Magnetic apparatus for metal casting

Abstract

Apparatus for metal casting using a magnetostatic field generator having at least one superconducting solenoid magnet disposed on at least one side of a mould or metal shell for setting up a heterogenious magnetostatic field of intensities at least in excess of 10,000 gauss in the liquid metal retained within the mould or metal shell to thereby agitate the liquid metal. The mageto-static field generator is accommodated within a cryostat, which is always held cool at a predetermined temperature by a special auxiliary cooling means.

Description

This invention relates to metal casting and, more particularly, to a process and apparatus for metal casting, which makes it possible to prevent or reduce macro- or micro- or micro-metallographic heterogenity resulting chiefly at the center of ingots or continuously casted billets from the gradual cooling, cooling or rapid cooling of liquid metal poured into the mould or liquid metal core remaining within the solidified metal shell. This is accomplished by applying magneto-static field at least in excess of 10,000 gauss to the liquid metal or liquid metal core during the solidification thereof, thereby to obtain an ingot or billet having a homogenious metallographic structure.

It is well known in the art that when liquid metal is solidified, the center of the resultant body or casting center is prone to a sort of heteroneious structure usually termed center porosity, center-segregation or inner-crack, this trend being particularly pronounced where the metal liquid is cooled in a still state through rapid cooling, as is disclosed in Japanese Patent Publication No. 33025/1972. It has also been known that this heterogenity may be eliminated or reduced only by agitating the liquid metal until the solidification thereof, and there have been proposed various types of means for causing the flow of the liquid metal by means of electromagnetic force.

The prior-art methods for electromagnetically causing the flow of liquid metal include: a rotating electromagnetic field method as disclosed, for instance, in Dukefriet Unkhans patent (Japanese Patent Publication No. 9962/1956); one using an electromagnetic field set up by three-phase alternating current as disclosed in Japanese Patent Publication No. 32486/1972; and one using a travelling electromagnetic field set up by alternating current as disclosed in U.S. Pat. No. 3,656,537. All these prior-art methods have resorted to a copper wire coil which is disposed to surround or disposed in the close proximity of the mold or metal shell and through which single or three phase alternating current is caused, whereby the liquid metal is electromagnetically agitated due to alternating magnetic field set up in it and eddy current induced in it.

In the above prior-art method, which use alternating current for obtaining the agitating force, very large current has to be passed through the copper wire coil. Therefore, the ampere turns of the copper wire coil has an extremely large value, so that water cooling or oil cooling is necessary to remove the Joule's heat. Also, the magnetic field generator coil as a whole is very large in size. Particularly, where the coil surrounds a large casting body, its operability will be limited due to the Joule's heat, and its cost as a practical unit will be enormous.

According to the invention, unlike the afore-mentioned prior-art techniques having resort to alternating magnetic field for obtaining strong agitating force, a very intensive magnetic-static field at least in excess of 10,000 gauss is set up in the liquid metal by a single magnet.

Thus, the size and cost of the magnetic field generator may be extremely reduced. While sufficient electromagnetic agitating effect can be obtained with a single magnetic field generator or magnet according to the invention, further strong electromagnetic agitating effect may be given to the liquid metal by a combination of a plurality of magneto-static field generators arranged such as to provide the most effective magnetic flux distribution. Thus, it is possible to obtain a strong agitating force that could not be obtained by the prior-art alternating current method with an extremely small and inexpensive apparatus as compared to the prior-art one.

In accordance with the invention, the magneto-static field is set up by direct current through a superconducting solenoid magnet. The superconducting solenoid is made of a superconducting metal having a character of offering zero electric resistance at the temperature of liquid helium (-268.9.degree.C) such as a niobium-tin intermetallic compound and a niobium-titanium alloy, and it is held at the termperature of liquid helium when it carries direct current. Since the solenoid offers zero electric resistance at the working temperature, it may use a very fine wire or filament and may be formed in a very small size with a large number of turns. Also, since the solenoid as a whole may be readily held at the liquid helium temperature, it is readily possible to produce a high magneto-static field of 70,000 to 100,000 gauss at the center of the solenoid.

The above and other objects, features and advantages of the present invention become more apparent from the following description in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a set-up using superconducting solenoid magnets according to the invention applied to a still casting process;

FIGS. 2 to 4 are schematic representations of set-ups according to the invention applied to a continuous casting process;

FIG. 5 shows a cryostat;

FIG. 6 shows a schematic liquid helium circulating system used for a continuous casting process according to the invention; and

FIGS. 7A and 7B are graphs showing sulphur and carbon assay content distributions in a billet obtained in accordance with the invention and a billet obtained without using any agitating electromagnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a set-up according to the invention with a superconducting solenoid magnet disposed in the vicinity of one side of a still mold for setting up an intensive magneto-static field in the liquid metal within the mold. In FIG. 1 reference numeral 1 designates a mould made from a non-magnetic material, and numeral 2 a solid metal shell surrounding liquid metal 3. Numeral 5 designates the superconducting solenoid magnet, which is disposed in the vicinity of one side of the mold 80 that the magnetic flux generated by it may act upon the liquid metal. The solenoid magnet 5 is supported within a cryostat 7, and in this embodiment it is held in position by a non-magnetic support 8. The cryostat 7 is a high performance insulated vessel having an evacuated double-wall insulating structure made of a non-magnetic metal. It is filled with liquid helium, whose surface level 9 is held at a substantially constant level. The liquid helium is replenished from a feed pipe 10, and evaporated helium is recirculated through a discharge pipe 11 to a helium liquifier. The cryostat has an upper flange 21 provided with a power lead terminal 12 for supplying power to the solenoid magnet 5. A heat insulator 15 is insterposed between the cryostat and the mold for protecting the wall structure of the cryostat. The magnetic flux 6 set up by the solenoid magnet and acting upon the liquid metal may be moved relative to the liquid metal by vertically oscillating the cryostat by means of a vertically movable stand 13 or by horizontally moving the mould by vertically movable stand 13 or by horizontally moving the mould by means of wheels 16 and rails 17, so that the intensive magneto-static field may efficiently act upon the liquid metal so as to evoke the flow thereof in the desired direction. Of course, it is desirous to this end to move both the cryostat and the mold simultaneously. Also, it is possible to move the cryostat in the horizontal direction and the mold in the vertical direction.

FIGS. 2 to 4 show other forms of the invention applied to the continuous casting process. In the Figures, the cryostat is not shown but is implied. In FIGS. 2 and 3, numeral 1' designates a bottomless water cooled mold, and numeral 2 a solidified metal shell enclosing non-solidified or liquid metal 3. The billet containing the liquid metal is withdrawn at a substantially constant speed in the direction of arrow 4. In both these arrangements, two solenoid magnets are provided with respectively opposite poles (N and S poles) directed toward the shell. In the example of FIG. 2, both the solenoid magnets 5 and 5-1 are disposed on the same side of the shell and at different vertical positions. With this arrangement, it is possible to vary the magnetic field intensity or magnetic flux density by varying the distance between the two magnets. In the example of FIG. 3, the two solenoid magnets 5 and 5-1 are disposed on opposite sides of the shell and at different vertical positions. With the disposition of the solenoid magnets with respect to the lateral direction of the billet as in the example of FIG. 2, an extremely improved agitating effect may be obtained.

In case of the continuous casting, the billet is withdrawn substantially at a constant speed, and this means that the solenoid magnet is always moved relative to the liquid metal. Thus, in this case the cryostat oscillating means as has been mentioned in connection with FIG. 1 is not needed. The agitating effect of the magneto-static field upon the liquid metal is attributable not only to the eddy current induced but also to the gradient of the intensity or flux density of the field in the liquid metal. In other words, the liquid metal about to solidify is subjected to the combined effect of eddy current and gradient of the field. In further detail, in case of FIG. 3 example, for instance, the billet is continuously withdrawn, so that the liquid metal enclosed within the solid shell proceeds usually at a speed of, for instance, 0.5 to 3 meters per minute. Due to its interaction with the magneto-static field, the liquid metal experiences forces tending to stop its movement. However, since there is a gradient of the field in the liquid metal, there also results a gradient of the intensity of the force acting upon the liquid metal, so that the liquid metal is agitated. Although a gradient of the field is formed solely with a single magnet, the magnetic flux pattern may be changed to suit the type of casting and various specifications of the ingot or billet to be produced by appropriately changing the number and disposition of the solenoid magnets as typically shown in the examples of FIGS. 2 to 4.

Also, the solenoid magnet for generating the magnetic field is designed by taking various conditions such as the intensity of the exerted force and the position of installation into consideration.

FIGS. 5 and 6 show examples of apparatus required for the execution of the invention. FIG. 5 shows an example of the cryostat. The illustrated cryostat, generally designated at 7, essentially consists of an upper part having a lower flange 22 and a lower part accommodating a solenoid magnet 5. Its interior is in communication with a vacuum pump not shown, through a pipe 23 and is held under a pressure of 10.sup.-5 mm Hg. Numeral 24 designates a magnet case, whose top communicates with a pipe 25. Liquid helium is supplied from a liquid helium feed pipe 10 to fill the case 24 and pipe 25 to a constant level 9. Evaporated helium is recirculated through a discharge pipe 11 to a helium liquifier as shown in FIG. 6. This example uses liquid nitrogen having a vaporization temperature of -195.8.degree.C as auxiliary cooling means to take up external heat with respect to the cryostat for ensuring steady cooling effect of the liquid helium. More particularly, a liquid nitrogen chamber 26 having a plurality of downwardly extending fine tubes 27 is provided within the upper part of the cryostat. Liquid nitrogen is supplied through a supply tube 29, and evaporated nitrogen is exhausted through an exhaust tube to the outside. The cryostat used for the invention is entirely made of such non-magnetic material as stainless steel 304. Although not shown, the actual apparatus will include a liquid level gauge, a thermometer, a vacuum gauge, power lead terminals for the magnet 5, a magnet position adjuster, a safety device and heat insulation means.

FIG. 6 shows an example of the liquid helium supply system employed for a continuous casting process of curved strand type. In this process, liquid metal is poured from a pouring system 31 into a bottomless water cooled mold 1'. Solenoid magnets 5 and 5-2 are arranged in a way as in the example of FIG. 3 on the path of the strand 2 emerging from the mold 1' directly below or in the secondary cooling zone. Liquid helium and liquid nitrogen are supplied to the cryostat and evaporated gas is recovered in the manner as described above. Only the recirculation of helium is shown. Helium gas stored under a pressure of about 150 atm. in helium gas bombs 33 and under a pressure of about 1 atm. in a helium gas tank 34 is supplied to a helium gas compressor 35, and pressurised helium gas from the compressor 35 is supplied to a high pressure helium gas tank 36, and thence to a helium liquifier 37. In the helium liquifier, the high pressure helium gas is subjected to heat exchange with liquid nitrogen and then passed through a gas expander, whereby it is rendered into liquid helium, which is supplied through a liquid helium tank 38 to the cryostat 7. Also, evaporated helium from the cryostat goes to the helium gas tank 34 either through the liquifier 37 or directly for recovery of liquid helium for recirculation. Although liquid helium is expensive, substantially no helium is lost in the course of the recirculation involving evaporation and liquefaction since the processed helium is recirculated through a closed loop. Also, in the iron making plants provided with apparatus for producing oxygen, liquid nitrogen is inexpensively and readily avialable as a by-product. Thus, no debit factor is found from the standpoint of the running cost.

The following experimental example demonstrates the effects attainable according to the invention.

EXPERIMENT

A quantity of 2,000 kilograms of molten low carbon steel, which was produced by a high frequency induction furnace, was casted with an experimental vertical continuous casting apparatus provided with a magneto-static field generator according to the invention. The chemical composition of the steel was 0.15 percent carbon, 0.30 percent silicon, 0.71 percent manganese, 0.010 percent phosphor, 0.012 percent sulphur and 0.030 percent soluble aluminum, the rest being iron. The resultant billet had a thickness of 130 millimeters and a width of 260 millimeters. The magneto-static field generator was disposed directly below the mold, with its magnets positioned in the proximity of one broader side of the billet, that is as in the arrangement in FIG. 2. The first half of the steel, namely 1,000 kilograms of steel, was casted without applying any magnetic field, and the field was applied for the remaining half. The billet obtained in this way, was sampled at its positions corresponding to the feed of 400 kilograms and 1,600 kilograms respectively from the start of the casting. From each of these samples assay test pieces were cut out at intervals of 2 to 10 millimeters in the direction of the thickness of the billet for examining the segregation of component elements. The results of the tests are shown in FIGS. 7A and 7B. It will be seen that the billet obtained by applying the magneto-static field resulted in very little segregation of carbon sulphur at the center compared to the billet obtained without any field applied. Also, it was confirmed that the magneto-static field applied is particularly effective in the refinement of the dendrite grain structure and capable of reducing the segregation coefficient down to well below 2.0. The specifications of the magnets used in this experiment were as follows:

Number of magnets: 2

Type and size of the magnets: Solenoid type magnet with an inner diameter of 50 mm, an outer diameter of 180 mm and a width of 50 mm

Number of turns of the solenoid: 22,000

Solenoid wire: Copper wire 0.4 mm in diameter and containing 40 sealed fine filaments of Nb-Ti alloy resin coating being provided after winding the wire.

Current: 30 amperes

Field intensity: 70,000 gauss at the center of solenoid and 22,000 gauss at the end of solenoid

Initial liquid helium consumption: 25 liters

As has been shown, by using the superconducting solenoid magnet, a magneto-static field of high flux density, namely in excess of 10,000 gauss which could never be attained by passing large current through the conventional solenoid coil, may be obtained in a large space inexpensively and with a very small unit to effectively exert an agitating force to the liquid metal. Also, as has been verified by experiments, it is possible to suppress segregation and center-porosity and reduce the segregation coefficient down to 2.0 or below. Further, in addition to the fact that the growth of dendrites can be repressed by the slight agitation of the metal liquid, crystalline precipitates or ionized concentrates of higher susceptibility than the original liquid metal are affected by the very intense magnetic field and tend to be attracted toward the higher flux density. While the above experimental results are obtained by using the experimental continuous casting apparatus, it may be readily understood that similar results may be obtained in the case of the set-up of FIG. 1 provided the mold and magnet are moved relative to each other, and application to large ingots is possible. (In this case, the mold should be made of a non-magnetic material such as stainless steel.)

From the above grounds, it is possible to rapidly cool the liquid metal without resulting in defects attributable to the central heterogenious structure. Thus, it is possible to adopt the rapid cooling system so as to increase the billet withdrawal speed and hence reduce the possibility of crack formation in the rolling process, so that the invention will greatly contribute to the betterment of the yield of products.

Claims

What is claimed:

1. An apparatus for casting metals comprising means for holding non-solidified metal, means for generating a heterogenious magneto-static field in said non-solidified metal, said generating means being disposed in the vicinity of one side of said holding means, and means for causing relative movement of said holding means and generating means to each other.

2. The apparatus according to claim 1, wherein said magneto-static field generating means includes at least one super-conducting solenoid magnet.

3. The apparatus according to claim 2, wherein said solenoid magnet is hollow and formed by winding a wire containing very fine super-conducting filaments sealed therein and has a resin coating provided after the winding of the wire.

4. The apparatus according to claim 1, wherein said holding means is a mold.

5. The apparatus according to claim 4, wherein said moving means includes means for vertically move said generating means.

6. The apparatus according to claim 4, wherein said moving means includes wheels carried by a wagon supporting said mold and rails to guide and support said wheels.

7. The apparatus according to claim 1, wherein said holding means is a shell formed as a result of solidification of said non-solidified metal.

8. The apparatus according to claim 7, wherein said moving means is a mechanism for continuously withdrawing said shell.

9. The apparatus according to claim 8, wherein the speed of withdrawal of said shell ranges from 0.5 to 3 meters per minute.

10. The apparatus according to claim 1, wherein said magneto-static field generating means can generate a magneto-static field of intensities above 10,000 gauss in said non-solidified metal.

11. The apparatus according to claim 1, wherein said magneto-static means includes a plurality of super-conducting solenoid magnets disposed on opposite sides of said holding means with opposite poles directed toward the opposite sides of said holding means.

12. The apparatus according to claim 1, wherein said magneto-static means includes a plurality of super-conducting solenoid magnets disposed on opposite sides of said holding means with like poles directed toward the opposite sides of said holding means.