EP1833628A1

EP1833628A1 - Apparatus for continuous casting of magnesium billet or slab using electromagnetic field and the method thereof

Info

Publication number: EP1833628A1
Application number: EP05821971A
Authority: EP
Inventors: Joon-Pyo Park; Myoung-Gyun Kim; Gyu-Chang Lee; U-Sok Yoon
Original assignee: Jeonnam Advanced Material Support Center; Research Institute of Industrial Science and Technology RIST
Current assignee: Jeonnam Advanced Material Support Center; Research Institute of Industrial Science and Technology RIST
Priority date: 2004-12-23
Filing date: 2005-12-22
Publication date: 2007-09-19
Also published as: US20080179038A1; JP2008525197A; WO2006068424A1; EP1833628A4

Abstract

An apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, and a method thereof compensates small solidification latent heat of magnesium, controls a solidification speed, and stirs molten metal within a mold via an electromagnetic field, thereby enabling continuous casting. The apparatus comprises a mold for continuous casting an as-cast billet or slab, a coil positioned outside the mold and adapted to allow electric current to be applied thereto, and a cooling nozzle positioned outside the mold and the as-cast billet or slab. During casting, electric current having a frequency of 2 ~ 1,000 D or electric current of 50 ~ 10,000 A having a frequency of 200 ~ 200,000 D is applied to the coil. A magnesium billet or slab having enhanced inner quality can be produced at a high casting speed without surface defects by applying a low frequency or high frequency electromagnetic field to the mold during continuous casting of magnesium.

Description

APPARATUS FOR CONTINUOUS CASTING OF MAGNESIUM BILLET OR SLAB USING ELECTROMAGNETIC FIELD AND

THE METHOD THEREOF

Technical Field

[1] The present invention relates to an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, which can compensate small solidification latent heat of magnesium, and control a solidification speed via application of a high or low frequency electromagnetic field, thereby enabling continuous casting, and a method thereof.

[2] More particularly, the present invention relates to an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, which can compensate small solidification latent heat of magnesium, and control a solidification speed by heating molten metal via application of a low frequency electromagnetic field to the molten metal upon continuous casting of magnesium, and which comprises a low frequency electromagnetic stirring device positioned below a mold, thereby producing a billet having an enhanced quality at high casting speeds without surface defects, and a method thereof.

[3]

[4] In addition, the present invention relates to an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, which is adapted to apply a high frequency electromagnetic field to a specific mold during continuous casting of magnesium, and comprises a low frequency electromagnetic stirring device positioned at a lower portion of the mold, thereby producing a billet having an enhanced quality at high casting speeds without surface defects, and a method thereof.

[5]

Background Art

[6] Magnesium alloy is light-weight metal, and widely applied to various fields from light-weight components for transportation to components in electronics for information and communication due to its excellent physical properties such as high specific strength, excellent electromagnetic interference (EMI) shielding, excellent heat dissipation and vibration damping capability. Since magnesium is rapidly solidified during casting due to its very small solidification latent heat, it cannot be cast via a general continuous casting method.

[7]

[8] In addition, since magnesium products have been generally manufactured by a die- casting method, there has been no patents or reports related to continuous casting of magnesium. As an exception, a twin roll continuous casting method is used to produce magnesium slabs.

[9] Meanwhile, since the magnesium alloy is rapidly solidified during casting due to its small solidification latent heat, and thus has various defects after solidification, there are difficulties in application of the magnesium alloy to continuous casting. For continuous casting of the magnesium alloy, it is necessary to significantly suppress cooling, and thus there is a limit in increasing casting speed.

[10] As a continuous casting method of a magnesium billet, a method called

MGACAST was developed by LKR in Austria. This method was designed for continuous casting of the magnesium billet by enhancing a conventional direct chill (DC) casting method used for continuous casting of aluminum. Since this method results in lots of defects on the surface of an as-cast billet, there are problems in that a scarfing process is required to remove the defects from the surface of the billet, and that casting speed is very low.

[H]

[12] Additionally, although a horizontal continuous casting method can be applied to manufacture of magnesium billets, it also creates lots of defects on the surface of the as-cast billet, and has a very low casting speed. [13]

Disclosure of Invention

Technical Problem

[14] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, which can prevent various surface defects from being created during casting, increase casting speed, reduce inner defects by forming an equiaxial structure, and enhance productivity, and to provide a method thereof.

[15]

[16] It is another object of the present invention to provide an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, which can reduce inner defects by forming an equiaxial structure via stirring of molten metal, and enhance productivity, and to provide a method thereof.

[17]

Technical Solution

[18] In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, comprising: a mold for continuous casting of an as-cast billet or slab; a coil positioned outside the mold and adapted to allow electric current to be applied thereto; and a cooling nozzle positioned outside the mold and the as-cast billet or slab.

[19]

[20] Preferably, low frequency electric current is applied to the coil, and the mold is provided with a replaceable ring at an upper portion inside the mold.

[21] Preferably, the ring is made of graphite.

[22] Preferably, the mold has a circular or rectangular cross-section, and the coil is positioned to surround the mold.

[23] Preferably, the cooling nozzle cools the mold, the coil, and the as-cast billet or slab by directly spraying water thereto.

[24]

[25] In accordance with another aspect of the present invention, there is provided a method for continuous casting of magnesium billets or slabs using an electromagnetic field with an apparatus for continuous casting of the magnesium billet or slab using the electromagnetic field according to the above aspects of the present invention, wherein electric current having a frequency of 2 ~ 1,000 D is applied to a coil of the apparatus during casting.

[26] In accordance with yet another aspect of the present invention, an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, comprises: a mold having a slit formed therein for continuous casting of an as-cast billet or slab; a coil positioned outside the mold and adapted to allow electric current to be applied thereto; and an electromagnetic stirring device positioned at a lower portion of the mold.

[27] Preferably, the mold has segments partitioned by the slit, and a cooling water passageway formed in the segments.

[28]

[29] Preferably, the slit is formed at a central region of the mold, and extends from below an upper end of the mold to above a lower end thereof.

[30] Preferably, the slit has a width of 0.2 ~ 0.3 D, and is filled with ceramic bonds to allow the electromagnetic field to be applied to molten metal within the mold while preventing leakage of the molten metal.

[31] Preferably, the mold has a circular or rectangular cross-section, and the coil is provided to surround the mold.

[32] Preferably, the apparatus further comprises a cooling nozzle between the mold and the electromagnetic stirring device.

[33] Preferably, the cooling nozzle cools the as-cast billet or slab, and the elec- tromagnetic stirring device by directly spraying water thereto.

[34]

[35] In accordance with yet another aspect of the present invention, there is provided a method for continuous casting of magnesium billets or slabs using an electromagnetic field with an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field according to other aspects of the present invention, wherein electric current of 50 ~ 10,000 A is applied at a frequency of 200 ~ 200,000 D to a coil of the apparatus, and electric current of 20 ~ 1,000 A is applied at a frequency of 2 ~ 100 D to an electromagnetic stirring device of the apparatus.

[36]

Advantageous Effects

[37] According to the present invention, the apparatus comprises a mold formed of copper or copper alloys having a good electrical conductivity, a coil positioned outside the mold to apply an electromagnetic field to the mold during casting of magnesium to perform induction heating of molten magnesium within the mold while reducing a contact pressure between the mold and a solidified cell by virtue of electromagnetic pressure, thereby preventing surface defects from being created during casting, and an electromagnetic stirring device positioned at a lower portion of the mold to increase casting speed and a depth of pool within the mold.

[38] In addition, the present invention employs a low frequency electromagnetic field upon magnesium casting, and comprises a mold formed of copper, copper alloys or aluminum having a good electrical conductivity and a coil positioned outside the mold to apply an electromagnetic field to the mold to perform induction heating of molten magnesium with the electromagnetic field while stirring the molten metal by means of the low frequency electromagnetic field, thereby enabling continuous casting.

[39] Exemplary construction and operation of the present invention will be described with reference to the drawings. However, it should be noted that the present invention is not limited to such a specific construction, and the following description is provided as an example for understanding of the present invention.

[40]

Brief Description of the Drawings

[41] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[42] Fig. 1 is a conceptual diagram illustrating an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field in accordance with a first embodiment of the present invention, which comprises a circular mold, and a coil adapted to allow low frequency electric current to be applied thereto;

[43] Fig. 2 is a plan view illustrating the apparatus in accordance with the first embodiment, which comprises the circular mold, and the coil adapted to allow the low frequency electric current to be applied thereto;

[44] Fig. 3 is a conceptual diagram illustrating an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field in accordance with a second embodiment of the present invention, which comprises a circular mold, and a coil adapted to allow high frequency electric current to be applied thereto;

[45] Fig. 4 is a plan view illustrating the apparatus in accordance with the second embodiment, which comprises the circular mold, and the coil adapted to allow the high frequency electric current to be applied thereto;

[46] Figs. 5a and 5b are views illustrating a modification of the apparatus in accordance with the second embodiment which comprises a rectangular mold;

[47] Fig. 6 is an explanatory view illustrating a direction of electric current and electromagnetic field generated in the mold of the apparatus in accordance with the present invention;

[48] Fig. 7 is an explanatory view comparing surfaces of molten metal achieved by a conventional casting technique and by applying low frequency electric current and high frequency electric current with the apparatus in accordance with the present invention; and

[49] Figs. 8a and 8b are photographs showing surfaces of magnesium billets produced by the conventional technique and by the apparatus in accordance with the present invention, respectively.

[50]

Best Mode for Carrying Out the Invention

[51] Preferred embodiments of the present invention will be described in detail with reference to the drawings.

[52] Fig. 1 is a conceptual diagram illustrating an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field in accordance with a first embodiment of the present invention.

[53]

[54] According to the first embodiment, an apparatus 1 for continuous casting of the magnesium billets or slabs using the electromagnetic field comprises a mold 11 for solidifying molten metal 15, a coil 12 positioned outside the mold 11 and adapted to allow low frequency electric current to be applied thereto, a ring 13 positioned on an inner upper surface of the mold 11 for controlling solidification of the molten metal and enabling smooth casting, and a cooling water spray nozzle 16 for cooling the ring 13, an as-cast billet or slab 14, and the mold 11.

[55] Fig. 2 is a plan view illustrating the apparatus for continuous casting of the magnesium billets or slabs using the electromagnetic field according to the first embodiment.

[56] Although the mold 11 and the coil 12 are illustrated as having a circular cross- section in the drawing, the mold 11 and the coil 12 may have a rectangular cross- section or others.

[57]

[58] When low frequency electric current is applied to the coil 13, magnetic field and electric current are induced in the molten magnesium 15, as shown in Fig. 6, within the mold 11 which is made of metal having a good electrical conductivity.

[59] Such induced current serves not only to heat the molten metal via Joule heating, but also to generate electromagnetic force in the molten metal via interaction with the magnetic field. As the molten metal is heated by the induced current, cooling effect by cooling water is reduced at an initial solidified cell of the molten metal, thereby enabling stable solidification of the molten metal to be achieved.

[60]

[61] The electromagnetic force generated within the molten metal 15 has a gradient that the electromagnetic force is large on the surface of the molten metal, and small at an inner portion of the molten metal, and causes the molten metal to be stirred within the mold, thereby enhancing the inner structure of the billet.

[62] The mold 11 is made of a material, such as aluminum or copper alloys, having a good electrical conductivity. The ring 13 made of graphite may be inserted into the mold 11 to provide lubrication effect conducive to good casting of the as-cast billet or slab while delaying cooling of the as-cast billet or slab.

[63]

[64] The ring 13 has a length from an upper portion of the mold to approximately 20mm below the surface of the molten metal, and a thickness of 2 D or more. The ring 13 can be replaced with new ones when being damaged.

[65] The ring 13 delays cooling of the molten metal, while reducing friction between the as-cast billet or slab and the mold by virtue of exothermic effect and heat transfer lowering effect of graphite resulting from interaction with the electromagnetic field.

[66] The low frequency electromagnetic field applied to the coil 12 has a frequency of 2

~ 1,000 D, which is determined according to the size of the as-cast billet or slab. The intensity of electric current is also determined by the size of the as-cast billet or slab, and casting speed.

[67] Cooling of the mold 11 and the as-cast billet or slab 14 is performed by direct cooling via the cooling water spray nozzle 16. Cooling water cools the mold 11, the ring 13 within the mold 11, and the coil 12 at the same time.

[68] Fig. 3 is a conceptual diagram illustrating an apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field in accordance with a second embodiment of the present invention.

[69] According to the second embodiment, an apparatus 100 for continuous casting of the magnesium billet or slab using the electromagnetic field comprises a mold 101 for solidifying molten metal 105, a coil 102 positioned outside the mold 101 and adapted to allow high frequency electric current to be applied thereto, an electromagnetic stirring device 103 positioned at a lower portion of the mold 101 for stirring the molten metal 105, and a nozzle 104 between the mold 101 and the stirring device 103 for cooling an as-cast billet or slab 106 by spraying a refrigerant.

[70]

[71] In the apparatus 100 for continuous casting of the magnesium billet or slab using the electromagnetic field according to the second embodiment, the mold 101 has a cooling water passageway 107 formed therein through which cooling water is circulated, and a manifold 108 attached thereto for supplying and discharging the cooling water.

[72] Fig. 4 is a plan view illustrating the apparatus 100 for continuous casting of the magnesium billet or slab using the electromagnetic field according to the second embodiment.

[73] Although the mold 101 and the coil 102 are illustrated as having a circular cross- section in the drawing, the mold 101 and the coil 102 may have a rectangular cross- section or others.

[74]

[75] The mold 101 is formed with slits 109 in a casting direction in order to allow a high frequency electromagnetic field to be applied to molten magnesium. As shown in Fig. 3, each of the slits 109 is formed in an intermediate region of the mold 101 such that each slit 109 extends from below an upper end of the mold 101 to above a lower end thereof. The length of the slits 109 is determined in the range not to provide an adverse influence on rigidity of the mold 101.

[76] Each of the slits 109 has a width of 0.1 D or more, and preferably in the range of 0.2

~ 0.3 D. The number of slits 109 is determined in the range not to provide an adverse influence on rigidity of the mold 101. Each slit 109 is provided for induction of high frequency, and preferably filled with ceramic bonds and the like to prevent leakage of the molten metal 105.

[77] Since the high frequency electromagnetic field has a very small penetration depth in the molten metal, it is difficult to apply the high frequency electromagnetic field into the mold 101 made of copper. Thus, the slits 109 are machined in the mold 101 to allow the high frequency electromagnetic field to be effectively applied to the molten metal 105. On the other hand, since the low frequency electromagnetic field has a large penetration depth in the molten metal, it can be applied to the molten metal within the copper mold. Thus, it is not necessary to form the slits in the mold.

[78]

[79] Figs. 5a and 5b are views illustrating a rectangular mold 101' of the apparatus for continuous casting of the magnesium billet or slab using the electromagnetic field according to the second embodiment. Specifically, Fig. 5a is a front view of the rectangular mold 101' in which slits 109' are formed in an intermediate region of the mold, and extend from below an upper end of the mold 101' to above a lower end thereof. The length of the slits 109' is determined in the range not to provide an adverse influence on rigidity of the mold 101'.

[80] Each of the slits 109' has a width of 0.1 D or more, and the number of slits 109' is determined in the range not to provide an adverse influence on rigidity of the mold.

[81]

[82] Fig. 5b is a cross-sectional view taken along line A-A' of Fig. 5a, in which an independent cooling water passageway 107' for cooling the mold 101' is formed in each segment 110' separated by the slits.

[83] For the apparatus 100 according to the second embodiment, when high frequency electric current is applied to the coil 102, magnetic field and electric current are induced in the molten magnesium 105, as shown in Fig. 6, within the mold 101 which is partitioned by the slits 109.

[84] Such induced current serves not only to heat the molten metal 105 via Joule heating, but also to generate electromagnetic force in the molten metal 105 via interaction with the magnetic field. The electromagnetic force increases a curvature of a surface S of the molten metal 105 (see Fig. 3) where the molten metal 105 contacts the mold 101, while reducing a contact pressure between the as-cast billet or slab 106 and the mold 101.

[85]

[86] With the apparatus of the embodiment, the surface of the molten metal is intensively heated via Joule heating, thereby enabling control of a solidification speed while increasing solidification latent heat, so that continuous casting can be performed without defects caused by uneven solidification and quenching.

[87] The apparatus 100 for continuous casting of the magnesium billet or slab using the electromagnetic field according to the second embodiment is further provided with a nozzle 104 attached to the lower portion of the mold 101 to cool the as-cast billet or slab 106 by directly spraying gas or water thereto. [89] In addition to a function of enabling the continuous casting by controlling cooling of the as-cast billet or slab 106 via the high frequency electromagnetic field, the apparatus of the second embodiment may further comprise an electromagnetic stirring device 103 attached to the lower portion of the mold 101 for stirring the molten metal 106 in order to provide a function of controlling the inner quality thereof.

[90] When the molten metal 105 is rotated by the rotational electromagnetic stirring device 103 in a circumferential direction within the mold 101, a columnar structure of the molten metal is broken so that the overall structure of the as-cast billet or slab 106 becomes an equiaxial structure, thereby providing a billet having a fine casting structure.

[91] A method for continuous casting of a magnesium billet or slab using the apparatus

100 according to the second embodiment will be described hereinafter.

[92] In order to provide an as-cast billet or slab 106 having good surface and inner quality, electric current of 50 ~ 10,000 A is applied at a high frequency of 200 ~ 200,000 D to the coil 102.

[93]

[94] In addition, electric current of 20 ~ 1,000 A is applied at a frequency of 2 ~ 100 D to the electromagnetic stirring device 103. The frequency for the coil or the electromagnetic stirring device is determined according to the size of the billet 106, and the intensity of electric current is also determined according to the size of the as-cast billet or slab and casting speed. According to the invention, although the as-cast billet or slab 106 is illustrated as having a circular or rectangular cross-section in the above description, the billet may have other shapes.

[95] Fig. 7 is an explanatory view comparing surfaces of molten metal achieved by a conventional casting technique and by applying low frequency electric current and high frequency electric current with the apparatus in accordance with the present invention.

[96]

[97] Referring to Fig. 7, in conventional casting operation, the surface of molten metal contacts the mold at a right angle, so that, as a billet is produced while moving downwardly, severe friction occurs between the molten metal and the mold, thereby deteriorating the surface quality of the as-cast billet or slab.

[98] On the other hand, in electromagnetic casting accompanied with application of high frequency electric current by the apparatus of the present invention, a high frequency electromagnetic field has a shallow penetration depth due to the properties of high frequency, and is thus concentrated on the surface of the molten metal, so that the surface of the molten metal has a very large radius of curvature. As a result, it is possible to reduce friction between the as -cast billet or slab and the mold. [99]

[100] Accordingly, since electromagnetic force is concentrated on the surface of the molten metal, it has small stirring effect with respect to the interior of the molten metal, thereby enabling the molten metal to be maintained stable. However, since the inner quality of the molten metal cannot be enhanced due to low stirring force with respect to the molten metal, the apparatus of the invention is provided with the electromagnetic stirring device to enhance the inner quality of the molten metal.

[101] In addition, in electromagnetic casting accompanied with application of low frequency electric current by the apparatus of the present invention, the low frequency electromagnetic field has a deep penetration depth, and thus interacts with a deep portion of the molten metal, thereby increasing the stirring effect. However, in comparison with the high frequency electric current, the low frequency electric current provides a lower effect of enhancing the surface quality. Irrespective of the lower surface quality enhancing effect, the apparatus of the invention employing the low frequency electric current has a merit of low installation costs.

[102]

[103] Figs. 8a and 8b are photographs showing surfaces of as-cast billets 106 produced after melting AZ31 magnesium alloy. Fig. 8a is the photograph showing the surface of the as-cast billet 106, to which an electromagnetic field is not applied, according to the conventional technique. The as-cast billet 106 has lots of surface defects, and cannot be produced by normal casting operation.

[104]

[105] Fig. 8b is the photograph showing the surface of the as-cast billet 106 to which an electromagnetic field is applied according to the present invention. The as-cast billet 106 has no surface defects, and can be produced by increasing casting speed to 0.4 m/ min or more.

[106]

Industrial Applicability

[107] As apparent from the above description, the present invention provides advantageous effects in that a magnesium or magnesium alloy as-cast billet or slab having enhanced inner quality can be produced at an increased casting speed without surface defects.

[108]

[109] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. It should be noted that these modifications, additions and substitutions are within the scope of the present invention. [HO]

Claims

[I] An apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, comprising: a mold for continuous casting of an as-cast billet or slab; a coil positioned outside the mold and adapted to allow electric current to be applied thereto; and a cooling nozzle positioned outside the mold and the as-cast billet or slab. [2] The apparatus according to claim 1, wherein low frequency electric current is applied to the coil, and the mold is provided with a replaceable ring at an upper portion inside the mold.

[3] The apparatus according to claim 2, wherein the ring is made of graphite.

[4] The apparatus according to claim 1, wherein the mold has a circular or rectangular cross-section, and the coil is positioned to surround the mold. [5] The apparatus according to claim 1, wherein the cooling nozzle cools the mold, the coil, and the as-cast billet or slab by directly spraying water thereto. [6] A method for continuous casting of magnesium billets or slabs using an electromagnetic field by an apparatus according to any one of claims 1 to 5, wherein electric current is applied at a frequency of 2 ~ 1,000 D to a coil of the apparatus during casting.

[7] An apparatus for continuous casting of magnesium billets or slabs using an electromagnetic field, comprising: a mold for continuous casting of a magnesium billet or slab, and having a slit formed therein; a coil positioned outside the mold and adapted to allow electric current to be applied thereto; and an electromagnetic stirring device positioned at a lower portion of the mold. [8] The apparatus according to claim 7, wherein the mold has segments partitioned by the slit, and a cooling water passageway formed in the segments. [9] The apparatus according to claim 8, wherein the slit is formed at a central region of the mold, and extends from below an upper end of the mold to above a lower end thereof. [10] The apparatus according to claim 8, wherein the slit has a width of 0.2 ~ 0.3 D, and is filled with ceramic bonds to allow the electromagnetic field to be applied to molten metal within the mold while preventing leakage of the molten metal.

[I I] The apparatus according to claim 7, wherein the mold has a circular or rectangular cross-section, and the coil is provided to surround the mold.

[12] The apparatus according to claim 7, further comprising: a cooling nozzle positioned between the mold and the electromagnetic stirring device.

[13] The apparatus according to claim 12, wherein the cooling nozzle cools the as- cast billet or slab, and the electromagnetic stirring device by directly spraying water thereto.

[14] A method for continuous casting of magnesium billets or slabs using an electromagnetic field with an apparatus according to any one of claims 7 to 13, wherein during casting, electric current of 50 ~ 10,000 A is applied at a frequency of 200 ~ 200,000 D to a coil of the apparatus, and electric current of 20 ~ 1,000 A is applied at a frequency of 2 ~ 100 D to an electromagnetic stirring device of the apparatus.