TECHNICAL FIELD

The present invention relates to a method for continuous or semicontinuous casting of metal or metal alloys in a cooled mould. The invention ensures that the temperature of the metal at the upper surface of the melt, the meniscus, is maintained sufficiently high while the metal is being cast in a mould in which a primary flow of hot metal melt is supplied to a cooled mould, which is open in both ends in the casting direction and this primary flow of melt is braked and split up by means of at least one static or periodic low-frequency magnetic field. The invention also relates to a device for carrying out the invented method.

BACKGROUND ART

During continuous or semicontinuous processes for casting of metals and metal alloys, a hot metal melt is supplied to a cooled mould which is open in both ends in the casting direc¬ tion. The mold is preferably water-cooled. During the passage through the mould, a cast strand is formed while the melt is being cooled by the water-cooled mould. When the cast strand leaves the mould, it comprises a solidified, self-supporting surface layer around a remaining residual melt . Melt can be supplied to the mould in a number of ways, by means of one or more tapping jets, by means of a casting tube which opens out below the upper surface of the melt present in the mould, or through channels which open out into the mould. Regardless of how the melt is supplied to the mould, the intention is to brake the inflow of hot melt, partly to avoid an uncontrolled inflow of hot melt containing slag particles or other non¬ metallic particles, partly to control secondary flows of hot melt arising in the non-solidified portions of the cast strand and hence also the heat distribution in the non-solidified portions of the cast strand as well as the solidification process. An uncontrolled inflow of melt in the non-solidified

portions of the cast strand results in problems both from the point of view of quality and production engineering. If in¬ flowing melt is allowed to flow into the mould in an uncon¬ trolled manner, its impulse will cause it to penetrate deep down into the non-solidified portions of the strand. This makes difficult the separation of particles contained in the melt. The particles adhere to the solidification front instead of being separated to the upper surface. In addition, the self-supporting surface layer is weakened which increases the risk of melt breaking through the surface layer formed in the mould. From, for example, Swedish patent application SE 436 251, it is known to arrange one or more static or periodic low-frequency magnetic fields in the path of the melt to brake and split up the inflowing melt. As already described, quality and production problems arise if hot melt is allowed to pene¬ trate deep down into the non-solidified parts of a cast strand in an unbraked manner. An uncontrolled secondary flow gives rise to problems both from the point of view of quality and production engineering in that the solidification process is not controlled. If the upwardly flowing secondary flows of hot melt towards the upper surface, the meniscus, become too weak, there is a risk that the meniscus freezes. A frozen meniscus entails production-engineering problems because the meniscus completely or partially solidifies. Such freezing may be reflected in the quality of the cast strand in the form of surface defects. The surface quality is affected in a negative way by too low a temperature of the melt near the meniscus when a reduced temperature at the meniscus gives 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 mould, adhering to the inner walls of the mould. Adherence to the mould results in surface defects and, in the worst case, in tearing off of the solidified self-supporting layer of the cast strand. Such tearing off may lead to the melt breaking through the shell of the strand and running out over the cast strand, downstream of the mould, 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. If the upward flows become too strong, formation of waves on the upper surface arises as a result of the turbulence, which pulls down slag from the upper surface into the melt with ensuing quality problems, among other things in the form of unwanted non-metallic particles trapped in the steel.

It is an object of the invention, when casting metal in a mould which is open in both ends in the casting direction, where a primary flow of hot metal melt is supplied to the mould, is braked and split up by means of at least one static or periodic low-frequency magnetic, to suggest a method by which it is ensured that the temperature of the metal at the upper surface of the melt, the meniscus, is maintained sufficiently high while the metal is being cast without risking turbulence and other unwanted flow phenomena at the meniscus and/or to ensure good conditions for lubrication between the melt/the mould and the cast strand/the mould, respectively, thus attaining considerable improvements with respect to quality and productivity.

It is another object of the invention to suggest a device for carrying out the invented method.

SUMMARY OF THE INVENTION

During casting of metal in a mould which is open in both ends in the casting direction and which is supplied with at least one primary flow of hot metal melt, wherein the incoming pri- mary flow of metal melt is braked and split up by means of a static or periodic low-frequency magnetic field acting across the casting direction, the desired control of the temperature of the melt adjacent the meniscus and the casting conditions in other respect are achieved by the provision, according to the invention, of at least one inductive heater adapted to apply a magnetic alternating field to act on the melt adjacent to the upper surface of the melt, the meniscus. The inductive heater may be of single-phase or polyphase design. The mag-

netic alternating field, which preferably is a high-frequency magnetic alternating field, acts on the melt and develops heat in the melt whereby the temperature of the melt ^" adjacent the meniscus may be controlled simultaneously, or alternatively the high-frequency magnetic alternating field develops com¬ pressive forces which act on the melt. By means of compressive forces, the pressure between the mould wall and the melt is reduced, thus significantly improving the conditions for lubrication. This gives improved surface quality of the cast strand and a possibility of increasing the casting speed without risking the surface quality. This is achieved prima¬ rily by combining these compressive forces, according to the invention, with the braking static or period low-frequency magnetic field. If only the high-frequency magnetic field is applied to develop compressive forces acting on the melt, there is a risk, at least when casting heavy metals or metal alloys such as steel, that the surface of the melt towards the mould does not become stable. The combination of at least one braking static or periodic low-frequency magnetic field, which in addition to the braking of inflowing melt also stabilizes the surface of the melt, with at least one high-frequency magnetic 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 semicontinuous casting.

A more controlled temperature distribution is obtained by the provision, according to one embodiment of the invention, of at least one further static or periodic low-frequency magnetic field to act on the non-solidified portions of the cast strand in the mould to control the secondary flows arising. These static or periodic low-frequency magnetic fields are adapted to act in the path of the secondary flows arising in such a way that a controlled temperature distribution is obtained in the non-solidified portions of the cast strand. In this way, both the quality, expressed for example in casting structure and inclusions, and the productivity, expressed as availabi-

lity and casting speed, are improved because a safe and reliable casting process can be maintained.

The same improved control of the temperature distribution is obtained by the provision, according to the invention, of at least one braking static or periodic low-frequency magnetic field which, in addition to the braking of inflowing melt, also stabilizes the surface of the melt, with at least one high-frequency magnetic field which develops compressive forces acting on this stable surface and/or heat in these parts of the melt, in combination with a thermally insulating sleeve, the sleeve being arranged at the inlet end of the mould or directly connected to the inlet end of the mould, so- called hot-top casting. During casting, the thermally insula- ting sleeve is filled at least partially with molten metal. To control the temperature of the melt present in the sleeve, the inductive heater is arranged on a level with the sleeve. The heater applies at least one magnetic alternating field to the molten metal present in the thermally insulated sleeve, to act on the melt adjacent the meniscus of the molten metal present in the thermally insulating sleeve. This results in develop¬ ment of heat in the melt which is located in the thermally insulated sleeve such that the temperature of this melt can be controlled. At the same time, or alternatively, the high- frequency magnetic alternating field develops compressive forces acting on the melt. These compressive forces reduce the pressure between the wall of the sleeve and the melt, thus significantly improving the conditions for lubrication. This provides significantly improved possibilities of utilizing the advantages offered by hot-top casting for increasing the sur¬ face quality of the cast strand, improving and controlling the casting structure of the strand, and increasing the casting speed without risking the casting quality. This is achieved primarily by the combination of at least one static or perio- die low-frequency magnetic field which stabilizes the surface of the melt while at the same time the compressive forces developed by the high-frequency magnetic field will act on this stable surface. If only the high-frequency magnetic field

were to be applied to develop compressive forces acting on the melt, there would be at risk, at least when casting heavy metals or metal alloys such as steel, that the pressure reduc¬ tion in the surface of the melt towards the sleeve would not be stable and would therefore collapse at certain points and/or at certain times, thus jeopardizing a good and even lubrication. The combination of at least one braking static or periodic low-frequency magnetic field, which acts on the melt in the mould, and at least one high-frequency magnetic field, which acts on the melt in the thermally insulating sleeve, provides considerable improvements with respect to quality and production engineering in relation to the prior art for this type of casting. In this way, it is possible to essentially avoid that melt present in the thermally insulating sleeve solidifies and, in particular, it can be avoided that a soli¬ dified surface layer is formed and adheres to the wall of the sleeve. Especially important is to prevent melt from solidi¬ fying and adhering to the transition from the sleeve to the cooled mould. Alternatively, this could have been achieved with an overtemperature of the incoming primary metal flow but this provides conditions in the mould/the sleeve which are both very difficult to control and hazardous, which easily results in catastrophic effects such as surface defects, poor casting structure and running out.

The above two embodiments can advantageously be combined to further improve the control of the temperature distribution of the melt during the casting process and, in particular, the temperature distribution in connection with the initial soli- dification stage adjacent the cooled mould in a continuous or semicontinuous casting process.

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 1 shows the invention as applied to casting in a conventional mould for

continuous or semicontinuous casting, and Figure 2 shows the invention as applied to a mould which is supplemented with an insulating sleeve which is partly filled with metal during the casting, so-called hot-top casting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiments of the invention shown in Figures 1 and 2, a mould 12 which is open in both ends in the casting direction is supplied with a primary flow of hot melt. During continuous and semicontinuous casting processes, at least one cast strand 11 is formed in the mould 12. The mould 12 is usually a water- cooled copper mould. Static or periodic low-frequency magnetic fields are adapted to act on the non-solidified portions 13 of the cast strand 11 and hence brake and split up the melt flowing into the mould 12 and prevent the primary flow of hot melt, which usually contains non-metallic particles, from penetrating deep down into the cast strand 11 and to control the flow in the non-solidified portions 13 of the strand. To apply the static or periodic low-frequency magnetic field, magnetic poles, which may be permanent magnets or, as shown in the figures, induction coils are supplied with direct current or a periodic low-frequency alternating current. The poles comprise a core 14a, 14b, 14c, 14d and windings 15a, 15b, 15c, 15d arranged around the core 14a, 14b, 14c, 14d. A magnetic return path 16a, 16b is arranged adjacent the cores 14a, 14b, 14c, 14d to connect the poles together as an external circuit, such that a return path 16a, 16b and cores 14a, 14b, 14c, 14d together with magnetic fields acting between the poles form a closed magnetic circuit. The first of the applied braking static or periodic low-frequency magnetic fields brakes and splits up the 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 of the splitting up, secondary flows arise and a more or less controlled circulation of melt in the non-solidified portions 13 of the cast strand 11. The second static or periodic low-frequency magnetic field con-

trols the secondary flow arising, among other things to ensure a sufficient heat supply to the upper surface 17 of the melt, the meniscus, and to prevent the meniscus 17 from solidifying, it is required that a sufficiently strong flow arises adjacent to the meniscus 17. This flow is controlled so as not to become so strong that casting powder and other non-metallic particles run the risk of being drawn down into the melt. To ensure that heat in a sufficient amount is supplied to the melt adjacent the meniscus 17 without risking that unwanted non-metallic particles are drawn down into the melt, according to the invention the static magnetic fields are supplemented with an inductive heater 18 arranged near the mould 12 on a level with the meniscus 17. The inductive heater 18 may be a single-phase or a polyphase heater. When the magnetic alterna- ting field, which preferably is a high-frequency magnetic alternating field, acts on the melt, heat develops in the melt so that the temperature of the melt adjacent the meniscus can be controlled. At the same time, or alternatively, the high- frequency magnetic alternating field develops compressive forces acting on the melt 13. The compressive forces reduce the pressure between the wall of the mould 12 and the melt 13 and thus improve the condition for lubrication significantly. If only a high-frequency magnetic field is applied to develop compressive forces acting on the melt, at least during casting of heavy metals or metal alloys such as steel, the pressure reduction is not reliable since the surface 20 of the melt against the mould is not sufficiently stable. The desired improvements from the points of view of quality and production engineering in relation to the prior art for continuous or semicontinuous casting are obtained according to the embodi¬ ment of the invention shown in Figure 1 by combining the first braking static field and the high-frequency magnetic field with at least one additional static or periodic low-frequency magnetic field to also control the secondary flow arising and stabilize the surface 20 of the melt such that the compressive forces which are developed by the high-frequency magnetic field will act on a stable surface 20. The combination of the braking static or periodic low-frequency magnetic fields and

at least one high-frequency magnetic field thus provides an improved surface quality of the cast strand 11 and a possibi¬ lity of increasing the casting speed without jeopardizing the surface quality.

The mould 12 in the embodiment shown in Figure 2 has, accor¬ ding to an alternative embodiment of the invention, been supplemented by a thermally insulating sleeve 19 at the inlet end of the mould. Thermally insulating sleeves 19 close to cooled moulds are used primarily in semicontinuous casting of weaker blanks such as extrusion blanks of copper or aluminium or alloys based on any of these substances. Usually, then, a plurality of blanks of this type are cast on a casting table by conducting the melt, which is tapped from a furnace or an intermediate container, to a plurality of water-cooled moulds disposed in the casting table. An insulating sleeve 19 is then placed over a cooled mould 12 as an extension of the mould 12 in the casting direction. This increases the possibilities of

- monitoring the structure of the cast strand, - improving the surface quality, and of

- raising the casting speed.

To achieve the above, however, great accuracy and skill of the machine operator are required, as well as a high and repeat¬ able quality of the insulating sleeves 19 and the mounting thereof. Critical points are to prevent the melt from starting to solidify in the sleeve 19 and to prevent a solidified surface layer from adhering to the sleeve 19. If a solidified surface layer is formed even in the sleeve 19, the desired improvement of the surface quality is jeopardized. It is especially critical, with disastrous consequences, if melt solidifies into a surface layer which adheres to the transi¬ tion from sleeve 19 to cooled mould 12. When the cast strand 11 then continues down in the mould 12, the surface layer is torn up and longitudinal surface defects arise; in the worst case, the solidified surface layer is torn off resulting in running out under the mould 12. Another reason for elongated surface defects is that a piece of the surface layer, which has solidified in the transition between sleeve 19 and mould

12, adheres thereto and gives rise to an elongated scratch along the cast strand 11.

By arranging an inductive heater 18, according to the embodi- ment of the invention shown in Figure 2, on a level with the thermally insulating sleeve 19, the above-mentioned critical moment is essentially eliminated by the high-frequency magne¬ tic field, acting on the melt in the sleeve, generating heat whereby the temperature of the melt 13 in the thermally insu- lating sleeve 19 is controlled such that the melt can be maintained in liquid state. At the same time, compressive forces are developed which act on the melt 13 and then pre¬ ferably on the surface 20 of the melt against the sleeve 19. Especially in combination with a braking static or periodic low-frequency magnetic field, these compressive forces provide improved conditions for lubrication between the melt 13 and the sleeve 19, the melt 13 and the mould 12 and the cast strand 11 and the mould 12 in that the pressure between the melt 13 and the sleeve 19 and between the melt 13 and the mould 12 decreases. The static magnetic field thus stabilizes the surface such that the compressive forces applied by the high-frequency magnetic field act evenly in time and space. Thus, by combining a braking static or periodic low-frequency magnetic field with a high-frequency magnetic field acting in the upper molten parts of the mould 12, according to the invention, the conditions for lubrication are improved while at the same time the formation of a first solidified surface layer is moved by controllable means from the sleeve 19 to the mould 12. This eliminates the risk of a solidified surface layer forming and adhering to the wall of the sleeve 19 and, similarly, melt is prevented from solidifying and adhering to the transition from the sleeve 19 to the cooled mould 12.