Process for heating steel products to a shaping temperature

Abstract

The invention relates to a method for heating steel products to a shaping temperature.

Description

[0001] Process for heat treating steel products to forming heat

[0002] The invention relates to a method for heat treating steel products to forming heat.

[0003] Steel products that are heated to forming heat and then soaked through include slabs that are heated to rolling heat, ingots, billets, or discs that are heated to rolling or forging heat and heat-treated in appropriate furnaces to a temperature of at least 950°C to achieve a fully austenitic structure, depending on the composition, which reduces forming resistance. In practice, directly heated heat treatment furnaces are standard practice. Heat treatment furnaces equipped with multiple burners have been fed with fossil fuels, such as natural gas, for decades. Since combustion takes place in the heat treatment furnace, direct heating can be used to create a reducing or oxidizing furnace atmosphere, depending on the set air ratio (lambda value of the fuel gas).The heat treatment furnace therefore contains the combustion gas from the burners, which has a composition with a high proportion of water (H2O) and, depending on the air ratio, oxygen (O2) and carbon dioxide (CO2) or hydrogen (H2) and carbon monoxide / carbon dioxide (CO / CO2). If a reducing furnace atmosphere is set with a lambda value < 1, the combustion gas contains carbon monoxide (CO gas) to protect the steel product to be heated from oxidation. However, a slightly oxidizing furnace atmosphere can also be set with a lambda value > 1. In this case, the combustion gas also contains oxygen, which causes a targeted oxidation of the steel product to be heated. The aim of the atmosphere setting in a directly heated heat treatment furnace is generally to ensure a scale-free surface when the steel product exits the furnace.The combustion gas is thus used as a “protective gas” against uncontrolled oxidation (= scaling).

[0004] As part of the globally demanded decarbonization, plants powered by fossil fuels are to be converted or converted to more environmentally friendly fuels or energy sources, such as hydrogen, in the future in order to reduce or ultimately eliminate the use of fossil energy. Decarbonization requires a reduction in the use of fossil fuels and energy sources, and thus, in turn, a reduction in CO2 emissions.

[0005] 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. For example, the applicant's plants are being newly built or converted to gradually replace fossil fuels with climate-neutrally produced hydrogen, see the applicant's website: https: / / www.thyssenkrupp.com / de / newsroom / pressemeldungen / pressetailseite / top-ober flachen-fur-die-automobilindustrie-thyssenkrupp-nimmt-neuen-hubbalkenofen-am-standort-duisburg-in-betrieb-156354

[0006] Furthermore, it is also known from the technical report on decarbonization "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 / 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 openly heated by means of 160 sidewall and ceiling radiant burners, whereby the integration of suitable burners, which have been successfully tested on a pilot scale using 100% H2 and also mixtures of H2 and natural gas, still needs to be implemented on an industrial scale after preparation.

[0007] The publication "The morphologies of oxide scale in a Si-containing steel", IOP Conf. Series: Materials Science and Engineering 244 (2017) 012026, doi:10.1088 / 1757-899X / 244 / 1 / 012026, describes a scale buildup on a Si-containing steel product that was annealed at 1050 or 1230 °C in an 11 vol.% water vapor-containing and oxidizing atmosphere.

[0008] In the case of direct combustion of hydrogen as fuel gas or of hydrogen components in the fuel gas, the combustion reactions with air and / or oxygen result in a higher furnace humidity or a higher water vapor partial pressure in the furnace atmosphere compared to conventional natural gas combustion.

[0009] Heat treatment of a steel product in a water vapor-containing atmosphere alters the grain layers in the microstructure, leading to rapid grain boundary oxidation. Due to the increased iron oxide formation (scaling), grain boundary oxidation can also occur more rapidly and penetrate deeper into the steel product.

[0010] Thus, the iron oxides and the scale layer develop differently at elevated water vapor partial pressures in the heat treatment atmosphere. The scale adheres more firmly than usual. Furthermore, the scale layer is significantly thicker or higher than usual under elevated water vapor partial pressure, so that impaired processability (thickness fluctuations, temperature fluctuations, etc.), surface defects, and material loss in the heat treatment furnace cannot be ruled out.

[0011] The object of the present invention is to further develop the process for heat treating steel products to forming heat in such a way that a positive influence can be exerted on the formation of scale.

[0012] This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

[0013] 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 1350 °C, wherein the furnace comprises a plurality of burners which are supplied with a hydrogen-containing fuel gas.

[0014] What is essential for the invention is that before entering the directly fired furnace chamber or the directly fired section of the furnace chamber, the steel product comprises a carbonaceous enrichment on its surface.

[0015] A switch from a fossil fuel (natural gas) to an alternative, hydrogen-containing fuel in a directly fired furnace chamber or a directly fired section of a furnace chamber used for heat treating a steel product to forming heat thus results in a changed furnace atmosphere with highly influential parameters regarding the material properties and surface finish of the steel product to be achieved. When hydrogen-containing fuel gases, which, for example, contain hydrogen in a proportion of between 10 and 100 vol. %, are burned, a larger amount of water vapor is generated compared to natural gas, which ensures a higher water vapor partial pressure in the furnace atmosphere. This results in a different type of oxidation (scaling), resulting in firmly adhering scale in the steel product during heat treatment due to oxygen-affinity elements.

[0016] On the surface of the steel product, which essentially contains at least C, Mn, and Si, the remainder being Fe and unavoidable impurities, an oxidizing atmosphere can be created by the combustion of the hydrogen-containing fuel gas, whereby a layer of iron oxides can form on the steel product, comprising a matrix of wustite and phases of magnetite and hematite. Additional Si-containing oxides can be incorporated within the wustite matrix as a result of heat treatment. An increasing water vapor content in the combustion gas or in the furnace atmosphere can thus cause the fayalite phases to be incorporated and thus displaced from the boundary layer plane into the wustite matrix. This can result in particularly stable adhesion of the scale to the surface of the steel product, which can either not be removed at all or only with great effort.Under certain circumstances, a substantially covering layer of hematite may also form, which also makes chemical removal, especially pickling, difficult or even impossible.

[0017] An increase in hydrogen in the fuel gas and thus an increase in the water vapor partial pressure in the resulting combustion gas must be counteracted by influencing the combustion gas through a targeted shift in the thermodynamic processes in the furnace chamber, in particular by influencing the chemical reaction between water vapor and steel sheet. By providing a carbonaceous enrichment on the surface of the steel product, which corresponds to near-surface and / or targeted carburization of the steel product surface, the water vapor can react primarily with the carbon on the surface of the steel product during heat treatment, thus positively influencing scale formation, which can be removed relatively easily in subsequent processes.Furthermore, a "darker" surface can be heated more quickly due to the carbon enrichment compared to non-"carburized" surfaces, thereby reducing the throughput time and thus energy and, in particular, CO2. The furnace with its directly fired furnace chamber or with its directly fired section of a furnace chamber can be designed and have corresponding means with which it is possible to pass the steel product to be heat-treated through the furnace either essentially continuously or almost continuously, or preferably discontinuously, i.e. a steel product is fed into the furnace, remains there until the forming heat is reached, and then leaves the furnace to be fed to a forming process in the hot state.

[0018] The furnace may have one furnace chamber with direct firing or multiple furnace chambers, at least one of which is directly fired, or several of which may be directly fired. However, the furnace may also have one or more furnace chambers, with at least a portion of at least one furnace chamber being 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 would also involve a complex manipulation of the product parameters.

[0020] The determination or recording of kiln humidity or water vapor partial pressure in a kiln atmosphere is familiar to those skilled in the art. This can be done, for example, by measuring the dew point using suitable measuring devices.

[0021] The hydrogen used at least in part in the fuel gas can, for example, be produced and provided in water electrolysis using renewable energies such as wind, water and / or sun.

[0022] The heat treatment of a steel product to forming heat is carried out in such a way that the temperature of the steel product upon removal from the furnace is between 950°C and 1350°C, in particular at least 1000°C, preferably at least 1050°C, more preferably at least 1100°C, more preferably at least 1150°C. The temperature of 1350°C should not be exceeded in order 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 1300°C, preferably to a maximum of 1280°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 device. Thus, the temperature of the steel product can be recorded 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 can certainly be higher.

[0023] The steel product can be in the form of a slab, an ingot, a billet or a disc.

[0024] The steel product contains at least elements such as C, Mn, Si, the balance Fe, and unavoidable impurities. The composition may include or consist of, in wt.%:

[0025] C: 0.001 to 0.9;

[0026] Mn: 0.05 to 12.0;

[0027] Si: 0.001 to 5.0;

[0028] N: max. 0.1 ;

[0029] S: max. 0.1 ;

[0030] P: max. 0.1 ; optionally one or more of the following elements:

[0031] AI: max. 2.0;

[0032] Ni: max. 0.5;

[0033] Cr: max. 1.5;

[0034] B: max. 0.01;

[0035] Ca: max. 0.01;

[0036] Mo: max. 0.5;

[0037] Ti: max. 0.2;

[0038] Nb: max. 0.2;

[0039] V: max. 0.2;

[0040] Rest Fe and unavoidable impurities.

[0041] Depending on the volume of the steel product to be soaked, 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 20 hours. 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 soak slabs, ingots, slabs, 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.

[0042] Alternatively, the furnace for heat-treating a steel product to forming heat can be a roller-hearth furnace designed to accommodate and heat through 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.

[0043] A steel product is cast from molten steel or a molten steel alloy into a slab, block, disc, or billet. Using a conventional continuous casting plant as an example, a molten steel or a molten steel alloy is traditionally poured into a mold and completely solidified into a strand, which is then drawn off and separated into several slabs of finite dimensions. The slabs are then allowed to cool to ambient temperature, particularly through natural cooling. For further processing, the slabs are (re)heated and thoroughly heated, for example, in a walking beam furnace or pusher furnace, to forming temperature, thus preferably to rolling temperature.The shaping preferably comprises hot rolling in a hot rolling mill, which may comprise at least one hot rolling stand or preferably several, preferably up to seven hot rolling stands in a hot rolling mill stage, wherein optionally one or more, for example up to three roughing stands may be arranged in front of a hot rolling stand or a hot rolling mill stage in the process direction.

[0044] Alternatively, the forming process may also include forging, so that the heat treatment of the steel product takes place in a furnace at forging heat.

[0045] The processes for heat treating steel products in the form of slabs, blocks, discs or billets to forming heat and thus also the design of corresponding furnaces are state of the art and therefore familiar to those skilled in the art. In particular, hydrogen can be contained in the fuel gas in a proportion of at least 20 vol.%. Preferably, hydrogen can be contained in the fuel gas in a proportion of at least 40 vol.%. Preferably, hydrogen can be contained in the fuel gas in a proportion of at least 60 vol.%. Particularly preferably, hydrogen can be contained in the fuel gas in a proportion of at least 80 vol.%. Further preferably, hydrogen can be contained in the fuel gas in a proportion of at least 98 vol.%. One embodiment comprises, for example, 100% use of hydrogen, in other words, the fuel gas consists of 100 vol.% hydrogen or almost 100 vol.%, with impurities in the fuel gas amounting to up to 0.5 vol.%, in particular up to 0.2 vol.-%, preferably less than 0.1 vol.%, are permitted, whereby impurities cannot be avoided technically or only with high equipment expenditure.

[0046] If the fuel gas does not consist entirely of hydrogen, it may contain, in addition to hydrogen, further proportions of methane (CH4) and / or carbon monoxide (CO) to make up to 100 vol.%, together with impurities which are permitted up to 0.5 vol.%, in particular up to 0.2 vol.%, preferably less than 0.1 vol.%.

[0047] For example, when using natural gas, the proportions of the main component methane can vary and thus also include other components such as ethane, propane, ethylene and butane individually or in combination.

[0048] Furthermore, the temperature of the burner flame also influences the temperature of the furnace atmosphere. The combustion temperature with ambient air and natural gas is approximately 1970 °C, and with ambient air and hydrogen, approximately 2130 °C. Combustion with oxygen and natural gas is approximately 2860 °C, and with oxygen and hydrogen, approximately 3080 °C.

[0049] The burners can be operated with an air ratio between 0.75 and 1.25. The air ratio can be in particular between 0.75 and 0.99, in particular between 0.80 and 0.98, preferably between 0.85 and 0.97, more preferably between 0.90 and 0.96, in order to avoid the presence of oxygen (compounds) in the combustion gas, or alternatively between 1 and 1.25, in particular between 1.01 and 1.20, preferably between 1.02 and 1.15, more preferably between 1.03 and 1.10, in order to control the amount of oxygen in the combustion gas for targeted scaling, for example with certain products. The oxygen-containing gas used to operate the burners can be air, for example ambient air, oxygen, or a combination of air and oxygen.The oxygen-containing gas and / or the fuel gas can be preheated before being fed to the combustion process 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.

[0050] The oxygen required for combustion can also be generated and provided by electrolysis using renewable energies (sun, wind, water, etc.).

[0051] According to one embodiment, the carbonaceous enrichment can be achieved thermally by diffusion of a carbon- or hydrocarbon-containing gas on the surface of the steel product. During annealing prior to the actual heat treatment of the steel product to forming heat, the steel product can be annealed either in line in a preliminary unit or in a section of the furnace upstream of the directly fired furnace chamber, or in a separate process step in a unit in a carbon-rich atmosphere at a temperature of at least 800°C, in particular of at least 850°C, preferably of at least 900°C, up to a maximum of 1000°C; in particular up to a maximum of 970°C, in order to form a carbon-rich layer by diffusion on the surface of the steel product.The annealing atmosphere can be generated, for example, with a slightly reducing carrier gas, which typically consists of 20% CO, 40% H2, and 40% N2, and a regulated enrichment gas containing a hydrocarbon, e.g., methane or propane.

[0052] The carbon-rich layer can extend from the surface to a depth of 1 μm or several μm into the steel product. On average along the extension, the carbon content in the carbon-rich layer is at least 20%, in particular at least 30%, preferably at least 40%, preferably at least 50% higher than the carbon content of the steel product and can be a maximum of 2.0 wt.%, in particular a maximum of 1.50 wt.%, preferably a maximum of 1.20 wt.%.

[0053] According to an alternative embodiment, the carbonaceous enrichment can be achieved by applying a carbon-containing solid to the surface of the steel product. Before heat-treating the steel product to forming heat, the steel product can be coated with a carbonaceous coating, for example, with a graphite layer or another carbon-rich coating, which is preferably at least temperature-resistant up to approximately 850 °C. During heat-treating to forming heat, the carbonaceous application not only carburizes the steel product surface but also serves as a sacrificial reactant for the increased water vapor content in the furnace atmosphere.

[0054] The "carburization" can, for example, extend from the surface to a depth of 1 pm into the steel product. On average along the extension, the carbon content in the carbon-rich layer is at least 20%, in particular at least 30%, preferably at least 40%, preferably at least 50% higher than the carbon content of the steel product and can be a maximum of 2.0 wt.%, in particular a maximum of 1.50 wt.%, preferably a maximum of 1.20 wt.%.

[0055] The invention is explained in more detail using the following embodiments in conjunction with the drawing.

[0056] Laboratory-scale investigations were conducted in a gas-fired furnace with a furnace chamber, where 100 vol.% hydrogen was used as the fuel gas, combusted with air at an air / fuel ratio of approximately 1.05 in the burner, resulting in a furnace atmosphere with a high water vapor partial pressure. A furnace humidity with a dew point of approximately 75 °C was measured.

[0057] Two thick steel samples, each measuring 10 x 10 x 4 cm, cut from conventionally cast slabs, were provided. One of these samples was coated with a graphite coating, "Fortindispersion TG 45" from TECHNOGRAFIT GmbH, with a thickness of approximately 30 μm, by hand using a paint roller and then dried in air. The residence time for both samples was approximately 60 minutes. The samples were heat-treated in a furnace and each removed at a temperature of approximately 1200 °C. After removal, the samples were quenched and prepared for analysis using light microscopy as required.

[0058] Figure 1 shows, in a cross-section (light micrograph), the structure of the scale on the surface of the sample without carbonaceous enrichment (top) and with carbonaceous enrichment (bottom). It is clearly visible that a continuous and fully developed scale layer has formed on the steel product in the upper micrograph, which is highly resistant to abrasion. The steel product in the lower micrograph, on the other hand, exhibits an interrupted, porous transition zone at the interface to the steel product, so that the scale layer can be removed using conventional and standard means, such as chemical and / or mechanical methods.

[0059] As expected, heat treatment of steel products to forming heat with increased water vapor partial pressure in the furnace atmosphere due to at least partial combustion of hydrogen in the fuel gas leads to a different scale buildup on the steel product. By providing a carbonaceous enrichment, the scale buildup can be positively influenced in a furnace atmosphere with a high water vapor partial pressure. This allows the existing downstream process chain, such as scale scrubbers in a hot rolling mill, to continue to be used without restrictions.

Claims

Claims 1. A method for heat treating a steel product to forming heat, wherein the heat treatment 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 1350 °C, wherein the furnace comprises a plurality of burners which are supplied with a hydrogen-containing fuel gas, characterized in that before entering the directly fired furnace chamber or the directly fired section of the furnace chamber, the steel product comprises a carbonaceous enrichment on its surface.

2. A process according to claim 1, wherein the carbonaceous enrichment is carried out thermally by diffusion of a carbon- or hydrocarbon-containing gas at the surface of the steel product.

3. A process according to claim 2, wherein the steel product has been annealed in a carbon-rich atmosphere at a temperature of at least 800 °C.

4. The method according to claim 1, wherein the carbonaceous enrichment is carried out by applying a carbon-containing solid to the surface of the steel product.

5. The method of claim 4, wherein the steel product has been coated with a carbon-containing coating.

6. A method according to any one of the preceding claims, wherein the steel product heat-treated to forming heat is subjected to hot rolling.

7. A method according to any one of claims 1 to 5, wherein the steel product heat-treated to forming heat is subjected to forging.

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