GB1575196A

GB1575196A - Continuous casting machine

Info

Publication number: GB1575196A
Application number: GB750178A
Authority: GB
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 1978-02-24
Filing date: 1978-02-24
Publication date: 1980-09-17

Description

(54) A CONTINUOUS CASTING MACHINE (71) We, SUMITOMO METAL INDUSTRIES LIMITED, a Japanese Corporation of 15, 5chome, Kitahama, Higashi-ku, Osaka City, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a continuous casting machine comprising a slab mould for the continuous shaping of a metal slab which is continuously withdrawn therefrom and a plurality of supporting metal rollers for supporting said slab along its direction of withdrawal and means for the agitation of unsolidified metal contained in the slab being cast by the mutual interaction of a current and magnetic field.

During the continuous casting of steel in the form of a slab, a segregation zone rich in carbon, sulphur and phosphorus is likely to be generated in the centre portion of the slab. The macroscopic structure of such a segregation zone may result in the slab having poor mechanical properties which will diminish its value. One way of reducing such an effect is to agitate unsolidified metal within the solidified surfaces of the slab during the continuous casting operation.

One method of effecting such agitation is to use a moving e.g. rotating magnetic field to provide a movement within the unsolidified metal in the same direction as the direction of movement of the magnetic field.

Another method is to use a stationary magnetic field and to pass a direct electrical current to the unsolidified molten metal, whereby the required movement is achieved in the unsolidified molten metal by the mutual action of this current and magnetic field.

According to the first method an agitating device must be set by removing a roller of a roller apron for the slab being cast the agitating device being provided with a special supporting means so as to prevent the slab from becoming curved due to pressure.

Such supporting means however make the whole structure complicated, and the supporting means must be rigid to prevent the bulging of the slab, since the pressure increases towards the lower part of the slab.

By this method therefore it is not practicable to provide more than one such agitating means by removing several rollers of the roller apron. It is further impossible to set the agitating means at a lower part of the slab being cast. Thus such a single agitation means must of necessity be constructed to exert a powerful agitating effect and since the agitation is effected only in one direction (the direction of the width of the slab), the agitated volume is liable to fluctuate in width.

The second method employs U-shaped magnets as the permanent magnet. Such Ushaped magnets are however impractical to set in the continuous casting machine because it is necessary to remove the guide rollers and thus the same disadvantage is present as in the first method. Furthermore in the second method the agitation will be effected in a large loop since the N poles and S poles of the magnets are on the same side in the width direction of the slab so that non-uniform width of agitation will occur.

The present invention is directed to an improvement in the arrangements previously used, whereby the N and S poles of magnets are arranged on opposite sides of the slab being cast so that the magnetic fields extend transversely of the path of travel of a slab being cast, enabling the agitated metal to move in small loops so that a substantially uniform agitation may be obtained.

The invention provides a continuous casting machine comprising a slab mould for the continuous shaping of a metal slab which is continuously withdrawn therefrom and a plurality of supporting metal rollers for supporting said slab along its direction of withdrawal and means for the agitation of unsolidified metal contained in the slab being cast by the mutual interaction of a current and magnetic field, said means comprising (a) a plurality of groups of permanent magnets disposed along the direction of travel of a slab being cast from the slab mould exit to a position where a slab being cast becomes completely solidified, each group of magnets comprising at least one pair of magnets the constituent magnets of which are disposed on opposite sides of a slab being cast the N-S axis of each magnet being orientated in the width direction of a slab being cast and are disposed, in relation to each other, so that the N pole of one magnet corresponds to the S pole of the other magnet of a pair so that a magnetic field extends transversely of the path of travel of a slab being cast, and (b) electrical brushes contacting at least two of the said rollers and connectable to a source of direct electrical current whereby a flow of direct current may flow longitudinally within a slab being cast.

in a particular embodiment of the present invention, the N-S direction of poles of each magnet on the same side of a slab being cast corresponds to the S-N direction of the next adjacent magnet in the direction of travel of a slab being cast. Preferably pairs of magnets are provided in each group thereof, extending along the width direction of a slab being cast. Preferably the NS direction of each magnet on the same side of a slab being cast corresponds to the S-N direction of an adjacent magnet of the group.

In particular the groups of permanent magnets may each be located between adjacent supporting rollers, so that the rollers of an existing machine need not be disturbed, ensuring a multi-step agitation resulting in a more uniform internal structure bf a cast slab.

The invention is hereinafter particularly described and illustrated in the accompanying drawings, of which Figure 1 is a perspective view showing the slab casting mould of a circular arc type continuously casting machine and associated support rollers, with one embodiment of agitating means according to the present invention; Figure 2A is a cross-sectional view of one embodiment of a set of corresponding groups of magnets according to the invention disposed on opposite sides of a slab casting; Figure 2B is a cross-sectional view of another embodiment of a set of corresponding groups of magnets according to the invention; Figure 2C is a schematic perspective view showing one arrangement of magnets in adjacent groups according to the invention; Figures 3A to 3C are side views of various embodiments of the invention showing the path of direct electrical current in a slab casting; Figure 4 is a schematic representation showing relationships between a magnetic flux density, direct current and electromagnetic force; Figure 5 is a schematic plan view showing the convection state of unsolidified molten metal within a slab casting; Figure 6 is a graph showing phosphorus segragation across a slab casting; Figure 7 is a side view of one embodiment of a supporting roller and associated electrical brush according to the invention Figures 8A to 8D are views showing embodiments of the configuration and arrangements of groups of magnets according to the invention, and Figure 9 is a schematic plan view showing the convention flow of unsolidified molten metal within a slab casting.

Referring to Figure 1 of the drawings, in a circular arc type continuously casting machine, 1 is a casting mould, 2 is a slab which is to be withdrawn in the direction of arrow D and 3 is one support roller of a set of support rollers. Groups of permanent magnets 4 are each located between adjacent support rollers.

As shown in Figure 2A or 2B, the sets of magnets in magnet groups 4 are so disposed that their N poles and S poles are opposed to each other, and lie adjacent to the surfaces of a slab on a long side 21 thereof, and the magnetic poles of permanent magnets correspondingly disposed on the opposite long side are of opposite to corresponding poles of magnets on the other long side. The simplest arrangement of magnet groups is shown in Figure 2B in which arrangement the agitated flow will describe a comparatively large loop.

The use of a single agitation zone however is liable to cause the solidified volume itself to solidify to produce a segregation zone called a "white band". The development of such negative composition segregation zones may be inhibited by dividing the agitation into separate parts. Thus in applying the present invention it is desirable to use a plurality of permanent magnets in each group of magnets, for instance as illustrated in Figure 2A, and more preferably a plurality of magnets are provided in each group to cover the entire width direction of a slab being cast. With such a plurality of magnets in each group, the poles preferably alternate in polarity along the width direction. In a particularly preferred embodiment, the poles of any one of said groups of magnets are together orientated in reverse direction to the poles of corresponding magnets in an adjacent group of magnets disposed along the direction of travel of a slab being cast as illustrated in Figure 2C.

As illustrated in Figures 1, 3A and 7, supporting rollers 7 above the uppermost group of magnets and supporting rollers 8 below the lowermost group of magnets are each provided with brushes 6 and 9 respectively, connected to a source of direct electrical current by lines 10 so that when such a current is passed it flows to the rollers 8 and brushes 9 through the unsolidified molten metal of the slab 2 from the brushes 6 and rollers 7, as shown in Figure 7. A spring 30 is provided at the rear of the brush 9 to maintain the contact of the brush with the roller. To prevent the current from leaking from one set of supporting rollers to another, the respective rollers are insulated from the continuously casting machine body at their bearings.

Other embodiments for the passage of direct currents are ilustrated in Figures 3B and 3C.

In these embodiments, if only the sets of supporting rolls between the sets of rolls 81 and 81l are insulated from the continuously casting machine, the current led into the slab from roll 71 or 711 will all pass through the part of the slab in which the magnetic field acts, with a reduced requirement for insulating parts.

The permanent magnets used may have residual magnetic flux density Br of 5 to 10keg and a coercive force Hc of 5 to 10 kOe. It is preferred that a magnet having a large maximum energy product (BrHc) max is used. Suitable and preferred magnets are rare earth metal, for instance YCo1, CeCoS, Pro6, SmCoS or SmPrCo5 magnets.

As shown in Figures 8A to 8D, the magnet may be of such shape as can be contained in the clearance between adjacent supporting rolls and two or more permanent magnets 4 may be fixed at suitable intervals inside a yoke 11 of a length substantially equal to the width of the slab 2. The yoke 11 is provided on the back surface with supporting arms 12 to be fixed to and supported by the casting machine. As shown in Figure 8D, in order to prevent a damage by convection heat from the slab, the permanent magnet may be used having a covering member 13 made of nonmagnetic substances for example 18-8 stainless steel. and provided with water passages 14 and 15 for cooling.

During continuous casting with arrangements according to the invention, a magnetic field 5 whose main direction is perpendicular to the casting direction due to the permanent magnets 4 opposed to each other will act on the slab 2 in the course of solidification in the roller apron part after emerging from the casting mould 1. The passage of the direct current in the casting direction interacts with the magnetic field 5 to provide, as indicated in Figure 4, an electromagnetic force F in the width direction of the slab at a right angle respectively to the direct current J, and the magnetic flux density B of the magnetic field 5 will act on the unsolidified molten metal within the slab being cast, and the electromagnetic force F will act on the unsolidified molten steel within the slab to provide an agitation as shown in Figure 9. Equiaxed crystals will be formed within the slab and the centre segragation will be reduced. Figure 9 illustrates the large magnetic loop produced by a magnet group consisting of a single magnet (Figure 2B) and to achieve sufficient agitation to provide equiaxed crystals, it will be necessary to effect a strong agitation. As hereinbefore described it is preferable to provide a plurality of magnets, particularly extending to total width of the slab. In such an arrangement the electromagnetic forces F1 and F2 of two sets of permanent magnets arranged above and below are formed so as to act in directions reverse to each other, as shown in Figure 5, and a partly rotating thrust f will be produced between them. By this thrust f the unsolidified molten metal may form a plurality of small loops in the casting direction which are convective to produce effective agitation.

The following practical Example of the invention is provided.

3 charges of a low carbon aluminiumsilicon killed steel composition 0.16% carbon, 0.3% silicon, 1.45% manganese, 0.018% phosphorus and 0.013% sulphur, balance iron, after being continuously refined in a 160-ton converter, were continuously cast in a circular arc type slab continuous caster with two slab moulds, at a teeming temperature of 1540"C. and casting speed of 0.8m/min, to form 240 tons of each slab of cross-section l90mmX 1600mm.

The first slab was solidified while being agitated in the unsolidified part using an arrangement of the type as shown in Figures 1 to 3, by passing a direct current of voltage of 20V and current 5500 Amps through the casting and using permanent magnets (SmCo5) of magnetic flux density of 1KG in the centre of the slab width in 4 groups separated by 450, 475, 530 and 560cm from the surface of the casting mould and setting current passing brushes on the rollers 10th and 15th from above the roller apron. The other slab was solidified in the ordinary manner without agitation.

Test pieces were cut out of slabs 20, 50 and 80 m after the beginning of the casting and sulphur prints of the cross-sectional areas and the composition distributions in the thickness directions of the slabs were taken and also the segregation states in the centre portions of the slabs were inspected.

The results are shown in the graph of Figure 6, which shows the distribution of phosphorous across the thickness of the slabs.

Curve b is a distribution curve of the sample of the first slab subjected to the present invention. Curve a is a distributing curve of the sample of the second slab not subjected to agitation. It is to be seen that a large phosphorous segregation was present in the centre portion of the slab not subjected to agitation, whereas substantially no segregation was present in the slab agitated according to the present invention.

WHAT WE CLAIM IS:- 1. A continuous casting machine comprising a slab mould for the continuous shap- ing of a metal slab which is continuously withdrawn therefrom and a plurality of supporting metal rollers for supporting said slab along its direction of withdrawal and means for the agitation of unsolidified metal contained in the slab being cast by the mutual interaction of a current and magnetic field, said means comprising (a) a plurality of groups of permanent magnets disposed along the direction of travel of a slab being cast from the slab mould exit to a position where a slab being cast becomes completely solidified, each group of magnets comprising at least one pair of magnets the constituent magnets of which are disposed on opposite sides of a slab being cast the N-S axis of each magnet being orientated in the width direction of a slab being cast and are disposed, in relation to each other, so that the N pole of one magnet corresponds to the S pole of the other magnet of a pair so that a magnetic field extends transversely of the path of travel of a slab being cast, and (b) electrical brushes contacting at least two of the said rollers and connectable to a source of direct electrical current whereby a flow of direct current may flow longitudinally within a slab being cast.

2. A machine as claimed in Claim 1, wherein the N-S direction of poles of each magnet on the same side of a slab being cast corresponds to the SN direction of the next adjacent magnet in the direction of travel of a slab being cast.

3. A machine as claimed in Claim 1 or Claim 2, wherein a plurality of pairs of magnets are provided in each group thereof, extending along the width direction of a slab being cast.

4. A machine as claimed in Claim 3, wherein the N-S direction of each magnet on the same side of a slab being cast corresponds to the S-N direction of an adjacent magnet of the group.

5. A machine as claimed in any of Claims 1 to 4, wherein the said groups of magnets are each located between adjacent said supporting rollers.

6. A continuous casting machine as claimed in Claim 1, as hereinbefore described with particular reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. areas and the composition distributions in the thickness directions of the slabs were taken and also the segregation states in the centre portions of the slabs were inspected. The results are shown in the graph of Figure 6, which shows the distribution of phosphorous across the thickness of the slabs. Curve b is a distribution curve of the sample of the first slab subjected to the present invention. Curve a is a distributing curve of the sample of the second slab not subjected to agitation. It is to be seen that a large phosphorous segregation was present in the centre portion of the slab not subjected to agitation, whereas substantially no segregation was present in the slab agitated according to the present invention. WHAT WE CLAIM IS:-

1. A continuous casting machine comprising a slab mould for the continuous shap- ing of a metal slab which is continuously withdrawn therefrom and a plurality of supporting metal rollers for supporting said slab along its direction of withdrawal and means for the agitation of unsolidified metal contained in the slab being cast by the mutual interaction of a current and magnetic field, said means comprising (a) a plurality of groups of permanent magnets disposed along the direction of travel of a slab being cast from the slab mould exit to a position where a slab being cast becomes completely solidified, each group of magnets comprising at least one pair of magnets the constituent magnets of which are disposed on opposite sides of a slab being cast the N-S axis of each magnet being orientated in the width direction of a slab being cast and are disposed, in relation to each other, so that the N pole of one magnet corresponds to the S pole of the other magnet of a pair so that a magnetic field extends transversely of the path of travel of a slab being cast, and (b) electrical brushes contacting at least two of the said rollers and connectable to a source of direct electrical current whereby a flow of direct current may flow longitudinally within a slab being cast.