Methods of controlling molten metal surface level in mold for continuous casting machine and method of producing 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 method for controlling the level of the molten metal in the mold of a continuous casting machine and a method for producing steel using the same, and more particularly to a method for controlling the level of the molten metal in the mold of a continuous casting machine that can reduce bulging-related fluctuations in the molten metal level during steady casting. [Background technology]

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

[0003] Conventionally, PI control has been widely used to control the molten steel level in the mold of this type of continuous casting machine. In this case, the control system is configured to achieve a mass balance between the molten steel being withdrawn from the mold while solidifying and the molten steel being supplied to the mold depending on the opening of the sliding nozzle.

[0004] Fluctuations in the molten metal level in the mold occur when this disturbance in the mass balance cannot be fully compensated for, and are believed to be mainly caused by flow fluctuations during drawing. The largest type of flow fluctuation during drawing is called bulging molten metal level fluctuation, which has a strong periodicity and is said to occur due to the vibration of the solidified shell of the slab between the drawing pinch rolls. It is also known that such bulging molten metal level fluctuations have a frequency determined by the drawing speed of the pinch rolls (casting speed) and the interval between the pinch rolls (roll pitch).

[0005] One example of a technique for suppressing this bulging-related mold level fluctuation is disclosed in Patent Document 1. This technique involves installing a disturbance compensator, extracting the bulging-related mold level fluctuation from the mold level signal, creating a phase-advanced signal, and using it for control.

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

[0007] Furthermore, Patent Document 3 discloses a technology in which a standing wave model is used to remove the effects of primary and secondary standing waves from the molten metal level signal, and then a disturbance compensator for bulging-related molten metal level fluctuations is used. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 10-314911 [Patent Document 2] JP 2007-253170 A [Patent Document 3] JP 2012-170999 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, the technology disclosed in Patent Document 1 requires a situation in which there is molten metal level fluctuation that makes the disturbance compensation effective in order to suppress the molten metal level fluctuation by the disturbance compensation. If there is no molten metal level fluctuation that makes the disturbance compensation effective, the problem is that the compensation has an adverse effect and worsens the molten metal level fluctuation.

[0010] Furthermore, in the technology shown in Patent Document 2, since disturbance compensation is not performed until the occurrence of molten metal level fluctuations for which disturbance compensation is effective, the molten metal level fluctuations do not worsen. However, once molten metal level fluctuations for which disturbance compensation is effective begin to occur, it is not possible to determine whether the cause of the current molten metal level fluctuations is the molten metal level fluctuations for which disturbance compensation is effective, or whether the molten metal level fluctuations have been worsened by disturbance compensation. Therefore, once molten metal level fluctuations for which disturbance compensation is effective occur, even if the molten metal level fluctuations for which disturbance compensation is effective subside, disturbance compensation continues, resulting in a problem that such disturbance compensation has an adverse effect on the molten metal level fluctuations, causing the molten metal level fluctuations to worsen.

[0011] Furthermore, while the technology disclosed in Patent Document 3 can eliminate the effects of standing waves, there is a problem in that disturbance compensation has a negative effect on molten metal level fluctuations caused by factors other than standing waves, such as molten metal level fluctuations due to feedback control, and the molten metal level fluctuations become worse.

[0012] The present invention has been made in consideration of the above circumstances, and has an object to provide a method for controlling the mold level of a continuous casting machine that can effectively suppress bulging-related mold level fluctuations when there is no mold level fluctuation that would make disturbance compensation effective, or when mold level fluctuations that would make disturbance compensation effective have occurred but have subsided, and also to provide a method for producing steel using such a control method. [Means for solving the problem]

[0013] That is, the gist of the present invention is as follows. 1. A method for controlling the molten metal surface level in the mold of a continuous casting machine, which detects the molten metal surface level in the mold of the continuous casting machine, outputs a sliding nozzle opening command provided in a tundish by PI control based on the detected molten metal surface level signal, and adds a bulging disturbance compensation amount to the output to adjust the amount of molten metal flowing into the mold, and determines whether to stop or continue adding the disturbance compensation amount based on spectral peak detection by frequency analysis of the molten metal surface level signal.

[0014] 2. The method for controlling the molten metal surface level in the mold of a continuous casting machine as described in 1 above, wherein the decision to stop adding the disturbance compensation amount is made when the peak value detected by the spectrum peak detection exceeds a first threshold value A and the difference between the ratio of the casting speed to the peak frequency detected by the spectrum peak detection and the ratio of the casting speed to the peak frequency at the time when adding the disturbance compensation amount is started exceeds a second threshold value X.

[0015] 3. The threshold value X is determined by the roll pitch d (mm) at which bulging-induced molten metal level fluctuations may occur, and the acceleration of the casting speed ΔVc (m / min 2 3. The method for controlling a molten metal level in a mold of a continuous casting machine according to claim 2, wherein X (mm) is given by the following equation 1 using Vc (m / min), Vd (m / min), and T (min) as the observation time for frequency analysis: X = ( d × ΔVc × T ) ÷ ( 2 × Vc ) ··· Formula 1

[0016] 4. A method for producing steel, comprising casting molten iron using the method for controlling the meniscus level in the mold of a continuous casting machine according to any one of 1 to 3 above. Effect of the Invention

[0017] According to the present invention, it is possible to determine whether the cause of the current molten level fluctuation is molten level fluctuation for which disturbance compensation is effective, or the result of molten level fluctuation being aggravated by disturbance compensation. Therefore, even when molten level fluctuation for which disturbance compensation is effective is not occurring, or when molten level fluctuation for which disturbance compensation is effective has occurred but then subsided, bulging-related molten level fluctuation can be effectively suppressed. The present invention can also provide a method for producing steel, which uses the above-mentioned method for controlling fluctuations in the molten iron level to cast stably molten iron. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram showing an example of equipment for carrying out the present invention. [Diagram 2] FIG. 1 is a diagram showing an example of an apparatus configuration for carrying out the present invention. [Diagram 3] FIG. 2 is a diagram showing an example of a processing flow according to the present invention. [Figure 4] 11A and 11B are diagrams showing an example of molten metal level fluctuations and SN opening output when molten metal level fluctuations for which disturbance compensation control is ineffective occur during the execution of disturbance compensation control according to the present invention. [Diagram 5] FIG. 4 is a diagram showing an example of setting a threshold value X in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. 1 and 2 are diagrams showing an example of equipment and device configurations for carrying out the present invention, in which 1 denotes molten steel, 2 denotes a ladle, 3 denotes a tundish, 4 denotes a submerged 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 surface level gauge, 10 denotes a PI controller, 11 denotes a sliding nozzle controller, 12 denotes a signal processor, 20 denotes a pinch roll, 21 denotes a secondary cooling zone, and 22 denotes a torch (cutter).

[0020] A tundish 3 is placed above the mold 5, a sliding nozzle 7 is placed at the bottom of the tundish, and a submerged nozzle 4 is placed below the sliding nozzle 7. Molten steel 1 filled in the tundish 3 is poured into the mold 5 through the sliding nozzle 7 and the submerged nozzle 4.

[0021] Next, the molten steel 1 poured into the mold 5 is cooled from the side of the mold and solidifies from the surface to form a shell 6, while being drawn downward from the mold 5 by the pinch rolls 20 to become a cast slab. At this time, the drawing speed is controlled to be approximately constant by a pinch roll control device (not shown).

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

[0023] The opening of the sliding nozzle 7 is controlled so as to balance the amount of molten steel poured into the mold and the amount withdrawn downward. This control is realized by changing the sliding nozzle opening setting using PI control so as to keep the molten steel level constant, based on a deviation signal between a signal obtained from a molten steel level gauge 9 that detects the molten steel level in the mold and a molten steel level setting.

[0024] Here, bulging-related mold level fluctuations are said to occur when the slab solidifies between the pinch rolls 20 that pull it out, bulging in a direction perpendicular to the pulling direction, and the bulged portion is pushed in by the pinch rolls 20 as the slab is pulled out, disrupting the mass balance of the molten steel in the mold. Such mold level fluctuations are characterized by periodicity, with the period being determined by the pinch roll interval and the casting speed.

[0025] The molten metal level signal is subjected to FFT analysis, and the frequency component having the maximum peak intensity in the frequency range determined from the pinch roll gap and the casting speed is determined as the bulging-related molten metal level fluctuation frequency. Since the pinch roll gap is not constant over the entire length of the continuous casting machine, the frequency determined from the pinch roll gap and the casting speed has a certain range.

[0026] In the present invention, when the bulging-type molten metal level fluctuation frequency falls within that certain range and its maximum peak intensity reaches or exceeds threshold value A, it is recognized that 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 certain range of the bulging-type mold level fluctuation frequency is the range between the minimum frequency fmin and the maximum frequency fmax. The minimum frequency fmin and the maximum frequency fmax are the minimum and maximum frequencies of possible unsteady bulging-type mold level fluctuations that are determined from the relationship between the casting speed and the roll pitch, as described in JP 2007-136480 A, and the minimum frequency fmin is expressed by "1÷(maximum roll pitch / current casting speed)", and the maximum frequency fmax is expressed by "1÷(minimum roll pitch / current casting speed)". However, in the present invention, the maximum peak intensity threshold value can be determined as either "equal to or greater than" the threshold value.

[0027] Furthermore, in the present invention, even while the disturbance compensation control output is being used for the molten metal level control, the molten metal level signal is subjected to FFT analysis, and if the maximum peak intensity exceeds threshold A and the difference between the ratio of the casting speed and the peak frequency at the time when the addition of the disturbance compensation amount to the ratio of the casting speed and the peak frequency detected by the spectrum peak detection starts exceeds threshold X, it is recognized that the disturbance compensation control is adversely affecting the molten metal level fluctuation, and the use of the disturbance compensation control output is stopped. The difference between the ratio of the casting speed and the peak frequency detected by the spectrum peak detection and the ratio of the casting speed and the peak frequency at the time when the addition of the disturbance compensation component is started to be performed is expressed by the following formula 2, where the casting speed is V1, the peak frequency is H1, the casting speed at the time when the addition of the disturbance compensation component is started to be V2, and the peak frequency is H2. (V1 / H1)-(V2 / H2)...Equation 2 As described above, in the present invention, the maximum peak intensity can be determined to be either "equal to or greater than" the threshold value.

[0028] Here, if the threshold value X is too small, when the casting speed is changed, even if bulging-related mold level fluctuations continue to occur, it is erroneously determined that the disturbance compensation control is adversely affecting the mold level fluctuations, and the use of the disturbance compensation control output is stopped.On the other hand, if the threshold value X is too large, even if the disturbance compensation control is adversely affecting the mold level fluctuations, it is not determined that the disturbance compensation control is adversely affecting the mold level fluctuations, and the use of the disturbance compensation control output is not stopped. Therefore, the threshold value X is determined based on the roll pitch d (mm) at which bulging-induced molten metal level fluctuations can occur and the acceleration of the casting speed ΔVc (m / min 2 ), casting speed: Vc (m / min), and observation time for frequency analysis: T (min), the value X (mm) is preferably given by the following formula 1: X = ( d × ΔVc × T ) ÷ ( 2 × Vc ) ··· Formula 1

[0029] Furthermore, the threshold value A may be set to a value that can distinguish between the peak value when no bulging-related mold level fluctuation occurs and the peak value when bulging-related mold level fluctuation occurs, by actually measuring the peak value in an actual machine.

[0030] In many cases where the disturbance compensation control adversely affects the molten metal level fluctuation, the frequency at which the molten metal level fluctuation has a maximum peak intensity is outside the frequency range determined by the pinch roll gap and the casting speed, and therefore the use of the disturbance compensation control output is not started again immediately after the use of the disturbance compensation control output is stopped.

[0031] In the present invention, when bulging-related mold level fluctuations occur at different bulging-related mold level fluctuation frequencies during disturbance compensation control, it is recognized that the disturbance compensation control is adversely affecting the mold level fluctuations, and the use of the disturbance compensation control output is stopped. However, if the different bulging-related mold level fluctuation frequencies are within a frequency range determined from the pinch roll gap and the casting speed, it is immediately recognized that bulging-related mold level fluctuations have occurred, and the disturbance compensation control output is used for mold level control using parameters corresponding to the different bulging-related mold level fluctuation frequencies.

[0032] 3 is a diagram showing an example of a processing flow in the present invention. First, processing is started in a processing start step (S100). Then, in the next data accumulation step (S101), a molten metal level gauge signal from the molten metal level gauge 9 in FIG. 2 is sampled at a constant cycle and accumulated 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 spectrum peak detection step (S103), the frequency at which the spectrum of the molten metal level is maximum and its peak value are detected from the data processed in the previous step. This frequency is the bulging-related molten metal level fluctuation frequency.

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

[0036] In the step of "Is the peak frequency within range?" (S105), it is determined whether the previously determined peak frequency is within the frequency range determined from the pinch roll gap and the casting speed (the certain range of the bulging-related molten metal level fluctuation frequency). If the peak frequency is outside the frequency range determined from the pinch roll gap and the casting speed, the process returns to the step of "Storage data" (S101). On the other hand, if the peak frequency is within the frequency range determined from the pinch roll gap and the casting speed, the disturbance compensation parameters are determined in the step of "Determine disturbance compensation parameters" (S106).

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

[0038] In step S111, threshold value B is used as the threshold value instead of threshold value A. The threshold value B can be set to a value lower than the peak value actually measured when there is molten metal level fluctuation as a result of the molten metal level fluctuation being aggravated by disturbance compensation. Here, it is desirable that the threshold value B be higher than the peak value of the mold level fluctuation at which the current cause of the mold level fluctuation is and at which disturbance compensation is effective. However, in reality, this is difficult because the peak value of the mold level fluctuation at which the current cause of the mold level fluctuation is and at which disturbance compensation is effective is often higher than the peak value when mold level fluctuation exists as a result of the mold level fluctuation being aggravated by disturbance compensation. Therefore, it is preferable to give priority to a value lower than the peak value when there exists molten metal level fluctuation as a result of the molten metal level fluctuation being aggravated by disturbance compensation.

[0039] In addition, for the molten metal level fluctuation for which disturbance compensation is currently effective as a cause of the molten metal level fluctuation, it is likely to be determined that the compensation is ineffective in step S111. However, in the present invention, even if it is determined that the compensation is ineffective in step S111, it is determined that the compensation is effective in the next step S112, and the disturbance compensation control continues, so there is no problem.

[0040] In the step of "Is disturbance compensation control having a negative effect?" (S112), 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 at the start of disturbance compensation control exceed a threshold value X. If the ratio does not exceed the threshold value X, it is determined that disturbance compensation control is effective, and the process returns to the step of "Data accumulation (S108)" while continuing such disturbance compensation control. On the other hand, if the ratio exceeds the threshold value X, it is determined that the disturbance compensation control is having a negative effect on the molten metal level fluctuation, and the disturbance compensation control is stopped in the step of "Stop disturbance compensation control" (S113), and the process returns to the step of "Data accumulation (S101)".

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

[0042] Furthermore, when a steel manufacturing method in which molten iron is cast using such a method for controlling the molten steel level in the mold of a continuous casting machine is adopted, splashing of molten steel outside the facility during casting can be suppressed, the transportation of steel can be stabilized, and steel can be produced with a high yield.

[0043] In addition, when the mold level fluctuation is large, there is usually concern about surface defects due to powder entrapment, etc., so it becomes necessary to perform surface machining of several mm at the slab stage. In other words, if bulging-related mold level fluctuation is suppressed, the concern about powder entrapment can be reduced and the need to perform surface machining of several mm at the slab stage can be reduced, leading to improved yields.

[0044] In the method for producing steel according to the present invention, any item not described in this specification can be performed by a conventional method. One example of such a method for producing steel is a method of casting molten iron using equipment such as that shown in FIG. That is, molten steel 1 is temporarily stored in a tundish 3, which maintains a constant casting speed, promotes the floating of inclusions, and performs continuous casting, from a ladle 2, and then the molten steel is guided through a nozzle into a mold 5 made of a copper alloy and equipped with internal water cooling and oscillation functions (up and down vibration). A level gauge for the molten steel is installed in the mold 5, and the nozzle opening is usually controlled so that the height of the molten steel remains constant. After leaving the mold, the slab is cooled by spray water (high-velocity water jet) in a secondary cooling zone 21, pulled out by pinch rolls 20, and the completely solidified slab is cut by a torch 22 to produce slabs. 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] FIG. 4 shows an example of the molten metal level fluctuation and the SN opening output when molten metal level fluctuations for which disturbance compensation control is ineffective occur while disturbance compensation control is being performed in an actual machine using the molten metal level control method in the mold of a continuous casting machine of the present invention. In the data accumulation step (S101), the molten metal level gauge signal from the molten metal level gauge 9 in FIG. 2 was sampled at a constant period (sampling period: 0.1 seconds, sampling period: 102.4 seconds, so that 0.1-second pitch data was subjected to FFT analysis at 1024 points) and accumulated as data. In addition, thresholds A and B were set to the peak values ​​obtained by FFT analysis of the 1024 points of molten metal level (% *1%=1.5mm) data. The peak value of threshold A, which is the control start condition, was set to 1500, and the peak value of threshold B, which is the control stop condition, was set to 1000.

[0046] As shown in Fig. 4, the melt level fluctuations increased from around 8060 seconds, and at around 8120 seconds, it was determined that the disturbance compensation control was having a detrimental effect and the disturbance compensation control was stopped, after which the melt level fluctuations decreased.

[0047] Figure 5 shows an example of setting the threshold value X. The horizontal axis of the graph is the ratio of the casting speed to the peak frequency when the disturbance compensation control started, and the vertical axis is the ratio of the current casting speed to the peak frequency. The dashed line is the line where the ratio of the casting speed to the peak frequency when the disturbance compensation control started is equal to the ratio of the current casting speed to the peak frequency, and any point on the vertical axis that is further away from this dashed line than the distance of the threshold value X is judged to be causing an adverse effect due to the disturbance compensation control, and the disturbance compensation control is stopped.

[0048] In this setting example, the threshold value X is set as follows: roll pitch d=380 mm, and the acceleration of the casting speed ΔVc=0.3 m / min. 2 The calculation was made using the following equation 3, assuming that the casting speed Vc was 1.5 m / min and the observation time for frequency analysis T was 1.7 min. X = (380 × 0.3 × 1.7) ÷ (2 × 1.5) ≒ 65 mm...Formula 3

[0049] In Fig. 5, the solid line indicates the threshold value X = 65 mm. In Fig. 5, the ◯ plots are plots immediately after the casting speed is decelerated while bulging-related mold level fluctuations are occurring in an actual casting machine. On the other hand, the × plots are plots when disturbance compensation control adversely affects mold level fluctuations in an actual casting machine.

[0050] As shown in FIG. 5, the solid line indicating the threshold value X=65 mm can separate the ◯ plots from the × plots. [Explanation of symbols]

[0051] 1. Molten Steel 2 ladle 3 Tundish 4 Submerged Nozzle 5 Mold 6. Shell 7 Sliding Nozzle 8 Water surface 9. Water Level Gauge 10 PI Controller 11 Sliding nozzle control device 12 Signal Processing Device 20 Pinch Roll 21 Secondary cooling zone 22 Torch (cutter)

Claims

1. A method for controlling the level of the molten metal surface in a mold of a continuous casting machine by detecting the level of the molten metal surface in the mold, outputting a sliding nozzle opening command provided in a tundish by PI control based on a signal of the detected level of the molten metal surface, and adding a bulging disturbance compensation amount to the output to adjust the amount of the molten metal flowing into the mold, The method for controlling a molten metal surface level in a mold of a continuous casting machine comprises determining whether to stop or continue adding the disturbance compensation amount based on detection of a spectrum peak by frequency analysis of the molten metal surface level signal.

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

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

4. A method for producing steel, comprising the steps of: casting molten iron using the method for controlling the molten iron level in a mold of a continuous casting machine according to any one of claims 1 to 3.

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