Method for heat treating steel products

Abstract

The invention relates to a method for heat-treating steel products according to claim 1.

Description

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

[0002] Steel products that are heated and heated through, particularly for forming, include slabs brought to rolling temperature, billets, ingots, or discs heated to rolling or forging temperature. These are heat-treated in appropriate furnaces to a temperature of at least 950 °C to achieve a fully austenitic microstructure, depending on the composition, which reduces forming resistance. In practice, directly heated furnaces are the standard. These furnaces, equipped with multiple burners, have been fueled with fossil fuels, such as natural gas, for decades. Since combustion takes place within the furnace, direct heating allows for the creation of a reducing or oxidizing furnace atmosphere, depending on the set air-fuel ratio (lambda value of the fuel gas).The furnace contains the flue gas from the burners, which has a composition with a high proportion of water (H₂O) and, depending on the air-fuel ratio, oxygen (O₂) and carbon dioxide (CO₂) or hydrogen (H₂) and carbon monoxide / carbon dioxide (CO / CO₂) as well as nitrogen / nitrogen oxides (N₂ / NOₓ). If a reducing furnace atmosphere is set with a lambda value < 1, the flue gas contains carbon monoxide (CO gas) to protect the steel product being heated from oxidation. However, a slightly oxidizing furnace atmosphere can also be set with a lambda value > 1. In this case, oxygen is also present in the flue gas, which causes targeted oxidation of the steel product being heated. The goal of atmospheric setting in a directly heated furnace is generally to achieve a scale-free surface when the steel product exits the furnace.The flue 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 or ammonia, in order to reduce or ultimately completely avoid the use of fossil energy and thereby achieve a reduction in CO2 emissions.

[0004] The steel processing industry is working at full speed to become climate-neutral as quickly as possible and thus meet the politically mandated climate targets. The applicant is also constructing new or retrofitting equipment to gradually replace fossil fuels with climate-neutral hydrogen or ammonia; see the applicant's website. https: / / www.thyssenkrupp.com / de / newsroom / pressemeldungen / pressedetailseite / top-oberflächen-fur-die-automobilindustrie-thyssenkrupp-nimmt-neuen-hubbalkenofen-am-standortduisburg-in-betrieb-156354.

[0005] Furthermore, it is also known from 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 / md wiretube / lib / all / lob / return download.cgi, see scenario 4 on page 22, that a furnace chamber of a walking beam furnace for the reheating of 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] Direct combustion of hydrogen or ammonia as fuel gas, or of hydrogen or ammonia components in the fuel gas, results in higher furnace humidity and / or a higher partial pressure of water vapor in the furnace atmosphere compared to conventional natural gas combustion, due to the combustion reactions with air and / or oxygen. Consequently, the formation of an unwanted, thicker, and more firmly adhering scale on the steel product cannot be ruled out as a result of this higher water vapor partial pressure. Furthermore, changes in crude steel production, such as the planned use of larger quantities of scrap, can lead to an increased introduction of impurities. These two factors contribute to the formation of thicker and more firmly adhering scale.Without countermeasures, this would lead to increased material loss (scale formation and required scale removal) and / or loss of efficiency (due to longer descaling times / additional processing steps). Furthermore, reduced surface quality could result from scale formation and removal.

[0007] The object of the present invention is to further develop the method for heat-treating steel products in such a way as to have a positive influence on scale formation.

[0008] This problem is solved by a method having the features of claim 1. Further embodiments are described in the dependent claims.

[0009] The teaching relates to a method for heating a steel product, wherein the heating of the steel product is carried out in a furnace with a directly fired furnace chamber or with a directly fired section of a furnace chamber, wherein the temperature of the steel product when removed from the furnace is between 500 °C and 1400 °C, wherein the furnace comprises several burners which are operated with a fuel gas containing 60 vol.% to 99.5 vol.% hydrogen or ammonia and an oxygen-containing gas and from which a flue gas is generated which fills the furnace chamber or the section of the furnace chamber.

[0010] Essential to the invention is that the fuel gas comprises a mixture containing or consisting of hydrogen and carbon monoxide with a proportion between 0.5 vol.% and 40 vol.%.

[0011] A change from a fossil fuel (natural gas) to an alternative, preferably hydrogen-containing fuel or ammonia, in a directly fired furnace chamber or a directly fired section of a furnace chamber for heat-treating a steel product results in a changed furnace atmosphere, which has a significant impact on the material properties and surface of the steel product. When hydrogen-containing fuel gases or ammonia 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 not only to a greater tendency for oxidation (scale formation) during heating by oxygen-affine elements in the steel product, but also to the formation of firmly adhering scale.

[0012] An increase in the proportion of hydrogen or ammonia in the fuel gas, and consequently an increase in the partial pressure of water vapor in the resulting flue gas, must be counteracted by selectively influencing the thermodynamic processes in the furnace chamber, particularly the chemical reaction between water vapor and the steel product. By introducing carbon monoxide in a concentration between 0.5% and 40% by volume into the fuel gas, a positive effect can be achieved on the reaction kinetics at the surface of the steel product being heat-treated. This has been observed to result in a compact and brittle scale buildup, which, for example, indicates a reduced incorporation of hydrogen as a reaction product in wüstite.

[0013] Therefore, the water / water vapor content in the flue gas can be advantageously reduced due to the carbon monoxide content in the fuel gas, and the reaction kinetics at the surface can be altered. This reduces oxidation processes of freely available oxygen and absorption processes of freely available hydrogen at the surface of the steel product being heat-treated, resulting in a significantly more compact and, in some cases, narrower scale buildup. Due to reduced hydrogen incorporation, less plastic wüstite, a so-called adhesive scale, can form within the scale. Consequently, optimized scale removal is possible, and fewer surface defects resulting from scale removal can occur.

[0014] The hydrogen used, at least partially, in the fuel gas can be produced and supplied, for example, in water electrolysis using conventional energies for electricity generation, such as natural gas, mixed gases, nuclear power, or renewable energies for electricity generation, such as wind, water and / or sun.

[0015] The furnace, with its directly fired furnace chamber or with its directly fired section of a furnace chamber, can be designed and have appropriate means by which it is possible to pass the steel product to be heat-treated either substantially continuously or almost continuously through the furnace, or preferably discontinuously, i.e., that a steel product is fed into the furnace, remains there until a predetermined temperature, preferably a forming temperature, is reached, and then leaves the furnace to be subjected to a preferred forming process in the hot state.

[0016] The furnace can have one furnace chamber with direct firing, or several furnace chambers, at least one of which is directly fired, or several of which may be directly fired. Alternatively, the furnace can have one or more furnace chambers, where at least a section of at least one furnace chamber is directly fired.

[0017] Decarbonization in a furnace with a directly fired furnace chamber or with a directly fired section of a furnace chamber for heat-treating a steel product to forming heat would therefore not only be a simple switch from fossil to non-fossil fuels, but also involves a complex influence on the product parameters.

[0018] Determining or measuring the humidity or 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 with suitable measuring devices.

[0019] The heat treatment of a steel product to forming temperature is carried out such that the temperature of the steel product upon removal from the furnace is between 500 °C and 1400 °C, in particular at least 600 °C or 700 °C, preferably at least 800 °C or 900 °C, more preferably at least 1000 °C or 1050 °C, and further preferably at least 1100 °C or 1150 °C. The temperature of 1400 °C should not be exceeded to avoid partial melting and / or excessive scaling of the steel product. For ecological and economic reasons, the temperature can be limited, in particular, to a maximum of 1350 °C, preferably to a maximum of 1330 °C. The temperature is measured, for example, on one side of the surface of the steel product, in particular with a pyrometer or other suitable measuring instruments. Thus, the temperature of the steel product can be determined using means known to those skilled in the art.The temperature in the furnace chamber, or rather the temperature of the atmosphere in the furnace chamber, can certainly be higher.

[0020] The steel product can be in the form of a slab, a block, a billet or a disc.

[0021] The steel product can be made of carbon steel.

[0022] Depending on the volume of the steel product to be heat-treated or heated, and in particular also depending on a target (shaping) temperature, the residence time in the furnace chamber or in the section of the furnace chamber can be between 10 minutes and 48 hours, in particular between 30 minutes and 30 hours, preferably between 1 hour and 24 hours.

[0023] The furnace with the directly fired furnace chamber or with the directly fired section of the furnace chamber for heat-treating a steel product, preferably to forming heat, can be a pusher beam furnace or a walking beam furnace designed for receiving and heating slabs, blocks, or billets. Depending on the volume of the steel product to be heat-treated, a residence time of, for example, between 30 minutes and 15 hours can be considered.

[0024] Alternatively, the furnace for heat-treating a steel product, preferably to forming heat, can be a roller hearth furnace designed for receiving and heating forgings. Depending on the volume of the steel product to be heat-treated, a residence time of between 2 and 18 hours can be considered.

[0025] 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 classically 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. For further processing, the slabs are reheated and heated through, for example, in a walking beam furnace or a pusher furnace, preferably to forming heat, and thus preferably to rolling heat.The forming process preferably comprises hot rolling in a hot rolling mill, which may include at least one hot rolling stand or preferably several, preferably up to seven, hot rolling stands in a hot rolling stage, wherein optionally one or more, for example up to three, roughing stands may be arranged in the process direction in front of a hot rolling stand or a hot rolling stage.

[0026] Alternatively, the shaping process can also include forging, so that the heat treatment of the steel product in a furnace can thus be carried out at forging heat.

[0027] The processes for heat-treating steel products in the form of slabs, blocks or billets, preferably at forming heat, and thus also the construction of corresponding furnaces, are state of the art and therefore familiar to the expert.

[0028] The hydrogen or ammonia content in the fuel gas can be at least 60 or 65% by volume, in particular at least 70 or 75% by volume, preferably at least 80 or 85% by volume, and more preferably at least 90 or 95% by volume. The hydrogen or ammonia content in the fuel gas can be up to 99.5% by volume. Impurities in the fuel gas of up to 0.5% by volume, in particular up to 0.2% by volume, and preferably less than 0.1% by volume, are permitted, provided that impurities cannot be avoided technically or only with considerable technical effort.

[0029] Therefore, in addition to hydrogen or ammonia, the fuel gas contains carbon monoxide in a proportion of at least 1.0 vol.%, preferably at least 1.5 vol.%. The carbon monoxide content of the fuel gas can be, in particular, a maximum of 35 vol.%, 30 vol.%, preferably a maximum of 25 vol.%, 20 vol.%, more preferably a maximum of 15 vol.%, 10 vol.%, and most preferably a maximum of 5 vol.%.

[0030] The burners can be operated with an air-fuel ratio between 0.75 and 1.25. The air-fuel ratio can be, in particular, between 0.75 and 0.99, especially between 0.80 and 0.98, preferably between 0.85 and 0.97, preferably between 0.90 and 0.96, in order to avoid the presence of oxygen (compounds) in the flue gas and, in particular, to obtain hydrogen in the flue gas; or alternatively, between 1 and 1.25, especially between 1.01 and 1.20, preferably between 1.02 and 1.15, preferably between 1.03 and 1.10, in order to control the amount of oxygen in the flue gas for targeted scaling, for example, in certain products.

[0031] Air, for example ambient air, oxygen, or a combination of air and oxygen can be used as the oxygen-containing gas for operating the burners. 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. Preheating can, for example, be limited to a maximum of 1000 °C. Preheating the fuel gas and / or the oxygen-containing gas can lead to an increase in the adiabatic flame temperature.

[0032] The oxygen that may be needed for combustion can also be generated and provided by electrolysis using conventional energy sources for electricity generation, such as natural gas, mixed gas or nuclear power, or by renewable energy sources (sun, wind, water, etc.).

[0033] The invention is explained in more detail with reference to the following exemplary embodiments in conjunction with the drawing.

[0034] Laboratory-scale investigations were conducted in a gas-fired furnace with a single combustion chamber. In one experiment, 100 vol% hydrogen was combusted with air at an air-fuel ratio of approximately 1.1 in the burner, resulting in a furnace atmosphere with a high partial pressure of water vapor. The measured furnace humidity had a dew point of approximately 75 °C. In the other experiment, 1.5 vol% carbon monoxide was combusted with 98.5 vol% hydrogen as a fuel gas, also with air at an air-fuel ratio of approximately 1.1. This resulted in a lower partial pressure of water vapor compared to the pure hydrogen combustion, with a measured furnace humidity of 70 °C.

[0035] Two thick steel samples, each measuring 10 x 10 x 4 cm, cut from conventionally cast slabs, were provided. The residence time for both samples was approximately 60 minutes. The samples were heat-treated in a furnace using the aforementioned fuel gas and removed at a temperature of approximately 1250 °C. After removal, the samples were prepared for analysis using light microscopy according to standard industry practices.

[0036] Figure 1Each image shows a partial top view of the samples. The image on the left clearly shows that pure hydrogen combustion causes flaking on the surface of the scale layer. This could be attributed to internal stresses resulting from hydrogen incorporation into and between the scale layers, and the associated differences in the structure of the scale layers. In contrast, a mixture with 1.5 vol% carbon monoxide results in an intact scale surface.

[0037] Also in Figure 2 The aforementioned results and findings are reflected in the respective macro images of the scale structure in partial sectional view. With the aforementioned mixed fuel gas, a flatter and more compact scale layer can be formed (see lower illustration), whereas pure hydrogen combustion leads to a superficial, fractured scale layer (see upper illustration).

Claims

1. A method for heat-treating a steel product, wherein the heat-treating of the steel product is carried out in a furnace with a directly fired furnace chamber or with a directly fired section of a furnace chamber, wherein the temperature of the steel product upon removal from the furnace is between 500 °C and 1400 °C, wherein the furnace comprises several burners which are operated with a fuel gas having a proportion of 60 vol.% to 99.5 vol.% hydrogen or ammonia and an oxygen-containing gas and from which a flue gas is generated which fills the furnace chamber or the section of the furnace chamber, characterized by the fact that The fuel gas comprises a mixture containing or consisting of hydrogen or ammonia and carbon monoxide with a proportion between 0.5 vol.% and 40 vol.%.

2. The method according to claim 1, wherein the proportion of carbon monoxide in the fuel gas is a maximum of 30 vol.%.

3. Method according to one of the preceding claims, wherein the proportion of carbon monoxide in the fuel gas is a maximum of 20 vol.%.

4. Method according to one of the preceding claims, wherein the proportion of carbon monoxide in the fuel gas is a maximum of 10 vol.%.

5. Method according to one of the preceding claims, wherein the proportion of carbon monoxide in the fuel gas is a maximum of 5 vol.%.

6. Method according to one of the preceding claims, wherein the temperature of the steel product when removed from the furnace is at least 700 °C.

7. Method according to one of the preceding claims, wherein the temperature of the steel product when removed from the furnace is at least 900 °C.

8. Method according to one of the preceding claims, wherein the temperature of the steel product when removed from the furnace is at least 1100 °C.

9. Method according to one of the preceding claims, wherein the heat-treated steel product is subjected to hot rolling.

10. Method according to any one of claims 1 to 8, wherein the heat-treated steel product is subjected to forging.

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