method according to claim 1 , wherein the supply of said windings (5) is carried out with alternate supply currents, having a controlled phase shift angle, suitable for creating three magnetic-field loops, of which the first (152) and the second (153) loops are generated by each inductor separately, and the third loop (151) is generated in common by the two inductors.

4. The method according to claim 3, wherein there are provided concentration of said electromagnetic forces in said area of impact (12) and protection of said gas jets from an overheating by means of high-conductivity shields (16, 160, 160') acting also as concentrators of flux.

5. The method according to claim 3, wherein the frequency of the supply currents is in a range comprised between 50 and 1000 Hz, preferably between 100 and 500 Hz.

6. The method according to claim 5, wherein the phase shift angle of the supply currents is comprised in the range ±180°, preferably equal to ±90°.

7. The method according to claim 6, wherein among the electromagnetic forces those acting in a direction substantially orthogonal to the flat metal product can be reversed in direction in a controlled way to keep said metal product in a centred position by means of a reversal oHhe phase angle between the supply currents of the two inductors.

8. The method according to claim 7, wherein there are provided detection of the position of the flat metal product by means of sensors (14) in a magnetic gap (13) between the two inductors and emission of a signal for possibly varying electrical parameters of supply of the two inductors.

9. A device for controlling the thickness of coating of a flat metal product (3), the product being suitable for defining a feeding direction when it exits from a bath of coating material in continuous hot-dip galvanization processes, comprising:

- two inductors, which can be supplied with single-phase non=continuos current, arranged respectively at the surfaces (11) of major extension of the flat metal product, each inductor having a substantially C-shaped magnetic core (4) and at least one winding (5), wound around said core, suitable for- producing electromagnetic forces acting on at least one surface (11) of the flat metal product; - nozzles (2), cooperating respectively with said inductors, suitable for producing at least one jet of gas acting on at least one surface (11) of the flat metal product, said device being characterized in that there are provided, within each inductor, supply means for supplying said nozzles (2) so that the action of said at least one jet of gas and of said electromagnetic forces is concentrated on one and the same area of impact (12) of the surface (11) of the flat metal product (3) in order to make more efficient the action of removal of the coating material in excess and to control oscillation of the strip itself.

10. The device according to claim 9, wherein the nozzles (2) are arranged in proximity of magnetic yoke poles (14', 14") of each magnetic core (4).

11. The device according to claim 9, wherein the nozzles (2) are defined by the configuration of magnetic yoke poles (14¹, 14") of each magnetic core (4) and there are provided partitions (30) at an inlet section of said nozzles (2) for equalizing the flow rate of gas within the nozzles themselves.

12. The device according to claim 9, wherein the nozzles (2) are defined by the configuration of high-conductivity concentrators of magnetic flux.

13. The device according to claim 10 or claim 12, wherein the nozzles (2) are arranged inside or outside the inductors.

14. The device according to claim 10 or claim-11 , wherein the poles of the magnetic yokes (14', 14") are tapered, and their shape is optimized for maximizing the electromagnetic force on the coating directed downwards and/or in a direction perpendicular to the flat metal product (3).

15. The device according to claim 10 or claim 11 or claim 12, wherein there are provided first high-conductivity concentrators of magnetic flux (16), for concentrating the flux in the area of impact (12), arranged between said supply means and the core (4), and possible second high-conductivity concentrators of magnetic flux (160', 160"), for concentrating the flux in the area of impact (12), arranged outside each magnetic core (4) and in proximity of said poles (14' , 14").