This invention relates to the continuous casting of

steel, and more particularly to electromagnetic stirring

of the still-molten and/or solidifying steel after pouring

into a mould.

It has been found desirable to provide such stirring

to many grades of cast steel for a number of reasons

perhaps the chief of which in general is to achieve

homogeneity of at least the major portion of the cross-

section of the steel strand, and to remove non-metallic inclusion and included gases.

It is an object of the present invention to provide an

improved electromagnetic stirring arrangement for use .in the

continuous casting of steel.

According to one aspect of the present invention there

is provided a method of continuously downwardly casting

steel, including pouring the molten steel into a continuous

casting mould and subjecting the still molten and/or

solidifying steel after pouring to a helical electromagnetic

force field. Such force can be upwardly helical

radially outer part of the molten steel spirals upwards a the radially central portion spirals downwards relative t the metal strand, and conversely for a downward helical force.

5 According to another aspect of the present invention

there is provided apparatus for the continuous downward

casting of steel including a continuouscasting mould and

electromagnetic stirring means located about the metal

strand path adjacent to the mould, the electromagnetic

10 stirring means being adapted to subject still molten and/

solidifying steel to a helical force. Such helical force can be upwards or downwards.

The helical stirring movement of the molten steel of

the present invention has useful application with regard

■ _j _5 any continuous cast steel, and its value can be exampled the continuous casting of aluminium treated steel. Two

noteworthy problems in the casting of such steel are the

need to avoid retaining inclusions of alumina in the stee as it is cast, and the need to ensure an outer surface la

20 around the strand free from included gas and entrapped sl

Previously proposed arrangements for horizontal,

rotational, stirring of the steel suffer from the dis¬

advantages, especially in the context of the above-mentio

aluminium treated steel, that inclusions tend to be trapp

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and retained below the surface of the steel by rotating

stirring action. In addition, with the commonly used

moulds of rectangular plan form, the rotational movement

of stirring tends to create turbulence and/or standing

waves at the mould corners which severely hinders the

removal of inclusions and produces undesirable surface

ripples on the strand surface. It is also desirable with

this form of stirring to ensure that the stirring force

penetrates to the central portions of the steel for

adequate stirring throughout, which imposes restrictions

on power supply frequencies.

Again previously proposed arrangements for a vertical

stirring of the steel suffer from the disadvantage that

considerable energy is required to reverse the movement of

the molten steel at the top and bottom of the movement

pattern, so that excessive power is required for the

electromagnetic arrangement to achieve satisfactory stirring movement.

The helical stirring of the present invention enables

all of these disadvantages to be overcome.

The electromagnetic stirring may be provided by means

of a plurality of electromagnetic coil assemblies located

about the steel strand path, each being directed towards

the strand path. The coil assemblies may be powered from a

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multi phased (e.g. three phased) electricity supply.

The coil assemblies may be arranged in a series of

rings about the steel strand path, adjacent assemblies i

successive rings being phase shifted to provide the over

helical force field. The amount of phase shift between successive rings may vary axially of the arrangement so as to provide a varied angle helical field.

Thus in one embodiment the phase shift may be much greater at the upper portions of the stirring means,

thereby providing a much more -rotational force at the upper and a much more vertical force at the lower end of

the steel being stirred. Such an arrangement has been found especially well adapted at avoiding the dis¬

advantages of prior arrangements whilst combining their

advantages. The series of rings may be capable of

relative rotational adjustment so as to adjust the angle

of the helical field. Alternatively or in addition the

coil assemblies in successive rings may be connected for

switching phase sequences to provide a phase shift to

adjust the angle of the helical force field.

Alternatively a shaded pole winding arrangement or

skewed winding arrangement can be used for providing the

helical force field.

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The electromagnetic stirring means may be disposed'

about the casting mould itself, or alternatively or

additionally may be provided about the strand path below

the mould.

We believe the arrangement of the invention is

particularly useful in casting steel of square sections of

less than about 200mm. width.

The helical stirring will be greatly facilitated if

the electromagnetic body force can be made small on or near

the strand axis by choosing an electromagnetic field con¬

figuration with a minimum of 2 pole pairs circumferentially,

and the smallest pole pitch axially commensurate with

achieving optimum performance.

With strands in the size range 150-200mm. (square

section widths) the relative value of input electrical

energy is preferably of the order, between circumferential

and axial of 0.2. This ratio preferably decreases for

smaller sizes of strand and increases for larger sizes.

In order that the invention may be more readily

understood, one embodiment thereof will now be described

by way of example with reference to the accompanying

drawing in which:-

Figure 1 is a diagrammatic sectional elevation of

continuous steel casting apparatus according to the

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invention; and

Figure 2 is a section of the line II - II of

Figure 1.

Referring to the drawings, 1 represents a tundish or similar vessel containing molten steel, located above and pouring steel 2 into a continuous casting mould 3.

As will be clearly seen from Figure 2, the mould 3 is

shown, for ease of illustration, as being of circular plan. In practice, however, the mould would often be of

a square or rectangular section. The mould is provided

in the usual way with a water cooling annulus 10.

In the usual manner, the steel 2 is withdrawn from

the mould as a solidifying strand 4 for subsequent

processing.

Located about the mould with annular end plates 8 are six sets of annular stator blocks 7 arranged one abov

the other, each block comprising six coil assemblies 5 disposed equi-angularly about the mould 3, each assembly 5 being provided with electrical windings (not shown).

With a three-phase supply it will be appreciated that the

six assemblies 5 in each block 7 may be connected to give

a two pole or a four pole arrangement.

Water cooling may be provided for the electromagnetic

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- - stirring means by means of interconnected water channels 6

disposed between and around assemblies 5. Water inlets 11 and outlets 12 are provided. Alternatively, water cooling

may be provided by means of hollow windings for the

5 assemblies 5 through which water is forced.

By means of suitable phase shifting between successive

stator blocks 7, by electrical connecting and/or by

mechanical alignment, a helical force field may be produced

having a predetermined upward angle within the mould. Since

0 the field is strongest adjacent the outer wall of the mould

the field will have the effect of creating an upward

helical stirring of metal around the radially outer portions

of the liquid metal, and a returning downward helical in the

radially inner portions of the liquid metal. m c- It will be understood that any required length of

stirring arrangement can be provided by using the required number of coil assemblies 5.

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