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 tap­ping jet of molten material entering the mold (9) directly or via a casting pipe (8). According to the invention a mag­netic field is created at the cast strand, in the direction of casting downstream of the mold (9) and acting trans­versely 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

[0001] 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 which is characterized by the features of the characterizing part of claim 1. The invention also relates to a device for carrying out the method.

[0002] 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.

[0003] 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).

[0004] 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.

[0005] 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.

[0006] 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.

[0007] To achieve this aim the invention suggests a method and a device 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.

[0008] Further developments of the method are characterized by the features of the additional Claims 2 and 3.

[0009] A device for carrying out the method according to the inven­tion is characterized by the feature of Claims 4.

[0010] Further developments of the device are characterized by the features of the additional Claims 5 and 6.

[0011] 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.

[0012] In a preferred embodiment 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. However, it is possible to place the brake means from a position immediately beneath the mold to ap­proximately 5 m beneath the meniscus, depending on the cast­ing conditions.

[0013] 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.

[0014] 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.

[0015] 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-molten parts of the blank far below the mold.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] The invention, both the method and the device, can be varied in many ways within the scope of the following claims.

Claims

1. 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 (9) directly or via a casting pipe (8), characterized in that a mag­netic field (10) is created at the cast strand, in the di­rection of casting downstream of the mold (9), acting trans­versely thereto and having its centre between 1.5 and 4 me­ters 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 (10).

2. Method according to Claim 1, characterized in that a DC-powered or low-frequency AC-powered magnetic field (10) is applied to the melt, in the latter case with a frequency of preferably below 1 Hz.

3. Method according to Claim 1, characterized in that a permanent-magnetic field is applied to the melt.

4. Device for carrying out the method according to any of the preceding claims, 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), characteriz­ed in that means for creating a magnetic field (10) are located at the cast strand, in the direction of casting downstream of the mold (9), said magnetic field being adapted to act transversely to the casting direction in or­der to reduce and split up the movements (12) of the melt.

5. Device according to Claim 4, characterized in that the means for generating the magnetic field (10) consist of DC-powered or low-frequency AC-powered coils, or of permanent magnets.

6. Device according to Claims 5 or 6, character­ized in that the means/coils for generating 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 (9).

Drawing