METHOD AND DEVICE FOR APPLYING A LAYER ONTO A FLAT STEEL PRODUCT

Description

The present invention relates to a method by which flat steel products can be coated with a zinc-aluminum-magnesium (ZnAlMg) based layer, e.g. as a protective coating. It also relates to a device configured to apply the method according to the invention.

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 placing the flat steel product 100 coming from a furnace into a zinc alloy melting bath 11, as shown in FIG. 1 using an exemplary device 150. In order to protect the flat steel product 100 from oxidation, it is typically introduced into the bath 11 on the inlet side E through a trunk 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 exit side A. As it emerges from this bath 11, the molten alloy film adhering to the front and rear sides of the flat steel product 100 is stripped to the target thickness (in the micrometer range) or to the target surface coating mass (in g/m²) by a gas jet from the gas nozzles 15 of a stripping nozzle device and the flat steel product 100 is then transferred to a cooling area 16. This continuous method is generally referred to as hot-dip coating.

Details of a suitable method and particularly suitable alloy compositions can be found, for example, in the published application WO 2014/033153 A1 of the applicant VOESTALPINE STAHL GMBH.

There is prior art that mentions water or water vapor in connection with the hot-dip coating of flat steel products. The corresponding documents are listed below and their content is briefly described where of interest here.

Armco Inc. patent EP0172682B1, filed in 1985, deals with the control of zinc vapor in connection with the hot-dip coating of an iron-based metal strip. An oxygen-reduced atmosphere containing a small amount of water vapor is provided in an enclosed area (comparable to the trunk 12 in FIG. 1) on the inlet side of the immersion bath. This small amount of water is intended to prevent the formation of zinc vapor on the surface of the immersion bath. The dew point of the gas used on the inlet side is set so that zinc vapor cannot form.

In the published patent application JP2020100886 A2 of the company Nippon, the objective is to produce a galvanized steel strip that has a surface with an increased coefficient of friction. In order to increase the coefficient of friction, water is sprayed under pressure onto the surface of the flat steel product after gas blowing. The particle size of the water droplets should be at least 0.07 mm and preferably more than 1.5 mm. The spraying of water droplets deliberately creates irregularities 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 from the present invention.

In addition to pure protection against corrosion, there are ever more stringent requirements in terms of the surface quality of zinc-coated flat steel products. The automotive industry in particular expects products that meet the highest surface requirements. However, the provision of homogeneous surfaces is not trivial.

The main problems here are often surface defects of the ZnAlMg layer. For example, a marbling (marble-effect), the “toothpick” or “beach pattern” defect can form on the ZnAlMg layer or slag formation can occur. There are patents (e.g. EP20130826634 AM/J.M.Mataigne; JP20080256208 NSSMC/Oohashi et al.) that attempt to eliminate similar surface defects (gloss effects or displaced oxide skins) by other means (reduction of the O₂ content in the surroundings of the wiper nozzle).

The task is therefore to provide a method and a corresponding device to coat flat steel products that have a particularly durable and robust protective effect in terms of corrosion, wherein the surface of the protective coating should be particularly homogeneous, very smooth and without marbling (without “marble-effect”) and/or toothpick defects (without “toothpick”). The aim is to achieve a surface quality that meets the highest customer requirements.

In addition, this method should be as energy-efficient, cost-effective, simple and reproducible as possible.

SUMMARY OF THE INVENTION

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

All embodiments involve the application of a (protective) layer to a flat steel product, the layer thickness of this layer being intended to correspond to a target thickness (according to a corresponding specification). This layer is produced by passing the flat steel product through a zinc alloy melt bath and blowing it off with gas on the exit side of the bath by means of a stripping nozzle device comprising at least one gas nozzle. 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 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 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.

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 nozzle lip gap if the current absolute local air humidity is reduced and vice versa, and/or
  • reducing the distance between the nozzle 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 adjusted in order to keep the target thickness of the layer to be applied essentially constant.

A device configured to apply a coating to a flat steel product comprises

• • a zinc alloy melt bath having an inlet side, an exit side and a deflector for guiding the flat steel product coming from the inlet side through the zinc alloy melt bath to the exit side at a strip speed,
  • a stripping nozzle device which comprises at least one nozzle lip gap and which is arranged in the area of the exit side in such a way that the still liquid layer on the flat steel product can be blown off with gas which emerges through the nozzle lip gap in order to blow off the layer to a target 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 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 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.

This device is adapted to or configured to make 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 nozzle lip gap if the current absolute local air humidity is reduced and vice versa, and/or
  • reducing the distance between the nozzle 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 essentially constant.

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 percent by weight) 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;
  • 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, the weight related content of each additional element in the metallic coating being less than 0.1%, and unavoidable impurities.

In some embodiments, the aluminum content (in percent by weight) may be greater than or equal to the magnesium content (in percent by weight).

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

By the combination of a precisely defined ZnAlMg alloying concept, monitoring or observation of the current absolute local air humidity and a targeted adjustment of the stripping process (or the corresponding method and/or system parameters or the stripping efficiency) a surface that shows no or negligible marbling can be produced.

In all embodiments, 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 exit side and the cooling area (if present), are in the range from 1 g/m³ to 300 g/m³, preferably in the range from 1.08 g/m³ to 51 g/m³, i.e. the method can be successfully carried out at an absolute local air humidity which is in the said range.

In all embodiments, the stripping nozzle device can optionally be followed by a strip stabilization device, which serves to automatically stabilize the movement of the flat steel product. In these embodiments, the values of the absolute local air humidity in the area between the stripping nozzle device and the strip stabilization device can be determined.

The method preferably comprises the following steps at an absolute local air humidity greater than 1 g/m³:

• • operating the alloy melt bath with 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 stripping 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 exit side of the alloy melt bath with the gas flowing through the nozzle lip gap in the direction of the flat steel product with a gas flow rate which is in the range of 200 to 8000 Nm³ per hour.

The ZnAlMg alloying concept defined above is based on numerous investigations and calculations. Within the specified limits of the ZnAlMg alloying concept, the technical teaching presented here has proved particularly successful.

In at least some of the embodiments, the absolute local air humidity is permanently measured.

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

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

Preferably, the method is carried out in all embodiments with a reduced bath temperature TB_red, which is in the range 420<TB_red<460 degrees Celsius. In this range, the formation of slag can also be reduced.

Specific tests have shown that the ZnAlMg alloying 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 efficiency).

The development of the new method, the particularly suitable ZnAlMg alloying concept and the targeted adjustment of the (method) parameters (or the corresponding stripping efficiency) is based on theoretical considerations, various simulations of stripping processes and numerous experiments.

The processes in the area of the stripping nozzle device and at the flat steel product are highly complex and depend on numerous (method) parameters and influencing variables (or the corresponding stripping efficiency). Therefore, the present method and device rely on some simplified assumptions and definitions in order to obtain reproducible results.

In all embodiments, the coated flat steel product can, as usual, be subjected to a dressing process or re-rolling process and/or a bending-stretch-straightening process after coating. Preferably, the overall degree of deformation for the coated flat steel product is between 0.5% and 2.5%, preferably between 0.7% and 1.7%. The dressing process or re-rolling step (with a deformation in the range of 0.7-1.7%) can further reduce the negative influence of marbling.

In all embodiments, 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 information sheet “Charakteristische Merkmale 095—Schmelztauchveredeltes Band und Blech”, 2010 edition, published by the Steel Information Center 40039 Düsseldorf.

Further advantageous embodiments of the invention form the objects of the dependent claims.

DRAWINGS

Embodiments of the invention are described in more detail below with reference to the drawings.

FIG. 1 shows a highly schematized representation of a known device for dip coating and stripping flat steel products (state of the art);

FIG. 2 shows a highly schematized representation of a first exemplary device in which the method of the invention is used;

FIG. 3A shows a highly schematized side view of a stripping nozzle device with only one nozzle in order to be able to define some of the method and system parameters relevant here (this illustration is not to scale);

FIG. 3B shows a schematic representation of the pressure curve P at the surface of the coating in relation to a position on the x-axis;

FIG. 3C shows a schematic representation of the shear force curve t at the surface of the coating in relation to a position on the x-axis;

FIG. 4 shows a summarizing graphical representation of numerous exemplary tests, with the stripping efficiency AWZ plotted on the ordinate axis and the absolute air humidity f on the abscissa;

FIG. 5 shows a highly schematized representation of a device in which the method of the invention is used, whereby two virtual cylinder volume segments are shown;

FIG. 6 shows a highly schematized representation of a further device of the invention in which the method of the invention is used;

FIG. 7A shows a highly schematized representation of a strip side of a flat steel product that has no marbling;

FIG. 7B shows a highly schematized representation of a strip side of a flat steel product that has a medium marbling;

FIG. 7C shows a highly schematized representation of a strip side of a flat steel product that has a 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. 8A shows details of Table 2 with test results;

FIG. 8B shows details of Table 2 with further test results;

FIG. 9 shows exemplary steps of the method described here.

DETAILED DESCRIPTION

The present invention relates to 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 on the upper side of the strip). This layer 10 is produced by passing the flat steel product 100 from an input side E to an output side A through a zinc alloy melt bath 11 and blowing it off with gas G on the exit side A by means of a stripping nozzle device 14, as shown by way of example in FIGS. 2, 3A, 6 and 7. The purpose of the stripping nozzle device 14 is to strip off the excess (still liquid) ZnMgAl layer (layer 10) as it exits the bath 11.

In the context of the invention, care must be taken to ensure that, on the one hand, the layer 10 does not essentially change, even if the absolute air humidity f of the environment changes, and that, on the other hand, no marbling occurs. In other words, the aim is to avoid marbling when the absolute local air humidity f changes, while at the same time substantially maintaining the target thickness of the layer 10.

In addition to the target thickness of the layer 10, the target (surface) coating mass of the layer 10 can also be specified in all embodiments. Typically, there is a narrowly specified 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 fulfills the specifications.

The processes in the area of the stripping nozzle device 14 and at the flat steel product 100 are highly complex and depend on numerous parameters and influencing variables.

In all embodiments, the 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 facing each other (if both sides of the strip are to be blown off). FIGS. 2, 6 and 7 show embodiments with two nozzles 15 and FIG. 3A shows an embodiment with only one nozzle 15.

In at least some of the embodiments, the method is performed and controlled such that the layer 10 per strip side of the flat steel product 100 has a target thickness that is within the tolerance window. Preferably, in all embodiments, the target thickness of the layer 10 per side of the strip is in the range of 3 to 30 μm, and particularly preferably in the range of 4.5 to 15 μm.

Preferably, in at least some of the embodiments, the target surface coating (coating mass per side of the strip; referred to as coating per side in Table 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 order to be able to reliably apply such a layer 10 which essentially corresponds to the target 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, the weight related content of each additional element in the metallic coating being less than 0.1%, and unavoidable impurities.

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 alloying concept:

• • the aluminum content (in percent by weight) is in the range between 2.09 and 2.66 percent by weight;
  • the magnesium content (in percent by weight) is in the range between 1.45 and 2.24 percent by weight;
  • 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, the weight related content of each additional element in the metallic coating being less than 0.1%, and unavoidable impurities.

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

In order to prevent the formation of marbling or to significantly reduce marbling, the absolute local air humidity f is determined continuously or from time to time (e.g. by direct or indirect measurement). This statement also applies to the prevention or reduction of toothpick formation.

Absolute air humidity f is a physical quantity that can be expressed, for example, in the unit g/m³. In other words, it is the mass of gaseous water in a standardized volume body having a volume of 1 m³. In other words, the absolute air humidity f indicates the content of water vapor in a volume body. f is used here as the formula symbol for absolute air humidity. In all embodiments, the absolute air humidity f can be estimated approximately from the air temperature TL and the relative air humidity r, whereby the following applies:

f = 13.235 · r T ⁢ L + 273.15 · 10 7.5 · TL T ⁢ L + 237.3

r relative air humidity in %
TL air temperature in ° C.

The absolute local humidity f can be measured directly or indirectly in all embodiments. Indirect measurement is understood here to mean, among other things, measuring the air temperature TL and the relative air humidity r and calculating/deriving the absolute local humidity f.

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 an area between the exit side A and the cooling area 16 (if present), are in the range of 1 g/m³ and less than 300 g/m³ in all embodiments. Preferably, the absolute local air humidity is in the range of 1.08 g/m³ to 51 g/m³ in all embodiments.

With an absolute local air humidity f greater than 1 g/m³ and less than 300 g/m³, the method enables the controlled application of the layer 10 to at least one side of the flat steel product 100 by selectively adjusting adjustable parameters (method 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 (still) essentially corresponds to the target thickness. This means, care is taken to ensure that a layer is still applied which corresponds to the target thickness (within tolerances) and which at the same time shows no or only very slight marbling

The adjustable parameters (method 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 nozzle lip gap 17 if the current absolute local air humidity f is reduced and vice versa, and/or
  • reducing the distance Z between the nozzle 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 additionally the flow rate D of the gas G is adjusted (automatically, for example by control) in order to keep the target thickness of the layer 10 to be applied essentially constant.

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, wherein care is also taken here to ensure that the target thickness of the layer 10 to be applied remains essentially constant.

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

A so-called target specification of the layer 10 to be applied can stipulate for all embodiments that the following specification(s) must be fulfilled:

• • coating layer of the layer 10 per side of the strip should be in the range 20 to 200 g/m², preferably in the range 30 to 100 g/m², and/or
  • target thickness of the layer (10) per side of the strip lying in the range of 3 to 30 μm, preferably in the range of 4.5 to 15 μm.

The bath temperature TB of the alloy melt bath 11 has an influence on the viscosity of the melt during the stripping process. An increased bath temperature TB leads to a reduced viscosity of the melt. If the adjustable parameters (method and system parameters) remain the same, more material would be stripped than desired. Therefore, when the bath temperature TB is increased, the other adjustable parameters (method and system parameters) are changed so that the layer 10 still has the target thickness. When increasing the bath temperature TB, for example, the flow rate D of the gas G is reduced to reduce the stripping effect in order to still achieve the same target thickness.

In all embodiments, the bath temperature TB of the alloy melt bath 11 is preferably 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.

For the operation of the alloy melting bath 11, a bath temperature TB in the specified temperature range is specified in all embodiments. Maintaining this temperature window (temperature range) is important, as more undesirable slag can form on the flat steel product 100 when working above the specified range.

The bath temperature TB can be adjusted in all embodiments, for example by means of an inductive heating device 30 (see FIGS. 2 and 6) or a resistance heater.

To avoid slag formation, the bath temperature TB is preferably reduced in all embodiments. The reduced bath temperature TB_red is preferably in the already mentioned range 420<TB_red<460 degrees Celsius.

When carrying out the method, in all embodiments it is preferably paid attention that

• • the thickness d of the nozzle 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 strip speed (v) is in a range between 50 and 200 m/min, preferably between 70 and 150 m/min.

The method functions particularly reliably within these ranges.

In all embodiments, a corresponding gas nozzle 15 has a longitudinal extension 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.

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.

In at least some of the embodiments, the absolute local air 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, for example to make adjustments to the adjustable parameters.

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 adjusted manually.

In at least some of the embodiments, the absolute local air humidity f is measured directly or indirectly.

The measurement of the absolute local air humidity f is not carried out directly at the impact line where the gas G hits the layer 10 to be stripped, since the gas mixture is relatively “dry” there (i.e. comprises little air humidity). In all embodiments, the absolute local air humidity f is preferably measured directly or indirectly in an area that is at least a normal (right-angled) distance of 20 cm from the impact line or from the flat steel product 100.

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

In all embodiments, sensors of the following design or mode of operation can be used as humidity sensors 51:

• • mechanically operating measuring sensors based on the expansion or contraction of (usually organic) measuring elements caused by humidity;
  • psychrometrically operating measuring sensors, whereby two identical, very precise thermometers are used, along which the gas flow to be measured is guided at a defined speed;
  • capacitive measuring sensors, e.g. comprising a humidity-sensitive capacitor with two flat electrodes;
  • dew point mirror hygrometer, which determines the air humidity with dew point mirrors, at which the condensation of water vapor is evaluated if the dew point falls below;
  • resistive measuring method in which, for example, the impedance of the alternating current resistance of a hygroscopic element is determined;
  • spectrometric measurement methods that, for example, measure the gaseous water content in the near or mid-infrared range (NIR or MIR) without contact.

All embodiments of the device 150 may comprise a controller 250. In all embodiments, this controller 250 may be designed as a computer supported automation and control unit and may comprise a human-machine interface, a computer and a database.

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

In these embodiments, the adjustment/change of the adjustable parameters (system or method parameters), or the stripping efficiency AWZ, can then be carried out by the overall system controller and/or by the controller 250.

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

In all embodiments, the current air humidity can be measured indirectly, for example, by determining the transmission rate of an optical path using a type of light barrier. The transmission rate is high in very dry air. The presence of gaseous water, on the other hand, hinders the transmission of light through the optical path and the transmission rate is lower.

In all embodiments, the indirect determination can be made 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 FIGS. 7A, 7B and 7C). A corresponding measurement of the surface property (ies) can, for example, be made optically before the cooling region 16 or after the cooling region 16 (e.g., by optically measuring the reflectivity of the surface of the layer 10). FIGS. 7D to 7G show exemplary photographs of coated flat steel products 100.

Preferably, an inert gas is used as gas G in all embodiments. Nitrogen or a nitrogen-containing gas mixture has proven to be particularly effective.

FIG. 3A shows a highly schematized side view of another stripping nozzle device 14 in order to be able to define the adjustable parameters (system and method parameters), or the stripping efficiency AWZ, more precisely. 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.

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, as well as the thickness d of the nozzle lip gap 17. The nozzle lip gap 17 serves as gas exit gap of the stripping nozzle device 14.

FIG. 3B shows a schematic representation of the gas pressure curve P that ensues along the front side of the flat steel product 100. The pressure P depends on the position on the x-axis. Ideally, the pressure curve P has the shape of a Gaussian curve, as indicated in FIG. 3B. This Gaussian curve can be used to determine the half-width at the pressure P_s/2, as shown, wherein P_s 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”, C. V. Tu et al., Experimental Thermal and Fluid Science 1996, 16, pages 364-373, Elsevier Science Inc.

The gas jet emerging from the nozzle 14, together with the force of gravity (if the flat steel product 100 is pulled vertically upwards out of 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 in relation 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 that acts 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 at the layer 10 to be stripped.

Tests have shown that the adjustable system and/or method parameters, or the stripping efficiency AWZ, can be adjusted within certain limits without significantly changing the target thickness and/or the surface weight (coating per strip side) of the layer 10.

It has also been shown that (in addition to the absolute local air humidity f), two variables that characterize the shear force profile that the stripping nozzle 15 generates on the strip 100 are decisive for the formation of the marbling. One is the maximum shear force τ_max and the other is the time t in which the strip-shaped flat steel product 100 passes the distance l between the shear force maxima (see FIG. 3C). The time t again depends on the strip speed v.

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

t = 120 v · - b 2 2 · ln ⁢ ( 0.5 ) ( 1 )

In all embodiments, the strip speed v is preferably in the range of 50 m/min to 200 m/min and particularly preferably between 70 and 150 m/min.

The equations describing the dynamic flow behavior of the gas G at the flat steel product 100 are very complex. This is due, among other things, to the fact that areas with laminar and turbulent flow patterns form on the layer 10 of the flat steel product 100 in the gas jet that exits through the nozzle lip gap 17 of the nozzle 15. In addition, the gas jet draws in ambient air, which is swirled with the gas G. Details can be found, for example, in the already mentioned publication “Wall Pressure and Shear Stress Measurements Beneath an Impinging Jet”.

Complex tests and statistical evaluations of the measured results did not provide any directly usable results with regard to a correlation between the stripping nozzle parameters and marbling due to the complex interrelationships. Only after additional systematic investigation of various internal and external influencing variables did a correlation emerge between the degree of marbling on layer 10, the stripping nozzle parameters and the environmental conditions of the test setup. Air humidity in particular shows an influence on the marbling on layer 10.

Further targeted investigations and the graphical processing of the results of these tests showed for the first time a correlation between the air humidity, the stripping nozzle parameters and the tendency to marbling. FIG. 4 shows a summarizing graphical representation of numerous tests, with the stripping efficiency AWZ (as a summary or generic term for the method and device parameters or stripping nozzle parameters for short) plotted on the ordinate axis and the absolute air humidity f in g/m³ on the abscissa.

By weighting and adding the two variables τ_max and τ, a so-called stripping efficiency AWZ can be determined, which can be compared directly with the absolute local air humidity ƒ, as follows (inequality (2.1)):

f > τ max + 500 · t - 636 1 ⁢ 4 ( 2.1 )

The right-hand side of the inequality (2.1) corresponds to the stripping efficiency AWZ, i.e., the following relation applies (2.2):

f > AWZ ( 2.2 )

Stripping efficiency AWZ summarizes the following method and device parameters:

• • the effective flow rate (quantity) D of the gas G over the strip width w per strip side,
  • the proportionality factor k,
  • the strip width w of the flat steel product 100,
  • the thickness d of the nozzle 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.

Further details on the stripping efficiency 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 AWZ. Before describing inequality (3), we refer further to FIG. 4.

The gray or black-filled symbols (squares, diamonds, triangles or circles) in FIG. 4 represent flat steel products 100 in which marbling has clearly formed on the surface of layer 10. The marbling is particularly strong in the black-filled symbols. The gray-filled symbols represent less pronounced marbling. The unfilled symbols, on the other hand, represent no or negligible marbling. Circular symbols stand for tests that were carried out with a nozzle lip gap thickness of d=0.8 mm, square symbols stand for tests with a nozzle lip gap thickness of d=0.9 mm, diamond-shaped symbols stand for a thickness of the nozzle lip gap of d=1 mm and triangular symbols stand for d=1.2 mm and upside-down triangular symbols stand for d=1.4 mm.

A straight line Ge was inserted into the graph in FIG. 4 as a dividing line in order to separate, as a first approximation, those tests with clear or medium marbling from those tests that show no or only negligible marbling. In the tests that lie to the right below the straight line Ge, there is no or only negligible marbling (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 efficiency AWZ (see also inequalities (2.1) and (3)).

A detailed evaluation of the test results suggests that the above-mentioned stripping efficiency AWZ can be adjusted as a function of the absolute local air humidity f in order to avoid the occurrence of undesirable marbling. In all embodiments, this adjustment of the stripping efficiency AWZ is preferably carried out in such a way that the target thickness of the layer 10, or the surface coating (mass) of this layer 10, does not change or hardly changes at all. This means, when changing the stripping efficiency AWZ, care is always taken to ensure that the layer 10 to be applied essentially has the target thickness.

However, it is important to ensure that when adjusting the stripping efficiency AWZ, in addition to avoiding marbling, other defects such as toothpick defects, beach pattern defects and so on are avoided or the formation of slag is reduced. For this reason, parameter ranges are specified here that have proven to be particularly effective.

As mentioned, the straight line Ge shown in FIG. 4 is 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/m³ and the stripping efficiency AWZ:

	d	Thickness of the nozzle lip gap	[mm]
	D	Gas flow rate per strip side	[Nm3/h]
	w	Width of the flat steel product 100	[mm]
	v	Strip speed	[m/min]
	2b	Half-width	[mm]
	k	Proportionality factor

f > 1 1 ⁢ 4 · ( D 2 · k · e - 0.5 · 1.251 · 10 4 w 2 · d 2 · 25.92 · - 2 · ln ⁡ ( 0.5 ) b 2 + 6 · 10 4 v · - b 2 2 · ln ⁡ ( 0.5 ) - 6 ⁢ 3 ⁢ 6 ) ( 3 )

Two exemplary approaches are described below, which serve to simplify the inequality (3):

1st approach: According to a 1st approach, a subdivision is made into three different curve areas (referred to here as cases 1.1 to 1.3). With such a subdivision, the right-hand side of the inequality (3) can be calculated in a simplified manner.

(Case 1.1). In a linear area, 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.

Z d < 5.2 → b = 1.9 · d 2 ⁢ and ⁢ ⁢ k = 1 Case 1.1

(Case 1.2) In a further area starting at approx. Z/d=5.2 and ending before Z/d=10, the following relations apply:

5.2 ≤ Z d < 10 → b = [ - ( Z d ) 4 · 3.22 · 10 - 4 + ( Z d ) 3 · 9.78 · 10 - 3 - ( Z d ) 2 · 8.39 · 10 - 2 + ( Z d ) · 2.72 · 10 - 1 + 1.62 ] · d 2 → k = - ( Z d ) 4 · 6.05 · 10 - 4 + ( Z d ) 3 · 2.2 · 10 - 2 - ( Z d ) 2 · 2.89 · 10 - 1 + ( Z d ) · 1.55 - 1.9 Case 1.2

(Case 1.3) In a further area starting at approx. Z/d=10, the following relations apply:

Z d ≥ 1 ⁢ 0 → b = 0.125 · Z ⁢ and ⁢ k = 6.5 · d Z Case 1.3

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

The differentiation between the three areas or cases mentioned still leads to complicated mathematical formulas, especially in the 2nd area (case 1.2).

2nd approach: Therefore, a 2nd approach is now described here, which serves to make the complicated inequality (3) easier to handle in practice. In this 2nd approach, a case differentiation is made with only two cases (cases 2.1 and 2.2), whereby the ratio Z/d is also used here for the case differentiation.

Z d < 7.6 → b = 1.9 · d 2 ⁢ and ⁢ k = 1 Case 2.1 Z d ≥ 7.6 → b = 0.125 · Z ⁢ and ⁢ k = 6.5 · d Z Case 2.2

The 2nd approach has the disadvantage that the separation between flat steel products 100 without marbling and flat steel products 100 with marbling is not entirely clear or unambiguous in the border area. The best results can be achieved if a case differentiation according to the 1st approach using the inequality (3) and/or the inequality (2.1) is/are evaluated (e.g. processed numerically by the controller 250).

In all embodiments or in at least some of the embodiments, the adjustment/adaptation of the adjustable parameters (system and/or method parameters), or the stripping efficiency AWZ, can now be carried out either using the inequality(ies) (2.1) and/or (3) using the 1st or 2nd approach. Or, as already mentioned, a numerical representation of the inequality(ies) (2.1) and/or (3) may also be used in the controller 250.

The adjustment/adaptation of the adjustable parameters (system and/or method parameters), or the stripping efficiency 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 predetermined tolerance limits, as defined, for example, by a target specification.

In all embodiments, the target specification can be predetermined by the manufacturer and/or customer or client.

Preferably, either inequality(ies) (2.1) and/or (3) or the formulae of the 1st approach or the formulae 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 method parameters) or the stripping efficiencies AWZ are provided in one or more tables. Using a “lookup” table, the controller 250 can then call up the correspondingly suitable adjustable parameters (system and/or method parameters) or stripping efficiencies AWZ for a currently valid absolute local humidity value f and adjust the device 150, or provide the machine operator with a numerical value for adjusting the thickness d of the nozzle lip gap (for example showing it on a display).

In all embodiments, adjustable parameters (system and/or method parameters) or stripping efficiencies 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 in partial ranges, as the respective maximum and minimum values originate from different tests. Only the respective maximum and minimum values were taken from Table 2 (FIG. 8A, 8B) and summarized here.

The nozzle height listed in Table 2 (FIG. 8A, 8B) is the vertical distance between the zinc bath level and the impact line 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.

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

	TABLE 1
	Parameter	Lower limit	Upper limit

	TB [° C.]	409	473
	Z [mm]	2.5	14.1
	d [mm]	0.8	1.2
	D [Nm3/h]	410	1775
	k	0.55	1.0
	w [mm]	1159	1614
	b [mm]	0.76	1.76
	v [m/min]	70	150

Table 2, which is divided into two parts (see FIGS. 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 (method and/or system parameters), or stripping efficiencies AWZ (as exemplary shown in FIG. 7A). The values in Table 2 of FIG. 8A were sorted according to ascending b.

Table 2 of FIG. 8B, on the other hand, shows tests in which layers 10 with medium (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 by ascending b.

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 that were produced with these adjustable parameters (method and system parameters) or stripping efficiencies AWZ lie below the straight line Ge in FIG. 4.

Excerpt from Table 2 (FIG. 8A)

	Absolute	Stripping
	air humidity (f)	efficiency
	g/m3	(AWZ)	Evaluation of the layer 10

	3.33	−8.59	no marbling
	4.2	−6.27	no marbling
	3.13	−5.68	no marbling
	3.33	−4.16	no marbling
	4.52	−3.76	no marbling
	2.93	−2.25	no marbling
	2.93	−0.60	no marbling
	13.14	−0.46	no marbling
	13.22	2.23	no marbling
	5.99	2.53	no marbling
	6.93	3.10	no marbling
	15.54	3.81	no marbling
	9.29	4.43	no marbling
	5.22	4.70	no marbling
	6.86	4.89	no marbling
	9.13	5.71	no marbling
	11.08	5.87	no marbling
	13.47	6.48	no marbling
	6.97	6.89	no marbling
	17.67	8.22	no marbling
	9.52	8.30	no marbling
	15.34	9.26	no marbling
	11.99	9.26	no marbling
	20.4	10.81	no marbling
	13.47	12.03	no marbling

The following pairs of values for the absolute air humidity f and the stripping efficiency AWZ can be extracted from Table 2 of FIG. 8B (sorted by ascending stripping efficiency AWZ), whereby all layers 10 that were produced with these adjustable parameters (method and system parameters) or stripping efficiencies AWZ lie above the straight line Ge in FIG. 4.

Excerpt from Table 2 (FIG. 8B)

	Absolute	Stripping
	air humidity (f)	efficiency
	g/m3	(AWZ)	Evaluation of the layer 10

	3.17	3.80	medium marbling
	3.76	6.43	medium marbling
	2.71	7.21	medium marbling
	5.95	8.14	medium marbling
	5.14	8.26	strong marbling
	3.76	9.10	strong marbling
	3.13	9.24	strong marbling
	5.99	9.56	strong marbling
	7.83	10.02	strong marbling
	8.96	10.25	medium marbling
	4.09	12.15	strong marbling
	7.83	12.51	strong marbling
	5.99	13.28	strong marbling
	11.47	13.36	medium marbling
	8.36	13.50	strong marbling
	3.87	14.58	strong marbling
	9.12	15.19	medium marbling
	4.21	16.43	strong marbling
	3.17	17.98	strong marbling
	3.17	20.02	strong marbling
	13.67	21.25	medium marbling

If marbling defects occur under a given absolute local air humidity f and under given method conditions, then in at least some of the embodiments for eliminating the marbling, the stripping efficiency AWZ (corresponds to the right-hand side of the inequality (2.1) and (3)) is reduced until this inequality is satisfied again.

Usually, it is not permissible to change the coating layer of the flat steel product 100 produced, as this is stipulated in the product specification. Provided that the thickness of the coating layer is kept constant, the reduction in the stripping coefficient AWZ can be brought about in various ways.

Three specific examples are used below to exemplary illustrate possible ways of changing the stripping coefficient AWZ.

Example 1

Reducing the Stripping Efficiency AWZ by Reducing the Nozzle Spacing Z (see Table 3).

Table 3 below shows two test examples 1.1 and 1.2, which demonstrate a reduction in the stripping efficiency AWZ by reducing the nozzle spacing Z from 10 mm to 8 mm.

	TABLE 3
				Nozzle			Gas flow
	Coating	Strip	Strip	lip	Nozzle		rate per
	per	width	speed	gap	spacing	Nozzle	side	Bath			Stripping
	side	(w)	(v)	(d)	(Z)	pressure	(D)	temperature			efficiency
	g/m2	mm	m/min	mm	mm	mbar	Nm3/h	° C.	b	k	(AWZ)

1.1	36.5	1315	100	1.0	10	539	1691	438	1.25	0.65	21.3
1.2	36.5	1315	100	1.0	8	390	1386	439	1.06	0.79	13.5

For the initial state (Table 3-Example 1.1) with a gas flow rate D of 1691 Nm³/h, a stripe width w of 1315 mm, a nozzle lip gap d of 1.0 mm, a nozzle spacing Z of 10 mm and a strip speed v of 100 m/min, the AWZ is calculated to be 21.3 according to inequality (3).

Based on Example 1.1, the nozzle spacing Z was reduced from 10 mm to 8 mm in order to reduce the stripping efficiency AWZ, whereby the strip width w, the nozzle lip gap d, the strip speed v, the bath temperature TB and the layer coating per side were left 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 the nozzle pressure and thus the gas flow rate per side D had to be reduced from 1691 to 1386 Nm³/h as a result of the change in the nozzle spacing Z in order to keep the thickness of layer 10 essentially constant.

If one now calculates the stripping efficiency AWZ with these new process parameters (Table 3, Example 1.2), the result is a value of 13.5. By reducing the nozzle spacing from 10 mm to 8 mm, the stripping efficiency AWZ could thus be reduced from 21.3 to 13.5 with unchanged layer coating.

Example 2

Reduction of the Stripping Efficiency AWZ by Reducing the Nozzle Lip Gap d

Table 4 below 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.

	TABLE 4
				Nozzle			Gas flow
	Coating	Strip	Strip	lip	Nozzle		rate per				Stripping
	per	width	speed	gap	spacing	Nozzle	side	Bath			efficiency
	side	(w)	(v)	(d)	(Z)	pressure	(D)	temperature			coefficient
	g/m2	mm	m/min	mm	mm	mbar	Nm3/h	° C.	b	k	(AWZ)

2.1	46.7	1455	100	1.2	7	237	1373	452	1.15	0.97	9.4
2.2	46.7	1455	100	1.0	7	252	1166	454	1.00	0.88	4.9

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

Based on this, the nozzle lip gap d was reduced from 1.2 mm to 1.0 mm in order to reduce the stripping efficiency AWZ, whereby the other parameters were essentially left constant.

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

By the changed nozzle lip gap d the ratio Z/d is shifted, which also changes the values therewith determined for b and k. If these new method parameters (Table 4, Example 2.2) are now used to calculate the stripping efficiency AWZ, the result is a value of 4.9. By reducing the nozzle lip gap from 1.2 mm to 1.0 mm, the stripping efficiency AWZ could thus be reduced from 9.4 to 4.9 with an unchanged layer coating.

Example 3

Reduction of the Stripping Efficiency AWZ by Increasing the Bath Temperature TB

Table 5 below shows two test examples 3.1 and 3.2, which show a reduction in the stripping efficiency by increasing the bath temperature TB from 439° C. to 455° C.

	TABLE 5
				Nozzle			Gas flow
	Coating	Strip	Strip	lip	Nozzle		rate per				Stripping
	per	width	speed	gap	spacing	Nozzle	side	Bath			efficiency
	side	(w)	(v)	(d)	(Z)	pressure	(D)	temperature			coefficient
	g/m2	mm	m/min	mm	mm	mbar	Nm3/h	° C.	b	k	(AWZ)

3.1	41.5	1615	100	1.0	7	336	1607	439	1.00	0.88	12.4
3.2	41.5	1615	100	1.0	7	280	1339	455	1.00	0.88	5.8

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

Based on this, the bath temperature TB was increased from 439° C. to 455° C. mm to reduce the stripping efficiency AWZ, leaving the other parameters essentially constant. In this example, the ratio Z/d remains unchanged, the herewith determined values for b and k also remain constant when the bath temperature TB is increased.

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 side of the strip had to be reduced from 1607 to 1339 Nm³/h by the automatic support control in order to keep the thickness constant. The reason for this is that the viscosity of the zinc melt decreases as the bath temperature TB increases, which means that lower stripping forces are required to strip the same amount of melt.

If one now calculates the stripping efficiency AWZ with these new method parameters (Table 5, Example 3.2), the result is a value of 5.8, i.e. the stripping efficiency AWZ could thus be reduced by increasing the bath temperature TB.

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 to the right and left parallel to the strip-shaped flat steel product 100 is excluded.

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

In FIG. 5, a part of a further device 150 is shown, 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. In the bath 11 the liquid zinc alloy (abbreviated here as ZnAlMg) is contained. Only a short length of the flat steel product 100, which has a strip shape, is shown after it has been dipped out of the bath 11. The flat steel product 100 is guided vertically out of the bath 11 in the direction of the x-axis between two opposing gas nozzles 15 of the stripping nozzle device 14. If the two gas nozzles 15 are arranged parallel to each other, then a center line ML can be defined between these nozzles 15. This center line ML is shown as a dashed line in FIG. 5. The center line ML lies in a nozzle plane DE (which is defined as the x-y plane).

The virtual cylinder body vZK in 3-dimensional space is formed around the center line ML. The “outer shell” of the virtual cylinder body vZK is concentric to the center line ML. All points of the “outer shell” have an equidistant distance ra to the center line ML (ra is the radius of the virtual cylinder body vZK).

The virtual cylinder body vZK includes at least the nozzles 15 of the stripper nozzle device 14 and has a virtual cylinder height vZH, which is 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 nozzle 15 (defined parallel to the y-axis).

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

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

Preferably, the absolute local air humidity f is determined or measured at several points of 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.

Preferably, in all or at least some of the embodiments, the absolute local air humidity f 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, this virtual cylinder body vZK having a volume of 2 m³.

In all embodiments, however, the absolute local air humidity f can also be defined in a volume range from 1 m³ to 10 m³ and measured there directly or indirectly, the measurement preferably not being carried out directly at the line of incidence of the gas G but at a y-line which runs parallel to the y-axis above the line of incidence.

In the embodiment of FIG. 2, the y-line described lies in the region between the nozzles 15 of the stripping nozzle device 14 and the lower inlet side of the cooling region 16. In the embodiment of FIG. 6, the y-line described lies above the line of incidence 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.

FIG. 6 shows a further embodiment of a device 150, whereby an approach for directly measuring the absolute local air humidity is also used here. The setup 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 exit side A of the bath 11 is shown. Two gas nozzles 15 are arranged parallel to the front and rear sides of the strip (the gas nozzles 15 extend into the plane of the drawing). As indicated schematically, each of the nozzles 15 is fed with the inert gas G by means of pumps P_g. The two pumps P_g are connected to the controller 250 in terms of control technology (or regulation technology) and the controller 250 can, for example, control the gas flow D per strip side. The corresponding connecting lines or pipes are labeled V5, V6, V7 and V8 in FIG. 6. An air supply unit with blower(s) and control valves is referred to here as pump P_g.

Preferably, all embodiments of the device 150 comprise a control of the flow rate D of the gas G (referred to as automatic layer control), which is designed such that a layer 10 with a substantially constant target thickness is always produced despite a change in the other adjustable parameters. For this purpose, the control system comprises at least one sensor (not shown) that measures the actual thickness of the layer 10 after the blowing. If the actual thickness is smaller than the target thickness, the control system reduces the flow rate D and vice versa.

Each of the nozzles 15 can be moved by a motor or actuator M parallel to the z-axis. The motors or actuators M are connected to the controller 250, as shown. The corresponding connecting lines or cables are labeled V9, V10, V11 and V12 in FIG. 6. Sensors are present (not shown) in order to be able to control the nozzle distance Z. This allows the nozzle spacing Z to be set and/or checked via the controller 250. The control of the nozzle distance Z can be laser-supported in all embodiments.

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. In the case of an inductive heater, the coil 30 of which is indicated in FIG. 6, the controller 250 can set 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 cables are labeled V13 and V14 in FIG. 6.

A virtual cylinder body vZK is also indicated in FIG. 6, the center line ML of which intersects the drawing plane slightly above the nozzles 15. In order not to overload the drawing, the position of the center line ML is shown by a small white circle on the strip-shaped flat steel product 100. Since the overall constellation is limited on the bottom side 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 cut at the top and bottom.

Here too, for example, two humidity sensors 51 (at least one on each side of the strip) are used in order to be able to determine the absolute local air humidity. The humidity sensors 51 are connected to the controller 250. The corresponding connection lines or cables are labeled V1, V2, V3 and V4 in FIG. 6.

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.

FIG. 9 shows exemplary steps of the method described here in the form of a flow chart. 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, on the basis of a target specification of the layer 10 to be applied.

Before, during or after setup S1, the current absolute local air humidity f is measured (directly or indirectly) (step S2). Then one of the inequalities or a “lookup” table is used to determine whether the condition f>AWZ is fulfilled (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 fulfilled (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 the AWZ so that the condition f>AWZ is fulfilled. When making adjustments, care is taken to ensure that the target specification of the layer 10 to be applied remains fulfilled.

In a similar form, the checking of the condition f>AWZ can be repeated from time to time during the application of the layer 10 in order to be able to react to changing absolute local air humidity f. If f has decreased, it is checked again (as in step S3) whether the condition f>AWZ is still fulfilled. If yes, then the application of layer 10 is continued. If not, then (analogous to step S1) adjustments can be made to the setup of the components and elements of the device 150, aiming to ensure that the condition f>AWZ is or remains fulfilled.

In all embodiments, a corresponding controller 250 may be configured or programmed 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
  • the stripping efficiency AWZ can be reduced by reducing the distance Z.

In all embodiments, a corresponding controller 250 can be configured or programmed in such a way that it is taken 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.

If f drops significantly and if no reasonable adjustment is possible within the target specification, the method can be interrupted. As part of such an interruption, the width d of the nozzle lip gap 17 can then be adjusted manually, for example (which is not possible automatically in the most devices 150).

REFERENCE SIGNS

(Protective) layer/(protective) coating	10
Zinc alloy melt bath/bath	11
Trunk	12
Roller	13
Nozzles/stripping nozzle device	14
Gas nozzle/Stripping nozzle	15
Cooling area	16
(Gas) nozzle lip gap	17
Inductive heating device	30
Humidity sensor	51
Flat steel product/Steel strips/Steel sheets/Strip	100
Coated flat steel product/Steel strip/Steel sheet	100, 10
Device	150
Controller	250
Exit side	A
Stripping efficiency	AWZ
Half-width	2b
Thickness of the nozzle lip gap	d
Effective gas flow over the bandwidth per band side	D
Nozzle plane/Stripping plane	DE
Inlet side	E
Absolute air humidity	f
Frequency generator	FG
Gas	G
Straight line	Ge
Proportionality factor	k
Curve	K1
Distance between the two shear force maxima	/
Motor/Actuator	M
Central line	ML
Pressure	P
Pressure progression	P(x)
Pump	Pg
Maximum pressure	PS
Distance/Radius	ra
Relative air humidity in %	r
Steps	S1, S2, . . .
Temperature	T
Bath temperature	TB
Reduced bath temperature	TBred
Air temperature in ° C.	TL
Shear force	τ
Maximum occurring shear force	τmax
Strip speed	v
Virtual cylinder height	vZH
Virtual cylinder body	vZK
Connections/Cables	V1, V2, V3, . . .
Strip width	w
Axis	x
Axis	y
Axis	z
Nozzle spacing	Z