DE69506225T2 - Electromagnetic retention of melt by current excitation - Google Patents

Description

Background of the invention

The present invention relates generally to devices and methods for electromagnetically containing molten metal, and more particularly to a device and method for preventing the escape of molten metal through the open side of a vertically extending gap between two horizontally spaced members between which the molten metal is located.

An example of an environment in which the present invention is intended to operate is a system for continuously pouring molten metal directly into strips, e.g., steel strips. Such a system typically includes a pair of horizontally spaced, counter-rotating rolls having a horizontally disposed, vertically extending gap therebetween for receiving the molten metal. The gap formed by the rolls tapers downwardly to the nip between the rolls. The rolls are cooled and in turn cool the molten metal as the molten metal moves downward through the nip and exits as a solid strip of metal at the nip between the rolls.

The gap has an open side adjacent to each end of a roll. At each open end of the gap, the molten metal is not contained by the rolls. To prevent molten metal from escaping through the open side of the gap, a dam must be used. Mechanical dams or seals have been used for this purpose, but they have disadvantages which are described in U.S. Patent No. 4,936,374 to Pareg [sic] and in U.S. Patent No. 4,974,661 to Lari, et al., the disclosures of which are incorporated herein by reference.

To overcome the disadvantages inherent in the use of mechanical dams and seals, efforts have been made to contain the molten metal at the open side of the gap by using an electromagnet having a magnetic core surrounded by an electrically conductive coil and having a pair of spaced magnetic poles located adjacent to the open side of the gap. The magnet is energized by the flow of a time-varying current (e.g., alternating current or fluctuating direct current) through the coil, and the magnet produces a time-varying magnetic field extending across the open side of the gap and between the poles of the magnet. The magnetic field can be either horizontal or vertical, depending on the arrangement of the poles of the magnet. Examples of magnets that produce a horizontal field are described in the above-referenced U.S. Patent No. 4,936,374 to Pareg [sic] and in U.S. Patent No. 5,251,685 to Praeg. Examples of magnets that produce a vertical magnetic field are described in the above-referenced Lari et al. U.S. Patent No. 4,974,661. The disclosures of these patents are incorporated herein by reference.

The time-varying magnetic field induces eddy currents in the molten metal adjacent to the open side of the gap. The induced eddy currents give rise to their own time-varying magnetic field, which provides a magnetic flux density at the open side of the gap that is in addition to the magnetic flux density provided by the magnetic field from the electromagnet. The resulting repulsive mass force is directed at the molten metal at the open side of the gap. The repulsive mass force can be expressed as can be expressed as follows:

f = J · B,

where

f = the repulsive mass force on the open side of the gap,

J = the peak induction current density in the molten metal,

B = the peak magnetic flux density due to (1) the magnetic field from the electromagnet and (2) the magnetic field from the induced eddy currents.

Another expedient for magnetically confining molten metal at the open end of a gap between a pair of rollers is to place a coil adjacent to the open side of the gap through which a time-varying current flows. This causes the coil to generate a magnetic field that induces eddy currents in the molten metal adjacent to the open side of the gap, resulting in a repulsive mass force similar to that described above in connection with using magnetic poles adjacent to the gap. Embodiments of a coil-type magnetic dam are described in Gerber et al. U.S. Patent No. 5,197,534 and Gerber U.S. Patent No. 5,279,350, and the disclosures are incorporated herein by reference.

Regarding the repulsive mass force, the integral of the same also indicates the mean magnetic repulsive pressure P which, in the case of the coil-like embodiment of a magnetic dam, can be expressed as follows:

P = kB²/4u, where

k = the coupling factor between the coil and the molten metal,

u = the magnetic permeability of air (and of molten metal),

B = the peak magnetic flux density (as described above) .

The coupling factor is typically less than 1. If the molten metal is steel and the frequency of the time-varying electric current is 3000 Hz, the coupling factor (k) can have approximate values anywhere between 0.18 and 0.90 depending on the geometry of the molten metal pool at the open side of the gap. The coupling factor decreases with increasing skin depth (i.e. penetration) of the induced eddy currents in the molten material. The skin depth increases with a decrease in frequency; thus a decrease in frequency leads to a decrease in the coupling factor (k), which in turn decreases the repulsion mass pressure (P).

To hold the molten steel, the repulsive mass pressure must be at least equal to the pressure that pushes the molten metal out through the open side of the gap between the rolls. The repulsive mass pressure (P) can be increased by increasing the peak magnetic flux density (B) produced by the dam, but increasing the flux density also increases the energy loss in the dam (energy consumed as heat). The average energy loss per unit area in the dam (PL) is expressed as follows:

PL = B²/(2u²δ),

where (6) is the skin depth in copper of which the containment coil is composed and (u) is the magnetic permeability of copper.

From the foregoing equation, it is apparent that the energy loss in the dam can be reduced by increasing the skin depth (δ), which can be increased by decreasing the frequency of the time-varying current. However, as noted above, decreasing the frequency decreases the coupling factor (k), which in turn produces a decrease in the repulsion mass pressure (P). It is desirable to (1) provide a repulsion mass pressure sufficiently high to hold the molten metal while (2) reducing the energy loss in the dam. In other words, one should provide a relatively high ratio of holding pressure to energy loss in the dam. This ratio (P/PL) can be expressed as follows:

P/PL = δku/2, where

(δ) is the skin depth in copper (the material of the coil)

(u) is the magnetic permeability of air (and of copper and molten steel) and

(k) is the coupling factor between the coil and the molten metal.

For descriptive purposes, the magnetic permeability (u) of air, copper and the molten steel can be assumed to be equal.

Summary of the invention

The above desirable properties are achieved by the invention as claimed in independent claims 1 and 17. Preferred embodiments are claimed in the dependent claims.

The energy loss in the dam is reduced without any significant decrease in the repulsive mass pressure. This is accomplished by using a conduction current in the molten metal. Such an arrangement has several advantages (described below) over an arrangement using only induced eddy currents in the molten metal to generate a magnetic confinement field.

The containment coil used in all embodiments of the present invention has a vertically disposed first containment coil portion facing the molten metal pool at the open side of the gap between the rolls of the continuous strip casting apparatus. The bottom of the first coil portion is electrically connected to the bottom of a vertically disposed second containment coil portion.

An upper electrode extends into the top of the molten metal bath adjacent to the open side of the gap. Additional lower electrodes or brushes (a) contact the solidified steel strip at a location just below the nip of the rolls adjacent to the open side of the gap or (b) contact the two rolls at that location or (c) contact both the strip and the rolls as in (a) and (b) in combination.

In all embodiments, a time-varying current is introduced into the first containment coil region. In one embodiment, all current from the power source (e.g., the secondary coil of a transformer) initially flows downward through the first coil region and is then split into two current flows: (a) one current flow is directed downward through the second containment coil region; (b) another current flow is directed to the lower electrodes or brushes just below the nip of the rolls and then flows as a conduction current through the molten metal pool to the upper electrode. trode upwards.

In another embodiment, the current from the power source is initially provided as two separate discrete current flows: (a) one current flow is directed through the first and second containment coil regions as described above; (b) the other current flow is initially directed to the above-mentioned lower electrodes or brushes and flows upward as a conduction current through the molten metal as described above.

In all embodiments, induced eddy currents as well as a conduction current are present in the molten metal pool. These eddy currents are induced in the molten metal pool by the magnetic field generated by the containment coil. The magnetic flux density (B) that generates the repulsive mass pressure for containing the molten metal pool includes three components: (1) the magnetic flux density due to the magnetic field generated by the current flowing through the containment coil; (2) the magnetic flux density due to the magnetic field generated by the induced eddy currents in the molten metal pool; and (3) the magnetic flux density due to the magnetic field generated by the conduction current flowing through the molten metal pool. The second component, i.e. (2), is essentially a factor lower in terms of the total magnetic flux density than in an arrangement in which the electrical currents in the molten metal bath are solely induced eddy currents.

As noted above, when the frequency of the time-varying current is reduced, there is a decrease in the energy loss in the coil of the dam; but there is also an increase in the skin depth (8), in the molten metal pool, of the induced eddy currents. This increase in the skin depth (penetration depth of the eddy currents) reduces the head factor (k) between the containment coil and the molten metal, which in turn reduces the repulsion mass pressure.

However, with respect to the conduction current in the molten metal bath, the penetration depth of the current (current distribution) is not so much a function of frequency, but rather a function of the arrangement of the electrodes. (If the time-varying conduction current is fluctuating direct current, a reduction in frequency has essentially no effect on the current distribution; if the time-varying conduction current is alternating current, a reduction in frequency has a significantly reduced effect on the current distribution than an arrangement with no conduction current in the molten metal.) Thus, a reduction in the frequency of the time-varying conduction current does not produce a significant change in the current distribution. Accordingly, there is no significant decrease in the coupling factor (k) (which decreases with increased skin depth).

Consequently, a decrease in frequency to reduce energy loss in the containment coil does not produce a decrease in the coupling factor associated with the line current; nor does it produce a significant decrease in the flux density due to the magnetic field generated by the line current. Any negative effect on the repulsive mass pressure from such a decrease in frequency would be substantially less than the negative effect resulting from a situation where the electrical currents in the molten metal pool were solely induced eddy currents.

Reducing the frequency of the time-varying current not only reduces the energy loss in the containment coil, but also reduces the energy loss in the molten metal.

The time-varying current produces a time-varying magnetic field with a corresponding frequency comprising cycles of increasing and decreasing magnetic flux density. The ability of the magnetic field to hold the molten metal can be adversely affected if the frequency of the time-varying current is reduced too much. The frequency cannot be reduced below a lower limit at which the time interval between the peak magnetic flux density for successive cycles of the time-varying magnetic field is too long to prevent molten metal from flowing out through the open side of the gap between the rolls.

For a given input current in the confinement system, the magnetic flux density produced by an arrangement according to the present invention using a conduction current in the molten metal bath is greater than the magnetic flux density produced by an arrangement in which the current in the molten metal bath consists solely of induced eddy currents.

Further features and advantages will be apparent from the claimed and disclosed apparatus and method or will become apparent to one skilled in the art from the following detailed description taken in conjunction with the accompanying schematic drawings.

Short description of the drawings

Fig. 1 is an end view of a continuous strip casting apparatus employing an embodiment of an electromagnetic containment device according to the present invention;

Fig. 2 is a plan view of a portion of the structure shown in Fig. 1,

Fig. 3 is an enlarged fragmentary end view of a portion of the assembly shown in Fig. 1;

Fig. 4 is an enlarged fragmentary end view similar to Fig. 3;

Fig. 5 is a schematic representation of an embodiment of the containment device using alternating current;

Fig. 6 is a schematic representation of another embodiment of the containment device using alternating current;

Fig. 7 is a fragmentary plan view of a portion of the containment device illustrating the direction of electrical currents and magnetic fields associated with the device;

Fig. 8 is an enlarged fragmentary end view symbolically illustrating another portion of the device; and

Fig. 10 is a schematic diagram illustrating an embodiment of the containment device using direct current.

Detailed description

Referring first to Figs. 1 to 3, there is generally indicated at 30 an electromagnetic containment device for preventing the egress of molten metal 38 through the open side 36 of a vertically extending gap 35 between two horizontally spaced members 31, 32 between which there is a bath 38 of molten metal.

The horizontally spaced elements comprise a pair of counter-rotating casting rolls of a continuous strip caster.

The casting rolls 31, 32 have a nip 39 therebetween at the bottom of the vertically extending nip 35. The mutually rotating rolls include means for solidifying metal from the molten bath 38 into a continuous strip 37 extending downwardly from the nip 39. The rolls 31, 32 are cooled in a conventional manner not described here. The bath 38 is typically molten steel.

However, with the containment device 30, the molten metal in the gap 35 would exit through the open side 36 of the gap 35. Although only one open side 36 and one electromagnetic containment device 30 are shown in the figures, it should be understood that there is one device 30 on each of the two open sides 36 of the gap 35.

Referring now to Figures 5-7 and 10, an electromagnetic containment device 30 includes an electrically conductive containment coil 40 adjacent the open side 36 of the gap 35. The coil 40 generates a first horizontal magnetic field that extends through the open side 36 of the gap 35 to the molten metal pool 38.

The coil 40 includes a vertically disposed first containment coil portion 41 facing the open side 36 of the gap 35 and a vertically disposed second containment coil portion 42 electrically connected to the first coil portion 41 at 43. The second coil portion 42 is located a distance behind the first coil portion 41 and faces the first coil portion 41.

Referring now to Figs. 3 to 6 and 10, the electromagnetic containment device 30 also includes brushes 46, 47 for electrically contacting at least one of (a) strips 37 and (b) casting rolls 31, 32 at a location below the roll gap 39 and adjacent the open side 36 of the gap 35. The apparatus 30 also includes an electrode 48 for electronically contacting the molten metal pool 38 at a location above the roll gap 39 and adjacent the open side 36 of the gap 35.

Referring to Figures 5, 6 and 10, the device 30 includes a transformer 50 including a primary coil 51 for receiving an input current and at least one secondary coil, e.g. 52 in Figure 6. In the embodiment of Figure 5, the secondary coil comprises a pair of separate, discrete coil portions 52a and 52b. In the embodiment of Figure 10, the secondary coil is in the form of a center tap coil, indicated at 53.

Referring again to the embodiment of Fig. 5, the vertically disposed first containment coil portion 41 has respective upper and lower ends 44, 45.

The vertically disposed second containment coil section 42 has respective upper and lower ends 54, 55. As previously noted, the transformer 50 includes a pair of separate, discrete secondary coil sections 52a and 52b. Each secondary coil section includes a pair of opposed coil terminals. A lead 56 electrically connects a terminal 70 of the secondary coil section 52b to the upper end 44 of the first containment coil section 41. A return lead 57 electrically connects the other terminal 71 of the secondary coil section 52b to the upper end 54 of the second containment coil section 42. A lead 58 electrically connects a terminal 60 of the secondary transformer coil section 52a to brushes 46, 47 via respective branch leads 58a, 58b (Fig. 3). A return line 59 electrically connects the other terminal 61 of the secondary coil area 52a of the transformer with an electrode 48.

The lines 56 and 57 comprise a first conductor means for directing a time-varying electrical current from the transformer 50 through the first coil region 41, in a first vertical direction (downward in Figure 5), and then through the second coil region 42 in a second vertical direction opposite to the first vertical direction, i.e., upward through the second coil region 42. In particular, current flows from the secondary coil region 52b of the transformer through the line 56, then downward through the first containment coil region 41, then through an electrical connection 43 connecting the bottoms 45, 55 of the coil regions 41 and 42, then upward through the second containment coil region 42, and then through the return line 57 to the secondary coil region 52b. The time-varying current flowing through the containment coil regions 41, 42 generates a first horizontal magnetic field adjacent to the open side 36 of the gap 35.

The electrically conductive line 58, branch lines 58a, 58b, brushes 46, 47, electrode 48 and electrically conductive return line 59 comprise a second conductor means for directing a time-varying electrical current from the secondary coil region 52a of the transformer vertically through the molten metal bath 38 as a conduction current adjacent to the open side 36 of the vertically extending gap 35. The flow of the conduction current through the bath 38 is in a direction opposite to that of the current flowing through the first containment coil region 41 (i.e., upward through the molten metal bath 38). This conduction current flow creates a second horizontal magnetic field adjacent to the open side 36 of the gap 35.

The directions of the waves generated by the first and second containment coils The currents flowing through the regions 41, 42 are shown at 62, 63, respectively, and the direction of the conduction current through the molten metal bath 38 is shown at 64 (Figs. 5 and 7). The directions of the first and second horizontal magnetic fields are shown at 65 and 66, respectively, in Fig. 7. The two magnetic fields flow in the same direction and reinforce each other.

The containment coil 40 and the first and second conductor means (as described above) comprise a device which, in the presence of the molten metal pool 38, cooperate to provide a magnetic repulsion pressure which forces the molten metal inwardly away from the open side 36 of the gap 35.

Referring now to the embodiment of Fig. 6, the secondary coil of the transformer comprises a single coil 52. In this embodiment, the lead 56 electrically connects one terminal 90 of the secondary coil 52 to the upper end 44 of the first containment coil section 41. The return lead 57 electrically connects the upper end 54 of the second containment coil section 42 to the other terminal 91 of the secondary transformer coil 52. The lower end 45 of the first containment coil section 41 is connected to the brushes 46, 47 by an electrical connection 43 and a pair of connection leads 68 (only one of which is shown in Fig. 6). The return line 59 connects the electrode 48 to a terminal 91 of the secondary transformer coil 52. As noted above, the terminal 91 is also connected to the return line 57, which in turn is connected to the upper end 54 of the second containment coil section 42.

In the embodiment of Fig. 6, the time-varying electrical current flows from the secondary coil 52 of the transformer through the line 56, then through the first containment coil lene region 42 downwards, then into the electrical connection 43 where the current is split. Part of the current flows upwards through the second containment coil region 42 and then back through the line 57 to the secondary coil 52 of the transformer. Another part of the current flows through the connecting lines 68 into the brushes 46, 47 and then through the molten metal 38 to the electrode 48 from which it flows back through a return line 59 to the secondary coil 52 of the transformer.

In the embodiment of Fig. 6, the directions of current flow through the containment coil regions 41, 42 and through the molten metal pool 38 are shown at 62, 63 and 64 in Figs. 6 and 7, respectively, and these directions are the same as the directions of current flow in the embodiment of Fig. 5.

The time-varying current flowing through coil 40 produces a first horizontal magnetic field having a direction indicated at 65 in Fig. 7; the time-varying conduction current flowing through molten metal pool 38 produces a second horizontal magnetic field having a direction indicated at 66 in Fig. 7. These are the same directions as the magnetic fields produced by the embodiment of Fig. 5. Similarly, the second horizontal magnetic field having a direction indicated at 66 in Fig. 7 amplifies the first horizontal magnetic field having a direction indicated by 65 in Fig. 7 to increase the magnetic repulsion pressure at the open side 36 of gap 35.

In all embodiments of the present invention, the conduction current flowing vertically through the molten metal bath 38 always flows in a direction 64 that is opposite to the direction 62 of the current flowing vertically through the first containment coil section 41. As a result of this relationship, the direction 66 of the conduction current flowing through the molten metal bath 38 The direction 65 of the horizontal magnetic field generated by the line current flowing is always the same as the direction 65 of the horizontal magnetic field generated by the coil 40. When the first and second horizontal magnetic fields flow in the same horizontal direction (65, 66 in Fig. 7), they reinforce each other.

In addition to the line current flowing through the molten metal pool 38, there may also be eddy currents in the molten metal pool 38 generated by the first horizontal magnetic field and flowing in the same direction 64 as the line current. The horizontal magnetic field generated by the induced eddy currents flows in the same direction 66 as the horizontal magnetic field generated by the line current flowing through the molten metal pool and amplifies the horizontal magnetic field generated by the line currents and by the time-varying current flowing through the containment coil 40.

Figs. 5 and 6 illustrate embodiments in which the time-varying current is alternating current. The time-varying electrical current may also be fluctuating direct current. An embodiment using fluctuating direct current is illustrated in Fig. 10. The embodiment of Fig. 10 is similar to the embodiment of Fig. 6, with certain differences. The similarities are not repeated. The differences are described below.

The transformer 50 in Fig. 10 has a secondary coil 72 with a center tap 73 connected to return lines 57 and 59. Each end of the secondary coil 72 is connected to a respective rectifier 74, 75, each of which is in turn electrically connected to line 56 for directing current into the upper end 44 of the first containment coil section 41. A fluctuating direct current from the rectifiers 74, 75 flows downward through the first containment coil section 41. When the current leaves the first containment coil section 41 at its lower end 45, the current is split into two parts: a first part of the split current is directed by the electrical connection 43 into the second containment coil section 42 and flows upwardly therethrough; a second part of the split current is directed by the electrical connections 68 and brushes 46, 47 into the molten bath 38 through which the second part of the current flows upwardly. The return line 57 connects current flowing from the upper end 45 of the second containment coil section 42 to the center tap 73 on the secondary transformer coil 72, and the return line 59 connects current flowing from the electrode 48 to the center tap 73.

In the embodiments of Figs. 6 and 10, a portion of the current flowing downward through the first containment coil region 41 also flows through the molten metal pool 38. On the other hand, in the embodiment of Fig. 5, no portion of the current flowing through the molten metal pool 38 flows through any portion of the containment coil 40.

The horizontal magnetic fields generated by the time-varying current flowing through the containment coil 40 and by the line current flowing through the molten metal pool 38 cooperate to provide a magnetic repulsion pressure that pushes the molten metal pool 38 inwardly away from the open side 36 of the gap 35.

Referring again to Fig. 7, a first containment coil region 41 includes a front surface 76, a back surface 77, and a pair of opposing sides 78, 79. A magnetic element 80, typically electrically insulated from the first coil region 41 by a thin insulating layer (not shown), surrounds the side 78, back surface 77, and side 79 of the first coil region. The magnetic element 80 is typically composed of conventional magnetic material and provides a low reluctance return path for the magnetic field generated by the time varying current flowing through the containment coil 40. The magnetic element 80 includes a pair of arm portions 81, 82, each located on a respective opposite side 78, 79 of the first coil portion 41 and extending toward the open side 36 of the gap 35. The magnetic element 80 also includes a rear connection portion 83 extending between the arm portions 81, 82 and located between the first and second containment coil portions 41, 42.

The structural designs that can be used for the coil sections 41, 42 are described in more detail in the above-referenced Gerber et al. U.S. Patent No. 5,197,534, which is incorporated by reference into the present application. However, unlike the device described in the above-referenced Gerber et al., '534 patent, the device of the present invention does not include a magnetic shield on the outside of the magnetic arm sections 81, 82. Such shielding has been used to confine the magnetic field generated by the containment coil to a space adjacent the open side 36 of the gap 35. This is important where reliance is placed on induced eddy currents in the molten metal pool 38 as the primary power source for the horizontal magnetic field generated by current flowing through the molten metal pool 38. In the present invention, however, line current is the primary source of current for the horizontal magnetic field created by current flowing through the molten metal bath 38. Accordingly, the magnetic shielding of Gerber et al.-'534 is not necessary.

As noted above, the electrode 48 is arranged between the casting rolls 31, 32 above the roll gap (Fig. 2).

The electrode 48 is made of an electrically conductive material that is resistant to high temperatures of the molten metal bath 38 into which the electrode 48 is at least partially immersed. The electrode 48 can, for example, be made of graphite.

The casting rollers 31, 32 can be composed of copper or a copper alloy or a ceramic material or austenitic (non-magnetic) stainless steel.

A casting roll composed of ceramic material is not very electrically conductive. In such a case, the relevant electrical connection to the molten metal bath 38 is via the brushes 46; 47 and the strip 37. Preferably, a spring is used to urge the brush into contact with the strip 37, and such a spring is symbolically shown at 85 in Fig. 9.

When the casting rolls are composed of an electrically conductive material, such as copper or copper alloy, the relevant electrical connection to the molten metal bath 38 includes the casting rolls. In other words, the relevant electrical connection is between a brush 46, 47 and a roll 31, 32; the connection may additionally be between a brush and the strip 37. When the relevant electrical connection is between a brush and a casting roll, a spring 86 is preferably used to press the brush into contact with the casting roll, and such a spring is symbolically shown at 86 in Fig. 8.

The brushes 46, 47 are composed of an electrically conductive material, such as graphite or phosphor bronze.

If a brush is made of metal (e.g. phosphor bronze), it can be cooled internally. If the brush is made of Graphite, cooling may be effected by using a brush holder which is internally cooled. Cooling arrangements of the types described in the preceding parts of this paragraph are within the skill of the art.

The foregoing detailed description has been given for convenience only, and no unnecessary limitations should be inferred from it, since modifications will be apparent to those skilled in the art.

Claims (22)

1. A casting system comprising a casting apparatus and an electromagnetic containment device (30), the casting apparatus comprising two horizontally spaced members (31, 32) defining a vertically extending gap (35) in which a pool of molten metal (38) is located, the gap having an open side (36) and the electromagnetic containment device (30) preventing the molten metal (38) from escaping through the open side (36), the electromagnetic containment device (30) further comprising electrically conductive containment coil means (40) adjacent the open side (36) of the gap (35) for generating a first horizontal magnetic field extending through the open side (36) of the gap (35) to the pool of molten metal (38), the containment coil means (40) comprising a vertically disposed first containment coil region (41) facing the open side (36) of the gap (35), and a vertically disposed second containment coil region (42) electrically connected to the first coil region (41) and located a distance behind the first coil region and facing the first coil region, the electromagnetic containment device (30) further comprising first conductor means (56, 57) for directing a time-varying electrical current through one of the coil regions in a first vertical direction and then through the other coil region in a second vertical direction opposite to the first vertical direction to create a first horizontal magnetic field adjacent to the open side (36) of the gap (35), the electromagnetic containment device (30) being characterized by: a second conductor means (46 to 48, 59 and either 58, 58a, 58b or 68) for directing the time-varying electrical current vertically through the molten bath (38) as a conduction current adjacent to the open side (36) of the vertically extending gap (35) in a direction opposite to that of the current flowing through the first coil region (41) to produce a second horizontal magnetic field adjacent to the open side (36) of the gap (35); wherein the containment coil means (41, 42), the first conductor means (56, 57) and the second conductor means (46 to 48, 59 and either 58, 58a, 58b or 68) cooperate in the presence of the molten bath (38) to provide a magnetic repulsion pressure which forces the molten metal (38) inwardly away from the open side (36) of the gap (35). 2. A casting system according to claim 1, characterized in that the horizontally spaced elements (31, 32) comprise a pair of counter-rotating casting rolls of a continuous strip casting apparatus; the casting rolls have a roll gap (39) between them at the bottom of the vertically extending gap (35); and the counter-rotating rollers (31, 32) have a device for solidifying the metal from the molten bath (38) into an endless strip (37) which extends downwards from the roll gap (39). 3. A casting system according to claim 2, and comprising: transformer means (50) including primary coil means (51) for receiving an input current and at least one secondary coil means (52 or 53) for connecting the first and second conductor means. 4. A casting system according to claim 3, characterized in that the second conductor means comprises: a brush means (46, 47) for electrically contacting at least one of (a) strips (37) and (b) a casting roll (31 or 32) at a location below the roll gap (39) and adjacent to the open side (36) of the gap (35); and an electrode device (48) for electrically contacting the melt bath (38) at a location above the roll gap (39) and adjacent to the open side (36) of the gap (35). 5. A casting system according to claim 4, characterized in that the vertically arranged first containment coil portion (41) has upper and lower ends (44, 45); the secondary coil device of the transformer (50) comprises a single coil (52); the first conductor means comprises means (56) for directing the current from the single secondary coil (52) of the transformer (50) into the upper end (44) of the first containment coil region (41); and the second conductor means comprises means (68) for electrically connecting the lower end (45) of the first insulation coil area (41) with the brush device (46). 6. A casting system according to claim 4, characterized in that each of the vertically arranged first and second containment coil portions (41, 42) has upper and lower ends (44, 54 and 45, 55); the secondary coil means of the transformer (50) comprises a pair of separate, discrete secondary coils (52a, 52b); each of the secondary coils (52a, 52b) of the transformer (50) includes a pair of terminals (60, 61 and 70, 72); the first conductor means (a) first means (56) for electrically connecting one terminal (70) of a secondary transformer coil (52b) to the upper end (44) of a vertically arranged containment coil section (41) and (b) second means (57) for electrically connecting the other terminal (71) of the one secondary transformer coil (52b) to the upper end (54) of the other vertically arranged containment coil section (42); the second conductor means comprises a lead (58 and either 58a or 58b) for electrically connecting a terminal (60) of the other secondary transformer coil (52a) to one of (i) the brush means (46, 47) and (ii) the electrode means (48); and the second conductor means further comprises a further lead (59) for electrically connecting the other terminal (61) of the other secondary transformer coil (52a) to the other of (i) the brush means (46, 47) and (ii) the electrode means (48). 7. A casting system according to claim 6, characterized in that the first electrical connection means of the first conductor means comprises means (56) for electrically connecting the one terminal (70) of the one secondary transformer coil (52b) to the upper end (44) of the first vertically arranged containment coil portion (41); and said one lead of said second conductor means comprises means (58 and 58a or 58b) for electrically connecting said one terminal (60) of said other secondary transformer coil (52a) to said brush means (46 or 47). 8. A casting system according to claim 4, characterized in that the electrode device (48) is arranged between the casting rolls (31, 32) above the roll gap (39) and comprises a device for at least partial immersion in the bath of molten metal (38). 9. A casting system according to claim 8, characterized in that the electrode device (48) is composed of graphite. 10. A casting system according to claim 4, characterized in that each casting roll (31, 32) is composed of at least one of (a) copper and (b) copper alloy; and the device (30) comprises a device (86) for bringing the brush device (46, 47) into contact with the roller (31, 32). 11. A casting system according to claim 4, characterized in that each casting roller (31, 32) is composed of a ceramic material; and the device comprises means (87) for bringing the brush means (46, 47) into contact with the strip (37). 12. A casting system according to claim 10 or 11, characterized in that the brush means (46, 47) is composed of one of (a) graphite and (b) phosphor bronze. 13. A casting system according to claim 4, characterized in that each casting roller (31, 32) is made of austenitic stainless steel. 14. A casting system according to claim 1, and comprising: a pair of opposing sides (78, 79) on the vertically arranged first containment coil portion (41); means (80) composed of a magnetic material for forming a low reluctance return path for the first magnetic field; wherein the device (80) composed of a magnetic material comprises (a) a pair of arm portions (81, 82), each located on a respective opposite side (78, 79) of the first containment coil portion (41) and extending in the direction of the open side (36) of the gap (35) and (b) comprises a connecting portion (83) extending between the arm portions (81, 82) and located between the vertically disposed first and second containment coil portions (41, 42); wherein the device has no magnetic shielding on the outside of the arm regions (81, 82). 15. A casting system according to claim 1, characterized in that, in use, the second conductor means generates the second horizontal magnetic field to enhance the first horizontal magnetic field and increase the magnetic repulsion pressure at the open side of the gap. 16. A casting system according to claim 1, characterized in that the time-varying electrical current is an alternating current. 17. An electromagnetic containment method for preventing the escape of molten metal (38) through the open side of a vertically extending gap (35) between two horizontally spaced members (31, 32) having molten metal (38) therebetween, the method comprising the steps of providing an electrically conductive containment coil device (40) having a first vertically disposed containment coil region (41) facing the open side (36) of the gap (35) and a vertically disposed second containment coil region (42) electrically connected to the first containment coil region (41) and located a distance behind the first containment coil region and facing the first containment coil region, and directing a time-varying electrical current through one (41) of the containment coil regions in a first vertical direction and then through the other (42) of the containment coil portions in a vertical direction opposite to the first vertical direction to produce a first horizontal magnetic field extending through the open side (36) of the gap (35) to the pool of molten metal (38), and the method is characterized by: the additional step of directing the time-varying electrical current vertically through the melt pool (38) as a conduction current adjacent to the open side (36) of the gap (35) in a direction opposite to that of the current flowing through the first coil region (41) to create a second horizontal magnetic field adjacent to the open side (36) of the gap (35); and in that the foregoing steps cooperate to provide a magnetic repulsion pressure which pushes the molten metal (38) inwardly away from the open side (36) of the gap (35). 18. A method according to claim 17, characterized in that the second horizontal magnetic field reinforces the first horizontal magnetic field to increase the magnetic pressure at the open side (36) of the gap (35). 19. A method according to claim 17 and comprising: Providing a return path (81, 83, 82) with a low magnetic resistance composed of a magnetic material for the first magnetic field and without a magnetic shield for the return path. 20. A method according to claim 18 and comprising: Directing the current through the first containment coil region (41) downward; splitting the current into two parts when the current leaves the first containment coil region (41); directing a first portion of the divided current upwardly through the second containment coil region (42); and directing a second portion of the divided stream upwardly through the bath of molten metal (38) as said line stream. 21. A method according to claim 17 and comprising: directing a first current flow downwardly through the first containment coil region (41); providing a second current flow that does not flow through the containment coil device (40); and directing the second current flow upwardly through the bath of molten metal (38) as said conduction current. 22. A method according to claim 17, characterized in that the time-varying electrical current is an alternating current.