JP2007136480A - Method for detecting unsteady bulging in continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately detect an unsteady bulging with a simple treating constitution and a low cost means by analyzing only a signal of molten metal surface level value. <P>SOLUTION: When an amplitude value in the molten metal surface level value in a mold for continuous caster exceeds a first threshold value and an amplitude value, which is obtained based on the roll pitch in the cast slab supporting rolls and the casting speed in a frequency characteristic in the above molten metal surface level value in the frequency range in the molten metal surface level variation to be generated, exceeds a second threshold value, it is judged that this is the unsteady bulging. In such way, the unsteady bulging can be detected with the high accuracy and low cost facility. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for detecting unsteady bulging of a slab in a continuous casting machine.

  In continuous casting of molten steel, there may be a long period of fluctuation in the mold surface in the mold. At this time, the cycle of the molten metal surface in the mold is several seconds to several tens of seconds, and the amplitude may reach several millimeters to several tens of millimeters. The cause of the long-term fluctuation of the molten metal surface can be considered as follows.

  That is, as schematically shown in FIG. 1, the relationship between the bulging of the solidified shell and the fluctuation of the molten metal surface in the mold, the solidified shell 10 is bulged in a convex shape between the rolls of the slab support roll 8 by static iron pressure. When the part (convex part) reaches the next slab support roll 8 by pulling out the slab, the bulging part does not return along the slab support roll 8 and part of the solidified shell 10 is deformed. As a result, a phenomenon occurs in which the slab is pushed back toward the unsolidified portion 11 while leaving the bulging portion. Such a bulging phenomenon is generally called “unsteady bulging”. If this unsteady bulging occurs in a continuous section of the same roll pitch (interval between adjacent slab support rolls) and at each roll part, the solidified shells of the slab are unsolidified over a wide area at once. The movement of being pushed back to the part side and, conversely, being pushed out between the rolls, periodically occurs as the slab is pulled out. This is considered to cause the above-described long-term fluctuation of the molten metal surface. Hereinafter, this hot water surface fluctuation is referred to as “unsteady bulging hot water surface fluctuation”. In FIG. 1, 2 is a mold, 8 is a slab support roll, 10 is a solidified shell, 11 is an unsolidified portion, and 12 is a molten steel surface.

  Unsteady bulging If the fluctuation of the molten metal surface is so severe that it is out of the controllable range of the molten metal surface control system, there is a risk of slab quality deterioration or breakout due to the inclusion of mold powder added to the mold. In order to accurately detect the occurrence of bulging and suppress unsteady bulging, it is necessary to operate at a reduced casting speed (also referred to as “slab drawing speed”).

In order to detect this unsteady bulging, for example, there is a method of detecting by measuring a molten metal surface level in a mold with an eddy current type level sensor or the like and evaluating a period when the fluctuation amount exceeds a predetermined threshold. . A technique for further increasing detection accuracy is disclosed in Patent Document 1. The detection method according to Patent Document 1 uses the fact that the period of unsteady bulging level fluctuation is determined by the roll pitch and casting speed of the secondary cooling zone, and the periodicity of the pinch roll motor current value and the level level. This is a detection method in which unsteady bulging has occurred when the frequency of any one of the value and the molten metal level control signal value or the frequency of the combined value of both coincides.
Japanese Patent Laid-Open No. 11-170021

  However, the fluctuation of the molten metal surface in the mold is not caused only by unsteady bulging, but is also caused by fluctuations in the amount of molten steel injected into the mold, fluctuations in the casting speed, and the like. That is, since the molten metal level fluctuation in the mold occurs as a composite result of a plurality of factors, it is detected that unsteady bulging has occurred from the cycle when the fluctuation level of the molten metal level in the mold exceeds a predetermined threshold value. This method cannot be detected with high accuracy.

  In addition, the method disclosed in Patent Document 1 is a method of taking a pinch roll motor current value and a hot water surface control signal value in addition to the hot water surface level signal and making a determination based on a plurality of signals, although the accuracy is improved. A device for capturing signals other than the molten metal level is required, and there is a problem that the processing device is complicated and the equipment cost increases.

  The present invention has been made in view of such circumstances, the purpose of which is to analyze only the signal of the level value in the mold, by a simple processing configuration and low-cost means, To provide a method for detecting unsteady bulging with high accuracy.

  The method for detecting unsteady bulging of a continuous cast slab according to the first invention for solving the above-mentioned problem is that the amplitude value of the molten metal surface level value in the mold of the continuous casting machine exceeds the first threshold value, and the molten metal surface In the frequency characteristics of the level value, when the amplitude in the frequency range of the level fluctuation that can be obtained, which is obtained based on the roll pitch and casting speed of the slab support roll, exceeds the second threshold value, unsteady bulging It is characterized by determining.

  According to a second aspect of the present invention, there is provided a method for detecting unsteady bulging of a continuous cast slab according to the first aspect, wherein the unsteady bulging is generated based on a frequency corresponding to an amplitude exceeding the second threshold. The position of the single support roll is obtained.

  According to the present invention, the amplitude of the fluctuation level of the molten metal level value signal exceeds the first threshold value, and the frequency characteristic of the molten metal level value is obtained based on the roll pitch of the slab support roll and the casting speed. The unsteady bulging can be detected with low-cost equipment and high accuracy because the unsteady bulging is determined when the amplitude in the frequency range of the generated molten metal level fluctuation exceeds the second threshold value. . In addition, by inputting the result of unsteady bulging detection as an operation condition, it is possible to expect an effect that an action such as a reduction in casting speed can be automatically performed without delay. Furthermore, according to the present invention, it is possible to estimate not only the occurrence of unsteady bulging but also the position of the slab support roll where unsteady bulging occurs, so the amount of secondary cooling water is changed, etc. Can be performed automatically, shortening the time to recover from unsteady bulging, reducing the entrainment of mold powder due to unsteady bulging surface variations, and reducing slab quality defects be able to.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 2 is a schematic side view of the periphery of the mold of the continuous slab casting machine used in carrying out the present invention.

  In FIG. 2, a tundish 1 is disposed at a predetermined position above the mold 2 of the slab continuous casting machine, and a sliding nozzle 3 and an immersion nozzle 4 are disposed at the bottom of the tundish 1. The molten steel 9 once retained is injected into the mold 2 through the sliding nozzle 3 and the immersion nozzle 4. The sliding nozzle 3 has a three-plate structure including an upper fixing plate 3a, a sliding plate 3b, and a lower fixing plate 3c. The sliding plate 3b is connected to the actuator 7, and the sliding plate 3b is connected to the actuator 7. By sliding with the upper fixing plate 3a and the lower fixing plate 3c in close contact with each other, the opening degree of the sliding nozzle 3 is increased and decreased, and the outflow amount of the molten steel 9 from the tundish 1 to the mold 2 is controlled. It is like that.

  In this case, the actuator 7 is actuated by a signal input from a hot water level control device 6 such as a PI control device for adjusting the opening degree of the sliding nozzle 3. An eddy current level sensor 5 for measuring the position of the molten steel surface 12 in the mold 2 is disposed above the molten steel surface 12 in the mold. It is input to the level control device 6. That is, the hot water surface level control device 6 continuously inputs a deviation signal between the hot water surface level detection signal obtained from the eddy current type level sensor 5 and a preset hot water surface level setting value. Based on this, an opening degree command for the sliding nozzle 3 is output to the actuator 7 so that the position of the molten steel surface 12 in the mold 2 is constant.

  Further, the measurement signal of the eddy current level sensor 5 is input to an unsteady bulging detection device 14 for detecting unsteady bulging. The unsteady bulging detection device 14 inputs a measurement signal of the eddy current type level sensor 5, and also inputs a signal input unit 15 for inputting a signal from the process computer 20 for managing the operation conditions of the continuous casting machine, An amplitude calculation unit 16 for calculating the amplitude of the measurement signal of the eddy current type level sensor 5 and an FFT (fast Fourier transform) calculation unit 17 for performing frequency analysis of the measurement signal of the input eddy current type level sensor 5 And a determination processing unit 18 for determining the presence or absence of occurrence of unsteady bulging based on the calculation results of the amplitude calculation unit 16 and the FFT calculation unit 17, and a determination result of the determination processing unit 18 for screen display or printing output. And a determination result output unit 19.

  The molten steel 9 injected into the tundish 1 from a ladle (not shown) is injected into the mold 2 through the immersion nozzle 4 in accordance with the opening determined by the sliding nozzle 3 installed at the bottom of the tundish 1. . A mold powder 13 serving as a heat retaining agent, an antioxidant, a lubricant with the mold 2 and the like is added to the molten steel surface 12 in the mold. The molten steel 9 poured into the mold 2 is cooled by the mold 2 and solidifies at the contact surface with the mold 2 to form a solidified shell 10, and an outer shell is the solidified shell 10 and the inside is an unsolidified portion 11. Is pulled out below the mold 2 by the driving force of the pinch roll while being supported by the slab support roll 8 comprising a guide roll and a pinch roll.

  In this case, the amount of molten steel injected into the mold 2 is set according to the opening degree of the sliding nozzle 3 as described above. That is, the actuator 7 adjusts the opening of the sliding nozzle 3 according to the opening command of the sliding nozzle 3 output from the molten metal level control device 6, and the amount of molten steel inflow from the tundish 1 is adjusted accordingly. That is, the molten metal level control device 6 generates a deviation signal between the molten metal level detection signal obtained from the eddy current level sensor 5 that detects the molten metal level of the molten steel in the mold and the preset molten metal level setting value. Based on this deviation signal, an opening degree command for the sliding nozzle 3 is output to the actuator 7 so that the molten steel surface level of the molten steel in the mold becomes constant.

  In such continuous casting operation, the unsteady bulging detection device 14 detects the presence or absence of occurrence of unsteady bulging based on the surface level value input from the eddy current level sensor 5. The unsteady bulging surface level fluctuation is caused by bulging the solidified shell 10 in a direction perpendicular to the drawing direction between the slab support rolls 8 such as a guide roll and a pinch roll for supporting the slab. Since the fluctuation occurs and vibration occurs, the mass balance of the molten steel in the mold is lost, and the fluctuation has a relatively long periodicity as a feature of the fluctuation. Specifically, it has a period determined by an interval between adjacent slab support rolls 8, that is, a roll pitch and a casting speed. That is, it has a period determined by a value obtained by dividing the roll pitch by the casting speed (roll pitch / casting speed).

  The procedure for determining unsteady bulging in the unsteady bulging detection device 14 will be described based on the processing flow shown in FIG.

  First, the operation conditions of continuous casting are input from the process computer 20 to the signal input unit 15 (step 1). In the calculation process of the present invention, data on the casting speed and the roll pitch of the slab support roll 8 is necessary. Therefore, at least these pieces of information are inputted and updated every time the operating conditions are changed.

  The signal input unit 15 inputs a detection signal of the molten metal level value input from the eddy current level sensor 5 at a predetermined sampling period tr (for example, 60 seconds) at a predetermined sampling period ts (for example, 1 second period). A / D (analog / digital) conversion and the signal is stored in a storage medium such as a memory (step 2).

  The sampling period ts needs to be set to an interval necessary for grasping the fluctuation state from the molten metal surface fluctuation period (or frequency) assumed from the relationship between the roll pitch and the casting speed. For example, when the minimum roll pitch of the slab support roll 8 of the continuous casting machine is 150 mm and the maximum casting speed is 3 m / min, the fluctuation level of the molten metal surface is “0.150 m (roll pitch) ÷ 3 m / min (casting speed). ) = 3 seconds ”, in order to capture the frequency variation, the sampling period ts may be set to a time that is ½ or less of the sampling theorem, that is, 1.5 seconds or less.

  In addition, the sampling period tr needs to capture data of the longest fluctuation period obtained from the relationship between the roll pitch and the casting speed. In addition, it is preferable to set in consideration of the number of data required for data processing of frequency analysis described later, a memory capacity for storing data, and the like.

  For example, when the maximum value of the roll pitch is 250 mm and the minimum value of the casting speed is 0.3 m / min, the fluctuation period is 50 seconds (0.25 m / 0.3 m / min = 50 seconds). For example, if the sampling period ts is 1 second, 50 points of one period or more, that is, a sampling period of 50 seconds is required. Further, considering the ease of the FFT calculation and the unit of the memory capacity, a power of 2 is preferable. Therefore, it is sufficient to set 64 points that are 50 or more and the minimum value of the power of 2.

  Then, the amplitude calculation unit 16 determines whether the amplitude value of the fluctuation amount is in the “normal” range or the “abnormal” range based on the input level signal (step 3).

Figure 4 is a diagram showing the concept of normality / abnormality determination, as shown in FIG. 4, on the basis of the data of the time variation of the molten metal surface level values, whether the amplitude is greater than the first threshold value X _th Judge with. In Figure 4, molten metal surface level setting value, that is based on the melt surface level X ₀ of the reference, the threshold X _th above and below provided, the range of "X ₀ -X _th" of "X ₀ + X _th" input The molten metal level value fluctuates to “normal” when the measured molten metal level value is included, or otherwise, that is, a range smaller than “X ₀ −X _th ” or a range larger than “X ₀ + X _th ”. Sometimes it is determined that there is a possibility of “abnormal”, that is, unsteady bulging. In the description of FIG. 4, the mold upper end position in the equipment specification is set to 0 mm position, and the lower side is set to a minus (negative) position. Therefore, X ₀ is a negative value, while X _th is a threshold value. Therefore, a positive (positive) value is displayed. Further, in the example of the _{molten metal} level shown in FIG. 4, since the amplitude exceeds the threshold value Xth, it is determined as abnormal.

  If it is determined as “abnormal” in step 3, the process moves to the FFT calculation unit 17. On the other hand, if it is determined as “normal”, the process moves to the determination processing unit 18, and unsteady bulging occurs. It is determined that there is none (step 8).

When it is determined as “abnormal” in step 3, the FFT calculation unit 17 performs frequency analysis by performing FFT (fast Fourier transform) processing on the molten metal level signal input in step 2 (step 4). The data to be subjected to the FFT processing is assumed to be fluctuation component data (deviation from X ₀ ) from the molten metal level X ₀ as a reference of the molten metal level in Step 3. That is, it is set to a value obtained by adding X ₀ that is a reference value of the molten metal surface level to actually inputted measurement value data (data output with the upper end of the mold in FIG. 4 as 0 mm).

  FIG. 5 is an example of data of the frequency characteristics of the amplitude corresponding to the frequency obtained by the frequency analysis in step 4. First, the determination processing unit 18 determines a frequency range Δf that can be generated as a fluctuating frequency of the molten metal surface level from the relationship between the roll pitch input in Step 1 and the casting speed currently being operated (range of fmin to fmax in FIG. 5). ) Is determined, and it is determined whether or not there is a frequency within the frequency range Δf in the frequency analysis data obtained in step 4 input from the FFT operation unit 17 (step 5). The frequency existing within the frequency range Δf is determined as unsteady bulging hot water level fluctuation. By this processing, the peak value other than the unsteady bulging level fluctuation based on the relationship between the casting speed and the roll pitch (from the minimum value to the maximum value) is excluded, and only the FFT peak value of the unsteady bulging level fluctuation is obtained. Extracted.

  The minimum frequency fmin and the maximum frequency fmax are the minimum frequency and the maximum frequency of the unsteady bulging surface level that can be determined, which are determined from the relationship between the casting speed and the roll pitch. The minimum frequency fmin is “1 ÷ (maximum Roll pitch / current casting speed) ”, and the maximum frequency fmax is represented by“ 1 ÷ (minimum roll pitch / current casting speed) ”.

Then, the determination processing unit 18 determines whether or not the amplitude value exceeds the second threshold Y _th within the frequency range Δf that can be generated (step 6). As shown in FIG. 5, when there is a peak exceeding the threshold _Yth within the frequency range Δf, it is determined that unsteady bulging has occurred (step 7). If there is no peak exceeding the threshold value Y _th within the frequency range Δf, it is determined that no generation of unsteady bulging (Step 8). The threshold value Y _th set here may be set based on the amplitude when unsteady bulging occurs by analyzing past operation data.

The determination result of the determination processing unit 18 is sent to the determination result output unit 19, and the determination result output unit 19 outputs that fact (step 9). Further, the frequency of the amplitude value exceeding the threshold value Y _th is determined based on the casting speed to which roll slab support roll 8 corresponds, and the roll pitch value and the slab support roll 8 of that roll pitch are determined. Also displays where the is. For example, an alarm sound or an alarm is displayed, and “unsteady bulging detection: roll pitch 200 mm” is displayed.

  In this case, data indicating the frequency of unsteady bulging that may occur based on the relationship between the roll pitch of the slab support roll 8 and the casting speed from directly under the mold of the continuous casting machine to the solidification completion position of the secondary cooling zone. By creating a table in advance, it is possible to quickly identify the location where unsteady bulging occurs. By doing in this way, it becomes possible to perform the handling process of unsteady bulging accurately and quickly, for example, by increasing the amount of secondary cooling water at that part.

  Thus, according to the present invention, the amplitude of the fluctuation amount of the hot water level value signal exceeds the first threshold value, and the frequency characteristics of the hot water surface level fluctuation obtained by frequency analysis of the hot water surface level value, When the amplitude in the frequency range of the level fluctuation (unsteady bulging) that can be generated, which is obtained based on the roll pitch and casting speed of the slab support roll 8, exceeds the second threshold, unsteady bulging Since it is determined, unsteady bulging can be detected with high accuracy and low cost equipment.

  In addition, by inputting the result of unsteady bulging detection as an operation condition, it is possible to expect an effect that an action such as a reduction in casting speed can be automatically performed without delay. Furthermore, according to the present invention, not only the occurrence of unsteady bulging but also the position of the slab support roll 8 where unsteady bulging occurs can be estimated, so the amount of secondary cooling water is changed. Can be performed automatically, shortening the time to recover from unsteady bulging, reducing the entrainment of mold powder caused by unsteady bulging hot water surface fluctuations, and reducing slab quality defects It becomes possible to do.

It is a figure which shows typically the relationship between the bulging of a solidification shell, and a molten metal surface fluctuation | variation. It is the side schematic diagram of the mold peripheral part of the slab continuous casting machine which implemented this invention. It is a figure which shows the determination processing flow of unsteady bulging in an unsteady bulging detection apparatus. Based on the first threshold value X _th, a diagram showing the concept of a determination of normality / abnormality. It is a figure which shows the example of the data which frequency-analyzed the amplitude.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tundish 2 Mold 3 Sliding nozzle 4 Immersion nozzle 5 Eddy current level sensor 6 Molten surface level control device 7 Actuator 8 Slab support roll 9 Molten steel 10 Solidified shell 11 Unsolidified part 12 Molten steel surface 13 Mold powder 14 Unsteady bulging detection Device 15 Signal input unit 16 Amplitude calculation unit 17 FFT calculation unit 18 Judgment processing unit 19 Judgment result output unit 20 Process computer

Claims (2)

  Occurrence that the amplitude value of the molten metal surface level value in the mold of the continuous casting machine exceeds the first threshold and is determined based on the roll pitch of the slab support roll and the casting speed in the frequency characteristics of the molten metal surface level value. A method for detecting unsteady bulging of a continuous cast slab, characterized by determining unsteady bulging when an amplitude in a frequency range of possible molten metal level fluctuation exceeds a second threshold value.   2. The continuous cast slab according to claim 1, wherein the position of the slab support roll in which unsteady bulging occurs is determined based on a frequency corresponding to an amplitude exceeding the second threshold. Unsteady bulging detection method.

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