Methods and facilities for suppressing vortices arising in tundishes or ladles during their respective discharge

Abstract

A method is provided for suppressing a vortex arising in a tundish or ladle at the lowering of the free surface of the melt below a critical level using a rotating magnetic field excited in the melt above an outflow pipe by an RMF inductor of a special design.

Description

This application claims the benefit of U.S. provisional patent application No. 60/609,697, filed Sep. 13, 2004, which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to vortex suppression in tundishes.

The lowering of the free surface of a melt occurring during the discharge of the melt from a tundish or ladle of a continuous casting plant (“CCP”) may create vortices in the replacement of the tundish or ladle immediately prior to the melt. This phenomenon typically occurs when the level of the melt is lower than normal. Such a low level may occur when the tundish is in the process of being emptied, for example. Slag floating on the melt surface is drawn into the vortex and gets into the mold of the continuous casting plant. Thus, a certain part of a continuous ingot contains slag inclusions and must be cut off and remelted later on. Cutting off the slag and remelting the slag increases production costs and decreases throughput.

Presently, an overwhelming majority of metals and alloys are cast on CCPs. Therefore, the problem of vortex suppression is highly urgent.

An attempt to solve this problem was made using ceramic boxes of rather complicated configuration mounted above the discharge hole (see, e.g., Sankaranarayanan et al. U.S. Pat. No. 5,382,003, entitled “Flow Control Device For The Suppression of Vortices”). The drawbacks of such a method are obvious, and include the necessity of the washing out and the further destruction of ceramics by the melt after a long-term operation of the tundish. As a result, melt fed into the mold can become essentially irregular.

Accordingly, it would be desirable to provide improved methods of vortex suppression.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide improved methods of vortex suppression.

A proposed method of vortex suppression of the invention uses a rotating magnetic field (“RMF”). This method does not involve the arrangement of any ceramic components inside the tundish and therefore is free from the above-mentioned drawbacks. Moreover, the parameters of RMF are easy to change and, hence, the process of vortex suppression using RMF can be easily controlled within broad limits.

The proposed method of vortex suppression is confirmed by the results of experiments conducted on vortex suppression by RMF performed on a low-temperature tundish model, wherein, as a melt, eutectic indium-gallium-tin alloy (InGaSn) has been used with a melting temperature of approximately 10° Celsius.

In accordance with one embodiment of the invention, there is provided a method of suppressing a vortex arising in a tundish or ladle at the lowering of the free surface of a melt below a critical level using a rotating magnetic field continuously excited by m-phase current (i.e., any suitable number of current phases or m-phase voltage) in the melt above an outflow pipe, wherein the direction of RMF rotation is opposite to the direction of melt rotation in the vortex.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the invention will be more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

FIG. 1 shows a vertical cross-section of a portion of a tundish adjacent to a discharge hole with an RMF inductor, in accordance with the invention;

FIG. 2 shows a horizontal cross-section of a three-phase RMF inductor with six explicit poles, taken from line A-A of FIG. 1;

FIG. 3 shows the configuration of pole pieces of a two-phase inductor with four explicit poles, in accordance with the invention;

FIG. 4 shows the configuration of pole pieces of a three-phase inductor with three explicit poles, in accordance with the invention;

FIG. 5 schematically illustrates induced current oscillations in the inductor windings, in accordance with the present invention;

FIG. 6 shows a schematic diagram of a tundish used during experiments on vortex suppression; and

FIG. 7 shows the results of experiments conducted on the tundish of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a tundish cover 1 (FIG. 1), inspection window 2 is made, and above this window 2, optical probe 3 may be mounted, which records the displacement of melt surface 4. When a rotating flow arises in the vicinity of a discharge hole 5, m-phase voltage may be applied to inductor 6 (FIG. 2). As a result, RMF is excited above discharge hole 5, which induces a rotating system of currents in the melt. Interaction of these currents with the RMF generates electromagnetic body forces (“EMBF”) that can either hinder or accelerate vortex formation above discharge hole 5, depending on the way of switching on inductor 6.

If the EMBF field is directed against the rotating flow arising at a melt level lowering below the critical value for a given tundish (or ladle), vortex formation is efficiently suppressed.

Typically sinusoidal waveforms of current are generated in an inductor (e.g., inductor 6) of the type described herein such that RMF is excited above discharge hole 5. In accordance with an other embodiment of the invention, instead of typical sinusoidal waveforms, superwaves may be generated and applied to inductor 6 when its windings are connected to a power supply (not shown).

FIG. 5 schematically illustrates the formation of doubly-modulated sinusoidal current oscillations (two-level SuperWaves). FIG. 5 illustrates low-frequency carrier wave 110 modulated, for example, by waves 120 and 130. Minor waves 120 and 130 have progressively higher frequencies (compared to major wave 110). Other modulation levels of even higher frequency may modulate major wave 110, but are not shown for clarity. This superwave is depicted in the time-domain in FIG. 5.

According to experimental results obtained on a low-temperature model, in some cases it is more efficient to apply frequency and/or amplitude modulated RMF or to change RMF rotation direction, intensity, or frequency with time.

FIG. 6 shows a schematic diagram of the tundish used in this experiment. It shows characteristics of level used for the evaluation of the efficiency of RMF effect on the process of funnel formation, wherein H_m is the initial height of liquid metal, H₀ is the melt height corresponding to funnel formation without RMF, and the relative change in critical level of funnel formation under the action of RMF, H_mf, is ΔH=(H₀−H_mf)/H₀.

Experimental results are shown in FIG. 7. The notation +t in FIG. 7 refers to the delay of the generator switching on after opening the stopper for melt discharge. (The initial level of the melt in all experiments was the same, with H_m being about 70 mm).

The current in the coils of a 3-phase inductor (see, e.g., FIGS. 1 and 2) in the case of harmonic RMF (see, e.g., the first set of data on the left of FIG. 7) varied according to the following law:
I=I_o sin (ω_ot+α),   (1)
where α is a temporal phase shift, and ω_o is the circular frequency of the current (in the experiments, ω_o=20 Hz). The delay +t varied from 0 to 2 seconds, I_o varied from 8 A to 13 A. Apparently, the greatest effect is observed at the maximal current in the absence of delay. It is noteworthy that in this case, maximal disturbance of the metal surface in the tundish is observed.

In the case of modulated RMF, the current in the inductor coils varied according to the following law:
I=I_o [1+e·sin (kω_ot+α)] sin (ω_ot+α),   (2)
where k is the multiplicity ratio between the carrier frequency and the modulation frequency, and e is the modulation depth. Therefore, the notation K3_—04_—11A_—+2s, for example, in FIG. 7 means that the current specified by (2) had the following parameters: k=3, e=0.4, I_o=11A, and +t=+2 seconds.

A facility realizing the proposed method constitutes explicit-pole inductor 6 (FIGS. 1 and 2) with the number of poles being a multiple to the number of phases m (in the case of two-phase current, the inductor may be made with 4 (items 7 in FIG. 3), 8, etc. poles; in the case of three-phase current, the inductor may be made with 3 (items 8 in FIG. 4), 6 (items 9 in FIG. 2), etc. poles). These poles may be located around the outflow pipe 10 (FIG. 1).

The magnetic circuit of inductor 6 preferably consists of ferromagnetic back 11 with explicit poles 9, 12 (FIG. 2) and coils 13 arranged on them (FIG. 1). If commercial frequency currents of about 50-60 Hz are applied, the magnetic circuit may preferably be made of sheet electrotechnical steel or in the form of thin-sheet jacket 14 (FIG. 2), preferably filled with iron powder 15 (FIG. 1) whose particles are electrically insulated. If low-frequency currents of about 2-10 Hz are applied, the magnetic circuit may preferably be cast from steel or cast iron.

Pole pieces 16 (FIG. 1), 7 (FIG. 3), 8 (FIG. 4) of various configurations may preferably be made of steel, iron, or laminated electrotechnical steel and arranged inside jackets 17 built in the lining 19 of the tundish bottom (FIG. 1). There is preferably a gap between the jacket 17 and the pole pieces, through which air may be blown in order to cool the pole pieces down to the temperature below the Curie point of the respective ferromagnetic.

The inductor may preferably be fixed to tundish jacket 17 using flange 20, which may preferably be made of nonmagnetic steel rigidly connected with the poles of the magnetic circuit.

Various types of circuitries and devices made of various materials can be used to implement the pump as described above according to the invention.

It will be understood, therefore, that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention, and the present invention is limited only by the claims that follow.

Claims

1. A method of suppressing a vortex arising in a tundish or ladle at the lowering of the free surface of a melt below a critical level using a rotating magnetic field (RMF) continuously excited by 5 m-phase current in the melt above an outflow pipe, wherein the direction of RMF rotation is opposite to the direction of melt rotation in the vortex.

2. The method of claim 1, wherein the direction of the RMF rotation is varied with a certain frequency and on-off time ratio.

3. The method of claim 1, wherein the RMF is amplitude and/or frequency modulated, wherein a modulation frequency exceeds a carrier frequency.

4. The method of claim 1, wherein the RMF is excited in a discontinuous manner with a certain on-off time ratio.

5. The method of claim 1, wherein the RMF is excited with a certain delay after the onset of free melt surface lowering, depending on the stationary melt level and conditions of vortex formation.

6. The method of claim 1, wherein the RMF intensity is varied during the melt discharge.

7. The method of claim 1, wherein the frequency of current exciting the RMF is varied during the melt discharge.

8. A facility realizing the method of any one of claims 1-7 constituting an explicit-pole inductor wherein the number of poles is a multiple to the number of current phases, comprising a magnetic circuit, windings, and pole pieces, which are mounted under the bottom of the tundish or ladle around the outflow pipe, wherein the magnetic circuit back is made of ferromagnetic material in the form of a flat disk with a central hole in which the outflow pipe is arranged, wherein the poles have a trapezoidal cross-section with windings which are perpendicular to the plane of the back, and wherein the pole pieces are air-cooled, made in the form of fragments of a hollow cone, and arranged in the lining of the bottom of the tundish or ladle around the discharge hole.

9. The facility according to claim 8, wherein the pole pieces of the inductor are made in the form of fragments of a hollow cylinder with a conical inner surface.

10. The facility of claim 8, wherein the magnetic circuit of the inductor is made of ferroceramics.

11. The facility of claim 8, wherein the magnetic circuit of the inductor is made in the form of a thin-wall jacket filled with iron powder consisting of electrically insulated particles.