Patent Grant
10926321
The present invention relates to the continuous casting of molten materials and in particular, a system and method for the continuous casting of molten materials.
In conventional casting of steel, mold oscillations are used to minimize friction and sticking of the solidifying shell, and to avoid shell tearing and liquid steel breakouts. Oscillation is usually achieved either hydraulically or via motor driven cams or levers which support and reciprocate the mold.
Conventional mold oscillation has many disadvantages, including the requirement for mechanical and hydraulic gear to oscillate the mold; a fixed refractory connection with the tundish is not possible if the mold is mechanically oscillated; partial oxidation of the metal due to surface perturbations; the requirement for loop control to maintain constant melt flow when mechanical oscillations are used; and the formation of oscillational marks on the surface of the cast product.
One solution to this problem is illustrated in U.S. Pat. No. 4,522,249, where a magnetic pulse is applied to a coil held in a mandrel, which, in turn, applies pressure to the mold and contracts the cross-sectional dimension of the molten metal. In this arrangement, it was found that the coil underwent physical deformation from the mandrel. While the coil is reparable or replaceable, repairs and/or replacement result in downtime of the system, leading to loss of production and replacement of the coil is costly. This mandrel arrangement has limited current carrying capacity; much lower than the current carrying capacity of the coil itself. Thus, induced counter currents in the mandrel resulted in reduced repelling forces. Additionally, internal cooling of the coil leads to several difficulties. For example, high flow rates are required to keep the coil cool, however, because of the coil enclosure, flow rate is limited.
There is a need to provide an improved apparatus and method for the continuous casting of molten material.
It is an object of the present invention to provide an improved apparatus and method for the continuous casting of molten material that overcome disadvantages of the prior art apparatuses and methods.
In accordance with an aspect of the invention, there is provided a method of continuous casting of a molten material, the method comprising the steps of: continuously feeding the molten material into an elongate molding cavity of an elongate mold, the elongate mold having an inner wall and an outer wall defining the cavity therein, an inlet at a first end of the elongate mold for receiving the molten material, an outlet at a second end of the elongate mold for outputting a solidifying billet of the molten material, the mold being constructed of an electromagnetic material; continuously flowing cooling water into an annular channel formed between the outer wall of the elongate mold and an inner surface of an electrical coil arranged in a helical direction around the outer wall of the elongate mold, the annular channel for receiving the continuously flowing cooling water from a water inlet and passing the continuously flowing cooling water therethrough to a water outlet to cool the coil, the elongate mold, and the molten material contacting the inner wall; continuously applying a pulsating current to the electrical coil, the pulsating current inducing a counter current in the elongate mold, the counter current causing a repelling force between the coil and the elongate mold thereby causing a flexure of the elongate mold; and removing the solidifying billet from the outlet of said elongate mold.
In an embodiment of the present invention, at the step of applying a pulsating current, the method further comprising simultaneously inducing electromagnetic forces via electromagnetic stirrers arranged substantially circumferentially around the elongate mold such that the electromagnetic forces cause the molten material to be stirred within the molding cavity.
In an embodiment of the present invention, at the step of applying a pulsating current, the method further comprising simultaneously inducing electromagnetic forces via electromagnetic stirrers arranged substantially circumferentially around the cast product beyond the exit end of the elongate mold.
In an embodiment of the present invention, at the step of applying a pulsating current, the method further comprising simultaneously inducing electromagnetic forces via electromagnetic stirrers arranged around the elongate mold such that the electromagnetic forces cause the molten material to be stirred within the molding cavity and arranged substantially circumferentially around the cast product beyond the exit end of the elongate mold.
In an embodiment of the present invention, the electromagnetic stirrers are placed around the mold in areas where the molten material is substantially still liquid, areas in which the mold is being pulsated where the molten material is solidifying and substantially mushy, and areas in which the mold is outside the pulsating magnetic field where the molten material is solidifying and substantially mushy.
In an embodiment of the present invention, the electromagnetic stirrers stir in a substantially longitudinal direction corresponding to a direction substantially parallel to the feeding of the molten material.
In an embodiment of the present invention, the electromagnetic stirrers stir in a substantially lateral direction corresponding to a direction substantially perpendicular to the feeding of the molten material.
In an embodiment of the present invention, the electromagnetic stirrers stir in a substantially helical direction.
In an embodiment of the present invention, the rapidly pulsating magnetic field has a pulse duration of about 1 millisecond to about 2 milliseconds and an intensity of about 1000 to about 5000 amperes peak.
In an embodiment of the present invention, the magnetic field has a pulse interval of about 10 to about 100 times per second.
In an embodiment of the present invention, the elongate molding cavity has a substantially circular cross-section.
In an embodiment of the present invention, the elongate molding cavity has a substantially rectangular cross-section.
In an embodiment of the present invention, the elongate molding cavity has a substantially dog-bone cross-section.
In an embodiment of the present invention, the molten material is selected from the group consisting of steel, aluminum, aluminum alloy, and aluminum based metal-matrix composite.
In an embodiment of the present invention, the electroconductive material is copper.
In accordance with an aspect of the present invention, there is provided an apparatus for continuous casting of molten material, said apparatus comprising: an elongate tube of electrically conductive material having an inner and an outer wall defining a molding cavity therein, the inner and outer walls having a first end having an inlet for receiving the molten material and a second end having an outlet for removing a solidifying billet formed from the molten material; an electrical coil with an inner surface and an outer surface, the electrical coil arranged to surround the outer wall of the elongate tube; an annular channel defined by the outer wall of the elongate tube and the inner surface of the electrical coil, the annular channel for receiving a flow of cooling water from a water inlet and passing the cooling water therethrough to a water outlet; and wherein when pulsating current passes through the electrical coil, a counter current is induced in the elongate mold causing a repelling force between the electrical coil and the elongate mold, thereby causing inward radial flexure of the elongate mold.
In an embodiment of the present invention, the apparatus further comprises electromagnetic stirrers arranged substantially circumferentially around the mold to induce electromagnetic forces to cause the molten material to be stirred within the molding cavity.
In an embodiment of the present invention, the apparatus further comprises electromagnetic stirrers arranged substantially circumferentially around the cast product beyond the exit end of the elongate mold.
In an embodiment of the present invention, the apparatus further comprises electromagnetic stirrers arranged around the elongate mold to induce electromagnetic forces to cause the molten material to be stirred within the molding cavity and arranged substantially circumferentially around the cast product beyond the exit end of the elongate mold.
In an embodiment of the present invention, the electromagnetic stirrers are placed around the mold in areas where the molten material is still substantially liquid, and areas in which the mold is being pulsated where the molten material is solidifying and substantially mushy, areas in which the mold is outside the pulsating magnetic field where the molten material is solidifying and substantially mushy.
In an embodiment of the present invention, the electromagnetic stirrers stir in a substantially longitudinal direction corresponding to a direction substantially parallel to the feeding of the molten material.
In an embodiment of the present invention, the electromagnetic stirrers stir in a substantially lateral direction corresponding to a direction substantially perpendicular to the feeding of the molten material.
In an embodiment of the present invention, the electromagnetic stirrers stir in a substantially helical direction.
In an embodiment of the present invention, rapidly pulsating magnetic field has a pulse duration of about 1 millisecond to about 2 milliseconds and an intensity of about 1000 to about 5000 amperes peak.
In an embodiment of the present invention, the magnetic field has a pulse interval of about 10 to about 100 times per second.
In an embodiment of the present invention, the apparatus further comprises compression rods to restrain the coil.
In an embodiment of the present invention, the elongate tube is arranged substantially horizontal.
In an embodiment of the present invention, the elongate molding cavity has a substantially circular cross-section.
In an embodiment of the present invention, the elongate molding cavity has a substantially rectangular cross-section.
In an embodiment of the present invention, the elongate molding cavity has a substantially dog-bone cross-section.
In an embodiment of the present invention, the molten material is selected from the group consisting of steel, aluminum, aluminum alloy, and aluminum based metal-matrix composite.
In an embodiment of the present invention, the electroconductive material is copper.
Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:
In pulse mold casting in accordance with the present invention, the need for mechanical oscillations is eliminated, by creating a near frictionless movement in the direction of casting by electromagnetically oscillating the mold perpendicular to the direction of casting. When oscillating the mold in this manner, the mold wall detaches itself from the solidifying shell of molten material, allowing the cast product to be withdrawn from the mold efficiently and easily, resulting in a uniform surface finish of the product.
Referring to the drawings,
Charge handling unit 102 feeds the solid or liquid material (not shown) into a melting furnace 106.
Induction coils (not shown) are placed around or inside the crucible of the furnace 106. The induction coils are used to stir the molten material 104 in the furnace 106. Induction induces flow streamlines in the molten material 104 which is electrically conducting, thereby mixing the alloying elements and promoting homogeneity in the molten material 104.
The melt is further purified through a process of degassing in the degassing unit 110. In one example, for aluminum alloys, the dissolved hydrogen is removed. In this case, a rotary impeller degasser (RIM) may be used. Because aluminum is extremely reactive when it comes in contact with moist air or wet tools, the water decomposes to release hydrogen in the melt. This dissolved hydrogen then causes casting defects like porosity. The chemical reaction is represented by the following equation: 2Al+3H2O═Al2O3+6H. Solubility of gaseous hydrogen falls sharply when aluminum solidifies, releasing excess hydrogen upon solidification which causes porosity.
In the example where the degassing unit 110 is a RIM, an inert or chemically inactive gas (argon, nitrogen etc.) is purged through a rotating shaft and rotor (not shown). The energy of the rotating shaft causes formation of a large number of fine bubbles providing very high surface area-to volume ratio. The large surface area promotes fast and effective diffusion of hydrogen into the gas bubbles resulting in equalizing activity of hydrogen in liquid and gaseous phases.
Turning back to
The continuous casting mold 10 in accordance with an embodiment of the present invention is shown in greater detail in
The elongate cavity 12 in the illustrated embodiment in
The tubular mold member 18 is constructed of any convenient electroconductive material in which a magnetic field may be induced and which maintains the solid state upon passage of the molten material therethrough. One suitable material of construction is copper, which may be alloyed with other metals to increase its toughness.
In the illustrated embodiments of
The outer surface 21 is wrapped with a current-carrying coil 36. The length of the coil 36 is determined by the solidification characteristics of the material, such as the metal or alloy being cast. The coil 36 is pulsed with a coil current in ‘pulsed form’, i.e., it is a sinusoidal form that is switched on and off. A typical pulsing frequency could be 10 pulses per second.
Between the coil 36 and the outer surface 21 of the mold member 18, is a narrow gap of from about 5 to about 6 mm. This gap acts as a cooling passage 20 which allows cooling liquid to be pumped therethrough at a high flow rate. The most common coolant is water. Water's high heat capacity and low cost makes it a suitable heat-transfer medium. This cooling water cools the outer surface 21 of the mold member 18 as well as the surface of the coil 36 that is facing the cooling passage 20. The water flow rate in the cooling passage is high enough so that it does not vaporize from the heat and cause any cavitation. Adjacent the inlet end 14 of the mold cavity 12, the upstream end of the cooling passage 20 communicates with a first annular cavity 22 defined by a water inlet housing 24 having an inlet passage 26 for the flow of fresh cooling water to the cavity 22 and thence to the cooling passage 20. Adjacent the outlet end 16 of the mold cavity 12, the downstream end of the cooling passage 20 communicates with a second annular cavity 28 defined by a water outlet housing 30 having an outlet passage 32 for the flow of used cooling water from the cavity 28. If desired, the cooling water may be caused to flow in the opposite direction through the cooling passage 20 by reversing the flow of water through the passages 26 and 32.
Because the coil 36 becomes heated due to the passage of the current, through resistive heat (also called ‘Joule heating’ or I2R type of heating), cooling of the coil is advantageous since this heat will continually build up with time as energy=power×time.
The mold member 18, in turn, is made of electroconductive material which also needs to be cooled so that in the casting direction, the material can progressively solidify since the solidification front moves from the inner mold wall 19 to the center of the mold cavity 12.
Turning back to
Returning to
The coil 36 may be a solid copper conductor that communicates with electrical power inlet and outlet wires (not shown), which, in turn, are connected to a source of pulsating current (not shown), to provide in cyclic manner, short bursts of current through the coil windings, thereby producing a short duration intense magnetic field. Each pulse through the coil 36 produces a force causing the coil 36 to deform in a longitudinal and lateral direction to the coil 36. Steel compression rods 33 in the coil housing 34 restrain the coil 36 in a longitudinal direction to prevent this deformation of the coil 36. In another embodiment, the coil 36 may be encased in a high strength material to withstand deformation and vibrations.
The pulses are generated by electromagnetic interactions. Electromagnetic fields are created by the passage of pulsed current in the coil 36 which encircles the mold member 18. This field causes an opposite and almost equal current to be induced in the mold member 18 (minus some electromagnetic decay and losses).
The interaction of these two currents (coil current and induced current) creates a repelling force between the coil 36 and the mold member 18. This force tends to displace the mold member 18 in a direction perpendicular to casting (that is, the direction of primary molten material flow and product withdrawal).
The force of repulsion is proportional to the product of the magnitudes of the coil current and the induced current in the copper mold:
Frepulsion∝i1×i2
where Frepulsion is the force of repulsion; i1 is the RMS magnitude of the coil current, and i2 is the RMS magnitude of the induced current in the mold member 18.
i1B=(50-75%) of i1A
Similarly,
i2B=(50-75%) of i2A
Table 1 provides a summary of exemplary process parameters, which not only enable the process but also allow for process customization, control, and flexibility. These parameters can be changed within a certain range, and can be optimized based on the type of product cast, casting parameters like speed, alloy cast, and required product properties.
In Table 1, the current is pulsed in the coil 36, in one embodiment, via a capacitor arrangement. The mold member 18 pulsates in the direction normal to its contact line with the solidifying shell of molten material. The pulsating action reduces the coefficient of friction to almost zero in the direction of product withdrawal. This electromagnetic pulsing obviates the need of a mechanically oscillated mold, allowing for a fixed refractory connection from the tundish 114 to the mold 10, as well and providing stable flow control and flow characteristics of the molten material from tundish 114 to mold 10, and reducing oxidation of the liquid material that enters from the tundish 114 to the mold 10. In addition, the process does not require loop control of the flow control that is required in conventional casting.
In operation, the process is outlined in
Cooling water continuously flows from inlet pipe 26 to the annular cooling passage 20 out to the outlet pipe 32 in step 54. The cooling water continuously flowing through the cooling passage 20 causes molten material closest to the internal wall 19 of the mold cavity 12 to cool and continuously solidify via heat transfer, while radially inwardly thereof, the material remains molten. It is preferable to use cooling water flow rates that are high enough so the cooling water does not boil.
In step 56, pulsed current is continuously passed through the coil 36 which encircles the mold member 18. This continuously pulsed current causes an induced current in the mold member 18. The interaction of these two currents (coil current and induced current) creates a repelling force between the coil 36 and the mold member 18. This force tends to displace the mold member 18 in a direction perpendicular to casting (that is, the direction of primary molten material flow and product withdrawal).
At a pulse frequency of 100 pulses per minute, a reduced current allows for a pulse every 1.3 mm. The breaking of dendrites may not be as effective at this frequency; however, the casting reliability and surface quality may increase.
The pulse creates rapid elastic movement of the mold member 18 causing mold member 18 to move slightly radially inwardly, thereby applying pressure to the solidifying shell. Since the molten material has a skin of solid material resulting from the cooling induced by the passage of cooling water through the cooling passage 20, the material does not relax to the same extent as the mold member 18 before the next pulse again induces radially inward movement of the mold member 18. As the material flows through the mold cavity 12, more of the cross-section of the material solidifies. Effectively, therefore, the solidifying material detaches from the inner wall of the mold cavity by the rapid reciprocal radial movement of the mold member 18.
Unlike U.S. Pat. No. 4,522,249, wherein the flowing molten material is subjected to flexure under the influence of the magnetic field which may create flow patterns in the molten material, in the present invention, the mold member 18 is flexed, moving the molten material away from the solidifying shell of the forming billet or bar 115 and flowing continuously in a single direction downstream within the mold cavity 12.
The flexure of the mold member 18 creates zero or near-zero friction between the inner mold wall 19 and the solidifying shell of the bar 115 to permit ready withdrawal from the mold cavity 12, in step 58, without the formation of significant surface imperfections or blemishes, thereby overcoming the problems of the prior art. The absence of surface defects permits the casting to be forwarded directly to a rolling mill or other forming methods.
Ultimately the material throughout the cross-sectional dimension solidifies enough so that a billet or bar of material, such as metal or metal alloy, is removed from the outlet 16 from the casting cavity 12 in step 58 in
In the examples of aluminum alloy 2024 or aluminum alloy 6061, for a bar having a cross section of 3680 mm2, a casting speed of 10.5 m/min (0.175 m/s) can be obtained through the exemplary casting mold 10 of
Cooling water flow rates are high enough that the cooling water will not boil due to the heat transfer from the mold member 18. Superheat temperature is rapidly extinguished and freezing of material begins near the mold entrance 14. For lower cooling rates, there is a lower overall heat transfer rate through the mold member 18, resulting in a higher liquid fraction in the bar at the mold exit 16. Controlling cooling flow rates and casting speed are combined to optimize the solid shell thickness of the bar at the mold exit 16. In the example of aluminum alloys 6061 and 2024, a bar of aluminum alloy 6061 will likely have a larger shell thickness at the mold exit 16, but is easier to chill cast than aluminum alloy 2024. Thus, with high enough water flow rates and a casting velocity corresponding to a throughput of 6-10 tonnes per hour, aluminum alloys 6061 and 2024 can be cast using horizontal direct chill casting.
The electromagnetic stirrers 62 may be placed in three different zones on the mold: i) where most of the molten material is still in a liquid form; ii) where the molten material is a substantial combination of solid and liquid or mushy within the area where the mold is being pulsed; or iii) in a substantially mushy area outside the area of the mold that is being pulsed. Stirring is effective if it is imposed on liquid material, less so on mushy material, and ineffective on solid material. The solid shell of the material being cast is growing in the direction of casting, as shown in
In another embodiment, the electromagnetic stirrers 62 may be placed in all three of these areas on the mold or any combination of these three areas to produce stirring in the longitudinal, lateral, or helical directions. The electromagnetic stirrer 62 placement can be adjusted for fluid dynamic and solidification characteristics.
In a further embodiment, electromagnetic stirrers 62 may be placed beyond the exit of the mold. In the continuous casting of alloys with large solidification temperature ranges, a substantial portion of the core of the solidifying bar 115 may be liquid. In such cases, stirring beyond the outlet end 16 may be beneficial if the electromagnetic stirrer is operated such that the electromagnetic fields can penetrate the large shell thickness at that location.
In some cases, helical/circumferential stirring may be more beneficial than longitudinal stirring and in some cases a combination of both, helical/circumferential and longitudinal stirring, may be useful.
The above-described embodiments and method may be used in the casting of metals including but not limited to steel, aluminum, copper, and their various alloys.
The above-described embodiments are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention, which is defined solely by the claims appended hereto.
It should be understood that the phrase “a” or “an” used in conjunction with the Applicant's teachings with reference to various elements encompasses “one or more” or “at least one” unless the context clearly indicates otherwise. Additionally, conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment.
While the Applicant's teachings have been particularly shown and described with reference to specific illustrative embodiments, it should be understood that various changes in form and detail may be made without departing from the scope of the teachings. Therefore, all embodiments that come within the scope of the teachings, and equivalents thereto, are claimed. The descriptions and diagrams of the methods of the Applicant's teachings should not be read as limited to the described order of elements unless stated to that effect.
While the Applicant's teachings have been described in conjunction with various embodiments and examples, it is not intended that the Applicant's teachings be limited to such embodiments or examples. On the contrary, the Applicant's teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art, and all such modifications or variations are believed to be within the scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 62/418,857 filed Nov. 8, 2016, which is hereby incorporated herein by reference.
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| Number | Date | Country |
|---|---|---|
| 1001862 | Feb 2002 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20180185908 A1 | Jul 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| 62418857 | Nov 2016 | US |
