EP2776600B1 - Process and apparatus for the hot-dip coating of a metal strip with a metallic coating - Google Patents

Description

The invention relates to a process for hot dip coating a metal strip with a metallic coating, in which the metal strip is passed in a continuous pass through a melt bath in which the thickness of the metallic coating present on its exit from the melt bath is adjusted by means of a scraping device, and wherein the slag present on the melt bath is expelled by means of a gas flow from the metal strip emerging from the melt bath. Typically, the metal strips coated in this manner are hot or cold rolled steel strips.

The invention also relates to a device for hot dip coating a metal strip with a metallic coating, said device comprising a melt bath, a conveyor for continuously passing the metal strip through the melt bath, a stripping device for adjusting the thickness of the metal present on its exit from the molten bath on the metal strip Coating and at least one nozzle for discharging a gas stream, which expels existing slag on the melt bath of the emerging from the melt bath metal strip.

The continuous hot dipping refinement of the type specified initially represents an industrially established, economically and ecologically sensible process principle with which metallic flat products can be coated with a metallic coating, for example for the purpose of corrosion protection. Thus, the hot dip finishing of a previously in-line recrystallization annealed metal strip with a Zn (hot dip galvanizing) or Al alloy coating (fire aluminizing) has a high importance for the production of starting material for sheet metal applications in the automotive, household appliance and mechanical engineering.

In the continuous hot dipping refinement, the annealed metal strip is passed through a melt bath consisting of a melt of the metal forming the respective coating or metal alloy forming the respective coating, and then deflected at least once within the melt bath via a roller system while being stabilized in its barrel until it comes out of the melt bath. Excess, still molten coating material is stripped off after leaving the coating of wiping nozzles. The stripping is usually done by blowing off by means of a gas stream. However, purely mechanical stripping systems are also in use.

During the dipping phase in the coating bath, something inevitably always dissolves from the steel material of the steel strip in the coating bath. At the same time, the molten coating bath is permanently in direct contact with the ambient air. Both do not lead to one avoidable slag formation in the melt bath. This slag floats on the melt bath as so-called "upper slag".

If top slag is entrained by the metal strip emerging from the coating bath, the coating quality can be permanently impaired by the resulting imperfections. For example, so-called "smear" or the tape is damaged by indentations when the entrained slag adheres to subsequent roles and cakes. This sometimes creates significant costs due to rework and devaluation of the coated metal strip. The discharge of larger lumps of top slag, so-called "bats", can even lead to cost-intensive roller damage to the usually in-line downstream skin pass mill.

The plant operator is thus faced with the constant challenge of avoiding the entrainment of upper slag by the coated metal strip as far as possible.

There are various known ways to avoid entrainment of slag by the metal strip emerging from the melt bath.

In the first place, here are manual-mechanical methods. In practice, the upper slag is removed from the coating bath at short intervals by employees with the aid of pull-off tools. This procedure has the disadvantage that the upper slag removal is discontinuous, thus always - albeit short - time windows exist in which upper slag can freely come into contact with the exiting metal strip. When manually removing the top slag from the immediate vicinity of emerging from the molten metal strip, the coating quality can also be affected by excessive swirling of the coating and by touching the metal strip with the stripping tool.

Likewise, so-called Abschlackeroboter are known, the motor-driven slag automatically deduct from the respective melt bath. Such Abschlackeroboter form the manual stripping and can not be placed on any hot dip coating plant due to the structural conditions.

Furthermore, so-called mirror rollers are used in practice, which are positioned parallel to the width axis of the exiting metal strip and remove the slag coming into contact with them, which adheres to its surface, from the melt bath. To this state of the art also belongs in the DE 10 2006 030 914 A1 described device in which a motor-driven working material strips the top slag at a uniform speed of the coating surface. The use of motorized mirror rollers or motorized scrapers allows a continuous operation. However, moving parts are in permanent contact with the coating bath. The industrial everyday life shows here that the aggressiveness of the molten coating bath a considerable Wear generated on such moving components. This applies to the coating of a steel strip with an Albasierten coating ("Feueralumierung").

A third way of preventing the slag from the metal strip emerging from the melt bath is by continuously circulating the coating bath and by establishing cooling zones, by means of which slag formation can be deliberately laid in surface areas of the melt bath which are remote from the strip run. The effectiveness of these measures can thereby be increased by directing the flows within the coating bath so that they act counter to the strip run. As a result, dissolved metal strip components are transported away from the metal strip. Procedures of this kind are in each case in the WO 2009/098362 A1 , of the WO 2009/098363 A1 , of the US 5,084,094 A1 , of the US Pat. No. 6,426,122 B1 and the US 6,177,140 B1 described. A problem with these methods is that they sometimes require very complex and expensive equipment, which in many cases can not be retrofitted to any existing hot-dip coating equipment. Furthermore, it shows that the required bath flow in industrial everyday life is very difficult to maintain permanently. Moreover, in the apparatus required to carry out these methods, many mechanical components come into direct contact with the molten plating bath and are accordingly subject to high wear.

When stripping superfluous coating material from the metal strip by means of Abreifdüsen, immediately above are positioned at the coating bath surface, results at high gas pressures and correspondingly high flow rates of the gas stream as a positive side effect that a deflected to the surface of the coating bath partial gas flow suppresses upper slag from the exiting metal strip. Wiping nozzles that do this are, for example, in the DE 43 00 868 C1 and the DE 42 23 343 C1 described. However, here the expulsion of the slag takes place from the emerging from the melt bath metal strip in each case in an uncontrolled, rather random manner. At low gas pressures, such as those set at low belt speeds or in the case of high coating thicknesses, the side effect "pushing the slag away from the metal strip emerging from the melt bath" does not occur.

From the JP 07-145460 Finally, it is known to arrange a nozzle carrier transversely to the metal strip emerging from the melt bath and parallel to the surface of the melt bath in such a way that the gas flowing out of it deposits the slag present on the melt bath through parallel to the strip and tangentially to the surface of the melt bath the roof surfaces of a pointed gable roof are driven laterally away from each other facing gas flows to the respective outer edge of the melt bath. The damming up there slag can then be mechanically removed. However, a disadvantage of this procedure, which is closest to the invention, is the dead space which inevitably arises in the area below the nozzle carrier. In this dead space slag can collect, which comes into contact with the emerging from the melt bath Band comes there and leads to significant surface defects in the bandwidth center. Another disadvantage of this approach is the fact that the gas streams of the nozzle beam are located for the most part at a great distance from the metal strip and therefore slag is driven at one point from the surface of the melt bath, at the no risk of penetration of the metal strip with slag consists. This leads to unnecessary gas consumption.

Against the background of the prior art described above, the object of the invention was to provide a method and an apparatus for hot dip coating of metal strips, which allow with simple and inexpensive means to avoid the contact of slag with the emerging from the melt bath metal strip and to ensure an optimal surface quality.

With regard to the method, this object has been achieved according to the invention in that such a method comprises the measures specified in claim 1.

With regard to the device, this object has been achieved according to the invention in that such a device has the features specified in claim 9.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

Accordingly, in a method according to the invention for hot dip coating a metal strip with a metallic coating, in accordance with the prior art described above, the metal strip is passed continuously through a melt bath, then the thickness of the metallic coating present on the metal strip as it emerges from the melt bath set a scraper and thereby driven away on the melt bath slag by means of a gas stream of the emerging from the melt bath metal strip.

According to the invention, a gas stream extending over the width of the metal strip is directed onto the surface of the melt bath by means of at least one nozzle arranged closely adjacent to the metal strip for expelling the slag.

Similarly, an apparatus according to the invention for hot dip coating a metal strip with a metallic coating comprises a melt bath, a conveyor for continuously passing the metal strip through the melt bath, stripping means for adjusting the thickness of the metallic coating present on the metal strip as it emerges from the melt bath, and at least a nozzle for discharging a gas stream expelling slag present on the melt bath from the metal strip emerging from the melt bath.

According to the invention, the nozzle for discharging the gas stream is now arranged closely adjacent to the metal strip and brings out a gas flow extending over the width of the metal strip and directed onto the surface of the melt bath.

Surprisingly, it has been shown that by means of a directed onto the surface of the melt bath, extending over the width of the respective emerging from the molten metal strip metal gas stream on the surface of the coating surface coating surface can be kept away from the exiting metal strip. The gas flow can be easily controlled and regulated. In particular, pressure and injection angle of the gas flow can be adapted to the coating bath, the desired coating thickness and the belt speed and always be selected so that the gas flow acts directly on the coating bath. As a result, the risk of surface defects due to contact of the coating with slag present on the molten bath is effectively minimized by simple means and in a particularly reliable manner.

As a result of the procedure according to the invention and the special design of a device according to the invention, a particularly low wear and also a low susceptibility to interference results. This results in a high level of maintenance and user-friendliness with minimized operating costs.

Another advantage of the invention is that existing hot dip coating systems with can be retrofitted with a device according to the invention little effort and can be operated in accordance with the invention. In this case, the invention can be used independently of the composition of the respective processed melt bath.

Advantageously, the gas flows are aligned so that a direct flow of the respective surface of the metal strip is avoided. By a direct flow, the band position of the metal strip could be destabilized in the Abstreifdüse.

Therefore, the gas flow discharged by the nozzle arranged according to the invention is optimally aligned in each case such that the gas flow is directed transversely to the conveying direction of the metal strip away from it. In this case, the gas flow is preferably oriented in such a way that it is aligned substantially at right angles to the surface of the metal strip assigned to the respective nozzle.

By a gas flow flowing away from the metal strip, it is ensured that, as far as possible, no impulse also directed transversely to the conveying direction of the metal strip but directed against the surfaces of the metal strip is caused by the gas flow.

In the case that the gas flow flows away from the associated surface of the metal strip, optimum effects result if the injection angle is in the range of 0-60 °, in particular 0-45 °. In such an orientation, it is ensured that the gas flow with a for driving away the slag sufficient impulse hits the surface area of the melt bath to be kept clear of slag.

The nozzle provided according to the invention for discharging the gas stream is preferably arranged as close as possible to the metal strip, wherein the distance between the nozzle and the strip in practice will each be chosen so that it also under the fluctuations of the strip layer occurring in practice no contact between the nozzle and the belt comes. For this, the distance between the nozzle and the metal strip is set in the range of 50 - 500 mm.

In many cases, it will not be possible to arrange the nozzle provided for discharging a gas stream according to the invention in the immediate vicinity of the associated surface of the metal strip. Instead, a certain minimum distance must be maintained. In such a case, at least a partial flow of the gas stream discharged from the nozzle is preferably directed against the metal strip. In this case, the gas flow is preferably oriented in such a way that the impact area lies on the surface of the melt bath in front of the metal strip, that is to say that the gas stream does not cover the surface of the metal strip. In this way, irregularities of the coating, which could be caused by a gas jet striking the metal strip in front of the scraping device, are avoided. In addition, it is avoided that the gas stream destabilizes the proper band position of the metal strip on its conveying path through the stripping device.

In practice, the respective gas stream may be air, a gas inert with respect to the melt bath, or a gas mixture formed from air and a gas inert to the melt bath. It has been found that pressurization of the gas flow supplied to the nozzles in the range of 1 to 15 bar leads to good results under the conditions prevailing in practice. The regulation and control of the gas flow can be done by the operator via adjustment of the horizontal, vertical and possibly axial alignment of the device according to the invention and the gas pressure. The pressure set in each case must be large enough on the one hand to drive away the upper slag from the surface of the emerging metal strip. The gas pressure should not exceed 15 bar, however, because at high pressures there is a risk that the surface of the coating bath is set by the impulse of the impinging gas in undesirable vibrations.

The "blown" upper slag can be skimmed off in a conventional manner at a sufficient distance from the exiting metal strip mechanically from the coating.

Experimental observations have shown that a particularly positive effect of the method according to the invention is achieved when N ₂ or another gas inert with respect to the metallic coating and the melt bath is used for the gas flow. This results from the fact that when using nitrogen or a comparably inert gas next to the pure expelled the Upper slag and the formation of new upper slag in the area swept by the gas stream is significantly reduced.

The metal strips processed in accordance with the invention are typically cold-rolled or hot-rolled steel strips.

For the melt baths, all of the metallic melts that can be applied by hot dip coating can be used. These include, for example, zinc or zinc alloy melts as well as aluminum or aluminum alloy melts.

As a nozzle for the purposes of the invention, for example, slot nozzles of known type are suitable. Also can be used as a nozzle acting as a slot nozzle slotted or perforated tube and a nozzle unit, which is occupied by two or more juxtaposed individual nozzles. Practical tests have shown that the invention can also be carried out with nozzle widths which are narrower than the width of the respective metal strip to be coated. Thus, in the case of a central arrangement with respect to the width of the metal strip, nozzles or nozzle arrangements are sufficient which extend over only 20% of the metal strip width. However, the nozzles or nozzle arrangements can also be wider than the metal strip to be coated. However, nozzle widths of more than 120% of the metal bandwidth would not make economic sense due to the increasing proportion of ineffective gas.

Optimal protection with simultaneously optimized process stability results if each surface of the metal strip is assigned a nozzle for expelling the slag.

Preferably, the pressure with which the gas flowing into the nozzle used according to the invention is applied is in the range from 1 to 15 bar.

Fig. 1

eine Vorrichtung zum Schmelztauchbeschichten eines Stahlbands in seitlicher Ansicht;

Fig. 2

einen vergrößerten Ausschnitt A von Fig. 1;

Fig. 3

eine der Fig. 2 entsprechende Darstellung der Vorrichtung gemäß Fig. 1 in einer alternativen Betriebsweise;

Fig. 4

eine der Fig. 2 entsprechende Darstellung der Vorrichtung gemäß Fig. 1 in einer weiteren alternativen Betriebsweise;

Fig. 5

die Vorrichtung gemäß Fig. 1 und 2 in einer Ansicht von oben.

The invention will be explained in more detail by means of exemplary embodiments. Each show schematically:

Fig. 1

a device for hot dip coating a steel strip in a side view;

Fig. 2

an enlarged section A of Fig. 1 ;

Fig. 3

one of the Fig. 2 corresponding representation of the device according to Fig. 1 in an alternative mode of operation;

Fig. 4

one of the Fig. 2 corresponding representation of the device according to Fig. 1 in a further alternative mode of operation;

Fig. 5

the device according to Fig. 1 and 2 in a view from above.

A device 1 for hot dip coating a metal strip M, which here for example to cold-rolled steel strip consisting of a corrosion-sensitive steel, comprises a melt bath 3 filled in a vessel 2 into which the metal strip M to be coated, previously brought to a sufficient immersion temperature in a known manner, is passed over a proboscis 4.

In the hot-dip bath 3, the metal strip M is deflected at a deflection roller 5 so that it emerges from the melt bath 3 in a vertically oriented conveying direction F. In this case, the metal strip M emerging from the melt bath 3 passes through a stripping device 7 arranged at a specific distance above the surface 6 of the melt bath 3. This comprises two stripping nozzles 8,9 formed as slit nozzles, one of which comprises a stripping gas stream AG1 on the one surface O1 of the metal strip M extending on one side between the longitudinal edges of the metal strip M, and the other of which directs a Abstreifgasstrom AG2 on the present on the opposite side of the metal strip M surface O2.

Under optimum operating conditions, the metal strip M emerging from the melt bath 3 is aligned such that its center layer ML aligned centrally between the surfaces 01, 02 lies in a vertically oriented plane H.

Between the wiper nozzles 8.9 of the stripping device 7 and the surface 6 of the melt bath 3, a nozzle 10, 11 is arranged on each side of the metal strip M at a distance d of 200 mm, each one extending over the width B of the metal strip M extending gas flow G1, G2.

The nozzles 10, 11 may be designed as conventional slot nozzles. In practice, however, have been tested as nozzles 10,11 air bars, which consisted of a tube with 20 mm inner diameter, were drilled in the spaced at intervals of 25 mm twelve cylindrical nozzle orifices with a diameter of 2 mm. The gas was supplied centrally. When tested in practice embodiment of the air bar used was about 300 mm wide and centered to 1370 mm amounting width B of the metal strip M aligned.

At the in Fig. 2 As shown, the outlet openings of the nozzles 10,11 are aligned so that a larger partial flow G11, G21 of the respective gas flow G1, G2 with its central axis Ga1 each with respect to the perpendicular to the surface of the melt bath 3 injection angle ß of about 30 ° directed to the surface of the melt bath 3 and flows there from the associated surface O1, O2 of the metal strip M in a direction substantially normal to the relevant surface O1, O2 pioneering flow direction. In contrast, a smaller partial flow G12, G22 of the respective gas flow G1, G2 is directed against the associated surface 01, 02 of the metal strip. In this case, the blowing angle β 'of this partial flow G12, G22, which is related to the perpendicular to the melt bath 3, is selected such that the boundary of the impact area X assigned to the metal strip M strikes the surface 6 of the melt bath 3 in the respective gas flow G1, G2 , at a close distance from the Metal band M ends. The provided with the metallic coating surfaces 01,02 of the metal strip M are not affected in this way by the associated gas flow G1, G2.

At the in Fig. 3 As shown, the nozzles 10,11 are set so that they do not deploy in the direction of the metal strip M directed streams G12, G22.

At the in Fig. 4 On the other hand, the nozzles 10, 11 are set in such a manner that they do not dispense any partial flows G11, G21 directed away from the metal strip M.

Regardless of whether the gas streams G1, G2 have been applied partially or completely to the metal strip M or away from it, the gas streams G1, G2 drive the slag S present on the melt bath 3 in a direction oriented transversely to the metal strip M from the metal strip M. away, so that they each collect in an uncritical for the metal strip M, sufficiently spaced area B1, B2 and from there mechanically, that can be removed manually or by means of a suitable, motor-driven device from the surface 6 of the melt bath 3.

For operational experiments, a N ₂ gas flow was blown by means of two arranged in the manner of the nozzles 10,11 air bar between the melt bath and stripping nozzles at a large industrial hot dip coating plant during the fire aluminizing. The coating bath contained 9.5 wt .-% Si, 2.5 wt .-% Fe and balance Al and traces of other elements and unavoidable impurities. The speed of the metal strip emerging from the melt bath was 38 m / min with a coating layer to be applied of min. 75 g / m ² per side of the metal strip M.

Blown-off top slag was manually-mechanically removed from the aluminum bath surface. Over a longer production period, surface defects due to entrained top slag could be effectively reduced or prevented.

Table 1 shows, for a slit nozzle arranged below the wiping nozzles in accordance with the invention, that this result of the result was not given if no gas flow was applied or if the boundary conditions given in accordance with the invention were abandoned.

REFERENCE NUMBERS

1

Device for hot dip coating

2

boiler

3

melt bath

4

trunk

5

idler pulley

6

Surface of the melt bath 3

7

stripping

8.9

wiping

10.11

jet

ß, ß '

supply angle

AG1, AG2

Abstreifgasströme

B

Width of the metal strip M

B1, B2

Portions of the surface 6 of the melt bath 3

d

distance

F

conveying direction

G1, G2

gas flows

G11-G22

Partial streams of the respective gas jet G1, G2

Ga1, Ga2

central axes of the gas flows G1, G2

H

vertically aligned plane of the middle layer ML

M

metal band

ML

center position

O1, O2

Surfaces of the metal strip M

S

slag

X

impingement

Table 1 attempt Gas pressure [bar] Blowing angle [°] Surface defects due to top slag? According to the invention? 1 no gas flow often No 2 0.5 45 often No 3 1.0 45 isolated Yes 4 2.0 45 isolated Yes 5 4.0 45 none Yes 6 6.0 50 isolated Yes 7 8.0 50 isolated Yes 8th 10.0 50 none Yes 9 12.0 50 none Yes 10 14.0 5 none Yes 11 16.0 5 often No 12 15.0 5 none Yes 13 10 direct flow of the metal strip often fluctuating in distribution No 14 10 70 often No 15 10 80 often No

Claims (14)

1. Method for hot-dip coating a metal strip (M) with a metal covering,

   - wherein the metal strip (M) is directed continuously through a melt bath (3),

   - wherein the thickness of the metal covering which is present on the metal strip (M) when it leaves the melt bath (3) is adjusted by means of a scraping device (7), and

   - wherein slag (S) which is present on the melt bath (3) is driven away from the metal strip (M) leaving the melt bath (3) by means of a gas flow (G1, G2),

   characterised in that, in order to drive away the slag (S) from the metal strip (M) by means of at least one nozzle (10, 11) arranged with a spacing (d) of from 50 - 500 mm from the surface (6) of the metal strip (3)associated therewith , a gas flow (G1, G2) which extends over the width (B) of the metal strip (M) is directed onto the surface (6) of the melt bath (3), so that the gas flow (G1, G2) expels the slag (s) present on the melt bath (3) in a direction directed perpendicular to the metal strip (M) away from the metal strip (M).
2. Method according to claim 1, characterised in that the gas flow (G1, G2) is produced in a direction directed transversely relative to the conveying direction (F) of the metal strip (M) and away from the metal strip (M).
3. Method according to claim 2, characterised in that the influx angle (β) at which the gas flow (G1, G2) strikes the surface (6) of the melt bath (3) is in the range from 0 - 60°.
4. Method according to claim 1, characterised in that at least a part-flow (G11-G22) of the gas flow (G1, G2) is directed against the metal strip (M).
5. Method according to claim 4, characterised in that the impact region (X) of the gas flow (G1, G2) on the surface (6) of the melt bath (3) is located in front of the metal strip (M).
6. Method according to any one of the preceding claims, characterised in that the nozzle (10, 11) extends over at least 20% of the width (B) of the metal strip (M).
7. Method according to any one of the preceding claims, characterised in that the respective gas flow (G1, G2) comprises air, a gas which is inert with respect to the melt bath (3) or a gas admixture which is formed by air and a gas which is inert with respect to the melt bath (3) .
8. Method according to any one of the preceding claims, characterised in that the slag (S) which is driven away by the respective gas flow (G1, G2) is removed from the melt bath (3) by means of a mechanically operated device.
9. Device for hot-dip coating a metal strip (M) with a metal covering,

   - having a melt bath (3),

   - having a conveying device for continuously directing the metal strip (M) through the melt bath (3),

   - having a scraping device (7) for adjusting the thickness of the metal covering present on the metal strip (M) when it leaves the melt bath (3),
   and

   - having at least one nozzle (10, 11) for producing a gas flow (G1, G2) which drives away slag (S) present on the melt bath (3) from the metal strip (M) leaving the melt bath (3),

   characterised in that the nozzle (10, 11) for producing the gas flow (G1, G2)is arranged with a spacing (d) of from 50 - 500 mm from the surface (6) of the metal strip (3) associated therewith and produces a gas flow (G1, G2) which extends over the width (B) of the metal strip (M) and which is directed onto the surface (6) of the melt bath (3), so that the gas flow (G1, G2) expels the slag (s) present on the melt bath (3) in a direction directed perpendicular to the metal strip (M) away from the metal strip (M).
10. Device according to claim 9, characterised in that a nozzle (10, 11) for driving away the slag (S) is associated with each surface (O1, O2) of the metal strip (M), respectively.
11. Device according to any one of claims 9 or 10, characterised in that the nozzle (10, 11) is a slot nozzle or a slotted pipe.
12. Device according to any one of claims 9 or 10, characterised in that the nozzle (10, 11) is formed by a nozzle bar in which a plurality of nozzle openings which are arranged with spacing from each other are provided.
13. Device according to claim 11 or 12, characterised in that the nozzle (10, 11) is arranged centrally with respect to the width (B) of the metal strip (M) leaving the melt bath (3).
14. Device according to any one of claims 9 to 13, characterised in that the nozzle extends over at least 20% of the width (b) of the metal strip (M).

Images