EP4545672A1 - Method for hot-dip coating a flat steel product and hot-dip coating installation - Google Patents

Abstract

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

Description

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

The process for hot-dip coating of flat steel products as well as the corresponding hot-dip coating systems for carrying out the process are state of the art, see for example EP 2 762 599 A1 , EP 3 109 338 A1 . In practice, directly fired heat treatment furnaces (also called DFFs) have become established for preheating and cleaning hot-rolled and cold-rolled flat steel products. These are typically fed with fossil fuels, such as natural gas. Since combustion takes place in the heat treatment furnace, direct heating can be used to create a reducing or oxidizing furnace atmosphere, depending on the set air ratio (lambda value of the combustion gas). The heat treatment furnace therefore contains the combustion gas from the burners, which contains a high proportion of water (H2O) and, depending on the air ratio, oxygen ( _O2 ) and carbon dioxide ( _CO2 ) or hydrogen ( _H2 ) and carbon monoxide/carbon dioxide (CO/ _CO2 ). As a rule, a reducing furnace atmosphere is set with a lambda value < 1, see also EP 2 762 599 A1 The combustion gas therefore contains carbon monoxide (CO) to protect the passing steel flat product from oxidation. In exceptional cases, however, a slightly oxidizing furnace atmosphere can be set using a lambda value > 1. In this case, the combustion gas also contains oxygen, which causes a targeted oxidation of the passing steel flat product, but only to the extent that these oxides can be reduced again later in the furnace of a hot-dip coating system. The goal of the atmosphere setting in the directly heated heat treatment furnace of a hot-dip coating system is to achieve a scale-free surface when the steel flat product exits the furnace and is immersed in the downstream metallic molten bath. The combustion gas is thus used as a "protective gas" against uncontrolled oxidation (= scaling).

As part of the globally demanded decarbonization, plants powered by fossil fuels are to be switched to more environmentally friendly fuels or energy sources, such as for example hydrogen, in order to reduce or ultimately avoid the use of fossil energy.

Decarbonization requires a reduction in the use of fossil fuels and energy sources and, in turn, a reduction in _CO2 emissions.

The object of the present invention is to further develop the hot-dip coating process in such a way that the use of fossil fuels is reduced or even avoided.

This object is achieved by a method having the features of claim 1 and by a hot-dip coating system having the features of claim 13. Further embodiments are described in the subclaims.

• Vorwärmen des Stahlflachprodukts auf eine Temperatur zwischen 400 °C und 950 °C in einem DFF-Ofen, wobei der DFF-Ofen über wenigstens einen Brenner verfügt, welcher mit einem Brenngas und einem sauerstoffhaltigen Gas betrieben wird, welche zu einem Verbrennungsgas verbrannt werden, wobei in Abhängigkeit von der Zusammensetzung des Brenngases und der Zusammensetzung des sauerstoffhaltigen Gases das Verbrennungsgas eine Zusammensetzung mit einem Wasserdampfpartialdruck aufweist;
• Erwärmen und/oder Halten des vorgewärmten Stahlflachprodukts bei einer Temperatur zwischen 400 °C und 950 °C;
• Abkühlen des warmen Stahlflachprodukts auf eine mindestens 50 K unterhalb bis maximal 50 K oberhalb einer Schmelzenbadtemperatur betragende Temperatur;
• Eintauchen des abgekühlten Stahlflachprodukts in ein metallisches Schmelzenbad mit einer Schmelzenbadtemperatur, um das Stahlflachprodukt mit einem metallischen Überzug mittels Schmelztauchen zu beschichten.

The first teaching relates to a method for hot-dip coating a hot-rolled or cold-rolled flat steel product comprising the steps:

• Preheating the flat steel product to a temperature between 400 °C and 950 °C in a DFF furnace, wherein the DFF furnace has at least one burner which is operated with a fuel gas and an oxygen-containing gas, 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;
• Heating and/or holding the preheated flat steel product at a temperature between 400 °C and 950 °C;
• Cooling the hot flat steel product to a temperature of at least 50 K below and a maximum of 50 K above a melt bath temperature;
• Immersing the cooled steel flat product into a metallic molten bath at a molten bath temperature in order to coat the steel flat product with a metallic coating by hot dipping.

It is essential for the invention that hydrogen is used in the fuel gas for the DFF furnace with a proportion of at least 10 vol.%, and that a low-water vapor and/or low-hydrogen gas is additionally added to the DFF furnace, whereby the added gas mixes with the combustion gas in such a way that a water vapor partial pressure of the mixture in the furnace atmosphere of the DFF furnace is lower than the water vapor partial pressure of the combustion gas.

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 flat steel products would result in a modified furnace atmosphere with highly influential parameters regarding the material properties and surface of the steel flat product being processed. When hydrogen-containing fuels are burned, a larger amount of water vapor is generated than with natural gas, which would result in a higher water vapor partial pressure in the furnace atmosphere. This results in a greater tendency for oxidation (= scale formation) during preheating by oxygen-affine elements in the flat steel product, which occurs particularly on the surface of the flat steel product. The presence of a higher water vapor partial pressure influences the bond between the scale and the flat steel product surface, or, in simple terms, the adhesion to the flat steel product surface. A scale layer (oxide layer) would also grow and/or be affected.

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 in high-strength steel flat products, which is known as "delayed fracture."

In a furnace atmosphere with a high water vapor content, the protective effect described above would be lost, resulting in widespread, uncontrolled oxidation (= scaling), which could no longer be reduced further in the hot-dip coating system, particularly in the holding zone and/or cooling zone of the furnace. The result would be insufficient adhesion of the metallic coating after leaving the molten bath, leading to significant quality degradation and even total failure of the flat steel product used.

Preheating a flat steel product in a steam atmosphere can change the grain layers in the microstructure, which can lead to undesirable, premature grain boundary oxidation, which in turn can cause coating and/or surface defects Due to the increased oxidation or scale formation, the formation of grain boundary oxidation can also occur more quickly and also penetrate deeper into the substrate.

Preheating a steel flat product in a steam atmosphere can also lead to a greater depth of decarburization, 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 the mechanical properties being outside the required range and can also lead to poorer surface properties.

Decarbonization in the application case of preheating a steel flat 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 manipulation of the product parameters.

An increase in hydrogen in the fuel gas and thus an increase in the water vapor partial pressure in the resulting combustion gas must be counteracted by "diluting" the combustion gas through targeted mixing with a low-water vapor/water vapor-free and/or low-hydrogen/water vapor-free gas in order to set a furnace atmosphere in the DFF furnace of a hot-dip coating system that has a lower water vapor partial pressure compared to the (pure) combustion gas, in particular to set a furnace atmosphere that largely corresponds to a conventional natural gas-fired furnace atmosphere in order to avoid having to unnecessarily change the existing process chain and to be able to essentially retain the standard process.

Low in water vapor means a water or water vapor content in the gas to be added of a maximum of 15.0 vol.%, in particular a maximum of 10.0 vol.%, preferably a maximum of 8.0 vol.%, more preferably a maximum of 5.0 vol.%, more preferably a maximum of 3.0 vol.%, more preferably a maximum of 1.50 vol.%, and in particular > 0.10 vol.%. Water vapor-free means that either no water or water vapor is present, or that the gas to be added may contain traces of up to a maximum of 0.10 vol.%.

Low-hydrogen means a hydrogen content in the gas to be added of a maximum of 7.0 vol.%, in particular a maximum of 5.0 vol.%, preferably a maximum of 4.0 vol.%, preferably a maximum of 2.50 vol.%, more preferably a maximum of 1.0 vol.%, more preferably a maximum of 0.50 vol.%, and in particular >0.10 vol.%. Hydrogen-free means that either no hydrogen is present or that the gas to be added may contain traces of up to a maximum of 0.10 vol.%.

The measure according to the invention allows a furnace atmosphere in the DFF furnace of a hot-dip coating system to be adjusted to correspond to or be adapted to the currently known level of natural gas-fired burners. The hydrogen used, at least in part, in the fuel gas can be generated and provided, for example, in water electrolysis using renewable energies such as wind, water, and/or solar energy. Any oxygen required can also be generated and provided by electrolysis using renewable energies (solar, wind, water, etc.).

The low-water vapor/free and/or low-hydrogen/free gas for mixing or adding can contain or consist of nitrogen ( _N2 ), argon (Ar), carbon dioxide ( _CO2 ), carbon monoxide (CO), or a mixture thereof. For example, inert gases are used. Other gases or mixtures of gases that contain no or relatively low amounts of water and/or water vapor and/or hydrogen, or no or relatively low amounts of hydrogen compounds, and that are suitable for preheating in the DFF furnace of a hot-dip coating system, can also be used accordingly.

The temperature for preheating the hot-rolled or cold-rolled flat steel product in the DFF furnace of a hot-dip coating line is generally between 400 °C and 950 °C, in particular between 500 °C and 900 °C, preferably between 600 °C and 850 °C. This temperature refers to the temperature of the flat steel product to which it is to be preheated. The furnace atmosphere temperature in the DFF furnace can certainly be higher.

The heating and/or holding of the preheated flat steel 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 at most 900 °C, wherein the DFF furnace (part) downstream part of the furnace for further heating and optional holding and thus annealing of the hot-rolled or cold-rolled flat steel product is provided with indirect firing, for example a radiant tube furnace (RTF) with an adjustable furnace atmosphere, preferably with a reducing furnace atmosphere.

The cooling of the warm or annealed flat steel product takes place to a temperature of 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 melt bath temperature, wherein the cooling furnace or part of the heat treatment furnace for cooling the hot-rolled or cold-rolled flat steel product is provided with indirect firing, quasi a radiant tube furnace (RTF), with an adjustable furnace atmosphere, preferably a reducing furnace atmosphere.

For example, the temperature on the surface of one side of the flat steel product is measured, in particular using a pyrometer or other suitable measuring device. Thus, the temperature of the flat steel product can be measured in any area of the hot-dip coating system using means known to those skilled in the art.

The immersion of the cooled steel flat product into a metallic molten bath with a molten bath temperature in order to coat the steel flat product with a metallic coating by means of hot dipping is essentially carried out in a protective gas atmosphere in a known manner.

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

The hot-dip coating system is preferably equipped with a horizontal furnace, but can alternatively be designed in a vertical design.

The process for hot-dip coating and thus also the construction of a hot-dip coating system is state of the art and therefore familiar to the expert.

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 for combustion to increase energy efficiency, for example, to at least 200 °C, in particular to at least 300 °C, preferably to at least 400 °C. 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.

The determination or recording of a water vapor partial pressure in a furnace atmosphere is familiar to those skilled in the art. This can be done, for example, by measuring the dew point using suitable measuring devices.

"Steel flat product" refers to manufactured sheets or similarly manufactured strips as rolled products made of a steel material, which can be either hot-rolled, i.e., a hot-rolled strip, or cold-rolled, i.e., a cold-rolled strip.

In particular, hydrogen can be present in the fuel gas in a proportion of at least 20 vol.%. Preferably, hydrogen can be present in the fuel gas in a proportion of at least 40 vol.%. Preferably, hydrogen can be present in the fuel gas in a proportion of at least 60 vol.%. Particularly preferably, hydrogen can be present in the fuel gas in a proportion of at least 80 vol.%. More preferably, hydrogen can be present in the fuel gas in a proportion of at least 98 vol.%. This embodiment comprises, for example, 100% use of hydrogen, in other words, the fuel gas consists of hydrogen, with impurities in the fuel gas being permitted at up to 0.5 vol.%, in particular up to 0.2 vol.%, preferably less than 0.1 vol.%, with impurities being technically impossible to avoid or only possible with considerable equipment expenditure.

If the fuel gas does not consist entirely of hydrogen, it may contain, in addition to hydrogen, further proportions of methane (CH ₄ ) and/or carbon monoxide (CO) to give 100 vol.%, together with impurities which are permitted up to 0.5 vol.%, in particular up to 0.2 vol.%, preferably less than 0.1 vol.%.

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

In order to avoid negatively impacting the energy of the combustion gas and/or even to increase energy efficiency, it may be advantageous if, according to one embodiment, the low-water vapor/free and/or low-water vapor/free gas is heated before being added to the DFF furnace. To essentially maintain the energy level of the combustion gas, the low-water vapor/free and/or low-water vapor/free gas is heated to a temperature that preferably corresponds to the temperature of the combustion gas between +/- 300°C. The temperature can thus correspond to a temperature window between minus and plus 300°C relative to the temperature of the combustion gas. The temperature of the combustion gas can be measured using means known to those skilled in the art.

In order to economically utilize the exhaust gas discharged from the DFF furnace of a hot-dip coating system, a mixed gas consisting of combustion gas and added gas with low/free water vapor and/or low/free hydrogen, it may be advantageous to use part or all of the exhaust gas to heat the low/free hydrogen and/or low/free water vapor gas before adding it. In this case, too, the means for exhaust gas utilization or heat transfer are known to those skilled in the art. Alternatively or additionally, the oxygen-containing gas and/or the fuel gas can also be heated/preheated accordingly.

Alternatively or in addition to the use of exhaust gases, the (additional) heating or preheating can also be carried out by other means, for example electrically, if a higher temperature level is required compared to the exhaust gas temperature.

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, and with ambient air and hydrogen it is approximately 2130 °C. Combustion with oxygen and natural gas is approximately 2860 °C, and with oxygen and hydrogen it is approximately 3080 °C.

The burner can be operated with an air ratio between 0.75 and 1.25. The air ratio can be adjusted between 0.75 and 0.99 to avoid the presence of oxygen (compounds). in the combustion gas, or alternatively between 1 and 1.25 in order to control the amount of oxygen in the combustion gas for targeted scaling, for example in certain products.

According to one embodiment, the flat steel product can be coated with a zinc-based coating. In addition to zinc and unavoidable impurities, the metallic molten 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. %, more 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. %, more preferably up to 5 wt. % in the coating. If improved corrosion protection is required, the metallic molten bath can contain or consist of magnesium with a content of at least 0.3 wt. %, in particular of at least 0.6 wt. %, preferably of at least 0.9 wt. %. Additionally or alternatively, aluminum may be present in addition to magnesium in a content of at least 0.1 wt.%, in particular at least 0.3 wt.%, for example to improve the bonding of the metallic coating to the flat steel product and, in particular, to substantially prevent the diffusion of iron from the substrate into the coating during heat treatment of the coated flat steel product, so that, for example, good adhesive properties can be ensured. The thickness of the metallic coating per side can be adjusted to between 1.5 and 60 µm, in particular between 2 and 50 µm, preferably between 3 and 30 µm, using known stripping nozzles arranged above the molten bath.

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

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

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.%.

The coating may also contain only zinc with small amounts of aluminum in addition to unavoidable impurities, also known by the designation "Z" in technical circles.

As unavoidable impurities, for example, elements from the group silicon, antimony, lead, titanium, calcium, manganese, tin, lanthanum, cerium and chromium may be contained individually or in combination with a total of up to 0.5 wt.%, in particular up to 0.3 wt.% in the metallic melt bath.

According to a further alternative embodiment, the flat steel 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.% silicon, optionally up to 4 wt.% iron, optionally up to 1.0 wt.% alkali or alkaline earth metals.

In a preferred variant, 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.%.

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.%.

In a preferred variant, 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 can in particular comprise at least 0.0015 wt.% calcium.

In another alternative embodiment, the flat steel product can be coated with an aluminum-based coating. The metallic molten 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.

The thickness of the metallic coating per side can be set between 1 and 60 µm, in particular between 2 and 50 µm, preferably between 3 and 50 µm.

As unavoidable impurities, for example, elements from the group antimony, lead, titanium, manganese, tin, lanthanum, cerium and chromium may be contained individually or in combination with a total of up to 0.5 wt.%, in particular up to 0.3 wt.% in the metallic melt bath.

The second teaching relates to a melt-exchange coating system, comprising a furnace and a pot for receiving a liquid metallic melt bath, wherein the furnace contains or consists of a section for preheating, a section for heating and/or holding and a section for cooling a continuously passing flat steel 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 can be combusted 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 the fuel gas, and that in addition at least one means is provided for adding a low-water vapor/water vapor-free and/or low-hydrogen/water vapor-free gas to the DFF furnace.

According to one embodiment, the means comprises at least one inlet nozzle, which is individually orientable and/or adjustable in spatial direction. This advantageously allows the inflow direction of the added gas into the DFF furnace to be influenced in order to force a forced flow within the DFF furnace through the outflow and/or the pulse, thus forcing a mixture with the combustion gas.

Additionally or alternatively, the combustion gas can also flow in via one or more burners with a separate geometric arrangement, in particular to achieve (faster) mixing with the additional gas added.

The invention is explained in more detail using the following exemplary embodiments in conjunction with the drawing:
The drawing shows the invention using the example of a schematic illustration.

Figure 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) Contains a section for preheating (11), a section for (further) heating and holding (12), and a section for cooling (13) a continuously passing flat steel product (1). The flat steel product (1) is preheated and (further) heated to a temperature between 400 °C and 950 °C. The heated flat steel product (1) is held at a temperature between preferably 600 °C and 950 °C. The warm flat steel product (1) is cooled to a temperature that is at least 50 K below and a maximum of 50 K above a molten bath temperature. Immersing the cooled steel flat product (1) into a metallic melt bath (S) having a melt bath temperature in order to coat the steel flat product (1) with a metallic coating by means of hot dipping, wherein a nozzle (14) is provided between the cooling section (13) and the pot (20), which nozzle ensures that the steel flat product (1) does not come into contact with oxygen or the ambient atmosphere before immersing it in the metallic melt bath (S).

The preheating section (11) is designed as a DFF furnace 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 combustion 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 .

Hydrogen can be provided partially or entirely as fuel gas (11.3). Additionally, at least one means (11.1) is provided for adding a water vapor-free and/or hydrogen-free gas (11.5) to the furnace atmosphere (11.11) of the DFF furnace (11). The hydrogen used at least partially in the fuel gas can be generated and provided, for example, in water electrolysis using renewable energies such as wind, water, and/or solar energy; this is not shown here.

Thus, for the DFF furnace (11), hydrogen is used in the fuel gas (11.3) with a proportion of at least 10 vol.%, and a water vapor-poor/free and/or hydrogen-poor/free gas (11.5) is additionally added to the DFF furnace (11), whereby the water vapor-poor/free and/or hydrogen-poor/free gas (11.5) mixes with the combustion gas (11.9) in such a way that a water vapor partial pressure of the Mixing in the furnace atmosphere (11.11) of the DFF furnace (11) is smaller than the defined water vapor partial pressure of the combustion gas (11.9).

The means comprises at least one inlet nozzle (11.1), which is individually orientable and/or adjustable, for example, in spatial direction. This allows, for example, the inflow direction (11.10) of the added gas to be specifically influenced such that the pulse forces a forced flow within the DFF furnace (11) and thus a mixture with the combustion gas (11.9).

In order to orientate oneself on a conventionally known furnace atmosphere in the DFF furnace (11) and to adjust it despite the use of hydrogen in the fuel gas, the volume of the fuel gas and the gas to be added can be determined in a standard manner, particularly depending on the volume of the DFF furnace (11). The volume of the fuel gas depends on the heat output required to preheat the passing flat steel product (1) and is based on a control of the material temperatures required for the preheating process. The volume of the gas to be added is determined from the volume of the fuel gas and thus from the chemical elements arising from combustion and the volume of the DFF furnace (11), corrected by corresponding analytical measurements of the furnace atmosphere (11.11).

Before being added (11.10) to the DFF furnace (11), the low-water vapor and/or low-water vapor and/or low-water vapor can be heated. The oxygen-containing gas (11.4), not shown, can also be preheated before combustion. An exhaust gas (11.7) can be removed from the DFF furnace (11), which can be partially or completely used to heat the low-water vapor and/or low-water vapor and/or low-water vapor using a suitable heat exchanger (11.6). Alternatively or additionally, the water vapor-poor/free and/or hydrogen-poor/free gas (11.5) can be heated, in particular additionally, for example by an electrical heating device (11.8), shown in dashed lines, with which a temperature increase of the water vapor-poor/free and/or hydrogen-poor/free gas (11.5) above the temperature of the exhaust gas (11.7) would also be possible.

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

Figures 3 and 4 Each shows a diagram when natural gas, assuming approximately 99 vol.% methane, is used as fuel with a proportion of between 0 and 100 vol.% hydrogen (abscissa). The left indicates no hydrogen and 100 vol.% natural gas, while the right indicates no natural gas and 100 vol.% hydrogen in the fuel gas. The oxygen-containing gas used for the burner was ambient air ( Figure 3 ) and oxygen ( Figure 4 ), whereby an air ratio of 1.1 was taken into account for the calculation.

Depending on the composition of the fuel gas, the components of the combustion gas (left ordinate) are also shown in the diagram. On the right ordinate, the combustion gas volume produced can be determined in ^m³ per ^m³ of fuel gas used, depending on the composition of the fuel gas.

The Figure 3 and 4 The results shown have been determined numerically and show the influence of non-fossil fuels, such as hydrogen in the fuel gas, on the composition of the combustion gas.

What is surprising is that when ambient air is used as the oxygen-containing gas for combustion, a reduction in the _CO2 content in the combustion gas is only possible with a hydrogen content of at least 35 vol.% in the fuel gas, see. Figure 3 . Furthermore, Figure 3 It is clear that a fuel gas consisting of 100 vol.% hydrogen cannot have a combustion gas volume of less than 2.5 m ³ per m ³ of fuel gas (= hydrogen) used.

However, Figure 4 When using oxygen as the oxygen-containing gas for combustion with 100% hydrogen as the fuel gas, the volume of the combustion gas corresponds essentially 1 to 1 to the volume of the fuel gas used. A reduction in the _CO2 content in the combustion gas can also be seen even with lower hydrogen contents (less than 35 vol%) in the fuel gas.

From a hydrogen content of 60% in the fuel gas, the water vapor partial pressure in the combustion gas begins to increase significantly. Figure 3 When hydrogen is burned with oxygen, the ratio is more extreme. With increasing volume fraction of hydrogen in the Ultimately, the water vapor partial pressure increases to a maximum when 100 vol.% hydrogen is used in the fuel gas. If 100 vol.% hydrogen is burned without diluting the furnace atmosphere, this negatively impacts the product properties of the flat steel product. Therefore, the water vapor content of the furnace atmosphere in the DFF furnace can be reduced accordingly by adding air, for example, to 20 vol.%. This would lead to an improvement in further processing properties. "Diluting," for example, with unheated air, would result in a temperature drop, which could potentially deprive the DFF furnace and thus the flat steel product of necessary preheating energy.

Claims (14)

A method for hot-dip coating a hot-rolled or cold-rolled flat steel product (1), comprising the steps: - Preheating the flat steel product (1) to a temperature between 400 °C and 950 °C in a DFF furnace (11), wherein the DFF 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), which are combusted to form a combustion 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 combustion gas (11.9) has a composition with a water vapor partial pressure; - heating and/or holding the preheated flat steel product (1) at a temperature between 400°C and 950°C; - cooling the hot flat steel product (1) to a temperature of at least 50 K below and at most 50 K above a melt bath temperature; - immersing the cooled steel flat product (1) in a metallic molten bath (S) having a molten bath temperature in order to coat the steel flat product (1) with a metallic coating by means of hot dipping; characterized in that for the DFF furnace (11) hydrogen is used in the fuel gas (11.3) with a proportion of at least 10 vol.%, and in that a low-water vapor and/or low-hydrogen/hydrogen-free gas (11.5) is additionally added to the DFF furnace (11), whereby the added gas (11.10) mixes with the combustion gas (11.9) in such a way that a water vapor partial pressure of the mixture in the furnace atmosphere (11.11) of the DFF furnace (11) is lower than the water vapor partial pressure of the combustion gas (11.9). Method according to claim 1, wherein hydrogen is contained in the fuel gas (11.3) in a proportion of at least 20 vol.%. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) in a proportion of at least 40 vol.%. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) in a proportion of at least 60 vol.%. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) in a proportion of at least 80 vol.%. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas (11.3) in a proportion of at least 98 vol.%. Method according to one of the preceding claims, wherein the water vapor-poor/water vapor-free and/or hydrogen-poor/hydrogen-free gas (11.5) is heated before adding the DFF furnace (11). Method according to claim 7, wherein the heating is carried out to a temperature which corresponds to the temperature of the combustion gas (11.9) between +/- 300 °C. Method according to one of claims 7 or 8, wherein an exhaust gas (11.7) is discharged from the DFF furnace (11), which exhaust gas is used partly or completely for heating the low-hydrogen/hydrogen-free and/or low-water vapor/water-free gas (11.5). Method according to one of the preceding claims, wherein the burner (11.2) is operated with an air ratio between 0.75 and 1.25. Method according to one of the preceding claims, wherein the flat steel product (1) is coated with a zinc-based coating. Method according to one of claims 1 to 10, wherein the flat steel product (1) is coated with an aluminum-based coating. Melt-exchange coating system (100), comprising a furnace (10) and a pot (20) for receiving a liquid metallic melt bath (S), wherein the furnace (10) has a section for preheating (11), a section for heating and/or holding (12) and a section for cooling (13) a continuously flowing steel flat product (1) or consists of these sections (11, 12, 13), 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 the oxygen-containing gas (11.4) can be combusted in the burner (11.2) to form a combustion gas (11.9) with which a furnace atmosphere (11.11) can be generated in the DFF furnace (11), characterized in that hydrogen can be provided at least partially as fuel gas (11.3), and that in addition at least one means (11.1) for supplying the DFF furnace (11) with a water vapor-free and/or hydrogen-free gas (11.5) is provided. Melt-exchange coating system according to claim 13, wherein the means comprises at least one inlet nozzle (11.1) which is individually alignable and/or adjustable in the spatial direction.

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