IN-MOLD ELECTROMAGNETIC STIRRING OF MOLTEN METAL DURING CASTING

Abstract: A system for electromagnetically stirring molten metal in an electrically conductive mold. An alternating electric current is applied to the walls (12) of the mold (2) and generates an electromagnetic field with the mold (21) to cause the stirring of the melt (8) therein. In one embodiment, the walls of the mold are continuous. In another, the mold wall has a non-conductive gap along the vertical length.

Description: IN-MOLD ELECTROMAGNETIC STIRRING OF MOLTEN METAL DURING CASTING This invention relates generally to the casting of molten metals. More particularly, this invention relates to the in-mold stirring of the molten metal during casting. The electromagnetic stirring (EMS) of molten metal during the continuous casting process is well known in the steel and aluminum industry. In an article by J. P. Birat and J. Chone entitled "Electromagnetic Stirring on Billet, Bloom and Slab Continuous Casters State of the Art in 1982", appearing in Continuous Casting Volume three, Pages 21 through 34, published by the Iron and Steel Society of AIME, 1984, several techniques for electromagnetic stirring during the continuous casting process of steel are described. Such techniques include beneath the mold (submold) stirring and in-mold stirring. In submold stirring, an electromagnetic stirrer is placed beneath the mold to generate a magnetic field which penetrates through the solidified shell of the cast metal and into the liquid melt core. Such a technique is described in an article by T. Adachi, M. Mizutani and K. Kimura entitled "Application of Electromagnetic Stirrers" appearing in Continuous Casting Volume three, Pages 79 through 85, published by the Iron and Steel Society of AIME, 1984. In-mold stirring involves the stirring of the melt while it is in the mold. This has been accomplished by placing stators, linear motors, or induction coils around the mold with the electro¬ magnetic field generated thereby passing through the wall of the molds as well as the solidified shell to accomplish the stirring. The use of an induction motor for this purpose is described in the article by M. Gray, A. McLean, G. Weatherly, R. J. Simcoe, R. Hadden and L. Beitelman entitled "Electromagnetic Stirring in the Mold During Continuous Casting" appearing in Continuous Casting Volume three. Pages 69 through 76, published by the Iron and Steel Society of AIME, 1984. The use of an inductive loop around the mold for stirring is described in an article by Ch. Vives and V. Forest entitled "CREM: A New Casting Process Part I: Fundamental Aspect", Light Metals. 1987. The use of an inductor for in-mold stirring is also described in the U.S. Patent 4,462,458 issued July 31, 1984 to Jacques Ruer. All liquid metal stirring processes involve the interactions between currents induced in the melt and the primary magnetic flux. In static field cases these body forces are normal to the current and flux vectors. Thus different senses of fluid motion can be achieved using different magnetic geometries. Alternatively traveling fields can be used to move liquid metal as in the case of stators and linear motors. Here, the liquid is caused to move via coupling to the traveling field that is derived from the pole-to-pole phase difference in the magnet. Two primary modes are of practical interest in continuous slab casting. These are linear motor arrays set in the plane of the mold liner about the casting axis. Such a geometry results in stirring about the casting axis. Alternatively, an inductor (current carrying, single turn) is situated about the axis of the casting direction close to the walls of the mold liner. This geometry results in recirculating, perimeter, stirring cells parallel and antiparallel to the casting direction. In both cases, stirring velocities are sought that can disturb the solidifying interface thereby and modifying the solidification structure. In submold stirring, it is necessary for the core of molten metal to extend below the bottom surface of the mold. In some cases, particularly in the case of the continuous casting of copper and copper alloys, the molten core, or sump, is of a similar depth as the mold or less and does not extend below the mold. This precludes the use of submold stirring techniques and requires the electromagnetic field to be applied in the mold. In electromagnetic stirring of the melt in the mold, previous techniques have required the electromagnetic field to pass through the mold liner. In casting the molten metal, particularly the continuous casting of copper alloys, the mold liner used is copper or a copper based alloy. Copper has high electrical conductivity which works to a detriment in that the copper mold liner walls tend to severely attenuate the magnetic flux of the electromagnetic field. Thus, the passing of the electromagnetic field through the mold wall is highly inefficient and requires extremely high power levels. This problem is described in an article by A. A. Tzavaras entitled "Solidification Control by Electromagnetic Stirring-State of the Art" appearing in Continuous Casting Volume Three, published by the Iron and Steel Society of AIME, 1984. It is an object of the present invention to provide a process and system for electromagnetically stirring molten metal while in the mold during casting which overcomes the problems set forth above. It is another object of the present invention to provide an improved process and system for electro¬ magnetically stirring of molten metal during casting which is relatively more efficient from an energy utilization standpoint. A more specific object of the present invention is the provision of a process and system for the electromagnetic stirring of the melt during the continuous casting of copper and copper based alloys. These and other objects of the present invention may be accomplished in accordance with the present invention through the provision of a system for electromechanically stirring molten metal during casting which comprises an electrically conductive mold having walls surrounding the molten metal and means for connecting the mold liner to a source of electric current for generating an electromagnetic field within the mold. A process according to the present invention for casting molten metal may comprise of providing an electrically conductive mold forming a wall around the molten metal, pouring the molten metal into the mold, and passing a current through the mold liner and producing an electromagnetic field within the mold to provide stirring for the molten metal therein. These and other features and advantages of the present invention will be better understood by reference to the following description of specific embodiments and to the accompanying drawings showing illustrative embodiments of the invention. In the course of the following detailed description reference will be made to the attached drawings in which: Figure 1 is a schematic view of the electro¬ magnetic stirring system of the present invention showing the mold liner in cross section; Figure 2 is a partial plan view of the system of Figure 1; Figure 3 is a top plan view of a mold liner showing a second embodiment thereof; Figure 4 is a partial sectional view taken along the lines of 4-4 of Figure 3; Figure 5 is a partial sectional view taken along the lines of 5-5 of Figure 4; and Figure 6 is an enlarged sectional view taken along the lines of 6-6 of Figure 4. Referring now to Figure 1, the improved system for in-mold stirring according to the present invention is shown in connection with the direct chill (DC) casting of metal and metal alloys and particularly copper and copper alloys. The DC casting process is done in vertical molds and produces large cross sectional slabs which are subsequently reheated and hot rolled into heavy gauge strip and coiled. The system includes a conventional mold or mold liner 2 which is fabricated from copper or a copper alloy. In a typical commercial process, the mold liner 2 would be one of several that are placed in a spray box (not shown) to produce slabs 4 of the desired length. According to the commercial process the molten metal is poured from a holding furnace into a pouring box and then through a spout into the individual mold liners. Generally, the mold liners are short, 8 to 20 inches, so that a hardened shell 6 forms against the interior surface of the mold. The interior of the cast metal within the mold is molten forming a melt 8 which may have a depth equal to or less than the depth of the mold liner 2 as shown in Figure 1. As the slab 4 exits from the open bottom of the mold in the direction indicated by the arrow in Figure 1, water is sprayed on it, cooling it and causing the contained molten metal to freeze until a continuously cast slab 4 of the desired length is produced. Typically, the mold liner 2 is rectangular in shape having an upper flange 10 extending outwardly from the downwardly extending walls 12 which form a smooth, uninterrupted internal wall surface 14 which surrounds and contains the metal and forms the opening 16. (The internal wall surface is formed by two relatively long side walls 15 and two relatively short end walls 17.) According to the embodiment shown in Figures 1 and 2, a pair of oppositely disposed plates 18 and 20 are mounted on and extend upwardly from the upper surface 22 of the mold liner 2, which serve to provide a bolting flange and electrical connection for electrical conductors 24 and 26. As shown, the plates 18 and 20 are located adjacent the short sides of the rectangular opening 16. The electrical conductors 24 and 26 connect each of the plates 18 and 20 to a power control system 28. The power control system 28 is in turn connected by suitable connectors 30 and 32 to a suitable power supply such as a 440 volt single phase 60 Hz incoming power supply to the plant. The power control system 28 is such that it takes the incoming power supply and converts it to the desired voltage and alternating current values for transmission to the mold liner 2. Any type of system can be used. For example, such system may include a saturable core reactor 34 and a transformer 36. The saturable core reactor 34 is connected between the incoming power supply and the transformer 36, while the output of the transformer is connected to the mold liner by electrical conductors 24 and 26. According to the embodiment shown, the power system 28 takes a 440 volt, 60 Hz incoming power supply and converts it to a low voltage, in the range of 0 to 10 volts, and a high alternating current, in the range of 1,000 to 20,000 amperes or greater which is transmitted to the mold liner 2, With the embodiment shown in Figure 1, one half of the current flows from the positive plate 18 at one of the end walls through one long side wall of the mold liner 2 to the negative plate 20 and the other half of the current flows from the positive plate 18 through the other long side wall to the negative plate 20. The current flows around the mold and its associated normal electromagnetic field induces current flow in the melt. The magnetic interaction between the induced current and the primary field results in a repulsive force normal to both vectors. This force acts to move the liquid away from the mold wall with resultant circular motion and stirring of the melt. The stirring is therefore driven about axes horizontal and parallel to the mold walls and about the full perimeter of the mold. Singularities such as the corners disturb the symmetry of these stirring patterns and result in advantageous turbulent patterns of flow. In the embodiment shown in Figure 3, the rectangular mold liner 38 is split adjacent the junction of one of the short end walls 40 and a long side wall 42 forming a gap as indicated by the reference numeral 44. A thin sheet or strip 46 of a dielectric material is mounted in the gap formed by the split and has an inner surface of the mold liner 38 to provide even surface continuity in the internal wall of the mold. The dielectric material may be any suitable nonconducting material such as mica, or the like which will interrupt current flow. With reference to Figures 4, 5, and 6, the gap 44 is closed and held in a clamped position by means of a plurality of stainless steel, socket headed threaded bolts 50 which are mounted in bores 52 drilled into the elongated side wall portion of the mold along a vertically extending reduced land 54 provided thereon. A heli-coil internally threaded insert 58 is screwed into the bottom of each of the bores 52 to receive the threaded ends of the bolts 50. A phenolic spacer 60 is provided between the heads of the bolts 50 and the surface of the land 54. The spacer 60 may be in the form of individual washers or a single elongated strip on the order of .125 inch thick. A tube 62, also of phenolic material, surrounds the shank 64 of each of the bolts 50 and extends from the land 54 on the mold liner to the gap 44. Although the spacers 60 and tubes 62 are preferably made of a phenolic material, any suitable non-conducting material may be used. The tightening of the bolts 50 into the inserts 58 serves to clamp the mold liner together with the mica sheet 46, spacer 60, and tubes 62 insuring a interruption in electrical conductivity at the gap 44. A pair of plates 66 and 68 are connected to and extend upwardly on either side of the gap in the mold liner from the upper surface thereof and serve as bolting flanges for attaching the positive and negative connectors 70 and 72 respectively to the power control system 28 (the same as shown in connection with the embodiment of Figure 1). The power control system serves to provide low voltage, high alternating current to the mold liner 38. As in the first embodiment, the voltage may vary from about 0 to about 10 volts and the alternating current should be at least 1,000 amperes, and range from 1,000 amperes to 20,000 amperes or greater. The application of the electric voltage to the mold liner 38 causes the current to flow from the positive terminal around the walls of the mold liner 38 to the negative terminal. Thus, the current flowing through any given wall portion of the liner is substantially equal to the current coming from the power control system, which is substantially twice the value of the current passing through the mold walls of the embodiment of Figures 1 and 2. The current flows around the mold and its associated normal electro- magnetic field induces current flow in the melt. The magnetic interaction between the induced current and the primary field results in a repulsive force normal to both vectors. This force acts to move the liquid away from the mold wall with resultant circular motion and stirring of the melt. The stirring is therefore driven about axes horizontal and parallel to the mold walls and about the full perimeter of the mold. Singularities such as the corners disturb the symmetry of these stirring patterns and result in advantageous turbulent patterns of flow. The electro- magnetic field should be at least as great as 150 gauss and may preferably be between 300 and 400 gauss or higher. By way of example, a low iron-copper alloy C195 was cast into a 2,600 pound casting using the split-mold electromagnetic stirring system of the present invention. The chemistry of a furnace sample of the alloy was 1.13% Fe, .87% Co, .50% Sn and the remainder essentially Cu. The mold liner used was a standard 9 inch liner for producing a rectangular bar. The liner was split through the flange and short wall section in the area adjacent the larger wall as shown in Figure 3. A mica sheet was provided in the gap and the mold clamped together by means of the bolts and liner also shown in Figure 3. The electric power to the mold was supplied through a power control system consisting of a step-down transformer and a saturable core reactor which regulated the incoming 440v, 60Hz power supply so that the output of the transformer was between 0 and 10 volts and the alternating current from 0 to about 8,000 amperes. The mold was filled with the molten alloy and the cast was commenced. The current through the mold was initiated and the bar was cast with the maximum field available for ten minutes which in this example was 160 gauss. It was found that the bar acrostructure was columnar near the surface with an equiaxed core. Both the columnar grains and equiaxed grains were smaller than generally observed on DC cast bars of this alloy. The patents and publications set forth in this specification are intended to be incorporated herein by reference in their entirety. While this invention has been described above in combination with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in the light of the foregoing description. According, it is intended to embrace such alternatives, modifications and variations as fall within this spirit and broad scope of the appended claims.

Claims: What is claimed is: 1. A system for electromagnetically stirring molten metal during casting characterized by: a. an electrically conductive mold (2) forming a wall (12) surrounding said molten metal; and b. means for connecting said mold to a source of electrical current for generating an electromagnetic field within the mold. 2. The system of claim 1 characterized in that said source of electric current supplied to said mold is at least 1,000 amperes. 3. The system of claim 2 characterized in that said electric current is in the range of from about 1,000 to about 20,000 amperes. 4. The system of claim 1 characterized in that said electromagnetic field generated by said current in said mold is at least 150 gauss. 5. The system of claim 4 characterized in that said electromagnetic field is at least 300 gauss. 6. The system of claim 4 characterized in that said mold is copper or a copper alloy. 7. The system of claim 1 characterized in that said mold has a non-conductive gap (44) therein along its vertical length, said current flowing from one side of said gap around said mold to the other side of said gap. 8. The system of claim 7 characterized in that said gap (44) is formed by a split in said mold and a dielectric material (46) positioned in said split. 9. The system of claim 8 characterized in that said gap is held closed by means of bolts (50) and further including means (60, 62) insulating said bolts from conducting current across said gap. 10. The system of claim 7 further characterized in that means (66, 68) on said mold spaced on either side of said power source to provide for current flow from one side of said gap around said mold to hte other side of said gap. 11. The system of claim 7 characterized in that said mold is rectangular having two relatively long side walls and two relatively short end walls and having an open bottom from which a cast slab of cast metal exits, said gap being provided in one of said end walls. 12. The system of claim 11 characterized in that said gap in said end wall is provided adjacent said side wall. 13. A process for casting molten metal characterized by: a. providing a mold including an electrically conductive mold liner (2) forming a wall around the molten metal; b. pouring said molten metal into said mold; c. passing a current through said mold and producing an electromagnetic field within the ° mold to provide stirring for the molten metal therein, 14. The process of claim 13 characterized in that the current is supplied to said mold is at least 1,000 amperes. 15. The process of claim 14 characterized in that said current is in the range of from about 1,000 to about 20,000 amperes. 16. The process of claim 11 characterized in that said supplying of said current to said mold generates an electromagnetic field in said mold of at least 150 gauss. 17. The process of claim 13 characterized in that said mold is rectangular with an open bottom with relatively long side walls (15) and relatively short end walls (17). 18. The process of claim 17 characterized in that said current flow supplied to said mold is divided between the two side walls. 19. The process of claim 17 characterized in that said current supplied to said mold flows in one direction around said mold. 20. The process of claim 19 characterized in that said current supplied to said mold flows from one end portion around said mold and t back to the same end portion.

Inventors: LEWIS BRIAN G, TUNGATT PAUL D, LEWIS, BRIAN, G., TUNGATT, PAUL, D.
Applicants: OLIN CORP, OLIN CORPORATION
Application year: 1990
Main classification: B22D11
IPC codes: B22D11