METHOD AND DEVICE FOR TREATMENT OF NON-SOLIDIFIED PARTS OF A CAST STRAND

Abstract: Method and device for treatment of the non-solidified parts of a cast strand, which is formed in a mold (9) from a tap­ ping jet of molten material entering the mold (9) directly or via a casting pipe (8). According to the invention a mag­ netic field is created at the cast strand, in the direction of casting downstream of the mold (9) and acting trans­ versely thereto. When the inflowing metallic melt passes through this magnetic field (10), the velocity of the melt is reduced and the stream of the incoming melt is divided (12).

Description: The present invention relates to a method for the treatment of the non-solidified parts of a cast strand according to the introductory part of claim 1. The invention also relates to a device for carrying out the method. A method and a device of this kind is known from the Patent Abstracts of Japan, Volume 10, No. 142, (M-481)(2199) 24.05.1986. EP-A-0 040 383 discloses a method for stirring of the above-mentioned kind. In this prior art method, the path of the tapping jet in the mold is arranged to pass through a static magnetic field produced by a permanent magnet or created by a flowing electrical direct current. When the inflowing metallic melt passes through this magnetic field, the velocity of the tapping jet is reduced and the tapping jet is being divided so that the effect of its impact on the rest of the melt in the mold is at least weakened. This publication also describes a device for carrying out the known method described. The above-mentioned known method has also been improved in such a way that magnetic poles, which produce the static magnetic field, are placed so close to the casting pipe or the mold that those parts of the melt which are located at the side of the incoming casting jet, and are thus not directly influenced thereby, are instead stirred by cooperation between the magnetic field which emanates from the magnetic poles and the currents induced when the tapping jet is slowed down (see EP-A-0 092 126). It has been found that the investment costs for a plant for these methods tend to increase because of the special design that is required for the mold together with the coils/magnets. The Patents Abstracts of Japan, Volume 10, No. 142, (M-481)(2199) 24.05.1986 discloses a continuous casting plant with a permanent magnet arranged immediately under the mold, with the field of the magnet being directed transversly to the longitudinal direction of the strand. The Patents Abstracts of Japan, Volume 11, No. 128, (M-583)(2575) 22.04.1987 discloses a continuous casting plant with two stirrers arranged on the small sides of the slab and disposed in the lower part of the supporting part of the mold. These stirrers too are positioned relatively close to the lower edge of the mold. Each stirrer generates a travelling magnetic field which travels in the longitudinal direction of the strand but opposite to the movement of the strand itself. If a casting machine is used for higher velocities (exceeding 2.0 m/min) or for thinner cast strands, it may be difficult, using these known methods, to obtain a purer steel because of the deeper penetration of non-metallic bodies entering with the tapping jet, among other things from a nozzle which is passed by Argon (Ar) bubbles, and because of a higher degree of meniscus changes and fluctuations in the surface of the melt in the mold, which may be difficult to control. The invention aims to provide a solution to the above-mentioned problems and other problems associated therewith, while at the same time utilizing the advantages of the prior art methods. To achieve this aim the invention suggests a method for treatment of non-solidified parts of a cast strand according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. A device for carrying out the method according to Claim 1 is characterized by the features of Claim 2. The magnetic field, which is applied on the melt, may be created by a flowing direct or low-frequency (less than 1 Hz) alternating current. The field may also be created by permanent magnets. This method provides a possibility of improving the braking intensity in relation to the geometry of the elements included in the casting device. The investment costs for the plant are reduced and the mold need not be modified with respect to coils and the like. The method can also be applied to thin slabs and to all types of steel alloys. A very intense brake power in relation to what has hitherto been possible can be obtained. The inventive concept can also be applied to slab casting at high velocities as well as to the casting of blooms. The coils (magnets) are positioned with their centre between 1.5 m and 4 m below the melt surface (the meniscus) in the mold, thus obtaining the most efficient braking. It is also possible to apply the method and the device in connection with continuous casting of billets, but the thrust of the invention centers on the casting of slabs, i.e. blanks having a greater width than thickness, for example of the order of magnitude of 2.0 x 0.2 m in cross section. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 continuous casting without the use of a magnetic brake, Figure 2 electromagnetic braking using known methods, Figure 3 braking in accordance with the invention. Figure 1 shows continuous casting of slabs without a magnetic brake. Melt arrives at an open-bottomed mold 2 via a tapping pipe 1 from a container or furnace, located above the mold. The movements of the gas bubbles are indicated by arrows 3. As can be seen, the casting jet penetrates the non-solidified parts of the blank far below the mold. Figure 2 shows the prior art method described above. Melt arrives at an open-bottomed mould 6 via a casting pipe 4. The movement of the melt, caused by the tapping jet from the pipe 4, is retarded by the magnetic field 5, and the movement of the melt is split up according to the arrows 7, thus obtaining a purer steel. Slag particles accumulate on the melt surface and the gas bubbles (Ar) are prevented from travelling deeper down into the melt. Figure 3 shows the method according to the invention. Melt arrives (possibly at high velocity) via a tapping pipe or nozzle 8. This pipe (nozzle) 8 is also passed by Ar-bubbles. The braking of the movement of the melt takes place downstream of the mold 9, that is, 1.5 to 4 m below the melt surface (meniscus) 11, by means of magnetic fields 10 produced by means of DC-powered or low-frequency (< 1 Hz) AC-powered coils or by permanent magnets. The movement of the melt is split up according to the arrows 12. A pure steel is obtained as end product, preferably slabs, possibly blooms. Also the device according to the invention is shown in Figure 3. The device is effective in reducing non-metallic in clusions, for example from powder, possibly from the mold, entering the melt. It is also effective to optimize the braking intensity in relation to the geometry of the elements included, such as the angle of incidence of nozzles and the position of the coils (magnets). The method and the device can also be used for distances below the mold extending from immediately below the mold to 5 m from the meniscus 11.

Claims: Method for treatment of the non-solidified parts of a cast strand being formed in a mold (9) from a tapping jet of molten material entering the mold directly or via a casting pipe (8), with a magnetic field created at the cast strand downstream of the mold and acting transversely to the longitudinal direction of the mold, characterized in that the strand is cast at a speed exceeding 2 m/min, and that the magnetic field (10), - a permanent-magnetic field, a DC-powered magnetic field or an AC-powered magnetic field with a frequency below 1 Hz -, is positioned with its centre between 1.5 and 4 m below the melt surface/meniscus (11) in the mold (9), the magnetic field reducing and splitting up the movement of the melt when passing through said magnetic field. Device for carrying out the method according to claim 1, comprising an open-bottomed mold (9) in a continuous casting machine and a ladle or container from which tapping into the mold is performed directly or via one or more casting pipes (8), with means for creating a magnetic field (10) located at the cast strand downstream of the mold and adapted to act transversely to the casting direction, characterized in that means for casting the strand at speeds exceeding 2 m/min are arranged, that the means for creating the magnetic field (10) consist of permanent magnets, DC-powered coils or AC-powered coils, that the means for creating the magnetic field (10) are placed with their centre at a distance of between 1.5 and 4 m below the melt surface/meniscus (11) in the mold said magnetic field being adapted to reduce and split up the movements (12) of the melt.

Inventors: ERIKSSON, JAN ERIK, SUZUKI, HIROSHI
Applicants: ASEA BROWN BOVERI AB
Application year: 1988
Main classification: B22D11
IPC codes: B22D11