CA1165970A - Electromagnetic shape control by differential screening and inductor contouring - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The disclosure relates to a non-magnetic screen for use in electromagnetic casting of molten castable materials. The screen includes a closed loop having at least one portion of a small radius of curvature and a locally changing orientation at the at least one portion as compared to portions of the screen adjacent the at least one portion.

Description

5~'71~3 This application is a division of application Ser. No. 353,504, filed ~une 6, 1980.
This invention relates to an improved process and apparatus for control of corner shape in continuous or semi-continuous electromagnetic casting of desired shapes, such as for example, sheet or rectangular ingots of castable materials.
The basic electromagnetic casting process had been known and used for many years for continuously or semi-continuously casting castable materials including, but not restricted to, metals and alloys and silicon or other similar semi-metals, metalloids, and semi-conductors.
- One of the problems which has been presented by elec-tromagnetic casting of sheet ingots has been the existence of large radius of curvature corners`thereon. Rounding off of corners in electromagnetic cast sheet ingots is a result of higher electromagnetic pressure at a given distance from the inductor near the ingot corners, where two proximate faces of the inductor generate a larger field. This is in contrast to lower elec~tromagnetic pressure at the same distance from the inductor on the broad face of the ingot remote from the corner, where only one inductor face acts.
There is a need to form small radius of curvature corners on sheet ingots so that auring rolling cross-sectional changes at the edges of the ingot are minimized. Larger radius of curvature corners accentuate tensile stress at the ingot edges during rolling which causes edge cracking and loss of material. Thus, by reduc-ing the radius of curvature of the ingot at the corners there is a maximizing in the production of useful material.
It has been found in accordance with the present invention that rounding off of corners in electromagnetically cast ingots can be made less severe or of smaller radius by bringing about a net downward displacement of the ~creening . 10052-MB

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current at the corners of a shield placed at the molten metal or allo~ input end o~ the casting zone and~or b~ contouring the field produclng induc-tor so as to enlarge the a~r gap bet~een the inductor and the ingot a~ areas between the inductor and the ingot corners. Thus, since undesirable rounding off of the corners results ~rom the action of excess electromagnetic force at the ingot corners, the desired modifica~ion of the field shape can be obtained by increased local screening o~ the field and/or ~ contouring the inductor at the corners.
Various embodiments of the present invention increase local screening of the electromagnetlc field by locally lncreasing shield depth, b~ locally providing deeper displace-ment of the shield, or by certain local changes in shield section or orientation.
PRIOR ART STATEMENT
Known electromagnekic casting apparatus comprises a three part mold consisting of a water cooled inductor~ a non-magnetic ~creen and a manifold for applying coollng water to the ingot being cast. Such an apparatus is exemplifled in U.S. Patent No. 3,467,166 to Getselev et al. Containment Or the molten metal is achieved ~ithout direct contact between the molten metal and any component of the mold. Solidification of the molten metal is achieYed by direct application of ~ater from the cooling manifold to the forming ingot shell.
In some prior art approaches the inductor is rormed as part of the cooling manifold so that the cooling manifold supplies both coolant to solidify the castin~ and to cool the inductor. See ~nited States Patent 4,0049631 to Goodrich et al.

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Non-~agnetic screens of the prior art are typlcall~
utilized ko properl~ shape the magnetic ~ieId for containing the molten metal, as exe~pll~ied in U.S. Patent 3,605,865 to &etselev. Another approach with respect to use o~ non~
magnetic screens is exempli~ied as well in U.S. Patent No.

3,985,179 to Cloodrich et al. Goodrich et al. ~17~ describes the use of a shaped inductor in conJunction with a screen to modify the electromagnetic forming field.
It is generally known that during electromagnetic casting the solidification front between the molten metal and the solidifyin~ ingot at the ingot surface should be maintained within the zone of maximum magnetic ~ield strength, i.e. the solidlfication front should be located withln the inductor.
If the solidification front extends above the inductor 3 cold folding is likely to occur. On the other hand, if lt recedes to below the inductor, a bleed-out or decantation o~ the liquid metal is likely to result. Getselev et al. '166 associate the coolant application manifold ~ith the screen portion of the mold such that they are arranged for simultaneous movement `~ 20 relatlYe to the lnductor. In U.S. Patent 4,1569451 to ~etselev a cooling medium is supplied upon the lateral face of the ingot in several cooling tiers arran~ed at varlous levels longitudinally o~ the ingot. Thus, depending on the pullin~ velocity of the ingot, the solldification front can be maintained within the inductor by appropriate selection of one of the tiers.
Another approach to improved ingot shape h~s included provisions of more uni~orm ~ields at conductor bus connections (Canadian Patent ~3Q,925 to Getselev).
~n electromagnetically casting rectangular or shee~

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ingots, the ingots are often cast with high radius of curvature ends or corners w~ich is indic~tive of the need for improved ingot shape control at the corners of such ingots.
Finally, United States Patent 3,502,133 to Carson teaches utilizing a sensor in a continuous or semi-continuous DC casting mold to sense temperature variations at a particular location in the mold during casting. The sensor controls application of coolant to the mold and forming ingot. Use of such a device overcomes instabilities with respect to how much extra coolant is required at start-up of the casting operation and jùst when or at what rate this excess cooling should be reduced. The ultimate purpose of adjusting the flow of coolant is to maintain the free2e line of the casting at a ; substantially constant location.
Carson '133 teaches that ingots having a width to thickness ratio in the order of 3 to 1 or more possess an uneven cooling rate during casting when coolant is applied periph-erall~ of the mo~d in a uniform mannerD ~o overcome this problem, Carscn '133 applies coolant to the wide faces of the ingot or/and the mold walls and not at all tor at least at a reduced rate) to the relatively narrow end faces of the ingot or/and the mold walls.

SUMMARY OF THE INVENTION
The present invention comprises a process and appar-atus for electromagnetic casting or castable materials inc-luding, but not restricted to, metals and alloys and silicon or other similar semi-metals, metalloids, and semi-conductors into rectangular or sheet ingots a~d other desired elements of shape control 9 having small radius of curvature corners or portions by modification of the electromagnetic field. In .. . . . . .. . .

r;~3 partlcular, a method and apparatus utilizing control or shaplng of the magnetic fi.eld by means o~ controlled or di~ferential field screenlng, particularly a~ the corners o~
rectangular ingots or other desired elements of shape ls clalmed. Control and shaping of the magnetic ~ield by means o~ contourlng of the electromagnetic inductor is also claimed.
In a ~urther embodiment, control or shaping of the magnetic fleld by difrerential screening and/or by inductor contouring ls combined with contoured lmpingement of a coolant about the surface of the ingot belng cast such ~nat the impinging coolant contac~s the ingot at a mlnimum peripheral ele~ation at or near the corners of the formi~g lngot.
According to the present invention, the desired modifi-cation of the field shape can be obtained by inductor contouring and/or by increased local screening of the electro-magnetlc field at the ingot corners, thereby making the rounding off of corners in electromagnetlc cast ingots less severe or of smaller radius.
In accordance with one embodiment of this invention, a desired modi~ication o~ the electromagnetlc field is obtained by contourlng the inductor so as to enlarge the gap between the inductor and the ingot at the lngot corners.
In accordance with another embodiment of this lnvention, increased local screening of the electromagnetic field at the ingot or deslred shape corner~ ls achleved by locally increasing the shleld depth at the corners.
In accordance with another pre~èrred embodiment of thls invention, increased local screenlng of the electromagnetic field at the desired shape or ingot corners 15 achieved by S~'7~

locally deeper displacement of the shield section at the corners.
In accordance with another ernbodiment of this invention, increased local screening is accomplished by locally changing the shield cross-section at the corners of the ingot or desired shape.
In accordance with yet another embodiment of this invention, increased local screening of the electromagnetic field at the ingot corners is achieved by locally altering the orientation of the shield at the ing~t corners.
All of the aforementioned screening embodiments of this invention operate via a net downward displacement of the screening current at the corners of the shield. It is of course understood that hybrids of locally increased shield depth, locally deeper displacement of the shield, local changes ln shield cross-section and local changes in shield orientation can also be utilized in accordance with the concepts of this invention.
Other ernbodiments of this invention contemplate the combining of the various modified screens with a contoured inductor and/or with a coolant manifold such that the effects of field control are enhanced by increased static head at the ingot carners brought about by impingement of coolant at a lower elevation at or near the corners of the inyot.
Accordingly, it is an object of this invention to pro-vide an improved process and apparatus for electromagnetic casting of castable materials into sheet ingots, or other de-sired elements of shape control, characterized by small radius of curvature corners or portions thereon.

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This and other ob~ects will become more apparent from the following descriptlon and drawings.
B~rEF DESCRI~TION OF THE D~AWIN~S
Figure 1 is a schematic cro~s-sectional representation of a prior art electromagnetic casting apparatus utilizing a uniform depth, cross-section and orientation non-magnetic shield.
Figure 2 is a perspective view of the prior art non-ma~netic shield o~ Figure 1.
Flgure 3(a) is a perspectiYe view of a non-magnetic shield in accordance with this inventlon showing increased local depth of the shield at the corners. Figure 3(b) ls a partial section through the ~ace of the shield of Figure 3(a) showing the shield positloned between an inductor and an ingot being cast.
Flgure 3(c) is a partial section through the corner of the shield, inductor and ingot of Fi~ure 3(b).
~igure 4~a) ls a perspective view of a non-magnetic shleld in accordance with another embodiment of this invention sAowing areas o~ locally deeper displacement of the shield at the corners. Figure 4(b) is a partlal section through the face of ~he shield of Figure 4(a) showing the shleld posit~oned between an inductor and an ingot being cast. Figure 4(c) is a partial section through the corner of the shield, inductor and ingot of Figure 4(b).
Figure 5(a) ls a perspectiYe view o~ a non-magnetlc ~hield in accordancP with another embodiment of this inventlon sho~ing areas o~ locally incllnation to the screen axis at the corners.
Figure 5(b) is a partial section through the face of the shield of Figure 5(a) showln~ the shield posltioned between an inductor and an ingot being cast. Figure 5(c) ls a partial 10052~
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section through the corner o~ the shield, indu~tor and ingot o~ Figure 5tb). Figure 5(d) is a bottom view o~ the shield of Figure 5(a).
Figures 6(a~ and 6(d) are top and bottom Yiews, respec-tlYely, of a non-magnetic shleld in accordance with another embodiment of this invention showing a ~hield o~ tapered section having increased thickness at the bottom of the screen corners. Figure 6(b) is a partial section throu~h the face of the shield of Figure 6(a) showing the shield positioned between an inductor and an ingot being cast. Figure 6(c) is a partial section through the corner o~ the shield, lnductor and ingot i ( of Figure 6(b).

Figure 7 is a partial schematic cross-sectional repre-sentatlon of the shield of Figure 3(a) belng utilized as part of a coolant manifold ln an electromagnetic casting apparatus.
Figure 8 is a partial schematlc cross-sectional repre-sentatlon of the shield of Figure 4(a) being utilized as part of a coolant manifold in an electromagnetic casting apparatus.
Figure 9 is a partial schematic cros~-sectional repre---20 sentation of the shield of Figure 5(a) being util~ zed as part o~ a coolant manifold ln an electromagnetlc castlng apparatus.
Flgure 10 ls a partlal schematic cross-sectlonal repre-sentation of a shield similar to the shield depic~ed ln Figures 6(a)-(d) belng utillzed as part of a coolant mani~old ln an electromagnetic casting appara~us.
Figure 11 is a partial top Ylew showing the isoflux llne contour for a prior art rectangular inductor.

Figure 12 i~ a partial top view showing the isoflux llne contour for a contoured inductor in accordance with one embQdiment of thls inYention.
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~igure 13 is a partial top Yiew showing a contoured inductor in accardance with another embodlment of this invention.-Figure 14 is a partial top view showing the iso~lux llne contour for a contoured inductor in accordance with yet another embodlment o~ this inYention.
DET~IL~D DESCRIPTION OF PRE~ERRED EMBODIMENTS
In all drawing ~igures alike parts are designated by alike numerals.
Re~erring now to FIGURE 1, there is shown therein a prlor art electromagnetic casting apparatus in accordance with U.S.
( Pa~ent 4,158,479.
The eLectromagnetic casting mold 10 is comprised of an inductor 11 which is water cooled; a coolant manifold 12 for applying cooling water to the peripheral sur~ace 13 o~ the metal being cast C; and a non magnetic screen 14. Molten metal is continuously introduced into the mold 10 during a casting run, in the normal manner using a trough 15 and down spout 16 and conven~ional molten metal head control. The inductor 11 is excited by an alternatlng current from a suitable power source (not shown).
The alternating curren~ ln the inductor 11 produces a magnetic fleld which interac~s with the molten metal head 19 to produce eddy currents therein. These eddy currents ln turn interact wlth the magnetic field and produce ~orces which apply a magnetic pressure to the molten metal hea~ 19 to contain lt so that it solldi~ies in a desired ingot cross-section, An air gap exls~s durlng casting, between the molten metal head 1~ and the inductor 11. The molten metal head''l9 is s~7~:3 formed or molded into the s~me general shape as. the inductor 1_ thereby proYiding the desired ingot cross-section. The inductor may have any known standard shape including circular or rectangular as required to obtain the deslred ingot C
cross section, but may also ln accordance with ~his invention be given a specific contour as depicted for example in Figures 12, 13, and 14.
The purpose of the non-magnetlc screen 14 is to fine tune and balance the magnetic pressure wi~h the hydrostatic pressure of the molten metal head 19 The non-magnetic screen 14 - comprises a separate element as shown and is not a part of the manifold 12 for applying the coolant.
Inltially, a conventional ram 21 and bottom block 22 ls held in the magnetic containment ~one of the mold 10 to allow the molten metal to be poured into the mold at the start of the castlng run. The ram 21 and bottom block 22 are then uniformly withdrawn at a deslred casting rate.
Solidiflcation of the molten metal which is magnetically contained in the mold lG is achieved by direct application of water from the cooling manifold 12 to the ingot surface 13.
The water is shown applied to the ingot surface 13 within the confines of the inductor 11. The water may be applled, however, to the ingot surface 1~ from above, withln or below the inductor 11 as desired.
The solldification front 25 of the castlng comprises the boundary between the molten metal head lg and the solidified ingot C. The location of the solidification front 25 at the lngot surface 13 results from a balance of the heat input from the superheated liquid metal 19 and the resistance . :~
3G heating ~rom the induced currents in the ingot surface layerg 10052-1~B
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with the lo~g-ltudinal heat extraction resultlng ~rom the cooling water application.
Coolant mani~old 12 is arranged aboYe the inductor 11 and includes t least one discharge port 28 at the end o~ extended portion 30 for d~recting the coolant against the surface 13 Or the lngot or casting. The discharge port 28 can comprise a slot or a plurality of lndividual ori~ices for d~recting the coolant again~t the surface 13 of the lngot C about ~he entlre periphery o~ that sur~ace.
Coolant manifold 12 is arranged for movement along vertlcally e~tending rails 38 and 39 a~ially of the ingot C
such that extended portion 30 and discharge port 28 can be moved between the non-magnetic screen 14 and the inductor 11.
Axial ad~ustment of the discharge por~ 28 posltion ls provlded by means of cranks 40 mounted to screws 41.
The coolant is discharged against the surface of the casting in the direction ind~cated by arrows 43 to define the plane of coolant applicat~on.
Figure 2 shows a prior art screen 14 o~ constant helght and sectlon a~ shown in Figure 1.- Rounding off o~ corners in electromagnetic casting of rectangular lngots and other shapes having corners from higher electromagnetic pressure at a glven distance ~rom the inductor near the corners, where two pro2imate races o~ the single turn lnductor generate field, as compared to the pressure at the same distance from the inductor on the broad faces of the illgOt or other shapes remote from the corner~ where only one inductor face acts. Solution to the problem may be sought ln accordance with thls invention through electromagnetic ~leld modl~icatlon. This in~ention relates to a method and apparatus which is utilized to control 10052-~B

or shape the magnetlc fleld by means of controlled or dl~ferential ~ield screenlng, partlcularly at the corner~ of rectangular ingots.
Use of screens for field modiflcation such as shown in Fi~ures 1 and 2 is known in the art. Getselev '865 descrlbes a screen or shleld in the form of a closed ring positioned wi~hin the induc~or with lts lower edge located approximately at the level of half of the helght o~ the inductor. The thickness o~ this shield is changed along its height in an a~ial or vertical direction to obtain a balance between the hydrostatic pressure and the electromagnetlc forces while maintaining a vertical side wall on the liquid immediately above the solidification front. This technique is designed to prevent ~ormation of a wave-shaped ingot surface due to varlations in its transverse dimensions. Accordingly~ shaping in thi~ form of screening is restricted to control of the liquid contour along the vertical axls of the castlng. No consideration ls given to shaping in the horizontal axis such as could be used for corner definltlon ln castlng of `20 rectangular ingots.
Since rounding o~ of lngot and other castlng shape corners results to a large extent from the action o~ excess electromagnetic force at the corner, the desired modiflcatlon of the field shape can be obtained by increased local screening of the f~eld at the corner~ In accordance wlth this invention, increased local screening can be achleved by locally increased shield depth, by locally deeper displacement of the shield, by locally changlng the shield sectionl or by locally changlng shield orientatlon. All o~ the abo~e embodiments operate via s~

a net, do~nward displacement of the screening current at the corners of the shield.
Figure 3~a) shows a non-magnetlc shield ln accordance with the present invention. Shield 32 is provlded wlth areas 34 of greater depth at the corners. Figure 3(b~ shows a partial section through a ~ace of ,nductor 11, screen ~a and ingot 20 while Figure 3(c) shows a partial section through a corner of these elements. For reference purposes elevatlon I-I is shown passlng through the critical point where liquid (L) - solid (S) front 37 intersects the periphery of ingot 20. It can be seen that at the ingot corners, Figure 3(c), screen ~2 pro~ects a greater depth with respect to elevation I-I than does the remainder of the screen along the faces of ingot 20, Fl~ure 3(b)~
Thls greater screen depth at the ingot corners causes the screening of more electromagnetic field from the ingot 20 at elevatlon I-I at the corners than along the faces of ingot 20.
Figure 4(a) shows a modification of the screen depicted in Figure 3(a). Screen 35 is provided ~ith greater depth 36 at the corners by displacement of the whole screen section downward at the corner locatlons. Figure 4tb) shows a sectlon through a face of inductor 11, screen 35 and lngot 20, while Fl~ure 4(c) shows a sectlon through the corner of these elements. The greater depth 36 of screen 35 as can be een in Figure 4(c) provides further enhanced screening at . .. .
elevation r-I at the corners of ingot ~0 than through the broad face depicted ln Figure 4(b).
Figures 5 (a) and 5 (d) illustrate another embodlment of this invention. Screen 52 is an inclined member of constant sectlon having a lower angle of inclination at the corners wlth respect to the axis of lngot 20. A~ can be seen from ~gi5~7~

Figure 5(b), a section through 'the ~ace of ingot 20, inductor ' 'li9 and screen-52, and Figure '5~c), a section through the corner o~ these elements, the ba,se of screen 52 nearest to elevation I-I is closest to inductor 1 at the corner of ingot 20. The closer a shield is to an inductor the more current is induced in the shield. Thus, the change in shield angle at the corners modulates the containment field at and near elevation I-I at the 'ingot corner depicted in Figure 5(c) more than along the ingot faces depicted by Figure 5~b~.
A further embodiment of a screen which can be utilized in accordance with this invention to provide modified (- screening at the ingot corners is depicted in Figures 6(a) through (d). Screen '54 is a tapered sectlon along the faces of the ingot 20 (Figure 6tb))- owe~er, screening of the corner at and near elevation I-I is increased by increasing the screen thickness at the bottom '56 o~ screen'54 as shown , in ~ection in Figure 6(c). I~ necessary~ the angle of taper can be reduced to zero. -Solution to the problem of rounded off corners caused by ~0 higher electromagnetic pressure near and' at ingot corners in electromagnetic casting may also be sought through metal head or pressure modlfication. Rounding off of corners in electromagnetic casting results in part from higher electro-magnetic pressure near and at the corners of the forming ingot and in part from excess cooling or higher heat extraction rates at the corners as a result of geometric and . ~ ... .. . --higher heat trans~er characteristics.

Prior art uniform rate and height peripheral coolant flow directed at the surface of a forming ingot leads to excess cooling at ingot corners and results in the solidif-ication front rising at the corners of the ingot as compared to the position of the solidification front along the faces of the forming ingot. Stated another way, the height of the solidifieation front from the point of coolant impingement at the corners of a uniformly cooled eleetromagnetically cast ingot is greater than the height of the solidification front from point of coolant impingement along the faces of the forming ingot. Thus, the combination of higher solidification front (lower head) and increased macJnetic pressure at the corners of the forming ingot causes the pushing of molten castable materials away from the eorners leading to a highly undesirable rounding off of the eorners.
Control of coolant application may also be utilized to produce controlled differential static head to thereby obtain refinement of ingot shapes at the corners, and in partieular to form smaller radius of eurvatures at ingot eorners~ This control is effeeted by selection of the rate and/or location of eooling water application to forming ingot shells. Rounding off of cornerls in electromagnetic casting ean be made less severe or of smaller raclius by con~ouring the water application rate and/or elevation so that the rate and/or elevation is a minimum at the corners of the ingot. Reduction of the water application rate anc~or lowerin~ of the application level serves to reduee the local heat extraetion rate along an ingot transverse cross-section line of constant height. This in turn lowers the position of the solidifieation front at the ingot corner and 10052~MB

correspondingly raises the metal static head or pPessu~e at the corneP. This increased' pressure results in the 'liquid metal approaching the inductor more closely at the corner and thus filling the corner to form a smaller radius of curvature at the corner before the increased static pressure is counterbalanced by ~he increased electromagnetic force.
- In a further embodiment of this inYention, aspects of two solutions to rounding off of ingot corners, namely solution khrough'electromagnetic fiel'd modification utilizing modified screens and solution through metal head or pressure modification by coolant control are combined in one apparatus and process. Figures 7 through 10 deoict utilization of the modified screens of this lnvention ln conJunction wlth or as part of a coolant manifold.
Figures 7 and 8 show screens 32 (Figures 3ta)) and '35 (Figure 4~a)) utilized as a part of or as an element of coolant manifolds' 18. Line''29 divides Figures 7 and 8 lnto sides (A) and (B)~ ~A) being a partial section through a face of the lngot'20, the inductor 11 and manifold 18, ~hile ~0 shield (B) represents a partial section t.hrough a corner of these elements. It can be seen that screer.s 32 and 35~ when utlllzed as a part of coelant mani~olds 18, serve the dual function of modifying and reducing the magnetic field at the corners of ingot 20 while simultaneously calsing a lowering of the elevatlon of impingement of coolant on the sur~ace 13 of ingot 209 thereby lowering the solidific~tion front 25 at the corners of ingot 20. In accordance wit'n the prlnciples discussed hereinabove 3 the combination of higher metal static head 19 and lower electromagnetic ~ield at the corners o~
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ingot 20 bring about added corner shaping and a reduction of the radius of curvature at the ingot corners.
Figure 9 shows screen '52 ~Figure 5ta)) utilized as part of or as an element of coolant manifold 18'. Again~ screen 52 is utilized as a part of manifold 1'8' to direct coolant flow at the surface 13 of ingot 20 such that the effects of increased screening at the corners, side tB~, would be enhanced by the lower elevation of water impingement on the surface of the ingot corner. The lower eleYation o~ impinge-ment of coolant at the ingot corners is brought about as a result of the shallow angle of screen '52 to the ingot surface at the corners thereof.
Finally, Figure 10 depicts a sli~ht variation of the screen depicted in Flgures 6(a) through 6td) utilized as part of a coolant manifold 18. Screen 54' directs coolant at ingot surface 13 at a lower elevation at the corners ('side' B) than at the broad faces of ingot 20 (side' A~. Thus, lncreased screening at the corners is enhanced by the lower elevation of coolant lmpingement and consequent lowering of solldlfi-cation front 25 at the ingot corners.
As an alternative or in addition to lower elevation coolant impingement, the mani~old and screens o~ this invention could be combined so as ~o deliver a lower rate of coolant application, lncluding a zero rate at the corners of the ingot. Such a lower rate also leads to a lowering of the solidification ~ront at the corners of the forming ingot leading to formation of corners haYing a smaller radius of curvature.
The mani~ol~s of this lnvention are typically constructed o~ non-metallic materials such as plastics 3 in particular 10052~

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relnforced phenolics,.whil~ th~ :screen~ in accordance with this invention are typicall~ con tructed o~ a non-~agneti.c metal ~uch as for example aus~enitlc ~tainless s~eel.
In acccrdance wlth another aspect of the present inven-tion, it has been found possible to reduce and control corner radlus in electromagnetically cast ingots by ~nductor shaping.
When an ingot is being cast with an electromagne~ic mold, the lngot will assume whatever shape is necessary to balance the hydros~atlc pressures against the containment ~orce. ~he 10 containment ~orce at any point is glYen by the vector product of the field (B) and the induced curren~.densit~ (J) 9 l.e.
( ~ the force is BxJ. Thus, that component 3c of the vector B
which contributes to the conta~nment .~orce is hereln denoted containment field. Since the current density ~J) is induced by the field (B), the containment force is roughly proportional to Bc Accordingly, to a first approximation a load with uniform head at equilibrlum in an EM mold will have a uni~orm Bc fleld around its perimeter at some eleYation Z above the solidification front. Wh tever shape the lines of constant i 20 con~ainment field map the load will conform to. Where the contours of containment ~ield Bc map into a rectangle, so will the load~ An exception to this general rule is found when a corner o~ radlus less than the penetration depth (~) e~i.sts. Here, current tends to short clrcult the corner.
Hence, at and near the corner J is reduced below what would be expected from the magnitude of the Bc field, and the ~orce Bc-J ls also reduced causing a fur~her bulglng efrect.
This bulging tend~ to ~urther reduce the corner radlus.
In accordance with this aspect o~ the present inventlon ln order to lmprove th~ corner shape of the containment ~ield ~,~g;S~

con~o~r lines, it is necessary to change the shape of the inductor in the vicinity o~ that corner.
Figure 11 shows a contai~ment field contour for a typical rectangular inductor, the inside surface 61 of which is shown in the drawing. As can be seen from the plot, the containment contour line 63 in the ~icinity of a corner, for e~ample corner 65, can be characterized by a curve with a maJor and minor radii, Rl and R2, respectively. Points A-A' mark the intersection of the two curves formed by Rl and R2 and serve as the reference for basic modification of the lnductor.
Points B-~' on the inductor face are opposite Poin~s A-A'. By C changlng the shape of the inductor t~ the shape of inductor 61' illustrated in Figure 12 3 wherein the inductor corners 62 are provided with a generally triangular cross-section, R
can be slgnificantly reduced with the contalnment contour more closely approaching the ideal containment contour 54.
As the parametric ratio dl/d2, with d2 being the normal alr gap~ increases~ R3 decreases asymptotically. By ad~usti~g the break points B-B t along the axis and ad~usting the radius ~ 20 dl/d2, corners wlth various degrees of curvature can be obtained.
To reduce the corner radii R3 in Figure 12 be~ond lts asymptotlc limit, an additional modification ~o the inductor corner is necessary. Such a modiflcation is shown in Flgure 13 wherein an inductor lnside surface 71 indicates the general shape of such ~ modi~ied inductor. In this modification the inductor corners 74 are provided so as to have a generally rectangular shaped cross-section. Again3 the parameters dl, d3, and B-B' are a ~unction o~ the normal air gap d2 desired and the ingot geometry. The as~mptotic limit of load corner 10052~
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radii o~ this modificatlon appears to be nearly an order of ~gnitude better than for the unmsdified prior art inductor 61 depicted ln Figure 11.
An analytical approach to the problem of obtalning ingots with small radii corners suggests an inductor ~rom 81 as ou~llned in Figure 14. As can now be seen, the inductors 61' and 71 shown in Figures 12 and 13 are piecewise l~near approx-imations to the inductor 81 ~n Flgure 14. The lnductor 81 ls shown provlded with generally rectangul~r shap~d cross~section corners 85 having curved transition sections 67 which ~oin the corners 85.to the sides 68 of lnd~ctor 81~ This inductor ! ~ produces a containment field contour 63" with nearly ideal corners. The actual curvature of the induc~or ls basically a function of desired ingot geometry, air gap d2 and the amount o~ ingot shrinka~e.
As stated hereinabove, corner~ of ingots which have been electromagnetically ca~t can be charac~erized by a curve having ma~or and minor radli Rl and R2, respectlvely~ Such an ingot can be utllized to determine the location of the points A-A', which points then ser~e as the baslc points ~or modiflcation o~ the inductor. Having determined the location of the polnts A-A', the polnts B-B' are then established on the inductor opposite points A-A'.
In the embodiment of Figure 12, it is desirable to make the value o~ dl signi~cantly greater than the Yalue of d2~
and at least twice as great as d2. In known electromagnetic castlng processes the value of d2 is typically between about 1~2 and 1-1~2 inches. Thus, the value o~ dl in accordance ~ith this inYention mlght range anywhere from about 1 lnch to lnfinity. For practical reasons, a preferred value of d 10052~ 3 would be in the range of 2 to 4 inchesO Re~errlng to Figure 1 ha~ing established the location o~ the polnts B-B' and the value of dl, ~he value of d3 becomes set implicitly and is seen to be approxi~ately equal to the distance between the point~ B-B'.
It should be noted that the optlmum contour for a gl~en EM casti~g process as exempli~ied by 63, 63', and 63'' in Figures 11, 12, and 14, respectively, is embedded lnto a famlly of non-opt~mum contours representing decreaslng contalnment ~lelds toward the lnterior of the inductor. Con~ours near the inductor will tend to simulate the shape of the inside perim-eter o~ the inductor whlle contours further removed from the lnside perimeter of the lnductor wlll tend to be elliptlc.
Typlcal EM casting lnductors have a height of from approximately 3/4 of an inch to 2 inches, and the inductors are typ~cally maintalned anywhere~ ~rom about 1/2 lnch to 1-1/2 inches from the forming ingot surface. The above descrlbed techniques for obtaining optimum contours of constant con-tainment flelds are most effective when applied to ~nductors ~~ 20 whose heights do not exceed about 10 times the gap between the inner surface of the inductor and the outer surface of the forming lngot.
Accordingly, corner control by inductor shaping can produce ingots wlth small radii corners, and this procedure constitutes an alterna~lve to using shield shape modifications.
However, it should be understood that elther method can be used singularly or in concert to produce ingots wlth improved corner definition.
A further advantage of the lnductor shaping procedure of thl~ inYention relates to inductor lead connections. Such i5~'71;~

lead connections are known to cause non uni~ormity o~ fleld and consequent ingot shape per~urbations (U.S. 3,70~,155 to Getselev)~ Such problems are readily solved by making the lead connectlons at a corner such as corners 66 an* 66' as shown in Flgures 12 and 14, respectively, wherein inductors''6'1' and 81 in accordance with th~s invention are shown attached to power sources 69 The increased separation of the lead connections ~rom the ingot sur~ace a~orded by this procedure serves to dlminish the field non-uniformity so produced to a negllglble le~el.
The novel method and apparatus of the present invention ; `- find applicability ln the electromagnetic casting of any . shapes wherein it is desired to ~orm portions thereon of low radlus of curvature.
It ls apparent that there ha.s been provided with thls invention a novel process and means ~or utilizing modi~ied inductor contours and/or modlfied local screening of electro-magnetic fields to o~taln refinement o~ lngot shape during elec~roma~netic casting which ~ully sat~sfy the ob~ect , means ~` `` 20 and advantages set forth hereinabove. While the invention has ; been descrlbed in combinatlon with specific embodlments thereo~, it is evident that many alternatlves, modlficatlo~s, and variations will be apparent to those skilled in the art i~
light of the foregolng description. Accordingly, lt is lntended to embrace all such alterna~ives, modl~ications, and variations as ~all within the spirit and broad scope o~ the appended claims.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A non-magnetic screen for use in electromagnetic casting of molten castable materials, said screen comprising a closed loop having at least one portion of small radius of curvature, said screen having a locally changing orient-ation at said at least one portion as compared to portions of said screen adjacent said at least one portion.

2. A screen as in claim 1 wherein said screen is an inclined member of constant section and said locally changing orientation comprises a variation in the angle of inclination of said screen with respect to the axis thereof.

3. A screen as in claim 1 wherein said screen is part of a coolant manifold.

4. A screen as in claim 1 wherein said loop is of a rectangular configuration and said at least one portion of small radius of curvature comprises the corners of said screen.