Method for heat-treating a steel product at a shaping heat

Abstract

The invention relates to a method for heat-treating a steel product at a shaping heat in a directly heated heat-treatment furnace, wherein: scale forms at least partially on the surface of the steel product as a result of the heat treatment; the steel product is removed from the heat-treatment furnace at a temperature of at least 950 °C and is at least partially descaled in order to be supplied for shaping; the scale is brought to a temperature of at least 1330 °C before descaling.

Description

[0001] Method for heat-treating a steel product to forming heat

[0002] The invention relates to a method for heat-treating a steel product to forming heat in a directly heated heat treatment furnace, wherein, as a result of the heat treatment, scale forms at least partially on the surface of the steel product, is removed from the heat treatment furnace at a temperature of at least 950 °C and is at least partially descaled in order to be used for forming.

[0003] Heat treatment furnaces, such as direct-fired furnaces (DFFs), are well-established furnaces used for the heat treatment of metals. These are typically fueled with fossil fuels, such as natural gas. 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). So-called by-product gases or mixtures thereof are also suitable as fuel gases, which are generated particularly in integrated steelworks and can therefore be utilized accordingly. The furnace atmosphere thus consists of the combustion gas produced by the burner(s), containing less than 35.0% water vapor by volume, and in particular less than 25% by volume.-% and, depending on the air ratio, may contain oxygen (O2) and carbon dioxide (CO2) or hydrogen (H2) and carbon monoxide / carbon dioxide (CO / CO2).

[0004] Oxygen-containing gases react with the steel surface, and at high temperatures, this reaction primarily oxidizes iron and other alloying elements such as manganese and silicon. The primary reaction product is iron oxide, also known as scale. Up to a temperature of approximately 575 °C, the scale on the steel surface consists of a magnetite phase (Fe3O4) facing the steel surface and a hematite phase (Fe2O3) facing away from the steel surface. With increasing temperatures, especially above 580 °C, a layer of wüstite (FeO) forms between the steel surface and the magnetite phase; see, for example, "The morphologies of oxide scale in a Si-containing steel," Zhou et al., IOP Conf. Series: Materials Science and Engineering 244 (2017) 012026, doi:10.1088 / 1757-899X / 244 / l / 012026.

[0005] As part of the globally mandated decarbonization efforts, plants powered by fossil fuels are to be converted to more environmentally friendly fuels or energy carriers, such as hydrogen, in order to reduce or ultimately eliminate the use of fossil energy. Switching from a fossil fuel, such as natural gas, to an alternative, hydrogen-containing fuel, or to hydrogen in general, would result in a changed furnace atmosphere, which in turn would significantly influence the material properties and surface area of ​​the resulting metal product, particularly steel. The combustion of hydrogen-containing fuels produces a larger quantity of water vapor compared to natural gas, leading to a higher partial pressure of water vapor in the furnace atmosphere.This results in a greater tendency for oxidation (scale formation) during heating due to oxygen-affine elements in the steel product, which occurs particularly on the surface of the steel product. The presence of a higher partial pressure of water vapor affects the bond between the scale and the steel product surface—simply put, its adhesion to the surface. At the same time, a higher partial pressure of water vapor leads to an increase in the scale layer thickness and an altered scale formation.These effects can have various impacts on the steel product: strongly adhering primary scale can lead to surface defects and thus to product failure; furthermore, it can be associated with poorer scale removal in the scale washer and thus with subsequent defects in further processing (for example, the incorporation of residual scale during hot rolling); in addition, the formation of a covering hematite layer can influence the pickling process and thus also lead to surface defects or product 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.

[0006] Heat treatment furnaces, which heat steel material in the form of slabs to forming temperature in so-called pusher and / or walking beam furnaces, are heated with natural gas or, if available and integrated into a 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 can therefore have a greater, and especially negative, impact on the surface quality of the steel material being formed. The scale produced in the heat treatment furnaces can adhere well or poorly depending on its composition. Consequently, it cannot be guaranteed that the installed capacity of a conventional scale scrubber is always sufficient to remove the primary scale before the slab is formed or rolled into hot strip in a hot rolling mill.

[0007] 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 / cgi-bin / mdwiretube / lib / all / lob / returndownload.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.

[0008] A measure to prevent negative scale formation when using a hydrogen-containing fuel gas is known from the prior art, namely that the furnace atmosphere is "diluted" with an additional gas during the combustion of a hydrogen-containing fuel gas in order to reduce the moisture in the furnace atmosphere, cf. for example DE 10 2022 118 249 Al.

[0009] The object of the present invention is to further develop the method for heating a steel material in such a way as to have a positive influence on the scale that inevitably forms on the surface of the flat steel product during the heating process.

[0010] This problem is solved by a method having the features of claim 1. Further embodiments are described in the dependent claims.

[0011] The invention relates to a method for heat-treating a steel product to forming temperature in a directly heated heat treatment furnace, wherein, as a result of the heat treatment, scale forms at least partially on the surface of the steel product, is removed from the heat treatment furnace at a temperature of at least 950 °C, and is at least partially descaled in order to be used for forming. A key aspect of the invention is that the scale is heated to a temperature of at least 1330 °C before descaling.

[0012] 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 heat treatment furnace is at least 950 °C, in particular at least 1050 °C, preferably at least 1100 °C, and more preferably at least 1150 °C. In principle, the temperature should not exceed 1500 °C to avoid partial / complete melting and / or excessive scaling of the steel product. For ecological and economic reasons, the temperature is limited, in particular, to a maximum of 1400 °C, preferably to a maximum of 1350 °C, more preferably to a maximum of 1325 °C, and further preferably to a maximum of 1300 °C. The temperature is measured, for example, on one side of the surface of the steel product, in particular with pyrometers or other suitable measuring instruments. These temperatures should not be exceeded in the core of the steel product.The core temperature of a steel product cannot be directly determined during normal operation. Therefore, the temperature of the steel product can be measured using methods known to those skilled in the art. The temperature in the heat treatment furnace, or rather the temperature of the atmosphere within the heat treatment furnace, may well be higher.

[0013] Scale formation depends on technological conditions, such as temperature, time, air content, and the chemical composition of the steel product. With increasing temperature, air content, and time, the scale on the surface of the steel product increases. Atmospheric humidity can also influence the increase (thickness) of the scale. The structure of the scale, or scale layer, is always similar, characterized by the following three distinct phases. The outermost layer, or upper phase, comprises hematite (Fe₂O₃, melting point 1565 °C), the middle layer, or middle phase, comprises magnetite (Fe₃O₄, melting point 1527 °C), and the lower layer, or lower phase, or the layer / phase closest to the steel product, comprises wüstite (FeO, melting point 1369 °C). This innermost phase is also the most unstable.

[0014] The inventors have surprisingly discovered that when the scale, which has formed at least partially on the surface of the steel product, particularly when it completely covers the surface, is subjected to a temperature increase to at least 1330 °C, particularly at least 1350 °C, preferably at least 1369 °C, and thus is at least partially melted on the surface of the steel product, since wüstite is the most unstable phase compared to the other phases in the scale. Through this at least partial melting of the scale and the subsequent solidification, at least a portion of the scale transforms into a glass-like structure, which is brittle and less porous. The adhesion of this "newly formed" scale to the surface of the steel product is surprisingly lower than that of "standard" scale, so that it flakes off the surface of the steel product even with minimal force and is therefore easier to remove.

[0015] This positive effect would also be noticeable when switching from a fossil fuel (natural gas) to an alternative, hydrogen-containing fuel. When hydrogen-containing fuels are burned, a larger amount of water vapor is produced 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 by oxygen-affine elements in the steel product, which forms on the surface of the steel product. 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 steel product surface. The scale layer (oxide layer) would also grow and / or be affected.

[0016] 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. Alternatively or additionally, the partial pressure of water vapor can also be calculated.

[0017] The steel product may contain or consist of at least elements such as C, Mn, Si, balance Fe, and unavoidable impurities. The composition may, in wt.%, include or consist of:

[0018] C: 0.001 to 0.9;

[0019] Mn: 0.05 to 12.0;

[0020] Si: 0.001 to 5.0;

[0021] N: max. 0.1 ;

[0022] S: max. 0.1 ; P: max. 0.1 ; optionally one or more of the elements:

[0023] AI: max. 2.0;

[0024] Mon: max. 1.0 ;

[0025] Ni: max. 1.0;

[0026] Cr: max. 1.5;

[0027] B: max. 0.01 ;

[0028] Ca: max. 0.01 ;

[0029] Note: max. 0.5;

[0030] Ti: max. 0.5;

[0031] V: max. 0.5;

[0032] Cu: max. 1.0;

[0033] Co: max. 0.5;

[0034] Sn: max. 0.5;

[0035] As: max. 0.2;

[0036] REM: max. 0.3;

[0037] Residual iron and unavoidable impurities.

[0038] Preferably, the steel product consists of carbon steel.

[0039] Furthermore, the temperature of the burner flame influences the temperature in the furnace atmosphere and the temperature of the furnace chamber. The flame temperature can be specified as follows:

[0040] - Propane / butane with air 1925 °C and with oxygen 2850 °C;

[0041] - Methane (natural gas) with air 1970 °C and with oxygen 2860 °C;

[0042] - Ethyne (acetylene) with air 2250 °C and with oxygen 3030 °C;

[0043] - Hydrogen with air 2130 °C and with oxygen 3080 °C.

[0044] The scale can be brought to a temperature preferably of at least 1380, 1390 °C, more preferably of at least 1400, 1410 °C, more preferably of at least 1420, 1430 °C.

[0045] Since only the scale should experience a temperature increase, and since, due to thermodynamic equilibrium, heat conduction and thus heat transfer from the scale to the near-surface layer of the steel product still occurs, and in order to prevent the steel product from melting, the temperature to which the scale is to be brought must not exceed 1538 °C, as this temperature corresponds to the melting point of iron. Because the steel product contains other alloying elements besides iron, such as carbon, manganese, and silicon, these also lower the melting point depending on their concentration. Therefore, the scale should not only be brought to a temperature below 1538 °C, but the temperature should be specifically below 1500 or 1490 °C, preferably below 1470 or 1460 °C, and more preferably below 1450 or 1440 °C.

[0046] To reduce a temperature increase in the near-surface layer of the steel product adjacent to the scale, which could lead to undesirable partial melting of the steel product surface and thus to mixing with the at least partially melted scale, it is provided, for example, that the scale is maintained at a temperature of at least 1369 °C for a duration of 1 s to 7200 s. The duration can be, in particular, at least 2, 3, or 5 s, preferably at least 7, 10, or 12 s, and more preferably at least 15, 17, or 20 s, in order to bring the scale to this temperature. The duration can be limited, in particular, to a maximum of 5000, 2000, or 800 s, preferably to a maximum of 500, 400, or 300 s, and more preferably to a maximum of 200, 100, 50, or 30 s, in order to avoid partial melting of the surface of the steel product.

[0047] For direct heating, a fuel gas containing 5 to 100 vol% hydrogen is preferably used. In particular, the hydrogen content of the fuel gas can be at least 10, 15, or 20 vol%, preferably at least 30, 35, or 40 vol%, more preferably at least 50, 55, or 60 vol%, and more preferably at least 65, 70, or 75 vol%, and further preferably at least 80, 85, or 90 vol%. The degree of decarbonization can be improved with an increasing hydrogen content, especially if the hydrogen used, at least partially, in the fuel gas is produced and supplied, for example, in water electrolysis using renewable energies such as wind, water, and / or solar power. The fuel gas can also consist of hydrogen with impurities in the fuel gas of up to 0.5 vol%, particularly up to 0.2 vol%, and preferably less than 0.1 vol%.-% are permitted, whereby impurities cannot be avoided technically or only with considerable equipment effort, so that one can still speak of 100 vol% hydrogen in the fuel gas. If the fuel gas does not consist entirely of hydrogen, it may contain additional proportions of methane (CH4) and / or carbon monoxide (CO) in addition to hydrogen, in order to achieve 100 vol% along with impurities, which are permitted up to 0.5 vol%, in particular up to 0.2 vol%, preferably less than 0.1 vol%.

[0048] 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.

[0049] 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 is chosen.

[0050] Any heat source suitable for raising the temperature of the scale can be used. According to one embodiment, at least one burner, or several burners, or at least one laser, or several lasers can act on the scale. Preferably, one or more burners are rigidly installed, and the steel product, which at least partially contains scale, is moved past it, for example, slowly, or remains stationary within its area of ​​effect, particularly for the aforementioned duration. Preferably, several burners are installed, whose flames can act on the scale, at least section by section.

[0051] The scale can be heated to a temperature of at least 1330 °C, particularly at least 1350 °C, preferably at least 1369 °C, within the heat treatment furnace. Preferably, the scale can also be heated to a temperature of at least 1330 °C, particularly at least 1350 °C, preferably at least 1369 °C, at the outlet of the heat treatment furnace. For this purpose, corresponding zones, preferably equipped with burners, can be arranged to ensure a high heat input and thus a corresponding temperature increase in the scale.

[0052] Alternatively, the scale can be heated outside the heat treatment furnace to a temperature of at least 1330 °C, in particular at least 1350 °C, preferably at least 1369 °C. For this purpose, burners or lasers can be used to achieve the corresponding temperature increase in the scale. For example, the burners or lasers can be positioned upstream of a scale remover, such as a scale scrubber, when viewed in the process direction of the steel product. Scale removers, such as scale scrubbers, are state of the art, and their operating principles are known to those skilled in the art.

[0053] The steel product can be in the form of a slab, a block, a disc or a billet.

[0054] Depending on the volume of the steel product to be heat-treated, and especially depending on the target forming temperature, the residence time in the heat treatment furnace can be between 10 minutes and 48 hours, particularly between 30 minutes and 24 hours, preferably between 1 hour and 12 hours.

[0055] The heat treatment furnace with direct heating for heat-treating a steel product to forming heat can be a pusher beam furnace or a walking beam furnace, designed to hold and heat slabs, blocks, discs, or billets. Depending on the volume of the steel product to be heat-treated or heated, a residence time of between 30 minutes and 6 hours can be considered.

[0056] Alternatively, a roller hearth furnace, designed for receiving and heating forgings, can be used for heat-treating a steel product to forming temperature. Depending on the volume of the steel product to be heat-treated or heated through, a residence time of between 2 and 18 hours can be considered.

[0057] A steel product is cast from molten steel or a molten steel alloy into a slab, ingot, 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. This strand is then drawn off and cut into several slabs of finite dimensions, after which the slabs are allowed to cool to ambient temperature, primarily through natural cooling. Alternatively, the slabs can also be used warm or hot, at temperatures up to 800 °C. For further processing, the slabs are heat-treated, reheated, and heated through, for example, in a walking beam furnace or a pusher furnace, to shaping heat, 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.

[0058] Alternatively, the shaping process can also include forging, so that the heat treatment or heating of the steel product in a furnace is carried out at forging temperature.

[0059] The processes for heat-treating or heating steel products in the form of slabs, blocks, discs or billets to forming heat, and thus also the construction of corresponding heat treatment furnaces, are state of the art and therefore familiar to the expert.

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

[0061] In laboratory-scale investigations, a gas-fired heat treatment furnace, a muffle furnace with a furnace volume of 0.04 m³, was used. 3 Two different fuel gas compositions, natural gas and hydrogen, were combusted with oxygen, each with an air-fuel ratio of 1.03. This resulted in different partial pressures of water vapor in the atmosphere of the heat treatment furnace, and thus different furnace humidity levels, with combustion with hydrogen leading to approximately twice the water vapor content compared to combustion with pure natural gas.

[0062] Several steel samples made of carbon steel S235 with dimensions of 10 x 10 x 3 cm were heat-treated or heated through for a residence time of 60 minutes each and taken at a temperature of approximately 1250 °C, measured using a pyrometer (at the scale surface).

[0063] The first group of steel samples, which had undergone combustion using both natural gas and hydrogen, were subjected to a scale scrubber with a nozzle pressure of up to 380 bar after heat treatment, corresponding to a laboratory scale. No descaling was performed on the second group of steel samples.

[0064] A third group of steel samples, treated with natural gas and hydrogen combustion, were subjected to an additional 30-second exposure to a burner or flame (fueled with natural gas) after heat treatment. An average temperature of approximately 1420 °C was measured at the scale surface using a pyrometer. These steel samples were then processed in a scale scrubber. A fourth group of steel samples also underwent additional flame treatment, as previously mentioned, but no descaling was performed.

[0065] Complete descaling of the steel samples (first part) exposed to hydrogen combustion was not possible. This was due to the fact that hydrogen combustion resulted in a different chemistry and structure in the scale compared to natural gas combustion. In particular, the scale layer was thicker compared to natural gas combustion and exhibited persistent adhesion to the surface of the steel sample.

[0066] A different result was achieved with the additional flame treatment (third part), in which the surfaces of the steel samples could be almost completely descaled even after hydrogen combustion. This was due to the fact that the additional flame treatment led to at least partial melting of the scale, and during the subsequent solidification, at least some of the scale was transformed into a glass-like structure, which was brittle and less porous.

[0067] Figure 1 shows the scale removed from steel samples that had been heat-treated by natural gas combustion, where the scale on the left (R) corresponded to the normal process and the scale on the right (I) underwent an additional temperature increase as described in the examples above.

[0068] Figure 2 shows in top view two scaled steel samples which were heat-treated with hydrogen combustion as described in the examples above, wherein the scale of the left steel sample (R) had no additional flame treatment and the scale of the right steel sample (I) had undergone additional flame treatment.

Claims

Patent claims 1. A method for heat-treating a steel product to forming heat in a directly heated heat treatment furnace, wherein, as a result of the heat treatment, scale forms at least partially on the surface of the steel product, is removed from the heat treatment furnace at a temperature of at least 950 °C and is at least partially descaled in order to be used for forming, characterized in that the scale is brought to a temperature of at least 1330 °C before descaling.

2. The method of claim 1, wherein the scale is brought to a temperature below 1538 °C.

3. Method according to one of the preceding claims, wherein the scale is held at a temperature of at least 1330 °C for a duration of 1 s to 7200 s.

4. Method according to one of the preceding claims, wherein a fuel gas with a proportion of 5 vol.% to 100 vol.% hydrogen is used for direct heating.

5. Method according to one of the aforementioned claims, wherein, in addition to a fuel gas, an oxygen-containing gas is used for direct heating and the combustion is carried out with an air-fuel ratio between 0.75 and 1.

25.

6. Method according to one of the preceding claims, wherein at least one burner or at least one laser acts on the scale to increase the temperature.

7. Method according to one of the preceding claims, wherein the scale is brought to a temperature of at least 1330 °C inside the heat treatment furnace.

8. Method according to one of the preceding claims, wherein the scale at the outlet of the heat treatment furnace is brought to a temperature of at least 1330 °C.

9. Method according to any one of claims 1 to 6, wherein the scale is brought to a temperature of at least 1330 °C outside the heat treatment furnace.

10. Method according to one of the preceding claims, wherein the steel product is a slab which is heat-treated to forming heat in a heat treatment furnace in the form of at least one walking beam furnace or at least one pusher furnace, in order to subsequently be hot-rolled.

11. Method according to any one of claims 1 to 9, wherein the steel product is a forging which is heat-treated in a heat treatment furnace in the form of at least one forging furnace to forming heat in order to be subsequently forged.

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