Method and device for applying a layer onto a flat steel product

Abstract

The invention relates to a device (150) for applying a layer onto a flat steel product (100), comprising: - a molten zinc alloy bath (11) having an entry side (E) and an exit side (A); - an air-knife device (14) which comprises at least one knife-edge gap and which is located in the region of the exit side (A) in such a way that the layer which is still in liquid form on the flat steel product (100) can be blown off by means of gas exiting through the knife-edge gap. The device (150) is designed to carry out at least one of the following adjustments: - raising the bath temperature of the molten alloy bath (11) if the current local absolute humidity decreases, and vice versa, and / or - reducing the width of the knife-edge gap if the current local absolute humidity decreases, and vice versa, and / or - reducing the distance between the knife-edge gap and the side of the flat steel product (100) if the current local absolute humidity decreases, and vice versa, and - automatically adjusting the flow rate of the gas in order to keep the target thickness of the layer to be applied substantially constant.

Description

voestalpine Stahl GmbH, Linz Method and device for applying a layer to a flat steel product

[0001] The present invention relates to a method for coating flat steel products with a zinc-aluminum-magnesium (ZnAlMg)-based layer, e.g., as a protective coating. It also relates to a device designed to implement the method according to the invention.

[0002] It is well known that flat steel products 100, such as steel strips or steel sheets, are coated with a 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 molten zinc alloy 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 a (zinc bath) roller 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 flat steel 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 coating mass (in g / m. 2 ) and the flat steel product 100 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 A1 of the applicant VOESTALPINE STAHL GMBH.

[0004] There is prior art that mentions water or steam in connection with hot-dip coating of flat steel products. The relevant documents are listed below, and their contents are briefly described where relevant.

[0005] Patent EP0172682B1 from Armco Inc., filed in 1985, concerns the control of zinc vapor during the hot-dip coating of an iron-based metal strip. On the inlet side of the dip bath, an oxygen-reduced atmosphere containing a small amount of water vapor is provided in an enclosed area (comparable to nozzle 12 in Fig. 1). This small amount of water vapor is intended to prevent the formation of zinc vapor at the surface of the dip bath. The dew point of the gas used at the inlet side is adjusted so that zinc vapor cannot form.

[0006] 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.

[0007] In addition to pure protection against corrosion, there are increasingly stringent requirements regarding the surface quality of zinc-coated flat steel products. The automotive industry, in particular, demands products that meet the highest surface requirements. However, providing homogeneous surfaces is no trivial task.

[0008] The main problems here are often surface defects in the ZnAlMg layer. For example, a marble effect, a toothpick, or a beach pattern defect can develop on the ZnAlMg layer, or slag can form. There are patents (e.g., EP20130826634 AM / JMMataigne; JP20080256208 NSSMC / Oohashi et al.) that attempt to eliminate similar surface defects (gloss effects or displaced oxide skins) using other means (reducing the O2 content in the vicinity of the wiping nozzle).

[0009] The task therefore arises of providing a process and a corresponding device for coating flat steel products that offer particularly durable and robust corrosion protection, while ensuring a surface of the protective coating that is particularly homogeneous, very smooth, and free of marbling (without the "marble effect") and / or toothpick defects (without "toothpick"). The goal is a surface quality that meets the highest customer requirements.

[0010] Furthermore, this process should be as energy-efficient, cost-effective, simple, and reproducible as possible. Summary of the invention

[0011] According to the invention, a continuous (hot-dip) process and a corresponding device are provided which allow a flat steel product to be provided with a metallic layer which can serve, for example, as a (protective) coating, said layer protecting the steel substrate of the flat steel product from external influences.

[0012] All embodiments involve the application of a (protective) layer to a flat steel product, whereby the layer thickness of this layer corresponds to a The desired thickness (according to a corresponding specification) should be achieved. This layer is created by passing the flat steel product through a molten zinc alloy bath and blowing it off with gas on the exit side of the bath using a wiping nozzle device comprising at least one gas nozzle.The zinc alloy of the zinc alloy melt pool has the following composition: - an aluminum content ranging between 1.0 and 3.5% by weight and preferably between 1.3 and 2.8% by weight, - a magnesium content ranging between 1.0 and 3.0% by weight and preferably 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-related content of each additional element in the metallic coating being less than 0.1%, and unavoidable impurities.

[0013] The method is characterized in that at least one of the following adjustable parameters is adjusted as follows: - Increasing the bath temperature of the alloy melt bath if the current absolute local air humidity is reduced and vice versa, and / or - Reducing the thickness of the die lip gap if the current absolute local air humidity is reduced and vice versa, and / or - Reducing the distance between the die lip gap and the side of the flat steel product if the current absolute local air humidity is reduced and vice versa, wherein additionally the flow rate of the gas is adjusted in order to keep the target thickness of the layer to be applied substantially constant.

[0014] A device designed for applying a layer to a flat steel product, comprising - a zinc alloy melt bath having an inlet side, an outlet side and a deflection to guide the flat steel product from the inlet side at a strip speed through the zinc alloy melt bath to the outlet side, - a wiping nozzle device comprising at least one nozzle lip gap and arranged in the region of the outlet side such that the still liquid layer on the flat steel product can be blown off with gas which exits through the nozzle lip gap in order to blow off the layer to a desired thickness, wherein the zinc alloy of the zinc alloy melt bath has the following composition: - an aluminum content which is in the range between 1.0 and 3.5% by weight and preferably in the range between 1.3 and 2.8% by weight, - a magnesium content which is in the range between 1.0 and 3.0% by weight and preferably in the range between 1.2 and 2.2% by weight, and - the remainder of the zinc alloy melt bath is zinc and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, Zr, Sr, La, Ce or Bi, wherein the weight-related content of each additional element in the metallic coating is less than 0.1%,and unavoidable impurities.

[0015] This device is set up or designed to carry out at least one of the following adjustments manually or automatically: - increasing the bath temperature of the alloy melt bath if the current absolute local air humidity is reduced and vice versa, and / or - reducing the thickness of the die lip gap if the current absolute local air humidity is reduced and vice versa, and / or - reducing the distance between the die lip gap and the side of the flat steel product if the current absolute local air humidity is reduced and vice versa, wherein in addition the flow rate of the gas is automatically adjusted in order to keep the target thickness of the layer to be applied substantially constant.

[0016] In all embodiments or at least some of the embodiments, a metallic bath is provided with a ZnAlMg alloy composed according to the following alloying concept: - the aluminum content (in percent by weight) is in the range between 1.0 and 3.5 percent by weight and preferably in the range between 1.3 and 2.8 percent by weight; - the magnesium content (in weight percent) is in the range between 1.0 and 3.0 weight percent and preferably in the range between 1.2 and 2.2 weight percent; - the remainder of the zinc alloy molten bath 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 additional element in the metallic coating being less than 0.1%, and unavoidable impurities.

[0017] The aluminum content (in weight percent) may be greater than or equal to the magnesium content (in weight percent) in some embodiments.

[0018] In all embodiments or at least some of the embodiments, the unavoidable impurities are in a range that is significantly less than 1 percent by weight (wt%), preferably the sum of all unavoidable impurities is less than 0.5 percent by weight.

[0019] The combination of a precisely defined ZnAlMg alloy concept, monitoring or observation of the current absolute local air humidity and a targeted adjustment of the stripping process (or the corresponding process and / or system parameters or the stripping efficiency), can produce a surface that shows no or negligible marbling.

[0020] The values ​​of the absolute local air humidity which, according to the invention, should be present in the immediate vicinity of the flat steel product in an area between the outlet side and the cooling area (if present), are in all embodiments in the range from 1 g / m³ to 300 g / m³, preferably in the range from 1.08 g / m³ to 51 g / m³, ie the process can be carried out successfully at an absolute local air humidity which is in the stated range.

[0021] In all versions, the stripping nozzle device can optionally be followed by a strip stabilizing device, which is used to automate Stabilizes the movement of the flat steel product. In these embodiments, the absolute local humidity values ​​can be determined in the area between the air knife device and the belt stabilizing device.

[0022] The method preferably comprises the following steps at an absolute local air humidity of greater than 1 g / m³: - Operating the alloy melt bath at a bath temperature TB in the range 400 < TB < 480 degrees Celsius, preferably in the range 409 < TB < 473 degrees Celsius, and particularly preferably in the range 420 < TB < 460 degrees Celsius, - Operating the wiping nozzle device with a nozzle distance from the flat steel product of 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 gas which flows through the nozzle lip gap in the direction of the flat steel product at a gas flow rate in the range of 200 to 8000 Nm³ per hour.

[0023] The ZnAlMg alloy concept defined above is based on numerous studies and calculations. Within the specified limits of the ZnAlMg alloy concept, the technical theory presented here has proven particularly successful.

[0024] In at least some of the embodiments, the absolute local humidity is measured permanently.

[0025] In at least some of the embodiments, the absolute local humidity is measured from time to time.

[0026] In at least some embodiments, the method is interrupted if the absolute local humidity is too low to adjust one or more of the adjustable parameters.

[0027] Preferably, in all embodiments, the process is carried out with a reduced bath temperature TB red carried out in the range 420 < TB red < 460 degrees Celsius. In this range, the formation of slag can also be reduced.

[0028] Based on concrete tests, it has been demonstrated that the ZnAlMg alloy concept does indeed lead to very good results. It has been shown that excellent results can be achieved by specifically adjusting the stripping process (or the corresponding stripping coefficient).

[0029] The development of the new process, the particularly suitable ZnAlMg alloy concept and the targeted adaptation of the (process) parameters (or the corresponding stripping efficiency) is based on theoretical considerations, various simulations of stripping processes and numerous tests.

[0030] The processes in the air knife device and on the flat steel product are highly complex and depend on numerous (process) parameters and influencing factors (or the corresponding air knife coefficient). Therefore, the present method and device rely on some simplified assumptions and specifications to obtain reproducible results.

[0031] In all embodiments, the coated flat steel product can, as usual, be subjected to a skin-pass or temper-rolling process and / or a bending-stretch-leveling process after coating. The total degree of deformation for the coated flat steel product is preferably between 0.5% and 2.5%, more preferably between 0.7% and 1.7%. The skin-pass or temper-rolling step (with a deformation in the range of 0.7% to 1.7%) can further reduce the negative impact of marbling.

[0032] Furthermore, the coated flat steel product can be treated with the usual transport protection measures such as oiling or other chemical treatment agents, as described in point 7 of the leaflet "Characteristic features 095 - Hot-dip coated Strip and Sheet", edition 2010, published by the Steel Information Centre 40039 Düsseldorf.

[0033] Further advantageous embodiments of the invention form the subject matter of the dependent claims. DRAWINGS

[0034] 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; FIG. 3A shows a highly schematic side view of a stripping nozzle device with only one nozzle in order to be able to define some of the process and system parameters relevant here (this representation is not to scale); FIG. 3B shows a schematic representation of the pressure curve P at the surface of the coating relative to a position on the x-axis; FIG. 3C shows a schematic representation of the shear force curve τ at the surface of the coating relative to a position on the x-axis; FIG.Figure 4 shows a summary graphic representation of numerous exemplary experiments, with the stripping efficiency factor AWZ plotted on the ordinate axis and the absolute humidity f plotted on the abscissa axis; Figure 5 shows a highly schematic representation of an apparatus in which the method of the invention is used, showing two virtual cylinder volume segments. FIG. 6 shows a highly schematic representation of a further device of the invention in which the method of the invention is used; FIG. 7A shows a highly schematic representation of a strip side of a flat steel product that has no marbling; FIG. 7B shows a highly schematic representation of a strip side of a flat steel product that has moderate marbling; FIG. 7C shows a highly schematic representation of a strip side of a flat steel product that has strong marbling; FIG. 7D shows a photo of a coated flat steel product without marbling; FIG. 7E shows a photo of a coated flat steel product with strong marbling including toothpick defects; FIG. 7F shows a photo of a coated flat steel product without marbling after skin passing; FIG. 7G shows a photo of a coated flat steel product with marbling including toothpick defects after skin passing; FIG.Figure 8A shows details of Table 2 with test results; Figure 8B shows details of Table 2 with further test results; Figure 9 shows exemplary steps of the method described here. Detailed description.

[0035] This concerns a method and a device 150 for applying a layer 10 to a strip-shaped flat steel product 100 (see Fig. 3A, where the layer 10 can be seen schematically on the upper strip side). This layer 10 is produced by guiding the flat steel product 100 from an inlet side E to an outlet side A through a molten zinc alloy bath 11 and blowing it off with gas G on the outlet side A by means of a wiping nozzle device 14, as shown by way of example in Figs. 2, 3A, 6 and 7. The purpose of the wiping nozzle device 14 is to strip off the excess (still liquid) ZnMgAl layer (layer 10) as it exits the bath 11.

[0036] Within the scope of the invention, care must be taken to ensure, on the one hand, that layer 10 remains essentially unchanged, even if the absolute humidity f of the environment changes, and, on the other hand, that no marbling occurs. In other words, the aim is to prevent marbling when the absolute local humidity f changes, while at the same time essentially maintaining the desired thickness of layer 10.

[0037] In addition to the target thickness of layer 10, the target (surface) coating mass of layer 10 can also be specified in all embodiments. Typically, there is a narrow tolerance range for the target thickness. As long as the layer 10 to be produced lies within the tolerance range(s), the layer 10 essentially meets the specifications.

[0038] The processes in the area of ​​the air knife device 14 and on the flat steel product 100 are highly complex and depend on numerous parameters and influencing factors.

[0039] In all embodiments, the air stripping nozzle device 14 comprises at least one gas nozzle 15 (if only one side of the strip is to be blown off) or two gas nozzles 15 positioned opposite each other (if both sides of the strip are to be blown off). Figures 2, 6, and 7 show embodiments with two nozzles 15, and Figure 3A shows an embodiment with only one nozzle 15.

[0040] In at least some embodiments, the process is carried out and controlled such that the layer 10 on each strip side of the flat steel product 100 has a target thickness that lies within the tolerance window. Preferably, the target thickness of the layer 10 on each strip side in all embodiments is in the range of 3 to 30 µm, and particularly preferably in the range of 4.5 to 15 µm.

[0041] Preferably, the target surface coating (coating mass per belt side; referred to as coating per side in Tables 8A and 8B) is in the range from 20 to 200 g / m² and particularly preferably in the range from 30 to 100 g / m² in at least some of the embodiments.

[0042] In order to be able to reliably apply such a layer 10, which essentially corresponds to the desired thickness, the zinc alloy of the zinc alloy melt bath 11 has the following composition in all or at least some of the embodiments: - an aluminum content which is in the range between 1.0 and 3.5 percent by weight and preferably in the range between 1.3 and 2.8 percent by weight, - a magnesium content which is in the range between 1.0 and 3.0 percent by weight and preferably in the range between 1.2 and 2.2 percent by weight, and - the remainder of the zinc alloy melt bath 11 is zinc and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, Zr, Sr, La, Ce or Bi, wherein the weight-related content of each additional element in the metallic coating is less than 0.1%, and unavoidable impurities.

[0043] In all embodiments or at least some of the embodiments, a metallic bath 11 is provided with a particularly preferred ZnAlMg alloy, which is composed according to the following alloy concept: - the aluminum content (in weight percent) is in the range between 2.09 and 2.66 weight percent; - the magnesium content (in weight percent) is in the range between 1.45 and 2.24 weight percent; - the remainder of the zinc alloy molten bath 11 is zinc and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, Zr, Sr, La, Ce or Bi, wherein the weight-related content of each additional element in the metallic coating is less than 0.1%, and unavoidable impurities.

[0044] In all embodiments, the aluminum content (in weight percent) can be equal to or greater than the magnesium content (in weight percent).

[0045] To prevent or significantly reduce the formation of marbling, the absolute local humidity f is measured continuously or periodically (e.g., by direct or indirect measurement). This statement also applies to preventing or reducing the formation of toothpicks.

[0046] Absolute humidity f is a physical quantity which is expressed in units such as g / m 3 This means that it is the mass of gaseous water in a standardized volume body with a volume of 1 m 3 In other words, the absolute humidity f indicates the water vapor content in a volume body. f is used here as the symbol for absolute humidity. The absolute humidity f can be approximately estimated for all embodiments from the air temperature TL and the relative humidity r, where: 13.235 ∙ ^ ^.^ ∙^^ .10 ^^^^^^,^ ^^ + 273,15 r relative humidity in % TL air temperature in °C

[0047] The measurement of the absolute local humidity f can be performed directly or indirectly in all embodiments. Indirect measurement here includes, among other things, measuring the air temperature TL and the relative humidity r and calculating / deriving the absolute local humidity f.

[0048] The values ​​of the absolute local air humidity f, which according to the invention should be present in the immediate vicinity of the flat steel product 100 in a region between the outlet side A and the cooling zone 16 (if present), are in all embodiments in the range of 1 g / m³ and less than 300 g / m³. Preferably, the absolute local air humidity in all embodiments is in the range of 1.08 g / m³ to 51 g / m³.

[0049] The process enables the controlled application of layer 10 to at least one side of the flat steel product 100 at an absolute local air humidity f that is greater than 1 g / m³ and less than 300 g / m³ by specifically adjusting adjustable parameters (process and system parameters). In all embodiments, care is taken to ensure that the adjustable parameters are adjusted in such a way that the layer 10 to be applied (continues) essentially corresponds to the target thickness. This means that care is taken to ensure that a layer is still applied that corresponds to the target thickness (within tolerances) and that, at the same time, exhibits no or only very slight marbling.

[0050] The adjustable parameters (process and system parameters) can be adjusted in all embodiments as follows: - Increasing the bath temperature TB of the alloy melt bath 11 if the current absolute local air humidity f is reduced and vice versa, and / or - Reducing the thickness d of the die lip gap 17 if the current absolute local air humidity f is reduced and vice versa, and / or - Reducing the distance Z between the die lip gap 17 and the side of the flat steel product 100 if the current absolute local air humidity f is reduced and vice versa, wherein in addition the flow rate D of the gas G is adjusted (automatically, for example by control technology) in order to keep the target thickness of the layer 10 to be applied essentially constant.

[0051] In addition, in all embodiments, the strip speed v at which the flat steel product 100 is moved out of the zinc alloy melt bath 11 can also be changed, whereby here too care is taken to ensure that the desired thickness of the layer 10 to be applied remains substantially constant.

[0052] Preferably, in all embodiments, several of these adjustable parameters (process and system parameters) are changed in a coordinated manner to ensure that the layer 10 applied corresponds to the target thickness. The mathematical relationships that apply here will be described later.

[0053] A so-called target specification of the layer 10 to be applied can specify for all embodiments that the following specification(s) must be met: - Coating application of the layer 10 per strip side should be in the range of 20 to 200 g / m 2 , preferably range 30 to 100 g / m2 , and / or - desired thickness of the layer (10) per strip side, which is in the range 3 to 30 ^m, preferably range 4.5 to 15 ^m.

[0054] The bath temperature TB of the alloy melt bath 11 influences the viscosity of the melt during the stripping process. An increased bath temperature TB leads to a reduced viscosity of the melt. With constant adjustable parameters (process and system parameters), more material would be stripped off than desired. Therefore, when the bath temperature TB is increased, the other adjustable parameters (process and system parameters) are changed so that the layer 10 continues to have the desired thickness. When the bath temperature TB is increased, for example, the flow rate D of the gas G is reduced to reduce the stripping effect while still achieving the same desired thickness.

[0055] Preferably, the bath temperature TB of the alloy melt bath 11 in all embodiments is in the range 400 < TB < 480 degrees Celsius, preferably in the range 409 < TB < 473 degrees Celsius, and particularly preferably in the range 420 < TB < 460 degrees Celsius. Within these range limits, the bath temperature TB can be adjusted to change the viscosity.

[0056] For the operation of the alloy melt bath 11, a bath temperature TB within the specified temperature range is specified for all designs. Maintaining this temperature window (temperature range) is important because unwanted slag accumulates in the Flat steel product 100 can be formed when working above the specified range.

[0057] In all embodiments, the bath temperature TB can be adjusted, for example, by means of an inductive heating device 30 (see Figs. 2 and 6) or a resistance heater.

[0058] To avoid slag formation, the bath temperature TB is preferably reduced in all embodiments. The reduced bath temperature TBred is preferably in the aforementioned range of 420 < TBred < 460 degrees Celsius.

[0059] When carrying out the method, in all embodiments it is preferably ensured that - the thickness d of the die lip gap 17 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 flow rate D is in the range from 200 to 8000 Nm³ per hour, and / or - the distance Z is in a range between 2 and 15 mm, preferably between 3 and 12 mm, and / or - the belt speed (v) is in a range between 50 and 200 m / min, preferably between 70 and 150 m / min.

[0060] The process works particularly reliably within these areas.

[0061] In all embodiments, a corresponding gas nozzle 15 has a length parallel to the y-axis. Preferably, in all embodiments, the nozzle 15 has an active length that corresponds to the strip width w of the strip-shaped flat steel product 100 (see also Fig. 5). The thickness d of the gas nozzle 15 is defined parallel to the x-axis.

[0062] The strip width w of the strip-shaped flat steel product 100 is preferably in the range from 500 mm to 2500 mm in all embodiments. Particularly preferably, the strip width w of the strip-shaped flat steel product is in the range from 800 mm to 1800 mm in all embodiments.

[0063] In at least some of the embodiments, the absolute local humidity f is measured permanently or from time to time and, if the absolute local humidity f is too low, the application of the layer 10 is interrupted in order, for example, to make adjustments to the adjustable parameters.

[0064] Since the thickness d of the nozzle lip gap 17 can only be adjusted manually in some of the devices 150, in at least some of the embodiments the process is stopped before the nozzle lip gap 17 is manually adjusted.

[0065] In at least some of the embodiments, the measurement of absolute local air humidity f is carried out directly or indirectly.

[0066] The measurement of the absolute local air humidity f is not performed directly at the impact line where the gas G encounters the layer 10 to be stripped, since the gas mixture there is relatively "dry" (i.e., contains little air humidity). The measurement of the absolute local air humidity f is preferably performed, in all embodiments, directly or indirectly in an area that is at least a normal (at a right angle) distance of 20 cm from the impact line or from the flat steel product 100.

[0067] Figs. 2 and 7 show an approach for directly measuring absolute local humidity. Device 150 comprises, for example, two humidity sensors 51 (at least one per belt side). In the schematic representation, each of these sensors 51 has two contacts, which can be connected, for example, to a controller 250. The corresponding connections or lines V1, V2, V3, V4 are shown by dashed lines in Figs. 2 and 6.

[0068] In all embodiments, sensors of the following design or functionality can be used as humidity sensors 51: - mechanically operating measuring sensors which are based on the humidity-induced expansion or contraction of (usually organic) measuring elements; - psychrometrically operating measuring sensors which use two identical, very accurate thermometers along which the gas flow to be measured is passed at a defined speed; - capacitive measuring sensors which, for example, comprise a humidity-sensitive capacitor with two flat electrodes; - dew-mirror hygrometers which determine the air humidity using dew-point mirrors in which the condensation of water vapor is evaluated when the temperature falls below the dew point; - resistive measuring method which, for example, determines the impedance of the alternating current resistance of a hygroscopic element; - spectrometric measuring methods which, for example, measure the gaseous water content without contact in the near or mid-infrared range (NIR or MIR).

[0069] All embodiments of the device 150 may include a controller 250. In all embodiments, this controller 250 may be configured as a computer-aided automation and control unit and may include a human-machine interface, a computer, and a database.

[0070] 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.

[0071] The adaptation / adjustment of the adaptable parameters (plant or process parameters), or the stripping efficiency AWZ, can then be carried out in these embodiments by the overall plant control and / or by the control 250.

[0072] When measuring the absolute local humidity indirectly, the humidity at or in the surrounding area of ​​the device 150 is not measured (e.g. within a virtual cylinder body vZK), but the current humidity is determined indirectly.

[0073] In all versions, the current humidity can be measured indirectly, for example, by using a type of light barrier to determine the transmission rate along an optical path. In very dry air, the transmission rate is high. The presence of gaseous water, on the other hand, impedes the transmission of light along the optical path, and the transmission rate is lower.

[0074] In all embodiments, the indirect determination can be performed by measuring the surface property(ies) of the coated flat steel product 100 (three examples of a coated flat steel product 100 are shown in Figures 7A, 7B, and 7C). A corresponding measurement of the surface property(ies) can be performed optically, for example, before the cooling area 16 or after the cooling area 16 (e.g., by optically measuring the reflectivity of the surface of the layer 10). Figures 7D to 7G show exemplary photographs of coated flat steel products 100.

[0075] In all embodiments, an inert gas is preferably used as gas G. Nitrogen or a nitrogen-containing gas mixture has proven particularly effective.

[0076] Fig. 3A shows a highly schematic side view of another air knife device 14 to more precisely define the adjustable parameters (plant and process parameters), or rather the air knife coefficient (AWZ). Important adjustable parameters are: - the thickness d of the nozzle lip gap 17, - the flow rate D of the gas G, - the distance Z between the nozzle lip gap 17 and the (strip) side of the flat steel product 100, - the strip speed v (running parallel to the x-axis) at which the flat steel product 100 is moved out of the zinc alloy melt bath 11.

[0077] Fig. 3A shows the nozzle distance Z between the nozzle 15 and the corresponding strip side (here the front side) of the flat steel product 100, and the thickness d of the nozzle lip gap 17. The nozzle lip gap 17 serves as a gas outlet gap of the wiping nozzle device 14.

[0078] Fig. 3B shows a schematic representation of the gas pressure curve P along the front side of the flat steel product 100. The pressure P depends on the position on the x-axis. The pressure curve P ideally has the shape of a Gaussian curve, as indicated in Fig. 3B. From this Gaussian curve, the half-width at the pressure PS / 2 can be determined, as shown, where PS represents the maximum pressure. 2b is the half-width in millimeters. A narrow gas jet is defined by a small half-width 2b. The larger (wider) the gas jet becomes, the larger the half-width 2b becomes. Further details can be found in the publication "Wall Pressure and Shear Stress Measurements Beneath an Impinging Jet", CV Tu et al., Experimental Thermal and Fluid Science 1996, 16, pages 364 - 373, Elsevier Science Inc.

[0079] 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, 6, and 7), exerts a shear force τ on the still-liquid layer 10. Fig. 3C shows a representation of the shear force τ relative to a position on the x-axis (the shear force τ was determined by the negative first derivative of the pressure profile in Fig. 3B). This is the shear force τ acting on the layer 10 to be stripped. The course of the shear force curve τ is, to a first approximation, symmetrical to the point x=0, τ=0 (if the strip speed v parallel to the x-axis is neglected). The nozzle 15 is located exactly above the x=0 position at a distance Z > 0. τ max defines the maximum shear force occurring on the layer 10 to be stripped.

[0080] Tests have shown that the adjustable plant and / or process parameters, or the stripping efficiency AWZ, can be adjusted within certain limits without significantly changing the target thickness and / or the basis weight (coating per belt side) of layer 10.

[0081] In addition, it has been shown that (besides the absolute local air humidity f) two variables characterising the shear force profile generated by the wiping nozzle 15 on the belt 100 are important for the formation of the marbling are decisive. On the one hand, there is the maximum shear force τ max and, on the other hand, the time t during which the strip-shaped flat steel product 100 passes through the distance ^ between the shear force maxima (see Fig. 3C). The time t, in turn, depends on the strip speed v.

[0082] There is a direct relationship between time t, belt speed v and half-width 2b, as expressed in equation (1):

[0083] In all embodiments, the belt speed v is preferably in the range from 50 m / min to 200 m / min and particularly preferably between 70 and 150 m / min.

[0084] The equations describing the dynamic flow behavior of gas G on the flat steel product 100 are very complex. This is due, among other things, to the fact that regions 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 gas G. Details can be found, for example, in the aforementioned publication "Wall Pressure and Shear Stress Measurements Beneath an Impinging Jet."

[0085] Due to the complex interrelationships, extensive tests and statistical evaluations of the measured results did not provide any directly usable results regarding a relationship between the air knife parameters and marbling. Only after systematically investigating various internal and external influencing factors did a relationship between the degree of marbling at layer 10, the air knife parameters, and the ambient conditions of the test setup become apparent. Humidity, in particular, shows an influence on marbling at layer 10.

[0086] Further targeted investigations and the graphical processing of the results of these experiments showed for the first time a correlation between the Humidity, the air knife parameters and the tendency to marbling. Fig. 4 shows a summary graphic representation of numerous tests, with the air knife efficiency factor AWZ (as a summary or generic term for the process and device parameters or, for short, air knife parameters) on the ordinate axis and the absolute air humidity f in g / m on the abscissa axis. 3 were applied.

[0087] By weighting and adding the two quantities τmax and t, a so-called stripping coefficient AWZ can be determined, which can be directly compared with the absolute local air humidity ^, as follows (inequality (2.1)):

[0088] The right side of the inequality (2.1) corresponds to the stripping coefficient AWZ, ie, the following relationship (2.2) applies: ^ > ^^^ (2.2)

[0089] The stripping efficiency AWZ summarizes the following process and device parameters: - the effective flow rate D of the gas G per strip side across the strip width w, - the proportionality factor k, - the strip width w of the flat steel product 100, - the thickness d of the die lip gap 17, - the half-width 2b, - the speed v (strip speed) of the flat steel product 100, or the time t, which is correlated with the speed v.

[0090] Further details on the stripping efficiency factor AWZ can also be found in inequality (3), which will be discussed later. The entire term on the right-hand side of this inequality (3) can also be used as the definition of the stripping efficiency factor AWZ. Before describing inequality (3), we refer to Fig. 4.

[0091] The gray or black filled symbols (squares, diamonds, triangles or circles) in Fig. 4 represent steel flat products 100 in which clearly visible marbling has formed on the surface of layer 10. The black filled symbols indicate particularly strong marbling. The gray filled symbols represent less strong marbling. The open symbols, on the other hand, represent no or negligible marbling. Circular symbols represent tests conducted with a die lip gap thickness d=0.8 mm, square symbols represent tests with a die lip gap thickness d=0.9 mm, diamond-shaped symbols represent a die lip gap thickness of d=1 mm, triangular symbols represent d=1.2 mm, and upside-down triangular symbols represent d=1.4 mm.

[0092] A straight line Ge was inserted into the graph of Fig. 4 as a dividing line to, as a first approximation, separate those tests with significant or moderate marbling from those showing no or only negligible marbling. In the tests located to the right below the straight line Ge, no or only negligible marbling occurs (a corresponding flat steel product 100 with a layer 10 without marbling is shown in Fig. 7A). The straight line Ge can be understood as a function of the stripping coefficient AWZ (see also inequalities (2.1) and (3)).

[0093] A detailed analysis of the test results suggests that the above-mentioned effective stripping factor (AWZ) can be adjusted depending on the absolute local air humidity f to prevent the occurrence of undesirable marbling. This adjustment of the effective stripping factor (AWZ) is preferably carried out in all embodiments in such a way that the target thickness of layer 10, or the surface area (mass) of this layer 10, does not change or hardly changes. This means that when changing the effective stripping factor (AWZ), care is always taken to ensure that the layer 10 to be applied essentially has the target thickness.

[0094] However, it is important that when adjusting the stripping coefficient (AWZ), care is taken to avoid marbling, as well as other defects such as toothpick defects, beach pattern defects, and so on, or to reduce the formation of slag. Therefore, parameter ranges that have proven particularly effective are specified here.

[0095] The straight line Ge shown in Fig. 4, as mentioned, represents only a first approximation. The actual relationships are very complex. The following formula representation (inequality (3)) shows a further mathematical description of the relationship between the absolute local air humidity f in g / m 3 and the effective stripping factor AWZ: d thickness of the nozzle lip gap [mm] D gas flow per strip side [Nm 3 / h] w Width of the steel flat product 100 [mm] v Belt speed [m / min] 2b Half-value width [mm] k Proportionality factor

[0096] In the following, two exemplary approaches are described that serve to simplify inequality (3):

[0097] 1st approach: According to the first approach, a subdivision into three different curve regions is made (here referred to as cases 1.1 to 1.3). This subdivision allows for a simplified calculation of the right-hand side of inequality (3).

[0098] (Case 1.1). In a linear range, the assumption applies: 2b / d=1.9 or b = 1.9d / 2. This simplification applies to the following relationship between the nozzle spacing Z and the width d of the nozzle lip gap 17: Z / d<5.2.

[0099] (Case 1.2) In a further range, which begins at approximately Z / d=5.2 and ends before Z / d=10, the following relationships apply:

[0100] (Case 1.3) In a further range, which begins at approximately Z / d=10, the following relationships apply:

[0101] In order to make the complicated inequality (3) manageable in practice, a case distinction with the three cases 1.1 to 1.3 is recommended, whereby the ratio Z / d is used for case differentiation.

[0102] The distinction between the three areas or cases mentioned above still leads to complicated mathematical formulas, especially in the second area (case 1.2).

[0103] Second approach: Therefore, a second approach is described here, which serves to make the complex inequality (3) easier to handle in practice. In this second approach, a case distinction is made with only two cases (cases 2.1 and 2.2), whereby the ratio Z / d is also used for case differentiation.

[0104] The second approach has the disadvantage that, in the borderline region, the distinction between steel flat products 100 without marbling and steel flat products 100 with marbling is not entirely clear or unambiguous. The best results can be achieved if a case distinction according to the first approach is evaluated using inequality (3) and / or inequality (2.1) (e.g., processed numerically by the controller 250).

[0105] The adjustment / adjustment of the adjustable parameters (plant and / or process parameters), or the effective stripping factor AWZ, can now be carried out in all embodiments or in at least some of the embodiments either using the inequality(s) (2.1) and / or (3) using the first or second approach. Or, as already mentioned, a numerical representation of the inequality(s) (2.1) and / or (3) can also be applied in the controller 250.

[0106] The adaptation / adjustment of the adaptable parameters (plant and / or process parameters), or the effective stripping factor AWZ, can now be carried out in all embodiments or in at least some of the embodiments in such a way that the surface coating of the layer 10 and / or the (coating) mass of the layer 10, and / or the target thickness of the layer 10 remains constant or within narrowly specified tolerance limits, as defined, for example, by a target specification.

[0107] For all designs, the target specification can be specified by the manufacturer and / or the purchaser or client.

[0108] Preferably, either inequality(s) (2.1) and / or (3) or the formulas of the 1st approach or the formulas of the 2nd approach are implemented in the controller 250 by software, or numerical values ​​for the absolute local humidity f and correspondingly Suitable adjustable parameters (system and / or process parameters), or the effective stripping coefficients (AWZ), are stored. Using a lookup table, the controller 250 can then retrieve the correspondingly suitable adjustable parameters (system and / or process parameters), or the effective stripping coefficients (AWZ), for a currently valid absolute local moisture value f and adjust the device 150, or specify a numerical value for the machine operator, for example, for adjusting the thickness d of the die lip gap (e.g., display it on a screen).

[0109] In all embodiments, adjustable parameters (plant and / or process parameters) or effective stripping coefficients (AWZ) in the following numerical or value ranges are preferably used. The individual numerical or value ranges in Table 1 are not correlated with each other, or only partially correlated, because the respective maximum and minimum values ​​originate from different tests. Only the respective maximum and minimum values ​​were taken from Table 2 (Figs. 8A, 8B) and summarized here.

[0110] The nozzle height mentioned in Table 2 (Fig. 8A, 8B) is the vertical distance between the zinc bath level and the line of impact of the gas jet on the layer 10 to be stripped. The nozzle pressure in Table 2, Fig. 8A, 8B is defined as the (over)pressure (relative to the ambient pressure) of the stripping gas G in the nozzle 15 in mbar.

[0111] The flow rate D of the gas G per belt side is typically in the range between 200 and 8000 Nm³ per hour in all embodiments.

[0112] Table 2, which is divided into two parts (see Figures 8A and 8B), shows specific numerical values ​​for the test results shown in Fig. 4. Table 2 of Fig. 8A shows tests in which layers 10 without (visible) marbling were produced by specifying suitable adjustable parameters (process and / or system parameters), or stripping efficiency factors (AWZ) (as shown by way of example in Fig. 7A). The values ​​in Table 2 of Fig. 8A were sorted in ascending order of b.

[0113] Table 2 of Fig. 8B, on the other hand, shows experiments in which layers 10 with moderate (as shown by way of example in Fig. 7B) or even strong marbling (as shown by way of example in Fig. 7C) were produced. The values ​​in Table 2 of Fig. 8B were also sorted in ascending order of b.

[0114] From Table 2 of Fig. 8A, the following pairs of values ​​for the absolute local air humidity f and the stripping efficiency AWZ can be extracted (sorted by ascending stripping efficiency AWZ), whereby all layers 10 generated with these adjustable parameters (process and plant parameters), or stripping efficiency AWZ, lie below the straight line Ge in Fig. 4.

[0115] From Table 2 of Fig. 8B, the following pairs of values ​​for the absolute humidity f and the stripping efficiency AWZ can be extracted (sorted by ascending stripping efficiency AWZ), whereby all layers 10 generated with these adjustable parameters (process and plant parameters), or stripping efficiency AWZ, lie above the straight line Ge in Fig. 4.

[0116] If marbling defects occur under a given absolute local air humidity f and under given process conditions, then in at least some of the embodiments to eliminate the marbling, the stripping efficiency AWZ (corresponds to the right-hand side of inequalities (2.1) and (3)) is reduced until this inequality is fulfilled again.

[0117] As a rule, it is not permitted to change the coating thickness of the produced flat steel product 100, as this is stipulated in the product specification. Provided the coating thickness is kept constant, the reduction of the stripping efficiency factor (AWZ) can be achieved in various ways.

[0118] Using three concrete examples, possibilities for changing the stripping index AWZ are illustrated below.

[0119] Example 1: Reduction of the effective scraping factor AWZ by reducing the nozzle distance Z (see Table 3).

[0120] The following Table 3 shows two test examples 1.1 and 1.2, which demonstrate a reduction in the effective stripping coefficient AWZ by reducing the nozzle distance Z from 10 mm to 8 mm.

[0121] For the initial condition (Table 3 – Example 1.1) with a gas flow D of 1691 Nm³ / h, the strip width w of 1315 mm, a nozzle lip gap d of 1.0 mm, the nozzle distance Z of 10 mm and the strip speed v of 100 m / min, the AWZ is calculated according to inequality (3) to a value of 21.3.

[0122] Based on Example 1.1, in order to reduce the effective stripping coefficient AWZ, the nozzle distance Z was reduced from 10 mm to 8 mm, while the strip width w, the nozzle lip gap d, the strip speed v, the bath temperature TB and the layer thickness per side were kept constant. The changed nozzle distance Z shifts the ratio Z / d, which also changes the values ​​determined for b and k. Table 3 shows that as a result of the change in the nozzle distance Z, the nozzle pressure and thus the gas flow rate per side D had to be reduced from 1691 to 1386 Nm³ / h by means of automatic layer control of the stripping nozzle system in order to keep the thickness of the layer 10 essentially constant.

[0123] Calculating the effective stripping coefficient (AWZ) using these new process parameters (Table 3, Example 1.2) yields a value of 13.5. By reducing the nozzle spacing from 10 mm to 8 mm, the effective stripping coefficient (AWZ) could be reduced from 21.3 to 13.5 while maintaining the same coating thickness.

[0124] Example 2: Reduction of the effective scraping factor AWZ by reducing the nozzle lip gap d.

[0125] The following Table 4 shows two test examples 2.1 and 2.2, which demonstrate a reduction in the stripping efficiency AWZ by reducing the thickness d from 1.2 mm to 1 mm.

[0126] For the initial condition (Table 4, Example 2.1) with a flow rate D = 1373 Nm³ / h, the strip width w = 1455 mm, a nozzle lip gap d = 1.2 mm, the nozzle distance Z = 7 mm and the strip speed v = 100 m / min, the AWZ is calculated according to inequality (3) to a value of 9.4.

[0127] Based on this, in order to reduce the effective stripping factor AWZ, the nozzle lip gap d was reduced from 1.2 mm to 1.0 mm, while the other parameters were kept essentially constant.

[0128] From Table 4 it can be seen that as a result of the change in the nozzle lip gap d, the nozzle pressure and thus the gas flow per side D of the automatic support control had to be reduced from 1373 to 1166 Nm³ / h in order to keep the thickness of the layer 10 constant.

[0129] The changed die lip gap d shifts the ratio Z / d, which also changes the resulting values ​​for b and k. Calculating the effective stripping coefficient (AWZ) using these new process parameters (Table 4, Example 2.2) yields a value of 4.9. By reducing the die lip gap from 1.2 mm to 1.0 mm, the effective stripping coefficient (AWZ) could be reduced from 9.4 to 4.9 while maintaining the same coating thickness. Example 3: Reducing the effective stripping coefficient (AWZ) by increasing the bath temperature (TB).

[0130] The following Table 5 shows two test examples 3.1 and 3.2, which were obtained by increasing the bath temperature TB from 439 0 C to 455 0 C demonstrate a reduction in the stripping efficiency AWZ.

[0131] For the initial condition (Table 5, Example 3.1) with a flow rate D = 1607 Nm³ / h, the strip width w = 1615 mm, a nozzle lip gap d = 1.0 mm, the nozzle distance Z = 7 mm and the strip speed v = 100 m / min, the AWZ is calculated according to inequality (3) to a value of 12.4.

[0132] Based on this, to reduce the stripping coefficient (AWZ), the bath temperature (TB) was increased from 439°C to 455°C mm, while the other parameters remained essentially constant. In this example, the ratio Z / d remains unchanged, meaning the resulting values ​​for b and k also remain constant when the bath temperature (TB) is increased.

[0133] Table 5 shows that, as a result of the change in bath temperature TB, the nozzle pressure and thus the gas flow rate D per strip side of the automatic support control had to be reduced from 1607 to 1339 Nm³ / h to maintain a constant thickness. The reason for this is that with increasing bath temperature TB, the viscosity of the zinc melt decreases, requiring lower stripping forces to strip the same amount of melt.

[0134] If the stripping efficiency factor AWZ is now calculated using these new process parameters (Table 5, Example 3.2), the resulting value is 5.8. This means that by increasing the bath temperature TB, the stripping efficiency factor AWZ could be reduced.

[0135] According to the present invention, the absolute local air humidity f is preferably defined as the absolute air humidity within a virtual cylinder body vZK, as shown schematically in Fig. 5. In this virtual cylinder body vZK, an area extending right and left parallel to the strip-shaped flat steel product 100 is excluded.

[0136] The virtual cylinder body vZK is composed of two virtual cylinder volume segments, which are delimited on the one hand by a virtual cylinder surface that concentrically or nearly concentrically encloses the gas nozzles 15. On the other hand, the cylinder volume segments are delimited by two planes running parallel to the steel flat product 100 on both sides of the steel flat product 100, each with a distance of, for example, s=20 cm from the steel flat product 100. The two cylinder volume segments together have a volume in the range of 1 m 3 up to 10m3 and preferably a volume of less than 2 m 3 . The measurement of the absolute local humidity f is preferably carried out in all embodiments directly or indirectly within the cylinder volume segments.

[0137] Fig. 5 shows part of a further device 150, wherein the two cylinder volume segments of the virtual cylinder body vZK are shown here. The bath 11 is shown here as a rectangular container that is open at the top. The liquid zinc alloy (abbreviated here as ZnAlMg) is located in the bath 11. Only a short length section of the flat steel product 100, which has a strip shape, is shown after emerging from the bath 11. The flat steel product 100 is guided vertically in the direction of the x-axis out of the bath 11 between two opposing gas nozzles 15 of the wiping nozzle device 14. If the two gas nozzles 15 are arranged parallel to each other, then a center line ML can be drawn between these Nozzles 15 are defined. This center line ML is shown in dashed lines in Fig. 5. The center line ML lies in a nozzle plane DE (which is defined as the xy plane).

[0138] The virtual cylindrical body vZK is formed in 3-dimensional space around the center line ML. The "outer shell" of the virtual cylindrical body vZK runs concentrically to the center line ML. All points of the "outer shell" are equidistant from the center line ML (ra is the radius of the virtual cylindrical body vZK).

[0139] The virtual cylinder body vZK encloses at least the nozzles 15 of the wiping nozzle device 14 and has a virtual cylinder height vZH defined parallel to the y-axis. Preferably, in all embodiments, the virtual cylinder height vZH corresponds to the strip width w of the flat steel product 100 and / or the length of the nozzles 15 (defined parallel to the y-axis).

[0140] The specified value ranges for the absolute local air humidity f can be defined for all embodiments within this virtual cylinder body vZK.

[0141] In all or at least some of the embodiments, the absolute local air humidity f is measured along a y-line that runs parallel to the flat steel product 100 and lies inside the virtual cylinder body vZK. This y-line runs parallel to the y-axis. The so-called nozzle plane (also called the wiping plane) is parallel to the yz-plane, and the center line ML lies within the nozzle plane.

[0142] Preferably, the absolute local air humidity f is determined or measured at several points on this y-line in all or at least some of the embodiments, and an average of the determined or measured values ​​is compared with the numerical range specified for the absolute local air humidity f.

[0143] Preferably, the absolute local air humidity f in all or at least some of the embodiments inside a virtual cylinder body vZK is in the range from 1 g / m³ to 300 g / m³, preferably in the range from 1.08 g / m³ to 51 g / m³, whereby this virtual cylinder body vZK has a volume of 2 m 3 has.

[0144] However, the absolute local humidity f can also be measured in a volume range of 1m 3 up to 10m 3 be defined and measured there directly or indirectly, whereby the measurement is preferably not carried out directly at the line of impact of the gas G but at a y-line which runs above the line of impact parallel to the y-axis.

[0145] In the embodiment of Fig. 2, the described y-line lies in the area between the nozzles 15 of the wiping nozzle device 14 and the lower inlet side of the cooling area 16. In the embodiment of Fig. 6, the described y-line lies above the impact line and is, for example, at least 20 cm away from the center line ML of the virtual cylinder body vZK, which is marked on the flat steel product 100 by a small white circle.

[0146] Fig. 6 shows a further embodiment of a device 150, which also uses an approach for directly measuring the absolute local humidity. The structure of the device 150 is similar to the device 150 shown in Fig. 2, therefore reference is also made to the description of Fig. 2. Only the area of ​​the outlet side A of the bath 11 is shown. Two gas nozzles 15 are arranged parallel to the front and rear belt sides (the gas nozzles 15 extend into the plane of the drawing). Each of the nozzles 15 is, as schematically indicated, supplied with air by means of pumps P gfed with the inert gas G. The two pumps Pg are connected to the controller 250 for control purposes, and the controller 250 can, for example, control the gas flow D per belt side. The corresponding connecting lines or pipes are designated V5, V6, V7, and V8 in Fig. 6. Pump Pg here refers to an air supply unit with blower(s) and control valves.

[0147] Preferably, all embodiments of the device 150 comprise a control of the flow rate D of the gas G (called automatic pressure control), which is designed so that despite a change in the other adjustable Parameters always produce a layer 10 with a substantially constant target thickness. For this purpose, the control system includes at least one sensor (not shown) that measures the actual thickness of the layer 10 after blowing. If the actual thickness is less than the target thickness, the control system reduces the flow rate D, and vice versa.

[0148] Each of the nozzles 15 can be moved parallel to the z-axis by a motor or actuator M. The motors or actuators M are connected to the controller 250, as shown. The corresponding connecting lines or cables are designated V9, V10, V11, and V12 in Fig. 6. Sensors (not shown) are provided to control the nozzle spacing Z. This allows the nozzle spacing Z to be adjusted and / or controlled via the controller 250. Control of the nozzle spacing Z can be laser-assisted in all embodiments.

[0149] In all embodiments, the controller 250 can also be connected to an inductive heater 30 or to an electrical resistance heater of the bath 11 in order to adjust the bath temperature TB. With an inductive heater, whose coil 30 is indicated in Fig. 6, the controller 250 can adjust the operating frequency for driving the coil(s) 30 via a frequency generator FG. Therefore, the frequency generator FG is connected to the controller 250 for control purposes, as indicated. The corresponding connecting lines or conductors are designated V13 and V14 in Fig. 6.

[0150] Also indicated in Fig. 6 is a virtual cylinder body vZK whose center line ML intersects the drawing plane slightly above the nozzles 15. To avoid overloading the drawing, the position of the center line ML is indicated by a small white circle on the strip-shaped flat steel product 100. Since the overall configuration is limited on the underside by the bath 11 and on the top side by the optional cooling area 16, the virtual cylinder body vZK here is a virtual cylinder body vZK that is cut at the top and bottom.

[0151] Here, for example, two humidity sensors 51 (at least one per band side) are used to measure the absolute local Humidity can be determined. The humidity sensors 51 are connected to the controller 250. The corresponding connecting lines or cables are labeled V1, V2, V3, and V4 in Fig. 6.

[0152] In such a device 150, as shown schematically and by way of example in Fig. 6, the method can be carried out particularly advantageously and with reproducible results.

[0153] Fig. 9 shows exemplary steps of the method described here in the form of a flowchart. Before the method for applying a layer 10 to a flat steel product 100 is carried out, the individual components and elements of the device 150 are set up (step S1). The device can be set up, for example, based on a target specification for the layer 10 to be applied.

[0154] Before, during, or after setup S1, the current absolute local air humidity f is measured (directly or indirectly) (step S2). Then, using one of the inequalities or a lookup table, it is determined whether the condition f > AWZ is met (step S3). If f is greater than AWZ (YES in the flow chart), the method for applying a layer 10 can start (step S4). If the condition f > AWZ is not met (NO in the flow chart), the method branches back to step S1. After branching back, adjustments can be made to the setup of the components and elements of the device 150, with the aim of reducing AWZ so that the condition f > AWZ is met. During the adjustment, care is taken to ensure that the target specification of the layer 10 to be applied continues to be met.

[0155] Similarly, checking the condition f > AWZ can be repeated from time to time during the application of layer 10 in order to be able to react to changing absolute local air humidity f. If f has decreased, a check is made again (as in step S3) to determine whether the condition f > AWZ is still met. If yes, then the application of layer 10 is continued. If no, then (analogous to step S1) adjustments can be made to the configuration of the components and elements of device 150. with the aim of ensuring that the condition f > AWZ is or remains fulfilled.

[0156] A corresponding control 250 can be designed or programmed in all embodiments such that - by increasing the bath temperature TB a significant reduction in the stripping efficiency AWZ is achieved, and / or - by reducing the thickness d of the nozzle lip gap 17 the stripping efficiency AWZ can be reduced, and / or - by reducing the distance Z the stripping efficiency AWZ can be reduced.

[0157] In all embodiments, a corresponding controller 250 can be designed or programmed to take into account that a change in the thickness d and / or the distance Z has no or only a small influence on the stripping efficiency AWZ if the ratio Z / d is small.

[0158] If f drops significantly and no meaningful adjustment is possible within the target specification, the process can be interrupted. During such an interruption, the width d of the nozzle lip gap 17 can then be manually adjusted, for example (which is not automatically possible in most devices 150).

[0159] Reference symbol:

Claims

Claims 1. Method for applying a layer (10) with a desired thickness to at least one side of a flat steel product (100), in which the flat steel product (100) is moved through a zinc alloy molten bath (11) and on the outlet side (A) of which gas (G) exits through a nozzle lip gap (17) of at least one gas nozzle (15) in the direction of the flat steel product (100) in order to blow off the layer (10) to the desired thickness, wherein the zinc alloy of the zinc alloy molten bath (11) has the following composition: - an aluminum content which is in the range between 1.0 weight percent and 3.5 weight percent and preferably in the range between 1.3 weight percent and 2.8 weight percent, - a magnesium content which is in the range between 1.0 weight percent and 3.0 weight percent and preferably in the range between 1.2 weight percent and 2.2 weight percent, and - a remainder comprising Zinc, optionally one or more of Si, Sb, Pb, Ti, Ca, Mn, Sn, Zr, Sr, La,Ce or Bi, selected additional elements, the weight-related content of each additional element being less than 0.1%, and unavoidable impurities, characterized in that at least one of the following adjustable parameters is adjusted as follows: - increasing the bath temperature (TB) of the alloy melt bath (11) if the instantaneous absolute local air humidity (f) decreases, and reducing the bath temperature (TB) of the alloy melt bath (11) if the instantaneous absolute local air humidity (f) increases, and / or - reducing the thickness (d) of the die lip gap (17) if the instantaneous absolute local air humidity (f) decreases, and increasing the thickness (d) of the die lip gap (17) if the instantaneous absolute local air humidity (f) increases, and / or - reducing the distance (Z) between the die lip gap (17) and the side of the flat steel product (100),if the current absolute local air humidity (f) decreases and increasing the distance (Z) between the die lip gap (17) and the side of the flat steel product (100), if the current absolute local air humidity (f) increases, wherein, in addition, the flow rate (D) of the gas (G) is adjusted in order to keep the target thickness of the layer (10) to be applied substantially constant.

2. Method according to claim 1, characterized in that the layer (10) meets the following specification(s): - Target surface area of ​​the layer (10) per strip side, which is in the range of 20 g / m 2 up to 200 g / m 2 , preferably range 30 g / m 2 up to 100 g / m 2, and / or - desired thickness of the layer (10) on each strip side, which lies in the range 3 m to 30 m, preferably in the range 4.5 m to 15 m.

3. Method according to claim 1 or 2, characterized in that the adjustment of the said parameters is carried out for the following range in order to prevent marbling and / or toothpick defects on the layer (10): - absolute local air humidity (f) in the range from 1 g / m³ to 300 g / m³, preferably in the range from 1.08 g / m³ to 51 g / m³.

4. Method according to one of claims 1 to 3, characterized in that a relationship between the current absolute local air humidity (f) and the adjustable parameters is predetermined on the basis of a stripping efficiency factor (AWZ), the following condition having to be met in order to avoid marbling and / or toothpick defects: f > AWZ.

5. Method according to claim 4, characterized in that the stripping efficiency (AWZ) is defined as follows: ^ ^^^ = 24.61 3639 ∙ − 45.42 ^ where: d is the thickness of the nozzle lip gap in mm D is the effective flow rate (volume) D of the gas G per strip side over the strip width (w) in Nm³ / hk is a unitless proportionality factor w is the strip width of the steel compartment product (100) in mm 2b is the half-width of the pressure distribution of the gas G on the strip in mm v is the belt speed in m / min.

6. Method according to claim 4, characterized in that the stripping efficiency (AWZ) is defined as follows: 1 ^ ^ ∙ ^ ∙ ^ ^^,^ ∙ 1,251 ∙ 10 ^ −2 ∙ ln 0.5 6 ∙ 10 ^ −^ ^ ^^^ = ∙ ^ ∙ ^ ( ) + ∙ − 6 ^ ∙ ^ ∙ 25.92 ^ ^ ^ 36^14 ^ ^ ^ 2 ∙ ln ( 0.5 )where: d is the thickness of the nozzle lip gap in mm D is the effective flow rate (quantity) D of the gas G per belt side over the belt width (w) in Nm³ / hk is a unitless proportionality factor w is the belt width of the steel compartment product 100 in mm 2b is the half-width of the pressure distribution of the gas G on the belt in mm v is the belt speed in m / min.

7. Method according to claim 5 or 6, characterized in that when determining the values ​​for the half-width at half maximum (b) and the proportionality factor (k) using the ratio of the distance (Z) to the thickness (d) of the nozzle lip gap (17), the following definitions apply: Case 1.1: ^ ^ ∙ ^ < 5.2 → ^ = 1.9 ^ ^^^ ^ = 1 Case 1.2: 5.2 ≤ ^ ^ < 10 ^ ^ ^ ^ = ^ ^ ∙ 3.22 ∙ 10 ^ ^ ^ ^ ^ → ^ ^ + ^ ^ ∙ 9.78 ∙ 10 ^^ − ^ ^ ∙ 8.39 ∙ 10 ^^ + ^ ^ ∙ 2.72 ∙ 10 ^^ + 1.62^ ∙ ^ ^ ^ ^ 2 → ^ = − ^ ^ ^ ^ ^ ^^ ∙ ^^ ^ ^^ ^ ^^^ 6.05 ∙ 10 + ^ ^^ ∙ 2.2 ∙ 10 − ^ ^^ ∙ 2.89 ∙ 10 + ^ ^^ ∙ 1.55 − 1.9 ^ → ^ = 0.125 ∙ ^ ^^^ ^ = 6.5 ∙ ^ 8. Method according to claim 5 or 6, characterized in that in determining the values ​​for the half-width at half maximum (b) and the proportionality factor (k) using the ratio of the distance (Z) to the thickness (d) of the nozzle lip gap (17) the following simplified definitions apply: ^ ^ Case 2.1: ^ < 7.6 → ^ = 1.9 ∙ ^ ^^^ ^ = 1 ^ ^ Case 2.2: ^ ≥ 7.6 → ^ = 0.125 ∙ ^ ^^^ ^ = 6.5 ∙ ^ 9. Method according to one of claims 1 to 8, characterized in that - the thickness (d) of the nozzle lip gap (17) is in a range between 0.5 mm and 5 mm, preferably between 0.6 mm and 2 mm, particularly preferably between 0.8 mm and 1.5 mm, and / or - the flow rate (D) is in a range from 200 Nm³ per hour to 8000 Nm³ per hour, and / or - the distance (Z) is in a range between 2 mm and 15 mm, preferably between 3 mm and 12 mm, and / or - the belt speed (v) is in a range between 50 m / min and 200 m / min, preferably between 70 m / min and 150 m / min. 10.Method according to one of claims 1 to 9, characterized in that the bath temperature TB of the molten alloy bath (11) is in the range 400 degrees Celsius < TB < 480 degrees Celsius, preferably in the range 409 degrees Celsius < TB < 473 degrees Celsius, and particularly preferably in the range 420 degrees Celsius < TB < 460 degrees Celsius.

11. Method according to one of claims 1 to 10, characterized in that the absolute local air humidity (f) applies in two virtual cylinder volume segments which are delimited on the one hand by a virtual cylinder surface which concentrically or almost concentrically encloses the at least one gas nozzle (15) and on the other hand by two planes running on both sides of the flat steel product parallel to the flat steel product (100) and at a distance (s) from the flat steel product (100), the cylinder volume segments together having a volume in a range of 1m. 3 up to 10 m 3 and preferably a volume of less than 2 m3 wherein the measurement of the absolute local humidity (f) is carried out directly or indirectly.

12. The method according to claim 11, characterized in that the absolute local humidity (f) is measured continuously or from time to time, and if the absolute local humidity (f) is too low, the method is interrupted in order to adjust one or more of the adjustable parameters.

13. The method according to one of claims 1 to 12, characterized in that the application of the layer (10) is controlled by a controller (250) such that, if the absolute local humidity (f) changes, one or more of the adjustable parameters are automatically and / or manually adjusted depending on the current absolute local humidity (f).

14. A device (150) for applying a layer (10) to a flat steel product (100), comprising - a zinc alloy molten bath (11) with an inlet side (E), an outlet side (A), and a deflection (13),in order to guide the flat steel product (100) coming from the inlet side (E) at a strip speed (v) through the zinc alloy melt bath (11) to the outlet side (A), - a wiping nozzle device (14) which comprises at least one nozzle lip gap (17) and which is arranged in the region of the outlet side (A) such that the still liquid layer (10) on the flat steel product (100) can be blown off with gas (G) which exits through the nozzle lip gap (17) in order to blow off the layer (10) to a desired thickness, wherein the zinc alloy of the zinc alloy melt bath (11) has the following composition: - an aluminum content which is in the range between 1.0 percent by weight and 3.5 percent by weight and preferably in the range between 1.3 percent by weight and 2.8 percent by weight, - a magnesium content which is in the range between 1.0 percent by weight and 3.0 percent by weight and preferably in the range between 1.2 percent by weight and 2.2 percent by weight,and - a residue comprising zinc, optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, Zr, Sr, La, Ce or Bi, wherein the, weight-related content of each additional element is less than 0.1%, and unavoidable impurities, characterized in that the device (150) is set up or designed to carry out at least one of the following adjustments manually or automatically: - increasing the bath temperature (TB) of the alloy melt bath (11) if the instantaneous absolute local air humidity (f) decreases and reducing the bath temperature (TB) of the alloy melt bath (11) if the instantaneous absolute local air humidity (f) increases, and / or - reducing the thickness (d) of the die lip gap (17) if the instantaneous absolute local air humidity (f) decreases and increasing the thickness (d) of the die lip gap (17) if the instantaneous absolute local air humidity (f) increases, and / or - reducing the distance (Z) between the die lip gap (17) and the side of the flat steel product (100),if the current absolute local air humidity (f) decreases, and increasing the distance (Z) between the die lip gap (17) and the side of the flat steel product (100) if the current absolute local air humidity (f) increases, wherein additionally the flow rate (D) of the gas (G) is automatically adjusted in order to keep the target thickness of the layer (10) to be applied substantially constant.

15. Device (150) according to claim 14, characterized in that the layer (10) to be applied meets the following specification: - Target surface area of ​​the layer (10) per strip side, which is in the range of 20 g / m, 2 up to 200 g / m 2 , preferably range 30 g / m 2 up to 100 g / m 2, and / or - target thickness of the layer (10) per strip side, which is in the range of 3 m to 30 m, preferably 4.5 m to 15 m.

16. Device (150) according to claim 14 or 15, characterized in that the adjustment of the adjustable parameters can be carried out for the following range in order to prevent marbling and / or toothpick defects on the layer (10): - absolute local air humidity (f) in the range of 1 g / m³ to 300 g / m³, preferably in the range of 1.08 g / m³ to 51 g / m³.

17. Device (150) according to one of claims 14 to 16, characterized in that it additionally comprises: - at least one humidity sensor (51) arranged in the local environment of the wiping nozzle device (14), - a controller (250) which can be connected to the at least one humidity sensor (51) by means of communication technology in order to continuously or from time to time obtain a measured value or signal from which a physical quantity can be derived which is directly related to the absolute local air humidity (f) in a local nozzle environment, wherein the controller (250) is designed to carry out at least one of the following steps or to trigger the implementation thereof: - changing the bath temperature (TB) of the alloy melt bath (11), and / or - changing the thickness (d) of the nozzle lip gap (17),and / or - changing the distance (Z) between the die lip gap (17) and the side of the flat steel product (100), - adjusting the flow rate (D) of the gas (G) in order to keep the target thickness of the layer (10) to be applied substantially constant.

18. Device (150) according to one of claims 14 to 16, characterized in that it comprises a controller (250) designed to adjust one or more of the adjustable parameters as a function of the current absolute local air humidity (f) or to trigger the adjustment thereof.

19. Device (150) according to one of claims 14 to 16, characterized in that it comprises a controller (250) which is designed to determine a stripping efficiency factor (AWZ), wherein the stripping efficiency factor (AWZ) defines a relationship between the current absolute local air humidity (f) and the adjustable parameters, and wherein the following condition must be met to avoid marbling: f > AWZ.

20. Device according to claim 18 or 19, characterized in that it comprises a motor or actuator (M) for each gas nozzle (15), which is connected to the controller (250) for control purposes in such a way that the controller (250) reacts to a changing absolute local air humidity (f) by reducing the distance (Z) if the current absolute local air humidity (f) decreases and reacts by increasing the distance (Z) if the current absolute local air humidity (f) increases.

21. Device according to claim 18 or 19, characterized in that it comprises a pump device (Pg) and a control loop for each gas nozzle (15) in order to automatically adjust the flow rate (D) of the gas (G) such that the target thickness of the layer (10) remains substantially constant.Device according to claim 18 or 19, characterized in that it comprises a bath heater (30) which is connected to the control system (250) in such a way that the control system (250) reacts to a changing absolute local air humidity (f) by adjusting the bath temperature TB, the bath temperature TB being in the range 400 degrees Celsius < TB < 480 degrees Celsius, preferably in the range 409 degrees Celsius < TB < 473 degrees Celsius, and particularly preferably in the range 420 degrees Celsius < TB < 460 degrees Celsius.

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