JPH0131979B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for stirring the unsolidified portion of a cast strand from a continuous casting machine that is formed in the mold and is provided with agitation by at least one electromagnetic stirring device or stirrer. Regarding the method. Conventionally, such stirrers are supplied with two-phase, three-phase or polyphase current.

Various methods are already known for stirring the unsolidified part of the cast strand. It is already known, inter alia, to stir the part at one or more points transversely to the casting process by means of an electromagnetic stirrer. One reason for such agitation is the desire to prevent segregation and tightening, so-called mini-ingots, in the finished billet. One problem with this type of agitation is the so-called white band, a band with negative segregation that forms when the molten metal or melt is moved rapidly by too vigorous agitation. (When steel solidifies, crystals, so-called dendrites, are formed, and aggregates of sulfur and carbon occur between these dendrites. With normal stirring, these aggregates are dispersed in the melt.) (as well as being replaced by other melts, thereby forming a so-called white band.) One way to solve this problem has already been proposed, which is to supply an electric stirring device with a multiphase current. However, the phase current of one phase differs in magnitude by at least 10% from the phase current of the other phase, and or the number of windings of the coils is different with respect to each other, so that the stirring is asymmetrical. There is. This results in a better casting structure than with symmetrical feeding. A stirring device or stirrer is usually
Although constructed symmetrically, it is also possible to make the coils of different phases in the stirrer asymmetrically, so that stirring asymmetry is obtained by symmetrical power supply.

The multiphase current may have a frequency of 0 to 10 Hz, usually associated with the stirring of so-called slabs, ie strands with a large cross section. The frequency can also be chosen between 10 and 30 Hz in the case of so-called blooms, ie medium-sized strands, and between 50 and 60 Hz in the case of so-called billets, ie small-sized strands. In the first mentioned case it is usually a matter of transverse stirring, so that the stirring direction is chosen approximately at right angles to the casting direction; in the second and third cases it is
Usually it is a matter of vertical stirring.

The object of the present invention is to provide a method of asymmetric stirring, in which two-phase currents with mutual phase displacements deviating from 90° or three-phase currents with mutual phase displacements deviating from 120° ( phase angle asymmetry) is 1
Stirring is performed asymmetrically by supplying the liquid to one or more stirrers.
According to such a method, for example, Japanese patent application
Benefits previously obtained with current asymmetric stirrers such as those described in 54-40242 or 1984-144923 are obtained.

The deviation in mutual phase displacement should preferably be 5°.

One of the disadvantages of this type of asymmetric stirring is the relatively poor utilization of the stirrer: that is, the core and windings are underutilized. One method of solving this problem is disclosed in Swedish Patent Application No. 7902317-2 filed by the applicant, in which more than one stirrer is used or one stirrers and one separate coil are used, in the first case two or all stirrs are also supplied with polyphase current, and the stirring from at least one of these is symmetrical. carried out, and the stirring from at least one other is carried out asymmetrically, such that the phase current in one phase differs in magnitude by at least 10% from the phase current in the other phase, and/or the coils have different numbers of turns with respect to each other. In this way, with a symmetrically powered stirrer, an asymmetrical stirring is obtained, in this case configured as explained below. Thus, symmetrical stirring is kinetic stirring,
(motoric stirring), while asymmetrical stirring causes turbulence in part. This helps reduce the viscosity of the melt and eliminates so-called white bands.

In one preferred embodiment of the invention, the texture of the strands cast in a continuous caster is greatly improved. This method provides that the stirring takes place by at least two stirrers, in which case the stirring by the first stirrer is carried out asymmetrically and the stirring by the second stirrer is carried out with the same number of phases as that in the first stirrer. , or with a different type of asymmetry from that of the first steerer, for example by phase currents in one phase differing in magnitude by at least 10% from phase currents in the other phases, or in coils with a different number of turns, respectively. or by supplying a combination of current and phase asymmetry to a symmetrically configured stirrer.

The first and/or second stirrer may be powered by a phase asymmetric current source, in which case the stirrer or stirrers may be configured symmetrically or asymmetrically.

The stirrer or stirrs may also be supplied with a current in one phase that differs in magnitude from the current in the other phase.

Overall, the combination of a current source with a stirrer asymmetry requires a current difference of 10% compared to the case of pure current asymmetry.

The present invention will be described in more detail below with reference to the accompanying drawings.

In the accompanying drawings, FIG. 4 shows current vectors of symmetrical three-phase currents (R, S, T), where achieving asymmetry of the current is achieved by connecting any of the current vectors R, S, T to the dashed line. It can be obtained by replacing it with the current vector (R', S', T') shown by

In the present invention, phase asymmetry is applied,
Figure 5 shows a partial angle from 120°, preferably over 5°,
Phase asymmetric current vector diagrams R, S, T with shifting mutual phase angles are shown. This phase-asymmetric source produces symmetrical vibrations, resulting in a better texture of the casting, for example when using an asymmetrical stirrer 1 (see FIG. 1) which stirs the unconsolidated part of the casting strand according to the invention. is obtained. α indicates mutual phase angle displacement.

FIG. 1 shows a transverse or vertical stirring in the case of billets, blooms or slabs and an electromagnetic stirring device or stirrer 1 supplied with a multiphase current. The stirrer 1 is configured to provide asymmetrical oscillations by being fed with a two-phase current with a mutual phase displacement that deviates from 90° or a three-phase current with a mutual phase displacement that deviates from 120°. has been done. The second stirring device or stirrer 2 is either powered in phase with the first one or is adapted to effect asymmetric stirring in a manner different from the first stirrer. These two stirrers 1, 2 are arranged opposite each other and are configured together to provide horizontal or vertical stirring. In certain special cases, the angles relative to the casting direction may be different for the two stirrers 1, 2. It is also possible for the two stirrs 1, 2 to be powered in both cases with phase asymmetric stirring.

The two stirrers 1 and 2 supplied with three-phase current shown in FIG.
come from casting 4 and have the same stirring direction, resulting in a pattern according to arrow 5. The second stirrer 2 has a mutual phase displacement and a phase current in one phase is at least as small as a phase current in the other phase.
Power is supplied with mutual phase current differences such that the magnitudes differ by 10%. In this case, two different methods are combined to achieve the asymmetry. The second stirrer 2 can be powered with a mutual phase displacement and with coils configured with a mutually different number of turns, in which case a combination of phase asymmetry and amplitude asymmetry is achieved. Ru.

FIG. 3 shows two stirrers 1, 2 placed longitudinally one after the other along the casting strand, for example in the case of billet casting. In this case too, the same mutual differences exist between the two stirrers 1 and 2 as in the devices shown in FIGS. 1 and 2. Therefore, not only vertical stirring as shown in FIG. 3, but also horizontal stirring is used for both stirrers 1 and 2. It is desirable to apply the mutual stirring force between the stirrers 1 and 2 in such a way as to create stirring inside the mold, ie to pass the stirring current through the upper mold itself and to eliminate the white band. It is important to adapt the stirring force in stirrer 1 to the stirring force in stirrer 2 so that either of these two stirring forces does not become too dominant and deteriorate the stirring effect. In embodiments with different coil turns, it is important that the amperage for at least one phase be offset by at least 10% from the amperage for the other phase. In this case, a magnetic traveling wave and a steady field are obtained.

[Brief explanation of drawings]

1 and 2 are schematic longitudinal cross-sectional views showing examples of two different stirring devices for carrying out the method of the invention, and FIG. 3 is another stirring device for carrying out the method of the invention. FIG. 4 is a diagram showing the state in the case of current asymmetric stirring, and FIG. 5 is a diagram showing the state in the case of phase asymmetric stirring. In the drawing, 1 and 2 are "stirring devices, or stirrers";
3 indicates "casting strand"; 4 indicates "mold".

Claims (1)

[Claims] 1. A method for stirring the unsolidified portion of a cast strand from a continuous casting machine that is formed in a mold 4, in which the stirring is performed by at least one electromagnetic stirring device. : Supplying one stirring device or several stirring devices with two-phase currents with mutual phase displacements that deviate from 90° or three-phase currents with mutual changes that deviate from 120° (phase angle asymmetry) A method for stirring the unsolidified part of a cast strand, characterized in that the stirring takes place asymmetrically. 2. In the method described in claim 1:
A method for stirring an unconsolidated part of a cast strand, characterized in that the stirring is carried out asymmetrically with a mutual phase displacement of at least 5°. 3. In the method according to claim 1 or 2, a phase angle asymmetric current is supplied to the stirring device, and a current asymmetric current having different magnitudes is supplied to the stirring device at least in two phases. by supplying a current in which the current in one phase differs in magnitude by at least 10% with respect to the current in the other phase, or the coil in at least one phase is A method for stirring an unsolidified portion of a cast strand, characterized in that stirring is carried out asymmetrically by supplying current to coils whose number of turns is different from that of the coils in phase. 4. In the method according to claim 1 or 2, the stirring is performed by at least two stirring devices, and a current having an asymmetric phase angle is supplied to the first stirring device; The stirring device is provided with an asymmetrical current having a different magnitude in at least two phases, the current in one phase differing in magnitude by at least 10% with respect to the current in the other phase. stirring by supplying a current, or by supplying a current to coils in which the coils in at least one phase have a different number of turns with respect to the coils in the other phase, and the unsolidified portion of the cast strand, characterized in that the stirring is carried out by supplying to the second stirring device electric currents having different magnitudes in at least two phases and whose phases are displaced from each other; How to stir. 5. In the method described in claim 4:
Stirring the unconsolidated part of the cast strand, characterized in that at least one of the first and second stirring devices is powered by a phase-asymmetric current source, and that the stirring device is therefore configured symmetrically or asymmetrically. Method. 6 In the method described in claim 4:
A method for stirring an unconsolidated part of a cast strand, characterized in that at least one of the stirring devices is further supplied with a current of one phase that is different from the other phase. 7 In the method described in claim 1:
Stirring is carried out by two stirring devices 1, 2, thus stirring the unsolidified part of the cast strand, characterized in that the stirring takes place in the same manner and asymmetrically in both stirring devices. Method.