JP2713030B2 - Method of controlling molten steel drift in continuous casting mold - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten steel for controlling a drift of molten steel by detecting a drift of a molten steel discharge flow in a mold in a continuous casting facility, judging an abnormality in slab quality due to the drift, and controlling the drift of the molten steel. The present invention relates to a drift control method.

[0002]

2. Description of the Related Art Generally, molten steel in continuous casting is injected into a mold by an immersion nozzle.

[0003] At this time, the nonmetallic inclusions in the molten steel are brought into the mold by the injection flow from the immersion nozzle, and most of them float on the surface of the molten metal. However, the remaining part is trapped in the slab, causing deterioration of the slab quality.

On the other hand, a flux for surface coating floats on the surface of the molten steel in the mold, and a part of the flux is also caught in the slab by the molten steel discharge flow from the immersion nozzle.
This also causes deterioration of the slab quality.

[0005] The trapping of these non-metallic inclusions and the amount of flux involved greatly vary depending on the state of molten steel flow in the slab. FIG. 9 is an explanatory view showing that, in the continuous slab casting machine, the immersion nozzle 2 is arranged at the center of the mold 1 and the molten steel flow discharged from the discharge holes 3a and 3b is the mold 1
It flows inside as shown by arrows 4 and 5.

More specifically, the velocity of the molten steel flow from the discharge hole 3 is reduced while flowing through the molten steel 6 in the mold, and the reverse flow is caused by the collision of the short sides 1a and 1b of the mold 1 with the side wall surface. Becomes One of the inversion flows is an upward flow 4 toward the molten metal surface.
A and 5A, and the other is a downward flow 4B and 5B going downward.
As a result, the vehicle is greatly decelerated during this time, and as a result, the upward flow 4A,
5A does not involve the flux 7 on the surface of the molten metal in the molten steel, and the downflows 4B and 5B do not reach deep into the slab, and the casting is performed with the quality of the slab improved.

However, such a relationship in FIG.
This is a favorable condition that occurs when the molten steel flows from 3a and 3b are equal, and the immersion nozzle 2 is controlled by the degree of throttle opening of a sliding nozzle (not shown) attached to the immersion nozzle 2 and the casting speed. When the falling molten steel flow fluctuates, or when non-metallic inclusions such as alumina adhere to the inner wall of the immersion nozzle 2, the uniform relationship between the molten steel flows from the left and right discharge holes 3a and 3b is broken. That is, a so-called drift occurs.

[0008] The situation of occurrence of drift at this time has the following two patterns. That is, the first drift pattern is, as shown in FIG. 10, a case where one of the left and right discharge holes 3a and 3b in the immersion nozzle 2 has a strong discharge flow. When such a drift occurs, of the molten steel flow in the mold, the upward flow or the downward flow becomes stronger on the side where the strong flow occurs, and the flux 7 is entrained or the non-metallic inclusions extend deep inside the molten steel 6. It is carried and cannot be levitated and is captured (see FIG. 12A).

On the other hand, the second drift pattern, as shown in FIG. 11, is a case where the discharge flow from the left and right discharge holes 3a and 3b in the immersion nozzle 2 periodically repeats strength. When such a drift occurs, when the strengths of the left and right discharge flows reach their respective peaks, as in the case of the first drift pattern described above, the entrainment of flux and non-metallic inclusions on the side where the strong flow has occurred (See FIG. 12A).

At the same time, since the intensity of the discharge flow periodically changes, the intensity of the upward reversal flow generated after the short-side wall collision also changes periodically. At this time, at the collision portion of the discharge reversal flow from the left and right discharge holes 3a and 3b near the immersion nozzle, the vortex caused by the change in the speed of the reversal flow (see FIG. 12B)
Is generated, and the flux on the surface of the molten metal is caught by the downward flow caused by the vortex.

As described above, due to the occurrence of drift, the frequency of flux entrapment and inclusion trapping increases, leading to deterioration of cast slab quality.

Conventionally, as a means for detecting and controlling such a drift, for example, as shown in Japanese Patent Application Laid-Open No. 2-207955, the difference between the water temperatures of the cooling water on the short sides of the left and right molds is compared. Detects the drift of molten steel flow. Alternatively, as shown in JP-A-3-275256 and JP-A-3-32457, a drift is detected from temperature information of a plurality of thermocouples installed on a long side and a short side of a mold, and this is detected. It is known that the problem is solved by an electromagnetic stirrer (EMS).

Further, Japanese Unexamined Patent Publication Nos.
As shown in JP-A-294053, eddy current sensors are installed on the left and right sides between the immersion nozzle in the mold and the short sides on both sides of the mold. A method has been proposed in which a drift is detected by comparing the frequency components of the above, and the detected drift is eliminated by an electromagnetic brake.

[0014]

However, in the above-described conventional drift detection method, the first drift pattern (hereinafter simply referred to as the first drift) in which the discharge flow in one of the left and right discharge holes in the immersion nozzle is strong. Pattern can be detected when it occurs, but a second drift pattern (hereinafter simply a second drift pattern) in which the discharge flow from the left and right discharge holes in the immersion nozzle periodically repeats strength and weakness occurs. In this case, detection becomes difficult.

That is, Japanese Patent Application Laid-Open Nos. 2-207955 and 3
In the cooling water temperature difference and the mold temperature measurement method disclosed in JP-A-32457 and JP-A-3-275256, the responsiveness to the flow change in the mold is insufficient, so that the left and right discharge flows are relatively small. It is impossible to accurately detect the drift when the strength is repeated in a short cycle (about 10 to 15 seconds).

Further, Japanese Patent Application Laid-Open No. Hei 3-294053 and
In the method disclosed in JP-A-32456, in this second drift pattern, the left and right level fluctuations have the same fluctuation components that are out of phase, so that the fast Fourier transform results of the left and right eddy current sensors are similar. As a result, the drift cannot be detected.

Furthermore, the electromagnetic brake application method for preventing the drift is also intended to eliminate the drift by increasing the applied current on the strong discharge flow side generated in the first drift pattern. It was impossible to respond to the occurrence of the pattern, that is, to repeat the strength on both the left and right sides.

The present invention has been made in view of the above-described circumstances, and has as its object the above-mentioned second drift pattern in which the left and right discharge flows periodically repeat the intensity, which has conventionally been difficult to detect and eliminate. Of the present invention is to provide a method for controlling the drift of molten steel in a continuous casting mold that can accurately detect and control the flow rate of molten steel.

[0019]

SUMMARY OF THE INVENTION According to the present invention, there is provided a method for controlling the drift of molten steel in a mold, comprising disposing an immersion nozzle having a molten steel discharge hole directed to a short side of the mold in the center of the mold. The length of the mold in the continuous casting facility
At both ends in the hand direction, one on each side of the immersion nozzle, a total of two
Each level gauge sensor (eddy current sensor) is installed
Measure the level of each molten steel, then
The difference in the level of the molten metal at both ends
Fourier transform to find the power spectrum for each frequency,
Power spectrum in high and low frequency bands
The pattern of molten steel drift in the mold (drift
The first non-uniform flow pattern where one of the discharge flows is stronger
The second, the discharge flow on the left and right periodically repeats strength
Drift pattern, where there is no drift but the level fluctuates on the left and right
) And the magnitude of the drift.

Next, two discharge holes in the immersion nozzle
Two electromagnetic brakes installed on the long side of the mold facing the
By this, either one of the left and right between the two level sensor
In the case of the first drift pattern where the discharge flow is strong, the discharge flow
More current depending on the magnitude of the drift on the strong side of
And the two electromagnetic brakes allow the
The discharge flow on the left and right between the sensors periodically repeats the intensity
In the case of the drift pattern of
More current on the strong side of the discharge flow according to the changing cycle
So that the current corresponding to the magnitude of the drift is periodically
Apply with strong or weak.

[0021]

According to the present invention, all types of drift can be detected with high accuracy by performing a fast Fourier transform on the difference between the level gauges installed between the immersion nozzle and both short sides.

Hereinafter, the level fluctuation and the drift, and the difference in the drift detection accuracy between the conventional method and the method of the present invention will be described.

When there is no drift and the level of the molten metal is stable, the output values from the level meters installed between the immersion nozzle and both short sides are Fourier-transformed, as shown in FIG. .

When a drift, which is the first drift pattern, occurs, the output value of the level meter increases on the side of the strong discharge flow, as shown in FIG. Then, on the weak discharge flow side, the level of the molten metal is stabilized, and becomes as shown in FIG.

When the drift, which is the second drift pattern, occurs, level fluctuations on both sides occur, and the output values from both level meters are as shown in FIG.

Although there is no drift, when the level controllability deteriorates, level fluctuations on both sides occur, and the output values from both level meters become as shown in FIG. 5 as in the case described above. .

As described above, of the above methods, the conventional method (reference, analysis by frequency component without considering the phase difference)
Cannot be distinguished from each other, so that the drift of the second drift pattern cannot be detected.

On the other hand, in the present invention, in the above cases, the levels on both short sides move almost synchronously, and the result of the Fourier transform of the level difference on both sides, as shown in FIG. The power spectrum also becomes smaller in the frequency band of.

In the present invention, in the above case, since the fluctuation on one side of the strong discharge flow side becomes large due to the drift, the result of the Fourier transform of the level difference on both sides is, as shown in FIG. The component appears as a large powers vector in the high frequency band.

Further, in the present invention, in the above-mentioned case, the phase is opposite during the alternating flow of the strong discharge flow alternately between the two short sides. In addition, a wave component due to the drift appears as a large power spectrum in a high frequency band. At the same time, the drift of the strong left and right discharge flows specific to the drift pattern occurs alternately, and the cycle at this time appears as a peak in a low frequency band (see FIG. 8).

As described above, according to the method of the present invention, it has become possible to accurately detect the drift of the second drift pattern, which has been difficult to detect conventionally. Of course, the drift of the first drift pattern, which can be detected by the conventional method at the same time, can also be detected by the method of the present invention.

Next, the method of controlling the occurrence of these drifts will be described with reference to the above-described method, in the case of the presence or absence of the drift, the degree of the drift, the drift patterns (first and second drift patterns), and the second drift pattern. Calculates the period, and, based on the result, prevents drifting using two electromagnetic brake devices installed on the long side of the mold corresponding to the two discharge holes of the immersion nozzle.

When a drift of the first drift pattern occurs, as in the conventional method, more applied current is applied to the strong side of the discharge flow, and the power spectrum obtained by Fourier-transforming the difference between the two-sided level meters is shown in FIG. It may be added until the state shown is approached.

When a drift of the second drift pattern occurs, the current applied to the electromagnetic brake is adjusted to a cycle in which the left and right discharge flow intensities of the drift of the second drift pattern are switched, and the current having a stronger discharge flow is adjusted. The current may be periodically increased and decreased so that a large amount of applied current is added, and the power spectrum of the level difference may be suppressed.

In the method of the present invention, the phase of the cycle in which the left and right discharge flow intensities of the first and second drift patterns in the first and second drift patterns are interchanged is determined by the difference between the two levels before the Fourier transform. Judge it.

[0036] For example, the H _W -H _E in FIG. 10 if increased compared to the case with no drift, in the first drift pattern strong discharge flow strength E side. Also, point H _W -H _E is changed placed from an increase to a decrease is, by the second drift patterns, gradually decreasing the discharge flow of the E side from the maximum, in that the discharge flow intensity of W side starts to rise is there.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for controlling the drift of molten steel in a continuous casting mold according to the present invention will be described with reference to the drawings.

An embodiment is shown in FIG. A level gauge sensor (for example, eddy current sensor) between both short sides of the immersion nozzle 11
a, 11b are installed. Each of this sensor level position H _W, detects the H _E. The difference H _W of the H _W and H _E
The -H _E in the computer 13 through the input device 12 to fast Fourier transform. Based on this result, an instruction is issued to the electromagnetic brake control unit 14 to control the current applied to the electromagnetic brakes 15a and 15b.

[0039] In the method of the present invention performs a sampling of the level value for each 0.1 sec, and a fast Fourier transform H _W -H _E value of the sampling data of 1200 points. Based on this result, a peak value is obtained in a frequency band in which the power spectrum increases when a drift occurs, for example, 0.3 to 0.5 Hz. In addition, the frequency band 0.0 in which the power spectrum increases due to a large swell at the time of occurrence of the drift, which is the second drift pattern.
The peak value and its frequency are obtained at 5-0.1 Hz.

According to the magnitude and frequency of these peak values,
The presence / absence of a drift and the cycle of the drift pattern and the second drift pattern are determined. Table 1 shows a specific example.

[0041]

[Table 1]

Of course, the value used for this determination is a value determined by casting conditions such as a mold size and a casting speed, and is obtained experimentally.

In the following, a description will be given of the case where a drift occurs. First, an applied current ΔI is applied to the strong side of the discharge flow according to the drift strength proportional to the peak value of 0.3 to 0.5 Hz. Here, the applied current of the electromagnetic brake is the same as that used in normal operation, but in this case, in addition to the normal, the applied current is increased by ΔI only on the strong side of the discharge flow. For example, as shown in Table 2 below.

[0044]

[Table 2]

At this time, in the case of the first drift pattern, the applied current is constantly applied to the strong side of the discharge flow in the same manner as in the conventional method (reference, only plus current, strength not to undulate). However, in the case of the second drift pattern, the frequency having the peak value of 0.3 to 0.5 Hz, ΔI × 2 shown in Table 2 above, and the frequency of the peak value of 0.05 to 0.1 Hz are periodically changed according to the peak value of 0.3 to 0.5 Hz. The applied current increase amount of the sine wave I = ΔI sin (2παt +
θ) may be added as shown in FIG. Where α
Is the undulation frequency of the second drift pattern within 0.05-0.1 Hz, t is time (sec) and θ is the initial phase.

At this time, by adding | I | to the strong side of the discharge flow, it is synchronized with the large undulation of the second drift pattern and controlled.

[0047]

According to the method for controlling molten steel drift in a continuous casting mold of the present invention, both steady drift (first drift pattern) and periodic drift (second drift pattern) can be detected. As well as control. As a result, the inclusion of inclusions in the slab and the entrainment of the powder are reduced, and the product defect of the slab in continuous casting can be greatly reduced (see FIG. 3).

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a continuous casting facility for implementing a method for controlling molten steel drift in a continuous casting mold according to the present invention.

FIG. 2 is a graph showing an electromagnetic brake application current control state when a second drift pattern occurs in the molten steel drift control method of the present invention.

FIG. 3 is a graph showing the effect of reducing defects in cast slab products by the molten steel drift control method of the present invention.

FIG. 4 is a graph showing an example of a fast Fourier transform of a level meter measurement value when there is no drift in the conventional method.

FIG. 5 is a graph showing an example of a fast Fourier transform of a level meter measurement value when a drift occurs in a conventional method.

FIG. 6 is a graph showing an example of a fast Fourier transform of a level meter measurement value when there is no drift in the method of the present invention.

FIG. 7 is a graph showing an example of a fast Fourier transform of a level meter measurement value when a first drift pattern occurs in the method of the present invention.

FIG. 8 is a graph showing an example of a fast Fourier transform of a level meter measurement value when a second drift pattern occurs in the method of the present invention.

FIG. 9 is a schematic view showing a molten steel discharge flow in a general continuous casting mold.

FIG. 10 is a schematic diagram showing a first drift pattern of a molten steel flow in a mold.

FIG. 11 is a schematic view showing a second drift pattern of a molten steel flow in a mold.

FIG. 12 is a schematic diagram showing a state of capturing inclusions and entraining powder when a drift occurs in a mold.

[Explanation of symbols]

1: short side of mold, 2: immersion nozzle, 3: nozzle discharge hole,
4, 5 ... molten steel discharge flow, 4A, 5A ... upward flow, 4B, 5B
... downflow, 6 ... molten steel, 7 ... flux, 8 ... level gauge sensors, H _W, H _E ... level position, A ... entrainment inclusions flux by the discharge flow, B ... inclusions flux entrainment by vortex.

Claims (1)

(57) [Claims] 1. Two level gauge sensors (eddy current sensors) are installed at both ends of a mold in a continuous casting facility in the longitudinal direction of a mold, one on each side of an immersion nozzle, and each molten steel level is measured. Then, the difference in the level of the molten metal at both ends in the longitudinal direction in the mold is obtained, and this difference is subjected to a fast Fourier transform to perform the analysis for each frequency.
Power spectrum, and calculate the high and low frequency bands.
Based on the magnitude of the power spectrum in the mold, the pattern of the molten steel drift and the magnitude of the drift in the mold are detected, and then two electromagnetic brakes installed on the long sides of the mold are opposed to the two discharge holes in the immersion nozzle. Thereby, in the case of the first drift pattern in which the discharge flow on either the left or right between the two level meter sensors is stronger,
Applying more current in response to the magnitude of drift, the by two electromagnetic brakes, the second polarization to the discharge flow of the left and right between both levels gauge sensor repeats periodically strength
In the case of the flow pattern , a current corresponding to the magnitude of the drift is periodically applied so as to apply more current to the strong side of the discharge flow in accordance with the cycle in which the left and right discharge flow intensities of the drift are switched. A method for controlling drift of molten steel in a continuous casting mold, characterized in that: