Device and method for humidity-controlled blow-off after the application of a layer to a flat steel product

Abstract

The invention relates to a device (150) and method for applying ZnAlMg layers or ZnAl layers to the front (V) and rear (R) side of a flat steel product (100), comprising the following steps: moving the flat steel product (100) from an input side to an output side (A) of a zinc-alloy melt pool; providing a dry gas flow (TG); providing a water vapour gas (WG); combining the dry gas flow (TG) and the water vapour gas (WG) in order to obtain a stripping gas (AG) mixture; determining the gas humidity of the stripping gas (AG); discharging the stripping gas (AG) through at least one gas nozzle (15) that functions to blow off the front side (V), and through at least one gas nozzle (15) that functions to blow off the rear side (R), in order to blow off the front side (V) and rear side (R) of the flat steel product (100) using the stripping gas (AG); wherein the one stripping gas flow (AG) is discharged which complies with at least one of the following two conditions (B1, B2): B1: the stripping gas (AG) has a moisture content, relative to a proportion of the gaseous water vapour gas (WG), of greater than 200ppm and less than 43700ppm, wherein the moisture content is preferably in the region of 500ppm and 9980ppm; B2: the stripping gas (AG) has a dew point (TP) of greater than - 390°C and lower than + 30°C, wherein the dew point (TP) is preferably in the region of - 290°C and + 70°C.

Description

Device and method for moisture-controlled blow-off after application of a layer to a flat steel product

[0001] The present invention relates to a device for coating flat steel products in a molten bath with a zinc (Zn) or zinc-aluminum-magnesium (ZnAlMg)-based layer, e.g., as a protective coating, and for blowing the layer off in a controlled manner on the exit side of the molten bath. It also relates to a corresponding method.

[0002] It is well known that flat steel products 100, such as steel strips or steel sheets, are coated with a zinc (Zn) or ZnAlMg alloy to improve their corrosion resistance. In practice, this is usually done by introducing the flat steel product 100 from a furnace into a zinc alloy molten bath 11, as indicated in Fig. 1 using an exemplary device 150. To protect the flat steel product 100 from oxidation, it is typically introduced into the bath 11 on the inlet side E through a nozzle 12 with an inert atmosphere. In the bath 11, the flat steel product 100 is deflected by one or more (zinc bath) rollers 13 and moved upwards out of the bath 11 on the outlet side A.When emerging from this bath 11, the alloy melt film adhering to the front and back of the steel flat product 100 is sprayed with a gas jet from the gas nozzles 15 of a wiping nozzle device to the target thickness (in the micrometer range) or to the target surface area (in g / m. 2 ) stripped (also called blow-off) and the steel flat product 100 is then transferred to a cooling area 16. This continuous process is generally called hot-dip coating.

[0003] Details of a suitable process and particularly suitable alloy compositions can be found, for example, in the published application WO 2014 / 033153 Al of the applicant VOESTALPINE STAHL GMBH.

[0004] The steel flat product 100 can then be roll-treated by subjecting it to a skin-pass process step and / or a bending-stretching process. Furthermore, the steel flat product 100 can be chemically post-treated. Steel flat products 100 (e.g., in the form of steel strips) for the automotive industry can be further processed, for example, by oiling. Oiling can be performed, for example, using an oiling machine.

[0005] It has been shown that a freshly hot-dip coated flat steel product 100 can exhibit surface defects or imperfections depending on the alloy composition and the specific process. On the one hand, zinc vapor can form above the dip bath 11 in a zinc melt bath, which can have a negative impact on the surface of the hot-dip coated flat steel product 100. On the other hand, the blow-off process on the exit side A of the melt bath 11 also influences the surface quality.

[0006] Patent EP0172682B1 from Armco Inc., filed in 1985, concerns the reduction or avoidance of oxygen in the vicinity of the air knife and in the area of ​​the steel strip emerging from the zinc bath, as well as the control of zinc vapor in connection with the hot-dip coating of an iron-based metal strip. On the exit side of the dip bath, an oxygen-reduced atmosphere is provided in an enclosed area, which also contains the air knife, instead of the normally present ambient air. Since zinc would evaporate rapidly in this atmosphere, a small amount of water vapor must be added to the oxygen-reduced atmosphere. The enclosed area is located directly above the molten bath surface and This creates a hermetically sealed chamber. The oxygen-reduced atmosphere in this chamber is intended to improve the stripping process, and the small water content is intended to prevent the formation of zinc vapor on the surface of the immersion bath. The moisture content in the hermetically sealed chamber is adjusted so that zinc vapor cannot form.

[0007] Published application DE2033847 discloses a device designed to regulate the coating thickness during the galvanizing of steel strip. A wide slotted nozzle is used to blow air, gas, or steam against the zinc-coated strip for the purpose of stripping. The nozzle slot is larger near the strip edges than near the center to account for the fact that the strip may have a curved shape on the exit side of the bath.

[0008] A device that uses wet steam in the form of a steam jet for stripping has been known for over 50 years, as described in the published application DE1521405 A1 by National Steel. The use of wet steam results in rapid cooling of the steel strip after hot-dip coating, with the blown wet steam causing the coating to solidify within a short period of time. It is proposed to use a condenser device designed to supply wet steam at the desired temperature and pressure.

[0009] The published patent application JP2020100886 A2 from Nippon aims to produce a galvanized steel strip with a surface with an increased coefficient of friction. To increase the coefficient of friction, water is sprayed onto the surface of the flat steel product under pressure after the gas has been blown off. The particle size of the water droplets should be at least 0.07 mm and preferably more than 1.5 mm. By spraying water droplets, irregularities are deliberately created on the surface of the steel strip. This document pursues a different objective, and the corresponding technical teaching thus goes in a completely different direction than the present invention.

[0010] In addition to preventing surface defects and imperfections and simply providing protection against corrosion, there are increasingly stringent requirements regarding the surface quality of Zn-coated flat steel products in general, and ZnAlMg-coated flat steel products in particular. The automotive industry, in particular, expects products that meet the highest surface requirements. However, providing homogeneous surfaces is not trivial.

[0011] The main problems here are often surface defects in the ZnAlMg layer. For example, "marbling," "toothpick," or "beach pattern" defects can develop on the ZnAlMg layer, or slag formation can occur. Patents exist (e.g., EP20130826634). AM / JMMataigne; JP20080256208 NSSMC / Oohashi et al.), which attempt to eliminate similar surface defects (gloss effects or displaced oxide skins) by other means (reduction of the Oz content in the vicinity of the wiping nozzle) than the present invention.

[0012] Similar surface defects may also occur in Zn layers that contain an Al content (the Al content may, for example, be less than 1 wt.%).

[0013] The task now arises of providing a device and a process for coating flat steel products with a ZnAlMg or ZnAl coating that offers particularly durable and robust corrosion protection. The surface of this coating should be particularly homogeneous and free of marbling (without "marbling effect") and / or toothpick defects (without "toothpick"). The goal is a surface quality that meets the highest customer requirements.

[0014] In addition, the device and method should consume as little energy as possible, be cost-effective to operate and robust in use. Summary of the invention

[0015] According to the invention, a corresponding device is provided which uses a continuous (hot-dip) process and which allows a flat steel product to be provided with a metallic ZnAlMg layer or a ZnAl layer, which can serve, for example, as a (protective) coating. This layer is intended to protect both sides of the steel substrate of the flat steel product from external influences. In the following, the corresponding immersion bath is referred to as a zinc alloy melt bath (or zinc melt bath for short), whereby the term zinc alloy melt bath is intended to include both a melt bath that contains predominantly zinc (Zn) and a small admixture of aluminum (Al) (e.g. less than 1 wt.%), and a melt bath that contains a ZnAlMg alloy. The layer to be applied to both sides is also referred to here as a Zn-containing (protective) layer.

[0016] A device for applying a ZnAlMg layer or a ZnAl layer to a flat steel product is proposed. In all embodiments, this device comprises: - a zinc alloy melt bath with an inlet side and an outlet side, - a gas supply designed to provide dry gas, - a steam device designed to provide gaseous steam, - means (e.g. a device) for determining gas humidity (or moisture content), - a stripping nozzle device which is fluidly connected to the gas supply and to the steam device in order to supply the stripping nozzle device with a stripping gas as a mixture of the dry gas and the gaseous steam, wherein the stripping nozzle device comprises at least one gas nozzle for blowing off the front side and at least one gas nozzle for blowing off the back side of the flat steel product with the stripping gas, o the gas nozzles are arranged in the region of the outlet side of the zinc alloy melt bath, o the means for determining gas moisture (or moisture content) are arranged in or on the stripping nozzle device in order to determine the moisture content of the stripping gas before or upon exit of the stripping gas towards the front or back of the flat steel product (e.g. by measuring), and o wherein at least one of the following two conditions B1, B2 is met: B1: the stripping gas (AG) has a moisture content or a proportion of gaseous water vapor (WG) that is greater than 200 ppm and less than 43,700 ppm, wherein the moisture content is preferably in the range from 500 ppm to 9,980 ppm, B2: the stripping gas (AG) has a dew point (TP) greater than - 39°C and less than + 30°C, with the dew point (TP) preferably being in the range between - 29°C and + 7°C.

[0017] The moisture content of the stripping gas can be defined for at least some embodiments of the device and the corresponding method by the volume fraction (referred to here as condition B1), which lies in the range between 200 ppm and 43,700 ppm. Preferably, the volume fraction lies in the range between 500 ppm and 9,980 ppm in at least some embodiments.

[0018] The moisture content of the stripping gas can be defined for at least some embodiments of the device and the corresponding method by a dew point (referred to here as condition B2) that is greater than -39°C and less than +30°C. Preferably, the dew point of the stripping gas lies in the parameter range of -29°C to +7°C in at least some embodiments.

[0019] It should be noted that conditions B1 and B2 define ranges that largely overlap. Only the boundary values ​​may deviate due to rounding.

[0020] Alternatively, the moisture content of the stripping gas can be defined for at least some of the embodiments of the device and the corresponding method by using a mixture of dry gas and gaseous water vapor that is controlled and modified such that it is always unsaturated (whereby condition B1 and / or B2 is / are also met in this alternative approach). The unsaturated stripping gas always contains water only in the vapor phase. In other words, a stripping gas is used whose proportion (moisture content) of gaseous water vapor is kept so low that the stripping gas is unsaturated with respect to the water vapor proportion. This means that the instantaneous dew point of the stripping gas is always lower than the instantaneous temperature of the stripping gas. This statement regarding the unsaturated state also applies to changing gas pressure and / or changing temperature of the stripping gas.

[0021] The unsaturated stripping gas can also be defined as unsaturated for all embodiments as long as it contains only superheated steam. In the unsaturated state, the stripping gas is a homogeneous, single-phase mixture containing only a gaseous phase (no solid or liquid). In the unsaturated state, the stripping gas has a relative humidity of less than 100%.

[0022] For the moisture content of the stripping gas, the dew point upper limit of +30°C - which corresponds to a volume fraction of 43700 ppm water in the stripping gas - is defined in order to avoid the condensation of water when the stripping gas emitted by the stripping nozzle mixes with the ambient gas (or the ambient air) in the area around the stripping nozzle.

[0023] The corresponding process and device are based on controlling the moisture content required at the point of impact of the stripping jet on the flat steel product in such a way that marbling and / or toothpick defects are avoided. This means that a sufficiently high moisture content must always be present in the stripping gas to enable stripping without the formation of marbling and / or toothpick defects. At the same time, however, the formation of condensate must be avoided, as described above. These two boundary or framework conditions result in a parameter window that is preferably observed in all embodiments in addition to conditions B1 and / or B2.

[0024] Preferably, the moisture content, i.e., the water vapor content, is controlled in the device and the corresponding method by monitoring the instantaneous dew point of the stripping gas and maintaining it within a suitably specified parameter window (condition B2). This prevents the formation of condensate and allows stripping to be carried out without the aforementioned errors. Condensing water would negatively impact the stripping process and the surface quality of the galvanized strip.

[0025] In all embodiments, a control or monitoring unit of the device can implement a humidity adaptation protocol in order to be able to react appropriately to changes in the current moisture content and to ensure compliance with conditions B1 and / or B2.

[0026] In all versions, the required moisture content is provided directly via the stripping gas. This means that the stripping gas essentially serves as a carrier or transport medium for the very small amount of water vapor required here.

[0027] In all embodiments - preferably in a gas supply line to the wiping nozzle - a water vapor gas stream is introduced into the dry wiping gas, here called dry gas stream, in order to mix the dry gas and the water vapor gas stream.

[0028] In all embodiments, the dry gas stream preferably comprises nitrogen or consists of nitrogen. In all embodiments, the dry gas stream may also contain another inert gas instead of nitrogen.

[0029] The moisture content of the stripping gas can be measured in all versions, e.g. with a humidity sensor (e.g. a thermal or capacitive dew point Sensor), which is arranged between the feed point for the gaseous water vapor and the nozzle opening of the wiping nozzle, can be measured and, in other possible embodiments, also controlled.

[0030] It is an advantage of this method, or rather the corresponding device, that it can effectively prevent marbling and / or toothpick defects without the need to install or mount additional, disruptive devices (e.g. a housing or enclosure according to the present applicant's European patent application EP22182309.9 (V08-0015P-EP / P219205 / VA23004) directly above the zinc bath area, or in the immediate vicinity of the wiping nozzles and in the area above the zinc bath). Access to the wiping nozzle and the zinc bath surface for periodic cleaning work, which is necessary for correct process control, is still ensured.

[0031] Furthermore, the process is very efficient in terms of media consumption, compared to a process that is the subject of the aforementioned European patent application EP 22182309.9, since only a fraction of the amount of water vapor is used to avoid marbling and / or toothpick defects.

[0032] All embodiments involve the application of a Zn-containing (protective) layer to a flat steel product, with the layer thickness being intended to correspond to a target thickness (according to a corresponding specification). This layer is created by passing the flat steel product through a molten zinc alloy bath and, on the exit side of the bath, blowing off the stripping gas in a controlled manner using a stripping nozzle device comprising at least one gas nozzle per side of the flat steel product.

[0033] The zinc alloy of the zinc alloy melt bath may preferably have the following composition in all embodiments, but is not limited to these compositions: - an aluminium content in the range between 1.0 and 3.0% by weight and preferably in the range between 1.3 and 2.8% by weight, - a magnesium content in the range between 1.0 and 2.5% by weight and preferably in the range between 1.2 and 2.2% by weight, and - the remainder of the zinc alloy melt pool is zinc and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, Zr, Sr, La, Ce or Bi, the weight content of each of these additional elements in the metallic coating being less than 0.1%, and unavoidable impurities.

[0034] The zinc alloy of the zinc alloy melt bath can preferably have the following composition in all embodiments but is not limited to these compositions: - an aluminium content of less than 1.0% by weight and preferably in the range between 0.1 and 0.5% by weight, and - the rest of the zinc alloy melt pool is zinc and unavoidable impurities.

[0035] Preferably, the following definitions apply to the plant parameters and / or process parameters in all embodiments: - the thickness of the nozzle lip gap (called the height of the nozzle opening) of the two nozzles is in a range between 0.5 and 5 mm, preferably between 0.6 and 2 mm, particularly preferably between 0.8 and 1.5 mm and / or - the effective flow rate of the stripping gas across the bandwidth in the range from 200 to 8000 Nm 3 per hour, and / or - the distance between the die lip gap and the front or back of the flat steel product is in a range between 2 and 15 mm, preferably between 3 and 12 mm, and / or - the belt speed is in a range between 50 and 200 m / min, preferably between 70 and 150 m / min.

[0036] It should be noted that under certain environmental conditions, marbling and / or toothpick defects may not occur. This may be the case, for example, if the ambient air is sufficiently humid (e.g., in the case of high humidity in summer). The ambient air is sucked in by the stripping gas that exits the nozzles and is swirled with the stripping gas. However, the occurrence of such surface defects also depends on numerous other parameters (e.g., the bath temperature). At low bath temperatures, the tendency for surface defects to form can increase even when the ambient air humidity is high. If - according to the invention - a suitable moisture content is ensured in the stripping gas itself, then the hot-dip coating and blow-off processes are largely independent of the currently prevailing and uncontrollable ambient conditions. This means that the hot-dip coating and blow-off processes become more robust against external influences.

[0037] All embodiments may comprise one or more of the following sensor configurations: at least one sensor for determining the ambient humidity of the air within the vicinity of the device, and / or at least one sensor for determining the ambient humidity of the air in the vicinity of the device (e.g. in the factory hall).

[0038] Furthermore, all embodiments comprise at least one means (e.g., implemented as a hardware device) for determining gas humidity or the moisture content in or at the stripping nozzle device in order to determine (e.g., measure) the gas humidity before or upon exit of the stripping gas (toward the front or back of the flat steel product).

[0039] Preferably, this means for determining gas humidity or moisture content, or a sensor of this means, is located in a gas supply line at a location that lies in the flow direction at a point after the merging / mixing of the dry gas stream and the water vapor gas stream.

[0040] Alternatively, this means for determining gas humidity or moisture content, or a sensor of this device, can be located in or on the gas nozzle.

[0041] In all embodiments, this means for determining gas humidity or moisture content, or a sensor of this device, can be located in a gas supply line and in the gas nozzle.

[0042] The method is characterized in all or at least some of the embodiments in that the ZnAlMg layer or the ZnAl layer is applied to both sides of a flat steel product according to a target specification by moving the flat steel product through a molten zinc alloy bath (ZnAl; ZnAlMg) and on the outlet side of which stripping gas exits through a nozzle lip gap of at least one gas nozzle in the direction of the front side and through a nozzle lip gap of at least one gas nozzle in the direction of the back side of the flat steel product in order to blow off the layers on both sides according to the target specification.

[0043] The process for applying ZnAlMg or ZnAl layers to the front and back of a flat steel product comprises the following steps: - Moving the steel flat product from an inlet side to an outlet side of a zinc alloy melt bath, - Providing a dry gas stream, - Providing a steam gas stream, - Combining the dry gas stream and the water vapor gas stream to obtain a stripping gas as a mixture, - Determination of the gas humidity or the moisture content of the stripping gas, - Emitting the stripping gas through at least one gas nozzle serving to blow off the front side and through at least one gas nozzle serving to blow off the back side, in order to blow off the front and back side of the flat steel product with the stripping gas, o wherein at least one of the following two conditions B1, B2 is met: B1 : the stripping gas (AG) has a moisture content or a proportion of gaseous water vapor (WG) which is greater than 200 ppm and is less than 43700 ppm, with the moisture content preferably being in the range 500 ppm to 9980 ppm, B2: the stripping gas (AG) has a dew point (TP) greater than - 39°C and less than + 30°C, with the dew point (TP) preferably being in the range between - 29°C and + 7°C.

[0044] In order to prevent marbling and / or the formation of toothpick defects in the ZnAlMg layer or ZnAl layer to be produced, the ambient humidity in the region of the device can optionally also be determined in at least some of the embodiments. Since the device, or rather the method, draws in ambient air during blow-off (as already mentioned), more precise adjustments to the gas humidity (moisture content) of the stripping gas can be made taking into account the currently prevailing ambient humidity. If the currently prevailing ambient humidity is particularly low, for example, the gas humidity of the stripping gas is usually very important to reliably prevent surface defects. Under "humid" ambient conditions, it is not always absolutely necessary to add water vapor to the stripping gas to reliably prevent the formation of marbling and / or toothpick defects.

[0045] In at least some embodiments, the gas humidity (moisture content) of the stripping gas is adjusted / adjusted through the use of a steam device by providing a correspondingly large flow rate of the steam gas stream to the actual flow rate of the provided dry gas stream and combining / mixing it with the dry gas stream. This means that in these embodiments, the flow rate of the steam gas stream is actively adjusted to the actual flow rate of the provided dry gas stream (referred to as control or regulation of the steam gas stream source).

[0046] In all embodiments, one can assume, as a first approximation, that the water vapor gas flow is negligible in relation to the dry gas flow. Therefore, the dry gas flow is practically non-existent. changes when water vapor is added. Adjusting the dry gas flow is therefore not mandatory. However, the dry gas flow can be reduced in all designs if the water vapor flow is increased (and vice versa).

[0047] In another embodiment, the gas humidity (moisture content) of the stripping gas is adjusted by adjusting both the flow rate of the dry gas stream and the flow rate of the water vapor gas stream. This can be done, for example, by using controllable gas valves in a dry gas supply and a water vapor gas supply. Or the discharge rate of the dry gas stream source and the water vapor gas stream source is controlled or regulated.

[0048] In some embodiments, the gas humidity (moisture content) of the stripping gas can be adjusted / adjusted by a mixing valve in the area where the two gas streams merge that adjusts one or both flow rates.

[0049] The near area of ​​the device is defined in at least some of the embodiments as a volume in a range of 1 m 3 up to 10 m 3 defined.

[0050] The environment of the device is defined in at least some of the embodiments as a volume that is larger than 10 m 3 .

[0051] In all embodiments, the device or the wiping nozzle system can comprise an automatic coating control system designed to automatically adjust the flow rate of the (wiping) gas to keep the target thickness of the layers to be applied substantially constant. The automatic coating control system is preferably designed to be capable of compensating for fluctuations in one or more system and process parameters.

[0052] In all embodiments or at least some of the embodiments, the unavoidable impurities of the alloys are in a range that is significantly smaller than 1 weight percent (wt%), Preferably, the sum of all unavoidable impurities is less than 0.5% by weight.

[0053] By combining a precisely defined bath composition with monitoring and / or adjusting the gas humidity (moisture content) of the stripping gas, a surface can be produced that exhibits no or negligible mottled appearance and no or negligible toothpick defects. During the production of the respective layers, the gas humidity (moisture content) of the stripping gas can be kept essentially constant or adjusted (e.g., if the humidity in the vicinity or surrounding the device changes) to obtain consistent layers (that are within the specified specifications).

[0054] In all embodiments, the air knife device can optionally be followed by a belt stabilizing device, which serves to automatically stabilize the movement of the flat steel product.

[0055] In all embodiments, the device is preferably operated in the following range(s): - alloy melt bath with a bath temperature TB in the range 400 < TB < 480 degrees Celsius, preferably in the range 409 < TB < 472 degrees Celsius, and particularly preferably in the range 410 < TB < 460 degrees Celsius, - Nozzle distance from the flat steel product, which is between 2 and 15 mm, preferably between 3 and 12 mm, - Blowing off the flat steel product on the outlet side of the alloy melt bath with the (stripping) gas, which is released by the The gas flows through the die lip gap towards the flat steel product with a gas flow rate that is in the range of 200 to 8000 Nm per meter of strip width 3 per hour.

[0056] The processes in the air knife device and on the flat steel product are complex and depend on numerous (process and system) parameters and influencing factors. Therefore, a control or Control of the device is based on some simplified assumptions and definitions.

[0057] The preferred specifications for the ZnAl and ZnAlMg alloy concepts defined above and the preferred specification of a moisture content of the stripping gas, which is defined, for example, by a dew point in the range between -39°C and +30°C, or by specifying ppm (e.g., between 200 ppm and 43,700 ppm), are derived from numerous studies. Within the specified limits of the alloy concepts defined here as examples and the moisture content according to conditions B1 and / or B2, the technical teaching presented here has proven particularly successful.

[0058] In addition to the means and methods described here for controlling and regulating the moisture content of the water vapor in the stripping gas, a cooling surface or cooling area can optionally be provided on or in the stripping nozzle device to provide a controlled area for condensation of excess water vapor, should it occur despite all measures. An outlet can also be provided in this area to allow condensate to be drained from time to time. The cooling surface or cooling area should always be cooler than the current dew point temperature of the stripping gas.

[0059] A device for condensing excess water vapor can also be used in the supply line for the water vapor gas.

[0060] In all embodiments, the flat steel product can be subjected to an annealing or tempering step at a temperature of approximately 765 °C (or at a lower or higher temperature) in a zinc alloy melt bath before hot-dip coating.

[0061] In all embodiments, the flat steel product can be cold rolled after hot-dip coating (for example, using smooth cold rolls and / or using skin-pass rolls with a special roughness).

[0062] In addition, the steel strip can be subjected to a bending-stretching process in-line in addition to the skin-passing process or alone in order to increase the flatness of the steel strip.

[0063] In all embodiments, the flat steel product in strip form, for example, as deep-drawing steel, mild steel, structural steel, or a higher-strength steel grade, can be cleaned of rolling oil and mill dust, rinsed with water, and dried in a so-called pretreatment or pre-cleaning process in the continuous hot-dip galvanizing plant using a combined dipping / brushing / electrolytic cleaning process. The cleaned and dried flat steel product in strip form then enters the annealing furnace of a continuous hot-dip coating line, where it is preheated, heated, and brought to the annealing temperature under protective gas. At the end of the annealing furnace, the flat steel product in strip form is cooled to a strip immersion temperature and dipped into the ZnMgAl alloy molten bath.After leaving the bath, the flat steel product in strip form is adjusted to the desired layer thickness using the stripping gas at the stripping nozzles according to the embodiments described and claimed here. The zinc alloy melt is solidified on the steel strip in a subsequent cooling tower.

[0064] Following the cooling tower, the flat steel product in strip form can be re-rolled inline (in the continuous hot-dip galvanizing plant) in a skin-pass mill and a specified roughness can be imprinted.

[0065] Following an in-line inspection for surface defects, which identifies surface defects such as marbling or toothpick defects, the flat steel product in strip form can be coated with corrosion protection and forming oil in an oiling machine and finally wound up on the coiler. The steel product in the form of a coiled steel strip can be coated with a lacquer in a coil coating line after winding on the coiler. Alternatively, the flat steel product in strip form, after bending, stretching, leveling and / or skin-passivating, is chemically treated with an organic / inorganic passivation layer using a coater. (coating device) and then dried. The steel flat product 100 is then inspected for surface defects and, if no marbling or toothpick defects are detected, is wound onto the reel.

[0066] All embodiments may include a PC or other computer to automatically control or regulate the moisture content of the stripping gas and / or to manually operate the stripping gas within a dew point window.

[0067] Further advantageous embodiments of the invention form the subject matter of the dependent claims. DRAWINGS

[0068] Embodiments of the invention are described in more detail below with reference to the drawings. FIG. 1 shows a highly schematic representation of a known device for dip coating and stripping flat steel products (prior art); FIG. 2 shows a highly schematic representation of a first exemplary device in which the method of the invention is used, wherein the wiping nozzle device here comprises only one nozzle for blowing off the front side of the flat steel product; FIG. 3 shows a highly schematic representation of a second exemplary device in which the method of the invention is used, wherein the wiping nozzle device here has a nozzle for blowing off the front side and for blowing off the back side of the flat steel product; FIG. 4 shows a highly schematic representation of two gas nozzles facing each other; FIG. 5 contains a table with numerous examples. Detailed description

[0069] The moisture content of gases can be described in different ways. Commonly used are the dew point in °C and the mass fraction of water per volume of gas in g / m 3(also called absolute humidity) and the volume fraction in ppm (parts per million parts, also ppm V). The dew point describes the temperature at which water vapor in a gas (here in the stripping gas AG) begins to condense. When the dew point temperature is reached, the gas can no longer absorb any additional water vapor, i.e. the gas is saturated with water vapor. The term frost point can be used for temperatures below 0 °C. However, we use the term dew point throughout here (even at negative temperatures). Pure nitrogen gas, which is used as stripping gas AG for venting, typically has a temperature of 10 to 30 °C. However, the nitrogen gas can also be heated before venting (e.g. to temperatures in the range 50 to over 200 °C). Warm stripping gas AG can absorb more water vapor, so the dew point can be higher. In cold stripping gas AG the situation is reversed.In addition, stripping gas AG can absorb less water vapor at high pressure than at lower gas pressures. It should also be noted that water vapor condensation usually occurs in the lines and components (e.g., in the nozzles 15) of the gas-carrying system.

[0070] Within the scope of the present invention, the moisture content of the stripping gas AG is preferably defined by setting a minimum dew point TPmin. The minimum dew point TPmin is -39°C for all embodiments that rely on adding a small amount of water vapor WG to the stripping gas AG, preferably the minimum dew point is TPmin = -29°C. In all embodiments, a maximum dew point TPmax can also be set, which is +30°C (which corresponds to approximately 43,700 ppm H2O in the stripping gas AG). To avoid the occurrence of surface defects, the moisture content of the stripping gas AG is set in all these embodiments so that the dew point TP of the stripping gas AG is always greater than TPmin = - 39°C or preferably TPmin = - 29°CDh, the formula TP > TPm applies in. In these embodiments, this requirement makes us independent of the ambient conditions (in the factory hall in which the device 150 is operated, the temperature and humidity of the ambient air can fluctuate considerably depending on the climate and the season).

[0071] To prevent condensation of water in the stripping nozzle device 14 in all embodiments, a temperature difference AT can be specified in all embodiments according to the formula TP = TAG - AT. The temperature difference AT can preferably be at least 5°C and particularly preferably at least 10°C. In this formula, TP defines the instantaneous dew point in the stripping gas stream AG, and TAG defines the instantaneous temperature of the stripping gas stream AG.

[0072] By specifying a temperature difference ΔT, for example, the fact that the tendency for condensation of the water vapor component increases when the pressure of the stripping gas AG increases can be taken into account. By specifying a temperature difference ΔT, which can be understood as a kind of safety margin, it can be ensured that condensation does not occur even with a slight increase in the pressure of the stripping gas AG. The safety margin also prevents condensation in the stripping gas AG due to fluctuations in the control of the added water vapor gas.

[0073] In addition, a maximum dew point of the stripping gas AG is specified for all versions, which at TP max = + 30°C (corresponding to approximately 43,700 ppm H2O in the stripping gas AG). The maximum dew point is preferably TPmax = + 7°C (sample number 74 in Table 1). For example, if the temperature TAG of the stripping gas is 18°C, the dew point TP of the stripping gas AG should be set to values ​​in the range -39°C < TP < 18°C ​​or, preferably, to values ​​in the range -29°C < TP < + 7°C.

[0074] If the optional safety margin AT of 10°C described above is also applied, then, for example, the dew point TP of the stripping gas AG at a gas temperature TAG of 18°C ​​is in the range - 39°C < TP < DAY - AT (with or preferably to values ​​in the range - 29°C < TP < + 7°C.

[0075] This relates, among other things, to a device 150 (see, for example, Fig. 2) which is designed to apply a layer 10 (see Fig. 4) to the front side V and back side R of a strip-shaped flat steel product 100. The components of the dipping bath 11 are not shown here (reference is made to Fig. 1, which shows exemplary components). The outlet side of the dipping bath 11 is identified in Fig. 2 by the letter A. The flat steel product 100 is moved vertically upwards in the direction of the arrow B. The wiping nozzle device 14 comprises at least one gas nozzle 15 for blowing off the front side V and at least one gas nozzle 15 for blowing off the back side R of the flat steel product 100. In Fig. 2, the wiping nozzle device 14 only comprises one gas nozzle 15 for blowing off the front side V for the sake of simplicity of illustration. The gas nozzle 15 for blowing off the back side R is designed accordingly.In the embodiment shown in Figure 3, two wiping nozzle devices 14 are provided, which comprise nozzles 15 located opposite one another.

[0076] The layers 10 on both sides V, R are created by guiding the flat steel product 100 from an inlet side E to an outlet side A through a molten zinc bath 11 (see, for example, Fig. 1) and blowing it off on the outlet side A using the stripping nozzle device 14 with (stripping) gas AG. The purpose of the stripping nozzle device 14 is to strip off the excess (still liquid) ZnMgAl layers or ZnAl layers (layers 10) on the flat steel product 100 in a controlled manner after it exits the bath 11 using the (stripping) gas AG.

[0077] It is important to ensure that the layers 10 are produced according to the (predetermined) specification (the specification defines, for example, the target thickness or the number of layers per side V, R), and that no marbling and / or toothpick defects occur. In some embodiments, it may be necessary, for example, to avoid these "defects" in the case of changing environmental conditions in the production area (e.g., in the factory hall). Even if the ambient humidity JG should change in the wider environment of the device 150 (e.g., in the factory hall) or in the immediate vicinity, the device 150 and the method according to the invention can ensure that no marbling and / or toothpick defects occur and that the layers 10 can continue to be produced according to specification.

[0078] In some embodiments, it may also be a matter, for example, of using the method and the device 150 in such a way that water vapor gas WG is only added to the dry gas TG if the ambient air humidity ZUG is too low.

[0079] The target surface coating (coating per belt side) can be in the range of 20 to 200 g / m 2 and particularly preferably in the range of 30 to 160 g / m 2 lay.

[0080] In all embodiments, the wiping nozzle device 14 comprises at least one gas nozzle 15 per side V, R (e.g. two gas nozzles 15 which are opposite each other, as indicated in Figs. 3 and 4).

[0081] The flow rate of the (wiping) gas AG, which is released through the nozzle lip gap 17 in the direction of the front V or rear R, is given here in Nm 3 specified (Nm 3stands for standard cubic meter. One standard cubic meter is the amount of stripping gas AG contained in a volume of one cubic meter. This applies at a temperature of 0 degrees Celsius and a pressure of 1.01325 bar.

[0082] The term dry gas TG refers to an inert gas with a dew point of approximately -70°C and below. This corresponds to a water vapor content of approximately 5 ppm and below. Thus, the dry gas TG has a very low residual moisture content (also called trace moisture) in a range that is typical for industrial gases. The dry gas TG used here can, in all versions, meet the requirements of the "Specification for Industrial Nitrogen", British Standard BS 4366: 1993. According to this standard, the water content of the gaseous Nitrogen dry gas TG (cf. paragraph 8) with a maximum of 10 / 10 6which corresponds to 1 ppm. Thus, in all embodiments, the dry gas TG should have a residual moisture content that is less than 1 ppm and preferably less than 5 ppm.

[0083] In all embodiments, the device 150 includes a dry gas supply or dry gas source 18 (see Fig. 2). For example, a gas tank, a gas cylinder, or a gas line (e.g., coming directly from the gas supplier or from an air separation device operating according to the Linde process, for example) can serve as source 18.

[0084] In all embodiments, the device 150 can also comprise, for example, two dry gas supplies or dry gas sources 18 (see Fig. 3), wherein each of the sources 18 is assigned to a respective wiping nozzle device 14. In all embodiments, the device 150 can also comprise, for example, one dry gas supply or dry gas source 18 that feeds both wiping nozzle devices 14.

[0085] In all embodiments, the device 150 comprises at least one water vapor device or source 50 (see Fig. 2). For example, an evaporator or a gas humidifier, for example by means of ultrasonic atomization, can serve as source 50. In all embodiments, this water vapor device or source 50 is fluidly connected to the gas nozzle 15, as indicated in Fig. 2. In the embodiment of Fig. 2, the source 50 is connected to a dry gas supply (line) 21 via a water vapor gas supply (line) 22. The two gas supply lines 21, 22 are fluidly connected to one another in a T-shaped region 19. The dry gas flow is represented by an arrow TG and the water vapor gas flow by an arrow WG.

[0086] Preferably, in all embodiments, one or more steam generators are used as water vapor device(s) or source(s) 50, which is / are designed as a pure steam generator that generates gaseous water vapor WG or a water vapor gas stream WG from purified or highly purified water.

[0087] In all embodiments, the steam device 50 can preferably comprise a pure steam generator and, for example, a valve that can regulate the steam gas flow WG through line 22. For this purpose, the pure steam generator and / or the valve can be connected in all embodiments to a controller of the device 150 and / or to the device for determining the gas humidity 20 or 26 (not shown). In all embodiments, the steam gas flow WG can also originate from a condensate recovery system.

[0088] In all embodiments, the device 150 can also comprise, for example, two water vapor devices or sources 50 (see Fig. 3), wherein each of the sources 50 is assigned to a respective wiping nozzle device 14.

[0089] In region 19 (see Figs. 2 and 3), the gas streams TG and WG are combined and mixed. The resulting gas mixture is referred to here as stripping gas AG. The stripping gas AG flows from region 19 through the gas nozzle 14 toward the front side V or rear side R of the flat steel product 100. The stripping gas AG, which exits through a (gas) nozzle lip gap 17 of the respective gas nozzle 14, is symbolized in Figs. 2 and 3 by three parallel arrows.

[0090] In the area 19 (see Fig. 2 and 3) in all embodiments, for example, a mixing chamber can be provided to mix the gases TG, WG.

[0091] To prevent the formation of marbling and / or toothpick defects or to significantly reduce marbling and toothpick defects, in all embodiments, the air humidity fuc in the vicinity and / or surroundings of the device 150 can be determined continuously or from time to time (e.g., by direct or indirect measurement) in order to be able to adjust the moisture content of the stripping gas AG accordingly when the ambient conditions change. However, this adjustment to the ambient conditions is optional.

[0092] Optionally, a water vapor gas stream WG can be added to the stripping gas AG in all embodiments only if the Environmental conditions themselves should not be sufficient (e.g. if the ambient air is too dry) to avoid these errors.

[0093] In all embodiments, the device 150 can comprise at least one device 20 designed and arranged accordingly to determine the moisture content in the wiping nozzle device 14. The device 20 is designed to determine the current moisture content of the wiping gas AG (e.g., in the form of signals or measured values ​​containing information on the current dew point TP and / or the moisture content in ppm or as absolute or relative humidity). In the embodiment shown in Fig. 2, the device 20 comprises two sensors 23, 24, both of which protrude into a gas line 25. In the embodiment shown in Fig. 3, the device 20 comprises a combined or integrated sensor 23 / 24 that protrudes into the respective gas line 25.

[0094] In all embodiments, the moisture content of the stripping gas AG can be determined additionally or alternatively before or upon exit of the stripping gas AG in the direction of the front V and / or rear R of the flat steel product 100.

[0095] Sensor 23 can be, for example, a humidity sensor, and sensor 24 a temperature sensor. Both sensors 23 and 24 are connected to a module 26 of device 20 via lines KV3 and KV4.

[0096] Combined or integrated sensors that measure the moisture content and temperature TAG of the AG can also be used in all embodiments. Fig. 3 shows an embodiment in which one combined or integrated sensor 23 / 24 is provided for each wiping nozzle device 14. The corresponding communication line is designated KV5.

[0097] In all versions, sensors can also be used to measure, for example, the pressure dew point as well as the absolute humidity content f and temperature TAG of the gas AG.

[0098] Digital and / or analog sensors can be used in all versions.

[0099] In all embodiments, a dew point measuring device can also serve as device 20.

[0100] Suitable humidity sensors include sensors based on the principle of absorption of electromagnetic waves (microwave absorption sensors) or sensors that detect a change in the dielectric constant (capacitive sensors). One example is a polymer sensor designed for measuring humidity in gases in the temperature range of interest here.

[0101] Sensors of the following design or functionality can be used as humidity sensors in all versions: - mechanically operating measuring sensors based on the moisture-induced expansion or contraction of (mostly organic) measuring elements; - psychrometric measuring sensors, using two identical, very accurate thermometers, along which the gas flow to be measured is guided at a defined speed; - capacitive measuring sensors, which include, for example, a humidity-sensitive capacitor with two flat electrodes; - Resistive measuring method, in which, for example, the impedance of the alternating current resistance of a hygroscopic element is determined; - Spectrometric measuring methods that measure the gaseous water content non-contact, for example in the near or mid-infrared range (NIR or MIR).

[0102] Instead of determining and controlling the water vapor content in the stripping gas AG using the dew point TP, the moisture content of the stripping gas AG can also be determined and processed in all versions by measuring the volume fraction in ppm (also ppm V). For this purpose, a measuring cell with a humidity sensor (e.g., a sensor that adsorbs the moisture in the gas AG and then electrolytically decomposes it) can be used as part of the device 20. It should be noted that the relationship between the dew point temperature in °C and the volume fraction in ppm is not linear, but exponential (see also equation (1)).

[0103] The following estimates / approximations are used to describe the relationship between relative humidity (r) and gases, as shown in Table 1. Relative humidity (r) can be used to express the moisture content of gases relative to the maximum possible saturation of the gas. r = 100% humidity means that no more water vapor can be absorbed by the gas. At r = 100%, the gas is saturated with water vapor. If the temperature of the gas is increased, the gas can absorb a larger amount of water, and the saturation vapor pressure of the gas increases. If the amount of water in the gas remains constant with an increase in temperature, the relative humidity (r) decreases. The absolute humidity in g / m 3) of gases and thus also the absolute humidity fuc of the ambient air used here are also temperature-dependent. The dew point in °C and the volume fraction of H2O in ppm, however, are temperature-independent and apply regardless of the gas type. Therefore, the dew point in °C and the volume fraction of H2O in ppm are preferred to indicate the moisture content of the stripping gas AG.

[0104] The examples shown in Table 1 (see Fig. 5) apply specifically to the alloy compositions and process conditions specified. Table 1 (see Fig. 5) contains the following columns from left to right: Sample number; dew point TP of the stripping gas AG in °C (with the examples in Table 1 sorted from lowest to highest dew point); volume fraction of water vapor H2O in the stripping gas AG in ppm; coating per side (10 layers per side V, R) in g / m 2; Strip speed of the steel flat product 100 (parallel to the movement arrow B) in m / min; the thickness of the die lip gap 17 in mm (called the height of the die opening); the horizontal distance between the die and strip (steel flat product 100) in mm; the die height, the vertical distance of the die to the zinc bath surface in mm; the die pressure in mbar; the bath temperature of the bath 11 in °C; the bath composition by specifying the proportions of Al and Mg in wt.%; the absolute ambient humidity fuG in g / m 3 The two columns on the far right contain information on marbling, where - a black dot symbolises an alloy / process example in which strong marbling was observed, - a grey dot symbolises an alloy / process example in which light to medium marbling was observed, and - a white dot symbolises an alloy / process example in which no marbling was observed.

[0105] Table 1 (see Fig. 5) shows that the use of dry nitrogen as a stripping gas in many alloy / process examples leads to the occurrence of sometimes severe mottled defects (specifically in the examples with sample numbers 1 to 43). Looking at the second and third columns (from the left) for the examples with sample numbers 1 to 43, the stripping gas AG is a dry nitrogen gas with a dew point TP between -77°C and -72°C and a moisture content between 1.8 ppm and 3.8 ppm. Such a nitrogen gas complies with the specifications of the aforementioned British Standard, as the residual moisture content is less than 1 ppm. Using such a dry nitrogen gas as a stripping gas AG, mottled defects cannot be reliably and reproducibly avoided, as can be seen from the two columns on the far right.

[0106] Only for samples with sample numbers 53 to 74 does strong marbling no longer occur. This significant reduction in strong marbling is achieved by adding a small amount of water vapor gas WG to the dry nitrogen gas TG, at a minimum of 208 ppm (sample number 53) and up to 9978 ppm (sample number 74). The dew point of the stripping gas AG for sample numbers 53 to 74 is between -39°C (sample number 53) and +7°C (sample number 74).

[0107] From this, a condition Bl can be derived as follows: The stripping gas AG should always have a moisture content, or a proportion of gaseous water vapor WG, that is greater than 200 ppm and less than 43700 ppm The lower limit of 200 ppm is derived from the test results shown in Table 1 (208 ppm rounded to 200 ppm), while the upper limit of 43,700 ppm ensures that no water condensation occurs in the vicinity of the wiping nozzle, as previously mentioned.

[0108] From this, a condition B2 can also be derived, as follows: The stripping gas AG should always have a dew point TP that is greater than -39°C and less than +30°C. The lower limit of -39°C is derived from the test results presented in Table 1, while the upper limit of +30°C ensures that no condensation of water occurs in the vicinity of the stripping nozzle.

[0109] Only for samples with sample numbers 58 to 74 does no marbling occur at all (with the exception of sample number 59). This significant reduction in marbling is achieved by adding a small amount of water vapor gas WG to the dry nitrogen gas TG, with a concentration of at least 552 ppm (sample number 58) and up to 9978 ppm (sample number 74). The dew point of the stripping gas AG for sample numbers 58 to 74 is between -29°C (sample number 58) and +7°C (sample number 74).

[0110] From this, a further preferred condition Bl can be derived, as follows: The stripping gas AG should always have a moisture content, or a proportion of gaseous water vapor WG, that lies in the range 500 ppm to 9980 ppm, whereby these ppm values ​​have been rounded up or down.

[0111] From this, the preferred condition B2 can be derived as follows: The stripping gas AG should always have a dew point TP that lies between - 29°C and + 7°C.

[0112] These specifications with regard to conditions Bl and B2 refer to nitrogen as the dry gas TG and to temperatures TAG of the stripping gas AG, which are in the range between 10°C and 30°C.

[0113] For the conditions specified under B2, the respective dew point TP of the stripping gas AG is below the current temperature TAG of the Stripping gas AG. This condition (TP < TAG) is referred to here as Condition B2.1. Applying Condition B2.1 has the advantage that it is independent of the temperature TAG of the stripping gas AG. For example, if the stripping gas AG has a temperature TAG of 27°C, then to fulfill Condition B2.1, the dew point TP of the stripping gas AG must be below +27°C.

[0114] The preferred condition B2.1 can also be defined as follows: between the temperature TAG of the stripping gas AG and the dew point TP of the stripping gas AG, there should always be an optional safety margin AT, as follows: TP < TAG - AT. This optional safety margin AT can be AT = 10°C in all embodiments. To fulfill this preferred condition B2.1, the dew point TP of the stripping gas AG should be below +17°C if the temperature TAG of the stripping gas AG is, for example, 27°C.

[0115] This condition B2.1 also applies when the gas pressure and / or temperature TAG of the stripping gas AG changes.

[0116] The additional condition B3 can also be defined, which specifies that the stripping gas AG is a gas that is in an unsaturated state.

[0117] This condition B3 also applies when the gas pressure and / or temperature TAG of the stripping gas AG changes.

[0118] If conditions B1 and / or B2 and / or B2.1 are observed with regard to the stripping gas AG, then you are on the safe side when it comes to marbling. Condition B3 is considered an additional condition that can be observed for all embodiments in addition to conditions B1 and / or B2 and / or B2.1.

[0119] By adhering to these conditions, the hot-dip coating and blow-off processes are stabilized. This means that these processes become more robust against disruptive (some of which cannot be influenced) Environmental conditions. Furthermore, the parameter window within which the method operates reliably is expanded.

[0120] For example, test series and the evaluation of process data have shown that the humidity in the area surrounding or in the immediate vicinity of the wiping nozzle has a significant influence on the occurrence of marbling defects. JG is used here as the symbol for the absolute humidity of the area surrounding or in the immediate vicinity of the wiping nozzle (also called ambient humidity). For all designs, the absolute humidity JG can be estimated from the air temperature TL of the area surrounding or in the immediate vicinity, as well as the relative humidity r of the area surrounding or in the immediate vicinity. The following formula applies: r relative humidity or air humidity in % fuG Humidity of the environment or the immediate area TL Air temperature of the surrounding or nearby area in °C.

[0121] As already described, the determination / measurement / monitoring of the ambient or nearby air humidity fuc and / or the air temperature TL can be performed directly or indirectly in all embodiments. Indirect measurement is understood here to include, among other things, measuring the air temperature TL and the relative air humidity r and calculating / deriving the absolute local air humidity fuc from them.

[0122] The current flow rate of the stripping gas AG can be adjusted automatically in a known manner in all embodiments (for example, by means of control technology through an automatic layer control of the device 150) in order to keep the target thickness or the layer per side of the layers 10 to be applied essentially constant if one or more of the system parameters and / or process parameters change. The supplied amount of water vapor gas stream WG must then be adjusted accordingly to ensure that the stripping gas AG complies with the conditions B1, B2, B2.1, B3 of the invention with regard to moisture content.

[0123] Fig. 4 also shows the nozzle spacing (defined parallel to the y-axis) between the nozzles 15 and the respective strip side (front side V, back side R) of the flat steel product 100, as well as the thickness (defined parallel to the x-axis) of the nozzle lip gap 17 (called the height of the nozzle opening). The nozzle lip gap 17 serves as the gas outlet gap of the wiping nozzle device 14. In Fig. 4, the thickness of the flat steel product 100 and the two layers 10 are exaggerated in order to schematically demonstrate in the spatial region X that the thickness of the layers 10 is reduced by blowing off with the wiping gas AG.

[0124] The gas jet emerging from the nozzle 14, together with gravity (if the flat steel product 100 is pulled vertically upwards from the bath 11, as shown, for example, in Figs. 2 and 3), exerts a shearing force on the still liquid layer 10. Due to the shearing force, the thickness of the layers 10 is reduced by blowing off with the stripping gas AG.

[0125] The equations describing the dynamic flow behavior of the gas AG on the flat steel product 100 are very complex. This is due, among other things, to the fact that areas of laminar and turbulent flow develop in the gas jet exiting through the nozzle lip gap 17 of the nozzle 15 at layer 10 of the flat steel product 100. Furthermore, the gas jet draws in ambient air, which is swirled with the stripping gas AG (therefore, adding water vapor gas WG to the dry gas TG may not be necessary under certain circumstances, especially at high ambient humidity fuc). Details can be found, for example, in the publications “Wall Pressure and Shear Stress Measurements Beneath an Impinging Jet”, CV Tu, DH Wood, Experimental Thermal and Fluid Science Volume 13, Issue 4, November 1996, Pages 364-373 and “Minimization of the N2 Dilution When Wiping in Air”, M. Dubois, in AISTech 2019-Proceedings of the Iron & Steel Conference, May 6-9, Association for Iron & Steel Technology, Warrendale, PA, 2019, Pittsburgh, USA.

[0126] It is important that by combining the dry gas TG and the water vapor gas stream WG, a stripping gas stream AG is generated that contains a very small but sufficiently high amount of water vapor to prevent the formation of surface defects and imperfections in the layers 10. Furthermore, there should not be too much water vapor in the stripping gas stream AG to prevent condensation and the formation of water droplets.

[0127] Further targeted investigations have shown a correlation between the moisture content of the stripping gas AG and the occurrence of such surface defects and defects, whereby the (ambient) air humidity fuc can also have an influence (if the ambient air humidity is high enough, surface defects and defects may not occur under certain circumstances). In Table 1 (cf. Fig. 5), all parameter ranges and sample numbers are highlighted in light gray (sample numbers 53 to 74), which enable immersion coating and controlled blow-off of the layers 10 without the occurrence of severe surface defects and defects. In Table 1 (cf. Fig. 5), all parameter ranges and sample numbers are highlighted in dark gray (sample numbers 58 to 74), which enable immersion coating and controlled blow-off of the layers 10 without the occurrence of surface defects and defects.

[0128] When operating the device 150, in all embodiments, it is preferably ensured that - the thickness of the nozzle lip gap 17 (called height of the nozzle openings) in a range between 0.5 and 5 mm, preferably between 0.6 and 2 mm, particularly preferably between 0.8 and 1.4 mm, and / or - the flow rate of the exhaust gas flow AG in the range of 200 to 8000 Nm 3 per hour, and / or - the nozzle distance (nozzle-band distance) of the nozzle 15 to the side V or R is in a range between 2 and 15 mm, preferably between 2.5 and 14.1 mm, and / or - the belt speed of the steel belt 100 is in a range between 50 and 200 m / min, preferably between 70 and 150 m / min.

[0129] Within these (value) ranges, the device 150 and the method operate particularly reliably.

[0130] In all embodiments, a corresponding gas nozzle 15 has a longitudinal extension (referred to as the nozzle width) perpendicular to the drawing plane of Figures 2, 3, and 4 (parallel to the z-axis in Figure 4). Preferably, in all embodiments, the nozzle 15 has an active nozzle width that corresponds at least to the strip width of the strip-shaped flat steel product 100. The strip width of the strip-shaped flat steel product 100 can, in all embodiments, be, for example, in the range from 500 to 2500 mm, preferably between 800 and 1800 mm, and particularly preferably in the range from 1159 mm to 1614 mm. For wider strip-shaped flat steel products 100, the active nozzle width also increases accordingly.

[0131] The nozzles are positioned at a variable vertical distance from the zinc bath surface. This distance is commonly referred to as the nozzle height. This distance is primarily adjusted depending on the speed of the passing strip and / or the desired zinc layer thickness. For all designs, the nozzle height can be, for example, between 230 and 500 mm.

[0132] All embodiments of the device 150 may include an optional controller 250, as indicated schematically and by way of example in Fig. 3. In all embodiments, this controller 250 may be designed as a computer-aided automation and control unit and may include a human-machine interface, a computer, and a database.

[0133] The controller 250, if present, can be connected to the means or devices for determining gas humidity 20 via communication links KV1, KV2 in all embodiments.

[0134] In all embodiments, the controller 250 may be part of the overall system control of the device 150, or it may be connected to the overall system control in all embodiments.

[0135] In all embodiments, the controller 250, if present, can have one or more analog and / or digital inputs to obtain information about the currently prevailing ambient conditions (e.g., the current air temperature T and / or the (absolute) air humidity fuc of the environment or the immediate vicinity). Based on this information, the controller 250 can, for example, reduce or increase (or even shut off) the metered addition of the water vapor gas stream WG in order to continue producing layers 10 with defect-free surfaces in the device 150.

[0136] In all embodiments, the controller 250, if present, can include communication connections that enable the controller 250 to reduce or increase the flow rate of the water vapor gas stream WG and / or to reduce or increase the flow rate of the dry gas stream TG. Alternatively, the controller 250 can, for example, use communication connections to adjust a mixing valve in the area of ​​the junction 19 of the two gas streams TG, WG as needed.

[0137] If, for example, the humidity of the ambient air ZUG is very low, the intake of dry ambient air can lead to the formation of marbling on layer 10. The invention can address this by switching on or automatically increasing the moisture content in the stripping gas AG.

[0138] First Embodiment: A cold-rolled flat steel product 100 in strip form, namely cold-rolled deep-drawn steel in strip form, is cleaned of rolling oil and rolling debris in at least some embodiments during a pretreatment in the continuous hot-dip coating system using a combined dip / brush / electrolytic cleaning process, and is rinsed with water and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip coating system, where it is preheated using a direct-fired furnace (DFF). heated and brought to an annealing temperature of 820°C in a radiant tube furnace under protective gas at a dew point of -40°C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of 450°C and immersed in the 430°C ZnMgAl alloy molten bath 11 for 3 s. After leaving the bath 11 on the outlet side A, the flat steel product 100 in strip form is sprayed at the stripping nozzles 15 of the stripping nozzle device 14 with dry stripping gas AG with a dew point of minus 73°C or 3 ppm H2O content to a specified target layer thickness of ZM90 (45 g / m 2per side V, R). The air knife nozzles 15 have a nozzle lip gap of 1.0 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of Fig. 4) of 6 mm on both sides of the flat steel product 100 in strip form. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the flat steel product 100. Subsequently, the flat steel product 100 in strip form is re-rolled in a skin-pass mill and a roughness of 1.4 pm is imprinted. After an inspection for surface defects, during which surface defects such as mottled areas and toothpick defects are detected, the flat steel product 100 in strip form is oiled in an oiling machine with 1.0 g / m 2Each side V and R is coated with corrosion protection and forming oil and finally wound on the coiler. In a further step, for example, in a so-called inspection line, the flat steel product 100 can be unwound in strip form, the defective area removed, and then the strip rewound.

[0139] Second embodiment: A cold-rolled flat steel product 100 in strip form, namely a cold-rolled deep-drawn steel in strip form, is cleaned of rolling oil and rolling debris by means of a combined dip / brush / electrolytic cleaning process during in-line pretreatment of the continuous hot-dip coating system, rinsed with water, and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip coating system, is preheated, heated by means of a direct-fired furnace (DFF), and heated to an annealing temperature of 820°C in a radiant tube furnace under protective gas at a dew point of -40°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 450°C and immersed for 3 s into the 430°C ZnMgAl alloy bath 11. After leaving the bath in the outlet area A, the steel flat product 100 in strip form is sprayed at the wiping nozzles 15 with humidified nitrogen (after admixing gaseous H 2 O vapor, as described and claimed here, to 310 ppm or -35°C dew point) to the target layer thickness of ZM90 (45 g / m 2per side). The air knife nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of Fig. 4) of 6 mm on both sides from the flat steel product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then skin-rolled in a temper-pass mill and a roughness Ra of 1.4 pm is applied. After inspection for surface defects, which determines the presence of mottle, the flat steel product 100 is coated in-line (in a continuous hot-dip coating system) with an oiling machine with 1.0 g / m 2Each side is coated with corrosion protection and forming oil and finally wound onto the coiler. In a further step, for example, in a so-called inspection line, the steel flat product 100 can be unwound in strip form, the defective area removed, and then the strip rewound.

[0140] Third embodiment: A cold-rolled deep-drawing steel strip is cleaned of rolling oil and rolling debris in-line during the pretreatment of the continuous hot-dip coating system using a combined dip / brush / electrolytic cleaning process, rinsed with water, and dried. The cleaned, dried steel flat product 100 in strip form enters an annealing furnace of the continuous hot-dip coating system, is preheated, heated using a direct-fired furnace (DFF), and brought to an annealing temperature of 820°C in a radiant tube furnace under protective gas at a dew point of -40°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy molten bath 11 of 450°C and immersed for 3 seconds in the 430°C ZnMgAI alloy bath 11. After leaving the bath in the outlet area A, the flat steel product 100 in strip form is sprayed at the wiping nozzles 15 with humidified nitrogen (after admixing gaseous H 2 O vapor, to 552 ppm or - 29°C dew point) the target layer thickness of ZM90 (45 g / m 2 per side). The air knife nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of Fig. 4) of 6 mm on both sides from the flat steel product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then re-rolled in a skin-pass mill and a roughness of 1.4 pm is imprinted. After inspection for surface defects, during which neither mottled nor toothpick defects are detected, the flat steel product 100 is oiled in-line with an oiling machine with 1.0 g / m 2coated on each side with anti-corrosive and forming oil and finally wound on the reel.

[0141] Fourth embodiment: A high-strength cold-rolled steel flat product 100 in strip form is cleaned of rolling oil and rolling debris during the pretreatment of the continuous hot-dip coating line using a combined dip / brush / electrolytic cleaning process, rinsed with water, and dried. The cleaned and dried steel flat product 100 enters an annealing furnace of the continuous hot-dip coating line, is preheated, heated using a direct-fired furnace (DFF), and heated to an annealing temperature of 800°C in a radiant tube furnace under protective gas at a dew point of -50°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAI alloy melt bath 11 of 490°C and immersed for 5s in the 430°C warm ZnMgAI alloy bath 11.After leaving the bath in the outlet area A, the flat steel product 100 in strip form is sprayed at the wiping nozzles 15 with humidified nitrogen (after admixing gaseous H2O vapor, to 3065 ppm or. - 9°C dew point) the target layer thickness of ZM90 (45 g / m 2 per side). The wiping nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are arranged at a horizontal distance (parallel to the y-axis of the Fig. 4) of 7.6 mm on both sides to the steel flat product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the steel flat product 100. The steel flat product 100 is then re-rolled in a skin-pass mill and a roughness of 1.3 pm is imprinted. After inspection for surface defects, during which neither mottled nor toothpick defects are detected, the steel flat product 100 is oiled in-line with an oiling machine with 0.8 g / m 2coated on each side with anti-corrosive and forming oil and finally wound on the reel.

[0142] Fifth embodiment: A structural steel, as a cold-rolled flat steel product 100 in strip form, is cleaned of rolling oil and rolling debris during the pretreatment of the continuous hot-dip coating line using a combined dipping / brushing / electrolytic cleaning process, rinsed with water, and dried. The cleaned and dried flat steel product 100 enters an annealing furnace of the continuous hot-dip coating line, is preheated, heated using a direct-fired furnace (DFF), and heated to an annealing temperature of 730°C in a radiant tube furnace under protective gas at a dew point of -45°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAI alloy melt bath 11 of 465°C and immersed for 4s in the 455°C warm ZnMgAI alloy bath 11.After leaving the bath in the outlet area A, the flat steel product 100 in strip form is sprayed at the wiping nozzles 15 with dry nitrogen (without admixture of gaseous H2O vapor; 3 ppm or - 75°C dew point) to the target layer thickness of ZM120 (60 g / m. 2 per side). The air knife nozzles 15 have a nozzle lip gap of 1.0 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of Fig. 4) of 7.7 mm on both sides from the flat steel product 100. In a connected cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then flattened in a bending-stretching-straightening machine and re-rolled in a skin-pass stand, and a roughness of 1.4 pm is imprinted. After inspection for surface defects, in which neither If neither marbling nor toothpick defects are detected, the steel flat product 100 is wound onto the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating line.

[0143] Sixth embodiment: A structural steel, as a cold-rolled flat steel product 100 in strip form, is cleaned of rolling oil and rolling debris during the pretreatment of a continuous hot-dip coating line using a combined dip / brush / electrolytic cleaning process, rinsed with water, and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip coating line, is preheated, heated using a direct-fired furnace (DFF), and heated to an annealing temperature of 730°C in a radiant tube furnace under protective gas at a dew point of -45°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAI alloy melt bath 11 of 465°C and immersed for 4s in the 455°C warm ZnMgAI alloy bath 11.After leaving the bath in the outlet area A, the flat steel product 100 in strip form is sprayed at the wiping nozzles 15 with humidified nitrogen (after admixing gaseous H2O vapor to 1470 ppm or -18°C dew point) to the target layer thickness of ZM120 (60 g / m. 2per side). The air knife nozzles 15 have a nozzle lip gap of 1.0 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of Fig. 4) of 7.7 mm on both sides from the flat steel product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then flattened in a bending-stretching-leveling machine and re-rolled in a skin-pass mill, where a roughness of 1.4 pm is imparted. After inspection for surface defects, during which neither mottled nor toothpick defects are detected, the flat steel product 100 is wound up on the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating line.

[0144] Seventh embodiment: A structural steel is produced as a cold-rolled flat steel product 100 in strip form in the pretreatment of a continuous The hot-dip coating system is cleaned of rolling oil and rolling debris using a combined dip / brush / electrolytic cleaning process, rinsed with water, and dried. The cleaned, dried steel flat product 100 enters an annealing furnace of the continuous hot-dip coating system, is preheated, heated using a direct-fired furnace (DFF), and heated to an annealing temperature of 730°C in a radiant tube furnace under protective gas at a dew point of -45°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy molten bath 11 of 465°C and immersed for 4 seconds in the 455°C ZnMgAl alloy bath 11. After leaving the bath in the outlet area A, the flat steel product 100 in strip form is sprayed at the wiping nozzles 15 with humidified nitrogen (after admixing gaseous H2O vapor to 5230 ppm or - 2°C dew point) to the target layer thickness of ZM120 (60 g / m 2per side). The air knife nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of Fig. 4) of 7.7 mm on both sides from the flat steel product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then flattened in a bending-stretching-leveling machine and re-rolled in a skin-pass mill, where a roughness of 1.4 pm is imparted. After inspection for surface defects, during which neither mottled nor toothpick defects are detected, the steel strip 100 is wound up on the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating line.

[0145] Eighth embodiment: A mild steel, as a cold-rolled flat steel product 100 in strip form, is cleaned of rolling oil and rolling debris in the pretreatment of a continuous hot-dip coating line by means of a combined dip / brush / electrolytic cleaning process, rinsed with water, and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip coating line, is preheated, heated by means of a direct-fired furnace (DFF), and heated in a radiant tube furnace under protective gas at a dew point of -50°C to the Annealing temperature of 720°C. At the end of the annealing furnace, the steel flat product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy molten bath 11 of 460°C and immersed for 3 s into the 455°C ZnMgAl alloy bath 11. After leaving the bath in the outlet area A, the steel flat product 100 in strip form is sprayed at the wiping nozzles 15 with dry nitrogen (without admixture of gaseous H2O vapor; 3 ppm or -74°C dew point) to the target layer thickness of ZM90 (45 g / m 2per side). The air knife nozzles 15 have a nozzle lip gap of 1.2 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis in Fig. 4) of 10 mm on both sides of the flat steel product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt solidifies on the flat steel product 100. The flat steel product 100 is then flattened in a bending-stretching-leveling machine and skin-passed in a skin-pass rolling mill, where a roughness of 1.0 pm is imparted. As part of an in-line chemical post-treatment, an organic / inorganic passivation layer is applied by means of a coater and dried. After inspection for surface defects, during which mottled or toothpick defects are identified, the steel strip 100 is wound up on the reel.In a further step, for example in a so-called inspection line, the flat steel product 100 can be unwound in strip form, the defective area can be separated out and the strip can then be rewound.

[0146] Ninth embodiment: A mild steel, as a cold-rolled flat steel product 100 in strip form, is cleaned of rolling oil and rolling debris in the pretreatment of a continuous hot-dip coating line using a combined dip / brush / electrolytic cleaning process, rinsed with water, and dried. The cleaned, dried flat steel product 100 enters an annealing furnace of the continuous hot-dip coating line, is preheated, heated using a direct-fired furnace (DFF), and brought to an annealing temperature of 760°C in a radiant tube furnace under protective gas at a dew point of -50°C. At the end of the annealing furnace, the flat steel product 100 in strip form is heated to the strip immersion temperature of the ZnMgAl- The alloy melt bath 11 is cooled from 460°C and immersed for 4s in the 455°C warm ZnMgAl alloy bath 11. After leaving the bath in the outlet area A, the steel flat product 100 in strip form is sprayed at the wiping nozzles 15 with humidified nitrogen (after admixing gaseous H2O vapor to 9980 ppm or + 7°C dew point) to the target layer thickness of, for example, ZM120 (60 g / m 2 per The air knife nozzles 15 have a nozzle lip gap of 1.2 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis in Fig. 4) of 10 mm on both sides of the flat steel product 100. In an attached cooling tower 16 (see Fig. 1), the zinc alloy melt solidifies on the flat steel product 100. The flat steel product 100 is then flattened in a bending-stretching-straightening machine. After inspection for surface defects, during which neither mottled nor toothpick defects are detected, the steel strip 100 is wound onto the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating line.

[0147] Reference symbol

Claims

Claims:

1. Device (150) for applying a ZnAlMg layer or a ZnAl layer (10) to a flat steel product (100), comprising: - a zinc alloy melt bath (11) with an inlet side (E) and an outlet side (A), - a dry gas supply (18) designed to provide a dry gas flow (TG), - a steam device (50) designed to provide steam gas (WG), - means (20) for determining gas humidity and / or moisture content, - a stripping nozzle device (14) which is fluidically connected to the gas supply (18) and to the steam device (50) in order to supply the stripping nozzle device (14) with a stripping gas (AG) as a mixture of the dry gas flow (TG) and the gaseous steam (WG), wherein o the stripping nozzle device (14) comprises at least one gas nozzle (15) for blowing off the front side (V) and at least one gas nozzle (15) for blowing off the back side (R) of the flat steel product (100) with the stripping gas (AG), o the gas nozzles (15) are arranged in the region of the outlet side (A) of the zinc alloy melt bath (11), o the means (20) for determining gas moisture and / or the moisture content is arranged in or on the stripping nozzle device (14) in order to determine the gas moisture before or upon exit of the stripping gas (AG) in the direction of front (V) or back (R) of the flat steel product (100), and wherein at least one of the following two conditions Bl,B2 is fulfilled:, Bl: the stripping gas (AG) has a moisture content, or a proportion of the gaseous water vapor (WG), which is greater than 200 ppm and less than 43700 ppm, wherein the moisture content is preferably in the range 500 ppm to 9980 ppm, B2: the stripping gas (AG) has a dew point (TP) greater than - 39°C and less than + 30°C, with the dew point (TP) preferably being in the range between - 29°C and + 7°C.

2. Device (150) according to claim 1, wherein the proportions of the dry gas stream (TG) and the water vapor gas (WG) for mixing the stripping gas (AG) are controlled such that an instantaneous dew point (TP) results in the stripping gas (AG) which is lower than the temperature (TAG) of the stripping gas stream (AG) in order to avoid condensation of water in the stripping nozzle device (14).

3. Device (150) according to claim 1 or 2, wherein according to the formula TP < TAG - AT a temperature difference AT is predetermined which is at least 5°C and preferably at least 10°C, wherein TP defines the instantaneous dew point in the stripping gas stream (AG) and TAG defines the instantaneous temperature of the stripping gas stream (AG).

4. Device (150) according to claim 1 or 2, characterized in that the stripping gas stream (AG) is defined by a dew point (TP) which is greater than - 39°C and less than + 30°C, wherein the dew point (TP) is preferably in the parameter range - 29°C to + 7°C if the temperature TAG of the stripping gas stream (AG) is in the range 10 to 30°C.

5. Device (150) according to one of claims 1-3, comprising: - a temperature sensor (24) to measure the instantaneous temperature (TAG) of the stripping gas flow (AG), - a control of the dry gas flow (18) and / or the water vapor gas flow (WG) in order to control the proportions of the dry gas flow (TG) and the water vapor gas (WG) for mixing the stripping gas (AG) in such a way that a dew point (TP) results in the stripping gas flow (AG) which is lower by a temperature difference (AT) than the instantaneous temperature (TAG) of the stripping gas flow (AG) in order to avoid condensation of water in the stripping nozzle device (14).

6. Device (150) according to one of claims 1 to 5, wherein a maximum dew point of + 30°C is specified in the stripping gas stream, even if the temperature of the stripping gas is more than 30°C 7. Device (150) according to one of claims 1 to 6, wherein the drying gas (TG) is nitrogen gas having a volume fraction of water vapor gas (WG) of less than 5 ppm, or a dew point (TP) of less than -70°C.

8. A method for applying ZnAlMg layers or ZnAl layers (10) to the front (V) and back (R) of a flat steel product (100), comprising the steps: - moving the steel flat product (100) from an inlet side (E) to an outlet side (A) of a zinc alloy melt bath (11), - Providing a dry gas stream (TG), - Providing a water vapor gas (WG), - Combining the dry gas stream (TG) and the water vapor gas (WG) to obtain a stripping gas (AG) as a mixture, - Determination of the gas humidity of the stripping gas (AG), - Emitting the stripping gas (AG) through at least one gas nozzle (15) which serves to blow off the front side (V) and through at least one gas nozzle (15) which serves to blow off the back side (R) in order to blow off the front side (V) and back side (R) of the flat steel product (100) with the stripping gas (AG), o wherein a stripping gas stream (AG) is emitted which satisfies at least one of the following two conditions B1, B2: Bl: the stripping gas (AG) has a moisture content, or a proportion of the gaseous water vapor (WG), which is greater than 200 ppm and less than 43700 ppm, wherein the moisture content is preferably in the range 500 ppm to 9980 ppm, B2: the stripping gas (AG) has a dew point (TP) greater than - 39°C and less than + 30°C, with the dew point (TP) preferably being in the range between - 29°C and + 7°C.

9. The method according to claim 8, wherein the proportions of the dry gas stream (TG) and the water vapor gas (WG) in the stripping gas (AG) are controlled such that an instantaneous dew point (TP) results in the stripping gas (AG) which is lower than the temperature (TAG) of the stripping gas stream (AG) in order to avoid condensation of water in the stripping nozzle device (14).

10. Experience according to claim 8 or 9, characterized in that the moisture content of the stripping gas stream (AG) is defined by a dew point (TP) which is greater than - 39°C and less than + 30°C, and wherein the dew point (TP) is preferably in the range -29°C to + 7°C, if the temperature TAG of the Stripping gas flow (AG) is in the range 10 to 30°C.

11. Method according to one of claims 8 to 10, characterized in that an admixture of the gaseous water vapor (WG) is only carried out if in the environment or in the vicinity of the outlet side (A) of the zinc alloy melt bath (11) there is an absolute ambient humidity (fuc) which is lower than a predetermined limit value.

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