Patent Grant
7722933
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.
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:
According to other features of the invention:
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:
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
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
This duct 13, also called “snout”, has, in the illustrative example shown in the figures, a rectangular cross-section.
The lower part 13a 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 13a 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 16a 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
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
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
According to a second embodiment, shown in
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
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 23a 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 23a 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.
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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3941906 | Bostroem | Mar 1976 | A |
| 4337929 | Evans | Jul 1982 | A |
| 5069158 | Rey | Dec 1991 | A |
| 6994754 | Dauchelle et al. | Feb 2006 | B2 |
| Number | Date | Country |
|---|---|---|
| 486 584 | Jun 1938 | GB |
| 06128711 | May 1994 | JP |
| 06128711 | May 1994 | JP |
| 06269913 | Sep 1994 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20060113354 A1 | Jun 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10416191 | Oct 2003 | US |
| Child | 11275935 | US |
