Method for controlling the molten metal level inside a continuous casting machine mold and method for manufacturing steel using the same.

Abstract

To provide a method of controlling a molten metal surface level in a mold for a continuous casting machine, capable of effectively suppressing a fluctuation of a bulging molten metal surface level even in either case where the molten metal surface fluctuation in which the outer disturbance compensation becomes effective has not occurred and where the molten metal surface fluctuation in which the outer disturbance compensation becomes effective has occurred but has subsided.SOLUTION: A method of controlling a molten metal surface level in a mold for a continuous casting machine determines, based on a detection of a spectrum peak by frequency analysis of a molten metal surface level signal, to stop or continue an addition in an amount of the outer disturbance compensation.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a control method for controlling the molten metal level in a mold of a continuous casting machine and a method for producing steel using the same. In particular, the present invention relates to a method for controlling a molten metal level in a mold of a continuous casting machine, which is capable of reducing bulging-type molten metal level fluctuation during steady casting.

Background Art

[0002] Generally, controlling the level of molten metal (hereinafter also referred to as molten steel) in the mold of a continuous casting machine is important not only for stable operation but also for ensuring the quality of slabs.

[0003] Conventionally, PI control has generally been widely used for controlling the molten metal level in the mold of this type of continuous casting machine. In this case, the control system is configured for the purpose of balancing the mass balance between the molten steel that is being drawn out from the mold while solidifying and the molten steel supplied to the mold depending on the opening degree of the sliding nozzle.

[0004] Fluctuation of the molten metal level in the mold occurs when the disturbance of this mass balance cannot be sufficiently compensated, and the main cause is considered to be the flow fluctuation during drawing. The largest one of such drawing flow fluctuations is what is called highly periodic bulging-type molten metal level fluctuation, which is said to be generated by the vibration of the solidified shell of the slab between the drawing pinch rolls. It is also known that such bulging-type molten metal level fluctuation has a frequency determined by the drawing speed of the pinch rolls (casting speed) and the interval between the pinch rolls (roll pitch).

[0005] As a technique for suppressing this bulging-type molten metal level fluctuation, for example, there is a technique disclosed in Patent Document 1. This technique installs a disturbance compensator, extracts the bulging-type molten metal level fluctuation from the molten metal level signal, generates a phase-advanced signal, and uses the signal for control.

[0006] Furthermore, Patent Document 2 discloses a technique in which disturbance compensation is performed only when the peak value detected by spectral peak detection through frequency analysis of the molten water level signal exceeds a threshold, and when the peak frequency corresponding to the peak value changes during disturbance compensation, the parameters of the disturbance compensator are changed.

[0007] Furthermore, Patent Document 3 discloses a technique that uses a standing wave model to remove the effects of primary and secondary standing waves from the molten metal level signal, and then uses a disturbance compensator for bulging molten metal level fluctuations. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 10-314911 [Patent Document 2] Japanese Patent Publication No. 2007-253170 [Patent Document 3] Japanese Patent Publication No. 2012-170999 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, the technology described in Patent Document 1 requires that a situation arise where disturbance compensation is effective in suppressing molten metal level fluctuations. If such fluctuations do not occur, it has the opposite effect and worsens the molten metal level fluctuations.

[0010] Furthermore, in the technology described in Patent Document 2, disturbance compensation is not performed until a molten metal level fluctuation in which disturbance compensation is effective begins to occur, so the molten metal level fluctuation does not worsen. However, once a molten metal level fluctuation in which disturbance compensation is effective begins to occur, it is not possible to determine whether the current molten metal level fluctuation is caused by a molten metal level fluctuation in which disturbance compensation is effective, or whether the molten metal level fluctuation has worsened as a result of disturbance compensation. Therefore, once a molten metal level fluctuation in which disturbance compensation is effective occurs, even if the molten metal level fluctuation in which disturbance compensation is effective subsides midway through, the disturbance compensation continues, and there is a problem that such disturbance compensation adversely affects the molten metal level fluctuation and worsens it.

[0011] Furthermore, while the technology described in Patent Document 3 can eliminate the effects of standing waves, it has the problem that disturbance compensation has a negative impact on molten metal level fluctuations caused by factors other than standing waves, such as molten metal level fluctuations due to feedback control, thus worsening the molten metal level fluctuations.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a method for controlling the molten metal level inside a continuous casting machine mold that can effectively suppress bulging molten metal level fluctuations when no molten metal level fluctuations that would enable disturbance compensation have occurred, or when molten metal level fluctuations that would enable disturbance compensation have occurred but have subsided. The present invention also aims to provide a method for manufacturing steel using such a control method. [Means for solving the problem]

[0013] In other words, the gist of the present invention is as follows: 1. A method for controlling the molten metal level inside a mold of a continuous casting machine, which involves detecting the molten metal level inside the mold of the machine, outputting a sliding nozzle opening command to the tundish via PI control based on the detected molten metal level signal, and controlling the molten metal level by adjusting the amount of molten metal flowing into the mold by adding a bulging disturbance compensation component to the output, and deciding whether to stop or continue adding the disturbance compensation component based on the detection of a spectral peak by frequency analysis of the molten metal level signal.

[0014] 2. The method for controlling the molten metal level inside a mold of a continuous casting machine according to claim 1, wherein the decision to stop adding the disturbance compensation is made when the peak value detected by spectral peak detection exceeds a first threshold A, and the difference between the casting speed and the peak frequency at the time the disturbance compensation was started to be added to the ratio of the casting speed to the peak frequency detected by spectral peak detection exceeds a second threshold X.

[0015] 3. The threshold X is defined as the roll pitch where bulging molten metal surface fluctuations can occur: d (mm), and the acceleration of the casting speed: ΔVc (m / min). 2 The method for controlling the molten metal level inside a mold of a continuous casting machine as described in item 2 above, wherein X (mm) is given by the following equation 1 using the casting speed: Vc (m / min) and the observation time of frequency analysis: T (min). X = ( d × ΔVc × T ) ÷ ( 2 × Vc ) ··· Formula 1

[0016] 4. A method for producing steel by casting molten iron using the method for controlling the molten iron level inside a continuous casting machine mold described in any one of items 1 to 3 above. [Effects of the Invention]

[0017] According to the present invention, it is possible to determine whether the current molten metal level fluctuation is caused by a molten metal level fluctuation for which disturbance compensation is effective, or whether the molten metal level fluctuation has been worsened by the disturbance compensation. Therefore, even if a molten metal level fluctuation for which disturbance compensation is effective has not occurred, or if a molten metal level fluctuation for which disturbance compensation is effective has occurred but then subsided, bulging molten metal level fluctuations can be effectively suppressed. Furthermore, the present invention can also provide a method for manufacturing steel that casts stable molten iron using such a method for controlling fluctuations in the molten iron level. [Brief explanation of the drawing]

[0018] [Figure 1] This figure shows an example of equipment for carrying out the present invention. [Figure 2] It is a diagram showing an example of an apparatus configuration for carrying out the present invention. [Figure 3] It is a diagram showing an example of a processing flow in the present invention. [Figure 4] It is a diagram showing an example of molten steel level fluctuation and SN opening output when molten steel level fluctuation for which disturbance compensation control is not effective occurs during execution of the disturbance compensation control of the present invention. [Figure 5] It is a diagram showing a setting example of the threshold value X in the present invention. Mode for Carrying Out the Invention

[0019] Hereinafter, an example of a mode for carrying out the present invention will be described with reference to the drawings. Figs. 1 and 2 are diagrams showing an example of the configuration of facility equipment and an apparatus for carrying out the present invention. In the drawings, 1 denotes molten steel, 2 denotes a ladle, 3 denotes a tundish, 4 denotes an immersion nozzle, 5 denotes a mold, 6 denotes a shell, 7 denotes a sliding nozzle (hereinafter partially abbreviated as SN), 8 denotes a molten steel surface, 9 denotes a molten steel level gauge, 10 denotes a PI controller, 11 denotes a sliding nozzle control device, 12 denotes a signal processing device, 20 denotes pinch rolls, 21 denotes a secondary cooling zone, and 22 denotes a torch (cutter), respectively.

[0020] The tundish 3 is disposed above the mold 5, the sliding nozzle 7 is disposed at the bottom of the tundish, and the immersion nozzle 4 is further disposed on the lower surface side of the sliding nozzle 7. The molten steel 1 filled in the tundish 3 is injected into the mold 5 through the sliding nozzle 7 and the immersion nozzle 4.

[0021] Subsequently, the molten steel 1 injected into the mold 5 is cooled from the side surface of the mold, solidifies from the surface to form the shell 6, and is drawn out below the mold 5 by the pinch rolls 20 to form a cast slab. At this time, the drawing speed is controlled to be substantially constant by a pinch roll control device (not shown).

[0022] Furthermore, the amount of molten steel injected into the mold 5 is determined by the opening degree of the sliding nozzle 7. The opening degree of the sliding nozzle is adjusted by driving an actuator (not shown), such as a hydraulic cylinder, via the sliding nozzle control device 11.

[0023] The opening of the sliding nozzle 7 is controlled to balance the amount of molten steel injected into the mold with the amount withdrawn downwards. This control is achieved by changing the sliding nozzle opening setting value using PI control, based on the signal obtained from the molten steel level gauge 9, which detects the mold molten steel level, and the deviation signal from the molten steel level setting value, so that the molten steel level remains constant.

[0024] Here, bulging molten metal level fluctuations are said to occur when the molten metal solidifies between the pinch rolls 20 used to draw out the slab, bulging in a direction perpendicular to the drawing direction. As the bulging portion is pushed in by the pinch rolls 20 during the drawing of the slab, the mass balance of the molten steel in the mold is disrupted. Furthermore, a characteristic of such molten metal level fluctuations is that they are periodic, with a period determined by the distance between the pinch rolls and the casting speed.

[0025] The molten metal level signal is subjected to FFT analysis, and the frequency component with the maximum peak intensity within the frequency range determined from the pinch roll spacing and casting speed is defined as the bulging molten metal level fluctuation frequency. Furthermore, since the pinch roll spacing is not constant over the entire length of the continuous casting machine, the frequency determined from the pinch roll spacing and casting speed has a certain range.

[0026] In this invention, when the bulging-type molten metal level fluctuation frequency falls within a certain range and its maximum peak intensity exceeds threshold A, it is recognized that a bulging-type molten metal level fluctuation has occurred, and the disturbance compensation control output is used for molten metal level control (disturbance compensation control). The above-mentioned range of bulging molten metal surface fluctuation frequencies refers to the range of the minimum frequency fmin and the maximum frequency fmax. Furthermore, the minimum frequency fmin and the maximum frequency fmax are the minimum and maximum frequencies of the transient bulging molten metal surface fluctuations that can occur, determined from the relationship between the casting speed and the roll pitch, as described in Japanese Patent Publication No. 2007-136480. The minimum frequency fmin is expressed as "1 ÷ (maximum roll pitch / current casting speed)", and the maximum frequency fmax is expressed as "1 ÷ (minimum roll pitch / current casting speed)". However, in this invention, the threshold for the maximum peak intensity can be determined as either "greater than or equal to" or "greater than or equal to".

[0027] Furthermore, in this invention, even while the disturbance compensation control output is being used for molten metal level control, the molten metal level signal is subjected to FFT analysis. If the maximum peak intensity exceeds threshold A, and the difference between the casting speed and the peak frequency ratio at the start of adding the disturbance compensation to the ratio of the casting speed to the peak frequency detected by the spectral peak detection exceeds threshold X, the system recognizes that the disturbance compensation control is adversely affecting the molten metal level fluctuations and stops using the disturbance compensation control output. Furthermore, the difference between the casting speed and the peak frequency ratio at the start of adding the disturbance compensation to the ratio of the casting speed and the peak frequency detected by the spectral peak detection is expressed by the following Equation 2, where V1 is the casting speed, H1 is the peak frequency, V2 is the casting speed at the start of adding the disturbance compensation, and H2 is the peak frequency. (V1 / H1)-(V2 / H2)...Formula 2 Furthermore, as mentioned above, in the present invention, the threshold for maximum peak intensity can be determined as either "greater than or equal to" or "greater than or equal to".

[0028] Here, if the threshold X is too small, even if bulging molten metal level fluctuations continue to occur, changing the casting speed will cause the system to mistakenly recognize that the disturbance compensation control is adversely affecting the molten metal level fluctuations, and will stop using the disturbance compensation control output. On the other hand, if the threshold X is too large, even if the disturbance compensation control is adversely affecting the molten metal level fluctuations, the system will not recognize that the disturbance compensation control is adversely affecting the molten metal level fluctuations and will not stop using the disturbance compensation control output. Therefore, the threshold X is the roll pitch d (mm) at which bulging molten metal surface fluctuations can occur, and the casting speed acceleration ΔVc (m / min). 2 It is preferable to use the value X (mm) given by the following equation 1, where the casting speed is Vc (m / min) and the observation time for frequency analysis is T (min). X = ( d × ΔVc × T ) ÷ ( 2 × Vc ) ··· Formula 1

[0029] Furthermore, the threshold A mentioned above should be set to a value that allows for the distinction between the peak value when bulging-type molten metal level fluctuations do not occur and the peak value when bulging-type molten metal level fluctuations occur, by actually measuring these two values ​​in the actual machine.

[0030] Furthermore, in many cases where disturbance compensation control negatively affects molten metal level fluctuations, the frequency at which the peak intensity of that fluctuation is greatest falls outside the frequency range determined by the pinch roll spacing and casting speed. Therefore, the use of the disturbance compensation control output will not be restarted immediately after it has been stopped.

[0031] If a bulging molten metal level fluctuation occurs at a different bulging molten metal level fluctuation frequency during disturbance compensation control, the present invention recognizes that the disturbance compensation control is adversely affecting the molten metal level fluctuation and stops using the disturbance compensation control output. However, if the different bulging molten metal level fluctuation frequencies fall within the frequency range determined from the pinch roll spacing and casting speed, the present invention immediately recognizes that a bulging molten metal level fluctuation has occurred and uses the disturbance compensation control output for molten metal level control with parameters corresponding to the different bulging molten metal level fluctuation frequencies.

[0032] Figure 3 shows an example of the processing flow in the present invention. First, in the processing start (S100) step, processing is started. Then, in the next data storage (S101) step, the molten metal level meter signal from the molten metal level meter 9 in Figure 2 is sampled at a constant period and stored as data.

[0033] In the FFT processing step (S102), a Fast Fourier Transform (hereinafter abbreviated as FFT) is performed on the accumulated data to convert the time series data into frequency components.

[0034] In the spectral peak detection (S103) step, the frequency and peak value of the spectrum at the molten metal level that is maximized from the data processed in the previous step are detected. This frequency is the bulging molten metal level fluctuation frequency.

[0035] In the Peak Value > Threshold? (S104) step, it is determined whether the previously calculated peak value exceeds a predetermined threshold A. If the previously calculated peak value is smaller than threshold A, the process returns to the Data Storage (S101) step.

[0036] Is the peak frequency within the range? (S105) In this step, it is determined whether the peak frequency obtained earlier is within the frequency range (a certain range of the bulging molten metal level fluctuation frequency) determined from the pinch roll spacing and casting speed. If the peak frequency is outside the frequency range determined from the pinch roll spacing and casting speed, the process returns to the data storage (S101) step. On the other hand, if the peak frequency is within the frequency range determined from the pinch roll spacing and casting speed, the disturbance compensation parameters are determined in the disturbance compensation parameter determination (S106) step.

[0037] In the disturbance compensation control start (S107) step, disturbance compensation control is performed using the previously determined disturbance compensation parameters. The following steps—data accumulation (S108), FFT processing (S109), spectral peak detection (S110), and peak value > threshold (S111)—are the same as the previously described data accumulation (S101), FFT processing (S102), spectral peak detection (S103), and peak value > threshold (S104) steps.

[0038] Furthermore, step S111 uses threshold B instead of threshold A. This threshold B can be determined by actually measuring the peak value when there is a molten metal surface fluctuation resulting from the deterioration of the molten metal surface fluctuation due to disturbance compensation, and setting a value lower than that peak value. Here, the threshold B should ideally be higher than the peak value of the current molten metal level fluctuation for which disturbance compensation is effective. However, in reality, this is difficult because the peak value of the current molten metal level fluctuation for which disturbance compensation is effective is often higher than the peak value when there is a molten metal level fluctuation resulting from the deterioration of the molten metal level fluctuation by disturbance compensation. Therefore, it is best to prioritize setting the water level to a value lower than the peak value when water level fluctuations exist as a result of exacerbating water level fluctuations due to disturbance compensation.

[0039] Furthermore, in step S111, it is easy to determine that the current molten metal level fluctuation is not affected by disturbance compensation, even if the disturbance compensation is effective for that fluctuation. However, in this invention, even if it is determined that the disturbance compensation is not effective in step S111, it will be determined to be effective in the following step S112, and the disturbance compensation control will continue, so there is no problem.

[0040] Is disturbance compensation control having an adverse effect? ​​(S112) In this step, it is determined whether the ratio of the current casting speed to the peak frequency and the ratio of the casting speed to the peak frequency when disturbance compensation control was started exceed a threshold X. If the threshold X is not exceeded, it is determined that the disturbance compensation control is effective, and the system returns to the data storage (S108) step while continuing the disturbance compensation control. On the other hand, if the threshold X is exceeded, it is determined that the disturbance compensation control is having an adverse effect on the molten metal surface fluctuation, and the system stops the disturbance compensation control in the disturbance compensation control stop (S113) step and returns to the data storage (S101) step.

[0041] As explained above, in the present invention, during disturbance compensation control, it is possible to determine whether the cause of the current molten metal level fluctuation is a molten metal level fluctuation for which disturbance compensation is effective, or a result of the molten metal level fluctuation being worsened by disturbance compensation, based on spectral peak detection by frequency analysis of the molten metal level signal. Therefore, even when no molten metal level fluctuation for which disturbance compensation is effective occurs (the molten metal level fluctuation is zero), and even when a molten metal level fluctuation for which disturbance compensation is effective has occurred but subsided, bulging molten metal level fluctuations can be effectively suppressed.

[0042] Furthermore, by employing a steel manufacturing method that uses this continuous casting machine mold level control method to cast molten iron, it is possible to suppress the scattering of molten steel outside the equipment during casting, stabilize the transport of steel, and produce steel with a high yield.

[0043] In addition, when the molten metal level fluctuates significantly, surface defects such as powder entrapment are usually a concern, requiring several millimeters of surface removal during the slab-making stage. In other words, suppressing bulging-type molten metal level fluctuations reduces concerns about powder entrapment and eliminates the need to remove several millimeters of surface material during the slab-making stage, thus leading to improved yield.

[0044] In addition, in the steel manufacturing method according to the present invention, any items not described herein can be handled by conventional methods. One example of a steel manufacturing method is casting molten iron using the equipment shown in Figure 1. Specifically, molten steel 1 is temporarily stored in a tundish 3, which maintains a constant casting speed, promotes the floating of inclusions, and enables continuous casting, from the ladle 2. Then, the molten steel is guided through a nozzle into a mold 5 made of copper alloy, which has an internal water cooling function and an oscillation function (up and down vibration). A level gauge for the molten steel is installed in the mold 5, and the nozzle opening is normally controlled so that the height of the molten steel remains constant. After leaving the mold, the slab is cooled in a secondary cooling zone 21 with spray water (high-speed water jet), the slab is pulled out by pinch rolls 20, and the fully solidified slab is cut by a torch 22 to produce a slab. The casting speed is preferably in the range of 1.2 to 3.0 m / min, the mold thickness is preferably in the range of 200 to 500 mm, and the mold width is preferably in the range of 800 to 2500 mm. [Examples]

[0045] Figure 4 shows an example of molten metal level fluctuations and SN opening output when disturbance compensation control using the molten metal level control method inside the mold of the continuous casting machine of the present invention is implemented in an actual machine, and disturbance compensation control is not effective due to molten metal level fluctuations. In the data storage (S101) step, the molten metal level meter signal from the molten metal level meter 9 shown in Figure 2 was sampled at a fixed period (sampling period: 0.1 seconds, sampling period: 102.4 seconds, thus performing FFT analysis on 1024 points of 0.1-second pitch data) and stored as data. Furthermore, thresholds A and B were defined as the peak values ​​obtained by performing FFT analysis on the 1024 points of molten metal level (% *1% = 1.5 mm) data mentioned above. The peak value for the control activation condition, which is threshold A, was set to 1500, and the peak value for the control deactivation condition, which is threshold B, was set to 1000.

[0046] As shown in Figure 4, the water level fluctuations increased from around 8060 seconds, and around 8120 seconds, the disturbance compensation control was stopped because it was determined that it was having a negative impact, after which the water level fluctuations decreased.

[0047] Figure 5 shows an example of setting the threshold X. The horizontal axis of the graph represents the ratio of casting speed to peak frequency at the start of disturbance compensation control, and the vertical axis represents the ratio of the current casting speed to peak frequency. The dashed line represents the line where the ratio of casting speed to peak frequency at the start of disturbance compensation control is equal to the ratio of the current casting speed to peak frequency. Points that are more than a distance of threshold X from this dashed line on the vertical axis are judged to be having an adverse effect, and the disturbance compensation control is stopped.

[0048] In this example setting, the threshold X is set with a roll pitch d = 380 mm and a casting speed acceleration ΔVc = 0.3 m / min. 2 Assuming a casting speed Vc = 1.5 m / min and an observation time T = 1.7 minutes for frequency analysis, the following equation 3 was used to determine the value. X = (380 × 0.3 × 1.7) ÷ (2 × 1.5) ≒ 65 mm...Formula 3

[0049] In Figure 5, the solid line represents the threshold X = 65 mm. In Figure 5, the ○ plots represent the point immediately after the casting speed was reduced while bulging molten metal level fluctuations were occurring in the actual machine. On the other hand, the × plots represent the point when disturbance compensation control adversely affected the molten metal level fluctuations in the actual machine.

[0050] As shown in Figure 5, the solid line representing the threshold X=65mm clearly separates the ○ plots from the × plots. [Explanation of Symbols]

[0051] 1 Molten steel 2 Ladle 3 Tan Dish 4 Immersion nozzle 5. Mold 6 Shells 7. Sliding nozzle 8. Surface of the water 9. Water level gauge 10 PI controller 11. Sliding nozzle control device 12 Signal Processing Devices 20 Pinch Roll 21 Secondary cooling zone 22 Torch (Cutter)

Claims

1. In a continuous casting machine, the molten metal level in the mold is detected, and based on this detected molten metal level signal, a sliding nozzle opening command is output via PI control to the tundish. A bulging disturbance compensation component is added to this output to adjust the amount of molten metal flowing into the mold, thereby controlling the molten metal level. A method for controlling the molten metal level inside a mold of a continuous casting machine, which determines whether the disturbance compensation is effective in controlling the molten metal level based on the relationship between the peak frequency detected by spectral peak detection through frequency analysis of the molten metal level signal and the casting speed, and the temporal change of said relationship, and decides whether to stop or continue adding the disturbance compensation.

2. The method for controlling the molten metal level inside a mold of a continuous casting machine according to claim 1, wherein the decision to stop adding the disturbance compensation is made when the peak value detected by spectral peak detection exceeds a first threshold A, and the difference between the casting speed and the peak frequency at the time the disturbance compensation was started to be added exceeds a second threshold X.

3. The threshold X is defined as the roll pitch at which bulging molten metal surface fluctuations can occur: d (mm), and the acceleration of the casting speed: ΔVc (m / min). 2 A method for controlling the molten metal level inside a mold of a continuous casting machine according to claim 2, wherein X (mm) is given by the following equation 1 using the casting speed: Vc (m / min) and the observation time of frequency analysis: T (min). X = (d × ΔVc × T) ÷ (2 × Vc) ...Formula 1

4. A method for producing steel by casting molten iron using the method for controlling the molten iron level inside a continuous casting machine mold according to any one of claims 1 to 3.

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