WO2008149219A2

WO2008149219A2 - Method and device for controlling the thickness of a coating on a flat metal product

Info

Publication number: WO2008149219A2
Application number: PCT/IB2008/001474
Authority: WO
Inventors: Fabio Guastini; Andrea Codutti; Milorad Pavlicevic; Alfredo Poloni
Original assignee: Danieli & C. Officine Meccaniche S.P.A.
Priority date: 2007-06-08
Filing date: 2008-06-09
Publication date: 2008-12-11
Also published as: CN101720361B; WO2008149219A3; ITMI20071167A1; EP2167698A2; CN101720361A

Abstract

Method and relative device to realise an operation of controlled removal of excess coating during the final stage of continuous hot dip galvanizing of a flat metal product, such as a steel strip for example, by means of the use of electromagnetic fields and possibly also of gas jets in such a manner to increase maximum productivity in the actual galvanizing lines and at the same time to improve the quality of the final product, in particular, by reducing and possibly eliminating the problem of 'splashing'. A further object of the invention relates to the possibility of efficaciously controlling the weight of the coating and the distribution uniformity thereof.

Description

METHOD AND DEVICE FOR CONTROLLING THE THICKNESS OF A COATING ON 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 duringjthe passage through the melted bath 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.

The same type of process can be used to coat metal strips with Zn-Al, aluminium and tin alloys.

The air knife is characterised by very narrow pressure distribution on the zone of impact, only a few millimetres wide, such as 3-5 mm, for example, and by the presence of a larger shear stress zone of action. The main effect of the pressure distribution is to generate a force, due to the gradient of pressure on the thickness of the liquid zinc, that abruptly cuts the fluid vein and reduces the thickness of the coating, returning any quantities of Zn in excess to the bath. The value of this force is at its maximum when the gas jet is perpendicular to the strip surface.

The value of the final coating thickness is also determined by the action of the shear stress generated on the strip by the gas that fiovvs-along the surface of the strip. This value is at its minimum when the gas jet is perpendicular to the strip surface. Another effect of the gas jet is to partially cool the strip and the Zn exiting from 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. International standards and market demand have established a discrete number of admissible coating thicknesses and the respective tolerances suitable for successive industrial applications. As well as producing the required thickness, it is also necessary-to obtain constant thickness levels and maximum uniformity in the- galvanized surface to guarantee coating quality and to reduce to a minimum the amount of Zn required to obtain a determined coating, thus providing an economical advantage.

Among the limits of air knife technology, the most important are the lack of uniformity of the applied coating and the limited strip speed resulting in a limited productivity. Because of the very limited application zone for pressure force, the variation in the Zn thickness is very abrupt and, according to the gas flow rate and the shear stress which depends strongly also on the inclination angle of the jet with respect to the strip surface, for a given final thickness of Zn deposited on the strip, there is 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 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 by forming "jet lines", as well as environmental safety because of the dust released, and this represents one of the main causes that limits productivity in actual galvanizing plants. Furthermore the released drops also soil the air knives themselves.

In particular the "splashing" phenomenon occurs when a certain gradient of coating thickness reduction has been exceeded, and therefore over a-determined feeding speed of the strip, and when the exit angle of the air or gas jet exiting in a downward direction and creating the impact with the strip surface, said angle being defined by the direction of the relative air-zinc speed downwards with respect to the vertical plane, exceeds a critical value, thus provoking the detachment of solid particles (ZnO) from the surface of the strip. For example, for a feeding speed equal to 190 metres/min and with a flow rate by each air knife of 1000 Nm³/h at a pressure of 0,5 bar, the critical angle is approximately 15°. Another problem 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. Another limit related- to- this- technology is- caused by the different fluid dynamics and thermal situation present on the centre of the strip with respect to the strip edges. In fact, this situation leads to a lack of uniformity in the thickness of the coating on the total width of the strip, but this 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 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. Further limits imposed by air knife technology are represented 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;

- since the feeding speed of the strip is fixed, the final thickness of the coating depends on the peak of the pressure gradient force, but the pressure of the air or gas 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;

- vice-versa, in the case where the final thickness of the coating is fixed at a relatively reduced value, since it is not-possible to increase the airpressure 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.

For this reason a method and relative device must be realised for controlling the thickness of a coating on metal products, exiting from a hot bath which are able to overcome the aforesaid drawbacks. Summary of the invention

One object of the present invention is to provide a method and relative device to realise an operation of controlled removal of the excess coating during the final continuous galvanizing stage by the hot dipping of a flat metal product, such as a steel strip, by means of the use of electromagnetic fields and possibly gas jets in such a manner to increase the maximum productivity of the actual galvanizing lines and at the same time improve the quality of the final product, in particular reducing and possibly eliminating the problem of "splashing". A further object of the invention relates to the possibility of efficaciously controlling the weight of the coating and the uniformity of the distribution thereof. In order to achieve the aforesaid objects, according to a first aspect of the present invention, there is provided a method for controlling the thickness of the coating on a flat metal product, the product defining a feeding direction when it exits from a melted coating material bath in a continuous hot dip galvanizing processy wherein there are provided first means for generating at least one alternate monophase magnetic field and second means for generating gas jets, suitable for producing gas jets directed on the surfaces of major extension of said product, both said means being placed in proximity of said surfaces, the method comprising according to claim 1 the following stages: a) generating gas jets, by said second means, at a first restricted area along the width of the metal product, on each of said surfaces, in order to locally cool the coating possibly overheated and to remove part of the coating by means of the action of pneumatic forces; b) generating at least one non continuous alternate monophase- magnetic field, by said first means, in proximity of said surfaces of the product, said field inducing a distribution of induced currents on the surfaces- -in order to produce electromagnetic forces cooperating with said pneumatic forces for the removal o^'f part of the coating, said electromagnetic forces being distributed on a second area along the width of the metal product, on each of said surfaces, said second area comprising said first restricted area.

A second aspect of the invention provides a device for controlling the thickness of the coating on a flat metal product, the product defining a feeding direction when it exits from a melted coating material bath in a continuous hot dip galvanizing process, comprising, according to claim 10, means for generating gas jets, suitable for producing gas jets directed on the surfaces of major extension of said product at a first restricted area along the width of the metal product, in order to locally cool the coating possibly overheated and to remove part of the coating by means of pneumatic forces; means for generating at least one non continuous alternate monophase magnetic field, suitable for producing electromagnetic forces cooperating with said pneumatic forces for the removal of part of the coating, said electromagnetic forces being distributed on a second area along the width of the metal product, on each one of said surfaces, comprising said first restricted area, both said means being positioned in proximity of said surfaces. Advantageously the method and the device according to the invention provide the use of inductors^ supplied with alternate monophase current, which are possibly able to act in cooperation with air knives and produce electromagnetic forces that act on the layer of liquid Zn in a direction mainly perpendicular or parallel to the surface of the strip and in a manner to be able to superimpose the effects of these forces on the reduction of the thickness of Zn over the gasdynamic or pneumatic force effects, at the same time benefiting from the cooling action of the air jets. In the case of electromagnetic forces or volume forces acting in a direction substantially perpendicular to the surface of the strip, it is possible to increase quality of productivity because, with the same total pressure applied to the Zn coating, given by the combination of magnetic and gasdynamic forces, the pressure-and the flow rate of the air knife can be maintained under the critical values over which the "splashing" phenomenon is triggered. In the case of electromagnetic forces or volume forces acting_r~instead, in a direction substantially parallel to the surface of the strip, it is possible to increase quality of productivity also because the distribution of these forces is such that it produces a more gradual variation in the Zn thickness with respect to the rather abrupt variation typical of air knives. In fact, by introducing magnetic field gradients in a vertical direction, that is parallel to the strip, it is possible to vary the volume forces acting on the coating in a gradual manner. In this way quality productivity can be increased since, as well as increasing the overall coating removal force, it is possible to maintain the value of the angle defined by the direction of the relative air-zinc speed in a downward direction with respect to the vertical plane, well below the value of the critical angle for a determined value of air knife pressure and flow rate. Advantageously this makes it possible to prevent or at least control the undesirable "splashing" phenomenon even at high strip feeding speed. Advantageously, since the electromagnetic forces acting on the Zn coating are proportional to the frequency and intensity of the imposed magnetic field, it is possible to optimise these parameters in such a manner to obtain the maximum of the forces on the Zn, inducing a restrained heating in the Zn and in the strip that favours the volume force action, as they make the liquid Zn more fluid, reducing the kinematic viscosity and the surface tension. Furthermore, in this way, overheating does not occur to the extent that will cause metallurgical problems during the coating process.

The present invention also resolves the problem of the accumulated Zn on the edges of the strip since the temperature of the Zn and of the strip are more uniform on the thickness of the strip.

In this manner it is possible to obtain a strip with a uniform coating thickness on the total surface, and consequently, to avoid problems with winding and successively with the flatness of the galvanized strip, as well as uniformity problems in the coating properties when the strip is subjected to successive treatments, such as a successive "galvannealing" treatment, for example. Furthermore the amount of Zn required for a determined coating can be reduced to a minimum-with consequential economic advantages.

Lastly, thanks to the advantageous combination of air knives and magnetic fields generating electromagnetic forces cooperating with the pneumatic pressure- forces, it is possible to reduce the pressure of the air supply, thus also reducing the problems linked with coating oxidation.

Possibly, it is also possible to apply alternate magnetic fields, in directions both parallel and perpendicular to the strip, in particular in the case of magnetic fields, produced by inductors, that do not produce a magnetic saturation of the steel strip. These magnetic fields have a intensity variable between 0,05 and 0,5 T in air, preferably less than or equal to 0,3T.

The method according to the present invention can be applied to control the thickness of the coating on steel strips exiting from a hot bath, for example a bath of zinc, Zn-Al alloys, aluminium, Al and tin alloys. Brief description of the figures

Further characteristics and advantages will be made clearer from the detailed description of preferred but not exclusive embodiments of the method and device according to the invention with the help of the appended drawings wherein:

Figure 1 shows a diagram of an immersion process of a strip in a melted metal bath with the successive application of air knives;

Figure 2a shows a portion of strip on which a first magnetic field is applied, having a first direction;

Figure 2b shows a side view of the strip with-the diagram of the induced currents and the volume forces produced by the magnetic field shown in Fig. 2a;

Figure 3 shows a schematic view of a first embodiment of the device according to the invention; Figure 4 shows a schematic view of a second embodiment of the device according to the invention;

Figure 5 shows a schematic view of a third embodiment of the device according to the invention;

Figure 6 shows, a schematic view of a fourth embodiment of the device of ther invention;

Figure 7a shows a portion of strip on-which is applied a second magnetic field having a second direction;

Figure Zb shows a side view of the strip- with the diagram of the induced currents of the volume forces produced by the magnetic field of Fig. 7a; Figure 8- shows a schematic view of a fifth embodiment of the device of the invention;

Figure 8a shows a schematic view of a sixth embodiment of the device of the invention;

Figure 9a shows a portion of strip on which are simultaneously applied two second magnetic fields having opposite directions;

Figure 9b shows a side view of the strip with the diagram of the induced currents and the volume forces produced by the magnetic fields of Fig. 9a;

Figure 10 shows a schematic view of a seventh embodiment of the device of the invention; Figure 11 shows an example of the distribution of the pneumatic and electromagnetic forces acting on a strip;

Figure 12 shows a comparison between simulation results on reduction of coating thickness using air knives only and combining the action of an alternate magnetic field with the knives;

Figure 13 shows a further example of distribution of the electromagnetic forces acting on a strip; Figure 14 shows a section of a variant of the device according to the present invention.

Detailed description of preferred embodiments of the invention A diagram illustrating the galvanizing process of a metal strip by immersion in a hot bath is shown in Fig. 1. The metal strip 1 , suitably thermally pre-treated in a non-oxidising/reducing atmosphere is immersed in the bath 2 of melted Zn and is guided out from the bath in a vertical direction, at a predetermined speed, by three rollers immersed in the bath.

Above the bath 2, at each side of larger extension of the strip, there are provided means for generating gas jets, comprising nozzles or air knives 4 suitable to produce jets or blades of air or othergas, such as steam or. N₂, and therefore pneumatic forces to reduce the thickness of Zn deposited on the strip. These jets act at a first restricted area along the width-of the strip on both its surfaces 11 ; said first area extends along the feeding direction of the strip for approximately 5 mm. The supply pressure of nozzles 4 is preferably comprised betweerrθ,1 bar and 1 bar.

In order to perform the method according to the present invention, a relative device comprising means for generating non-continuous, alternate monophase electromagnetic fields in order to remove the excess coating material by means of the electromagnetic forces induced on the layers of strip coating, said means being advantageously possibly combined with the aforesaid means for generating gas jets.

A first embodiment of the method according to the present invention provide the generation of a non-continuous, alternate monophase uniform magnetic field B and having a direction substantially parallel to the feeding direction X of the strip, that is a vertical direction, as illustrated in Fig. 2a.

This magnetic field B induces induced electric currents 6 in both strip 1, for example made of ferromagnetic steel, and in the layers of zinc coating. Because of the greater electrical conductivity of the zinc with respect to the steel for predetermined frequency values, that depend on the thickness of the strip and on the presence or lack of magnetic saturation of the strip itself, the currents tend to concentrate on the coating surface. These currents 6 flow along the surface of the Zn coating transversally with respect to the feeding direction of the strip. The interaction between these induced currents 6 and the inducing magnetic-field B generates electromagnetic forces 7 that act on the coating mainly in a direction perpendicular to the surfaces 11. These electromagnetic forces 7 act on the coating of the strip in a manner similar to the pneumatic forces of the air knives or nozzles 4 in order to reduce the thickness of the Zn at a second area that substantially coincides with the first limited area, removing and sending back the excess Zn.

For typical thicknesses of strips used in galvanizing processes, it is advantageous to use_supply frequencies for the magnetic field higher than 100 Hz and lower than -500 kHz in order to be able to concentrate the electrical current on the surfaces of the strip and .of the Liquid Zn. Said supply frequency preferably ranges between 800Hz and 200^:kHz for a strip thickness of between 0,25 to 4 mm, in oder to avoid problems of overheating on the strip-but exclusively to produce a wiping action. In particular, for the efficiency of the prevalently vertical magnetic flux solutions, it is preferable that the ratio between the thickness of the strip and the depth of the current penetration in the strip, depending on the supply frequency of the coils or windings, has a value comprised between 0,5 and 20, preferably between 1 and 4. Instead, the intensity of the alternate magnetic field is preferably comprised between 0,005 and 0,5 T in air in the zone between the strip and the magnetic yoke poles or the coils.

In order to avoid problems of. local overheating on the strip, that is increases in temperature higher than 80⁰C, preferably no higher than 50⁰C, the frequency and intensity of the coil supply current must be selected in a manner such that the thermal flow transferred to the strip be less than 10 MW/m² , preferably no higher than 0,95 MW/m².

This first embodiment of the method according to the invention can be realised by means of a device comprising, in a first variant, one or more coils or windings 8 wound around the strip 1 and supplied with an alternate monophase current in a manner to create an longitudinal alternate magnetic field B inside the coils, as illustrated in Fig. 3. The air knives 4 are advantageously arranged in proximity to the coil 8, preferably at half-height of the winding. A second variant of the device, illustrated in Fig. 4, provides means for generating electromagnetic fields comprising two inductors, for example each one being composed of one or more windings or coils 9 wound around a core or ferromagnetic yoke 10, substantially having a C shape, while the means for generating gas jets comprise, for each inductor, a support and supply structure for the nozzles 4, comprising a feed manifold 12 for the gas and the same nozzles, positioned in proximity to each surface of major extension 11 of the steel strip 1 in exit from the melted coating material bath. In this manner it is possible to maximise the action of cooling and removal of the gas jet. The ferromagnetic cores 10, having a substantially C shape, are lamination stack or compact type- and produced in ferromagnetic or magneto-dielectric, or ferritic material, while the coils 9-are positioned opposite one another on each side of the steel strip 1 and can be cooled with water. There is provided the control of the alternating magnetic field frequency according .to the type and quality of the coating to be removed. Advantageously since the support structure, comprising-the feed manifold 12 and the nozzles 4, is positioned inside the ferromagnetic cores 10, the superposition of the gas jets over the action zone of the magnetic forces is always guaranteed. The nozzles 4, positioned in proximity of the magnetic yoke poles of each ferromagnetic core 10, are placed inside the inductors. In order to realise the alternate monophase magnetic field B, having a direction substantially parallel to the feeding direction X of the strip, an alternate current flows through the coils 9 with a phase shift angle between currents equal to 180° in a manner such that there is only a longitudinal magnetic flux generated by the magnetic flux loops 13, 13' circulating on each inductor. Advantageously by varying the arrangement, the number of the turns along the vertical axis or feeding direction of the strip, and/or the form of the ferromagnetic yoke 10 it is possible to also vary the distribution of the electromagnetic forces 7 on the liquid Zn coating in a more gradual manner compared to the usual narrow distribution produced by the air knives 4. This method also helps to resolve the "splashing" problem since the variation of the thickness of the strip occurs at a second area or zone, along the width of the strip, more extensive than the first restricted area of application of the pneumatic forces.

For example, by placing the turns of the coil 8 so that they are positioned closer to the strip at the top and_cjraduaiiy further away from the strip at the bottom, as illustrated in Fig. 5, and/or by providing a decreasing number of turns along the vertical plane towards the melted coating material bath, that is from top to bottom as illustrated in Fig. 6, otherwise by providing ferromagnetic yokes that concentrate the field lines differently, electromagnetic forces 7 decreasing in the direction opposite to the strip feeding direction are obtained. In the embodiment illustrated in Fig. 5 the flare angle β of the coil 8 with respect to the vertical plane is preferably between 0° and 60°. The embodiments of the device shown in figures 3, 5 and 6 can be provided on the upper part with Coanda effect air blades (not shown) or nozzle air blades (such as those indicated by reference 4) suitable to direct the jets in a downward direction to cool the^~action zone of-the electromagnetic forces onihe. strip- in order to prevent undesirable overheating of the Zn which would trigger uncontrolled formation of Fe-Zn alloys.

A second embodiment of the method according to the invention provides the generation of an alternate monophase non uniform magnetic field B' having a direction substantially perpendicular to the feeding direction X of the strip, that is the vertical direction, as illustrated in Fig. 7a. This magnetic field B' induces induced electric currents 6', both into the strip 1 , for example made of ferromagnetic steel, and in the layers of zinc coating, that flow along the width of the strip and that close laterally on the edges of the strip in the vertical direction as better illustrated in Fig. 7b. The interaction between these induced currents 6' and the inducing magnetic field B^! generates electromagnetic forces 7', 7" that act on the coating mainly in a direction substantially parallel to the surfaces 11. Since the magnetic field B' is not uniform, there are produced on the coating surface and the strip surface forces T directed downwards which are greater than the forces 7" directed upwards in order to favour the removal of the excess coating towards the bath.

In this case, for the typical thicknesses of strip used in galvanizing processes, it is advantageous to use magnetic field supply frequencies higher than 5 Hz and lower than 5 kHz in order to be able to concentrate the electric current on the strip surface and on the liquid Zn surface. Said supply frequency preferably ranges between 100Hz and 1000 Hz, more preferably between 200 Hz and 480 Hz, for a strip thickness of between 0,25 to 4 mm, in order to avoid problems of overheating on the strip but exclusively to produce a wiping action. Instead, the intensity of the magnetic field B¹ is preferably comprised between 0,005 and 0,5 T in air in the zone between the strip and the magnetic yoke poles or the coils.

This second embodiment of the method according to the invention can be realised by .means of a device, illustrated in Fig. 8, comprising two inductors, each one composed, for example, of one or more windings or coils 9' wound around a core or ferromagnetic yoke 10'. The two parts of the yoke 1Q^, shown in Fig. 8, each arranged at one surface of major extension of the strip 1, are advantageously conneεted-on a horizontal plane perpendicular to- the sheet in order to close and maximise the magneticiiuxτ The inclination of the poles 15 with respect to the vertical plane is defined by an angle y advantageously comprised between 0° and 60°.

The means for generating gas jets comprise for each inductor a support and supply structure for supporting and supplying nozzles 4', comprising a gas feed manifold 12', placed outside the ferromagnetic yoke 10'. The nozzles 4' are arranged immediately above said inductors and slightly inclined in a downward direction to ensure that the gas jet zone coincides with the action zone of the electromagnetic forces in order to maximise both the cooling action of the air and the removal of the coating. This solution allows an easier access for cleaning nozzles since the upper part thereof is unencumbered. Therefore, in order to concentrate and direct the forces on a precise point, magnetic fields are advantageously applied, said fields varying along the strip feeding direction and in particular being more intense in the zone where it is desiderable to concentrate to a larger extent the reduction forces for reducing the Zn thickness, and having an intensity decreasing in the adjacent zones. The greater the intensity variation of field B' along the vertical direction, the greater the possibility of concentrating the electromagnetic forces acting in the downward direction.

A variant for realising said second -embodiment of the method according to the invention provides the use of a series or winding of non-uniformly distributed turns 70 of the type illustrated in Fig. 8a. The turns 70, arranged on one side only with respect to the feeding direction of the strip, are wound in order to define axes perpendicular to said direction and an internal surface inclined, with respect to a vertical plane, of a angle preferably comprised between 0° and 60°. A further embodiment of the method according to the invention is illustrated in the diagrams of Figures 9a and 9b, according to which another manner for creating strong magnetic field gradients in a vertical direction, that is parallel to the. strip, in order to create- in the coating the volume forces directed mainly in a downward direction, is to cross two magnetic fields B¹, orthogonal with respect to the_ surfaces 11 of the strip, having opposite directions.

In both cases shown-in-F-igures 7a, 7b and 9a, 9br, the gradient of the field, that is the variation oHts- intensity, in vertical direction determines the variation of -the electromagnetic forces applied to the Zn and, consequently, their action more or less gradual on the reduction of the coating thickness.

This third embodiment of the method according to the invention can be realised by means of a device, such as that shown in Fig. 10, which is completely identical to that described above and illustrated in Fig. 4. In order to realise the alternate monophase magnetic field B', that passes through the feeding direction X of the strip in a substantially orthogonal direction, an alternate current is sent through the coils 9 with a phase shift angle between the currents equal to 0° in a manner such that there is a single magnetic flux crossing the strip twice in opposite directions, said flux being generated by the magnetic flux loop 13" common to the two inductors.

The use of ferromagnetic yokes or cores with suitably shaped poles allows to shape the magnetic field. In particular the inclination of the poles with respect to the vertical direction, that is the feeding direction of the strip, must be comprised between 0° and 60° in order to be effective.

With reference to the devices shown in figures 4 and 10, a further variant of the invention provides the variation of the phase shift angle between the currents in the range of ±180°, with values different than 0° and 180°, in order to generate magnetic fluxs longitudinal and transversal with respect to the feeding direction of the strip, having an intermediate intensity comprised between the minimum and maximum values. Figure 13 illustrates the situation that occurs in the zone comprised between strip 1 and the respective magnetic yoke poles of the ferromagnetic cores or yokes 10, substantially having a C shape, in the case in which the magnetic fields produced by the inductors do not produce magnetic saturation on the strip (possible if the intensity of the field in air < 0,3T). Advantageously, because of the ferromagnetic properties of the steel strip, the lines of magnetic field B' in proximity of the strip surface, and therefore initie thin layer of Zn, are perpendicular to the strip in the entry and exit zones of the strip from the inductors. The component of the magnetic field B^! perpendicular to the strip reacts with the induced currents on- the plane of the Zn thickness in order to produce forces T advantageously directed downwards, that contribute to a more gradual removal of the coating, thus limiting the "splashing" phenomenon. Instead, the component of the magnetic field B parallel to the strip reacts with the induced currents that flow on the surface of the Zn coating transversally with respect to the feeding direction of the strip. The interaction between these induced currents 6 and the inducing magnetic field B generates electromagnetic forces 7 that act on the coating mainly in a direction perpendicular to the surfaces 11. These electromagnetic forces 7 act on the coating of the strip in a manner similar to the pneumatic forces of the air knives or nozzles 4 in order to reduce the Zn thickness at an area that substantially coincides with the restricted area of action of said pneumatic forces, removing and returning back the excess Zn. In all cases in which there is provided a magnetic field component parallel to the strip feeding plane, in order to avoid problems of local overheating on the strip, and therefore increases in temperature higher than 80°C, preferably no higher than 50°C, the frequency and intensity of the coil supply current must be selected so that the thermal flow transferred to the strip is lower than 10 MVWm² , preferably no higher than 0,95 MVWm².

The same situation occurs in the zone comprised between strip 1 and the coils wrapping the strip, as for example shown in Figure 3, in particular in the case in which there is provided a magnetic core wound externally around= the coil to increase the concentration of the magnetic fields B and B'.

With reference to the devices illustrated in Figures 4 and 10, a variant can provide that the magnetic core or yoke 10 can also perform the function of "air knife". This is possible because the polar expansions or magnetic poles 14', 14", can be appropriately shaped to define the nozzles 4 adapted to generate gas jets, as in the example in Fig. 14. In this variant, advantageously there are provided bulkheads 30, or slots, at the inlet section of said nozzles 4 which are conceived to equalise the flow rate inside the nozzles themselves. In this case, therefore, the nozzles 4 are defined by the configuration of the polar expansions 14', 14" and have a passage orifice which, when seen in transversal section (Fig. 14), has a shape tapering along the feeding direction of the strip. In the embodiment shown in Fig. 14, in particular, said passage orifice comprises- two successive tapering stretches defining directions incident with one another. In this case the distance between the magnetic yoke poles 14', 14", respectively the upper one and the lower one, is comprised between 0,5 and 5 mm.

Advantageously, in order to reduce the induction heating of the support and supply structure of the gas knives, placed inside each ferromagnetic core 10 and comprising the manifold 12 and possibly the nozzles 4, it can be provided at least one high electrical conductivity shield, placed between said structure and the core 10, that performs two functions:

- preventing air knife overheating by induction,

- and concentrating the magnetic flux directly in the zone where the gas jet acts. Additional high electrical conductivity shields can be provided, placed outside each ferromagnetic core and in proximity of the magnetic yoke poles, in order to reduce the induction heating on the strip 1 and on the coating layer 11 , when the temperatures become excessive for the process. By means of this appropriate positioning of the external shields the magnetic flux reduction can be limited in the zone where the gas jet acts in order to maintain the efficiency of the removal system of the excess coating material.

Said shields also act as magnetic field concentrators in the space between strip and magnetic core, partially increasing the local efficacy of action of said field on the strip.

According to a further variant, the aforesaid electromagnetic shields, inside or outside the magnetic cores, can be shaped in a manner that they form the nozzles for the gas jets. Therefore, in this case, the nozzles are defined by the configuration of the electromagnetic shields.

Fig. 11 shows an example of distribution of the volume force or Lorentz force 20 obtainable by imposing an alternate magnetic field at 200Hz in comparison with the pneumatic pressure force 21 generated by an air knife. It can be seen that the distribution of the volume forces 20 generated by the variable, magnetic field is not concentrated in a reduced space as are the pneumatic forces 21, typically acting on an area with a height of approximately 5 mm; but is much more extensive. In Fig. 11 the volume forces 20, useful for the coating removal, that is pushing downwards, extend for approximately 150 mm along the strip. This-fact favours a more gradual variation in the Zn thickness on the strip. Furthermore, it is possible to superimpose the action zones of the alternate magnetic fields -and of the pneumatic forces, taking in account the nature of the sources of the variable magnetic field, without the need to slope the gas jet, thus maximising the action of said jet and reducing the shear stress dangerous for triggering the "splashing" phenomenon. Figure 12 shows a comparison between simulation results about a reduction of coating thickness using air knives only and combining with these the action of an alternate magnetic field. It can be seen how the effect of the additional magnetic field (line 22) is able to both reduce the final thickness with respect to that obtainable with the air knife only (line 23), and provoke a more gradual thickness reduction that does not trigger the "splashing" phenomenon.

By superimposing in this way the gasdynamic or pneumatic forces, concentrated under the gas jet, and the electromagnetic forces distributed more gradually along the strip, it is possible to obtain a more gradual reduction of the thickness of Zn in order to be able to operate under all the "wiping" operation conditions with angles lower than the critical angle which provokes the undesirable "splashing" phenomenon. Advantageously the method according to the invention ailows to operate at a strip feeding speed comprised-between 1 and 5 metres/sec.

A further advantage is represented by the fact that the heating induced by the currents 6, 6' is such that it contrasts the cooling effect caused by the action of the gas jets or air knives, whereby the air knives or nozzles 4, 4' must be provided above or at most in correspondence with the impact zone of said jets. In this manner the strip can be maintained in motion at a temperature that is substantially equal to the temperature at the.exitfrom the bath 2 until it reaches the impact zone of the jets, in this manner avoiding the zinc surface solidification in proximity of the nozzles, in fact, the surface of the strip that could be subject to the risk of solidification is that just under- the nozzles, that is under the impact zone of the air jets, having a width- approximately equal to that of the strip and a height ranging from a few millimetres to 10 mm which corresponds with the pressure peak of the gas jet. Lastly, a variant of the method according to the invention provides only the-use of the means for generating electromagnetic fields, and thus without providing the use of air knives for the removal of excess coating material. This last variant can be used advantageously for strip feeding speed up to approximately 3 metres/sec with the advantage of being able to avoid all the problems linked with the use of air knives and thus obtaining a higher quality. All the embodiments of the present invention are characterised by a vertical distribution of the coils or of the terminal parts of the magnetic yokes, in proximity of the strip feeding plane, ranging between 10÷100 mm in order to concentrate the electromagnetic force along a stretch of strip that extends in longitudinal direction for 5-150 mm. Compared to devices that exploit a "travelling magnetic field", with this distribution it is possible to obtain electromagnetic forces with an increase in maximum intensity equal to 20%. In this manner it is also possible to take advantage of the cooling action of the concentrated pneumatic jets more efficiently.

Claims

1. Method for controlling the coating thickness of a flat metal product, the product defining a feeding direction when it exits from a melted coating material bath in a continuous hot dip galvanizing process, wherein there are provided first means for generating at least one alternate monophase magnetic field and second means for generating gas jets, suitable for producing gas jets-directed on the surfaces of major extension (11) of said product, both said means being placed in proximity of said surfaces, the method comprising the following stages: a) generating gas jets, by said second means, at a first restricted area along the width of the metal product, on each of said surfaces, in order to locally cool the coating possibly overheated and to remove part of the coating by means of the action of pneumatic forces; b) generating at least one non continuous alternate monophase magnetic field (B, B'), by said first means, Jn proximity of said surfaces of the product, said field inducing a distribution of induced- currents on the surfaces in order to produce electromagnetic forces cooperating with said pneumatic forces for the removal of part of the coating, said electromagnetic forces being distributed on a second area along the width _αf the metal product, on each of said surfaces, said second area comprising said first restricted area.

2. Method according to claim 1 , wherein the alternate monophase magnetic field (B) is uniform and has a direction substantially parallel to the feeding direction of the product in such a manner that the electromagnetic forces produced (7) act on the coating material mainly in a direction perpendicular to the surfaces (11) at said second area substantially coinciding with said first restricted area.

3. Method according to claim 1 , wherein said at least one alternate monophase magnetic field (B') is non uniform and has a direction substantially perpendicular to the feeding direction of the product in such a manner that the electromagnetic forces produced (7^!) act on the coating material mainly in a direction parallel to the surfaces (11) at said second area which has a greater extension than the first area along said feeding direction.

4. Method according to claim 2, wherein said alternate magnetic field has a supply frequency such that it is obtained a predetermined ratio between thickness of the metal product and depth of penetration of the induced currents in the product itself, preferably between 0,5 and 20.

5. Method according to claim 4, wherein said supply frequency is between 100 Hz and 500 kHz, preferably between 800 Hz and 200 kHz, and the magnetic field has an intensity between 0,005 and 0,5T.

6. Method according to claim 3, wherein two alternate monophase magnetic fields (B¹) are generated, said fields having directions substantially perpendicular to the feeding direction of the product and opposite to one another.

7. Method according to claim 3, wherein the supply frequency of the magnetic field is higher than 5 Hz and lower than 5 kHz, preferably between 100 Hz and 1000 kHz, and the magnetic field has an intensity between 0,005 and 0,5T.

8. Method according to claim 1 , wherein there is provided the variation of the phase shift angle between the currents generating said at least one non continuous -magnetic field (B, B') within the range ±180°, with values different than 0° and 180°, in order to generate magnetic fluxs-longitudinal and transversal with respect-to. the feeding direction of the product.

9. Method-according to claim 8, wherein, in the case of magnetic fields produced by the first means that do noLproduce magnetic saturation on the flat metal product, in the zone- comprised between the feeding direction of the product-and said first means, there are produced first electromagnetic forces (7) acting on the coating material mainly in a direction perpendicular to the surfaces (11) and second electromagnetic forces (7') acting on the coating material mainly in a direction parallel to surfaces (11) in order to contribute to a more gradual coating removal.

10. Device for controlling the coating thickness of a flat metal product, the product defining a feeding direction (X) when it exits from a melted coating material bath in a continuous hot dip galvanizing process, comprising means for generating gas jets, suitable for producing gas jets directed on the surfaces of major extension (11) of said product at a first restricted area along the width of the metal product, in order to locally cool the coating possibly overheated and to remove part of the coating by means of pneumatic forces; means for generating at least one non continuous alternate monophase magnetic field (B, B^!), suitable for producing electromagnetic forces cooperating with said pneumatic forces for the removal of part of the coating, said electromagnetic forces being distributed on a second area along the width of the metal product, on each one of said surfaces, comprising said first restricted area, both said means being positioned in proximity of said surfaces.

11. Device according to claim 10, wherein said means for generating at least one magnetic fieid comprising one or more coils (8) wound around the feeding direction (X), and said means for generating gas jets comprise nozzles (4) arranged in proximity of the one or more coils (8).

12. Device according to claim 11 , wherein the turns of the coil (8) define a flare angle (β) of the coil itself with respect the vertical plane, preferably between 0 and 60°, in order to obtain electromagnetic forces (7) decreasing in the direction opposite to the metal product feeding direction.

13. Device according, to claim 11 , wherein the number of turns of the coil ^"(8) decreases along the vertical plane towards the melted coating-material bath, in order to obtain electromagnetic forces (7) decreasing in the direction opposite-to the metal product feeding-direction.

14. Device according to claim 10, wherein said means for generating at least one magnetic field comprise two inductors, comprising one or more coils (9, 9') wound around a ferromagnetic yoke (10, 10').

15. Device according to claim 14, wherein said means for generating gas jets comprise, for each inductor, a support and supply structure for supporting and supplying nozzle (4, 4'), the structure comprising a gas feed manifold (12, 12'), placed inside or outside the respective electromagnetic yoke.

16. Device according to claim 15 wherein said nozzles (4, 4') are placed in proximity of magnetic yoke poles of each ferromagnetic yoke (10, 10'), inside or outside the inductors, and said ferromagnetic yoke poles (10') can have a surface inclined, with respect to the feeding direction of the metal product, of an angle between 0° and 60°.