GB2059834A - Electromagnetic stirring of liquid metal in continuous casting - Google Patents

Abstract

Stirring of the non-solidified portion 9 of a cast strand 1 coming from a continuous casting machine is performed by means of two electromagnetic stirrers 2, 3 located substantially opposite to each other at the cast strand 1. There is a phase difference of 180 DEG between the two stirrers, and preferably each of the stirrers produces an asymmetrical stirring effect. <IMAGE>

Description

SPECIFICATION Method of, and means for, continuous casting This invention relates to a method of stirring the non-solidified melt in a cast metallic strand issuing from a continuous casting machine, the strand being formed in a mould and stirring of the melt being performed by means of electromagnetic stirrers. The invention also relates to means for carrying out the method.

In the field of continuous casting, the cast products are variously referred to as slabs, blooms or billets, largely depending on the cross-sectional area of the product, slabs being of larger cross-sectional area than blooms, and blooms being of larger cross-sectional area than billets. In the present specification all these products are included in the term "cast strand".

Various ways of stirring the non-solidified portion of a cast strand have been proposed.

For example, it has been proposed to effect such stirring with one or more electromagnetic stirrers disposed transversely of, or along the casting direction, in one or more places.

One of the reasons for this is the desire to prevent constrictions, so-called mini-ingots, and segregations in the finished product and the desire to have as uniform a solidification process as possible. However, when using electromagnetic stirrers according to the hitherto known proposals, the use of too low a power or too high a frequency may result in too low a rate of flow in the non-solidified portion of the cast strand. The present invention aims to overcome this problem.

According to the invention, a method of stirring the non-solidified melt in a cast metallic strand coming from a continuous casting machine, the strand being formed in a mould and stirring being performed by means of electromagnetic stirrers, is characterised in that the melt is stirred by means of two stirrers, located substantially opposite to each other adjacent to the cast strand or in the mould, the travelling fields having the same direction and there being a phase difference of 1 80 between the two stirrers. In this way the two stirrers are able to cooperate in such a way that a sufficiently high rate of flow and a sufficiently efficient stirring in the non-solidified portion of the melt are obtained.According to a further aspect of the invention, in means for carrying out the method and comprising a continuous casting machine with a mould and at least two electromagnetic twophase, three-phase or multiphase stirrers for stirring of the non-solidified portion of the cast strand, one pair of stirrers, positioned opposite to each other, have the same direction of stirring and are connected for mutual synchronous stirring, with a phase displacement of 180 .

In a preferred embodiment of the means, one stirrer or each stirrer of the pair is constructed so as to obtain asymmetrical stirring, i.e. a certain turbulence is obtained from the stirrer and has the effect of avoiding so-called white bands in the cast product. For example, the current strength in one phase of the stirrer may deviate at least 10 per cent from the corresponding current strength in at least one adjacent phase, or the phase coils may be constructed asymmetrically in a corresponding manner. Such an asymmetrical stirrer is described in British Patent Application 7911703 (Published Application No. 2020561).

The invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a sectional view of a cast strand from a continuous casting plant, showing supporting means for the strand and electromagnetic stirrers for the sump of non-solidified melt in the strand, and Figure 2 is a view similar to Fig. 1 but showing electromagnetic stirrers of a different kind.

Fig. 1 shows a cast strand 1 with a sump 9 of non-solidified melt, the strand being supported on opposite sides by rollers 8 and 11 after issuing from a mould (not shown) in the direction of the arrow A. At least some of the rollers 8, 11 are located between the salient poles of two multi-phase electromagnetic stirrers 2, 3. The rollers between the stirrer poles should, of course, be made of non-magnetic material to prevent the field of the stirrers from being screened or short-circuited. The stirring direction of the two stirrers 2, 3 is the same, being indicated by the two arrows B.

The cast strand may be a billet, for example of circular cross-section, or a slab of rectangular cross-section. In the latter case, the stirrers 2, 3 are suitably located opposite to each other at the broad sides of the slab. The two stirrers are adapted to operate synchronously, which means that the field originating in the stirrer 2 is sucked up into the stirrer 3. A phase difference of 1 80' between the two stirrers is maintained. To bring about turbulence in the stirring, the stirrers are constructed for asymmetrical stirring, for example in one of the ways described in Published British Patent Application No. 2020561. By using the two stirrers 2, 3, with a phase difference of 180 between their stirring effects, the asymmetrical stirring effect is amplified.

The stirring frequency employed may be from 0-10 Hz in the case of slabs, from 10-30 Hz in the case of blooms and from 50-60 Hz in the case of billets.

Fig. 2 shows the employment of stirrers 4 and 7 with non-salient poles. Feed rollers 5, 6 are placed between the stirrers and the cast strand 10, these rollers being made of a non magnetic material. However, in some cases, the stirrers may be located between rollers instead.

In the embodiments described above, the stirrers may be supplemented with other pairs of stirrers or with individual stirrers, and the direction of stirring may be transversely of or along the direction of casting, or at an acute angle to the direction of casting.

By employing the method according to the invention a synchronous stirring with amplified asymmetry is obtained, and too low a rate of flow in the sump of non-solidified melt is avoided.

Claims (6)

1. A method of stirring the non-solidified melt in a cast metallic strand coming from a continuous casting machine, the strand being formed in a mould and stirring being performed by means of electromagnetic stirrers, characterised in that the melt is stirred by means of two stirrers located substantially opposite to each other adjacent to the cast strand or in the mould, the travelling fields from the two stirrers having the same direction but there being a phase difference of 180 between the two stirrers.

2. Means for carrying out the method according to claim 1, comprising a continuous casting machine with a mould and at least two electromagnetic two-phase, three-phase or multi-phase stirrers for stirring the non-solidified portion of the cast strand, in which one pair of stirrers, positioned opposite to each other, have the same direction of stirring and are connected for mutual synchronous stirring, with a phase displacement of 180 .

3. Means according to claim 2, in which said one pair of stirrers are constructed for asymmetrical stirring.

4. Means according to claim 3, in which in each of said stirrers the current strength in one phase deviates at least 10 per cent from the corresponding current strength in at least one adjacent phase.

5. Means according to claim 3, in which in each of said stirrers the phase coils are asymmetrically constructed.

6. Means according to claim 2, constructed and arranged substantially as herein described with reference to, and as illustrated in, Fig. 1 or Fig. 2 of the accompanying drawing.