Method and device for hot-dip coating a metal strip with a metal covering

Abstract

The invention relates to a device and a method for hot-dip coating a metal strip with a metal covering, wherein the metal strip is directed continuously through a melt bath, wherein the thickness of the metal covering present on the metal strip when it leaves the melt bath is adjusted by means of a scraping device, and wherein slag which is present on the melt bath is driven away from the metal strip leaving the melt bath by means of a gas flow. To prevent slag from coming into contact with the metal strip leaving the melt bath, the invention drives away the slag from the metal strip by means of at least one nozzle which is arranged in close proximity to the metal strip, that a gas flow which extends over the width of the metal strip is directed onto the surface of the melt bath.

Claims

1. A device for hot-dip coating a metal strip with a metal covering, comprising a melt bath, a conveying device for continuously directing the metal strip through the melt bath, a scraping device above a surface of the melt bath for adjusting the thickness of the metal covering present on the metal strip when it leaves the melt bath, and at least one nozzle for producing a gas flow, wherein the nozzle for producing the gas flow is arranged in close proximity to the metal strip and the gas flow extends over the width of the metal strip and is directed onto the surface of the melt bath, the gas flowing from the at least one nozzle for producing gas flow does not contact a surface of the metal strip and is directed away from the surface of the metal strip in a direction that is at an angle to the metal strip, the at least one nozzle is arranged with a spacing of 50-500 mm from the surface of the metal strip, and the at least one nozzle extends over at least 20% of the width of the metal strip.

2. The device according to claim 1, wherein the nozzle for driving away the slag is associated with each surface of the metal strip.

3. The device according to claim 1, wherein the nozzle is a slot nozzle or a slotted pipe.

4. The device according to claim 1, wherein the nozzle is formed by a nozzle bar in which a plurality of nozzle openings are arranged with spacing from each other.

5. The device according to claim 3, wherein the nozzle is arranged centrally with respect to the width of the metal strip leaving the melt bath.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in greater detail below with reference to embodiments. In the schematic drawings:

(2) FIG. 1 is a side view of a device for hot-dip coating a steel strip,

(3) FIG. 2 is an enlarged cut-out A from FIG. 1;

(4) FIG. 3 shows the device according to FIG. 1 corresponding to FIG. 2 in an alternative operating mode;

(5) FIG. 4 shows the device according to FIG. 1 corresponding to FIG. 2 in another alternative operating mode;

(6) FIG. 5 is a top view of the device according to FIGS. 1 and 2.

DESCRIPTION OF THE INVENTION

(7) A device 1 for hot-dip coating a metal strip M in which it is, for example, a cold-rolled steel strip comprising a corrosion-sensitive steel, comprises a melt bath 3 which is introduced in a vessel 2, in which the metal strip N which is intended to be coated and which has previously been brought to an adequate immersion temperature in a known manner is directed via a nozzle 4.

(8) In the hot-dip bath 3, the metal strip M is redirected on a redirection roller 5 in such a manner that it is discharged from the melt bath 3 in a vertically orientated conveying direction F. In this instance, the metal strip M being discharged from the melt bath 3 passes through a scraping device 7 which is arranged with a specific spacing above the surface 6 of the melt bath 3. This scraping device 7 comprises in this instance two scraping nozzles 8, 9 which are constructed as slot nozzles and of which one directs a scraping gas flow AG1 onto one surface O1 of the metal strip M, which surface extends at one side between the longitudinal edges of the metal strip M, and the other of which directs a scraping gas flow AG2 onto the surface O2 present at the opposing side of the metal strip M.

(9) Under optimum operating conditions, the metal strip M being discharged from the melt bath 3 is orientated in such a manner that the centre position ML thereof which is orientated centrally between the surfaces O1, O2 is located in a vertically orientated plane H.

(10) Between the scraping nozzles 8, 9 of the scraping device 7 and the surface 6 of the melt bath 3, there is arranged at each side of the metal strip M with a spacing d of 200 mm a nozzle 10, 11 which produces a gas flow G1, G2 which extends over the width B of the metal strip M, respectively.

(11) The nozzles 10, 11 may be constructed as conventional slot nozzles. However, there have been tested in practice as nozzles 10, 11 air bars which comprised a pipe having an inner diameter of 20 mm and in which twelve cylindrical nozzle openings each having a diameter of 2 mm were drilled, with a spacing of 25 mm, respectively. The gas supply was carried out centrally. In the embodiment tested in practice, the air bar used was approximately 300 mm wide and orientated centrally with respect to the 1370 mm width B of the metal strip M.

(12) In the operating method illustrated in FIG. 2, the discharge openings of the nozzles 10, 11 are orientated in such a manner that a relatively large part-flow G11, G21 of the respective gas flow G1, G2 is directed onto the surface of the melt bath 3 with the centre axis Ga1 thereof in each case at an influx angle of approximately 30 in relation to the perpendicular relative to the surface of the melt bath 3, and at that location flows from the associated surface O1, O2 of the metal strip M in a flow direction which is directed away substantially normally with respect to the respective surface O1, O2. A smaller part-flow G12, G22 of the respective gas flow O1, O2 is in contrast directed against the associated surface O1, O2 of the metal strip. In this instance, the influx angle of this part-flow G12, G22 in relation to the perpendicular relative to the melt bath 3 is selected in such a manner that the border of the impact region X associated with the metal strip M, in which region the respective gas flow G1, G2 strikes the surface 6 of the melt bath 3, terminates with little spacing in front of the metal strip M. The surfaces O1, O2 of the metal strip M provided with the metal covering are thus not touched by the associated gas flow G1, G2.

(13) In the operating method illustrated in FIG. 3, the nozzles 10, 11 are adjusted in such a manner that they do not produce any part-flows G12, G22 which are directed in the direction of the metal strip M.

(14) In the operating method illustrated in FIG. 4, the nozzles 10, 11 are in contrast adjusted in such a manner that they do not produce any part-flows G11, G21 directed away from the metal strip M.

(15) Regardless of whether the gas flows G1, G2 have been produced partially or completely so as to be directed onto the metal strip M or away from it, the gas flows G1, G2 drive the slag S present on the melt bath 3 away from the metal strip M in a direction orientated transversely with respect to the metal strip M so that they accumulate in each case in a region B1, B2 which is non-critical for the metal strip M and which is sufficiently spaced apart, and from there can be removed mechanically, that is to say, manually or by means of a suitable motor-driven device, from the surface 6 of the melt bath 3.

(16) For operational tests, on a large-scale industrial hot-dip coating installation, during the hot aluminium coating, an N.sub.2 gas flow was blown between the melt bath and scraping nozzles by means of two air bars arranged in the manner of the nozzles 10, 11. The coating bath contained 9.5% by weight of Si, 2.5% by weight of Fe and the balance being Al and traces of other elements and inevitable impurities. The speed of the metal strip being discharged from the melt bath was 38 m/min at a layer thickness to be applied of a minimum 75 g/m.sup.2 per side of the metal strip M.

(17) Upper slag which was blown away was removed from the aluminium bath surface in a manual/mechanical manner. Over a relatively long production time, surface defects resulting from carried-along upper slag was able to be effectively reduced or prevented.

(18) Table 1 shows for a slot nozzle which is arranged in a manner according to the invention below the scraping nozzles that this good result was not achieved if no gas flow was applied or if the peripheral conditions provided for according to the invention were deviated from.

LIST OF REFERENCE NUMERALS

(19) 1 Device for hot-dip coating 2 Vessel 3 Melt bath 4 Nozzle 5 Redirection roller 6 Surface of the melt bath 3 7 Scraping device 8, 9 Scraping nozzles 10, 11 Nozzles , Influx angle AG1,AG2 Scraping gas flows B Width of the metal strip M B1, B2 Regions of the surface 6 of the melt bath 3 d Spacing F Conveying direction G1,G2 Gas flows G11-G22 Part-flows of the respective gas flow G1, G2 Ga1,Ga2 Centre axes of the gas flows G1, G2 H Vertically orientated plane of the central position ML M Metal strip ML Central position O1,O2 Surfaces of the metal strip M S Slag X Impact region

(20) TABLE-US-00001 TABLE 1 Gas Surface defects as In accordance pressure Influx angle a result of upper with the Test [bar] [] slag? invention? 1 no gas flow frequently No 2 0.5 45 frequently No 3 1.0 45 sporadically Yes 4 2.0 45 sporadically Yes 5 4.0 45 none Yes 6 6.0 50 sporadically Yes 7 8.0 50 sporadically Yes 8 10.0 50 none Yes 9 12.0 50 none Yes 10 14.0 5 none Yes 11 16.0 5 frequently No 12 15.0 5 none Yes 13 10 direct frequently with No application of significant flow to the fluctuations in metal strip distribution 14 10 70 frequently No 15 10 80 frequently No