Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-08-13
2003-09-16
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S502000
Reexamination Certificate
active
06619377
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the continuous casting of metals. More specifically, it relates to electromagnetic devices fitted into continuous casting molds and acting on the liquid metal present in said molds.
The use of electromagnetic fields to have an influence on the movements of the liquid steel in continuous casting molds of any format is in practice at the present time. The main objectives of imposing rotating electromagnetic fields (in the case of casting blooms and billets of square or slightly rectangular cross section) or traveling electromagnetic fields (in the case of casting slabs of rectangular cross section, the width of which is much larger than the thickness) are to homogenize the solidifying structures over the entire cross section of the product and to improve the surface finish of the product, together with its cleanliness from the standpoint of inclusions, especially near its surface. When casting slabs continuously, it is also known to impose static electromagnetic fields in the mold in order to stabilize the meniscus (i.e. the free surface of the molten metal in the top of the mold). This stabilization makes it possible to increase the product casting rate and therefore the productivity of the continuous caster. The electromagnetic devices producing this effect are known as “electromagnetic brakes”.
The known uses of electromagnetic fields in continuous casting molds have, for the moment, not been sufficient to solve completely satisfactorily all the problems of cast product quality. Among these persistent problems, mention may be made of the following:
the improvement in surface quality of the as-cast products, which corresponds to the reduction in the number of surface cracks and in the depth of oscillation ripples;
the improvement in the subshell cleanliness of the cast product, which corresponds to a reduction in the size of the “solidification hooks” which form during oscillation of the mold, these hooks being potential sites for the trapping of inclusions and gas bubbles present within the liquid metal in the mold, and also to the elimination of inclusion pickup by the solidification front, benefiting from the effect of this front being “washed” by the liquid metal entrained by the electromagnetic stirring (the mechanisms relating to these problems will be described in detail below);
the achievement of meniscus stability sufficient to guarantee optimum lubrication of the mold/solid metal interface by the covering slag which, in the liquid state, infiltrates therein so that this improved lubrication results in casting rates significantly greater than the usual rates.
Solving these problems satisfactorily would result in an increase in productivity of the caster and of the entire steel works. In addition to the increase in casting rate already mentioned, it would reduce the frequency of crack removal operations (in which the surface of the product is ground in order to eliminate defects therein) and would thus increase the proportion of products having a sufficient quality to be sent directly to the hot rolling mill. However, no currently known technique allows all the aforementioned qualitative objectives to be met simultaneously in an optimum fashion. In addition, the known techniques for achieving one or other of the these objectives are either expensive or require tricky implementation as they are very sensitive to the other casting conditions. Among these, apart from the methods mentioned above involving magnetic fields, mention may be made of systems which apply nonsinusoidal oscillations on the mold, embossed molds having a controlled hot-face roughness, covering slags of optimized composition, etc.
SUMMARY OF THE INVENTION
The object of the invention is to provide a process and a plant for the continuous casting of metals, which meet the productivity and quality objectives expected by operators of casters for continuously casting metals, especially steel.
With these objectives in mind, the subject of the invention is a process for the vertical continuous casting of metal products in a mold having cooled plates joined together, in which process the region of the meniscus of the liquid metal present in the mold is subjected to the action of an axial alternating magnetic field, collinear with the direction of casting, tending to impose on said meniscus a domed overall shape, characterized in that said region of the meniscus is also subjected to a continuous magnetic field directed transversely to the direction of casting in order to allow the shape of said meniscus to be stabilized.
The subject of the invention is also a plant for the vertical continuous casting of metals, comprising a mold having cooled plane plates joined together, of which two are long, facing one another in order to define a casting space, which plant is of the type having an electromagnetic coil supplied with AC current and surrounding the mold in the region of the meniscus of the liquid metal which is present therein so as to produce therein an alternating magnetic field directed along the casting axis, characterized in that it also includes an electromagnetic inductor which produces a continuous magnetic field passing through the long plates of the mold in the region of the meniscus perpendicular to the casting axis .
As will have been understood, the invention consists in creating at least two electromagnetic fields in the liquid metal present within the continuous casting mold, these fields acting simultaneously on said metal in the region of the meniscus. On of these fields is an axial alternating field and the other is a transverse continuous field, both being exerted in the region of the meniscus. They are produced by means of fitted inductors or inductors producing their effect near the meniscus.
Schematically speaking, the alternating field collinear with the casting axis is used to “dome” the meniscus, that is to say to define the convex dome shape that it naturally assumes on contact with the walls of the mold, while the transverse continuous field acts as an electromagnetic brake in order to reduce the local geometrical irregularities at the surface of this meniscus, resulting in subjacent convection currents generated by this alternating field.
Theoretically, applying a single alternating magnetic field might suffice by itself to obtain a smooth domed meniscus. This is because the electromagnetic force generated on the liquid metal has both:
a confining surface component which tends to push the periphery of the meniscus away from the sides of the mold, and therefore to “hollow” it around the border, smoothing out its surface. This force is especially active at high frequency; and
a stirring volume component which, because of the configuration of the convective currents in the liquid metal that it causes (annular stirring with the metal rising in the center of the mold), “swells” the central part of the meniscus. This force is, in contrast, especially active at low or medium frequency. Moreover, it is for this reason that it is the cause of surface instabilities. The maximum effect of this stirring force is obtained at a medium frequency, namely around 200 Hz to be specific, but in any case less than 500 Hz, whatever the nature or the thickness of the mold or the format of the metallurgical product cast.
It is these two conjugate actions—peripheral repulsion and stirring with central rising (which actions could be obtained from one and the same pulsating magnetic field—which give the meniscus a desired defined domed shape.
By the same token, but for the purpose of solidifying the electromagnetically confined metal, that is to say metal away from any physical contact with the cooled sides of the mold, it has already been proposed to create a magnetic environment within the mold, consisting of the superposition of two axial fields, that is to say both fields being directed along the casting axis, one being periodic (the confining field) and the other being constant in order to produce radial vibration forces in the confined liquid metal.
Delannoy Yves
Etay Jacqueline
Galpin Jean-Marie
Gardin Pascal
Garnier Marcel
Lin Kuang Y.
The Japan Research and Development Center for Metals
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