WO1993004801A1

WO1993004801A1 - Method and apparatus for the electromagnetic stirring of molten metals in a wheel caster

Info

Publication number: WO1993004801A1
Application number: PCT/US1992/007329
Authority: WO
Inventors: Julian Szekely; Alfredo Riviere
Original assignee: Julian Szekely; Alfredo Riviere
Priority date: 1991-08-29
Filing date: 1992-08-28
Publication date: 1993-03-18

Abstract

In a wheel caster (10) stirring AC coils are provided, which are positioned immediately above a wheel mold (11) of the caster (10) so as to effect stirring of a molten metal portion in the wheel mold (11) in a direction perpendicular to the direction of casting.

Description

METHOD AND APPARATUS FOR THE ELECTROMAGNETIC STIRRING OF MOLTEN METALS IN A WHEEL CASTER

Cross-Reference to Related Application

This application is a continuation-in-part of application Serial No. 482,526 filed February 21, 1990.

Field of the Invention This invention relates to a method and apparatus for the electromagnetic stirring of molten metals in a wheel caster for use in a continuous bar casting systems where the casting operation involves relatively thin cross sections.

Background of the Invention

It is known that electromagnetic stirring can substantially improve the properties of continuously cast steel products, such as billets, blooms and slabs which involve relatively thick cross sections. Electromagnetic stirring is indeed quite extensively practiced in the steel industry for this purpose. Typically the sections stirred are quite large, in the order of at least 100 x 100 mm and larger, such as 300 x 300 mm blooms or 25 x 2,000 mm slabs. The benefits attributed to stirring are the rapid dissipation of superheat, the promotion of eguiaxed as compared to columnar structures and the minimization of segregation in alloyed materials. (See Electromagnetic

Stirring During the Continuous Casting of Steel, Literature Search and Installation Survey. February, 1986, The

Association of Iron and Steel Engineers. U.S.A.).

The use of electromagnetic stirring in the aluminum industry is much more recent; here again large sections, such as ingots are being stirred and the perceived benefits are grain refining and a minimization in the extent of microsegregation. (See Vives et al., French Patent No. 8414740) .

In all these applications, relatively large sections (at least 100 mm x 100 mm) are being stirred and access to the molten metal by the electromagnetic field is readily accomplished.

More specifically, in the electromagnetic stirring of steel castings, the stirring coils have typically been placed just outside the copper molds, in which case very low frequencies must be employed, so that the field is not intercepted by the highly conductive copper. Alternatively, the specimen may be stirred upon exiting the mold ("below the mold stirring") , higher frequencies may be employed and the coils may be placed quite close to the billet, slab or bloom.

A common feature of the alternative, more recent technologies employing such casters, roll casters and belt casters is that the solidified product exiting the caster is either a bar, typically 50 x 60 mm or of a comparable dimension, or a plate, of approximately 10-20 mm thick and 100-1,000 mm wide.

The direct casting of bar and plates is being practiced in the aluminum, copper and related industries and these technologies are currently being developed for steel. Electromagnetic stirring offers many potential attractions for improving the quality of continuously cast products. More specifically, in the roll and plate casting of aluminum, segregation would be avoided, which is particularly critical for the more highly alloyed grades, and grain refining could be accomplished. Similar improvements can be realized for copper, copper alloys and steel.

Up to the present time, the electromagnetic stirring of the molten metal or melt in these direct casting operations has not been practiced. One may speculate that among the reasons for this situation are the following factors:

- Until recently there has been only a poor understanding of the mechanism of electromagnetic stirring. - In order to properly implement the use of electromagnetic stirring in elongated bars and sheets, a need exists for precise knowledge of the causal relationships between stirring power and liquid motion, in the molten metal. - In order for electromagnetic stirring to be effectively used, one needs to precisely know the domain of the molten portion of the bar, slab or plate.

Major practical difficulties exist in implementing electromagnetic stirring schemes which require significant modification of the original casting equipment.

The closest prior art of which the applicants are aware which addresses the problems of stirring in continuous casting devices are the following:

- U.S. Patent No. 2,782,473 to Brennan discloses continuous casting from a crucible through an enclosing die and from a continuous wheel caster, utilizing a high frequency coil to maintain the metal in a molten state.

- U.S. Patent No. 4,372,369 to Flemings, et al., teaches a wheel caster with mechanical agitation to produce an agitation zone to prevent formation of an interconnected dendritic network.

- U.S. Patent No. 3,656,537 to Von Starck teaches a method for continuously casting slabs and plates using an open ended mold and a traveling field inductor to induce agitation.

- U.S. Patent No. 3,693,697 to Tzavaras discloses the continuous casting of ingots involving stirring utilizing a coil energized with a source of alternating current. - U.S. Patent No. 3,882,923 to Alberny et al., teaches an apparatus for casting a metal slab using an electrical inductor connected to a multiphase current source.

- U.S. Patent 3,981,345 to Alberny et al. , teaches the use of induction formed electromagnetic stirring in the casting of slabs, particularly in the steel industry.

- U.S. Patent No. 4,106,546 to Sundberg relates to the continuous casting of steel strands, involving the use of induced current to stir the molten metal within the cooling strand. - U.S. Patent No. 4,139,048 to Andersson teaches continuous casting of metal strands involving the use of a magnetic stirrer positioned outside of some of the rollers used therein.

- U.S. Patent No. 4,158,380 to Sasaki et al. , involves the continuous casting of steel slabs with the use of agitation induced by magnetic forces.

U.S. Patent No. 4,429,731 to Delassus is directed to continuous^" slab casting using an inductor capable of concentrating the magnetic flux to enable a strong magnetic field to be applied to a relatively thick cross sectioned article being cast.

The prior art which is being practiced using electromagnetic stirring in continuous casting systems may be summarized by stating that electromagnetic stirring is being extensively practiced in the steel industry in the casting of large sections, such as slabs, billets and blooms. Practice has shown that the stirring arrangements used for these different shapes differ significantly, such as the use of linear stirrers for slabs, and circular stirrers in most_.cases of billets and blooms.

Prior art devices used for electromagnetic stirring may be of a so-called wheel caster type. European patent 289,433 issued to Meyer discloses a solidification process of liquid metal in a casting wheel or wheel caster. Meyer teaches subjecting metal, in the process of solidifying in a groove of the wheel of the caster, to electromagnetic forces of variable intensity in the direction which is parallel to the direction of displacement of the cast blank. The electromagnetic forces are provided by inductors with sliding motors, such as linear motors placed above a metal ribbon which is guided by pulleys and encloses part of the length of the groove of the wheel caster. The linear motors are fed with three-phase alternating current (AC) and produce, in a metal molten portion undergoing solidification, stirring in the direction of casting, e.g. the direction of rotation of the wheel. Meyer suggests that the linear motors be placed in respect to the wheel such that the armature recesses thereof, in which the electrical windings propagating electromagnetic forces when fed with a polyphase current are housed, extend at an acute angle between 45 and 90° to the direction of displacement of the wheel. In the arrangement proposed by Meyer, the stirring of the molten metal portion is carried out in the direction which is parallel to the direction of casting or at the most, forms an angle not exceeding 45° with the casting direction. This stirring is effected due to the arrangement of the recesses and windings either perpendicular to the casting direction or at an angle from 90 to 45 degrees thereto.

fiiimrnary of the Invention

It is an object of the present invention to provide a wheel caster for electromagnetic stirring of high efficacy.

It is a further object of the present invention to provide a wheel caster in which stirring is carried out in the direction perpendicular to the direction of casting in a wheel caster. This is obtained by placing electromagnetic stirring means, e.g. linear motors so that their armature recesses or grooves form the angle of 0° with the casting direction. In other words, the armature grooves extend parallel to the casting direction.

Stirring in the direction perpendicular to the direction of displacement of the blank, e.g. casting direction, in a wheel caster of the type under discussion will produce better grain refinement and will result in a better metal quality product.

Highly alloyed grades of aluminum and of other metals may be obtained by wheel casting carried out by the method according to the present invention. The stirring of bars in casters of the type under consideration has been difficult until now for the following reasons:

- Stirring is caused by the establishment of force field gradients in the melt. In conventional continuous casting the amount of metal to be agitated is large and the extent of the molten regions is well established, so that stirring should be readily accomplished.

- In the metal casting carried out in a wheel caster the molten metal portion being agitated is small so that the establishment of significant field gradients require precise coil design. Furthermore, the small dimensions of the molten regions to be stirred make it quite difficult to establish the desired recirculatory flow pattern. - In realizing agitation in wheel casters systems yet another group of problems arise regarding the location of the stirring coils. In conventional wheel casters for continuous casting, the stirring coils either tend to surround the mold, bar, billet, slab or bloom to be cast, or alternatively a .linear stirring arrangement is provided, as detailed above. In such casting applications there are major difficulties in arranging the coils such that an appropriate field can be produced. The reason for this is that access to the molten regions is severely restricted by the construction of the wheel caster.

According to the present invention, a wheel caster is provided for the electromagnetic stirring of the melt region for casting a molten metal into bars, wherein the casting operation involves relatively thin cross sections. The wheel caster comprises AC electromagnetic stirring means located in the wheel caster in proximity to the molten metal portion of the wheel caster in such a manner as to minimize segregation and to effect grain refining in the as cast metal bar or sheet.

The molten material processed in the apparatus into a cast is selected from the group consisting of aluminum, copper, zinc and alloys thereof.

The electromagnetic stirring means may comprise one or more electromagnetic coils having multiphase electrical connections so as to permit their operation in such a manner as to effect stirring within the molten metal being cast in the direction perpendicular to the direction of displacement of the blank. The electromagnetic stirring means may be constructed to create stirring velocities within the molten metal being cast of from about 0.3 to about 2.0 m/sec.

The electromagnetic stirring means of the present invention is capable of generating a magnetic field of from about 300 to 3,000 gauss and electromagnetic forces of from about 500 to about 5,000 N/m . The electromagnetic stirring means may operate at a frequency of from about 30 to about 3000 Hz.

In continuous wheel casters of the present invention, the electromagnetic stirring means may be located immediately above the hold down strip located above the casting wheel.

The ring of the casting wheel and the entire casting wheel may be fabricated from non-magnetic stainless steel. The aforementioned objects, features and advantages of the invention will, in part, become obvious from the following more detailed description of the invention, taken' in conjunction with the accompanying drawings, which form an integral part thereof.

Brief Description of the Drawings

Fig. 1 is a schematic representation of a typical conventional wheel caster arrangement;

Fig. 2 is a schematic representation of a conventional wheel caster provided with a stirrer effecting stirring in the direction of casting;

Fig. 3 is a cross-sectional view of a steel or aluminum bar to be manufactured in the method according to the present invention;

Figs. 3a to 3c schematically represent the velocity fields in a transversely stirred bar in a wheel caster according to the present invention; Fig. 4 is a schematic side view of the wheel caster with a stirrer according to the present invention;

Fig. 5 is a schematic front view of the wheel caster of Fig. 4;

Fig. 6 is a view of the stirrer taken along line A-A of Fig. 5; and

Fig. 7 is a schematic view of the armature of the AC coil of the stirrer of Fig. 6, showing the arrangement of grooves for receiving windings.

Detailed Description of the Preferred Embodiment

With reference to the figures, Figs. 1 and 2 show a typical wheel casting apparatus used in the production of aluminum bars. In practice the casting wheel has a zirconium - copper ring, which moves with a linear velocity in the range of 0.2 - 0.3 m/s. The bar itself may have a cross section such as represented in Fig. 3, with a typical cross sectional area in the 2,000 mm² range. In order to provide stirring, it was proposed to install a linear stirrer in the space above the hold-down strip, as represented in Fig. 2. The stirrer itself will have approximately the same width as the wheel (approximately 70 - 80 mm) and have a length sufficient to cover an arc of about 60 degrees from the vertical, along the length of the wheel. For a typical wheel caster application this would correspond to a length of approximately 1 meter.

The metal in a liquid state is fed into a groove between a periphery of the wheel of the caster and a hold- down strip which is cooled in the process of cooling by suitable cooling means. The metal gradually solidifies along the groove. In the process of solidification, the metal becomes detached from the hold-down strip. As the molten metal enters the groove of the caster it is subjected to stirring by electromagnetic forces which impart to the liquid metal a movement at a predetermined speed. In the conventional arrangement shown in Fig. 2, a linear motor is employed to stir the metal in the direction of displacement of the cast which coincides with the direction of rotation of the wheel. In the conventional arrangement, windings and their recesses respectively are perpendicular to the direction of casting.

With the stirring in the direction of casting, it is, however, very difficult to impart linear motion to a long, thin body of liquid metal in the direction of the major axis of the body. Such stirring would lead to flow instabilities which would be difficult to control. Furthermore, the generation of linear motion in the direction of casting would result in the disruption of the meniscus in the molten metal, which is undesirable.

Referring now to Figs. 4 to 7, a wheel caster generally designated at 10 is shown, which includes a wheel mold 11 referred to as a caster groove hereinabove, into which mold a liquid metal is fed via a metal inlet similar to the conventional method and using a caster such as shown in Fig. 2. AC stirrers 12 each including magnetic yokes 14 and an AC coil 16 are arranged over the wheel mold as shown in Fig. 5 (only one stirrer is shown in the drawing) . The stirrer 12 is suspended from a cooling water manifold 18. AC coil 16 may be, for example, a three-phase coil with 60 turns per phase. Current of 200 A is applied to the AC coil to propagate required magnetic forces. A DC current may be used in the unit for braking the stirring.

As shown in Figs. 6 and 7, recesses 20 in a coil armature 22, which recesses accommodate windings (not shown) of the coil, extend in the direction of casting so that the windings of the coil are parallel to the casting direction whereby magnetic stirring forces extend in the direction perpendicular to the casting direction. The stirring coils of the stirrers are designed so as to provide a body force in the region of 500 - 5,000 N/m³ in the melt and an associated magnetic flux density in the range of 300 - 3,000 gauss. The resultant velocity fields for transverse stirring to be expected are represented in Fig. 3A. In a preferred application of the technique, an alternating stirring arrangement is selected to develop the maximum turbulence in the melt. The preferred frequency would be in the range of 50 - 1000 Hz for aluminum and 30 - 300 Hz for steel. The typical alternating stirring arrangement requires stirring in one direction for about 0.2 - 2.0 seconds, pausing for about 0.2 - 2 seconds and then stirring in the opposite direction for 0.2 - 2.0 seconds.

A DC current may be used for braking the melt flow. In the practical application of the stirring arrangement of the present invention, the steel belt or ring which rides on the copper ring on the wheel of the caster would be replaced with a non-magnetic stainless steel ring or belt and preferably the entire wheel would be constructed of non-magnetic stainless steel.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purpose of the disclosure, which modifications do not constitute departures from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method of casting molten metal into bar shapes having relatively thin cross sections, comprising the steps of feeding the molten metal to a wheel mold provided at a periphery of a continuous wheel caster, positioning electromagnetic stirring means in proximity to a molten metal portion located in said wheel mold and arranging said magnetic stirring means so as to effect stirring perpendicular to a direction of casting of said metal, said magnetic stirring means generating a magnetic field of from about 300 to 3,000 gauss to thereby minimize segregation and effect grain refining in a cast metal bar or sheet.

2. A method according to claim 1, wherein the molten metal to be cast is aluminum, or copper, or zinc, or steel, or alloys thereof.

3. A method according to claim 1, wherein said stirring means is operated to create stirring velocities within the molten metal being cast of from about 0.2 to about 2.0 m/sec.v

4. A method according to claim 1, wherein said stirring means include AC coils operated at a frequency of from 30 to 3,000 Hz and generating electromagnetic forces of from about 500 to about 5,000 N/m³.

5. In a wheel caster, a device for the electromagnetic stirring of a melt region in a wheel mold located at a periphery of a wheel caster for continuously casting a molten metal into sheet shapes of substantially thin cross sections, said device comprising electromagnetic stirring means positioned above said wheel mold in the proximity of a molten metal portion located therein so as to effect stirring in a direction perpendicular to a direction of casting said metal, said stirring means generating a magnetic field of from about 300 to 3,000 gauss to thereby minimize segregation and to effect grain refining in a cast metal bar or sheet.

6. A wheel caster according to claim 7, wherein said stirring means are electromagnetic AC coils operated at a frequency of from about 30 to 1000 Hz.

7. A wheel caster according to claim 5, wherein said electromagnetic stirring means is positioned immediately above a hold-down strip provided at said wheel.

8. A wheel caster according to claim 5, wherein a ring of the wheel caster is fabricated from non-magnetic stainless steel.

9. A wheel caster according to claim 5, wherein the entire wheel caster is fabricated from non-magnetic stainless steel.