JPH0985407A - Method for controlling molten steel level in mold in continuous casting machine - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To accurately control the molten steel level in a mold in a continuous casting machine under any operating condition. SOLUTION: When controlling the molten steel level in the mold 3 while detecting the level of molten steel injected from the tundish 2 into the mold 3 by adjusting a sliding nozzle 4, the control gain is different. P
The ID control system, the observer control system, and the robust control system are switched according to the respective threshold values of the pouring speed, the residual molten steel amount in the ladle, and the tundish weight. As a result, the level of molten steel in the mold 3 can be accurately controlled by the control method most suitable for the current operating conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The molten steel level injected into a mold by adjusting an actuator from a tundish can be detected using a molten steel level meter, and a plurality of level control methods can be selected to switch the molten steel level in the mold. The present invention relates to improvement of a molten steel level control method in a mold in a controlled continuous casting machine.

[0002]

2. Description of the Related Art Generally, molten steel manufactured by a converter or an electric furnace is poured into a mold of a continuous casting machine, and a cast piece is drawn out while solidifying in the mold. In such continuous casting, if the molten steel surface fluctuates rapidly in the mold, it may cause mold powder entrainment or turbulence of the molten steel flow, causing surface defects or cracks in the slab or causing breakout. . Therefore, it is important to control the molten steel injection amount by an actuator such as a sliding nozzle or a stopper provided in the tundish to suppress the variation amount and the variation speed of the molten steel level in the mold.

When a sliding nozzle is used, a molten steel level control in the mold is performed by providing a molten steel level meter in the mold, a sliding nozzle at the bottom of the tundish, and a level control device for detecting the level signal detected by the molten steel level meter. In particular, proportional operation (P) and integral operation (I) are the main components, which are output to the sliding nozzle actuator by appropriate differential operation (D), and the nozzle opening is performed according to the stroke amount. The amount of molten steel injected from the tundish into the mold is controlled so that the molten steel level is constant by adjusting the degree in an analog manner.

As mentioned above, the PID control is used to control the molten steel level in the mold of the continuous casting machine. However, there is a change in the operating conditions such as the pouring speed, which is determined by the amount of molten steel poured from the tundish into the mold, the amount of residual molten steel in the ladle when changing the ladle for continuous casting, and the tundish weight, that is, the molten steel weight and molten steel level. In addition, there is nozzle clogging due to alumina adhesion to the immersion nozzle attached to the upper nozzle of the tundish or the lower part of the sliding nozzle, or peeling of the adhered alumina, the behavior of the gas blown into the nozzle part, the wear of refractory, the equipment The gain of the molten steel level control system in the mold may change due to various unpredictable disturbances such as deterioration. Therefore, stable continuous casting cannot be achieved by the conventional molten steel level control in the mold by PID control.

More recently, demands for improving product quality and yield have become stricter year by year. On the other hand, however, a further increase in casting speed is required, and the problems associated therewith are increasing. . As one of them, for example,
Even when the casting from the tundish to the mold is stable, in the high-speed casting, the molten steel flow discharged from the two-hole type immersion nozzle in a substantially horizontal direction is large, so that the molten steel surface in the mold is wavy at a low frequency. The molten steel level meter detects this as a disturbance and makes the molten steel level control by the PID control unstable, so the solution is a problem.

In order to deal with such a problem, recently, advanced control to which modern control theory is applied has also been used. For example, Iron and Steel, Vol.79, No.5 (19
93) -40, it was analyzed that the molten steel surface wave in the mold consisted of a combination of long-period wave and short-period wave, and the molten steel surface velocity in the mold was measured by measuring the wave height of the long-period wave near the short side. By estimating and selecting casting conditions so that the melt level variation index, which is in a linear relationship with this estimated long-period wave, falls within the appropriate range, a continuous casting mold that achieves stable continuous casting without powder slag inclusion A report entitled "Casting Conditions and Effects of Internal Molten Steel Flow" is disclosed.

CAMP-ISIJ, Vol.3, No.4 (1990)
-1126 includes a feedback control of molten steel surface level, automatic correction of control operation according to the response delay state of the hydraulic control system, feedforward control of pouring speed, and a control unit that optimizes the control gain by an evaluation function. A report entitled "Casting Machine Mold Level Control System" is disclosed.

[0008]

However, even if the advanced control resistant to disturbances disclosed in the above-mentioned report is used, it is difficult to predict changes in operating conditions, nozzle clogging, wear of refractories, etc. as described above. It was almost impossible to obtain stable control results under all operating conditions with one control method when the gain changed due to such disturbance. If a stable control result is not obtained, the quality of the slab will deteriorate,
If it is not cared for at the stage of the steel slab, it cannot be sent to the next process, resulting in an increase in the fuel consumption rate, an increase in the maintenance cost, a decrease in yield, and a rise in the manufacturing cost.

By the way, a plurality of level control methods can be selected while the level of molten steel injected from the tundish into the mold is detected by adjusting the sliding nozzle, and a plurality of level control methods can be selected. There are known methods for controlling the level of molten steel. First, in Japanese Patent Laid-Open No. 58-159962, the target level value is made to follow the actually measured level value during manual operation before switching to automatic control, and the separation type drive device of the stopper moving device contacts the stopper moving device. There has been proposed a molten steel level control method in continuous casting in which the measured level value at that time is used as a control target value to shift to automatic level control. However, this method is based on switching from manual operation to automatic control, and is not suitable for completely automatic switching of the mold level control system.

Further, in Japanese Unexamined Patent Publication No. 4-270050, when the deviation from the target value of the molten steel level in the mold is used as a reference, the main molten metal amount control system is mainly used when the deviation is large, and the drawing speed is mainly used when the deviation is small. A method for controlling the level of molten steel surface in a mold using a control system has been proposed. According to this method, it may not be possible to flexibly deal with the various disturbances as described above, and it is difficult to achieve stable molten steel level control in the mold under all operating conditions.

The present invention solves the above-mentioned problems of the prior art, can cope with the various disturbances as described above, and achieves stable molten steel level control under all operating conditions, and particularly An object of the present invention is to provide a method for controlling the level of molten steel in a mold in a continuous casting machine that is suitable for automatic switching according to the control method.

[0012]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a plurality of levels by detecting the level of molten steel injected from a tundish into a mold by adjusting a sliding nozzle while using a molten steel level meter. In a molten steel level control method in a continuous casting machine for controlling a molten steel level in a mold by selectively switching control methods, the plurality of level control methods are set as a plurality of level control methods having different input-output control gains. , Continuous casting, characterized in that a specific level control method is selectively switched from among the plurality of level control methods according to one or a combination of thresholds of pouring speed, residual molten steel amount in ladle, and tundish weight. This is a method for controlling the level of molten steel in a mold in a machine.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In order to adjust the flow rate of molten steel injected from a tundish into a mold, the circular nozzle hole provided in a fixed plate of a sliding nozzle and the circular nozzle hole provided in a slide plate have a large or small overlap. It is necessary to change the nozzle opening area through which molten steel flows.

Incidentally, when the slide plate provided on the sliding nozzle is moved from fully closed to fully open, the molten steel flow rate with respect to the nozzle opening is S as shown in FIG.
It changes with character characteristics. As you can see from this figure,
When the slide plate of the sliding nozzle is moved in an analog manner to increase the position (opening) from fully closed to fully open, there is little change in the molten steel flow rate with respect to the opening when the opening of the sliding nozzle is small, The characteristic is that the flow rate with respect to the opening increases almost proportionally when the opening becomes large to some extent, and the molten steel flow rate changes little with respect to the nozzle opening when the opening approaches full opening.

Looking at the operating conditions from the viewpoint of controlling the level of molten steel in the mold, at the start of casting of the continuous casting machine, the casting speed by injecting molten steel from the tundish into the mold is low, and therefore the opening of the sliding nozzle is It is controlled in a small place. Therefore, even if the sliding nozzle is moved a little, there is little change in the molten steel flow rate. Further, when the ladle is replaced in continuous casting, the weight of molten steel in the tundish is small and the molten steel level is low, and the static pressure of molten steel is low, so the flow rate of molten steel is low.

During normal operation in which the molten steel level in the tundish is high, the control gain is adjusted to reduce the output control gain with respect to the input, so that the molten steel flow rate does not change much even if the sliding nozzle is moved a little. Furthermore, during high-speed casting, the molten steel surface discharged from the immersion nozzle due to the molten steel flow in the mold swells at a low frequency, which becomes a disturbance.

If the control gains are the same, only the control output is generated for the deviation between the set level value and the detection level value. Since the molten steel level in the tundish is high during steady operation, the gain is adjusted to be small. Therefore, even if there is the same deviation between the set level value and the detection level value, the flow velocity injected into the mold decreases when the molten steel level in the tundish decreases. Therefore, even if the sliding nozzle is moved by the same opening degree, the weight of molten steel to be injected cannot keep up, and the molten steel level in the mold is lowered. For this reason, when the molten steel level in the tundish is lowered, a control method in which the control gain is set high, that is, a gain that outputs more control output with respect to the deviation between the set level value and the detection level value is provided. Unless a control method is used, stable control results cannot be obtained.

Therefore, in the present invention, as described above, a plurality of level control methods are set as a plurality of level control methods having different input-output control gains, and the level control method is a control method that best matches each operating condition. It is switched so that it can be selected. Especially, if the control method with the highest control accuracy is automatically switched, the quality of the cast piece can always be stably produced by the automatic control.

The present invention will be specifically described below with reference to the drawings. FIG. 1 is an explanatory diagram showing a control mechanism used for controlling molten steel level in a mold of a continuous casting machine according to the present invention, which is particularly suitable for automatic control. In FIG. 1, a ladle 1 filled with molten steel is placed above a tundish 2, and the molten steel in the ladle 1 is injected into the tundish 2 via a sliding nozzle 15 and a long nozzle 16 provided at the bottom of the ladle 1. To be done. At this time, for example, when the weight of molten steel poured from the ladle 1 into the tundish 2 reaches a set weight (level), the sliding nozzle 4 provided at the bottom of the tundish 2 is opened to remove the molten steel in the tundish 2. It flows down through the two-hole type immersion nozzle 17 and is injected into the mold 3 that vibrates up and down at a predetermined cycle. Powder is supplied into the mold 3 to cover the surface of the molten steel injected into the mold 3 to prevent oxidation of the surface of the molten steel and heat dissipation from the surface, and a slab 18 along the inner wall surface of the mold 3. It flows in between and, and acts as a lubricant between them.

The molten steel injected into the mold 3 is cooled by contact with the water-cooled inner wall surface of the mold 3 and solidified, and becomes a slab 18 whose outside is covered with a solidification cell. It is held between the support rolls 19 arranged, and is continuously pulled out by the rotation of a pinch roll (not shown). The molten steel level value in the mold 3 continuously detected by the molten steel level meter 5 arranged above the mold 3 is input to the process input unit 14 of the level controller 9 via the level meter amplifier 7. As the molten steel level meter 5, here, an eddy current is generated to generate a magnetic field on the molten steel surface in the mold 3 to form a magnetic field, and the strength of this magnetic field is measured to detect the molten steel level in the mold 3. Although the molten steel level meter of the type was used, it is not limited to this, and another level meter such as a radiant molten steel level meter which emits γ-rays may be used.

The sliding nozzle 4 provided at the bottom of the tundish 2 is operated by an actuator 6, and the nozzle opening is measured by a nozzle opening meter (position meter) 13, and the measured opening value is a level controller. 9 is input to the process input unit 14. And basically 5 levels of molten steel
The molten steel level value in the mold 3 detected by is input to the process input unit 14 and compared with the level set value input in advance here, and when there is a level deviation between the detected molten steel level value and the set level value. The molten steel level in the mold 3 is controlled by adjusting the opening of the sliding nozzle 4 while measuring the nozzle opening meter 13 so as to eliminate the level deviation.

In the present invention, various operating conditions such as the weight of molten steel in the ladle 1, the weight of molten steel in the tundish 2 and the pouring speed are collected in the operating condition collection unit 10 of the level control device 9 and these operating conditions are collected. Based on the PID control method, the observer control method, and the robust control method, the current optimum control method is selected, and the selected control method is input to the process input unit 14.

As shown in FIG. 3, the PID control system is mainly intended for a steady pouring stage in which the sliding nozzle opening is in an intermediate stage and the change in molten steel flow rate with respect to the nozzle opening adjustment is proportional and large. At this time, the gain of the control output with respect to the input is made small, and for example, the gain parameter is set to the control output 1 for the input 1.
Further, the observer control method is applied at the beginning of pouring when the sliding nozzle opening is small and the change in the molten steel flow rate due to the nozzle opening adjustment is small.At this time, the gain of the control output with respect to the input is increased. For example, the gain parameter is set to control output 1.8 for input 1.

Furthermore, the robust control system is applied when the opening of the sliding nozzle 4 becomes large and the change of the molten steel flow rate due to the nozzle opening adjustment is small, that is, when the casting speed is high. At this time, P
It is necessary to make the gain of the control output with respect to the input larger than that of the ID control method. However, since the opening of the sliding nozzle 4 is close to full opening and high speed casting is performed, 2
Due to the violent action of the molten steel discharged from the hole-type immersion nozzle 17 into the mold 3, the molten steel surface swells at a low frequency, so that the level value detected by the molten steel level meter 5 frequently changes. Due to the influence, if the opening adjustment by the sliding nozzle 4 is performed with a gain comparable to the observer control method, the gain becomes too large and there is a risk that the molten steel level in the mold 3 may be destabilized. Therefore, during high-speed casting with a large sliding nozzle nozzle opening,
The gain is set to an intermediate level between the PID control system and the observer control system, and for example, the gain parameter is set to 1.5 for the control output gain for input 1.

As described above, the optimum condition selected by the operating condition collection unit 10 is input to the process input unit 14 and the selected PID control system, observer control system, or robust control system is output. The switching unit 11 is instructed. The output switching unit 11 switches the control gain so that the commanded control method is achieved. The molten steel level value in the mold 3 detected by the molten steel level meter 5 in the state where the control system is switched in this way is amplified through the level meter amplifier 7 and then input to the process input unit 14. The process input unit 14 compares the preset level value input in advance, and when there is a deviation between the detected molten steel level value and the set level value, the deviation is calculated based on the gain of the selected control method, and Is input to the output switching unit 11, where switching to a predetermined output gain is performed.

Further, the output switching unit 11 obtains the oil amount corresponding to the level deviation from the inputted molten steel set level value in the mold 3, and this oil amount is instructed to the servo valve 12 via the servo amplifier 8 and the servo valve 12 is operated. By operating the valve 12, a proper amount of oil is supplied to the actuator 6. The operation of the actuator 6 causes the sliding nozzle 4 to move to the nozzle opening gauge 13
The opening degree is controlled while detecting the nozzle opening value input to the process input unit 14 from. By controlling the opening of the sliding nozzle 4 as described above, the molten steel level control in the mold 3 is always achieved under the control method with the highest control accuracy.

Here, as shown in Table 1, examples of the operating conditions and control system of the continuous casting machine correspond to when the casting speed is large, the T / D level is stable, and the amount of molten steel in the ladle 1 is low. The control method can be used properly.

[0028]

[Table 1]

In Table 1, the casting speed B is less than 1.0 m / mim, for example, and the ladle molten steel weight is less than 50t when the ladle is replaced from the next ladle injection until the T / D level automatic control is entered. The observer control method is adopted. In the steady pouring speed region (B is 1.0 m / mim or more and C is less than 1.8 m / mim), the PID control method is adopted, and the high pouring speed region (C is
For 1.8m / mim or more), a robust control system is selected and continuous casting is performed by a continuous casting machine.

Next, referring to FIG. 2, the tundish (T
/ D) Molten steel weight, mold (M / D) molten steel level, and casting speed are each set to 100% as the maximum value, and will be explained by the time transition in the case of actual% of each. With the weight of molten steel in the tundish 2 in FIG. 1 reaching the 80% level as shown in FIG. 2, the sliding nozzle 4 is opened and the molten steel in the tundish 2 is passed through the immersion nozzle 17 to the mold 3 Inject into the inside. In the region (1) at this time, since the opening of the sliding nozzle 4 is small, the observer control method is input to the process input unit 14 by the selection in the operating condition collecting unit 10.

Therefore, observer control is input from the process input section 14 to the output switching section 11, and the output switching section 11
A command by which the gain by the observer control method, that is, the control output with respect to the input is large, is sent to the servo valve via the servo amplifier 8.
Communicate to 12. As a result, the actuator 6 is operated and the sliding nozzle 4 is opened with a large gain, so that the molten steel level in the mold 3 is promptly raised.

At the casting speed threshold of 1.0 m / mim which has entered the region (2) due to the rapid increase of the casting speed, the process input section 14 receives the command from the operation condition collecting section 10 and the observer control method with a large gain is used. Switch to a PID control method with a small gain. Then, the molten steel level meter 5 detects the molten steel level in the mold 3, and the opening degree of the sliding nozzle 4 is measured by the nozzle opening degree meter 13 so as to eliminate the deviation between the set level value and the detected level value. However, the molten steel level in the mold 3 is controlled by adjusting the gain under a small gain.

Since the molten steel is continuously supplied from the ladle 1 to the tundish 2 even while the molten steel is being poured from the tundish 2 into the mold 3, the weight of the molten steel in the tundish 2 in the region (2) is almost constant. It is held constant and fluctuations in the casting speed are suppressed. When the ladle is replaced and the casting is continuously performed, when the molten steel weight in the ladle 1 becomes less than or equal to the threshold value 50t, the process input unit is instructed by the operation condition collecting unit 10
The observer control method is input to 14. In the area (3) from the amount of residual molten steel in the ladle 1 of less than 50t in the ladle 1 to the automatic T / D level turning on after the next ladle is exchanged, the tank 1 is temporarily turned off. The molten steel supply to the dish 2 will be stopped. For this reason, the weight of molten steel in the tundish 2 decreases and the level drops. Therefore, in this region (3), an observer control system with a large gain is used to prevent the casting speed from decreasing. As a result, even when the ladle is replaced, continuous casting can be continued without causing the molten steel level in the mold 3 to deviate from the set level value.

When the ladle exchange in the area (3) is completed, the process input section 14 receives a P
The ID control method is input and the area (4) is entered. This area
In (4), under the next molten steel injection from the ladle 1 to the tundish 2, the P with the small gain is obtained as in the PID control.
The molten steel level in the mold 3 is continuously cast at a steady casting speed by the ID control without deviation from the set level value.

Next, the pouring speed is increased so that the pouring speed becomes a threshold value.
When it reaches 1.8 m / min, the robust control method is input to the process input unit 14 by the command from the operating condition collection unit 10,
Enter (5). Thus, in the region (5), continuous casting is performed under high speed casting to prevent the molten steel level in the mold 3 from deviating from the set level value under the robust control method, which is a gain at an intermediate level between the observer control method and the PID control method. To be done.

As described above, according to the present invention, a plurality of level control methods having different input-output gains are switched to the optimum control method based on the operating conditions, so that stable casting is performed as shown in FIG. It can be seen that the molten steel level in the mold 3 does not deviate from the set value at the speed and can be controlled with high accuracy. In the above description, the case where the PID control method, the observer control method, and the robust control method are combined for control has been described.
It is also possible to control by a combination of the D control method and a combination of the observer control method and the robust control method. Furthermore, it is possible to incorporate other control methods such as the advanced control method.

[0037]

As described above, according to the present invention, the control system having different input-output gains is switched to the optimum control system according to each operating condition. Even in the above, the molten steel level in the mold can be accurately performed with a stable pouring speed. As a result, it is possible to greatly reduce troubles such as product downgrade and breakout due to the entanglement of the mold powder due to the fluctuation of the molten steel level in the mold and the disturbance of the molten steel flow.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a control mechanism used for mold control of a continuous casting machine according to the present invention.

FIG. 2 is a diagram showing changes over time in T / D molten steel weight, M / D molten steel level, and casting speed.

FIG. 3 is a characteristic diagram showing a relationship between a sliding nozzle opening and a molten steel flow rate.

[Explanation of symbols]

 1 Ladle 2 Tundish 3 Mold 4 Sliding Nozzle 5 Molten Steel Level Meter 6 Actuator 7 Level Meter Amplifier 8 Servo Amplifier 9 Level Control Device 10 Operating Condition Collection Section 11 Output Switching Section 12 Servo Valve 13 Nozzle Openness Meter 14 Process Input Section 15 Sliding nozzle 16 Long nozzle 17 Immersion nozzle 18 Cast slab 19 Support roll

Claims (1)

[Claims] 1. A molten steel level in a mold is controlled by selectively switching a plurality of level control methods while detecting the level of molten steel injected into a mold from a tundish by adjusting a sliding nozzle. In the molten steel level control method in a mold in a continuous casting machine, the plurality of level control methods are set as a plurality of level control methods having different input-output control gains,
A continuous casting machine characterized by selectively switching a specific level control method from the plurality of level control methods according to one or a combination of thresholds of pouring speed, residual molten steel amount in ladle, and tundish weight. For controlling molten steel level in mold in.