Industrial electric heating furnaces – Induction furnace device – With means for manipulation of the charge or melt
Reexamination Certificate
2001-12-03
2003-09-09
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Induction furnace device
With means for manipulation of the charge or melt
C373S152000
Reexamination Certificate
active
06618426
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to installations of induction processing of molten metallic materials, for example for stirring, motion organization, and/or metallic material formation. The present invention more specifically relates to such installations using an inductive crucible and, among these, installations in which the crucible is intended not only for organizing the induction stirring of a molten metal, but also for heating up this metal by induction. The present invention applies to such inductive crucible installations, be they continuous casting installations or not.
2. Description of Related Art
The present invention will be described hereafter in relation with a cold inductive crucible, but it should be noted that it more generally applies to any other installation in which molten metal is in a magnetic field. Among inductive crucible installations, a cold crucible is often preferred to a crucible made of a refractory material when the metallic material to be elaborated requires a high degree of purity, since a cold crucible minimizes the contamination of the processed material.
FIG. 1
schematically shows a conventional installation for the formation of a metallic material by induction in a cold inductive crucible.
Such an installation includes a crucible
1
cooled down by circulation of water inside its wall, and intended for containing the material to be molten. In
FIG. 1
, the details constitutive of the crucible walls have not been shown for clarity. In particular, the means of circulation of the cooling liquid in the wall thickness have not been illustrated.
An inductor, generally a winding
2
, surrounds crucible
1
and is connected, by its two ends, across a single-phase A.C. voltage generator
3
. A capacitor C is connected in parallel on generator
3
, that is, across winding
2
.
The metallic material to be molten in the crucible is introduced therein, for example, in the form of chips m. In the case of a continuous casting crucible, chips m are continuously introduced, generally via a spout
4
.
The magnetic field caused by inductor
2
heats up the material contained in crucible
1
. The frequency of the A.C. excitation circuit of inductor
2
especially depends on the diameter of crucible
1
and on the electric resistivity of the material contained therein. As known, the electric resistivity of the material and the excitation frequency of inductor
2
condition the electromagnetic skin depth (&dgr;). The desired skin depth depends on the applications. For example, in the case of an ingot mould, the desired skin depth is the smallest possible while being sufficient to avoiding adversely affecting the thermal yield in view of the cold crucible walls. As a specific example, for a crucible having a diameter on the order of 10 centimeters, it is generally desired to have an electromagnetic skin depth on the order of from 1 to 10 millimeters.
It should be noted that, although this has not been shown in
FIG. 1
, a cold crucible is a sectorized crucible, that is, a crucible provided with vertical sectors isolated from one another to avoid a looping of the induced currents on the crucible periphery. As illustrated in
FIG. 1
, in a crucible supplied by a single-phase generator, the free surface of the metal melt appears in the form of a liquid dome, the profile of which results from the balance between the hydrostatic pressure and the electromagnetic pressure coming from single-phase inductor
2
. Further, on this free surface, there exists an electromagnetic stirring force which is a force with an always centrifugal radial component at the melt surface, as illustrated by the arrows in FIG.
1
. Thus, chips m coming from supply system
4
and falling by gravity into the melt are systematically, at their arrival, driven from the center to the periphery, and thus towards the cold walls of crucible
1
.
Such a circulation is unfavorable to the thermal yield of the system and to the ingestion of chips m in the melt. This may even affect the surface state of the solidified ingot obtained by continuous casting in the lower portion (not shown) of crucible
1
.
It would be desirable to invert the stirring direction at the melt surface so that the chips of the material to be melted are driven towards the center of the melt, and thus improve the mixing. For this purpose, it can be devised to form, around the crucible, a system of multiphase inductors creating a drift magnetic field, generating an ascending electromagnetic force in the thickness of the electromagnetic skin, and thus at the crucible periphery. This force then results in having the liquid-metal of the melt rise at the periphery in the thickness of the electromagnetic skin, and by conservation of the flow rate, having the liquid metal fall at the center of the melt. A centripetal radial electromagnetic stirring force is thus obtained, which is propitious to the driving of the supply chips, at their arrival on the melt, no longer towards the periphery, but conversely towards the center of the melt where they are immediately ingested and molten.
However, the implementation of such a principle poses several problems which have caused, up to now, that this solution is not industrially viable in practice.
A first problem is due to the need to have a multiphase induction generator, and thus two voltage generators shifted in phase with respect to each other.
For an installation in which the crucible has a sufficiently large diameter (on the order of thirty centimeters) enabling its supply by a generator operating on the frequency of the A.C. electric network (50 or 60 hertz), it is necessary to have a multiphase power supply (di- or triphasic) to implement this principle of inversion of the melt stirring direction. Thus, this prevents the simple connection of the installation on a single-phase electric power supply.
A similar problem arises in other cases, for example, the duct of relatively small diameter of an electromagnetic pump.
The problem is increased for induction generators of medium frequency (on the order of ten kilohertz) for which it is necessary to have an electronic power circuit to form the generator ((
3
,
FIG. 1
) to supply inductor
2
with a current of frequency different from the distribution frequency.
In such applications, which more specifically relate to crucibles or containers of small diameter, it would then be necessary to have an electronic power circuit for each phase, which considerably increases the installation cost. In particular, this requires the multiplication of the number of power components in accordance with the number of phases. Further, the synchronization of the generators with the phase of the A. C. power supply provided by the distribution network is made all the more difficult as the frequency of the induction generator and the number of phases are high.
SUMMARY OF THE INVENTION
The present invention aims at overcoming the disadvantages of conventional installations with an inductive crucible. The present invention aims, in particular, at providing a novel installation enabling the stirring of the molten metal melt, at will, in a centripetal direction or in a centrifugal direction.
More generally, the present invention aims at providing a novel solution to problems of stirring by induction in molten metal containers.
The present invention also aims at providing a novel solution for performing a multiphase generation which is economically viable. The present invention aims, in particular, at providing a solution that does not require multiplication of the power switches for applications requiring a medium-frequency induction.
The present invention also aims at providing a solution that can be supplied from a single-phase electromagnetic source.
The present invention further aims at providing a solution that poses no problem of synchronization of the different phases with respect to one another.
To achieve these objects, the present invention provides an installation for processing by induction a metallic mate
Centre National de la Recherche Scientifique
Duane Morris LLP
Hoang Tu Ba
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