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

[0001]The present invention relates to a device by which flat steel products can be coated with a zinc (Zn) or zinc-aluminum-magnesium (ZnAlMg) based layer, e.g. as a protective coating. It also relates to a corresponding method.

[0002]It is well known that flat steel products 100, such as steel strips or steel sheets, are coated with a zinc (Zn) or ZnAlMg alloy to improve their corrosion resistance. In practice, this is usually done by placing the flat steel product 100 coming from a furnace into a zinc alloy melt 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 side of the flat steel product 100 is stripped to the target thickness (in the micrometer range) or to the target surface coating (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.

[0003]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.

[0004]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.

[0005]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.

[0006]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.

[0007]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.

[0008]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 stripping nozzle).

[0009]Similar surface defects can also occur under certain circumstances with Zn coatings that contain a small proportion of Al (typically less than 1 wt. %). These coatings are referred to here as ZnAl coatings.

[0010]The task is therefore to provide a device for coating flat steel products with a ZnAlMg layer or a ZnAl layer that have a particularly durable and robust protective effect in terms of corrosion, wherein the surface of the protective coating should be particularly homogeneous 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.

[0011]In addition, the device should consume as little energy as possible, be cost-effective to operate and robust in use.

SUMMARY OF THE INVENTION

[0012]According to the invention, a corresponding device is provided which uses a continuous (hot-dip) process and which allows a steel flat product to be provided with a metallic ZnAlMg layer or a ZnAl layer which can serve, for example, as a (protective) coating. This layer is intended to protect the steel substrate of the flat steel product from external influences. In the following, the corresponding immersion bath is referred to as a zinc melt bath, wherein the term zinc alloy melt bath is intended to comprise both a melt bath containing mainly zinc (Zn) and a small admixture of aluminum (Al) (typically less than 1% by weight), as well as a melt bath containing a ZnAlMg alloy. The layer to be applied is also referred to here as the Zn-containing (protective) layer.

[0017]All embodiments relate to the application of a Zn-containing (protective) layer to a flat steel product, the 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.

[0025]In all or at least some of the embodiments, the device is characterized in that it can be set up or prepared prior to applying and blowing off the layer to produce and coat a flat steel product according to specification with the ZnAlMg layer or the ZnAl layer.

[0038]The system parameter(s) and method parameter(s) define a so-called stripping efficiency AWZ, which is essentially constant during the application of the layer (as long as the corresponding parameters do not change). However, small variations in the stripping efficiency AWZ are possible.

[0052]In at least some of the embodiments, the stripping efficiency is defined as follows:

$\mathrm{AWZ}=24,61·\frac{D^2·k}{w^2·d^2·b}+3639·\frac{b}{v}-45,42.$

[0053]In at least some of the embodiments, the stripping efficiency is defined as follows:

$\mathrm{AWZ}=\frac{1}{14}·\left(\frac{D^2·k·e^{-0,5}1,251·{10}^4}{w^2·d^2·25,92}·\sqrt{\frac{-2·\ln \left(0,5\right)}{b^2}}+\frac{6·{10}^4}{v}·\sqrt{\frac{-b^2}{2·\ln \left(0,5\right)}}-636\right).$

[0061]The following definitions apply when determining the values for the half-width and the proportionality factor using the ratio of the distance to the thickness of the nozzle lip gap:

$\begin{array}{cc}\frac{Z}{d}<5,2\to b=1,9·\frac{d}{2}\mathrm{and}k=1& \mathrm{case}1.1\end{array}$$\begin{array}{cc}5,2\le \frac{Z}{d}<10\to b=\left[-{\left(\frac{Z}{d}\right)}^4·3,22·{10}^{-4}+{\left(\frac{Z}{d}\right)}^3·9,78·{10}^{-3}-{\left(\frac{Z}{d}\right)}^2·8,39·{10}^{-2}+\left(\frac{Z}{d}\right)·2,72·{10}^{-1}+1,62\right]·\frac{d}{2}\to k=-{\left(\frac{Z}{d}\right)}^4·6,05·{10}^{-4}+{\left(\frac{Z}{d}\right)}^3·2,2·{10}^{-2}-{\left(\frac{Z}{d}\right)}^2·2,89·{10}^{-1}+\left(\frac{Z}{d}\right)·1,55-1,9& \mathrm{case}1.2\end{array}$$\begin{array}{cc}\frac{Z}{d}\ge 10\to b=0,125·Z\mathrm{and}k=6,5·\frac{d}{Z}& \mathrm{case}1.3\end{array}$

[0062]In order to prevent marbling and/or the formation of toothpick defects of the ZnAlMg layer or ZnAl layer to be produced, in at least some of the embodiments the absolute local humidity f is determined in the close range of the controlled water vapor atmosphere. Preferably, in all embodiments, the absolute local humidity f of the controlled water vapor atmosphere is determined within a predefined virtual cylinder volume which surrounds or encloses the flat steel product in the region of the at least one gas nozzle. In this case, this predefined, virtual cylinder volume defines the close range. Since mixing of the gases occurs close to the front and rear side of the flat steel product, an area parallel to the front and rear side of the flat steel product is excluded when defining the virtual cylinder volume. Preferably, the virtual cylinder volume is limited for this purpose by two planes, each of which has a distance s/2 from the flat steel product.

[0063]If the determined surrounding humidity f_UG is greater than the stripping efficiency (i.e. if f_UG>AWZ), the layer can be produced without marbling or without the formation of toothpick defects. If the determined surrounding humidity f_UG is currently lower than the stripping efficiency (i.e. if f_UG<AWZ), the absolute local humidity f within the controlled water vapor atmosphere is deliberately increased until the absolute local humidity f in the close range is greater than the stripping efficiency (f>f_UG and f>AWZ). Only then is a coating produced without marbling or without the formation of toothpick defects.

[0064]In at least some of the embodiments, the absolute local humidity f of the controlled water vapor atmosphere is generated by using the water vapor device. That is, the water vapor device is configured to selectively increase the absolute local humidity f within the controlled water vapor atmosphere, that is, in the close range.

[0065]In all embodiments, the controlled water vapor atmosphere is defined in a close range of the water vapor device, or the controlled water vapor atmosphere is provided in a close range of the water vapor device. In all embodiments, this close range can be defined by a predefined, virtual cylinder volume which extends on both sides of the flat steel product and which also surrounds or encloses the at least one gas nozzle. In all embodiments, however, this volume can optionally also be defined by a housing or an enclosure that surrounds the flat steel product on both sides. If a housing or enclosure is used, the inner area defined in this way is referred to as the close range.

[0066]In at least some of the embodiments, the close range of the water vapor device comprises a volume in a range from 1 m³ to 10 m³ and preferably a volume of at least 2 m³, wherein when determining (by direct or indirect measurement) the absolute local humidity within the close range, it must be taken into account that it may be drier directly at the flat steel product than in areas further away from the flat steel product due to mixing of the gaseous water vapor with the stripping gas.

[0067]In all embodiments, the absolute local humidity is preferably selectively increased in a close range of the water vapor device (if the surrounding humidity f_UG should be less than AWZ), wherein the monitoring or control of the current absolute local humidity is performed by means of direct or indirect measurement at a distance of more than 20 cm from the flat steel product to avoid the significantly drier area.

[0068]In all embodiments, the device or stripping nozzle system may comprise an automatic coating control configured to automatically adjust the flow rate of the (stripping) gas to maintain the target thickness of the coating to be applied essentially constant. The automatic coating control is preferably configured to be able to compensate for fluctuations in one or more system parameters and method parameters.

[0069]The aluminum content (in percent by weight) may be equal to or greater than the magnesium content (in percent by weight) in all embodiments using a ZnAlMg alloy melt bath.

[0070]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.

[0071]The combination of a precisely defined bath composition, of a monitoring or observation of the surrounding humidity f_UG and optionally also of the current absolute local humidity f in the close range of the water vapor device and a targeted adjustment of the absolute local air humidity f in the close range of the water vapor device can produce a surface that shows no or negligibly low marbling and no or negligibly low toothpick defects. During the production of a corresponding coating, the stripping efficiency AWZ is kept essentially constant to obtain a consistent coating (which is within the predetermined specification).

[0072]In all embodiments, the values of the absolute local air humidity f, which according to the invention should be present in the close range of the flat steel product, are in the range of 1 g/Nm³ to 300 g/Nm³, preferably in the range of 2.71 g/Nm³ to 50 g/Nm³, which means that the device can be reliably operated at an absolute local humidity f which is in the stated value range.

[0073]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.

[0078]The specification of the ZnAl and ZnAlMg alloy concepts defined above is the result of numerous investigations and calculations. Within the specified limits of the alloy concepts defined here, the technical teaching presented here has proved particularly successful.

[0079]In at least some of the embodiments, the surrounding moisture f_UG is permanently measured.

[0080]In at least some of the embodiments, the surrounding moisture f_UG is measured from time to time.

[0081]In at least some of the embodiments, the absolute local humidity f is permanently measured at close range.

[0082]In at least some of the embodiments, the absolute local humidity f in the close range is measured from time to time.

[0083]Preferably, in all embodiments, the device is operated at a 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.

[0084]Specific tests have shown that the method described here does indeed lead to very good results. It has been shown that excellent results can be achieved through targeted (pre-)adjustment of the stripping process (or the corresponding stripping efficiency AWZ) as long as the absolute local air humidity f is set or specified higher than the stripping efficiency AWZ in the close range.

[0085]The development of the new method and the targeted adaptation of the absolute air humidity f in the close range to the (method and system) parameters (or to the corresponding stripping efficiency AWZ) is based on theoretical considerations, various simulations of stripping processes and numerous tests.

[0086]The processes in the area of the stripping nozzle device and at the flat steel product are highly complex and depend on numerous (method and system) parameters and influencing variables (or the corresponding stripping efficiency). The device therefore relies on some simplified assumptions and specifications in order to obtain reproducible results.

[0087]By specifically increasing the absolute local humidity f within the controlled water vapor atmosphere (referred to as close range), additional degrees of freedom are gained for the definition or specification of the method and system parameters.

[0088]Further advantageous embodiments of the invention form the objects of the dependent claims.

DRAWINGS

[0089]Embodiments of the invention are described in more detail below with reference to the drawings.

[0090]FIG. 1 shows a highly schematized representation of a known device for dip coating and stripping flat steel products (state of the art);

[0091]FIG. 2A shows a highly schematized representation of a first exemplary device in which the method of the invention is used;

[0092]FIG. 2B shows a highly schematized representation that is used to define the virtual cylinder volume;

[0093]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);

[0094]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;

[0095]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;

[0096]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;

[0097]FIG. 5 shows a highly schematized sectional view of a further device of the invention in which the method of the invention is used;

[0098]FIG. 6A shows a highly schematized representation of a strip side of a flat steel product that has no marbling;

[0099]FIG. 6B shows a highly schematized representation of a strip side of a flat steel product that has a medium marbling;

[0100]FIG. 6C shows a highly schematized representation of a strip side of a flat steel product that has a strong marbling;

[0101]FIG. 7 shows a table with exemplary test results of the method described here;

[0102]FIG. 8 shows exemplary steps of the method described here.

DETAILED DESCRIPTION

[0103]It is about a device 150 (see for example FIG. 2A) for applying a layer 10 to a strip-shaped flat steel product 100 (see FIG. 2B, where a layer 10 can be vaguely recognized on both sides). This layer 10 is produced by passing the flat steel product 100 from an input side E to an exit side A through a zinc alloy melt bath 11 and blowing it off with (stripping) gas G on the exit side A by means of a stripping nozzle device 14. The purpose of the stripping nozzle device 14 is to strip off the excess (still liquid) ZnMgAl layer or ZnAl layer (layer 10) at the flat steel product 100 after exiting the bath 11.

[0104]In the context of the invention, care should be taken to ensure that the layer 10 is produced according to (predetermined) specification (the specification defines, for example, the target thickness), and that no marbling and/or no toothpick defects occur/occur. More precisely, it is a matter of avoiding these “defects” in the event of changing surrounding conditions in the production area (e.g. in the production hall). Even if the surrounding air humidity f_UG should change in the surrounding of the device 150, it is ensured by the device 150 according to the invention that no marbling and/or toothpick defects occur and that the layer 10 continues to be produced according to specification.

[0105]In all embodiments, the specification can, for example, specify the target thickness of the layer 10 and/or the target (surface) coating of the layer 10. 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 requirements of the specification.

[0106]In at least some of the embodiments, the device 150 is operated 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 of the specification. 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.

[0107]Preferably, in at least some of the embodiments, the target surface coating (coating per side of the strip) is in the range of 20 to 200 g/m² and particularly preferably in the range of 30 to 100 g/m².

[0108]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.

[0109]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. 2A and 5 show embodiments with two nozzles 15 and FIG. 3A shows an embodiment with only one nozzle 15. The flow rate D of the (stripping) gas G, which is discharged through the nozzle lip gap 17 in the direction of the front or rear side, is indicated here in Nm³. (Nm³ stands for standard cubic meter). A standard cubic meter is the quantity of a gas G contained in a volume of one cubic meter. This applies at a temperature of 0 degrees Celsius and a pressure of 1.01325 bar.

[0121]In order to prevent the formation of the marbling and/or toothpick defects or to significantly reduce the marbling and toothpick defects, the absolute local air humidity f in the close range NB and/or the surrounding air humidity f_UG is preferably determined continuously or from time to time in all embodiments (e.g. by direct or indirect measurement).

[0122]In all embodiments, the device 150 comprises a water vapor device 50 (see FIGS. 2A and 5), which will be described in more detail. This water vapor device 50 is preferably located in the area of the exit side A of the bath 11 in all embodiments.

[0123]In at least some of the embodiments, the absolute local air humidity f in the close range NB (see FIGS. 2A, 5) of the water vapor device 50 is specifically adjusted (increased). In FIGS. 2A and 5, the corresponding devices 150 comprise humidity sensors 51 which are arranged in the close range NB or which protrude into the close range NB in order to be able to control the absolute local air humidity f in the close range NB continuously or from time to time. The control of the absolute local air humidity f in the close range NB enables a targeted control of the air humidity f in the close range NB.

[0124]However, in all embodiments, the humidity sensor(s) 51 for determining the absolute local air humidity f may also be arranged at a different location of the device 150.

[0125]In all embodiments, the device 150 may comprise at least one humidity sensor 56 for determining the surrounding air humidity f_UG, as shown in FIG. 2A and FIG. 5 top right.

[0126]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 proportion of gaseous water WG in a standardized volume body having a volume of 1 m³. In other words, the absolute air humidity f indicates the content of gaseous water vapor in a volume body. f and f_UG are used here as the formula symbol for the absolute air humidity. In all embodiments, the absolute air humidity f and f_UG can be estimated approximately from the air temperature TL and the relative air humidity r, whereby the following applies:

[0129]In all embodiments, the measurement/monitoring of the absolute local air humidity f can be carried out directly or indirectly (preferably within the close range NB). 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. Accordingly, the surrounding air humidity f_UG can also be determined in all embodiments by measuring the air temperature TL and the relative air humidity r of the surrounding air and calculating/deriving the absolute surrounding air humidity.

[0130]The values of the absolute local air humidity f, which according to the invention should be present in the vicinity of the flat steel product 100 in a close range NB 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 f is in the range of 2.71 g/m³ to 50 g/m³ in all embodiments.

[0131]The device 150 enables the targeted adaptation/adjustment of the absolute local air humidity f in the close range NB in a value range of 1 g/m³ to 300 g/m³.

[0132]For this purpose, in all embodiments, the device 150 can comprise at least one steam generator DG in or at the close range NB. In FIG. 2A, an embodiment with two steam generators DG is shown, which are placed at the upper edge of the close range NB close to the front and rear sides of the flat steel product 100. FIG. 5 shows an embodiment with four steam generators DG, which are placed outside the close range IB and which introduce gaseous water vapor WG into the close range NB via gas lines 54 and inlet bridges 53 near the front and rear side of the flat steel product 100.

[0133]Preferably, in all embodiments, one or more steam generator(s) DG is/are used, which is/are adapted as high-purity steam generator, which generate gaseous water vapor WG from purified or highly purified water.

[0141]In all embodiments, care is taken to ensure that the system parameters and/or method parameters are specified in such a way that the layer 10 to be applied essentially corresponds to the specification. That means, care is taken that a layer 10 is applied and blown off which, for example, corresponds to the target thickness (within tolerances) and which at the same time shows no or only very slight marbling and no or only very slight toothpick defects.

[0142]In all embodiments, the current flow rate D of the gas G can be automatically adjusted in a known manner (for example, control-wise by an automatic coating control) in order to keep the target thickness of the layer 10 to be applied essentially constant if one or more of the system parameters and/or method parameters should change.

[0143]The mathematical relationships that can be used here during setup/preparation will be described below. It will also be described how a so-called stripping efficiency AWZ can be defined by the method and system parameters.

[0144]FIG. 3A also 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 outlet gap of the stripping nozzle device 14.

[0145]FIG. 3A additionally shows in purely schematic form the supply of gaseous water vapor WG according to the invention (by two block arrows to the right and left of the nozzle 15).

[0151]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.

[0152]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. 2A and 5), exerts a shear force t on the still liquid layer 10. FIG. 3C shows a representation of the shear force t in relation to a position on the x-axis (the shear force t was determined by the negative first derivative of the pressure profile of the pressure profile in FIG. 3B). This is the shear force t that acts on the layer 10 to be stripped. The course of the shear force curve t 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.

[0153]There is a direct relationship between the time t in which the strip-shaped flat steel product 100 passes through the distance/between the shear force maxima (see FIG. 3C), the strip speed v and the half-width 2b, as expressed in equation (1), as expressed in equation (1):

$\begin{array}{cc}t=\frac{120}{v}·\sqrt{\frac{-b^2}{2·\ln \left(0,5\right)}}& \left(1\right)\end{array}$

[0154]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.

[0155]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 surrounding 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”. In addition to the surrounding air, the gas jet also draws in gaseous water vapor WG.

[0156]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, air humidity and the marbling due to the complex interrelationships. Only after systematic investigation of various internal and external influencing variables did a correlation emerge between the degree of marbling on layer 10 and the surrounding conditions of the test setup. Air humidity in particular shows an influence on the marbling.

[0157]Further targeted investigations and the graphical processing of the results of these tests showed for the first time a correlation between the (surrounding) air humidity and the marbling. FIG. 4 shows a summarizing graphical representation of numerous tests, with the absolute air humidity f in g/m³ plotted on the ordinate axis and the stripping efficiency AWZ (as a summary or generic term for the method and device parameters) plotted on the abscissa.

[0158]By weighting and adding the two variables τ_max and t (see also FIG. 3C and equation (1)), the stripping efficiency AWZ, which can be compared directly with the absolute surrounding air humidity f_UG, can be determined as follows (inequality (2.1)):

$\begin{array}{cc}{f}_{\mathrm{UG}}>\frac{{\tau }_{\max }+500·t-636}{14}& \left(2.1\right)\end{array}$

[0159]The right-hand side of the inequality (2.1) corresponds to the stripping efficiency AWZ, i.e., the following relation applies (2.2):

$\begin{array}{cc}{f}_{\mathrm{UG}}>\mathrm{AWZ}& \left(2.2\right)\end{array}$

[0160]In at least some of the embodiments, the stripping efficiency is defined as follows:

$\begin{array}{cc}\mathrm{AWZ}=24,61·\frac{D^2·k}{w^2·d^2·b}+3639·\frac{b}{v}-45,42.& \left(2.3\right)\end{array}$

[0161]In at least some of the embodiments, the stripping efficiency is defined as follows:

$\begin{array}{cc}\mathrm{AWZ}=\frac{1}{14}·\left(\frac{D^2·k·e^{-0,5}1,251·{10}^4}{w^2·d^2·25,92}·\sqrt{\frac{-2·\ln \left(0,5\right)}{b^2}}+\frac{6·{10}^4}{v}·\sqrt{\frac{-b^2}{2·\ln \left(0,5\right)}}-636\right).& \left(2.4\right)\end{array}$

[0169]When determining the values for the half half-width b and the proportionality factor k using the ratio of the nozzle distance Z to the thickness d of the nozzle lip gap 17, the following definitions apply (whereby three cases 1.1, 1.2 and 1.3 are distinguished):

$\begin{array}{cc}\frac{Z}{d}<5,2\to b=1,9·\frac{d}{2}\mathrm{and}k=1& \mathrm{case}1.1\end{array}$$\begin{array}{cc}5,2\le \frac{Z}{d}<10\to b=\left[-{\left(\frac{Z}{d}\right)}^4·3,22·{10}^{-4}+{\left(\frac{Z}{d}\right)}^3·9,78·{10}^{-3}-{\left(\frac{Z}{d}\right)}^2·8,39·{10}^{-2}+\left(\frac{Z}{d}\right)·2,72·{10}^{-1}+1,62\right]·\frac{d}{2}\to k=-{\left(\frac{Z}{d}\right)}^4·6,05·{10}^{-4}+{\left(\frac{Z}{d}\right)}^3·2,2·{10}^{-2}-{\left(\frac{Z}{d}\right)}^2·2,89·{10}^{-1}+\left(\frac{Z}{d}\right)·1,55-1,9& \mathrm{case}1.2\end{array}$$\begin{array}{cc}\frac{Z}{d}\ge 10\to b=0,125·Z\mathrm{and}k=6,5·\frac{d}{Z}& \mathrm{case}1.3\end{array}$

[0170]Reference is made below to FIG. 4. The black-filled circles in FIG. 4 represent flat steel products 100 in which a marbling has clearly formed on the surface of layer 10 and the gray-filled circle represents a flat steel product 100 in which a medium marbling has formed. The unfilled circles, on the other hand, represent no or negligible marbling. All the tests shown were carried out with a nozzle lip gap thickness of d=1.0 mm.

[0171]A total of five test pairs (examples 1-5) are shown in FIG. 4. The corresponding method and system parameters as well as the stripping efficiency AWZ and the absolute air humidity f, which was measured in the close range NB, are listed in Table 1. Table 1 is shown as FIG. 7. The third last column of Table 1 indicates whether the water vapor device 50 was switched on to increase the local air humidity. Black, gray and white circles are used in Table 1 in accordance with FIG. 4. The penultimate column indicates in text form whether there was a strong marbling, medium marbling or no marbling and the last column on the far right of Table 1 indicates whether the condition of the inequality f>AWZ was fulfilled.

[0172]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 marbling from those tests that showed no or only negligible marbling. In the tests that lie above the straight line Ge, no or only negligible marbling occurred (a corresponding flat steel product 100 with a layer 10 without marbling is shown in FIG. 6A). The straight line Ge can be understood as a function of the stripping efficiency AWZ and is defined as follows:

$f=\mathrm{AWZ}$

[0173]A detailed evaluation of the test results shows that for a given stripping efficiency AWZ, it is possible to ensure that the inequality (2.2) is always fulfilled during production by specifically increasing the absolute local air humidity f in the close range NB.

[0174]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.

[0175]In all embodiments, a bath temperature TB in the specified temperature range is specified for operating the melt bath 11. Maintaining this temperature window (temperature range) is important, as more undesirable slag can form at the flat steel product 100 when working above the specified range.

[0176]The bath temperature TB can be predetermined in all embodiments, for example by means of an inductive heating device 30 (see FIGS. 2A and 5) or a resistance heater.

[0177]To avoid slag formation, the alloy melt bath 11 is preferably operated at a reduced bath temperature TB_red in all embodiments. The reduced bath temperature TB_red is preferably in the already mentioned range 420<TB_red<460 degrees Celsius.

[0178]The bath temperature TB is an important parameter and can preferably be set/preset relatively freely within the aforementioned temperature limits in all embodiments, if at the same time other method and system parameters are adjusted so that the AWZ remains essentially constant and if the air humidity in the close range NB is adjusted so that the condition of the inequality f>AWZ is still fulfilled.

[0184]The device 150 operates particularly reliably within these (value) ranges.

[0185]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 (called nozzle length DL) that corresponds to the strip width w (see FIGS. 6A-6C) of the strip-shaped flat steel product 100. The thickness d of the gas nozzle 15 is defined parallel to the x-axis (see FIG. 3A).

[0186]In all embodiments, the strip width w of the strip-shaped flat steel product 100 is in the range of 500 mm to 2500 mm and particularly preferably in the range of 1159 mm to 1614 mm.

[0187]The In at least some of the embodiments, the absolute local air humidity f in the close range NB is measured permanently or from time to time and if the absolute local humidity f“threatens” to drop below a threshold defined by AWZ in the close range NB, the air humidity is increased by using the water vapor device 50. For this purpose, the steam generator(s) DG can increase the output of gaseous water vapor, or the steam generator(s) DG can be turned on to produce gaseous water vapor.

[0188]In at least some of the embodiments, a steam generator DG preferably comprises a steam generator and, for example, a valve 55 (see FIG. 5) that can control the flow of gas through the conduit 54. For this purpose, the steam generator and/or the valve 55 may be connected to the controller 250 in all embodiments (not shown).

[0189]The absolute local air humidity f is preferably not measured directly at the impact line where the gas G strikes the layer 10 to be stripped, as the gas mixture is relatively “dry” there (i.e. contains little air humidity). Instead, the absolute local air humidity f is preferably measured directly or indirectly in all embodiments in an area that is at least 20 cm s/2 away from the line of impact. In the embodiment shown in FIG. 2A, the humidity sensors 51 are accordingly positioned a small distance above the nozzles 15 and the impact lines. In the embodiment of FIG. 5, the moisture sensors 51 are located a small distance below the nozzles 15 and the impact lines.

[0190]In FIG. 2B, a virtual cylinder volume vZV is shown as an example in schematic form. This predefined, virtual cylinder volume vZV surrounds or encloses the flat steel product 100 in the area of the at least one gas nozzle 15 (not shown in FIG. 2B). In this case, the predefined virtual cylinder volume vZV defines the close range NB. Since mixing of the gases WG and G occurs close to the front and rear side of the flat steel product 100, the definition of the virtual cylinder volume vZV excludes an area parallel to the front and rear side of the flat steel product 100, as shown in FIG. 2B. Preferably, the virtual cylinder volume vZV is limited for this purpose by two planes, each of which has a distance s/2 from the flat steel product 100.

[0191]FIGS. 2A and 5 show an approach for directly measuring the absolute local air humidity f and the surrounding air humidity f_UG. The device 150 comprises, for example, two humidity sensors 51 (at least one on each side of the strip) and a humidity sensor 56. In the schematic representation, each of these sensors 51, 56 has two contacts, which can be connected, for example, to a controller 250. The corresponding connections or lines V1, V2, V3, V4, V5, V6 are shown in FIGS. 2A and 5 by dotted lines.

[0199]When indirectly measuring the absolute local air humidity f or the surrounding air humidity f_UG, the air humidity is not measured directly, but the current air humidity is determined indirectly (e.g. optically).

[0200]The indirect determination of the absolute local air humidity f can be carried out in all embodiments 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. 6A, 6B and 6C). A corresponding measurement of the surface property(ies) can be made optically, for example, before the optional cooling region 16 or after the optional cooling region 16 (e.g., by optically measuring the reflectivity of the surface of the layer 10). If there is little or no marbling, the reflectivity is above a predetermined limit value. If the reflectivity should decrease (e.g. if it falls below a tolerance limit), the air humidity in the close range NB can be adjusted so that the inequality f>AWZ is fulfilled again.

[0201]All embodiments of the device 150 may comprise a controller 250. In all embodiments, this controller 250 may be configured as a computer-supported automation and control unit and comprise a human-machine interface, a computer and a database.

[0202]The In all embodiments, the controller 250 can be part of the overall system controller of the device 150, or it can be connected to the overall system controller in all embodiments.

[0203]The setup/preparation of the device 150 by adjusting the parameters (system or method parameters) can then be carried out by the overall system controller and/or by the controller 250 in these embodiments.

[0204]In all embodiments, the aforementioned stripping efficiency AWZ can be calculated by the controller 250 and/or by the overall system controller. The corresponding formulas are described below. In all embodiments, however, the stripping efficiency AWZ can also be determined by external means (e.g. by means of a workstation computer).

[0205]Preferably, an inert gas is used as the (stripping) gas G in all embodiments. Nitrogen or a gas mixture containing nitrogen has proven to be particularly effective.

[0206]To ensure that the target thickness of the layer 10, or the coating per side of the strip, does not change or hardly changes at all, care is taken to ensure that the system and method parameters, or the stripping efficiency AWZ, remain essentially constant. In examples 1 and 2, AWZ remains constant (see Table 1 in FIG. 7). In examples 3, 4 and 5, AWZ remains essentially constant (see Table 1 in FIG. 7).

[0207]However, it is important that care is taken when specifically increasing the absolute local air humidity f in the close range NB to ensure that also other defects, such as the formation of toothpick defects, slag and so on, are avoided in addition to marbling.

[0208]If, for example, the humidity of the surrounding air f_UG is very low, the intake of dry surrounding air can lead to the formation of marbling on layer 10 (if f_UG is less than AWZ). This is where the invention comes in by automatically increasing the absolute local air humidity f in the controlled water vapor atmosphere in the close range NB of the flat steel product 100, while AWZ is kept essentially constant.

[0209]Preferably, the inequality (2.2) is implemented in the controller 250 by software, or pairs of numbers for the stripping efficiencies AWZ and for the corresponding minimum humidity f_min to be specified are stored in one or more tables. Using a “lookup” table, the controller 250 can then determine a minimum value f_min for the absolute local air humidity value f for a currently valid stripping efficiency AWZ and transfer it to the water vapor device 50. The water vapor device 50 then generates an absolute local humidity value f in the close range NB that is greater than f_min. In these embodiments, f_min corresponds more or less to the straight line Ge in FIG. 4.

[0210]In all embodiments, parameters (system and/or method parameters) in the following numerical or value ranges are preferably used. The individual numerical or value ranges in Table 2 are not correlated with each other, or only in partial ranges, as the respective maximum and minimum values originate from different test examples. Only the respective maximum and minimum values were taken from Table 1 (FIG. 7) and summarized here.

[0211]Table 1 (see FIG. 7) shows specific numerical values for the test results shown in FIG. 4. Table 1 in FIG. 7 shows five examples of tests in which layers 10 without (visible) marbling were produced by specifying suitable method and/or system parameters, or stripping efficiencies AWZ, and by increasing an absolute local humidity value f in the close range NB (as shown as an example in FIG. 6A). In the first two columns of Table 1, the coating per side and the strip width w of the flat steel product 100 are given according to the specification.

[0212]In the test examples shown graphically in FIG. 4 and shown in Table 1 (FIG. 7), the following principle was applied. If marbling defects occur under a given surrounding air humidity f_UG and under given method and system conditions, the absolute local air humidity f in the close range NB (corresponding to the left-hand side of inequality (2.2)) can be increased in accordance with the invention until inequality (2.2) is fulfilled, at least in the close range NB, in order to eliminate the marbling.

[0213]The method and system conditions, i.e. the process and stripping nozzle parameters, can be kept unchanged, with the exception of the usual variations in production, as can be seen in Table 1 (FIG. 7).

[0214]Table 1 uses five examples 1-5 to illustrate the use of a water vapor device 50 to increase the absolute local humidity f in the close range NB.

[0215]In the specific examples 1-5, the nozzle spacing Z, the nozzle lip gap d and the strip speed v were kept constant, the coating, the strip width w, the nozzle pressure, the flow rate D and the bath temperature TB were not deliberately changed and are only subject to the usual variations in production. This means that in each of the examples, the stripping efficiency AWZ calculated therewith remains essentially unchanged (±10%). Preferably, the device 150 is operated in all embodiments in such a way that the calculated stripping efficiency AWZ changes by a maximum of ±5% when the absolute local air humidity f in the close range NB is increased and the method and system parameters are adjusted.

[0216]The initial situation for example 1 is shown in Table 1 under 1.1 (Table 1, line 1.1): With the given method and stripping nozzle parameters, this results in a stripping efficiency AWZ of 9.6 according to inequality (2.2) at a currently prevailing absolute surrounding air humidity f_UG of 3.8 g/m³. The condition from inequality (2.2) for marbling-free production, namely f>AWZ, is therefore not fulfilled. In fact, strong marbling defects also occurred in production under these conditions. FIG. 4 shows the corresponding black circle at AWZ=9.6 and f=3.8 g/m³.

[0217]Based on this, the local absolute air humidity f in the close range NB was raised from 3.8 to 20.6 g/m³ using a water vapor device 50, wherein the method and stripping nozzle parameters were kept within the usual process scatter (Table 1, line 1.2). After the targeted increase in air humidity f, the condition from inequality (2.2) for marbling-free production, namely f>AWZ, was fulfilled. In fact, no more marbling defects occurred on layer 10 under these conditions. FIG. 4 shows the corresponding white circle at AWZ=9.6 and f=20.6 g/m³. A block arrow with the designation 1.1→1.2 marks this 1st example.

[0218]Examples 2-5 are to be understood analogous to example 1. A block arrow with the designation 2.1→2.2 identifies the 2nd example and a block arrow with the designation 3.1→3.2 identifies the 3rd example. Examples 4 and 5 are also marked accordingly in FIG. 4.

[0219]The following pairs of values for the stripping efficiency AWZ and the absolute local air humidity f can be extracted from Table 1 in FIG. 7 (sorted by ascending stripping efficiency AWZ). All values for the absolute local air humidity f are greater than the respective AWZ (in FIG. 4, the corresponding white-filled circles are above the straight line Ge).

[0220]From Table 1 in FIG. 7, the following pairs of values can also be extracted for the stripping efficiency AWZ and the absolute air humidity f (sorted by ascending stripping efficiency AWZ). All values for the absolute local air humidity f are smaller than the respective AWZ (in FIG. 4, the corresponding circles filled in black or gray are below the straight line Ge).

[0221]FIG. 5 shows a further embodiment of a device 150, wherein an approach for directly measuring the absolute local air humidity f in the close range NB and the surrounding air humidity f_UG in the vicinity of the device 150 is also used here. The setup of the device 150 is similar to the device 150 shown in FIG. 2A, therefore reference is also made to the description of FIG. 2A. In contrast to FIG. 2A, however, the device 150 of FIG. 5 has a housing 52. Such a housing 52 is optional, since it is sufficient for the invention if the water vapor device 50 generates a sufficiently high, absolute local air humidity f in the close range NB.

[0222]The liquid ZnAlMg alloy or ZnAl alloy is located in the bath 11, which is shown here as a rectangular container that is open at the top. Only a short length section of the flat steel product 100, which has a strip shape, is shown in FIG. 5 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 at strip speed v between two opposing gas nozzles 15 of the stripping nozzle device.

[0223]The water vapor device 50 here comprises the aforementioned housing 52, which is defined here, for example, by the shape of an approximately cylindrical body in 3-dimensional space. In FIG. 5, this approximately cylindrical housing 52 of the water vapor device 50 is schematically indicated. The housing 52 here consists of two housing halves that are arranged symmetrically to the x-axis. The close range NB defined in this way lies quasi between the two halves of the housing.

[0224]The housing 52 of the water vapor device 50 can be divided into four quadrants in the schematic sectional view. Here, the water vapor device 50 comprises each one steam generator DG per quadrant (i.e. two steam generators DG per housing half). Each of the steam generators DG is arranged outside the close range NB (or outside the housing 52). As indicated schematically, each steam generator DG can introduce gaseous water vapor WG into the close range NB via a corresponding gas line 54, a valve 55 and an inlet bridge 53. In the sectional view of FIG. 5, it can be seen that each inlet bridge 53 is fluidically connected to the close range NB via at least one passage opening in the housing 52. In this way, gas can flow from the respective steam generator DG, through the line 54, the valve 55 and the inlet bridge 53 into the close range NB. The housing 52 can optionally have a series of such passage openings parallel to the y-axis, so that the gas WG can evenly distribute in the close range NB.

[0225]In this embodiment, the housing 52 of the water vapor device 50 sits directly at the nozzles 15 of the stripping nozzle device and has a housing height that is defined parallel to the x-axis. Preferably, the housing length, which is defined parallel to the y-axis, corresponds in all embodiments to at least the strip width w of the flat steel product 100 and/or the length DL of the nozzle 15 (also defined parallel to the y-axis).

[0226]The housing 52 of the water vapor device 50 can also have a different shape in all embodiments.

[0227]As schematically indicated, each of the nozzles 15 is fed with the inert (stripping) gas G by means of pumps P_G. The two pumps P_G are connected to the controller 250 in terms of control (or regulation) and the controller 250 can thus, for example, control the gas flow D per strip side. The corresponding connection lines or conduits are labeled V7, V8, V9 and V10 in FIG. 5. The pump P_G is referred to here, for example, as a blower with control valves.

[0228]Preferably, all embodiments of the device 150 comprise a control of the flow rate D of the gas G (referred to as automatic coating control), which is configured such that a layer 10 with an essentially constant target thickness is always produced despite a change in the other method and system parameters. For this purpose, the control comprises at least one sensor (not shown) that measures the actual thickness of the layer 10 after the excess molten zinc has been blown off. If the actual thickness is less than the target thickness, the control reduces the flow rate D and vice versa.

[0229]In all or at least some of the embodiments, the absolute local air humidity f in the close range NB can be measured along a horizontal y-line (which is, for example, at least s/2=20 cm away from the flat steel product 100) which runs parallel to the flat steel product 100 and parallel to the y-axis. In the embodiment of FIG. 2A, the horizontal y-line described lies approximately in the area between the nozzles 15 of the stripping nozzle device 14 and the two steam generators DG. In the embodiment of FIG. 5, the described y-line lies between the exit side A of the bath 11 and the nozzles 15 (here a small distance below the nozzles 15).

[0230]Each of the nozzles 15 can be moved parallel to the z-axis by a motor or actuator (not shown). The motors or actuators can be connected to the controller 250. Sensors are present (not shown) in order to be able to control the nozzle spacing Z. This allows the nozzle distance Z to be set and/or controlled via the controller 250. The control of the nozzle distance Z can be laser-supported in all embodiments. If the housing halves of the housing 52 are mechanically connected to the nozzles 15, the housing halves can be moved in solidarity with the nozzles.

[0231]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. 5, 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 connection lines or conduits are labeled V11 and V12 in FIG. 5A.

[0232]Preferably, in all or at least some of the embodiments, the water vapor device 50 is configured to specify an absolute local air humidity f in the local range NB in the range of 1 g/m³ to 300 g/m³, preferably in the range of 2.71 g/m³ to 50 g/m³, wherein the water vapor device 50 is preferably only switched on or switched in when f_UG<AWZ.

[0233]In all embodiments, the volume of the close range NB (e.g. defined by the virtual cylinder volume vZV or by the housing 52) of the water vapor device 50 can be between 1 m³ and 10 m³.

[0234]FIG. 8 shows exemplary steps of a method that can be carried out in the described devices 150. FIG. 8 shows the steps 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 or prepared (step S1). The device 150 can be set up or prepared, for example, on the basis of a target specification of the layer 10 to be applied. Setting up or preparing includes defining and (pre-)setting the method and system parameters.

[0235]Before, during or after set-up S1, the corresponding stripping efficiency AWZ is determined (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 to step S5. In step S5, the air humidity f in the close range NB is increased by the water vapor device 50. And it is then checked again in step S3 whether the condition f>AWZ is now fulfilled.

[0236]Optionally, the method and/or system parameters can also be slightly adjusted in an intermediate step, wherein this adjustment is preferably carried out in all embodiments in such a way that AWZ remains essentially constant.

[0237]Similarly, 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 surrounding conditions. If f in the surrounding or in the close range NB of the device 150 has decreased, it should be 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 the air humidity f in the close range NB can be increased (analogous to step S5).

[0238]If f_UG drops significantly in the vicinity of the device 150 and if no reasonable adjustment is possible within the target specification, the process can be interrupted.

REFERENCE SIGNS