Patent Application
20170291217
The present disclosure relates to a cold crucible used during electromagnetic casting, and, in more detail, to a cold crucible for electromagnetic continuous casting for manufacturing a material having poor workability to form an ingot having a shape similar to that of a panel, using a relatively small amount of energy. In particular, the cold crucible proposed in the present disclosure provides an effective technique for manufacturing an ingot of a light metal required to have high purity.
Electromagnetic casting (EMC) is the most common process used in melting and casting metals using electromagnetic fields. In the technique described above, alternating current (AC) electricity is applied to a cold crucible to generate changes in magnetic fields, thereby forming an induced current on a surface of a metal to be melted. A metal may be melted due to Joule heating generated by the induced current. Such a direct melting method, by electromagnetic induction, may allow a material, such as a metal, to be melted in a short period of time, thereby providing relatively high productivity.
The induced current may interact with a magnetic field to generate electromagnetic force (Lorentz force) in molten metal. Even if a direction of a coil current is changed, the generated electromagnetic force may always be directed toward a center of an interior of a cold crucible, according to Fleming's left-hand rule. Due to a pinch effect, such as electromagnetic pressure, contact between molten metal and an internal wall of a cold crucible may be prevented. Thus, with molten metal not in contact with a cold crucible, molten metal may be melted, and casting without a mold is possible. Contamination of a material is suppressed, quality of an ingot is improved, and a mold is neither consumed nor replaced, thereby reducing equipment costs and improving productivity.
However, in terms of EMC through electromagnetic induction, in a case in which a distance between an internal wall surface of a cold crucible and a center thereof is not uniform, electromagnetic force is not applied to a surface of molten metal in a uniform manner, thereby causing the molten metal to be unstable . In a case in which the molten metal is unstable, the molten metal may slop and come into contact with an internal wall of a cold crucible, and the manufacturing of ingots having a desired shape may be difficult.
However, in order to manufacture ingots having various forms, cold crucibles are required to be altered to have various forms besides a circular shape. In order to facilitate molding, such as forging, rolling, or the like, of a material having poor workability, cold crucibles are required to be changed to have various forms similar to a flat form, so that ingots similar to a panel shape can be manufactured from the time at which ingots are cast using EMC. Thus, the distance between the internal wall surface of cold crucibles and the center thereof may not be maintained to be uniform.
Thin ingots may be cast using a nozzle having a flat shape, while the form of the cold crucibles is not changed, and cold crucibles are maintained in a conventional form during EMC of the related art. However, due to problems such as the excessive consumption of energy required in order to maintain a greater volume of molten metal than is necessary in a molten state, the need for a design of a nozzle that is excellent in terms of form, material and the like, while having durability, excessive costs may be incurred. Thus, cold crucibles for electromagnetic casting having a form the same as that of an ingot are still required.
In detail, in a case in which ingots of light metals having high purity are manufactured using a continuous electromagnetic casting method, a large amount of effort is required to prevent contact between molten metal and the internal wall surface of cold crucibles. For example, in the case of continuous electromagnetic casting of steel having a relatively greater specific gravity and not demanding high purity, a melting operation is performed in a separate furnace, and only a solidification process is performed in a cold crucible for electromagnetic casting. In this case, the function of cold crucibles for electromagnetic casting is used to prevent defects, such as oscillation marks, on the surface of ingots, from being generated, by allowing for light contact between a cold crucible and molten metal. However, in order to produce ingots of a light metal having high purity, melting and solidification of a raw material may be performed simultaneously in a cold crucible for electromagnetic casting. Ina case in which a molten metal pool is in contact with the wall surface of a water-cooling cold crucible, a continuous melting operation is impossible. In particular, since light metals have relatively low specific heat and heat capacity, and ease of the solidification thereof is facilitated, a continuous casting process maybe more difficult. Ina case in which the supply of excessive power allows continuous melting to occur, an inflow of impurities into the molten metal from a cold crucible may not be prevented, so that an ingot of a metal having high purity may not be manufactured. Thus, in order to manufacture light metals continuously to have a flat, thin, slab shape, having high purity without included impurities, specially designed cold crucibles for electromagnetic casting are required.
An aspect of the present disclosure may provide a cold crucible for electromagnetic casting, controlling electromagnetic force applied to a surface of molten metal from an internal wall surface of a cold crucible and, in detail, controlling electromagnetic force applied to a surface of molten metal formed using a light metal, in order to stabilize the molten metal in the cold crucible and to cast an ingot having a uniform-size particle, as well as to change a form of the cold crucible into various forms during electromagnetic casting.
According to an aspect of the present disclosure, a cold crucible for thin slab continuous casting of light metal with high-purity, in which an alternating current (AC) electricity is applied to a coil wound around an outer circumferential surface of a cold crucible; an edge of the cold crucible is formed of a plurality of segments, in order for a portion of the AC electricity, which is applied, to be applied to the cold crucible; and a plurality of slits are disposed between the plurality of segments. In the cold crucible for thin slab continuous casting of light metal with high-purity, a horizontal, cross-sectional shape of the cold crucible is non-circular; and a ratio (d2/d1) of a width d2 of a segment in a central side portion of the cold crucible disposed closest to a center of the cold crucible, to a width dl of a segment in an end portion of the cold crucible, disposed farthest from the center of the cold crucible, based on the center of the cold crucible, toward which electromagnetic force applied to a surface of molten metal in the cold crucible from an internal wall surface of the cold crucible is directed, is 1.5 or more to 2.0 or less.
A width of the plurality of segments may be 20 mm or more to 50 mm or less.
A ratio (d2/T) of the width d2 of the segment in the central side portion of the cold crucible, to a thickness T of the plurality of segments, may be less than or equal to 1.8.
The molten metal may be formed using a light metal having a specific gravity of less than or equal to 5.
The horizontal, cross-sectional shape of the cold crucible may be oval, rectangular, or polygonal.
The horizontal, cross-sectional shape of the cold crucible maintains a uniform shape from an uppermost portion to a lowermost portion of the cold crucible, thereby minimizing an amount of the molten metal remaining in a molten state in the cold crucible.
According to an aspect of the present disclosure, electromagnetic force applied to a surface of molten metal from an internal wall surface of a cold crucible may be controlled by designating a ratio of a width of a segment in an end portion of a cold crucible to that of a segment in a central side portion thereof as a critical ratio, so that, during EMC, even though a form of the cold crucible may be changed to have various forms, and the form of the cold crucible may be the same as that of the molten metal. In addition, since the molten metal may be stabilized, the molten metal in the cold crucible may be prevented from slopping, thereby removing a risk of the molten metal coming into contact with an internal wall surface of the cold crucible. Furthermore, since the minimum amount of molten metal may be formed, energy may be saved. In detail, a flat, thin slab may be continuously manufactured to have a high purity, without impurity contamination, by using molten metal of a light metal.
Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein, and those skilled in the art and understanding the present disclosure can easily accomplish retrogressive inventions or other embodiments included in the scope of the present disclosure by the addition, modification, and removal of components within the same scope, but those are construed as being included in the scope of the present disclosure. Like reference numerals will be used to designate like components having similar functions throughout the drawings within the scope of the present disclosure.
All terms, including technical and scientific terms, to be used hereinafter have the same meaning as that commonly understood by those skilled in the art. Terms defined in advance are additionally construed as having a meaning conforming to relevant technical documents and the contents presently disclosed and are not construed as an ideal or official meaning, as long as they are not defined as such.
Here, in order to apply AC electricity to the cold crucible 1, AC electricity is applied thereto in such a manner that a coil is wound around an outer surface of the cold crucible 1. A magnetic field penetrates through a slit by the AC electricity, thereby directly applying a primary induced current to molten metal 4. In addition, a secondary induced current is formed in each segment 6. AC electricity is also applied to a surface of the molten metal 4 by AC electricity applied to the segment 6, thereby forming electromagnetic force between the segment 6 and the molten metal 4.
In
However, in a case in which all structures are maintained in a manner the same as that of a related art cold crucible, and only a shape of a horizontal cross section of a cold crucible is formed to be non-circular, molten metal assumes a form for maintaining linear balance, in order to minimize surface energy of the molten metal moving in a manner the same as a fluid in the cold crucible, and to minimize internal energy due to a volume of a fluid of molten metal. Plateau-Rayleigh instability is a theory describing the case above. In a case in which molten metal having an elongated form is present, as illustrated in
Furthermore, a small difference in distribution of an induced current generated and flowing in molten metal causes a difference in electromagnetic force applied to molten metal, so that a form of molten metal may be changed. Since a change in the form of molten metal allows a difference in distribution of the induced current to be greater, the change tends to make the form of the molten metal different from that of the cold crucible. In more detail, the induced current generated in molten metal has a relatively high current density in a long side, rather than a corner or a short side, in a rectangular conductor, due to characteristics in which the induced current is intensively concentrated on a surface of molten metal by a skin effect, and the induced current flows to minimize electrical resistance. The small difference in distribution of a current causes the difference in electromagnetic force to be greater; the difference in electromagnetic force makes the change in the form of molten metal greater; and the change in the form of molten metal makes a difference in distribution of a current greater. Ultimately, the molten metal results in having a polyhedral form significantly different from that of an interior of the cold crucible. Thus, the form of molten metal in the cold crucible having a non-circular, horizontal cross section is not formed to be the same as that of the cold crucible in a manner the same as the molten metal 4 of
Hereinafter, the present disclosure will be described in detail through a specific experimental example, but the scope of rights of the present disclosure is not limited by the experimental example.
An analytic process of designing a cold crucible using theories related to electromagnetics and electricity, in order to implement the present disclosure, is described as follows.
=V/R [Formula 3]
In Formula 3, I is an induced current applied to a segment, while Vis induced electromotive force applied to a segment. Since resistance R is proportional to a distance which an electric current flows, while induced electromotive force V is proportional to a cross-sectional area through which magnetic flux penetrates, based on Formula 3, a primary induced current I in the small segment 11 is expressed using Formula 4
I=V/R
D
2/4D=D/4, [Formula 4]
while, based on Formula 3, a primary induced current I in the large segment 12 is also expressed using Formula 5
I=V/R=ND
2/2D(1+N)=ND/2(1+N). [Formula 5]
Thus, in molten metal of a conductive metal in the cold crucible, a secondary induced current is generated by the primary induced current generated in the cold crucible.
In the meantime, since electromagnetic force F, generated on a surface of molten metal, is proportional to J×B, the electromagnetic force F may be expressed as below:
F
J×B [Formula 6]
In Formula 6, J is a primary current applied by the cold crucible, while B is magnetic flux density generated by the primary current.
In the meantime, according to a change in a level of an induced current generated in the small segment 11 and the large segment 12, based on Formula 4 and Formula 5, strength of a magnetic field generated on the surface of molten metal in the cold crucible varies, as expressed in Formula 7, below.
B=−L·dI/dt [Formula 7]
In Formula 7, Lisa constant value, dI is an amount of change of an electric current, and dt is a change in time.
In other words, a relative ratio of a magnetic field strength between a magnetic field strength B(α), generated at a position α of
B(β)/B(α)ND/2(1+N)/D/4=2N/1+N [Formula 8]
Thus, in a case in which Formula 8 is inserted into Formula 6 to calculate a relative ratio between the electromagnetic force at the position α and that at the position β, J applied to molten metal in the cold crucible are equal, so that the relative ratio between electromagnetic force at the position α and that at the position β may be 2N/(1+N), equal to a relative ratio between a magnetic field at the position α and that at the position β. In other words, in a case in which N is a number greater than 1, those skilled in the art could expect that electromagnetic force generated in the large segment 12 would be relatively greater than that generated in the small segment 11 of the cold crucible. Thus, the inventors may control electromagnetic force applied in a direction from the surface of molten metal toward the central portion in the cold crucible in such a manner that an interval between slits, that is, a width of a segment, is controlled to be a non-uniform interval, in order to solve problems in which molten metal is unstable in a non-circular cold crucible, and the ingot having a desired form and quality cannot be manufactured. In other words, in order to prevent a slab form from being changed into a janggu form during continuous casting of a light metal having a form of a thin slab, the interval between slits disposed adjacent to the central side portion of the cold crucible should be wider than that in an end portion.
Electromagnetic force according to the actual number of slits and an interval of segments of a cold crucible has been measured.
Table 2 provides conditions related to the cold crucible of Experimental Examples 1 to 5.
1. Melting material: Ti and Ti alloy
2. Cold crucible form:
3. Cold crucible material: Cu
4. Melting method: non-contact type electromagnetic induction melting of cold crucible
5. Condition of melting operation
1) Input power of induction coil: 10 to 75 kW
2) Frequency: less than or equal to 15 kHz
3) Condition in melting chamber: maintaining an argon (Ar) gas atmosphere (Ar purging) at one atmospheric pressure after vacuum decompression (10−3 torr or less)
4) Melting time: 5 to 10 minutes
6. Thickness of segment: 20 mm
With reference to
In a case in which Ti, a type of a light metal, is, in actuality, continuously cast to have a thin slab shape of 20 mm in width, if instability corresponding to about 12.5% of a width of the cold crucible is generated (d=20 mm, 1=7d, δ=d/8), pressure Pc in a concave central side portion of molten metal is higher, by about 410 N/m2, than pressure Pe in a convex end portion, based on Formula 1 and Formula 2. Thus, in a case in which an interval between slits in the end portion of the cold crucible is smaller than an interval between slits in the central side portion, thereby causing the difference in electromagnetic pressure to be greater than 410 N/m2, the molten metal may initially have a flat shape, conforming to a cavity in the cold crucible. In a case in which a condition satisfying the case described above is expressed, in
Thus, in order to solve a problem in which molten metal is unstable in the cold crucible having a non-circular shape, and an ingot having a desired form and quality cannot be manufactured, the inventor sets a ratio of the width of a segment in the end portion of the cold crucible to that of a segment of the central side portion thereof as the critical ratio, thereby controlling electromagnetic force applied in a direction from the surface of molten metal toward the center thereof in the cold crucible. In a case in which electromagnetic force applied in a direction from the surface of molten metal toward the center thereof in the cold crucible is controlled, anisotropy of a form may be caused when molten metal is formed in the cold crucible. Thus, a form of molten metal may be properly controlled.
In general, in a case in which the width of the segment of the cold crucible is significantly wide, efficiency of a process is relatively low, due to a magnetic field blocking effect, and distribution of electromagnetic pressure in the cold crucible is non-uniform, so that the width of the segment of the cold crucible may be less than or equal to 50 mm. In addition, since, in a case in which the width of the segment is significantly narrow, proper strength of the segment as a member is difficult to be secured, and, further, a cooling channel is difficult to be formed in a segment, the cold crucible is difficult to be manufactured at an affordable price. Thus, in general, the width of the segment may be greater than or equal to 20 mm.
In a case in which an interval between slits is relatively wide, as illustrated in
where γ refers to surface energy of molten metal, while r1 and r2 refer to principal radii of curvature of a surface of molten metal.
Thus, according to characteristics of the present disclosure, electromagnetic force applied to the surface of molten metal formed using a light metal such as Ti may be controlled to maintain molten metal to have the same form as that of the cold crucible, while molten metal may be manufactured to have a shape similar to a panel shape, thereby facilitating molding, such as forging, rolling, and the like.
The cold crucible 21 of
Since a cold crucible for electromagnetic casting has a non-circular, horizontal, cross-sectional shape, an ingot having a desired form may be manufactured from the time of EMC. In particular, in a case in which the ingot is manufactured to have a form similar to a panel shape, molding, such as forging, rolling, and the like, of a material having poor workability may be facilitated. A non-circular shape thereof described above may include one shape among an ellipse, a rectangle, and a polygon.
As described in the Background Art of the present disclosure, the ingot having a form similar to a panel shape may be manufactured in such a manner that a form of the cold crucible is not changed, and only a nozzle in a lower portion of the cold crucible is flattened. However, in a design described above, a greater volume of molten metal is formed than is necessary, so that an amount of molten metal included in the cold crucible is increased. Thus, a relatively large amount of energy to melt the molten metal is required, in order for the cold crucible to melt and store a relatively large amount of molten metal. In addition, since molten metal is required to float, in order not to be in contact with the lower portion of the cold crucible, larger amounts of energy may be required.
In a case in which a relatively narrow nozzle is formed in the lower portion of the cold crucible, and the volume of molten metal is formed to be greater than that of the nozzle in the cold crucible, even if the form of the cold crucible is changed to have any form, a phenomenon in which molten metal is formed to be different from the form of the cold crucible, or molten metal is unstable, does not occur.
Thus, the present disclosure may be valuably used in the cold crucible to maintain a uniform form of molten metal from an uppermost portion to a lowermost portion of the cold crucible, in order to save energy needed to melt and float molten metal in the cold crucible.
While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2014-0137560 | Oct 2014 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2015/008868 | 8/25/2015 | WO | 00 |
