Apparatus and method for degassing melts in continuous casting plants

Abstract

The invention relates to an apparatus (1) and to a method for degassing a melt in a continuous casting plant, said apparatus comprising at least one container (2), wherein the container (2) can be divided into an upper region (2.1) and a lower region (2.2), wherein the upper region (2.1) of the container (2) has a first inlet (3) for a pouring stream (A) of the melt, and the lower region (2.2) of the container (2) has an outlet (4) for a molten bath (B) of the melt that collects in the lower region (2.2), wherein the upper region (2.1) of the container (2) has a second inlet (5) for an inert gas and an outlet (6) for generating a vacuum, at least in the upper region (2.1) of the container (2), and wherein the upper region (2.1) and the lower region (2.2) of the container (2) form a shared volume.

Description

[0001] Title:

[0002] Device and method for degassing melts in continuous casting plants

[0003] Area:

[0004] The present invention relates to a device and a method for degassing melts in continuous casting plants.

[0005] State of the art:

[0006] In the future, electric steel production via electric arc furnaces and the use of directly reduced iron (DRI) of various grades as feedstock will become increasingly important as steelmaking processes decarbonize. The necessary degassing, particularly the reduction of dissolved hydrogen, nitrogen, and / or oxygen in the liquid steel, will therefore become increasingly important to ensure the quality of the steel produced.

[0007] Devices and methods for degassing melts in steel production are known from the state of the art; their function is to remove dissolved gases, such as hydrogen or nitrogen, from the liquid steel.

[0008] The Ruhrstahl-Heraeus process (RH process), for example, is a secondary metallurgy process in which liquid steel is subjected to a continuous vacuum treatment for degassing. The goal of this treatment is to produce steel with greater purity and to remove hydrogen or nitrogen.

[0009] Another state-of-the-art process is vacuum degassing (VD). In vacuum degassing, unwanted dissolved gases are removed from the liquid melt in the ladle under vacuum. During VD treatment, the ladle contents are additionally purged with argon. This movement leads to rapid and homogeneous mixing of the steel alloy in the ladle and accelerated degassing. Under the influence of different vacuum levels in the VD system, gases such as hydrogen and nitrogen are reduced, and non-metallic inclusions from the steel are separated into the slag. The vacuum degassing (VOD) process is also known from the state of the art.The vacuum refining process, based on vacuum degassing, is particularly suitable for high-chromium stainless, acid-resistant, and heat-resistant steel (RSH steels), as it allows for the almost complete removal of carbon from the melt in high-chromium grades. This is made possible by injecting additional oxygen into the melt at reduced pressure in the VOD system.

[0010] However, the disadvantages of the aforementioned processes are the high investment costs and high operating costs for the acquisition and maintenance of the equipment. Another significant disadvantage of the aforementioned processes is that they require their own processing time or treatment time for the melt during the ongoing production process, which accordingly extends the overall production time of the steel.

[0011] Furthermore, the state-of-the-art solutions require high investment costs and also generate high operating costs. Combining the degassing process of the pouring stream and the melt pool also reduces the process time of the production process, or rather, the treatment time of the melt in the secondary metallurgy, thus reducing the overall steel production time accordingly compared to the state-of-the-art devices.

[0012] Object of the invention:

[0013] The object of the invention is therefore to provide a device and a method with which the treatment time of a steel melt in the production process can be reduced.

[0014] Invention:

[0015] This object is achieved by the features of independent claim 1. Further advantageous embodiments of the invention are specified in the dependent claims. The features listed individually in the dependent claims can be combined with one another in a technologically expedient manner and can define further embodiments of the invention.

[0016] The device according to the invention for degassing melts in a continuous casting plant comprises at least one container, wherein the container can be divided into an upper region and a lower region, wherein the upper region of the container has a first inlet for a pouring jet of the melt and the lower region of the container has an outlet for a molten pool of the melt collecting in the lower region, wherein the upper region of the container has a second inlet for an inert gas and an outlet for generating a vacuum at least in the upper region of the container, and wherein the upper region and the lower region of the container form a common volume.

[0017] The device according to the invention allows a casting stream to be degassed before entering the mold, thus eliminating the need for prior degassing in the secondary metallurgy or at least reducing the duration or depth of the vacuum. Compared to conventional casting stream degassing systems, the combination of the casting stream degassing system with the continuous casting distributor reduces the overall height, allowing retrofitting into an existing continuous casting system.

[0018] Furthermore, the device allows for sequential casting of multiple ladles and / or individual casting of a batch in the continuous casting plant. The elimination of the charging valve supports the improved or reduced height of the facility.

[0019] The inert gas is initially used as a purge gas to flush residual gases such as hydrogen, oxygen, nitrogen, and other gases that come into contact with the melt from the vessel volume. The inert gas ring integrated according to the invention in the CSD system forms at least a partial gas film / layer over the melt pool. This protects the melt in the CSD system from the air atmosphere, for example, during a ladle change.

[0020] The division of the vessel into an upper and a lower section is defined such that the upper section represents the part of the vessel in which the melt is present as a pouring stream. The lower section of the vessel, on the other hand, is defined such that it receives the melt in the form of a molten pool or forms the molten pool. In this respect, the lower section can also form the continuous casting distributor. The sections within the vessel are not locally defined and can have a different volume distribution depending on the fill level. Furthermore, the upper and lower sections of the vessel do not necessarily represent a structural division. The interior of the vessel can also have different refractory linings.

[0021] The numerical terms used below ("first," "second," etc.) serve to distinguish between multiple similar objects, quantities, or processes and do not imply any interdependence and / or sequence of these objects, quantities, or processes. Should a dependence and / or sequence be required, this is explicitly stated here or will be obvious to the person skilled in the art upon studying the specifically described embodiment.

[0022] Preferably, the first inlet is located on the upward-facing side of the container. To increase the process time for degassing the pouring stream, the first inlet, through which the melt enters the container, is located at the highest point of the container.

[0023] The pouring stream is directed vertically downwards, for example, although an inclined arrangement is also conceivable to increase the drop distance while maintaining a constant height and thus slightly increasing the degassing rate. Alternatively, the pouring stream can also be directed laterally into the vessel to create an extended pouring stream.

[0024] To further facilitate the flow of the melt out of the container, particularly against the internal vacuum pressure of the container, the outlet is preferably arranged on the underside of the container. This ensures that, despite the vacuum, sufficient pressure is exerted by the melt's own weight on the melt at the outlet to allow it to flow out of the container. The flow rate can also be adjusted by the amount of purge gas used or by adjusting the negative pressure. To ensure vacuum degassing of both the pouring stream and the melt pool, the container is preferably designed as a gas-tight pressure chamber. Therefore, the container must withstand the external pressure created by the vacuum and maintain its structural integrity at high process temperatures.

[0025] The inlets and outlets can preferably be closed in a gas-tight manner, with the first inlet also preferably being designed to create a gas-tight flange connection with a non-stressed pan. Furthermore, the vacuum that can be generated in the container preferably has a pressure of less than 100 mbar, more preferably less than 10 mbar, and even more preferably less than 1 mbar. In order to keep the pressure in the container as constant as possible and to reduce the energy required for a vacuum pump, the interior of the container is sealed as well as possible against the external pressure. However, since the container is provided with inlets and outlets, it is necessary for these to seal the container airtight in certain process situations in order to maintain the constant vacuum in the container.

[0026] An airtight flange connection to the ladle prevents air from entering the vessel's interior, both during a single filling of the vessel and during serial filling (or sequential casting of multiple batches) with multiple ladles. Tests and experience have shown that a pressure of less than 100 mbar, preferably less than 10 mbar, and more preferably less than 1 mbar, is necessary to achieve effective degassing of the melt. To increase the effect and achieve increased melt purity, lower negative pressures are preferable.

[0027] Preferably, the melt pool collected in the lower region of the vessel has a free surface opposite the melt-free space in the upper region of the vessel. A free surface is defined as an interface between a gas and a liquid. Consequently, the area of ​​the melt pool that comes into direct contact with the vacuum is the free surface of the melt pool.

[0028] The drop height of the pouring stream between the first inlet and the lower region of the container is preferably between 1 m and 5 m, more preferably between 1 m and 3 m, and even more preferably between 1 m and 2 m. Effective degassing of the pouring stream can be ensured from a drop height of 1 m. As the drop height increases, the pouring stream fans out further, increasing the system volume.

[0029] The device preferably has a minimum installation height of 1.5 m and a maximum installation height of 10 m. The installation height according to the invention refers to the external dimensions of the system. By combining the CSD chamber and the distribution trough, the installation height is reduced.

[0030] This means that the device can be retrofitted into existing systems without significantly increasing the vertical height of the existing system. Furthermore, the hall height of an existing production facility may be sufficient, so that no modifications to the hall are necessary during a conversion. However, the lower limit of the device is limited by the fact that a certain minimum height is required to enable high-quality degassing of the pouring stream.

[0031] The inert gas blown into the vessel is preferably argon, nitrogen, or a mixture of the two. Alternatively, other typical inert gases are also conceivable, such as carbon dioxide. Furthermore, the purge gas and / or an additional wire, e.g., nitrogen or CaSi wire, can be fed into the melt using an additional attachment on the purge gas supply. Particular attention must be paid to the wire feed system to ensure that the wire feeds are gas-tight or that the vacuum pump is designed to accommodate the leakage rates of the wire feed system. The performance of the vacuum pump can be adjusted to suit different operating modes by controlling the extraction rate.

[0032] A pleated bag preferably surrounds the connection point between the first inlet and a non-stressed pan. A pleated bag provides a simple and flexible seal between a pan and the device. Positional deviations of the pan relative to the first inlet are thus still tolerable within limits. The container is preferably formed from a CSD chamber and a vacuum distribution channel, or from a CSD chamber and a vacuum intermediate container. Alternatively, the container is preferably designed as a distribution channel or as an intermediate container, with the selection of the various embodiments depending on the desired connection process.

[0033] In addition to the device, the invention also comprises a method according to the invention for operating a device for degassing melts in a continuous casting plant according to one of claims 1 to 12, in which the following steps are carried out: a) introducing the inert gas into the container (2) via the second inlet (5) b) generating a constant vacuum inside the container (2) via the outlet (6) c) introducing the melt into the container (2) via the first inlet (3) and degassing the pouring stream thus created d) degassing the molten bath (B) collecting in the container (2) e) once a certain fill level in the container (2) has been reached, opening the outlet (4) to discharge the melt from the container (2), preferably into a non-stressed mold.

[0034] To ensure consistent continuous casting quality, it is first necessary to purge the volume of reactive gases. This is done, as explained in step a), by blowing in the inert gas. Therefore, in this step, the inert gas represents a purge gas. A purge gas is a generally inert gas that is passed as a steady stream through a process chamber, in this case the volume of the vessel. It is intended to ensure the cleanest possible process chamber, so that as few unwanted and uncontrolled chemical reactions as possible occur with residual gases and the melt.

[0035] After the volume of the container has been flushed, a vacuum is created in the container by the vacuum pump. The melt is poured into the container through the first inlet, whereby the melt is sucked into the container from the flanged ladle by gravity and the vacuum. This process first degasses the pouring stream and then degasses the molten bath via its free surface. The reduced melt is then fed via the outlet, for example, to a mold. It is important that the melt spends a certain time in the container to ensure sufficient degassing. The cross-sections of the inlet and outlet are dimensioned or controlled in such a way that when filling from multiple ladles the molten bath in the container preferably does not exceed or fall below a minimum or maximum fill level.

[0036] Preferably, step b overlaps with step a. To ensure optimal purging of the volume and reduce the amount of reactive gases in the vessel, the vacuum pump can evacuate the vessel while the inert gas is being injected. To reduce process costs, it is also conceivable to use different inert gases, with a gas that is more favorable than argon being used for the purging process, and argon being injected only subsequently.

[0037] To maintain consistently high quality in the continuous casting process, the internal pressure of the vacuum is preferably kept constant, even when the fill level of the molten bath changes. The negative pressure generated in the container is preferably less than 100 mbar, preferably less than 10 mbar, and more preferably less than 1 mbar. Tests and experience have shown that a pressure of less than 100 mbar is necessary to achieve degassing of the melt. To increase the effect and achieve increased melt purity, lower pressures are preferable.

[0038] The following three figures are attached to the description:

[0039] Fig. 1 : schematic sectional view of the inventive

[0040] Device according to a first embodiment;

[0041] Fig. 2: schematic sectional view of the inventive

[0042] Device according to a second embodiment

[0043] Fig. 3: a schematic sectional view of the device according to the invention, according to a third embodiment

[0044] The invention is described in detail below with reference to the figures. In all figures, identical technical elements are provided with identical reference numerals. Fig. 1 shows a schematic representation of a device 1 according to the invention for degassing melt in a continuous casting plant according to a first embodiment. The device 1 according to Fig. 1 is a container 2 comprising a CSD chamber 2.3 and an intermediate container 2.4 or a tundish 2.4. In the following, reference will be made only to the intermediate container 2.4, although this term can also be replaced by a tundish 2.4 for all three proposed embodiments.

[0045] The CSD chamber 2.3 is located above the intermediate vessel 2.4 and is open at the bottom. The intermediate vessel 2.4 has an opening on its upward side that complements the open bottom of the CSD chamber 2.3. The downwardly open side of the CSD chamber 2.3 and the opening on the top of the intermediate vessel 2.4 are aligned with each other and connected to the outside in a gas-tight manner at their connecting area. The interconnected CSD chamber 2.3 and the intermediate vessel 2.4 together form a single volume. The entire volume, the inner lining, and the seals are designed so that the melt level can also rise into the area of ​​the CSD chamber.

[0046] A pouring nozzle 3.1 with a ladle gate valve 3.2 is arranged on top of the CSD chamber 2.3. The melt is introduced from a ladle (not shown) into the CSD chamber 2.3 and the intermediate vessel 2.4 via the pouring nozzle 3.1. The ladle gate valve 3.2 is arranged on the pouring nozzle 3.1 and, depending on its position, is designed to prevent or allow the flow of melt from the ladle (not shown) through the shroud 3.1 into the volume of the VSD chamber 2.3 and the intermediate vessel 2.4, or to adjust the melt flow.

[0047] A dip pipe 4.1 and a distributor slide 4.2 are arranged on the underside of the intermediate container 2.4. The melt collected as molten pool B in the intermediate container 2.4 is discharged into a mold (not shown) via the dip pipe 4.1 or a pouring pipe. The distributor slide 4.2 is arranged on the dip pipe 4.1 and, depending on its position, is designed to prevent or allow the flow of melt from the intermediate container 2.4 through the dip pipe 4.1 into the mold, or to adjust the melt flow. In addition, a vacuum pipe 6.1 and another valve 6.2 are arranged laterally on the CSD chamber 2.3. One end of the vacuum pipe 6.1 opens into the interior of the CSD chamber 2.3 and its other end is connected to a vacuum pump (not shown). The air is at least partially evacuated from the volume formed by the CSD chamber 2.3 and the intermediate container 2.4 by the vacuum pump and via the vacuum pipe 6.1.If a negative pressure of, for example, less than 1 mbar is reached, valve 6.2 switches accordingly until a set internal pressure threshold is exceeded again. If this pressure is exceeded, valve 6.2 activates automatically. It is also conceivable that the vacuum pump is continuously running. Valve 6.2 is designed to prevent fresh air from entering the volume from outside when the vacuum pump is idle.

[0048] When closed, the pan slide 3.2, the distributor slide 4.2 and the valve 6.2 enable an airtight closure of the container 2. In other words, the penetration of air through the pan slide 3.2, the distributor slide 4.2 or through the valve 6.2 is largely prevented.

[0049] The CSD chamber 2.3 is designed to receive a pouring stream A and allow it to fall over a certain height through the evacuated space into a molten pool B. The melt pouring into the molten pool B is collected in the intermediate container 2.4. The intermediate container 2.4 is dimensioned such that the molten pool B forms a surface C that is free from the evacuated space. The vacuum removes the gaseous impurities from the pouring stream A and, via the free surface C of the molten pool B. To promote this effect, the intermediate container 2.4 has a greater horizontal extent than the CSD chamber 2.3, while the VSD chamber 2.3 has a greater vertical extent than the intermediate container 2.4. However, the fill level is not limited to the volume of the intermediate container 2.4. Should the molten pool B exceed the volume of the intermediate container 2.4, the CSD chamber 2 serves as the fill level.3 at least partly also as a receiving space for the melt pool B.

[0050] Although not shown in detail in Fig. 1, an inert gas is blown into the volume through another inlet 5. The inert gas is used as a purge gas to flush residual gases such as hydrogen, nitrogen, and other gases that come into contact with the melt from the pouring stream volume. Furthermore, the inert gas forms a gas film over the melt pool B, preventing air introduced, for example, by a ladle change or a ladle gate valve 3.2 that is not sealing optimally, from reaching the melt and contaminating it.

[0051] To preferably facilitate a streamlined melt flow and avoid creating flow shadows in the melt pool B, the CSD chamber 2.3 is located at the upper left end of the intermediate container 2.4, with the dip tube 4.1 located at the lower right end of the intermediate container 2.4. This ensures that the incoming and outgoing melt has a homogeneous residence time in the device 1.

[0052] Fig. 2 shows a schematic representation of a device 1 according to the invention for degassing melts in continuous casting plants according to a second embodiment. The second embodiment of the device 1 differs from the first embodiment only in that the intermediate container 2.4 is provided with a dip pipe 4.1 at both lower ends and each dip pipe 4.1 is provided with a distributor slide 4.2. In order to achieve a uniform volume flow of melt through both dip pipes 4.1, the CSD chamber 2.3 according to the second embodiment is arranged centrally on the intermediate container 2.4. Furthermore, it is possible to add additional dip pipes for multiple strands of a continuous casting plant.

[0053] Fig. 3 shows a schematic representation of a device 1 according to the invention for degassing melts in continuous casting plants according to a third embodiment. The third embodiment of the device 1 differs from the first and second embodiments in that the third embodiment dispenses with a CSD chamber. For this purpose, the intermediate container 2.4 is dimensioned such that it provides both sufficient height for the open pouring stream A and sufficient volume for the melt collecting in the intermediate container 2.4. In other words, the intermediate container 2.4 thus also fulfills the function of a CSD chamber. In order to hermetically seal the intermediate container 2.4, its opening on the top is closed by a plate 7 that rests hermetically on the intermediate container 4. According to this embodiment, the pouring nozzle 3.1 and the associated ladle slide 3.2 are arranged on the plate 7. In addition, the vacuum tube 6.1 and the associated valve 6.2, as well as the inlet 5 for the inert gas, are arranged in an upper region 2.1 of the intermediate container 2.4. The advantage of the embodiment according to Fig. 3 is that the molten pool B is provided with a constant free surface C even at high fill levels. Thus, a continuous and level-independent diffusion of gases, at least from the molten pool B, is possible.

[0054] What the second and third embodiments have in common is that the intermediate container 2.4 is provided with a dip tube 4.1 at both lower ends, and each dip tube 4.1 is provided with a distributor slide 4.2. However, a design with only one dip tube 4.1 or even additional dip tubes 4.1 is also conceivable in all of the illustrated embodiments.

[0055] Reference symbol meaning

[0056] 1 device

[0057] 2 containers

[0058] 2.1 upper area

[0059] 2.2 lower area

[0060] 2.3 CSD chamber

[0061] 2.4 Intermediate container

[0062] 3 first entry

[0063] 3.1 Pouring nozzle

[0064] 3.2 Pan slider

[0065] 4 Outlet

[0066] 4.1 Immersion tube

[0067] 4.2 Distribution valve

[0068] 5 second entrance

[0069] 6 Outlet

[0070] 6.1 Vacuum tube

[0071] 6.2 Valve

[0072] 7 plate

[0073] A pouring stream

[0074] B Melt bath

[0075] C Free surface

Claims

Patent claims:

1. Device (1) for degassing melts in a continuous casting plant with a continuous casting distributor, comprising at least one container (2), wherein the container (2) can be divided into an upper region (2.1) and a lower region (2.2), wherein - the upper region (2.1) of the container (2) has a first inlet (3) for a pouring stream (A) of the melt and the lower region (2.2) of the container (2) has an outlet (4) for a melt pool (B) of the melt collecting in the lower region (2.2); and - the upper region (2.1) of the container (2) has a second inlet (5) for an inert gas and an outlet (6) for generating a vacuum; and - wherein the upper region (2.1) and the lower region (2.2) of the container (2) form a common volume.

2. Device (1) according to claim 1, characterized in that the first inlet (3) is arranged on the upwardly directed side of the container (2).

3. Device (1) according to one of the preceding claims, characterized in that the outlet (4) is arranged on the underside of the container (2).

4. Device (1) according to one of the preceding claims, characterized in that a negative pressure compared to the ambient pressure of less than 100 mbar, preferably less than 10 mbar, more preferably less than 1 mbar can be generated in the container (2).

5. Device (1) according to one of the preceding claims, characterized in that the melt pool (B) collected in the lower region (2.2) of the container (2) has a free surface (C) opposite the melt-free space of the upper region (2.1) of the container (2).

6. Device (1) according to one of the preceding claims, characterized in that the fall height of the pouring jet (A) between the first inlet (3) and the lower region (2.2) of the container (2) is between 1 m and 5 m, preferably between 1 m and 3 m, more preferably between 1 m and 2 m.

7. Device (1) according to claim 6, characterized in that the device (1) has a minimum installation height of 1.5 m and a maximum installation height of 10 m.

8. Device (1) according to one of the preceding claims, characterized in that the container (2) is designed as a gas-tight pressure chamber.

9. Device (1) according to one of the preceding claims, characterized in that the inlets (3, 5) and outlets (4, 6) can be closed gas-tight.

10. Device (1) according to one of the preceding claims, characterized in that the first inlet (3) is designed to produce a gas-tight flange connection with a non-stressed pan.

11. Device according to one of the preceding claims, characterized in that a pleated bag surrounds the connection point between the first inlet (3) and a non-stressed pan.

12. Device (1) according to one of the preceding claims, characterized in that the container (2) is formed from a CSD chamber (2.3) and a vacuum distribution trough (2.4) or a CSD chamber (2.3) and an intermediate container (2.4).

13. Device (1) according to one of claims 1 to 12, characterized in that the container (2) is a distribution channel (2.4).

14. A method for operating a device for degassing melts in a continuous casting plant according to one of claims 1 to 13, in which the following steps are carried out: a) introducing the inert gas into the container (2) via the second inlet (5); b) generating a constant vacuum within the container (2) via the outlet (6); c) introducing the melt into the container (2) via the first inlet (3) for degassing the resulting pouring stream (A); d) degassing the molten bath (B) collecting in the container (2); e) once a certain fill level in the container (2) has been reached, opening the outlet (4) to discharge the melt from the container (2), preferably into a non-loaded mold.

15. Method according to claim 14, characterized in that Step b. overlaps with step a.

16. Method according to one of claims 14 or 15, characterized in that the vacuum is kept constant even when the filling level of the melt bath (B) changes.

17. Method according to one of claims 14 or 16, characterized in that the negative pressure generated in the container (2) is a pressure of less than 100 mbar, preferably less than 10 mbar, more preferably less than 1 mbar.

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