Method for heat treating a steel product

Abstract

The invention relates to a method for heat-treating a steel product, wherein the steel product is placed in a directly heated heat treatment furnace (1) comprising several gas burners which are supplied with hydrogen as fuel gas, in which the steel product is heated to a target temperature (Tz) between 1000 °C and 1500 °C, characterized in that the steel product is electrically preheated in a system (2) to a temperature (T) below the target temperature (Tz) of between 1000 °C and 1500 °C and optionally kept at this temperature.

Description

[0001] The invention relates to a method for heat-treating a steel product.

[0002] Heat treatment furnaces, such as directly heated furnaces ( d direct f ired fDirect-fired furnaces (DFFs), also known as blast furnaces, are established furnaces used for the heat treatment of metals. They are typically fueled with fossil fuels, such as natural gas. Since combustion takes place within the furnace, a reducing or oxidizing furnace atmosphere can be achieved by direct heating, depending on the set air-fuel ratio (lambda value of the fuel gas). So-called by-product gases or mixtures thereof are also suitable as fuel gases, which are generated particularly in integrated steelworks and can thus be utilized accordingly. The furnace atmosphere is therefore the combustion gas produced by the burner(s), which contains less than 30% water vapor by volume, particularly less than 25% by volume, and, depending on the air-fuel ratio, may contain oxygen (O₂) and carbon dioxide (CO₂) or hydrogen (H₂) and carbon monoxide / carbon dioxide (CO / CO₂).

[0003] Against the backdrop of the globally mandated decarbonization, the proportion of fossil fuels in industrial plants is to be reduced in the future in favor of CO₂-neutral fuels or energy carriers. For example, the use of hydrogen as a fuel gas in the steel industry is intended to contribute to reducing emissions during production. When switching the combustion process from a fossil fuel (natural gas) to a sustainable, hydrogen-based fuel, significant changes occur in the furnace atmosphere, which are associated with massive scale growth (see, among others, the report "D2.1 - Impact of H₂ heating on product quality, yield, and refractory," available at the following link). https: / / hyinheat.eu / wp-content / uploads / 2024 / 07 / D2.1-Impact-of-H2-heating-on-product-quality-yield-and-refractory PU.pdf, as well as being associated with increased hydrogen ingress into the steel. This is caused by the increased water content in the flue gas produced during hydrogen combustion, which influences / accelerates the oxidation and absorption processes at the surface. These effects can have various consequences for the steel product and its further processing: Strongly adhering and thick scale can lead to problems during removal (scale scrubbers) and thus provoke subsequent errors in further processing (e.g., rolling in residual scale during hot rolling), a poorer surface quality which can lead to devaluation or even product failure, a massive hydrogen input which can lead to embrittlement effects during further processing or even component failure, furthermore, a scale layer or an increased scale layer thickness can also lead to yield losses which can have a negative impact on the energy balance.

[0004] Heat treatment furnaces, which heat / through-heat steel products, preferably in the form of slabs, in so-called pusher and / or walking beam furnaces, are heated with natural gas or, if available and integrated into a so-called steelworks complex, can be operated with gases (by-product gases) or gas mixtures such as coke oven gas, blast furnace gas, converter gas, etc. A conversion to combustion with hydrogen therefore has a stronger, and in particular negative, impact on the surface quality of the steel product being heat-treated, or rather, increases the hydrogen input into the steel product.

[0005] Furthermore, the technical report "Strategies for the Decarbonization of Reheating and Heat Treatment Processes in the Steel Industry" by Wuppermann et al., pp. 16 to 25, published on September 22, 2023 at https: / / www.tube.de / cgibin / mdwiretube / lib / all / lob / returndownload.cgi, states (see Scenario 4 on page 22) that a furnace chamber of a walking beam furnace for reheating slabs is heated openly using 160 side-wall and ceiling radiant burners, whereby the integration of suitable burners, which have been successfully tested on a trial scale using 100% H₂ and also mixtures of H₂ and natural gas, still needs to be implemented on an industrial scale after preparation.

[0006] The object of the present invention is to further develop the method for heating a steel product in such a way that it can have a positive influence on CO2 emissions and at the same time not have a detrimental effect on scale formation in the heat treatment process or reduce hydrogen input in the heat treatment process.

[0007] This problem is solved by a method having the features of claim 1. Further embodiments are described in the dependent claims.

[0008] The invention relates to a method for heat-treating a steel product, wherein the steel product is placed in a directly heated heat treatment furnace comprising several gas burners which are supplied with hydrogen as fuel gas, in which the steel product is heated to a target temperature between 1000 °C and 1500 °C.

[0009] Essential to the invention is that the steel product is electrically preheated in a system to a temperature below the target temperature, which is between 1000 °C and 1500 °C, and optionally kept at this temperature.

[0010] Scale formation depends on the technological conditions, such as temperature, air-fuel ratio, time, and the chemical composition of the steel product. With increasing temperature, increasing air-fuel ratio, and longer processing time, the scale on the surface of a steel product increases. The partial pressure of water vapor in the furnace atmosphere can also influence the increase (thickness) of the scale. With increasing water vapor partial pressure in the furnace atmosphere, the hydrogen input into the steel product undergoing heat treatment also increases, and this effect can be intensified by increasing process and product temperatures and decreasing air-fuel ratios.

[0011] The effects described above can be mitigated by the selective application of different heat treatment technologies. This is based on the strong temperature and fuel gas dependency of scale formation and hydrogen input. With increasing temperature, the effects of increased scale formation and hydrogen input during hydrogen combustion intensify. Therefore, the selective application of different heat treatment technologies is proposed to reduce these effects.

[0012] The steel product can be in the form of a slab, a block, a disc or a billet.

[0013] The heat treatment of a steel product is carried out by first electrically preheating the steel product in a system to a temperature corresponding to a target temperature between 1000 °C and 1500 °C, optionally maintaining this temperature, and then placing the steel product into a directly heated heat treatment furnace comprising several gas burners supplied with hydrogen as fuel gas, in which the steel product is heated through to the target temperature (Tz) between 1000 °C and 1500 °C. Before further processing, the (hot) steel product can preferably be descaled. Further processing can, for example, include at least one hot rolling or at least one forging. The target temperature can be, in particular, at least 1050 °C, preferably at least 1100 °C, more preferably at least 1150 °C, and most preferably at least 1200 °C.In principle, the target temperature of 1500 °C should not be exceeded to avoid partial / complete melting and / or excessive scaling of the steel product. For ecological and economic reasons, the target temperature can be limited to a maximum of 1450 °C, preferably to a maximum of 1400 °C, more preferably to a maximum of 1350 °C, and most preferably to a maximum of 1300 °C. The temperature / target temperature is measured, for example, on one side of the surface of the steel product, particularly with pyrometers or other suitable measuring instruments. The aforementioned target temperatures should also not be exceeded or fallen below in the core of the steel product, although the core temperature of a steel product cannot be directly determined during normal operation. Generally, the temperature of the steel product can be determined using methods known to those skilled in the art.The temperature in the heat treatment furnace, or rather the temperature of the furnace atmosphere in the heat treatment furnace, can certainly be higher.

[0014] The hydrogen for the fuel gas can, for example, be produced and supplied via water electrolysis using renewable energies such as wind, water, and / or solar power, or it can be of non-fossil origin to promote decarbonization in the steel industry. While the fuel gas consists of hydrogen, impurities of up to 0.5% by volume, particularly up to 0.2% by volume, and preferably less than 0.1% by volume, are permissible, provided that such impurities cannot be avoided entirely or only with considerable technical effort, so that the fuel gas can still be described as containing 100% hydrogen by volume.

[0015] According to one embodiment, the steel product is preheated to a temperature between 850 °C and 1200 °C. The temperature can be at least 900 °C, preferably at least 950 °C, and more preferably at least 1000 °C. The temperature can be limited to a maximum of 1150 °C, preferably to a maximum of 1100 °C, and more preferably to a maximum of 1050 °C.

[0016] Partial decarbonization can take place during the heat treatment of steel products by switching from complete heat treatment in heat treatment furnaces using fossil fuel gases to partial heat treatment, specifically preheating with electricity.

[0017] Furthermore, (partial) decarbonization during heat treatment can also occur when switching from a fossil fuel gas to hydrogen in the heat treatment furnace. Combusting hydrogen produces a larger quantity of water vapor 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 heat treatment due to oxygen-affine elements in the steel product, which form on the surface. The presence of a higher partial pressure of water vapor can affect the bond between the scale and the steel product surface—in simpler terms, its adhesion to the surface. The scale layer (oxide layer) would also grow and / or be altered. There is also a greater tendency for hydrogen to be incorporated.These adverse effects occur only to a small extent or can be kept negligible in their impact if the temperature or the temperature range is not exceeded.

[0018] The electrical preheating of the steel product can advantageously be carried out outside the heat treatment furnace. This eliminates the need to expose the system to the thermal influences of the heat treatment furnace and / or to active cooling, which would in turn reduce the efficiency of the electrical preheating.

[0019] According to one embodiment, the electrical preheating of the steel product can be carried out inductively. Advantageously, inductors can be configured, particularly above a roller conveyor, over which the steel product can be guided and which can optionally be stopped. Depending on their size and power, several inductors can be arranged in series, over which the steel product to be preheated can be guided. Inductive heating units are known and economically viable. Through so-called longitudinal field heating and / or transverse field heating in conjunction with an optimal frequency and / or power, the temperature of the steel product can be influenced and adjusted accordingly across its width and / or length and / or thickness.In longitudinal field heating, the steel product can be passed by an open inductor, by several inductors arranged in series, and / or by several inductors arranged around the circumference of the steel product. The resulting eddy currents flow parallel to the surface of the steel product within the current penetration depth. In transverse field heating, the steel product can be passed through a coil or several coils arranged in series. In particular, the temperature of the steel product can be measured before inductive preheating, and the surface and core temperatures of the steel product can be equalized by means of inductive preheating in order to provide a substantially homogeneous temperature within the steel product for the subsequent through-heating. Such designs are state of the art.

[0020] According to an alternative or additional embodiment, the electrical preheating of the steel product can be carried out conductively. In this case, the steel product can be brought into contact with a contact plate on at least one side, or preferably on both sides. The contact plates act as electrodes, which are connected to a power source, for preheating the steel product. Such embodiments are prior art.

[0021] The electricity required for inductive and conductive preheating can ideally be generated and supplied using renewable energy sources such as wind, water, and / or solar power, thus reducing CO2 emissions. Alternatively, the electricity can also be sourced from nuclear power plants.

[0022] According to an advantageous design, the system can be operated under normal atmospheric conditions. This eliminates the need for additional enclosure and / or flooding with an inert gas. Thus, the ambient atmosphere prevails within the system.

[0023] The temperature of the burner flame affects the temperature in the furnace atmosphere. The flame temperature, for example at an air-fuel ratio of 1, can be specified as follows: Propane / butane with air 1925 °C and with oxygen 2850 °C; methane (natural gas) with air 1970 °C and with oxygen 2860 °C; ethyne (acetylene) with air 2250 °C and with oxygen 3030 °C; hydrogen with air 2130 °C and with oxygen 3080 °C.

[0024] Combustion can be adjusted with an air-fuel ratio between 0.75 and 1.25. In particular, the air-fuel ratio can be at least 0.90, preferably at least 0.95, and more preferably at least 1.0. In particular, the air-fuel ratio can be a maximum of 1.35, preferably a maximum of 1.30, and more preferably a maximum of 1.25. Particularly preferably, to avoid harmful emissions during combustion as much as possible, an air-fuel ratio between 1.02 and 1.20 can be selected.

[0025] Depending on the volume of the steel product to be heat-treated, and in particular also depending on the temperature to which the steel product is to be heated, the residence time in the heat treatment furnace can be between 10 minutes and 48 hours, in particular between 20 minutes and 24 hours, preferably between 30 minutes and 12 hours.

[0026] The heat treatment furnace with direct heating for heat-treating a steel product can be a pusher beam furnace or a walking beam furnace, designed to hold and heat-treat slabs, blocks, discs, or billets. Depending on the volume of the steel product to be heat-treated or heated through, a residence time of between 10 minutes and 6 hours can be considered.

[0027] The steel product can also be fed into the system warm or hot, at a temperature of up to 800 °C, particularly up to 700 °C, preferably up to 600 °C. For example, the steel product may still have a certain temperature, such as that resulting from the manufacturing process, without cooling down to room temperature. The minimum temperature can be 200 °C.

[0028] The steel product may, for example, comprise a composition in wt.% or consist of: C: 0,001 bis 0,9; Mn: 0,05 bis 12,0; Si: 0,001 bis 5,0; N: max. 0,1; S: max. 0,1; P: max. 0,5; optionally one or more of the following elements: Al: max. 2,0; Mo: max. 1,0; Ni: max. 1,0; Cr: max. 4,5; B: max. 0,01; Ca: max. 0,01 Nb: max. 0,5; Ti: max. 0,5; V: max. 0,5; W: max. 0,5; Cu: max. 1,0; Co: max. 0,5; Sn: max. 0,5; As: max. 0,2; REM: max. 0,3; Residual iron and unavoidable impurities.

[0029] Preferably, the steel product consists of carbon steel.

[0030] A steel product is cast from molten steel or a molten steel alloy into a slab, block, or billet. Using a known continuous casting plant as an example, molten steel or a molten steel alloy is conventionally poured into a mold and solidifies completely into a strand, which is then drawn off and cut into several slabs of finite dimensions. Finally, the slabs are allowed to cool to ambient temperature, primarily through natural cooling. Alternatively, the slabs can also be used or supplied warm or hot at a temperature of up to 800 °C, particularly up to 700 °C, preferably up to 600 °C, and preferably at a temperature of at least 200 °C.The slabs are preheated electrically, inductively and / or conductively, in a preheating system and then heated to a target temperature, which can essentially correspond to the rolling temperature, for example in a walking beam or pusher furnace. After leaving the walking beam or pusher furnace, the slabs are fed to a hot rolling mill and hot-rolled. The hot rolling mill can comprise at least one hot rolling stand or preferably several, preferably up to seven, hot rolling stands in a hot rolling section. Optionally, one or more, for example up to three, roughing stands can be arranged upstream of a hot rolling stand or hot rolling section in the process direction.

[0031] The processes for heat-treating steel products in the form of slabs, blocks, discs or billets, and thus also the construction of corresponding heat treatment furnaces and electrical preheating systems, are state of the art and therefore familiar to experts.

[0032] The invention is explained in more detail with reference to the following exemplary embodiments in conjunction with the drawing.

[0033] Figur 1 The upper part shows a schematic diagram of the heat treatment of a steel product (not shown here). The steel product is electrically preheated in a system (2) to a temperature (T) below a target temperature (Tz) between 1000 °C and 1500 °C and optionally held at this temperature. It is then placed in a directly heated heat treatment furnace (1) comprising several gas burners supplied with hydrogen as fuel gas (not shown). In the heat treatment furnace (1), the steel product is heated to the target temperature (Tz) between 1000 °C and 1500 °C. The arrows on the left and right represent the process direction and the flow direction of the steel product to be heat-treated, respectively.

[0034] The steel product not shown is preferably a slab and the heat treatment furnace (1) is a walking beam furnace or a pusher furnace.

[0035] In the lower part of the Figur 1Figure 2 shows a diagram of the temperature profile of a steel product to be heated, in the direction of the process, encompassing the heat treatment in the system (2) and in the heat treatment furnace (1). In the system (2), which is preferably separate from the heat treatment furnace (1), the electrical preheating of the steel product can thus be advantageously carried out outside the heat treatment furnace (1). The electrical preheating of the steel product in the system (2) is carried out either conductively or, preferably, inductively. The electrical preheating in the system (2) is performed to a temperature (T) between 850 °C and 1200 °C and can optionally be maintained at this temperature. For example, normal atmosphere prevails in the system (2). In the heat treatment furnace (1), the steel product is heated through to a target temperature (Tz) between 1000 °C and 1500 °C.

[0036] Not shown is that the slab may, for example, already have a temperature between 200 °C and up to 800 °C, so that the slab can be brought up to temperature more quickly in the plant (2).

[0037] In a series of tests, hydrogen combustion experiments were conducted in a laboratory furnace. Steel products in the form of sheet metal pieces of grade S420MC were used. Hydrogen was used as the fuel gas in all tests, with an air-fuel ratio of 1.1. The following tests were carried out: Heating and percolating samples from room temperature to a target temperature of 1300 °C in a directly heated laboratory oven with a total residence time of 1 hour, → Reference samples; Preheating samples from room temperature to a temperature of 1100 °C using an inductor and then percolating them in a directly heated laboratory oven with a total residence time of 35 minutes to a target temperature of 1300 °C, → Samples A; Preheating samples from room temperature to a temperature of 900 °C using an inductor and then percolating them in a directly heated laboratory oven with a total residence time of 40 minutes to a target temperature of 1300 °C, → Samples B; Preheating warm samples from 600 °C to a temperature of 1100 °C using an inductor and then percolating them in a directly heated laboratory oven with a total residence time of 35 minutes to a target temperature of 1300 °C, → Samples C.

[0038] After sampling, the samples were quenched and deep-frozen. The hydrogen content in the sheet metal samples was analyzed using thermal desorption mass spectrometry (TDMS), a standard method for hydrogen measurement. Depending on the heat treatment, diffusible hydrogen content was determined to be on average 1.75 ppm (reference samples); 1.14 ppm (samples A); 1.42 ppm (samples B); and 1.09 ppm (samples C). The structure of the scale is always similar, characterized by the following three distinct phases: the outermost layer (upper phase) comprises hematite, the middle layer (middle phase) comprises magnetite, and the lower layer (lower phase), the layer / phase closest to the steel product, comprises wüstite. A fayalite phase may also be present along the interface.Furthermore, with increasing temperature not only was a visible increase in scale observed, but a different type of scale also formed, due to the high water vapor content > 30 vol.% in the furnace atmosphere, whereby an incorporation and thus a displacement of fayalite phases from the boundary layer plane into a wüstite matrix was detected in a microscopic examination.

[0039] It has been shown that a positive effect can be achieved if steel products are first electrically preheated in a plant to a temperature below a target temperature between 1000 °C and 1500 °C and optionally held at this temperature, and then placed in a directly heated heat treatment furnace comprising several gas burners which are supplied with hydrogen as fuel gas, in which they are subsequently heated through to the target temperature between 1000 °C and 1500 °C.

Claims

1. A method for heat-treating a steel product, wherein the steel product is placed in a directly heated heat treatment furnace (1) comprising several gas burners which are supplied with hydrogen as fuel gas, in which the steel product is heated to a target temperature (Tz) between 1000 °C and 1500 °C, wherein the steel product is a slab and the heat treatment furnace (1) is a walking beam furnace or a pusher furnace, characterized by the fact that The steel product is electrically preheated in a plant (2) to a temperature (T) below the target temperature (Tz) which is between 1000 °C and 1500 °C and optionally kept at this temperature.

2. The method of claim 1, wherein the steel product is preheated to a temperature (T) between 850 °C and 1200 °C.

3. Method according to one of the aforementioned claims, wherein the system (2) is separated from the heat treatment furnace (1) and thus the electrical preheating of the steel product is carried out outside the heat treatment furnace (1).

4. Method according to one of the preceding claims, wherein the electrical preheating of the steel product in the system (2) is carried out inductively.

5. Method according to one of the preceding claims, wherein the electrical preheating of the steel product in the system (2) is carried out conductively.

6. Method according to one of the preceding claims, wherein normal atmosphere prevails in the plant (2).

7. Method according to one of the preceding claims, wherein the steel product is fed into the system (2) warm at a temperature between 200 °C and 800 °C.

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