Patent EA-004449-B1 (EA)
Title: Method and plant for hot process and continuos dip coating of a metal strip
Inventor: DAUCHELLE DIDIER
IPC: C23C2/26
Filed: 2001-11-07
Abstract: 1. Process for the continuous dip-coating of a metal strip (1) in a tank (11) containing a liquid metal bath (12), in which process the metal strip (1) is made to run continuously, in a protective atmosphere, through a duct (13), the lower part (13a) of which is immersed in the liquid metal bath (12) in order to define with the surface of the said bath, and inside this duct (13), a liquid seal (14), the metal strip (1) is deflected around a deflector roller (15) placed in the metal bath (12) and the coated metal strip (1) is wiped on leaving the metal bath (12), characterised in that a natural flow of the liquid metal from the surface of the liquid seal (14) is set up in two overflow compartments (25; 29) made in the said duct (13) and each having an internal wall (20; 26) which extends the duct (13) in its lower part and facing each side of the strip (1), the upper edge (21; 27) of each compartment (25; 29) being positioned below the said surface and the drop in height of the liquid metal in the compartments (25; 29) 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 level of liquid metal in the said compartments (25; 29) is maintained at a level below the surface of the liquid seal (14). 2. Plant for the continuous hot dip-coating of a metal strip (1), of the type comprising: a tank (11) containing a liquid metal bath (12), a duct (13) through which the metal strip (1) in a protective atmosphere runs and the lower part (13a) of which duct (13) is immersed in the liquid metal bath (12) in order to define with the surface of the said bath (12), and inside this duct (13), a liquid seal (14), a roller (15), placed in the metal bath (12), for deflecting the metal strip (1) and means (16) for wiping the coated metal strip (1) on leaving the zinc bath (12), characterised in that the duct (13) is extended, in its lower part (13a) and facing each side of the strip (1), by an internal wall (20; 26) directed towards the surface of the liquid seal (14) and the upper edge (21; 27) of which internal wall is positioned below the said surface, the said walls (20; 26) forming two compartments (25; 29) for overflow of the liquid metal, provided with means (30) for maintaining the level of liquid metal in the said compartments (25; 29) at a level below the surface of the liquid seal (14) in order to set up a natural flow of the liquid metal from this surface towards these compartments (25; 29), the drop in height of the liquid metal in the said compartments being 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. 3. Plant according to claim 2, characterised in that the drop in height of the liquid metal in each compartment (25, 29) is greater than 100 mm. 4. Plant according to claim 2, characterised in that the internal wall (20; 26) of each compartment (25; 29) has a lower part flared out towards the bottom of the tank (11) and an upper part parallel to the metal strip (1). 5. Plant according to claim 2 or 3, characterised in that the upper edge (21; 27) of the internal wall (20; 26) of each compartment (25; 29) is straight. 6. Plant according to claim 2 or 3, characterised in that the upper edge (21; 27) of the internal wall (20; 26) of each compartment (25; 29) comprises, in the longitudinal direction, a succession of hollows (22) and projections (23). 7. Plant according to claim 6, characterised in that the hollows (22) and the projections (23) are in the form of circular arcs. 8. Plant according to claim 6 or 7, characterised in that the difference in height between the hollows (22) and the projections (23) is between 5 and 10 mm. 9. Plant according to claim 6 or 7, characterised in that the distance between the hollows (22) and the projections (23) is of the order of 150 mm. 10. Plant according to any one of the preceding claims, characterised in that the upper edge (21; 27) of the internal walls (20; 26) of each compartment (25; 29) is tapered. 11. Plant according to any one of the preceding claims, characterised in that the internal wall (20; 26) of each compartment (25; 29) is made of stainless steel and has a thickness of between 10 and 20 mm for example. 12. Plant according to claim 2, characterised in that the means for maintaining the level of liquid metal in the compartments (25; 29) are formed by a pump (30) connected on the suction side to each of the said compartments via a connecting pipe (31; 33) and provided on the delivery side with a pipe (32) for discharging the withdrawn liquid metal into the volume of the bath (12). 13. Plant according to any one of the preceding claims, characterised in that it includes means (35) for displaying the level of liquid metal in each compartment (25; 29). 14. Plant according to claim 13, characterised in that the display means are formed by a reservoir (35) placed outside the duct (13) and connected to the base of each compartment (25; 29) via a connection pipe (36; 37). 15. Plant according to claims 12 and 14, characterised in that the point where the pump (30) is connected to each compartment (25; 29) lies above the point where the reservoir (35) is connected to each compartment (25; 29). 16. Plant according to claim 14, characterised in that the reservoir (35) forms a buffer container of liquid metal for each compartment (25; 29). 17. Plant according to claim 14, characterised in that the reservoir (35) is equipped with a liquid metal level detector. 18. Plant according to any one of the preceding claims, characterised in that the duct (13) is extended, in its lower part (13a) and facing each lateral edge of the metal strip (1), by an internal wall (40) directed towards the surface of the liquid seal (14), whose upper edge (41) is positioned below the said surface and forming a liquid metal overflow compartment (42).