JPH03294053A - Method for controlling drift flow of molten steel in continuous casting mold - Google Patents

Abstract

PURPOSE:To control drift flow of molten steel in a mold with high accuracy by detecting molten steel surface level at both sides of a submerged nozzle, comparing power spectra thereof and controlling electromagnetic braking device. CONSTITUTION:Level meters 11a, 11b are set to the prescribed height from the molten steel surface at between the submerged nozzle 2 and short sides 1a, 1b at both mold sides. High speed Fourier transform is executed to the measured signals with an arithmetic device 13 to obtain the power spectra. By comparing both frequency components, existence of the development of drift flow of the molten steel in the mold 1, the developing direction of drift flow, and the degree of drift flow, are decided. According to this result, impressed current to the electromagnetic braking devices 15a, 15b set to the long sides 1c, 1d in the mold, is controlled. By the electromagnetic force generated with the impressed current, the flowing velocity of molten steel flow discharged from discharging holes 3a, 3b in the submerged nozzle 2 is adjusted to control the drift flow of molten steel.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for controlling the drift of molten steel in a continuous casting mold.

<Prior art> In general, non-metallic inclusions in molten steel during continuous casting are carried into the inside of the slab by the injection flow of molten steel, and most of them float above the molten metal surface, but the remaining part is It is trapped in the slab as it is, causing deterioration of the quality of the slab. The amount of non-metallic inclusions that are trapped varies greatly depending on the flow of molten steel in the form of a slab during pouring. , is on the rise.

Therefore, in continuous casting, the immersion nozzle is shaped with a discharge hole on the short side so that the molten steel flow discharged from the immersion nozzle does not reach deep into the slab, and the molten steel flow is suspended on the surface of the self-molten steel of the mold. The discharge hole is oriented slightly downward to avoid getting the surface coating flux involved.

FIG. 4 is an explanatory diagram thereof, and in the continuous slab casting machine, the immersion nozzle 2 is arranged at the center of the mold 1, and its discharge holes 3a and 3b are located on both short sides 1a of the mold 1.

1 bll and discharged from the discharge holes 3 a and 3 b flows within the mold 1 as shown by arrow 4.5. That is, the molten steel flow from the discharge hole 3 reduces its speed while flowing through the molten steel 6 stored in the casting 111, and
A reverse flow occurs due to collision with each short side of the fall wall.

One side of this reverse flow is an upward flow 4A, 5A toward the hot water surface,
The other flow becomes downward flows 4B and 5B, and as a result of being greatly decelerated during this time, the upward flows 4A and 5A do not involve the Franks on the surface of the hot water into the hot water, and the downward flows 4B
, 5B are cast to improve the quality of the slab by preventing it from reaching deep into the slab.

However, the relationship shown in Fig. 4 is a good situation that occurs when the molten steel flow from both ridge holes 3a and 3b is equal.
If there is fluctuation in the flow of molten steel descending through the immersion nozzle 2 due to the throttle opening or casting speed of the sliding nozzle (not shown) attached to the immersion nozzle 2, or if there is non-metallic material such as alumina on the inner wall of the immersion nozzle 2. If something sticks, f4J from the left and right discharge holes 3a, 3b
, ml flow collapses, and the flow of molten steel from either side becomes stronger, resulting in so-called drifting.

When such a drift occurs, the upward flow or downward flow becomes stronger on the side of the molten steel flow in the mold where strong flow occurs, which may be caused by flux entrainment or downward flow reaching deep inside the slab. Internal defects will occur, leading to deterioration in slab quality.

Conventionally, as a means for controlling the drift of molten steel described above, a plurality of thermocouples are placed on the wall surfaces of the short sides of the left and right molds at predetermined intervals in the vertical direction, as disclosed in, for example, Japanese Patent Laid-Open No. 62-252650. The level difference between the left and right sides is detected from the temperature information, and the level difference is measured using an electromagnetic stirring device (EM).
S) and how to eliminate it by activating
As disclosed in Japanese Patent No. 649, various methods have been proposed for eliminating the difference in level between the left and right sides by independently controlling the amount of gas blown into the submerged nozzle.

<Problem to be solved by the invention> However, in the former method using an electromagnetic stirring device disclosed in JP-A No. 62-252650, the relationship between the degree of drift and the stirring force to eliminate it is not specifically specified. If a constant stirring force is used, it does not take into account temporal changes in molten steel flow (guynamics), and there is a problem with control accuracy.

In addition, regarding the latter case of JP-A-62-252649, the authors note that it is not possible to effectively control molten steel drift with minute flow rate differences such as those seen in the experimental examples in the detailed explanation section of the invention in the same publication. The inventors have confirmed f1 through experiments.

On the other hand, the drift detection method used in both is also disclosed in Japanese Patent Application Laid-Open No. 62-93054, but as shown in FIG.
A plurality of thermocouples 9a and 9b are buried in the wall surface at a predetermined interval in the vertical direction, and the level difference Δ between the left and right scenes is determined from the temperature information.
By understanding h, the difference in the flow rate of molten steel flowing out from the left and right discharge holes 3a and 3b of the immersion nozzle 2 is used as an index. The generation mechanism is, as mentioned above,
The molten steel flows 4, 5 from the discharge holes 3a, 3b are directed to the respective short sides 1a, l.
This is a partial upheaval of the molten steel scene due to the upward flows 4A and 5A that occur when they collide with b, and the level on one of the short sides does not change uniformly over the entire surface, so it can be easily detected as a level difference Δh. That is difficult.

That is, for example, when the flow of molten steel from the discharge hole 3b on the right side of the immersion nozzle 2 is strong, as shown in FIG. A level difference Δh occurs compared to the bath surface. However, this raised portion 8 does not occur along the wall surface of the short side 1a of the mold 1, but resides at the position where the upward flow 5A reaches the molten steel bath surface, and is usually located away from the wall surface as shown in the figure. This will occur in some parts.

Therefore, if you try to detect a difference in level using the thermocouple 9b buried in the wall surface, it will only be possible when the raised part 8 due to the upward flow 5A reaches the wall surface, so it cannot be detected at that stage. There is a problem that detection is not possible, and even if detection is possible, the accuracy is poor.

In addition, JP-A-62-197255 discloses a method of detecting the drift of molten steel by arranging two eddy current level needles between the immersion nozzle and each short side of the mold on both sides to determine the level deviation. However, when using this eddy current level meter, it is a prerequisite to always install the level meter at the location where the amount of upheaval is the greatest, but in practice, it is technically difficult to do so due to the following reasons. Have difficulty.

In other words, in continuous casting, it is necessary to change the casting width frequently, but if the level needle is installed in a fixed position, it will not necessarily match the position of the raised part, so it will be possible to accurately detect the height of the raised part. You will not be able to do so. Therefore, if the level needle is made to be movable to accommodate changes in the width of the mold, the level meter will have to be reinstalled each time, resulting in problems such as loss of installation accuracy and changes in the level needle characteristics. The accuracy of detecting minute level differences may be reduced.

Furthermore, when processing measurement signals using an eddy current level meter as a two-way type, it is assumed that the absolute measured values match, and it is essential to improve the absolute precision of each level needle in terms of measurement. . However, the measurement accuracy of the level meters actually used generally differs slightly from one another, and the locations where they are installed are in adverse environments with extremely high temperatures, which has a large effect on the accuracy of the measurement. .

Therefore, it is difficult to maintain four level meters at the same level or with high accuracy, so when such level meters are used, the accuracy of detecting drifting flow will be dominated by their measurement accuracy. , it is difficult to achieve accurate detection.

It is possible to increase the number of eddy current type level meters installed to cope with fluctuations in the position of the maximum protrusion due to changes in the width of the mold. There is a problem of mutual interference, so it is not an appropriate countermeasure.

The present invention was made in order to solve the problems of the prior art as described above, and it detects drifting of molten steel with high accuracy in a continuous casting mold, and detects drifting in the direction and degree of drifting. The purpose is to provide a method for controlling drift.

Means for Solving the Problems> The present invention provides continuous casting when a submerged nozzle having a discharge hole for molten steel facing the short side of the mold is disposed in the center of the mold. By disposing at least one level meter between each of the short sides, fast Fourier transforming each level measurement value detected by the level needle to obtain a power spectrum, and comparing the frequency components of the two, It is possible to determine the presence or absence of drifting of molten steel in the mold, the direction and degree of drifting, and control the amount of current applied to two electromagnetic brake devices installed on the long sides of the mold according to the results. This is a method for controlling the drift of molten steel in a continuous casting mold.

<Function> The present inventor has conducted intensive research on molten steel drift control, and first conducted a temperature measurement experiment on a mold copper plate and a multi-point level measurement experiment in the mold to detect drift in molten steel. ! ! As a result of searching for an index that could be used to detect drift, we found that the method of detecting drift by comparing the frequency components of the measured values of level needles provided on both short sides, which the applicant had already filed in Japanese Patent Application No. 1-165423, was found to be better. , compared with the two previous examples, namely the method for detecting drifting of molten steel using a thermocouple as disclosed in JP-A No. 62-93054 and the method for detecting drifting of molten steel using four eddy current level meters as disclosed in JP-A-62-197255. It was found that the current phenomenon was well expressed.

In addition, regarding the drift control of the molten steel, two of the submerged nozzles
By using two electromagnetic brake devices on the left and right corresponding to each discharge hole, more current can be applied in the direction where the drift is occurring, effectively suppressing the discharge flow velocity of the melt flow. The present invention was completed by combining these findings.

That is, according to the present invention, the state of change in the level due to the uplift of the molten steel scene caused by the upward flow generated when the molten steel flow discharged from the immersion nozzle collides with both the left and right short side walls is detected, and the changes are detected by fast Fourier transform. By determining the power spectrum and comparing their frequency components, it is possible to determine the presence or absence of molten steel aluminum flow in the mold, the direction and degree of drifting, and the two installed in the mold according to the degree of drifting. Since the amount of current applied to each electromagnetic brake device is individually controlled, the drift of molten steel in the mold can be controlled with high precision.

Here, a description will be given of how the waveform of the power spectrum changes depending on the presence or absence of drift.

Figures 2 (a) and (b) show an example of the waveform distribution of the power spectrum when measuring the left and right levels of the molten steel scene without drifting, and there is no fluctuation in the molten steel scene level. At this time, the waveform distributions of the left and right power spectra are almost in phase and there is no big difference.

Note that level fluctuations due to scene elevation due to the upward flow of molten steel can be detected as fluctuations in the power spectrum in a high frequency range, since the fluctuations are more severe on the drifting side than on the anti-drifting side.

In addition, Figures 3 (a) and (b) show an example of the waveform distribution of the power spectrum when measuring the left and right levels of the molten steel scene when drifting occurs. There are significant differences in the distribution. In other words, the level on the left side does not change much compared to the normal state as shown in Figure 3 (Fig. As shown in (b), it can be seen that the power spectrum in the high frequency range increases dramatically.Thus, by comparing the waveform distribution of the left and right power spectra in the high frequency range, we found that there is a strong upward flow on the right side of the power spectrum where the power spectrum is high. Therefore, it can be determined that the molten steel is drifting.

<Examples> Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration diagram showing an embodiment of a molten steel drift control device according to the method of the present invention. In addition, in the figure, the same parts as in the conventional example are given the same reference numerals.

As shown in the figure, a submerged nozzle 2 is arranged in the center of a mold 1 with its discharge holes 3a and 3b facing the short sides 1a and Ib of the mold.
Ilb is immersed in the nozzle 2 and each short side 1a of the mold on both sides,
lb at a predetermined height from the molten steel scene.

The measurement signals from each of the level meters 11a and 11b are input to an input device 12 such as an A/D converter, and are further subjected to fast Fourier transform by a calculation device 13 such as a microcomputer to calculate the power spectrum. The amount of current applied to the two electromagnetic brake devices 15a and 15b installed on the long side 1c and fd side of the mold 1 is controlled via the 1tia brake control device 14 according to the difference. In these electromagnetic brake devices 15a and 15b, the immersion nozzle 2 is
discharge hole 3a.

The flow velocity of the molten steel flow discharged from 3b is adjusted to control the drift of molten steel.The calculation results of the calculation device 13 are displayed on a display device 16 such as a CRT.

Next, the operation of the molten steel drift control device configured as described above will be explained. The measurement signals from each level meter 118° 11b are sent to the calculation device 13 to detect whether or not molten steel drift has occurred based on the high frequency difference in the power spectrum. When it is determined that a drift has occurred, the direction and degree of the drift is determined, and the electromagnetic brake is applied depending on the degree of the difference.
The difference ΔI (-1a-1b) between the applied currents la and Ib output from the device 14 to the electromagnetic brake devices 15a and 15b is adjusted.

In other words, when there is no drift, these applied currents 1a and Ib are determined by the casting operating conditions such as the type of mesh, but when a drift occurs, the applied currents 1a and Ib are determined by the molten steel discharged from the discharge hole on the side where the drift occurs. The flow rate is faster than that of the other outlet.

Therefore, in order to equalize the flow speed of the molten steel flow discharged from the discharge holes 3a and 3b of the submerged nozzle 2 by applying more current to the discharge hole with a faster flow velocity to suppress the discharge flow velocity of the molten steel, a biased flow is applied. The drift is controlled by adding the difference Δ■ to the applied current output to the electromagnetic brake device on the side of the discharge hole where the discharge hole is occurring. (In the example shown in FIG. 1, the molten steel flow 5 is the object to be controlled.) In the above embodiment, one level needle is provided on the left and one on the left, but the present invention is not limited to this. It goes without saying that if a plurality of each of these types of sensors are provided, the accuracy of detecting drifting currents will be further improved.

The method of the present invention was applied to continuous casting to control drifting flow. That is, 512 points are sampled at a period of 0.4 seconds, and the level measurement values obtained from both level meters 11a and llb are fast Fourier transformed to derive their respective power spectra, and then the frequencies with large power spectra are derived. Several points were extracted and their average values were derived, and the one with the larger average frequency value was determined to have a drift. (This is based on the experimental result that the fluctuation period of the level signal on the drift side is shorter than that of the other level signal, that is, it contains more high frequency components.) Next, calculate The difference Δf between the average frequency values is calculated, a signal corresponding to the value is output as 1 to the magnetic brake control device 14, and the magnetic brake control device 214 applies an application [current difference Δ!] according to the signal of Δf. The molten steel drift was controlled by adding 1 in the direction in which the drift occurred to the applied current of the magnetic brake device (15a or 15b) and outputting the result.

As for the relationship between the frequency average value difference Δf and the applied current difference Δ■ optimal for suppressing drifting, values previously obtained through online experiments were used as shown in Table 1.

As a result, drifting of the molten steel could be suppressed, and the quality of the slab could be significantly improved.

The characteristic diagram showing an example of the waveform distribution of the power spectrum on the right side of Table 1,
FIGS. 4 and 5 are explanatory diagrams showing conventional examples.

<Effects of the Invention> As explained above, according to the present invention, the levels of the left and right molten steel surfaces are measured, their power spectra are compared to detect drift, and the difference in frequency is used to detect the ii electromagnetic Since the braking device is controlled, the drift of molten steel in the mold can be controlled, which greatly contributes to improving the quality of slabs.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of a drift control device according to the method of the present invention, FIGS. 2(a) and 2(b) are characteristic diagrams showing an example of the waveform distribution of left and right power spectra when there is no drift,
Figures 3 (a) and (b) show left la when drifting occurs,
lb...short side, 2...immersion hole 3b...discharge hole, 6...molten steel 11a, llb...level meter, 12...13...
Arithmetic unit, 14...it magnetic brakes 15a, 15b...
・Magnetic brake device 1...mold. Zulu, 3a. 8... Protuberance. input device. Key control column device. I6...Display device.

Claims (1)

[Claims] When performing continuous casting by disposing a submerged nozzle having a discharge hole for molten steel facing the short side of the mold in the center of the mold, at least one level meter is installed between the submerged nozzle and each short side of the mold on both sides thereof. By fast Fourier transforming each level measurement value detected by the level meter to obtain a power spectrum and comparing the frequency components of the two, it is possible to determine whether or not molten steel drift occurs in the mold. A method for controlling the drift of molten steel in a continuous casting mold, the method comprising: determining the direction and extent of the flow, and controlling the amount of current applied to two electromagnetic brake devices installed on the long sides of the mold according to the results. .