EP2748348A1

EP2748348A1 - Plant for coating flat metal products by means of continuous hot dipping and relative coating process

Info

Publication number: EP2748348A1
Application number: EP12769740.7A
Authority: EP
Inventors: Alfredo Poloni; Zamir POMARE'
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2011-08-24
Filing date: 2012-08-22
Publication date: 2014-07-02
Anticipated expiration: 2032-08-22
Also published as: EP2748348B1; WO2013027179A1; CN103890218A; KR20140048330A; ITMI20111544A1; KR101596609B1; US20140193586A1

Abstract

A plant for coating a strip (5) by means of continuous hot dipping, comprising a containment tank (2) to contain molten zinc; a coating vessel (3) receiving the molten zinc, arranged above the containment tank and provided with an opening (4) on a bottom thereof for passing the strip; a chamber (6) arranged below the coating vessel and provided with guide rollers (7, 8) to guide the strip towards said opening; wherein the containment tank is arranged below the guide rollers so that chamber and containment tank form a single place, and wherein there is provided an intercepting and conveying system (9), arranged within the chamber, to intercept and convey the molten zinc leaking from said opening into the containment tank, said intercepting and conveying system being configured to bypass and wind the guide rollers and the strip being fed towards said opening.

Description

PLANT FOR COATING FLAT METAL PRODUCTS BY MEANS OF CONTINUOUS HOT DIPPING AND RELATIVE COATING PROCESS

Field of the invention

The present invention relates to a plant for coating, by means of continuous hot dipping, flat metal products, such as steel strips, with a coating metal, which may be, for example, Zn or alloys containing Zn, and relates also to a relative coating process by means of continuous hot dipping of flat metal products.

State of the art

A known process for coating a steel strip provides that the strip is passed vertically through a bath of molten metal maintained in semi-levitation by means of an alternating magnetic field.

The conventional hot galvanizing process is continuous and normally requires, as a preliminary step, the pre-treatment of the steel strip and the preheating thereof with a careful control of the temperature of the strip prior to the application of the coating. The pre-treatment - chemical and thermal - improves the adhesion of the coating on the strip, and the pre-treatment step may be both a preliminary heating operation in a controlled atmosphere and a fluxing operation in which the strip is immersed in a reducing inorganic flux, to coat the surface of the strip with a protective film which prevents the oxidation thereof.

When the steel strip is subjected to preliminary heating in a controlled atmosphere, it may enter the coating bath at a high temperature which, in the case of a coating bath composed of Zn or Zn alloys, may also be equal to that of the metal bath. This type of hot galvanizing process provides a coating step carried out in a semi- levitated bath contained in a vessel with an opening on the bottom in order to allow the strip to pass upwards. The strip is guided by means of one or two deflector rollers placed below the molten coating bath. The strip enters the bath normally from below with a substantially vertical direction and then goes through the molten coating material. Subsequently, the strip is extracted from the metal bath in a vertical direction.

The known systems which use magnetic Ievitation of the bath disadvantageously involve disturbances on the lower meniscus of the bath and consequent inevitable leakages of the coating liquid through the opening on the bottom of the vessel containing the bath. The probability of leakages of molten material through the lower meniscus increases with the oscillation of the strip. Furthermore, leakages of molten material are always present in the start-up or shut-down step of the plant and in any emergency situation in which it is necessary to stop the plant.

Disadvantageously, the coating liquid which leaks from the vessel containing the bath falls onto the strip below, which is at a standstill or being fed towards the vessel, and on the guide rollers of the strip itself. The adhesion of drops of coating liquid on both the strip and the guide rollers cause the formation of imprints on the strip, deteriorating the quality of the final product.

Furthermore, the drops of coating liquid which adhere to the guide rollers make frequent shut-downs of the plant necessary over the course of time, in order to replace dirty rollers and clean them for subsequent reuse.

A known coating plant is described in the document US6290776B1. This plant provides sophisticated sealing systems in order to limit, during the start-up and shut-down steps of the plant only, the falling of molten metal from the opening of the funnel towards the guide rollers of the strip, conveying the leaked motel metal into at least one collecting tank provided above the guide rollers. There are also provided special moving means, e.g. hydraulic, for moving the sealing systems (Example 5). This solution, in addition to being complex and costly, does not allow the conveying of the leakages of liquid metal which are always present even during the steady state operation of the plant.

Another known plant is described in the document US5702528A. This plant provides conveying means for conveying the molten metal exiting from the opening of the funnel which only enter into action in cases of emergency, when the application of the magnetic field is interrupted and the molten metal is drained from the funnel.

The need was therefore felt to provide a plant for coating flat metal products by means of continuous hot dipping and a relative coating process which allow the aforementioned drawbacks to be overcome.

Summary of the invention

The primary object of the present invention is to provide a plant for coating flat metal products, for example steel strips, by means of continuous hot dipping in a vessel containing a molten coating bath placed above the guide rollers of the strip, said plant having a simple and cost-effective structure, configured for an optimal management of leakages of coating liquid from the bottom of said vessel, in both the start-up or shut-down step of the plant or in any emergency situation in which it is necessary to stop the plant, and during the steady state operation of the plant. Another object of the present invention is to provide a plant which allows the superficial quality of the final product to be improved, maintenance operations to be reduced and the speed of the strip to be coated to be increased, increasing the productivity of the plant.

A further object of the invention is to provide a relative coating process, by continuous hot dipping of flat metal products, having a greater efficiency than known processes.

The present invention, therefore, proposes to achieve the objects discussed above by providing a plant for coating flat metal products by means of continuous hot dipping which, in accordance with claim 1 , comprises a containment tank for containing molten coating material; a coating vessel adapted to receive and contain molten coating material, arranged above said containment tank and provided with an opening on a bottom thereof for passing the flat metal product to be coated; a chamber arranged below said coating vessel and provided with guide rollers to guide the flat metal product towards said opening; wherein the containment tank is arranged below said guide rollers and within said chamber whereby chamber and containment tank form a single place, and wherein there is provided an intercepting and conveying system arranged within the chamber, between the coating vessel and the containment tank, and configured to intercept and convey into said containment tank the molten coating material which leaks from said opening, said intercepting and conveying system having a structure defining a cavity thereof, facing the containment tank; within said cavity being at least partially housed at least one first guide roller adapted to divert the flat metal product upon its entry into the chamber, and being completely housed at least one second guide roller, placed above said first guide roller, adapted to divert and stabilize the flat metal product being fed towards the opening, whereby said guide rollers and the flat metal product, being fed towards the opening, are completely protected from the falling of drops of molten coating material during the steady state operation of the plant or from the falling of large quantities of molten coating material in both the start-up or shut-down step or in the maintenance or emergency phases.

A second aspect of the present invention provides a process for coating flat metal products by means of continuous hot dipping which, according to claim 14, comprises the following steps:

- providing an inert atmosphere in said single place at a higher pressure than the pressure outside said place;

- feeding the flat metal product into the chamber, guiding it towards the coating vessel by means of the guide rollers,

- coating the flat metal product within said coating vessel;

wherein it is provided that the molten coating material which leaks from the opening of the bottom of the coating vessel is intercepted and conveyed into the containment tank by means of the intercepting and conveying system provided in the chamber, so as to protect the guide rollers and the flat metal product, being fed towards said opening, which are found within the cavity of the structure of said intercepting and conveying system.

Advantageously, the intercepting and conveying system for the coating liquid which leaks from the coating vessel is configured so as to protect both the strip being fed towards said vessel and the guide rollers and to directly recover said coating liquid, conveying it into the containment tank which, in a preferred embodiment, may coincide with the melting tank for the ingots of coating material. The recovered coating liquid is drained from said containment tank with a pump, in order to take it back to the coating vessel, also known as a funnel.

The intercepting and conveying system, or receiving and conveying system, defines channels which diverge from each other in the direction from said coating vessel to said containment tank. In this way, it is possible to recover the coating liquid without dirtying strip and guide rollers whereby imprints are not created on the strip, improving the superficial quality thereof. Furthermore, the operation of cleaning the guide rollers and the walls of the chamber is no longer necessary. In a preferred embodiment, the intercepting and conveying system is provided with two deflectors, the aim of which is to protect the underlying strip and guide rollers, deflecting roller and stabilizer roller, from the drops of molten metal material which leak from the bottom part of the electromagnetic device which includes an inductor and the coating vessel which contains the molten metal bath. The shape of these deflectors causes these drops to flow therealong and leads them into the containment tank, also known as a zinc-pot. The two deflectors define a gap through which the strip passes in proximity of the passing area between the chamber or thermal machine and the electromagnetic device or magnetic machine.

In particular, the aforementioned divergent channels are defined by the pair of deflectors, the upper ends of which, parallel or not parallel to each other, define the aforementioned gap for passing the strip in a vertical direction, and by a structure which is external to said deflectors and independent of the latter.

Advantageously, a kinematic mechanism is provided for adjusting the gap between the deflectors which is also capable of performing an immediate closure in the case of emergency.

The chamber, actively heated by appropriate heating means, connects the containment tank to an electromagnetic levitation device or simply electromagnetic device (electromagnetic galvanizer in the case that the coating is zinc-based), and is configured so as to increase the compactness of the entire plant.

The temperature of the single place, defined by the chamber and the containment tank, is advantageously constantly maintained at around 450-490°C, that is the maintenance temperature of the zinc-based coating material in the molten state, to favour optimal conditioning of the strip and therefore a better process of adhesion of the zinc to the strip. In this way, all the elements in said single place, i.e. internal walls and fixed and/or movable members, will be maintained at the aforementioned temperature.

Advantageously, the atmosphere in said single place is an atmosphere of inert gas, e.g. nitrogen, introduced with a greater pressure than the external atmospheric pressure so as to prevent the presence of oxygen and therefore reduce the formation of DROSS. A further advantage is that the bearings of the guide rollers are on the outside of the chamber, preventing that they are subjected to the high temperatures of the chamber place and allowing possible maintenance and/or replacement operations without excessive loss of time.

The dependent claims describe preferred embodiments of the invention.

Brief description of the figures

Further features and advantages of the invention will become clearer in light of the detailed description of preferred but not exclusive embodiments of a plant for coating a flat metal product by means of continuous hot dipping, shown by way of non-limiting example with the aid of the attached drawings in which:

Figure 1 shows a first sectional side view of an embodiment of the plant of the invention;

Figure 1a shows an enlargement of a part of the first sectional side view in Figure 1 ;

Figure 2 shows a second sectional side view of the embodiment of the plant in Figure 1 ;

Figure 3 shows a side view of a first part of the plant in Figure 1 ;

Figure 4 shows a perspective view of said first part of the plant;

Figure 5 shows a cross-section according to the plane H-H and the plane B-B of the view in Figure 3;

Figure 6 shows a top view of a second part of the plant of the invention;

Figure 7 shows a sectional view according to the plane A-A of said second part of the plant in Figure 6;

Figure 8 shows a perspective view of said second part of the plant of the invention; Figure 9 shows a top view of said first part and second part of the plant of the invention.

The reference numbers in the figures identify the same elements or components. Detailed description of a preferred embodiment of the invention

With reference to the Figures, it is shown a preferred embodiment of plant, globally indicated with the reference number 1 , for coating a flat metal product by means of continuous hot dipping. In the course of this description, reference is made, by way of example, to a galvanizing plant for a metal strip, e.g. made from steel, whereby the coating material is zinc-based, e.g. molten zinc or zinc alloys in the molten state, and the electromagnetic device is an electromagnetic galvanizer. However, the plant of the invention can also be used for continuous coating of a metal strip with other metal materials, e.g. Al-based, alloys of Al, Mg, Si, Sn, Pb and alloys thereof.

Said plant 1 comprises:

- a containment tank 2 for containing molten zinc,

- a coating vessel 3 receiving molten zinc, arranged above said containment tank 2 and provided with an opening 4 on a bottom thereof for passing the strip 5, - a chamber 6 arranged below said coating vessel 3 and provided with guide rollers 7, 8 to guide the strip 5 towards said opening 4.

The containment tank 2 is advantageously arranged below the guide rollers 7, 8 and is contained within the chamber 6 whereby chamber 6 and containment tank 2 form a single place, and there is further provided an intercepting and conveying system 9, arranged within chamber 6 between coating vessel 3 and containment tank 2, in order to intercept and convey into said containment tank 2 the molten zinc exiting from the opening 4 of the coating vessel 3.

The intercepting and conveying system 9 is advantageously configured to bypass and wind the guide rollers 7, 8 and the strip 5 being fed towards the opening 4, so as to protect them from the falling of drops of molten zinc during the steady state operation of the plant or from the falling of large quantities of molten zinc in both the start-up or shut-down step and in the maintenance or emergency phases.

In particular, the intercepting and conveying system 9 has a structure configured so as to define a cavity thereof, with the mouth of said cavity facing downwards, i.e. towards the containment tank 2. Within said cavity there is partially housed at least one guide roller 7 which diverts the strip 5 upon its entry into the chamber 6. Within said cavity there is also completely housed at least a further guide roller 8, placed above the guide roller 7 and which diverts and stabilizes the strip being fed towards the opening 4. With this configuration, guide rollers 7, 8 and strip 5, being fed towards the opening 4, are completely protected from leakages of molten zinc in any operating condition.

Said intercepting and conveying system 9 comprises in a preferred variant: - a pair of deflectors 12 which protect strip 5 and guide rollers 7, 8, each deflector 12 being arranged at a respective side of the vertical feeding plane X of the strip 5 and connected to two opposite lateral walls 6' of the chamber 6 by means of a support shaft 17 integral with the deflector 12 and in turn flanged to the walls 6' (Figures 5 and 9);

- and a structure 20, external to the deflectors 12 and integrally fixed to two opposite lateral walls 6' of the chamber 6 (Figures 1a, 9), defining together with the deflectors 12 channels for intercepting and conveying towards the containment tank 2.

The deflectors 12 are preferably arranged symmetrically with respect to said vertical feeding plane X, at least with the upper ends 13 thereof.

These upper ends 13 of the deflectors 12 define a gap for passing strip 5 in a vertical direction. This gap may vary from zero, in the case in which the deflectors come to rest against each other in the absence of strip, up to at least a value equal to the thickness of the strip to be coated plus a value between 0.5 and 5 mm for each side of the plane X, during steady state operation.

Advantageously, said upper ends 13 of the deflectors, made of steel, may be coated in a ceramic material, e.g. added or obtained in the shape of either ceramic wear plates, or in Inconel steel to reduce the risk of scratches on the strip in the event of slipping.

In addition to protecting the strip and guide rollers, the deflectors 12 also perform the function of stabilizing the strip. The electromagnetic field of the galvanizer 10 may in fact be subject to oscillations, also due to a lowering of current, inducing vibrations and oscillations on the strip. The passage between the upper ends 13 of the deflectors 12 may function as a mechanical striker and reduce the degree of oscillations induced during the transit of the strip. There is therefore an effect of stabilization of the strip upstream of the electromagnetic galvanizer 10, further optimizing the galvanizing process. In fact, stabilizing in known plants is only provided downstream of the galvanizer.

As better shown in Figure 3, the deflectors 12 have lower ends. 14 which are substantially parallel to each other and more spaced from each other with respect to the upper ends 13. In fact, there is provided a central portion 15 of the deflectors 12, which is substantially flat and inclined by a predetermined angle greater than zero, preferably between 35 and 45°, more preferably still between 40 and 43°, with respect to the vertical feeding plane X of the strip 5. Said central portion 15 of the deflectors 12 connects an upper end 13 to the respective lower end 14, defining the aforementioned cavity, whereby both the deflecting roller 7 and the underlying stabilizer roller 8 are covered by the pair of deflectors 12. Said lower ends 14 are positioned so as to at least partially cover the deflecting roller 7, having a greater diameter than the stabilizer roller 8 and arranged below it.

Advantageously, adjustment means are provided for adjusting the gap of the deflectors which are configured to adjust the value of said gap according to the thickness of the strip to be coated and the vibrations of the strip.

The adjustment means for adjusting the gap of the deflectors comprise a kinematic mechanism which allows the adjustment of the gap until reaching immediate closure of the deflectors in the case of emergency.

This kinematic mechanism comprises, for example, a system of manual or automated linkages 16 guided by threaded bushings 18 which allow extremely precise, even micrometric, adjustment of the gap. Actuating the linkages allows two support shafts 17, arranged parallel to the vertical feeding plane X, to be rotated in a synchronous and specular manner with respect to the feeding plane X of strip 5. Each support shaft 17 is integrally fixed to a respective deflector 12, e.g. by means of fixing flanges 52, preferably at the lower part of the central portion 15, i.e. in proximity of the lower end 14. Each shaft 17 is inserted at the ends on guide flanges 50, fixed to an block 51 connected to the walls 6' (Figure 5).

In a preferred variant, the system of linkages 16 comprises a series of cams 19 connected by means of threaded bushings 18 with very small pitch, e.g. around 1 mm, which, being actuated, allow a lever arm which leads to a significant opening of the deflectors 12. The system of linkages 16 is advantageously configured so that the synchronous and specular rotation of the support shafts 17 about the axis thereof determines a spacing between the upper ends 13 of the deflectors, said spacing being specular with respect to said feeding plane X of the strip 5. This means that, while in the configuration with the gap at zero the upper ends 13 of the deflectors 12 are parallel, upon actuation by the system of linkages 16 said upper ends 13 move apart so as to no longer be parallel to each other, i.e. forming an angle which differs from zero with respect to the vertical feeding plane X of the strip.

In a second variant (not shown), there may be two distinct systems of linkages which provide the adjustment of the gap by means of the disjointed and no longer synchronous movement of the two deflectors.

In a third variant, the system of linkages may be replaced for example with a system of eccentrics so that the deflectors are not made to rotate but rather made to individually or simultaneously translate, always keeping their inclination constant with respect to the axis X, whereby the upper ends 13 of the deflectors 12 remain parallel to each other with respect to the vertical feeding plane X of the strip.

A further variant provides that the deflectors can both rotate and translate, e.g. thanks to the combination of a system of eccentrics and a system of linkages. As indicated above, the structure 20 defines, together with the deflectors 12, conveying channels 22 (Figure 1a) for conveying the molten zinc exiting from the coating vessel 3 towards the containment tank 2. Said conveying channels 22 are laterally delimited by lugs 23, show for example in Figure 4, protruding upwards at the sides of the deflectors 12. In a preferred variant, said lugs 23 are arranged at the lateral edges of the deflectors 12, following the shape of said deflectors. A first end of the lugs 23 is arranged at the lower area of the upper ends 13 of the deflectors; a second end is arranged at and follows the shape of the lower ends 14 of the deflectors.

Said structure 20 is configured so as to define, at the upper end, a longitudinal flared opening 21 , which extends transversally to the chamber 6 for substantially about the entire width of said chamber, within which the upper ends 13 of the deflectors are arranged (Figures 1a and 9). Said longitudinal flared opening 21 functions as a funnel-like element or open expansion tank for intercepting large quantities of liquid zinc exiting from the bottom of coating vessel 3, in both the start-up or shut-down step and in the maintenance or emergency phases. In the start-up step, for example, prior to reaching the so-called support threshold or head above which the molten zinc is supported by the magnetic field in the coating vessel 3, the molten zinc is not yet adequately supported and is discharged from the opening 4. Said support threshold or head is defined according to the size of the coating vessel 3 and the intensity of the magnetic field. In the case of maintenance or emergency, e.g. in the case of loss of the magnetic field, the longitudinal flared opening 21 allows the intercepting and management of the large quantity of molten zinc in the funnel which is entirely discharged.

Structure 20 (Figures 7, 8) is composed of two half elements 20', independent from the deflectors 12 and integrally fixed to two opposite lateral walls 6' of the chamber 6 (Figure 9) so that the upper ends 27 thereof are in contact with each other by means of respective lateral support plates 28. Below said lateral support plates 28, lateral drainage channels 29 are provided in each half element 20'. Since the deflectors 12 are arranged at the centre of the narrowing area 37 (Figure 6, 9) of the longitudinal flared opening 21 , in the space between the two pairs of lateral support plates 28 in contact with each other, and extend as far as the lower ends 14 with constantly the same width L (Figure 9), said lateral drainage channels 29 advantageously allow the conveying of that part of molten zinc which goes beyond the width of said deflectors, and therefore beyond the width of the conveying channels 22.

At the lower end, on the other hand, the structure 20 is configured so as to define longitudinal end channels 24, 25 (Figures 1 a, 7), the outlet section of which is in proximity of a funnel-like element 26 which collects the molten zinc discharged from the conveying channels 22 and from the lateral drainage channels 29 in order to pour it into the containment tank 2 below. Funnel-like element 26 is optional and, in the case that it is not provided, the discharged molten zinc passes directly into the containment tank 2.

The conveying channels 22, each arranged on a respective side of the vertical feeding plane X, substantially follow the external profile of the deflectors 12 in the stretch between said longitudinal flared opening 21 and said end channels 24, 25. In a preferred variant, shown in Figures 1 a, 7 and 8, the end channel 25 has a bridge-like configuration 40 whereby the molten zinc which arrives from the conveying channels 22 and the lateral drainage channels 29 is laterally diverted and discharged into the funnel-like element 26 or directly into the containment tank 2 so as to prevent dirtying the strip entering the chamber 6 and moving towards the deflecting roller 7. The span of said bridge is advantageously greater than the width of the strip 5 entering the chamber 6.

This end channel with bridge-like configuration is therefore arranged at least at the area between the deflecting roller 7 and the lateral opening 41 (Figure 1a) of the chamber 6 through which the strip 5 enters said chamber after the pre-treatment. Below the respective upper end 27, the half elements 20' of structure 20 have (Figures 7, 8) a respective central portion 36 and a respective lower end 35. Said central portion 36 has a profile which substantially corresponds to that of the central portion 15 of the corresponding deflector 12 and is suitably spaced therefrom whereby the conveying channels 22 are defined, delimited at the sides by the lugs 23.

The chamber 6, which includes and connects the containment tank 2 to an electromagnetic galvanizer 10, which in turn includes an inductor 11 and the coating vessel 3, is advantageously provided with heating means for maintaining the place comprising said chamber and containment tank 2 at a predetermined temperature, substantially equal to a maintenance temperature of the coating material in the molten state. The temperature of all the elements of the chamber, such as internal walls and fixed and/or movable members, is for example maintained constant at around 460-490°C according to the chemical composition of the ingots of coating material. The strip 5 arrives in chamber 6 already preheated, at a temperature substantially equal or greater than said maintenance temperature of the coating material in the molten state.

The fact that the containment tank 2 for the molten zinc is provided in the same place as the chamber 6 determines a contribution of energy from the containment tank or zinc-pot 2 to quickly bring the entire single place up to the predetermined temperature.

The guide rollers 7, 8, made from steel, are advantageously provided with a coating of ceramic material or chromium oxide or boron nitride, or obtained by means of welding filling material (e.g. with transferred arc). In this way, preventing the creation of irregularities of Fe-Zn alloy, which is extremely hard and difficult to remove, the adhesion of possible drops of liquid zinc on the guide rollers is prevented and therefore the formation of imprints on the strip due to said irregularities on the roller. Other elements, e.g. the walls of chamber 6, may be coated with ceramic tiles.

First heating means of chamber 6 are provided in at least one of the two guide rollers 7, 8.

In a first variant, the deflecting roller 7, of cylindrical shape, is provided externally with a hollow cylindrical jacket 7', coaxial to roller 7. Said hollow cylindrical jacket T is provided with a plurality of longitudinal hollows or holes 30, obtained in the thickness of said jacket, for housing respective electrical resistors connected to appropriate electrical power means. The longitudinal hollows 30 preferably have the axis thereof parallel to the rotation axis of the deflecting roller 7, are arranged along a cylindrical lateral surface and the number thereof and the distance between one hollow and the next are appropriately defined so as to guarantee a homogeneous temperature on the external surface of the roller 7 on which the strip 5 runs.

A second variant (not shown), on the other hand, provides that the deflecting roller 7, of hollow cylindrical shape, is provided internally with at least three fixed resistors or a radial flame burner, arranged in the hollow part of the roller 7.

In both the variants, the drops of molten zinc which might possibly fall on the external surface of the deflecting roller 7 do not adhere to said heated external surface but are maintained liquid and slide away, falling into the funnel-like element 26 or directly into the containment tank 2.

In all variants, the stabilizer roller 8 may also be provided with heating means and may, in a limit case, coincide with a suitably coated resistor.

The guide rollers 7, 8 are advantageously mounted on detachable hubs. The hollow jacket T represents the wear area of the deflecting roller system, while both the guide rollers 7, 8 advantageously have the respective bearings 31 , 32 on the outside of the chamber 6, thus allowing possible maintenance and/or replacement operations without excessive loss of time since it is not necessary to dismantle the entire machine.

Second heating means of the chamber 6 are also provided in different points of the chamber.

Said second heating means are preferably heating plates 33, e.g. provided in the thickness thereof with longitudinal hollows or holes 34 for housing respective electrical resistors connected to appropriate electrical power means.

In a preferred variant, said heating plates 33 are arranged in proximity of the central portions 36 and in proximity of the lower ends 35 of the half elements 20' of structure 20 of the intercepting and conveying system (see Figures 1 and 7).

Other heating plates 33 may be provided in proximity of external surfaces of funnel-like element 26 (Figure 1 ) and containment tank 2 (Figures 1 and 2).

The chamber 6 is advantageously provided with two sealing systems and the atmosphere therein is inert, thanks to the presence of inert gas, i.e. nitrogen, introduced and maintained at a greater pressure than the external atmospheric pressure so as to prevent the presence of oxygen and reduce the formation of DROSS.

A first sealing system (not shown) is provided at a lateral opening 41 (Figure 1 a) of the chamber 6 through which the strip 5 enters said chamber after the pre- treatment.

A second sealing system, on the other hand, consists of the liquid zinc in the coating vessel 3, which does not allow leaking of the inert gas present in chamber 6.

At the passing area between chamber 6 and coating vessel or funnel 3, there is advantageously provided a channel 45, preferably thermally isolated, e.g. with ceramic material, through which the strip 5 passes. Said channel 45 has a first end fixed at the bottom of the coating vessel 3 and communicating with the opening 4 of the latter, and a second end which passes through the upper wall of the chamber 6, thus communicating with said chamber.

Flared opening 21 and the entire intercepting and conveying system 9 advantageously have a width L' which is greater than both width L of the deflectors 12 (Figure 9) and the width of the opening 4 of the coating vessel 3. The width of the channel 45 is at least equal to the width of the opening 4 and less than the width L' of the flared opening 21 .

With regard to feeding molten zinc into the coating vessel 3, the latter can receive the molten zinc from the containment tank 2 which may also constitute the melting tank for the zinc ingots. In the start-up step, the zinc ingots are loaded directly into the containment tank 2, resting in a mobile drawer moved with hydraulic or pneumatic means.

The entire chamber 6, which includes containment tank 2, is advantageously configured in modules so as to be able to add a smelting furnace which gives the possibility to continuously load zinc. Said smelting furnace will be provided with a respective recirculation pump, arranged in the crucible so as to be immersed in the molten coating material fed into the furnace.

Alternatively, the containment tank 2 may be a different tank from the melting tank for the zinc ingots from which the pure molten zinc is taken in order to introduce it into the coating vessel 3 or funnel. In this way, the delivery step, in which the pure molten zinc is sent from said melting tank to the coating vessel 3, is kept separate from the step of recovering the molten zinc exiting from the coating vessel 3. The recovered molten zinc may have impurities whereby it could be necessary to retreat and/or re-filter it by means e.g. of a sedimentation tank suitably heated for purifying molten coating material. However, pump 40, visible in Figure 2, is configured to work below the discharge level in the containment tank 2 whereby it always sucks in proximity of the bottom where there is clean molten zinc since the DROSS gathers on the surface in said containment tank.

With reference now to Figure 1 , a continuous strip 5 is unrolled from a skein (not shown) and is subjected to a conventional pre-treatment upstream of the first sealing system. After the pre-treatment, the strip 5 enters the chamber 6 and is directed by means of the guide rollers 7, 8 towards the longitudinal opening or slot 4, provided on the bottom of the coating vessel 3 of electromagnetic galvanizer 0. The opening 4 allows the introduction of the strip 5 inside the molten zinc bath provided in the vessel 3, the bath defining an upper surface or upper meniscus. The strip 5 moves along a direction which extends through the bath. The movement of strip 5 through the bath allows the coating of said strip with a layer of molten zinc of which the bath is composed. A coated strip emerges from the bath downstream of the upper surface thereof. The coating vessel 3 has an open upper end through which the coated strip moves upwards.

A control device for controlling the thickness of the coating, of the pneumatic or electromagnetic type, typically used to obtain the desired basic weights on the strip, is positioned above the coating vessel 3. Downstream of the control device, a reel (not shown) is placed, on which the cooled coated strip is wound into a skein which is then removed by the reel.

During the entire coating process, an inert atmosphere is advantageously provided in the single place, defined by chamber 6 and containment tank 2, at a higher pressure than the pressure outside said place, and there is further provided the conveying of the molten zinc, exiting from the opening 4 of the bottom of containment tank 3, into the containment tank 2 by means of the intercepting and conveying system 9 provided in chamber 6.

In the steady state operation of the plant, the drops of molten zinc emerging from the opening 4 will slide on the strip 5 until arriving on the deflectors 12, which cause these drops to follow the conveying channels 22 and finally the end channels 24 and 25.

In the case of start-up or shut-down of the plant, the abundant quantity of molten zinc emerging from the opening 4 will go through the channel 45 and flow into the longitudinal flared opening 21 , subsequently following the conveying channels 22 and the end channels 24 e 25.

The temperature within said single place is maintained during the steady state operation at a level which is substantially equal to a maintenance temperature of the coating material in the molten state.

Claims

1. A plant for coating a flat metal product (5) by means of continuous hot dipping, comprising

- a containment tank (2) for containing molten coating material,

- a coating vessel (3) adapted to receive and contain molten coating material, arranged above said containment tank (2) and provided with an opening (4) on a bottom thereof for passing the flat metal product to be coated,

- a chamber (6) arranged below said coating vessel (3) and provided with guide rollers (7, 8) to guide the flat metal product (5) towards said opening (4),

wherein the containment tank (2) is arranged below said guide rollers (7, 8) and within said chamber (6) whereby chamber and containment tank form a single place,

and wherein there is provided an intercepting and conveying system (9) arranged within the chamber (6), between the coating vessel (3) and the containment tank (2), and configured to intercept and convey into said containment tank (2), the molten coating material which leaks from said opening (4),

said intercepting and conveying system having a structure defining a cavity thereof facing the containment tank (2); within said cavity being at least partially housed at least one first guide roller (7) adapted to divert the flat metal product (5) upon its entry into the chamber (6), and being completely housed at least one second guide roller (8), placed above said first guide roller (7), adapted to divert and stabilize the flat metal product being fed towards the opening (4), whereby said guide rollers (7, 8) and the flat metal product (5), being fed towards the opening (4), are completely protected from the falling of drops of molten coating material during the steady state operation of the plant or from the falling of large quantities of molten coating material in both the start-up or shut-down step and in the maintenance or emergency phases.

2. A plant according to claim 1 , wherein said intercepting and conveying system (9) defines channels (22) which diverge from each other in the direction from said coating vessel (3) to said containment tank (2).

3. A plant according to claim 2, wherein said channels (22) are defined by a pair of deflectors (12), the upper ends of which (13) define a passing gap for passing the flat metal product (5) in a vertical direction, and by a structure (20) which is external to said deflectors and independent of these latter.

4. A plant according to claim 3, wherein each deflector (12) is arranged at a respective side of a vertical feeding plane (X) of the flat metal product (5) and is connected to two opposite lateral walls (6') of the chamber (6).

5. A plant according to claim 4, wherein said structure (20) is configured so as to define, at the respective upper end, a flared opening (21 ) which extends transversally to the chamber (6) and within which the upper ends (13) of the deflectors (12) are arranged.

6. A plant according to any one of the claims from 3 to 5, wherein there are provided adjustment means for adjusting said passing gap.

7. A plant according to claim 6, wherein said passing gap adjustment means are configured to move two support shafts (17) of the deflectors (12), arranged parallel to the vertical feeding plane (X), each support shaft (17) being integrally fixed to a respective deflector (12) and connected to two opposite walls (6') of the chamber (6).

8. A plant according to any one of the preceding claims, wherein there are provided heating means arranged within said chamber (6).

9. A plant according to claim 8, wherein there are provided first heating means on at least one of the guide rollers (7, 8), said first heating means preferably being electrical resistors housed in a plurality of hollows (30) provided in a hollow jacket (7') of the first guide roller (7).

10. A plant according to claim 9, wherein there are provided second heating means in proximity of the external walls of said structure (20), said second heating means preferably being heating plates (33).

11. A plant according to any one of the preceding claims, wherein the supports of the guide rollers (7, 8) have bearings on the outside of the chamber (6).

12. A plant according to any one of the preceding claims, wherein the coating vessel (3) is part of an electromagnetic device (10) suitable to keep a bath of molten coating material in semi-levitation.

13. A plant according to any one of the preceding claims, wherein there is provided a channel (45), communicating at a first end with the opening (4) of the coating vessel (3) and at a second end with the chamber (6), suitable for passing the flat metal product (5) from said chamber (6) to said coating vessel (3).

14. A coating process for a flat metal product by means of continuous hot dipping, executable by means of a plant according to claim 1 , the process comprising the following steps:

- feeding the flat metal product (5) into the chamber (6), guiding it towards the coating vessel (3) by means of the guide rollers (7, 8),

- coating the flat metal product within said coating vessel (3);

wherein it is provided that the molten coating material which leaks from the opening (4) of the bottom of the coating vessel (3) is intercepted and conveyed into the containment tank (2) by means of the intercepting and conveying system (9) provided in the chamber (6) so as to protect the guide rollers (7, 8) and the flat metal product (5), being fed towards said opening (4), which are found within the cavity of the structure of said intercepting and conveying system.

15. A process according to claim 14, wherein it is provided that said single place is maintained at a suitable temperature to maintain the coating material in the molten state.