WO1999021670A1

WO1999021670A1 - Device for casting of metal

Info

Publication number: WO1999021670A1
Application number: PCT/SE1998/001905
Authority: WO
Inventors: Anders Lehman
Original assignee: Abb Ab
Priority date: 1997-10-24
Filing date: 1998-10-21
Publication date: 1999-05-06
Also published as: SE9703892L; SE9703892D0; SE515793C2

Abstract

A device for continuous or semi-continuous casting of metal comprising a cooled mold (12) and an inductive heater (15) multi-turn coil arranged as an integral part of the mold at the top end of the mold.

Description

Device for casting of metal.

TECHNICAL FIELD

The present invention relates to a device for continuous or semi-continuous casting of metal or metal alloys into an elongated strand. The device comprises a cooled continuous casting mold and an induction heater arranged at the surface of the molten metal in the top end of the mold, at the meniscus. The invention ensures that the temperature at the meniscus and other casting conditions determining the initial solidification conditions in the mold are controlled such that a cast product, a strand, exhibiting an improved surface characteristics, a controlled cast structure, a low level of entrapped inclusions and other defects is produced at maintained or increased productivity.

BACKGROUND ART

During continuous or semi-continuous casting of metals and metal alloys, a hot metal melt is supplied to a cooled mold which is open in both ends in the casting direction. The mold is cooled, preferably water-cooled and preferably surrounded and supported by a structure of support beams. The support beams and the mold comprise internal cavities or channels in which the coolant, water, flows during casting. As the metal passes through the mold it is solidified and a cast strand is formed. When the cast strand leaves the mold, it comprises a solidified, self-supporting surface layer or shell around a remaining residual melt. Melt is supplied to the mold in a number of alternative ways, either by means of a free tapping jet, a casting tube which opens out below the upper surface of the melt present in the mold, a submerged entry nozzle, or through a channel which guides the melt to the mold. For casting of steel the two first mentioned alternatives are predominantly used while the third is essentially used for casting of non-ferrous alloys such as copper and aluminum based alloys. Generally it can be said that the surface conditions and of course the cast structure is highly dependent on the conditions of initial solidification. But also metal cleanliness will depend on the conditions in the top end of the mold, i.e. the locations at which the metal starts to solidify.

The conditions of initial solidification is dependent on a number of factors influencing each other in a complex manner, such as;

- Metal flow in the upper part of the mold;

- Lubrication between the mold and the melt/cast strand;

- Heat losses and overall thermal conditions at the meniscus: - Thermal conditions and heat dissipation at the front of solidification; and

- Oscillation, if any, of the mold.

The metal flow in the non- solidified parts of the cast strand can be divided into- a primary flow in the form of jets of hot melt entering the mold and a secondary flow pattern developed in the mold. As the primary jets enter the mold they will interact with the melt and the partly solidified strand in the mold and/or any magnetic fields applied to brake and split the primary jets and control the secondary flow pattern or applied to stir the melt in the mold. The secondary flow and in particular the amount of upward flow to the meniscus and the velocity of this flow will effect the thermal conditions at the meniscus, the surface quality, the cast structure and also the cleanliness of the metal.

A lubricant, such as a glass or glass forming composition or oil, is continuously supplied to the upper surface of the melt in the mold. The lubricant serves many purposes amongst others will it prevent the skin of the cast strand first developed from sticking to the mold wall. Normal adherence between oscillation show as so called oscillation marks. Should the solidified skin stick or adhere more severely to the mold it will show as severe surface defects and in some cases as ripping of the first solidified skin. For casting of large dimension strands of steel the lubricant is predominantly a so-called mold powder comprising glass-producing compounds that will melt by the heat at the meniscus. The mold powder is often continuously added to the upper surface of the melt in the mold during casting, as an essentially solid free flowing paniculate powder. The composition of a mold powder is customized. Thereby the powder will melt at a desired rate and lubrication will be provided at the desired rate to ensure lubrication. A too thick layer of lubricant between mold and cast strand will also affect the solidification conditions and surface quality in an undesired way, thus the thermal conditions at the meniscus need to be controlled.

Heat losses and overall thermal conditions at the meniscus can be effected by a thermal insulation provided by some mold powders. But the thermal conditions at the meniscus are predominantly controlled by the secondary flow in the mold. It is also known to install heaters, in particular inductive heaters, adjacent to the meniscus. The use of thermal heaters will be described more in detail in the following. If high thermal losses are not compensated by a supply of heat the upper surface can start to solidify, causing severe disturbances in the casting process and destroying the quality of the cast product in most aspects. Thus, it is also for this reason advantageous with a closely and accurately controlled thermal situation at the meniscus. Thermal conditions and heat dissipation at the front of solidification are predominantly controlled by the flow pattern in the mold. The flow pattern can be effected by the rate of melt coming in to the mold by the primary jets, the casting speed, the superheat of the hot melt supplied to the mold, the mold characteristics etc. As the conditions at the solidification front is dependent on the thermal conditions at the upper end of the mold it would also from this point be advantageous with a closely and accurately controlled thermal situation at the meniscus.

Oscillation is primarily applied to ensure that the cast strand leaves the mold. However minor surface defects so-called oscillation marks are normally formed on the cast strand. These oscillation marks also effect the structure of the cast strand as inclusions often are trapped at them. It is known that the compressive forces generated by a high frequency electromagnetic field will act to separate the melt from the mold. The contact between the melt and mold during initial solidification of the skin will thus be minimized and the feed of lubricant will be improved hereby further improving the surface quality of the cast strand. From the foregoing it is clear that the use of an inductive heater at the meniscus offers advantages as it reduces the contact between melt and mold before and at the stage of initial solidification and improves the conditions for a controlled supply of mold lubricant. As an extra benefit the use of an inductive heater at the top end of the mold creates possibilities whereby the importance of the oscillation is reduced.

It is, from the foregoing, clear that it is advantageous to arrange an inductive heater at the top end of a continuous casting mold. It will provide improved conditions to maintain and control the temperature of the metal at the upper surface of the melt, the meniscus, sufficiently high during casting and the same time reduce the risk for sticking, reduce oscillation mark and provide improved conditions for mold lubrication. It is known to arrange such an inductive heater adjacent to the meniscus in order to provide the desired control of the temperature of the melt and provide the compressive forces that acts to separate the melt from the mold. The inductive heater may be of single-phase or poly-phase design. Preferably a high- frequency magnetic alternating field is applied. By means of the compressive forces generated by the high frequency magnetic field, the pressure between the mold wall and the melt is reduced, thus the conditions for lubrication are significantly improved. This surface quality of the cast strand can thereby be improved and the casting speed can be increased without risking the surface quality and the importance of the oscillation is reduced. The inductive heater can if desired be combined with other electromagnetic devices acting on the melt in the cast strand or the mold such as an electromagnetic brake as described in the foregoing but the inductive heater can also be applied independent. US-A-5 375 648 further discusses how to arrange a high- frequency induction heater at the top end of the mold. This heater can of course be used on its own or in combination with other means for controlling the thermal situation with the mold. It has been suggested to arrange induction heaters within the mold just above the meniscus but a coil at this position is subjected to adverse conditions and thus easily damaged. Therefore it is preferred to arrange the induction heater outside the mold at the top end of the mold. However, as the mold for several reasons preferably are made of copper, which has a high electrical conductivity, measures has to be taken to reduce the eddy-current losses in the mold and thereby ensure that the electromagnetic field penetrates the mold and efficiently heat the metal in the mold. It is known that a reduction of the frequency of the magnetic field or a reduction of the thickness of the mold plates will increase penetration, but neither of these methods are attractive. A reduced frequency will induce movements or undesired flows in the melt, such movements will disturb the preferred stable flow situation in the melt at the meniscus. A reduced wall thickness will impair the mechanical properties of the mold risking damage to the mold which eventually can cause metal penetration into the internal channels or cavities in the mold plates in which coolant, such as water, flows. Such contact between coolant and hot melt is likely to be detrimental and the casting must be stopped for repair. Further it has been suggested to replace the copper mold with a mold made from a material exhibiting a lower electrical conductivity, for example a Ni-Cr-Fe or a Ni-Cr-Co alloy.

The most widely accepted way to increase the penetration of a high frequency magnetic field through a mold and into the melt is to use a mold which is slitted in the casting direction at the top end of the mold, i.e. at level with the high frequency induction heater. Such a mold configuration is known as a cold crucible. The slitted mold will reduce the eddy-current losses and increase the heat efficiency as the current paths for the electrical currents induced in the mold by the applied magnetic field is cut. To accomplish the desired increase in heat efficiency a large number of slits is required. Thus a compromise is often done where the number of slits is kept at a reasonable level and compensated with an increase in the size and rated power of the heater. An increase in rated heater power typically show as an increase in the number of turns in the heater coil. Thereby ensuring that a sufficient amount of heat is supplied to the melt, however, at the cost of low heat efficiency.

OBJECT OF THE INVENTION

It is an object of the invention to provide a device for continuous casting of metal, comprising a continuous casting mold and a high frequency inductive heater at the top end of the mold which provides improved conditions for the initial solidification of the cast metal in the mold. Preferably the device exhibits an increased heat efficiency, so that when casting metal the temperature of the metal at the upper surface of the melt, the meniscus, is maintained sufficiently high without risking damage to the support means and other mold parts vulnerable to excess heating comprised in the device. Further the device shall generate compressive forces acting to separate the melt from the mold whereby oscillation marks are essentially eliminated or substantially reduced and the overall conditions for a good and even supply of mold lubrication are improved. The elimination or substantial reduction of oscillation marks further improves the cast structure and removal of inclusions as inclusions and/or defects are trapped at the oscillation marks.

It is another object to provide a device with reduced eddy-current losses in the mold plates such that the heat efficiency is increased. Thereby providing a device comprising an inductive heater with reduced size and power so that the mold plates and any support frames and mold parts comprised in the continuous casting device are not damaged by the excess heating.

It is further an object of the present invention to provide a continuous casting device with a high frequency inductive heater that operates at a sufficiently high frequency. By a sufficiently high frequency movements such as turbulence and other unwanted flow phenomena at the meniscus are avoided and desired compressive forces acting to separate the melt from the mold are generated. Thereby good thermal, flow, lubrication and overall conditions are provided at the top end of the mold.

It is further an object to provide a continuous casting device that ensures;

- Good lubrication conditions, characterized by a stable and even supply of lubricant, between the melt/the mold and the cast strand/the mold respectively;

- Good control of the thermal conditions at the meniscus and during initial solidification, whereby freezing of the meniscus or development of a so-called deckel, i.e. that the metal at the meniscus starts to solidify, is avoided;

- Good conditions for a clean metal, i.e. a flow at the meniscus essentially free from disturbances so that no mold powder or the like is dragged down into metal and entrapped in cast product but gas, oxide or other particulate impurities in the metal are allowed to float to the surfaces where it is taken care of by the mold powder; and thus attaining considerable improvements with respect to quality and productivity.

SUMMARY OF THE INVENTION

To achieve this the present invention suggests a device for continuous or semi- continuous casting of metal according to the preamble of claim 1, which is characterized by the features of the characterizing part of claim 1. The device comprises a cooled mold and an inductive heater multi-turn coil arranged at the top end of the mold. The inductive heater coil is according to the present invention arranged as an integral part of the mold. Preferably the mold is surrounded and supported by a structure of support beams. These beams comprise a system of internal cavities and or channels which supplies the mold with a coolant such as water. The channels in the support beams are connected to channels in the mold through which coolant flows during casting.

According to one embodiment which is predominantly used for casting of larger size strands such as slabs or blooms the mold is comprised of mold plates, preferably four mold plates. The mold plates are arranged to form an essentially rectangular mold and according to this embodiment of the present invention the inductive heater coil, or a part of the inductor heater coil, is arranged as an integral part of a mold plate. As this integrally arranged heater coil in contrast to a heater coil arranged outside the mold the eddy-current losses is essentially eliminated. At least are they substantially reduced as some losses still might occur as a result of eddy-currents induced in the most adjacent mold parts, i.e. the parts of the mold in mechanical contact with the coil. To reduce these minimal losses a few slits might be cut in these parts of the mold, such slits shall be oriented in the casting direction. As the losses are substantially reduced and the heat efficiency is substantially improved the size and power rating of the heater can be reduced. This will most often show as a reduction in turns of the integral heating coil according to the present invention compared to a heating coil according to prior art for the same size of mold. As the power rating is reduced the risk of damaging the mold and any support beam through excessive heating is substantially reduced. The mold will also be more mechanically stable as essentially all slits are eliminated. Also the need to consider the electrical conductivity of the mold plates is eliminated and the mold plates can be chosen based primarily on mechanical and thermal properties. As the losses are reduced electromagnetic fields with a higher frequency can be used whereby the heat efficiency can be further improved.

The inductive heater multi-turn coil comprises preferably a conductor winding where the wound, at least in part, is covered by an electrical insulation. The electrical insulation is needed to separate and electrically insulate each turn in the coil from each other turn and also to separate and electrically insulate the coil conductor from the other adjacent mold parts and from the metallic melt in the mold and thereby substantially reducing or essentially eliminating short-circuits or leakage-currents between turns. Thuds is according to one embodiment the electrical insulation arranged between each turn and outside the two outermost turns of the coil. Preferably an electrical insulation is also arranged between the inductive heater coil and the melt contained in the mold. In some instances the lubricant alone is sufficient to ensure the electrical insulation between coil and melt, provided that the glasslike lubri- cant is electrically insulating and that it is present over the whole contact surface between melt and coil. The electrical insulation must be capable of operating at the high temperatures prevailing at the top end of the mold. Therefore it preferably comprises inorganic insulation materials chosen from the following group of materials, an oxide, a silicate such as a glass or a natural or man-made flake material, a nitride or a combination of these. The electrical insulation shall preferably also be capable of withstanding chemical attacks from or interaction with the metallic melt or the lubricant. An electrical insulation comprising mica have been found suitable so has a coating applied to the conductor by thermal spraying and comprising an oxide, a silicate or glass forming material, a nitride or a mixture of any of these materials. An insulation comprising mica is commercially available in many forms such as tapes, sheets, yarn etc. that can be wound or formed in other ways around the conductor or inserted between the turns. According to one embodiment the heater coil is separated from the melt in the mold by a thin division such as a coating, a foil or the like, applied to the inside of the mold at level with the insulated inductive heater multi-turn coil to reduce the thermal and chemical demands on the electrical insulation chosen so that the electrical properties can be given a more significant role in the determination of a suitable insulant or insulating coating. Such a separation is applied to the inside of the mold at level with the insulated inductive heater multi-turn coil. A coating applied to the insulated inductive heater coil is suitably applied by thermal spraying.

The inductive heater coil preferably comprises a tube conductor with a hollow internal cavity arranged as a flow channel for a coolant. Of course is the present invention also applicable for devices where the inductive heater is cooled indirectly by the mold coolant.

To ensure a stable flow pattern in the non-solidified parts of the cast strand and especially at the meniscus the high-frequency inductive heater is combined with a further electromagnetic field generating device such as a stirrer or an electromagnetic brake. The high- frequency electromagnetic field applied by the heater develops heat and compressive forces acting on the melt at the meniscus, but to ensure the beneficial effects of this it is often advantageous to combine the heater with other electromagnetic devices that ensures a stable and controlled flow in the non-solidified parts of the cast strand. The application of one or more electromagnetic brakes applying one or more static or periodic low-frequency magnetic fields, will ensure that an incoming primary flow of hot melt is braked and split, it will also act to stabilize the flow at the surface of the melt. The magnetic brakes can be arranged to apply magnetic fields at one or more levels arranged one after the other in the casting direction, and with varying configuration of the magnetic fields over the width of the cast strand. A combination of such a magnetic brake with at least one high-frequency magnetic heating field which develops compressive forces acting on this stable surface and/or heat in these parts of the melt provides considerable improvements from the points of view of quality and production engineering relative to the prior art for continuous or semi-continuous casting. Also the combination of an inductive heater according to the present invention and a magnetic stirrer applying an alternating magnetic traveling field that induces a stable flow circulation in the mold is advantageous.

A more controlled temperature distribution in the non-solidified parts of the cast strand and especially at the meniscus is obtained with a continuous casting device according to the present invention. In this way, both the quality, expressed e.g. as the casting structure and/or number of inclusions, and the productivity, expressed as availability, yield, and/or casting speed, are improved because a safe and reliable casting process can be maintained.

As the incoming primary flow of hot melt is supplied to the mold a secondary flow is developed in the non-solidified parts of the strand. Regardless of if or how this secondary flow is controlled the use of a device according to the present invention ensures a stable and controlled thermal situation at the meniscus whereby a frozen meniscus is avoided. At the same time compressive forces are acting on the melt to separate it from the mold, whereby the contact melt/mold within the area of initial solidification is reduced and the conditions for a stable and sufficient supply of any mold lubricant is improved. The temperature control provided by the present invention safeguards against a frozen meniscus or a deckel. A complete or partial solidification of the meniscus entails a severe production-engineering problem that often results in production stoppage. A partial freezing is even when minor reflected in the quality of the cast strand in the form of surface defects. The surface quality is affected in a negative way also by a reduced temperature at the meniscus as this shows as a lower rate of melting of casting powder supplied to the upper surface of the melt and hence inferior protection against the melt/cast strand, during its passage through the mold, adhering to the inner walls of the mold. Adherence to the mold results in surface defects and, in the worst case, in tearing off of the solidified self-supporting shell. A torn shell may when it comes to worst lead to a breakout through the shell such that melt runs out over the cast strand, downstream of the mold, whereby the casting process has to be stopped and a time-consuming work be initiated of cleaning the casting machine from metal flowing out before it may be restarted. The compressive forces generated by the high frequency heater to act on the melt and separate it from the mold provides conditions for a substantial improvement in surface quality as it;

- Reduces the contact melt/mold at the place of the initial forming of the solidified shell and thereby reduces the size and number of oscillation marks;

- Improves the feed of mold lubricant which provides an overall improvement of surface quality. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained in greater detail and be exemplified by means of preferred embodiment with reference to the accompanying figures;

Figure la shows a cut along the casting direction through the top end of a device for continuous casting according to prior art with an inductive heater arranged at the top end of the mold;

Figure lb show a cut transverse to the casting direction for the same mold;

Figure 2 shows a cut along the casting direction through the top end of a device for continuous casting according to one embodiment of the present invention with an inductive heater arranged as a integral part of the mold at the top end of the mold; and

Figure 3 shows a cut along the casting direction of a device according to one further embodiment of the present invention where the inductive heater have been complemented with an electromagnetic brake.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The devices for continuous casting of metal shown in the Figures la, lb, 2 and 3 all comprises a continuous casting mold 12. A continuous casting mold is open in both ends in the casting direction and is arranged with cooling means, preferably the mold comprises a system of internal cavities or channels wherein a coolant flows during operation, and means for ensuring that the formed cast strand continuously leaves the mold. The cooled mold is continuously supplied with a primary flow of hot melt, the hot metal is cooled and a cast strand 11 is formed in the mold 12. The mold 12 is usually a water-cooled copper mold. The mold is preferably surrounded and support by a structure of support beams 30 as shown in figure 3. The mold 12 and any support beam 30 comprises internal cavities or channels, not shown, in which the water, flows during casting. During casting a primary flow of hot melt is supplied to the mold 12. Melt is supplied to the mold 12 either by means of a free tapping jet, a casting tube which opens out below the upper surface 16 of the melt present in the mold, a submerged entry nozzle 31 as shown in figure 3, or through a channel or system of feeder channels or runners which guides the melt to the mold 12. For casting of steel the two first mentioned alternatives are predominantly used while the third is essentially used for casting of non-ferrous alloys such as copper and aluminum based alloys e.g. in the form of extrusion billets. As the metal passes through the mold 12 it is cooled and solidified whereby a cast strand 10 is formed. When the cast strand 10 leaves the mold 12, it comprises a solidified, self-supporting surface shell around a remaining residual melt. Generally it can be said that the surface conditions and of course the cast structure is highly dependent on the conditions of initial solidification. But also metal cleanliness will depend on the conditions in the top end of the mold, i.e. the locations at which the metal starts to solidify. To control the thermal situation at the top end of the mold 12 is in all figures an inductive high- frequency heater 14, 15 arranged at this top end at level with the top surface of the melt in the mold, the meniscus 16.

The heater 14 according to prior art as shown in figure la and lb is arranged outside the mold 12 and the high frequency magnetic field generated by the heater 14 has to penetrate the mold 14 and into the melt. To reduce the eddy-current losses in the mold 14 and improve the heat efficiency the mold 12 is slitted in the casting direction at level with the heater 14. The slits a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p are best shown in figure lb. In contrast to this the heater 15 according to the present invention is included as an integral part in the mold as shown in figures 2 and 3. Hereby is the eddy-current losses essentially eliminated or at least substantially reduced as some losses still might occur as a result of eddy-currents induced in the most adjacent mold parts, i.e. the parts of the mold in mechanical contact with the coil. To reduce these minimal losses a few slits might be cut in these parts of the mold, such slits shall be oriented in the casting direction. As the losses are substantially reduced and the heat efficiency is substantially improved the size and power rating of the heater 15 can be reduced. This will most often show as a reduction in turns of the heating coil 15. As the power rating is reduced the risk of damaging mold 12 and any support beam 30 through excessive heating is substantially reduced. The mold 12 will also be more mechanically stable as essentially all slits are eliminated or as in embodiments where measures have been taken to further reduce eddy current losses in the mold the number of slits are substantially reduced. Also the need to consider the electrical conductivity of the mold 12 is eliminated and the mold 12 can be chosen based primarily on mechanical and thermal properties. As the losses are reduced electromagnetic fields with a higher frequency can be used whereby the heat efficiency is further improved. The inductive heater 15 may be a single-phase or a poly-phase heater. When the high frequency magnetic alternating field is applied to act on the melt, heat is developed in the melt so that the temperature of the melt adjacent to the meniscus 16 can be controlled. At the same time compressive forces acting on the melt are developed by the high frequency alternating field. The compressive forces reduce the pressure between the mold 12 and the melt and thus improve the condition for lubrication significantly. Improvements obtained in relation to the prior art devices for continuous or semi-continuous casting comprising externally and separately arranged inductive heaters are obtained using the embodiment of the invention shown in Figure 2 relates to many quality and productivity aspects such as;

- Heat efficiency;

- More mechanically stable mold; - Cleanliness; Surface quality; Controlled cast structure;

- Reduced down-time; and

- Provisions to increase casting speed and /or reduce oscillation.

To further improve the quality of the cast product and/or productivity the device according to some embodiments of the present invention is complemented with other electromagnetic devices such as an electromagnetic mold stirrer or an electromagnetic brake. One such embodiment is included as example and shown in figure 3. Of course can the present invention be used with or without such a further electromagnetic device and in any combination with one or more devices of that kind or other devices or measures adopted to increase the control of the flow and/or the thermal situation in the cast strand 10 during solidification and especially during the initial solidification at the top end of the mold. An electromagnetic brake is arranged to apply static or periodic low-frequency magnetic fields to act on the non-solidified portions of the cast strand 10 and hence brake and split up the melt flowing into the mold 12 and prevent the primary flow of hot melt, which usually contains non-metallic particles, from penetrating deep down into the cast strand 10 and to control the flow in the non-solidified portions of the strand 10. The brake comprises magnetic poles which may be permanent magnets or, as shown in figure 3, induction coils. The magnetic fields according to the embodiment shown in figure 3 are applied at one level downstream of the nozzle ports. The present invention is of course applicable for electromagnetic devices disposed in other ways. Including devices comprising electromagnetic brakes applied to act in two or more levels, the levels being arranged one after the other in the casting direction. The coils are supplied with direct current or a periodic low-frequency alternating current. Each pole comprises a core 24 associated to a winding 25. Preferably each winding 25 is arranged around one core 24. A magnetic return path 26 is arranged to form a one or more closed magnetic circuits. Each closed magnetic circuit comprising at least one or more cores 24, one or more return paths 26 and the magnetic field applied over the mold 12. A first static or periodic low-frequency magnetic field is applied at level with or downstream of the nozzle port through which the primary flow of hot melt is supplied, to brake and split this incoming primary flow of hot melt, thus reducing the risk of slag being drawn into the melt while at the same time creating good conditions for separation of non-metallic particles. As a consequence, a secondary flow is developed showing a more or less controlled circulation or flow pattern in the non-solidified portions of the cast strand 10. A second static or periodic low- frequency magnetic field is applied upstream of the first field at a level between the nozzle ports and the meniscus to control and stabilize the secondary flow and among other things to ensure a sufficient heat supply to the meniscus 16. To prevent the meniscus 16 from solidifying, it is required that a sufficiently amount of hot melt flows upwards to and along the meniscus 16 This flow must however be limited so as not to become so strong that casting powder and other non- metallic particles run the risk of being drawn down into and entrapped in the melt. To ensure that a sufficient amount of heat is supplied to the melt adjacent to the meniscus 16 without risking that unwanted non-metallic particles are drawn down into the melt, according to the present invention the static magnetic fields are supplemented with an inductive heater 15 arranged as an integral part of the top end of the mold 12 at level with the meniscus 16. The inductive heater 15 may be a single-phase or a polyphase heater. When the high-frequency magnetic alternating field is applied to act on the melt, heat is developed in the melt so that the temperature of the melt adjacent to the meniscus 16 can be controlled. At the same time compressive forces acting on the melt are developed. The compressive forces reduce the pressure between the wall of the mold 12 and the melt and thus improve the condition for lubrication significantly. The combination of the electromagnetic brake 20 and the inductive heater 15 integrally arranged within the mold ensures the beneficial results of the heater 15 as it will act on a melt which exhibit a stable and controlled flow pattern in the non-solidified parts of the cast strand and especially at the meniscus 16. The combination of the brake 20 and the heater 15 that according to the present invention is arranged as an integral part of the top end of the mold 12 provides an improved surface quality of the cast strand 10, an improved control of the initial solidification and the cast structure and a possibility to increase the casting speed without jeopardizing the quality of the cast product.

Claims

1. A device for continuous or semi-continuous casting of metal comprising a cooled mold and an inductive heater multi-turn coil arranged at the top end of the mold, characterized in that the inductive heater coil is arranged as an integral part of the mold.

2. A continuous casting device according to claim 1, characterized in that the mold comprises mold plates arranged to form an essentially rectangular mold and that the inductive heater coil or a part of the inductor heater coil is arranged as an integral part of a mold plate.

3. A continuous casting device according to claim 1 or 2, characterized in that the inductive heater multi-turn coil comprises a wound conductor that at least in part is covered by an electrical insulation.

4. A continuous casting device according to claim 3, characterized in that an electrically insulation is arranged between each turn to separate and electrically insulate them from each other and outside the two outermost turns of the coil to separate and electrically insulate the coil from the adjacent mold parts.

5. A continuous casting device according to claim 3 or 4, characterized in that the inductive heater coil is electrically insulated from the melt contained in the mold.

6. A continuous casting device according any of claims 3 to 5, characterized in that the electrical insulation is capable of operating at high temperatures and comprises materials chosen from the following group of materials, an oxide, a silicate such as a glass or a natural or man-made flake material, a nitride or a combination of these.

7. A continuous casting device according to claim 6, characterized in that the electrical insulation comprises a natural silicate material, such as mica.

8. A continuous casting device according to claim 6, characterized in that the electrical insulation comprises a coating applied to the conductor by thermal spraying.

9. A continuous casting device according to any of the preceding claims, characterized in that the inductive heater coil comprises a tube conductor with a hollow internal cavity arranged as a flow channel for a coolant.

10. A continuous casting device according to any of the preceding claims, characterized in that the heater coil is separated from the melt in the mold by a thin division such as a coating, a foil or the like, applied to the inside of the mold at level with the insulated inductive heater multi-turn coil.

11. A continuous casting device according to claim 10, characterized in that a coating is applied to the inside of the mold at level with the insulated inductive heater multi- turn coil by thermal spraying.

12. A continuous casting device according to any of the preceding claims, characterized in that a further electromagnetic device is arranged to apply at least one electromagnetic field to control the flow pattern in the non-solidified parts of the strand.

13. A continuous casting device according to claim 12, characterized in that an electromagnetic brake is arranged to apply at least one static or periodic low frequency electromagnetic field to act in the way of an incoming primary flow of hot melt supplied to the mold such that this primary flow is braked and split and to control the secondary flow pattern developed in the non-solidified parts of the strand.

14. A continuous casting device according to claim 12 or 13, characterized in that two or more electromagnetic brakes are arranged to apply static or periodic low frequency electromagnetic fields to act in one or more levels arranged after each other in the casting direction.

15. A continuous casting device according to claim 12, 13 or 14, characterized in that an electromagnetic stirrer is arranged to apply at least one alternating electromagnetic traveling field to act upon the melt in mold and induce a circulating flow pattern in the non- solidified parts of the cast strand.