Patent Publication Number: US-7722933-B2

Title: Method and installation for dip coating of a metal strip, in particular of a steel strip

Description:
This is a continuation of application Ser. No. 10/416,191, filed Oct. 21, 2003, now U.S. Pat. No. 6,994,754. The entire disclosure of the prior application, application Ser. No. 10/416,191, is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a process and a plant for the continuous hot dip-coating of a metal strip, especially a steel strip. 
     In many industrial applications, steel sheet is used which is coated with a protective layer, for example for corrosion protection, and usually coated with a zinc layer. 
     This type of sheet is used in various industries to produce all kinds of parts, in particular visual parts. 
     To obtain this kind of sheet, continuous dip-coating plants are used in which a steel strip is immersed in a bath of molten metal, for example zinc, which may contain other chemical elements, such as aluminium and iron, and possible addition elements such as, for example, lead, antimony, etc. The temperature of the bath depends on the nature of the metal, and in the case of zinc the temperature of the bath is around 460° C. 
     In the particular case of hot galvanising, as the steel strip runs through the molten zinc bath, an Fe—Zn—Al intermetallic alloy with a thickness of a few tens of nanometres forms on the surface of the said strip. 
     The corrosion resistance of the parts thus coated is provided by the zinc, the thickness of which is controlled usually by air wiping. The adhesion of the zinc to the metal strip is provided by the layer of the aforementioned intermetallic alloy. 
     Before the steel strip passes through the molten metal bath, this steel strip firstly runs through an annealing furnace in a reducing atmosphere where the purpose is to recrystallise it after the substantial work hardening resulting from the cold-rolling operation and to prepare its surface chemical state so as to favour the chemical reactions necessary for the actual dip-coating operation. The steel strip is heated to about 650 to 900° C. depending on the grade, for the time needed for recrytallisation and surface preparation. It is then cooled to a temperature close to that of the bath of molten metal by means of heat exchangers. 
     After it has passed through the annealing furnace, the steel strip runs through a duct, also called a “snout”, containing an atmosphere which protects the steel, and is immersed in the bath of molten metal. 
     The lower part of the duct is immersed in the bath of metal in order to define, with the surface of the said bath and inside this duct, a liquid seal through which the steel sheet passes as it runs through the said duct. 
     The steel strip is deflected by a roller immersed in the zinc bath. It emerges from this metal bath and then passes through wiping means used to regulate the thickness of the liquid metal coating on this steel strip. 
     At the moment when the strip is extracted from the bath, it passes through the surface of the zinc bath, which is covered with zinc oxide and with dross coming from the steel strip dissolution reaction. 
     To prevent the particles from being entrained by the strip, the surface of the bath, accessible by the operators, is periodically cleaned in such a way that the strip does not entrain particles. 
     However, this manual cleaning procedure does not permanently guarantee the cleanliness of the surface of the bath and the absence of particles periodically rising from the bath to the point where the steel strip is extracted. 
     Thus, the coated steel strip has visual defects which are magnified or revealed during the zinc wiping operation. 
     This is because the foreign particles are retained by the air wiping jets before the said particles are ejected or broken up, thus creating streaks of lesser thickness in the liquid zinc having a length ranging from a few millimeters to a few centimeters. 
     One solution for avoiding these drawbacks consists in cleaning the surface of the liquid seal by pumping off the zinc oxides and dross coming from the bath. 
     These pumping operations allow the surface of the liquid seal to be cleaned only very locally at the point of pumping and their effectiveness and range of action are very low, which does not guarantee that in particular the region where the steel strip leaves the liquid zinc bath is completely cleaned. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a process and a plant for the continuous dip-coating of a metal strip which make it possible to avoid the abovementioned drawbacks and to achieve a very low density of defects required by customers desiring surfaces free of visual defects. 
     The subject of the invention is a process for the continuous dip-coating of a metal strip in a tank containing a liquid metal bath, in which process the metal strip is made to run continuously, in a protective atmosphere, through a duct, the lower part of which is immersed in the liquid metal bath in order to define with the surface of the said bath, and inside this duct, a liquid seal, the metal strip is deflected around a deflector roller placed in the metal bath and the coated metal strip is wiped on leaving the metal bath, characterised in that, in the region where the strip leaves the liquid metal bath, the liquid metal is isolated from the surface of the said bath in an isolating enclosure and the metal oxide particles and intermetallic compound particles are recovered by the liquid metal flowing from this region into the said enclosure, the drop in height of the liquid metal in this enclosure being determined in order to prevent metal oxide particles and intermetallic compound particles from rising as a countercurrent to the flow of liquid metal, and the said particles are extracted from this enclosure. 
     The subject of the invention is also a plant for the continuous hot dip-coating of a metal strip, of the type comprising:
         a tank containing a liquid metal bath,   a duct through which the metal strip in a protective atmosphere runs and the lower part of which duct is immersed in the liquid metal bath in order to define with the surface of the said bath, and inside this duct, a liquid seal,   a roller, placed in the metal bath, for deflecting the metal strip and   means for wiping the coated metal strip on leaving the metal bath, characterised in that it comprises, on the one hand, in the region where the strip leaves the liquid metal bath, an enclosure for isolating the liquid metal in this region with respect to the surface of the bath and for recovering the metal oxide particles and intermetallic compound particles by the liquid metal flowing from this region into the said enclosure, the drop in height of the liquid metal in the enclosure is greater than 50 mm in order to prevent the metal oxide particles and intermetallic compound particles from rising as a countercurrent to the flow of liquid metal and, on the other hand, means for extracting the said particles from this enclosure.       

     According to other features of the invention:
         the drop in height of the liquid metal in the enclosure is greater than 100 mm;   the enclosure surrounds the metal strip and has a bottom and two concentric walls making between them a compartment and defining, in the upper part of the said enclosure, an opening, the upper edge of the external wall being positioned above the surface of the liquid metal bath and the upper edge of the internal wall being positioned below the said surface;   the internal wall of the enclosure has a lower part flared out towards the bottom of the tank and an upper part parallel to the metal strip;   the means for extracting the particles are formed by a pump connected, on the suction side, to the compartment of the enclosure via a connecting pipe and provided, on the delivery side, with a pipe for discharging the withdrawn liquid metal towards the rear of the tank;   the plant includes means for positioning the metal strip with respect to the upper edge of the internal wall of the enclosure.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from the description which follows, given by way of example, with reference to the appended drawings in which: 
         FIG. 1  is a schematic side view of a continuous dip-coating plant according to the invention; 
         FIG. 2  is a view on a larger scale of the enclosure placed at the point where the strip leaves the galvanizing plant, according to the invention; 
         FIG. 3  is a sectional view on the line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a schematic side view of a first embodiment of the upper edge of the internal wall of the enclosure; 
         FIG. 5  is a schematic side view of a second embodiment of the upper edge of the internal wall of the enclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, a description will be given in the case of a plant for the continuous galvanising of a metal strip. However, the invention applies to any continuous dip-coating process in which surface pollution may occur and for which a clean liquid seal must be maintained. 
     Firstly, on leaving the cold-rolling mill train, the steel strip  1  passes, in a reducing atmosphere, through an annealing furnace (not shown) for the purpose of recrystallising it after the substantial work hardening resulting from the cold rolling, and to prepare its chemical surface state so as to favour the chemical reactions needed for the galvanising operation. 
     The steel strip is heated in this furnace to a temperature of between, for example, 650 and 900° C. 
     On leaving the annealing furnace, the steel strip  1  passes through a galvanising plant, shown in  FIG. 1  and denoted by the overall reference  10 . 
     This plant  10  comprises a tank  11  containing a bath  12  of liquid zinc which contains chemical elements such as aluminium and iron and possible additional elements such as lead, antimony etc. 
     The temperature of this liquid zinc bath is around 460° C. 
     On leaving the annealing furnace, the steel strip  1  is cooled to a temperature close to that of the liquid zinc bath by means of heat exchangers and is then immersed in the liquid zinc bath  12 . 
     As shown in  FIG. 1 , the galvanising plant  10  includes a duct  13  within which the steel strip  1  runs in an atmosphere which protects the steel. 
     This duct  13 , also called “snout”, has, in the illustrative example shown in the figures, a rectangular cross-section. 
     The lower part  13   a  of the duct  13  is immersed in the zinc bath  12  so as to define with the surface of the said bath  12 , and inside this duct  13 , a liquid seal  14 . 
     Thus, the steel strip  1  on being immersed in the liquid zinc bath  12  passes through the surface of liquid seal  14  in the lower part  13   a  of the duct  13 . 
     The steel strip  1  is deflected by a roller  15 , usually called the bottom roller, placed in the zinc bath  12  and, on leaving this zinc bath  12 , the coated steel strip  1  passes through wiping means  16  which consist, for example, of air spray nozzles  16   a  and which are directed towards each side of the steel strip  1  in order to regulate the thickness of the liquid zinc coating. 
     Thus, as shown in  FIGS. 1 and 2 , the plant includes, in the region  17  where the strip  1  leaves the liquid zinc bath  12 , an enclosure  20  for isolating the liquid zinc in this region  17  with respect to the surface of the bath  12  and for recovering the zinc oxide particles and intermetallic compound particles by the liquid zinc flowing from this region  17  into the said enclosure  20 , as will be seen later. 
     The enclosure  20  surrounds the metal strip  1  and has a bottom  21  and two concentric walls, an external wall  22  and an internal wall  23  respectively, making between them a compartment  24 . The walls  22  and  23  define, in the upper part of the enclosure  20 , an opening  25 . 
     As shown in  FIG. 2 , the upper edge  22   a  of the external wall  22  is positioned above the surface of the liquid zinc bath  12  and the upper edge  23   a  of the internal wall  23  is positioned below this surface. 
     The drop in height of the liquid metal in the enclosure ( 20 ) is determined in order to prevent the metal oxide particles and intermetallic compound particles from rising as a countercurrent to the flow of liquid metal and this drop is greater than 50 mm and preferably 100. 
     Preferably, the internal wall  23  has a lower part flared out towards the bottom of the tank  11 . The walls  22  and  23  of the enclosure  20  are made of stainless steel and have a thickness of between 10 and 20 mm for example. 
     According to a first embodiment, shown in  FIG. 4 , the upper edge  23   a  of the internal wall  23  is straight and preferably tapered. 
     According to a second embodiment, shown in  FIG. 5 , the upper edge  23   a  of the internal wall  23  of the enclosure  20  comprises, in the longitudinal direction, a succession of hollows  26  and projections  27 . 
     The hollows  26  and the projections  27  are in the form of circular arcs and the difference in height “a” between the said hollows and the said projections is preferably between 5 and 10 mm. In addition, the distance “d” between the hollows  26  and the projections  27  is, for example, of the order of 150 mm. 
     Again in this embodiment, the upper edge  23  of the internal wall  23  is preferably tapered. 
     As shown in  FIG. 1 , the plant also includes means for extracting the particles collected in the compartment  24  of the enclosure  20 . 
     These extraction means are formed by a pump  30  connected, on the suction side, to the compartment  24  via a connecting pipe  31  and provided, on the delivery side, with a pipe  32  for discharging the withdrawn zinc into the volume of the bath  12 . 
     Moreover, the plant includes means for positioning the steel strip  1  with respect to the upper edge  23   a  of the internal wall  23 , which positioning means consist of two horizontal rollers  35  and  36  placed on each side of the strip and offset with respect to each other. 
     In general, the steel strip  1  penetrates the zinc bath  12  via the duct  13  and the liquid seal  14 , and this strip entrains the zinc oxide particles and intermetallic compound particles coming from the bath, thus creating visual defects in the coating. 
     These particles, in supersaturation in the liquid zinc bath  12 , have a lower density than that of liquid zinc which rises to the surface of this bath and especially in the region  17  where the strip leaves. 
     Thus, at the moment of extraction of the strip  1 , on leaving the liquid zinc bath  12 , this steel strip passes through the region  17  which is covered with zinc oxide and intermetallic compound particles. 
     To avoid this drawback, the region  17  where the steel strip  1  leaves is reduced by the internal wall  23  of the enclosure  20  which surrounds the steel strip  1  and the surface of the liquid zinc isolated in this region  17  flows into the compartment  24  of the enclosure  20 , passing over the upper edge  23   a  of the internal wall  23  of the said enclosure  20 . 
     The particles which float on the surface of the liquid zinc region  17  and which are the cause of visual defects are entrained into the compartment  24  of the enclosure  20  and the liquid zinc contained in this compartment  24  is pumped so as to maintain a depressed level sufficient to allow the natural flow of the zinc from this region  17  towards this compartment  24 . 
     In this way, the free surface of the region  17  where the coated steel strip  1  leaves is isolated by the internal wall  23  of the enclosure  20  and this liquid zinc surface is permanently replenished and the liquid zinc sucked up by the pump  30  from the compartment  24  is injected into the zinc bath  12  at the rear of the tank  11  by the discharge pipe  32 . 
     By means of the effect thus created, the coated steel strip runs, on leaving the liquid zinc bath  12 , through a permanently cleaned surface of liquid zinc and emerges from this zinc bath with the minimum of defects. 
     The flow of zinc into the compartment  24  of the enclosure  20  is adjusted by raising the level of the zinc bath  12  by putting zinc ingots into the tank  11 . 
     According to a variant, the flow of zinc into the compartment  24  may be adjusted by varying the vertical position of the enclosure  20  with respect to the surface of the zinc bath  12 . For this purpose, this enclosure  20  may be fitted with height adjustment means for adjusting its vertical position. These means consist, for example, of at least one hydraulic or pneumatic cylinder or any other suitable component. 
     When the level decreases in the compartment  24 , this corresponds to a slight reduction in the amount of zinc flowing into this compartment  24  and therefore in the level of zinc in the region  17 . 
     This reduction is due to the zinc consumed by the steel strip  1  and by the skimming of the surface of the zinc bath  12 . 
     By virtue of the plant according to the invention, the density of defects on the coated surfaces of the steel strip is substantially reduced and the surface quality thus obtained of this coating meets the criteria required by customers desiring parts whose surfaces are free of visual defects. 
     The invention applies to any metal dip-coating process.