Method for hot-dip coating a flat steel product and hot-dip coating installation

Abstract

The invention relates to a method for hot-dip coating a flat steel product (1) and a hot-dip coating system (100).

Description

[0001] The invention relates to a method for hot-dip coating a flat steel product and to a hot-dip coating system.

[0002] The process for hot-dip coating steel flat products, as well as the corresponding hot-dip coating equipment for carrying out the process, is state of the art; see, for example, EP 2 762 599 A1, EP 3 109 338 A1. In practice, directly fired heat treatment furnaces (DFFs) have become established for preheating and cleaning hot-rolled and cold-rolled steel flat products. These are typically fueled with fossil fuels, such as natural gas. Since combustion takes place within the heat treatment furnace, a reducing or oxidizing furnace atmosphere can be achieved by means of direct heating, depending on the set air-fuel ratio (lambda value of the fuel gas).The combustion gas from the burners is present in the heat treatment furnace. This gas contains a high proportion of water and, depending on the air-fuel ratio, oxygen (O₂) and carbon dioxide (CO₂) or hydrogen (H₂) and carbon monoxide / carbon dioxide (CO / CO₂). A reducing furnace atmosphere is typically set with a lambda value < 1 (see also EP 2 762 599 A1). This results in carbon monoxide (CO gas) being present in the combustion gas to protect the steel product from oxidation. In exceptional cases, however, a slightly oxidizing furnace atmosphere can be set with a lambda value > 1. In this case, oxygen is also present in the combustion gas, causing targeted oxidation of the steel product, but only to the extent that these oxides can be reduced again later in the furnace process of a hot-dip coating system.The aim of the atmosphere control in the directly heated heat treatment furnace of a hot-dip coating system is to obtain a scale-free surface when the flat steel product exits the furnace and enters the downstream metallic melt bath. The combustion gas is thus used as a "protective gas" against uncontrolled oxidation (=scaling).

[0003] As part of the globally required decarbonization, plants powered by fossil fuels are to be converted or retrofitted in the future to more environmentally friendly fuels or energy carriers, such as hydrogen, in order to reduce or ultimately avoid the use of fossil energy.

[0004] Decarbonization requires a reduction in the use of fossil fuels and energy sources, and consequently a reduction in CO2 emissions.

[0005] The object of the present invention is to further develop the hot-dip coating process in such a way as to reduce or even avoid the use of fossil fuels.

[0006] This problem is solved by a method having the features of claim 1 and by a hot-dip coating system having the features of claim 12. Further embodiments are described in the dependent claims.

[0007] The first teaching relates to a method for hot-dip coating a hot-rolled or cold-rolled steel flat product, comprising the steps of: - Preheating the steel flat product to a temperature between 400 and 950 °C in a DDF furnace, wherein the DDF furnace has at least one burner which is operated with a fuel gas and an oxygen-containing gas, wherein the oxygen-containing gas can be air, oxygen, or a combination of air and oxygen, which are combusted to form a combustion gas, wherein, depending on the composition of the fuel gas and the composition of the oxygen-containing gas, the combustion gas has a composition with a water vapor partial pressure, wherein the burner is operated with an air-fuel ratio between 0.75 and 1.25; - Heating and / or holding the preheated steel flat product at a temperature between 400 and 950 °C;- Cooling the warm steel flat product to a temperature at least 50 K below and at most 50 K above a melting bath temperature; - Immersion of the cooled steel flat product into a metallic melting bath at a melting bath temperature in order to coat the steel flat product with a metallic coating by hot-dip coating.

[0008] Essential to the invention is that hydrogen in the fuel gas with a proportion of at least 10 to 100 vol.% is used for the DFF furnace, wherein if the fuel gas does not consist entirely of hydrogen, it may contain further proportions of methane and / or carbon monoxide in addition to hydrogen, and that the furnace atmosphere in the DFF furnace is brought into contact with at least one drying agent, which establishes a water vapor partial pressure in the furnace atmosphere of the DFF furnace that is lower compared to the water vapor partial pressure of the combustion gas.

[0009] The second teaching relates to a melt exchange coating plant comprising a furnace and a pot for receiving a liquid metallic melt bath, wherein the furnace contains or consists of a preheating section, a heating and / or holding section, and a cooling section for a continuously passing steel flat product, wherein the preheating section is designed as a DFF furnace, wherein the DFF furnace has at least one burner which can be supplied with a fuel gas and an oxygen-containing gas, wherein the fuel gas and oxygen-containing gas are combustible in the burner to form a combustion gas with which a furnace atmosphere can be generated in the DFF furnace, wherein hydrogen can be provided at least partially as a fuel gas, and wherein at least one desiccant is provided for contact with the furnace atmosphere of the DFF furnace.

[0010] A switch from a fossil fuel (natural gas) to an alternative, hydrogen-containing fuel in a DFF furnace in a hot-dip coating plant for preheating and, in particular, cleaning hot-rolled or cold-rolled steel flat products would result in a changed furnace atmosphere, which would have a significant impact on the material properties and surface of the steel flat product passing through. When hydrogen-containing fuels are burned, a larger quantity of water vapor is generated compared to natural gas, resulting in a higher partial pressure of water vapor in the furnace atmosphere. This leads to a greater tendency for oxidation (scale formation) during preheating by oxygen-affine elements in the steel flat product, which occurs particularly on the surface of the steel flat product.The presence of a higher water vapor partial pressure affects the bond between scale and the steel flat product surface—in simpler terms, the adhesion to the steel flat product surface. A scale layer (oxide layer) would also grow and / or be affected.

[0011] In particular, the steel flat product would be very sensitive to an increase in the water vapor partial pressure in furnace atmospheres during preheating. This can also promote undesirable hydrogen ingress into the steel flat product, leading to problems, especially in high-strength steel flat products, which is known as "delayed fracture" or "hydrogen-induced cracking".

[0012] In a furnace atmosphere with a high water vapor content, the protective effect described above would be lost, leading to widespread, uncontrolled oxidation (scaling) that could not be reduced further in the hot-dip coating process, particularly in the holding and / or cooling zones of the furnace. The result would be insufficient adhesion of the metallic coating after exiting the molten bath, leading to significant quality losses and potentially even total failure of the flat steel product.

[0013] Preheating a flat steel product in a steam atmosphere can alter the grain structure, potentially leading to undesirable, premature grain boundary oxidation, which in turn can cause coating and / or surface defects. Due to the accelerated oxidation and scale formation, grain boundary oxidation can also occur more rapidly and penetrate deeper into the substrate.

[0014] Preheating a steel flat product in a steam atmosphere can also lead to a greater decarburization depth, which means that the properties of a hot-dip coated steel flat product are also affected, particularly adversely. This can manifest itself, for example, in mechanical properties falling outside the required range and can also lead to poorer surface properties.

[0015] Decarbonization in the application case of preheating a flat steel product in a DFF furnace in a hot-dip coating plant would therefore not only be a simple switch from fossil to non-fossil fuels, but would also involve a complex influence on the product parameters.

[0016] An increase in hydrogen in the fuel gas, and thus a corresponding increase in the water vapor partial pressure in the resulting flue gas, must be counteracted by selectively dehumidifying and / or drying the flue gas using at least one desiccant. This is necessary to establish an oven atmosphere in the DFF oven of a hot-dip coating system that has a lower water vapor partial pressure compared to the (pure) flue gas. In particular, an oven atmosphere can be established that largely corresponds to a conventional natural gas-fired oven atmosphere, so as not to have to change the existing process chain unnecessarily and to essentially maintain the standard process.

[0017] The inventive measure allows for the adjustment of an oven atmosphere in the DFF oven of a hot-dip coating system to a level comparable to, or adaptable to, that of currently known natural gas-fired burners. The hydrogen used, at least partially, in the fuel gas can be produced and supplied, for example, by water electrolysis using renewable energies such as wind, water, and / or solar power. Any oxygen required can also be produced and supplied by electrolysis using renewable energies (solar, wind, water, etc.).

[0018] The temperature for preheating the hot-rolled or cold-rolled steel flat product in the DFF furnace of a hot-dip coating plant is essentially between 400 °C and 950 °C, particularly between 500 °C and 900 °C, preferably between 600 °C and 850 °C, where this temperature refers to the temperature of the steel flat product to which it is to be preheated. The furnace atmosphere temperature in the DFF furnace can certainly be higher, for example between 750 and 1400 °C.

[0019] The heating and / or holding of the preheated steel flat product takes place at a temperature between 400 °C, in particular between 500 °C, preferably between 600 °C, preferably between 700 °C and 950 °C, in particular a maximum of 900 °C, wherein a downstream part of the furnace following the DFF furnace (part) is provided with indirect firing for further heating and optional holding and thus annealing of the hot-rolled or cold-rolled steel flat product, for example a radiant tube furnace (RTF) with an adjustable furnace atmosphere, preferably with a reducing furnace atmosphere.

[0020] The cooling of the hot or annealed steel flat product is carried out to a temperature at least 50 K, in particular at least 40 K, preferably at least 30 K, preferably at least 20 K below to a maximum of 50 K, in particular a maximum of 40 K, preferably a maximum of 30 K, preferably a maximum of 20 K above a melting bath temperature, wherein the cooling furnace or part of the heat treatment furnace for cooling the hot-rolled or cold-rolled steel flat product is provided with indirect firing, essentially a radiant tube furnace (RTF), with an adjustable furnace atmosphere, preferably a reducing furnace atmosphere.

[0021] For example, the temperature at the surface on one side of the steel flat product is measured, in particular using a pyrometer or other suitable measuring instruments. Thus, the temperature of the steel flat product can be recorded in any area of ​​the hot-dip coating plant using methods known to those skilled in the art.

[0022] The immersion of the cooled steel flat product into a metallic melt bath at a melt bath temperature, in order to coat the steel flat product with a metallic coating by means of hot-dip coating, is carried out essentially under a protective gas atmosphere in a known manner.

[0023] The furnace in a preferably continuous hot-dip coating plant is in particular divided into three stages, the first stage being designed as a DFF furnace for preheating and optional cleaning of the hot-rolled or cold-rolled steel flat product.

[0024] The hot-dip coating system is preferably equipped with a horizontally designed oven, but can alternatively be designed in a vertical configuration.

[0025] The hot-dip coating process, and therefore also the construction of a hot-dip coating system, is state of the art and thus familiar to experts.

[0026] The oxygen-containing gas for operating the burner can be air, for example ambient air, oxygen, or a combination of air and oxygen. The oxygen-containing gas and / or the fuel gas can be preheated before being fed to the combustion chamber to increase energy efficiency, for example to at least 200 °C, in particular to at least 300 °C, preferably to at least 400 °C. The preheating can, for example, be limited to a maximum of 500 °C. Preheating the fuel gas and / or the oxygen-containing gas can lead to an increase in the adiabatic flame temperature.

[0027] Determining or measuring the partial pressure of water vapor in a furnace atmosphere is familiar to those skilled in the art. This can be done, for example, by measuring the dew point with suitable measuring devices.

[0028] The term "flat steel product" refers to manufactured sheets or similarly constructed strips as rolled products made from a steel material, which can be either hot-rolled, essentially a hot-rolled strip, or cold-rolled, essentially a cold-rolled strip. In particular, hydrogen can be present in the fuel gas with a proportion of at least 20% by volume. Preferably, hydrogen can be present in the fuel gas with a proportion of at least 40% by volume. More preferably, hydrogen can be present in the fuel gas with a proportion of at least 60% by volume. Particularly preferably, hydrogen can be present in the fuel gas with a proportion of at least 80% by volume. Further preferably, hydrogen can be present in the fuel gas with a proportion of at least 98% by volume. This configuration includes, for example, the use of 100% hydrogen; in other words, the fuel gas consists of hydrogen, with impurities in the fuel gas up to 0.5% by volume, and in particular up to 0.2% by volume.-%, preferably less than 0.1 vol.%, are permitted, whereby impurities cannot be avoided technically or only with high equipment costs.

[0029] If the fuel gas does not consist entirely of hydrogen, it may contain additional proportions of methane (CH₄) and / or carbon monoxide (CO) to achieve a hydrogen content of 100% by volume, along with impurities permitted up to 0.5% by volume, in particular up to 0.2% by volume, preferably less than 0.1% by volume. Traces of carbon dioxide (CO₂) up to 1.5% by volume may also be present.

[0030] For example, when using natural gas, the proportions of the main component methane can vary and may also include other components, such as ethane, propane, ethene and butane, individually or in combination.

[0031] Additionally or alternatively, blast furnace gas, for example containing or consisting of one or more of the components coke oven gas, blast furnace gas, converter gas, smelter gas, etc., can be considered as a fuel alongside hydrogen.

[0032] The temperature of the flue gas or the furnace atmosphere can be measured using methods known to experts.

[0033] Furthermore, the temperature of the burner flame also influences the temperature of the furnace atmosphere. The combustion temperature with ambient air and natural gas is approximately 1970 °C, with ambient air and hydrogen approximately 2130 °C, with oxygen and natural gas approximately 2860 °C, and with oxygen and hydrogen approximately 3080 °C.

[0034] The air ratio can be between 0.75 and 0.99, in particular to avoid the presence of oxygen (compounds) in the flue gas, or alternatively between 1 and 1.25, to control the amount of oxygen in the flue gas for targeted scaling, for example in certain products.

[0035] The desiccant is preferably hygroscopic. It is particularly preferred that the desiccant be temperature-resistant up to 1500 °C, and more preferably has a melting point > 1600 °C. The desiccant has a water adsorption capacity of at least 20%, particularly at least 30%, preferably at least 35%, and, for example, up to a maximum of 80% by weight of the desiccant. This means that with the use of 100 kg of desiccant, a water adsorption capacity of at least 20% by weight, etc., can be ensured. If, for example, a water adsorption capacity of approximately 45% is possible, this would correspond to a water content of 45 kg and a total weight of 145 kg in the aforementioned example. The water adsorption capacity is determined, for example, under standard conditions, particularly at room temperature.

[0036] According to one embodiment, the drying agent can contain or consist of silicon dioxide. The melting point of silicon dioxide is approximately 1710 °C.

[0037] According to one embodiment, the drying agent can contain or consist of aluminum oxide. The melting point of aluminum oxide is approximately 2070 °C.

[0038] It may also be a combination of silicon dioxide and aluminum oxide, for example with a mixing ratio between 5:95 and 95:5.

[0039] In order to reduce the water vapor partial pressure in the furnace atmosphere, a quantity or volume of a desiccant can be provided which is able to reduce the water vapor partial pressure in the furnace atmosphere by at least 30%, in particular at least 40%, preferably at least 50% and, for example, a maximum of 100% compared to the water vapor partial pressure of the flue gas.

[0040] The desiccant can be arranged in a suitable receiving device, which allows for spacing between the individual granules or grains of the desiccant. This increases the surface area of ​​the desiccant compared to, for example, a bulk material, thus promoting water absorption. The individual granules or grains can be spherical with a diameter between 0.05 and 10 mm. The desiccant can be arranged in special receiving devices that are designed to allow flow. Preferably, several receiving devices are provided, which are alternately fed into the oven via a suitable airlock device to reduce the water vapor partial pressure or used for drying during regeneration. Three or more receiving devices are particularly preferred.

[0041] Once the desiccant reaches saturation, i.e., its maximum water adsorption capacity, it can be regenerated relatively easily; that is, the absorbed water can simply evaporate while still warm. Since the desiccant is already at a temperature corresponding to the temperatures prevailing in the furnace, for example, between 750 °C and 1400 °C, and is therefore hot, it must be exposed to an atmosphere with lower humidity (or water vapor partial pressure) to allow it to dry again. This atmosphere can be ambient air or another gas with low humidity, for example, < 30% relative humidity, or even no humidity at all, such as an inert gas. To accelerate the regeneration process, the desiccant can be exposed to a flowing gas or ambient air.

[0042] The desiccant can, for example, be equipped with a color indicator that changes color depending on the moisture content. This indicator can be used, for instance through monitoring, to determine when the desiccant needs to be replaced. The replacement is preferably carried out during operation.

[0043] According to an alternative embodiment, the desiccant, in the form of granular particles or granules, can be blown into the DFF oven via at least one inlet nozzle, which can be individually aligned and / or adjusted in the spatial direction, along with a carrier gas, preferably a dry protective gas, such as nitrogen. This advantageously allows the inflow direction of the carrier gas for the desiccant to be influenced in the DFF oven, so that the outflow and / or the impulse forces a forced flow within the DFF oven and thus contact with the flue gas.

[0044] Additionally or alternatively, the carrier gas can also be introduced via one or more burners with a separate geometric arrangement, in order to achieve, in particular, (faster) contact with the blown-in desiccant.

[0045] Alternatively, the desiccant can also be introduced into the atmosphere of the DFF furnace via a suitable airlock, for example a rotary valve.

[0046] The desiccant, blown in or introduced as an example, leaves the DFF furnace with the exhaust gas. For this to work, a special filter device would need to be used in the exhaust system of the DFF furnace to filter the blown-in desiccant and separate it from the exhaust gas. This would allow the desiccant to be reused and reintroduced into the cycle.

[0047] According to one embodiment, the steel flat product can be coated with a zinc-based coating. In addition to zinc and unavoidable impurities, the metallic melt bath can contain or consist of additional elements such as aluminum with a content of up to 15 wt.%, in particular up to 10 wt.%, preferably up to 8 wt.%, preferably up to 5 wt.%, and / or magnesium with a content of up to 15 wt.%, in particular up to 10 wt.%, preferably up to 8 wt.%, preferably up to 5 wt.%. If improved corrosion protection is required, the metallic melt bath can contain or consist of magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt.%. Additionally or alternatively, aluminum can be added to magnesium with a content of at least 0.1 wt.%, in particular at least 0.3 wt.%.-% must be present to, for example, improve the bonding of the metallic coating to the flat steel product and, in particular, to essentially prevent the diffusion of iron from the substrate into the coating during heat treatment of the coated flat steel product, thus ensuring, for example, good adhesion. The thickness of the metallic coating on each side can be set between 1.5 and 60 µm, particularly between 2 and 50 µm, preferably between 3 and 30 µm, using known stripping nozzles arranged above the melt pool.

[0048] If the metallic melt bath contains or consists of magnesium within the aforementioned limits, aluminum within the aforementioned limits and the remainder being zinc along with unavoidable impurities, the resulting metallic coating on the steel flat product is known in the trade as zinc-magnesium (ZM) or Zn-Al-Mg.

[0049] In a preferred variant, the aluminium content in the metallic melt bath is 1.1 to 8 wt.%, in particular 1.2 to 5 wt.%.

[0050] In a preferred variant, the magnesium content in the metallic melt bath is 1.1 to 8 wt.%, in particular 1.2 to 5 wt.%.

[0051] The coating may also contain only zinc with small amounts of aluminum alongside unavoidable impurities, also known in specialist circles as "Z".

[0052] Unavoidable impurities, such as elements from the group consisting of silicon, antimony, lead, titanium, calcium, manganese, tin, lanthanum, cerium and chromium, may be present individually or in combination in the metallic melt bath up to a total of 0.5 wt.%, in particular up to 0.3 wt.%.

[0053] According to a further alternative embodiment, the steel flat product can be coated with an aluminum-based coating. The metallic melt bath can contain or consist of, in addition to aluminum and unavoidable impurities, optionally up to 15 wt.% Si, optionally up to 4 wt.% Fe, and optionally up to 1.0 wt.% alkali or alkaline earth metals.

[0054] In a preferred embodiment, the silicon content in the metallic melt bath is either 0.2 to 4.5 wt.% or 7 to 13 wt.%, in particular 8 to 11 wt.%.

[0055] In a preferred variant, the optional iron content comprises 0.2 to 4.5 wt.%, in particular 1 to 4 wt.%, preferably 1.5 to 3.5 wt.%.

[0056] In a preferred embodiment, the optional content of alkali or alkaline earth metals comprises 0.01 to 1.0 wt.% magnesium, in particular 0.1 to 0.7 wt.% magnesium, preferably 0.1 to 0.5 wt.% magnesium. Furthermore, the optional content of alkali or alkaline earth metals may in particular comprise at least 0.0015 wt.% calcium.

[0057] In a further alternative embodiment, the steel flat product can be coated with an aluminum-based coating. The metallic melt bath can contain or consist of, in addition to aluminum and unavoidable impurities, 2 to 24 wt.% zinc, 1 to 7 wt.% silicon, optionally 1 to 8 wt.% magnesium if the silicon content is between 1 and 4 wt.%, and optionally up to 0.3 wt.% total lead, nickel, zirconium, or hafnium.

[0058] The thickness of the metallic coating on each side can be set between 1 and 60 µm, in particular between 2 and 50 µm, preferably between 3 and 30 µm.

[0059] Unavoidable impurities, such as elements from the group consisting of antimony, lead, titanium, manganese, tin, lanthanum, cerium and chromium, may be present individually or in combination in the metallic melt bath up to a total of 0.5 wt.%, in particular up to 0.3 wt.%.

[0060] The invention is explained in more detail with reference to the following exemplary embodiments in conjunction with the drawing.

[0061] The drawing shows the invention using a schematic illustration as an example. Figure 1Figure 1 shows a hot-dip coating system (100) comprising a furnace (10) and a pot (20) for receiving a liquid metallic melt bath (S). The furnace (10) includes a preheating section (11), a holding section (12), and a cooling section (13) for a continuously passing steel flat product (1). The steel flat product (1) is preheated and held at a temperature between 400 and 950 °C. The heated steel flat product (1) is held at a temperature preferably between 600 and 950 °C. The warm steel flat product (1) is cooled to a temperature at least 50 K below and at most 50 K above the melt bath temperature.Immersion of the cooled steel flat product (1) into a metallic melt bath (S) at a melt bath temperature in order to coat the steel flat product (1) with a metallic coating by means of hot-dip coating, wherein a nozzle (14) is provided between the cooling section (13) and the pot (20) which ensures that the steel flat product (1) does not come into contact with oxygen or the ambient atmosphere before immersion in the metallic melt bath (S).

[0062] The preheating section (11) is designed as a DFF oven and is described in detail in Figure 2 outlined. The DFF furnace (11) has at least one burner (11.2) which can be supplied with a fuel gas (11.3) and an oxygen-containing gas (11.4), wherein the fuel gas (11.3) and oxygen-containing gas (11.4) can be combusted in the burner (11.2) to form a flue gas (11.9) with which a furnace atmosphere (11.11) can be generated in the DFF furnace (11), cf. Figure 2, which is a schematic sectional view in direction II, see. Figure 1 .

[0063] Hydrogen can be provided partially or completely as fuel gas (11.3). The hydrogen used, at least partially, in the fuel gas can be produced and provided, for example, in water electrolysis using renewable energies such as wind, water, and / or solar power (not shown here). At least one desiccant (20) is provided for contact with the furnace atmosphere (11.11) of the DFF furnace (11).

[0064] Thus, hydrogen in the fuel gas (11.3) is used for the DFF furnace (11) with a proportion of at least 10 to 100 vol%, whereby if the fuel gas (11.3) does not consist entirely of hydrogen, it may contain additional proportions of methane and / or carbon monoxide, and that the furnace atmosphere (11.11) in the DFF furnace (11) is brought into contact with at least one desiccant (20), which results in a water vapor partial pressure in the furnace atmosphere (11.11) of the DFF furnace (11) that is lower compared to the water vapor partial pressure of the flue gas (11.9).

[0065] The desiccant (20) can be arranged in a receiving device (not shown) and contains or consists of silicon dioxide or aluminum oxide. The desiccant (20) can be introduced into the DFF oven (11) via suitable means (22) in a receiving device to reduce the water vapor partial pressure in the oven atmosphere (11.11) in a working position. When the desiccant (20) in contact with the oven atmosphere (11.11) reaches saturation, it is discharged from the DFF oven (11) via the means (22) to dry in a regeneration position. During this regeneration, the hot desiccant (20) can be exposed to the environment or, for example, actively aerated with ambient air to accelerate the regeneration process.Simultaneously or prior to the discharge of the contacting desiccant (20), a further desiccant (20) is introduced into the DFF oven (11) to ensure a continuous reduction of the water vapor partial pressure. The desiccant (20) is shown by way of example in three receiving devices. Each device is provided with a locking device (not shown).

[0066] Alternatively or additionally, the desiccant (20) can be arranged outside the DFF oven (11), for example in one or preferably several receiving devices, wherein the desiccant (20) is brought into contact with the gas of the oven atmosphere (11.11) via lines and preferably via a suction inlet (21) and, after contact, is returned to the DFF oven (11) at a reduced water vapor partial pressure. When the desiccant (20) in contact with the oven atmosphere (11.11) reaches saturation, another desiccant (20), not shown here, is supplied with the gas of the oven atmosphere (11.11) according to the aforementioned procedure. The saturated desiccant (20) is regenerated by exposing the hot desiccant (20) to ambient air or a substantially dry gas to dry it.

[0067] In the form of a dashed line, the desiccant (20) can also be granular particles or granules with a carrier gas (12) blown into the DFF oven via at least one inlet nozzle, which can be individually aligned and / or adjusted in the spatial direction.

[0068] With the furnace atmosphere (11.11) set according to the invention, preheating of the hot-rolled or cold-rolled flat steel product (1) is possible without the disadvantages of altered or different oxidation / scale formation on the surface of the flat steel product (1) despite the use of non-fossil fuels, if hydrogen with proportions between 10 and 100 vol.% in the fuel gas (11.3) is used.

[0069] In laboratory-scale investigations, different fuel gas compositions, such as natural gas (a) and mixtures of natural gas and hydrogen with 25 vol% (b), 50 vol% (c) and 75 vol% (d) as well as 100% hydrogen (e), were burned with oxygen and an air ratio of approximately 0.95 in a gas-operated furnace with a furnace volume of 0.04 m³. This resulted in different furnace atmospheres with different furnace humidities and increasing water vapor partial pressures.

[0070] For each furnace process parameter, three thick steel samples, each measuring 10 x 5 x 0.4 cm, were cut from hot-rolled strip material. These samples were provided and each was burned with one of the aforementioned fuel gas compositions (a) to (e). The residence time for all samples was approximately 15 minutes, during which time the samples were thoroughly heated in the furnace and removed at a temperature of approximately 700 °C. After removal, the samples were quenched and deep-frozen. The hydrogen content of the samples was analyzed using thermal desorption mass spectrometry (TDMS), a standard method for hydrogen measurement (see Table 1), with mean values ​​given below. Table 1 Fuel gas a b c d e Hydrogen in [ppm] < 0,4 0,57 0,74 1,01 1,57

[0071] As a result of the reaction processes, significantly more water is produced with increasing hydrogen content in the fuel gas than in conventional combustion processes with natural gas. The increasing water vapor partial pressure (furnace humidity) leads to an increase in surface reactions, which include the dissociation of H₂ to H₂ as well as the adsorption and absorption of H₂.

[0072] Further samples of the same dimensions were combusted with 100% hydrogen in the fuel gas and oxygen with an air-fuel ratio of approximately 0.95. Different amounts of silicon dioxide were introduced into the furnace atmosphere as a desiccant to reduce the water vapor partial pressure. The amounts used were 300 g and 650 g. The residence time for all samples was approximately 15 minutes, during which time the samples were thoroughly heated in the furnace and removed at a temperature of approximately 700 °C. After removal, the samples were quenched and deep-frozen. The hydrogen input into the samples was analyzed using TDMS. The average hydrogen input for the samples containing 300 g of desiccant in the furnace atmosphere was 1.4 ppm, and for the samples containing 650 g of desiccant, it was 0.86 ppm. The average hydrogen input without desiccant was 1.57 ppm.

[0073] Heating steel products to forming temperatures with an increased water vapor partial pressure in the furnace atmosphere, due to at least partial combustion of hydrogen in the fuel gas, leads, as expected, to a significant hydrogen ingress into the steel product. By including at least one drying agent and bringing it into contact with the furnace atmosphere, thereby reducing the water vapor partial pressure and / or the dew point in the furnace atmosphere, the hydrogen ingress is significantly reduced. This, in turn, reduces the hydrogen ingress and the associated potential embrittlement and further processing problems.

Claims

1. A method for hot-dip coating a hot-rolled or cold-rolled steel flat product (1) comprising the steps of: - preheating the steel flat product (1) to a temperature between 400 and 950°C in a DDF furnace (11), wherein the DDF furnace (11) has at least one burner (11.2) which is operated with a fuel gas (11.3) and an oxygen-containing gas (11.4), wherein the oxygen-containing gas (11.4) may be air, oxygen or a combination of air and oxygen, which are combusted to form a flue gas (11.9), wherein, depending on the composition of the fuel gas (11.3) and the composition of the oxygen-containing gas (11.4), the flue gas (11.9) has a composition with a water vapor partial pressure, wherein the burner (11.2) is operated with an air ratio between 0.75 and 1.25; - Heating and / or holding the preheated steel flat product (1) at a temperature between 400 and 950°C; - Cooling the warm steel flat product (1) to a temperature at least 50 K below and at most 50 K above a melting bath temperature; - Immersing the cooled steel flat product (1) in a metallic melting bath (S) at a melting bath temperature in order to coat the steel flat product (1) with a metallic coating by hot-dip coating; . characterized by the fact thatfor the DFF furnace (11) hydrogen in the fuel gas (11.3) with a proportion of at least 10 to 100 vol% is used, wherein if the fuel gas (11.3) does not consist entirely of hydrogen, it may contain additional proportions of methane and / or carbon monoxide, and that the furnace atmosphere (11.11) in the DFF furnace (11) is brought into contact with at least one desiccant (20) which establishes a water vapor partial pressure in the furnace atmosphere (11.11) of the DFF furnace (11) which is lower compared to the water vapor partial pressure of the flue gas (11.9).

2. The method according to claim 1, wherein hydrogen is contained in the fuel gas (11.3) with a proportion of at least 20 vol.%.

3. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) with a proportion of at least 40 vol.%.

4. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) with a proportion of at least 60 vol.%.

5. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) with a proportion of at least 80 vol.%.

6. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) with a proportion of at least 98 vol.%.

7. Method according to any of the preceding claims, wherein the drying agent (20) contains or consists of silicon dioxide.

8. Method according to any of the preceding claims, wherein the drying agent (20) contains or consists of aluminium oxide.

9. Method according to one of the preceding claims, wherein the desiccant (20) is provided in an amount capable of reducing the water vapor partial pressure in the furnace atmosphere by at least 30% compared to the water vapor partial pressure of the flue gas (11.3).

10. Method according to one of the preceding claims, wherein the steel flat product (1) is coated with a zinc-based coating.

11. Method according to any one of claims 1 to 9, wherein the steel flat product (1) is coated with an aluminium-based coating.

12. Melt-exchange coating plant (100) comprising a furnace (10) and a pot (20) for receiving a liquid metallic melt bath (S), wherein the furnace (10) includes or consists of a preheating section (11), a heating and / or holding section (12), and a cooling section (13) of a continuously passing steel flat product (1), wherein the preheating section (11) is designed as a DFF furnace, wherein the DFF furnace (11) has at least one burner (11.2) which can be supplied with a fuel gas (11.3) and an oxygen-containing gas (11.4), wherein the fuel gas (11.3) and oxygen-containing gas (11.4) are combustible in the burner (11.2) to form a flue gas (11.9) with which a furnace atmosphere (11.11) can be produced in the DFF oven (11), characterized by the fact that at least one desiccant (20) is provided for contact with the furnace atmosphere (11.11) of the DFF furnace (11).

13. Melt exchange coating system according to claim 12, wherein the desiccant (20) can be brought into a working position and into a regeneration position via means (22).

14. Melt exchange coating system according to claim 12, wherein the desiccant (20) can be blown into the DFF furnace as granular particles or granules with a carrier gas (12) via at least one inlet nozzle.

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