DEVICE AND METHOD AND DEVICE FOR MOISTURE-CONTROLLED BLOW-OFF AFTER THE APPLICATION OF A LAYER ONTO A FLAT STEEL PRODUCT

Description

The present invention relates to a device with which flat steel products can be coated in a melt bath with a zinc (Zn) or zinc-aluminum-magnesium (ZnAlMg) based layer, e.g. as a protective coating, and can be blown off in a controlled manner on the exit side of the melt bath. It also relates to a corresponding method.

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 input 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 galvanization.

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.

The flat steel product 100 can subsequently be roll-treated by subjecting it to a temper rolling process step and/or a bending-stretching process. The flat steel product 100 can also be chemically post-treated. Flat steel products 100 (e.g. in the form of steel strips) for the automotive industry can, for example, be further processed by oiling. The oiling can be carried out using an oiling machine, for example.

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

Armco Inc. patent EP0172682B1, filed in 1985, deals with the reduction or elimination of oxygen in the surrounding of the stripping nozzle and in the area of the steel strip exiting the zinc bath and the control of zinc vapor in connection with the hot-dip galvanization of an iron-based metal strip. On the exit side of the dip bath, an oxygen-reduced atmosphere is provided instead of the ambient air normally present in an enclosed area in which the stripping nozzle is also located. As zinc would evaporate strongly in this atmosphere, a small amount of water vapor must be added to the oxygen-reduced atmosphere. The enclosed area is located directly above the surface of the melt bath and thus forms a hermetically sealed space. The oxygen-reduced atmosphere in this space is intended to improve the stripping process and the small proportion of water is intended to prevent the formation of zinc vapor on the surface of the dip bath. The moisture content in the hermetically enclosed space is adjusted so that zinc vapor cannot form.

A device is known from the published application DE2033847 which is adapted to regulate the layer thickness when galvanizing steel strips. A wide slotted nozzle is used to blow air, gas or steam against the zinc-coated strip for the purpose of stripping. The nozzle slot is larger in the area of the strip edges than in the area of the strip center in order to take into account the fact that the strip may have a curved shape on the exit side of the bath.

A device that uses wet steam in the form of a steam jet for stripping has been known for over 50 years from the published application DE1521405 A1 from the company National Steel. The use of wet steam results in rapid cooling of the steel strip after hot-dip galvanization, wherein the blown wet steam causes the coating to solidify within a short period of time. It is proposed to use a condenser device that is adapted to supply wet steam at the desired temperature and desired pressure.

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. By the spraying of water droplets irregularities are deliberately created on the surface of the steel strip. This document pursues a different objective and the corresponding technical teaching thus goes in a completely different direction than the present invention.

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

The main problems here are often surface defects of the ZnAlMg layer. For example, marblings (marble-effect), “toothpick” or “beach pattern” defects 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 surrounding of the stripping nozzle) than the present invention.

Similar surface defects can also occur under certain circumstances with Zn layers that contain a proportion of Al (the proportion of Al can for example be less than 1 wt. %).

The task is therefore to provide a device and a method 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.

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

SUMMARY OF THE INVENTION

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 both sides of the steel substrate of the flat steel product from external influences. In the following, the corresponding dip bath is referred to as a zinc alloy melt bath (or short 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) (e.g. less than 1% by weight), as well as a melt bath containing a ZnAlMg alloy. The layer to be applied on both sides is also referred to here as the Zn-containing (protective) layer.

A device for applying a ZnAlMg layer or a ZnAl layer to a flat steel product is proposed. In all embodiments, this device comprises:

• • a zinc alloy melt bath having an input side and an exit side,
  • a gas supply configured to provide dry gas,
  • a water vapor device configured to provide gaseous water vapor,
  • means (e.g. a device) for determining gas humidity (or moisture content),
  • a stripping nozzle device which is fluidically connected to the gas supply and to the water vapor device in order to supply a stripping gas to the stripping nozzle device as a mixture of the dry gas and the gaseous water vapor, wherein
    • the stripping nozzle device comprises at least one gas nozzle for blowing off the front side and at least one gas nozzle for blowing off the rear side of the flat steel product with the stripping gas,
    • the gas nozzles are arranged in the area of the exit side of the zinc alloy melt bath,
    • the means for determining gas humidity (or the moisture content) are arranged in or at the stripping nozzle device in order to determine (e.g. by measuring) the moisture content of the stripping gas before or when the stripping gas exits in the direction of the front or rear side of the flat steel product, and
  • wherein at least one of the following two conditions B1, B2 is fulfilled:
    • B1: the stripping gas (AG) has a moisture content or a proportion of gaseous water vapor (WG) which is greater than 200 ppm and less than 43700 ppm, the moisture content preferably being in the range of 500 ppm to 9980 ppm, B2: the stripping gas (AG) has a dew point (TP) which is greater than −39° C. and less than +30° C., the dew point (TP) preferably being in the range between −29° C. and +7° C.

The moisture content of the stripping gas for at least some of the embodiments for the device and the corresponding method may be defined by the volume fraction (herein referred to as condition B1), which is in the range between 200 ppm and 43700 ppm. Preferably, the volume fraction for at least some of the embodiments is in the range 500 ppm to 9980 ppm.

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

It should be noted that by the B1 and B2 conditions areas that largely overlap are defined. Only the boundary values may deviate due to rounding.

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

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

The upper dew point limit of +30° C. is defined for the moisture content of the stripping gas—this corresponds to a volume fraction of 43700 ppm water in the stripping gas—in order to avoid the condensation of water when the stripping gas emitted by the stripping nozzle mixes with the surrounding gas (or the surrounding air) in the area around the stripping nozzle.

The corresponding method and the device are based on the controlled specification of the moisture content required at the point of impact of the stripping jet at the flat steel product in such a way that marbling and/or toothpick defects are avoided. In other words, there must always be a sufficiently high moisture content in the stripping gas in order to be able to carry out the stripping without the formation of marbling and/or toothpick defects. At the same time, however, as described above, the formation of condensate must be avoided. These two boundary or framework conditions result in a parameter window that is preferably complied with in all embodiments in addition to conditions B1 and/or B2.

Preferably, in the device and the corresponding method, the moisture content, i.e. the water vapor content, is controlled by monitoring the current dew point of the stripping gas and keeping it within a suitably predetermined parameter window (condition B2). In this way, the formation of condensate can be prevented and stripping can be carried out without the defects mentioned. Condensing water would have a negative effect on the stripping process and the surface quality of the galvanized strip.

In all embodiments, a control or control unit of the device can implement a moisture adjustment protocol in order to be able to react to changes in the current moisture content in a suitable form and to ensure compliance with conditions B1 and/or B2.

In all embodiments, the respectively required moisture content is provided directly via the stripping gas. In other words, the stripping gas serves as a carrier or transport medium for the very small amount of water vapor required here.

In all embodiments-preferably in a gas supply line to the stripping nozzle—a water vapor gas stream is introduced into the dry stripping gas, referred to here as the dry gas stream, in order to mix the dry gas and the water vapor gas stream.

Preferably, in all embodiments, the dry gas stream comprises nitrogen or consists of nitrogen. In all embodiments, the dry gas stream may also comprise another inert gas instead of nitrogen.

In all embodiments, the moisture content of the stripping gas can be measured, for example with a humidity sensor (e.g. a thermal or capacitive dew point sensor), which is arranged between the feed point for the gaseous water vapor and the nozzle opening of the stripping nozzle and can also be regulated in further possible embodiments.

It is an advantage of this method, or the corresponding device, that marbling and/or toothpick errors can be effectively avoided without the need for building up or mounting additional, disruptive devices (e.g. a housing or enclosure according to the European patent application EP22182309.9 of the present applicant (V08-0015P-EP/P219205/VA23004) directly above the zinc bath area, or in the immediate vicinity of the stripping nozzles and in the area above the zinc bath. Access to the stripping nozzle and the zinc bath surface for periodic cleaning work, which is necessary for correct process management, remains possible.

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

All embodiments involve 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 steel flat product through a zinc alloy melt bath and blowing it off on the exit side of the bath with the controlled “moistened” stripping gas by means of a stripping nozzle device comprising at least one gas nozzle per side of the steel flat product.

In all embodiments, the zinc alloy of the zinc alloy melt bath can preferably have the following composition, is however not limited to those compositions:

• • an aluminum content being in the range between 1.0 and 3.0 percent by weight and preferably in the range between 1.3 and 2.8 percent by weight,
  • a magnesium content being in the range between 1.0 and 2.5 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.

In all embodiments, the zinc alloy of the zinc alloy melt bath can preferably have the following composition, is however not limited to those compositions:

• • an aluminum content being less than 1.0 percent by weight and preferably in the range between 0.1 and 0.5 percent by weight, and
  • the remainder of the zinc alloy melt bath is zinc and unavoidable impurities.

Preferably, the following definitions apply to the system parameters and/or method parameters in all embodiments:

• • the thickness of the nozzle lip gap (referred to as height of the nozzle opening) of both nozzles is in a range between 0.5 and 5 mm, preferably between 0.6 and 2 mm, particularly preferably between 0.8 and 1.5 mm and/or
  • the effective flow rate (D) of the stripping gas over the strip width is in the range of 200 to 8000 Nm³ per hour, and/or
  • the distance between the nozzle lip gap and the front or rear side of the flat steel product is in a range between 2 and 15 mm, preferably between 3 and 12 mm, and/or
  • the strip speed is in a range between 50 and 200 m/min, preferably between 70 and 150 m/min.

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

All embodiments may have one or more of the following sensor constellations:

• • at least one sensor for determining the humidity of the surrounding air within the close range of the device, and/or
  • at least one sensor for determining the humidity of the surrounding air in the surrounding area of the device (e.g. in the factory hall).

In addition, all embodiments comprise at least one means (e.g. realized as a hardware device) for determining gas humidity or the moisture content in or at the stripping nozzle device in order to determine (e.g. measure) the gas humidity before or at the exit of the stripping gas (in the direction of the front or rear side of the flat steel product).

Preferably, this means for determining gas humidity or the moisture content, or a sensor of these means, is located in a gas supply line at a location which is located in the direction of flow at a point after the dry gas stream and the water vapor gas stream have merged/mixed.

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

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

In all or at least some of the embodiments, the method is characterized in that the ZnAlMg layer or the ZnAl layer is applied to both sides of a flat steel product according to a target specification by moving the flat steel product through a zinc alloy melt bath (ZnAl; ZnAlMg) and, on the output side thereof, stripping gas exits through a nozzle lip gap of at least one gas nozzle in the direction of the front side and through a nozzle lip gap of at least one gas nozzle in the direction of the rear side of the flat steel product in order to blow off the layers on both sides in accordance with the target specification.

The method for applying ZnAlMg layers or ZnAl layers on the front and rear side of a flat steel product comprises the following steps:

• • moving the flat steel product from an input side to an exit side of a zinc alloy melt bath,
  • providing a dry gas stream,
  • providing a water vapor gas stream,
  • combining the dry gas stream and the water vapor gas stream to obtain a stripping gas as a mixture,
  • determining the gas moisture or the moisture content of the stripping gas,
  • discharging the stripping gas through at least one gas nozzle used for blowing off the front side and through at least one gas nozzle used for blowing off the rear side in order to blow off the front side and rear side of the flat steel product with the stripping gas,
    • wherein at least one of the following two conditions B1, B2 is fulfilled:
      B1: the stripping gas (AG) has a moisture content or a proportion of gaseous water vapor (WG) which is greater than 200 ppm and less than 43700 ppm, the moisture content preferably being in the range 500 ppm to 9980 ppm,
      B2: the stripping gas (AG) has a dew point (TP) which is greater than −39° C. and less than +30° C., the dew point (TP) preferably being in the range between −29° C. and +7° C.

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 surrounding air humidity in the area of the device can optionally also be determined. Since the device or the method draws in surrounding air during blow-off (as already mentioned), more precise adjustments can be made to the gas humidity (moisture content) of the stripping gas, taking into account the surrounding air humidity currently present. If the current surrounding air humidity is particularly low, for example, the gas humidity of the stripping gas is usually very important in order to reliably prevent surface defects. In “humid” surrounding conditions, it is not always absolutely necessary to add water vapor to the stripping gas in order to reliably prevent the formation of marbling and/or toothpick defects.

In at least some of the embodiments, the gas humidity (moisture content) of the stripping gas is set/adjusted by using a water vapor device by providing a correspondingly large flow rate of the water vapor gas stream to the actual flow rate of the dry gas stream provided and combining/mixing it with the dry gas stream. I.e., in these embodiments, the flow rate of the water vapor gas stream is actively adjusted to the actual flow rate of the dry gas stream provided (called control or regulation of the water vapor gas stream source).

In all embodiments, it can be assumed in a first approximation that the water vapor gas flow is negligibly small in relation to the dry gas flow. Therefore, the dry gas flow is practically not changed when water vapor is added. It is therefore not absolutely necessary to adjust the dry gas flow. However, the dry gas flow can be reduced in all embodiments if the water vapor gas flow is increased (and vice versa).

In another part of the embodiments, the gas humidity (moisture content) of the stripping gas is set/adjusted by adjusting both the flow rate of the dry gas stream and the flow rate of the water vapor gas stream. This can be done, for example, using adjustable gas valves in a dry gas supply and in a water vapor gas supply. Or the output quantity of the dry gas flow source and the water vapor gas stream source is controlled or regulated.

In some of the embodiments, the gas humidity (moisture content) of the stripping gas can be set/adjusted by a mixing valve adjusting one or both flow rates in the area where the two gas streams combine.

In at least some of the embodiments, the close range of the device is defined as a volume in a range from 1 m³ to 10 m³.

In at least some of the embodiments, the surrounding of the device is defined as a volume that is greater than 10 m³.

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

In all embodiments or at least some of the embodiments, the unavoidable impurities of the alloys 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.

With the combination of a well-defined bath composition together with monitoring and/or adjusting the gas humidity (moisture content) of the stripping gas, a surface can be produced that shows no or negligible marbling and no or negligible toothpick defects. During production of the respective layers, the gas humidity (moisture content) of the stripping gas can be kept substantially constant or adjusted (e.g., as the humidity in the close range or surrounding of the device changes) to obtain consistent layers (that are within the predetermined specification).

In all embodiments, the stripping nozzle device may optionally be followed by a strip stabilizing device that is used to automatically stabilize the movement of the flat steel product.

In all embodiments, the device is preferably operated in the following range(s):

• • alloy melt bath with a bath temperature TB in the range 400<TB<480 degrees Celsius, preferably in the range 409<TB<472 degrees Celsius, and particularly preferably in the range 410<TB<460 degrees Celsius,
  • nozzle distance from the flat steel product, which is between 2 and 15 mm, preferably between 3 and 12 mm,
  • blowing off the flat steel product on the exit side of the alloy melt bath with the (stripping) gas, which flows 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 per meter of strip width.

The processes in the area of the stripper nozzle device and on the flat steel product are complex and depend on numerous (method and system) parameters and influencing variables. Therefore, control or regulation of the device can be based on some simplified assumptions and specifications.

The preferred specifications for the ZnAl and ZnAlMg alloying concepts defined above and the preferred specification of a moisture content of the stripping gas, which is defined, for example, by a dew point lying in the range between −39° C. and +30° C., or by the specification of ppm (e.g. between 200 ppm and 43700 ppm), result from numerous investigations. Within the specified limits of the alloying concepts defined here by way of example and the moisture content according to condition B1 and/or B2, the technical teaching presented here has proved particularly successful.

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

However, a device for condensing excess water vapor can also be used in the supply line for the water vapor gas.

In all embodiments, the flat steel product may be subjected to an annealing or tempering step at a temperature of about 765° C. (or at a lower or higher temperature) prior to hot-dip galvanization in a zinc alloy melt bath.

In all embodiments, the flat steel product may be cold rolled after hot-dip galvanization (for example, using smooth cold rolls and/or using skin-pass rolls with special roughness).

Furthermore, the steel strip can additionally be subjected to a temper rolling process or alone in-line to a bending-stretching process in order to increase the flatness of the steel strip.

In all embodiments, the flat steel product in strip form, for example as deep-drawing steel, mild steel, structural steel, steel of a higher-strength steel grade, in each case in strip form, can be cleaned of rolling oil and rolling abrasion in a so-called pre-treatment or pre-cleaning in the continuous hot-dip galvanization plant by means of a combined immersion/brushing/electrolytic cleaning, rinsed with water and dried. The cleaned, dried flat steel product in strip form then enters the annealing furnace of a continuous hot-dip galvanization line, where it is preheated, heated and brought to the annealing temperature under inert gas. At the end of the annealing furnace, the flat steel product in strip form is cooled to a strip immersion temperature and immersed in the ZnMgAl alloy melt bath. After exiting the bath, the flat steel product in strip form is adjusted to the target coating thickness with the stripping gas at the stripping nozzles in accordance with the embodiments described and claimed here. In a subsequent cooling tower, the melt zinc alloy on the steel strip is brought to solidification.

Following the cooling tower, the flat steel product in strip form can be re-rolled in-line (in the continuous hot-dip galvanization line) in a skin pass mill and a predetermined roughness can be applied.

After an in-line inspection for surface defects, in which, for example, surface defects such as marbling or toothpick defects are detected, the flat steel product in strip form can be coated with corrosion protection and forming oil in an oiling machine and finally wound on the reel. The steel product in the form of a coiled steel strip can be coated with a lacquer in a strip coating machine after being wound onto the reel. Alternatively, the flat steel product in strip form is coated with an organic/inorganic passivation layer by means of a coater (coating device) after bending-stretch-straightening and/or finishing in a chemical post-treatment and then dried. Then the flat steel product 100 is inspected for surface defects and wound on the reel if no marbling or toothpick defects are detected.

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

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, the stripping nozzle device comprising herein only one nozzle for blowing off the front side of the flat steel product;

FIG. 3 shows a highly schematized representation of a second exemplary device in which the method of the invention is used, the stripping nozzle device comprising herein each one nozzle for blowing off the front side and for blowing off the rear side of the flat steel product;

FIG. 4 shows a highly schematized representation of two gas nozzles facing each other;

FIG. 5 contains a table with numerous examples.

DETAILED DESCRIPTION

The moisture content of gases can be described in different ways. It is common to specify the dew point in ° C., the mass fraction of water per volume of the gas in g/m³ (also known as absolute humidity) and the volume fraction in ppm (parts per million, also known as ppm V). The dew point describes the temperature at which water vapor begins to condense in a gas (here in the stripping gas AG). When the dew point temperature is reached, the gas can no longer absorb any additional water vapor, i.e. the gas is saturated with water vapor. The term frost point can be used for temperatures that are below 0° C. However, we use the term dew point throughout (even at negative temperatures). Pure nitrogen gas, which is used as stripping gas AG for blowing off, typically has a temperature of 10 to 30° C. However, the nitrogen gas can also be heated before blowing off (e.g. to temperatures in the range of 50 to over 200° C.). Warm stripping gas AG can absorb more water vapor, so the dew point can be higher. The opposite is true for cold stripping gas AG. In addition, stripping gas AG can absorb less water vapor under high pressure than at lower gas pressure. It should also be noted that condensation of the water vapor usually occurs in the lines and components (e.g. in the nozzles 15) of the gas-carrying system.

In the context of the present invention, the moisture content of the stripping gas AG is preferably defined by setting a minimum dew point TP_min. In all embodiments that rely on adding a small proportion of water vapor WG to the stripping gas AG, the minimum dew point TP_min is −39° C., preferably the minimum dew point is TP_min=−29° C. In all embodiments, a maximum dew point TP_max can also be set, which is +30° C. (which corresponds to approx. 43700 ppm H₂O in the stripping gas AG). In order to avoid the occurrence of surface defects, the moisture content of the stripping gas AG is adjusted in all these embodiments so that the dew point TP of the stripping gas AG is always greater than TP_min=−39° C. or preferably TP_min=−29° C. This means, the formula TP>TP_min applies. In these embodiments, this specification means that we are independent of the surrounding conditions (in the factory hall in which the device 150 is operated, the temperature and humidity of the surrounding air can fluctuate significantly depending on the climate and the time of year).

In order to avoid condensation of water in the stripping nozzle device 14 in all embodiments, a temperature difference ΔT can be specified in all embodiments according to the formula TP=T_AG−ΔT. The temperature difference ΔT can preferably be at least 5° C. and particularly preferably at least 10° C. In this formula, TP defines the current dew point in the stripping gas flow AG and T_AG defines the current temperature of the stripping gas flow AG.

By specifying a temperature difference ΔT, for example, the fact that the tendency for condensation of the water vapor component increases when the pressure of the stripping gas AG is increased can be taken into account. By specifying a temperature difference ΔT, which is to be understood as a kind of safety margin, it can be ensured that condensate is not formed even if the pressure of the stripping gas AG increases slightly. The safety margin also prevents condensation in the stripping gas AG due to fluctuations in the regulation of the metered water vapor gas.

In addition, in all embodiments, a maximum dew point of the stripping gas AG is specified, which is at TP_max=+30° C. (which corresponds to approx. 43700 ppm H₂O in the stripping gas AG). Preferably, the maximum dew point is at TP_max=+7° C. (sample number 74 in Table 1). If, for example, the temperature T_AG of the stripping gas is 18° C., the dew point TP of the stripping gas AG is therefore be set to values in the range −39° C.<TP<18° or preferably to values in the range −29° C.<TP<+7° C.

If the optional safety margin ΔT of 10° C. described above is now additionally applied, then, for example, the dew point TP of the stripping gas AG at a gas temperature T_AG of 18° C. is to be set to values in the range −39° C.<TP<T_AG−ΔT (with T_AG−ΔT=18° C.-10° C.=8° C.)→−39° C.<TP<+8° C. or preferably to values in the range −29° C.<TP<+7° C.

Among other things, this relates to a device 150 (see, for example, FIG. 2) which is adapted for applying a layer 10 (see FIG. 4) to the front side V and rear side R of a strip-shaped flat steel product 100. The components of the dip bath 11 are not shown here (reference is made to FIG. 1, which shows exemplary components). The exit side of the dip bath 11 is identified by the letter A in FIG. 2. The flat steel product 100 is moved vertically upwards in the direction of arrow B. The stripping nozzle device 14 comprises at least one gas nozzle 15 for blowing off the front side V and at least one gas nozzle 15 for blowing off the rear side R of the flat steel product 100. In FIG. 2, the stripping nozzle device 14 comprises only one gas nozzle 15 for blowing off the front side V due to the simpler representation. The gas nozzle 15 for blowing off the rear side R is adapted accordingly. In the embodiment shown in FIG. 3, two stripping nozzle devices 14 are provided, which comprise opposing nozzles 15.

The layers 10 on both sides V, R are produced by passing the flat steel product 100 from an input side E to an exit side A through a zinc melt bath 11 (see e.g. FIG. 1) and blowing it off on the exit side A by means of the stripping nozzle device 14 with (stripping) gas AG. The purpose of the stripping nozzle device 14 is to strip off the excess (still liquid) ZnMgAl layers or ZnAl layers (layers 10) at the flat steel product 100 in a controlled manner with the (stripping) gas AG after it emerges from the bath 11.

Care must be taken to ensure that the layers 10 are produced in accordance with the (predetermined) specification (the specification defines, for example, the target thickness or the coating per side V, R) and that no marbling and/or no toothpick defects occur/occurs. Some of the embodiments may, for example, be concerned with avoiding these “errors” in the event of changing surrounding conditions in the production area (e.g. in the factory hall). Even if the surrounding air humidity f_UG should change in the wider surrounding of the device 150 (e.g. in the factory hall), or in the immediate close range, the device 150 and the method according to the invention can ensure that no marbling and/or toothpick defects occur/occurs and that the layers 10 can continue to be produced according to specification.

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

In at least some of the embodiments, the target surface coating (coating per side of the strip) can be in the range from 20 to 200 g/m² and particularly preferably in the range from 30 to 160 g/m².

In all embodiments, the stripping nozzle device 14 comprises at least one gas nozzle 15 per side V, R (e.g. two gas nozzles 15 facing each other, as indicated in FIGS. 3 and 4).

The flow rate of the (stripping) gas AG, which is emitted through the nozzle lip gap 17 in the direction of the front V or rear R, is indicated here in Nm³ (Nm³ stands for standard cubic meter). A standard cubic meter is the amount of stripping gas AG contained in a volume of one cubic meter. This applies at a temperature of 0 degrees Celsius and a pressure of 1.01325 bar.

The term dry gas TG is used here to refer to an inert gas that has a dew point of approx. −70° C. and below. This corresponds to a water vapor content of approx. 5 ppm and below. The dry gas TG therefore has a very low residual moisture content (also known as traces of moisture) in a range that is normal for industrial gases. In all embodiments, the dry gas TG used here can meet the requirements of the “Specification for Industrial nitrogen”, British Standard BS 4366:1993, for example. According to this standard, the water content of the gaseous nitrogen dry gas TG (cf. paragraph 8) is set at a maximum of 10/10⁶, which corresponds to 10 ppm. Thus, in all embodiments, the dry gas TG should have a residual moisture content that is less than 10 ppm and preferably less than 5 ppm.

In all embodiments, the device 150 comprises a dry gas supply or dry gas source 18 (see FIG. 2). For example, a gas tank, a gas cylinder or a gas line (coming, for example, directly from the gas supplier or from an air separation apparatus operating, for example, according to the Linde process) may serve as source 18.

In all embodiments, the device 150 may also comprise, for example, two dry gas supplies or dry gas sources 18 (see FIG. 3), wherein one of the sources 18 is associated with each stripping nozzle device 14. In all embodiments, the device 150 may also comprise, for example, a dry gas supply or dry gas source 18 that feeds both stripping nozzle devices 14.

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

Preferably, in all embodiments, one or more steam generator(s) is/are used as steam device(s) or source(s) 50, which is/are adapted as high-purity steam generator(s) that generate gaseous steam WG, or a water vapor gas stream WG, from purified or highly purified water.

In all embodiments, the water vapor device 50 may preferably comprise a pure steam generator and, for example, a valve capable of regulating the stream of water vapor gas WG through the conduit 22. For this purpose, in all embodiments, the high purity steam generator and/or the valve may be connected to a controller of the device 150 and/or to the device for determining the gas humidity 20 or 26 (not shown). In all embodiments, the water vapor gas stream WG may also originate from a condensate recovery system.

In all embodiments, the device 150 may also comprise, for example, two water vapor devices or sources 50 (see FIG. 3), wherein each of the sources 50 is associated with each stripping nozzle device 14.

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

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

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

Optionally, in all embodiments, a water vapor gas stream WG can be supplied to the stripping gas AG only if the surrounding conditions per se should not be sufficient (e.g. if the surrounding air should be too dry) in order to avoid these errors.

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

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

The sensor 23 may, for example, be a humidity sensor and the sensor 24 may be a temperature sensor. Both sensors 23, 24 are connected for the means of communication to a module 26 of the device 20 via lines KV3, KV4.

In all embodiments, combined or integrated sensors can also be used, which measure the moisture content and the temperature T_AG of AG. FIG. 3 shows an embodiment in which one combined or integrated sensor 23/24 is provided for each stripping nozzle device 14. The corresponding communication line is labeled KV5.

In all embodiments, sensors can also be used which measure, for example, the pressure dew point and the absolute moisture content f and temperature T_AG of the gas AG.

Digital and/or analog sensors can be used in all embodiments.

In all embodiments, a dew point meter can also serve as device 20.

Suitable moisture sensors include, for example, sensors based on the principle of absorption of electromagnetic waves (microwave absorption sensors), or which determine a change in the dielectric constant (capacitively operating sensors). An example of this is a polymer sensor adapted for measuring the humidity of gases in the temperature range of interest here.

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

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

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

The present context is based on the following estimates/approximations, as shown in Table 1. The relative air humidity r can be used to indicate the proportion of moisture in gases in relation to the highest possible saturation of the gas. r=100% humidity means that no more water vapor can be absorbed in the gas. At r=100%, the gas is saturated with water vapor. If the temperature of the gas is increased, the gas can absorb a larger amount of water and the saturation vapor pressure of the gas increases. If the amount of water in the gas remains constant when the temperature is increased, the value of the relative humidity r decreases. The absolute humidity f (in g/m³) of gases and thus also the absolute humidity f_UG of the surrounding air used here are also temperature-dependent. The dew point in ° C. and the volume fraction of H₂O in ppm, on the other hand, are temperature-independent and apply regardless of the gas type.) The dew point in ° C. and the volume fraction of H₂O in ppm are therefore preferably used here to specify the moisture content of the stripping gas AG.

The examples shown in Table 1 (see FIG. 5) apply specifically to the alloy compositions and method conditions indicated in each case. Table 1 (see FIG. 5) contains the following columns from left to right: Sample number; dew point TP of the stripping gas AG in ° C. (whereby the examples in Table 1 are sorted from the lowest to the highest dew point); volume fraction of water vapor H₂O in the stripping gas AG in ppm; coating per side (layer 10 per side V, R) in g/m²; strip speed of the flat steel product 100 (parallel to the movement arrow B) in m/min; the thickness of the nozzle lip gap 17 in mm (called height of the nozzle opening); the horizontal distance between nozzle and strip (flat steel product 100) in mm; the nozzle height, the vertical distance of the nozzle to the zinc bath surface in mm; the nozzle pressure in mbar; the bath temperature of the bath 11 in ° C.; the bath composition by specifying the proportions of Al and Mg in wt. %; the absolute surrounding air humidity f_UG in g/m³. The two columns on the far right contain information on marbling, where

• • a black dot symbolizes an alloy/method example in which strong marbling was detected,
  • a grey dot symbolizes an alloy/method example in which light to medium marbling was found, and
  • a white dot symbolizes an alloy/method example in which no marbling was detected.

It can be seen from Table 1 (see FIG. 5) that the use of dry nitrogen as a stripping gas in many alloy/method examples results in the occurrence of sometimes severe marbling defects (specifically in the examples with sample numbers 1 to 43). If one looks at the second and third column (from the left) for the examples with sample numbers 1 to 43, the stripping gas AG is a dry nitrogen gas with a dew point TP between −77° C. and −72° C. and a moisture content of between 1.8 ppm and 3.8 ppm. Such a nitrogen gas meets the requirements of the aforementioned British Standard, as the residual moisture is less than 10 ppm. Marbling cannot be reliably and reproducibly avoided with such a dry nitrogen gas as stripping gas AG, as can be seen from the two columns on the far right.

Only for the examples with the sample numbers 53 to 74 does strong marbling no longer occur. This clear reduction in strong marbling is achieved by adding a small proportion of the water vapor gas WG with at least 208 ppm (sample number 53) and up to 9978 ppm (sample number 74) to the dry nitrogen gas TG. The dew point of the stripping gas AG for sample numbers 53 to 74 is between −39° C. (sample number 53) and +7° C. (sample number 74).

A condition B1 can be derived from this, as follows: The stripping gas AG should always have a moisture content, or a proportion of the gaseous water vapor WG, which is greater than 200 ppm and less than 43700 ppm. The lower limit of 200 ppm is derived from the test results shown in Table 1 (208 ppm rounded down to 200 ppm), the upper limit of 43700 ppm ensures that there is no condensation of water in the surrounding of the stripping nozzle, as previously mentioned.

A condition B2 can also be derived from this, as follows: The stripping gas AG should always have a dew point TP that is greater than −39° C. and less than +30° C. The lower limit of −39° C. results from the test results shown in Table 1, the upper limit of +30° C. ensures that there is no condensation of water in the surrounding of the stripping nozzle.

Only for the examples with sample numbers 58 to 74 does marbling no longer occur at all (with the exception of sample number 59). This significant decrease in marbling is achieved by adding a small proportion of the water vapor gas WG with at least 552 ppm (sample number 58) and up to 9978 ppm (sample number 74) to the dry nitrogen gas TG. The dew point of the stripping gas AG for sample numbers 58 to 74 is between −29° C. (sample number 58) and +7° C. (sample number 74).

A further preferred condition B1 can be derived from this, as follows: The stripping gas AG should always have a moisture content, respectively a proportion of the gaseous water vapor WG, which is in the range 500 ppm to 9980 ppm, whereby these ppm specifications were rounded down or up.

The preferred condition B2 can also be derived from this, as follows: The stripping gas AG should always have a dew point TP that is in the range between-29° C. and +7° C.

These specifications with regard to conditions B1 and B2 relate to nitrogen as the dry gas TG and to temperatures T_AG of the stripping gas AG, which are in the range between 10° C. and 30° C.

For the conditions specified under B2, the respective dew point TP of the stripping gas AG is therefore below the current temperature T_AG of the stripping gas AG. This condition (TP<T_AG) is referred to here as condition B2.1. The application of condition B2.1 has the advantage that it is independent of the temperature T_AG of the stripping gas AG. For example, if the stripping gas AG has a temperature T_AG of 27° C., then the dew point TP of the stripping gas AG must be below +27° C. in order to fulfill the condition B2.1.

The preferred condition B2.1 can also be defined as follows: there should always be an optional safety margin ΔT between the temperature T_AG of the stripping gas AG and the dew point TP of the stripping gas AG, as follows: TP<T_AG−ΔT. This optional safety margin ΔT can be ΔT=10° C. in all embodiments. In order to fulfill this preferred condition B2.1, the dew point TP of the stripping gas AG should be below +17° C. if the temperature T_AG of the stripping gas AG is, for example, 27° C.

This condition B2.1 also applies with changing gas pressure and/or changing temperature T_AG of the stripping gas AG.

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

This condition B3 also applies with changing gas pressure and/or changing temperature T_AG of the stripping gas AG.

If the conditions B1 and/or B2 and/or B2.1 are complied with in relation to the stripping gas AG, then one is on the safe side in terms of marbling. Condition B3 is an additional condition that can be complied with in all embodiments in addition to conditions B1 and/or B2 and/or B2.1.

By complying with these conditions, the process of hot-dip galvanization and blow-off is stabilized. I.e., these processes become more robust against disturbing surrounding conditions (some of which cannot be influenced). In addition, the parameter window in which the method works reliably is extended.

In series of tests and in the evaluation of process data, it has been shown that the air humidity in the surrounding or in the close range around the stripping nozzle has a significant influence on the occurrence of marbling defects. f_UG is used here as a formula symbol for the absolute air humidity of the surrounding or the close range around the stripping nozzle (also called surrounding air humidity). In all embodiments, the absolute air humidity f_UG can be estimated accordingly from the air temperature T_L of the surrounding or the close range and the relative air humidity r of the surrounding or the close range. The following formula is used here:

f UG = 13 . 235 · r T L + 273.15 · 10 7.5 · T L T L + 237.3

• • r relative humidity or air humidity in %
  • f_UG air humidity of the surrounding or the close range
  • T_L air temperature of the surrounding or the close range in ° C.

The determination/measurement/monitoring of the air humidity f_UG of the surrounding or of the close range and/or of the air temperature T_L can, as already described, be carried out directly or indirectly in all embodiments. Indirect measurement is understood here to mean, among other things, measuring the air temperature T_L and the relative air humidity r and calculating/deriving the absolute local air humidity f_UG from this.

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

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

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 and 3), exerts a shearing force on the still liquid layer 10. The shear force reduces the thickness of the layers 10 by blowing them off with the stripping gas AG.

The equations describing the dynamic flow behavior of the gas AG 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 stripping gas AG (for this reason, it may not be necessary to add water vapor gas WG to the dry gas TG if the surrounding air humidity f_UG is high). Details can be found, for example, in the publications “Wall Pressure and Shear Stress Measurements Beneath an Impinging Jet”, C. V. Tu, D. H. Wood, Experimental Thermal and Fluid Science Volume 13, Issue 4, November 1996, Pages 364-373 and “Minimization of the N2 Dilution When Wiping in Air”, M. Dubois, in AISTech 2019—Proceedings of the Iron & Steel Conference, May 6-9, Association for Iron & Steel Technology, Warrendale, PA, 2019, Pittsburgh, USA.

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

Further specific investigations have shown a correlation between the moisture content of the stripping gas AG and the occurrence of such surface defects and errors, wherein the (surrounding) air humidity f_UG can also have an influence (if the surrounding air humidity is high enough, surface defects and errors may not occur under certain circumstances). In Table 1 (cf. FIG. 5), a summary of all parameter ranges or sample numbers are highlighted in light gray (sample numbers 53 to 74), which enable dip coating and controlled blow-off of the layers 10 without severe surface defects and errors occurring. In Table 1 (cf. FIG. 5), all parameter ranges or sample numbers are highlighted in dark gray (sample numbers 58 to 74), which enable dip coating and controlled blow-off of the layers 10 without surface defects and errors occurring.

When operating the device 150, care is preferably taken in all embodiments that

• • the thickness of the nozzle lip gaps 17 (referred to as the height of the nozzle openings) is in a range between 0.5 and 5 mm, preferably between 0.6 and 2 mm, particularly preferably between 0.8 and 1.4 mm, and/or
  • the flow rate of the exhaust gas stream AG is in the range of 200 to 8000 Nm³ per hour, and/or
  • the nozzle distance (nozzle-strip distance) of nozzle 15 to side V or R is in the range between 2 and 15 mm, preferably between 2.5 and 14.1 mm, and/or
  • the strip speed of the steel strip 100 is in a range between 50 and 200 m/min, preferably between 70 and 150 m/min.

The device 150 and the method operate particularly reliably within these (value) ranges.

In all embodiments, a corresponding gas nozzle 15 has a length extension (called nozzle width) perpendicular to the drawing plane of FIGS. 2, 3 and 4 (parallel to the z-axis in FIG. 4). Preferably, in all embodiments, the nozzle 15 has an active nozzle width which corresponds at least to the strip width of the strip-shaped flat steel product 100. In all embodiments, the strip width of the strip-shaped flat steel product 100 can be, for example, in the range of 500 to 2500 mm, preferably between 800 and 1800 mm, and particularly preferably in the range of 1159 mm to 1614 mm. In the case of wider strip-shaped flat steel products 100, the active nozzle width also increases accordingly.

The nozzles are positioned at a variable vertical distance from the zinc bath surface. This distance is commonly referred to as nozzle height. This distance is primarily set as a function of the speed of the passing strip and/or the zinc coating layer to be set. In all embodiments, the nozzle height can be between 230 and 500 mm, for example.

All embodiments of the device 150 may comprise an optional controller 250, as schematically and exemplarily indicated in FIG. 3. In all embodiments, this controller 250 may be adapted as a computerized automation and control unit and may comprise a human-machine interface, a computer and a database.

In all embodiments, the controller 250, if present, may be connected to the mean(s) or device(s) for determining gas humidity 20 via communication links KV1, KV2.

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

In all embodiments, the controller 250, if present, may have one or more analog and/or digital inputs to obtain information about the currently prevailing surrounding conditions (e.g., the current air temperature T_L and/or the (absolute) air humidity f_UG of the surrounding or the close range). Based on this information, the controller 250 can, for example, reduce or increase (or even switch off) the metering of the water vapor gas stream WG in order to continue to produce layers 10 with defect-free surfaces in the device 150.

In all embodiments, the controller 250, if present, may comprise communication links that allow the controller 250 to reduce or increase the flow of the water vapor gas stream WG, and/or to reduce or increase the flow of the dry gas stream TG. Alternatively, the controller 250 can, for example, adjust a mixing valve in the area where the two gas streams TG, WG merge 19 according to the situation via communication links.

If, for example, the air humidity of the surrounding air f_UG is very low, the intake of dry surrounding air can lead to the formation of marbling at the layer 10. This is where the invention can come into play by switching on or automatically increasing the moisture content in the stripping gas AG.

First embodiment example: A cold-rolled flat steel product 100 in strip form, namely cold-rolled deep-drawn steel in strip form, is cleaned of rolling oil and rolling abrasion in at least some of the embodiments in a pretreatment in the continuous by means hot-dip galvanization system of a combined immersion/brushing/electrolytic cleaning and rinsed with water and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip galvanization system, where it is preheated, heated by means of a direct-fired furnace (DFF) and brought to an annealing temperature of 820° C. in a radiant tube furnace under protective gas at a dew point of −40° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of 450° C. and immersed in the 430° C. warm ZnMgAl alloy melt bath 11 for 3s. After leaving the bath 11 on the exit side A, the flat steel product 100 in strip form is adjusted to a predetermined target layer thickness of ZM90 (45 g/m² per side V, R) at the stripping nozzles 15 of the stripping nozzle device 14 using dry stripping gas AG with a dew point of minus 73° C. or 3 ppm H₂O content. The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (referred to as the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 6 mm on both sides to the flat steel product 100 in strip form. The zinc alloy melt is solidified on the flat steel product 100 in a connected cooling tower 16 (see FIG. 1). The flat steel product 100 in strip form is then rolled in a skin-pass mill and a roughness of 1.4 μm is imprinted. After an inspection for surface defects, during which surface defects such as marbling and toothpick defects are identified, the flat steel product 100 in strip form is coated with 1.0 g/m² of anti-corrosive and forming oil per side V, R in an oiling machine and finally wound up on the reel. In a further step, for example in a so-called inspection line, the flat steel product 100 in strip form can be unwound, the defective area separated and the strip then wound up again.

Second embodiment example: A cold-rolled flat steel product 100 in strip form, namely a cold-rolled deep-drawn steel in strip form, is cleaned of rolling oil and rolling abrasion, rinsed with water and dried as part of an in-line pretreatment of the continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 820° C. in the radiant tube furnace under protective gas at a dew point of −40° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 450° C. and immersed for 3s in the 430° C. warm ZnMgAl alloy bath 11. After leaving the bath in the exit area A, the flat steel product 100 in strip form is set at the stripping nozzles 15 with humidified nitrogen (after admixture of gaseous H₂O vapor, as described and claimed here, to 310 ppm or −35° C. dew point) to the target layer thickness of ZM90 (45 g/m² per side). The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 6 mm on both sides of the flat steel product 100. In a connected cooling tower 16 (see FIG. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then rolled in a skin-pass mill and a roughness Ra of 1.4 μm is imprinted. After inspection for surface defects, during which the presence of marbling is determined, the flat steel product 100 is coated in-line (in a continuous hot-dip galvanization system) with an oiling machine with 1.0 g/m² per side with anti-corrosion and forming oil and finally wound up on the reel. In a further step, for example in a so-called inspection line, the flat steel product 100 can be unwound in strip form, the defective area can be separated and the strip can then be rewound.

Third embodiment example: A cold-rolled deep-drawing steel in strip form is cleaned in-line in the pretreatment of the continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning of rolling oil and rolling abrasion, rinsed with water and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 820° C. in the radiant tube furnace under protective gas at a dew point of −40° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 450° C. and immersed for 3s in the 430° C. ZnMgAl alloy bath 11. After leaving the bath in the exit area A, the flat steel product 100 in strip form is set to the target layer thickness of ZM90 (45 g/m² per side) at the stripping nozzles 15 with humidified nitrogen (after admixing gaseous H₂O vapor, to 552 ppm or −29° C. dew point). The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 6 mm on both sides of the flat steel product 100. The zinc alloy melt is solidified at the flat steel product 100 in a connected cooling tower 16 (see FIG. 1). The flat steel product 100 is then rolled in a skin-pass mill and a roughness of 1.4 μm is imprinted. After inspection for surface defects, during which neither marbling nor toothpick defects are found, the flat steel product 100 is coated in-line with an oiling machine with 1.0 g/m² per side with anti-corrosive and forming oil and finally wound up on the reel.

Fourth embodiment example: A high-strength cold-rolled flat steel product 100 in strip form is cleaned of rolling oil and rolling abrasion in the pretreatment of the continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning, rinsed with water and dried. The cleaned, dried flat steel product 100 enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 800° C. in the radiant tube furnace under protective gas at a dew point of −50° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 490° C. and immersed for 5s in the 430° C. warm ZnMgAl alloy bath 11. After leaving the bath in the exit area A, the flat steel product 100 in strip form is adjusted to the target layer thickness of ZM90 (45 g/m² per side) at the stripping nozzles 15 with humidified nitrogen (after admixture of gaseous H₂O vapor, to 3065 ppm or −9° C. dew point). The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 7.6 mm on both sides of the flat steel product 100. In a connected cooling tower 16 (see FIG. 1), the zinc alloy melt is solidified at the flat steel product 100. The flat steel product 100 is then rolled in a skin-pass mill and a roughness of 1.3 μm is imprinted. After inspection for surface defects, during which neither marbling nor toothpick defects are found, the flat steel product 100 is coated in-line with an oiling machine with 0.8 g/m² per side with anti-corrosion and forming oil and finally wound up on the reel.

Fifth embodiment example: A structural steel is cleaned as a cold-rolled flat steel product 100 in strip form in the pretreatment of the continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning of rolling oil and rolling abrasion, rinsed with water and dried. The cleaned, dried flat steel product 100 enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 730° C. in the radiant tube furnace under protective gas at a dew point of −45° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 465° C. and immersed for 4s in the 455° C. warm ZnMgAl alloy bath 11. After leaving the bath in the exit area A, the flat steel product 100 in strip form is set at the stripping nozzles 15 with dry nitrogen (without admixture of gaseous H₂O vapor; 3 ppm or −75° C. dew point) to the target layer thickness of ZM120 (60 g/m² per side). The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 7.7 mm on both sides from the flat steel product 100. The zinc alloy melt is solidified at the flat steel product 100 in a connected cooling tower 16 (see FIG. 1). The flat steel product 100 is then made flat in a bending-stretching-straightening machine and then rolled in a skin-pass mill and a roughness of 1.4 μm is imprinted. After inspection for surface defects, during which neither marbling nor toothpick defects are found, the flat steel product 100 is wound up on the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating system.

Sixth embodiment example: A structural steel is cleaned as a cold-rolled flat steel product 100 in strip form in the pretreatment of a continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning of rolling oil and rolling abrasion, rinsed with water and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 730° C. in the radiant tube furnace under protective gas at a dew point of −45° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 465° C. and immersed for 4s in the 455° C. warm ZnMgAl alloy bath 11. After exiting the bath in the exit area A, the flat steel product 100 in strip form is set to the target layer thickness of ZM120 (60 g/m² per side) at the stripping nozzles 15 with humidified nitrogen (after admixing gaseous H₂O vapor to 1470 ppm or −18° C. dew point). The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 7.7 mm on both sides of the flat steel product 100. In a connected cooling tower 16 (see FIG. 1), the zinc alloy melt is solidified at the flat steel product 100. The flat steel product 100 is then made flat in a bending-stretching-straightening machine and re-rolled in a skin pass mill, and a roughness of 1.4 μm is imprinted. After inspection for surface defects, during which neither marbling nor toothpick defects are found, the flat steel product 100 is wound up on the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating system.

Seventh embodiment: A structural steel is cleaned as a cold-rolled flat steel product 100 in strip form in the pretreatment of a continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning of rolling oil and rolling abrasion, rinsed with water and dried. The cleaned, dried flat steel product 100 enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 730° C. in the radiant tube furnace under protective gas at a dew point of −45° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 465° C. and immersed for 4s in the 455° C. warm ZnMgAl alloy bath 11. After exiting the bath in the exit area A, the flat steel product 100 in strip form is set to the target layer thickness of ZM120 (60 g/m² per side) at the stripping nozzles 15 using humidified nitrogen (after admixing gaseous H₂O vapor to 5230 ppm or −2° C. dew point). The stripping nozzles 15 have a nozzle lip gap of 1.0 mm (height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 7.7 mm on both sides of the flat steel product 100. In a connected cooling tower 16 (see FIG. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then made flat in a bending-stretching-straightening machine and rolled in a skin pass mill, and a roughness of 1.4 μm is imprinted. After inspection for surface defects, in which neither marbling nor toothpick defects are found, the steel strip 100 is wound up on the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating system.

Eighth embodiment example: A mild steel is cleaned as a cold-rolled flat steel product 100 in strip form in the pretreatment of a continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning of rolling oil and rolling abrasion, rinsed with water and dried. The cleaned, dried flat steel product 100 in strip form enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 720° C. in the radiant tube furnace under protective gas at a dew point of −50° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 460° C. and immersed for 3s in the 455° C. warm ZnMgAl alloy bath 11. After exiting the bath in the exit area A, the flat steel product 100 in strip form is set at the stripping nozzles 15 with dry nitrogen (without admixture of gaseous H₂O vapor; 3 ppm or −74° C. dew point) to the target layer thickness of ZM90 (45 g/m² per side). The stripping nozzles 15 have a nozzle lip gap of 1.2 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 10 mm on both sides of the flat steel product 100. The zinc alloy melt is solidified on the flat steel product 100 in a connected cooling tower 16 (see FIG. 1). The flat steel product 100 is then flattened in a bending-stretching-straightening machine and rolled in a skin-pass mill, and a roughness of 1.0 μm is imprinted. As part of an in-line chemical post-treatment, an organic/inorganic passivation layer is applied and dried using a coater (coating device). After inspection for surface defects, during which marbling and toothpick defects are identified, the steel strip 100 is wound up on the reel. In a further step, for example in a so-called inspection line, the flat steel product 100 can be unwound in strip form, the defective area can be separated out and the strip can then be wound up again.

Ninth embodiment example: A mild steel is cleaned as a cold-rolled flat steel product 100 in strip form in the pretreatment of a continuous hot-dip galvanization system by means of a combined immersion/brushing/electrolytic cleaning of rolling oil and rolling abrasion, rinsed with water and dried. The cleaned, dried flat steel product 100 enters an annealing furnace of the continuous hot-dip galvanization system, is preheated, heated by means of a directly fired furnace (DFF) and brought to the annealing temperature of 760° C. in the radiant tube furnace under protective gas at a dew point of −50° C. At the end of the annealing furnace, the flat steel product 100 in strip form is cooled to the strip immersion temperature of the ZnMgAl alloy melt bath 11 of 460° C. and immersed for 4s in the 455° C. warm ZnMgAl alloy bath 11. After exiting the bath in the exit area A, the flat steel product 100 in strip form is set at the stripping nozzles 15 with humidified nitrogen (after admixing gaseous H₂O vapor to 9980 ppm or +7° C. dew point) to the target layer thickness of, for example, ZM120 (60 g/m² per side). The stripping nozzles 15 have a nozzle lip gap of 1.2 mm (called the height of the nozzle opening) and are positioned at a horizontal distance (parallel to the y-axis of FIG. 4) of 10 mm on both sides of the flat steel product 100. In a connected cooling tower 16 (see FIG. 1), the zinc alloy melt is solidified on the flat steel product 100. The flat steel product 100 is then made flat in a bending-stretching-straightening machine. After inspection for surface defects, during which neither marbling nor toothpick defects are found, the steel strip 100 is wound up on the reel. The steel strip 100 is then coated with a lacquer in a continuous strip coating system.

REFERENCE SIGNS

	(Protective) layer/(protective) coating	10
	Zinc melt bath/zinc alloy melt	11
	bath/(dipping) bath
	Trunk	12
	Roller	13
	Nozzles/stripping nozzle device	14
	Gas nozzle/Stripping nozzle	15
	Cooling area (e.g. cooling tower)	16
	(Gas) nozzle lip gap	17
	Dry gas supply/dry gas source	18
	Combining/mixing of gas	19
	streams/area of combining
	Means/device for determining gas	20
	humidity/dew point measuring device
	Dry gas supply (line)	21
	Water vapor gas supply (line)	22
	Sensor	23
	Sensor	24
	Gas line	25
	(Processing) module	26
	Water vapor device/source	50
	Flat steel product/Steel	100
	strips/Steel sheets/Strip
	Coated flat steel product/Steel	100, 10
	strip/Steel sheet
	Device	150
	Controller	250
	Exit side	A
	Stripping gas	AG
	Direction arrow	B
	Conditions	B1, B2,
		B2.1, B3
	Temperature difference	ΔT
	Input side	E
	(Absolute) humidity/(absolute)	f
	moisture/(absolute) moisture content
	(Absolute) surrounding air	fUG
	humidity/(absolute) air humidity
	Communication connections	KV1, KV2,
		KV3, KV4, KV5
	Relative (air) humidity	r
	Rear side	R
	Temperature of the stripping gas	TAG
	Dry gas	TG
	Air temperature (surrounding	TL
	or close range)
	Dew point of the stripping gas	TP
	Maximum dew point of	TPmax
	the stripping gas
	Minimum dew point of	TPmin
	the stripping gas
	Bath temperature	TB
	Front side	V
	Gaseous water vapor	WG
	Room space	X
	Coordinates	x, y, z