Device for vertical continuous casting

Abstract

The invention relates to a device for vertical continuous casting, having a casting pit (1) which, together with a die (2), forms an outlet region (3) for a metal strand (4) forming a liquid metal column. In order to design such a device in such a way that, despite simple design measures, undesired segregation effects in the edge regions or on the surface of the cast metal are effectively reduced even below the solidification point (20), it is proposed that the outlet region (3) is closed off in a gas-tight manner and in order to reduce the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point (20) of the metal strand (4), and the pressure in the outlet region (3) is provided with a fresh gas supply (8) and an exhaust gas discharge (9) for setting a casting pit pressure and for discharging a coolant.

Description

[0001] Device for vertical strand casting

[0002] Technical area

[0003] The invention relates to a device for vertical continuous casting, comprising a casting pit which, together with a mold, forms an outlet area for a metal strand forming a liquid metal column.

[0004] State of the art

[0005] During vertical continuous casting of molten metals, particularly aluminum melts, a fundamental difficulty is preventing the formation of material defects on the surface of the solidified metal. During the casting process, a material zone with an increased concentration of alloying elements can develop in the edge region, i.e. in the area directly beneath the surface of the solidified metal. This is why this edge region can have significantly different properties than the material further inside. This effect, known in casting technology as "exudation", "edge zone segregation" or "inverse segregation", is attributable to a metallostatic overpressure in the liquid metal column during the casting process. This metallostatic overpressure can subsequently lead to undesirable dendrite-shaped grain structures or cold runs in the edge region, as well as to surface defects in the solidified metal.

[0006] US20220062973A1 describes a "hot-top" process frequently used in vertical continuous casting. Needle valves are used to regulate the pressure of the gas cushion formed by the pouring gas supply above the solidification point of the liquid metal column. This allows increased pressure buildup above the solidification point, which would otherwise escape intermittently due to gas escaping between the mold wall and the solidified melt, to be diverted via these valves, thus preventing any impact-induced microstructural anomalies. However, the disadvantage is that the pouring gas can flow freely into the outlet area, so that material defects can continue to occur in the edge region or on the surface of the solidified metal below the solidification point.

[0007] Description of the invention

[0008] The invention is therefore based on the object of designing a device of the type described above in such a way that, regardless of the overall height of the mold, undesirable segregation effects in the edge areas or on the surface of the cast metal are effectively reduced even below the solidification point, despite simple design measures.

[0009] The invention achieves the stated object in that the outlet region is sealed in a gas-tight manner and, in order to reduce the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point of the metal strand, and the in particular atmospheric pressure in the outlet region, is provided with a fresh gas supply and an exhaust gas discharge for setting a casting pit pressure and for discharging a coolant, in particular converted into the gaseous phase.

[0010] The invention is based on the finding that the metallostatic pressure which acts as a driving force for the formation of undesirable segregation effects can be effectively compensated for by applying gas pressure directly to the surface of the solidified metal strand or ingot in the outlet region. The pouring pit pressure, i.e. the air or gas pressure present in the outlet region and immediately surrounding the metal strand, is regulated to an excess pressure compared to the ambient pressure outside the pouring pit in such a way that the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point of the metal strand, and the atmospheric pressure in the outlet region in particular is reduced. To achieve high surface and edge zone quality of the solidified metal strand, it is recommended that this differential pressure is less than 20 mbar, in particular less than 10 mbar.Particularly good results can be achieved with a differential pressure of less than 5 mbar. A crucial prerequisite for reliable gas pressure control in the outlet area to reduce segregation effects is that the outlet area is sealed gas-tight. However, this creates the problem that the water-based coolant, for example, initially partially evaporates as the metal column surface cools, and the resulting coolant vapor cannot escape from the gas-tight outlet area. Consequently, not only the suddenly increasing vapor pressure in the outlet area, but also, particularly in connection with aluminum melts, the unstoppable exothermic reaction of the water vapor with the aluminum leads to surface defects and, in the worst case, to a risk of bursting of the mold or casting table.Against this background, the exhaust gas discharge according to the invention offers the additional effect that the coolant, in particular water vapor, which has been converted into the gas phase, can be discharged from the outlet area. As a result, the water vapor cannot escape unhindered into the environment outside the outlet area of ​​the casting pit, thus also preventing unwanted moisture from entering the machine hall. Both the fresh gas supply and the exhaust gas discharge can have corresponding control fittings such as valves or damper slides. A corresponding control or regulating unit can be provided for gas pressure regulation. Particularly reliable discharge conditions for the exhaust gas are achieved if the exhaust gas discharge has a discharge ventilation unit for the forced discharge of the exhaust gas. In addition, the method according to the invention offers the further advantage that the frequency and intensity of so-called "bleed-outs" can be significantly reduced.In the foundry industry, the term “bleed-out” refers to a pressure-driven breakthrough of the liquid metal through the solidified edge zone, which can subsequently lead to the liquid metal flowing freely into the casting pit or to direct contact of the liquid metal with the cooling medium, thus representing an additional risk of accident. As a result of the measures according to the invention, existing vertical continuous casting plants, in particular hot-top plants, can be adapted with comparatively little effort and without the need for complex conversion work or modifications to the casting table, which would fundamentally change its structural design. In the event of a malfunction of the fresh gas supply or exhaust gas discharge, the device according to the invention enables the fresh gas supply or exhaust gas discharge to be interrupted manually, if necessary in this case.Exhaust gas removal means that the casting process itself does not have to be interrupted, but can be continued in the sense of a conventional vertical or direct chill continuous casting process (“DC continuous casting process”).

[0011] For the purposes of the invention, fresh gas is preferably understood to be ambient air, which is drawn in, for example, from the machine hall. In the case of high relative humidity, it can also be provided that the ambient air is dehumidified before being fed as fresh gas to the outlet area in the casting pit.

[0012] Favorable design conditions arise when the outlet area is sealed gas-tight by a housing. The housing only needs to have receptacles for the corresponding mold bodies, whereby the gas-tight closure of the outlet area is achieved when the molds are filled. The housing is preferably designed so that the area between the casting table and the casting pit is enclosed from the ambient atmosphere, with the casting table itself forming a corresponding housing section.

[0013] During the casting process, the high exit velocity and the drop height of several meters of the coolant in the casting pit, as well as the devices for extracting the resulting steam, typically result in a noise level that is sometimes hazardous to health at the casting plant's installation site or in the machine shop, necessitating the wearing of hearing protection. This makes conversations and, consequently, any necessary instructions for process adjustments between plant operators difficult. Against this backdrop, it has been shown that an enclosure with a sufficiently solid construction or soundproofing design can significantly reduce the noise level.

[0014] To better monitor the casting process, it is proposed that the enclosure incorporate a viewing window whose interior is positioned in the flow path of the fresh gas and / or exhaust system. This provides a clear view of the exit area without fogging due to the coolant vapor generated during the casting process.

[0015] For safety reasons, it is recommended that pressure relief devices, such as pressure relief valves, be provided in the outlet area. This ensures that a specified maximum overpressure relative to the ambient pressure is not exceeded. The maximum overpressure preferably corresponds to the metallostatic pressure of the liquid metal column at the target fill level of the corresponding mold.

[0016] In order to not only create the conditions for advantageous material quality in the sensitive edge region of the molten metal above the solidification point, but also to improve the pressure control conditions in the outlet region, it is proposed that the mold have a casting gas supply, preferably arranged in the solidification region of the metal strand, to seal the mold outlet opening from the outlet region. The casting gas supply creates a gas cushion in the region of the liquid meniscus of the molten metal immediately above the solidification point, which counteracts the metallostatic pressure in this region. In addition, the gas cushion created, which flows out in particular downwards into the outlet region, forms a barrier for any fresh gas introduced into the outlet region through the fresh gas supply or for coolant vapor that arises in the outlet region as a result of the cooling process during casting.

[0017] The invention also relates to a method for vertical continuous casting using a device according to the invention. In order to achieve improved material quality, which is in particular free from casting defects, after the mold has been filled to a minimum fill level, the pressure in the outlet region is regulated via the fresh gas supply and the exhaust gas discharge to an overpressure relative to the ambient pressure such that the difference between the metallostatic pressure of the liquid metal column and the pressure in the outlet region is reduced. The set overpressure is preferably more than 10 mbar, more preferably more than 20 mbar, and particularly preferably more than 30 mbar. Conversely, if the overpressure is too high, exceeding 60 mbar, the material and the tightness, in particular of any housing, are subjected to comparatively severe stress, which in turn entails an increased risk of accidents.

[0018] In order to create casting conditions that are favourable to the material quality and, in particular, to reduce the risk of steam-related reactions in the exit area, it is suggested that at least 50 m 3 , preferably from 50 to 1000 m 3 , particularly preferably from 200 to 1000 m 3 Exhaust gases are removed from the casting pit.

[0019] To reduce the risk of moist exhaust gas escaping through the mold openings at the end of a casting process, it is recommended that the overpressure established in the casting pit during the casting process be reduced, especially at the latest when the minimum fill level in at least one mold is reached. Preferably, the casting pit is regulated to a slight negative pressure of approximately 0 to 1 mbar compared to ambient pressure at the end of a casting process. Mold fill levels can generally be monitored using known measuring systems such as optical, acoustic, capacitive, conductive, or mechanical.

[0020] Brief description of the invention

[0021] The drawing shows an example of the subject matter of the invention.

[0022] Fig. 1 is a schematic sectional view of a device according to the invention with the casting cylinder extended at the beginning of the casting process and the casting cylinder retracted at the end of the casting process,

[0023] Fig. 2 a sectional detailed view of a filled mold body on a larger scale,

[0024] Fig. 3 is a photographic detail of the surface of an aluminium ingot of alloy EN-AW6082 cast using a conventional DC continuous casting process,

[0025] Fig. 4 is a light microscopic micrograph of the aluminum ingot shown in Fig. 3,

[0026] Fig. 5 is a photographic detail of the surface of an aluminium ingot of the alloy EN-AW6082 cast using a method according to the invention,

[0027] Fig. 6 is a light microscopic micrograph of the aluminum ingot shown in Fig. 5 and

[0028] Fig. 7 is a diagram obtained from EDX spectroscopic analyses of the edge zones shown in Fig. 4 and 6, respectively, showing the respective silicon content for each edge zone as a function of the surface depth.

[0029] Ways to implement the invention

[0030] A device according to the invention for vertical continuous casting comprises a casting pit 1, which, together with a plurality of molds 2, forms an exit area 3 for the metal strands 4, for example in the form of round bars, assigned to the respective mold 2. A casting table 5, which receives the molds 2, is arranged above the casting pit 1. As schematically indicated in Figure 1, a starting device 6, mounted on a casting cylinder and having corresponding starting heads for the molds 2, is lowered during the casting process from an initial base position into a final position shown in dash-dotted lines, forming the metal strands 4.

[0031] The outlet area 3 is sealed gas-tight by a housing 7. In order to adjust the casting pit pressure during the casting process in such a way as to compensate for the difference between the metallostatic pressure in the area of ​​any solidification point of a metal strand and the atmospheric pressure in the outlet area 3, as well as to remove any vaporous coolant generated during the casting process, the outlet area 3 is provided with a fresh gas supply 8 and an exhaust gas outlet 9. Both the fresh gas supply 8 and the exhaust gas outlet 9 each comprise corresponding fans 10 and control valves 11. The latter can be designed as flap valves, for example. Furthermore, the fresh gas supply 8 and the exhaust gas outlet 9 can be connected to a control unit (not shown in detail).

[0032] A mold 2 shown in Fig. 2, for example, has a mold body 12 that accommodates a nozzle plate 13. To enable primary or contact cooling for the molten metal introduced into the mold 2 via the casting table 5, a tread body 14 forming a running surface is arranged in the region of the developing liquid meniscus of the molten metal, which tread body radially delimits the liquid meniscus with respect to the longitudinal axis of the metal strand 4. For secondary cooling, coolant channels 15 are provided in the mold body 12, which open into corresponding coolant outlets and release the liquid coolant, preferably water 16, onto the surface of the metal strand 4, partially generating coolant vapor.

[0033] As shown in Fig. 2, the metal strand 4 is essentially divided into a liquid region 17 formed by the molten metal and a solid region 18 formed by the solidified metal, with the dot-dash line 19 defining the boundary between these regions. Accordingly, a solidification point 20 forms on the surface of the metal strand 4 in the contact area between the liquid meniscus and the tread body 14.

[0034] To prevent coolant vapor from entering the mold area above the solidification point 20 or even escaping from the casting table 5 through the mold inlet and coming into contact with the liquid metal, the mold 2 has a casting gas supply 21, with a corresponding supply channel provided in the mold body 12. The dry casting gas introduced via the casting gas supply 21 penetrates the running surface body 14, which in this case is porous and made of graphite, and forms a gas cushion 22 above the solidification point 20. Additionally, the mold 2 can also include a corresponding lubricant supply 23.

[0035] 3 and 5 and Fig. 4 and 6 refer to surface photographs and microscopic micrographs of aluminum ingots from the same batch, which were obtained on the one hand by a conventional DC continuous casting process, i.e. carried out without measures to regulate the pressure in the casting pit (Figs. 3 and 4), and on the other hand by a process according to the invention (Figs. 5 and 6). The aluminum ingots each have a diameter of 9 inches and were cast from the alloy EN-AW6082. In the case of the conventional process, the differential pressure, i.e. the difference between the metallostatic pressure of the liquid metal column at the level of the solidification point 20 and the pressure in the outlet region 3, was approximately 35 mbar. In contrast, the differential pressure in the case of the inventive process was approximately 1-3 mbar, with the counterpressure, i.e. the overpressure set in the outlet region 3, being approximately 33-35 mbar.

[0036] As can be seen in Fig. 3, the surface of the aluminum ingot obtained by a conventional process has undesirable unevenness that is clearly visible to the naked eye. This also correlates with the micrograph shown in Fig. 4, which shows an inverse segregation zone that is clearly visible up to a depth of approximately 150 pm. In contrast, the surface of the aluminum ingot obtained by a process according to the invention is significantly smoother and more uniform, as can be seen in Fig. 5. In the corresponding micrograph in Fig. 6, the inverse segregation zone is visible up to a depth of approximately 80 pm, although the number and density of the optically recognizable phases is significantly lower than that in the micrograph according to Fig. 4.

[0037] Finally, Fig. 7 shows a diagram obtained during an energy-dispersive X-ray spectroscopy (EDX) analysis of the aforementioned aluminum ingots from Figs. 3 to 6, in which the mass fraction of silicon in the total mass of the respective aluminum ingot is plotted against the surface depth. According to curve 24, it can be seen that the silicon content in the aluminum ingot obtained from the conventional process is significantly increased up to a surface depth of approximately 150 pm, whereas according to curve 25, no significant inverse segregation is discernible in the edge zone in the aluminum ingot obtained from the inventive process.

Claims

Patent claims 1. Device for vertical continuous casting, with a casting pit (1) which, together with a mold (2), forms an outlet area (3) for a metal strand (4) forming a liquid metal column, characterized in that the outlet area (3) is sealed in a gas-tight manner and, in order to reduce the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point (20) of the metal strand (4), and the pressure in the outlet area (3), is provided with a fresh gas supply (8) and an exhaust gas discharge (9) for setting a casting pit pressure and for discharging a coolant.

2. Device according to claim 1, characterized in that the outlet area (3) is sealed gas-tight by a housing (7).

3. Device according to claim 2, characterized in that the housing (7) has a viewing window, the inside of which is arranged in the flow path of the fresh gas and / or exhaust gas device.

4. Device according to one of claims 1 to 3, characterized in that the mold (2) has a casting gas supply (21) for sealing the mold outlet opening from the outlet region (3).

5. Method for vertical continuous casting with a device according to one of claims 1 to 4, characterized in that after filling the mold (2) to a minimum filling level, the pressure in the outlet region (3) is regulated via the fresh gas supply (8) and the exhaust gas discharge (9) to an overpressure compared to the ambient pressure in such a way that the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point (20), and the pressure in the outlet region (3) is reduced.

6. Method according to claim 5, characterized in that the pressure in the outlet region (3) is regulated via the fresh gas supply (8) and the exhaust gas discharge (9) to an overpressure relative to the ambient pressure such that the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point (20), and the pressure in the outlet region (3) is less than 20 mbar.

7. Method according to claim 6, characterized in that the difference between the metallostatic pressure of the liquid metal column, in particular at the level of the solidification point (20), and the pressure in the outlet region (3) is less than 5 mbar.

8. Method according to one of claims 5 to 7, characterized in that the overpressure set in the outlet region (3) is more than 10 mbar.

9. Method according to claim 8, characterized in that the overpressure set in the outlet region (3) is more than 30 mbar.

10. Method according to one of claims 5 to 9, characterized in that per 1 ton of cast metal at least 50 m 3 Exhaust gas is discharged from the casting pit (1 ).

11. Method according to one of claims 5 to 10, characterized in that the overpressure in the casting pit (1) is reduced when the minimum filling level in the at least one mold (2) is undershot.

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