Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-08-31
2002-09-17
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S466000, C164S504000
Reexamination Certificate
active
06450241
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for continuous or semi-continuous casting of metal or metal alloys into an elongated strand. The strand is cast using a device comprising a cooled continuous casting mold and an inductive coil arranged at the top end of the mold. The coil being supplied with a high frequency alternating current from a power supply. The invented method ensures that temperature and other casting conditions determining the initial solidification conditions in the mold are controlled such that a cast product, a strand, exhibiting an improved surface characteristics, a controlled cast structure, a low level of entrapped inclusions and other defects is produced at maintained or increased productivity. The present invention also relates to a device including the continuous casting mold, the coil, a power supply unit with control means suitable for the invented method.
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 preferably water-cooled and typically 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. The conditions of initial solidification is dependent on a number of factors influencing each other in a complex manner, such as;
Metal flow in the upper part of the mold;
Lubrication between the mold and the melt/cast strand;
Heat losses and overall thermal conditions at the meniscus;
Thermal conditions and heat dissipation at the front of solidification; and
Oscillation, if any, of the mold.
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. Should the solidified skin stick or adhere more severely to the mold it will show as 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 inductive HF heaters 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 the same time generate compressive forces acting to separate the melt and the mold, thereby reducing the risk for sticking, reducing oscillation marks and in general provide improved conditions for mold lubrication. The improved lubrication is primarily attributed to the compressive forces acting to separate the melt from the mold. The inductive heater 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 preferably applied to ensure that the cast strand leaves the mold. However minor surface defects, so-called oscillation marks are normally formed on the cast strand upon contacts between mold and strand during the formation of the skin. These oscillation marks also effect the structure of the cast strand as inclusions often are trapped at them. 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 thereby further improving the surface quality of the cast strand. Thus the use of multi-turn coil supplied with a high frequency alternating current and arranged at the meniscus is believed to provide a means to eliminate or at least substantially reduce the oscillation marks and thereby improving surface quality, internal structure, cleanliness and also productivity.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method 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 is improved. In particular it is an object of the present invention to control the high frequency magnetic field, which is applied to act on the melt at the top end of the mold, such that the generated compressive forces acting to separate the melt from the mold ensuring a stable feed of mold lubricant into the interface between the mold and the strand and a formation of a lubricating film in the interface. Thereby can surface defects such as internal oscillation marks and any defects or productivity concern associated with them be essentially eliminated or at least substantially reduced. 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 preamble of claim
1
, which is characterized by the features of the characterizing part of claim
1
. Further developments of the method are characterized by the features of additional claims
2
to
13
.
It is further an object of the present invention to provide a continuous casting device comprising a cooled continuous casting mold, oscillation means, a multi-turn inductive coil supplied with a high frequency alternating current and a power supply unit with current control means to generate and control the high frequency magnetic field applied to act on the melt at the top end of the mold.
In particular the casting device shall comprise means to control the alternating current supplied to the high frequency magnetic field generating device such that casting conditions and operating parameters are optimized to accomplish quality improvements and/or productivity improvements. In particular shall the casting device be arranged w
ABB AB
Dykema Gossett PLLC
Elve M. Alexandra
Tran Len
LandOfFree
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