GB2061783A

GB2061783A - Electromagnetic stirring in continuous casting

Info

Publication number: GB2061783A
Application number: GB8034977A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1979-10-31
Filing date: 1980-10-30
Publication date: 1981-05-20
Also published as: GB2061783B; SE7909003L; CH648776A5; JPS5677052A; SE418934B; DE3039491A1

Abstract

In a method of stirring the non- solidified melt 6 in a cast strand 5 from a continuous casting machine, the strand 5 is formed in a mould 3 and the stirring is performed by means of at least one electromagnetic stirrer 7, which is suitably straight and provides an asymmetrical stirring effect. The flow of melt in or downstream of the mould 3 is measured by a measuring instrument 9, and the measuring signal thus obtained is employed for controlling the asymmetry of the stirrer in such a way that when a reduced flow of melt is measured, the asymmetry is increased, and vice versa. With Fig. 1). <IMAGE>

Description

SPECIFICATION Continuous casting method This invention relates to a method of stirring the non-solidified melt in a cast strand from a continuous casting machine.

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".

It has already been proposed (see published British Patent Applications Nos. 2020560 and 2020561) to stir the non-solidified melt in a cast strand from a continuous casting machine by means of an asymmetrically acting multiphase electro-magnetic stirrer. The current in one phase may, for example, be allowed to deviate by at least 10 per cent from the current in another phase, or the phases may be designed with different numbers of winding turns.

Asymmetrical stirring results in an improved cast structure, and it is known that it also results in turbulent stirring. As a matter of fact this is the simplest way to obtain turbulence, and the only known alternative is to switch on and off the supply current to the stirrer according to a certain programme.

The result of turbulence in the non-solidified melt in a cast strand is a lowering of the viscosity, so that the flow produced by the stirrer extends further upstream of the stirrer, and possibly into the casting mould, compared with non-turbulent stirring.

The turbulence also ensures that not only will growth nuclei in the melt survive, but that there will be multiplication of the growth nuclei, resulting in a more even solidification (i.e. a more uniform solidification front). A correct amount of nuclei results in a substantially constant two-phase area in the cast strand (i.e. the transition area between the liquid and solid phases) and few, if any, segregations (e.g. white bands) will occur. If the two-phase area in the melt is reduced the risk of white bands occurring will increase.

Many growth nuclei may increase the extent of said two-phase area. The present invention aims to obtain a balance between the extent of the two-phase area and the extent of the stirring.

According to the invention, a method of stirring the non-solidified melt in a cast strand from a continuous casting machine, the strand being formed in a mould and the stirring being performed by means of at least one electro-magnetic stirrer which provides asymmetrical stirring in the non-soidified melt, is characterised in that the flow of melt in or downstream of the mould is measured and the measuring signal thus obtained is used for controlling the asymmetry of the stirrer in such a way that when a reduced flow of melt is measured, the asymmetry is increased, and vice versa.

The method according to the invention provides the possibility of controlling the stirring so that a substantially constant rate of flow of the melt is obtained, and it is not necessary to deal with the difficult problem of measuring the extent of the aforementioned two-phase area downstream of the mould. When a reduction of the measured flow is detected, the measured signal gives rise to increased asymmetry of the stirrer, so that the turbulence as well as the rate of flow of the melt increase.

On the other hand, when an increase of the measured flow rate is detected, the measuring signal gives rise to decreased asymmetry of the stirrer, so that the turbulence and rate of flow of the melt decrease.

The invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a diagrammatic sectional view of a continuous casting machine operated by the method according to the invention, and Figure 2 is a circuit diagram of part of the machine of Fig. 1.

Fig. 1 shows a steel or iron melt being tapped into a downwardly open, cooled mould 3, in which a cast strand 5 is formed, which gradually solidifies, from the surface inwardly, as the strand moves downstream of the mould. The present invention is concerned with a method of stirring the non-solidified melt 6 in the strand 5 employing an electromagnetic stirrer 7. As usual the stirrer is fed with two-phase or three-phase current at a frequency of from 0.5 to 4 Hz, preferably from 1 to 2 Hz, when the strand 5 is a slab, at a frequency of about 25 Hz when the strand 5 is a bloom and at a frequency of from 50 to 60 Hz when the strand 5 is a billet.

The stirrer 7, which suitably is straight and located outside a support roller 8, which should be constructed of non-magnetic material, is designed for asymmetrical stirring, for example with a current that is 10 per cent higher or lower in one phase than in another, but the coils may also have different numbers of turns in different phases to achieve the same asymmetrical effect. The stirrer may also be designed so that the phase difference in the two-phase case deviates from 90 and in the three-phase case deviates from 120". The aforementioned two-phase area may be varied in different ways by changing the degree of asymmetry of the stirring, and in some cases even a change to symmetrical stirring may be made.

The flow in the melt 6 is measured in any suitable manner, for example employing one or more measuring instruments 9 close to the mould 3 which is/are arranged to emit a signal proportional to the rate of flow of the melt 6. This flow measurement may be based on a magnetic, mechanical, chemical or optical method or be performed in any other manner within the scope of available technique. For example, the or each measuring instrument 9 may be of an inductive nature in which the melt 6 forms part of the iron core, so that the inductance of the instrument changes as the rate of flow of the melt varies.

It is also possible to measure the flow in the melt 6 using photocells, from the output signals of which may be derived a signal which is dependent on the speed of the melt 6.

The signal emitted by the measuring instrument or instruments 9 is fed to a control or regulating device 10, which in turn is adapted to control the asymmetry of the stirrer 7 (see Fig. 2).

It is desired to maintain a substantially constant rate of flow in the melt 6 so as to obtain an even quaiity and absence of white bands and segregations in the cast strand. If the flow changes for some reason, this is indicated by the measuring instrument(s) 9, and the asymmetry of the stirrere is then adjusted so that the turbulence is changed and the desired rate of flow is restored again.

In the case of reduced rate of flow, for example, the asymmetry is increased, and vice versa. As mentioned, the asymmetry may be adjusted by changing phase currents or phase angles. A combination of phase angle change and current change is also possible; or the asymmetry may be changed in some other manner. It is also possible to combine the asymmetrical stirrer with a symmetrical stirrer in a manner known per se.

Claims

1. A method of stirring the non-solidified melt in a cast strand from a continuous casting machine, the strand being formed in a mould and the stirring being performed by means of at least one electromagnetic stirrer which provides asymmetrical stirring in the non-solidified melt, characterised in that the flow of melt in or downstream of the mould is measured and the measuring signal thus obtained is used for controlling the asymmetry of the stirrer in such a way that when a reduced flow of melt is measured, the asymmetry is increased, and vice versa.

2. A method according to claim 1, in which the phase current in one phase of the stirrer deviates from the current in at least one other phase, the degree of deviation being controlled with the aid of said signal.

3. A method according to claim 1, in which the stirrer has a different number of winding turns in one phase relative to at least one other phase, and the current in the different phases is varied in response to changes in said signal for the purpose of varying the asymmetry of the stirrer.

4. A method according to claim 1, in which the stirrer is supplied with two-phase or three-phase current, the phase-angle difference, which is 90 and 120 , respectively, in the case of symmetrical supply, then being' changed for the purpose of achieving asymmetry of the stirrer, the phase angle being further changed in response to changes in said signal for the purpose of varying the asymmetry of the stirrer.

5. A method of stirring the non-solidified melt in a cast strand from a continuous casting machine, substantially as hereinbefore described with reference to the accompanying drawing.