Method for vertical continuous casting

Abstract

A method for vertical continuous casting is described, with an apparatus comprising a casting pit (1) which, together with a mold (2), forms an outlet region (3) for a metal strand (4) forming a column of liquid metal. In order to design an apparatus of the type described at the outset in such a way that, irrespective of the construction height of the mold and despite simple structural measures, undesirable segregation effects in the edge areas or on the surface of the cast metal are effectively reduced even below the solidification point, it is proposed, that after filling the mold (2) to a minimum level, the air or gas pressure present in the outlet region (3) and immediately surrounding the metal strand (4) is adjusted via the fresh gas supply unit (8) and the waste gas exhaust (9) to an overpressure with respect to ambient pressure, such that the difference between the metallostatic pressure of the column of liquid metal, and the pressure in the outlet region (3) is reduced for the reduction of segregation effects as well as vapor-form cooling medium arising during the casting process is discharged from the outlet region (3).

Description

Method for vertical continuous castingTechnical FieldThe invention relates to a method for vertical continuous casting, with an apparatus comprising a casting pit which, together with a mold, forms an outlet region for a metal strand forming a column of liquid metal.Background of the InventionDuring vertical continuous casting of metal melts, such as in particular aluminum melts, a fundamental difficulty lies in preventing the formation of material defects on the surface of the solidified metal. For instance, during the casting process in the edge area, i.e., in the region directly below the surface of the solidified metal, there may be a material zone with an increased concentration of alloying elements, whereby this edge area can exhibit significantly different properties compared to the more interior material. This effect, known in casting technology as "exudation", "zone segregation" or "inverse segregation" (English: inverse segregation), is attributable to the metallostatic pressure of the column of liquid metal during the casting process. As a result of the excess metallostatic pressure, undesirable dendritic grain structures or cold laps may subsequently occur in the edge area, as well as surface defects on the solidified metal.US20220062973A1 discloses a "hot-top" process frequently used in connection with vertical continuous casting, wherein needle valves are provided for regulating the overpressure of the gas cushion formed by a casting gas supply above the solidification point of the column of liquid metal. As a result, an increased pressure buildup above the solidification point, which would otherwise exit intermittently through gas escape between the mold wall and the solidified melt, is diverted via these valves, thereby preventing any pressure-related microstructural anomalies. A disadvantage thereof, however, is that the casting gas can flow freely into the outlet region, so that material defects in the edge area or on the surface of the solidified metal can still occur below the solidification point.Summary of the InventionThe invention is therefore based on the problem of designing a method of the type described at the outset in such a way that, irrespective of the construction height of the mold and despite simple structural measures, undesirable segregation effects in the edge areas or on the surface of the cast metal are effectively reduced even below the solidification point.The invention solves the stated problem in that after filling the mold to a minimum level, the air or gas pressure present in the outlet region and immediately surrounding the metal strand is adjusted via the fresh gas supply unit and the waste gas exhaust to an overpressure with respect to ambient pressure, such that the difference between the metallostatic pressure of the column of liquid metal, and the pressure in the outlet region is reduced for the reduction of segregation effects as well as vapor-form cooling medium arising during the casting process is discharged from the outlet region.The invention is based on the finding that the metallostatic pressure acting as a driving force for the formation of undesirable segregation effects can be effectively compensated if a gas pressure loading acts in the outlet region directly on the surface of the solidified metal strand or billet. In this case, the casting pit pressure, i.e., the air or gas pressure present in the outlet region and immediately surrounding the metal strand, is regulated to an overpressure with respect to ambient pressure prevailing outside the casting pit such that the difference between the metallostatic pressure of the column of liquid metal, in particular at the level of the solidification point of the metal strand, and the in particular atmospheric pressure in the outlet region is reduced. For a high surface and edge area quality of the solidified metal strand, it is recommended that this differential pressure be 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. As a decisive prerequisite for enabling reliable gas pressure regulation in the outlet region for reducing segregation effects, it is necessary that the outlet region be sealed in a gas-tight manner. However, the problem arises here that the cooling medium, which may be water-based in one example, initially partially vaporizes while cooling the surface of the metal column, whereby the resulting cooling medium vapor cannot escape from the gas-tightly sealed outlet region. Consequently, not only does the abruptly increasing vapor pressure in the outlet region lead to, but precisely in connection with aluminum melts, the exothermic reaction of the water vapor with the aluminum that is not prevented therein leads to surface defects and, in the worst case, to a bursting risk for the mold or the casting table. Against this background, the waste gas exhaust according to the invention provides the additional effect that the cooling medium transferred into the gas phase, in particular water vapor, is extracted from the outlet region. As a result, the water vapor cannot escape unimpeded into the environment outside the outlet region of the casting pit, thereby also avoiding undesirable moisture ingress into the machine hall. Both the fresh gas supply unit and the waste gas exhaust may comprise corresponding control valves such as, for example, valves or slide valves. For the gas pressure regulation, a corresponding control or regulating unit may be provided. Particularly reliable discharge conditions for the waste gas result if the waste gas exhaust comprises a discharge ventilation unit for forced extraction of the waste gas. Furthermore, the method according to the invention provides the further advantage that the frequency and intensity of so-called "bleed-outs" can be significantly reduced. The term "bleed-out" denotes in foundry technology a pressure-driven breakthrough of the liquid metal through the solidified edge area region, which subsequently may lead to a free outflow of the liquid metal into the casting pit or to a direct contact of the liquid metal with the cooling medium and thus would represent an additional accident risk. As a result of the measures according to the invention, existing vertical continuous casting installations, in particular hot-top installations, can be adapted with comparatively little effort, without costly conversion work or modifications to the casting table being required that would fundamentally change its structural configuration. In the event of a malfunction of the fresh gas supply unit or waste gas exhaust, the apparatus according to the invention allows, subject to a possibly necessary manual interruption of the fresh gas supply unit or waste gas exhaust in this case, that the casting process itself need not be interrupted but can be continued in the manner of a conventional vertical or Direct-Chill (DC) continuous casting process ("DC continuous casting process").In the sense of the invention, ambient air, which is blown in, for example, from the machine hall, is understood to be preferred as fresh gas. In the case of high relative humidity, it may also be provided that the ambient air is dehumidified before being supplied as fresh gas to the outlet region in the casting pit.Favorable structural conditions result if the outlet region is sealed in a gas-tight manner via an enclosure. The enclosure merely needs to have mounts for corresponding mold bodies, with the gas-tight closure of the outlet region resulting in the filled state of the molds. It is preferred that the enclosure be configured such that the region between the casting table and the casting pit is enclosed toward the ambient atmosphere, wherein the casting table itself forms a corresponding enclosing section.Usually, during the casting process due to the high discharge velocity or the several meters high drop height of the cooling medium in the casting pit, as well as by devices for extracting the resulting water vapor, an occasionally health-hazardous noise level may arise at the installation site of the casting system or in the machine hall, which makes wearing hearing protection necessary. Consequently, conversations and thus possibly necessary instructions for process adaptation between the system operators are made more difficult. Against this background, it has been shown that an enclosure with a correspondingly massive construction or acoustically insulating design can significantly reduce the noise level.In order to monitor the casting process better, it is proposed that the enclosure comprise a viewing window, the inner side of which is arranged in the flow path of the fresh gas supply and / or waste gas exhaust. As a result of these measures, the viewing window provides free visibility into the outlet region without fogging up due to the cooling medium vapor generated during the casting process.For safety reasons, it is recommended that overpressure discharge devices, for example overpressure valves, be provided in the outlet region. This ensures that a predetermined maximum overpressure with respect to ambient pressure is not exceeded. The maximum overpressure preferably corresponds to the metallostatic pressure of the column of liquid metal at the nominal fill level of the corresponding mold.In order to enable not only the prerequisites for advantageous material quality in the sensitive edge area of the metal melt above the solidification point, but also to improve the pressure regulation conditions in the outlet region, it is proposed that the mold comprise a casting gas supply preferably arranged in the solidification region of the metal strand for sealing the mold outlet opening with respect to the outlet region. Via the casting gas supply, a gas cushion is generated in the region of the liquid meniscus of the metal melt immediately above the solidification point, which counteracts the metallostatic pressure in this region. Furthermore, the generated gas cushion, flowing out in particular downward into the outlet region, forms a barrier for any fresh gas introduced into the outlet region via the fresh gas supply unit or for cooling medium vapor that arises as a result of the cooling process during casting in the outlet region.In order to achieve improved material quality, which is in particular free of casting defects, after filling the mold to a minimum fill level, the pressure in the outlet region is regulated via the fresh gas supply unit and the waste gas exhaust to an overpressure with respect to ambient pressure such that the difference between the metallostatic pressure of the column of liquid metal and the pressure in the outlet region is reduced. The set overpressure is preferably more than 10 mbar, even more preferred more than 20 mbar, particularly preferred more than 30 mbar. With an excessively high overpressure of more than 60 mbar, conversely, the material and tightness, in particular of any enclosure, are comparatively heavily stressed, which in turn entails an increased risk of accidents.In order to create favorable casting conditions for material quality and in particular to reduce the risk of vapor-induced reactions in the outlet region, it is proposed that at least 50 m³, preferably from 50 to 1000 m³, particularly preferably from 200 to 1000 m³ of waste gas be extracted from the casting pit per 1 tonne of cast metal.In order to reduce the risk of moist waste gas flowing out through the mold openings that become free at the end of a casting process toward the end of a casting process, it is recommended that the overpressure set in the casting pit during the casting process, in particular at the latest when falling below the minimum fill level in the at least one mold, be reduced. Preferably, the casting pit is regulated at the end of a casting process to a slight negative pressure of approx. 0 to 1 mbar with respect to ambient pressure. The mold fill levels can essentially be monitored by means of known measuring systems such as optical, acoustic, capacitive, conductive, or mechanical measuring systems.Brief Description of the DrawingsThe subject matter of the invention is illustrated by way of example in the drawing. It shows:Fig. 1 a schematic sectional view of an apparatus according to the invention with an extended casting cylinder at the beginning of the casting process and an indicated retracted casting cylinder at the end of the casting process,Fig. 2 a sectioned detailed view of a filled mold body on a larger scale,Fig. 3 a photographic detail image of the surface of an aluminum billet of alloy EN-AW6082 cast by means of a conventional DC continuous casting process,Fig. 4 a light-microscopic cross-sectional image of the aluminum billet shown in Fig. 3,Fig. 5 a photographic detail image of the surface of an aluminum billet of alloy EN-AW6082 cast by means of a method according to the invention,Fig. 6 a light-microscopic cross-sectional image of the aluminum billet shown in Fig. 5 andFig. 7 a diagram obtained from EDX spectroscopic analyses of the edge areas shown in Fig. 4 and 6, respectively, showing the respective silicon content as a function of the surface depth for each edge area.Detailed Description of the InventionAn apparatus according to the invention for vertical continuous casting comprises a casting pit 1 which, together with several molds 2, forms an outlet region 3 for the metal strands 4 assigned to the respective molds 2, for example in the form of round billets. Above the casting pit 1, a casting table 5 is arranged, which accommodates the molds 2. As schematically indicated in Fig. 1, a start-up device 6 mounted on a casting cylinder and having corresponding start-up heads for the molds 2 is lowered during the casting process from an initial base position into an end position illustrated with dash-dotted lines under formation of the metal strands 4.The outlet region 3 is sealed in a gas-tight manner via an enclosure 7. In order to generally set the casting pit pressure during the casting process so as to compensate for both the difference between the metallostatic pressure in the region of a possible solidification point of a metal strand and the atmospheric pressure in the outlet region 3, and to discharge vapor-form cooling medium arising during the casting process, the outlet region 3 is provided with a fresh gas supply unit 8 and an waste gas exhaust 9. Both the fresh gas supply unit 8 and the waste gas exhaust 9 comprise corresponding fans 10 and control valves 11, respectively. The latter may be designed as, for example, flap valves. Furthermore, the fresh gas supply unit 8 and the waste gas exhaust 9 can be connected to a control unit not shown in detail.A mold 2 shown in Fig. 2 comprises, for example, a mold body 12 which accommodates a nozzle plate 13. In order to enable primary or contact cooling for the metal melt introduced into the mold 2 via the casting table 5, a runner body 14 forming a runner surface is arranged in the region of the liquid meniscus of the metal melt being formed, which limits the liquid meniscus radially with respect to the longitudinal axis of the metal strand 4. For secondary cooling, cooling medium channels 15 are provided in the mold body 12, which open into corresponding cooling medium outlets and discharge the liquid cooling medium, preferably water 16, to the surface of the metal strand 4 while partially generating cooling medium vapor.As shown in Fig. 2, the metal strand 4 is divided essentially into a liquid region 17 formed by the metal melt and a solid region 18 formed by the solidified metal, wherein the dash-dotted line 19 indicated defines the boundary between these regions. In the contact region between the liquid meniscus and the runner body 14, a solidification point 20 accordingly forms on the surface of the metal strand 4.In order to prevent cooling medium vapor from entering the mold region above the solidification point 20 or even escaping through the mold inlet out of the casting table 5 and coming into contact with the liquid metal, the mold 2 comprises a casting gas supply 21, wherein a corresponding supply channel is provided in the mold body 12. The dry casting gas introduced via the casting gas supply 21 penetrates through the runner body 14, which is porous in this case and manufactured from graphite, and forms a gas cushion 22 above the solidification point 20. Additionally, the mold 2 may also comprise a corresponding lubricant supply 23.Fig. 3 and 5 respectively and Fig. 4 and 6 relate to surface images respectively microscopic cross-sectional images of aluminum billets of the same batch, which were obtained on the one hand by a conventional DC continuous casting process carried out, i.e., without pressure regulation measures in the casting pit (Fig. 3 and 4), and on the other hand by a method according to the invention (Fig. 5 and 6). The aluminum billets each have a diameter of 9 inches and were cast from alloy EN-AW6082. In the case of the conventional process, the differential pressure, i.e., the difference between the metallostatic pressure of the column of liquid metal at the level of the solidification point 20 and the pressure in the outlet region 3, was approx. 35 mbar. In contrast, the differential pressure in the case of the method according to the invention was approx. 1-3 mbar, wherein the counterpressure, i.e., the overpressure set in the outlet region 3, was approx. 33-35 mbar.As can be seen in Fig. 3, the surface of the aluminum billet obtained by a conventional process exhibits undesirable unevenness which is clearly visible to the naked eye. This also correlates with the cross-sectional image shown in Fig. 4, from which an inverse segregation zone is discernible which is distinctly pronounced to a depth of approx. 150 µm. In contrast, the surface of the aluminum billet obtained by a method according to the invention is significantly smoother and more uniform, as can be seen from Fig. 5. On the relevant cross-sectional image in Fig. 6, the inverse segregation zone is discernible to a depth of approx. 80 µm, wherein however the number or density of optically discernible phases is significantly lower compared to those from the cross-sectional image according to Fig. 4.Fig. 7 finally shows a diagram which was obtained in the course of an energy-dispersive X-ray spectroscopy analysis (EDX analysis) of the aforementioned aluminum billets from Fig. 3 to 6, and wherein the mass fraction of silicon relative to the total mass of the respective aluminum billet is plotted against the surface depth. According to profile curve 24, it is shown that the silicon content in the aluminum billet obtained from the conventional process is significantly increased up to a surface depth of approx. 150 µm, whereas according to profile curve 25, no significant inverse segregation in the edge area is discernible in the aluminum billet obtained from the method according to the invention.

Claims

1. Method for vertical continuous casting using an apparatus, which apparatus comprises a casting pit (1) which, together with a mold (2), forms an outlet region (3) for a metal strand (4) forming a column of liquid metal, characterized in that, after filling the mold (2) to a minimum level, the air or gas pressure present in the outlet region (3) and immediately surrounding the metal strand (4) is adjusted via the fresh gas supply unit (8) and the waste gas exhaust (9) to an overpressure with respect to ambient pressure, such that the difference between the metallostatic pressure of the column of liquid metal, and the pressure in the outlet region (3) is reduced for the reduction of segregation effects as well as vapor-form cooling medium arising during the casting process is discharged from the outlet region (3).

2. Method according to claim 1, characterized in that the difference between the metallostatic pressure of the column of liquid metal, in particular at the solidification point (20), and the pressure in the outlet region (3) is less than 20 mbar.

3. Method according to claim 2, characterized in that the difference between the metallostatic pressure of the column of liquid metal, in particular at the solidification point (20), and the pressure in the outlet region (3) is less than 5 mbar.

4. Method according to one of claims 1 to 3, characterized in that the overpressure set in the outlet region (3) is greater than 10 mbar.

5. Method according to claim 4, characterized in that the overpressure set in the outlet region (3) is greater than 30 mbar.

6. Method according to one of claims 1 to 5, characterized in that at least 50 m³ of waste gas are extracted from the casting pit (1) per 1 tonne of cast metal.

7. Method according to one of claims 1 to 6, characterized in that the overpressure in the casting pit (1) is reduced when the minimum level in the at least one mold (2) falls below.

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