GB2092464A

GB2092464A - Continuous casting

Info

Publication number: GB2092464A
Application number: GB8136091A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1978-04-05
Filing date: 1979-04-04
Publication date: 1982-08-18
Also published as: CH635529A5; FR2421701A1; JPS54134032A; SE410940B; GB2092464B; IT1119053B; SE7803818L; DE2911842A1; IT7967702A0; GB2020560A; SE410940C; GB2020560B; FR2421701B1; ES479252A1

Abstract

A method of stirring the non- solidified melt 6 in a cast metallic strand 5 from a continuous casting machine, the strand being formed in a mould 3, open at the bottom, and the stirring being performed by at least one electro- magnetic stirrer 7 disposed adjacent to the strand 5. The or each stirrer 7 is supplied with multiphase current at a frequency of from 0.5 to 4 Hz, preferably from 1 to 2 Hz, and effects a stirring of the non-solidified melt 6 upstream of the stirrer 7 and extending up into the mould 3. In addition, the or each stirrer 7 is arranged to provide an asymmetrical stirring effect, for example by arranging for the current supplied to one phase of the stirrer to deviate by at least 10 per cent from the current supplied to another phase. This method of casting contributes to breaking down the fringe crystal zone and the achieving of a finer grained, equiaxial, globular structure in the finished casting. <IMAGE>

Description

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GB 2 092 464 A

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SPECIFICATION Continuous casting method

5 This invention relates to a method of stirring the non-solidified melt in a cast metallic strand from a continuous casting machine, the strand being formed in a mould and stirring of the melt being performed by means of at least one electromagnetic 10 stirrer.

Various methods are known for stirring the non-solidifed melt in a cast strand. For instance, it is already known for this purpose to employ one or more electromagnetic stirrers at one or more points 15 along the cast strand, which stirrer(s) produce a stirrihg effect either in the longitudinal or transverse direction of the cast strand. One problem with this type of stirring, when casting steel, is that if the melt is agitated too violently, there is formed in the 20 finished cast product what is known as a white band, i.e. a band with negative segregation. When steel solidifies, crystals, known as dendrites are formed, and concentrations of sulphur and carbon are obtained between these dendrites. With the hitherto 25 known methods of electromagnetic stirring, these concentrations are spread into the melt and modify the structure of the finished product which is manifested by the occurence of the afore-mentioned white band.

30 The present invention aims to provide a method of stirring the non-solidified melt in a cast metallic strand which avoids the problem mentioned above.

According to the invention a method of stirring the non-solidified melt in a cast metallic strand from a 35 continuous casting machine, the strand being formed in a mould, open at the bottom, and the stirring being performed by at least one electromagnetic stirrer arranged across the direction of casting, is characterised in that said at least one 40 stirrer is supplied with multiphase current of a frequency of from 0.5 to 4Hz, preferably of from 1 to 2 Hz, and with such effect that the stirring flow in the melt extends up into the mould, and in that the stirring is performed in such away that the coil(s) of 45 one phase of said at least one stirrer deliver(s) a stirring effect which is different from the stirring force delivered by the coil(s) of at least one other phase of the stirrer by an amount equivalent to supplying the coil(s) of one phase of a stirrer having 50 phase coils designed to deliver symmetrical stirring forces with a current which deviates by at least 10 per cent from the current required therein to deliver the symmetrical stirring forces.

By employing a frequency in the range of from 0.5 55 to 4 Hz, a satisfactory speed of flow in the melt upstream of the stirrer(s) is achieved without causing turbulence in the melt. The higher flow upstream of the stirrer(s) increases the life of the dendrite growth nuclei, and these nuclei are carried with the 60 melt flow from the hot region in the mould. This means that the globularzone in the centre of the strand increases noticeably in size as well as becoming more finely grained. The "white band" which appears in the cast product if stirring with a higher 65 frequency is employed, is no longer apparent to any significant extent, partly due to the almost total lack of turbulence during the stirring.

One way of achieving the desired asymmetrical stirring effect is to cause the phase current in one phase of said at least one stirrer to deviate at least 10 per cent from the phase current in another phase. If, on the other hand, said at least one stirrer has a symmetrical current supply, the desired asymmetrical stirring effect may be produced, for example, by arranging for one coil of the stirrer to have an ampere-turns numberwhich deviates at least 10per cent from the ampere-turns number of another coil of the same stirrer.

The invention will now be described, byway of example, with reference to the accompanying drawing, in which

Figure 1 is a schematic sectional view of part of a continuous casting machine employing the method according to the invention.

Figure 2 is a schematic side view of cast strands from a continuous casting machine illustrating the flow pattern in the non-solidified melt in the strand employing different frequencies, and

Figure 3 is a schematic cross-sectional view through cast strands from a continuous casting machine illustrating the flow pattern in the non-solidified melt in the strand in the region of a stirrer, again employing different stirring frequencies.

Figure 1 shows steel or iron melt being tapped at 4 into a cooled mould 3, open at the bottom, in which the cast strand 5 is moulded and gradually solidifies, first at the surface. Within the cast strand there is a sump 6 of non-solidified melt.

A short distance downstream of the mould 3 there is arranged a multiphase, electromagnetic stirrer 7, which may be single-sided, double-sided or cylindrical in relation to the cast strand 5. Support rollers 8 of non-magnetic material may possibly be placed between the stirrer and the cast strand, see Figure 3. The stirrer 7 is disposed transversely of the cast strand 5 to produce a stirring effect in the direction indicated by the arrow D in Figure 1, which results in a flow of melt in the sump 6 as indicated by the arrows.

The stirrer 7 is arranged to produce an asymmetrical stirring effect. For example, the surrent strength in one phase of the stirrer may deviate by at least 10 per cent from that in another phase. The asymmetrical stirring effect may also be obtained by adding ampere-turns to one phase or by changing the mutual inductance from phase to phase in order to obtain the same asymmetry as the 10 percent deviation in phase current in one phase in relation to another phase.

The left-hand side of Figure 2 shows the stirring effect produced when the stirrer is supplied with current and this will result in melting of the dendrites, the formation of fissures and "white band" and generally an unsatisfactory structure in the finished casting.

When the stirrer is operated with current in the frequency range of from 0.5 to 4 Hz, the flow pattern shown at the right-hand side of Figure 2 can be achieved, and by suitably adjusting the current to the stirrer the flow of melt in the sump 6 can be caused

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GB 2 092 464 A

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to flow up into the mould 3 as shown in Figure 1.

With this lower frequency, the speed of the travelling field will decrease proportionally, resulting in less turbulence. This means that the considerable return 5 flow seen in the left-hand side of Figure 2 just below the stirrer will disappear, or be considerably reduced, and the melt flow will extend up into the mould 3. This means that melt at almost liquidus temperature will flow from the sump 6 into the 10 mould 3 and therefore decrease the melt temperature in the mould. This means in turn that the many growth nuclei which are normally formed in the mould 3 are not melted down, but have a chance of surviving in orderto build up an equiaxial or 15 globular structure. The growth nuclei in the mould will be carried down into the sump 6 below the mould, and since the temperature here is at liquidus temperature or just below, these nuclei will survive and grow. This will contribute to breaking down the 20 fringe crystal zone and the achieving of a finder grained, equiaxial, globular structure in the finished casting. Since the flow alterations in the sump 6 occur extremely smoothly, there will be substantially no "white bands" or similar imperfections in the 25 casting structure. The growth nuclei which are carried down with the melt flow into the sump 6 will be deposited on the dendrite points, which assists in reducing the flow in the inter-dendrite region.

Acertain eddy formation underthe stirrer is 30 desired in orderto counteract segregation tendencies. This is obtained when the melt flow is deflected upwardly and downwardly in the sump 6.

The asymmetrical component of the stirring superimposed on the main symmetrical stirring effect 35 means that a transverse effect is obtained on the main stirring forces, which gives rise to an increase in turbulence in the sump 6. This asymmetrical stirring results in a decreased columnar zone in the finished casting, an increased globular zone, less 40 central segregation and no white bands, i.e. a final product of high quality.

in the specification of our co-pending Application No. 7911702, from which the present application is divided, there is claimed a method of stirring the 45 non-solidified melt in a cast metallic strand from a continuous casting machine which is similar in some respects to the method according to the present invention.

Claims

50 CLAIMS

1. A method of stirring the non-solidified melt in a cast metallic strand from a continuous casting machine, the strand being formed in a mould, open 55 at the bottom, and the stirring being performed by at least one electromagnetic stirrer arranged across the direction of casting, characterised in that said at least one stirrer is supplied with multiphase current of a frequency of from 0.5 to 4 Hz and with such effect 60 that the stirring flow in the melt extends up into the mould, and in that the stirring is performed in such a way that the coii(s) of one phase of said at least one stirrer deliver(s) a stirring force which is different from the stirring force delivered by the coil(s) of at 65 least one other phase of the stirrer by an amount equivalent to supplying the coil(s) of one phase of a stirrer having phase coils designed to deliver symmetrical stirring forces with a current which deviates by at leastper cent from the current required 70 therein to deliver the symmetrical stirring forces.

2. A method according to claim 1, in which the frequency of the current is from 1 to 2 Hz.

3. A cast metallic strand when produced by the method claimed in claim 1 or claim 2.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1982.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.