EP2870268B2 - Method and device for avoiding surface defects caused by zinc dust in a continuous strip galvanising process - Google Patents

Description

The invention relates to a method for avoiding surface defects on galvanized metal strip caused by zinc dust in continuous strip galvanizing, in which metal strip heated in a continuous furnace is moved under protective gas through a furnace nozzle and immersed in a zinc bath, according to the preamble of claim 1. Furthermore the invention relates to a device for avoiding surface defects on galvanized metal strip caused by zinc dust in continuous strip galvanizing, according to the preamble of claim 7.

A plant for the continuous hot-dip galvanizing of steel strip consists, among other things, of a continuous furnace, a zinc bath (melt bath), a device for adjusting the zinc coating thickness and a subsequent cooling device. The steel strip is continuously annealed in the continuous furnace. The desired mechanical properties of the base material are set by recrystallization of the steel. In addition, iron oxides formed in a preheating zone are reduced. In a cooling zone following the continuous annealing furnace, the strip is cooled under protective gas (HNX) to a temperature close to the melt bath temperature. The protective gas is intended to prevent the annealed strip from oxidizing before galvanizing, which would significantly impair the adhesion of the zinc layer. The protective gas The connecting piece between the annealing furnace and the zinc bath is called the furnace trunk.

In a conventional furnace snout of a continuous strip galvanizing line, deposits of zinc dust usually occur, which fall in larger pieces onto the zinc bath and/or the steel strip, particularly when the system is subjected to vibrations, thus causing surface defects (galvanizing defects). It was recognized that the steel strip moving in the direction of the zinc bath entrains protective gas downwards in the snout, with the entrained protective gas absorbing zinc vapor on the surface of the zinc bath, which condenses or resublimates on the colder inner walls of the snout when the entrained protective gas rises and turns into dust there settles.

From the JP 7157853 (A ) A device for removing zinc vapor in a snout of a continuous strip galvanizing line is known. In order to remove the zinc vapor that forms on the surface of the zinc bath, the furnace nozzle is provided with injection openings (circulation openings) and suction openings arranged vertically underneath. In a first exemplary embodiment, a single injection opening is arranged in the nozzle wall facing the upper side of the steel strip, and a single suction opening is arranged vertically below it. Accordingly, in the nozzle wall facing the underside of the steel strip, there is also a single blow-in opening and a single suction opening vertically underneath. In a second embodiment, a single injection opening is arranged in a side wall of the snout, while two suction openings are provided vertically below them, which are designed as longitudinal slots in tubes, which the Penetrate the side wall of the snout and extend across the entire width of the steel strip on the top and bottom of the steel strip.

With the one from the JP 7157853 (A ) known device, a relatively large amount of zinc vapor or zinc dust must be removed from the suctioned inert gas. Because of the design and arrangement of the injection openings and suction openings, it can be assumed that this known device promotes the absorption of zinc vapor by the protective gas entrained by the steel strip and the propagation of zinc vapor in the furnace nozzle.

The present invention is based on the object of specifying a method and a device of the type mentioned at the outset with which the absorption of zinc vapor by the protective gas contained in the furnace snout and the propagation of zinc vapor in the furnace snout can be significantly minimized.

This object is achieved by a method having the features of claim 1 or by a device having the features of claim 7.

In the method according to the invention, the upper and lower sides of the metal strip to be galvanized (eg steel strip) are also exposed to protective gas via injection openings in the furnace nozzle. Shielding gas loaded with zinc vapor and/or zinc dust is sucked off via suction openings which are arranged on both sides of the metal strip adjacent to the injection openings. According to the invention, a large number of injection openings are formed and arranged in the furnace nozzle in such a way that the air flowing out of these injection openings Shielding gas with an impact angle in the range of 70° to 110°, preferably 80° to 100°, particularly preferably about 90°, is directed onto the surface of the metal strip facing the respective injection opening. In addition, the distance between the respective injection opening of the plurality of injection openings and at least one suction opening assigned to it from the plurality of suction openings is selected and the flow rate of the protective gas exiting from the respective injection opening is controlled in such a way that entrainment occurs when the metal or steel strip moves is counteracted by protective gas in the direction of the zinc bath. According to the invention, the distance between the respective injection opening and the at least one suction opening assigned to it is selected to be less than or equal to 25 cm.

In the device according to the invention, the furnace nozzle is therefore provided with blow-in openings through which protective gas can be applied to the top and bottom of the metal strip, suction openings for sucking off protective gas laden with zinc vapor and/or zinc dust being arranged adjacent to the blow-in openings. According to the invention, a large number of injection openings are designed and arranged in the furnace nozzle in such a way that the protective gas flowing out of these injection openings is sprayed at an angle of incidence in the range from 70° to 110°, preferably 80° to 100°, particularly preferably approx. 90° is directed towards the surface of the metal strip facing the respective blowing-in opening, the distance between the respective blowing-in opening of the plurality of blowing-in openings and at least one suction opening assigned to it from the plurality of suction openings being selected in such a way that at a predetermined or specifiable flow rate of the protective gas emerging from the respective injection opening counteracts any entrainment of protective gas in the direction of the zinc bath that occurs when the metal strip moves. The distance between the respective injection opening and the at least one suction opening assigned to it is less than or equal to 25 cm.

The invention is based on the idea of influencing the flow conditions of the protective gas, particularly near the strip, in such a way that the aforementioned entrainment of protective gas is minimized and/or the condensation or resublimation of zinc vapor on the walls of the nozzle is prevented. In contrast to the one from the JP 7157853 (A ) known device, the aim of the present invention is to prevent the formation of protective gas laden with zinc vapor in advance by minimizing the entrainment of the protective gas in the direction of the zinc bath. For this purpose, the invention proposes an interruption or blocking of the protective gas (current of protective gas) entrained by the metal strip by using a gas lock or gas curtain effect.

An advantageous embodiment of the method according to the invention provides that the inert gas supplied via the injection openings is previously heated to a temperature of at least 500°C, preferably at least 550°C. With this configuration, the re-sublimation of zinc dust in the furnace nozzle can be prevented even more effectively, since the heated protective gas flow supplied via the injection openings keeps the zinc vapor produced on the zinc bath surface in the gaseous state.

Accordingly, a preferred embodiment of the device according to the invention provides that the suction openings are connected to the injection openings via a return line having at least one suction fan, the return line having at least one heating device for heating the protective gas to a temperature of at least 500° C., preferably at least 550° C is provided.

The flow of inert gas that is uniformly introduced into the nozzle over a large area and essentially over the entire width of the nozzle simultaneously represents a heating medium for the blowing/suction device and prevents cold zones in the nozzle that would lead to zinc dust precipitation. Due to the disclosed temperature control in the area of the snout, there is no sublimated zinc dust in the snout. Rather, the zinc vapor contained in the protective gas is removed before it can sublimate into dust grains.

The method according to the invention is preferably carried out in such a way that the temperature of the gas cloud in the spatially higher part of the nozzle is higher than the temperature in the spatially lower immersion region of the strip. This minimizes thermal turbulence in the trunk.

A further advantageous embodiment of the method according to the invention is characterized in that the protective gas is blown in via the injection openings and the protective gas is sucked off via the suction openings in at least three stages, which are arranged one after the other in the direction of strip travel, with each of the stages consisting of a series of at least five, preferably at least seven Injection openings and a row of at least five, preferably at least seven suction openings is formed. In this way, a particularly effective blocking of the protective gas entrained by the strip to be galvanized can be achieved. In particular, due to the relatively high number of injection openings and suction openings, a rather gentle, low-turbulence blowing flow of protective gas can be generated, so that excessive, uncontrollable turbulence of the protective gas and increased strip vibrations are avoided. This multi-stage arrangement of the injection openings and suction openings allows the concentration of the zinc vapor in the protective gas and thus the partial pressure of the zinc vapor to be gradually reduced to an uncritical level.

For this purpose, a preferred embodiment of the device according to the invention provides that the injection openings and the suction openings are formed in at least three stages, which are arranged one after the other in the direction of strip travel, with each of the stages consisting of a row of at least five, preferably at least seven, injection openings and one Row of at least five, preferably at least seven suction openings is formed.

A further advantageous embodiment of the method according to the invention is characterized in that the protective gas volume flow supplied via the injection openings is set equal to the protective gas volume flow sucked off via the suction openings or is set to a value which is at most 5% below the sucked protective gas volume flow. Due to the same or almost the same volume flows of supplied and extracted inert gas and the mentioned preferred uniform distribution of injection points and The gas turbulence in the nozzle is reduced to a minimum.

In order to block or interrupt the protective gas flow entrained by the moving metal strip as effectively as possible while at the same time minimizing the turbulence of the protective gas, it is advantageous if, according to a further preferred embodiment of the device according to the invention, the injection openings and the suction openings are arranged in a matrix. In this context, it is also favorable if the injection openings are offset relative to the suction openings—viewed in the direction of strip travel and over the strip width. The injection openings and the suction openings of the device according to the invention are preferably arranged at equal distances from one another.

The distance between the respective injection opening (injection nozzle) and the at least one suction opening assigned to it is preferably less than 15 cm, and particularly preferably less than or equal to 10 cm.

In order to achieve a low-turbulence interruption of the protective gas flow entrained by the moving metal strip or to achieve the most uniform possible distribution of the injection points and suction points, a further preferred embodiment of the device according to the invention provides that the injection openings are on prong-like branches of a comb-shaped blowpipe structure and the suction openings on prong-like branches of a Comb-shaped suction tube structure are formed, the prong-like branches of the comb-shaped blower tube structure and the prong-like branches of the comb-shaped suction tube structure intermesh.

If the flow of protective gas is heated by means of a gas heater before it is blown in, preferably to a temperature in the range from 450 to 600°C, the above-mentioned configuration also has the effect that a very uniform surface temperature distribution is established on the pipeline system composed of the comb-shaped pipe structures during operation , wherein the surface temperature of the pipe system arranged in the nozzle when the protective gas flow is heated to a temperature in the range from 450 to 600° C. above the dew point or resublimation temperature of zinc. In particular, the heating of the piping system with heated protective gas prevents the occurrence of temperature peaks and thus unwanted gas convection or gas turbulence.

In this context, a further advantageous embodiment of the device according to the invention provides that the comb-shaped blowing tube structure and the comb-shaped suction tube structure are thermally insulated from the furnace nozzle by thermal insulation.

According to a further preferred embodiment of the method according to the invention, the furnace nozzle is heated to a temperature of at least 400° C., preferably at least 450° C., at least in a region which extends from the zinc bath to the injection openings and/or suction openings. In addition to a heating device provided for this purpose, or as an alternative to this, this lower region of the furnace nozzle can also be provided with thermal insulation according to a preferred embodiment of the device according to the invention. This makes it possible to achieve that the relevant walls or wall sections of the furnace nozzle are warmer than the temperature at which the condensation or resublimation of zinc vapor begins.

Further preferred and advantageous developments of the invention are indicated in the appended claims.

Fig. 1

eine Längsschnittansicht eines Abschnitts eines erfindungsgemäß ausgeführten Ofen-Rüssels einer kontinuierlichen Bandverzinkung;

Fig. 2

eine Querschnittansicht des Ofen-Rüssels entlang der Schnittlinie II-II in Fig. 1;

Fig. 3

eine in einem Ofen-Rüssel gemäß Fig. 1 angeordnete Blas-Saugvorrichtung in Draufsicht mit zugeordneter Rückführleitung, die mit einem Absaugventilator, einer Zinkabscheidevorrichtung und einer Heizeinrichtung zum Erwärmen des von Zink gereinigten, einzublasenden Schutzgases versehen ist;

Fig. 4

eine weitere Längsschnittansicht eines Abschnitts eines erfindungsgemäß ausgeführten Ofen-Rüssels einer kontinuierlichen Bandverzinkung;

Fig. 5

eine Draufsicht auf einen Längsabschnitt des zu verzinkenden Metallbandes in einem Abschnitt des Ofen-Rüssels der Fig. 4; und

Fig. 6

den Abschnitt des Ofen-Rüssels gemäß Fig. 4 in einer perspektivischen Darstellung.

The invention is explained in more detail below with reference to a drawing showing several exemplary embodiments. They show schematically:

1

a longitudinal sectional view of a portion of a furnace snout designed according to the invention of a continuous strip galvanizing;

2

a cross-sectional view of the furnace snout along section line II-II in 1 ;

3

one in a furnace proboscis according to 1 arranged blow-suction device in top view with associated return line, which is provided with an exhaust fan, a zinc separator and a heating device for heating the protective gas to be blown in, cleaned of zinc;

4

a further longitudinal sectional view of a section of a furnace snout of a continuous strip galvanizing designed according to the invention;

figure 5

a top view of a longitudinal section of the metal strip to be galvanized in a section of the furnace snout of FIG 4 ; and

6

according to the section of the furnace trunk 4 in a perspective view.

In the drawing, a furnace trunk 1 of a continuous strip galvanizing (hot-dip galvanizing) is outlined. A metal strip 2 to be galvanized, preferably steel strip, is annealed in a continuous furnace (not shown) and fed to a zinc bath 3 under protective gas (HNX). The strip 2 dips diagonally downwards into the zinc bath 3 and is deflected upwards by a roller 4 arranged in the zinc bath. The bath temperature is typically in the range of about 440 to 470°C. On exiting the bath 3, the strip 2' entrains a quantity of liquid zinc well in excess of the desired coating thickness. The excess coating material that is still liquid is scraped off the top and bottom (front and back) of the coated strip 2' by means of flat air jet nozzles 5 extending across the strip width.

In the furnace nozzle 1, part of the protective gas is entrained in the direction of the zinc bath 3 by the movement of the strip. In order to prevent the inert gas that is entrained from absorbing zinc vapor on the zinc bath surface, which deposits as zinc dust on the colder inner wall surfaces of the nozzle 1 and can cause surface defects on the galvanized strip 2' if it is in larger pieces on the strip 2 and/or zinc bath 3 falls, the trunk 1 is provided with a special blowing-suction device 6.

The blowing suction device 6 according to the invention has a branched line system 7.1, 7.2 with a large number of Injection and suction openings 7.11, 7.21, by means of which protective gas is circulated in the end area of the nozzle 1, ie near the zinc bath 3, in such a way that the protective gas flow entrained by the strip 2 is interrupted as far as possible, but without causing increased strip vibrations. For this purpose, the blowing-in and suction openings 7.11, 7.21 are arranged in the direction of movement of the belt 2 in such a way that each blowing-in opening 7.11 is close to at least one suction opening 7.21, whereby the protective gas that has been blown in is sucked off again in the immediate vicinity, thus preventing uncontrollable turbulence of the protective gas .

The blow-suction device 6 comprises an upper part 6.1 and a lower part 6.2, with the upper part 6.1 extending across the entire width of the upper side of the belt (front side), while the lower part 6.2 extends across the entire width of the lower side of the belt (back side). The upper part 6.1 and the lower part 6.2 can each have a box-like design and are accordingly referred to as a blower/suction box or blower/suction box. The respective blowing and suction box (6.1, 6.2) is divided by partitions 7.3 into a branched blowing chamber 7.1' with blowing branches 7.10 running parallel to one another and a branched suction chamber 7.2' with suction branches 7.20 running parallel to one another. An injection branch 7.10 can be located directly next to a suction branch 7.20, in that both branches 7.10, 7.20 are separated from one another by the same partition wall 7.3. The subdivision into a branched blowing chamber 7.1' and a branched suction chamber 7.2' can be realized, for example, by a meandering or folded partition 7.3 or by meandering partitions that are joined together in a gas-tight manner at their abutting ends be as in figure 5 is sketched. Connecting pieces 7.41, 7.51 for connecting at least one return line 8, which is connected to a suction blower, suction fan 9 or the like and defines or enables a gas circuit (cf. 3 ).

The connection piece 7.51 for suction of the protective gas is arranged below the connection piece 7.41, through which the protective gas is supplied (see also 6 ). This ensures that the flow of the inert gas that is blown in is always or essentially only directed downwards, as a result of which a flow of zinc vapor from the zinc bath into the nozzle 1 is effectively prevented.

As in the figures 5 and 6 shown, preferably at least two connection pieces 7.41 for blowing in protective gas open into the upper main chamber section 7.4 of the respective blower/suction box 6.1 or 6.2, while the lower main chamber section 7.5 of the blower/suction box 6.1 or 6.2 preferably has at least two connection pieces 7.51 for sucking off inert gas loaded with zinc vapor is provided. The connecting pieces 7.41 of the upper main chamber section 7.4 are arranged at a distance from one another transversely to the direction of travel of the belt. Likewise, the connecting pieces 7.51 of the lower main chamber section 7.5 are spaced apart from one another transversely to the direction of travel of the belt.

The injection and suction branches 7.10, 7.20 are provided with a large number of openings (nozzles) 7.11, 7.21, which serve as injection openings and suction openings, respectively. These openings (nozzles) 7.11, 7.21 are arranged or designed in such a way that the protective gas flowing out of the blow-in openings 7.11 is directed or hits the surface of the strip 2 facing the blow-in opening at an angle of incidence in the range of 70° to 110°, preferably 80° to 100°. The injection nozzles 7.11 are preferably designed in such a way that the protective gas flowing out of them is directed essentially at right angles to the surface of the strip (cf. 2 and 4 ). The distance between the respective injection nozzle 7.11 and at least one suction opening 7.21 assigned to it is selected in such a way that at a predetermined or specifiable flow rate of the injected protective gas, the entrainment of protective gas in the direction of the zinc bath 3 that occurs when the strip 2 is moving is effectively interrupted or at least minimized .

The entrainment of protective gas caused by the strip movement contributes to a "natural gas movement". The natural gas movement is also driven by the usually existing temperature difference between the relatively hot protective gas entrained by the strip 2 above the zinc bath 3 and the colder protective gas in the upper area of the nozzle 1. The interruption or blocking of this natural gas movement according to the invention also means that it is carried along or the transport of zinc vapor from the zinc bath surface 3.1 to the upper nozzle area is interrupted or at least minimized.

In order to achieve a blocking effect that is as uniform as possible for the gas movement in the direction of belt travel and for the upward gas movement along the inside of the nozzle walls, without this resulting in increased belt vibrations, at least five, preferably at least seven, particularly preferably at least ten injection openings (nozzles) 7.11 arranged distributed over the width of the strip 2.

At least one suction opening 7.21 is located in the immediate vicinity of each injection opening 7.11. The injection openings 7.11 and the suction openings 7.21 are arranged in a matrix. The blowing in and sucking off thus takes place in several stages, preferably in at least three stages. The injection openings 7.11 are arranged offset to the suction openings 7.21 in the direction of strip travel and across the strip width (cf. figure 5 ). The injection openings 7.11 and the suction openings 7.21 are preferably arranged at an equal distance from one another.

A large amount of protective gas can be exchanged via the gas injection channels 7.10 without a large amount of gas being transported in the direction of strip travel. Advantageously, the band 2 is not excited to vibrate as a result. At the same time, the undesired transport of zinc vapor from the immersion region of the strip 2 into the upper part of the snout 1 is not supported by the gas flow.

The alternating arrangement of injection nozzles 7.11 and suction nozzles 7.21 ( 3 ) the nozzle cross-section can be completely flowed through in the transverse direction. Shielding gas not yet loaded with zinc dust mixes with shielding gas loaded with zinc dust and is extracted in close proximity.

As in 3 is outlined, the blowing-suction device 6 or the blowing-suction box 6.1, 6.2 can also be designed in such a way that the injection openings 7.11 on prong-like branches 7.10 of a comb-shaped blowpipe structure 7.1 and the Suction openings 7.21 are formed on prong-like branches 7.20 of a comb-shaped suction tube structure 7.2, the prong-like branches 7.10 of the comb-shaped blower tube structure 7.1 and the prong-like branches 7.20 of the comb-shaped suction tube structure 7.2 interlocking. This configuration enables the distance between the injection openings 7.11 and the suction openings 7.21 to be adjusted by shifting the comb-shaped blowing tube structure 7.1 relative to the comb-shaped suction tube structure 7.2.

In addition to the suction blower or suction fan 9, a zinc separation device 10 for cleaning the protective gas loaded with zinc vapor and/or zinc dust is integrated in the return line 8. The zinc separating device 10 is preferably provided with a cooling device which effects a re-sublimation of zinc vapor. The resulting zinc dust can be separated from the protective gas by means of a separating device and fed into a collection container 10.1.

The gradual blowing in of cleaned or unloaded inert gas and the extraction of inert gas laden with zinc vapor and/or zinc dust in the immediate vicinity of the injection points reduces the concentration of zinc vapor and/or zinc dust in the inert gas in nozzle 1 and thus the partial pressure of the zinc vapor gradually down to an uncritical level. The gradual reduction of the content of zinc vapor and zinc dust in the protective gas laden with it is in 4 schematically sketched, with the serpentine arrows Z for zinc vapor, the straight arrows G indicate the flow direction of the protective gas in the trunk 1 and in the blowing suction device (blasting suction box) and the "point clouds" D represent zinc dust. It is to see that the content of zinc vapor and zinc dust gradually decreases from the zinc bath surface 3.1 in the direction of the annealing furnace.

Before being blown in, the cleaned inert gas stream is heated by means of a gas heater 11, for example to a temperature in the range from 450 to 600.degree. The snout 1 with the blowing and suction device or the blowing and suction boxes 6.1, 6.2 is heated by this gas flow in such a way that at no point in the snout 1 does the dew point or resublimation temperature of zinc vapor fall below.

The gas injection channels 7.10 run along the longitudinal axis of the strip or longitudinal axis of the nozzle and parallel to the suction lines 7.20 arranged between them. In combination with the suction lines 7.20, the gas injection channels 7.10 cover a longitudinal section of the strip 2 completely or essentially completely both on the underside of the strip and on the upper side of the strip. This causes a uniform surface temperature of the blower/suction device or blower/suction boxes 6.1, 6.2, the surface temperature being above the dew point or resublimation temperature of zinc vapor.

The device 6 according to the invention is designed as a push-pull system. Hot protective gas is blown into the nozzle 1 at a slight overpressure via the injection openings 7.11 in order to generate cross-flows at the injection openings 7.11 (outlet points). The flow of inert gas blown in is adjusted to be equal to or slightly below the amount of gas flow extracted by means of a measuring and control device. For example, per hinge side (blow-suction box 6.1 or 6.2) the inert gas flow blown in is about 150 Nm ³ /h at approx. 600°C, while the inert gas flow extracted per strip side including zinc vapor is approx. 200 Nm ³ /h.

In order to minimize heat losses, the main blowing chamber (main blowing line) 7.1 and the injection branches (gas injection channels) 7.10 and preferably also the main suction chamber 7.2 and the suction branches (suction lines) 7.20 are thermally insulated from the trunk construction by a heat insulating layer. The snout 1 is also provided with an outer thermal insulation 12 in order to keep the inside of the snout walls at a temperature greater than 300°C.

The lowermost part of the snout 1, i.e. the snout end piece 1.1 located between the blowing suction device and the zinc bath 3, is preferably provided with thermal insulation 13. The thermal insulation 13 ensures that the walls or wall sections of the trunk provided with it are warmer than the dew point or resublimation temperature of the protective gas-zinc vapor mixture during operation of the galvanizing plant. The thermal insulation 13 is formed, for example, from mineral wool and/or ceramic plates and surrounds the trunk end piece 1.1, preferably in the form of a jacket.

Furthermore, a further embodiment of the invention provides that the snout end piece 1.1 is provided with a heating device (not shown) in addition to or as an alternative to the heat insulation 13.

The furnace nozzle 1 designed according to the invention can be divided into three areas A, B and C with regard to the protective gas (cf. 1 ).

The area A includes the end piece 1.1, which is preferably provided with thermal insulation 13. In this area A there is a relatively high zinc vapor load with little gas movement. The surface temperature of the trunk 1 is above 440° C. in this area.

Area A is followed by area B, which is equipped with the blowing and suction device according to the invention (e.g. in the form of blowing and suction boxes 6.1, 6.2). Area B serves as a separation lock or gas curtain. It interrupts the "natural gas flow", in particular the entrainment of protective gas caused by the strip movement in the direction of the zinc bath 3, by blowing in cleaned hot protective gas while at the same time sucking off gas laden with zinc vapor in close proximity to the injection points 7.11. Due to the multi-stage arrangement of the injection nozzles 7.11 and suction nozzles 7.21, the concentration of zinc vapor is gradually reduced in area B. The surface temperatures of the blow and suction boxes 6.1, 6.2 and the insides of the trunk 1 are above the dew point or resublimation temperature of zinc vapor, i.e. above 400°C.

Area C follows above area B. Area C is characterized by a low zinc vapor content in the protective gas. The surface temperature of the inside of the nozzle is more than 300°C in area C, which prevents condensation or resublimation of the zinc vapor that is still present in the protective gas to a small extent.

The implementation of the invention is not limited to the exemplary embodiments described above. Rather, numerous variants are possible, which also make use of the invention specified in the attached patent claims in the case of designs that deviate from the exemplary embodiments illustrated in the drawing. For example, the mutually parallel injection branches 7.10 and suction branches 7.20 of the blowing suction box 6.1, 6.2 or the "tines" of the comb-shaped blowing tube structure 7.1 and the comb-shaped suction tube structure 7.2 can also be aligned transversely to the direction of belt travel. Which of these variants is implemented depends on the course of the main lines for the protective gas supply and suction in relation to the orientation of the trunk 1 and the relevant mounting options.

Claims (16)

1. Method for avoiding surface defects, which are caused by zinc dust, on a galvanized metal strip in continuous strip galvanization, in which metal strip (2) heated in a continuous annealing furnace is moved through a snout (1) in protective furnace gas and is immersed into a zinc bath (3), in which, in the snout (1), the upper side and the lower side of the metal strip (2) are acted upon by protective furnace gas via injection openings (7.11), and in which protective furnace gas loaded with zinc vapour and/or zinc dust is extracted via extraction openings (7.21) which are arranged on both sides of the metal strip (2) adjacent to the injection openings (7.11), characterized in that a multiplicity of the injection openings (7.11) are configured and arranged in the snout (1) in such a manner that the protective furnace gas streaming out of said injection openings (7.11) is directed onto that surface of the metal strip (2) which faces the respective injection opening (7.11) with an angle of impact within the range of 70° to 110°, preferably 80° to 100°, wherein the distance between the respective injection opening (7.11) of the multiplicity of the injection openings (7.11) and at least one extraction opening (7.21) assigned thereto from the multiplicity of the extraction openings (7.21) is selected in such a manner, and the flow velocity of the protective furnace gas emerging from the respective injection opening (7.11) is controlled in such a manner, that an entraining of protective furnace gas, which occurs during movement of the metal strip (2), in the direction of the zinc bath (3) is opposed, the distance between the respective injection opening (7.11) and the at least one extraction opening (7.21) assigned thereto is selected smaller than or equal 25 cm.
2. Method according to Claim 1, characterized in that the protective furnace gas supplied via the injection openings (7.11) is heated beforehand to a temperature of at least 500°C, preferably at least 550°C.
3. Method according to Claim 1 or 2, characterized in that the injection of protective furnace gas via the injection openings (7.11) and the extraction of protective furnace gas via the extraction openings (7.21) is carried out in at least three stages which are arranged consecutively in the strip running direction, wherein each of the stages is formed from a series of at least five, preferably at least seven, injection openings (7.11) and a series of at least five, preferably at least seven, extraction openings (7.21) .
4. Method according to one of Claims 1 to 3, characterized in that the snout (1) is heated to a temperature of at least 400°C at least in a region which extends from the zinc bath (3) as far as the injection openings (7.11) and/or extraction openings (7.21) .
5. Method according to one of Claims 1 to 4, characterized in that the volumetric flow of protective furnace gas supplied via the injection openings (7.11) is adjusted to be identical to the volumetric flow of protective furnace gas extracted via the extraction openings (7.21), or is adjusted to a value which lies at maximum 5% below the extracted volumetric flow of protective furnace gas.
6. Method according to one of Claims 1 to 5, characterized in that the extracted protective furnace gas loaded with zinc vapour and/or zinc dust is cleaned by means of a zinc separating apparatus (10) .
7. Apparatus for avoiding surface defects, which are caused by zinc dust, on galvanized metal strip in continuous strip galvanization, in which metal strip (2) which is to be galvanized and is heated in a continuous annealing furnace is moved through a snout (1) in protective furnace gas and is immersed into a zinc bath (3), wherein the furnace pipe (1) is provided with injection openings (7.11) via which the upper side and the lower side of the metal strip (2) can be acted upon by protective furnace gas, and wherein extraction openings (7.21) for extracting protective furnace gas loaded with zinc vapour and/or zinc dust are arranged adjacent to the injection openings (7.11), characterized in that a multiplicity of the injection openings (7.11) are configured and arranged in the snout (1) in such a manner that the protective furnace gas streaming out of said injection openings (7.11) is directed onto that surface of the metal strip (2) which faces the respective injection opening (7.11) with an angle of impact within the range of 70° to 110°, preferably 80° to 100°, wherein the distance between the respective injection opening (7.11) of the multiplicity of the injection openings (7.11) and at least one extraction opening (7.21) assigned thereto from the multiplicity of the extraction openings (7.21) is selected in such a manner that, at a predetermined or predeterminable flow velocity of the protective furnace gas emerging from the respective injection opening (7.11), an entraining of protective furnace gas, which occurs during movement of the metal strip (2), in the direction of the zinc bath (3) is opposed, the distance between the respective injection opening (7.11) and the at least one extraction opening (7.21) assigned thereto is selected smaller than or equal 25 cm.
8. Apparatus according to Claim 7, characterized in that the extraction openings (7.21) are connected to the injection openings (7.11) via a return line (8) having at least one extraction ventilator (9), wherein the return line (8) is provided with at least one heating device (11) for heating the protective furnace gas to a temperature of at least 500°C, preferably at least 550°C.
9. Apparatus according to Claim 8, characterized in that the return line (8) is provided with a zinc separating apparatus (10).
10. Apparatus according to Claims 7 to 9, characterized in that the injection openings (7.11) for injecting protective furnace gas and the extraction openings (7.21) for extracting protective furnace gas are configured in at least three stages which are arranged consecutively in the strip running direction, wherein each of the stages is formed from a series of at least five, preferably at least seven, injection openings (7.11) and a series of at least five, preferably at least seven, extraction openings (7.21) .
11. Apparatus according to one of Claims 7 to 10, characterized in that the injection openings (7.11) and the extraction openings (7.21) are arranged in the form of a matrix.
12. Apparatus according to one of Claims 7 to 11, characterized in that the injection openings (7.11) are arranged offset with respect to the extraction openings (7.21), as viewed in the strip running direction and over the strip width.
13. Apparatus according to one of Claims 7 to 12, characterized in that the injection openings (7.11) and the extraction openings (7.21) are arranged uniformly spaced apart from one another.
14. Apparatus according to one of Claims 7 to 13, characterized in that the injection openings (7.21) are formed on teeth-like branches (7.10) of a comb-shaped blow pipe structure (7.1) and the extraction openings (7.21) are formed on teeth-like branches (7.20) of a comb-shaped suction pipe structure (7.2), wherein the teeth-like branches (7.10) of the comb-shaped blow pipe structure (7.1) and the teeth-like branches (7.20) of the comb-shaped suction pipe structure (7.2) intermesh.
15. Apparatus according to Claim 14, characterized in that the comb-shaped blow pipe structure (7.1) and the comb-shaped suction pipe structure (7.2) are thermally insulated in relation to the furnace pipe (1) by heat insulation.
16. Apparatus according to one of Claims 7 to 15, characterized in that the snout (1) is provided with heat insulation (13) and/or a heating device at least in a region (1.1, A) which extends from the zinc bath (3) as far as the injection openings (7.11) and/or extraction openings (7.21).

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