The invention concerns a method for coating the surface of a product, especially a strip-shaped product, for example, nonferrous metal strip or steel strip, with at least one metal coating by passing the product through at least one molten metal bath space that contains the molten coating material. The invention also concerns a device for carrying out the method.
In conventional hot dip coating of strip (referred to here as Method 1) with Zn, Zn—Al, Al, or Al—Si alloys, the strip runs in the coating section from an annealing furnace under conditions of air exclusion into the molten metal and is deflected vertically and stabilized by various arrangements of nondriven rollers (see
A disadvantage of Method 1 is that the rollers and the bearings of the rollers are located within the molten material, and all parts are exposed to chemical attack by the molten material. The service life of the parts that are used within the molten material is limited. In addition, a large volume of molten material with a correspondingly large dip bath is necessary to accommodate the rollers and all of the bath equipment. 200 to 400 t of molten zinc are customary in hot dip galvanizing. Due to this large volume, rapid regulation of the temperature and alloy composition of the melt is not possible. Large fluctuations of the specified parameters must be accepted and sometimes result in loss of quality, since measures related to the production of the alloy and those related to influencing the strip quality are carried out in the same tank and thus affect one another.
Another disadvantage is that the production speed cannot be increased to realize an economical plant output (about 180 m/min), especially in the case of thin strip <0.5 mm. One reason for this is that relative motion can develop between the rollers located in the bath and the strip. If the tension is increased in an effort to avoid this problem, there is the risk of strip breakage. This results in scrap and prolonged plant shutdowns.
The jet stripping system located above the zinc hot dip bath further limits the maximum strip advance speed of a hot dip galvanizing installation (see
So-called vertical hot dip galvanizing is well known as an advanced method for the hot dip coating of ferritic steel strip made of soft unalloyed steels and is described in various patents, such as EP 0 630 421 B1, EP 0 630 420 B1, and EP 0 673 444 B1.
In this method (referred to here as Method 2), the strip passes from bottom to top through a working tank filled with molten metal composed of zinc and/or Al alloys after it has been subjected to a heat treatment. The strip enters the molten bath under conditions of air exclusion. The volume of molten metal (about 2-5 t of molten zinc) is much smaller than in Method 1. The qualitative problems described above also do not occur, since the measures related to the production of the alloy are carried out in a reservoir located alongside the line, while measures to influence the strip quality are carried out separately in the working tank.
The working tank and the furnace chamber located below it are connected by a gastight ceramic duct, which is about 800 mm high and has a passage width for the strip of only a maximum of 20 mm. The working tank is sealed at the bottom to prevent molten metal from flowing down into the furnace chamber by means of a seal produced within this duct by two inductors arranged at the side of the duct or strip. These inductors induce an electromagnetic traveling field, which produces an upwardly directed force that prevents the molten metal from flowing down. This inductive system acts like a pump, so that exchange of the melt in the duct is ensured.
Method 2 is characterized by the fact that, at least in the coating area up to the hot dip bath, significantly higher strip speeds on the order of 300 m/min can be realized even with thin steel strip, since there are no rollers in the coating tank.
After the strip has passed through the coating unit from bottom to top at a temperature, e.g., in the case of hot dip galvanizing, of about 460° C., the desired thickness of the metal coating is adjusted a short distance above the hot dip bath by the jet stripping process, as in Method 1. This process is comparable to the process used in Method 1 and involves the blowing of compressed air or nitrogen.
As in Method 1, the jet stripping process in Method 2 also limits the maximum possible strip speed when thin coatings are being applied. However, Method 2 offers greater degrees of freedom for the galvanizing parameters of melt temperature and viscosity and alloy composition, which likewise affect the coating thickness. For this reason, it is to be expected that a higher strip speed can be selected in Method 2 than in Method 1 for the same coating thickness. In contrast to Method 1, Method 2 has not yet been tested on the industrial scale. So far only pilot plant trials with narrow strip have been conducted. These trials were successful.
However, an obstacle to an increase in speed is presented by the fact that the strip subsequently must be cooled below 300° C. in the upwardly traveling strand before the first deflection. If the temperature is higher, there is the danger that metallic particles will grow on the first contact roller or deflecting roller in the cooling tower and cause irreparable surface defects on the material.
The cooling is usually produced by several successive air cooling lines. However, the cooling effect or, more precisely, the cooling rate, is limited by the medium and cannot be increased at will on a fixed length of line (e.g., two times 15 m) with the use of air as the cooling medium. With increasing strip speed or with increasing mass throughput, the cooling lines must be lengthened. However, it then becomes necessary to raise the upper deflecting roller in the cooling tower of a hot dip coating installation.
In installations that are operated by Method 1, the height of the upper deflecting roller is usually 30-60 m. In the case of Method 2, it would be necessary, at high strip speeds, to lengthen the cooling lines accordingly, and the height of the cooling tower would have to be increased to about 80-90 m. This requires higher capital expenditures for buildings and foundations.
The free-running, unstabilized strip length in the tower would thus increase, and the strip flow would be destabilized, so that vibrations may occur, and the product quality may be adversely affected. The use of other cooling media in the upwardly traveling strand is problematic, and so far this problem has not been solved on the industrial scale.
Another problem, which concerns the electromagnetic seal used in Method 2, is that the forces that act on the liquid melt also act on the ferritic strip. Undesired contact of the strip with the duct due to the magnetic forces of the sealing inductors is possible only by additional expensive measures. This requires additional stabilizing coils and expensive automatic control technology.
The objective of the present invention is to avoid the specified disadvantages of Methods 1 and 2 and to create a high-speed hot dip coating installation without a cooling tower, which combines the least possible construction expense with optimized capital investment costs and high plant output with the best production quality.
This objective is achieved with a method of the type described in the introductory clause of claim 1 by sealing the molten metal bath space by means of rotating permanent magnets. The sealing of the molten metal bath space by rotating permanent magnets is considerably more reliable and less expensive than an electromagnetic solution, and significantly less power is needed for the rotation than for an electromagnetic seal, which is an advantage especially in the event of a power failure.
Refinements of the method are described in the dependent claims. A device and refinements of this device for carrying out the method of the invention are the objects of additional claims.
The invention is described below with reference to several embodiments shown schematically in the drawings.
In accordance with
By contrast, the present invention proposes two adjacent rotors 5, 5′. The rotors are tubes 6, 6′ made of materials that are resistant to heat and molten metal, preferably ceramic materials. Rollers, on whose cylindrical surface permanent magnets 4 are mounted, rotate inside these tubes 6, 6′, whose diameters may be freely selected. The rotors 5, 5′ can be adjusted to the melt or to the strip. It is also possible to close the gap 7 when the installation is shut down or is being started up.
Permanent magnets are significantly less expensive than electromagnetic sealing by means of coils or inductors, and much less power is required for the rotation than for an electromagnetic seal, which is an advantage especially in the event of a power failure.
In addition, much higher field strengths can be produced with permanent magnets (by a factor of 3) than by the electromagnetic method. These high field strengths and the resulting higher forces are needed for the stripping process for adjusting the desired coating thickness on the steel strip. In the previously known methods, this adjustment must be accomplished by additional stripping jets.
Additional measures within the magnetic seal and stripping are no longer required in the method of the invention, since the region of the narrowest passage of the strip 1 through the sealing unit is now only a few millimeters. Furthermore, the strip can be supported at much shorter lengths than in the previously known Methods 1 and 2, since the strip 1 can be immediately cooled and deflected into a water bath 9 directly below the sealing unit. The support length in the present invention is preferably only about 5,000 mm, whereas in Method 1 it is about 8-10 times greater, and in Method 2 it is greater still.
Another advantage of the method of the invention is that the surface of the molten metal, preferably the molten zinc, in the coating area is within a protective gas atmosphere, which preferably consists of a nitrogen/hydrogen mixture, so that interfering oxidation of the molten zinc cannot occur. In the previously known Methods 1 and 2, this can be accomplished only with considerable additional expense. Furthermore, in the previous methods, it is necessary for the surface of the zinc bath to be accessible for certain types of manual work. In the present invention, access to the surface of the hot dip bath for the purpose of removing particles of oxidized metal is unnecessary.
In the embodiment in
The incoming strip 1 to be coated passes through a tension roller 17 and then through a lock 18, which hermetically seals the protective gas atmosphere prevailing inside the hot dip coating installation from the ambient, oxygen-containing atmosphere.
In the galvanizing chamber 14 which follows, the strip 1 is vertically deflected by guide rollers 13 towards the coating section 19. Upon entering the coating station 19, the strip 1 passes vertically from top to bottom through the bath of molten metal 3 maintained in the gap 7 between the rotors 5, 5′ and thus receives the desired coating.
At the lower end of this hot dip bath 3, in a gap formed between spaced rotors 5, 5′, the molten metal is prevented from running out at the bottom by magnetic forces of magnetic fields or traveling magnetic fields of the rotating permanent magnets 4. The rotors 5, 5′ are located inside the tubes 6, 6′ that surround them. The coating station 19 is surrounded on the outside by a duct-like housing and holds the rotors 5, 5, which are spaced a variable distance apart. They are surrounded by the tubes 6, 6′, which are made of materials that are resistant to heat and molten metal, especially nonmagnetic materials and preferably ceramic materials.
The permanent magnets 4 rotate inside these tubes 6, 6′.
The molten metal required for coating, which must be continuously replenished, is conveyed in controlled amounts by a metal pump 12 from a reservoir 8, in which it is conditioned, into the gap 7 between the rotors 5, 5′. The strip 1, which is coated in the gap 7, passes through the gap at the lower end and then passes in succession through an arrangement 15 for air stabilization and an arrangement 16 for water cooling.
After it has passed through the water bath 9 and a tension roller 20, it is removed from the installation for further use or treatment.
The additional
(a) in a start-up situation, and
(b) during shutdown after operation.
(a) Start-Up Situation:
(b) During Shutdown after Operation:
Number | Date | Country | Kind |
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1048158.6 | Sep 2001 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP02/10741 | 9/25/2002 | WO |