JPH0675758B2 - Mold level control method and apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method and apparatus for controlling a molten metal level in a mold in continuous casting of steel.

[Conventional technology]

The conventional control system is basically based on feedback control for controlling the opening degree of the tundish nozzle based on the signal from the level meter in the mold. As a control method,
To ensure that the molten metal level from the top of the mold is always constant, monitor the molten metal level with a vortex type or thermocouple type molten metal level sensor, and adjust the tundish nozzle opening according to the displacement from the target molten metal level. It is controlled by a controller.

When the casting is stable and no disturbance is introduced into the control meter, the molten metal level control accuracy is stable, and for example, the controlled level within the target molten metal level ± 5 mm can be obtained. However, the level control becomes unstable during unsteady casting with disturbance such as fluctuation of casting speed and fluctuation of molten steel weight in tundish. For this reason, in recent years, feedback control with feed-forward compensation has been applied, which compensates for control accuracy by tracking the casting speed, the molten steel weight in the tundish, and the like, and capturing these signals.

However, although feedforward control is possible for disturbance factors that can be tracked, such as the casting speed and the weight of molten steel in the tundish, if the disturbance signal cannot be tracked, the control system tends to become unstable.

Recently, warm cooling and high-speed casting have been adopted for the purpose of manufacturing high quality and high temperature slabs, but under such conditions, hot water caused by unsteady bulging of the slab is required. Surface variation is likely to occur.

This unsteady bulging is difficult to monitor at all times, and the fluctuation amount and fluctuation speed of the periodic level fluctuation caused by it are generally large. Therefore, even if the PID constants are properly set, the control system May become unstable, making it impossible to control the molten metal surface.

As an example of measures to prevent fluctuations in the molten metal surface caused by unsteady bulging, there is an example reported in "Iron and Steel" 73 (1987), S913. According to the report, once unsteady bulging occurs, the casting speed and the level of the molten metal in the mold divergently increase. As a countermeasure against this, it is effective to prevent unsteady bulging by making the roll pitch non-periodic. I am trying.

Also, as a measure to improve the level control accuracy of mold hot water,
It is also reported by "Iron and Steel" 73 (1987), S979 that auto-tuning is effective, and that the surface quality of stainless slabs is significantly improved.

[Problems to be Solved by the Invention]

Such unsteady bulging is caused by peritectic reaction steel (0.10-0.
15% C) and ferritic stainless steel are particularly noticeable, and may occur as periodic abnormal melt surface fluctuations during casting. The features are as follows.

1) It does not occur immediately after the start of casting, but once it occurs for a while, it increases divergently.

2) The molten metal fluctuation period is several tens of seconds, but it varies depending on the casting speed and is a constant pitch in terms of casting length, which corresponds to the roll pitch of the continuous casting machine.

3) The fluctuation level of the molten metal may exceed ± 5 mm.

4) The ascending / descending speed of the molten metal surface is 1 to 3 mm / sec, which is much faster than the normal speed of 1 mm / sec or less.

According to the research conducted by the authors, the periodical fluctuation of the molten metal level is caused by the unsteady bulging of the slab, because it is synchronized with the variation cycle of the slab withdrawal resistance and the bulging. That is, it can be understood that the unsteady bulging occurs between the segment rolls and the molten steel in the solidified shell is pushed up in a periodic manner to change the molten metal level in the mold. However, the method reported in the above-mentioned "Iron and Steel" 73 (1987), S913 has a problem that a large-scale facility remodeling such as making a segment roll into a small-diameter split roll is required, and thus it can be easily adopted. It cannot be done.

Further, there is a problem that the above-mentioned unsteady bulging cannot be solved by the means similarly reported by "Iron and Steel" 73 (1987), S979.

[Means for Solving the Problems]

The present invention has been made to solve the above-mentioned problems, and has a cycle and an amount of fluctuation of an abnormal molten metal level (hereinafter, referred to as abnormal molten metal level variation) that periodically occurs for some reason. Is calculated, and the tundish nozzle opening is adjusted in advance so as to cancel the fluctuation of the molten metal level in accordance with the cycle and the amount of fluctuation of the abnormal molten metal level fluctuation, thereby reducing the responsiveness of the feedback control. It compensates and improves the level control accuracy.

FIG. 1 shows an embodiment of the present invention. In the figure, 1 is a tundish, 2 is a tundish nozzle, 3 is a mold, 4 is a roll, 5 is a slab, 6 is a melt level sensor, 7 is a nozzle driving device, 8 is a nozzle opening meter, and 9 is a nozzle opening. Degree control device, 10 is an adder, 11 is an output signal correction device, 12 is a molten metal level calculation device, and 13 is a controller.

As the molten metal level control method, PID control with feedforward compensation for abnormal molten metal level fluctuation is used. The output signal of the PID can be calculated by the following equation (1) by the controller 13 based on the signal from the bath level sensor 6. Here, the first term, the second term, and the third term on the right side are P operation, I operation, and D operation, respectively.

The controller output MV thus obtained is sent to the nozzle opening control device 9 via the adder 10 and adjusts the injection flow rate for which the opening command is issued to the nozzle drive device 7. In parallel, the command opening is compared with the nozzle opening measured by the nozzle opening meter 8 to drive and control the nozzle so that the actual nozzle opening matches the command opening.

The feedforward control in the present invention is performed by calculating the correction output in the output signal correction device 11 in FIG. 1 according to the procedure described below. That is, the signal from the molten metal level sensor 6 is sampled by the molten metal level calculation device 12, and the rising edge (PVmax), the falling edge (PVmin) and the fluctuation cycle (TmaxN-TmaxN-1) of the periodical molten metal level fluctuation are sampled. Calculation processing is performed as shown in FIG. The output signal correction device 11 compares the fluctuation of the molten metal surface with a preset reference value, and the following (2)
When the conditions of the expressions (3) and (3) are satisfied, the fluctuation of the molten metal surface is determined as an abnormal fluctuation of the molten metal surface.

PVmaxN-PVminN≥α (2) β≤TmaxN-TmaxN-1≤γ (3) where α is the reference constant for fluctuation amount (mm) and β and γ are reference constants for period (sec) , Any value can be set respectively. For example, when the level fluctuation with a fluctuation amount of 6 mm or more and a cycle of 20 to 35 seconds is defined as an abnormal fluctuation, α = 6, β = 20, and γ = 35 are set.

[Action]

When it is determined that there is an abnormal change in molten metal level, the output signal correction device 11 calculates a corrected output MV1 based on the following equation (4).

MV1 = (MVmax−MVmin) × δ (4) Here, δ is a positive number of δ ≦ 1. This correction output MV1
Is added to the output MV of the controller 13 by the adder 10 to set the combined output MV2 as shown in FIG.

MV2 = MV + MV1 (5) The combined output MV2 thus obtained is finally sent to the nozzle opening control device 9 and output to the nozzle drive device 7. As for the output timing of the correction output MV1 to the adder, the output is turned on at the timing of PVmax (Tmax) and turned off at the timing of PVmin (Tmin) as shown in FIG.

By the above repeated operation, when the fluctuation of the molten metal level becomes small and the conditions of the expressions (2) and (3) are not satisfied, MV1 = 0 and MV2 = MV, and the normal PID control is resumed. In the present description, the output of the adder 10 is sent to the nozzle opening control device 9 for control, but it is also possible to directly control the nozzle drive device 7 by the output of the adder 10.

According to the method described above, the level fluctuation prevention effect can be obtained by a simple and inexpensive modification of only the level control system without requiring a large-scale facility modification for preventing unsteady bulging.

Further, since it is not necessary to take measures such as lowering of casting speed for suppressing slab bulging and strengthening of secondary cooling, a great effect can be obtained in operation. Further, the present invention is applicable to a wide range of utilization because it can be applied to an abnormal level change caused by some other type of facility disturbance other than bulging.

〔Example〕

Examples will be described below. A stainless steel post slab having a cross-sectional size of 250 mm x 1,300 mm was cast at a casting speed of 0.90 m / min using a curved continuous casting machine with a radius of curvature of 12 mR. The level control in the mold was performed using the level control system shown in FIG. 1 based on the signal from the vortex type mold level sensor. A general-purpose microcomputer is incorporated in the output signal correction device 11, and α = 6 mm, β = 20 sec, γ = 35 sec, δ = as constants of the equations (2), (3) and (4).
I set 0.5.

The molten metal level control accuracy according to the present invention is shown in FIG. 3 together with the comparative example by the conventional PID control to which the present invention is not applied. The rate of abnormal fluctuations in the level of metal that exceeds ± 3 mm is
Compared to 19.5% in the case of conventional PID control, it is significantly improved to 0.9% when the method according to the present invention is applied,
The level control accuracy of the molten metal is significantly improved.

In addition, the conventional PID that does not output correction for abnormal fluctuation
In control casting, a differential transformer was installed in the guide roll band to measure the unsteady bulging of the cast slab. As a result, as shown in FIG. 4, there is a strong correlation between the amount of bulging and the amount of fluctuation of the molten metal measured at the same time, and it is found that the fluctuation of the molten metal is caused by the unsteady bulging. It was confirmed that the application of the present invention prevented abnormal fluctuation of the molten metal surface.

〔The invention's effect〕

By applying the present invention, the level control accuracy of the molten metal in the mold is improved, and the surface flaws such as vertical crack flaws, oscillation marks, and slag of the slab are greatly reduced, and subcutaneous inclusions due to the inclusion of mold powder and deoxidized products. Was significantly reduced. As a result, it became unnecessary to inspect and maintain the cast slab, and it was possible to secure a product surface flaw level equal to or higher than the conventional level even without maintenance. As a result, the slab refinement yield and the energy saving rate in direct feed rolling were improved.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a molten metal level control system of the present invention, and FIG.
FIG. 4 is an explanatory diagram showing a method of calculating a corrected output and a combined output according to the present invention, FIG. 3 is a diagram showing an embodiment of the present invention, and FIG. 4 is a graph showing an unsteady bulging amount and a level change amount in a comparison row. It is a figure which shows the relationship of. 1: Tundish, 2: Tundish nozzle, 3: Mold, 4: Foot roll, 5: Cast slab, 6: Liquid level sensor, 7: Nozzle drive device, 8: Nozzle opening meter, 9: Nozzle opening control apparatus,
10: Adder, 11: Output signal correction device, 12: Level change calculator, 13: Controller.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Nakahashi 12 Nakamachi, Muroran-shi, Hokkaido Inside Nippon Steel Co., Ltd. Muroran Works (56) Reference JP-A-1-317668 (JP, A)

Claims (2)

[Claims] 1. In a molten metal level control in a continuous casting mold, a variation amount and a period of a periodically varying molten metal level are calculated, and the variation amount is larger than a preset reference value and the period is set. If it is within the range, the correction output of the tundish nozzle opening is calculated from the output actual product of the PID controller so as to cancel the periodical fluctuation of the molten metal level, and this corrected output is continuously increased. Mold level control method with feedforward compensation, characterized in that a combined output is obtained by adding to the output of the PID controller from the end to the falling end, and the tundish nozzle opening is adjusted by the combined output. . 2. A melt level sensor (6) for detecting a melt level fluctuation amount in a mold, and a signal from the melt level fluctuation sensor.
A controller (13) for performing PID control, a melt level fluctuation calculation device (12) for outputting a correction signal to the output of the controller (13), an output signal correction device (11) and an adder (10), Mold hot water in which a correction output signal is added to the output signal of the controller (13) so as to cancel the fluctuation of the molten metal level, and the corrected output signal is output to the nozzle opening control device (9) for control. Surface level control device.