Electromagnetic stirrer for continuous casting

Abstract

 Electromagnetic stirrer for continuous casting, comprising at least ore iron core (2) provided with a number of coils (1). The iron core is provided with a damping plate (3) of electrically conducting material, forming a closed magnetic circuit, intended to damp the propagation of the magnetic field in the direction in which the damping plate is placed in relation to the iron core (1, 2).

Description

[0001] The present invention relates to an electromagnetic stirrer for continuous casting according to the precharacterising part of claim 1.

[0002] In continuous casting, especially in continuous casting moulds in which a stirrer is arranged to stir the liquid parts of the cast strand, a problem arises in that part of the magnetic field takes its path through the iron structure of the surrounding mould or the continuous casting machine. This may result in difficulties in obtaining a sufficient stirring power from the stirrer.

[0003] The present invention aims to provide a solution to the above problem and other problems associated therewith.

[0004] To achieve this goal the invention suggests an electromag­netic stirrer for continuous casting according to the intro­ductor part of claim 1, which is characterized by the fea­tures of the characterizing part of claim 1.

[0005] Further developments of the invention are characterized by the features of the additional claims. By damping the field leaking out and directing it towards the interior of the stirring region, a greater part of the available magnetic field can be utilized for stirring pur­poses.

[0006] In a preferred embodiment the iron core and the damping plate are built in a circle or in a rectangle or in a square so as to form a closed or partially closed magnetic circuit and - with respect to the damping plate - a closed electric circuit.

[0007] However, the damping plate can sometimes be divided into several parts, separated by air gaps (interruptions), which is also possible with the iron core thus obtaining a par­tially closed electric circuit for eddy currents in the damping plate and a partially closed magnetic circuit, re­spectively.

[0008] A coil-wound stirrer can be used, especially in mould stir­rers in which the stirrer is built into a mechanical iron structure. A considerably increased efficiency is obtained with such a stirrer design.

[0009] The coils can be arranged to surround the iron core, possi­bly also the damping plate, and be supplied from a single-­phase, two-phase, three-phase, or multi-phase a.c. source.

[0010] The invention can be applied to continuous casting plants for billets and blooms (from 60 mm square up to about 400 x 600 mm), the frequency being 2-20 Hz.

[0011] The damping plate should have a thickness of at least one-­third of the depth of penetration, which is defined as

in which ρ is the resistivity of the damping plate material, ω=2nf (angular frequency) and µx·µo the permeability. The material in the damping plate is copper, silver, or an alloy of any of these metals.

[0012] By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in

Figure 1 a circular mould stirrer,

Figure 2 a rectangular mould stirrer, and

Figure 3 an alternative to Figure 1.

Figure 1 shows a circular stirrer, supplied with three-phase current (R, S, T), for horizontal stirring in a cast strand at the mould.

[0013] The coils 1 are pairwise connected in opposition to each other (R- and R+, S- and S+, T- and T+; see, i.a., at 1 (R+)). The direction of the magnetic field is shown at the arrows in the iron core 2. The iron core is here an undi­vided, circular iron core, but it may also be with interrup­tions.

[0014] The coils 1 surround the iron core 2 but may also surround the surrounding damping plate 3 along with the core (see Figure 3). As will be seen in Figure 1, the coils consist of a number of sub-coils 1 (R, S, T + and -).

[0015] The damping plate 3 has a thickness of at least one-third of' the penetration depth δ and is made of an electrically highly conductive metal, such as copper or silver or an alloy of any of these metals. By the influence of the damping plate 3, the undesired out­wardly protruding part of the magnetic field from the iron core 2 is dampened by the damping plate and thus deflected towards the interior of the cast strand.

[0016] Figure 2 shows a rectangular stirrer supplied with two-phase current (R+ and R-, S+ and S-). Here the iron core 5 is square-shaped, but it may also be rectangular-shaped and may possibly be provided with interruptions. A coil is here shown on each leg of the square iron core 5 which is sur­rounded by a similarly square damping plate 6. Also this damping plate may be provided with interruptions and is to be made of electrically conducting material, such as silver or copper (see above).

[0017] For the rectangular mould stirrer according to Figure 2, the field is shown at the moment when the R-phase has maximum current and the S-phase zero current (90° phase angle).

[0018] Figure 3 shows a circular stirrer in which the coils 7 sur­round both the iron core 2 and the damping plate 3. The damping plate may be provided with interruptions or air gaps so that the stirrer can be constructed from several sub-­stirrers, which, inter alia, facilitates the mounting.

[0019] The stirrers are preferably of a coil-wound kind but may, of course, also be formed with a different winding.

[0020] The stirrer according to the invention may be varied in many ways within the scope of the appended claims.

Claims

1. Electromagnetic stirrer for continuous casting comprising at least one iron core (2;5) provided with a number of coils (1), characterized in that the iron core (2;5) is provided with a damping plate (3;6) of electrically con­ducting material, forming a closed magnetic circuit, in­tended to damp the propagation of the magnetic field in the direction in which the damping plate (3) is placed in rela­tion to the iron core (2).

2. Stirrer according to claim 1, characterized in that the damping plate surrounds the iron core in such a way that the radially outwardly protruding portions of the magnetic field are subjected to dampening.

3. Stirrer according to claim 1 or 2, character­ized in that the iron core and the damping plate are constructed in a circle or a rectangle or a square so as to form, respectively, a closed or partially closed magnetic circuit and a closed magnetic circuit.

4. Stirrer according to any of the preceding claims, cha­racterized in that the stirrer coil is built up of several sub-coils and composed such that the iron core forms a closed or partially closed magnetic circuit with small air gaps between the sub-coils and that the damping plate or plates is or are formed with interruptions between the sub-coils.

5. Stirrer according to and of the preceding claims, cha­racterized in that the coils are arranged to sur­round the iron core and are supplied from a single-phase, two-phase, three-phase, or multi-phase a.c. source.

6. Stirrer according to.any of the preceding claims, cha­racterized in that the coils are arranged to surround the iron core and the damping plate and are sup­plied from single-phase, two-phase, three-phase, or a multi-­phase a.c. source.

7. Stirrer according to any of the preceding claims, cha­racterized in that the damping plate is made of copper or silver or an alloy of any of these materials.

8. Stirrer according to any of the preceding claims, cha­racterized in that the thickness of the damping plate is at least one-third of the depth of penetration at a chosen frequency of the a.c. supply source.

9. Stirrer according to any of the preceding claims, cha­racterized in that it is arranged in or at a con­tinuous casting mould.

Drawing