Metal founding – Process – With measuring – testing – inspecting – or condition determination
Reexamination Certificate
2000-05-04
2002-12-17
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
With measuring, testing, inspecting, or condition determination
C164S466000, C164S502000, C164S154100
Reexamination Certificate
active
06494249
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for casting of metals, and in particular to a method for continuous or semi-continuous casting of a strand in a mold, wherein the flow of metal in non-solidified parts of the cast strand is acted on and controlled by at least one static or periodically low-frequency magnetic field applied to act upon the molten metal in the mold during casting. The present invention also relates to a device for carrying out the invented method.
BACKGROUND ART
In a process for continuous or semi-continuous casting, a metallic melt is chilled and formed into an elongated strand. Depending on its cross-section dimensions, the strand is called a billet, a bloom or a slab. During casting a primary flow of hot metal is supplied to a chilled mold wherein the metal is cooled and at least partly solidified into an elongated strand. The cooled and partly solidified strand leaves the mold continuously. At the point where the strand leaves the mold, it includes at least a mechanically self-supporting skin surrounding a non-solidified center. The chilled mold is open at both of its ends in the casting direction and is preferably associated with means for supporting the mold and means for supplying coolant to the mold and the support. The chilled mold preferably includes four mold plates, preferably made of copper or other material with a suitable heat conductivity. The support means are preferably beams with internal channels for supply of coolant, normally water, thus such support beams are often called water beams. The water beams are arranged around and in good thermal contact with the chilled mold to fulfill its double function of supporting and cooling the mold.
The hot primary metal flow is supplied either through a nozzle submerged in the melt (closed casting), or through a free tapping jet (open casting). These two alternative methods create separate flow situations and effects how and where the magnetic field(s) is/are applied. If the hot primary metal flow is allowed to enter the mold in an uncontrolled manner, it will penetrate deep in the cast strand, which is likely to negatively effect its quality and productivity. Non-metallic particles and/or gas might be drawn in and entrapped in the solidified strand. An uncontrolled hot metal flow in the strand might also cause flaws in the internal structure of the cast strand. Also a deep penetration of the hot primary flow might cause a partial remelt of the solidified skin, such that melt penetrates the skin beneath the mold, causing severe disturbance and long down-time for repair. To avoid or minimize these problems and improve the production conditions, according to the disclosure in European Patent Document EP-A1-0 040 383, one or more static magnetic fields can be applied to act on the incoming primary flow of hot melt in the mold to brake the incoming flow and split it up to create a controlled secondary flow in the molten parts of the strand. The magnetic field is applied by a magnetic brake which includes one or more magnets. Favorably, an electromagnetic device, i.e., a device comprising one or more windings such as a multi-turn coil wound around a magnetic core, are used. Such an electromagnetic brake device is called an EMBR.
According to the disclosure in European Patent document EP-B1-0 401 504, magnetic fields are applied in two levels, arranged one after the other in the casting direction, during casting with a submerged entry nozzle (closed casting). The magnets include poles having a magnetic band area covering essentially the whole width of the cast strand and one first level is arranged above and one second level below the outlet ports of the submerged nozzle. Further, EP-B1-0 401 504 teaches that the magnetic flux should be adopted to the casting conditions, i.e., the strand or mold dimensions and casting speed. The magnetic flux and the magnetic flux distribution are adopted to ensure a sufficient heat transport to the meniscus to avoid freezing while at the same time the flow velocity at the meniscus is limited and controlled so that the removal of gas or inclusions from the melt is not put at risk. A high uncontrolled flow velocity at the meniscus might also cause mold powder to be drawn down into the melt. It is also suggested in this document that an optimum range exists for the flow velocity at the meniscus, see FIG.
9
. It is suggested that the magnetic flux density over the mold is adopted before a casting operation based on the specific conditions assumed to prevail during the coming cast operation. To accomplish this EP-B1-0 401 504 suggests a mechanical magnetic flux-controlling device which is arranged to move the magnetic poles in essentially their axial direction to change the distance between the poles comprised in one cooperation pair and arranged facing each other on opposite sides of the mold, see FIG.
15
and column
8
, lines
34
to
50
. Such a mechanical magnetic flux-controlling device must however be extremely rigid to accomplish a stable magnetic flux density, especially when subject to the large magnetic forces prevailing under operation of the brake while at the same time being capable of small movements to accomplish the adjusting changes in flux density required as the flux density has a high sensitivity to changes in the distance between the poles. Such a mechanical magnetic flux density-controlling device requires a combination of heavy gauge material, rigid construction and small movements in the direction of the magnetically field, which will be hard and costly to accomplish. According to one alternative embodiment the mechanical flux density device is formed by partial substitution of the poles by non-magnetic material such as stainless steel, i.e., by a change in the configuration of the poles and thereby an alteration of the pattern of the magnetic flux in the mold before each cast. Similar ideas as to the configuration of the poles, are discussed in other documents such as EP-Al-577 831 and WO 92/12814. The patent document WO 96/26029 teaches the application of magnetic fields in further levels including one or more levels at or just downstream of the exit end of the mold to further improve the control of the secondary flow in the mold. Flux density-controlling devices of these types based on reconfiguration and/or movements of the poles by mechanical means must be complemented with means for securing the magnet core or partial cores to withstand the magnetic forces and is thus intended for presenting the magnetic flux density and adopted to casting conditions predicted to prevail during a forthcoming casting, and it will include costly and elaborative development work to use such devices for on-line regulation of the magnetic flux density.
According to European Patent Document EP-A1-0 707 909, the flow velocity at the meniscus shall be set within a range of 0.20-0.40 m/sec for a continuous casting method wherein a primary flow is supplied to mold through a nozzle capable of controlling the incoming flow and wherein a static magnetic field having a substantially uniform magnetic flux density distribution over the whole width of the mold is applied to act on the metal in the mold. It further teaches that the flow at the meniscus can be held within this range by setting several parameters such as;
the angle of the port(s) in the submerged nozzle;
the position of the nozzle ports) within the mold;
the position of the magnetic field; and
the magnetic flux density.
The angle and position of the nozzle port(s) as well as the position of the magnetic field(s) are determined and preset before the start of casting and the magnetic flux is controlled according to one out of two different algorithms. The choice of algorithm to be used is dependent on the position of the magnetic field relative the primary flow, i.e., if the primary flow out of the nozzle port(s) traverses a magnetic brake field or not before hitting the side-wall. The algorithms) are based on one measured value only, the flow velocity at the meniscus when no magnetic field is a
Eriksson Jan-Erik
Hällefält Magnus
Kollberg Sten
Petersohn Carl
Tallbäck Göte
ABB AB
Dykema Gossett PLLC
Lin Kuang Y.
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