CA1165969A - 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 substantially closed loop having at least one portion of a small radius of curvature and a locally changing cross-section at the at least one portion as compared to portions of the screen adjacent the at least one portion.

Description

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 seml-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 o~
higher electromagnetic pressure at a given distance from the inductor near the ingot corners, where two proximate faces of the inductor generate a ~arger field. This is in contrast to lower electromagnetic pressure at the same distance from the inductor on the broad face of the ingot remote from the corner, where o~ly one inductor face acts.
There is a need to form small radius of curvature corners on sheet ingots so that during 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 reducing 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 screening ~ 10052-MB
~i5~6"3 current at the corner~ of a shield placed at the molten metal or alloy inpu~ end o~ the casting zone and~or b~ contouring the field produclng inductor so as to enlarge the a~r gap between the inductor and the ingot a~ areas between ~he lnductor and the ingot corners. Thus, since undesirable rounding off of the corners results from the action of excess electromagnetic force at the lngot corners, the desired modification of the ~ield shape can be obtalned by lncreased local ~creening of the field and/or by contouring ~he inductor at the corners.
- 10 Various embodiments of the present invention increase local screening of the electromagnetic field by locally increasing shield depth~ b~ locally providing deeper displace-ment of the shield, or by certain local changes in shield section or orientation.
PRIOR ART STATEMENT
Kno~n electromagnetlc castin~ apparatus comprises a three part mold consisting of a water cooled inductor, a non-magnetic screen and a mani~old for applyin~ cooling water to the ingot being cast. Such an apparatus is exempllfled in U.S. Patent No. 3,467,166 to Getselev et al. Containment of the molten metal is achieved without direct contact between the molten metal and any component of the mold. 5O1idiflcation of the molten metal is achieYed by direct application of water from the cooling manifold to the formlng ingot shell.
In some prior art approaches the inductor ls formed as part of the coollng manl~old so that the cooling manlfold supplies both coolant to solidify the castln~ and to cool the lnductor. See United States Patent 4 3 004,631 to Goodrich et al.

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Non-magnetic screens of the priQr art are typically utllized to ~roperl~ shape the magne~1c ~ieId for con~aining the molten metal, as ex~mpll~ied in UOS. Patent 3,6Q5,865 to Getselev. Another approach with respect to use o~ non-magnetic screens is e~emplified as well in U.S. Patent NoO
3,985,179 to ~oodrich et al. Goodrich et al. ?179 describes the use of a shaped inductor in con~unction with a screen to modify the electromagnetic forming field.
It is generally known that during electromagnetic casting the solidifi~ation front between the molten metal and the solidiPying ingot at the ingot surface should be maintalned within the zone of maximum magnetic ~ield strength, i.e. the solidification front should be located within the inductor.
If the solidification front extends above the inductor, cold ~olding is likely to occur. On the other hand, if it recedes to belo~ the inductor, a bleed-out or decantation of the liquid metal is llkely to result. Getselev et al. '166 associate the coolant application manifold ~ith the screen portion of the ; mold such that they are arranged ~or simultaneous movement ~ 20 relatiYe ~o the inductor. In U.S. Patent 4,156,451 to Getselev a cooling medium is supplied upon the lateral face o~ the ~ngot in several cooling tiers arran~ed at various levels longitudinally of the ingot. Thus, depending on the pulling velocity of the ingot, the solidification front can be maintained within the inductor by appropriate selection of one of the tiers.
Another approach to improved ingot sh~pe h~s lncluded proYisions of more uniform fields at conductor 'DUS connections (Canadian Patent 930,925 to Getsele~)~
In electromagnetically casting rectangular or sheet 6S~6~

ingots, the ingots are often cast with high radius of curvature ends or corners which is indicative 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 insta~ilities with respect to how much extra coolant is required a~ start-up of the casting operation and just when or at what rate this e~cess cooling should be ~ reduced. The ultimate purpose of adjusting the ~low of `~ coolant is to maintain the freeze line of the casting at a su~stantially 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-erally of the mold in a uniform manner. ~o overcome this problem, Carson '133 applies coolant to the wide faces of the ingot or/and the mold walls and not at all (or at least at a reduced rate) to the relatively narrow end faces of the ingot or/and the mold walls.
SUMMARY OF THE INVEN~ION
The present invention comprises a process and appar-atus for electromagnetic casting of 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 and other desired elements of shape control, having small radius of curvature corners or portions by modification of the electromagnetic ~ield. In 10052-1~B

particular, a method and apparatus utilizing control or shaping of the magnetic field by means of controlled or di~erentlal field screening, particularly at the corners o~
rectangular ingots or other desired elements of shape is claimed. Control and shaping of the magnetic fleld by means of contouring o~ the electromagnetlc inductor is also claimed.
In a fur~her embodiment, control or shaping of the magnetic field by dif~erential screening and/or by inductor contouring is combined wlth contoured impingement of a coolant about the surface o~ the ingot being cast such that the implnging coolant contacts the ingot at a minimum perlpheral elevation at or near the corners o~ the forming lngot.
According to the present inventlon, the desired modi~l-cation o~ the field shape can be obtalned by inductor contouring and~or by increased local screening of the electro-magnetlc field at the lngot corners, thereby ma~ing the roundlng of~ o~ corners ln electromagnetlc cast ingots less severe or of smaller radius.
In accordance with one embodiment of this invention, a desired modi~ication of the electromagnetic field is obtained by contouring the inductor so as to enlarge the gap between the inductor and the ingot at the ingot corners.
In accordance wlth another embodlmen~ o~ this inventlon, increased local screening of the electromagnetic field at the ingot or desired shape corners ls achieved by locally increasing the shield depth at the corners.
In accordance with another preferred embodiment of this lnvention, increased local screening o~ the electroma~netic fleld at ~he desired shape or ingot corners is achieved by iS9~

locally deeper displacement of the shield section at the corners.
In accordance with another embodiment 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 in shield cross-section and local changes in shield orientation can also be utilized in accordance with the concepts of this invention.
Other embodiments of this invention contemplate the combining of the various modi~ied 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 corners brought a~out by impingement of coolant at a lower elevation at or near the corners of the ingot.
~ ccordingly, 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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S~36~3 This and other ob~ects will become more apparent from the following description and drawings.
..~ _~
Flgure 1 ls a schematic cross-sectional representation of a prior art electro~agnetic casting apparatus utilizing a uniform depth, cross-section and orientation non-magnetic shield.
Figure 2 is a perspective vlew o~ the prior art non-magnetlc shield o~ Figure 1.
Figure 3(a) ls a perspectiYe view of a non-magne~ic shield in accordance with this lnvention showing increased local depth of the shield at the corners. Figure 3(b) is a partial section through the face Or the shield of Figure 3(a) showin~ the shield positioned between an inductor and an ingot being cast.
Flgure 3(c) ls a partial section through the corner o~ the shleld, inductor and in~ot of Figure 3(b).
Flgure 4(a) is a perspective ~ie~ Or a non-magnetlc shield in accordance with another embodiment o~ this lnvention sho~ing areas o~ locally deeper displacement of the shi~ld at the corners. Figure 4(b) is a partlal section tArough the face of the shield of Figure 4(a) showing the shield positioned between an inductor and an ingot belng cast. Figure 4(c) is a partlal sectlon through the corner of the shield, inductor and ingot of Figure 4(b).
Figure 5(a) is a perspectiYe view o~ a non-magnetic shield ln accordance with another embodiment of this invention showing areas of locally inclination to the screen axis at the corners.
Figure 5(b) is a partial sectlon through the ~ace o~ the shield of Figure 5(~) showing the shield positioned between an inductor and an ingot being cast. Figure 5(c) is a partlal r7~

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sec~ion through the corner o~ the shield, inductor and ingot of Figure 5Cb). Figure 5~d) is a bottom vlew o~ ~he shleld o~
Figure 5(a).
Figures 6~a) and 6(d) are top and bottom views, respec-tively, of a non-magnetic shield in accordance with another embodiment o~ this invention showing a shield of tapered section havlng increased thlckness at the bottom o~ the screen corners. Figure 6(b) is a partial sectlon through the ~acè 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 throu~h the corner of the shleld, inductor and lngot of Figure 6(b).
Figure 7 is a partlal schematic cross-sectional repre-sen~ation of the shield of Figure 3(a) being utilized as part of a coolant manifold ln an electromagnetic casting apparatus.
Figure 8 is a partial schematic cross-sectional repre-sentation of the shield of Figure 4(a) being utilized as part of a coolant mani~old in an electromagnetic castin~ apparatus.
Figure 9 is a partial schematic cross sectional repre-sentation of the shield of Figure 5(a) being ut~llzed as part of a coolant mani~old in an electromagnetlc castlng apparatus.
Figure 10 is a partial schematlc cross sectional repre-sentatlon o~ a shield similar to the shield depicted ln Figures 6(a)-(d) being utilized as part of a coolant mani~old ; ln an electroma~ne~ic casting appara~us.
Figure 11 ls a partlal top Yiew showlng the iso~lux llne contour for a prior art rectangular inductor.
Flgure 12 is a p~rtial top view showing the lso~lux line contour for a contoured inductor in 2ccordance wlth one embodiment o~ this in~ention.

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Flgure 13 is a partial top view showing a cont.oured inductor in accordance with another embodiment of thl's inventlon .
Figure 14 1~ a partial top view showing the lsoflu~ line contour for a contoured lnductor ln accordance wlth yet another embodiment of this inYention.
DETAIL~D DESCRIPTION OF PREFERRED EMBODIMENTS
In all drawing figures alike parts are designated by alike numerals.
Ref'errlng now to FIGURE 1, there is shown therei~ a prlor art electromagnetic casting apparatus in accordance with U.S.
I ( Paten~ 4,158,479.
The electromagnetic castin~; mold 10 is comprised o~ an inductor 11 which is water cooled; a coolant manifold 12 for applying cooling water to the peripheral surface 13 of the metal belng cast C; and a non-ma.gnetlc screen 14. Molten metal is continuously lntroduced into the mold 10 during a castlng ~, run, in the normal manner using a trough 15 and down spout 16 and conventlonal molten metal head controlO The inductor 11 is excited ~y an alternatlng current from a suitable power source ' ~not shown).
The alternating current ln the inductor 11 produces a magnetic ~ield which interacts wlth the molten metal head 19 to produce eddy currents therein. These eddy currents in turn lnteract wi'h the magnetic field and produce forces which apply a magnetic pressure to the molten metal head''l9 to contain it so that it solldifie~ in a desired ingot cross-sectlon.
An air gap e~ists during casting~ between the molten metal head 1~ and the inductor 11. The molten metal head''l9 1s ~ti~

formed or molded lnto the same general shape as the induc~or 11 thereby proYiding the deslred ingot cross-section. The inductor may haYe any known standard shape includlng circular or rectangular as required to obtain the desired lngot C
cross-sectton, but may also ln accordance wlth this invention be glven a specific contour as depl cted for example ln Figures 12 3 13, and 14.
The purpose of the non-magnetic screen 14 is to fine tune and balance the magnetic pressure wl~h the hydrosta~ic pressure of the molten metal head 19. The non-magnetic screen 14 comprises a separate e~ement as shown and is not a part of the manifold 12 for applying the coolant.
Initially~ a conventional ram 21 and bottom block 22 is held in the magnetlc containment zone o~ the mold 10 to allow the molten metal to ~e poured into the mold at the start o~
the casting run. The ram 21 and bottom block 22 are then uniformly withdrawn at a desired casting rate.
Solidi~ication of the molten metal which is magnetlcally contained ln the mold lG is achie~ed by direct appllcation of water from the cooling manlfold 12 to the ingot surface 13.
The water is shown applied to the lngot surface 13 within the con~ines o~ the inductor 11. The water may be applied, however, to the ingot surface 13 from above, within or below the inductor 11 as desired.
m e solidlficatlon front 25 of the casting comprises the boundary between the molten metal head 19 and the solidlfled lngot C. The locatlon of the solldiflcation front 25 at the lngot surfaee 13 results ~rom a balance of the heat lnput from the superheated liquld metal 19 and the resistance heating from ~he lnduced currents ln the lngot sur~ace layer, ;

10052`l~B

~ 3 with the longitudinal heat extractlon resulting ~rom the cooling water application.
Coolant manifold 12 is arranged abo~e the inductor 11 and lncludes a~ least one discharge port 28 at the end of extended portion 30 ~or directing the coolant against the surface 13 of the ingot or castlng~ The discharge port 28 can comprise a slot or a plurality o~ individual orifices for directing the coolant again~t the surface 13 of the lngot C about the entire periphery of that surface.
Coolant manifold 12 is arranged for movement along vertically extending rails 38 and 39 a~ially of the ingot C
such that extended portion 30 and discharge port 28 can be mov~d between the non-magnetic screen 14 and the inductor 11.
Axlal ad~ustment of the discharge port 28 positlon is provided by means of cranks 40 mounted to screws 41.
The coolant is discharged against the surface of the casting in the dlrection ind+cated by arrows 43 to de~ine the plane o~ coolant application.
Figure 2 shows a prior art screen 14 of constant height and section as shown in Figure 1. Rounding off o~ corners in electroma~netic casting of rectangular ingots and other shapes having corners from higher electromagnetic pressure at a ~iven distance from the inductor near the corners, where two pro~lmate faces of the slngle turn inductor generate fleld, as compared to the pressure at the same dlstance from the inductor on the broad faces of the ingot or other sh2pes remote from the corner, where only one inductor face acts. Solution to the problem may be sought ln accordance with this lnvention through electromagnetlc ~ield modification. This lnYention relates to a method and apparatus which is utilized to control - iO052-MB

or shape the mag~etic ~ield by means of controlled or differential ~ield screening, particularly a~ the corner~ o~
rec~angular ingots~
Use o~ screens ~or field modification such as shown in ~lgures 1 and 2 is known in the art. Getselev '865 describes a sc~een or shield in the form o-f ? closed ring positioned wlthln the inductor with its lower edge located approximately at the level of half of the height of the inductor. The thickness of thls 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 solidi~lcation ~ront. Thls technlque is designed to preven~ formation of a wave-shaped ingot sur~ace due to variations in its transverse dimensions. Accordlngly, shaping in this form o~ screening is restricted to control o~ the llquid contour along the vertical axls o~ the casting. No consideratlon is given to shaping in the horlzontal axis such as could be used for corner definition in casting of `20 rectangular ingots.
Since rounding off o~ ingot and other casting shape corners results to a large extent from the action of excess electromagnetic force at the corner, the desired modl~lcation of the field shape can be obtained by increased local screening of the f~eld at the corner. In accordance with this invention, increased local ~creenlng can be achleved by locally increased shield depth, by locally deeper displacement o~ the shield, by locally changing the shield section, or by locally chansing shield orien~ation. All of the above embodiments operate via 3o ~;t~

a net downward displacement of the screening current at the corners of the shleld.
Figure 3(a~ shows a non-magnetic shield in accordance with the present invention. Sh~eld 32 ls provlded with areas 34 of greater depth at the corners. Figure 3(b~ shows a partial section through a face of inductor 11, screen 32, and ingot 20 while Figure 3(c) shows a partial section through a corner of these elements. For reference purposes elevatlon I I ls shown passing through the crltical 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 32 pro~ects a greater depth with respect to elevation I-I than does the remainder of the screen along the faces o~ ingot 20, Figure 3(b), This greater screen depth at the ingot corners causes the screening of more electromagnetic field from the ingot 20 at elevation I-I a~ the corners than along the faces of ingot 20.
Figure 4(a) shows a modi~ication of the screen depicted ln Figure 3(a). Screen 35 is provided with greater depth 36 at the corners by displacement of the whole screen section downward at the corner locations. Figure 4(b~ shows a section through a face of lnductor 11~ screen 35 and ingot 20, while Fi~ure 4(c) shows a ~ection through the corner of these elements. The greater depth 36 of screen 35 as can be seen in Figure 4(c) provides further enhanced screening at elevation r-I at the corners of ingot 20 than through the broad face deplcted in Figure 4(b).
Figures 5(a) and 5(d) illustrate another embodiment of this invention. Screen 52 ls an inclined member of constant section having a lower angle o~ inclination at the corners with respect to the axis o~ ingot 20. As can be seen ~rom .. . .. .. . ....

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Figure 5(b), a section through'the 'face of ingot 20, inductor ''li, and screen'52, and Figure '5~c), a section through the corner o~ these elements, the base o~ screen 52 nearest to elevation _-I is closest to inductor ll at the corner of ingot 20. The closer a shieId 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 ele~ation 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 ln accordance with this invention to provide modified ! ( screening at the ingot corners is depict~d in Figures 6(a) through (d). Screen 54 is a tapered section 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 of screen 54 as shown in section in Figure 6~c). If necessary, the angle of taper can be reduced to zero.
Solutlon 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 modification. 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 geometrlc and higher heat transfer 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 solidification front from the point of coolant impingement at the corners of a uniformly cooled electromagnetically 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 magnetic pressure at the corners of the forming ingot causes the pushing of molten castable materials away from the corners leading to a highly undesirable rounding off of the corners.
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 particular to form smaller radius of curvatures at ingot corners. This control is effected by selection of the rate and/or location of cooling water application to forming ingot shells. Rounding off of corners in electromagnetic casting can be made less severe or of smaller radius by contouring the water application rate ~nd/or elevation so that the rate andfor elevation is a minimum at the corners of the ingot. Reduction of the water application rate and~or lowering of the application level serves to reduce the local heat extraction rate along an ingot transverse cross-section line of constant height. This in turn lowers the position of the solidification front at the ingot corner and correspondingly raises the ~etal 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 ls counterbalanced by the increased electromagnetic force.
In a further embodiment of this inYention~ aspects of two solutions to rounding oP~ of ingot corners, namely solution through èlectromagnetic ~ield modification utilizing lQ modi~ied screens and solution through metal head or pressure modification by coolant control are combined in one apparatus ( and process. Figures 7 through 10 de~ict utiliæation o~ the modified screens of this invention in con~unction with or as part o~ a coolant maniPold.
Figures 7 and 8 show screens 32 (Figures 3(a))~ and 35 tFigure 4~a)) utilized as a part oP or as an element of coolant manifolds 18. Line 29 divides Figures 7 and 8 into sides (A~ and (B), (A) being a partial sectlon through a face of the ingot 20, the inductor 11 and manifold 18, while ( ~0 shield tB) represents a partial section t~lrough a corner of these elements. It can be seen that screers 32 and 35, when utilized as a part of coolant manifolds 18~ serve the dual function of modifylng and reducing the magnetic field at the corners of in~ot 20 while simultaneously calslng a lowering Or the elevation of lmpingement of coolant on the surface 13 of ingot 20, thereby lowering the solidific~tion front 25 at the corners of ingot 20. In accordance Wit'l the principles discussed hereinaboYe, the combination of hieher metal static head 19 and lower electromagnetlc Pield at the corners oP
3o 10052~

ingo~''20 bring about added corner shaping and a reduction of the radius of curvature at the ingot corners~
Flgure 9 show~ scree'n '52 ~Figure 5(a)) utilized as part o~ or as an element of coolant manlfold 18 t . Again, screen 52 is utilized as a part of manifold 1'8' to direct coolant flow at the surface '_ of ingot 20 such that the effects of increased screening at the corners, side ~B~, would be enhanced by the lower eleYation of water impingement on the surface of the ingot corner. The' lower eleYation of 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 slight variation of the screen depicted in Figures 6(a) through 6(d) utilized as part of a coolant manifold 18. Screen'54' directs coolant at ingot surface 13 at a lower elevation at the corners (slde' B) than at the broad faces of ingot 20 (side A~. Thus, increased screening at the corners is enhanced by the lower eleYation c of coolant impingement and consequent lowering of solldlfi-cation fron~ 25 at the ingot corners.
As an alternatiYe or in addition to lower elevation coolant lmpingement, the manl~old and screens o~ this invention could be combined so as to deliver a lower,rate of coolant application, includlng a zero rate at the corners of the ingot. Such a lower rate also leads to a lowering of the solidification front at the corners of the forming ingot leading to formation of corners having a smaller radius o~
curvature.
The manifolds o~ this invention are typically constructed of non-metalllc materials such as plastics, in particular 1~052~

re~n~orced phenolic~, whil~ th~ screens in accordance with thls invention are typicall~ constructed o~ a non-~agnetic metal such as for example austenitic stainless steel.
In accordance ~ith ano~her aspect of the present inven tion, it h~s been ~ound possible to reduce and control corner radius in electromagnetically cast ingots by inductor shaping.
When an ingot is being cast with an electromagnetic mold, the ingot will assume ~hatever shape ls necessary to balance the hydrostatic pressures against the containment force. The containment force at any point is gi~en b~ the vector product o~ the field tB) and the induced current density (J), i.e.
. the force is BxJ. Thus, that component Bc of the vector B
whlch contributes to the containment force is hereln denoted contai~ment field. Since the current density ~J) is induced by the field ~B), the containment force is roughly proportional to BC2. Accordingly, to a flrst approximatlon a load with unl~orm head at equilibrium in an EM mold will ha~e a unlform Bc field around i~s perlmeter at some eleYakion Z above the solidification front. Whatever shape the lines of constant 20 contalnment field map the load wlll conform to. Where the contours o~ containment ~ield 8c map into a rectangle9 so will the load. An exception to thls general rule ls ~ound when a corner of radius less than the penetration depth (~) exists. Here, current tends to short circult the corner.
Hence, at and near the corner J is reduced below what would be expected ~rom the magnitude of the Bc field, and the force Bc~J i~ also reduced causlng a further bulging effect.

Thls bulging tends ~o further reduce the corner radlus.
In accordance with this aspect o~ the present invention in order to improve the corner shape of the containment field 100~2~MB

contour llnes, lt is necessary to change the shape of the inductor in the vicinity of that corner.
Figure 11 shows a contalnment field contour for a typical rectangular inductor, the inslde surface 61 of which is shown in the drawing. As can be seen from the plot, the containment contour line 63 in the vicinity o~ a corner, for example corner 65, can be characterized by a cur~e with a ma~or and minor radii, Rl and R2, respectively. Points A-A' mark the intersectlon o~ the two cur~es formed by Rl and R2 and serve as the re~erence for basic modificatlon o~ the induc~or.
Points B-B' on the inductor face are opposite Points A-A'. By changing the shape of the inductor to the shape of inductor 61' illus~ra~ed ln Figure 12, whereln the lnductor corners 62 are provlded with a generally tr~angular cross-section, Rl can be significantly reduced with the containment contour 63' more closely approaching the ldeal contalnment contour 64.
As the parametric ratio dl/d2, with d2 being the normal alr gap, increases, R3 decreases asymptotlcally. By ad~usting the break points B-B' along the axis and adJusting the radiu~
dl/d2, corners wlth various degrees of curvature can be obtalned.
To reduce the corner radii R3 ln Flgure 12 beyond lts asymptotlc llmit, an additlonal modi~lcation to the inductor corner is necessary. Such a modlflcation is shown ln Flgure 13 whereln an lnductor inside surface 71 lndicates the general shape o~ such ~ modi~ied lnductor. In this modlfication the inductor corners 74 are provided so as to have a generally reGtangular shaped cross~-section. Again, the parameters dl, d3, and B-B' are a function of the normal air gap d2 desired and the ingot geometry. The asymptotic limit o~ load corner --19~

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radii o~ this modi~lcation appears to be nearly an order o~
m~gnitude bet~er than ~or the u~modifie~ prior ar~ inductor 61 depicted ln Figure 11.
An analytical approach to the problem of obtain~ng ingots with small radii corners suggests an induc~or from 81 as outlined in Figure 14. As can now be seen, the inductors 61' and 71 shown in Figures 12 and 13 are piecewise llnear approx imations to the inductor 81 in Figure 14. The inductor 81 is shown pro~ided with generally rectangular shaped cross section corners 85 having curved transition sections 67 which ~oin the corners 85.to the sides 68 o~ ind~ctor 81. This inductor ( produces a containment field contour 63'' with nearly ideal corners. The actual curvature of the inductor is basically a function o~ deslred ingot geometry, air gap d2 and the amount of lngot shrinka~e.
As stated hereinabove, corners of ingots which have been electromagnetically cast can be characterized by a curve having ma~or and minor radii Rl and R2, respectively. Such an ingot can be utillzed to determine the locatlon of the points A-A', which points then serve as the ~asic polnts ~or modification o~ the inductor. Having determined the location of the points A-A', t~e points B-B' are then established on the inductor opposite points A A'.
In the embodlment of Figure 12, it is desirable to make the value of dl significantly greater than the value o~ d2~
and at least twlce as great as d2. In known electromagnetlc cast~ng proces~e~ the value of d2 i~ typlcally between about 1/2 and 1-1/2 inches. Thus, the value of dl in accordance ~ith this in~ention might ranæe anywhere ~rom about 1 inch to in~inity. For practical reasons~ a pre~erred value o~ dl ~0--would be in the range o~ 2 to 4 inches. Referring to Flgure 1 ha~ing established the location of the points ~ B' and the value o~ dl, the value o~ d3 becomes set implicitly and is seen to be approxi~ately equal to the distance between the point3 B~B'.
It should be noted that the optimum contour for a given EM castln~ process as exemplified by 639 63', and 63" ln Figures 11, 12, and 14, respectively, is embedded into a family of non-optimum contours represen~ing decreasing containment fields toward the interior of the inductor. Contours near the inductor will tend to simulate the shape o~ the inslde perim-eter of the inductor while contours further removed from the lnside perlmeter o~ the inductor wlll tend to be elliptic.
Typical EM casting inductors have a heigh~ of ~rom approxlmately 3/4 o~ an lnch to 2 inches, and the inductors are typically maintained anywhere ~rom about 1/2 lnch to 1-1/2 inches from the ~orming ingot surface. The above de~cribed techniques for obtalning optlmum contours o~ constant con-tainment ~lelds are most e~ective when applied to lnduc~ors ~- 20 whose heights do nok exceed about 10 times the gap between the inner sur~ace of the inductor and the outer sur~ace o~ the forming ingot.
Accordingly, corner control by inductor shaping can produce ingots wlth small radii corners, and this p~ocedure constitutes an alternative to using shield shape modi~ications.
However, lt should be understood that either method can be ; used singularly or ln concert to produce ingots with lmproved corner definitlonO
A further advantage o~ the inductor shaping procedure of this invention relates to inductor lead connection~. Such 1 0 0 5 2 ~

tj~

lead connectlons are known to cause non-unlformlty of ~ield and consequent ingot shape perturbations (U.S. 3~7a~,155 to Getsel~v). Such problems are readily solved by making ~he lead connections at a corner such as corners 66 and 66' a shown in Figures 12 and 14, respectively, wherein inductors 51' and'81 in accordance with this invention are shown at~ached to power sources 69. The lncreased separation of the lead connections ~rom the ingot surface a~orded by this procedure serves to diml nish the ~ield non-unlformity so produced to a negllgible level.
The novel method and apparatus of the present invention find applicabllity in the electromagnetlc casting o~ any shapes wh~reln'it is deslred to form portions thereon of low radius of curvature.
It is app~rent that there has been provlded with this inventlon a novel process and means ~or utilizing modl~led lnductor contours and/or modifled local screening of electro-magnetic fleld~ to obtain refinement of ingot shape during electromagnetic casting which fully satis~y the ob~ect~, means and ad~antages set forth herelnabove. While the lnvention has been described in comblnation with speclflc embodiments thereo~, lt ls evident that many alternatives, modlfications, and variations wlll be apparent to those skllled in the art in light of the foregoing description. Accordingly, it ls intended to embrace all such alternatives, modifications, and variations as fall within the spiriS and broad scope of the appended claims.

3o

Claims (5)

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 substantially closed loop having at least one portion of small radius of curvature, said screen having a locally changing cross-section 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 has a greater depth at said at least one portion as compared to portions of said screen adjacent said at least one portion.

3. A screen as in claim 1 wherein said screen has a thicker bottom at said at least one portion as compared to portions of said screen adjacent said at least one portion.

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

5. 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.