EP2167697B1 - Method and device for controlling the thickness of coating of a flat metal product - Google Patents
Method and device for controlling the thickness of coating of a flat metal product Download PDFInfo
- Publication number
- EP2167697B1 EP2167697B1 EP08762808A EP08762808A EP2167697B1 EP 2167697 B1 EP2167697 B1 EP 2167697B1 EP 08762808 A EP08762808 A EP 08762808A EP 08762808 A EP08762808 A EP 08762808A EP 2167697 B1 EP2167697 B1 EP 2167697B1
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- EP
- European Patent Office
- Prior art keywords
- metal product
- flat metal
- gas
- magnetic
- strip
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- 238000000576 coating method Methods 0.000 title claims abstract description 68
- 239000011248 coating agent Substances 0.000 title claims abstract description 67
- 229910052751 metal Inorganic materials 0.000 title claims description 26
- 239000002184 metal Substances 0.000 title claims description 24
- 238000000034 method Methods 0.000 title claims description 21
- 230000005291 magnetic effect Effects 0.000 claims abstract description 79
- 239000000463 material Substances 0.000 claims abstract description 30
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- 238000001514 detection method Methods 0.000 claims 1
- 239000011701 zinc Substances 0.000 abstract description 19
- 229910000831 Steel Inorganic materials 0.000 abstract description 18
- 239000010959 steel Substances 0.000 abstract description 18
- 239000011247 coating layer Substances 0.000 abstract description 16
- 239000010410 layer Substances 0.000 abstract description 12
- 229910052725 zinc Inorganic materials 0.000 abstract description 10
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 abstract description 9
- 238000009826 distribution Methods 0.000 abstract description 7
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- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 238000007598 dipping method Methods 0.000 description 2
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- YVIMHTIMVIIXBQ-UHFFFAOYSA-N [SnH3][Al] Chemical compound [SnH3][Al] YVIMHTIMVIIXBQ-UHFFFAOYSA-N 0.000 description 1
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- HXFVOUUOTHJFPX-UHFFFAOYSA-N alumane;zinc Chemical compound [AlH3].[Zn] HXFVOUUOTHJFPX-UHFFFAOYSA-N 0.000 description 1
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Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/14—Removing excess of molten coatings; Controlling or regulating the coating thickness
- C23C2/16—Removing excess of molten coatings; Controlling or regulating the coating thickness using fluids under pressure, e.g. air knives
- C23C2/18—Removing excess of molten coatings from elongated material
- C23C2/20—Strips; Plates
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/14—Removing excess of molten coatings; Controlling or regulating the coating thickness
- C23C2/24—Removing excess of molten coatings; Controlling or regulating the coating thickness using magnetic or electric fields
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/50—Controlling or regulating the coating processes
- C23C2/52—Controlling or regulating the coating processes with means for measuring or sensing
- C23C2/524—Position of the substrate
Definitions
- 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.
- 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 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.
- 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 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.
- 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.
- 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.
- EP 0525387 A1 discloses a method for controlling coating weight on a hot-dipping steel strip, with the provision of flowing a high-frequency current strong enough to magnetically saturate the steel strip through a pair of high-frequency current conducting paths to induce a high-frequency current of an opposite phase in the steel strip, so that a magnetic pressure acting on surfaces of the steel strip is generated by interaction of the induced high-frequency current with a high-frequency current of the high-frequency current conducting paths.
- WO 2006/006911 A1 , WO 2007/004945 A1 and BE 1011059A6 disclose the combined use magnetic fields and gas fets on the same area of a hot difs coated elongated metallic element to control the thickness of the metallic element.
- FR 2754545 A discloses a pair of electromagnetic induction coils for controlling coating on hot-diffed metal strip. A screen for confining the electromagnetic field is associated with each induction coil.
- 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.
- a method for controlling the thickness of coating of a flat metal product according to claim 1.
- a second aspect of the invention provides a device for controlling the thickness of coating of a flat metal product, suitable for defining a feeding direction when-it exits from a bath of coating material in continuous hot-dip galvanization processes in accordance with claim 7.
- 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.
- 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.
- 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.
- oscillation or deformation of the strip during feeding thereof is thus avoided.
- 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.
- 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.
- 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.
- a coating is obtained of much smaller and more uniform thickness than in current plants, as well as higher production rates.
- 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.
- 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 bar and 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.
- the ensemble of the device constituted by the inductors together with the gas manifold 1 and the gas nozzles 2 can be inclined according-to 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.
- 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.
- 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.
- the inductors of the device of the invention it is possible to:
- the gas knives advantageously perform the function of control of the temperature, preventing an excessive induction heating both of the coating 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.
- the coils 5 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 100 and 500 Hz.
- 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.
- the invention provides the possibility of using just the means for generating electromagnetic fields individually.
- 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.
- 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.
- the magnetic yoke itself performs also the function of air knife.
- the polar expansions or poles 14' and 14" are shaped appropriately in order to define the nozzles 2 adapted to generate the gas jets.
- 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.
- said passage orefice comprises two successive tapered stretches defining mutually incident directions.
- the distance between the magnetic yoke poles 14', 14", top and bottom respectively is comprised between 0,5 and 5 mm.
- 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.
- 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 .
- 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.
- 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.
- 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.
- 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 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.
- 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.
- 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/m 3 ; 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.
- 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.
- 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.
- the steel strip 3 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.
- the currents in the windings have a phase shift comprised in the range ⁇ 180°
- 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.
- 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.
- 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 can be used also for measuring the position of the strip and for generating the signal necessary for the power source to change 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 of the 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 curves 240 and 241 represent the trend as the phase shift ⁇ , respectively, of the Maxwell force and of the sum of the Maxwell force and of the force of repulsion 24 vary.
- 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:
- 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.
- 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.
- 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.
- a magnetic material having a high electrical resistance e.g., iron or laminated steel, ferrite or magneto-dielectric material.
- 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.
- the nozzles 2 will be defined by the configuration of the electromagnetic shields.
- 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.
- 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 knives. 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.
Abstract
Description
- 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.
- 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 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.
- 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 final 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 problem described above, are unable, however, to increase the current maximum productivity of galvanizing lines, since the volume force produced by said devices on the layer 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 - 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".
EP 0525387 A1 discloses a method for controlling coating weight on a hot-dipping steel strip, with the provision of flowing a high-frequency current strong enough to magnetically saturate the steel strip through a pair of high-frequency current conducting paths to induce a high-frequency current of an opposite phase in the steel strip, so that a magnetic pressure acting on surfaces of the steel strip is generated by interaction of the induced high-frequency current with a high-frequency current of the high-frequency current conducting paths.>
WO 2006/006911 A1 ,WO 2007/004945 A1 andBE 1011059A6
FR 2754545 A - 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.
- 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, according to
claim 1. - A second aspect of the invention provides a device for controlling the thickness of coating of a flat metal product, suitable for defining a feeding direction when-it exits from a bath of coating material in continuous hot-dip galvanization processes in accordance with claim 7.
- 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.
- 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 inFigure 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. - 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 acore 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 supplyingnozzles 2, comprising agas 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 bar and 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 thecoils 5 are arranged facing one another on each side of thesteel 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 thegas nozzles 2 can be inclined according-to 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 thenozzles 2, are arranged within theferromagnetic 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". Thenozzles 2, arranged in proximity of themagnetic yoke poles 14', 14" of eachferromagnetic 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 the coating layers 11 and of the
steel strip 3. In this way, then, the induced currents never overheat the coating layers 11 and thesteel 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, thesteel 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 therange 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 theferromagnetic cores 4, i.e. the distance between the common branches of the magnetic flux, is as small as is allowed by thenozzles 2 that generate the gas knife, which are arranged advantageously inside or outside the inductors in proximity of saidpoles 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 orpoles 14' and 14" are shaped appropriately in order to define thenozzles 2 adapted to generate the gas jets. In said variant,partitions 30, or slots, are advantageously provided at the inlet section of saidnozzles 2, said slots having the purpose of equalizing the flow rate within the nozzles themselves. Thenozzles 2, in this case, are therefore defined by the configuration of thepolar 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 ofFigure 3 , in particular, said passage orefice comprises two successive tapered stretches defining mutually incident directions. In this case, the distance between themagnetic 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 thecore 4, as illustrated inFigure 3 , which is substantially C-shaped. Thewindings 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 ofFigure 1 , the lines ofmagnetic flux 15 generated by thecoils 5 in theferromagnetic core 4 and outside the core (dispersed flux). - Each inductor creates its own loop of
magnetic flux poles 14', 14" of theferromagnetic core 4, as illustrated in the right-hand part ofFigure 2 , and acommon loop 151 of magnetic flux embracing both of theferromagnetic cores 4, as illustrated in the left-hand part ofFigure 2 . Thus, themagnetic flux strip 3 in a substantially vertical direction (right-hand part ofFigure 2 ), i.e. tangentially to the surfaces, and simultaneously the loop of commonmagnetic flux 151, which flows between the two inductors, passes twice through thesteel strip 3 in directions substantially perpendicular to thesurfaces 11 of thestrip 3 and opposite each other according to thearrows 18', 18", visible inFigure 6 , respectively in the area of the top magnetic poles 14' and in the area of the bottommagnetic poles 14". -
Figure 6 shows that the component of the magnetic flux oriented in a direction perpendicular to thestrip 3 induces in the strip two loops of induced current 17', 17", which surround, respectively, the magnetic flux indicated by thearrows 18', 18". These twocurrent loops 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 inducedcurrents 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 thecurrent loop 17' with themagnetic flux 18", in order to maximize the intensity thereof the shape of the twomagnetic yoke poles 14', 14" is, in both of the variants, tapered and optimized for increasing to a maximum the intensity of current in theloop 17' and for concentrating themagnetic flux 18" on thestrip 3 and on the coating layers 11. In this way, the undesirable electromagnetic forces directed upwards 20', produced by the interaction of thecurrent loop 17" with themagnetic 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 magnetic flux 19 of each ferromagnetic core 4 (associated to the loops ofmagnetic flux Figure 2 ) produces theelectromagnetic forces 21, which have an orientation perpendicular to the surfaces of the strip, as illustrated inFigure 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, themagnetic flux 19 of each inductor induces acurrent loop 22 that surrounds thestrip 3 and the coating layers 11. The interaction between thecurrents 22 and theelectromagnetic flux 19 creates transverseelectromagnetic forces 23', 23", which are substantially perpendicular both to thestrip 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 thestrip 3. From the difference between theforces 23' and 23" there can be generated a resultant force perpendicular to thestrip 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 thestrip 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 theelectromagnetic forces 23', 23" with the area ofimpact 12 of the gas jets. - Represented in
Figure 4 is a graph of the longitudinalelectromagnetic forces 20', 20" and transverseelectromagnetic 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 transversemagnetic 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 inFigure 2 on the left), themagnetic fluxes 18', 18" join in acommon loop 151 of magnetic flux, and the longitudinalelectromagnetic 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
Figure 2 on the right), the loop ofmagnetic flux 151 vanishes and there remains only the longitudinalmagnetic flux 19, generated by theloops - 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 themagnetic 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 thestrip 3 from the centre of themagnetic 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 twomagnetic fluxes integral forces 21 and the difference between the transverseelectromagnetic forces 23' and 23" that tends to move thestrip 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 theforces 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 thestrip 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, orinductive type 14, as illustrated inFigure 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 thestrip 3 deviates from the position centred in thegap 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 position of the strip and for generating the signal necessary for the power source to change 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. Thecurve 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. Thecurves - 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 theforces Figure 6 for countering the ferromagnetic attraction exerted by the poles of the inductor, is maximum on thestrip 3 when the phase shift is approximately 90°. - In the same
Figure 7 , thecurves 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 ϕΔ = ±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 themanifold 1 and possibly thenozzles 2, at least one-high-conductivityelectrical shield 16 can be provided, arranged between said means and the core 4 (as illustrated inFigure 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 eachferromagnetic core 4 and in proximity of the poles of themagnetic yokes 14', 14", in order to reduce the induction heating on thestrip 3 and on thecoating 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 thearea 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 thenozzles 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, thenozzles 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 knives. 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 (9)
- 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 adapted to be supplied with single-phase alternate or impulsive 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 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, activating said inductors with said alternate or impulsive current with frequency in a range comprised between 100 and 500 Hz thereby 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 the oscillations of the flat metal product (3), characterized in that first high conductivity shields (16) are arranged inside each ferromagnetic core (4) to protect the gas jets from overheating and to concentrate the magnetic flux directly in said area of impact (12), and in that second high conductivity shields (160', 160") are provide outside each ferromagnetic core to reduce the induction heating on the flat metal product (3).
- The method according to claim 1, wherein the alternate or impulsive supply currents have 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,
- The method according to claim 2, wherein the phase shift angle of the supply currents is comprised in the range ±180 °, preferably equal to ±90°.
- The method according to claim 3, 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 centered position by means of a reversal of the phase angle between the supply currents of the two inductors.
- The method according to claim 4, 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.
- 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-continuous 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 nozzles being arranged in proximity of magnetic yoke poles (14', 14") of each magnetic core (4);- supply means, provided within each inductor, 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,said device being characterized in that
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 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"). - The device according to claim 6, wherein the nozzles (2) are defined by the configuration of magnetic yoke poles (14', 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.
- The device according to claim 6, wherein the nozzles (2) are arranged inside or outside the inductors.
- The device according to claim 7 or 8 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)
Applications Claiming Priority (2)
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IT001166A ITMI20071166A1 (en) | 2007-06-08 | 2007-06-08 | METHOD AND DEVICE FOR THE CONTROL OF THE COATING THICKNESS OF A METAL METAL PRODUCT |
PCT/IB2008/001472 WO2008149218A2 (en) | 2007-06-08 | 2008-06-09 | Method and device for controlling the thickness of coating of a flat metal product |
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EP2167697B1 true EP2167697B1 (en) | 2012-08-08 |
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EP3587613A4 (en) * | 2017-02-24 | 2020-01-01 | JFE Steel Corporation | Continuous molten metal plating apparatus and molten metal plating method using said apparatus |
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RU2482213C2 (en) * | 2008-09-23 | 2013-05-20 | Сименс Фаи Металз Текнолоджиз Сас | Method and device to squeeze liquid coating metal at outlet of tank for application of metal coating by submersion |
CN114269033A (en) * | 2016-09-27 | 2022-04-01 | 诺维尔里斯公司 | Rotary magnet thermal induction |
CN111334737A (en) * | 2020-04-13 | 2020-06-26 | 西安泰力松新材料股份有限公司 | Photovoltaic solder strip tin plating system and tin plating method |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
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GB2010917A (en) * | 1977-12-15 | 1979-07-04 | Australian Wire Ind Pty | Controlling metal coatings on wire strip and the like emerging from metal baths |
Family Cites Families (7)
Publication number | Priority date | Publication date | Assignee | Title |
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SE341651B (en) * | 1969-05-19 | 1972-01-10 | Asea Ab | |
KR950000007B1 (en) * | 1991-06-25 | 1995-01-07 | 니홍고오깡가부시끼가이샤 | Method of controlling coating weight on a hot-dipping steel strip |
FR2754545B1 (en) * | 1996-10-10 | 1998-12-11 | Maubeuge Fer | METHOD AND DEVICE FOR SPINNING A CONTINUOUSLY COATED OR TEMPERED COATED METAL STRIP |
BE1011059A6 (en) * | 1997-03-25 | 1999-04-06 | Centre Rech Metallurgique | Method of coating a steel strip by hot dip galvanising |
SE527507C2 (en) * | 2004-07-13 | 2006-03-28 | Abb Ab | An apparatus and method for stabilizing a metallic article as well as a use of the apparatus |
SE528663C2 (en) * | 2005-06-03 | 2007-01-16 | Abb Ab | An apparatus and method for coating an elongated metallic element with a layer of metal |
SE529060C2 (en) * | 2005-06-30 | 2007-04-24 | Abb Ab | Thickness-controlling device for metallic coating on elongated metallic strip comprises second wiper associated with respective electromagnetic wiper and designed to apply jet of gas to strip |
-
2007
- 2007-06-08 IT IT001166A patent/ITMI20071166A1/en unknown
-
2008
- 2008-06-09 EP EP08762808A patent/EP2167697B1/en not_active Not-in-force
- 2008-06-09 WO PCT/IB2008/001472 patent/WO2008149218A2/en active Application Filing
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2010917A (en) * | 1977-12-15 | 1979-07-04 | Australian Wire Ind Pty | Controlling metal coatings on wire strip and the like emerging from metal baths |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP3587613A4 (en) * | 2017-02-24 | 2020-01-01 | JFE Steel Corporation | Continuous molten metal plating apparatus and molten metal plating method using said apparatus |
US11162166B2 (en) | 2017-02-24 | 2021-11-02 | Jfe Steel Corporation | Apparatus for continuous molten metal coating treatment and method for molten metal coating treatment using same |
Also Published As
Publication number | Publication date |
---|---|
ITMI20071166A1 (en) | 2008-12-09 |
WO2008149218A8 (en) | 2009-04-16 |
EP2167697A2 (en) | 2010-03-31 |
WO2008149218A2 (en) | 2008-12-11 |
WO2008149218A3 (en) | 2009-01-29 |
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