Method for heat treating steel products to forming heat

Abstract

The invention relates to a method for heat-treating steel products (1) to forming heat.

Description

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

[0002] Steel products that are heated to forming temperatures 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. Because 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). 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 control 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, in order to reduce or ultimately completely avoid the use of fossil energy and thus in turn reduce 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; see the applicant's website: htt ps: / / 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 as a fuel gas, or hydrogen 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. This higher partial pressure of water vapor can lead to the undesirable formation of thicker and more firmly adhering scale on the steel product. 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 result in increased material loss (scale formation and the necessary scale removal) and / or a loss of efficiency (due to longer descaling times / additional processing steps).Furthermore, reduced surface quality could result from scale formation and scale removal.

[0007] The object of the present invention is to further develop the method for heat-treating steel products to forming heat in such a way that a positive influence can be exerted 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 to forming heat, 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 950 °C and 1400 °C, wherein the furnace comprises several burners which are operated with a fuel gas containing 0 vol.% to 100 vol.% hydrogen 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, before entering the directly fired furnace chamber or the directly fired section of the furnace chamber, the steel product is applied to its surface, at least section by section, a layer comprising or consisting of at least one carbonate.

[0011] Carbonate is a salt of carbonic acid, which can comprise a primary carbonate, known as hydrogen carbonate, and / or a secondary carbonate, which is chemically different from the primary carbonate. Salts of carbonic acid are known and can include magnesium carbonate (MgCO₃), calcium carbonate (CaCO₃), potassium carbonate (K₂CO₃), sodium carbonate (Na₂CO₃), ammonium carbonate ((NH₄)₂CO₃), iron carbonate (FeCO₃), zinc carbonate (ZnCO₃), or mixtures thereof.

[0012] A change from a fossil fuel (natural gas) to an alternative, preferably hydrogen-containing, fuel in a directly fired furnace chamber or a directly fired section of a furnace chamber for heat-treating a steel product to forming heat 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, which, for example, have hydrogen content between > 0 and 100 vol.%, 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.

[0013] The combustion of hydrogen-containing fuel gas on the surface of the steel product creates an oxidizing atmosphere, leading to the formation of a layer of iron oxides. This layer comprises a matrix of wüstite and phases of magnetite and hematite. Heat treatment can cause additional silicon-containing oxides (fayalite) to become embedded within the wüstite matrix. An increasing proportion of water vapor in the flue gas or furnace atmosphere can thus cause the fayalite phases to be incorporated and shifted from the boundary layer plane into the wüstite matrix. This can result in particularly strong adhesion of the scale to the surface of the steel product, which is either impossible or extremely difficult to remove. In some cases, a virtually covering hematite layer can also form, which further complicates or even prevents chemical removal, especially pickling.

[0014] An increase in hydrogen in the fuel gas, and consequently an increase in the water vapor partial pressure in the resulting flue gas, must be counteracted by influencing the flue gas through a targeted shift in the thermodynamic processes within the furnace chamber, particularly by influencing the chemical reaction between water vapor and the steel product. This is achieved by applying a layer, at least partially, consisting of, for example, calcium carbonate (CaCO₃), to the surface of the steel product. During heating, the calcium carbonate (CaCO₃) burns off or decomposes, producing calcium oxide (CaO) and carbon dioxide (CO₂). The released carbon dioxide forms a protective film that shields the surface, at least partially, from oxygen ingress.In the furnace, for example at a surface temperature of the steel product between 600 °C and 900 °C, carbon dioxide is released from the calcium carbonate and forms a protective gas curtain near the surface, temporarily shielding it from the humid furnace atmosphere. This does not completely prevent scale formation, but at least inhibits it temporarily, so that the processes CaCO₃ → CaO + CO₂ are not overwhelmed. The heat treatment of steel products up to a temperature of approximately 600 °C is protected from the (humid) furnace atmosphere by the (at least partially) overlying layer of calcium carbonate, as this layer provides a barrier effect and thus prevents contact with the (humid) furnace atmosphere.The decomposition of the layer comprising or consisting of calcium carbonate begins above 600 °C, whereby the surface of the steel product is protected both by the coating resulting from the decomposition to calcium oxide and by the released carbon dioxide. Above, for example, 900 °C, the decomposition is largely complete, so that no more carbon dioxide is released and a moderate protective effect can only be provided by a coating of residual calcium carbonate and calcium oxide.

[0015] Other carbonates or mixtures thereof mentioned above can also be applied.

[0016] The hydrogen used, at least partially, in the fuel gas can be produced and supplied, for example, in water electrolysis using renewable energies such as wind, water and / or sun.

[0017] 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 the forming heat is reached, and then leaves the furnace to be subjected to a forming process in the hot state.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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 950 °C and 1400 °C, in particular at least 1000 °C, preferably at least 1050 °C, more preferably at least 1100 °C, and further preferably at least 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 is 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 the temperature of the atmosphere in the furnace chamber, may well be higher.

[0022] The steel product can be in the form of a slab, a block, a billet or a disc.

[0023] The steel product can be made of carbon steel.

[0024] Calcium carbonate can be applied, at least section by section, preferably completely, to the surface of the steel product in suspension, particularly aqueous suspension, and / or in dispersion, particularly aqueous dispersion. The application process can be carried out with suitable equipment and is therefore prior art and thus familiar to those skilled in the art. Application can be carried out by spraying, dipping, brushing, or coating. A drying step can optionally be performed after application.

[0025] The at least one carbonate can be present in suspension, particularly in aqueous suspension, with a particle size between 1 and 100 µm (a fine suspension) or with a particle size between 101 and 1000 µm (a coarse suspension). The at least one carbonate can also be present in dispersion, particularly in aqueous dispersion, with a particle size smaller than 1 µm. Furthermore, a mixture of the aforementioned particle sizes is also conceivable.

[0026] Preferably, the at least one carbonate is present in a concentration of 30 g / l to 700 g / l, the remainder being water and unavoidable impurities. The concentration can be, in particular, at least 50 or 70 g / l, preferably at least 100 or 120 g / l, more preferably at least 150 or 170 g / l, particularly preferably at least 200 or 220 g / l, further preferably at least 250 or 270 g / l, and in particular at most 650 or 600 g / l, more preferably at most 550 or 500 g / l, more preferably 450 or 400 g / l.

[0027] The layer, comprising or consisting of at least one carbonate, can have a thickness (after application) between 1.0 µm and 60.0 µm. In particular, the thickness can be between at least 4.0 µm and a maximum of 58.0 µm, preferably between 5.0 and a maximum of 55.0 µm.

[0028] To improve heat transfer, the layer can also contain a coloring component to achieve a darker color. For example, carbon black, carbon, iron oxide, and / or manganese oxide can be added. The coloring component can be present in the layer at a concentration of between 0.1 and 20 wt.%, particularly up to 15 wt.%, preferably up to 10 wt.%.

[0029] Depending on the volume of the steel product to be heated, and in particular also depending on the target forming 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.

[0030] The furnace with the directly fired furnace chamber, or with the directly fired section of the furnace chamber, for heat-treating a steel product to forming heat can be a pusher beam furnace or a walking beam furnace, designed to receive and heat slabs, blocks, or billets. Depending on the volume of the steel product to be heat-treated, a residence time of between 30 minutes and 15 hours, for example, can be considered.

[0031] Alternatively, the furnace used to heat-treat a steel product to forming temperature can be a roller hearth furnace, which is designed to hold and heat 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.

[0032] 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. 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. For further processing, the slabs are reheated and heated through to forming temperature, preferably rolling temperature, in a walking beam furnace or a pusher furnace.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.

[0033] Alternatively, the shaping process can also include forging, so that the heat treatment of the steel product in a furnace is carried out at forging heat.

[0034] The processes for heat-treating steel products in the form of slabs, blocks or billets to forming heat, and thus also the construction of corresponding furnaces, are state of the art and therefore familiar to the expert.

[0035] In particular, hydrogen can be present in the fuel gas at a proportion of at least 10 or 20 vol%. Preferably, hydrogen can be present in the fuel gas at a proportion of at least 30 or 40 vol%. More preferably, hydrogen can be present in the fuel gas at a proportion of at least 50 or 60 vol%. Particularly preferably, hydrogen can be present in the fuel gas at a proportion of at least 70 or 80 vol%. Further preferably, hydrogen can be present in the fuel gas at a proportion of at least 98 vol%. One embodiment, for example, includes the use of 100% hydrogen; in other words, the fuel gas consists of 100 vol% hydrogen or nearly 100 vol%, wherein impurities in the fuel gas of up to 0.5 vol%, in particular up to 0.2 vol%, preferably less than 0.1 vol%, are permitted, and where impurities cannot be avoided technically or only with considerable equipment effort.

[0036] If the fuel gas does not consist entirely of hydrogen, it may contain additional proportions of methane (CH₄) and / or carbon monoxide (CO) to achieve a hydrogen content of 100% by volume, along with impurities permitted up to 0.5% by volume, in particular up to 0.2% by volume, preferably less than 0.1% by volume. Traces of carbon dioxide (CO₂) up to 1.5% by volume may also be present.

[0037] 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.

[0038] Additionally or alternatively, blast furnace gas, for example containing or consisting of one or more of the components coke oven gas, blast furnace gas, converter gas, smelter gas, etc., can be considered as a fuel gas alongside hydrogen.

[0039] Furthermore, the temperature of the burner flame also influences the temperature of the furnace atmosphere. The combustion temperature with ambient air and natural gas is approximately 1970 °C, with ambient air and hydrogen approximately 2130 °C, with oxygen and natural gas approximately 2860 °C, and with oxygen and hydrogen approximately 3080 °C.

[0040] 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, more specifically between 0.80 and 0.98, preferably between 0.85 and 0.97, more preferably between 0.90 and 0.96, to avoid the presence of oxygen (compounds) in the flue gas, or alternatively between 1 and 1.25, more specifically between 1.01 and 1.20, more preferably between 1.02 and 1.15, more preferably between 1.03 and 1.10, to control the amount of oxygen in the flue gas for targeted scaling, for example in certain products.

[0041] 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.

[0042] The oxygen that may be needed for combustion can also be generated and provided by means of electrolysis using renewable energies (sun, wind, water, etc.).

[0043] The invention is explained in more detail with reference to the following exemplary embodiments in conjunction with the drawing.

[0044] The drawing shows the invention using a schematic illustration as an example. Figure 1 Figure 1 shows a furnace (10) for heating a steel product (1) to forming heat. The furnace (10) can be designed as a walking beam furnace, pusher furnace, or roller hearth furnace for receiving and heating slabs, blocks, discs, billets, or forgings. The furnace (10) has a directly fired furnace chamber or a directly fired section of a furnace chamber. The directly fired furnace chamber or the directly fired section of the furnace chamber of the furnace (10) comprises several burners (not shown) which can be supplied with a fuel gas (not shown) containing 0 vol.%, in particular at least 10 vol.% to 100 vol.% hydrogen, and an oxygen-containing gas (not shown), and from which a flue gas is generated that fills the furnace chamber or the section of the furnace chamber.

[0045] Figure 1further shows that the thoroughly heated steel product (1), for example in the form of a slab, can be removed from the furnace (10) and fed to a further processing, for example a hot rolling mill (100) for hot rolling, in order to produce a hot strip (1').

[0046] Furthermore, an exemplary temperature profile over a length of the furnace (10) of a steel product (1) to be heated is also shown, with a target temperature (Tz) which preferably corresponds to the temperature of the steel product (1) when removed from the furnace (10). The temperature ranges between 600 and 900 °C, in which the combustion or decomposition of the layer comprising or consisting of preferably calcium carbonate takes place, are also shown as examples in dashed lines.

[0047] Other carbonates mentioned above, and mixtures thereof, can also be applied. A coloring component can also be added and included in the layer.

[0048] Laboratory-scale investigations were conducted in a gas-fired furnace with a single combustion chamber. 100% hydrogen by volume was used as the fuel gas, mixed with air at an air-fuel ratio of approximately 1.05 in the burner. This resulted in a furnace atmosphere with a high water vapor partial pressure. A furnace humidity with a dew point of approximately 75 °C was measured.

[0049] Two thick steel samples, each measuring 10 x 10 x 4 cm, cut from conventionally cast slabs, were provided. One sample was coated with calcium carbonate in an aqueous suspension with a particle size between 101 and 1000 µm and a concentration of 350 g / l, the remainder being water and unavoidable impurities. The coating was applied by hand to a thickness of approximately 30 µm using a paint roller and then air-dried. The residence time for both samples was approximately 60 minutes. The samples were heat-treated in an oven and removed at a temperature of approximately 1200 °C. After removal, the samples were prepared according to standard procedures for analysis by light microscopy.

[0050] Figure 2Each figure shows macro images of the scale in a top view. The left image corresponds to scale formation after the application of a layer comprising or consisting of calcium carbonate to the surface of the sample, while the right image shows a standard setup with high furnace humidity. It is clearly visible that the steel product in the right image has a more open-pored scale layer compared to the left image. Furthermore, it was found that pure hydrogen combustion resulted in a dominant and very pronounced scale layer, which proved to be very resistant to chemical and / or mechanical removal. In contrast, the steel product treated according to the invention exhibited a moderate scale layer, which could be removed relatively easily using conventional and commonly used methods.

[0051] The heat treatment of steel products to forming temperatures with elevated water vapor partial pressure in the furnace atmosphere leads, as expected, to a different and / or dominant scale buildup on the steel product due to at least partial combustion of hydrogen in the fuel gas. By providing at least a partial layer consisting of or comprising calcium carbonate, scale buildup can be positively influenced in a furnace atmosphere with high water vapor partial pressure. This allows the subsequent existing process chain to be used without restrictions, such as with conventional scale scrubbers in a hot rolling mill.

Claims

1. A method for heat-treating a steel product (1) to forming heat, wherein the heat-treatment of the steel product (1) is carried out in a furnace (10) with a directly fired furnace chamber or with a directly fired section of a furnace chamber, wherein the temperature of the steel product (1) when removed from the furnace (10) is between 950 °C and 1400 °C, wherein the furnace comprises several burners which are operated with a fuel gas containing 0 vol.% to 100 vol.% hydrogen 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 Before entering the directly fired furnace chamber or the directly fired section of the furnace chamber, the steel product must have a layer on its surface comprising or consisting of at least one carbonate, at least in sections.

2. The method according to claim 1, wherein the at least one carbonate is present in an aqueous suspension with a particle size between 1 and 100 µm.

3. Method according to one of the preceding claims, wherein the at least one carbonate is present in an aqueous suspension with a particle size between 101 and 1000 µm.

4. Method according to one of the preceding claims, wherein the at least one carbonate is present in an aqueous dispersion with a particle size of less than 1 µm.

5. Method according to one of the preceding claims, wherein the at least one carbonate is present in a concentration of 30 g / l to 700 g / l, the remainder being water and unavoidable impurities.

6. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas with a proportion of at least 20 vol.%.

7. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas with a proportion of at least 50 vol.%.

8. Method according to one of the preceding claims, wherein hydrogen is contained in the fuel gas with a proportion of at least 70 vol.%.

9. Method according to one of the preceding claims, wherein the steel product (1) heat-treated to forming heat is subjected to hot rolling (100).

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

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