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
  • C23C2/003—Apparatus
  • C23C2/0035—Means for continuously moving substrate through, into or out of the bath
• • 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

Definitions

• the present invention relates to a method for coating a metallic strip by dipping, that is to say by continuous scrolling through a liquid metal.
• the invention also relates to a device for implementing this method.
• Dip galvanizing of a steel strip generally comprises two main operations, namely annealing the strip and depositing the coating.
• the corresponding installation therefore comprises two large parts, namely an annealing oven and a galvanizing tank.
• annealing The purpose of annealing is to prepare the strip, in particular its surface, for the following coating operation. In particular, it recrystallizes the work hardened structure in the case of a cold rolled strip; it also operates, in all cases, cleaning and brightening the surface by means of an appropriate atmosphere, generally reducing, comprising hydrogen and nitrogen.
• conventional galvanizing consists in passing a steel strip through a bath of molten zinc and then, at the exit of this bath, adjusting the thickness of the zinc layer by wringing using air knives or nitrogen under pressure.
• the strip is guided by several rollers, in particular a first roller, called the bottom roller, entirely immersed in the zinc, under which it passes before rising vertically towards the surface of the bath, and other rollers, called stabilization, deflection or guide rollers, often partially or even completely submerged near the upper surface of the liquid metal bath, intended to ensure stabilization, possibly the flatness of the coated strip, before it is spun.
• a first roller called the bottom roller
• stabilization, deflection or guide rollers often partially or even completely submerged near the upper surface of the liquid metal bath, intended to ensure stabilization, possibly the flatness of the coated strip, before it is spun.
• rollers motorized or free, come into contact with the surface of the moving band and can thus cause surface defects. They must therefore be frequently dismantled to be inspected, repaired or even replaced, since their condition considerably influences the quality of the zinc coating. This results in frequent shutdowns of the production line and a significant loss of productivity. Furthermore, those skilled in the art are well aware of the difficulty of adjusting and maintaining the composition of the coating metal. It is hardly ever a pure metal, but rather an alloy containing different elements in determined quantities but variable from one type to another, intended to improve the performances of coatings, such as for example the additions of aluminum in galvanizing baths.
• the mattes that is to say aggregates of particles of intermetallic compounds, formed inter alia by the dissolution of the extreme surface of the steel strips by the bath of liquid metal, also cause appearance defects well known, and must therefore be regularly eliminated. This results in wasted time and metal, which adversely affects the productivity and profitability of conventional processes.
• the object of the invention is to propose a method for coating a metallic strip by dipping, which makes it possible to remedy the aforementioned drawbacks, in particular removing all submerged rollers and ensuring a constant composition of the liquid metal in the coating area.
• it proposes a device for implementing this method.
• a method for coating a metallic strip by dipping by means of a liquid metal in which said strip is passed through a volume of said liquid metal, is characterized in that one circulates said strip in a thin channel, in that the liquid metal is introduced into said thin channel at least on either side of said strip, and in that a movement of circulation of said liquid metal is created relative to to said strip, inside said thin channel. It is then found that the coating is therefore carried out by contacting the surface of the metal strip with a flow of liquid metal rather than coating the said surface by immersion in a bath of liquid metal.
• said thin channel is supplied with liquid metal from a separate source, constituted for example by a separate reservoir; such a separate tank is advantageously disposed higher than said thin channel, so as to supply at least partially by a gravimetric effect.
• a separate source constituted for example by a separate reservoir
• the separate tank is advantageously disposed higher than said thin channel, so as to supply at least partially by a gravimetric effect.
• Part or all of the supply can however also be provided by one or more pumps.
• the liquid metal is subjected to at least one treatment before introducing it into said thin channel.
• said treatment can consist in filtering the liquid metal before introducing it into the thin channel. This reduces the risk of depositing floating particles, such as intermetallic compounds, on the surface of the strip.
• the chemical composition and / or the temperature of the liquid metal are measured at determined intervals, preferably continuously, and, if necessary, this chemical composition and / or this temperature is adjusted before introducing the metal. liquid in said thin channel. It is thus possible to maintain at least substantially constant these conditions governing the dip coating operation of the metal strip.
• the supply of said thin channel is carried out in a closed circuit. This avoids any contact of the liquid metal with the ambient air, and therefore any risk of deterioration of the liquid metal, in particular by oxidation.
• said thin channel is supplied with liquid metal under an independently adjustable pressure on each face of the strip.
• the liquid metal feed pressure is adjusted, as the case may be, by varying the level difference between the thin channel and the separate reservoir, or by modifying the pressure supplied by the pumps. food.
• the speed of circulation of the stream of liquid metal with respect to the strip, inside said thin channel is between 0.1 m / s and 5 m / s.
• a device for implementing this method essentially comprises a thin channel, means for circulating the strip to be coated through said thin channel, as well as means for supplying said thin channel with liquid metal and for creating a circulation movement. liquid metal relative to the strip, inside the thin channel.
• the section of the thin channel according to the invention is determined so as to allow the passage of the widest strip which must be coated therein, without requiring an excessive flow of liquid metal.
• the width of the thin channel according to the invention is generally between 1 m and 2 m. It goes without saying that this width could be increased or decreased depending on particular circumstances.
• the height of the thin channel defines the thickness of the layer of liquid metal in circulation, and therefore also influences the flow of liquid metal in the thin channel. This height is preferably less than 6 cm, and more preferably less than 3 cm, so as to be able to limit the flow of liquid metal in the thin channel.
• the thin channel according to the invention may have a shape either planar or curved, in particular partially cylindrical. In the latter case, the curvature must be limited so as not to unnecessarily complicate the trajectory and guiding of the strip.
• the strip to be coated circulates through said channel under the action of the traction exerted by the drive systems of the coating line. Its trajectory is determined by guide means, generally rollers, arranged outside the channel, upstream and downstream thereof, so as not to damage the surface of the strip during the coating operation.
• the strip must therefore be kept at a minimum distance from the walls of the channel, in order to avoid any risk of contact with them.
• this distance is between 1 mm and 15 mm, depending on various parameters such as the speed and width of the strip, the nature of the liquid metal, the length and / or the shape of the path of the thin channel. .
• the thin channel according to the invention comprises on the one hand means for supplying liquid metal, and on the other hand means for creating a movement of circulation of the liquid metal with respect to the strip, inside the thin channel .
• Said supply means comprise at least one separate tank, with a capacity much larger than the thin channel; this reservoir is connected to the thin channel by a supply circuit leading to means for introducing the liquid metal into the thin channel, on either side of the strip which circulates there.
• introduction means can be constituted either by injectors arranged at at least one of the ends of the channel, or by injectors formed in the walls of the channel parallel to the plane of the strip, or even by a combination of these two types of injectors.
• the separate reservoir can be raised relative to the thin channel, or the supply circuit can comprise at least one pump.
• the device also comprises means for controlling and adjusting the temperature and / or the composition of the liquid metal, means for adding fresh liquid metal as well as means for filtering the liquid metal before it introduction into the thin channel.
• the channel is provided with sealing means, such as airlocks at its ends, allowing the passage of the strip but preventing the leakage of liquid metal.
• the channel can advantageously be integrated into a closed circuit, also comprising at least one supply reservoir, possibly raised, a circuit for supplying the thin channel, a circuit for recovering excess liquid metal, means for treating the liquid metal, and optionally at least one liquid metal circulation pump.
• the method and the device of the invention make it possible to coat a strip by immersion in a current of liquid metal animated at high speeds relative to the strip. These speeds are higher than those which result from the running of the strip in a bath of liquid metal at rest.
• Fig. 1 recalls the principle of a conventional dip coating installation of the prior art
• Fig. 2 shows the principle of the process of the present invention
• the Fig. 3a illustrates a first case of application of the method of the invention, with a thin rectilinear practically horizontal channel
• Fig. 3b illustrates a second case of application of the method of the invention, with a thin curved channel in the form of a parabola
• Fig. 3c illustrates a third case of application of the method of the invention, with a thin channel approaching the usual trajectory of the strip in a bath of liquid metal
• Fig. 4 shows different arrangements of the liquid metal supply orifices, formed in the walls of the thin channel (Fig. 4a, b, c)
• Fig. 5 illustrates different possible configurations of the thin channel with its liquid metal supply circuit (Fig. 5a, b, c, d).
• Embodiments of the Invention Figure 1 shows a conventional dip galvanizing device for a steel strip, integrated in a treatment line.
• a strip to be coated 1 is treated in a continuous annealing oven 2, from which it leaves by a tube 3 to plunge into a bath of molten zinc 4.
• the strip 1 passes under a bottom roller 5, completely submerged in liquid zinc 4, then it is guided by stabilization rollers 6 immersed near the upper surface of the zinc bath 4, before leaving the zinc bath along a vertical path.
• the excess zinc is drained by means of blades of gas, generally air or nitrogen, under pressure 7.
• the strip 1 then rises vertically to an upper deflection roller 8, which is situated high enough for the zinc coating is solidified before reaching it.
• Strip 1 then continues on its way in the processing line.
• the strip to be coated 1 circulates through a bath of liquid zinc at rest 4, and it is in direct contact with several immersed rollers 5, 6.
• the principle of the process of the invention is illustrated in FIG. 2.
• the strip to be coated 1 coming from a continuous annealing furnace not shown, circulates through a thin channel 10 in which a stream of metal has been created liquid 9.
• the channel 10 is rectilinear and slightly inclined upwards, and the circulation of the liquid metal is ensured by a pump 1 1.
• the strip 1 and the stream of liquid metal 9 circulate in the upward direction, indicated by arrows. Sealing systems, not shown, are provided at the inlet and outlet ends of the strip 1 in the channel 10.
• FIG. 3 shows how it is possible to control, at least in part, the position of the strip in a thin channel, according to the method of the invention.
• a movement of circulation of the liquid metal for example molten zinc, is created in the channel 10, on either side of the strip 1.
• This current preferably flows at high speed. , and is inevitably accompanied by pressure gradients between the strip 1 and the walls of the channel 10.
• the hydraulic paths that the liquid metal follows in the channel 10 can be designed so as to create pressure differences between the faces of the strip and thereby influence the position of the band in the channel. Indeed, even if they are of small amplitude, these pressure differences act on large surfaces, namely the surface of the two faces of the strip in the channel, and they can thus generate considerable forces.
• a pressure difference between the two faces of the strip can be obtained in several ways: by creating different liquid metal flow rates on the two faces, while maintaining sufficient circulation speeds; > by modifying the distance between the strip and the wall of the channel: at equal metal flow, a reduction in this distance leads to an increase in pressure; by differentiating the injection of liquid metal between the two faces of the strip, as regards both the injection pressure and the number and / or the arrangement of the injectors.
• FIG. 3a illustrates the simple case of a thin rectilinear channel 12, practically horizontal, in which the strip 1 circulates at equal distance from the two walls.
• the pressures 13 exerted on the lower face of the strip are higher than those 14 applied on the upper face, in order to support the weight of the strip 1 while maintaining the latter in position centered in the channel 12.
• the channel 15 has the shape of a parabola, corresponding to the so-called “chain” shape that naturally takes the strip 1 suspended freely between two upper rollers not shown.
• Such a parabolic trajectory of the strip 1 can be maintained by permanent control of the speed of the strip upstream and downstream of the channel 15. It is also maintained thanks to the pressures that the liquid metal exerts on the strip; if the pressures 16 on the internal face are higher than those 17 exerted on the external face, they are even capable of compensating for a traction in the strip.
• it is particularly easy to seal the channel by collecting the liquid metal overflowing at the two ends of the channel to reintroduce it into the lower part, for example using pumps 18.
• FIG. 3c shows a channel 15 of a shape close to that of the usual path of a strip 1 in a dip coating installation of the type illustrated in FIG. 1.
• the strip 1, which undergoes the traction exerted by the treatment line is supported by the application of pressures 19 higher on the internal face than those 20 exerted on the external face.
• the strip 1 can be guided in a controlled manner both at the inlet and at the outlet of the channel, by differential pressures 21, which advantageously replace, that is to say without contact, the guide rollers 6 of figure 1.
• the channel is easily sealed by collecting excess liquid metal at the two ends of the channel to reintroduce it into the lower part, for example using a common pump 22.
• the thin metal is supplied with liquid metal preferably by orifices in the walls of the channel, so as to distribute the pressures acting on the faces of the strip as uniformly as possible.
• FIG. 4 shows various possible arrangements of these supply orifices in a portion of a channel wall 23.
• the orifices can be either slots 24 parallel and / or perpendicular to the direction 25 of movement of the strip (Fig. 4a), or orifices, for example circular 26, arranged in lines also parallel to the direction 25 of movement of the strip (Fig. 4b), or even orifices, for example circular 27, distributed regularly, for example in staggered rows, in the wall of the canal (Fig. 4c).
• FIG. 5 illustrates different possible configurations of the liquid metal circuit, with a treatment unit, possibly combined with a raised external tank.
• FIG. 5a a horizontal rectilinear channel 12, traversed by a strip 1, is supplied directly by means of a pump 28.
• the liquid metal is collected at the ends of the channel 12 and returned by the pump 28, d first to a processing unit 29 and then directly into the channel 12.
• FIG. 5b also represents a horizontal rectilinear channel 12 traversed by a strip to be coated 1.
• This channel 12 is fed by gravity from a raised reservoir 30 of large capacity, while the excess liquid metal is recovered at the ends of the channel 12 and returned, still by gravity, to a collection tank 32. From this, the liquid metal is then returned to the raised tank 30 by means of a pump 31.
• a treatment unit 29 is placed between the tank 30 and the channel 12 and / or between the pump 31 and the reservoir 30.
• FIG. 5c The configuration of FIG. 5c is similar to that of FIG. 5a, with the difference that the channel 33 follows here the path of the strip 1 in a conventional dip coating line.
• the excess liquid metal is recovered at the raised ends of the channel 33 and returned to the low point of this channel, through a pump 28 and a treatment unit 29.
• FIG. 5d illustrates an arrangement, similar to that of FIG. 5b, but where the channel 33 follows the path of the strip 1 in a conventional dip coating line.
• the channel 33 is fed directly at its low point by gravity, from a raised reservoir 30 of large capacity; the excess liquid metal is collected at the raised ends of the channel 33 and returned, also by gravity, to a collection tank 32. From this, the liquid metal is returned to the raised tank 30 by means of a pump 31
• a processing unit 29 is provided between the reservoir 30 and the channel 33, and / or between the pump 31 and the reservoir 30.
• the present invention offers numerous advantages, as regards both the process itself and the quality coated products.
• the continuous circulation of the liquid metal between the coating channel and a separate tank makes it possible to process the liquid metal continuously.
• mattes that is to say the particles of intermetallic compounds which float in the liquid metal and which are liable to deposit on the strip.
• This elimination can be done by any suitable method, such as filtering, decantation, centrifugation, etc., in particular in the processing unit mentioned above.
• the quality of the coating depends in particular on the reactivity between the coating metal and the surface of the strip to be coated.
• high-strength steels intended to be galvanized for the automotive industry have a surface that is weakly reactive with regard to zinc; this results in a lack of wettability which can lead to serious coating defects.
• the high relative speed between the strip and the liquid metal in the coating channel causes an improvement in the reactivity between the liquid metal and the strip, and contributes to eliminating the abovementioned coating defects.
• the introduction of a significant relative speed, for example from 0.5 m / s to 1 m / s, between the strip and the liquid metal in the channel contributes to reducing the risk of sticking to the strip, of particles suspended in the liquid metal. This effect is in addition to that of the matte removal operation mentioned above.
• the method of the invention restores the symmetry between the two faces of the strip, owing to the fact that neither of the two faces of the strip is no longer in contact with rollers.
• the actual duration and the conditions of exposure of the two faces of the strip to liquid metal are identical.
• the device of the invention is much simpler than conventional dip galvanizing devices. It requires less liquid metal, it offers greater flexibility of operation and it allows obtaining better quality coated products. Finally, it can be integrated, without significant difficulty, into an existing processing line, as shown in Figure 3c.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Coating With Molten Metal (AREA)
• Continuous Casting (AREA)
• Chemically Coating (AREA)

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• 2001
  • 2001-04-03 BE BE2001/0216A patent/BE1014093A3/fr not_active IP Right Cessation
• 2002
  • 2002-03-28 ES ES02719575T patent/ES2242008T3/es not_active Expired - Lifetime
  • 2002-03-28 WO PCT/BE2002/000047 patent/WO2002083969A1/fr not_active Application Discontinuation
  • 2002-03-28 AT AT02719575T patent/ATE296905T1/de not_active IP Right Cessation
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