WO2002038310A1

WO2002038310A1 - Device for casting metal

Info

Publication number: WO2002038310A1
Application number: PCT/SE2001/002433
Authority: WO
Inventors: Lars Nordekvist; Conny Svahn; Eleonor Olsson; Tord Lindblad; Per Larsson; Magnus HALLEFÄLT
Original assignee: Abb Ab
Priority date: 2000-11-08
Filing date: 2001-11-06
Publication date: 2002-05-16
Also published as: SE0004082D0; AU2002212924A1

Abstract

A device for continuous or semi-continuous casting of metal comprising a mold (2) with an inner wall (3) and an outer wall (5) and an electromagnetic device (14), wherein said electromagnetic device (14) is arranged in between said inner wall (3) and said outer wall (5).

Description

Device for casting 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 electromagnetic device such as an induction coil arranged at the top end of the mold, at the meniscus.

BACKGROUND ART

During continuous or semi-continuous casting of metals and metal alloys, a hot metal melt is supplied to a cooled mold that 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 can comprise internal cavities or channels in which the coolant, e.g. water, flows during casting. As the metal passes through the mold it solidifies 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. The surface finish and the cast structure of the cast strand are highly dependent on the conditions of initial solidification which depends on the conditions in the top end of the mold, i.e. the locations at which the metal starts to solidify. The surface quality of the cast strand can be improved by minimizing the contact between the melt and the mold during initial solidification of the skin of the cast strand. This can be achieved by oscillating the top of the mold by an induction coil in the region of the meniscus. The compressive forces generated by the oscillating electromagnetic field from the induction coil act to separate the melt from the mold. These forces reduce the contact between melt and mold before and during initial solidification whereby the surface quality of the cast strand can thereby be improved and the casting speed can be increased without risk of negatively influencing the surface quality. The induction coil 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 or an electromagnetic stirrer but the induction coil can also be independent. US-A-5 375 648 further discusses how to arrange a high-frequency induction coil at the top end of the mold. This coil can of course be used on its own or in combination with other means for controlling the thermal situation within the mold. It has been suggested to arrange induction coils 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 coil outside the mold at the top end of the mold. However, as the molds preferably are made of copper, which has a high electrical conductivity, measures has to be taken to reduce 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 is attractive. A reduced frequency will induce movements or un- desired flows in the melt, and 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.

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 an electromagnetic device at the top end of the mold which provides improved conditions for the casting of the cast metal in the mold. Preferably the device exhibits an increased efficiency in generating compressive forces acting to separate the melt from the mold.

SUMMARY OF THE INVENTION

The present invention provides 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 mold and an electromagnetic device, for example in the shape of an induction coil, arranged at the top end of the mold. According to a first embodiment of the present invention, the electromagnetic device is arranged between the inner and outer walls of the mold. Preferably, at least one side of the electromagnetic device is cooled by a system of internal cavities or channels, wherein these cavities or channels supply the mold with a coolant such as water. Here, a cooling medium is located between said inner wall and said outer wall, thereby at least partly surrounding and effecting the temperature of the electromagnetic device.

As this electromagnetic device is closer to the melt than the prior art electro- magnetic device arranged outside the mold the eddy-current losses are substantially reduced. However, some losses still may 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 electromagnetic device. To reduce these minimal losses a few slits may be cut in these parts of the mold and such slits shall preferably be oriented in the casting direction. As the losses are substantially reduced and the efficiency is substantially improved, the size and power rating of the electromagnetic device can be reduced. This will most often show as a reduction in the number of windings of the electromagnetic device according to the present invention compared to an electromagnetic device according to prior art for the same size of mold. The mold can also be made more mechanically stable as many eddy-currents reducing slits can be eliminated.

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, where: Figure 1 a shows a plane view from above of a device for continuous casting according to prior art with an induction coil arranged at the top end of the mold; Figure lb shows a section along the line I-I of figure la; Figure 2a shows a plane view from above of a device for continuous casting according to one embodiment of the present invention with an induction coil arranged in the body of the mold at the top end of the mold;

Figure 2b shows a section along the line II-II of figure 2a

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The devices for continuous or semi-continuous casting of metal shown in the

Figures la, lb, 2a, 2b all comprise a casting mold 2. A continuous or semi-continuous casting mold is open at both ends in the casting direction and is usually provided with mold cooling means. The mold 2 has an inner wall 3, which the melt flows against, and an outer wall 5. Preferably, the mold 2 comprises a mold body 4 that can be made of plates or a tube and the inside wall of which forms the inner wall 3 of the mold 2 that contacts the melt. Preferably, said mold body 4 is cooled by cooling means such as a cooling jacket 6. The cooling jacket 6 can comprise a system of concentric tubes such as back-up tube 8 and outer back-up tube 10 that together form internal cavities or channels 13 wherein a coolant flows during operation. The outer wall 5 of the outer back-up tube 10 of the cooling jacket 6 forms the outer wall 5 of the mold 2. The cooled im er wall 3 of the mold 2 is continuously supplied with a primary flow of hot melt, which is cooled by contact with the inner wall 3 leading to a cast strand being formed in the mold 2. Casting molds 2 are usually water-cooled copper molds and they typically have a square or rectangular cross-section although circular cross-sections can also be used. The mold can be made up of several segments which are held together by the surrounding supporting structure (shown schematically by dotted lines in figures lb and 2b) and the surrounding backup tubes 8, 10. The mold 2 has internal cavities or channels 13', 13" in which cooling liquid, such as water, flows during casting. The cooling liquid flows through an inlet channel 13 ' which is situated nearest to the interior surface of the mold 2 between mold body 4 and inner back-up tube 8, and an outlet channel 13" which is situated nearest to the exterior surface of the mold 2 between the inner back-up tube 8 and an outer back-up tube 10. During casting a primary flow of hot melt is supplied to the mold 2. This melt can be supplied to the mold 2 either by supply means (not shown), such as 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, a channel or system of feeder channels or runners which guides the melt to the mold 2. As the metal passes through the mold 2 it is cooled and solidified whereby a cast strand is formed. When the cast strand leaves the mold 2, 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 which occur at the top of mold 2. To control and improve the vibrational situation at the top end of the mold 2 an induction coil 14 is arranged at this top end substantially level with the top surface of the melt in the mold, i.e. the meniscus 16 (shown by a broken line in the figures). Typically, the coil vibrates at a frequency of about 200 Hz.

The coil 14 according to prior art as shown in figure 1 is arranged outside the mold 2 at a small distance from the outer back-up tube 10, so that there is an air gap 17 between the coil 14 and the outer back-up tube 10. The magnetic field generated by the coil 14 has to cross the air gap 17 and then penetrate the mold body 4 before it enters the melt. To reduce the eddy-current losses in the mold body 4 and to improve the efficiency, the mold body 4 is provided with slots 19 in the casting direction substantially at the same level as the coil 14.

Figures 2a) and 2b) show a first embodiment of the present invention, in which parts corresponding to similar parts in the prior art device of figures la) and lb) have been given the same reference numerals. As shown in figure 2a) and 2b), the coil 14 is contained in an enclosure 20 arranged between the inner back-up tube 8 and the outer back-up tube 10 of the mold 2. Between the inner back-up tube 8 and the mold body 4 is a cooling fluid inlet channel 13'. The outer wall 5 of the outer back-up tube 10 forms the outer wall 5 of the mold 2. In this em- bodiment, enclosure 20 is comprised of a watertight duct 20 with a rectangular or other suitable cross-section adapted to fit the shape of the coil 14. Three sides of duct 20 are formed by a U- section body 22, the fourth side of duct 20 being formed by the outer surface of the inner backup tube 8 to which the U-section body 22 is attached. Duct 20 extends around the whole of the periphery of the inner back-up tube 8 and it is mounted at a level that corresponds to the position where oscillating is required, which is usually in the vicinity of the melt meniscus. Outer backup tube 10 is provided with a bulge 24 around duct 20. The bulge 24 is so dimensioned that a cooling fluid outlet channel 13" is formed between the outer surfaces of U-shaped channel 22 and the inside surface of outer back-up tube 10. In this way, the duct 20 and coil 14 are cooled by the cooling fluid in the cooling channels 13', 13". This helps prevent the windings of coil 14 from overheating. By positioning the coil 14 between the inner back-up tube 8 and the outer back-up tube 10 the distance from the coil to the inner surface of the mold body 4 (and consequently the melt) is considerably reduced. Depending on the thickness of the outer back-up tube 10 and the thickness of the inner back-up tube 8, the distance can be reduced to one half or less of the prior art distance. This reduces the eddy-current losses in the mold. As the losses are substantially reduced, the efficiency is substantially improved and therefore the size and power rating of the coil 14 can be reduced. This will most often show as a reduction in the number of windings in the coil 14. As the power rating is reduced, the risks of damaging mold 2 through excessive heating are substantially in the reduced. Naturally, the mold 2 can be provided with eddy current reducing slots 19 in the casting direction substantially at the same level as the coil 14.

As the coil 14 is wound against and around, and therefor in supporting contact with, the outer wall of the inner back-up tube 8 it can provide some support to the back-up tube 8, that is to say it reinforces back-up tube 8 which can therefore be made thinner, at least in the areas in contact with the coil 14. In the event that the inner back-up tube is made thinner then this reduction in the thickness of inner back-up tube 8 advantageously brings the coil 14 even closer to the melt. It should be understood that the coil of the electromagnetic device is in direct or indirect contact with the inner back-up tube 8.

In another embodiment of the invention, it is conceivable to wind the coil around the outer surface of the mold body 4 in the region where the meniscus of the melt is intended to be. This brings the coil even closer to the meniscus and has the further advantage that the coil can contribute to holding together the segments that together form the mold body and can mechanically support the mold body. In order to avoid short circuiting, the coil must be electrically insulated from the mold by insulating means, for example in the form of an intermediate electrically insulating layer.

In yet another embodiment of the invention, another electromagnetic device such as an electromagnetic brake is combined with the coil of the invention. This additional electromagnetic device is consequently also brought closer to the melt and acts more efficiently.

In a further embodiment, several separate induction coils are wound at different vertical levels around the mold body between the inner and outer walls of the mold.

In yet a further embodiment of the invention the mold lacks a cooling fluid outlet channel, i.e. the cooling fluid is allowed to flow freely down the outside of the mold once it reaches the top of the cooling fluid inlet channel. In this case the coil may be wound around the outer wall of the mold body or around the outer wall of the cooling fluid inlet channel.

It is conceivable for the inventive features of two or more of the above embodiments to be combined into a single device, e.g. providing several separate induction coils combined with several electromagnetic brakes.

Claims

1. A device for continuous or semi-continuous casting of metal comprising a mold

(2) with an inner wall (3) and an outer wall (5) and an electromagnetic device (14), characterized in that said electromagnetic device (14) is arranged in between said inner wall

(3) and said outer wall (5).

2. A device according to claim 1, characterized in that a cooling medium is located between said inner wall (3) and said outer wall (4), thereby at least partly surrounding and effecting the temperature of the electromagnetic device (14).

3. A device according to claim 2, characterized in that said electromagnetic device (14) is arranged in a water-tight duct (20) wherein said duct (20) is arranged between said inner wall (3) and said outer wall (5).

4. A device according to claim 3, characterized in that said duct (20) has a substantially U-shaped cross-section and is attached to the outer wall of an inner back-up tube (8).

5. A device according to any of the previous claims, characterized in that said electromagnetic device (14) is arranged between a cooling fluid inlet channel (13') and a cooling fluid outlet channel (13").

6. A device according to any of claims 1-5, characterized in that the electro- magnetic device (14) comprises a coil which is wound around an inner back-up tube (8).

7. A device according to any of claims 1-4, characterized in that said mold (2) has a mold body (4) surrounded by a cooling means (6), wherein said electromagnetic device (14) is tightly wound around said mold body in order to mechanically support said mold body (4).

8. A device according to any of the previous claims, characterized in that said mold (2) is provided with cooling means (6) and that said electromagnetic device (14) is positioned between said mold body (4) and said cooling means (6).

9. A device according to any of the previous claims, characterized in that said electromagnetic device is electrically insulated from said mold by an electrically insulating layer.

10. A device according to any of the previous claims, characterized in that it comprises a plurality of vertically spaced electromagnetic devices.

11. A device according to any of the previous claims, characterized in that the electromagnetic device (14) comprises an induction coil for generating compressive forces acting to separate a melt from the mold (2).