BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to methods and apparatus for carrying out strip casting of metals. The present invention has particular applicability to twin roll casting, but may also be applied to other continuous or quasi-continuous casting processes, such as belt casting, block casting and DC (direct chill) casting.

Related art

The present inventors have proposed the use of an electromagnetic edge dam for aluminium twin roll casting. Relevant discussion of this is set out in the reference McBrien and Allwood (2013).

Twin roll casting involves feeding liquid metal between two counter-rotating chilled rolls, where the metal solidifies and forms a sheet of uniform thickness and width. The liquid metal is commonly confined in a fixed ceramic feed system with mechanical 'edge dams' setting sheet width. These must be replaced after each cast, or when sheets of different widths are to be cast.

It is desirable to limit the yield loss of metal between the casting process and the formation of the final product.

McBrien and Allwood (2013) proposed a moveable electromagnetic (EM) edge dam to be used in a twin roll casting process. The non-contact nature of EM containment compared to known mechanical solutions implies a longer casting time may be achieved, while the geometry of the EM edge dam was designed so that the width of the coil may be changed by a simple displacement of the edge dam during casting.

SUMMARY OF THE INVENTION

The present inventors consider that the known methods of strip casting, such as twin roll casting, could be further improved. In particular, in a first development of the invention, they consider that the ability more closely to control the cross sectional form (i.e. the cross sectional shape and/or the cross sectional area) of the solidified metal strip would have significant commercial implications, allowing the casting of strip of a shape which is closer to a desired final shape than the known approaches. This in turn would allow less of the cast strip to be wasted when trimming the cast strip to the desired final shape. The first development of the present invention has been devised in order to address the fact that the known approaches do not provide a satisfactory solution to this problem.

Preferably, the present invention reduces, ameliorates, avoids or overcomes this problem.

In a general aspect, the first development of the invention provides control of the molten metal pressure in a molten metal feed during strip casting in coordination with movement of a dam in order to vary the cross sectional form of the molten metal feed to the rollers and thus the cross sectional form of the solidified metal strip.

Accordingly, in a first preferred aspect, the first development of the present invention provides a continuous casting apparatus for casting a metal strip with a cross sectional form which varies along the length of the strip, the continuous casting apparatus comprising:

opposing cooling means;

a molten metal feed system positionable to provide a molten metal feed for solidification between the opposing cooling means to form a solidified metal strip along a length direction;

a form adjustment system comprising at least one dam to determine, at least in part, the cross sectional form of the molten metal feed to the opposing cooling means, and thereby to determine the cross sectional form of the solidified metal strip, wherein the dam is moveable during operation of the apparatus to vary the cross sectional form of the molten metal feed to the opposing cooling means,

the continuous casting apparatus further comprising:

a molten metal pressure control system, operable to control the pressure of the molten metal in the molten metal feed during operation of the apparatus in coordination with movement of the dam.

In a second preferred aspect, the first development of the present invention provides a continuous casting method for casting a metal strip with a cross sectional form which varies along the length of the strip, the method comprising:

providing a molten metal feed for solidification between two opposing cooling means to form a solidified metal strip along a length direction;

operating a form adjustment system comprising at least one dam to determine, at least in part, the cross sectional form of the molten metal feed to the opposing cooling means, and thereby to affect the cross sectional form of the solidified metal strip, wherein the dam is moved during operation of the device to vary the cross sectional form of the molten metal feed to the opposing cooling means,

operating a molten metal pressure control system to control the pressure of the molten metal in the molten metal feed during casting in coordination with movement of the dam.

The first and/or second aspect of the first development of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.

Preferably, the present invention is used in twin roll casting. In this case, the opposed cooling means are rolls. Twin roll casting is particularly well suited, because the

downstream deformation (i.e. the deformation applied to the strip, subsequent to twin roll casting, in order to manufacture a final product) is typically relatively small. Therefore subsequent rolling of the strip is not usually carried out, or only carried out to a minor extent. This means that the irregular cross sectional form of the strip is not significantly elongated.

Alternatively, the present invention may be applied to other continuous or quasi-continuous casting processes, such as belt casting, block casting and DC (direct chill) casting.

The cross sectional form of the cast strip includes the cross sectional shape and/or the cross sectional area of the cast strip. In this art, the term "cross sectional profile" is typically reserved to describe variations in the thickness of the strip across its width. This terms is therefore included within the scope of the term "cross sectional shape".

Preferred embodiments of the invention can therefore be used to change, during casting of a continuous strip, the cross sectional form of the strip, for example by increasing the width of the strip and/or decreasing the width of the strip and/or including holes in the strip. It is intended that the variation in the cross sectional form of the strip does not consist only of uniform variations (across the width of the strip) of the thickness of the strip. Such variations in thickness may be achieved, for example, by changing the spacing and velocity of the rolls and/or solidification length along the rolls during a casting run.

It is preferred that the width of the strip is at least 500mm, more preferably at least 1000mm. The width of the strip is typically not greater than 2000mm. The thickness of the strip is preferably at least 1 mm, more preferably at least 2mm. The thickness of the strip may be up to 10mm. There is no particular restriction on the length of the strip. In practice, the maximum length of the strip is dictated by the available metal to be cast and the ability of the manufacturer to handle the cast strip, e.g. by taking the cast strip onto a coiler.

Preferably, the dam is an AC electromagnetic field dam provided by at least one electromagnet. Preferably the electromagnetic field dam is operated at a frequency of at least 0.5kHz. More preferably, the electromagnetic field dam is operated at a frequency of at least 1kHz. The electromagnetic field dam may be operated at a frequency of up to 100kHz. More preferably, the electromagnetic field dam is operated at a frequency of up to 50kHz, or up to 30kHz.

Preferably, the electromagnetic field dam is operable to provide a magnetic field strength (magnetic flux density) of at least 25mT within the molten metal feed.

Preferably, the electromagnet which provides the electromagnetic field dam is operable to provide at least 1000At (ampere-turns) of magnetomotive force.

The electromagnet preferably has a flux concentrator and current-carrying windings, in a known manner. The flux concentrator preferably has a horseshoe-shape, or C-shape, with a gap provided in order to fit the flux concentrator around the feed tip. The shape of the flux concentrator is adapted to conform with the shape of the rolls near the feed tip, which is discussed below. The electromagnet is preferably oriented so that the arms of the horseshoe-shape, or C-shape, meet behind the feed tip, along the longitudinal direction of the cast strip. This allows the dam to be moved within a wide range along the feed tip, transversely to the direction of casting.

The molten metal feed system typically includes a feed tip. This typically conveys the molten metal to the opposed cooling means. There may be provided a reservoir of molten metal. This may be in fluid communication with the feed tip via a conduit. The reservoir, conduit and/or feed tip may be provided with suitable heating and/or insulation in order to maintain the molten metal at a desired temperature before solidification.

Ignoring losses in the conduit and feed tip (which is appropriate particularly in the case of the relatively small flow rate in twin roll casting for example), the static pressure of molten metal in the feed tip is substantially the same as the static pressure of molten metal in the reservoir at the same height as the feed tip. The pressure of molten metal at the feed tip can therefore be controlled by controlling the pressure of molten metal in the reservoir. Conveniently, this can be done by controlling the level of molten metal in the reservoir. One way to do this would be to raise or lower the reservoir with respect to the feed tip. However, this would require a flexible conduit, and so this is not particularly preferred. A more preferred option is to displace the molten metal in the reservoir, in order to control the position of the level of the molten metal in the reservoir compared with the feed tip.

A particularly preferred arrangement has a displacement body arranged to be pushed into the reservoir. A suitable displacement body is sized and shaped to as to fit into the reservoir to leave a suitable space for the molten metal in the reservoir. A suitable displacement body is insulated and/or actively heated in order to limit its effect on cooling the molten metal. Pushing the displacement body into the reservoir displaces the molten metal, thereby changing the level of the molten metal in the reservoir. In turn, this adjusts the static pressure of the molten metal in the reservoir and in the feed tip.

An advantage of using a displacement body to control the pressure compared with, for example, restricting the flow of molten metal along the conduit, is that it is possible to achieve fast and precise adjustments of the molten metal pressure.

Preferably, when the dam is moved so as to increase the width of the strip, the molten metal pressure is increased. It is considered that this provides an advantage by more quickly filling the space in the feed tip which previously was occluded by the dam. This allows a faster and more certain increase in width of the strip.

Preferably, once the dam has been moved to increase the width of the strip and the width has increased to the desired amount, the molten metal pressure is reduced. For example, the molten metal pressure may be reduced to a level corresponding to the level used before the width of the strip was increased.

Preferably, when the dam is moved so as to decrease the width of the strip, the molten metal pressure is decreased. It is considered that this provides an advantage by reducing the force acting against reduction of the strip width. This allows a faster and more certain decrease in width of the strip.

Preferably, once the dam has been moved to decrease the width of the strip and the width has decreased to the desired amount, the molten metal pressure is increased. For example, the molten metal pressure may be increased to a level corresponding to the level used before the width of the strip was decreased.

In this way, it is preferred that the molten metal pressure control system is used to adjust the molten metal pressure during movement of the dam, so as to increase the speed of reliable change of the cross sectional form of the strip.

Furthermore, the molten metal pressure control system can be used to maintain substantially constant molten metal pressure where it is required to maintain a substantially constant cross sectional form of the strip, e.g. constant width.

Preferably, the method allows substantially a step change in the cross sectional form of the strip. For example, the method may allow the width of the strip to be changed by at least 10% over a distance of 30cm along the longitudinal (casting) direction of the strip. In some embodiments, the method allows steeper width changes to be achieved. For example, it is possible to achieve a change of width of the strip of at least 0% over a distance of 10mm or less along the longitudinal (casting) direction of the strip. Even greater changes of width can be achieved. For example, it is possible to achieve a change of width of the strip of up to 50% over a distance of 10mm or less along the longitudinal (casting) direction of the strip. In this case, the absolute change in width is from 130mm to 65mm. Here, the edge dam moves at a speed of about 100mm/s, which

is much greater than demonstrated by Smith et al. (2004) with a mechanical dam (1.5mm/s).

The dam may be moveable in the sense, for example, of an electromagnet being moveable transversely along the feed tip. However, it is possible to provide an array of at least two dams at different positions which are capable of being selectively switched into operation. The effect of switching from one dam to the other has the effect of moving the damming position. Thus, this is equivalent to moving the dam. There may be provided an array of more than two dams, for example, three, four, five, six or more. The actual positions of the dams may be fixed with respect to the feed tip, but selectively switching the dams on and off provides an effectively moveable dam. Preferably, these dams are EM dams.

The dam may be an edge dam, in the sense that control of the edge dam controls the position of the edge of the cast strip. There may be provided edge dams at opposing sides of the cast strip.

However, it is not necessarily essential for the dam to be an edge dam. This is because the inventors have realised that the operation of a dam with flow of molten metal on each side of the dam makes the dam act as a diverter, diverting the flow of motel metal out of a particular region in which the diverter acts. Where the diverter is not at the external edge of the molten metal flow, operation of the diverter can cause the formation of an opening in the cast strip. Movement of the diverter can cause a corresponding change in the shape of the opening as casting continues. Further movement of the diverter, and/or deactivation of the diverter can close up the opening.

The diverter can be an EM diverter, having a structure and operational capabilities as describe above in relation to the EM dam. There may be provided one or more moveable diverters. Alternatively there may be provided an array of two or more static diverters which can be switched into and out of operation as for the array of static dams described above.

The inventors have realised that pressure control may, however, not be necessary when using a diverter (although it may be preferred). Accordingly, in a second development of the invention, the inventors have considered further possible improvements that could be made to strip casting. They have realised that it is possible to affect the cross sectional form of the strip not only in terms of the positions of the edges of the strip, but also in terms of locating holes in the strip. "Holes" here can be voids through the thickness of the strip that are enclosed or partially open. In preferred embodiments, they are enclosed.

Controlling the cross sectional form of the strip in this way has advantages in the sense of reducing wastage when the desired product includes a hole in the strip. Therefore, again, control of the cross sectional form (i.e. the cross sectional shape and/or the cross sectional area) of the solidified metal strip has significant commercial implications, allowing the casting of strip of a shape which is closer to a desired final shape than the known approaches. This in turn allows less of the cast strip to be wasted when trimming the cast strip to the desired final shape. The second development of the present invention has been devised in order to address this problem. Preferably, the present invention reduces, ameliorates, avoids or overcomes this problem.

In a general aspect, the second development of the invention provides operation of a diverter to part the molten metal feed laterally in order to vary the cross sectional form of the molten metal feed to the rollers and thus the cross sectional form of the solidified metal strip, thereby providing at least one hole in the solidified metal strip.

Accordingly, in a first preferred aspect, the second development of the present invention provides a continuous casting apparatus for casting a metal strip with a cross sectional form which varies along the length of the strip, the continuous casting apparatus comprising:

opposing cooling means;

a molten metal feed system positionable to provide a molten metal feed for solidification between the opposing cooling means to form a solidified metal strip along a length direction;

a form adjustment system comprising at least one diverter, wherein the diverter is operational to part the molten metal feed laterally in order to vary the cross sectional form of the molten metal feed to the rollers and thus the cross sectional form of the solidified metal strip, thereby providing at least one hole in the solidified metal strip.

In a second preferred aspect, the second development of the present invention provides a continuous casting method for casting a metal strip with a cross sectional form which varies along the length of the strip, the method comprising:

providing a molten metal feed for solidification between two opposing cooling means to form a solidified metal strip along a length direction;

providing a form adjustment system comprising at least one diverter, and operating the diverter to part the molten metal feed laterally in order to vary the cross sectional form of the molten metal feed to the rollers and thus the cross sectional form of the solidified metal strip, thereby providing at least one hole in the solidified metal strip.

The first and/or second aspect of the second development of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features, and/or any one or, to the extent that they are compatible, any combination of the optional features set out with respect to the first development.

In particular, preferred features of the dam set out with respect to the first development may be applied to the diverter of the second development. For example, the diverter is preferably an electromagnetic diverter. It may be moveable. More than one may be provided, in order to generated the required variation in cross sectional form for the cast strip. An array of two or more diverters may be provided. These may be static, the required variation in cross sectional form for the cast strip being provided by suitable control of diverters in the array.

Optionally, there is provided a molten metal pressure control system, operable to control the pressure of the molten metal in the molten metal feed during operation of the apparatus in coordination with operation of the diverter. Where the diverter is operated to divert molten metal away from a particular region, the diversion may be assisted by a corresponding reduction in static pressure of the molten metal in the feed system. This is advantageous when the overall cross sectional area of the strip is reduced by the operation of the diverter. Similarly, when the diverter is switched off or otherwise operated to increase the overall cross sectional area of the strip, increasing the static pressure of the molten metal in the feed system can assist in filling the required area. These changes in molten metal pressure can be achieved as set out above in relation to the first development.

Unlike an edge dam, it is intended that the diverter operates to allow a flow of molten metal on each transverse side. It is therefore necessary to consider how the molten metal should reach each side. It is possible to provide more than one feed conduit from the molten metal reservoir. A first feed conduit may supply molten metal to one transverse side of the diverter and a second feed conduit may supply molten metal to the other transverse side of the diverter. Where more than one diverter is provided, there may be provided corresponding feed conduits for each transverse side of each diverter.

Where the diverter is moveable, providing an array of feed conduits corresponding to each possible position of the diverter may be impractical. In this case, at least one bypass conduit may be provided. The bypass conduit may be operational to allow molten metal to reach a transverse side of the diverter distal from a main feed conduit to the feed tip.

In the case of an EM diverter, the bypass conduit may be a conduit which substantially shields molten metal within the bypass conduit from the EM field. For example, the bypass conduit may be a conduit formed within the feed tip. The bypass conduit may be formed of an electrically conductive material, e.g. a metal such as a refractory metal.

The present inventors have recognised that the operation of a diverter to divert molten metal flow may present a more significant challenge compared with the operation of an edge dam. This is due to the diverter having to push molten metal out of the required location within the body of the molten metal flow, rather than at an edge position. There may therefore be provided one or more diverter assistance features. These may be provided, for example, within the feed tip. They may have a fixed position. In the case of an EM diverter, a suitable diverter assistance feature is a structural feature which allows the EM field generated by the EM diverter to be concentrated in the feed tip. Typically, the concentration of the EM field is coincident with the position of the diverter assistance feature. The effect of this is that as the EM field generated by the EM diverter grows, the EM field is concentrated at the diverter assistance feature and a void is nucleated in the molten metal. This void grows due to the concentration of the EM field in the void, diverting the molten metal and forming an opening.

A suitable diverter assistance feature is a structural feature which reduces or blocks the flow of molten metal at that feature, but allows the EM field to penetrate more easily than the EM field can penetrate the molten metal. For example, a diverter assistance feature can be provided by a projection of a non-ferromagnetic material (e.g. a non-electrically conductive material such as a ceramic) within the feed tip. A suitable projection can project forwardly from a rear internal face of the feed tip. Additionally or alternatively, a suitable projection can project upwardly or downwardly from an internal face of the feed tip corresponding to a major surface of the cast strip.

Once a suitable opening is formed in the molten metal, the diverter can be controlled (e.g. moved) to control the shape of the hole.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a schematic cross sectional view of twin roll caster showing the area of solidification.

Fig. 2 shows a schematic partial cross sectional view of the EM edge dam arrangement of Whittington et al (1998).

Fig. 3 shows a schematic side view of the EM edge dam arrangement of McBrien and Allwood (2013).

Fig. 4 shows a schematic perspective partial cross sectional view of the EM edge dam arrangement of McBrien and Allwood (2013).

Fig. 5 shows a schematic perspective view of the experimental arrangement used in McBrien and Allwood (2013) to determine the static pressure of molten aluminium retained by the EM edge dam.

Fig. 6 shows a schematic perspective view of an experimental arrangement used in the present work, showing the feed system and the EM edge dam.

Fig. 7 shows a circuit diagram to illustrate a parallel-resonant combination of inductor

(the EM edge dam) and capacitors that magnify the signal from a signal generator used in a power supply to provide the current to an EM edge dam used in the present work.

Fig. 8 shows a graphical comparison of magnetic field measurements from earlier low frequency tests and the power supply used in the present work (1700At applied to EM edge dam for all frequencies). FEA stands for finite element analysis.

Fig. 9 shows a view of a cast strip and the effect on the width when the EM dam is switched on.

Fig. 10 shows a stiffness plot of the EM edge dam when stationary, including lines of best fit for the data.

Fig. 1 1 shows a view of a cast strip and the effect on the width when the EM dam is switched on and off repeatedly.

Fig. 12 shows measurements of cast strip width with moving EM edge dam and variable pressure head, compared with a target width.

Fig. 13 shows the spread of ultimate tensile strength and elongation for tensile test specimens with and without EM edge dam (tested according to ASTM B557-06, 1" gauge length, 0.5mm/min).

Fig. 14 shows the hardness variation across the width of normal and EM cast strip (tested according to ASTM E92-92 with a 10kg load).

Fig. 15 shows through-thickness micrographs of cast strip: (a) longitudinal view, centreline of strip, no EM edge dam (EMED) (b) longitudinal view, centreline of strip, EMED on (c) transverse view, edge of strip, no EMED (d) transverse view, EMED on, near edge (e) transverse view, EMED on, far edge.

Fig. 16 shows the measured width and thickness for cast strips produced in successful (green) and unsuccessful (red) uses of the EM edge dam

Fig. 17 shows a simplified 2D slice for modelling balance of forces during casting.

Fig. 18 shows the contribution of surface tension to containment for the model of Fig. 17. Fig. 19 shows views of cast strips and the step response of width to stationary EM edge dam input.

Fig. 20 shows a plot of the FEA calculation of the change in pressure on the aluminium free surface as the width changes.

Fig. 21 shows a plan view of the apparatus of a preferred embodiment, indicating a proposed mechanism for stirring.

Fig. 22 shows a schematic perspective view of the molten metal feed apparatus according to an embodiment of the invention.

Figs. 23 and 24 show schematic sectional views of the displacement of the molten metal level by movement of the displacement body.

Figs. 25-27 illustrate the interaction of EM edge dam coil current, position, and pressure head to achieve desired changes in sheet width.

Fig. 28 illustrates an alternative embodiment using an array of static EM edge dams.

Fig. 29 shows a schematic plan view of the feed tip, cast strip and EM diverter of an embodiment for forming holes in the cast strip.

Fig. 30 shows an alternative embodiment to Fig. 29.

Fig. 31 shows a longitudinal cross sectional view of the embodiment of Fig. 30.

Fig. 32 shows a schematic cross sectional perspective view through the feed tip of a modified embodiment for forming holes in the cast strip.

Fig. 33 shows an alternative embodiment to that of Fig. 32.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER OPTIONAL FEATURES OF THE INVENTION

Significant amounts of aluminium are cast and then cut away in the process of making irregularly-shaped products because the supply chain is configured to make stock products which are all regular shapes. Better integration of the supply chain for sheet metal products is possible where an electromagnet is used to manipulate the profile of sheet metal in twin roll casting. Below, the first experimental trials are presented, during which one edge of the sheet was controlled and moved by the electromagnet.

The aluminium supply chain is split into two distinct parts: the metals industry, which produces aluminium from ore and then casts and rolls the metal to make stock products such as coils of sheet, and the manufacturing industries which take these stock products and reshape them to make consumer products, for example car doors. This makes the supply chain subtractive; a large fraction of the metal cast is removed and does not reach the final consumer product. This loss may be quantified by the yield which is the ratio of metal in the final product to the original mass of metal cast. Cullen and Allwood (2013) calculate the average yield across all aluminium products as 60%, and in a case study of an aluminium car door Milford et al. (2011 ) found a yield of 40%, with half of the metal subtraction attributable to the rectangular sheet being cut to create a door and window outline in the blanking and stamping processes. Therefore, the ability to cast the outline of irregular sheet products directly would create an opportunity for a significant improvement in yield.

The preferred embodiments of the invention build on existing efforts to cast nearer to net thickness by adding extra controls to allow the profile of irregular sheet products to be cast directly. The most established direct sheet casting process, twin roll casting, is taken as the starting point. As illustrated in Fig. 1 , in twin roll casting (TRC), sheets are cast directly by feeding liquid metal 20 through a refractory (e.g. ceramic) feed tip 10 between two counter- rotating cooled rolls 12, 14 (the opposed cooling means). As soon as the liquid metal touches the rolls it starts to form a solid shell which grows as it moves towards the roll bite, marked as line B. The shells on the top and bottom roll meet at a solidification point 18 just before the roll bite and from there the sheet 16 is deformed as it is in the hot rolling process. A cross section of the solidification region is shown in Fig. 1. The casting direction is direction C. The sump depth is marked as 22.

An electromagnetic (EM) edge dam can be used to manipulate the metal by applying a pressure along the sump during casting, allowing for control of the edge of the metal so varying the width of the cast sheet. As discussed in more detail below, an EM edge dam can be used on each edge of the strip and/or EM actuators can be added to cast strip with holes (requiring additional modifications to the metal feed). As a first step, in this disclosure the process is demonstrated by controlling one edge of a cast strip on a laboratory scale twin roll caster.

The methods of setting and changing width in conventional twin roll casting processes and the principles of electromagnetic containment are described below, and the opportunity of using an EM edge dam for width control identified.

The twin roll casting process is described in detail in the text by Ferry (2006). Liquid aluminium is fed into the back of a TRC via a refractory feed tip, which fully contains the metal until it is solidified. The top and bottom pieces end at a fixed setback from the roll bite determined by the desired solidification length. Two edge pieces protrude further towards the roll bite to provide a physical barrier to the liquid metal, thereby acting as static mechanical edge dams. In order to vary the width of the strip, the casting process must be halted, and either a new feed tip with a different width inserted or refractory plugs used to reduce the width of the aperture in the existing feed tip.

Smith et al. (2004) propose and demonstrate on a pilot caster the Fata Hunter Optiflow system, which separates the edge dam from the feed tip so it may slide inside the tip, transversely along the width. A graphite seal prevents liquid aluminium from leaking through the gap and the edge dam is actuated to give a controlled width. Without stopping casting, they demonstrated a width increase of 200mm incrementally over two hours of casting, at a maximum rate of 1.5mm/s. The Optiflow system is designed to cast sequential coils of sheet at different widths without interrupting casting, but problems could be encountered when trying to move the edge dam at much faster rates: can the graphite maintain a good seal in the feed tip, will its lifetime be compromised by rapid motions, and how does the moving edge dam interact with the partially solidified shell when decreasing the width? No follow up report has been made in the literature.

With all mechanical edge dams, there is a sliding contact between the solid shell which is moving forward and the static metal-facing surface of the edge dam. Friction and unwanted heat transfer out of the strip leads to defects at its edges. In particular edge cracks form via the mechanism described by Monaghan et al. (1993). The extra heat transfer through the edge dam causes solidification to occur earlier at the edges of the strip than at the centre, and therefore when the strip is rolled the edges are deformed more leading to cracking, particularly with hard alloys. This is a common problem in aluminium twin roll casting and as a result all industrial casters have edge trimming downstream to remove the cracked area, normally 20-30mm from the total width of 2000mm (Romano and Romanowski, 2009).

Given these defects, electromagnetic (EM) containment has already been proposed and demonstrated for use in aluminium twin roll casting. The principle, derived in more detail by Davidson (2001 ), involves applying an AC magnetic field tangentially to the surface being contained. At a suitably high frequency (on the order of kHz), the alternating field may only diffuse a small distance into the metal (the 'skin depth'). An electrical current is induced in the surface of the metal, and the interaction of the applied magnetic field with this current produces a magnetic pressure that acts to repel the metal from the field. The average magnetic pressure, Pm, is given in Equation (1). μο is the permeability of free space and Bo is the magnitude of the magnetic field.

Equation (1 )

The use of EM edge dams in twin roll casting has been demonstrated on a laboratory scale by Whittington et al. (1998) for horizontal aluminium TRCs, and in a theoretical design proposed by Gerber (2000) for vertical steel casters. The geometry of the

Whittington EM edge dam and its magnetic field are shown in Fig. 2. The Whittington design is a horseshoe-shape core 30 bolted to the side of the caster, which uses the fact that the steel rolls 32, 34 are magnetic to direct flux 36, 38 into the roll bite. The distribution of the magnetic field is such that an increase in pressure in the aluminium would cause the field to bunch up and increase in strength, so that the arrangement is inherently stiff and stable.

The Whittington EM edge dam was operated at 16-30kHz with up to 4000At applied, and successfully contained one edge of a cast strip. A small width variation of 3mm was noted when changing the current applied to the EM edge dam during start-up, but because the field attenuates quickly away from the magnet large changes in width would be impossible by varying current alone. The operating frequency was chosen based on an optimisation for stiffness; at this frequency the change in width with pressure in the aluminium is minimised. The skin depth in the aluminium is 0.6mm. With 4000At applied,

the EM edge dam requires water cooling to extract heat generated by eddy currents and hysteresis in the core.

The Gerber design uses a wedge shaped conductor with no flux concentrator, with a magnetic field generated in concentric circles around it. The geometry of the conductor is designed so that the field becomes strongest at the roll bite, where the static pressure is greatest, so that the free liquid metal surface is approximately vertical.

Both EM edge dam designs are unsuitable for rapid and large variations in width because they cannot easily be moved transversely along the rolls without impacting liquid metal. A different geometry was proposed by McBrien and Allwood (20 3), and this design is shown in Figs. 3 and 4. Like the Whittington EM edge dam, a horseshoe-shaped electromagnet 40 is used, but rotated by 90° and located behind the feed tip, pointing in the casting direction C. The horseshoe is profiled to fit around the feed tip and to direct the magnetic field into the roll bite area via the surface of the rolls 42, 44, allowing for it to move transversely (parallel to the rotational axis of the rolls) directly to control width. This EM edge dam design was tested at frequencies of 5kHz and 15kHz with a low melting point alloy, and it was suggested that a lower frequency was required to increase flux density at the roll bite to improve strength and stiffness of containment.

Beyond containment, the interaction of EM fields and liquid metal can be used to produce a wider range of effects. In a review of industrial applications, Li (1998) identified uses in transporting metal (valves, brakes, and pumps), stirring to distribute solutes (in continuous casting of steel) or for melting metal. In industry, the use of EM for containment is primarily through EM fields replacing copper moulds in the DC casting process, where the alternate cooling conditions and stirring create a more uniform microstructure so that scalping of the cast billet to remove the surface is reduced. The CREM ('Casting, Refining, ElectroMagnetic') process described by Vives (1989) and the 'Electromagnetic Roll Casting' process by Mao (2003) both use the stirring effect of lower frequency magnetic fields (10-50Hz) to refine the microstructure of the cast metal.

Applied at the solidification point, stirring disrupts the solid-liquid interface and distributes nucleation sites widely. In both cases, the grain refinement observed was no better than that achieved by adding a dedicated grain refining additive.

The McBrien and Allwood (2013) EM edge dam design consists of a 4.75 turn copper coil, and a horseshoe-shaped core that fits around the caster feed nozzle. See Figs. 3 and 4. The core 40 has profiled ends 46, 48 that match the roll radius so that flux 50 can be effectively linked into them. The magnetic field lines are concentrated through the core and directed into the ferromagnetic rolls, forming a loop by jumping across the air gap between them. As with the Whittington (1998) design, the field in the air gap provides containment of the liquid metal. This EM edge dam may be moved parallel to the rolling axis of the rolls to achieve the desired width changes.

The EM edge dam design of McBrien and Allwood (2013) is constrained by several external factors. It must fit within an existing lab scale twin roll caster, and is therefore subject to constraints in roll radius and material. A Statipower BSP12 power supply, which has a suitable current rating (up to 3000 A) and operates in approximately the correct frequency range (15-30 kHz) was used for the preliminary trials reported in McBrien and Allwood (2013). A further geometry constraint is the height of the feed nozzle, which depends on the sheet thickness and the need for sufficient insulation to prevent freezing.

The EM edge dam of McBrien and Allwood (2013) used a core made from an

experimental material manufactured by Fluxtrol Inc, 'Fluxtrol EM'. It is an iron doped plastic, which reduces internal heating due to eddy currents created in the oscillating magnetic field. Despite this, cooling is still required. An internal water flow is provided via cooling channels (not shown) machined on the inner surface of both halves of the core. These halves are glued together to provide a seal, with water fed in through hoses at the back of the core.

The base for the experiments in McBrien and Allwood (2013) is a representation of the area of the twin roll caster that has an important effect on the distribution of the magnetic field. Two experiments were carried out; firstly, measurements of the magnetic field were made, and secondly the edge dam was tested with liquid metal to determine the limit of pressure that could be contained.

To measure the magnetic field, a search coil was constructed with a copper wire wound around a ceramic former. The average flux density through the coil can be inferred from its area and open circuit voltage. With the EM edge dam held statically, the search coil was placed at various positions between the rolls to measure the distribution of the flux density.

Wood's metal, which melts at 70°C, was used to verify the EM edge dam's performance with liquid metal. Referring now to Fig. 5 (in which the rolls are not shown), a fixed volume of Wood's metal fills a ceramic nozzle 51 from a polycarbonate reservoir 52, with the roll gap plugged and sealed at various offsets from the roll bite. The EM edge dam 54 is moved between the rolls, via actuator 56, applying a magnetic pressure to the Wood's metal and causing it to flow back into the reservoir. The pressure head increases up to the limit that can be applied by the EM edge dam, and the relative motion of the metal and EM edge dam indicates the stiffness of the edge dam.

The experimental results of the magnetic field distribution measurements and the static pressure containment tests are described below. The EM edge dam was operated at 384 A and 16.3 kHz.

Measurements of the magnetic field distribution indicated that the edge dam has a low stiffness.

The maximum pressure determined in the static pressure containment tests is equivalent to approximately 5 mm of aluminium.

The experiments reported in McBrien and Allwood (2013) shows that the operation of the system would be affected by the flux density near the roll bite, limiting the overall pressure that may be contained, and the stiffness of the EM edge dam, affecting the stability of the edge during casting.

The flux density, and therefore the pressure that may be contained, attenuates with distance from the EM edge dam core. A number of options exist to increase roll bite flux density; current to the EM edge dam may be increased to increase the strength of the field everywhere. With the design set out in McBrien and Allwood (2013), saturation of the core will limit the gain beyond a current of approximately 800 A, and a higher current will generate more heat in the core, increasing cooling requirements or limiting the operation time. A more attractive option is to reduce the frequency of operation, which increases the thickness of the skin effect in the rolls allowing more flux to be carried. With current increased to 800 A and frequency reduced to 3 kHz, a roll bite flux density of 60 mT may be achieved, generating a magnetic pressure equivalent to 30 mm Al head. This is low, but sufficient for horizontal twin roll casting operations.

Low stiffness of the EM edge dam is an inherent disadvantage of this geometry because of the orientation of the EM edge dam. With a requirement for the ability to execute large changes in width, this would appear to be the only possible orientation and therefore the low stiffness must be accepted. In practice, low stiffness may cause oscillations in the edge position during casting and limit the rate of change of width when changing between sheets of different sizes. To mitigate this effect, a low overall pressure in the liquid metal would be required, which will reduce heat transfer to the rolls and potentially the stability of the casting process.

Further experimental work has been carried out in order to show how relatively rapid changes in the width of the cast strip can be achieved.

The experiments were carried out on a lab-scale horizontal TRC. The caster is a smaller version of industrial scale units, with small diameter rolls (320mm) and a narrow working section (120mm sheet width compared to 2000mm for the largest industrial casters). The rolls are made from H13 hot working tool steel, which has magnetic relative permeability around 680 (Smithells Metal Reference, 2004). The primary use of this caster is to conduct metallurgical experiments which require an undeformed

microstructure, so it is designed with a low stiffness. The top roll can move upwards so as not to apply a large rolling force, meaning the strip microstructure is as close to the as-cast state as possible. The EM edge dam and other equipment were designed specifically to fit this caster.

The EM edge dam 60 is a copper coil wrapped around a flux concentrator made from Fluxtrol 100. Fluxtrol 00 is an iron-doped plastic which has a relative permeability of 120 with reduced heat generation due to the minimisation of eddy currents. Despite this, the core must still be water cooled via an internal channel. As shown in Fig. 4, the concentrator geometry is profiled to direct flux into the roll surface where it is carried forward towards the roll bite, and fits around the feed tip. The transverse position of the EM edge dam is controlled via a linear actuator.

The feed system and the EM edge dam are shown in Fig. 6. The feed tip 62 must be non-conductive and non-magnetic so as to be transparent to the magnetic fields generated by the EM edge dam. It is made from N17, a calcium silicate refractory material which is commonly used in TRC feed tips. The feed tip was designed to be as thin as possible so that the EM edge dam could be placed closer to the roll bite, thereby increasing the strength of the magnetic field along the sump. Two mechanical edge dams 64, 66 are integrated in the feed tip - one to provide containment on the uncontrolled edge, and one beside the EM edge dam for use during start-up and to provide a failsafe situation if the EM edge dam switches off.

CLAIMS

1. A continuous casting apparatus for casting a metal strip with a cross sectional form which varies along the length of the strip, the continuous casting apparatus comprising:

opposing cooling means;

a molten metal feed system positionable to provide a molten metal feed for solidification between the opposing cooling means to form a solidified metal strip along a length direction;

a form adjustment system comprising at least one dam to determine, at least in part, the cross sectional form of the molten metal feed to the opposing cooling means, and thereby to determine the cross sectional form of the solidified metal strip, wherein the dam is moveable during operation of the apparatus to vary the cross sectional form of the molten metal feed to the opposing cooling means,

the continuous casting apparatus further comprising:

a molten metal pressure control system, operable to control the pressure of the molten metal in the molten metal feed during operation of the apparatus in coordination with movement of the dam.

2. A continuous casting apparatus according to claim 1 , wherein the continuous casting apparatus is a twin roll casting apparatus.

3. A continuous casting apparatus according to claim 1 or claim 2, wherein the dam is an AC electromagnetic field dam provided by at least one electromagnet.

4. A continuous casting apparatus according to any one of claims 1 to 3, wherein the molten metal feed system includes a feed tip in fluid communication with a molten metal reservoir via a conduit, the pressure of molten metal at the feed tip being controlled by controlling the pressure of molten metal in the reservoir.

5. A continuous casting apparatus according to claim 4, wherein the pressure of the molten metal in the molten metal feed is controlled by controlling the level of molten metal in the reservoir.

6. A continuous casting apparatus according to claim 5, wherein the level of molten metal in the reservoir is controlled by control of displacement of molten metal in the reservoir.

7. A continuous casting apparatus according to any one of claims 1 to 6, wherein the dam is moveable transversely to the length direction of the strip.

8. A continuous casting apparatus according to any one of claims 1 to 7, wherein there is provided an array of at least two dams at different positions, the dams being capable of being selectively switched into operation.

9. A continuous casting apparatus according to any one of claims 1 to 8, wherein the dam is an edge dam, operable to control the position of the edge of the cast strip.

10. A continuous casting apparatus according to any one of claims 1 to 8, wherein the dam is a diverter, operation to divert the flow of motel metal to form an opening in the cast strip.

11. A continuous casting method for casting a metal strip with a cross sectional form which varies along the length of the strip, the method comprising:

providing a molten metal feed for solidification between two opposing cooling means to form a solidified metal strip along a length direction;

operating a form adjustment system comprising at least one dam to determine, at least in part, the cross sectional form of the molten metal feed to the opposing cooling means, and thereby to affect the cross sectional form of the solidified metal strip, wherein the dam is moved during operation of the device to vary the cross sectional form of the molten metal feed to the opposing cooling means,

operating a molten metal pressure control system to control the pressure of the molten metal in the molten metal feed during casting in coordination with movement of the dam.

12. A continuous casting method according to claim 11 , including the step of moving the dam transversely to the length direction of the strip.

13. A continuous casting method according to claim 11 or claim 12, wherein the molten metal feed system includes a feed tip in fluid communication with a molten metal reservoir via a conduit, the method further comprising controlling the pressure of molten metal at the feed tip by controlling the pressure of molten metal in the reservoir.

14. A continuous casting method according to claim 13, wherein the pressure of the molten metal in the molten metal feed is controlled by controlling the level of molten metal in the reservoir.

15. A continuous casting method according to claim 14, wherein the level of molten metal in the reservoir is controlled by control of displacement of molten metal in the reservoir.

16. A continuous casting method according to any one of claims 11 to 15, wherein, when the dam is moved so as to increase the width of the strip, the molten metal pressure is increased.

17. A continuous casting method according to claim 16, wherein, once the dam has been moved to increase the width of the strip and the width has increased to the desired amount, the molten metal pressure is reduced.

18. A continuous casting method according to any one of claims 11 to 17, wherein, when the dam is moved so as to decrease the width of the strip, the molten metal pressure is decreased.

19. A continuous casting method according to claim 18, wherein, once the dam has been moved to decrease the width of the strip and the width has decreased to the desired amount, the molten metal pressure is increased.

20. A continuous casting apparatus for casting a metal strip with a cross sectional form which varies along the length of the strip, the continuous casting apparatus comprising:

opposing cooling means;

a molten metal feed system positionable to provide a molten metal feed for solidification between the opposing cooling means to form a solidified metal strip along a length direction;

a form adjustment system comprising at least one diverter, wherein the diverter is operational to part the molten metal feed laterally in order to vary the cross sectional form of the molten metal feed to the rollers and thus the cross sectional form of the solidified metal strip, thereby providing at least one hole in the solidified metal strip.

21. A continuous casting apparatus according to claim 20, wherein the continuous casting apparatus is a twin roll casting apparatus.

22. A continuous casting apparatus according to claim 20 or claim 21 , wherein the diverter is an AC electromagnetic field diverter provided by at least one electromagnet.

23. A continuous casting apparatus according to any one of claims 20 to 22, wherein the diverter is moveable transversely to the length direction of the strip.

24. A continuous casting apparatus according to any one of claims 20 to 23, wherein there is provided an array of at least two diverters at different positions, the diverters being capable of being selectively switched into operation.

25. A continuous casting apparatus according to any one of claims 20 to 24, wherein there is provided a molten metal pressure control system, operable to control the pressure of the molten metal in the molten metal feed during operation of the apparatus in coordination with operation of the diverter.

26. A continuous casting apparatus according to any one of claims 20 to 25, wherein the molten metal feed system includes a feed tip in fluid communication with a molten metal reservoir via a conduit.

27. A continuous casting apparatus according to any one of claims 20 to 26, wherein in operation, the diverter operates to provide a flow of molten metal on each transverse side of the diverter and a first feed conduit supplies molten metal to one transverse side of the diverter and a second feed conduit supplies molten metal to the other transverse side of the diverter.

28. A continuous casting apparatus according to any one of claims 20 to 27, wherein at least one bypass conduit is provided for molten metal to bypass the diverter to reach a transverse side of the diverter distal from a main feed conduit to the feed tip.

29. A continuous casting apparatus according to claim 28, wherein the diverter is an EM diverter, and the bypass conduit is a conduit which substantially shields molten metal within the bypass conduit from the EM field.

30. A continuous casting apparatus according to any one of claims 20-29, wherein the apparatus includes one or more diverter assistance feature.

31. A continuous casting method for casting a metal strip with a cross sectional form which varies along the length of the strip, the method comprising:

providing a molten metal feed for solidification between two opposing cooling means to form a solidified metal strip along a length direction;

providing a form adjustment system comprising at least one diverter, and operating the diverter to part the molten metal feed laterally in order to vary the cross sectional form of the molten metal feed to the rollers and thus the cross sectional form of the solidified metal strip, thereby providing at least one hole in the solidified metal strip.

32. A continuous casting method according to claim 31 , including the step of moving the diverter transversely to the length direction of the strip.