Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-09-01
2002-01-22
Elve, M. Alexandra (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S418000, C164S443000, C164S459000, C164S485000
Reexamination Certificate
active
06340049
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a device for continuous or semi-continuous casting of metal or metal alloys into an elongated strand, where the strand is cast using a device comprising a cooled continuous casting mold and an induction coil arranged at the top end of the mold. The coil is supplied with a high frequency alternating current from a power supply. The invented device exhibits improved mechanical strength.
BACKGROUND ART
During continuous or semi-continuous casting of metals and metal alloys, a hot metal melt is supplied to a cooled continuous casting mold, i.e. a mold which is open in both ends in the casting direction. The mold is typically water-cooled and surrounded and supported by a structure of support beams. Melt is supplied to the mold where the metal is solidified and a cast strand is formed as it is passed through the mold. A cast strand leaving the mold, comprises a solidified, self-supporting surface layer or shell around a residual melt. Generally it can be said that conditions of initial solidification is critical for both quality and productivity. A lubricant is typically supplied to the upper surface of the melt in the mold. The lubricant serves many purposes, amongst others it will prevent the skin of the cast strand first developed from sticking to the mold wall. Normal adherence between oscillation show as so called oscillation marks. Should the solidified skin stick or adhere more severely to the mold it will show as severe surface defects and in some cases as ripping of the first solidified skin. For large dimension strands of steel the lubricant is predominantly a so-called mold powder comprising glass or glass forming compounds that is melted by the heat at the meniscus. The mold powder is often continuously added to the upper surface of the melt in the mold during casting, as an essentially solid, free flowing particulate powder. The composition of a mold powder is customized. Thereby the powder will melt at a desired rate and lubrication will be provided at the desired rate to ensure lubrication. A too thick layer of lubricant between mold and cast strand will also affect the solidification conditions and surface quality in an undesired way, thus the thermal conditions at the meniscus need to be controlled. For smaller strands and for non-ferrous metals oil, typically vegetable oil, or grease is used as lubricant. Irrespective of what type of mold lubricant is used it should preferably be fed into the interface cast strand/mold at an even rate sufficient to form a thin uniform film in the interface to avoid surface defects originating from adherence between mold and strand. A too thick film might cause uneven surface and disturbs the thermal situation.
Heat losses and overall thermal conditions at the meniscus are predominantly controlled by the secondary flow that is developed in the mold. The use of an inductive HF heater or another HF-device used for electromagnetic casting, an EMC-device, for influencing the thermal situation at the top end is discussed in e.g. U.S. Pat. No. 5,375,648 and in earlier not yet published Swedish Patent Application No. SE9703892-1. High thermal losses are compensated by a supply of heat to the upper surface, either by a controlled upward flow of hot melt or by a heater, otherwise the meniscus can start to solidify. Such a solidification will severely disturb the casting process and destroy the quality of the cast product in most aspects.
A high frequency inductive heater arranged at the top end of a continuous casting mold will provide means to improve the capability to control the temperature of the metal at the upper surface of the melt, the meniscus, and at the same time generate compressive forces acting to separate the melt and the mold, thereby reducing the risk for sticking, reducing oscillation mark and in general provide improved conditions for mold lubrication. This technique, which today is referred to as electromagnetic casting, EMC, for an improved lubrication and thus improved surfaces is primarily attributed to the compressive forces acting to separate the melt from the mold. The inductive heater or coil may be of single-phase or poly-phase design. Preferably a high-frequency magnetic alternating field is applied. The compressive forces, generated by the high frequency magnetic field, reduce the pressure between the mold wall and the melt, whereby the conditions for lubrication are significantly improved. Surface quality of the cast strand is improved and the casting speed can be increased without risking the surface quality. Oscillation is primarily applied to ensure that the cast strand leaves the mold. As the compressive forces act to separate the melt from the mold they will minimize any contact between the melt and mold during initial solidification of the skin and improve the feed of lubricant hereby further improving the surface quality of the cast strand. The use of an induction coil supplied with a high frequency alternating current and arranged at the meniscus is believed to provide a means to substantially improve surface quality, internal structure, cleanliness and also productivity. To increase the penetration of a high frequency magnetic field through a mold and into the melt it is known to use a Cu-mold which at the top end of the mold, i.e. at level with the high frequency induction heater is slitted in the casting direction. The slitted mold will reduce the eddy-current losses and increase the heat efficiency as the current paths for the electrical currents induced in the mold by the applied magnetic field is cut. Such molds, known as cold crucible molds, are used for other purposes and are typically used as billet molds and the like, i.e. molds for small sized strands typically with an essentially square cross-section of 200×200 mm or less. The Cu-mold is favorable due to its high heat conductivity and high electrical conductivity but has a short coming in its mechanical strength.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a device for continuous casting of metal strand, wherein the conditions for the initial solidification of the cast metal in the mold are improved and in particular the conditions for mold lubrication are improved by the use of an EMC that exhibits low electromagnetic losses. In particular it is an object of the present invention to provide a device comprising a so called cold crucible mold, i.e. a mold that in its top end is slitted and thereby divided into segments which exhibit an improved mechanical integrity without any increase in the induced power losses.
A continuous casting device according to the present invention shall ensure good conditions for initial solidification within a mechanically stable mold for use together with EMC, wherein a good and controlled thermal flow, lubrication and overall conditions at the top end of the mold is provided, thus attaining considerable improvements with respect to quality and productivity. This is accomplished by the present invention, which according to one aspect provides a method for continuous or semi-continuous casting of metal according to the pre-amble of claim
1
, which is characterized by the features of the characterizing part of claim. Further developments of the device are characterized by the features of additional claims
2
to
21
. It is also an object of the present invention to provide a use of such continuous casting device, which is defined in claim
22
.
DESCRIPTION OF THE INVENTION
A device for continuous or semi-continuous casting of metal where hot melt is supplied to a cooled continuous casting mold, the melt is cooled and at least partly solidified to a strand which is extracted from the mold and further cooled and solidified downstream of the mold and which comprises an induction coil arranged at the top end of the mold is to reduce the induced power losses typically arranged with the top end of the mold slitted into a plurality of mold segments where each slot between two mold segments is filled with a partition comprising an electric
Eriksson Jan-Erik
Kroon Tord
Lehman Anders
Svensson Erik
ABB AB
Dykema Gossett PLLC
Elve M. Alexandra
Tran Len
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